Fairchild 9015 User's Guide

Application Note 9015 July, 2000

A180W, 100KHz Forward Converter Using QFET

by I.S. Yang

Introduction The inherent performance advantage of power MOSFETs makes their use very attractive in switched mode power supplies. The fundamental advantage of the power MOSFET is the operation at fast switching speeds and high frequencies. Higher operating frequency allows further reduction in the size of the filter and magnetic components. The recent introduction of Fairchild Semiconductor’s QFET power MOSFETs provide a significant improvement in the reliability of switching power supplies. This application note illustrates ways in which QFET power MOSFETs may be used in switched mode power supplies. It describes a 100KHz, 180W simple off-line power supply with three regulated DC voltages.

Description of the SMPS The power supply employs a single ended forward topology. (see Figure 3) The targeted per- formance figures for the supply are as follows:

Input voltage: 220V AC Output Voltage and current: VO1 = +5V/20A

VO2 = +12V/6A VO3 = -12V/0.8A

Full load efficiency: 75% (79% at half load) Switching frequency: 100KHz Output Regulation: 5% (1/2 full load to full load)

The control circuit is implemented by the use of the current-mode controller IC, KA3842 which is very popular in controlling power supplies. A single 800V/7A rated power QFET in a TO-220 package is used as the main switching device. The performance of a power supply is depen- dent on various factors such as layout and transformer construction etc. Because of this the above performance data should be regarded as a guide to what may be achieved. The stated purpose of this application note is not to offer an optimum design but to provide some guidelines on using QFETs for switched mode power supplies.

Basic Circuit Description In order to take full advantage of the characteristics of the power MOSFET, it is necessary to review the basic circuit concepts of single ended forward converter. The basic schematic for the operation of a single ended forward converter is shown in Figure 1. The operating wave- forms describe the idealized voltage and current across the MOSFET. When the MOSFET turns on, the current is transferred from the primary DC power source through the secondary transformer to the output circuit. During the MOSFET’s off period, the magnetizing energy in the primary transformer winding is discharged into the clamp and reset circuits. There are two main techniques, RCD type and reset winding, to properly clamp and reset the forward con-

Rev. B, July 2000