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Very HOT Mosfet in ZVS

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James

Sat Apr 30 2011, 05:58PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

It has to be physically rather large, and it should be gapped ferrite or powdered iron because it carries some DC. I use a powdered iron toroid about an inch in diameter wound to 100uH. I got them from filters in old switching power supplies from work.

buttim

Wed May 04 2011, 01:02PM

Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10

PC power supplies? The problem is I don't have anything for measuring inductance. I'll have to buy it.
I *know* I'm abusing of your patience, but I'm getting lost in online catalogs and I wonder if you could suggest an item from some online seller. I usually buy from Farnell, they deliver to a shop in my town.
Would a "choke" like this (

be good?

James

Wed May 04 2011, 05:13PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

The power supplies came from one of our older digital video recorder products, they are functionally similar to small PC power supplies though.

The catalog page is in a language I don't speak, but at a glance it looks like it ought to be suitable.

You can build a LC meter without too much difficulty, and this will also allow you to accurately measure much smaller capacitors than you can with a cheap DMM. I have the AADE LC IIB but there are quite a few similar designs out there that are cheaper.

OnFire

Thu May 12 2011, 04:51AM

Registered Member #3814 Joined: Wed Apr 06 2011, 07:35AM
Location: Simi Valley, CA
Posts: 1

I buy toroid cores off eBay for ~$2.50 each. Ferrites are heavy, so the shipping charges reflect that. Example eBay listing

My MOSFETs and their passive heat sinks have melted sections of several of my breadboards. I finally pulled them off the boards and mounted them on CPU heatsinks that came with a 12V 0.9W fan already attached. They cost ~$3.25. I got them from a surplus electronics store called All Electronics in Van Nuys, CA. No more melted boards. I connect the MOSFETs to the boards with soldered on bell wire.

I made the mistake of mounting 2 IRFP460's on the same aluminum heat sink recently. Took me a few hours trouble shooting to figure out they were coupling through the heat sink.

Inducktion

Thu May 12 2011, 11:17AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

OnFire wrote ...

I buy toroid cores off eBay for ~$2.50 each. Ferrites are heavy, so the shipping charges reflect that. Example eBay listing

My MOSFETs and their passive heat sinks have melted sections of several of my breadboards. I finally pulled them off the boards and mounted them on CPU heatsinks that came with a 12V 0.9W fan already attached. They cost ~$3.25. I got them from a surplus electronics store called All Electronics in Van Nuys, CA. No more melted boards. I connect the MOSFETs to the boards with soldered on bell wire.

I made the mistake of mounting 2 IRFP460's on the same aluminum heat sink recently. Took me a few hours trouble shooting to figure out they were coupling through the heat sink.

Err....You know IRFP460C's are the most god awful slow mosfet available that I've seen? The fall time is 1 uS....That's incredibly slow. Buy the actual international rectifier ones, they're probably cheaper and perform a LOT better than the ones by fairchild.

buttim

Thu May 19 2011, 01:16PM

Registered Member #3853 Joined: Thu Apr 28 2011, 06:16PM
Location:
Posts: 10

Thanks again everybody for taking your time to help giving ideas and uploading images.
My MOSFETs are out of the breadboard, each on a separate alluminium dissipator (no fan though). I had very little spare time recently, so I could not get a new inductor. But I found a few toroid cores which should be big enough (1 inch diameter). How many windings do you think will be enough to get an acceptable inductance value?

Another question. I use quite thin cables to connect the FETs to the rest of the circuit. Can be this the cause for the overheating?

Ash Small

Thu May 19 2011, 06:01PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

buttim wrote ...

Another question. I use quite thin cables to connect the FETs to the rest of the circuit. Can be this the cause for the overheating?

Generally, the bigger the wires the better, for two reasons. First, less heating in the wires due to resistance. Second, larger wires help to dissipate more heat (although I'm not sure if this makes much difference to MOSFET's, It's something I do when 'pushing' diodes, for example)

Also, it is generally accepted that the wire should be wrapped around the leg of the MOSFET (or other terminal) 7 times. (I believe some reccomend 5 times). This also reduces resistance at the points of contact where the wire is soldered. Care needs to be taken not to overheat the MOSFET (or other component) when soldering thicker wires, as more heat is required. I usually use some form of heatsink between the solder joint and body of component, eg pliers clamped onto the leg. I also tend to use pliers to compress the wire onto the leg before soldering (after fluxing) for better contact. Other 'tricks' can be used, such as flattening out the wire, if multi-stranded, before winding around the leg.

The main cause of excessive heating, though, is using a wire that is too thin.

What is the average amperage through each MOSFET?

ConKbot of Doom

Thu May 19 2011, 07:46PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Ash Small wrote ...

buttim wrote ...

Another question. I use quite thin cables to connect the FETs to the rest of the circuit. Can be this the cause for the overheating?

Where did you hear that? Sounds like something from wire-wrapping that (mistakenly?) got moved into practice for soldering? I know solder doesnt conduct as well as copper, but a good joint should be quite thin, and even if your joint is only 2-3 wire diameters (still pretty short for a lap joint) then the cross section of the solder joint would be quite large compared to the wire cross section. Plus mosfet leads are usually on the wimpy side compared to their current rating, and what you would use for copper wire to carry the same current.

For my NASA hand soldering training class for flight hardware/ground support systems, all the wire to device/post/lead joints were usually perpendicular, with as little as a 1/4 turn of the wire around the post/lead for some joints, upto 3/4 on others, surprisingly minimal compared to what I was expecting, and even the amount of solder used was minimal, so the joint could be inspected afterwards by other people.

In practice with my own stuff, I'd do a plain lap joint with the wire flat against the lead so there is minimal gap filling with the solder between the wire and lead. Though mind your mounting as this could put naughty amounts of stress/strain on the mosfet leads/case.

No problem if it has been tested to be effective or make a difference, but it just seems like it couldnt make that much of a difference, as the cross section of the wire wouldnt change, and be quite low compared to the large cross section the solder joint would have.

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