lnk623 Power Integrations, Inc., lnk623 Datasheet - Page 8

• Manufacturer Part Number: lnk623
• Description: Energy-efficient, Off-line Switcher With Accurate Primary-side Constant-voltage Cv Control
• Manufacturer: Power Integrations, Inc.

Rev. C 12/08
Drain Clamp
Recommended Clamp Circuits
Components R1, R2, C3, VR1 and D5 in fi gure 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 refl ected output voltage
(V
conduction during the fl yback voltage interval when the output
diode is conducting. The value of R1 should be the largest value
that results in acceptable setting 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.
OR
8
). This is to clip any turn off spike on the drain but avoid
RCD Clamp, Low Power or Low Leakage Inductance Designs.
LNK623-626
R
C2
R
D
C1
C1
PI-5107-110308
C
C1
RCD Clamp With Zener Bleed. High Power or High Leakage Inductance Designs.
Figure 10. Clamp Circuit (top). Feedback Pin Voltage (bottom).
Black Trace: D
Gray Trace: D
C1
C1
is a 1N4007G (standard recovery, trr = 2 us)
is a FR107 (fast type, trr = 500 ns)
D
R
C2
R
C2
C2
D
R
C1
C1
R
D
C1
C1
PI-5108-110308
C
PI-5107-110308
C1
C
C1
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