AN9008 Datasheet, PDF (2/9 Pages) Fairchild Semiconductor – The Use of QFETs in a Flyback Converter

:
QFET
FQP7N60
Figure 1: Gate Charge Improvement
Balanced with gate charge improvement, the on-resistance [R_ds(on)] goes down by about 20 per-
cent with respect to previous devices versus drain current. Figure 2 shows the improvement of on-
resistance in a QFET compared with a MOSFET.
1.10
Conventional MOSFET
QFET (FQP7N60)
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
1
2
3
4
5
6
7
Drain Current : Id [A]
Figure 2: On-resistance Improvement vs. Drain Current
The combined improvement of gate charge and on-resistance in this 60 watt flyback converter
leads to a more efficient system because of the reduced turn-off conduction loss. It is worth
emphasizing that QFETs offer designers significant improvements in terms of lower overall system
cost due to lower gate driver requirements, a smaller heat sink, and narrower PCB. Table 1 illus-
trates the features which are useful in flyback converters and other applications.
Table 1: Q_g and R_ds(on) Improvements
Voltage Rating
600V
600V
Device
Conventional Part
FQP7N60(QFET)
On-resistance
1.2 Ω
1.0 Ω
Gate Charge
65 nC
38 nC
Package
TO-220
TO-220
Rev D, July 2000
2