US00925791 OB2

(12) United States Patent (10) Patent No.: US 9.257,910 B2

Lindberg-Poulsen et al. (45) Date of Patent: Feb. 9, 2016
(54) ISOLATED BOOST FLYBACK POWER (56) References Cited
CONVERTER
U.S. PATENT DOCUMENTS
(75) Inventors: Kristian Lindberg-Poulsen,
Copenhagen N (DK); Ziwei Ouyang, 4,339,792 A * 7/1982 Yasumura ............... GOSF 1,325
323,248
Holte (DK); Gökan Sen, Holte (DK) 4,864,478 A 9, 1989 Bloom
(73) Assignee: Danmarks Tekniske Universitet, Kgs. 4,876,638 A * 10/1989 Silva ....................... HO1F 29, 14
323,250
Lyngby (DK) 5,208,739 A 5/1993 Sturgeon
5,436,818 A 7, 1995 Barthold
(*) Notice: Subject to any disclaimer, the term of this 6,496,389 B1* 12/2002 Yasumura ........... HO2M 1/4258
patent is extended or adjusted under 35 363.21.02
U.S.C. 154(b) by 208 days. 7,034,647 B2 * 4/2006 Yan ......................... HO1F 27/38
336,178
(21) Appl. No.: 14/131,034 2004/0218404 A1 11, 2004 Yan
2009,0196073 A1* 8, 2009 Nakahori ................ HO2M 3/28
363, 17
(22) PCT Filed: Jun. 11, 2012 2009, 0231885 A1* 9, 2009 Won ........................ HO1F 38.10
363, 17
(86). PCT No.: PCT/EP2012/06102O 2010/0067263 A1* 3/2010 Qian ..................... HO2M 3/285
363,21.12
S371 (c)(1), 2011/014.9613 A1 6/2011 Lanni
(2), (4) Date: Apr. 28, 2014 FOREIGN PATENT DOCUMENTS
(87) PCT Pub. No.: WO2013/004453 EP 1659678 A2 8, 2006
PCT Pub. Date: Jan. 10, 2013 OTHER PUBLICATIONS

(65) Prior Publication Data International Search Report and Written Opinion for PCT/EP2012/
061020, dated Apr. 29, 2013.
US 2014/0241012 A1 Aug. 28, 2014 * cited by examiner
Primary Examiner — Gary L. Laxton
Related U.S. Application Data (74) Attorney, Agent, or Firm — Peter A. Nieves; Sheehan
(60) Provisional application No. 61/505,205, filed on Jul. 7, Phinney Bass + Green PA
2011. (57) ABSTRACT
(30) Foreign Application Priority Data An isolated boost power converter comprises a magnetically
permeable multi-legged core (102) comprising first and sec
Jul. 7, 2011 (EP) ..................................... 11 172997 ond outer legs (132:136) and a center leg (134) having an air
gap (138) arranged therein. A boost inductor (Lboost) is
wound around the center leg (134) or the first and second
outer legs (132: 136) of the magnetically permeable multi
(51) Int. Cl. legged core (102). The boost inductor (Lboost) is electrically
HO2M3/335 (2006.01) coupled between an input terminal (104) of the boost con
HO2M 7/5387 (2007.01) verter and a transistor driver (106) to be alternatingly charged
HO2M I/OO (2007.01) and discharged with magnetic energy. A first and second
series connected secondary transformer windings (SW1;
(52) U.S. Cl. SW2) with a center-tap (116) arranged in-between are wound
CPC .......... H02M3/335 (2013.01); H02M 3/33523 around the first and second outer legs (132:136), respectively,
(2013.01); H02M 3/33553 (2013.01); H02M of the magnetically permeable multi-legged core (102). In a
2001/0064 (2013.01) first discharge state, the magnetic energy stored in the boost
(58) Field of Classification Search inductor (Lboost) is discharged by directing a discharge cur
CPC. H02M 3/3376; H02M 7/5387: G05F 1/325:
rent from the boost inductor through a primary transformer
GO5F 1/33
winding (PW1; PW2) and in a second discharge state, the
magnetic energy stored in the boost inductor (Lboost) is
USPC ............ 363/15, 16, 17, 95, 98: 323/250, 251, discharged by discharging a magnetic flux through the first
323/362 and second secondary transformer windings (SW1; SW2).
See application file for complete search history. 14 Claims, 8 Drawing Sheets
U.S. Patent Feb. 9, 2016 Sheet 1 of 8 US 9,257,910 B2

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U.S. Patent Feb. 9, 2016 Sheet 2 of 8 US 9,257,910 B2

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U.S. Patent Feb. 9, 2016 Sheet 6 of 8 US 9,257,910 B2

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U.S. Patent Feb. 9, 2016 Sheet 8 of 8 US 9,257,910 B2

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 US 9,257,910 B2
1. 2
SOLATED BOOST FLYBACK POWER input terminal and a driver to be alternatingly charged and
CONVERTER discharged with magnetic energy. The driver has a driver
output coupled to a primary transformer winding wound
The present invention relates to an isolated boost power around the first and second outer legs of the magnetically
converter comprising a magnetically permeable multi-legged permeable transformer core and the driver is configured to
core comprising first and second outer legs and a center leg generate a primary Voltage to the primary transformer wind
having an air gap arranged therein. A boost inductor is wound ing in accordance with a driver control signal. First and sec
around the center leg or the first and second outer legs of the ond series connected secondary transformer windings with a
magnetically permeable multi-legged core. The boost induc center-tap arranged in-between are wound around the first
tor is electrically coupled between an input terminal of the 10 and second outer legs, respectively, of the magnetically per
boost converter and a transistor driver to be alternatingly meable multi-legged core and a rectification circuit is coupled
charged and discharged with magnetic energy. A first and to respective outputs of the first and second secondary trans
second series connected secondary transformer windings former windings to provide a rectified converter output volt
with a center-tap arranged in-between are wound around the age, V. In a first discharge state, the magnetic energy stored
first and second legs, respectively, of the magnetically per 15 in the boost inductor is discharged by directing a discharge
meable multi-legged core. In a first discharge State, the mag current from the boost inductor through the primary trans
netic energy stored in the boost inductor is discharged by former winding. In a second discharge state, the magnetic
directing a discharge current from the boost inductor through energy stored in the boost inductor is discharged by discharg
a primary transformer winding and in a second discharge ing a magnetic flux through the first and second secondary
state, the magnetic energy stored in the boost inductor is transformer windings.
discharged by discharging a magnetic flux through the first The ability provided by the present isolated boost power
and second secondary transformer windings. In this manner, converter to discharge magnetic energy stored in the boost
the first and second secondary transformer windings may inductor through the first and second secondary transformer
replace the traditional separate flyback winding utilized for windings provides a novel solution to start-up of isolated
start-up purposes of isolated boost power converters. 25 boost power converters/DC-DC converters allowing these to
operate below 50% duty cycle, D, of a Pulse Width Modulated
BACKGROUND OF THE INVENTION (PWM) driver control signal. Consequently, the output volt
age range at V can be extended down to Zero without
Isolated boost power converters are generally accepted as a utilizing the traditional separate flyback winding because the
highly efficient converter topology or architecture for high 30 first and second secondary transformer windings provide the
power converters with low input Voltage and high output functionality of the separate flyback winding of prior art
voltage. Isolated boost power converters are very useful for boost power converters. The omission of the traditional sepa
DC-DC voltage conversion in a diverse range of applications rate flyback winding leads to a significant simplification of
Such as fuel cell converters, electric vehicles applications and electric and magnetic circuit design, reduces component
avionic applications. However, a disadvantage of prior art 35 count, improves reliability, reduces the price and size of the
isolated boost power converters is the need for a so-called power converter and decreases manufacturing costs. Further
flyback winding during a start-up phase or state of the power more, power conversion efficiency during start-up, and for
converter. During start-up, a duty cycle of a Pulse Width operation below 50% duty cycle of the PWM driver control
Modulated (PWM) control signal applied to a driver circuit signal, is improved without affecting normal boost operation
must be ramped-up slowly to avoid excessive in-rush cur 40 because copper material is not wasted on the traditional fly
rents. During ramp-up of the duty cycle, it starts at a value back winding. It is also noticeable that secondary transformer
much less than 0.5 which means that the driver circuit is winding normally is designed for low winding resistance Such
placed in an open or cut-off state during a cycle of the PWM that the additional flyback winding functionality requires no
control signal without any low impedance path to a positive or modification of an existing secondary transformer winding.
negative input Voltage terminal or rail. This situation leads to 45 The normal low resistance of existing secondary transformer
excessive Voltage spikes across the boost inductor(s) which windings also means that its use as flyback winding in accor
spikes may exceed the rated break-down Voltage of semicon dance with the present invention can increase power effi
ductor devices, such as MOS transistors, of the driver circuit ciency compared with the traditional separate flyback wind
so as to destroy these. This problem has previously been ing. Furthermore, the second discharge state also allows
addressed by adding a flyback winding and a flyback diode to 50 stored magnetic energy in the boost inductor to be discharged
the isolated power converter providing a discharge path for or dissipated during error states of the isolated boost con
energy stored in the boost inductor. However, the addition of Verter which e.g. may be caused by Switching errors in the
a flyback winding has numerous drawbacks as the flyback driver or an output short circuit condition.
winding is a separate power transferring element that is rela The skilled person will understand that the term "isolated
tively costly, adds to size and increases component count of 55 does not imply that the input and output sides of the present
the boost power converter. isolated boost power converter necessarily are galvanically
isolated by the magnetically permeable multi-legged core
SUMMARY OF THE INVENTION even though they may be so in numerous embodiments of the
invention. Respective negative terminals or rails of the recti
A first aspect of the invention relates to an isolated boost 60 fied output Voltage V, and the input voltage V, may be
power converter comprising a magnetically permeable multi electrically coupled to each other, for example through a
legged core comprising first and second outer legs and a shared GND connection Such that a galvanic connection is
center leg having an air gap arranged therein. An input termi established between the input/primary and output/secondary
nal is adapted for receipt of an input Voltage, V. A boost sides of the isolated boost power converter.
inductor is wound around the center leg or the first and second 65 According to a preferred embodiment, during the second
outer legs of the magnetically permeable multi-legged core discharge State the first and second secondary transformer
where the boost inductor is electrically coupled between the windings are coupled in parallel from the center-tap arranged
 US 9,257,910 B2
3 4
between first and second series connected secondary trans The number of windings of the first and second second
former windings to the rectified converter output Voltage, ary transformer windings is preferably Substantially
V. This allows discharge current to be directed to the rec identical to allow effective decupling between the boost
tified converter output voltage by both half-windings so as to inductor and the first and second secondary transformer
minimize conductive losses in the secondary transformer windings in the first discharge state.
winding. The magnetic flux flowing through the first and As previously mentioned the first and second secondary
second secondary transformer windings, generated by the transformer windings are preferably configured to discharge
magnetic energy stored in the boost inductor, is converted to
respective discharge currents flowing through the first and the magnetic energy stored in the boost inductor by Supplying
second secondary half windings SW and SW. Thereby, 10
a discharge current to the rectified converter output Voltage,
power or energy is transferred to the rectified converter output V, so as to transfer energy to the output. In this scheme, the
Voltage so as to control the latter when the output Voltage is first and second secondary transformer windings actjointly as
below a minimum Voltage required for normal boost mode a flyback winding.
operation. According to an advantageous embodiment of the inven
The present isolated boost converter is preferably config 15 tion, the magnetically permeable multi-legged core com
ured such that the second discharge state is automatically prises:
entered in response to a reversal of magnetic flux rate in the a center leg, having an air gap arranged therein,
boost inductor. The reversal of magnetic flux rate may be
caused by the driver entering a non-conducting or OFF-state a first outer leg and a second outer leg; The boost inductor
for example when a duty cycle of a pulse width modulated is magnetically coupled to the center leg to store the
driver control signal is less than 0.5. Under these conditions, magnetic energy therein and the first and second second
a resulting drop in charging current flowing into the boost ary transformer windings are wound around the first and
inductor will cause the reversal of the magnetic flux rate. The second outer legs, respectively. This topology of the
driver may for example enter the non-conducting state when magnetically permeable multi-legged core may com
all transistors of the driver simultaneously are in non-con prise a conventional El core. The air gap is well-suited
ducting states. The driver may comprise a half-bridge or an 25 for storage of the magnetic energy due to its high reluc
H-bridge with two or four MOS transistors, respectively. The tance or low magnetic permeability and preferably has a
H-bridge or full-bridge transistor driver may have first and height between 0.1 mm and 10 mm. In one embodiment,
second complementary driver outputs coupled to respective the boost inductor is wound around the center leg while
ends of the primary transformer winding. When all transistors in other embodiments windings of the boost inductor is
of the half-bridge or full-bridge transistor driver are placed in 30 split into two series connected half-windings wound
non-conducting states by the pulse width modulated driver around respective ones of the first and second outer legs.
control signal, a voltage across the driver rapidly rises to a In the latter embodiment, the boost inductor is magneti
destructive level due the continued effort of the boost inductor cally coupled to the air gap by a suitable magnetically
to force current towards the driver. This undesired rise of permeable structure of the magnetically permeable
boost inductor Voltage takes place during start-up of the iso 35 multi-legged core.
lated boost converter where the duty cycle D of the pulse In yet another embodiment of the invention which com
width modulated driver control signal must be initialized to a prises the center leg with the air gap and the first and second
value below 0.5, preferably close to zero, to avoid large in outer legs, the primary transformer winding comprises first
rush currents. However, the automatic entry into the second and second series connected half-windings wound around the
discharge state provided by this embodiment eliminates the 40 first and second outer legs, respectively, of the magnetically
generation of destructive boost inductor Voltage spikes by permeable multi-legged core. Preferably, the first and second
discharging the magnetic energy stored in the boost inductor series connected half-windings have an identical number of
as the discharge currents running through the first and second windings and the first and second series connected secondary
secondary transformer windings. The automatic entry of the transformer windings likewise have an identical number of
second discharge state may be caused by the above-men 45 windings.
tioned reversal of the flux rate through the boost inductor due In this embodiment, the first half-winding of the primary
to a sudden decrease of boost inductor current. The decrease transformer winding and the first secondary transformer
of boost inductor current may be caused by the off-state entry winding are both wound around the first outer leg of the
of the driver or by an error condition. magnetically permeable multi-legged core. Likewise, the
In accordance with a preferred embodiment of the inven 50 second half-winding of the primary transformer winding and
tion, the boost converter is configured to change between the the second secondary transformer winding are both wound
first and second discharge states by selectively coupling and around the second outer leg of the magnetically permeable
decoupling the boost inductor from the secondary trans multi-legged core to achieve good magnetic coupling
former windings Such that: between the primary winding and second winding.
during the first discharge state, magnetically decoupling 55 The skilled person will understand that the present isolated
the boost inductor from the first and second secondary boost power converter may comprise many different primary
transformer windings to deliver the magnetic energy to side circuit topologies in addition to the previously men
the primary transformer winding: tioned half-bridge and full-bridge drivers. In one embodi
during the second discharge state, magnetically coupling ment, the primary side comprises a first boost inductor and a
the boost inductor to the first and second secondary 60 second boost inductor such that the first boost inductor is
transformer windings through a shared flux path in the coupled between the input terminal and a first transistor driver
magnetically permeable multi-legged core. The shared output. The first transistor driver output is coupled to a first
flux path may comprise two separate shared flux paths end or first winding output of the primary transformer wind
such that a first shared flux path runs between the boost ing. The second boost inductor is coupled between the input
inductor and the first secondary transformer winding 65 terminal and a second transistor driver output. The second
and a second shared flux path runs between the boost transistor driver output is coupled to a second end or second
inductor and the second secondary transformer winding. winding output of the primary transformer winding. The first
 US 9,257,910 B2
5 6
and second transistor driver outputs may comprise respective leg of the second magnetically permeable multi-legged
drain or collector terminals of a MOS or bipolar transistor. core to store magnetic energy therein. The second boost
The use of at least two boost inductors is advantageous inductor being electrically coupled between the input
because this reduces driver component count for example by terminal and a second driver to be alternatingly charged
reducing the number of semiconductor Switches that may and discharged with magnetic energy. The second driver
comprise respective transistor Switches. having a second driver output coupled to a second pri
In another embodiment, the boost inductor comprises a mary transformer winding wound around a first outer leg
first half-winding and a second half-winding of the primary and a second outer leg of the second magnetically per
transformer winding to provide an integrally formed boost meable transformer core. The second driver is config
inductor and primary winding which leads to improved cop 10 ured to generate a second primary Voltage to the second
per utilization. In this embodiment, the magnetic energy primary transformer winding in accordance with the
stored in the boost inductor is directly transferred to the first driver control signal. The boost power converter further
and second secondary transformer windings by a magnetic comprises first and second secondary transformer wind
flux through the magnetically permeable multi-legged core. ings wound around the first outer leg and the second
In the above-discussed embodiments with a separate primary 15 outer leg, respectively, of the second magnetically per
transformer winding and boost inductor, magnetic energy is meable multi-legged core. The first secondary trans
initially stored in the boost inductor and subsequently former winding of the second magnetically permeable
released or discharged, during the first discharge state, as multi-legged core is coupled in series between the rec
discharge current flowing through the primary transformer tification circuit and the output of the first secondary
winding to induce a primary side Voltage therein. transformer winding of the first magnetically permeable
In a number of embodiments of the isolated boost power multi-legged core. The second secondary transformer
converter a rectifying element is electrically coupled to the winding of the second magnetically permeable multi
center-tap to conduct a discharge current, during the second legged core is coupled in series between the rectification
discharge State, from the first and second secondary trans circuit and the output of the second secondary trans
former windings to the rectified converter output Voltage, 25 former winding of the magnetically permeable multi
V. The discharge current is induced by the magnetic flux legged core, or first magnetically permeable multi
generated by the boost inductor flowing through the first and legged core, such that:
second secondary transformer windings. The rectifying ele in the first discharge state, the respective magnetic energies
ment is preferably electrically coupled to a predetermined stored in the first and second boost inductors are dis
electric potential of the boost converter such as a power 30 charged by directing respective discharge currents from
Supply rail, including ground, a negative DC Supply rail or a the respective boost inductors through the respective
positive DC supply rail, of the secondary side of the isolated primary transformer windings,
boost power converter. The rectifying element may be in the second discharge state, the respective magnetic ener
required if the rectification circuit comprises a full-bridge gies stored in the respective boost inductors are dis
rectifier or a voltage doubler because during the first dis 35 charged by discharging respective magnetic fluxes
charge state, the center-tap Voltage differs from both the rec through the respective first and second secondary trans
tified converter output Voltage and a negative rectified con former windings.
Verter output Voltage. According to the latter embodiment of the invention, the
Alternatively, the rectification circuit may comprise a cen first and second secondary transformer windings of the first
ter-tapped rectifier in accordance with a preferred embodi 40 magnetically permeable multi-legged core are electrically
ment of the invention Such that the rectifying element in series coupled to the rectification circuit in an indirect manner
with the center-tap can be avoided. According to this embodi through the respective ones of the first and second secondary
ment, the center-tap is electrically connected to a negative transformer windings of the magnetically permeable multi
rectified converter output voltage or the rectified converter legged core. The first and second secondary transformer
output Voltage, V. 45 windings are therefore coupled in series such that the rectified
the respective outputs of the first and second secondary converter output Voltage, V, is doubled in a symmetrical
transformer windings are coupled to an opposite output architecture or topology of transformer windings mounted on
Voltage relative to the center-tap Voltage through first the magnetically permeable multi-legged core and the first
and second rectifying elements. Each of the first and magnetically permeable multi-legged core. The magnetically
second rectifying elements preferably comprises a semi 50 permeable multi-legged cores may be provided as separate
conductor diode or diode-coupled transistor. parts, for example arranged in abutment or proximate to each
As previously mentioned, the rectification circuit may other, or as an integrally formed core element which has a
comprise a Voltage multiplier for example a Voltage doubler shared magnetically permeable structure or leg. In a preferred
circuit to increase the level of the rectified converter output embodiment, the magnetically permeable multi-legged cores
Voltage, V. The skilled person will understand that the 55 share a common magnetic flux path extending through a
rectifying element and/or the rectification circuit each may shared magnetically permeable leg. In the latter embodiment,
comprise one or more semiconductor diode(s), diode the magnetically permeable multi-legged cores may advan
coupled transistor(s) or synchronously controlled transistor tageously be configured to provide magnetic flux cancellation
Switch(es). Each of the semiconductor diodes may comprise or Suppression in the shared magnetically permeable leg. This
a MOS diode, a bipolar diode, a Schottky diode or any com 60 feature saves magnetic materialso as to reduce material costs
bination thereof. and size of the isolated boost power converter. The above
According to one advantageous embodiment or variant of discussed embodiments of the present invention based on the
the invention discussed above with the center leg surrounded first magnetically permeable multi-legged core and the mag
by the first and second outer legs, the isolated boost power netically permeable multi-legged core possess numerous
converter comprises: 65 favourable characteristics: scalability by the addition of fur
a second magnetically permeable multi-legged core and a ther magnetically permeable multi-legged cores and associ
second boost inductor magnetically coupled to a center ated primary side and secondary side transformer windings
 US 9,257,910 B2
7 8
and drivers. This property is highly beneficial because the of the isolated boost power converter in accordance with the
isolated boost power converter can readily be adapted to a first embodiment of the invention during a charging Subinter
whole range of applications with varying power transfer val of boost mode operation,
capacities. Thus, saving R&D design efforts and time, reduc FIGS. 3a) and 3b) illustrate schematically an electrical
ing design risk, reducing manufacturing costs etc. In addition, circuit diagram and a magnetic circuit diagram, respectively,
the current rating of each semiconductor switch of the first of the isolated boost power converter in accordance with the
and second drivers can be halved for a given current handling first embodiment of the invention during a first discharge state
capacity due to the split of input current between the first, of the boost mode operation,
second and possibly further drivers. FIGS. 4a) and 4b) illustrate schematically an electrical
The driver control signal may comprise a PWM signal 10 circuit diagram and a magnetic circuit diagram, respectively,
having an adjustable duty cycle, D. The adjustable duty cycle of the isolated boost power converter in accordance with the
may be used to set a desired or target DC level of the rectified first embodiment of the invention during a charging Subinter
converter output Voltage, V. The duty cycle is preferably val of a start-up mode.
set to a value between 0.5 and 1.0 after exiting or leaving a FIGS. 5a) and 5b) illustrate schematically an electrical
start-up state or mode, i.e. during normal boost mode opera 15 circuit diagram and a magnetic circuit diagram, respectively,
tion of the isolated boost converter. The duty cycle, D, may be of the isolated boost power converter in accordance with the
set or controlled in connection with a closed loop feedback first embodiment of the invention during a second discharge
control scheme for controlling any of the state variables of the state of the start-up mode wherein magnetic energy stored in
isolated power converter, such as the boost inductor current or a boost inductor is discharged by discharging a magnetic flux
the rectified converter output Voltage, V. through secondary transformer windings,
Another aspect of the invention relates to a method of FIG. 6a) is an electrical circuit diagram of an isolated boost
generating a rectified converter output Voltage, V, from an power converter with integration of a boost inductor and
input Voltage, V, by an isolated boost power converter primary transformer windings in accordance with a second
according to any of the preceding claims. The method com embodiment of the invention,
prises steps of 25 FIG. 6b) is an electrical circuit diagram of an isolated boost
generating a pulse width modulated driver control signal, power converter with a center-tapped rectification circuit in
Supplying the pulse width modulated driver control signal accordance with a third embodiment of the invention,
to the driver, FIG. 7 is an electrical circuit diagram of an isolated boost
gradually increasing a duty cycle, D, of the pulse width power converter with an integrally formed dual core topology
modulated driver control signal from below 0.5, prefer 30 in accordance with a fourth embodiment of the invention; and
ably below 0.1, to a value above 0.5, preferably between FIG. 8 is a graph depicting measurement data from an
0.55 and 0.99, experimental isolated boost power converter in accordance
adjusting the duty cycle, D, to a desired value to reach a with the first embodiment of the invention.
desired or target AC voltage waveform or DC voltage
level at the rectified converter output voltage, V. As 35 DETAILED DESCRIPTION OF PREFERRED
previously explained, before the isolated boost converter EMBODIMENTS
reaches its normal operating state, a start-up mode or
state is required. During the start-up phase or mode of The embodiments described in detail below are particu
the isolated boost power converter the duty cycle of the larly well-suited for power converters providing DC voltage
Pulse Width Modulated (PWM) driver control signal 40 amplification or step-up. However, the skilled person will
applied to the driver must be ramped-up slowly to avoid understand that power converter in accordance with the
excessive in-rush currents. During ramp-up of the duty present invention are highly useful for other types of appli
cycle, it preferably starts at a value much less than 0.5 cations both in step-up and step down Voltage converting
which means that the driver is placed in an off-state or applications.
cut-off state during a certain time interval of a cycle of 45 FIG. 1a) illustrates schematically an electrical circuit dia
the PWM signal. The off-state means the driver lacks a gram of an isolated boost power converter 100 in accordance
low resistance electric path to the input Voltage or a with a first embodiment of the present invention. The isolated
negative input voltage rail Such as ground, GND. How boost power converter 100 comprises a magnetically perme
ever, thanks to the ability of the first and second second able multi-legged core in form of a three legged El core 102.
ary transformer windings to discharge the magnetic flux 50 The three legged El core 102 comprises a center leg 134
stored in the boost inductor, the present isolated boost surrounded by a first outer leg 132 and a second outer leg 136
power converter can enter and exit the start-up mode so in a mirror-symmetrical layout or structure about a central
as to establish an initial output Voltage as V, without vertical axis extending through the center leg 134. The center
any need for a separate flyback winding or other auxil leg 134 comprises an air gap 138 which allows magnetic
iary start-up circuitry. 55 energy of a boost inductor, L, to be stored therein. The
isolated boost power converter 100 comprises an input termi
BRIEF DESCRIPTION OF THE DRAWINGS nal 104 for receipt of an input voltage, Vin. The input voltage
may be a DC voltage between 5 Volt and 100 Volt. The boost
A preferred embodiment of the invention will be described inductor, L, is arranged or wound around the center leg
in more detail in connection with the appended drawings, in 60 134 of the three legged El core 102 and electrically coupled
which: between the input terminal 104 and a transistor driver 106 to
FIGS. 1a) and 1b) illustrate schematically an electrical be alternatingly charged and discharged with magnetic
circuit diagram and a magnetic circuit diagram, respectively, energy through the transistor driver 106. The primary and
of an isolated boost power converter in accordance with a first secondary windings are both split into two half-windings
embodiment of the invention, 65 distributed between the first and second outer legs 132, 136 so
FIGS. 2a) and 2b) illustrate schematically an electrical that the flux from the boost inductor, L, is decoupled from
circuit diagram and a magnetic circuit diagram, respectively, the transformer function in normal boost operation. The
 US 9,257,910 B2
9 10
transformer winding outputs orports are marked a, b, c and d rails of the output Voltage V, and the input Voltage may be
in both the electrical circuit diagram of FIG. 1a) and the electrically coupled to each other, for example through a
magnetic diagram of FIG.1b). The transistor driver 106 com shared GND connection without compromising the desired
prises four NMOS transistors S-S coupled as a full-bridge boost inductor discharge functionality of the first and second
or H-bridge Such that a first driver output, at a shared junction secondary transformer windings of the present isolated boost
or node in-between the NMOS transistors S and S is elec power converter 100. A rectifying element in form of a semi
trically coupled to the first winding output, a, of a primary conductor diode, D, is coupled between the center-tap 116
transformer winding or primary side transformer winding and the negative rectified converter output voltage 121 to
PW. A second driver output, at a shared junction or node facilitate a flow of discharge current through the first and
in-between NMOS transistors S and S is electrically 10 second secondary transformer windings, SW and SW,
coupled to the second winding output, b, of the primary respectively. The discharge current Subsequently flows
transformer winding PW. The primary transformer wind through the rectification circuit 118 and to the output terminal
ing PW-2 is split as mentioned above so as to comprise two or node 119 providing the rectified converter output voltage,
series connected half-windings PW and PW (refer to FIG. 1 V, so as to transfer energy to the output, facilitating start-up
b)) wound around the first and second outer legs, 132, 136, 15 of the power converter 100 as explained in further detail
respectively, of the El core 102. below.
The transistor driver 106 consequently generates a primary The transfer characteristic of the isolated boost converteris
Voltage across the primary winding PW in accordance with set by a duty cycle, D, of the Pulse Width Modulated (PWM)
a driver control signal. Such as a pulse width modulated driver control signal, during normal boost operation accord
control signal, adapted to individually control the Switching ing to:
of four semiconductor switches implemented as NMOS tran
sistors S-S. Each of the NMOS transistors S-S is switched
between conducting and non-conducting states, i.e. on-state Vout it. in V (Equation 1)
or off-state, in accordance with an individual driver control = 3 p - Vol 2
signal applied to a gate terminal of the NMOS transistor. The 25
isolated boost power converter 100 comprises first and second wherein:
series connected secondary transformer windings, SW and
SW, respectively, having a center-tap or midpoint 116 Virectified DC output voltage of the boost converter,
arranged in-between them. The first and second series con V, the DC input voltage to the boost converter;
nected secondary transformer windings, SW and SW, 30 D-a duty cycle of the PWM control signal at each transistor
respectively, are wound around the first outer leg 132 and the input of the driver and defined as: T.T.,
ova. of a single PWM
second outer leg 136, respectively, i.e. separate legs of the El period;
core 102 such that the second secondary transformer winding n=transformer turns ratio set by the number of secondary
SW is arranged on the same outer leg 136 as the second transformer windings divided by the number of primary
half-winding PW, of the primary transformer winding. Like 35 transformer windings.
wise, the first secondary transformer winding SW is When
arranged on the same outer leg 132 as the first half-winding
PW of the primary transformer winding. A voltage transfer
ratio between the primary and secondary sides of the trans
former function provided by the El core 102 is set by a turns 40
ratio, n, between the number of secondary side transformer
windings relative to the number of primary side transformer the isolated boost converter may be in the start-up phase and
windings. In the present embodiment, this turns ratio, n, the duty cycle, D, of the Pulse Width Modulated (PWM)
equals the number of secondary transformer windings of SW driver control signal below 0.5.
and SW combined divided by the number of primary trans 45 FIG. 2a) illustrates schematically an electrical circuit dia
former windings of PW and PW combined. The turns ratio, gram 100 of the isolated boost power converter during a first
n, may naturally vary with requirements of a particular appli subinterval or state boost mode operation. Circuit elements
cation, in particular whether the boost converter is intended to that are not carrying current have been dimmed to clarify the
function as a step-up or step-down converter. The turns ratio, operation during the first state. Generally, when the duty cycle
n, is preferably set to value between 0.25 and 100 such as 50 of the driver control signal, D, exceeds 0.5, the isolated boost
between 1.0 and 64. The integration of the boost inductor, power converter 100 operates as a normal isolated boost
Ls, the primary transformer winding PW-2 and the first converter. The boost mode operation can be divided into two
and second series connected secondary transformer wind subintervals: The first state or boosting subinterval where all
ings, SW and SW on the common or shared El core 102 is the NMOS transistors S-S are in placed in respective con
often referred to as “integrated magnetics’ in the art. 55 ducting states or on states. A second state of the boost mode
A rectification circuit 118 is electrically coupled to respec operation is an energy transfer subinterval where two of the
tive outputs of the first and second secondary windings to NMOS transistors, either S-S or S-S are in conduction
provide a rectified converter output Voltage, V, between states simultaneously as illustrated on FIG.3a) and FIG. 4a).
rectified converter output Voltage, V, 119 and a negative In the latter state or Subinterval boost inductor current is
rectified converter output Voltage 121. A Supply capacitor C 60 allowed to flow through the primary transformer winding
or C is coupled between these converter output voltages or PW, such that energy or power is transferred through the El
rails to Suppress ripple Voltages at the output of the rectifica core 102 to the rectified output voltage. The positive direction
tion circuit 118 and provide an energy reservoir stabilising the of any currents is indicated by arrows on electrical conductors
output Voltage, V. In the depicted isolated boost power or wires and relevant Voltage polarities indicated by +/- signs.
converter 100, the input side or primary side and secondary 65 FIG. 2b) shows the magnetic diagram, including flux rate
side are galvanically isolated by the Elcore 102. However, the induced by the boost inductor winding: dep/dt (pL', shown by
skilled person will understand that the negative terminals or fat dotted lines 112a and 112b. The rising current in L.
 US 9,257,910 B2
11 12
corresponds to a positive Voltage drop of the input Voltage, winding Voltages and currents are reversed, which still results
V. The DC flux is excluded from the analysis and the illus in D, being reverse biased by one-half of V. It is also
trated drawing as this is only relevant for magnetic Saturation worthwhile to notice that in both cases, the flux rate induced
and power loss considerations of the Elcore 102. The induced by the boost inductor, L, is not coupling to the primary and
magnetic flux (pL' splits evenly between the first and second secondary transformer windings PW, and SW, SW,
outer legs 132, 136, respectively, inducing Voltage drops on respectively, of the El core 102 such that the boost inductor is
the transformer windings with polarities indicated by the magnetically decoupled from the transformer operation. Fur
indicated +/- signs at respective winding outputs a, b, c and d thermore, the current in the boost inductor, L, is falling, seen
following from the right hand rule. The polarities across the as a negative Voltage drop across the boost inductor in both
two series connected primary side half-windings PWI, PW 10 FIGS. 3a) and b).
are opposite, and the Voltages cancel and likewise for induced FIGS. 4a) and b) show schematically an electrical circuit
Voltages across the two series connected secondary side half diagram and a magnetic circuit diagram, respectively, during
windings SW1 and SW2, so it can be concluded that the boost a charging Subinterval of a start-up mode. The input Voltage,
inductor L or L is not coupled to the transformer function Vin, is charging the boost inductor while also transferring
of the El core 102 during this state or subinterval. Further 15 energy to the rectified output Voltage through the transformer
more, it is also evident that D, is reverse biased in the illus operation of the El core 102 of the isolated boost power
trated subinterval of boost mode operation. converter 100. This charging subinterval can be viewed as a
FIG.3a) illustrates schematically the electrical circuit dia hybrid boost and energy transfer subinterval. The MOS tran
gram of the isolated boost power converter 100 during a first sistors S.S. of the driver 106 and rectifying diodes D and D.
discharge state, or transformer energy transfer Subinterval, of are conducting. The operation is identical to the second Sub
the boost mode. Circuit elements that are not carrying current interval discussed above in connection with FIGS. 3a) and
have been dimmed to clarify the operation in this state. Dur 3b) except current in the boostinductor, L, or L is increas
ing the illustrated energy transfer Subinterval, the two series ing as indicated by the reversed polarity of the Voltage across
connected primary side half-windings PWI, PW or primary the boost inductor. The term “hybrid indicates that the boost
winding PW of the El core based transformer is connected 25 inductor, L, and the primary transformer winding, PW, are
in series with the boost inductor. Such that the magnetic both active Such that magnetic energy is loaded into the boost
energy stored in the boost inductor is discharged by a dis inductor, L, at the same time as energy is transferred to the
charge current flowing through the primary transformer rectified output Voltage through the transformer operation
winding such that energy is transferred to the rectified output between the primary and secondary windings PW-2, and
Voltage. The current direction through the primary winding 30 SW, SW, respectively, of the El core 102. Consequently,
PW is alternated for every other subinterval, such that during the first discharge state, the flux rate (pL induced by the
either NMOS transistors S. S. and rectifying diodes D, and boost inductor, L, is decoupled from the secondary trans
D, or NMOS transistors S. S. and rectifying diodes D, and former windings SW, SW of the El core 102 such that the
D are conducting. FIG.3a) shows the subinterval where the boost inductor is Substantially magnetically decoupled from
NMOS transistors S. S. and rectifying diodes D, and D, are 35 the transformer operation.
conducting. FIGS. 5a) and 5b) illustrate schematically an electrical
FIG.3b) shows the magnetic diagram, including a flux rate, circuit diagram and a magnetic circuit diagram, respectively,
dp/dt (pL', induced by the boost inductor L. A first flux the isolated boost power converter 100 during a second dis
path associated with the boost inductor extends around the charge state. Magnetic energy stored in a boost inductor, L, is
center leg 134, the first outer leg 132 and the air gap 138 as 40 discharged by discharging a magnetic flux through secondary
illustrated symbolically by fat dotted line 112a. Likewise, a transformer windings SW, SW. Circuit elements that are
second flux path extending around the center leg 134, the not carrying current have been dimmed to clarify the opera
second outer leg 132 and the air gap 138 is illustrated sym tion during the second discharge state of start-up mode.
bolically by fat dotted line 112b. The flux rate induced by the During the second discharge state, which can be viewed as
primary winding PW, dep/dt (p. is shown by an outer fat 45 a second Subinterval of the start-up mode, the magnetic
dotted line 114 illustrating how the flux circulates clockwise energy stored in the boost inductor, L, is discharged by cir
around an outer closed path or loop 114 around the outer culating the stored magnetic flux through the first and second
periphery of the El core 102 including the first and second secondary transformer windings SW, SW. When all the
outer legs 132,136, respectively. As seen in FIG.3b), the flux NMOS transistors S-S of the driver circuit 106 are turned
rate (p, flows in a low reluctance outer path of the El core 102 50 off, i.e. non-conducting, the boost inductor current commu
avoiding to travel across the air gap 138 due to its high nicates to the first and second secondary transformer wind
reluctance or low magnetic permeability. Using the right hand ings SW, SW, through the flyback diode D, as shown in
rule and the depicted voltage polarity on the two half-wind FIG. 5a) by a magnetic coupling 108 as symbolically indi
ings PWI, PW of the primary winding, it can be seen that cated on FIG.5a). A shared magnetic flux path comprising the
these two flux rates are in the same direction along the outer 55 first and second flux paths 112a, 112b (refer to FIG. 5b))
flux path depicted by the fat dotted line 114. In the same extending around the first and second outer legs, 132, 136,
manner, it is evident that the flux rate (p, couples to the respectively, magnetically coupling the boost inductor, L, to
secondary side half-windings SW and SW such that the first and second secondary transformer windings SW,
induced current flows from winding output d to the winding SW. The cutoff or non-conducting state of the NMOS tran
output c, forward biasing the rectifying diodes D1 and D2. 60 sistors of the driver leads to a Suddenly dropping boost induc
From this it follows that output voltage at the mid-point 116 tor current. The drop of boost inductor current results in a
between the secondary side half-windings is positive with reverse in the respective rates of fluxes /2(pL at the first and
approximately one-half of the rectified converter output volt second flux paths 112a, 112b whereby the polarity of voltage
age, V. Such that D, is reverse biased. across the boost inductor is reversed (compared to FIG. 4b)).
When instead NMOS transistors S. S. and rectifying 65 This flux rate reversal of /2(pL induces a voltage drop across
diodes D and D are conducting, a corresponding analysis each of the secondary side half-windings SW and SW.
applies due to symmetry. In the latter case, all transformer There is a positive Voltage drop from secondary winding
 US 9,257,910 B2
13 14
output c to D, as well as from secondary winding output d to multi-legged core 702a, 702b is mirror symmetrical about a
rectifying diode Dr. Consequently, a discharge current, I can horizontal symmetry axis 750. Each of the El cores 702a,
now travel from secondary winding output c through D and 702b comprises a center leg 734a, 734b, respectively, sur
into the load resistance R, continuing back through D. Simi rounded by a first outer leg 732a, b and a second outer leg
larly, a discharge current can travel from secondary winding 736a,b in a mirror-symmetrical layout or structure about a
output d through Ds into R, and back through D. In effect, central vertical axis extending through a middle of the center
during the second discharge State of the start-up mode, the legs 734a, 734b.
two secondary half windings SW and SW are working or Each of the center legs 734a, 734b comprises an air gap
coupled electrically in parallel from the center-tap 116 to the 738a, 738b which allows magnetic energy of an associated
rectified output Voltage V, through respective rectifying 10 boost inductor, L. and L. respectively, to be stored
diodes D and D. So as to jointly discharge the magnetic therein. The isolated boost power converter 700 comprises an
energy in the boost inductor. This magnetic energy is largely input terminal 104 for receipt of an input Voltage, V, which
stored in the air gap 138. As illustrated, the magnetic energy for example may be a DC voltage between 5 Volt and 100 Volt.
stored in the boost inductor is converted to discharge currents The first and second boost inductors, L. and L. are
flowing through the first and second secondary half windings 15 both coupled to the input voltage at terminal 704. A first
SW and SW to the rectified converter output voltage, V. H-bridge transistor driver 706a is coupled to the first boost
through the shared magnetic flux path 112a, 112b. Conse inductor, L, and a second H-bridge transistor driver
quently, the boost inductor, L, is magnetically coupled to the 706b is coupled to the second boost inductors, L. Each
first and second secondary transformer windings SW and of the first and second three legged El cores 702a, 702b have
SW. In effect, transferring energy to the rectified output associated primary and second transformer windings, PW,
Voltage during the start-up mode and allowing a gradual tran PW, and SW, SW, and PW., PW, and SW, SW,
sition towards a state of normal boost mode operation for the respectively, in a topology similar to the topology discussed
isolated boost power converter 100. above in detail in connection with the first embodiment of the
FIG. 6a) is an electrical circuit diagram of an isolated boost invention. However, only the center-tap 716 in-between the
power converter 600 in accordance with a second embodi 25 first and second series connected secondary transformer
ment of the invention. The isolated boost power converter 600 windings, SW, and SW, respectively, is coupled to a rec
is similar to the previously described isolated boost converter tifying element in form of semiconductor diode D, The first
100 except for the reversal of the polarity of the rectifying and second series connected secondary transformer wind
diode D, and an accompanying reversal of a winding orienta ings, SW, SW, of the second El core 702b are not con
tion of the boost inductor L. relative to the winding ori 30 nected to a center-tap but each half-winding output is coupled
entation depicted on FIG. 1b). Like features have been in series with the corresponding secondary half-winding of
marked with corresponding numerals to assist the compari the first Elcore 702a. The isolated boost power converter 700
son. The skilled person will notice that this configuration of comprises a shared single rectification circuit 719 coupled to
the rectifying diode D, works similarly to the above-described respective winding outputs of the first and second secondary
configuration, in that when the primary transformer winding 35 windings SW, and SW. The rectification circuit 719 is
is inactive, a decreasing flux in the center leg will cause D, to configured as a full-wave rectifier comprising four rectifying
be forward biased in either case. diodes D-D to produce a rectified converter output Voltage
FIG. 6b) is an electrical circuit diagram 640 of an isolated V between positive and negative output Voltage terminals
boost power converter with a center-tapped rectification cir or nodes 719, 721, respectively.
cuit 648 in accordance with a third embodiment of the inven 40 It is accordingly evident that the first and second primary
tion. The transistor based full-bridge driver 606, the primary transformer windings PW, and PW, are both coupled to
and secondary windings PWI, PW and SW, SW, respec separate drivers 706a and 706b, respectively, while the pair of
tively, and the El core 102 itself are preferably all identical to first secondary transformer windings SW and SW are
the same features of the above-discussed first embodiment of coupled in series between the center tap 716 and the rectifi
the present isolated power converter. However, in the present 45 cation circuit 718 and the pair of second secondary trans
embodiment, the rectification circuit only comprises two rec former windings SW, and SW, likewise are coupled in
tifying diodes D and D coupled from a first winding output series between the center tap 716 and the rectification circuit
of the first half-winding SW and second winding output of 718. This topology has the beneficial effect that voltage
the second half-winding SW, respectively, to a rectified con amplification, i.e. the ratio between the input voltage V, and
Verter output Voltage, V, at positive and negative output 50 the rectified converter output Voltage V, is doubled com
nodes 619, 621, respectively. However, as a center-tap voltage pared to the topology disclosed on FIGS. 1-5 due to the series
at node 616 arranged in-between the first and second second connected pairs of first and second secondary windings. In
ary transformer half-winding SW, SW is always held at the addition, the shared “I” leg 740 provides further magnetics
negative rectified converter output Voltage on output node integration so as to reduce material costs, decrease size and
621, which may be ground level GND, there is not any need 55 increase efficiency due to flux cancellation in the shared “I”
to add a rectifying diode D, in series with the center-tap 616 leg 740.
like in the previously discussed embodiments. Furthermore, the first and second series connected second
FIG. 7 is an electrical circuit diagram of an isolated boost ary transformer windings, SW, SW are configured to dis
power converter 700 with a dual core topology comprising an charge magnetic energy stored in the first boost inductor,
integrally formed multi-legged El core 702a, 702b in accor 60 L. in a manner similar to one used in the above-men
dance with a fourth embodiment of the invention. The iso tioned first embodiment of the invention by a shared magnetic
lated boost power converter 700 comprises an integrally flux path comprising a first magnetic flux path extending
formed magnetically permeable multi-legged core inform of through the first center leg 734a, and the first outer leg 732a
a first three legged El core 702a and a second three legged El and a second magnetic flux path extending through the first
core 702b that share a common “I’leg 740 such that the entire 65 center leg 734a, and the second outer leg 736a. Likewise, the
magnetically permeable multi-legged core is a unitary struc first and second series connected secondary transformer
ture. The structure of the integral magnetically permeable windings, SW, SW, of the upper El core 702b are config
 US 9,257,910 B2
15 16
ured to discharge magnetic energy stored in the second boost in a first discharge state, the magnetic energy stored in
inductor, L, through a shared magnetic flux path in the the boost inductor is discharged by directing a dis
upper El core 702b. charge current from the boost inductor through the
FIG. 8 is a graph 800 depicting measurement data from an primary transformer winding; and
experimental isolated boost power converter in accordance in a second discharge State, the magnetic energy stored in
with the above-described first embodiment of the invention. the boost inductor is discharged by discharging a mag
The experimental isolated boost power converter had the netic flux through the first and second secondary trans
following key data: former windings.
Windings on L2 2. An isolated boost power converter according to claim 1,
it primary transformer windings, PW=2 10
wherein the first and second secondary transformer windings
it secondary transformer windings, SW+SW-8 are coupled in parallel from the center-tap between first and
DC input voltage, V-25 Volt second series connected secondary transformer windings to
Load resistance, R=68.2 ohm the rectified converter output Voltage, V.
Core type: ELP64 available from manufacturer EPCOS AG. 3. An isolated boost power converter according to claim 1,
Core material: N87 15
Air gap height (at center leg) 0.5 mm. wherein the second discharge State is automatically entered in
The graph data were acquired by maintaining V, at 25 Volt response to a reversal of magnetic flux rate in the boost
and sweeping a duty cycle, D, of a Pulse Width Modulated inductor.
(PWM) driver control signal from 0 to 0.75 and then back to 4. An isolated boost power converter according to claim 1,
0 over a time period of 4 seconds. The time variable is indi configured to:
cated along the x-axis 805. The corresponding duty cycle, D. during the first discharge state, magnetically decoupling
is indicated on the right-hand vertical scale 803 and the mea the boost inductor from the first and second secondary
sured rectified converter output voltage, V indicated on the transformer windings to deliver the magnetic energy to
left-hand vertical scale in Volts. The substantially linear cor the primary transformer winding:
relation between the duty cycle and the rectified converter 25 during the second discharge state, magnetically coupling
output Voltage, V, is evident, and is for D-0.5 in accor the boost inductor to the first and second secondary
dance with equation (1) above. It is also noticeable that the transformer windings through a shared flux path in the
rectified converter output voltage, V is continuous across magnetically permeable multi-legged core.
the boundaries at D=0.5between start-up mode operation and 5. An isolated boost power converter according to claim 1,
normal boost mode operation. For D=0.75, the following 30
wherein the first and second secondary transformer windings
electrical data were measured, input current (at V)=27.94 A. are configured to discharge the magnetic energy stored in the
V=205.11 Volt, output current—3.006 A. boost inductor by Supplying a discharge current to the recti
Hence, the present measurement data confirms the capa fied converter output Voltage, V. So as to transfer energy to
bility of the present experimental isolated boost power con
verter to start-up (D-0.5) and proceed to normal boost opera 35 the output, wherein the first and second secondary trans
tion (D-0.5) in a well-behaved manner without any need for former windings act as flyback windings.
a separate flyback winding or other dedicated Start-up cir 6. An isolated boost power converter according to claim 1,
cuitry. wherein the boost inductor is magnetically coupled to the
center leg to store the magnetic energy therein.
The invention claimed is: 40 7. An isolated boost power converter according to claim 1,
1. An isolated boost power converter, comprising: wherein:
a magnetically permeable multi-legged core comprising the primary transformer winding comprises first and sec
first and second outer legs and a center leg having an air ond series connected half-windings arranged around the
gap arranged therein; first and second outer legs, respectively;
an input terminal for receipt of an input Voltage, V; 45 the first and second series connected half-windings hav
a boost inductor being wound around the center leg or the ing an identical number of windings; and
first and second outer legs of the magnetically perme the first and second series connected secondary trans
able multi-legged core; former windings have an identical number of windings.
the boost inductor being electrically coupled between the 8. An isolated boost power converter according to claim 1,
input terminal and a driver to be alternatingly charged 50 wherein the driver comprises a full-bridge transistor driver
and discharged with magnetic energy; having first and second complementary driver outputs
the driver having a driver output coupled to a primary coupled to respective ends of the primary transformer wind
transformer winding wound around the first and second ing.
outer legs of the magnetically permeable transformer 9. An isolated boost power converter according to claim 1,
core; 55 comprising a second boost inductor,
the driver being configured to apply a primary Voltage to the boost inductor being coupled between the input termi
the primary transformer winding in accordance with a nal and a first transistor driver output coupled to a first
driver control signal; end of the primary transformer winding; and
first and second series connected secondary transformer the second boost inductor being coupled between the input
windings with a center-tap arranged in-between and 60 terminal and a second transistor driver output coupled to
wound around the first and second outer legs; respec a second end of the primary transformer winding.
tively, of the magnetically permeable multi-legged core; 10. An isolated boost power converter according to claim 1,
and further comprising a rectifying element electrically coupled
a rectification circuit electrically coupled to respective out to the center-tap to conduct a discharge current, during the
puts of the first and second secondary transformer wind 65 second discharge state, from the first and second secondary
ings to provide a rectified converter output Voltage, V; transformer windings to the rectified converter output volt
wherein age, Vouz.
 US 9,257,910 B2
17 18
11. An isolated boost power converter according to claim 1, the second secondary transformer winding of the second
wherein the rectification circuit comprises a center-tapped magnetically permeable multi-legged core is coupled
rectifier wherein: in series between the rectification circuit and the out
the center-tap is electrically connected to a negative recti put of the second secondary transformer winding of
fied converter output voltage or the rectified converter 5 the magnetically permeable multi-legged core such
output Voltage, V; and that:
the respective outputs of the first and second secondary in the first discharge state, the respective magnetic
transformer windings are coupled to an opposite output energies stored in the first and second boost induc
Voltage relative to the center-tap Voltage through first tors are discharged by directing respective dis
and second rectifying elements. 10
charge currents from the respective boost inductors
12. An isolated boost power converter according to claim 1, through the respective primary transformer wind
further comprising: ings; and
a second magnetically permeable multi-legged core; in the second discharge state, the respective magnetic
a second boost inductor magnetically coupled to a center energies stored in the respective boost inductors are
leg of the second magnetically permeable multi-legged 15
discharged by discharging respective magnetic
core to store magnetic energy therein, fluxes through the respective first and second sec
the second boost inductor being electrically coupled ondary transformer windings.
between the input terminal and a second driver to be 13. An isolated boost power converter according to claim
alternatingly charged and discharged with magnetic 12, wherein the first magnetically permeable multi-legged
energy;
the second driver having a second driver output coupled to core and the second magnetically permeable multi-legged
a second primary transformer winding wound around a core share a common magnetic flux path extending through a
first outer leg and a second outer leg of the second shared magnetically permeable leg.
magnetically permeable transformer core; 14. A method of generating a rectified converter output
the second driver being configured to generate a second 25 Voltage, V, from an input voltage, V, by an isolated boost
primary Voltage to the second primary transformer power converter according to any of the preceding claims,
winding in accordance with the driver control signal; comprising steps of:
and generating a pulse width modulated driver control signal;
first and second secondary transformer windings wound Supplying the pulse width modulated driver control signal
around the first outer leg and the second outer leg, 30 to the driver;
respectively, of the second magnetically permeable gradually increasing a duty cycle, D, of the pulse width
multi-legged core, wherein: modulated driver control signal from below 0.5 to a
the first secondary transformer winding of the second value above 0.5; and
magnetically permeable multi-legged core is coupled adjusting the duty cycle, D, to a desired value to reach a
in series between the rectification circuit and an out 35 desired or target AC or DC voltage at the rectified con
put of the first secondary transformer winding of the Verter output Voltage, V.
magnetically permeable multi-legged core; and