Switchting Power Supply: potting compound/ silicone elastomer

[mfenner]

I am repairing a PSU from a Samsung computer monitor and I need the support of the swarm intelligence.

I noticed that there are some MOSFETs (here: in the PFC stage) whose legs are completely wrapped in potting compound. I do understand that this compound is used to hold mechanical components in place, such as axial resistors. Also, this can be used to prevent audible chirping of inductors.

However, I would like to know why the legs of the MOSFETs are completely wrapped inside the elastomer. Very strange is that a diode mounted to the same heat sink, connected to the same voltage is not covered at all.
- Does this compound has an effect when the MOSFET fails? (e.g. to prevent flame/plasma from spraying all over the board)
- Does the compound prevent dust accumulating between the legs? (which would result in a catastrophic damage...)

And, more important: Do I need to restore this compound after repair?

Thank you for the support,
Michael.

[T3sl4co1l]

The resistors might actually be for power dissipation reasons.  Power resistors tend to get pretty toasty when run at ratings, in air, and the radiant heat crisps the board -- not outside of ratings, mind, or at least I don't think darkened phenolic or soldermask has much impact in the grand scheme of things, but it's not a great sign, and is an acceleration/degradation process.

That assumes the compound is thermally conductive, which, it should be, but depends what it's filled with and how much.

Did they get any goop into the PCB slot there?  Might be just for better creepage around the device package itself.  I've never seen it required for TO-220s at 400V (what's the capacitor, 400, 450V?), or in general really, but it does look like it squidged around the leads well enough, if nothing else.

Could also be more of an environmental thing: keep dust away, potential conductive residues.

Same would apply to the (flyback?) device on the other heatsink, moreso even as it'll have a peak voltage probably 600-800V.

Hmm, I wonder if this partly explains the use of caps (like https://www.digikey.com/en/products/detail/t-global-technology/CP23-TO220-21-5-11-4-5-8-0-45/3878145 ); yes for increasing creepage to the heatsink (even for full-pak types), but even just the dust coverage might be useful in some cases -- if that's one of the reason they're doing it.

Tim

[Poroit]

I've seen this before and it was only for mechanical reasons to keep components in place particularly in portable equipment.

Some times the person applying the sealant gets carried away with how much they use.

[indeterminate]

Insects
its a cheep way to protect the fets's gate

[mfenner]

Thank you for the answers - to all of you.

Maybe it is all of these: Gate protection and some poor-mans-heatsink.

There is one argument that strongly supports the heat sink theory: In the top right corner, next to black 13V output connector, you see another place of this white snot - above the two blue output capacitors and to the left of the connector.
Below - you can not see them - there are two parallel (!) high-power resistors (I think these are shunt resistors). They are perfectly placed: axial resistors laying down on the PCB (and not vertically mounted, as the others). They do not need any mechanical support, since they can not vibrate nor get bent. However,  they have a "hat" of this compound, only connected to the resistors, not connected anywhere else. Why would they do that?
The only reason can be that this is a form of heat sink for the resistors... (otherwise this would be an insulation for these resistors, which would only be good to shorten the product lifetime)

Below the gates where we find the compound, the PCB is brownish. I thought this is because of the "heat-insulating" compound, but it can be vice-versa. Since these transistors get very hot and degrade the PCB, they have installed this material to provide a heat conductive path to the heatsink. Only for protecting the gates they could have used much less compound, but it is so much that it flowed also under the heatsink. If we assume that the material is heat-conducting, this would make perfect sense: It would be a form of a of cooler for the pins.
Originally I thought the PCB is brown because of the compound... And yes, for sure this is also a good protection for the gates - this now also makes perfect sense to me.

The capacitor is a 450V type - which could also be just good design, right? And yes, the compound has also smeared into the slot of the PCB. Not too deep, but enough to completely fill the air gap and restore the creepage path that they wanted to cut. I do not know if this was intended...

[Phil1977]

It´s quite common for parts that are in forced air cooling or convection cooling connected to the outside world. It just prevents dust and goo from collecting where a strong electric field together with heat attracts it. And insects are for sure a good reason too.