# Thread: Hold up cap capacitance when not considering hold up time

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## Hold up cap capacitance when not considering hold up time

I know that cap in pfc sectiom does the smoothing job and 'holds up' when the main is out.
The cap's capacitance seems to be determined by the hold up time.
But how much capacitance is relatively(to ATX spec 16ms capacitance) needed when not considering the hold up time(only smoothing purpose)?
30%? 50%?

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This mostly depends on wattage, circuit design and performance expectations. PSU with 5 ms hold up time (equals 30% required capacity versus 16.6 ms) still can do absolutely fine in all other tests.

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Reviewers sometimes states 'adequate capacitance for specific wattage'.
How is it calculated, and any equations or sheet?

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https://www.infineon.com/dgdl/Infine...4a62c75a923b05
Found a design guide and equations...

What numbers for each variables?

P0=wattage
thold=16.6
V0=output voltage=380V I guess?
what are V0min and delta V?

6. Years ago, in the days before PFC was required for computer power supplies, we had a customer that wanted a longer hold up time.

This was a power supply that used a voltage doubler circuit. The power supply had two green 820 mfd capacitors. (I think they were ELNA capacitors) The technician replaced the capacitors with two 1,000. mfd capacitors.

The hold up time decreased. I measured the capacitance of the old 820 mfd at 875 mfd. The new 1,000 mfd measured at 850 mfd.

We ended up finding out all other input capacitors measured about 15 percent low. The capacitors tolerance was +- 20 percent so they all met spec.

Keep this in mind when sizing capacitors.

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