Electronic – TO-220 rectangular washers

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The recommended way to mount a TO-220 package is to use a spring clip or bar pressing on the body of device. But the package was designed to be mounted using a bolt through the hole in the heatsink, and the manufacturers provide mounting instructions for doing so.

The mounting instructions frequently (Phillips, Alpha&Omega, Vishay, IR, Infineon, TI, ON semi etc) contain the suggestion that a rectangular washer be used, particularly when trying to mount fullpack (insulated) packages. For example, infineon, Mounting considerations for TO220 Full PAK.

… but the recomendation is as old as the package, and the same diagram seems to have been copied down from some ancient original source.

The main purpose of the washer is to protect the package: a secondary purpose is load spreading. This is not the insulated washer used for isolation beneath a standard TO-220 package.

Does anyone still use rectangular washers on TO220 packages? Does anyone supply TO-220 rectangular washers as a standard part? Have you ever seen a rectangular washer on the tab of a TO-220? If so, when?

Best Answer

The use of the rectangular washer helps to spread the pressure from the bolt head evenly across the tab. The tabs are typically copper and can deform as the bolt is tightened.

The rectangular washer would typically be steel and less susceptible to deformation as the bolt is tightened.

Without the rectangular washer any deformation can cause the tab to get decreased surface contact with the heat sink surface which increases the case to heatsink thermal impedance.

Related Solutions

Electronic – TO-220 case-to-sink thermal resistance

Total thermal resistance from junction to air is

Rja = Rjc + Rcs + Rsa.

Where: a = air, j = junction, c = case, s = (heat)sink Rjc & Rsa are reasonably fixed by component choices. Rcs is somewhat more variable - see below.

If your data sheet does not give junction to case thermal resistance (usually Rjc or similar)* then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.

Your comment about varying with heatsink suggests you don't really understand the subject. Finding one of the many many good tutorials on the net would be advised.

Thermal resistances are like electrical resistance - they can be added in series.Heat flows from junction to case outer, from case to sink (cia washer and thermal paste, and from heatsink to air. So you get

      Rthermal = Rjc + Rcs + Rsa

      Trise jc = Rjc x Watts etc

      Trise = Tjc + Tcs + Tsa

see prior paragraph for meanings.

Rjc is specified by the device manufacturer.
Rcs is set by thermal washer material, heatsink compound, pressure of mounting etc. It should be a minor contributor.
Rsa is set by heatink design and size, air flow etc.

Temperature rise = Rja x Watts.

Search eg www.digikey.com for TO220 and look at a few Tjc ratings.

This datasheeet

http://ww1.microchip.com/downloads/en/DeviceDoc/22057a.pdf

gives an unusually large number of Rjc and Rja ratings for various package versions of the same part and is a good starting point. For the TO220 package Rjc is 2 C/W.

Rja here is from about 30 to 60 C/W but the heatsinks are "wimpy". Large heatsinks with fans can be around 1 C/W - much less with much care.

A well heatsunk TO220 can manage Tja = 10C/W without too much effort. But it can be far worse without due effort.

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Edited:

The initial paragraph originally read as follows. It is evident that I wrote "case to sink" but intended "junction to case".

If your data sheet dies not give case to sink thermal resistance (usually Rjc or similar) then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.

Electronic – TO-220 voltage regulator, heatsink and mounts

Here is a site that shows the uninitiated how to mount components on heat-sinks (amongst other things). Towards the end they touch-on mylar insulating washers (like the one you have). For a simple regulator you don't need this insulating plate providing you make sure the heatsink doesn't touch parts of the circuit that are not at 0v (centre pin on most regulators)

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Related Solutions

Electronic – TO-220 case-to-sink thermal resistance

Electronic – TO-220 voltage regulator, heatsink and mounts

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