Rev D, July 2000

July, 2000

1

AN9008

The Use of QFETs in a Flyback Converter

By Il Soo Yang

Introduction

Power supply designers face many challenges in designing more efficient and cost-effective power

supplies. Efficiency is a major consideration in designing switching power supplies. Many factors in

the design process such as the input filter capacitance, transformer core geometry and construc-

tion, output rectifier, and switching device etc., affect the efficiency of switching power supplies.

Among the losses all components generate, switching device losses occupy about 30%. Hence,

selecting MOSFETs with optimum efficiency and high reliability is very crucial in power supply

design. This application note compares the key characteristics, power losses, and efficiency of the

new QFET and a conventional MOSFET in a 60 watt flyback converter operated at 180 to 265

VAC.

QFET Characteristics

Almost all the power supplies used in TVs, VCRs, PCs, fax machines, and other home appliances

rely on a switching circuit to convert the AC wall power to DC power or DC to AC. Thus, they are

referred to as switched mode power supplies. To obtain high efficiency, it is crucial for designers to

select switching MOSFETs to give very low losses in the circuits. MOSFETs must exhibit low con-

duction and switching losses with safety qualifications. Fairchild Semiconductor, in extending its

commitment to develop high quality MOSFETs, now offers new high efficiency QFETs for switched

mode power supply applications.

A power QFET, rated at 600V and used in a 60 watt flyback converter, features a gate charge rat-

ing which is 45 percent lower than existing devices for improved switching and drive efficiency. Fig-

ure 1 compares the new QFET FQP7N60 with its conventional MOSFET counterpart. By using

unified singular well stripe technology, the Miller capacitance of the new QFET is reduced by about

40 percent.