# How calculate the input filter capacitors for Half bridge SMPS

1. ## How calculate the input filter capacitors for Half bridge SMPS

Hello guys,

In sch attached how to calculate the filter capacitors C4 and C5 ? to a power of 1200W.

Can anyone help?

Thanks •

2. ## Re: How calculate the input filter capacitors for Half bridge SMPS

whatever value keeps the voltage at the mid point within say 1% of the nominal. Also, ensure no overly high ripple current in them. •

3. ## Re: How calculate the input filter capacitors for Half bridge SMPS Originally Posted by OrciBorg In sch attached how to calculate the filter capacitors C4 and C5 ? to a power of 1200W.
Do you mean C4 & C7? Those appear to supply the entire circuit.

They are replenished every 1/100 (or 1/120?) Hz.
Suppose they are 3mF.
And suppose the load calculates as 100 ohms.
Then the RC time constant is .003 x 100 (or 0.3 sec).

Voltage drops 63% during one time constant.
Or 6.3% during a tenth of that time (approx).
Then calculate the drop during 1/100 sec.

Adjust the Farad value up or down until you get a satisfactory figure.

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4. ## Re: How calculate the input filter capacitors for Half bridge SMPS

A decent rule of thumb will get you close:
C = I x dT/dV

C (in uF) = Amps x microseconds (between mains half cycles) divided by desired peak to peak ripple.

Lets see, assume 1,200 watts at 340v dc (= 3.5A) and 34v p/p ripple.
Assume 100 Hz from rectifier = 10,000uS hold up time between cycles
This is for both C4 and C7 as they are in series across the dc rail.

C = 3.5A x 10,000uS / 34v p/p ripple
1,029uF total

C5 and C7 will EACH need to be 2,000uF for about 10% ripple in 220v bridge configuration.
Voltage doubler configuration will require a bit more, but it gets complicated.

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