mosfet driver

Discussion in 'General Electronics Chat' started by Pete51, Oct 25, 2006.

  1. Pete51

    Thread Starter New Member

    Oct 25, 2006
    3
    0
    Sorry for my English.Can you help me with my problem with mosfet?
    It is N channel mosfet IRF 830 (100 W,max U DS=500V) I need it in power switching- two identical mosfets with load ...Gates are driven with cmos astable circuit/one mos is on another off.I have not driver but between gate and ground resistor.Capacitance of Gate /source is 810 pF.at low frequency it goes well,but at khZ freq. are problems,although I changed resistor(lower value) there is also C in one cycle acumulates energy,in another dissipates.
    U gs > Udd. thanks for your help
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    What kind of problem? And could you post the circuit for the astable block?
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I learned the hard way that drivers are essential at higher frequencies. Drivers help the gate charge and discharge faster.
     
  4. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Which CMOS ? 4000 series or what ?
    What's the supply voltage for the CMOS ?
    Didn't you use a gate resistor (to the CMOS output) ? :eek:


    That would also be a bad place to put a driver ;) Place it between CMOS output and gate.


    Input capacitance of IRF830 is 610 pF.


    At 1kHz or at 999kHz ?


    The gate to ground resistor ? (That will only worsen things).


    Please tell us the voltages, not just their relative amplitudes.

    You need to give precise specs, if you want precise answers.

    If driven by a 4000 series CMOS run from 12V, a very optimistic switching time would be around 370nS (each flank).
    A bad combination, with low current driver and *high* demanding MOSFET I'd say - any (good) reason for these choices ?

    If U_gs is larger than U_dd, then you're either trying to kill the MOSFET's or have chosen a really really really bad MOSFET for the job !

    Dedicated driver chips are capable of peak currents of around 4A !

    What exactly are you trying to accomplish ?
     
  5. Pete51

    Thread Starter New Member

    Oct 25, 2006
    3
    0
    Thanks for your opinions. Astable circuit was made from 2 NANDs and 2 "identical" R and C .It is standard circuit. I know that driver goes between gate and output from generator.But this was made without driver.The using of R between gate and 0 V was somewhere on net. Those NANDs were CMOS
    4011. Frequency- 47 kHZ. supply voltage of generator 9 V.Mosfet U dd 5 V.
    Simply I want one separated circuit to charge C and another to discharge it to load.You maybe say strange circuit.We want to compare and know effecienty of storage E into C... But possible problem is that potencial on source must be less than on gate.and if C charges the potencial may change.
    If you know some other circuits instead this one,please help. In one phase charge the C and no current goes to load,in another phase C discharges into load and C is off from U dd. Thanks
     
  6. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    If you really don't want a driver, at least use a (non-CMOS) 555, as it has about 20 times the output power over a 4011.
    Better MOSFET's would be hard NOT to find for this app.
    Go for Logic Level devices with low R_ds_on and low gate charge.
    Further, you really need to use both an N-channel and a P-channel device to drive it properly, but remember that the P-ch. device will have somewhat higher R_ds_on and be generally weaker compared to it's N-ch. "equivalent".


    Use the 555 and put a resistor of 47R to 56R from it's output (pin3) to the gate. Don't use g-s resistors.
    That will give you much steeper flanks.


    All the more reason to get Logic Level devices. As it is, you bang your head on the roof even before you meet the Miller plateau, so it's bound to go haywire.
     
  7. Pete51

    Thread Starter New Member

    Oct 25, 2006
    3
    0
    Thanks for your help. I used 555 in similar circuit /there was bipolar NPN switch.(MJE 13009).Here I need two oposite signals- Y and Y(neg).on picture.But i didnt
    find 555 circuit as astable circ. ,only as square generator.Do you know some schema with 555 as astable?I dont work with electronic long.My hobby is chemistry and physics. because for me it is important on one gate + U
    and on the second 0 V.I think that mosfet dont need much current.when U gate decrease to 0 V,there is that sad 610 pF and it last long time to discharge it through extremely high R of gate.Thus I used R to ground.But what if instead it I use - voltage ?Will it discharge faster ?
     
  8. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    That's the same thing !!


    Please re-read what I wrote. You need a P-channel as well as an N-channel device (a complimentary pair), since if you use only N-ch devices, you can't drive the gate of the "upper" one with around 6V more positive than its source.


    A very common mistake, made by people far more into electronics than you, but YOU know it's wrong, since you just read my former post ;)


    That's not the way, and your lack of a gate resistor (to the driving signal), has probably driven the CMOS into semi-latch-up.


    What if you followed my advice from my previous post and got a working circuit ? Then you wouldn't have to ask questions that was allready answered... MOSFET's needs HUGE amount's of current for fast shifting, end of story, no more "What if's", just pump as much current as possible into and out of it's gate. It's just like charging and discharging a capacitor, and to shift it, you need to drive it through the Miller plateu, which for common MOSFET's is around 6V. For Logic Level MOSFET's, it is somewhere between 1.5V and 3.5V usually, so they're much better suited for your circuit.
     
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