Capacitor calculation in push pull power supply

[newtonisback]

Hello there,

I am designing push pull power supply with one input(20-366 VDC) to multiple (4) outputs.
1. 24V DC (35mA)
2. 5V(35 mA)
3. 13V(35 mA)
4. -13V(35 mA)

I used full wave rectifier. Now problem is....How to calculate values of filter capacitors for rectifier side and all output side.

I found that ripple current is depends on cap.

Is there any specific calculation for calculating perfect calculation for the capacitance.?

Please help me solve this.

 

[BradtheRad]

The RC time constant is one avenue to try.

Example... given these specs:

* you permit 3% ripple.
* load is 40 ohms.
* smoothing capacitor is replenished every 1/1000 second.

The time constant (TC) is R * C. Defined as the time for output level to drop 63%.

You permit 3% drop in 1/1000 second. This translates to 63% drop in .021 sec.

TC = R * C.
Or, C = TC / R.

.021 / 40 = 525 uF. Value for smoothing capacitor.

There is another formula, based on amperes going in and out of the capacitor, per cycle.

 

[chuckey]

Err, Brad surely the value depends on the ripple frequency and the wave form, somehow? if you are using a squarewave inverter with full wave rectification, no cap is needed?
Frank

 

[BradtheRad]

Right, a rectified square wave becomes DC, more or less. There will be less need for a smoothing capacitor.

I was only guessing at what waveform he will have. Say, the ordinary pulsed or triangle waveform, where the capacitor powers the load during idle gaps.