Author Topic: Adding heatsink with electrical isolation  (Read 5630 times)

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Offline EEEnthusiastTopic starter

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Adding heatsink with electrical isolation
« on: December 11, 2019, 12:45:38 pm »
Folks,
What is the best way to mount a power transistor (TO-220 non isolated tab) to the metal case of the power supply in order to act as a good heat sink. It should be electrically isolate from the case as well.

It should be assembly friendly as the TO-220 package is PCB mount. The transistor cannot be connected to the board using wires. I was thinking of using two heatsinks, one on the transistor and the other on the case. After the case is closed, the two heat sinks would sit close to each other acting like a small heat exchanger. Like below. Any experience with this, or a better solution?
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Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #1 on: December 11, 2019, 12:53:24 pm »
Air gap is one of the best heat ... insulator.

Whats wrong with thermal pad ?

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Re: Adding heatsink with electrical isolation
« Reply #2 on: December 11, 2019, 12:59:52 pm »
The thermal pad on the PCB would mean more heat dissipation on the PCB, causing temperature fluctuations. I need to keep the PCB at the most stable temperature and hence need to take the heat off the board.
Air gap could be a good insulator, but with enough fins interacting with each other, should the thermal gap place a big restriction in the heat exchange?
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Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #3 on: December 11, 2019, 01:33:53 pm »
Thermal pad to the metal body, then secure the TO220 on it.

Offline brabus

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Re: Adding heatsink with electrical isolation
« Reply #4 on: December 11, 2019, 01:51:55 pm »
Linn Amplifiers use the case as heatsink, by screwing the transistors to an anodized block of alluminium. The thick anodization acts as electrical insulator and good heat conductor at the same time.

In your case I would definitely use a traditional sil-pad, it's an industry standard. I don't see how you could induce temperature fluctuations on the PCB with a sil pad: if it's correctly placed you will notice no difference in comparison with a direct alluminium mount.
 

Offline T3sl4co1l

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Re: Adding heatsink with electrical isolation
« Reply #5 on: December 11, 2019, 01:53:10 pm »
How exactly does a thermal pad increase the power (watts) dissipated by the circuit?

And how would an air-insulated heatsink not do the same thing..?

Typical solution is thermal pad with bolts and shoulder washers, or even better: spring clips, which can be screw mounted, press-in, or lever actuated.

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Re: Adding heatsink with electrical isolation
« Reply #6 on: December 11, 2019, 01:58:25 pm »
Well, the thermal pad on the PCB would increase the effective area of the radiator allowing the component to cool faster.  My assumption was that the gap between the two heat sinks could conduct the heat well as the effective area of heat exchange is larger.
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Offline GeorgeOfTheJungle

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Re: Adding heatsink with electrical isolation
« Reply #8 on: December 11, 2019, 02:02:59 pm »
Mica (0.1mm thick):
http://duckduckgo.com/?q=mica+to220+site:ebay.com

Alumina has the best thermal conductivity (20-25W/m.k), but it's thicker (0.635mm):
http://duckduckgo.com/?q=Alumina+Ceramic+t0220+site:ebay.com

« Last Edit: December 11, 2019, 02:50:50 pm by GeorgeOfTheJungle »
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Offline wraper

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Re: Adding heatsink with electrical isolation
« Reply #9 on: December 11, 2019, 02:06:19 pm »
http://duckduckgo.com/?q=mica+to220+site:ebay.com
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.



« Last Edit: December 11, 2019, 02:14:50 pm by wraper »
 
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Online Gyro

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Re: Adding heatsink with electrical isolation
« Reply #10 on: December 11, 2019, 02:06:53 pm »
Aside from agreeing with the others that the correct way to do it is to use a thermal pad between the TO220 package and the case...

Assuming that you did manage to achieve sufficient thermal coupling between the elements of your large area air-gap heat exchanger, how would you manage to ensure that the mechanical clearances were maintained well enough to achieve proper electrical isolation?
Best Regards, Chris
 

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Re: Adding heatsink with electrical isolation
« Reply #11 on: December 11, 2019, 02:09:41 pm »
The electrical isolation is guaranteed by the precision in the assembly process only. The TO-220 package height above the PCB is fixed and so is the heat sink height. The other heat sink on the case is also fixed at a constant location by marking it.
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Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #12 on: December 11, 2019, 02:18:49 pm »
Just make a suitable metal insert between the TO220 and the case,  use good thermal pad to insulate it electrically to the metal case, but thermally coupled thru the pad.

This has been done for many many decades, why you sound like you are trying to reinvent the wheel, but yet using non circle shaped wheel ?  :-//

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Re: Adding heatsink with electrical isolation
« Reply #13 on: December 11, 2019, 02:30:00 pm »
My issue is with the case style. The case I am using is a aluminium extrusion with open ends on two sides. The PCB slides into the case from the sides. So there is no easy way for me to access the package once the PCB is inside.
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Offline MagicSmoker

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Re: Adding heatsink with electrical isolation
« Reply #14 on: December 11, 2019, 07:03:04 pm »
My issue is with the case style. The case I am using is a aluminium extrusion with open ends on two sides. The PCB slides into the case from the sides. So there is no easy way for me to access the package once the PCB is inside.

Well, you've boxed yourself in pretty good, then <rimshot>!

You might be able to transfer enough heat from a board-mounted, but physically separate, heatsink to the aluminum enclosure by using a stirring fan - basically, a fan inside the box that blows the air around inside. This can be surprisingly effective, but you do need to have a relatively large enclosure surface area and it needs to made of aluminum, not steel and definitely not plastic.

 
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Offline T3sl4co1l

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Re: Adding heatsink with electrical isolation
« Reply #15 on: December 11, 2019, 10:33:29 pm »
Thermal pads can be used in those enclosures, with moderate success, but yes, you've found one good reason why they are to be avoided where much power dissipation is required.

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Online jbb

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Re: Adding heatsink with electrical isolation
« Reply #16 on: December 11, 2019, 11:50:04 pm »
SIL pads and similar are great. Their conductivity seems to be a bit lower than mica + grease, but mica + grease is delicate and messy.  I’m guessing you’re not after maximum performance given you’re using an enclosure heatsink.

Could you put the TO220 right at the end of the board so that you can slip a clip over it once the PCB has slid all the way into the enclosure?

How much voltage does the insulator need to stand off? If it short circuits, would you expose a user to dangerous voltages?
 

Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #17 on: December 12, 2019, 01:04:15 am »
The electrical isolation is guaranteed by the precision in the assembly process only. The TO-220 package height above the PCB is fixed and so is the heat sink height. The other heat sink on the case is also fixed at a constant location by marking it.

My issue is with the case style. The case I am using is a aluminium extrusion with open ends on two sides. The PCB slides into the case from the sides. So there is no easy way for me to access the package once the PCB is inside.

As you said you can do precision assembly process, although not used at extrusion style case, below are examples used a lot at common switching psus, this is from Meanwell.

I'm guessing this technique can be implemented in your case, all you need is just to precisely drill a hole at the case for the screw, and provide a metal insert sandwiched between the case and the TO-220 body as below.

The gray box is a TO-220 encased inside a box style thermal pad (cap) as above Wraper's example post, as this is a HV transistor, its secured using a strong "L" shaped steel with a hole (threaded) for the screw, when fastened will make the L steel pressing the TO-220 body, and its secured/fastened from "outside" of the case.




Same as above, this is the schottky diode using the same securing style. You see clearly a big piece of metal insert behind.

« Last Edit: December 12, 2019, 01:29:34 am by BravoV »
 

Online andy3055

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Re: Adding heatsink with electrical isolation
« Reply #18 on: December 12, 2019, 02:09:28 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.
 
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Offline EEEnthusiastTopic starter

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Re: Adding heatsink with electrical isolation
« Reply #19 on: December 12, 2019, 02:09:43 am »
SIL pads and similar are great. Their conductivity seems to be a bit lower than mica + grease, but mica + grease is delicate and messy.  I’m guessing you’re not after maximum performance given you’re using an enclosure heatsink.

Could you put the TO220 right at the end of the board so that you can slip a clip over it once the PCB has slid all the way into the enclosure?

How much voltage does the insulator need to stand off? If it short circuits, would you expose a user to dangerous voltages?
The voltage is well below 24V. I think the idea to move the TO-220 to the edge of the board would work. I would still be able to access the TO-220 after the PCB is slid into the slot.
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Offline EEEnthusiastTopic starter

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Re: Adding heatsink with electrical isolation
« Reply #20 on: December 12, 2019, 02:12:14 am »
Thank you all for the great inputs.. I would try out these and let you know which one worked the best in this "case".
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Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #21 on: December 12, 2019, 02:27:02 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.

Offline EEEnthusiastTopic starter

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Re: Adding heatsink with electrical isolation
« Reply #22 on: December 12, 2019, 02:30:42 am »
I would rather use a case which has a top side open access. Let me not compromise the thermal integrity over aesthetics. This would enable me to mount the TO-220 on the TOP lid and connect that to the PCB using wires.
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Offline BravoV

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Re: Adding heatsink with electrical isolation
« Reply #23 on: December 12, 2019, 02:37:05 am »
Instead of this kind of style ...




Why not just use this below 2 pieces splitted extrusion style enclosure, your problem will be solved nicely.

 
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Re: Adding heatsink with electrical isolation
« Reply #24 on: December 12, 2019, 03:13:36 am »
Thanks, I was going to use that same one you recommend...
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