EPR-21 datasheet(7/32 Pages) POWERINT

7 / 32 page

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.

Page 7 of 32

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