This document describes how to reduce switching losses in a step-up converter using QFETs
Efficiency is critical in switching mode power supply design, particularly as frequencies increase. This document introduces the Fairchild Semiconductor QFET family of power MOSFETs, which feature low on-resistance (Rds(on)) and gate charge (Qg) characteristics to achieve lower switching and drive losses compared to conventional devices.
The note details a specific 70W step-up converter design typically used in C-Monitors. It compares the performance of the QFET model FQP10N20 against a conventional MOSFET, demonstrating that the QFET reduces power losses through faster switching times and lower conduction resistance. The document includes circuit diagrams, waveform comparisons, efficiency graphs, and a parts list for the reference design.
| Input Voltage (DC) | 50V max |
|---|---|
| Output Voltage | 120V + 1% |
| Maximum Output Load | 70W |
| Switching Frequency | 80KHz |
| Efficiency (Full Load) | 90% |
| Output Capacitor | 500uF |
The QFET provides 2~4% higher efficiency, with differences becoming more noticeable as frequencies increase.
The reference design uses the FQP10N20 QFET.