# Choosing capacitor for the filter of a three phase full wave rectifier

I am designing a power management unit (PMU), which takes the power from an alternator (alternator output: three phases, 30-85VAC, 35-130Hz, Max current: 20A).

I want to reduce the ripple using capacitors (Between the rectifier and a buck converter).

I would like to reduce the ripple to 500 mV. I found a formula for three phase rectifiers, but I am not sure if it is correct, because the value is too high.

The formula I found is:

C = I/6fVpp, where 'I' is the current, 'f' is the line frequency, and 'C' is the filter capacitance.

The most unfavorable situation is with the lowest frequency and maximum current:

C = 18A/(6·35Hz·0.5V) = 0.171 F = 171 mF.

Does anyone know if this formula is correct?

Thank you.

• You should add a serial inductor just before the capacitors ... also to limit inrush current. Commented Nov 21, 2022 at 11:47
• I don't understand, you have there LTspice, you've created the schematic, you have calculated the value, what's stopping you from pressing "run" and test for yourself? Here are a few tips: if you mean to have the source with a positive sequence then you need 0,-120,120 degrees for phases, you don't need 3 caps in parallel (just use one with equivalent value), it will help to use either a .model for diodes or select one from the database, and, finally, use a current source as the load with the flag load. Commented Nov 21, 2022 at 11:48
• Also, the ground on the sources side should not be tied directly to the sources -- use a 1Meg (or so) valued resistor to ground. If you'll plot the current you'll see why. Or use the same approach for the load side, but not with a common ground. Commented Nov 21, 2022 at 11:52
• In other words, with two grounds where you have them, diodes are shorting out the supplies. Commented Nov 21, 2022 at 11:56
• Ok, thank you. But I put the image just to get the idea. Commented Nov 21, 2022 at 11:56