IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL.15, NO. 3, MAY 2000 479 An Improved Self-Resonant PWM Forward Converter Célio de Pádua Dias, Adriano Alves Pereira, Valdeir José Farias, João Batista Vieira, Jr., Member, IEEE, and Luiz Carlos de Freitas, IEEE, Member Abstract—This paper presents a forward converter with a nondissipative cell which provides a soft switching converter operation. This approach is based on the principle of self-res- onance, that is: An auxiliary voltage source feeds the resonant circuit, charging a capacitor which provides the condition for zero voltage switching (ZVS) turn on and turn off of the switches. The complete operating principle, relevant equations, simulation, and experimental results are presented. Index Terms—Converters, resonance, resonant power conver- sion. I. INTRODUCTION I N THE last few years, size, weight, cost, and electromag- netic interference (EMI) reduction in the switching mode power supply (SMPS) has been the goal of industry and aca- demic research. To achieve these features in SMPS it is neces- sary to increase switching frequency and keep a high efficiency converter operation. These operational features are not so easily achieved in hard switching power converter technology. So, for this reason the lossless commutation converters approach is often used and during the last few years many soft switching converters have been developed such as zero current switching (ZCS) and zero voltage switching (ZVS) quasiresonant converters which are reported in [1]. However, these converters present severe load limitations due to the current and/or voltage peak on the switches as well as variable switching frequency which imposes requirements for large output filters to attend to the lowest switching frequency of operation. In order to overcome these limitations many others, con- verters operating with fixed frequency pulse width modulation (PWM) and without losses of commutation have been presented in a great number of publications all over the world. Looking for the same objectives mentioned above, we devel- oped some lossless commutation cells which give emphasis to that presented in [2]. It was improved three times and presented in [3]–[5]. Using these approaches a forward prototype converter shown in Fig. 1 was built in our laboratory and detailed in [7]. This forward converter had the following operation properties. Manuscript received April 8, 1998; revised November 16, 1999. This work was supported by Thornton-Inpec Eletrônica Ltda., Siemens S. A., FAPEMIG, CNPq, and CAPES. Recommended by Associate Editor, J. Thottuvelil. C. P. Dias, V. J. Farias, J. B. Vieira, Jr., and L. C. de Freitas are with the Centro de Ciências Exatas e Tecnologia, Departamento de Engenharia Elétrica, Universidade Federal de Uberlândia, Uberlândia 38400-902, Brazil (e-mail: fre- itas@ufu.br). A. A. Pereira is with the Departamento de Computação, Universidade Federal de Goiás, Catalao 75.704-020, Brazil. Publisher Item Identifier S 0885-8993(00)03388-3. 1) Soft switching for full load range. 2) Conduction losses are almost the same as those observed in the hard PWM converter. 3) High switching frequency with high efficiency. 4) High power density. 5) Low level noise. 6) Soft switching for a full load range. As shown in Fig. 1 the converter topology is composed of a forward converter which includes a non dissipative commu- tation cell. This cell is obtained by using an auxiliary output voltage source that feeds a resonant circuit at the beginning of each switching cycle. During one switching cycle, switches , and com- mutate without losses in the ZVS or ZCS form. 1) The problem of this topology is a high voltage level on the auxiliary switch and the necessity to implement an auxiliary voltage source using an auxiliary winding on of the high frequency forward transformer. In order to over- come these two negative operational aspects this paper proposes a new improvement to the forward converter of Fig. 1. II. PROPOSED FORWARD CONVERTER Fig. 2 shows the simplified schematic circuit of the proposed forward converter that operates without commutation losses. One can see, in this new converter, that the auxiliary voltage source is connected from the negative terminal of the main voltage source to feed the resonant circuit . This topological construction leads to the following advantages when compared to the converter of Fig. 1. 1) The voltage on the switch is not greater than . 2) The voltages on the main switch and diode are equal to the auxiliary voltage source. 3) The auxiliary voltage source can be obtained by using a filter capacitor (Fig. 11), instead of the voltage source , that will charged by the magnetizing current of the transformer during the turn off of the switches , and . A suitable design of the transformer could guarantee that capacitor charges until the desired value . So it is not necessary to have an auxiliary winding on of the high frequency forward trans- former to implement the auxiliary voltage source as in the converter of Fig. 1. One can see in the circuit of Fig. 11 the input voltage source, switches , magnetizing inductance of the transformer, and capacitor consti- tute a non isolated buck-boost converter that provides the auxiliary voltage source . So, in order to provide an auxiliary voltage source with about 60% of , the max- imum duty cycle must be lower than 0.38. Moreover, like 0885-8993/00$10.00 © 2000 IEEE