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U4R1A

Fri Dec 17 2010, 11:42PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

What would be the overall effect of stacking to mosfets(irfp250n)? I'm sure there is multiple ways of doing this but in this case drain->drain/source->source/gate->gate. Would this double the overall current rating or leave some burnt silicon? I've googled this 15 different ways and haven't found any thing so its either a bad idea or not useful. Anyone have any insight?

***OK.. I found the info I was looking for so would this help a zvs driver out on reducing the heating of the fets?***

Patrick

Sat Dec 18 2010, 12:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i should start by saying -- im not sure.

However for other purposes i have done this before, i found the most difficult problem was how to arrange short traces between devices while still keeping them attatched to a heat sink, the best way i found was to put single transistors in between fins on the heatsink, then parallel the pins closely.

if it will work or not, depends on V, I , trace layout, and of course freq, the higher in freq - the more difficult.

...

Sat Dec 18 2010, 12:53AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You can parallel mosfets, however you run into issues relating to current sharing. Basically, even 2 mosfets from the same run will have very slightly different gains, so one will draw more current than the other. This problem is further exacerbated bu the fact that the gain changes with temperature, and at low gate voltages (as would be the case for switching losses) the hotter the device is the more current draws--which causes it to draw more current, and get hotter, etc. One solution is to simply put them both on the same heatsink so that the temperature is more or less constant between the two devices, but then the negative temp coefficient associated with higher gate voltages (as would be the case for I^2R losses) which means that one of the devices will always run a little cooler.

In general, for mosfets at least, it works fine to just wire them up in parallel and hope for the best. If you notice one device running significantly hotter (or as it more likely, it randomly dies on you) then you can rethink things and go buy a proper mosftet.

However, this may all be a moot point, since a ZVS driver really shouldn't run the mosfets very hot at all. What kind of heatsink do you have currently? Even a modest heatsink, such as that from an ATX power supply, should be sufficient for operation with any TV style flyback transformer.

Adam Munich

Sat Dec 18 2010, 01:11AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

... wrote ...
the hotter the device is the more current draws--which causes it to draw more current, and get hotter, etc.

I believe mosfets draw less current the hotter they get, while BJTs draw more.

Edit: Yep, just found this on the IR website.

"In general, semiconductors have a negative temperature coefficient, with the one notable exception of the power MOSFET in that its temperature coefficient is positive"

U4R1A

Sat Dec 18 2010, 04:01AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

@ ... My ZVS runs super cool til >65volts but I've seen several posts of hot fets on ZVS drivers so I figured I ask around about some solutions.

Mattski

Sat Dec 18 2010, 06:55AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Yep, look at figure 4 in the datasheet, the on-resistance increases with temperature, which means there will be some negative feedback to prevent thermal runaway, but it doesn't mean they'll share current equally. Thermal runaway is when the current draw increases with temperature, so if a device in a pair heats up it will draw more current, thus heating up more, until it overheats and fails. One way this is prevented is by putting a resistor on the emitter to provide a voltage drop that increases with current as negative feedback.

If you want the current to be shared better you can always manually find a reasonably matched pair out of the parts you have available by doing some manual characterization.

GeordieBoy

Sat Dec 18 2010, 07:48PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Do MOSFETs exhibit thermal runaway??? The real answer is "It depends!"

It is true that the "fully on resistance" typically called Rds(on) increases with increasing temperature. So two paralleled devices should share current if they are hard switched and they are both fully enhanced when they are meant to be on.

However, whilst Rds(on) has a positive temperature coefficient, the gate-source threshold voltage typically has a negative temperature coefficient. This means that given a gradually increasing gate voltage, the hottest MOSFET in a parallel pair will turn on first and hog the full load current.

As for transconductance, this can have either a positive or negative temperature coefficient depending on where you are operating in the curve.

So in summary, MOSFETs are far from immune to thermal runaway. This is particularly so if they are operated in the linear region or in things like constant current circuits where IÂ²R losses increase with increasing Rds(on)

Switching circuits using MOSFETs are the most immune to thermal runaway because the devices do not linger in the linear region where the negative temperature coefficient of Vgs(th) causes mismatched currents. However, the hottest device will still turn on first, and turn off last, thereby still taking a larger slice of the load current.

Just thought i'd dispell this common myth about MOSFETs and thermal runaway.

-Richie,

U4R1A

Sat Dec 18 2010, 08:29PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Tons of good info. What I'm getting out of this is you can run multiple mosfets in parallel and its possible that one will hog more power then the other and get a little warmer. But the question will putting 2 mosfets in parallel(even worse case 1 hogging more power) allow for overall cooler mosfets then just one at the same load?

ScotchTapeLord

Sat Dec 18 2010, 09:00PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Paralleling devices won't create more heating (unless something is horribly wrong with the rest of the circuit).
It's just incorrect to assume that two paralleled 20A semiconductors make a 40A equivalent, or even a 30A equivalent, or even a 25A equivalent, depending on how poorly the current distribution is.

ubuntupokemoninc

Sat Dec 18 2010, 09:17PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

test the resistance threw the fet when theirs power to the gate, then add resisters to make them equal,

that's what i did for my zvs, i have 4 irfp250's

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