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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
I have a DC-AC power modified sine wave inverter that I managed to blow one of the output half bridges on. While I have it torn to pieces, I figure now is the best time to do any additional mucking around I may want to do. I wanted to upgrade the primary side MOSFETs in addition to replacing and upgrading the output H-bridge mosfets.
Please pardon the lack of schematic, but its a pretty text-book design, and I just wanted to make sure my intuition was right before I break out the soldering iron.
Its a push-pull converter running at 30KHz to generate a ~130VDC bus on the secondary side, which feeds the output H-bridge switched at 60Hz. It has the typical +12V going to the primary center tap, paralleled N-FETs to 0V on each outer leg of the primary winding. The only unexpected thing is a 1.3uF 630V film cap array in series with the secondary before it goes to a full bridge rectifier (I traced it out, twice, full bridge and series cap, not a voltage multiplier) I believe that the series cap and either leakage inductance or additional secondary inductance is what makes the push-pull operate in ZVS. See waveforms below.
Yellow and blue are the VGS waveforms, and Green and Magenta are the VDS waveforms (The gently sloping, ringing ones, for any colorblind folks) for the primary side mosfets.
The turn on (below) looks OK as far as I can tell, only Vds is only 4V when it switches on, and it goes from Vgs = 0V to above the miller plateau in 114ns. Which isnt that bad for operating at 30KHz?
The turn off is a bit slower, taking ~300ns to go from the miller plateau to below Vgs= 2V. But Vds is also rising lazily.
Note that these waveforms are all unloaded, since half the output stage is cut out after the FETs went shorted. (plugging in that cell phone charger must have had one hell of a turn on surge )
For the upgrades, I was thinking that between the zero voltage switching, moderate-low operating frequency, and comparatively fast switching times keeping switching losses low, that replacing the primary FETs with ones of a similar gate charge and a reduced RDSon would be of more benefit than replacing the FETs with ones with a similar RDSon, and a significantly reduced gate charge. Am I barking up the right tree here?
Also, In my tracing it out, I noticed there is no output inductor installed, only a wire link and the silkscreen for an inductor. I'm assuming that plugging in the cell phone charger that killed it while the inverter was on shorted whatever mosfet was on due to dI/dT, and that mosfets mate got shorted on the following half cycle. Installing an inductor in the 20-500uH range in the inductor spot on the PCB should take care of this hopefully?
Finally, for upgrading the output FETs, since they are hard-switched, something with lower gate charge, reverse recovery time and reverse recovery charge will be the most beneficial in keeping them from getting blown by load transients?
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Yep, you definetly want the lower R(ds on) MOSFETs for the primary, the gate drive has been designed for those gate characteristics, and may not achieve low voltage switching with other MOSFETs. A lower R(ds) will be fine, however, and under ZVS operation the majority of losses will be MOSFET conduction losses.
I would just use the lowest R(ds) MOSFETs you can find for the secondary, at 60Hz you don't have to worry about gate charge and the inductor should be fine for suppressing surges. The addition of a PP capacitor across the output after the inductor will also help.
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Advice on component selection on inverter overhaul
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