# On the Development of High Power DC-DC Step-Down Converter with Energy Recovery Snubber

Authors: Alok Singh; Mangesh B. Borage; Sunil R. Tiwari; A. C. Thakurta
Journal: Advances in Power Electronics
 (2012)

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## Abstract

The effect of switching losses on the efficiency of a switch mode power converter and methods adopted for its improvement using an energy recovery lossless snubber has been presented. A comparative analysis of various types of soft switching techniques along with effects of dissipative and nondissipative snubbers on efficiency of the converter has been carried out before zeroing in on the selected scheme. The selected snubber serves the dual function of a turn-on and turn-off snubber and thereby reducing the switching losses both during turn-on and turn-off transients, resulting in improved efficiency of the converter. A detailed design procedure of the snubber for high-power applications taking into account various effects such as diode reverse recovery, diode voltage stress, and minimum and maximum duty cycle limits, has been presented in this paper. Importance of practical aspects in layout to minimize wiring inductance is also highlighted. A high-power prototype of buck converter has been developed to experimentally validate the theoretical design and analytical observations.


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## Body

## 1. Introduction

Switch mode power supplies are popular because of higher efficiency, smaller size, and lighter weight [1, 2]. Low weight and smaller size comes about because operation is significantly at a higher frequency range, and thus the magnetic components are quite smaller in size and weight [3]. High switching frequencies in turn results in large switching losses and stresses in power device because during the turn-on and turn-off transients the power device has to withstand high voltage and current simultaneously [4, 5]. Large di/dt and dv/dt transitions also induces significant amount of electromagnetic interference (EMI) in the circuit which in turn pollute the utility systems [6]. This problem, if left unattended, may result in lower efficiency and larger size of converters owing to increased size of the heat sinks and EMI filters. Efforts are made in order to alleviate the aforementioned limitations and to enable operation at still higher frequencies while maintaining better conversion efficiency [7].Snubbers are an essential part of power electronic systems. They are small network of passive components which forms a part of power switching circuits to reduce switching losses and stresses in power devices [8]. But, as the switching losses in the main switch are dissipated in form of heat in snubber resistors, the overall efficiency of the converter is deteriorated. Various soft switching techniques have been proposed in the  literature [1, 9, 10] to obtain soft switching of power devices while at the same time maintaining higher overall efficiency. But this is achieved at the cost of increased design complexity of power and control circuits and thereby increasing circuit cost and deteriorating the system reliability [5].Energy recovery snubber poses a solution to the problem as it is simpler, reliable, and cheaper than most of the soft switching techniques and also helps in reducing the switching losses without deteriorating the overall system efficiency. The energy stored in an energy recovery snubber is either moved back to the input or ahead to the output in a nondissipative manner [5, 6, 8, 11, 12]. Energy recovery snubbers can be further classified as turn-on, turn-off, and combined function snubbers. Turn-on snubbers are basically nondissipative current snubbers which control the rate of rise of current in the main switch at turn-on transient and thereby achieving zero current switching (ZCS) at turn-on transient of the switch [8, 13], while turn-off snubbers are nondissipative voltage snubbers which control growth rate of voltage across the switch during turn-off transient and thus help in achieving zero voltage switching (ZVS) at turn-off transient [8, 14–17]. The combined function snubber, also called as turn-on and turn-off snubber, serves the dual function of achieving ZCS turn-on and ZVS turn-off of the switch during turn-on and turn-off transients, respectively, and hence includes the functionality of both turn-on and turn-off snubber in a single circuit [5]. An energy recovery turn-on and turn-off snubber is thus selected for further investigation and implementation in a high power dc-dc converter.Although, synthesis and optimization of various snubbers have been widely reported in the literature for low power dc-dc converters [1, 5, 6, 9–19], the issues related to design, optimization, application, and practical aspects pertaining to their use in high power dc-dc converters are not reported. This paper presents the design and optimization of energy recovery snubber with wide operating range for high power dc-dc converters. The presented work is based on the analytical results reported in [5]. Brief operation of the energy recovery snubber is presented in Section 2 followed by design parameters of dc-dc converter in Section 3, design and optimization of energy recovery snubber in Section 4, simulation and experimental results in Section 5, and conclusions in Section 6.


## 2. Operation of Energy Recovery Snubber

Snubber shown in Figure1 is an energy recovery turn-on plus turn-off snubber also called as 3D-2C-1L snubber. The snubber network has been called so because it has three diodes, two capacitors, and one inductor. Growth rate of reverse recovery current is controlled by snubber inductor Ls, while growth rate of voltage across switch is controlled by snubber capacitor Cs. Energy recovery is achieved by discharging the buffer capacitor Cb to the output. The key waveforms of the buck converter with energy recovery snubber are shown in Figure 2 [5]. During turn-on process, the diode recovery current is the major cause of the switching losses and EMI. The rate of rise of current and peak reverse recovery current is restricted by the snubber inductor Ls to suppress the switching loss. During turn-off process, the rate of rise of drain-source voltage is restricted by the snubber capacitor Cs to obtain ZVS turn off and to reduce EMI noises.Figure 1
Buck converter with energy recovery snubber.Figure 2
Key waveforms of the buck converter with energy recovery snubber.The freewheeling diode is also commutated with ZVS during both turn-on and turn-off. The energy absorbed in snubber inductorLs and snubber capacitor Cs is transferred to Cb instead of being dissipated, which is subsequently transferred to the output. Detailed operation of this snubber with various modes of operation is given in [5].


## 3. Design Parameters of DC-DC Converter

A high-power prototype of buck converter with maximum output voltage and current ratings of 450 V and 55.5 A, respectively, has been developed to experimentally validate the theoretical design of energy recovery snubber. The design parameters of the buck converter are given in Table1.Table 1
Prototype specifications.
Maximum output power
25 kW
Switching frequency
25 kHz
Filter inductor,Lm
800μH
Filter capacitor,Co
3μF
Filter damping capacitor,Cd
15μF
Filter damping resistor,Rd
15 ΩThe insulated gate bipolar transistor (IGBT) module used in the prototype is SKM100GAR123D [20].


## 4. Design and Optimization of Energy Recovery Snubber

Design and optimization of energy recovery snubber are described in six steps as given below. Subsequently, a design flow diagram for snubber parameters calculation is also presented in   Figure3.Figure 3
Design flow diagram for snubber parameters calculation.Step 1 (Choice ofCs).
The snubber capacitorCs shown in Figure 1 is mainly responsible for ZVS turn-off of switch S1. Thus, the discharging time of Cs should be quite larger than the fall time tf of the switch to ensure proper ZVS turn-off of the switch. When the switch S1 turns off, the output current Io flows through D2 to discharge Cs to the output. During this period, capacitor discharges linearly. If the time taken by Cs to discharge completely from the value of Vd to zero is td, then, capacitance Cs can be written in terms of output current Io, input dc voltage Vd and discharge time td as follows:
(1)Cs=IotdVd.
As typically the fall timetf of the switch is around 100 ns. Thus, to ensure proper ZVS turn-off of the switch, discharge time td is taken to be approximately five-times the fall time of switch tf, that is, 500 ns. Therefore, for (Io)max = 55.55 A, (Vd)min= 504 V, and td=500 ns, the value of snubber capacitance Cs comes out to be 0.055 μF from (1). The nearest available value of snubber capacitance used in the practical circuit is 0.047 μF.Step 2 (Choice ofLs).
When the switchS1 shown in Figure 1 is turned on, the growth rate of the drain current is restricted by the snubber inductor to softly turn on the switch. During this duration, rate of fall of main diode current iD1, which is also equal to the rate of rise of switch current, is given as follows:
(2)diD1dt=VdLs.
Values ofdiD1/dt calculated for various values of Ls and Vd=600 V using (2) have been listed in column 2 in Table 2. Column 3 in Table 2 shows the estimated values of reverse recovery current Irr flowing through main diode D1 for the corresponding values of diD1/dt listed in column 2. These values have been estimated from Irr v/s diDf/dt curve given in the datasheet of IGBT module [20].Table 2
Calculation of snubber parameters.
Ls (μH)
di/dt (A/us)
I
rr
(A)
C
b
(μF)
Condition
I
sw
,
pk
(A)
Min. duty cycle limit
Max. duty cycle limit
1
600
35
0.328565
−7504.5
TRUE
182.6
0.028185861
0.969112264
2
300
22
0.323911
−5808
TRUE
144.7242
0.040937446
0.962884825
3
200
20
0.328112
−4356
TRUE
129.0297
0.051743372
0.955668148
4
150
18
0.329851
−2836
TRUE
119.4543
0.06115151
0.945936337
5
120
16
0.329561
−1260
TRUE
112.7242
0.069572661
—
6
100
15
0.330828
273
FALSE
107.9057
0.077569317
—
7
85.714286
15
0.334903
1728.5
FALSE
104.4218
0.085468737
—
8
75
15
0.338977
3184
FALSE
101.634
0.093078882
—
9
66.666667
15
0.343051
4639.5
FALSE
99.34127
0.100457752
—
10
60
15
0.347125
6095
FALSE
97.41529
0.107647168
—Step 3 (Choice ofCb).
Buffer capacitanceCb is expressed in terms of Ls, Cs, Vd, Irr, and VCb,p as follows [5]:
(3)Cb=LsIrr2+CsVd2VCb,p2.
As the voltage stress of freewheeling diodeD1 is equal to Vd plus VCb,p, thus the peak voltage appearing across buffer capacitor Cb is capped at 235 V to keep the voltage rating of D1 within limits (maximum voltage rating of diode D1is 1200 V). Hence, the peak voltage appearing across diode D1 is given by. 							(4)VD1,p=Vd+VCb,p≅850V.
Thus, values ofCb calculated for corresponding values of Ls, Cs, Vd, Irr, and VCb,p are listed in column 4 in Table 2.Step 4 (Checking for Condition).
The necessary condition which ensures the proper ZCS turn-on and ZVS turn-off operation of circuit is given in [5] and can also be written as follows:
(5)12LsIo2-12LsIrr2-12CsVd2<0.
Column 5 in Table2 calculates the left hand side (LHS) of inequality given in (5) and the result is TRUE if LHS is negative, while FALSE otherwise.Step 5 (Check forDmin and Dmax).
Maximum and minimum duty cycle limits are imposed on the buck converter due to finite time taken by the snubber circuit for its operation. During this duration, the normal operation of buck converter is suspended. Maximum duty cycle limit occurs due to the snubber circuit operation at switch turn-off transition, while minimum duty cycle limit occurs due to the snubber circuit operation at switch turn-on transition. Thus, the duty cycle limits can be obtained by calculating the time taken by the snubber both during turn-on and turn-off transitions.
Various stages of operations have been discussed in [5]. Out of 8 stages of operation of buck converter circuit described in [5], snubber is active during 6 stages (t0<t<t1),(t1<t<t2), (t2<t<t3), (t4<t<t5), (t5<t<t6), and (t6<t<t7). Time duration of all these 6 stages are calculated here for values of Ls varying from 1 μH to 10 μH, Cs=0.047 μF, Vd=600 V, peak voltage across buffer capacitor VCb,p=235 V, and output current Io=56 A. To do this, expressions for time durations for these stages are derived based on equations describing voltages and currents given in [5]. They are summarized as follows.
Time durationTd1 of stage 1 is given by
(6)Td1=(t1-t0)=LsVd(Io+Irr).
Time durationTd2 of stage 2 is given by
(7)Td2=(t2-t1)=1ω1{sin⁡-1⁡(CsVdCb(IrrZ1)2+Vd2)+tan-1⁡VdIrrZ1},
where Z1 and ω1 are given in (8),
(8)Z1=Ls(Cs+Cb)CsCb,ω1=Cs+CbLsCsCb.
Time durationTd3 of stage 3 is given by
(9)Td3=(t3-t2)=1ω2tan-1⁡(CbIs2Z2CsVd),
where Is2, Z2 and ω2 are given by (10), and (11), respectively:
(10)Is2=VdZ1sin⁡⁡(ω1Td2)+Irrcos⁡⁡(ω1Td2),(11)Z2=LsCb,ω2=1LsCb.
Time durationTd4 of stage 5 is given by
(12)Td4=(t5-t4)=CsVdIo.
Time durationTd5 of stage 6 is given by
(13)Td5=(t6-t5)=1ω2sin⁡-1⁡(IoZ2VCb,p).
Time durationTd6 of stage 7 is given by
(14)Td6=(t7-t6)=CsVdIocos⁡⁡(ω2Td5),
where Td5 is given by (13).
The switchS1 must be on from t=t0 at least till t=t3 when ILs becomes zero. This governs the minimum duty cycle limit which is given by
(15)Minimumdutycycle⁡limit(Dmin⁡)=Td1+Td2+Td3Ts,
where Td1,Td2, and Td3 are given by (6), (7), and (9) respectively, and Ts is the switching time period. Values of minimum duty cycle limit calculated for Ts=40 μs, corresponding to the switching frequency of 25 kHz are listed in column 7 in Table 2.
The switchS1 must be maintained in off condition from t=t4 to at least t=t7 when the buffer capacitor Cb is discharged completely. This dictates maximum duty cycle limit given by
(16)Maximumdutycycle⁡limit(Dmax⁡)=1-(Td4+Td5+Td6Ts).Step 6 (Calculation for Ratings).
Peak value of the drain current of switchS1 appears when VCb+VCs becomes equal to Vd and is obtained by the summation of output current Io and peak snubber inductor current ILs,p which is given by [5]:
(17)ILs,p=Vd2+(IrrZ1)2Z1,
where,
(18)Z1=Ls(Cs+Cb)CsCb.
Thus, peak switch current is given by(19)Isw,p=Io+ILs,p.
Therefore, from (17), (18), and (19), peak value of switch current Isw,p is calculated for corresponding values of Ls, Cs, Cb,Vd, Irr, and Io and listed in column 6 in Table 2.
For the buck converter prototype, specifications of which are given in Table1, iterative calculations have been done for buffer capacitance of snubber Cb, peak current rating of switch Isw,pk, and minimum and maximum duty cycle limits for various values of snubber inductance Ls ranging from 1 μH to 10 μH while taking Cs=0.047 μF, Vd=600 V, peak voltage across buffer capacitor VCb,p=235 V, and output current Io=56 A as shown in Table 2. Highlighted values in the table show the selected snubber parameters.Irr shown in table is the estimated reverse recovery current of the main diode D1.
Values of maximum duty cycle limit calculated forTs=40 μs are listed in column 8 in Table 2. It is important to note that the choice of snubber components, therefore, also governs the operating range of the converter in addition to various voltage and current stresses described previously.
Snubber parameters, therefore, should be selected after making a tradeoff between the maximum permissible switch current rating, diode voltage rating, and operating duty cycle range provided that the circuit satisfies the inequality given in (5) which ensures proper soft switching operation of snubber over that range.
Based on the calculations illustrated in Table2, the following component values are chosen:Ls=4 μH,Cs=0.047 μF,Cb=0.33 μF,Isw,p=119.45 A,VD1,p=850 V,VCs,p=616 V, ICs,p=55.5 A, andVCb,p=235 V, ICb,p = 55.5 A.
Diodes APT60D100B are used for snubber diodes [21]. Inductor Ls is developed in EE65.32.18 core with 4 turns and an air gap of 1.4 mm.


## 5. Simulation and Experimental Results

Simulation results for voltage and current waveforms across switch in the presence of energy recovery snubber are shown in Figures4 and 5, respectively, during on-time and off-time transitions for input dc voltage, Vd=616 V and output current, Io=56 A. From Figures 4 and 5, respectively, it is observed that energy recovery snubber helps in achieving ZCS turn-on and ZVS turn-off of switch S1 at turn-on and turn-off transients.Figure 4
ZCS turn-on of switchS1.Figure 5
ZVS turn-off of switchS1.Simulation results for comparison of efficiency of buck converter with and without dissipative and nondissipative snubbers are shown in Figure6 in which x-axis represents duty cycle variation from 0.1 to 0.9, while y-axis represents efficiency of buck converter. Dissipative snubber used in simulation consists of two cells one of which is responsible for achieving ZCS turn-on of the switch, while the other one ensures its ZVS turn-off. The turn-on snubber cell is placed in series with the switch and consists of an inductor (4 μH) connected in parallel with the series combination of a resistor (5 Ω) and an ideal diode. While the turn-off snubber cell is placed across the switch and consists of a capacitor (0.047 μF) placed in series with a parallel combination of a resistor (68 Ω) and an ideal diode. Nondissipative snubber components used in simulation are same as summarized in Section 4. From Figure 6 it is observed that efficiency of the buck converter reduces when a dissipative snubber is used to limit the switching losses. This is because of the fact that in dissipative snubbers switching losses in power devices are transferred to the dissipative component in the snubber thereby reducing overall efficiency of the converter. But, in contrast to this, nondissipative snubber reduces the switching losses by temporarily storing the energy of one switching transient in a buffer and then transferring it either back to the input or ahead to the output during the next switching transient in a nondissipative manner and thereby, reducing the switching losses while at the same time increasing the efficiency of the converter. The improvement in efficiency with energy recovery snubber is evident in simulation waveforms of Figure 6.Figure 6
Efficiency comparison of buck converter with dissipative and nondissipative snubbers.The simulation waveforms shown in Figures4 and 5 are valid only for ideal circuits which are free of any wiring inductances. But, in practical circuits, these wiring inductances play a significant role in introducing undesirable effects on the circuit operation. This can be demonstrated by introducing some wiring inductance in series with the input dc supply of simulation circuit. Figure 7 shows the simulation waveforms with the effects of wiring inductance of 100 nH introduced in the simulation circuit.Simulation waveforms with effect of wiring inductance forD=0.5, Vd=616 V, and Io=56 A: gate pulses, voltage and current in S1, voltage and current in D1, and current in Ls.
(a)
(b)
(c)
(d)From Figure7, it is observed that the wiring inductances have significant undesirable effect on the circuit performance. During turn-off transition, ringing is observed in switch voltage and current waveforms which results in an overlap between the two during the transition interval and thereby resulting in switching loss at turn-off transition as shown in Figure 7. Ringing is also observed in switch current after turn-on transition which disrupts the normal functioning of the buck converter during this interval as shown in Figure 7. Thus, significant steps should be taken during the development phase of the converter to minimize these wiring inductances by making a judicious arrangement for the assembly of snubber and other components of the converter. This can be done by prudently placing the components containing high frequency currents as close to each other as possible and thereby minimizing the wiring length which in turn results in reduced wiring inductance. Another way to reduce wiring inductance is to use twisted cables or to use flat wide conductors and using copper strips.Figures8(a) and 8(b) shows the experimental waveforms of the voltage across the switch (vs1) and current through it (is1) at turn on and turn off transitions, respectively, for D=0.5 and Vd=200 V. From Figure 8, it is clear that the switch turns on under ZCS condition and turns off under ZVS condition which is in agreement with the theoretical analysis of switch voltage and current waveforms and also with the simulation results presented in Figure 7.Experimental waveforms ofvs1 (50 V/div) and is1 (10 A/div) at (a) turn-on transition and (b) turn-off transition. In both parts, Ch3 shows gate pulse for switches (vge-s1, 20 V/div) and XScale is 250 ns/div.
(a)
(b)Figures9(a) and 9(b) shows the experimental waveforms of main diode D1 voltage(vD1)and snubber inductor Ls current (iLs) at turn on and turn off transitions respectively for D=0.5 and Vd=200 V.Experimental waveforms ofvD1 (50 V/div) and iLs (10 A/div) at (a) diode turn-on transition and (b) diode turn-off transition. In both parts, Ch3 shows gate pulse for switches (vge-s1, 20 V/div) and XScale is 500 ns/div.
(a)
(b)Since snubber inductorLs is connected in series with the main diode D1, as shown in Figure 1, a part of snubber inductor current flows through main diode D1 both during turn-on and turn-off transitions. As, it is difficult to monitor current waveform in main diode, snubber inductor current is observed here to demonstrate the ZVS turn-on and ZVS turn-off of the main diode D1. From Figure 9, it is clear that the main diode D1 also turns-on and turns-off under ZVS condition which is in agreement with the theoretical analysis of main diode voltage and current waveforms and also with the simulation results presented in Figure 7.Figure10(a) shows the snubber inductor Ls current waveform for full cycle of operation while Figure 10(b) shows the voltage and current waveforms of main diode D1 and output filter inductor Lm, as shown in Figure 1, respectively for D=0.5 and Vd=200 V. Figure 10(a) demonstrates that the snubber inductor current waveform is in accordance with the theoretical analysis and also with the simulation results presented in Figure 7. While Figure 10(b) shows that the voltage and current waveforms respectively of D1 and Lm are in agreement with the theoretical analysis of basic buck converter circuit.Experimental waveforms of (a)iLs (10 A/div) with XScale of 5 μs/div and (b) vD1(50 V/div), iLm(5 A/div) with XScale of 10 μs/div. In both parts, Ch3 shows gate pulse for switches (vge-s1, 20 V/div).
(a)
(b)Figure11 shows the graph for efficiency comparison of buck converter with and without nondissipative snubber. Graph is plotted for input dc voltage of 100 V because full load operation of circuit could be detrimental for the devices in absence of snubber.  It is quite clear from this graph that the efficiency of buck converter improves by using nondissipative snubber in the circuit, which is also in agreement with the simulation result as shown in Figure 6.Figure 11
Measured efficiency of buck converter with and without nondissipative snubber atVd=100 V.


## 6. Conclusions

Snubbers are essential part of switching power electronic circuits as these helps in reducing switching losses, stresses on semiconductor devices as well as EMI. Various types of soft switching snubbers are available in literature out of which an energy recovery snubber is selected for high power dc-dc step-down converter owing to its distinct advantages over other types. For high power applications, optimization of snubber parameters is done by making a tradeoff between the maximum permissible switch current rating, diode voltage rating and operating duty cycle range provided that the circuit satisfies an inequality which ensures proper soft switching operation of snubber over that duty cycle range. As wiring inductance brings about undesirable effects in the circuit operation, efforts should be made to minimize the wiring inductance by judicious selection of circuit layout. Experimental waveforms shows that the energy recovery snubber helps in reducing switching losses as well EMI by achieving ZCS turn-on and ZVS turn-off of switchS1 and also ZVS turn-on and turn-off of diode D1 without deteriorating overall efficiency of the converter. Practical effects of the wiring inductance on circuit performance are also demonstrated by experimental waveforms.



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*Source: 806738-2012-07-02.xml*