IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 1,JANUARY 2010 105 Zero-Voltage-Transition PWM Converters With Synchronous Rectifier Ehsan Adib, Student Member, IEEE, and Hosein Farzanehfard, Member, IEEE Abstract—In this paper, a family of zero-voltage-transition (ZVT) pulsewidth-modulated converters with synchronous recti- fier (SR) is introduced. The SR decreases the conduction losses, while it increases the achieved soft switching range. In this family of converters, zero-voltage-switching (ZVS) condition is attained for the main and rectifier switches. Also, zero-current switching is achieved for the auxiliary switch. In addition, the applied ZVS technique can eliminate the reverse recovery losses of the recti- fier switch body diode. The ZVT buck converter with SR is ana- lyzed, and the presented experimental results confirm the theoret- ical analysis. Index Terms—Synchronous rectifier (SR), zero-voltage switch- ing (ZVS). I. INTRODUCTION I N NONISOLATED converters with low-voltage source, where MOSFET is the preferred device, synchronous recti- fier (SR) can be applied to decrease conduction losses of the rec- tifying diode. Also, the same can be accomplished for isolated converters with low output voltages. However, the main problem is the reverse recovery time of SR body diode. Since, usually the reverse recovery time of SR body diode is greater than 150 ns, applying SR can considerably increase the switching losses and electromagnetic interferences (EMIs). Therefore, soft switching techniques can be employed to reduce the switching losses and EMIs. Among various soft switching techniques, zero-voltage- transition (ZVT) and zero-current-transition (ZCT) techniques can provide soft switching condition for regular pulsewidth- modulated (PWM) converters [1]–[18]. In ZCT technique, an inductor is placed in series with con- verter main switch or main diode to provide soft switching con- dition at switch turn-on instant. Before switch turn-off instant, an auxiliary switch is turned on and the main switch current is re- duced to zero [1], [2]. In ZVT technique, a capacitor is placed in parallel with the main switch to provide soft switching condition for switch turn-off. In this technique, soft switching condition for switch turn-on is achieved by an auxiliary switch, which dis- charges the snubber capacitor across the main switch [3]–[18]. Therefore, when the auxiliary switch is turned on in ZVT con- verters, it first provides the converter inductor current and the converter main diode turns off. Then, the snubber capacitor of main switch is discharged in a resonance fashion. Thus, ZVT techniques create a time gap between the conduction time of the Manuscript received March 12, 2009; revised May 11, 2009. Current version published January 29, 2010. Recommended for publication by Associate Editor K. Ngo. The authors are with the Department of Electrical and Computer Engi- neering, Isfahan University of Technology, Isfahan 84156-83111, Iran (e-mail: adib.ehsan@gmail.com; hosein@cc.iut.ac.ir). Digital Object Identifier 10.1109/TPEL.2009.2024153 Fig. 1. ZVT buck converter with SR. main switch and diode. Therefore, ZVT technique is a proper solution for reverse recovery problem of converter main diode. Consequently, by applying ZVT techniques to regular convert- ers with SR, the losses related to the reverse recovery time of SR body diode can be reduced significantly. Several ZVT techniques have been introduced previously [3]–[18]. One of these techniques is applied to PWM converters with SR to improve efficiency and overcome the SR reverse recovery problem [18]. However, in these converters, additional current stress is applied to the main switch. Also, the auxiliary switch voltage stress is high. In [19]–[21], SR is applied to soft switching isolated converters to improve their efficiency. Among the ZVT techniques, the method introduced in [4] has the best efficiency [22]. ZVT technique in [4] has better efficiency in comparison to other ZVT methods, since only half a resonance cycle occurs in the auxiliary circuit to discharge the snubber capacitor. Also, in this method, the auxiliary switch voltage stress is less than others, which results in low capacitive turn-ON losses of this switch. But, in other ZVT methods, in order to dis- charge the snubber capacitor in the auxiliary circuit, sometimes two resonances occur simultaneously (like dual-resonance ZVT converters [8], [9]) or a complete resonance cycle occurs (like ZVT converters with resonant auxiliary circuit [6], [7]). How- ever, the method of [4] can only provide zero-voltage switching at operating duty cycles higher than 0.5. Furthermore, in prac- tice, the soft switching range of the converters with this method is limited to duty cycles approximately higher than 0.6. In this paper, by applying SR to these converters, the soft switching range is increased while the converter conduction losses have decreased. Also, in order to improve the soft switching range, the switching algorithm is modified. In this paper, the ZVT buck converter with SR is introduced and analyzed in Section II. In Section III, design considerations are discussed. Experimental results presented in Section IV con- firm the theoretical analysis. Other ZVT converters with SR are introduced in Section V. 0885-8993/$26.00 © 2010 IEEE