Author Topic: What the best way to mount HIGH voltage semiconductors to grounded coolers?  (Read 6375 times)

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Offline Le_Bassiste

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #25 on: November 18, 2018, 09:42:12 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.
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:



An assertion ending with a question mark is a brain fart.
 

Offline Zero999

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #26 on: November 18, 2018, 10:06:50 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.
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.


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

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

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Also would recommend a softer dielectirc between the torroid and the tensioning washer.
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.

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also are those earth ground lugs star washered?
I doubt it.

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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.
I don't know what you mean. I'm not familiar with that.
 

Online coppercone2

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #27 on: November 18, 2018, 10:28:37 pm »
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.
 

Offline Wolfram

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #28 on: November 19, 2018, 11:45:15 am »
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.
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:

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.
 
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Online coppercone2

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #29 on: November 19, 2018, 01:38:21 pm »
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
 

Offline Wolfram

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #30 on: November 19, 2018, 05:44:00 pm »
They didn't specify it. This is a company that also does not provide any mechanical data on their mechanical products, and no thermal data on their thermal products so that's to be expected. Needless to say, we don't deal with them any more. The reduced capacitance from alumina with similar thermal performance reduced switching losses and EMI challenges, so there's no great motivation to make a kapton solution work in this application.
 

Offline 001Topic starter

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #31 on: November 19, 2018, 06:23:06 pm »
Hey Hey

Ho to do it without rocket since?
 

Offline stj

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #32 on: November 20, 2018, 03:14:35 am »
did i miss it, or was the working voltage not mentioned??
i'm thinking berilium-oxide spacers - old-school tech.
 

Offline Le_Bassiste

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #33 on: November 20, 2018, 11:57:12 am »
Hey Hey

Ho to do it without rocket since?

 :-DD if i get it correctly from your opening post, the important keywords are "600V" and "grounded heatsink".
hence i  assume that your application is mains-connected and requires double or reinforced isolation, so you have to consult the applicable safety standards for your application wrt voltage categories, etc.
have a look at silicone tubing for power transistors,
https://thermal.henkel-adhesives.com/en/eimea/thermal-materials-products/product-detail/sil-pad-400.html
mounting clips (already mentioned by others in this thread) and heatsinks that are made to accomodate those clips.
https://www.digikey.de/en/product-highlight/a/aavid-thermalloy/max-clip-heat-sinks
again, keep in mind that you will have to use double or reinforced isolation for your application.







« Last Edit: November 20, 2018, 12:00:31 pm by Le_Bassiste »
An assertion ending with a question mark is a brain fart.
 

Offline N2IXK

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #34 on: November 20, 2018, 04:19:46 pm »
Beryllium Oxide "thermal links" were used on conduction cooled vacuum tubes, where the anode may be operating at thousands of volts above ground.

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Offline Zero999

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Re: What the best way to mount HIGH voltage semiconductors to grounded coolers?
« Reply #35 on: November 21, 2018, 10:55:59 am »
Beryllium Oxide "thermal links" were used on conduction cooled vacuum tubes, where the anode may be operating at thousands of volts above ground.


And back then, electrical safety standards weren't so strict.

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Hey Hey

Ho to do it without rocket since?

 :-DD if i get it correctly from your opening post, the important keywords are "600V" and "grounded heatsink".
hence i  assume that your application is mains-connected and requires double or reinforced isolation, so you have to consult the applicable safety standards for your application wrt voltage categories, etc.
have a look at silicone tubing for power transistors,
again, keep in mind that you will have to use double or reinforced isolation for your application.
If the heatsink is grounded then reinforced/double insulation is unnecessary. Only basic insulation is required and whilst it does have to fulfil certain regulatory requirements, above functional insulation, they're nowhere near as strict as double/reinforced insulation which would only be required if the heatsink isn't earthed and the user may come into electrical contact with it.

Reinforced insulation typically needs to be double that of basic insulation. Here are some links about creepage and clearances which is relevant to heatsinking and PCBs.

https://www.infineon.com/dgdl/Infineon-ApplicationNote_MOSFET_CoolMOS_Electrical_safety_and_Isolation-AN-v01_00-EN.pdf?fileId=db3a30433d1d0bbe013d20e0cbf017fe
http://www.reo.co.uk/files/safety_7_-_low_voltage_directive.pdf
http://blog.optimumdesign.com/clearance-and-creepage-rules-for-pcb-assembly
https://www.frontdoor.biz/HowToPCB/HowToPCB-extra/CreepageandClearance.pdf
https://www.silabs.com/documents/public/application-notes/AN583.pdf

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https://thermal.henkel-adhesives.com/en/eimea/thermal-materials-products/product-detail/sil-pad-400.html
mounting clips (already mentioned by others in this thread) and heatsinks that are made to accomodate those clips.
https://www.digikey.de/en/product-highlight/a/aavid-thermalloy/max-clip-heat-sinks
At a glance it doesn't appear that those solutions will be applicable for basic insulation at 600V, but it depends on the materials used. Read the data sheets in great detail.
« Last Edit: November 21, 2018, 12:25:37 pm by Hero999 »
 


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