7004 Datasheet, PDF (6/12 Pages) Bourns Electronic Solutions – Network Interface Device

becomes a current source that forward biases diodes D1 and D2, returning the energy stored
in the transformer’s primary to the input capacitor. No snubbers or clamps are required, and
the maximum off voltage the primary side FET will see is the input voltage, plus a couple of
diode drops. Therefore, the 80V FDS3570 is useful in designs that will never have the input
exceed of 76V. The 100V FDS3670 will provide a little more design margin if desired. All wave-
forms are essentially similar to Figure 5 except the FET’s drain voltages, which are shown in
Figure 7.
V_IN
+
C1
Isolation
Input
Q2
Gate
Source
GND
D4 _{D3 T1}
D1
U1
PWM
Gate
Q1
GND
L1
D2
V_OUT
+
C2
RTN
Figure 6. Two transistor forward converter.
V_GS(Q₁TQ₂)
V_SD(Q₁TQ₂)
-V_IN
-Can Vary From
0 to V_IN
Figure 7. Voltage and current waveforms for two transistor forward converter.
Note that the primary current is the same as the single transistor design example above.
However, the FDS3570 has an on-resistance of only 15% than that of the 200V FDS2670
(20mΩ vs. 130 mΩ). Even with two transistors in series, the net on-resistance is still much
lower than the single high-voltage transistor, so a higher power level can be obtained.
Efficiency will also be several percentage points higher than the single transistor design since
the energy that would be dissipated in snubbers and clamps is, in this case, recovered to the
input capacitor. Using the FDS3670’s maximum RMS current rating of 9 amps, an assumed
efficiency of 83% with 36V in, and a maximum 45% duty factor, gives a maximum output
power of 180 watts. Again, it is permissible to parallel FETs in an effort to lower the conduction
losses and raise the useable output power of the converter.
©2001 Fairchild Semiconductor Corporation
6
Rev. A, June 2001