7004 Datasheet, PDF (2/12 Pages) Bourns Electronic Solutions – Network Interface Device

V_IN
+
C1
U1
PWM
Gate
GND
T1
Q1
D1
V_OUT
+
C2
RTN
Figure 1. Simplified flyback regulator schematic.
For 36- to 72-V input designs, the FDS3670, 100V MOSFET is the lowest voltage part that
should be used. Even then, the drain voltage will have to be carefully controlled with some
form of primary side clamp circuit. If the input supply can be expected to experience transient
voltages on the order of 100V, the 150V FDS2570 should be considered. It is important to
note, two devices would have to be connected in parallel to maintain the same power level that
can be achieved with a single FDS3670.
The off-state voltage is determined by the input voltage, output voltage, transformer turns ratio,
transformer leakage inductance, and snubber/clamp circuit. Ignoring the effects of leakage
inductance, the off voltage is equal to the input voltage plus the output voltage multiplied by the
transformer turns ratio. As an example, assuming a 1:1 turns ratio and a 5V output, the off-
state drain voltage is equal to Vin plus 5V. The effects of leakage inductance complicate the
matter considerably. The energy stored in the leakage inductance is equal to LI²/2, where L is
the leakage inductance and I is the peak primary current. This energy appears as a voltage
impressed on the various stray capacitances around the circuit. The energy is equal to CV²/2
and, therefore, V_pk=I_pk(L/C)^1/2. The problem is complicated by the fact that most of the stray
capacitances are nonlinear and vary greatly as a function of the instantaneous voltage
applied. As such, an empirical approach to designing the snubbers and clamps is generally
used. Figure 2 shows typical waveforms for the flyback regulator. Generally in a 48V
application, the maximum useful power level for a flyback design will be around 20 to 30 watts.
Above that, consider a single transistor forward converter as shown in Figure 3.
©2001 Fairchild Semiconductor Corporation
2
Rev. A, June 2001