Clamp Circuit,Values of R,C

I would recommend using our free design software (PI Expert)to aid you with your design. If you are interested in estimating a RCD clamp circuit, follow this procedure.

To find the resistor value:
R = (V^2)/(1/2*LI^2F)
where:
V is the average voltage on the clamp capacitor
L is the primary leakage inductance
I is the peak primary current
F is the switching frequency

for example:
If the reflected voltage on the primary (VOR) is 100 Volts (Vout*Npri/Nsec)and you want the average clamp voltage to be 125 volts above this, then: V = 225 Volts.
This voltage must be above VOR and this voltage plus the bus voltage must be less than the desired peak drain voltage. If the bus voltage is 375 volts and V is allowed to be 225 volts, then the peak drain voltage is 375+225= 600 volts.

If the leakage inductance is 10 uH and the peak drain current is 2 amps. In this example the switching frequency is 100 kHz.

Then: R = (225^2)/(1/2 *10uH * 2^2 * 100 kHz) = 253 k Ohms

C is estimated to be C = 10* (L *(I^2)/(V^2)= 10*(10uH *2A^2)/225^2) = 7.9 nF

This gives you starting values, check your design under all operating conditions for safe peak drain voltage.

Z. Cochrane

Thank you very much... Your Design guide was very helpful.,

I want the equation to calculate the r and c value in a RCD clamp circuit to reduce the overvoltage. The expected dc bus voltage is 680v and the overvoltage obtained is 1600v

Hi Rashmi B,

Thank you very much for choosing Power Integrations.

Could you please provide me with additional information regarding your power supply design?

1. Application of power supply?

2. Isolated or non-isolated

3. Input voltage range?

4. How many output voltage rails?

5. Output voltage and current for each rail?

6. Continuous output power

7. Peak power requirement if any?

8. Preferred PI part?

Input voltage is 480v, non isolted power supply and i dont have other information. I am designing the parallel RC clamp circuit across the dc bus link capacitor of adjustable speed drive to reduce the overvoltage across it.

Hi Rashmi B.

May I know what topology is used for your PSU? If you have a working schematic, I could help your review your design as well. In any case, I am attaching a document which can help you in sizing your clamp.