Heatsinking high power diodes

On a sunny day (Fri, 27 Jul 2018 15:04:48 -0400) it happened Bob Engelhardt wrote in :

now add the heatsink Rth.

I'd say NO.

-:)

My math correct? Its hot here...

fredag den 27. juli 2018 kl. 21.06.00 UTC+2 skrev Bob Engelhardt:

I doubt it. Use four, each in parallel each with a separate wire to help balance things?

No, those are, maybe not wirebonds, but maybe a thin tab or something. Wont be good for much more heat than the plastic itself, which is to say, not much.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

Have you considered something like a LT4320 and 4 MOSFETs. The solution might be cheaper and very much lower losses.

That sounds much safer. At high currents, the diodes will get substantially ohmic, with a positive TC, so the pairs will share pretty well even if they are strapped hard in parallel.

The LTC part requires an output capacitor, which might not be good for welding. A magnetic amplifier-based synchronous rectifier I described here a while ago doesn't have this limitation. Best regards, Piotr

Best regards, Piotr

Thanks for the comments. I have used a heat sink for the cathodes on both sides of the negative half. Another level of hokey, but not too bad.

I didn't say, but my use of these diodes is because I have them. Scrounged from God-knows-where & sitting in my collection.

Since you already have the diodes, I would say go for it. The welding is i ntermittent so it is not like you are going to have 125 amps continuous.

It would be good if you directed some of the air from the fan to flow over the heat sinks. If you do not have some heat sinks, let me know. I have s ome heat sinks that might work. They are 4 inches by 4 inches by 1.25 inche s with mounting for a couple of TO3 devices on each heat sink.

Dan

Do you have 4 of these packages ?

Why not use them all and connect the diodes in a packet in parallel. This will reduce the Vf for each diode and hence power dissipation. It also helps getting rid of the heat, since the devices are far away from each other.

Assuming Vf of 1 V/diode at 70 A, the peak dissipation would be 125 W/package, but since the diode conducts only half the time, the average dissipation is 63 W/package. Assuming Tj=175 C and Ta=40 C, the Rthja must be less than 2.1 C/W and since Rthjc takes out 0.45 C/W, so the heat sink must be better than 1.7 C/W, not an unreasonable number. With four such heatsinks, you can easily get rid of 250 W total.

Do _not_ use a short jumper between the two diode anodes on a package and then a single wire to the other connection points. Instead use separate wires from each anode to the connection point. This will add some (unavoidable) resistance to the connections, but this small resistance helps balancing the current between the two diodes in a package.

Dan, thanks for the offer. In my collection I also had a big 'ole heat sink that I cut up. And a fan from a junked welder. I also got an inductor from that welder that I'll be using.

Pictures and schmeatic are interesting but could you please consider using Dropbox or other less embarrassing site for posting pictures.

MK

Embarassing? What's wrong with this one? It works OK without the need to log in and I see only my content, without any ads, etc. If you have a better logon-free one, please name it.

Best regards, Piotr

I have a 350lb Oxford oil-cooled job here which I use for making up chassis as and when I need them. It can run off 3 or 1 phase and has a max output of 320 Amps which it will quite happily supply all day long. They made these for the shipyards and mine is probably the smallest model they made, since it will 'only' weld up to 1" thick steel plate. It was born in 1973 and it'll still be going strong in another 45 years. Made in England, see. :P

Sorry Piotr,

First time I looked at you images there was a border around it with ads for hair loss products and well endowed young ladies looking for friends.

I looked just now and it was just your stuff !

I don't think it was my fevered imagination or even previous browsing history - so an unknown quirk of the internet unless anyone else has a suggestion.

Sorry again for impugning your web service !

I use Dropbox - does it make you log on ?

(This should be a harmless dog picture.)

MK

Anything made in England in the 1970s has got to leak oil.

Cheers

Phil "former 1977 Triumph owner" Hobbs

When Honda started making motorcycles, they split the crankcase

Of course some of it was worthwhile, e.g. the 1952 Vincent Black Lightning:

Cheers

Phil Hobbs