Dielectric Withstand of TO-220 with isolated heat sink.

[ZappHappy]

The MFG rating of a typical power resistor in a TO-220 is 1500V. This is the breakdown
between the resistive element and the heat sink or any mounting parts and case, not
the limiting element voltage of the resistor.

If we look at this as a 3 terminal device, where one terminal is the breakdown (gap) could
I not double the breakdown when using 2 resistors or more on a floating heat sink as long
as the potential difference across the string doesn't exceed the individual breakdown ?

Ignoring max V across 2 equal resistors the gaps should be in series, will this double overall case
to case max difference if heat sink tabs are connected ?

 

[KlausST]

Hi,

only in idealistic case ... where the "heatsink" has
* zero capacitance against 0V, 300V and earth GND
* infinite resistance / impedance to 0V and 3000V
This is not realistic.

We don´t know whether the voltage is DC, and if AC what frequency and waveform.
In case of DC.. when a spark happenss (due to ESD for example) the spark may continue and cause a danger for life and even may cause fire.

... nor de we know heatsink sizes, protecticve earth availability, environment, accessability, casing, dirt/dust ... ... and so on ... this all needs to be considered to find a useful and safe solution.

***
If you can´t find suitable resistors .. then I´d mount these resistors on individual (bigger) aluminum or copper plates .... and with the use of suitbale insulating material ..on the bigger heatsink.

For sure this increases overall capacitance to the heatsink .. but due to the wider heat transfer area it may compensate for the increased thermal resistance caused by the barrier.

Found some information online (No guarantee)
If true then a 125um thick capton foil can widthstand more than 15kV. For sure one would not use it for 15kV, one needs to add margin.

Klaus

 

[ZappHappy]

Thank you,

I guess I should have put the 1500V RMS, but yes we don't really know what it might be subjected to..

But I guess you answered me "in an idealistic case"

I see this as adding 2 gaps in series, this should increase the breakdown voltage, perhaps not linearly, but
some increase should be expected...

My solution will be to put alumina heat sink isolators, this might decrease thermal efficiency so
I was hoping to eliminate them as my actual max voltage is so close to the margins..

 

[KlausST]

Nothing useful...