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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, I'm building an inverter which should make 12V at some 100W from a 24V lead-acid battery (20-30V input). The circuit is attached. The problem is that the MOSFET is heating more than it should, right now it's loaded with just some 3-4amps on the output and I've calculated the conduction loss of the transistor to some 0.2W. But it's heating more, few watts I would say.
So it must be switching or avalanche loss right? The gate turns off very quick so I wouldn't think that's the problem, and since the drain is coupled via the 2200uF cap and the schottky diode directly to the output filter cap, there can't be much avalanche loss either. Increasing the frequency increased the dissipation.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
hboy007: It works, why shouldn't it? The first cap is for power rails decoupling, the other is "leakage inductance killer"- it capacitively couples the drain to the output and eliminates the effect of imperfect coupling between the 2 coils.
Edit: Yes the core is below saturation, the current ramps up linearily before switching off.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Sorry I forgot to mention the resistor wattages. The 0.036 shunt gets pretty warm, from five 0.18ohm 2watt resistors in parallel. Also the 0.47ohm 1W resistor gets hot but ok.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635
If you think it's avalanche loss then swapping the FET would eliminate or confirm that suspicion... If the voltage is getting that high, the turn-on loss from the snubber dumping its charge into the FET might be quite considerable...
I think it might be a combination of a few things, like the snubber, perhaps avalanching as you mentioned, and possibly even CEMF. The diode in there seems suitable but it may be worth putting an external one on the outside and then checking out temperatures, if all else fails.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
What is this circuit called again, a SEPIC or Cuk converter, or what?
Anyway it's probably switching losses. Try shorting out the 10 ohm resistor/diode network and see if the FET runs cooler.
The 1N4148 has a relatively high forward voltage on account of its gold doping. It might not turn the FET off cleanly right below threshold. Maybe try a Schottky instead.
Also, the UC3843 might not deliver enough drive current to switch a large MOSFET fast.
If it was the snubber to blame, then by the maximum power transfer theorem, the snubber resistor would have to be dissipating the same heat as the MOSFET, or more.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I don't know how it's called.
The UC384x delivers 1A peak, I think thats more than enough. I think the gate waveform is as sharp as it can get. The frequency is 38kHz.
Ive put the diode and the transistor on the same heatsink and can't be bothered to put them on separate ones now, as this project needs to be finished ASAP But with 60W on the output, the two semiconductors are dissipating at least 5W, more likely 10W (a fairly sized PC PSU heatsink gets to 70C). That's pretty bad, I hoped for a better efficiency, with the 8mOhm ON-resistance MOSFET... so I'll call it 8amp PMPO , 5 amp continuous.
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24V/12V inverter, MOSFET gets hotter than it should
, 5 amp continuous.