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Re: [usa-tesla] Fast opto's ?

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  • Chris Swinson
    Slight change of topic, I was going to use a opto to drive a low side fet driver s inputs, but using it on the high side.
    Message 1 of 8 , Feb 1, 2011
      Slight change of topic, I was going to use a opto to drive a low side fet driver's inputs, but using it on the high side.
      Really its just a matter of level shifting. Even though the TC4422 driver does not seem to output under the simulation, the inputs are working.
      V5 is just a squarewave generator, the fet driver is is actually powered by C1, I had to simplify the half bridge a lot as the simulator kept crashing :-(
      When V5 goes high, it turns on Q1 which forces the input of the fet driver to -200V, though I cant do that else it would blow the driver, so I place D1 then so the voltage will never fall below about 0.4V below the "ground" connection of the driver.
      When V5 goes low, Q1 turns off, and R4 pulls the driver input to its 12V rail.
      Simulation seems to work fine, but the driver never seems to output anything, in anycase, my actual problem is I am inverting V5 with Q1 which I did not want to actually do. I've tried placing a PNP in there, actually had it in all ways and it never seems to work correctly.
      Where R1 is, I am actually swapping this manually to -200V, then GND, then +200V and +12V, all works with the NPN in there, but does not work with a PNP, seems as the voltage rises on R1 the voltage drops to about 600mV and it never turns on/off the fet driver.
      So wondering if anyone knows why a PNP will not work ? If anything they make a inverting driver too, but would be better to drive it without inverting the line in the first place, as I will be inverting a inverted line which is a a little pointless!
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