7004 Datasheet, PDF (5/12 Pages) Bourns Electronic Solutions – Network Interface Device

V_GS(Q1)
V_DS(Q1)
I_D(Q1)
I_D1
I_D2
I_L1
PWM'ED EDGE
-V_SPIKE
-V_RESET
-V_IN
I_OUT
SW
ON
V_{SW 0}
V_SAT
2V_IN
I_{PRI 0}
I_MIN
I_PK
TIME
TIME
Figure 5. Voltage and current waveforms for a single transistor forward converter.
The average switch current in a Forward converter is equal P_OUT/ηV_IN where P_OUT is the max-
imum output power, η is the efficiency, and V_IN is the minimum input voltage. With a maximum
rating of 3 amps for the FDS2670 and assuming an efficiency of 80%, the maximum output
power that can be obtained with a single primary-side FET is approximately 80W. Utilizing two
FDS2670 devices in parallel are therefore capable of a theoretical output power of about 160
watts. Note the instantaneous peak switch current is going to be 2.5 to 5 times higher than the
average current, depending on the input voltage range. The wider the input voltage range, the
higher the peak switch current.
Two Transistor Forward Converters
For still higher power levels, up to about 200 watts, the two transistor forward converter of
Figure 6 is a good choice. The basics of the circuit’s operation are similar to the single
transistor design. The difference is that the leakage inductance’s energy, and therefore, the
drain voltage, is no longer unconstrained. During the off state, the transformer’s primary
©2001 Fairchild Semiconductor Corporation
5
Rev. A, June 2001