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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a half bridge inverter. Two mosfets are connected in parallel to each leg of the inverter. I find when I am done that the heatsink for one leg of the inverter (the high side) is hotter than the other. The high side is the one closest to the positive DC voltage and comes before the load. Is this normal?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Either the RMS currents seen by the two pairs of Mosfets are different or their on resistances are different. If you used matched devices of the same type from the same manufacturer then it is likely the mismatch is in the currents. If the duty ratio is uneven then the top devices could be conducting for longer than the bottom ones. Is your converter really a half-bridge with equal switching times and AC coupled load? Or is it really a synchronous buck coverter. In the latter case the lower transistors only act as synchronous rectifiers so may not get as hot depending on how long the current freewheels for.
Also, if one pair of MOSFET gets hotter their on resistance increases. This increase in resistance causes a further increase in dissipation and more heating! That's why all devices should really be mounted on the same heatsink to ensure equal junction temperatures and even current sharing.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I've only felt a difference when I am running currents over 35A. Even then, the difference is not large. What I am actally feeling is the heat sink. The mosfets, themselves, feel the same.
I got the mosfets in the same group. Is there a code buried within them to tell if they are from the same lot? I did not see anything other than the model number.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
IamSmooth wrote ...
I've only felt a difference when I am running currents over 35A. Even then, the difference is not large. What I am actally feeling is the heat sink. The mosfets, themselves, feel the same.
maybe the thermal resistence form transistor case to heatsink isnt the same for both, tighten the screw or clamp on the one that seems to get hotter. if the mosfets feel the same temp, and only differ at higher currents i would tend to beliveve the problem to be mechanical not electrical. also i use an infra-red non-contact temperature sensor so i don thave to guess buy touch.
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unequal mosfet heating
