QFETs are a new type of MOSFET that offers high efficiency and low losses. They can be used in switch-mode 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.
This document is an application note titled 'The Use of QFETs in a Flyback Converter' published by Fairchild Semiconductor in July 2000. It evaluates the efficiency and cost-effectiveness of using QFETs, specifically the FQP7N60 model, in switched mode power supply designs compared to standard MOSFETs. The text highlights that switching device losses account for approximately 30% of total losses, making the selection of low-loss devices crucial.
The note details the characteristics of the FQP7N60, noting a 45% reduction in gate charge and a 40% reduction in Miller capacitance compared to conventional devices. It presents a design for a 60-watt flyback converter operating at 80kHz with inputs ranging from 180VAC to 265VAC. The document includes circuit diagrams, waveform comparisons showing reduced turn-off loss and faster switching times, and a parts list for the reference design.
| Output Power | 60 watt |
|---|---|
| Input Voltage | 180 VAC to 265 VAC |
| Switching Frequency | 80 kHz |
| Output Voltage A | 160V +5% 0.3A |
| Output Voltage B | 15V +5% 0.8A |
| Efficiency | 75% |
| Gate Charge | 38 nC |
| On-resistance | 1.02 |
QFETs offer significant improvements in efficiency and lower overall system cost due to reduced gate driver requirements, smaller heat sinks, and narrower PCBs.
The FQP7N60 features a gate charge rating that is 45 percent lower than existing devices.
The reference 60 watt flyback converter operates at a switching frequency of 80kHz.