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A New Soft-Switching Technique for Buck, Boost, and Buck~Boost Converters

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A New Soft-Switching Technique for Buck, Boost, and Buck~Boost Converters IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 6, NOVEMBER/DECEMBER 2003 1775 A New Soft-Switching Technique for Buck, Boost, and Buck–Boost Converters Yingqi Zhang, Student Member, IEEE, and Paresh C. Sen, Fellow, IEEE Abstract—A new soft-switching technique for buck, boost, and buck–boost converters using coupled inductors is proposed in this paper. The principles of operation of these converters are analyzed in detail. An additional winding is added on the same core of the main inductor for the purpose of commutation. By using hysteresis current control, zero-voltage switching conditions are ensured over a wide load range. The main inductor current is kept in contin- uous conduction mode with small ripple, which allows high output power and small filter parameters. Also, the switching frequency can be kept constant when the load changes. Prototypes of buck, boost, and buck–boost converters have been built to verify the pro- posed concept. The experimental results are presented and they verify the analysis. Fig. 1. ZVRT buck converter. Index Terms—Coupled inductor, hysteresis current control, zero-voltage switched pulsewidth-modulation (ZVS PWM) con- verters. I. INTRODUCTION WITCHING-MODE power supplies are widely used Fig. 2. Bidirectional inductor current. S in industrial, residential, and aerospace environments. The basic requirements are small size and high efficiency. voltage across the main switch reduces to zero so that the body High-switching-frequency operation is necessary to achieve diode conducts before the main switch is applied a gate signal. small size. However, the switching loss will increase as the The main switch achieves ZVS conditions and there is no switching frequency is increased. To solve this problem, soft recovery problem of diode. However, there exists turn-on loss switching techniques are necessary. The zero-voltage-switched for the auxiliary switch. (ZVS) technique and zero-current-switched (ZCS) technique In the zero-current-transition converter [4], the auxiliary are two commonly used soft switching methods. By adopting switch is turned on before the main switch is turned off. these techniques, either voltage or current is zero during Resonance occurs in the auxiliary resonant tank so that the switching transitions, which largely reduces the switching loss main switch current reduces to zero prior to turn-off. However, and also increases the reliability of the power supplies. the auxiliary switch is turned off at high current. Improved Quasi-resonant converters (QRCs) [1], [2], were introduced zero-current-transition converters [5] enable both the main to overcome the disadvantages of PWM converters operating at switch and the auxiliary switch to be turned on and off under high switching frequency. In QRCs, switches can be turned on zero current conditions. However, the auxiliary switches are at zero voltage or turned off at zero current so that switching triggered twice in one switching cycle and, hence, the control losses are zero and the efficiency is high. However, the switches circuit is complicated. in QRCs have to withstand high voltage stress or high current A commutation cell [6] was introduced in pulsewidth-mod- stress. Therefore, these techniques can only be used in low- ulation (PWM) dc-to-dc converters. Either a transformer or an power applications. autotransformer is used to implement the commutation source. In the zero-voltage-transition converter [3], the auxiliary ZVS conditions are achieved for the main switch. Also, the pro- switch is turned on before the main switch is turned on. The posed converters operate at constant switching frequency. How- ever, the auxiliary switch has turn-on loss. In consequence, they Paper IPCSD 03–090, presented at the 2002 Industry Applications Society are not suitable for very high switching frequency. Annual Meeting, Pittsburgh, PA, October 13–18, and approved for publication A simple solution to soft switching PWM converters is zero- in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial Power voltage resonant transition (ZVRT) converters [7], [8] where Converter Committee of the IEEE Industry Applications Society. Manuscript submitted for review November 1, 2002 and released for publication July 7, the main inductor is comparatively small. One such ZVRT con- 2003. verter is shown in Fig. 1. To achieve ZVS conditions, the in- The authors are with the Department of Electrical and Computer En- ductor current is controlled so that it is bidirectional (positive gineering, Queen’s University, Kingston, ON K7L 3N6, Canada (e-mail: [email protected]; [email protected]). and negative). The inductor current is shown in Fig. 2. The draw- Digital Object Identifier 10.1109/TIA.2003.818964 back of ZVRT converters is large ripple in the inductor current, 0093-9994/03$17.00 © 2003 IEEE 1776 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 6, NOVEMBER/DECEMBER 2003 Fig. 3. ZVS buck converter using coupled inductors. Fig. 4. ZVS—boost converter using coupled inductors. which makes the conduction loss higher compared with the con- ventional PWM converters. Also, it needs large filter parame- ters. Since all output filter capacitors have equivalent series re- sistance (ESR), large ripple current will generate high loss in the capacitors. High ripple current will also cause high magnetic core loss. For the sake of efficiency, the inductor current should have small ripple. This technique is only suitable in low-power applications. The concept of ZVS converters using coupled inductors was proposed [9]. Some preliminary investigations on the buck converter based on simulation were reported. With some modification of ZVRT converters, the switching current can still be bidirectional so that ZVS conditions are retained and at the same time, the main inductor current is in continuous con- duction mode with small ripple. The proposed technique helps to overcome the drawbacks of the ZVRT converter discussed Fig. 5. ZVS—buck–boost converter using coupled inductors. above. It allows small filter parameters and high output power. In this paper, the operational modes of the proposed buck, boost and buck–boost converters are analyzed in detail. To achieve ZVS conditions in the switches and fast response, hysteresis current control is used in these converters. By using this method, the bidirectional current has the average value de- termined by the output of the voltage loop, while its minimum value is always less than zero to achieve ZVS conditions. Also, the switching frequency can be kept constant when the load changes. To verify the proposed concept, prototypes of the proposed ZVS buck, boost, and buck–boost converters are built. Experi- Fig. 6. Waveforms of i and i . (a) Mode 1: conducts. (b) Mode 2:v , v , mental results are presented in this paper. They agree with the g , and g resonant. (c) Mode 3: h conducts. (d) Mode 4: conducts. (e) theoretical analysis. ZVS conditions are achieved over a wide Mode 5:v , g , and g resonant. (f) Mode 6: h conducts. (g) Mode 7: load range. and h conduct. II. PRINCIPLES OF OPERATION OF THE PROPOSED ripple. The waveforms for and for the buck converter are SOFT–SWITCHING TECHNIQUE shown in Fig. 6. These waveforms are similar for boost and The proposed ZVS-buck, ZVS-boost and ZVS-buck–boost buck–boost converters. converters are shown in Figs. 3–5, respectively. Inductors The current is bidirectional, which ensures ZVS conditions and are tightly coupled on the same ferrite core. The polar- for the two MOSFETs. Small ripple current in allows higher ities of the inductors and are marked as in the schematic output power and lower requirements of the output filter capac- circuits to ensure that the voltage across the coupled inductor itors. can be used as the commutation source for soft switching of the MOSFETs. A. Operational Modes The inductor is small while the main inductor is com- As an example, the proposed buck converter is analyzed. paratively large. The current is controlled to be bidirectional There are seven modes in one switching cycle. The equivalent (positive and negative) while the output current has small circuits for these modes are shown in Fig. 7. ZHANG AND SEN: NEW SOFT-SWITCHING TECHNIQUE FOR BUCK, BOOST, AND BUCK–BOOST CONVERTERS 1777 (a) (b) (c) (d) (e) (f) (g) Fig. 7. Equivalent circuits for each mode. Mode 1 [ ,Fig. 7(a)]: At , the switch is turned is negative. The current in begins to decrease due to on and the diode is off. The inductor is in series the voltage across . Because is comparatively small, with . The current will rise linearly at the slope of its current will decrease much faster than that of .As . When the current reaches the long as is on, the gate signal can be applied to so current reference that is determined by the output of that can be turned on at zero voltage. voltage regulation loop, the switch is turned off. During this mode, Mode 2 [ , Fig. 7(b)]: When is turned off at , resonance occurs between the inductors and the (1) snubber capacitors . During this interval, the ca- pacitor is charged while is discharged. At , the voltage across becomes zero and begins to con- where duct. Mode 3 [ , Fig. 7(c)]: When conducts, the voltage (2) across the output inductor changes its polarity. Be- cause inductors and are tightly coupled, the voltage (3) 1778 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 6, NOVEMBER/DECEMBER 2003 (4) Equations (13)–(15) show that the current in rises much faster than that in . At the same time, current in (5) will decay. (6) Mode 7 [ , Fig. 7(g)]: The diode is off and is still on. Currents and continue to increase. At , the By substituting (2)–(6) into (1), the current slopes in the current will be equal to so that current in inductor three inductors can be derived as follows: becomes zero.
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