19-Nov-02
EPR-21 – 5 V, 30 W DC-DC Converter
R13 applies a small amount of preloading to the supply output to prevent the voltage at
the auxiliary winding of L1 from collapsing at zero load. R13 is set to provide a minimum
of 8 V at the cathode of D1 at zero load.
4.3
Output Feedback
R10 and R11 divide down the supply output voltage and apply it to the reference pin of
error amplifier U3. U3 drives optocoupler U2 through resistor R6 to provide feedback
information to the CONTROL pin of U1. The optocoupler output also provides power to
U1 during normal operating conditions. D3 and C13 apply drive to the optocoupler during
supply startup to eliminate output voltage overshoot. D3 isolates C13 from the supply
feedback loop after startup. R7 discharges C13 when the supply is off. R8 provides bias
current to U3.
C6, C14, C16, R4, R6, R9, and R12 all play a role in compensating the power supply
control loop. C6 rolls off the gain of U1 at a relatively low frequency. R4 provides a zero
to cancel the phase shift of C6. R6 sets the gain of the direct signal path from the supply
output through U2 and U3. C14 and R9 roll off the gain of U3. R12 and C16 provide
phase boost near the output filter (L2, C10-11) resonant frequency to improve phase
margin and stability.
4.4 Construction
The EP21 is constructed with surface mount components using an aluminum clad circuit
board. The printed circuit board is an effective heat spreader, and allows attachment of a
heat sink on the back of the board for operation at high ambient temperature. Tantalum
and ceramic capacitors are used instead of conventional electrolytic capacitors to enable
operation at extreme ambient temperatures.
Power Integrations, Inc.
Tel: +1 408 414 9660 Fax: +1 408 414 9760
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