Capacitor Insulation Resistance

Baluncore said:

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The &#;self-healing property&#; of metalised polymer film capacitors operates by allowing the metal film to fuse near an insulation failure and so electrically isolate the problem area.

That requires the capacitor be used in a low impedance power circuit, one that is able to deliver sufficient energy to isolate a failure. Following the healing there will still be a very small leakage current.

For that reason insulation resistance (IR) takes a dynamic value, switching from below one ohm to tens of megohms as the self-healing cycles occur.

Self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance.

Where a series chain of capacitors is employed for high voltage, you must assume that one capacitor may be short circuited at the time, and that voltage balancing resistors will share the voltage fairly across the remaining capacitors. The IR during normal operating conditions will be significantly greater than the voltage sharing resistors.Since 10.000 has three trailing zeros, the stop should be interpreted as a thousands separator. Elsewhere in the data sheet trailing zeros are removed after a decimal point.

I will make sure there is enough headroom for one of the capacitors to be shorted without over-voltaging the other capacitors.

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That document states that the electrode layers are 1/th the diameter of a human hair. With the layers being so thin, it doesn't seem like it would take an unreasonable amount of energy to oxidize the area around a short. If a large current does not flow, that should mean the capacitor has relatively low leakage current and is functioning properly shouldn't it? As far as the fixed capacitance part, what type of circuit wouldn't be better off with a capacitance that decreases slightly after a fault compared to failing completely? From my understanding, the self healing property is a benefit because the percent of capacitance lost is proportional to the area vaporized which is very low considering the many many layers and the small area vaporized.

Also, according to that document, it appears that film capacitors have lower leakage, in general, than ceramic capacitors. DC/DC converters often use film capacitors because of their self healing properties and the low leakage (I thought). I would think a capacitor with high leakage would not be as desirable for a DC-link capacitor actually. So I'm having a hard time understanding the statement that self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance. One of the only places I an see this type of capacitor not being used as often is a resonant converter that requires a set capacitance to maintain resonance. Those values are typically small and wouldn't benefit from a large capacitance value anyway so something like a C0G(NP0) would be used.

That's just my current understanding so hopefully you can tell me where that logic is off?Using the decimal as a thousands separator would make the IR more in the range of what I would expect. 750 MOhm sounds a lot better than 750 kOhm.

That document states that the electrode layers are 1/th the diameter of a human hair. With the layers being so thin, it doesn't seem like it would take an unreasonable amount of energy to oxidize the area around a short. If a large current does not flow, that should mean the capacitor has relatively low leakage current and is functioning properly shouldn't it? As far as the fixed capacitance part, what type of circuit wouldn't be better off with a capacitance that decreases slightly after a fault compared to failing completely? From my understanding, the self healing property is a benefit because the percent of capacitance lost is proportional to the area vaporized which is very low considering the many many layers and the small area vaporized.Also, according to that document, it appears that film capacitors have lower leakage, in general, than ceramic capacitors. DC/DC converters often use film capacitors because of their self healing properties and the low leakage (I thought). I would think a capacitor with high leakage would not be as desirable for a DC-link capacitor actually. So I'm having a hard time understanding the statement that self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance. One of the only places I an see this type of capacitor not being used as often is a resonant converter that requires a set capacitance to maintain resonance. Those values are typically small and wouldn't benefit from a large capacitance value anyway so something like a C0G(NP0) would be used.That's just my current understanding so hopefully you can tell me where that logic is off?Using the decimal as a thousands separator would make the IR more in the range of what I would expect. 750 MOhm sounds a lot better than 750 kOhm.

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