AN9008 Datasheet, PDF (5/9 Pages) Fairchild Semiconductor – The Use of QFETs in a Flyback Converter

Figure 5 compares the waveforms of a conventional MOSFET with the new QFET(FQP7N60) at
turn-off without the additional clamped circuit (R5, C6, and D2), and the high conduction diode (D4)
for gate discharging (refer to Figure 3).
Id(0.5A/div)
Conventional
MOSFET
Vds(100V/div)
QFET
/div
Figure 5: Turn-off Improvement at Rated Conditions (V_in=220VAC, P_out=60W)
Note that the switching time of the QFET is faster than that of the conventional MOSFET because
of the reduction of gate charge by at least 45 percent. Figure 6 shows the difference in turn-off loss
between both MOSFETs without clamped circuits and the conduction diode, D4. The turn-on loss
in the crossover losses is very small and can be negligible. The turn-off loss period is due to the
finite switching time of the MOSFET which is directly related to the gate charge.
Conventional
MOSFET
QFET
/div
Figure 6: Turn-off Loss Improvement
5
Rev D, July 2000