Power Supply Heat sink questions

[Henry89789]

I have built several power supplies using 7809, 7812 and LM317  voltage regulators. I am most concerned about a heat sink for the LM317 which gets pretty hot pretty quickly. I have heat sinks and silicone heat sink thermal dielectric compound but I have also read about using mylar insulators or washers between the IC and the heat sink. So I have several questions:
a.  Is the dielectric compound between the IC and heat sink enough for a power supply?   
b. Are the mylar insulators or other insulating washer necessary?   
c. Or are mylar insulators required for some applications and not others?
d. Are the mylar insulators used together with the compound or alone depending on the application?
e. Can insulating washers be improvised using some materials other than mylar (like plastic) ?
I would appreciate if someone could clarify these issues for me. Thanks.

[greaser_au]

Hi Henry,

Bear in mind that the tab on TO-220 devices is *usually* connected to the centre pin. In the case of the LM317, this is Vout, so if your tab is not insulated from the heatsink, the heatsink will be at Vout also. Larger heatsinks could potentially act as an antenna in electrically/electromagentically environments and inject noise into the regulated rail.

a: it depends what the risk of something shorting to your heatsink, and the PSU case shielding in extreme RF environments. I'd say -  no, it likely isn't enough as this will leave your heatsink at Vout potential.
b: A mica or 'sil-pad' washer and insulating bush is cheap insurance  against (a)   
c: it depends on the application - back to (a) again!
d: It depends on what you use for washers:  a 'sil-pad' washer is soft and is designed to not require it. The thermal resistance of mica washers will significantly benefit from application of a sensibly minimal amount of heatsink compound.
e: some thermal resilience is necessary - plastic will melt or harden/degrade over time at elevated temperatures. I have quite often seen a nylon screw/nut used instead of a bush, and in many cases after a few years of service, the bolts have melted or broken.

Note that there is a temperature gradient from the junction to the heatsink. The device ('junction')  in a bare TO-220 will experience a temperature rise of about 50 degrees C per watt of power dissipated by the device.

For the maths behind it see http://en.wikipedia.org/wiki/Thermal_resistance

david

[R.G.]

David had most of it dead on. But let me amplify a bit.

a.  Is the dielectric compound between the IC and heat sink enough for a power supply?   

No.

b. Are the mylar insulators or other insulating washer necessary? 

Yes. One possible alternate is to insulate the heat sink from the chassis. This is actually better thermally, but has the risk that something touching the heat sink will short the power supply.

c. Or are mylar insulators required for some applications and not others?

Unless (1) the heat sink is completely insulated from chassis and the rest of the circuit or (2) the part of the component tied to the heat sink is at chassis ground voltage, they are always required. David's comment about the tab being the center pin is correct for all the TO-220s I know of, but as a more general rule, for NPN type devices, which all standard regulators are, the tab and center pin is connected to the most-negative voltage on the transistor or IC. This is why the output is connected here for the LM317, ground for all the 78xx regulators, and Vin for the 7900 negative regulators. The reason this is so has to do with how the silicon is diffused and the devices made. The most-negative voltage is attached to the unused back side of the chip/die, and this is what is bonded to the thermal tab.

d. Are the mylar insulators used together with the compound or alone depending on the application?

Never use just an insulator. This has to do with the fact that the IC/transistor, heat sink, and insulator are not perfectly smooth. There are peaks and valleys, and the peaks keep the surfaces from making intimate, complete contact. Air is held in the valleys, and air is a good thermal insulator if it can't move around - just ask a goose. The heat sink goo fills the spaces with something that conducts heat better than air.

e. Can insulating washers be improvised using some materials other than mylar (like plastic) ?

No. As David said, plastics flow/creep under continuous pressure. Paper isn't a good electrical insulator, IS a good thermal insulator, and is too thick for what thermal properties it does have.

Sorry - the real answer is to do it right. Either isolate the heat sink from chassis/ground and bolt the device to it, or order the insulators. If you order something like Sil-Pad insulators, they are a rubber loaded with metal oxide particles that are nearly as thermally conductive as an insulator with goo. I like them a lot for thermal applications that are not super critical. I wound up picking up about 100 each of TO-220 and TO-3 Sil-Pads over the years as I found them on surplus offers cheap, because I know I'll eventually use them. But you may not project ever needing these again.

[Henry89789]

Thanks for the replies and the detailed info. I definitely want to do this the right way (that is why I come here) especially when it involves potentially dangerous voltages. I went online and found that NTE 7809, etc. ICs come with what looks like a sil-pad insulator and a small plastic washer.

I just have a couple of follow up questions: As I understand it, the sil pad insulator goes between the heat sink and the metal tab on the IC; Should I put the dielectric compound on both sides of the sil-pad insulator?  And, where does that small washer go? Thanks.

[greaser_au]

Henry,

yes, put the insulating sheet between the device and the heatsink. Put a small amount of heatsink compund on both sides of the insulating pad. in my factory days, to pass inspection it had to be *just* visible all around the device.

The small plastic washer is actually a bush and it has a small raised boss on one side.  Put the boss in the TO-220 hole (on the opposite side of the device form the heatsink - this is intended to insulate the screw from the tab.
see the assembly diagram here: http://www.infineon.com/dgdl/Processing_TO.pdf?folderId=db3a304314dca38901154a72e3951a65&fileId=db3a30431936bc4b011938532f885a38. Figure 10 (left) on page 18 shows it in all the gory detail!

One thing: take a little bit of care using silicone heatsink compound, it can irritate eyes. ...and pretty much forget about ever gluing or painting something it has touched (for completeness:  you CAN get a cleaning agent - auto crash repairers use a specific cleaner to remove silicone polishes).

david

[R.G.]

I went online and found that NTE 7809, etc. ICs come with what looks like a sil-pad insulator and a small plastic washer.

I just have a couple of follow up questions: As I understand it, the sil pad insulator goes between the heat sink and the metal tab on the IC; Should I put the dielectric compound on both sides of the sil-pad insulator? 

yes, put the insulating sheet between the device and the heatsink. Put a small amount of heatsink compund on both sides of the insulating pad.

The big question here is whether the insulator is a Sil-Pad (rubbery, opaque, and notably thicker than a piece of note paper), or mica (clear and rigid) or mylar/kapton sheet (flexible, but "hard" feeling and as thin or thinner than note paper). It's hard for me to imagine NTE putting a Sil-Pad with their parts, but it could happen. Sil-Pad and its ilk were invented to make using heat sink goo unnecessary. The rubbery nature is to let it conform to the microscopically bumpy nature of the surfaces under pressure, as the goo was intended to do. Hence the rubbery nature and thickness. The metal oxides in it were to enhance the thermal conductance of the material.

If it's rubbery, no goo. If it's mica or kapton film, use goo. The washer is to keep the screw from shorting to the tab, as GA said.

[Henry89789]

Thanks for the replies. You all have explained what I need to know.

R.G.: Based on your description, I would say that the insulator that came with the NTE is not sil-pad, but rather mica because it is clear and rigid. I disassembled a broken inverter and found (a really nice big heat sink for the LM317 power supply) but also strips of a rubbery material between the metal tabs and the heat sink which sounds like sil-pad.   

Greaser:  Thanks for the link to the article. It is excellent and basically has all one needs to know about this topic.