Balancing High Voltage Capacitor Charging

[MuhScopeBroke]

Hello, I am working on a very high voltage application where I need to balance charge 4 capacitors to ~400V each. The capacitors are laid out in series and I am having trouble coming up with a solution to balance the charge on all 4. I've looked into the following without any luck.

A zener diode won't work due to temperature drift, the initial tolerance of a zener at 1% causes inaccuracy too large for my requirements.
IC's that can act as a comparator don't work at such high voltages either.

I'm sure there's some clever solution out there but I need help finding it

[PartialDischarge]

You will have to give more details, like is it only DC or DC with ripple, capacitors type, exactly what voltage.
How do you know they are not balanced? measured with a DMM?

[MuhScopeBroke]

I'm using electrolytic caps charged with DC. The specific ratings I'm going for are +/- 5% of the 400V coming from each cap. The reason I need to balance the caps is not because I know they are unbalanced but more that I need to be certain they aren't.

[PartialDischarge]

It seems you don't want to give much details of your applications or details, so all I can tell you is that if resistive balancing does not work for you, then there are active means based on transistors placed across them. This for example

https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/ertl_IET-PE_2008_.pdf

[T3sl4co1l]

To what end really?  Are they rated 400V and you just want to keep them happy?  Will you be dis/charging them rapidly or anything?

There may be counterindications.  If the latter applies, better to let them distribute according to value.  Remember, electrolytics have quite loose tolerances, from both manufacture and age.  Charging to equal voltages, then discharging suddenly, will result in reversal of the smaller values.  How much, depends on the imbalance and depth of discharge.

It might also just not be necessary; electrolytics tend to leak more near the voltage rating.  This does decrease over time (reforming dielectric) but it's effectively setting its own voltage limit, and they can distribute accordingly.  Not that I'd recommend stacking electrolytics completely naked like this, but just that there's an argument (if a weak one) for doing so.  The downside is, if the temperatures or aging aren't distributed evenly, some will eventually start to leak more, and hog and overvolt the others.  For which a resistor divider can help swamp out the imbalance, extending useful life before failure (but not preventing it, as leakage will continue to increase over time*).  Meanwhile you're burning that power in the resistors all the time, so it's poor economy to use low values.  Resistors tend to be chosen based on rated leakage, while consuming under a watt each, say.

*Or not, but capacitance drifts down, and ESR up.  Whatever the failure mode happens to be.

Tim

[gbaddeley]

Hi scope broke,
Did you look into the most common and easiest method, using resistors?

[MuhScopeBroke]

Thanks for the replies. I looked into the solution MasterTech linked and that looks very interesting. The only problem I can imagine is that there would be too many components, but that's much more negotiable that alternatives. The capacitors are 650uF +/- 10% so getting resistors large enough to be sure the cap is getting charge current may be an issue as well.

The issue passive resistor dissipation has is that at such a high voltage, losses are very high which isn't ideal.