ZVZCS, dual, two-transistor forward DC-DC converter with peak voltage of Vin/2, high input and high power application

This paper presents a zero voltage and zero current switching (ZVZCS), dual, two-transistor forward converter (DTTFC) for high input voltage and high power application. Two identical two-transistor forward converters (TTFC) are connected in series and coupled through a single transformer using two primary windings, because of which it operates like a full bridge and makes it suitable for high power application. It imposes only half the input voltage across each of the four switches. This DTTFC has advantage of high reliability as compared to the full or half bridge converters. A modified pulse width modulation (PWM) controller is proposed for DTTFC to minimize the circulating RMS current flowing through the transformer and switching devices. This is without any extra hardware in series with primary of the transformer or in secondary. Conduction losses, being proportional to the square of the conducting current in a transformer and MOSFET, are important in high power converters operating at high currents. Since switching losses are also important in high frequency switching application, in DTTFC two switches, out of four switches, are turned-on and off at zero voltage condition due to the current in leakage inductance of a transformer. The remaining two switches are turned-off at zero voltage and turned-on at zero current condition due to transformer leakage inductance and minimized circulating current, over a wide range of load current. The advantages of this simple circuit topology is high efficiency, high reliability, low component count, and low cost, makes the new converter attractive for high input voltage, high power applications. In this paper we discuss the operating states, switching transitions, method of control, experimental results and associated waveforms of a MOSFET based practical 60 V, 50 A, 100 kHz, dual, two-transistor forward DC-DC converter operating at 750 V input bus. The measured efficiency was 95.07% at full load with maximum efficiency of 96.6% at 70% load.