2

Rev. B, July 2000

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:

Figure 1. Basic Circuit and Idealized Waveforms

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.

220V AC line

110V AC line

150W

FQP6N70

FQP6N50, FQP6N60

250W

FQP6N80, FQP7N80

FQP9N50, FQP12N60

300W

FQP7N80, FQP6N90

FQP12N60

Table 1: QFET Selection

BV

dss

V

in max

()

1

1D

max

–

------------------------

leakage

induc

ce

tan

spike











+

≥

I

dmax

()

1.2P

o

ηV

in min

()

---------------------------------------------

≥

Control

Circuit

T1

L1

Vin

Switching

MOSFET

voltage

across

MOSFET

current in

secondary

transformer

Current

in MOSFET

current in

clamp

circuit

0

0

0

0

Vds

Vout

R

C

D

1

1

2

1

2 1

2

Input Voltage

Output Power