Safety unit BQ77PL900 chargepump control for back to back high side NCH's MosFets

Discussion in 'The Projects Forum' started by Michael Hentschel, Jun 14, 2012.

  1. Michael Hentschel

    Thread Starter New Member

    Jun 14, 2012
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    Hi, my name is Michael, I'm from Germany and I'm new here. :)

    I need a little bit assistance with a problem regarding a safety unit with a TI BQ77PL900 that I've developed. This chip usually drives high side back to back Pch-MosFets. Due to the problems / disadvantages with PCH-MosFet's compared with NCH-MosFet's I replaced the PCH's with NCH's. The problem now is to drive the Fet's, 'cause VGS must be ca. 10 Volts higher than souce to turn them on. I solved this problem with an Supertex HT0440 using both sections of that chip to drive DCH & CHG as you can see the attached file/schematic. But tis causes problems. Staticly (via the host mode in the GUI of the BQ77PL900) everything works, dynamically, unfortunately not. The problem appears to be the HT0440. E.G. will not shut off even with over-voltage condition. Does anyone have a solution already - possibly even a discrete - for a chargepump control for back to back high side NCH's MosFets?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    When it does not "turn off", what are the voltages at:

    A - GND
    B - GND
    DA - SOURCE
    CA - SOURCE ?

    Is the HT0440 driven with pulses, if so what frequency (A & B)?
     
  3. Michael Hentschel

    Thread Starter New Member

    Jun 14, 2012
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    Hi,
    Voltages are:
    A=B=Vreg1=+5VDC -> GND
    CA=DA +10 to 11VDC -> Back to back Source of T1.x/T2.x

    I suppose this problen has something to do with a.) the internal clock of the HT0440, and b.) that Pin 3 and 6 is tied together at the spurces of the MosFets; although input and both outputs are isolated / decoupled in the IC.
    This is the ubiquitous problem of the difference between theory and practice.;)

    Regards,
    Mike
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    So are you saying the output voltages are always between 10 and 11V, independently of the input signal A and B?
     
  5. Michael Hentschel

    Thread Starter New Member

    Jun 14, 2012
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    That's what the DMM shows. Meanwhile I have substituted 1x HT0440 by two (both outputs in parallel on each circuit), means one for each transistor bank. Now over voltage cutoff works, but no release so far with the BQ77PL900 at the moment. Crazy stuff this integrated circuits, means never rely on Datasheets, Applikation notes and at least not on the manufacturers. Bugs everywhere :(

    Mike
     
  6. Michael Hentschel

    Thread Starter New Member

    Jun 14, 2012
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    kann nur besser werden...

    By the way, do you speak german???
     
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