HIGH-FREQUENCY PWM dc dc converters have been

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1 256 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014 A Novel ZVT-ZCT-PWM Boost Converter Nihan Altintaş, A. Faruk Bakan, and İsmail Aksoy Abstract In this study, a new boost converter with an active snubber cell is proposed. The active snubber cell provides main switch to turn ON with zero-voltage transition (ZVT) and to turn OFF with zero-current transition (ZCT). The proposed converter incorporating this snubber cell can operate with soft switching at high frequencies. Also, in this converter all semiconductor devices operate with soft switching. There is no additional voltage stress across the main and auxiliary components. The converter has a simple structure, minimum number of components, and ease of control as well. The operation principle and detailed steady-state analysis of the novel ZVT-ZCT-PWM boost converter are given. The presented theoretical analysis is verified exactly by a prototype of 100 khz and 1 kw converter. Also, the overall efficiency of the new converter has reached a value of 97.8% at nominal output power. Index Terms DC DC converter, soft switching, zero-current transition (ZCT), zero voltage transition (ZVT). I. INTRODUCTION HIGH-FREQUENCY PWM dc dc converters have been widely used in power factor correction, battery charging, and renewable energy applications due to their high power density, fast response, and control simplicity. To achieve high-power density and smaller converter size, it is required to operate converters at high switching frequencies. However, high-frequency operation results in increased switching losses, higher electromagnetic interference (EMI), and lower converter efficiency. Especially, at high frequencies and high power levels, it is necessary to use soft-switching techniques to reduce switching losses [1] [22]. In the conventional zero-voltage transition (ZVT)-PWM converter [1], the main switch turns ON with ZVT perfectly with by means of a snubber cell. On the other hand the main switch turns OFF under near zero voltage switching (ZVS). The main diode turns ON and OFF with ZVS. The auxiliary switch turns ON with near zero-current swutching (ZCS) and turns OFF with hard switching. The operating of the circuit is dependent on line and load conditions [12]. To solve the problems in the conventional ZVT converter, many ZVT converters are suggested [4] [7], [11] [14], [17], [18]. In [17] and [18], the main switch turns ON with ZVT and the auxiliary switch operates by Manuscript received May 6, 2012; revised July 9, 2012, November 20, 2012, and February 27, 2013; accepted March 5, Date of current version July 18, Recommended for publication by Associate Editor S. Choi. The authors are with the Department of Electrical Engineering, Yildiz Technical University, Istanbul 34220, Turkey ( naltin@yildiz.edu.tr, fbakan@yildiz.edu.tr, iaksoy@yildiz.edu.tr). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPEL soft switching. The main switch turns OFF with near ZVS and soft switching depends on load current. In [23] [26], active clamp ZVT is realized. It is required to use two main switches. Zero-current transition (ZCT) is not implemented. To obtain active clamp two auxiliary switches are used. Additionally, the converter requires a special design transformer and two rectifier diodes. In the conventional ZCT-PWM converter [2], the main switch turns OFF under ZCS and ZVS. The auxiliary switch turns ON with approximate ZCS. The operation of the circuit depends on circuit and load conditions. When the main switch turns ON reverse recovery current flows through the main diode and a short circuit occurs between the main switch and the diode. The auxiliary switch turns OFF by hard switching and the parasitic capacitors of the switches discharge through the switches [12]. A lot of ZCT converters are submitted to solve the problems in conventional ZCT converter [2], [3], [13], [19]. In [13] and [19], the main switch turns OFF with ZCT without increasing the current stress of the main switch and the auxiliary switch operates by soft switching. The voltage stress across the main diode is high. The operation intervals depends on load current. In order to solve the problems of ZVT and ZCT converters, ZVT ZCT PWM dc dc converters that combines the ZVT and ZCT methods are suggested [9], [15], [16]. In these converters, the main switch turns ON and turns OFF with zero voltage and zero current, respectively. Besides the auxiliary switch turns ON and turns OFF by soft switching. In [9], the main switch turns OFF and turns ON with ZCS and ZVS. The main diode turns ON and turns OFF with ZVS. The drawbacks of the converter can be given as follows; the input voltage must be smaller than half of the output voltage for soft-switching operation, there is an additional current stress on the main switch, transition intervals take long time and cause conduction losses over one switching cycle. In [15], the main switch turns ON with zero voltage transition and turns OFF with zero current transition. There are no additional voltage and current stresses in the main switch and the main diode. A magnetic coupled inductance is used in the circuit. If the magnetic coupling is not good, parasitic oscillations and losses occur due to the leakage inductance. In this study, a novel active snubber cell, which overcomes most of the problems of the conventional ZCT-PWM converter [2] is proposed. The main contribution of this study is the modification of the control technique in the conventional ZCT- PWM converter. ZVT and ZCT properties are obtained from the normal ZCT converter without making any change in the circuit topology. In the proposed converter the main switch turns ON with ZVT and turns OFF with ZCT. All of the semiconductor devices operate under soft switching. The proposed converter /$ IEEE

2 ALTINTAŞ et al.: NOVEL ZVT-ZCT-PWM BOOST CONVERTER 257 Fig. 1. Circuit scheme of the proposed novel ZVT-ZCT-PWM boost converter. has simple structure and low cost. The operation principles and theoretical analysis of the proposed converter are verified with a prototype of a 1 kw and 100 khz boost converter. II. OPERATION MODES AND ANALYSIS A. Definitions and Assumptions The circuit scheme of the proposed ZVT-ZCT-PWM boost converter circuit is shown in Fig. 1. In this circuit, V i is input voltage source, V o is output voltage, L F is main inductor, C F is output filter capacitor, S 1 is main switch and D F is main diode. The main switch consist of a main transistor T 1 and its body diode D 1. The snubber circuit shown with dashed line is formed by snubber inductor L s, a snubber capacitor C s and auxiliary switch S 2. T 2 and D 2 are the transistor and its body diode of the auxiliary switch, respectively. The capacitor C r is assumed to be the sum of the parasitic capacitor of S 1 and the other parasitic capacitors incorporating it. In the proposed converter, it is not required to use an additional C r capacitor. During one switching cycle, the following assumptions are made in order to simplify the steady-state analysis of the circuit shown in Fig. 1. Input and output voltages and input current are constant, and the reverse recovery time of D F is taken into account. In the equations, semiconductor devices and resonant circuits are assumed ideal for simplification. B. Operation Modes of the Converter One switching cycle of the proposed novel ZVT-ZCT-PWM boost converter consist of eleven modes. In Fig. 2(a) (k), the equivalent circuit diagrams of the operation modes are given respectively. The key waveforms concerning the operation modes are shown in Fig. 3. The detailed analysis of the proposed circuit is presented below. Mode1[t 0 <t<t 1 : Fig. 2(a)]: At the begining of this mode, the main transistor T 1 and auxiliary transistor T 2 are in the OFF state. The main diode D F is in the ON state and the input current I i flows through the main diode. At t = t 0,i T 1 = 0, i Ls = i T 2 = 0, i DF = I i,v Cr = V o and v Cs = V Cs0 The initial voltage of snubber capacitor V Cs0 is constituted by the efficiency of the resonant circuit. Soft-switching range of the circuit depends on the initial voltage of C s. Soft switching depends on the value of V Cs0. The main diode D F is in the ON state and conducts the input current I i.att = t 0, when the turn on signal is applied to the gate of the auxiliary transistor T 2, mode 1 begins. A resonance starts between snubber induc- Fig. 2. Equivalent circuit schemes of the operation modes in the proposed novel ZVT-ZCT-PWM boost converter. (a) t 0 <t<t 1. (b) t 1 <t<t 2. (c) t 2 <t<t 3.(d)t 3 <t<t 4.(e)t 4 <t<t 5. (f) t 5 <t<t 6.(g)t 6 < t<t 7. (h) t 7 <t<t 8. (i) t 8 <t<t 9. (j) t 9 <t<t 10. (k) t 10 <t< t 11 = t 0. tances L s and snubber capacitor C s. Due to the resonance T 2 current rises and D F current falls simultaneously. The rise rate of the current is limited because of the L s snubber inductance connected serially to the auxiliary switch. So that the turn on of the auxiliary switch is provided with ZCS. For this interval, the following equations can be written: i Ls =(V o V Cs0 ) sin ω s(t t 0 ) (1) L s ω s v Cs = V o (V o V Cs0 )cosω s (t t 0 ). (2) In these equations ω s = 1 L s C s (3)

3 258 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014 Fig. 3. Key waveforms concerning the operation stages in the proposed converter. At t = t 1, snubber capacitor voltage v Cs is charged to V Cs1,i T 2 reaches I i and i DF falls to zero. When i DF reaches I rr,d F is turned OFF and this stage finishes. In this stage, T 2 is turned ON with ZCS due to L s. D F is turned OFF with nearly ZCS and ZVS due to L s and C r. At the end of this mode i Ls = i T 2 = I i + I rr (4) v Cs = V Cs1 (5) can be written. Mode 2 [t 1 <t<t 2 : Fig. 2(b)]: Before t = t 1,i T 1 = 0, i Ls = i T 2 = I i + I rr,i DF = 0, v Cr = V o and v Cs = V Cs1 are valid. The main transistor T 1 and the main diode D F are in the OFF state. The auxiliary transistor is in the ON state and conducts the sum of the input current I i and the reverse recovery current of D F. At t = t 1, a resonance between parasitic capacitor C r, snubber inductor L s and snubber capacitor C s starts. The equations obtained for this mode are given as follows: i Ls = I i + I rr cos ω r (t t 1 ) (V Cs1 V o ) ω r L s sin ω r (t t 1 ) v Cr = (V Cs1 V o )cosω r (t t 1 )+V Cs1 (6) L s ω r I rr sin ω r (t t 1 ). (7) In the previous equations 1 ω r = (8) L s C r At t = t 2,v Cr becomes 0 and this stage is finished. Thus, the transfer of the energy stored in the parasitic capacitor C r to the resonant circuit is completed. At this time the diode D 1 is turned ON with nearly ZVS and this stage ends. The capacitor C r is assumed the sum of the parasitic capacitor of S 1 and the other parasitic capacitors incorporating it. In the proposed converter, it is not required to use an additional C r capacitor. At the end of this mode i Ls = i T 2 = I Ls2 (9) v Cs = V Cs2 (10) Mode 3 [t 2 <t<t 3 : Fig. 2(c)]: Just after the diode D 1 is turned ON at t 2,i T 1 = 0, i Ls = i T 2 = I Ls2,i DF = 0, v Cr = 0 and v Cs = V Cs2 are valid at the begining of this mode. In this mode, the resonant which is between the snubber inductance L s and snubber capacitor C s continues. For this resonance i Ls = I Ls2 cos ω s (t t 2 ) V Cs1 sin ω s (t t 2 ) ω s L s (11) v Cs = V Cs1 cos ω s (t t 2 )+L s ω s I Ls2 sin ω s (t t 2 ) (12) are achieved.

4 ALTINTAŞ et al.: NOVEL ZVT-ZCT-PWM BOOST CONVERTER 259 At the beginning of this mode the voltage of C r becomes zero, so that the diode D 1 is turned ON and conducts the excess of snubber inductance L s current from the input current. The period of this stage is the ZVT duration of the main transistor so that this interval is called ZVT duration. In this mode, control signal is applied to T 1 while D 1 is in the ON state in order to provide ZVT turn ON of T 1.Att = t 3, this stage ends when the snubber inductance L s current falls to input current, and D 1 is turned OFF under ZCS. At the end of this mode i Ls = i T 2 = I Ls3 = I i (13) v Cs = V Cs3 (14) Mode 4 [t 3 <t<t 4 : Fig. 2(d)]: This mode begins when the diode D 1 turns OFF. At the begining of this mode, i T 1 = 0, i Ls = i T 2 = I Ls3 = I i,i DF = 0, v Cr = 0, and v Cs = V Cs3 are valid. The main transistor is turned ON with ZVT and its current starts to rise. The resonant between snubber inductance L s and snubber capacitor C s continues. For this mode, the following equations are derived: i Ls = I i cos ω s (t t 3 ) V Cs4 sin ω s (t t 3 ) ω s L s (15) v Cs = V Cs4 cos ω s (t t 3 )+L s ω s I i sin ω s (t t 3 ). (16) At t = t 4, the main transistor current reaches to the input current level and i Ls becomes zero. The current through the auxiliary transistor becomes zero and this mode ends by removing the control signal of the auxiliary transistor. At the end of this mode i Ls = i T 2 = I Ls4 =0 (17) v Cs = V Cs4 (18) Mode 5 [t 4 <t<t 5 : Fig. 2(e)]: This mode begins when the auxilary transistor T 2 is perfectly turned OFF under ZCT. For this mode, i T 1 = I i,i Ls = i T 2 = I Ls4 = 0, i DF = 0, v Cr = 0, and v Cs = V Cs4 In the beginning of this mode the diode D 2 is turned ON with ZCS and its current starts to rise. The resonant between snubber inductance L s and snubber capacitor C s still continues. However, i Ls becomes negative, so the current through the main transistor is higher than the input current in this mode. The equations can be expressed as follows: i Ls = V Cs4 sin ω s (t t 4 ) ω s L s (19) v Cs = V Cs4 cos ω s (t t 4 ). (20) At t = t 5, the main transistor current decrase to the input current level and i Ls becomes zero. i D 2 becomes zero and it is turned OFF under ZCS. At the end of this mode i Ls = i T 2 = I Ls5 =0 (21) v Cs = V Cs5 (22) Mode6[t 5 <t<t 6 : Fig. 2(f)]: At the begining of this mode, i T 1 = I i,i Ls = i T 2 = I Ls4 = 0, i DF = 0, v Cr = 0, and v Cs = V Cs5 In this mode, the main transistor continues to conduct the input current I i and the snubber circuit is not active. This mode is the ON state of the conventional boost converter. The ON state duration is determined by the PWM control. For this mode i T 1 = I i (23) can be written. Mode7[t 6 <t<t 7 : Fig. 2(g)]: At the begining of this mode, i T 1 = I i,i Ls = i T 2 = 0, i DF = 0, v Cr = 0, and v Cs = V Cs5 are valid. At t = t 7, when the control signal of the auxiliary transistor T 2 is applied, a new resonance between snubber inductance L s and snubber capacitor C s starts through C s L s T 2 T 1.The equations can be expressed as follows: i Ls = V Cs5 sin ω s (t t 5 ) ω s L s (24) v Cs = V Cs5 cos ω s (t t 5 ). (25) Due to the snubber inductance L s, the auxiliary transistor T 2 is turned ON with ZCS. The current which flows through the snubber inductance rises and the main transistor current falls due to the resonance, simultaneously. At t = t 7, when the curent of T 2 reaches to the input current level, the main transistor current becomes zero and this mode finishes. At the end of this mode i Ls = i T 2 = I Ls7 = I i (26) v Cs = V Cs7 (27) Mode8[t 7 <t<t 8 : Fig. 2(h)]: At the begining of this mode, i T 1 = 0, i Ls = i T 2 = I i,i DF = 0, v Cr = 0, and v Cs = V Cs7 This mode starts at t = t 7 when T 1 current falls to zero. D 1 is turned ON with ZCS. If T 1 is turned OFF when D1 is ON, T1 turns OFF with ZVS and ZCS. The resonance started before continues by through C s L s T 2 D 1. D 1 conducts the excess of i Ls from the input current. For this mode, the following equations are derived: i Ls = I i cos ω s (t t 8 ) V Cs7 sin ω s (t t 8 ) ω s L s (28) v Cs = V Cs7 cos ω s (t t 8 )+L s ω s I i sin ω s (t t 8 ). (29) Just before t = t 8,i D 1 falls to zero. i D 1 reaches I rr at t = t 8 and turns OFF, and this stage ends. At the end of this mode i Ls = i T 2 = I Ls8 = I i I rr (30) v Cs = V Cs8 = V Cs0 (31) Mode 9 [t 8 <t<t 9 : Fig. 2(i)]: This mode begins when D 1 is turned OFF under ZCS. For this mode, i T 1 = 0, i Ls = i T 2 = I Ls8 = I i I rr,i DF = 0, v Cr = 0, and v Cs = V Cs8 = V Cs0 A resonance between parasitic capacitor C r, snubber inductor L s, and snubber capacitor C s starts at t = t 8. At t = t 9,i Ls falls to zero and the capacitor C r is charged from zero to V Cs8 with this resonance. This mode ends by

5 260 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014 Fig. 4. Variation of I S 1max with L s for different C s values. Fig. 5. Variation of V Cs0 with L s for different C s values. removing the control signal of the auxilary transistor T 2.The auxilary transistor T 2 is turned OFF with ZCS. For this mode, the following equations are derived: i Ls = I i I rr cos ω r (t t 8 ) V Cs8 sin ω r (t t 8 ) (32) ω r L s v Cr = V Cs8 V Cs8 cos ω r (t t 8 )+L s ω r I rr sin ω r (t t 8 ). (33) At the end of this mode i Ls = i T 2 = I Ls9 =0 (34) v Cs = V Cs9 = V Cs0 (35) Mode 10 [t 9 <t<t 10 :Fig.2(j)]:At t = t 9,i T 1 = 0, i Ls = i T 2 = I Ls9 = 0, i DF = 0, v Cr = V Cs8, and v Cs = V Cs9 = V Cs0 During this mode, C r is charged linearly under the input current. For this mode Fig. 6. Variation of the t ZVT with L s for different C s values. v C r = V Cs9 + I i C r (t t 9 ) (36) can be written. At instant t 10, when the voltage across the C r reaches output voltage V o, the main diode D F is turned ON with ZVS and this mode finishes. Mode 11 [t 10 <t<t 11 = t 0 : Fig. 2(k)]: At t = t 10,i T 1 = 0, i Ls = i T 2 = 0, i DF = 0, v Cr = V o, and v Cs = V Cs0 This mode is the OFF state of the conventional boost converter. During this mode, the main diode D F continues conducting the input current I i and the snubber circuit is not active. The duration of this mode is determined by the PWM control. For this mode i DF = I i (37) can be written. Therefore, at the moment t = t 11 = t 0, one switching cycle is completed and another switching cycle starts. III. DESIGN PROCEDURE In order to design the proposed ZVT-ZCT-PWM boost converter, the characteristic curves are obtained by simulations and given in Figs The component values used in snubber cell Fig. 7. Variation of t ZCT with L s for different C s values. can be determined from these curves. The characteristic curves are obtained depending on L s and C s at nominal output power. From Fig. 4, it is seen that the maximum value of the main switch current I S 1max decreases when the value of L s snubber inductance increases. It decreases slightly when the value of C s snubber capacitance increases. In Fig. 5, the initial voltage of the snubber capacitor decreases with increasing C s, and increases with increasing L s. In Fig. 6, the ZVT duration of the main switch is shown depending on L s and C s. From the figure, it is seen that the ZVT interval decreases when L s and C s increases. In Fig. 7, the variation of the ZCT duration of the main switch is given. The

6 ALTINTAŞ et al.: NOVEL ZVT-ZCT-PWM BOOST CONVERTER 261 ZCT duration increases when C s and L s increases. The ZCT duration strongly depends on the resonance between L s and C s. The smallest values of L s and C s components are preferred from the characteristic curves. If the selected component values are high, the sum of the transient intervals and conduction losses increase. We have to take into account that current stress of the main switch should remain at reasonable level. A. Design Procedure 1) The capacitor C r is assumed to be the sum of parasitic capacitor of the main switch and the other parasitic capacitors incorporating it. 2) An additional current stress of the main switch can be acceptable as much as three times of maximum input current. 3) The initial voltage of snubber capacitor C s depends on the losses of the resonant circuit. For simplicity, these losses are not taken into account in the design procedure. If the value of C s decreases, the initial voltage of snubber capacitor C s increases. Initial energy of the C s should be high enough to provide soft switching of the main switch. 4) To turn OFF the main switch with ZCT, the duration of t ZCT should be longer than fall time of the main switch (t f 1 ). This can be defined as follows: t ZCT t f 1. (38) 5) The snubber inductance can be selected to provide the following conditions with reference to [15]. Here, t r2 is rise time of the auxiliary switch. V cs1 is assumed constant in t r2 duration. V o V cs1 L s t r2 I i max. (39) In order to give an idea about the selection of the components of the snubber cell, a design example of the snubber cell is given below: 1) The snubber capacitor C r is the sum of the parasitic capacitor of S 1 and the other parasitic capacitors incorporating it. The value of C r is approximately 1nF. 2) To turn ON auxiliary switch with ZCS, the required series inductor L s is calculated from (39) as ( ) L s μh. (40) 5 According to Fig. 7, in order to decrease ZCT duration, the value of L s is selected as the smallest possible value as 2 μh. 3) The resonant capacitor C s is determined depending on the transient intervals. The sum of the transient intervals is selected to be smaller than 20% of the switching period according to [9]. The transient intervals are t 05 and t 69. The intervals t 04 and t 89 are very small in the switching period. Thus, the sum of the transient intervals can be assumed as the sum of t 45 and t 68, and it is equal to the resonant period t R t R =2π L s C s (41) t R <T s (20/100) (42) C s <= 50 nf. The value of C s is selected as 33 nf to obtain appropriate ZVT duration as seen in Fig. 6. To small ZVT duration causes complexity in the practical implementation. 4) With the selected component values the current stress of the main switch is smaller than three times of the nominal current. From Figs. 6 and 7 the ZVT duration is 120 ns and the ZCT duration is 700 ns. IV. CONVERTER FEATURES By means of the snubber cell, the switching power losses of main switch, auxiliary switch, and main diode are reduced. The switching losses are not dissipated on the snubber cell. There is only a small amount of circulation energy loss, which only takes a resonant period. This causes a little increase in the conduction losses of the switches. The features of the proposed ZVT ZCT PWM boost converter can be summarized as follows: 1) All of the semiconductor devices are both turned ON and turned OFF under soft switching. The main switch is perfecetly turned ON and OFF with ZVT and ZCT, respectively. The main diode is both turned ON and OFF with ZVS and ZCS respectively. The auxiliary transistor is turned ON with near ZCS, and turned OFF with ZCT. Also, the other devices operate with soft switching. 2) All of the semiconductor devices are not subjected to any additional voltage stress. 3) The current stress of main switch is acceptable levels. The main diode is not subjected to any current stress. 4) The converter has a simple structure and low cost. The structure of the proposed converter is simpler than the ZVT-ZCT-PWM converters in the literature. 5) Soft-switching conditons are maintained at very wide line and load ranges. 6) The converter can operate at considerably high frequencies and acts as a normal PWM converter. Also the circulating energy is quite small. 7) The sum of the transient intervals is a very little part of the switching cycle. 8) The main and the auxiliary switches have a common ground, and this provides simplicity in the control. 9) The proposed new snubber cell can be easily applied to the other basic PWM converters and to all switching converters. 10) The proposed converter does not require any additional passive snubber cells. In the periodic steady state, the total area under the main inductor voltage waveform is zero. The theoretical waveforms are given in Fig. 3. The output voltage equation is obtained from V LF waveform as V o = V in (T s t a t b ) (T s D T s t b ). (43)

7 262 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014 TABLE I SOFT-SWITCHING CAPABILITIES OF THE ZCT AND THE ZVT-ZCT CONVERTERS In this equation, t a = t 12 and t b = t t a and t b are transient intervals in the proposed converter. In order to simplify the output voltage equation, the transient waveforms are assumed to be linear as shown in Fig. 3. The dependency of the output voltage to the load current increases as the load decreases. The output voltage expression of the novel converter is the same with the boost converter operating in continuous mode, if the transient intervals are neglected. The transient intervals are small compared with the switching period. In order to evaluate the efficiency, the amount of losses should be taken into account η = P o /(P o + P loss ). (44) The main losses in the converter are conduction (P cond ) and switching losses (P SW ) of the semiconductor devices, and inductor losses P loss = Pcond S 1 + P sw S 1 + P cond S 2 + P sw S 2 + P cond DF + P sw DF + P inductor. (45) In the proposed converter, switching losses are eliminated by means of soft switching. Inductor losses are very small and they can be neglected. Thus, the losses can be approximated as P loss = Pcond S 1 + P cond S 2 + P cond DF. (46) In the proposed converter, there is a little increase in the conduction losses as compared with the conventional hard switched boost converter. Due to the elimination of the switching losses the efficiency is higher than the conventional one. The soft-switching capabilities of the standard ZCT-PWM converter and the proposed converter are compared in Table I. In order to provide ZVT and ZCT for the main switch, the auxiliary circuits are operated twice in a switching period, both at turn ON and turn OFF processes. In the turn ON process of the main switch, the energy of the parasitic capacitor C r is transferred to the resonance circuit. The capacitor is discharged, body diode of the main switch turns ON, the main transistor turns ON with ZVT thus, no switching loss occurs in the turn ON process. V. EXPERIMENTAL RESULTS A prototype of a 1 kw and 100 khz boost converter shown in Fig. 8 was performed to verify the theoretical analysis of the proposed ZVT ZCT PWM boost converter. The photograph related to the experimental circuit is given in Fig. 9. The proposed converter is established by adding the developed snubber cell to the conventional boost converter. Fig. 8. Experimental circuit of a 1 kw and 100 khz ZVT-ZCT-PWM boost converter. Fig. 9. Experimental circuit. TABLE II NOMINAL VALUES OF THE SEMICONDUCTOR DEVICES IN THE PROPOSED CONVERTER The L F main inductance is large enough to be considered as a constant current. Some nominal values of the semiconductor devices are listed in Table II with reference to the datasheets of the manufacturers. The experimental results are shown in Fig. 10 at light load and in Fig. 11 at nominal load. In Figs. 10, 11(a) and (b), the control signals of the main and auxiliary switchs are shown. The control signal of the auxiliary switch is applied before about 150 ns and removed after about 400 ns with regard to the turn ON signal of the main switch. Similarly, the control signal of the auxiliary switch is applied before about 300 ns with regard to the turn OFF signal of the main switch. The signal of the auxiliary switch has a constant width of 600 ns. From the voltage, current, and control waveforms of the main switch S 1, given in Fig. 11(a), it can be seen that S 1 is operated under soft switching at both turn ON and turn OFF processes. There is no overlap between voltage and current waveforms of the main switch. During the turn ON and turn OFF process of the main switch, its body diode is turned ON. Therefore, ZVT turn ON and ZCT turn OFF processes are perfectly realized for the main switch. Also, from the waveforms it is seen that there is no any additional voltage stress across the main switch and the current stress is acceptable. Because of the current stress, the conduction loss of the main transistor increases slightly. In Fig. 11(b), the voltage, current, and control waveforms of the auxiliary switch are shown. Auxiliary transistor is activated

8 ALTINTAŞ et al.: NOVEL ZVT-ZCT-PWM BOOST CONVERTER 263 Fig. 10. Experimental waveforms at light load (1μs/div). (a) v S 1 (200 V/div), i S 1 (5 A/div), and v G 1 (20 V/div). (b) v S 2 (200 V/div), i S 2 (5 A/div), and v G 2 (20 V/div). (c) v DF (200 V/div) and i DF (5 A/div) (d) v o (200 V/div), v Ls (200 V/div), and v Cs (200 V/div). Fig. 11. Experimental waveforms at nominal load (1μs/div). (a) v S 1 (200 V/div), i S 1 (10 A/div), and v G 1 (20 V/div). (b) v S 2 (200 V/div), i S 2 (10 A/div), and v G 2 (20 V/div). (c) v DF (200 V/div) and i DF (10 A/div). (d) v o (200 V/div), v Ls (200 V/div), and v Cs (200 V/div).

9 264 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014 Fig. 12. Efficiency comparison of the proposed ZVT-ZCT converter, ZCT converter, and PWM boost converter. in both ZVT and ZCT processes of the main switch so it is switched twice in a switching period. In the both operations, the conduction time of the auxiliary transistor is nearly 600 ns. In the ZVT and ZCT processes of the main switch, the auxiliary switch is turned ON under near ZCS and is turned OFF with ZCT and near ZCS, respectively. Because of the resonance circuit loss, the peak current through the auxiliary switch in the ZCT interval is lower than the ZVT interval. As seen from the waveforms, there is no additional voltage stress across the auxiliary switch and it operates under soft switching. The ZVS turn ON and ZCS turn OFF processes of the main diode are shown in Fig. 11(c). It can be seen that there is no any additional voltage and current stresses on the main diode. The waveforms of the output voltage, resonant inductance voltage, and resonant capacitor voltage are shown in Fig. 11(d). It can be seen that, the voltage across C s is returned to initial value at the end of one switching period. From the experimental results, it is also observed that the soft-switching conditions of the novel converter are maintained for the overall load ranges. From Fig. 12 it can be seen that the efficiency values of the novel converter are much higher than that of the hard switching converter. Also, there are important differences between the proposed ZVT ZCT PWM converter and the standard ZCT PWM converter. These are listed in Table I. When the proposed converter is operated as ZCT PWM converter the efficiency decreases due to the turn ON switching losses of the main IGBT. At 100 khz operation frequency and at nominal load, the turn ON switching loss of the main IGBT is around 10 W. Besides that, the auxiliary switch and the main diode turns OFF under hard switching. Thus, the efficiency of the ZCT converter is lower than the proposed ZVT-ZCT-PWM converter. The overall efficiency of the proposed converter is measured about 97.8% at the nominal output power. As a result, it can be clearly seen that the predicted theoretical analysis and operation principles of the novel ZVT ZCT PWM boost converter are experimentally verified. VI. CONCLUSION In this study, a PWM boost converter with a novel active snubber cell has been analyzed in detail. This active snubber cell provides ZVT turn on and ZCT turn OFF together for the main switch of the converter. Also, the proposed snubber cell is implemented by using only one quasi-resonant circuit without an important increase in cost and complexity. In the proposed converter, all semiconductor devices are switched under soft switching. In the ZVT and ZCT processes, the auxiliary switch is turned ON under ZCS and is turned OFF with ZCT and near ZCS, respectively. There is no additional voltage stress across the main and auxiliary switches.the main diode is not subjected to any additional voltage and current stresses. The operation principles and steady-state analysis of the proposed converter are presented. In order to verify the theoretical analysis, a prototype of the proposed circuit is realized in the laboratory. The novel ZVT-ZCT-PWM boost converter using the proposed snubber cell has desired features of the ZVT and ZCT converters. It is observed that the operation principles and the theoretical analysis of the novel converter are exactly verified by experimental results taken from the converter operating at 1 kw and 100 khz. Additionally, at nominal output power, the converter efficiency reaches approximately to 97.8%. REFERENCES [1] G. Hua, C. S. Leu, Y. Jiang, and F. C. 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