Does anyone have any experience with Semikron Skiip power modules?
Specifically - with the variability (or not) of contact resistance with age?

I'd be interested in anyone else's experiences with highish current sense
resistors in power modules that use spring loaded contacts.
Also anyone's experiences of Semikron Skiip modules.

The Skiip modules mount a plate of thick film components on a  fan cooled
substrate and connect this to the PCB with back to back spring loaded
contacts. For each contact point one end of a contact arm connects to the
power module and the other end connects to the main PCB.  The whole assembly
is filled with heat conductive goo with a nature not unlike overly tenacious
silly putty.. A single bolt clamps the whole assembly together.

For high power circuits (up to at least 20 amps) this is apparently a
reliable means of connection.
HOWEVER the module I am interested in has a 0.01 ohm (10 milli-ohm) sense
resistor on the substrate. This is in the source lead of an IGBT. The "hot"
end (IGBT source has its own contact to take the circuit out of the module.
BUT the bottom of the sense resistor does NOT have its own Kelvin contact -
there are effectively 3 ground connectors used to return source current to
the PCB ground plane and this is apparently used as the analog ground for
the return circuit from the sense resistor to the current monitoring IC.
Semikron have a discussion paper that suggests the contacts have around 13
milli-ohm resistance each so 3 in parallel give about 4 milliohm. My
measurements infer a contact resistance of about 4 or 5 milliohm per contact
in a relatively new assembly. In any sane circuit that would be a negligible
amount of resistance, but in this case it is 40% of the sense resistor. As
long as this remained constant all could be OK BUT if the contact resistance
increases with age then "there will be trouble". It seems to.

The machines that these modules are used in have a decreasing load trip
point with age. They also have other bad problems but these are  outside the
scope of this message. The Semikron modules are proprietary, having been
made specially for the original manufacturer. I'm pretty sure (not my direct
concern) that the response to faults is the offer to buy a new controller.
There's quite a lot of money tied up in this but new controllers would be
uneconomic. (fwiw - I'm doing this job at the request of my major client.)

The mechanical construction makes it very hard to access the internals of
the module when operating and the metal used (looks like a copper alloy but
doesn't behave like it) refuses to be soldered. I have managed to solder to
it with evil spitting grotty marvellous aluminium solder.
I have found a way to hard ground the bottom of the sense resistor but it's
less than pleasant and carries some risk due to the need to open, de-goo,
modify using a significant amount of heat and reassemble the thick film
module.



        Russell McMahon

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