6

Rev D, July 2000

The turn-off loss area of QFET(FQP7N60) is half that of of the MOSFET.

During turn on and off, there is a short period when there is a significant overlap of voltage and cur-

rent across the MOSFET. Figure 5 shows that the QFET(FQP7N60) has a shorter overlap period

than the conventional MOSFET, resulting in a lower loss (Figure 6). In Figure 7 the efficiencies of the

converter are calculated without D4 (high conduction diode, refer to Figure 3) operating at rated con-

ditions of 220 VAC input voltage and 60 watt output as a function of frequency. As shown in Figure 7,

the QFET (FQP7N60) design is more efficient than its conventional MOSFET counterpart. The

advantage of QFET design is more pronounced as the switching frequency of the power supply

increases. These waveforms clearly demonstrate that faster switching translates into lower switch-

ing loss and much better efficiency.

Summary

To ensure high efficiency and reliable performance of the flyback converter, or any other converter,

the designer must ensure that the MOSFET operates effectively with lower on-resistance and gate

charge in the system. In this application note, that QFET(FQP7N60) flyback design demonstrates

higher efficiency than the previous MOSFET design because of the improvement of on-resistance

and gate charge. The other series of Fairchild’s QFETs with high voltage ratings (600, 800, and

900 V) allow designers to improve the performance of a switching mode power supply by a signifi-

cant reduction in gate charge and on-resistance.

40

60

80

100

120

140

0.70

0.75

0.80

0.85

0.90

QFET (FQP7N60)

Conventional MOSFET

Frequency [kHz]