9015 Datasheet, PDF (2/7 Pages) List of Unclassifed Manufacturers – QUAD NOR GATE

verter’s primary transformer to prevent a potential saturation problem. In this application note
a RCD type clamp circuit is used. According to the different values of RCD components, the
clamp voltage is developed variably in the primary winding. Usually, high voltage with twice the
input DC voltage is used to obtain the maximum 50% duty ratio for resetting the transformer
flux during the MOSFET’s off period at low input DC voltage and full load. That means the
peak voltage of twice the primary DC voltage is developed across the MOSFET during the off
period. For a nominal line of 220V AC, the peak voltage could reach to at least 660V. This is
the reason for a minimum voltage rating of at least 800V.
Simple equations for selecting power MOSFET are as follows:
leakage 
BV_dss
≥
V_{i n ( m a x ) 1}------–----_D-¹----_m-----_a--_x--
+


i
n
duc


s
p
i
ke
tan
ce




I_d(max) ≥ _η-----_V----_i--_n---₍-¹-_m---^.-_i-²-_n---^P₎--_D--^o---₍--_m----_a---_x---₎
Vin _R
T1 ¹
C
D
2
1
21
2 ^Vout
L1
Vds
0
0
Switching
0
Control ₁ MOSFET
Circuit
0
Figure 1. Basic Circuit and Idealized Waveforms
voltage
across
MOSFET
Current
in MOSFET
current in
clamp
circuit
current in
secondary
transformer
Input Voltage
Output Power
150W
250W
300W
220V AC line
FQP6N70
FQP6N80, FQP7N80
FQP7N80, FQP6N90
Table 1: QFET Selection
110V AC line
FQP6N50, FQP6N60
FQP9N50, FQP12N60
FQP12N60
QFETs appropriate for the respective power outputs are shown in Table 1. Fairchild Semicon-
ductor’s power MOSFET QFETs provide more useful characteristic ratings to the forward con-
verter and other topologies by the reduction of on- resistance, gate charge, and improved
reliability.
2
Rev. B, July 2000