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What the best way to mount HIGH voltage semiconductors to grounded coolers?
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Le_Bassiste:

--- Quote from: Wolfram on November 17, 2018, 08:35:40 pm ---Those kapton foils are nothing but trouble in high voltage applications, especially in cases with high dv/dt. I optimistically used some to isolate some SiC MOSFETs and diodes in a 750 V buck converter, and they failed after a short time in operation. Further testing gave consistent failures of this material. The manufacturer of this material (Aavid in this case) blamed the surface finish on our heatsinks, but they don't actually publish any information on the recommended surface finish for kapton foil insulators. Swapping them with alumina pads solved the problem completely.

--- End quote ---
interesting.  :-+  can you elaborate on this? i.e., what was the isolation thickness, what switching frequency were you using, what slew rate on VDS did you observe, what was the heatsink temperature, and how long was the operational time of your converter before the isolation broke down?
apologies for being nosy here, but i find your information extremely valuable and i'd like to understand the actual circumstances of the isolation break down. :popcorn:



Zero999:

--- Quote from: coppercone2 on November 18, 2018, 08:52:29 pm ---I don't think solder is permitted for earth ground connections officially, because of mating cycles and flex of the ground pin if the cord is fucked up. The solder lug on the chassis is a bit more acceptable because that point is not being flexed, but you obviously still need excellent strain relief so the motion does not transfer through the ground wire from the plug (which is less if you use multi strand wire).

and the earth solder joint in the lower picture looks down right suspicious. I think you should at least make a half hook through the hole to solder it with good clearance to the pin if you insist on soldering it, it looks like its bearly  attached. Did you clean and flux the joint (perhaps with a brass brush?). The wetting on your solder joints looks horrid but I am not sure if its a camera thing.
--- End quote ---
I agree: the earth should be crimped, but there's no flex. It's connected to the mains via an IEC connector.

I can't remember whether I wetted the joints, as this was well over ten years ago.



--- Quote ---Also, where are the fuses at?
--- End quote ---
They're in the black holders which aren't very clear on the pictures.


--- Quote ---cant hurt to put the levitating rectifier on a bracket either.
--- End quote ---
I agree, it might come off, if dropped.


--- Quote ---Also would recommend a softer dielectirc between the torroid and the tensioning washer.
--- End quote ---
If I remember rightly there is a rubber or foam washer between both the large tensioning washer and the other side of the toroid but they're not visible on the photograph.


--- Quote ---also are those earth ground lugs star washered?
--- End quote ---
I doubt it.


--- Quote ---This one is unnecessary but there is a kind of cheap trick to wire-pcb junctions, where you drill a hole away from any conductors and zip-tie the wire to the PCB through the holes or between the hole and the edge of the PCB. It works very well I had a hobby project that got yanked very hard and it held up fine without damage to the PCB.

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I don't know what you mean. I'm not familiar with that.
coppercone2:
you drill a hole next to the edge of the PCB so you can put a zip tie into the hole and around the PCB to clamp down on the wire and you can drill another hole to loop the wire through also.
Wolfram:

--- Quote from: Le_Bassiste on November 18, 2018, 09:42:12 pm ---
--- Quote from: Wolfram on November 17, 2018, 08:35:40 pm ---Those kapton foils are nothing but trouble in high voltage applications, especially in cases with high dv/dt. I optimistically used some to isolate some SiC MOSFETs and diodes in a 750 V buck converter, and they failed after a short time in operation. Further testing gave consistent failures of this material. The manufacturer of this material (Aavid in this case) blamed the surface finish on our heatsinks, but they don't actually publish any information on the recommended surface finish for kapton foil insulators. Swapping them with alumina pads solved the problem completely.

--- End quote ---
interesting.  :-+  can you elaborate on this? i.e., what was the isolation thickness, what switching frequency were you using, what slew rate on VDS did you observe, what was the heatsink temperature, and how long was the operational time of your converter before the isolation broke down?
apologies for being nosy here, but i find your information extremely valuable and i'd like to understand the actual circumstances of the isolation break down. :popcorn:

--- End quote ---

This particular test was done with 25 µm material (4200 V AC breakdown rating), the switching frequency was 75 kHz, the slew rate was around 30 V/ns, temperature was around 60 degrees on the heatsink, and it failed after around two hours. There were actually two failures, the first one was between the heatsink and the diode. This one went unnoticed. Then the second failure, between a transistor and the heatsink, tripped the converter protection. The failure left some nice craters in the backs of the transistor and diode, but the converter was still working afterwards.
coppercone2:
the surface  finish they should give should be specified in Ra (roughness) and flatness inch/mm (if its not flat enough you get uneven pressure distribution).

They seriously could not provide it when asked? So long you gently stone the surface before hand and give it a wipe down there should be no penetration.

I wanna know lol
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