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4hv.org :: Forums :: Tesla Coils
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One MOSFET gets hotter than the other.

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plazmatron
Sat Oct 04 2008, 04:00AM Print
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I have been tinkering with SSTC`s for quite a while now, and have observed something odd. In some of the SSTC half bridges I have built, I have noticed that one MOSFET becomes significantly hotter than the other. I have even observed this in commercial Half bridges, specifically those in plasma lamps.
Is there some reason for this? (bad design perhaps), and how can it be prevented?
I was thinking they are possibly resistive losses, from a MOSFET not turning hard on? Maybe a couple of extra turns on the GDT?

Thanks
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Dr. Dark Current
Sat Oct 04 2008, 12:03PM
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I experience this too, once in a HV transformer driver, everything was perfectly symmetrical even the drive waveform, and one FET was no matter what still getting hotter...

I have no idea as to why this happens.


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MRacerxdl
Sat Oct 04 2008, 02:23PM
MRacerxdl Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
Have you put the two on the same heatsink with mica?
I was have that problem, because I forgot to insulate the fets from heatsink.
That shorts one fet (that doesnt get any heat) and stay the other switching all current (gets hotter)
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Reaching
Sat Oct 04 2008, 03:42PM
Reaching Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
i think this problem is due to the small differences of each mosfet. its a mass product and none of the mosfets even the same type have the same specs. you can measure the gate capacitance of a halfbridge brick and sometimes you have 2 or 3 nanofarads difference between each igbt gate. the rds on of the mosfet may be different too or the gate capacitance.
if the gate cap is different but the drive of each mosfet is equal, one is switching on/off faster or slower and so on, causing shootthroo or more switching losses on turn on/off. thats the only explanation i can think of.
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MRacerxdl
Sat Oct 04 2008, 06:11PM
MRacerxdl Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
I think the better think to do, its buy the two fets of the same lot of production.
For example, I sampled two IGBT's from ST that have same gate capacitance and Emitter-Collector capacitance.
But I think that is not a easy think to do, because you cant see the lot number from the factory.
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plazmatron
Sat Oct 04 2008, 08:17PM
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Well so far I havent had any catastrophic failures, its just one of those little irritations. My heastsinks are good, the noticeable heat difference is on the MOSFET body itself. Maybe I worry too much, it probably is just differences in manufacture.
So long as my Silicon isnt in danger of exploding, I dont suppose there is much point demanding matching pairs from the manufacturer smile
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Wolfram
Sun Oct 05 2008, 09:42AM
Wolfram Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Are the heatsinks the same temperature? Maybe the thermal impedance between MOSFET and heatsink is different?


Anders M.
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GeordieBoy
Sun Oct 05 2008, 08:37PM
GeordieBoy Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> ...everything was perfectly symmetrical even the drive waveform, and one FET was no matter what still getting hotter...

The explanation for this behaviour is quite simple... Switched Mode power MOSFETs have two parameters that are influenced by die temperature. These are the Vgs(th) threshold voltage (where the device just starts to turn-on,) and the Rds(on) resistance (AKA The minimum device resistance when the channel is fully enhanced.)

As die temperature rises the gate-source threshold voltage falls and the Rds(on) resistance rises. If the load current is a fixed current, then the first phenomenon makes the load current flow in the hot device for a longer time because this device turns on earlier and turns off later.

The second phenomenon directly increases conduction losses due to the increased channel resistance. This effect usually dominates and can lead to thermal runaway if thermal managment is bad in a design.

A final thing worth considering is that the performance of co-packaged fast-diodes can also be adversely affected by increases in die temperature. Forward voltage usually drops, decreasing conduction losses with increasing temperature. Conversely reverse recovery time can increase with temperature, increasing switching losses. Thermal runaway of fast diodes is possible if the diode dissipation is switching loss dominated.

-Richie,
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