# 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). – Adam Lawrence Aug 16 '13 at 19:06
• look at ESR (equivalent series resistance) of a capacitor. – JIm Dearden Aug 16 '13 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. – Diego C Nascimento Aug 16 '13 at 19:14
• Very good. As long as the switches are up for it, you should be OK. – Adam Lawrence Aug 16 '13 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. – Diego C Nascimento Aug 16 '13 at 19:17

Capacitor choice is largely a matter of losses, yes. The capacitance value needs to be large enough to keep the midpoint more-or-less-fixed, with low ESR and ESL also being important. Under some transient conditions the midpoint can (and will) move around, so it's important to keep derating in mind as well. 50% voltage derating is well within reason (using 400V caps with a total DC input of 400V, for instance).

For AC-application capacitors (i.e. film caps), the ESR is often implied in the dissipation factor datasheet parameter, which is the relationship between ESR (resistive loss) and reactance:

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

$\omega = 2 \pi f$

It will usually be specified at a given frequency, which allows you to calculate the ESR.

Electrolytic capacitors generally have large ESL by nature of their construction vs. film capacitors, limiting their usefulness at high frequency. Film capacitors also have some self-healing capability under dielectric breakdown scenarios which make them a 'robust' choice in half-bridge designs.

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