Rev. E 09/09

8

LNK623-626

www.powerint.com

Drain Clamp

Recommended Clamp Circuits

Components R1, R2, C3, VR1 and D5 in figure 4 comprise the

clamp. This circuit is preferred when the primary leakage

inductance is greater than 125 μH to reduce drain voltage

overshoot or ringing present on the feedback winding. For best

output regulation, the feedback voltage must settle to within 1%

at 2.1 μs from the turn off of the primary MOSFET. This requires

careful selection of the clamp circuit components. The voltage of

VR1 is selected to be ~20% above the reflected output voltage

(V

This is to clip any turn off spike on the drain but avoid

conduction during the flyback voltage interval when the output

diode is conducting. The value of R1 should be the largest value

that results in acceptable settling of the feedback pin voltage and

peak drain voltage. Making R1 too large will increase the

discharge time of C3 and degrade regulation. Resistor R2

dampens the leakage inductance ring. The value must be large

enough to dampen the ring in the required time but must not be

too large to cause the drain voltage to exceed 680 V.

If the primary leakage inductance is less than 125 μH, VR1 can

be eliminated and the value of R1 increased. A value of 470 kΩ

with an 820 pF capacitor is a recommended starting point.

Verify that the peak drain voltage is less than 680 V under all

line and load conditions. Verify the feedback winding settles to

an acceptable limit for good line and load regulation.

Effect of Fast (500 ns) versus Slow (2 μs) Recovery

Diodes in Clamp Circuit on Pulse Grouping and Output

Ripple.

A slow reverse recovery diode reduces the feedback voltage

ringing. The amplitude of ringing with a fast diode represents

8% error in Figure 10.

Figure 9.

RCD Clamp, Low Power or Low Leakage Inductance Designs.

RCD Clamp With Zener Bleed. High Power or High Leakage Inductance Designs.

Figure 10. Effect of Clamp Diode on Feedback Pin Settling. Clamp Circuit (top).

Feedback Pin Voltage (bottom).

C

C1

R

C1

R

C2

D

C1

PI-5107-110308

D

C2

R

C2

R

C1

C

C1

D

C1

PI-5108-110308

C

C1

R

C1

R

C2

D

C1

PI-5107-110308

Black Trace: D

Gray Trace: D