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MOSFET driver killed with no particular reason

Discussion in 'Electronic Design' started by [email protected], Apr 18, 2007.

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  1. Guest

    hi to everyone. I mysteriously killed 5 MOSFET half bridge drivers and
    wish to see if anyone can help.

    I'm making a N-MOSFET H bridge circuit that switch a sine wave output
    using unipolar PWM for an inverter project. The gate driver I choose
    was IR21834 and I began testing half of the bridge on breadboard
    according to the datasheet circuit. Connecting input lines manually to
    V+ or GND to produce the output, the lower MOSFET would first work,
    but it always happened that somehow after changing the circuit such as
    connecting driver input here and there or switching the value of
    boostrap capacitor, at a random time the chip would go crazy . The
    chip would feel hot touch, followed by increase in current drawn, all
    of which signifying an internal short circuit. It seems the hide side
    circuit is causing problem, since the low side output would usually
    still functioning afterwards.

    I believe such short circuit behavior can only be caused by shorting
    high side output to ground or to V+, and I swear I didn't, nor did I
    connect bootstrap diode and capacitor wrongly (I redid the circuit
    several times and same error can't happen over and over again). I
    thought it might be chip design problem but as I tried chip from other
    manufacture, L6388 from ST, the same happened. Now I'm really
    threatened as I don't want to kill any more chips. Although I might
    find out the reason myself but that would probably take another 10
    chips dedicated to destroy, and this is why I ask for help here.

    While not expecting a direct reason to the problem, I am REALLY REALLY
    glad if someone who have killed MOSFET driver before (for whatever
    reasons) to share his/her story, and also tell me what to avoid when
    working with drivers, thousands thanks in advance!
  2. Read ALL the application notes carefully. Sounds like latchup
    (probably from poor layout).

    Best regards,
    Spehro Pefhany
  3. Guest

    This picture shows what I did:
    (NOTE: bootstrap diode is built in)

    Plus what Spehro said as latchup really fall into my mind, as I have a
    "feel" that chip internal logic crashed somehow. As I'm already using
    decoupling cap, can you explain a bit more on that thx.
  4. Guest

    Avoid drivers - :) Hrmph!

    My first guess is that the driver is way too fast for it's own good so you get
    ground bounce and probably a latchup. Or the chip oscillates. Both problems are
    due to the layout and decoupling - if there is an application note on how
    capacitors should be placed and wires routed then I.M.E. this should be followed
    without fail - because this will be the only way that *beeep* chip ever worked
    (once, in the lab).

    The second guess is that the MOSFET is oscillating, overvolting and killing the
    gate then blowing the driver. This is sort-of a layout thing but not always.
    Usually I stick a resistor between 10 and 47 Ohms in series with the gate to
    preempt any oscillation.

    A discrete bipolar driver curcuit is not more expensive, it is much more
    tolerant of abuse and layout and the circuit itself is more robust too. Them
    sales people like to boast the biggest slew rates on their powerpoint slides -
    while most of the real power electronics applications do not need kV/nsec at the
    driver level it at all.
  5. brushhead

    brushhead Guest

    What kind of de-coupling cap are you using? You need something with a
    very low impedance, in order to supply the current spike for the gate of
    the MOSFET.

    The other thing is, what is your PWM frequency? You might be buggering
    the chip because it's power will be similar to f*c*v^2, as a direct

  6. Guest

    I'm using electrolytic plus a green cap for decoupling.
    A fairchild application note
    suggests the gate resistor to be 4.7ohms. So for 10 to 50 ohms of Rg
    suggested by frithiof, would it be too large that may slow down rise
    and fall time by too much?

    Also it sounds good idea to buy discrete high and low side drivers as
    it debugging isolated devices will be easier. Thankyou
  7. brushhead

    brushhead Guest

    I would have though that that would be OK. Increasing the value of tge
    gate resistor will increase the switching losses in the FET, but will
    reduce the peak current required from the driver; simple Ohm's law that one.

    I would not have though that adding discreets will help you though.
    Herpahs understanding the current in the FET would be more beneficial.
    Have you scoped what the gate drive waveform looks like yet? That might
    tell you all you need to know. Also what does the D-S voltage look
    like. Is there a lot of overshoot because of a bad layout? Is the
    voltage source (CD?) properly de-coupled?

    There are loads of things to look at, but one thing my experiences have
    taught me is that gate drive is everything to a circuit. You may be
    ringing the gate-source capacitance like hell which is causing enourmous
    losses int he FET and then killing it.


  8. I saw the other thread in s.e.misc. If you would have cross posted this
    (instead of multiposting) it would have been easier to keep it all straight.

    At any rate, is that 100uF cap your Cboot? If so, shouldn't it be more
    like .1uF (100nF)?
  9. James Arthur

    James Arthur Guest

    I'm curious Frithiof, what sort of bipolar driver do you prefer?
    Something like this?:

    .. Vcc
    .. ---
    .. |
    .. |
    .. |/
    .. +----|
    .. | |>.
    .. | |
    .. >-+--R1--+--R2--> to FET
    .. | |
    .. | |<'
    .. +----|
    .. |\
    .. |
    .. |
    .. ===
    .. GND

    I may be driving some big -- okay, 'medium' -- FETs, would need gate
    drivers, and the cross-conduction thing is otherwise an annoyance.

    With regards,
    James Arthur
  10. Mark

    Mark Guest

    Connecting input lines manually to
    Are you saying that you manually moved the driver input wires by hand
    while the power was on?

  11. qrk

    qrk Guest

    Wow, you really mean breadboard. Superstrips are unsuitable for these
    types of circuits due to the fast switching speeds. The long
    interconnection wires can have enough inductance to produce voltage
    spikes that exceed the ratings of your devices. This sort of layout
    frequently causes gate punch-through on the FETs.

    I notice you have a lack of bypass capacitors. You need good quality
    bypass at the driver chip and the FETs. These capacitors need to have
    shortest possible leads to reduce the lead inductance. You can get
    circuits like your present breadboard layout to work if you bypass
    properly. Don't expect to drive much of a load with this setup.

    This sort of circuit could benefit using much better breadboard layout
    practices. Use perf board and copper tape for your ground and power.
    Use SMT bypass capacitors in the range of 100nF to 1uF around the
    driver and the FETs. You also need bulk capacitance from an
    electrolytic capacitor (use a low esr type for switching power
    supplies). Your bootstrap capacitor is generally in the range of

    Try make your interconnections as short as possible. That could help.
  12. Guest

    Sorry I'm newbie of using google group, how might I do cross post
  13. Guest

  14. Guest

    Please ignore this question, I just found out how, and sorry for my
    previous multiposting
  15. brushhead

    brushhead Guest

    Well it's how you learn. I don't cross post, simply for avoiding
    encurring the wrath of the Usenet community.


  16. Rich Grise

    Rich Grise Guest

  17. legg

    legg Guest

    A plug-in breadboard?

    Dont't use these for power control circuits.

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