ECE 463/ECE 663

Spring 2024

Mehrdad Kazerani
Electrical & Computer Engineering
University of Waterloo
©Mehrdad Kazerani, 2024
 LECTURE 12-2

Switch-Mode Operation, Snubber Circuits and

Soft Switching

4-May-24 2
 Switch-Mode Operation (Hard Switching)
 In switch-mode operation, the switch is turned on and off under full load
current and full source voltage.
 If we assume the current and voltage transitions during turn-on and turn-
off processes to be linear, the freewheeling diode to be ideal, and stray
inductances to be negligible, no overshoot is observed in the current and
voltage switching waveforms.

A controllable Switch in a circuit

[1]

Switching Waveforms [1]

4-May-24 3
 Switch-Mode Operation (Cont.)
 In real-life situation, the diode cannot be assumed ideal and stray inductances
cannot be neglected. As a result, overshoots in the voltage and current
switching waveforms can be observed. This implies that switches with
higher ratings are required to withstand the overvoltages and overcurrents.

Equivalent
v L L Circuit

A controllable Switch in a circuit

with stray inductances
[1]

Switching Waveforms [1]

4-May-24 4
 Switch-Mode Operation (Cont.)
 Assume the switch to be ON before t0 and iC = Io.
 At t0, OFF-command is received by the switch.
 vCE starts rising at a slope determined by the switch and switch driver
characteristics.
 At t1, vCE = Vd. Diode becomes forward biased and starts conducting and
sharing the load current with the switch.

v L L

Switching Waveforms
A controllable Switch in a circuit
[1]
with stray inductances
[1]

4-May-24 5
 Switch-Mode Operation (Cont.)
 iC starts falling, contributing to an additional LdiC /dt voltage across the
switch.
diC diC
vCE = Vd − L ( 0)
dt dt
 At t3, when iC = 0, LdiC /dt = 0 and vCE settles at Vd.
 After t3, the diode carries Io. This situation remains unchanged till the
switch receives an ON-command at t4.

vL L

Switching Waveforms
A controllable Switch in a circuit [1]
with stray inductances [1]

4-May-24 6
 Switch-Mode Operation (Cont.)
 At t4, ON-command is received by the switch.
 iC starts rising at a slope determined by the switch and switch driver
characteristics. The load current Io starts being transferred from the diode
to the switch.
 Due to diC / dt > 0, vCE suddenly drops to:
diC diC
vCE = Vd − L (  0)
dt dt
vCE stays at the reduced level as long as iC is rising.

vL L

Switching Waveforms [1]

A controllable Switch in a circuit
with stray inductances [1]

4-May-24 7
 Switch-Mode Operation (Cont.)
 iC exceeds Io due to the reverse recovery of the diode. iC reaches its peak
value when the freewheeling diode recovers at t5 .
iC , peak = I o + I rr
 Upon recovery, the reverse voltage across the diode starts rising and the
voltage across the switch starts falling.
 At t6, the reverse voltage across the diode is almost equal to Vd and
vCE=VON . This situation remains unchanged till the switch receives an
OFF-command.

v L L

Switching Waveforms [1]

A controllable Switch in a circuit
with stray inductances [1]

4-May-24 8
 Switch-Mode Operation (Cont.)
 As seen, the switch turns on and off the entire load current under the
entire source voltage. This mode of operation is known as Switch-Mode
Operation.
 The problems with switch-mode operation are:
– High switch stresses
– High switching power losses
– Electro-Magnetic Interference (EMI)

v L L

Switching Waveforms
A controllable Switch in a circuit
[1]
with stray inductances
[1]

4-May-24 9
 Switch Stress
 Switch stress is a result of:
– Simultaneous occurrence of high voltage and high current values
– High diSW / dt at turn-on
– High dvSW / dt at turn-off
 In switch-mode operation,
– High values of voltage and current happen at the same time. This
requires a large safe operating area (SOA).
– The rates of rise of current and fall of voltage at turn-on are high.
– The rates of fall of current and rise of voltage at turn-off are high.

iSW Idealized
switching
Turn-off trajectory

SOA Turn-on

vSW
Safe Operating Area (SOA), A [1]
characteristic of semiconductor switch
Switching trajectory in switch-mode operation

4-May-24 10
 Switch Stress (Cont.)
 To reduce switch stress,
– Avoid simultaneous occurrence of high switch voltage and current values.
– Reduce peak values of switch current and voltage during switching.
– Limit diSW / dt at turn-on.
– Limit dvSW / dt at turn-off.

iSW Idealized
switching
Turn-off trajectory

SOA Turn-on

vSW
Safe Operating Area (SOA), A
characteristic of semiconductor switch
Switching trajectory in switch-mode operation
[1]

4-May-24 11
 Switch Stress (Cont.)
 Using the properties of inductor and capacitor, circuits called Snubbers
can be designed and used around switches to avoid simultaneous
occurrence of high voltage and current values as well as high dv/dt and
high di/dt.

A di/dt
snubber
[1]
iSW
Turn-off Idealized
switching
trajectory
A dv/dt
Turn-on snubber
vSW
Switching trajectory improvement by snubbers

4-May-24 12
 Switching Power Losses
 High switching frequency results in:
– Better input/output waveform quality
– Reduced inductor, capacitor and transformer size  reduced converter size
and weight  Increased Power Density.
 In switch-mode operation, for purely-inductive load,
1
PSW = Vd I o (tc (on ) + tc (off ) ) f SW
2
fSW switching losses size of heat-sink converter size and
weight power density
 Conclusion: overall size of converter is not necessarily reduced by
increasing switching frequency. To limit switching losses, while taking
advantage of high switching frequency, proper measures have to be taken
in the power circuit and control.

[1]
[1]
4-May-24 13
 Switching Power Losses (Cont.)
 Simultaneous occurrence of high values of voltage and current during
cross-over times at turn-on and turn-off is the main reason for high
switching power losses and not being able to work at high switching
frequencies.
 Using snubber circuits, the peaks of the switch voltage and current can
be separated, and the switch losses can be reduced.
 As snubbers are dissipative in nature, the power losses are shifted from
the switch to the snubber circuit, but they are not eliminated. As a result,
snubbers are not able to reduce overall losses and improve efficiency.
They are used to reduce switch stress and prolong switch lifetime. They
also allow switches to operate at a higher switching frequency than they
can in hard switching. To improve the situation, regenerative snubbers
or soft switching techniques have to be used.

[1] [1]

4-May-24 14
 Electro-Magnetic Interference (EMI)
 In switch-mode operation, high fSW is possible if switching time (tc(on)
+ tc(off) ) is small, i.e., if fast switches are used.
 Fast switch  small rise times and fall times for current and voltage
 high di/dt and high dv/dt  EMI.
 di/dt and dv/dt and therefore, EMI, can be controlled using snubbers.
 In order to minimize the effect of EMI on neighbouring circuits, EMI
filters and special converter enclosure designs are used.

[1] [1]

4-May-24 15
Towards High Switching Frequency & High Efficiency-1
 In order to work at high switching frequency,
the problems with:
– Switch stress,
– Switching losses, and
– EMI
have to be solved.
 Snubbers:
A di/dt
– Snubbers are circuits composed of
snubber
resistors, inductors, capacitors and
diodes that are placed around switches.
– The fact that capacitors resist rapid
changes in voltage is used to limit A dv/dt
switch dv/dt. snubber
– The fact that inductors oppose rapid
changes in current is used to limit switch di/dt.
– Snubbers are dissipative in nature. iSW
Idealized
– Snubbers shift the power losses from Turn-off
switching
the switch to the snubber circuit. trajectory
– With snubbers, operating at high switching
Turn-on
frequency is made possible, but overall
vSW
efficiency is not improved.
4-May-24 16
Towards High Switching Frequency & High Efficiency-2
 Soft Switching:
– In soft switching, a combination of special converter topologies and switching strategies is
used to turn the switches on and off under zero voltage and/or zero current. Theoretically,
this results in zero switching power loss.
– Zero voltage switching (ZVS) is a condition that implies switching under zero voltage.
– Zero current switching (ZCS) is a condition that implies switching under zero current.
– Some converter topologies have inherent soft switching capability.
– In most of the cases, auxiliary circuits are needed to make soft switching possible.
– Resonant circuits are used to create zero crossings for switch voltage and current so that the
switching can be performed under zero voltage or zero current.
– Soft switching reduces switch losses, dv/dt, di/dt, and EMI, and enables the switch to use
only a small part of the SOA. It makes operating at high switching frequency possible.

iSW iSW
Turn-off Idealized Turn-off Idealized
switching switching
trajectory trajectory
Turn-on Turn-on
vSW
vSW

Switch- Mode With Snubber With Soft Switching

[1]

4-May-24 17
 Snubber Circuits
 Snubber Circuits:
– Shape the switching trajectory of the semiconductor device.
– Reduce switch stress.
• Limit vSW at turn-off.
• Limit iSW at turn-on.
• Limit dvSW /dt at turn-off (limit dv/dt during re-application of forward blocking
voltage in SCRs).
• Limit diSW /dt at turn-on.
– Act as an alternative to selecting a device of higher ratings.
– Shift switch losses to snubber circuit.
– Do not result in a gain in overall efficiency.
 Snubber Circuit Classification:
– Unpolarized R-C Snubbers:
• Protect diodes and SCRs.
• Limit peak voltage and maximum dv/dt.
– Polarized* R-C Snubbers (Turn-Off Snubbers):
• Shape turn-off trajectory of controllable switches.
• Clamp voltages applied to semiconductor devices to safe levels.
• Limit dv/dt during turn-off.
– Polarized L-R Snubbers (Turn-on Snubbers):
• Shape turn-on trajectory of controllable switches.
• Limit di/dt during turn-on.
*Note: Polarized implies inclusion of a diode in the snubber circuit.

4-May-24 18
 Snubbers for Controllable Switches
Turn-on Snubber-1
 Turn-on Snubbers:
– Turn-on snubbers are used to reduce turn-on switching losses and
– Limit the peak reverse recovery current of freewheeling diode.
 The main component in the turn-on snubber is a series inductor that
limits the rate of rise of switch current during turn-on process.

A controllable switch with turn-on snubber

4-May-24 19
 Turn-on Snubber-2
 At turn-on, as long as the switch current is rising, the voltage across the switch
stays at a reduced level below Vd. The drop in switch voltage is:
di Io
vS = Ls = Ls
dt tri
 If the snubber inductance is large enough, the drop in switch voltage can be
approximated to Vd. Therefore,
di Vd
for large Ls =
dt Ls
This means that slope of switch current is controlled by Ls. Note that the reduction
of di/dt results in avoiding the simultaneous occurrence of high voltage and high
current and thus, avoiding large switching losses at turn-on.

iS iS

vS vS

[1]
A controllable switch
with turn-on snubber Switch current and voltage waveforms at turn-on
4-May-24 20
 Turn-on Snubber-3
 During switch ON-state, energy is stored in Ls. This energy is given
by: 1
WON =2
Ls I o
2
 This energy will be dissipated in Rs during OFF-state of the switch.
 Ls and Rs are designed based on the following criteria:
– Turn-on snubber causes an over-voltage across the switch at turn-off.
The magnitude of the maximum over-voltage is:
vS ,max = Rs I o
For safe operation of the switch, Rs should
be selected such that

Vd + Rs I o  VOFF − state , Limit

Note that Vd + Rs I o = VOFF − state , Limit iLs

gives the largest possible Rs .

A controllable switch
with turn-on snubber

4-May-24 21
 Turn-on Snubber-4
 Turn-On Snubber Recovery: The current in Ls must decay to
below 10% of Io during the shortest OFF-state period of the switch so
that the turn-on snubber will be effective again in the next turn-on.
During OFF-state,
t
− Ls
iLs = I o e ,  =

Rs
For iLs to drop below 0.1Io during switch
OFF-state,
iLs
tOFF − state ,min  2.3

This is the condition for the

recovery of the turn-on snubber.
A controllable switch
Note: Ls must be able to carry Io.
with turn-on snubber

4-May-24 22
 Turn-off Snubber-1
 Turn-off snubber;
– Reduces switch stress during turn-off by reducing dv/dt and
– Keeps switch voltage low while switch current is high during turn-off
process. As current drops, voltage is allowed to rise. Avoiding simultaneous
occurrence of high voltage and high current is the key to reduction of
switching losses.
 The main element of turn-off snubber is a capacitor that limits the rate
of rise of voltage across the switch at turn-off to a safe value.

A controllable switch with turn-off snubber

4-May-24 23
 Turn-off Snubber-2
 When the switch receives an OFF-command, its resistance rises. As a
result, load current Io find an easier path through Ds, Cs, and the source.
As the switch current falls, the capacitor current rises. As a result, the
capacitor voltage builds up as well.
 Note that the capacitance of the snubber capacitor determines the rate of
rise of voltage across the switch. The capacitor can be designed in such a
way that switch dv/dt is maintained below the limit.
iS iS iS

[1]
A controllable switch Switch current and voltage waveforms at turn-off
with turn-off snubber

4-May-24 24
 Turn-off Snubber-3
 Depending on the size of the capacitor, three different cases can happen:
– Small Cs: Capacitor voltage reaches Vd before capacitor current reaches Io. As
soon as Capacitor voltage reaches Vd, the freewheeling diode will be forward
biased. The capacitor current drops to zero and diode conducts the current Io - iS.
– Base Cs: Capacitor voltage reaches Vd at the same time as capacitor current
reaches Io. As soon as Capacitor voltage reaches Vd, the freewheeling diode will
be forward biased. The capacitor current drops to zero and diode current jumps
to Io.
– Large Cs: Capacitor current reaches Io before capacitor voltage reaches Vd.
Switch current falls to zero and capacitor will take the entire load current. The
capacitor charges up to Vd, capacitor current falls to zero and diode current
jumps to Io. i i i
S S S

[1]
A controllable switch with turn-off snubber
Switch current and voltage waveforms at turn-off

4-May-24 25
 Turn-off Snubber-4
 Finding Base Cs (Cs1):
Io Io 1 t 1 Io 2
– If t fi = i
, then C = t v
, and Cs = v SW =  C
i dt = t .
di t fi
s
Cs 0 s 2C s t fi
dt
iCs rises as long as vCs (= vS )Vd & iCs  I o

Cs1 can be obtained by replacing vCs by Vd and t by tfi in the vCs equation as:
I o t fi
C s1 =
2Vd
iS iS iS

[1]
A controllable switch
with turn-off snubber Switch current and voltage waveforms at turn-off

4-May-24 26
 Turn-off Snubber-5
 Improved Turn-off Trajectory:

A controllable switch
with turn-off snubber

iS
iS iS iS

vS

Switch current and voltage waveforms at turn-off Turn-off trajectory for different Cs values
[1] [1]

4-May-24 27
 Turn-off Snubber-6
 Turn-off Snubber Design:
– Objectives:
• Improving turn-off trajectory
• Having no adverse effect on turn-on process
– Without turn-off snubber, switch current would drop to Io abruptly upon
recovery of freewheeling diode at switch turn-on.
– With turn-off snubber, the energy stored in Cs during turn-off causes a
current through Rs and the switch.

A controllable switch
with turn-off snubber

4-May-24 28
 Turn-off Snubber-7
 Turn-off snubber with Cs only:
– At turn-on, before diode recovers, the switch voltage is equal to Vd.
– Upon diode recovery, the switch voltage starts dropping. As the snubber
capacitor has been charged to Vd during turn-off and is at a higher potential
with respect to the switch, the capacitor starts discharging through the switch,
generating a large discharge current. This continues till switch voltage reaches
0. Note that the switch should be able to carry the snubber capacitor discharge
current. IGBTs usually use only a snubber capacitor as dv/dt snubber.

Controllable switch with turn-off Switch current and voltage waveforms at turn-off
snubber (capacitor only)
[1]
[1]

4-May-24 29
 Turn-off Snubber-8
 Turn-off Snubber with Cs , Ds , and Rs:
– At turn-on, before diode recovers, the switch voltage is equal to Vd.
– Upon diode recovery, the switch voltage starts dropping. As the snubber
capacitor has been charged to Vd during turn-off and is at a higher potential
with respect to the switch, the capacitor starts discharging through Rs and the
switch. Almost all of the energy stored in the capacitor will be dissipated in
Rs, with almost no dissipation in the switch as the switch voltage has already
dropped to zero. The peak value of the discharge current can be designed to be
less than the peak of diode reverse recovery current so that the switch does not
experience any current above the level that it would experience without a
turn-off snubber. In this way, the adverse effect of turn-off snubber on turn-
on process is eliminated. v
S

iS

Switch current and voltage

waveforms at turn-off
Controllable switch
with turn-off snubber [1]

4-May-24 30
 Turn-off Snubber-9
 Turn-off Snubber Design: Design of Cs
– Cs is designed such that at the largest iCs, which corresponds to the largest
dvCs /dt, the switch dv/dt does not exceed the set limit. If Cs is large enough
so that capacitor voltage rises to Vd right at the moment or after the capacitor
current reaches Io, the largest capacitor current, which corresponds to the
largest capacitor dv/dt, will be equal to Io.
dvCs dvSW 1 1
max = max = iCs ,max = Io
dt dt Cs Cs
Io
 Cs = (k  1, safety factor)
1 dvSW
lim
k dt
iS iCs

vS vCs
Io Io

Controllable switch tfi

with turn-off snubber
Waveforms of switch current, snubber capacitor current and
snubber capacitor voltage (or switch voltage) at turn-off

4-May-24 31
 Turn-off Snubber-10
 Turn-off Snubber Design: Design of Rs
– When the diode reverse recovery is complete, the voltage across the
switch falls below Vd and the snubber capacitor, which has been
charged to Vd during switch off-state, starts discharging into the
snubber resistor. The resistance of snubber resistor Rs is designed
based on the following criterion:
vS
Vd
(= iRs , peak ) + I o  I S ,rated
Rs
Vd
– Note that R (= iRs , peak ) + I o = I S ,rated
s
gives the smallest possible Rs. iS

Switch current and voltage

waveforms at turn-off
Controllable switch
with turn-off snubber

[1]
4-May-24 32
 Turn-off Snubber-11
 Turn-off Snubber Design: Improving Design of Cs
– Another criterion based on which Cs design can be improved is minimizing the overall
switch + snubber loss. As Cs gets larger, the turn-off switching loss is reduced due to
wider separation of large values of switch voltage and current. On the contrary, as Cs
gets larger, the losses in the snubber resistor are increased. Assuming that all the energy
stored in the snubber capacitor during turn-off is dissipated in snubber resistor during
on-state, the losses in the snubber resistor can be expressed as:
1
WR = Cs Vd2  Cs
2
– The following figure shows the energy loss in the switch and snubber resistor and sum
of these two components of loss versus capacitance of snubber capacitor. It is desirable
to have a snubber design that leads to the lowest total energy loss that is practical.
W
1
Vd I o t f
2

Wtotal = WS + WR
WR

[1]

WS

Controllable switch
with turn-off snubber Energy Loss in turn-off snubber versus Cs

4-May-24 33
 Turn-off Snubber-12
 Turn-off Snubber Design: Fine-tuning of design
– The initial selections of Rs and Cs have to be refined to make sure
that Cs is discharged to a low voltage (e.g., below 0.1Vd) during
the shortest ON-state time in operation so that turn-off snubber
will be effective again in the next turn-off.
– During ON-state period, the snubber capacitor voltage dies away
exponentially. t
−
vCs = Vd e  ,  = RsCs
– For the capacitor voltage to reach below 0.1Vd in the shortest
ON-state period, one must have:
tON − state ,min  2.3
This is the condition for the recovery of the turn-off snubber.

4-May-24 34
 Example
 In the circuit shown below, the IGBT switch has the following
parameters:
VDSM = 600 V, IDM = 100 A, dvDS lim = 800 V/s, and trv = 0.4 s.
dt
The circuit parameters are: Vd = 400 V, Io = 75 A and fsw = 20
kHz. The duty cycle d varies in the range of 0.2 to 0.6. Assume
the diode to be ideal.
(a) Is a turn-off snubber required for the IGBT?
(b) If yes, draw a turn-off snubber circuit for the switch.
(c) Design a turn-off snubber with a safety factor of 1.5 to dvDS dt .
(Assume that the snubber capacitor is large enough so that the
capacitor current reaches Io before capacitor voltage reaches Vd.)
(d) Does the snubber have enough time to recover?

4-May-24 35
 Solution
 (a)
v
i

Vd
Io

trv
Without turn-off snubber,
dvSW Vd 400 dvDS
= = −6
= 1000 V /  s  lim (= 800V /  s )
dt trv 0.4 10 dt

Therefore, a turn-off snubber is required to reduce the rate of rise of

voltage below the limit.

4-May-24 36
 Solution (Cont.)
 (b)

IGBT with turn-off snubber

4-May-24 37
 Solution (Cont.)
 (c)
It is assumed that Cs>Cs1. Therefore, with a turn-off snubber, the
following diagram can be drawn.
iS iCs

vCs
Io Io

tfi
dvCs dvSW 1 1
max = max = iCs ,max = Io
dt dt Cs Cs

For safety factor of 1.5 to dv/dt, choose a capacitance value that results
in: dvC 1 dvSW
s
max = lim
dt 1.5 dt
Io 75
 Cs = = = 0.14110−6 F = 0.141 F = 141nF
1 dvSW 1
lim (800V /  s )
1.5 dt 1.5

4-May-24 38
 Solution (Cont.)
 (c), Cont.
Vd
For Rs, Io + = I DM
Rs ,min
400
 75 + = 100
Rs ,min
400
 Rs ,min = = 16 
 (d) 25
For recovery, one must have:
tON − state ,min  2.3 s ,  s = Rs Cs

But,
 s = RsCs = (16)  (0.14110−6 ) = 2.256 10−6 s
1
tON = dTs , Ts =
fs

4-May-24 39
 Solution (Cont.)
 (d), Cont.
As d varies between 0.2 and 0.6, tON-state varies between 0.2Tsw
and 0.6Tsw. One must have:
tON − state,min  2.3 s
But,
0.2 0.2
tON −state,min = 0.2Ts = = = 10 −5
s = 10 s
f s 20 103

2.3 s = 2.3  2.256 10−6 = 5.19 10−6 s = 5.19 s

Since
tON − state,min (= 10  s ) 2.3 s (= 5.19  s )

the turn-off snubber will recover.

Note: If the snubber does not have enough time to recover, one
has to go back and modify the design.

4-May-24 40
 Over-Voltage Protection using Metal-Oxide Varistors
(MOVs)
 Metal-Oxide Varistors (MOVs) are devices (normally made of Zinc-
Oxide material) whose resistance varies with the applied voltage.
 MOV represents an open circuit as long as the voltage across it is at the
normal level.
 MOV represents a low resistance when experiencing an over-voltage. In
this case, the MOV provides a low-resistance path for the current and
maintains a safe voltage across the thyristor.
 MOVs are also used in power systems as surge arresters connected
between the power lines and the ground at the supporting poles to protect
the equipment against over-voltages caused by lightning and switching.

A switch with turn-off snubber and MOV

4-May-24 41
Snubber Requirements of Controllable Switches

 All three types of snubbers can be used with controllable switches

at the same time.
 The most common combination is:
– Turn-on snubber
– Turn-off snubber
– MOV
 Snubber requirements vary from one switch type to another. The
trend in manufacturing semiconductor devices has been to reduce
snubber requirements of the switches. To do this, the switch has to
have high dv/dt and di/dt capabilities.
 IGBTs feature low snubber requirements. Usually, a small
capacitor, together with an MOV, across the collector-emitter
terminals would be sufficient.

4-May-24 42
 Soft Switching-1
 Soft Switching is the process of turning controllable switches on and off at
zero voltage and/or current to reduce switching losses and allow high-
frequency operation.
 Zero-Voltage Switching (ZVS): Switching at zero voltage.
 Zero-Current Switching (ZCS): Switching at zero current.
 Soft switching requires special converter topologies and switching
strategies.
 Soft switching results in:
– Lower switch stress,
– Lower switching losses,
– Lower EMI, and
– Possibility of high switching frequency operation at high efficiency.
 Some converter topologies have inherent soft switching capability. In other
topologies, soft switching is implemented through auxiliary means.
 New advances in snubber circuit design have led to development of
lossless snubbers or regenerative snubbers with the capability of
recovering the energy that would otherwise be lost in snubber resistors.
The recovered energy is returned to the input side or transferred to the
output side, resulting in improved efficiency.

4-May-24 43
 Soft Switching-2
 Inherent Soft Switching Capability: A full-bridge isolated dc-dc
converter operated under phase-shift modulation technique has an
inherent capability for zero-voltage switching.
 The differences between the converter circuit shown below and the full-
bridge isolated dc-dc converter presented before are in the small capacitors
connected across the switches and the inductor connected in series with the
primary winding of the transformer.
 Phase-shift modulation technique is based on creating a phase shift
between the control signals of diagonal switches that are turned on and off
together in the conventional full-bridge isolated dc-dc converter under
bipolar voltage switching. By controlling this phase shift, widths of the
positive and negative pulses (or the dead-time between successive positive
and negative pulses) in vo1 are controlled. As a result, the widths of the
positive pulses in the rectified, unfiltered output voltage vo2 are controlled.
This, in turn, controls the average output voltage Vo.

+
+ v02
v01 -
-

[3]
An isolated full-bridge DC-to-DC Converter under phase-shift modulation

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 Soft Switching-3
 In the following, the control signals of the switches together with the
waveform of the voltage vo1 are shown.

+
v01
-
[3]

An isolated full-bridge DC-to-DC Converter under phase-shift modulation

Dead time
S1
t
S4
t
S3 
t
S2
t
v01
t

Switch control signals and H-bridge output voltage waveform

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 Soft Switching-4
 Inherent Soft Switching Capability:
– Objective: ZVS
– Ls keeps current flowing during switch over.
– Situation under study: S1 and S3 are ON. It is intended to turn S1 off and turn
S4 on.
– Assume iLs > 0.
– Scenario:
• iS1 falls.
• iLs is transferred to C1 across S1. C1 charges from 0 to Vin through the path
composed of dc source, C1, Ls, transformer primary winding and S3.
• C4 across S4 is discharged through the path composed of C4, Ls, transformer
primary winding and S3 in the meanwhile.
• S4 is turned on when vS4=vC4=0. This implies ZVS at turn-on.
– Note that iLs should be large enough to discharge C4 before S4 is turned on.
Therefore, Ls should be large. This can cause voltage drop in the circuit.

C1 C2

[3]
C4 C3

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 Soft Switching-5
 Soft Switching through Auxiliary Means:
– Resonant-Mode Converters are a class of converters which use L-C resonant
circuits to shape the voltage and current waveforms, i.e., produce zero-
crossings for the switch voltage or current at the moment of turn-on or turn-
off, to achieve ZVS and/or ZCS.
– Different combinations of converter topologies and switching strategies have
been developed to achieve this goal.
– Note that soft-switching techniques do not guarantee soft switching over the
entire operating range.

A Resonant-mode dc-to-dc converter

Resonant circuits
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A Regenerative Snubber for Boost Converter-1

[4]

4-May-24 48
A Regenerative Snubber for Boost Converter-2

1st turn-off
1

[4]

turn-on
2

2nd turn-off
3

4-May-24 49
 A Regenerative Snubber for Boost Converter-3
 4.5-kW proof-of-concept laboratory prototypes for hard-switched and
regenerative snubber converter were built and tested in the Power
Electronics Lab, at the University of Waterloo.
 Turn-on losses were negligible due to Silicon Carbide diodes.
 Turn-off losses were reduced by regenerative snubber.
 Auxiliary switches enjoyed soft turn-on and turn-off.
 Converter efficiency at 70 kHz was almost the same as that of hard-
switched converter at 30 kHz.
 Over 25% reduction in mass was achieved due to inductor size reduction as
a result of operation at a higher switching frequency.
 The two converters
had similar costs.

[4]

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A Regenerative Snubber for Boost Converter-4

Experimental Setup

[4]

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A Regenerative Snubber for Boost Converter-5

Experimental Results

[4]

Hard-Switched Converter Regenerative Snubber Converter

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A Regenerative Snubber for Boost Converter-6

Experimental Results

[4]

4-May-24 53
Reference:
[1] Mohan, Undeland and Robbins, Power Electronics: Converters,
Applications, and Design, 3rd Edition, John Wiley & Sons, Inc.,
2003.
[2] R.S. Ramshaw, Power Electronics and Semiconductor Switches,
Chapman & Hall, 1993.
[3] Prof. P. Jain’s Course Notes of “Design of Power Electronic
Circuits” (Queens University).
[4] J. Bauman and M. Kazerani “A Novel Capacitor- Switched
Regenerative Snubber for DC/DC Boost Converters”, in IEEE
Transactions on Industrial Electronics, Vol. 58, Issue 2, February
2011, pp. 514 – 523.

4-May-24 54