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Old 11th Jan 2012, 6:46 am   #25
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: capacitors in series - pitfalls?

I think that connecting capacitors in series is not such an issue as it may seem, certainly for electrolytics.

If we assume equal capacitors for simplicity, the leakage current through each capacitor is always identical (they are in series). It should not be assumed that the leakage current is a linear function of the voltage across the capacitor. The voltage across the "leakier" capacitor will be less than if it had half the total voltage across it and, due to this, its leakage current will always be well within specification, assuming good capacitors. Voltage balance is not important in a simple series configuration.

In fact, in a series configuration the leakage current goes up for the less leaky capacitor and down for the more leaky capacitor, as the voltage distributes itself between the capacitors, but the crux of the matter is that the leakage current is always well within spec by this process. Even when the voltage rating of one capacitor is exceeded, the leakage current is kept within spec by the other capacitor. If the leakage of an over-voltage capacitor does start to increase, let us say, the voltage across it will reduce and this brings down that leakage - there is a stabilising mechanism in the system.

As has been pointed out, most SMPS use electrolytics in series. In these circuits the voltage across the series electrolytics must be balanced due to the requirements of the following circuitry. The balancing is implemented by resistors (they must be good quality and have adequate power rating) across each of the capacitors. This serves to swamp the leakage. If one of these fails the voltage balance is destroyed and the following circuitry malfunctions. This is a rare fault and in my experience even this does not result in capacitor failure despite the use of resistors potentially causing an out-of-specification leakage current under resistor failure.

Personally I favour the use of resistors in parallel with the capacitors - you can understand what is happening, how the voltage divides - they make the designer feel good and in control. However, the reality is that they are generally redundant unless the following circuit requires balance - series capacitors look after themselves without help, provided the sum of their voltage ratings is adequate.

Peter
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