Which gets hotter???

[everell]

Seems that opinion polls are popular on this forum, so thought I would start one. Not sure how to set up the voting for this one, so will just ask the questions, get some opinions, and reasons from the experts on this forum.

Question number one: which gets hotter when computer power supply is running, the main switching transistor heat sink or the output diodes heat sink?

Question number two: which gets hotter, the main switching transformer or the output toroid transformer/coil?

No, these are NOT trick questions. Heat is the big enemy of aluminum capacitors. So it would be good to know what the bigger heat sources are so that capacitor placement can be better.

[c_hegge]

Re: Which gets hotter???

1. I would say the secondary heatsink as it is nearly always bigger the primary one.

2. No idea. I NEVER open the case on a PSU when it is plugged in. I've been electrocuted one too many times.

[everell]

Re: Which gets hotter???

As a safety issue, I don't recommend checking temperature of heat sinks and transformers with power ON. So anyone checking temperature of parts inside psu, turn OFF power and wait a minute or two for the electrons to drain off. Parts that get HOT will retain their heat for a few minutes.

[Toasty]

Re: Which gets hotter???

Dammit!

Where did I put that thermal imaging camera...

[370forlife]

Re: Which gets hotter???

I would say in the old Half-bridge designs the primary heatsink gets the hottest.

In a single or two transistor forward design, it depends. Also depends on how the secondary heatsink is set up.

If the secondary uses two schottky's for the 12v and/or 5v then they will stay cooler, in which case the primary would definitely be hottest, especially if it uses Half-bridge instead of the more efficient single or two transistor forward.

I would also imagine the primary transformer gets hotter than the secondary chokes.

[ratdude747]

Re: Which gets hotter???

depends on which arcs first (powmax and the like only)!

[Per Hansson]

Re: Which gets hotter???

Well actually I think the coil on the secondary side ranks high
In the PSU's I have seen where the fan have failed this coil has usually turned black!

However I have also seen the primary side actually desolder one of the transistors legs (resulting in big bang and nice fireworks) so I guess that is the hottest side afterall!

[Th3_uN1Qu3]

Re: Which gets hotter???

Well actually I think the coil on the secondary side ranks high
In the PSU's I have seen where the fan have failed this coil has usually turned black!

That's normal, those cores like high temperatures. If the wire it was wound with didn't, well that's another problem.

However I have also seen the primary side actually desolder one of the transistors legs (resulting in big bang and nice fireworks) so I guess that is the hottest side afterall!

That wouldn't happen if they stopped using those crap thermal pads. I'm trying to build a 2-transistor forward SMPS to power an audio amplifier. I use the heatsink and isolation pads from a crappy ATX supply. Without a fan, the heatsink sits at 80C - but the switching transistors are at 120C!!! With a fan, the heatsink sits at 35C and the transistors at 60C. That's still a 25C difference which means the thermal interface material is absolute junk. I'll have to find some mica washers.