MOSFET gate drive from 100VAC line

Discussion in 'The Projects Forum' started by Dyslexicbloke, Sep 4, 2010.

  1. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Hi folks,
    This post is related to my previous one
    http://forum.allaboutcircuits.com/showthread.php?p=273818#post273818
    but is an entirely different problem hence the new thread.

    I have a generator control circuit that drives a FET (N-Channel Enhancement mode) with a PWM signal to regulate the field current.

    The circuit is needs to be supplied from a stator winding that, due to a couple of small permanent magnets on the field rotor, only outputs about 12VAC at start-up. Obviously this is quite enough to adequately drive the FET gate but once energised, when the FET turns on, the same stator winding produces anything up to 130VAC and will be delivering several amps, all be it pulse width modulated via the FET, to the field rotor.

    My drive circuit is currently simple and ineffective:-
    Across the rectified output of the stator winding I have a large dropping resistor in series with a 10V zener with the fet gate at their junction. The FET source and zener cathode share the ground rail.
    To turn the FET off I have to sink all the current available from the resistor.


    There are several big issues with this configuration that I am aware of but cant work out how to cure.
    1. The high impedance of the circuit, imposed by the maximum zener current, makes the FET turn on slow. (Probably, I am finding it hard to measure)
    2. My drive circuit has to cope with the entire potential gate drive current whilst discharging the gate again I suspect slow.
    3. The drive circuit must have a very high impedance during power-up to allow the gate to charge initially without any positive assistance.
    4. The system constantly dissipates a relatively large amount of power, which is exactly what the FET is supposed to avoid the need for.
    I have tried several other configurations but cant get any of them, even the ones that didn't crash and burn, to turn the FET on at start-up.


    I am sure I am just missing something simple here …
    A voltage reg that will drop 100V or so at a couple mA perhaps?
    (I did think of using a depletion mode FET but can't find one that will handle the voltage / power)
    Please help if you can, I am stuck
    Thanks for reading
    Al
     
    Last edited: Sep 4, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Al,
    As I mentioned in your other thread, posting a schematic (preferably in .png format) of where you are now would be an enormous help.
     
  3. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
    420
    19
    As you can see the issue is the huge voltage range I am forced to work with ....
    I know what I have now isn't any good.

    For the purposes of this thread please assume the output of U1b is a clean PWM signal with a base frequency of about 200Hz and a duty cycle of about 30%.

    Reducing the value of the pullup resistors would help startup but dissipate more power and slow down the turnoff.
    I am aware that a buffer is required between U1b and the gate but I don't know what or how !
    Thanks for looking
    Al
     
  4. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
    420
    19
    I am a little surprised that no-one has an opinion .. :-(

    There are obviously some seriously clever people contributing here.

    Am I asking the wrong questions or am I just so far off the mark as to sound silly?

    I have no intention of shoving these posts continually back to the top, that’s just rude but I hope you will all forgive this one indulgence.

    We live off grid, have little money and this stuff is really important to me.
    Thanks folks
    Al
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, you fooled me by drawing the MOSFET upside-down; I mistakenly viewed it as a P-ch in the other thread.

    I'm wondering why you used a MOSFET with such a high Vdss rating? 400V seems rather excessive. All other things equal, selecting a MOSFET with a Vdss rating of 200v would decrease your gate charge requirements considerably.

    Anyway, I guess I need to re-draw your schematic in LTSpice so that it better conforms to generally accepted standards, which makes schematics much easier to understand.

    Basically, inputs come from the left, and outputs go towards the right - which you have largely accomplished.

    However, more positive voltages are towards the top, and more negative voltages are towards the bottom. I guess for convenience, you placed a ground rail in the middle of the schematic, and placed the MOSFET drain-up. This makes it very easy to overlook the arrow on the gate and mistake it for a P-channel MOSFET, which I did. Then I started wondering why the pair of 1N5408 diodes were drawn inverted ... until that dim bulb started to glow ... ;)
     
  6. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
    420
    19
    Hay ... slorry about the lack of LTSpice I had a quick look at it and realised I needed to learn how to use the thing before I had any chance of getting a schematic out of it much less running a symulation.
    I couldnt even work out how to find / add models that did not appear to be there.
    Anyway .... I'l work on that.
    (I drew did this in OO Draw)

    The FET is a salvage I have a couple of this particular type to be going at.
    If the final design will benifit from something with a lower VDS capability, and hence lower gate charge requirement I will just go get one!!

    If it is easier for you please feel free to pick one of these threads and ignore the other one .... I am easy, and will be happy to post a redirect.

    Once again thanks.
    Al
     
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