A True ZCZVT Commutation Cell for PWM Converters Carlos Marcelo De Oliveira Stein, Student Member, IEEE, and Hélio Leaes Hey, Member, IEEE

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 1, JANUARY 2000 185 A True ZCZVT Commutation Cell for PWM Converters Carlos Marcelo de Oliveira Stein, Student Member, IEEE, and Hélio Leaes Hey, Member, IEEE

Abstract—This paper introduces a true zero-current and disadvantages as mentioned in [7] and [22]. Recently, an zero-voltage transition (ZCZVT) commutation cell for dc–dc improved ZCT technique was presented in [22]. In this pro- pulsewidth modulation (PWM) converters operating with an posal, all switches commutates under soft switching. How- input voltage less than half the output voltage. It provides zero-current switching (ZCS) and zero-voltage switching (ZVS) ever, the main switch and the main diode have a high peak simultaneously, at both turn on and turn off of the main switch current stresses. and ZVS for the main diode. The proposed soft-switching The aim of this paper is to introduce a true zero-current and technique is suitable for both minority and majority carrier zero-voltage transition (ZCZVT) commutation cell for dc–dc semiconductor devices and can be implemented in several dc–dc PWM converters. The commutation cell provides ZCS and ZVS PWM converters. The ZCZVT commutation cell is placed out of the power path, and, therefore, there are no voltage stresses on simultaneously, at both turn on and turn off of the main switches power semiconductor devices. The commutation cell consists of a and ZVS for the main diodes. few auxiliary devices, rated at low power, and it is only activated The proposed soft-switching technique is suitable for both during the main switch commutations. The ZCZVT commutation minority and majority carrier semiconductor devices, and can cell, applied to a boost converter, has been analyzed theoretically be implemented in any member of the dc–dc PWM converter and verified experimentally. A 1-kW boost converter operating at 40 kHz with an efficiency of 97.9% demonstrates the feasibility of family. The auxiliary shunt resonant network of the ZCZVT the proposed commutation cell. commutation cell is placed out of the power path, and, therefore, there is no voltage stresses on power semiconductor devices. Index Terms—High-performance dc–dc power conversion, IGBT’s, zero-current–zero-voltage switching (ZCZVS). The operation of the ZCZVT commutation cell applied to a boost converter is theoretically analyzed in Section II. A design guideline and a design example are presented in Section III. I. INTRODUCTION In Section IV, simulation and experimental results on a 1-kW HE OVERALL performance of pulsewidth modula- prototype using IGBT’s as both main and auxiliary switches are T tion (PWM) converters can be improved by the use of presented. The last section summarizes the conclusions drawn soft-switching techniques. These techniques allow operation from this investigation. at higher switching frequencies resulting in higher power densities without penalizing the efficiency [1]–[13]. II. PRINCIPLE OF OPERATION There are two main soft-switching approaches, that are the A. The ZCZVT PWM Boost Converter zero-current switching (ZCS) [1]–[8] and the zero-voltage switching (ZVS) [9]–[13]. The choice depends on the semi- Fig. 1(a) shows the ZCZVT PWM boost converter. It differs conductor device technology that will be used. For example, from a hard-switching PWM boost converter by the presence of MOSFET’s present better performance under ZVS. This is an additional shunt resonant network formed by two resonant because under ZCS the capacitive turn-on losses increase the capacitors and , a resonant inductor , a bidirectional switching losses and the electromagnetic interference (EMI). auxiliary switch and two auxiliary diodes, and On the other hand, insulated gate bipolar transistors (IGBT’s) . The main features of this topology are as follows. present better results under ZCS which can avoid the turn-off • There are no additional voltage stresses on power semi- losses caused by the tail current [3]. conductor devices. Nevertheless, the ZCS techniques proposed in the liter- • Commutation under ZCS and ZVS at both turn on and turn ature present some drawbacks such as significant voltage off for the main switch, whenever . stress on the main diode, which increases the conduction • Commutation under ZCS at turn on and under ZCS and losses, and the presence of the resonant inductor in se- ZVS at turn off for the auxiliary switch. ries with the main switch, which increases the magnetic • The output rectifier is commutated under ZVS and losses. These drawbacks are not present in the ZCT tech- its reverse recovery is minimized. nique, proposed in [8]. On the other hand, it presents other • The ZCZVT PWM commutation cell is placed out of the main power path, and it is activated during the switching transitions only. Manuscript received February 5, 1998; revised June 29, 1999. Recommended by Associate Editor, J. Thottuvelil. The authors are with the Federal University of Santa Maria, B. Operation Principles UFSM-CT-DELC, 97105-900 Santa Maria, RS, Brazil (e-mail: [email protected]). To simplify the analysis, the input filter inductance and the Publisher Item Identifier S 0885-8993(00)00382-3. output filter capacitor are assumed large enough, and, there-

Fig. 1. ZCZVT PWM boost converter.

Fig. 2. Operation stages.

fore, the input current and the output voltage of the converter way until it reaches . At this time, the diode turns on. are considered constant over one switching cycle. The simpli- The resonant inductor current and the resonant capacitor fied circuit diagram is presented in Fig. 1(b). As shown in Fig. 2, voltage can be expressed as follows: 14 operating stages exist during one switching cycle, which are described as follows. (1) Stage 1— : The active switches are off, and the input current flows through the output rectifier . During this stage, the resonant capacitors voltages and are (2) clamped at ( for ) and , respectively. Stage 2— : At , the auxiliary switch is turned where on under ZCS. The current increases due to the resonance between and . The voltage evolves in a resonant and DE OLIVEIRA STEIN AND HEY: TRUE ZCZVT COMMUTATION CELL FOR PWM CONVERTERS 187

Fig. 3. Theoretical waveforms.

The duration of this resonant stage is equal to where

(3) and

The duration of this resonant stage is defined by Stage 3— : During this stage, the current increases linearly up to , when the output rectifier is (8) turned off under ZCS and ZVS. The resonant inductor current is given by Stage 5— : The current decreases linearly until it reaches , when is turned off. To achieve soft (4) commutation for the main switch , its turn-on signal should be applied while the diode is conducting. The inductor where is when . The time interval of this stage current can be expressed as is given by (9) (5) where is when . The time interval of this stage is equal to Stage 4— : The current continues to increase due to the resonance between and . When the voltage reaches zero, the diode turns on. The resonant in- (10) ductor current and the resonant capacitor voltage can be expressed as follows: Stage 6— : At , the main switch is turned on under ZCS and ZVS condition. The current continues to ramp down until it reaches zero and the current through main (6) switch reaches . The resonant inductor current is given by

(7) (11) 188 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 1, JANUARY 2000

Fig. 4. Relationship between k and k with k as parameter.

The time interval of this stage is equal to The time interval of this stage is equal to

(12) (18)

Stage 7— : The capacitor and the inductor form a half-cycle resonance through the main switch and the Stage 10— : During this stage, the diode is on diode , which reverses the polarity of the voltage . and the main switch can be turned off under ZCS and ZVS. During this stage, the auxiliary switch can be turned off When reaches the input current again, the diode turns under ZCS and ZVS conditions. The resonant inductor current off. The resonant inductor current and the resonant ca- and the resonant capacitor voltage can be ex- pacitor voltage can be expressed as follows: pressed as follows: (19) (13)

(20) (14) where is when . The time interval of this The time interval of this stage is equal to stage is equal to (15) (21) Stage 8— : The operation of the circuit at this stage is similar to that of the hard-switching PWM boost converter. Stage 11— : At , the resonant capacitor voltage The input current flows through the main switch . begins to increase due to the resonance between , Stage 9— : At , is turned on again under ZCS. and . When the resonant capacitor voltage As the current increases due to the resonance between reaches the diode turns on. The resonant inductor cur- and , the current through the main switch decreases rent and the resonant capacitors voltages and at the same rate since the sum of the two is equal to input current can be expressed as follows: . This stage ends when the current through the main switch reaches zero. At this time, the diode turns on again. The resonant inductor current and the resonant capacitor voltage can be expressed as follows: (22)

(16)

(17) (23) DE OLIVEIRA STEIN AND HEY: TRUE ZCZVT COMMUTATION CELL FOR PWM CONVERTERS 189

(30)

(24) where is when (31)

where is when . The time interval of this stage is equal to

(32) and where is when . Stage 14— : During this stage, the capacitor is linearly charged up to by the input current. At this moment the output rectifier turns on, beginning another switching The time interval of this stage is equal to cycle. The resonant capacitor voltage is given by

(25) (33) The time interval of this stage is equal to where is when . Stage 12— : The voltage continues to in- (34) crease due to the resonance between and . When reaches zero, the diode turns on again and turns off. The resonant inductor current and the resonant capacitor Fig. 3 shows the theoretical waveforms of the ZCZVT PWM voltage can be expressed as follows: boost converter. C. Soft Commutation Conditions 1) Main Switch: In order to achieve commutation under (26) ZVS and ZCS at turn on for the main switch (stage 5) and at turn off for the auxiliary switch (stage 7), the following inequality should be satisfied:

(35) (27) This constrain may be undesirable in some applications such where is when . The time interval of this as in power-factor-correction circuits. However, it could be re- stage is equal to (28), given at the bottom of the page. laxed by the use of an auxiliary voltage source which would Stage 13— : At , the voltage begins to ensure the required energy for the proper operation of the aux- decrease due to the resonance between , and , until iliary circuit. reaches zero, when turns off. During this stage, the In order to achieve commutation under ZVS and ZCS at turn auxiliary switch can be turned off under ZCS and ZVS. The off for the main switch (stage 9), the following inequality current and the voltages and can be ex- should be satisfied: pressed as follows: (36)

or (29) (37)

where

(28) 190 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 1, JANUARY 2000

maximum value of the input inductor filter current; minimum value of the input voltage source; a factor that guarantees the inequality (36). 2) Auxiliary Switch: The auxiliary switch is activated twice per period, as described in the beginning of this section. The auxiliary switch turn on under ZCS is ensured since the resonant inductor is connected in series with it. On the other hand, to achieve turn off under ZCS and ZVS the diode must conduct during this commutation. During the first turn off this condition is always satisfied, because the diode conducts the resonant current during the stage 7. The conduction of the diode during the second turn off depends on the existence Fig. 5. Power stage circuit. of the stage 13. Unfortunately, there is no close-form solution for the duration of the stage 13. However, it is possible to find TABLE I numerically the ratio between the resonant capacitors and UTILIZED COMPONENTS AND PARAMETERS IN SIMULATION AND IN THE BREADBOARDED CONVERTER , named , for a given and (dc voltage conversion ratio), which ensures the existence of stage 13. Fig. 4 gives the values of as a function of for different values of .

III. DESIGN GUIDELINES AND EXAMPLE In this section, a design procedure and an example of how to determine the component values of the proposed ZCZVT PWM boost converter are given. The input data are defined as follows: • output power: W; • output voltage: V; • input voltage: V (±10%); • approximate efficiency: %; • ripple of the input filter inductance: . 1) From the input data it is possible to achieve the dc voltage nF conversion ratio, which is given by where the parameter must be chosen greater than one. It is worth mentioning that the selection of a large value of would increase the current stresses in the circuit. In this design example, was initially defined equal to The dc voltage conversion ratio was defined to satisfy 1.3 to compensate for the intrinsic losses of the practical (35). setup. By taking a commercial value for of 33 nF, 2) The resonant inductor is calculated to control its di/dt results in . rate and, therefore, to minimize the reverse recovery of 5) The next step is to define the value of the resonant capac- the output diode. In this design example, this value was itor . From Fig. 4, with the values of the parameters µ chosen equal to 40 A/ s and is obtained the value of parameter , which is the ratio of . The procedure to obtain the Fig. 4 is explained in detail in [21]. With and , the parameter is 3) With the values of the output power, the minimum input equal to 1.05. Therefore, the capacitor is defined as voltage and the approximate efficiency, it is possible to follows: define the input power and the maximum dc input current nF

W Commercial value utilized: nF. Therefore, . A. 6) From (18) is defined the maximum value to fall time of the main switch , which is given by 4) From (37) is calculated the value of the resonant capacitor , which is given by

ns DE OLIVEIRA STEIN AND HEY: TRUE ZCZVT COMMUTATION CELL FOR PWM CONVERTERS 191

Fig. 6. Waveforms for the ZCZVT PWM boost converter.

In this example design, the fall time of the main switch equal to 20% of the switching period. Therefore, the max- must be smaller than 400 nF to assure its turn off under imum switching frequency is given by ZVS and ZCS. 7) From (3), (5), (8), (10), (12), (18), (21), (25), (28), (32), and (34) is defined the sum of the resonant time intervals, kHz which must be to involve a small fraction of the switching period. In this example design, this sum has been chosen 192 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 1, JANUARY 2000

The switching frequency was assumed equal to 40 kHz.

IV. SIMULATION AND EXPERIMENTAL RESULTS Following the design example shown in the preceding section, a 40-kHz 1-kW ZCZVT PWM boost converter has been simulated with Microsim PSpice using ideal components, and a prototype has been implemented to verify the operation and the performance of the proposed ZCZVT commutation cell. The power circuit is shown in Fig. 5. Its main parameters are sum- marized in Table I. The active switches were implemented with a UFS (ultrafast switches) series IGBT’s from Harris Semicon- ductor, which present built-in antiparallel hyperfast diodes. The main switch was a HGTP7N60C3D (600 V, 7 A), and the auxiliary switch was a HGTP3N60C3D (600 V, 3 A). The output rectifier and the auxiliary diode used were a hyperfast diodes Fig. 7. Efficiency of the boost converter. RHRP870 (700 V, 8 A) from Harris Semiconductor. Fig. 6 shows the simulation and the experimental waveforms. They confirm the previously mentioned analysis. As can be seen • Soft switching for all power semiconductor devices is in Fig. 6(a), the commutations of the main switch occurs truly achieved. The main switch commutates under ZCS and without losses, i.e., under ZCS and ZVS simultaneously. This is ZVS simultaneously at turn on and turn off. Thus, it is a very interesting feature of the proposed ZCZVT commutation suitable for both minority and majority carriers semicon- cell. The maximum voltage across the main switch is equal to ductor device applications such as MOSFET’s, IGBT’s, the output voltage. MCT’s, etc. The auxiliary switch commutates under ZCS Fig. 6(b) shows that the auxiliary switch is turned on at turn on and under ZCS and ZVS at turn off. The output under ZCS and turned off under ZCS and ZVS. Since the reso- rectifier commutates under ZVS and its reverse recovery nant inductor control the rate of the output rectifier, it is minimized. helps to minimize the reverse recovery losses of this diode. • Taking into account the experimental results, the converter From Fig. 6(c), it can be seen that the maximum voltage operates practically without ringing and with low across the output rectifier is equal to output voltage and it is and on the power devices, which can reduce the commutated under ZVS at turn on and ZCS and ZVS at turn EMI emission. off. • The converters are regulated by the conventional PWM The experimental results show that the converter operates technique at constant frequency. with very low ringing and with low and , reducing • Among several soft-switching techniques presented in its EMI emission. Owing to this, in the breadboarded converter the literature, mainly the ZCS techniques, the proposed it was not necessary to use any clamp circuit. ZCZVS commutation cell is a candidate and can be Fig. 7 shows the measured efficiency of the boost converter implemented in any member of the PWM family. with the proposed ZCZVT commutation cell as function of the output power, whose value was equal to 97.9% at full load (1 REFERENCES kW). Fig. 7 also includes the efficiency curve of the same circuit without the proposed commutation cell for comparison pro- [1] K. Wang, G. Hua, and F. C. Lee, “Analysis, design, and experimental results of ZCS-PWM boost converters,” in IEEJ IPEC Rec., 1995, pp. poses. Without the proposed commutation cell the converter ef- 1197–1202. ficiency at full load was 91.5%. [2] G. Ivensky, D. Sidi, and S. Ben-Yaakov, “A soft switcher optimized for IGBT’s in PWM topologies,” in Proc. IEEE Applied Power Electronics Conf., 1995, pp. 900–906. V. C ONCLUSIONS [3] R. Rangan, D. Y. Chen, J. Yang, and J. 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Gomes, “A high-power high-frequency As shown by theoretical analysis and experimental results, ZCS-ZVS-PWM buck converter using a feedback resonant circuit,” in the main features obtained are as follows. IEEE Power Electron. Specialists Conf. Rec., 1993, pp. 330–336. [7] R. C. Fuentes and H. L. Hey, “An improved ZCS-PWM commutation • The ZCZVT PWM commutation cell is placed out of the cell for IGBT’s applications,” in IEEE Applied Power Electron. Conf., main power path, and, therefore, there are no additional 1997, pp. 805–810. [8] G. Hua, E. X. Yang, Y. Jiang, and F. C. Lee, “Novel zero-current-transi- voltage stresses on power semiconductor devices. More- tion PWM converters,” in IEEE Power Electron. Specialists Conf. Rec., over, it is activated during the switching transitions only. 1993, pp. 538–544. DE OLIVEIRA STEIN AND HEY: TRUE ZCZVT COMMUTATION CELL FOR PWM CONVERTERS 193

[9] G. Hua and F. C. Lee, “Soft-switching techniques in PWM converters,” [22] H. Mao, F. C. Y. Lee, X. Zhou, H. Dai, M. Cosan, and D. Boroyevich, in Int. Conf. Ind. Electron., Control and Instrumentation, 1993, pp. “Improved zero-current transition converters for high-power applica- 637–643. tions,” IEEE Trans. Ind. Applicat., vol. 33, pp. 1220–1232, Sept./Oct. [10] A. Elasser and D. A. Torrey, “Soft switching active snubbers for dc/dc 1997. converters,” IEEE Trans. Power Electron., vol. 11, pp. 710–722, Sept. 1996. [11] Y. Jiang, G. Hua, E. X. Yang, and F. C. Lee, “Soft-switching of IGBT’s with the help of MOSFET’s in bridge-type converters,” in IEEE Power Electron. Specialists Conf. Rec., 1993, pp. 151–157. [12] K. Chen and T. A. Stuart, “A study of IGBT turn-off behavior and switching losses for zero-voltage and zero-current switching,” in Proc. Carlos Marcelo de Oliveira Stein (S’95) was born IEEE Applied Power Electron. Conf., 1992, pp. 411–418. in Santiago, RS, Brazil, in 1970. He received the B.E. [13] G. Hua, C. S. Leu, Y. Jiang, and F. C. Y. Lee, “Novel zero-voltage- and M.S. degrees in electrical engineering in 1996 transition PWM converters,” IEEE Trans. Power Electron., vol. 9, pp. and 1997, respectively, from the Federal University 213–219, Mar. 1994. of Santa Maria, Rio Grande do Sul, Brazil. He is cur- [14] K. M. Smith and K. M. Smedley, “A comparison of voltage mode soft rently working toward the Ph.D. degree in electrical switching methods for PWM converters,” in Proc. IEEE Applied Power engineering at the Federal University of Santa Maria. Electron. Conf., 1996, pp. 291–298. His research interests include power switching [15] J. Qian, A. Khan, and I. Batarseh, “Turn-off switching loss model and converters, power-factor-correction techniques, and analysis of IGBT under different switching operation modes,” in Int. soft-switching techniques. Conf. Ind. Electron., Control and Instrumentation, 1995, pp. 240–245. Mr. Stein is currently a member of the Brazilian [16] K. Wang, F. C. Lee, G. Hua, and D. Borojevic, “A compara- Society of Power Electronics (SOBRAEP). tive study of switching losses of IGBT‘s under hard-switching, zero-voltage-switching and zero-current-switching,” in IEEE Power Electron. Specialists Conf. Rec., 1994, pp. 1196–1204. [17] I. Husain and M. Ehsani, “Analysis of high power soft-switching dc–dc converters using basic three-terminal structures,” in Int. Conf. Ind. Elec- tron., Control and Instrumentation, 1995, pp. 252–257. [18] R. L. Steigerwald, “A review of soft-switching techniques in high per- Hélio Leaes Hey (M’88) was born in Santa Maria, formance dc power supplies,” in IECON, 1995, pp. 1–7. RS, Brazil, in 1961. He received the B.S. degree from [19] R. L. Steigerwald, R. W. De Doncker, and M. H. Kheraluwala, “A com- the Catholic University of Pelotas, Brazil, in 1985 parison of high-power dc–dc soft-switched converter topologies,” IEEE and the M.S. and Ph.D. degrees from the Federal Uni- Trans. Ind. Applicat., vol. 32, pp. 1139–1145, Sept./Oct. 1996. versity of Santa Catarina, Santa Catarina, Brazil, in [20] R. C. Fuentes and H. L. Hey, “A comparative analysis of the behavior 1987 and 1991, respectively. and of the switching losses for a group of ZCS-PWM converters using From 1989 to 1993, he was with the Federal IGBT’s,” in IEEE Power Electron. Specialists Conf. Rec., 1997, pp. University of Uberlândia, Brazil. Since 1994, he has 972–977. been with the Federal University of Santa Maria, [21] C. M. de O. Stein, “Concepção, Análise e Projeto de Conversores Rio Grande do Sul, Brazil, where he is currently CC-CC PWM com Comutações em Zero de Corrente e Zero de Tensão, a Professor. He is also an Editor of the Brazilian Simultaneamente,” M.Sc. dissertation (in Portuguese), Federal Univ. Power Electronics Journal. His research interests include power switching Santa Maria, Santa Maria, Brazil, Dec. 1997. converters, power-factor-correction techniques, and soft-switching techniques.