Zero-Voltage-Transition Topologies for Ciik Converters

Ching-Jung Tseng Chern-Lin Chen

Power Electronics Laboratory
Department of Electrical Engineering
National Taiwan University
Taipei, Taiwan
Abstract - With continuous input and output current, have been proposed in recent years [5-9]. These ZVT
wide output voltage range and small output filter, Cuk topologies use auxiliary switches and other passive
topology has gathered more and more attention in recent components to form shunt resonant snubber cells.
years. Soft switching is especially important to a Cuk Switching losses and EM1 noises are reduced because
converter because the power handling capability the shunt resonant snubber cell helps the power switch
requirements of semiconductor devices are higher than
those of other topologies. Several kinds of zero-voltage- to commutate under zero-voltage-switching during
transition (ZVT) topologies newly proposed can be short ZVT transient time. The operation principles are
applied to Cuk converters to achieve soft switching. identical to common PWM topologies during the rest
These ZVT topologies combine both merits of time. Control strategies and design rules of PWM
conventional PWM converters and resonant convcrtcrs. topologies can be directly applied to ZVT topologies.
The power switches in the ZVT topologies commutate Advantages of tlie conventional PWM converters and
under zero-voltage-switching by aid of resonant snubber resonant converters are both maintained in ZVT
cells during short ZVT time. Circuit operations are converters. Several kinds of ZVTF topologies can be
identical to common PWM topologies during rest time. applied to Cik converters to reduce switching losses
Four classes o f ZVT topologies for Cuk converters with
power MOSFET's employed as power switches are and EM1 noises. Four classes of ZVT topologies for
discussed in this paper. Qualitative descriptions and Cik converters with power MOSFET's employed as
experimental results are presented to illustrate each class power switches are discussed in this paper. Qualitative
o f ZVT topology. descriptions and experimental results are presented to
illustrate each class of ZVT topology.

correctors
I. INTRODUCTION

Most DCDC converters and power factor

use pulse-width-modulated (PWM)
techniques as control. PWM technique prevails against
other control techniques for its high power capability
and ease of control. Higher power density and faster
lK.r,cu
PoiPt

0.

0
Boost ( = 1-0 1

Buck [ = D

0.5
/ ( = Dl1-D) 1

1 Duty

transient response of PWM converters can be achieved Po : Output power, Pt : (Switch V stress)*(Switch 1 stress)
by increasing the switching frequency. However, as the Fig. 1. Switch utilizz~tionfactors iii switching circuits
switching frequency increases, so do the switching
losses and EM1 noises. High switching losses reduce
the power handling capability and serious EM1 noises 11. THE ZVT TOPOLOGIES FOR CUK
interfere the control of PWM converters. CONVERTERS

Cbk converters have been widely used as dc-dc To eliminate switching losses and EM1 noises in
converters and power factor correctors since first Chk converters when power MOSFET's are used as
introduced in 1977 [1-31. It is praised for the power switches, several kinds of ZVT topologies are
continuous input and output current, ripple-free input proposed recent years. Among them, four classes of
current, small output filter and wide output voltage ZVT topologies are deemed desirable to C i k converters.
range. However, as shown in Fig. 1, the switch Circuit struclures and qualitative characteristics of
utilization factor of the Chk topology is much lower these four ZVT Ctik converters are presented in this
than the buck topology and the boost topology [4]. In section.
other words, tlie power handling capability
requirements of the semiconductor devices of a Cuk A. clrrss A zvr io^
converter are much higher than those of a buck The ZVT topology first introduced in [ 5 ] can be
converter or a boost converter with the same output applied to a Cilk converter as shown in Fig. 2(a). It is
power. Reduction of switching losses and EM1 noises named by class A ZVT topology hereafter in this paper.
are especially important to a Chk converter. It differs from a conventional PWM Clik converter by
possessing an additional resonant snubber cell
To eliminate switching losses and EM1 noises, a consisting of a resonant inductor (Lr), a resonant
number of zero-voltage-transition (ZVT) topologies capacitor (G)% an auxiliary switch (S2) and an auxiliary

o I 998 IEEE
0-7a03-44a9-a/ga1$10.00 930
 ~

diode (Dz).The resonant capacitor C, also incorporates resulted switching losses, but also the dv/dt EM1
the output capac tance of tlie power MOSFET. An noises significantly reduce the performance of the
additional diode i i series with the resonant inductor or class A ZVT Cbk converter.
the auxiliary switch is required in practical use. This .Discharge of resonant inductor limited
diode can prevent the resonant inductor from The main switch SI has to turn off after
resonating with output capacitance of the auxiliary discharge of tlie resonant inductor L, is
switch after the auxiliary diode is turned off. Key accomplished. It lengthens the minimum duty cycle
waveforms of the class A ZVT topology are shown in of the main switch. Long minimum duty cycle not
Fig. 2@). only increases niinimuni acceptable output voltage
range but also iiiakes tlie class A topology not
suitable to be employed under wide duty range
applications.
*An additional saturable reactor required
In the class A ZVT topology, the main diode and
the ausiliary diode are essentially in parallel. When
F
(a) Circuit dia ‘ani ofthe class A ZVT Chk coovzitzr
the main diode is conducting, a certain percentage
of current will flow through the auxiliary diode.
This undesirable feature will cause considerable
switching loss when the auxiliary switch turns on.
An additional saturable reactor placed in series with
the resonant inductor is required.

B. Cluss U ZVI’ I’opology

A modification of the class A ZVT topology
proposed by the authors yields the class B ZVT
topology iis shown in Fig. 3(a). The major difference
lo Tiiiie between the class A and class B ZVT topologies is the
discharge path of the resonant inductor. The anode of
Fig. 2. ‘TCuk auxiliary diode Dz is connected to the output instead of
converter tlie anode of main diode DI. Several distinct
advantages are acquired but no additional component is
The features of the class A ZVT topology are
needed compared with tlie class A ZVT topology. The
summarized as follows:
control IC’s UC3855 and ML4822 designed for class A
Advantages:
ZVT topology can be used in this topology as well. Key
*Minimum vo tage and current stresses of switches
waveforms of the class B ZVT topology are shown in
and diodes
The voltagi: and current stresses of the main Fig. 3@).
switch are iden:ical to those in an ideal lossless Crik
converter. They are even smaller than those of a
hard switching Clik converter considering voltage
and current spikes during transients. The auxiliary
switch has the same voltage stress as the main
switch while tlie current stress is less than that of
the main switch because it only handles sinal1
amounts of resonant transition energy 681. Voltage
and current strmes of the main diode are the same
as the main switch and those of the auxiliary diode
are the same as the auxiliary switch. The active and
passive switche:s in the class A ZVT topology are
subjected to minimum voltage and current stresses.

Disadvantages:

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 resonant capacitors ( G I and Cr2), a resonant inductor
(L,) and an auxiliary switch (S2). The resonant
The features of the class B ZVT topology are capacitor GI incorporates the output capacitance of the
summarized as follows: main power MOSFET. Body diodes of the main switch
Advantages: and the auxiliary switch are also utilized. Key
Robust discharge of resonant inductor waveforms of the class C ZVT topology are shown in
Since the discharge current of resonant inductor Fig. 4(b).
Lr does not flow through the main switch SI, SI can
turn off before L, is completely discharged.
Minimum duty cycle of the main switch is tlie
shortest compared with other ZVT topologies. This
desirable feature makes the class B ZVT topology
especially suitable to be employed with variable
duty, such as PFC applications. Since the duty (a) Circuit diagram ofthe class C ZVT Cuk converter
cycle of the main switch determines the output
voltage range for linear operation, the output
voltage range of the class B ZVT topology is also vil
vs I
the widest. The saturable reactor in the class A
61
ZVT topology is no longer necessary in the class B
topology due to robust discharge of L,. v,,
'SI
niinuni ZVT time
Unlike other ZVT topologies, discharge time of "VI

the resonant inductor is not involved in ZVT time 'DI

VCd
in the class B ZVT topology. The main switch
4,
handles all the current from LI and L2 immediately bllL1'3 '4 'J'6 Lo Time
after ultrashort ZVT time. The voltage and current (b) Key wavefoiim ol'the class C ZVT Cuk converter
waveforms of the switches in the class B ZVT Fig. 4. Circuit dingam and key waveforms ofthe class C ZVT' C6k
topology are essentially square-waves except during converter
ultrashort ZVT time. Control and design
techniques of PWM converters can be best applied The features of the class C ZVT topology are
to the class B ZVT topology. Compared with class summarized as follows:
A ZVT topology, a larger L, can also be used to Advantages:
0 Soft switching for all semiconductor devices
reduce reverse recovery loss of D I with the same
ZVT time. It decreases not only the turn-on loss but The major disadvantage of the class A and class
also the current stress of SZ. B ZVT topologies is that the auxiliary switches and
diodes operate under hard switching. In the class
Disadvantages: C ZVT topology, the auxiliary switch turns off after
@Hardswitching for the auxiliary switch and diode its body diode is conducting to achieve ZVS turn-off.
As in the class A ZVT topology, the auxiliary All semiconductor devices are commutated under
switch and diode in the class B ZVT topology do soft switching.
ONO need of isolated drive circuit
not operate under soft switching.
@Increasedvoltage stresses of auxiliary switch and Unlike the other three topologies, drive circuit of
diode thc class C ZVT topology requires no isolation.
Compared with the class A ZVT topology, the Since the synchronization problem between control
voltage stresses of both the auxiliary switch SZ and signals of the main switch and the auxiliary switch
the auxiliary diode D2 increase from V, + V, to V, + is very important in ZVT topologies, reliability is
2V0. Since the high voltage stresses appear for only much improved without the requirement of isolated
a short time period with the current stresses drive circuit. Circuit cost is also reduced due to
unchanged, semiconductor devices with only saving of the isolated drive circuit.
slightly larger voltage ratings are suffkient.
D isadvaii titges:
C. Class C ZVT Topology .Energy accumulated in resonant snubber cell
The ZVT topology first introduced to be applied It can be seen from Fig. 4(b) that the energy in
to a boost converter [6] can also be applied to a Chk the resonant snubber cell keeps constant from tz to
converter as shown in Fig. 4(a). It is mined by class to because either the voltage across or the current
C ZVT topology hereafter in this paper. It differs from through the snubber cell is ideally zero. However,
a conventional PWM C& converter by possessing an €rom to to t?, the energy increases because both the
additional resonant snubber cell consisting of two voltage and the current are positive. It is clear that
tlie energy in the resonant snubber cell accumulates

932
 during to - t.~.every switching cycle. Since the
snubber energy is stored i n Crz, power dissipative
components are required to prevent unacceptable
high voltage xross Cr2. This undesirable feature
reduces not only efficiency but also practicability.
.Turn-on current spike of the main switch (a) Circuit diagram oftlie class D ZVT Cuk converter

Penalty fc r applying soft switching to all

semiconductor devices is the turn-on current spike VI1

of the main switch SI. It increascs conduction "0

losses and sonietinies increases the current stress of VS I

Is1
SI. Although the current spike is approximately
vs2
twice of the ,nput current during turn-on. The
IS2
increment of current stress of SI can be reduced to
V,
only 33.3% of the average current with properly 1

'Dl
selected induc:or values of LI and L2. It is much
vu1
less then that in discontinuous conduction mode
$32
where the current stress of SI is over 200% of the VCd
average current. However, the increased conduction 'U
losses indeed rsduce the efficiency and the increased lU'l'Z L, 14 l,la1, lo Tune
current stress cf S i raises the circuit cost. (b) Key wavefotms oftlie class D ZVT Cuk converter
.Increased voltage stress of the auxiliary switch Fig. 5 . Circuit dingram and kry wavefomis ofthe class D ZVT CGk
conveiler
The voltag,e stress of auxiliary switch S2 is
increased by VCR. Since VCr? varies with wide It IS shown in Fig. j(b) that the current through
range, voltage stress of S2 is increased to at most L, ( 1 ~ and
~ ) the voltage across C, (Vc,) are reset to
two times of tl-at of Si. It not only increases circuit zero bcfore S i turns on in every switching cycle
costs but also c ecreases stability. Although the energy temporarily stored in L, and C,
still varies during different switching cycle, it is
reset to zero every time before SI turns on. No
D. Cluss D ZVT Topology energy can be accumulated in the resonant snubber
The class I> ZVT topology proposed by the cell. The problem of the class C ZVT topology is
authors can be applied to a Clik converter as shown in resolved by adding an auxiliary diode D2. The
Fig. 5(a). It also can be seen as the amelioration of the increased voltage stress of the auxiliary switch of
class C ZVT topology. An auxiliary diode D2 is added the class C ZVT topology is also prevented. This
to discharge resonant capacitor Cr2. The energy in the desirable fealure guarantees the class D ZVT
resonant snubber cell is reset to zero every switching topology eligible to be employed under long-term
cycle by adding D?. The increased voltage stress of the operation.
auxiliary switch isr also prevented in the class D ZVT
topology. Diode D3 is added to clamp the voltage D i ~ i ~ ltilges:
vi~
across S2 to the output voltage when SI is on. It .Turn-on current spike of the main switch
prevents high vo tage spike of S2 due to resonant As described in the class C ZVT topology,
ringing when body diode of S2 is turned off at t4. This penalty for applying soft switching to all
diode can sometimes be removed since the voltage semiconductor devices in the class D ZVT topology
spike of S2 is genterally small. As in the class C ZVT is the turii-on current spike of the main switch SI.
topology, all semicronductor devices in the class D ZVT .Increased voltage stress of the auxiliary diode
topology, inchdin,: D2 and D3, also operate under soft Although the voltage stress of the auxiliary
switching. Key waveforms of the class D ZVT switch S2 is not increased as in the class C ZVT
topology are shown in Fig. 5@). topology, the voltage stress of the auxiliary diode D2
is increased by V02. Since Vcr2 in the class D ZVT
The features of lhe class D ZVT topology are topology is limited to a comparatively small value,
summarized as follows: voltage stress of D2 does not increase too much.
Advantages: Besides, the increased voltage stress of D2 appears
.Soft switchin<;for all semiconductor devices for only a short period of time. A power diode with
As in the class C ZVT topology, all slightly larger voltage rating is suffkient.
semiconductor devices in the class D ZVT topology
also operate under soft switching. Switching losses
and EM1 noises are reduced to minimum. 111. EXPERIMENTAL RESULTS
.Energy in the resonant snubber cell reset every
switching cycle

I 933
 (a) Vsi and Isi (b) Vsz and Isz (a) Vsi and Isi

t
(c) Vcr and ILr
I
L , ,
i
f
(c) VCrZ and ILr
Fig 6 . Key oscillograms of the class A ZVT Cuk converter Fig. 8. Key oscillograms of the class C ZVT Cuk converter.
Vsi,Vsz,Vcr : lOOV/div; ISi,IS2,ILr : 4Ndiv; Time 2us/div VSI,VSZ: 100Vidiv; Vcrz : SOV/div; IsI,Is~,ILI: 4Aidiv; Time Zusidiv.

I , , I 1 , I
(a) Vsi and Isi (b) Vs2 and Isz

i,, , , :

(c) V ~ r 2and ILr

Fig. 9. Key oscillograms of the class D ZVT Cuk converter
Vsi,Vsz : lOOVidiv, Vcr2 : 50V/div; Isi,Isz,ILr~4Mdiv; Time 2us/div

i
i
1
Fig. 10 Commutation oscillograms ofthe hard switching Cuk converter
Vsi 100V/div, ISI 4Aidiv, Time 2usldiv

these four circuits are listed in Table I. A hard

switching C& converter with the same specifications is
also built. The efficiency at 400W loading of the class
A, B ,C and D ZVT topologies and the hard switching
In order to provide an experimental comparison of counterpart are 95.0%, 95.2%, 94.3%, 94.7% and
the topologies described in this paper, prototypes of 88.7%, respectively.
four 400W, 200V DC output, 50kHz ZVT Cbk
converters have been built in laboratory to veri3 the
analysis presented. The components specifications of

934
 Key oscillograms of the class A, B, C and D ZVT conduction losses and current stress of the main switch.
topologies are shown in Fig. 6, Fig. 7, Fig. 8 and Fig. 9, Option should be made between the increased
respectively. Switch commutation oscillograms of the switching losses aiid EM1 noises caused by hard
hard switching one are also shown in Fig. 10. It is switching of S2 (topologies A and B) and the increased
clear seen from the oscillograms of VSI and ISI that the conduction losses and current stresses caused by turn-
main switch in each topology operates under ZVS. The on current spikes of SI (topologies C and D). Since
major difference cif the coniniutation waveforms of the discharge path of the resonant converter is modified in
main switches is that ISI of the class C and class D the class B ZVT topology, shorter ZVT time and
ZVT topologies lave turn-on current spikes. The robuster discharge of the resonant inductor make the
current spikes increase conduction losses and current class B ZVT topology superior to the class A ZVT
stresses of the main switches. Although no current topology. Since the class D ZVT topology prevents the
spikes appear on Is1 in the class A and class B ZVT energy from being accumulated in the snubber cell by
topologies, the auxiliary switches operate under hard adding an auxiliary diode, the class D ZVT topology is
switching. It increases switching losses and EM1 deemed superior to the class C ZVT topology.
noises. Option should be made between the increased
switching losses and EM1 noises caused by hard Table 11. Cornparison of four ZVT C i k converters.
switching of S2 (topologies A and B) and the incrcased
conduction losses and current stresses caused by turn-
on current spikes of Si (topologies C and D).

Comparing the waveforms shown in Fig. 6 and

Fig. 7, it can be seen that the waveforms of the class A
and class B ZVT topologies are quiet similar. However,
since the class B ZVT topology has robuster discharge
n Energy acculllulatcd
Isolated drive circuit
Sntiirable reactor
Specilied control IC
1
I
no
y6S
yes
yes
I
I
no
y eS
no
yes
I
I
yes
no
no
no
I no

no
no
of resonant inductor and shorter ZVT time, it is Lr discharge liniited by S 1 yes no yes yes
recommended com ared with the class A ZVT topology.
Comparing the waveforms shown in Fig. 8 and Fig. 9,
it can also be sertn that the waveforms of the class REFRENCES
C and class D ZVT topologies are quiet similar. S. Cilk and I<. D. Middlebrook, "A new optimum topology
Although the class C ZVT topology is easily to be swit~liiiigDC-to-DC converter," PESC'77, pp. 160-179.
13. 'f. Lin and Y.S. Lee, "Powcr-factor correction using Cuk
implemented because the drive circuit requires no coiivciters in discontinuous-capacitor-volk~gemode operation,"
isolation, it has a serious problem that the energy is IEEK 7i.tins. Irrd. Elccwuii.. vol. 44, no. 5 , pp. 648-653. Oct.
accumulated in the resonant snubber cell. This 1997.
problem is solved in the class D ZVT topology by M. Iizr~i;ii~drz,C. Agiiiliw. 1. Aricu. J. Sebiwtian iund J. IJceda,
"Comparative analysis of boost and buck-boost derived topologies
simply adding an auxiliary diode. An optional diode used as power hctor correctors," IECON95, pp.335-340.
D3 can also be addod to prevent the voltage spike across N. Mohiui, T. Undelaiid and W. Robbins, Power Electronics:
Sz caused by rescmant ringing. The class D ZVT Converters, Applications andDesign. Wiley. 1989, pp. 98-99.
(3. Hua, C. S. Leu, Y. Jiang and 1:. C. Lee, "Novel zero-voltage-
topology is reconmended compared with the class
transition PWM coiiverteis," IEEE Trons. Power Electron., vol.
c ZVT topology. 9,no. 2, pp. 213-219, Mar. 1994.
G. Moschopoulos, P. Jain and G. Joos, "A novel zero-voltage-
switched PWM boost converter," PESC'95, pp. 694-700.
nl. CONCLUSION G. Moschopoulos, P. Jain, Y. F. Liu and G . Joos, "A zero-
voltage- switched PWM boost converter with an energy
feedforward auxiliary circuit," PESC'96, pp. 76-82.
Four classes of newly proposed ZVT topologies G , Iiua tuid 1:. C. Lee, "Sof-switching teclmiques in PWM
for Ctik converters are presented in this paper conveilers," IEEE Trans. lnd. Electron., vol. 42, no. 6, pp. 595-
Comparison of these four ZVT topologies IS 603, Dec. 1995.
K. M. Smith, Jr. and K. M. Smedley, "A comparison ofvoltage-
accomplished thrc ugh qualitative descriptions and mode soft-switching methods for PWM conveiteis," IEEE Trans.
experimental wavei'orins. Characteristics of each ZVT PowrrElectron., vol. 12, no, 2, pp. 376-386, Mar. 1997.
topology are sunilrarized as shown in Table 11. The
major disadvantage of the class A and class B
topologies is the hard switching of the auxiliary switch
and diode. It increases switching losses and EM1
noises. Contrarilj-, soft switching is applied to all
semiconductor devices, including auxiliary switches
and diodes, in the class C and class D topologies,
However, the penally IS the turn-on current spikc of the
main switch, which becomes the major disadvantage of
the class C and ;lass D topologies. It increases

I 935