Soft Switching of IGBTs in Lagging Lag of ZVT Phase Shift DC/DC Converter
ZELJKOVIC Sandra
Soft Switching of IGBTs in Lagging Lag of ZVT Phase Shift DC/DC
Converter
Sandra Zeljkovic, Tomas Reiter
Infineon Technologies AG
Am Campeon 1-12
Neubiberg, Germany
E-Mail: Sandra.Zeljkovic@infineon.com
URL: http://www.infineon.com
Dieter Gerling
Universität der Bundeswehr München
Werner-Heisenberg Weg 39
Neubiberg, Germany
E-Mail: Dieter.Gerling@unibw.de
URL: http://www.unibw.de
Keywords
HV to LV DC/DC converter, hybrid electric vehicle, phase shift full bridge ZVT DC/DC converter,
high speed IGBTs, lagging leg.
Abstract
The additional effort to achieve zero voltage transition (ZVT) in the lagging leg of frequently used
ZVT phase shift full bridge converter can be avoided by designing the converter with ‘high speed’
trench fieldstop IGBTs. Thanks to their reduced turn-off but at the same time low turn-on losses, the
lost of ZVT in the lagging leg is not anymore critical to converter’s efficiency. Moreover, it can be
beneficial due to their improved ‘conduction to switching loss’ ratio. Based on that conclusion, a
simple method to maximize their efficiency by minimizing the resonant inductance is proposed.
Introduction
In hybrid and electric vehicles, the 14V network is supplied from the high voltage (HV) battery
through an isolated DC/DC converter [1]. One of the most common topologies for this application is
zero voltage transition (ZVT) phase shift (PS) full bridge (FB) DC/DC converter (Fig. 1(a)). Turn-on
losses of HV side switches in this topology are completely or partially eliminated by turning the
switches on at zero voltage. The idea behind is to achieve soft switching using parasitic elements output capacitances of switches and leakage transformer inductance. In the practical implementation,
external inductors are used to extend the range of currents at which ZVT is achieved in converter’s
lagging leg [2], [3]. This design consideration proved especially beneficial for superjunction (SJ)
MOSFETs (commonly used switches for the range of switching frequencies around 100 kHz and
medium blocking voltage, e.g. 600V). Modern IGBT series, which are nowadays alternative to SJ
MOSFETs in 100 kHz switching frequency range, are not their direct replacement regarding the
converter design and operation. Still, mentioned considerations are often simply transferred to the
converter design with IGBTs, which does not always bring expected results. In [4], ‘high speed’ trench
fieldstop IGBTs are successfully applied in ZVT PS FB converter at switching frequency of 100 kHz
and initial studies on differences in converter design compared to application of SJ MOSFETs are
done in [5].
In this paper, the effect of external inductor Lext used to extend the range of currents where ZVT is
achieved in the lagging leg is investigated. The results show that ‘high speed’ IGBTs exhibit better
efficiency during lagging leg transition when Lext is avoided and only relatively small leakage
inductance Lleak of the transformer is used instead. This is the consequence of the ratio between
conduction and switching losses of IGBTs, which differs from the ratio known for SJ MOSFETs when
they are applied in this topology. Although Lext helps to achieve ZVT for wider load range in
converter’s lagging leg, and reduces in that way the turn-on losses completely, it affects negatively
both the turn-off losses in the lagging leg and conduction losses in the freewheeling period.
For the design of the converter using optimized ‘high speed’ IGBTs, the difference in loss balance
compared to previously used technologies should be understood, so that the converter can better utilize
the advantages of chosen switch technology. These advantages mean the opportunity to improve the
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Soft Switching of IGBTs in Lagging Lag of ZVT Phase Shift DC/DC Converter
ZELJKOVIC Sandra
efficiency avoiding additional external elements (inductor in this case), thus to reduce the converter
cost and complexity.
The paper is organized as follows: it has been investigated how the value of Lres affects the three
following loss mechanisms in the lagging leg of the HV-side H-bridge: turn-on losses in Section 2,
turn-off losses in Section 3, and losses in the freewheeling period in Section 4. As a conclusion, the
overall impact on the converter's efficiency will be examined.
IGBT-based ZVT Phase Shift Full Bridge Converter
The detailed explanation of ZVS PS FB converter’s operation can be found in many references, e.g.
[6]-[7]. As a basis for understanding the mechanism of switching and conduction losses in HV side Hbridge, current of transformer primary winding is analytically expressed. This model is used later in
the paper for losses analysis. Fig. 1(b) shows the sequence of gate signals over a switching period as
well as transformer primary current waveform. Three main states in the operation of H-bridge are
power transfer, freewheeling period and ‘loss of duty cycle’ that occurs at the beginning of each half
of Tsw. The operation of two H-bridge leg (leading and lagging leg, see Fig. 1(a)) differs during one
half-period: the transition in the leading leg occurs between power transfer and freewheeling and
transition in the lagging leg occurs between freewheeling and ‘loss of duty cycle’ period. Current
waveform (Fig. 1(b)) in period from t1 to t6 (except short transition periods) is described in Table I.
Fig. 1 (a) ZVT phase shift full bridge DC/DC converter for (H)EVs (b) Transformer primary current
in one switching period Tsw and corresponding gate signals of HV side H bridge switches. In
experimental measurements presented in this paper, the measurement trigger point in reference to the
rest of Ts is marked
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Soft Switching of IGBTs in Lagging Lag of ZVT Phase Shift DC/DC Converter
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Effect of Lres on IGBT turn-on losses in the lagging leg
Numerous solutions have been proposed for the problem of losing ZVT in the lagging leg at light
loads. The most frequently used, due to its simplicity, is still an external resonant inductor Lext in
addition to Lleak (Fig. 1(a)) (e.g. in [7]). Its purpose is to extend the range of currents in which ZVT is
achieved in the lagging leg (thus eliminates turn-on losses). Unfortunately, additional Lext brings along
several disadvantages: increased number of components, higher loss of duty cycle (which may lead to
the need for lower transformer turns ratio, resulting in higher values of primary current) and higher
voltage stress on secondary side switches. The main condition to achieve the ZVT of lagging leg (1) is
that available energy in the resonant inductance (Lres = Lleak + Lext) has to be higher than the energy
required for charging capacitances that take part in this transition. Here Coss is a sum of non-linear
output capacitances of low- and high-side switch in lagging leg, Ctr is the capacitance of transformer
primary winding).
(1)
1
4
1
( Lleak + Lext ) I 2frw,end > CossVin2 + CtrVin2
2
3
2
TABLE I Overview of conduction periods and switching transitions of IGBT switches
and diodes in HV H-bridge during the one half of switching period Tsw (the other half is
symmetric)
Part of Tsw
Loss of
duty cycle
t1 - t2
Leading leg
Power transfer
transition
time
t2 - t3
Lagging leg
Freewheeling
transition
time t5 – t6
t 4 – t5
t3 - t4
Beginning
of a
0
d loss ⋅
T
2
T
− t lag _ trans
2
T
+ t lead _ trans
2
D⋅
T
2
D⋅
D⋅
T
+ t lead _ trans
2
T
− t lag _ trans
2
period
End of a
period
Conducting
d loss ⋅
T
2
S1, S4
D⋅
T
2
S1, S4
T
2
S1, D2
switches
Switching
Turn-on S1
Turn-off S4
Turn-off S1
events
v in
⋅t
L res
( I out , ave −
k out ⋅ D eff ⋅
2
n
Itr(t)
v in
− v out
n
⋅ (t − t 2 )
Lout
n
T
2)
−
+
req
⋅t
(Imin −ΔIlead_trans) ⋅ e Lleak
req
v + v − ⋅t
+ d0 ce0 (e Lleak −1)
req
req = rce + rd + R par
The effects of Lres on the losses in IGBT and freewheeling diode are investigated using the prototype
converter (whose details are given in Appendix). First, the process of successful ZVT in the lagging
leg is described.
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When the condition from (1) is fulfilled, the switch in the lagging lag, S3, turns on at Vce ≈ 0. Fig.
2(a) presents the turn-on of S3 in the lagging leg measured when ZVT is achieved thanks to the high
enough value of Lext. In period P1 in Fig. 2(a), output capacitance of the switch is discharged prior to
the occurrence of gate voltage. When the gate voltage reaches threshold value, current through the
U opH
switch starts to rise with the slope L res . This phase is the ‘loss of duty cycle’. In this period,
secondary winding of the transformer is still shorted by the rectifier diodes. Two of diodes (body
diode of switches SR1, SR3 or SR2, SR4) stop conducting when their current falls to 0, which happens
at the beginning of period P4 in Fig. 2(a). Period P4 is the power transfer. In the prototype converter
used to obtain the measurements of described event in Fig. 2(a), Lext of 2.2uH is used in addition to
Lleak of transformer winding of 1.3uH.
In the opposite case investigated here, when ZVT of S3 cannot be achieved (in the designs with low
value of Lres or in light load conditions in designs with high value of Lres), a certain amount of turn-on
losses occurs. However, turn-on losses mechanism in this case differs from the one in hard switching
converters (e.g. in motor drive inverter). In typical hard switching converter with clamped inductive
load, at turn-on, IGBT has to take over the full load current from the diode, so that the losses will also
be affected by diode reverse recovery. In case of turn-on in lagging leg of ZVT PS FB converter, two
differences are present. The switch which is turning on does not have to take over the current from the
opposite freewheeling diode, so there will be no reverse recovery losses. Furthermore, if ZVT of
lagging leg is not achieved, the energy stored in Lleak of transformer winding is spent; its current falls
to 0. When S3 is turned on, ‘loss of duty cycle’ period starts and current rises from 0 with limited slope
(4). Such mechanism of turn on losses is mathematically described in [7].
Turn-on event of S3 is presented in Fig. 2(b), measured on the same prototype in the same operating
condition as in Fig. 2(a), but without any Lext. During the period P1 in Fig. 2(b), while switch is still
off, its output capacitance is being discharged. When no more energy is available in Lleak, voltage rises
back to the value of DC link (P2). When gate voltage occurs (after the dead-time is over), output
capacitance has to be discharged again. The portion of discharge current can be clearly distinguished
in the current waveform in period P3, and is superposed to the primary winding current ‘ramp’. It can
be noticed that the current bump in period P3 (that occurs during the charging of intrinsic switch
capacitance) in Fig. 2(b) does not exist in Fig. 2(a). This is the consequence of the capacitance
discharge prior to the occurrence of gate voltage. Furthermore, the duration of period P3 in both
figures differs. In Fig. 2(a), due to the higher Lres applied, the loss of duty cycle is longer.
Fig. 2 (a) Turn-on of the switch in lagging leg when ZVT is achieved (gate voltage occurs when Vce
is already 0V). Operating point: UHop = 200V, ILout = 115A Lext = 2.2uH, Lleak = 1.3uH.
(b) Turn-on of the switch in lagging leg when ZVT is not achieved (gate voltage occurs while Vce is
equal to the DC link value). Operating point: UHop = 200V, ILout = 115A Lext = 0uH, Lleak = 1.3uH
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Turn-on losses are measured in the prototype without Lext over the range of load current for the input
voltage of 200V and presented in Fig. 5(a). The curve of turn-on losses is rather flat, due to the fact
that the switch capacitance is voltage dependent and current slope during dloss is not dependent on the
load current.
The common understanding of the effect of ZVT on the gate voltage is the absence of Miller plateau
[8]. Such understanding comes from the behavior of conventional vertical MOSFETs, but as can be
noted in the gate voltage in Fig. 2(a), the plateau in Vge is visible although the ZVT is successfully
achieved (i.e. Vce = 0 before Vge increases). Just a slight difference in the waveforms of Vge in Figs.
2(a) and 2(b) can be noted when turn-on events with and without ZVT are compared. Such behavior is
the consequence of IGBT structure with two different pn junctions, one at the emitter and the other at
the collector side. When IGBT is still in the blocking state with Vce = 0, the built-in voltages over
these two junctions are compensating each other. As soon as Vge reached the value where channel
opens, the pn junction at emitter side transits to the conducting state and this change in internal voltage
has further feedback on Vge, known as Miller plateau.
Effect of Lres on IGBT turn-off losses in the lagging leg
Fig. 3 (a) Turn-off of the switch in lagging leg when ZVT is not achieved. Operating point: UHop =
200V, ILout = 115A Lext = 0uH, Lleak = 1.3uH; (b) Primary winding current during the lagging leg
transition when Lext = 2.2uH applied in addition to Lleak = 1.3uH of primary winding (black trace) and
when no Lext applied so that only Lleak is present during transition (blue trace). Operating point: UHop =
200V, ILout = 115A Lext = 0uH, Lleak = 1.3uH
When ZVT is not achieved in the lagging leg (the test-case without Lext considered in this work,),
not only turn-on but also turn-off losses are affected. Fig. 3(a) is an example of the turn-off event of
switch S1 when there is not enough energy in Lres to achieve ZVT. When, on the other hand, there is
enough energy in Lres to discharge the output capacitance of S1, Vce falls to zero before all the energy
from inductor is spent. Switch is turned off and the rest of the primary winding current is taken over
by the freewheeling diode. Difference in IGBT turn-off currents with and without Lext is presented in
Fig. 3(b). Current during the freewheeling period is higher in case when Lext is applied, and
consequently, the Ic,turn-off of S1 is higher. Based on equations from Table I, turn-off current can be
analytically determined. The comparison of measured and calculated values over the range of load
currents in presented in Fig. 4(a).
Furthermore, turn-off voltage Vce, turn-off differs in test-cases with and without Lext as well. In Fig. 3(a)
in case when all the energy is spent from Lleak, turn-off will happen at Vce lower than UopH (marked on
Fig. 3(a)). In the test-case with Lext, when ZVT is achieved, turn-off happens at full input voltage UopH.
Thus, beside reduced Ic,turn-off, Vce,turn-off is also smaller compared to the test-case with Lext. In 5(a),
energy of turn-off losses for two considered test-cases is estimated based on the data-sheet value for
energy of turn-off losses, using measured values of Ic,turn-off and Vce,turn-off.
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Fig. 4 (a) Currents Ic,turn-off with ZVT achieved (black) and ZVT not achieved (blue). Calculated
values are dots and measured values are lines. (UHop = 200V).
(b) Voltages Vce,turn-off of S1 when ZVT achieved (black trace) and not achieved (blue trace). The results
are experimentally obtained at UHop = 200V.
Effect of Lres on the conduction losses in the freewheeling of IGBT-based
converter
Conduction losses in the freewheeling period (t4 to t5 in Fig. 1(b)) are also affected by the value of
Lres. Not only Ic,turn-off of S1 is higher, but also RMS values of freewheeling currents are increased, and
thus the conduction losses in the freewheeling (Fig. 5(b)). The effect is more stressed in IGBT-based
design (as the freewheeling diode has to conduct) than in SJ MOSFET-based one, where MOSFET
channel can be turned on and increase in conduction losses is less remarkable.
Fig. 5 (a) Comparison of switching energies of IGBT during the lagging leg transition for cases with
and without Lext. This transition occurs twice per switching period. (UHop = 200V). (b) Comparison of
conduction losses in primary winding circuit during the freewheeling period (values calculated based
on the model from Section 1 for UHop = 200V).
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Conclusion: Losses minimization by reduction of Lres in of IGBT-based
converter
When all effects are summarized, it can be concluded from Fig. 6 that ‘high speed’ trench fieldstop
IGBTs are more efficient with minimized Lres in lagging leg (only considerable small transformer
Lleak). With large enough Lext, the turn-on losses are eliminated indeed, but turn-off losses as well as
conduction losses in freewheeling are increased to the extent that total switching losses in the lagging
leg transition are increased. At higher load currents, effect of reduced efficiency when Lext is used is
more significant.
no Lext
Lext = 2.2µH
Lext = 3.3µH
94
93
without Lext ZVT not
achieved
ZVT achieved
92
ZVT achieved
91
20
40
60
80
100
120
140
Output Current [A]
Fig. 6 Efficiency of ‘high speed’ IGBT-based design of ZVT PS FB converter (no auxiliary supply
included) with blue and green for two values of Lext (applied to achieve ZVT of lagging leg) and red
without Lext (where natural zero current switching in the lagging leg is achieved). Operating
conditions: UHop = 200V; IoutL of 150A corresponds to the Ic, turn-off of 20A, and IoutL of 75A corresponds
to the Ic, turnoff of 10A.
The described behavior has not been experienced in ZVT PS FB topology with SJ MOSFETs, as
they exhibit lower turn-off losses compared to ‘high speed’ IGBTs due to absence of tail current
phenomena. Furthermore, due to larger chip area required for the same current rating, intrinsic
capacitance of the MOSFET is more significantly affecting turn-on losses than in case of ‘high speed’
IGBTs.
Thus, elimination of turn-on losses in the lagging leg of ‘high speed’ IGBT-based converter by
increasing Lres will not increase the converter’s efficiency. Reduction of Lres to the value of only
moderate Lleak of the transformer will result in boosted efficiency of ‘high speed’ IGBT-based
converter. Additionally, components number, converter’s cost and complexity will be reduced while
the behavior of IGBT switches will still be enhanced.
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Soft Switching of IGBTs in Lagging Lag of ZVT Phase Shift DC/DC Converter
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Appendix: Details of Prototype Converter
Operating
conditions
min
UopH
160V
UoutL
IoutL
typ
max
360V
14V
0A
170A
PoutL
2.4kW
fsw
100kHz
Component
Controller IC
TI UCC28950
HV switches
F4-50R07W1E3_B11A
LV switches
2 IPB019N08 per switch in H bridge
rectifier
Transformer
Transformer TDK T6973-A2
(9:1, 1.3uH)
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Output ind.
TDK T7921-51
Input capacitor
10uF
Output cap.
1000uF
1.7uH
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References
[1] S.M.N.Hasan, M.N.Anwar, M.Teimorzadeh, D.P.Tasky, “Features and challenges for Auxiliary Power
Module (APM) design for hybrid/electric vehicle applications”, IEEE Vehicle Power and Propulsion Conference
(VPPC) 2011, 6-9 Sept. 2011
[2] L.H.Mweene, C.A.Wright, M.F.Schlecht, “A 1 kW, 500 kHz front-end converter for a distributed power
supply system”, Applied Power Electronics Conference and Exposition 1989, Fourth Annual Conference
Proceedings 1989. pp.423-432, 13-17 March 1989
[3] R.Redl, L.Balogh, D.W.Edwards, “Optimum ZVS Full-Bridge DC/DC Converter with PWM Phase-Shift
Control: Analysis, Design Considerations, and Experimental Results”, Applied Power Electronics Conference
and Exposition 1994, Ninth Annual Conference Proceedings 1994. pp.159-165, 13-17 February 1994
[4] T.Reiter, S.Zeljkovic, “Design of an automotive 2.5kW HV to LV DC/DC converter using HighSpeed
IGBTs”, Elektrik/Elektronik in Hybrid und Elektrofahrzeugen und elektrisches Energiemanagement, Miesbach,
Germany 2012
[5] S.Zeljkovic, T.Reiter, D.Gerling, “Switching Behavior of IGBTs in Phase Shift Full Bridge ZVT DC/DC
Converter”, PCIM Europe, Nuremberg, Germany 2013
[3] J.A.Sabate, V.Vlatkovic, R.B.Ridley, F.C.Lee, B.H.Cho, “Design considerations for high-voltage high-power
full-bridge zero-voltage-switched PWM converter“, Applied Power Electronics Conference and Exposition
1990, Fifth Annual Conference Proceedings 1990, pp.275-284, 11-16 March 1990
[6] F.C.Lee, M.M.Jovanovic, J.A.Sabate, “A comparative study of a class of full bridge zero-voltage-switched
PWM converters”, Applied Power Electronics Conference and Exposition 1995, Tenth Annual Conference
Proceedings 1995., pp.893-899, 5-9 March 1995
[7] Fei Zhou, Xinmin Jin, Yibin Tong, Xuezhi Wu, Xiuyuan Yao, “A turn-on switching losses study in a ZCT
soft-switching converter”, 7th International Power Electronics and Motion Control Conference (IPEMC) 2012,
vol.3, no., pp.1607,1610, 2-5 June 2012
[8] M. Kazimierczuk and D. Czarkowski, Resonant Power Converters. Wiley-IEEE Press, 2011.
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