Matching or not hexfreds?

[PauloPT]

Hi!

I'm building a PSU and I want to use Hexfreds soft recovery diodes for the rectification. My question is: do I have to match the 4 diodes?

Thank you.

Regards,
Paulo.

[SY]

No.

There, I made your life easier.

[PauloPT]

Thanks SY!

[PauloPT]

Another question: the diodes are HFA25PB60 rated 25A continuos forward current. for more of less 5A do I need heatsinks? Can´t figure thAt out from the datasheet...

Thanks!

[Francois G]

Hi Paolo,

It depends on the current and voltage drop across the Mosfets, but you most likely would need heatsinking.

Use Ohms law to calculate the heat dissipation necessary. For example if you drop 10 volts at 5 amps you will need to get rid of 50 watts - that requires a good sized sink.

If you want to actually design a specific size you will need to decide the acceptable temperature rise in the heat sink above ambient temp and look at the specs to calculate the heat flow across junctions and allowable junction temps. The junction where the heat is generated has to be kept below a given temperature, and that depends on how fast the heat is conducted away, which depends on how easily the various material boundaries are crossed and the size and charateristics of the heatsink.

There are some websites that show these calculations - just ask Dr. Google.

Most of the time I set up a test and just see which of the heatsinks I have feel acceptable after an hour or so. As a guide, the heatsink is acceptable (to me) if it feels uncomfortably hot, but not so hot that I can't keep my hand on it. Obviously the cooler the better.

Good luck!

[PauloPT]

Hi Francois,

These aren't mosfets, they are diodes. Isn't the voltage drop across a diode 0,6 ~ 0,7V regardless the current across them?

My guess would be that I need heatsinks but I se many pictures on the net of these kind of rectifiers without heatsink like in here:

DIY power supply NT-E

Would this be fine?

Thanks!

[godfrey]

Hi Paulo

I would vote for heatsinks, but it depends what you're using it for.

For the same output power, a class B (or AB) amp draws a fairly low average current from the supply, whereas a Class A amp draws a much higher current continuously.

Here's my best shot at putting numbers to it:

From the datasheet:
Forward voltage drop could be up to about 1.5V
Max junction temperature is 150 degrees C.
Thermal resistance junction to ambient (i.e. no heatsink) is 40 degrees/watt.

So:
Max temperature rise is about 120 degrees (150 - 30 ambient)
Max power dissipation is about 3 watts (120 / 40)
Max current is about 2 amps average (3W / 1.5V)

That's per diode, so with a bridge I'd expect double that i.e. about 4 amps could be drawn without frying the diodes. Maybe a couple more depending on the actual voltage drop.

Cheers - Godfrey

p.s. Here's the datasheet I looked at: http://www.schuro.de/Daten/IR/hfa25pb60.pdf

[PauloPT]

Originally Posted by godfrey

Hi Paulo

I would vote for heatsinks, but it depends what you're using it for.

Thanks godfrey!

I'll go for heatsinks.

Cheers,
Paulo.