9015 Datasheet, PDF (1/7 Pages) List of Unclassifed Manufacturers – QUAD NOR GATE

Application Note 9015
A180W, 100KHz Forward Converter Using QFET
July, 2000
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: V_O1 = +5V/20A
V_O2 = +12V/6A
V_O3 = -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-
1
Rev. B, July 2000