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LNK626PG Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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LNK626PG Datasheet(HTML) 8 Page - Power Integrations, Inc. |
8 / 18 page 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 OR). 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 C1 is a FR107 (fast type, trr = 500 ns) Gray Trace: D C1 is a 1N4007G (standard recovery, trr = 2 us) |
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