Author Topic: Adding heatsink with electrical isolation  (Read 5626 times)

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Online andy3055

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Re: Adding heatsink with electrical isolation
« Reply #25 on: December 12, 2019, 04:07:58 am »
Rather than complicating the process by precision engineering the “heat exchange” type h/sinks, why not mount the transistor (with proper electrical insulation-I would use mica/plastic washer/thermal compound) in a convenient position that does not interfere with the PCB and have the leads soldered to 3 wires that end up in a plug? This is a common practice in a lot of things. Examples can be seen in most Tek scopes. As long as you have a tight fitting plug, I don’t see any issue.

See attached image where you can see the transistor on the left of the EHT transformer on a 465M scope.

Not possible as the case style is where the PCB slide in type thru front / rear openings.

Why not? Look at his original drawing.
 

Online andy3055

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Re: Adding heatsink with electrical isolation
« Reply #26 on: December 12, 2019, 04:10:42 am »
Thanks, I was going to use that same one you recommend...

You still have to decide how you are going to mount the transistor. Changing the type of enclosure does not resolve your original issue.
 

Offline David Hess

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Re: Adding heatsink with electrical isolation
« Reply #27 on: December 12, 2019, 11:38:08 am »
I strongly suggest to not use mica for mains-case insulation as it sometimes cracks. Thick-ish silicone pad is much safer solution. Also don't use screw+plastic washer. It's a very weak point. Use square silicone pad without any holes + clip. Or even better, use silicone cap over whole transistor instead of pad.

The only time I have seen mica fail is when it was damaged by being folded.  It is pretty delicate but there are other insulating pad materials if something more rugged and easy to use is required.  I have seen alumina crack even in normal service.

A clip to apply pressure at the center of the package is one of the best mounting attachments because it is least likely to bend the die frame but it can be difficult to get enough force.

The screw attachment requires insulation and tends not to have enough compliance but works very well if a Belleville washer is used.  Split washers do not apply enough force.  Because of poor availability of Belleville washers, I tend to use several wave washers stacked together to get enough force.

Motorola published an excellent application note discussing power semiconductor mounting considerations:

https://www.nxp.com/files-static/rf_if/doc/app_note/AN1040.pdf
« Last Edit: December 12, 2019, 11:41:27 am by David Hess »
 
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Online nctnico

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Re: Adding heatsink with electrical isolation
« Reply #28 on: December 12, 2019, 01:32:06 pm »
Another option is to drill a (countersunk) hole in the flat bottom of the case and have a slot in the PCB so the TO220 can be mounted with the legs up onto the solder side. That way you can slide the PCB in and use a screw to fix the TO220 to the case (with whatever isolator you want to use).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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Re: Adding heatsink with electrical isolation
« Reply #29 on: December 12, 2019, 04:47:02 pm »
Another option is to drill a (countersunk) hole in the flat bottom of the case and have a slot in the PCB so the TO220 can be mounted with the legs up onto the solder side. That way you can slide the PCB in and use a screw to fix the TO220 to the case (with whatever isolator you want to use).

That works but long term reliability will be compromised if too much tension and compression force is applied along the length of the leads.  Solder joints are not mechanically strong and will eventually crack if this happens.  Changing force on the leads should always be in bending so lots of mechanical compliance is available and forces are low.  This is a real problem with TO-3 parts mounted rigidly between a printed circuit board and separate chassis.

There are some heavy leaf spring type sockets for TO-220 style packages but they are of questionable long term reliability.  Better is to solder a three wire stranded pigtail directly between the leads and printed circuit board although this brings up lead length issues.  Collet socket pins are available which will fit a TO-220 lead but I have never tried them.
 

Online nctnico

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Re: Adding heatsink with electrical isolation
« Reply #30 on: December 12, 2019, 08:48:07 pm »
Another option is to drill a (countersunk) hole in the flat bottom of the case and have a slot in the PCB so the TO220 can be mounted with the legs up onto the solder side. That way you can slide the PCB in and use a screw to fix the TO220 to the case (with whatever isolator you want to use).

That works but long term reliability will be compromised if too much tension and compression force is applied along the length of the leads.
For clarity: I meant bending the leads and soldering them into through-holes but I see this wasn't clear from my text. I'm aware bends are required to have some form of strain relief from thermal cycles.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 


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