Very hot rectifier

[The Master]

Hi, I've made a circuit to control some powerful RGB LEDs. The total load is less than 5A and it's powered by an 80VA 12V transformer through a Kbpc2502 rectifier that should be able to handle 200V at 25A.

I would have thought the rectifier could handle the required current with very little effort but, after burning my finger, I tested it and it's running at 103 degrees C. I know this is within the operating temperature range but it seems a bit excessive considering I'm running only a 5th of the rated current through it. I just want to check, is this normal?

[Charlie]

What sort of heat sink are you using, and how is it mounted? it will be dissipating 5-10 Watts at your 5A load....

[The Master]

There isn't currently a heatsink. I thought the metal casing would be more than enough but I will be looking for a suitable one now. I just find it hard to believe that it can get so hot when nowhere near it's limit. Imagine what would happen if I was drawing 25A!

[AvionicsMaster1]

According to what I read in the datasheet, at 103C the max output is 10A. That means to me you're doing something wrong. If you've got this thing mounted to the case made of metal you've probably have a good enough heat sink since it looks like it's operating at near max current. It doesn't specifiy the size or type of heat sink though I think it's necessary.

Me thinks, and I'm very often wrong, you might oughta look at mounting, wiring or assembly to find your excess draw. I'd also like to see your schematic if that's possible.

[The Master]

I've connected my multimeter in and the current draw is hovering around 3.7A with the LEDs on full power.

Just to clarify, I meant the metal casing around the rectifier itself. I haven't attached anything else to it that could act as a heatsink.

I haven't got a schematic available but it's a simple circuit. The rectifier feeds into 2 capacitors in parallel (2200uF) then through some transistors into the LEDs. There is a WS2801 chip with it's own 100nF ceramic cap for each LED but these chips draw very little current. The 3.7A I measured was taken from the output of the rectifier and represents the total load of the entire circuit.

[Charlie]

This is something so fundamental, the datasheet doesn't even bother to mention it, assuming no one would connect a power device without a heat sink.
If you look at the curve, it tells you you can do 25 A if you keep the core under 55 degrees C. It's up to you to do that with a properly calculated heat sink.
Here's a good paper to start: http://www.ti.com/lit/an/slva462/slva462.pdf