# Selecting capacitor for voltage-divider of half-bridge high power SMPS

How to dimension the capacitor for the voltage-divider of a half-bridge high power (>5KW) SMPS power supply?

The question is more on type of capacitor and current rating than the capacitance.

I have read some projects using electrolytic capacitors, even some computer power supply uses this approach and the capacitors are the DC Link ones too!

But as I know this is not a good choice, its better to use polymer, right?

But if you are lucky enough to find the manufacturer specifications, it's difficult to get any info about the current the capacitor will support. Some papers says that's this is based mostly on dissipation of the capacitor, I have even made an spreadsheet to calculate that dissipation, I will post if necessary, but I didn't know if this is the way to go. simulate this circuit – Schematic created using CircuitLab

• Are you sure a full-bridge isn't a better choice, given the power level? The current stress in the switches is double vs. a full-bridge (because the transformer voltage is halved). Aug 16, 2013 at 19:06
• look at ESR (equivalent series resistance) of a capacitor. Aug 16, 2013 at 19:06
• @Madmanguruman that's for a welding machine. There's a paper from Fairchild (AN-9742) (although its for a ~3.5KW machine) saying that the major manufactured ones uses half-bridge topology. As its easier to design a half-bridge (except mostly for that) I'm going this approach. The switches are IGBTs of near 100A anyway. Aug 16, 2013 at 19:14
• Very good. As long as the switches are up for it, you should be OK. Aug 16, 2013 at 19:15
• @JImDearden I'm don't think thats the way, in my spreadsheet the dissipation by ESR is relatively low compared to dielectric dissipation. Aug 16, 2013 at 19:17

$DF = \omega C \cdot ESR = \dfrac{1}{Q}$
$\omega = 2 \pi f$