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Simple mosfet question

Discussion in 'Electronic Design' started by Andrew, Aug 2, 2007.

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  1. Andrew

    Andrew Guest

    I have a simple circuit where I want to convert a digital input from
    0-5V to an inverted signal from 0-12V. ie, 0V into the circuit = 12V
    out, and 5V into the circuit = 0V out.

    I don't have the means to get a schematic up, but I've tried to draw
    it with text below (but it probably won't show up right), so I will
    describe the simple circuit and I think it will be clear:

    There is a 2N7000 N-ch mosfet transistor, with the source tied to
    ground. The gate is tied directly to the input. The drain is tied to
    a resistor, 10k, and the other side of the 10k resistor is tied to a
    battery, +12V. The output is connected to the drain, and is used to
    feed the clock inputs of 2 CMOS ICs. There is a 470pF filtering
    capacitor tied from the output to ground.

    My question is, can anyone see how this circuit would blow the
    transistor? Is this an OK way to hook it up? Should I maybe add
    another 10k resistor between drain and output (before the cap) to
    better protect the transistor? I realize that doing this would bring
    the output voltage down very slighlty, but that's fine if it is needed
    to protect the transistor.

    I'm asking because I've hooked up the circuit more than a few times,
    and it always works at first / for a while, but sometimes the
    transistor blows unexpectedly and I don't know why yet. I think it is
    because I was damaging the transistor during install (with ESD, I was
    not using protection at the time, but I learned my lesson!), but I
    want to make sure it isn't actually a circuit problem too.

    Sorry that I can't get a schematic up, I don't have a place to upload
    files while I'm at work. I hope I have made it clear enough :)

    \ 10k
    | |
    INPUT _____| |__
  2. Tim Wescott

    Tim Wescott Guest

    Make your pictures with a fixed-width font, then we'll all be able to read

    Putting a resistor in series with the cap will spare your transistor, but
    you don't need to make it 10K. You could probably get away with 200 ohms
    or less if I recall the capabilities of that transistor correctly.

    If you're driving it from CMOS a small resistor from gate to input would
    spare the driver from the capacitive load, but may not be necessary.

    Tim Wescott
    Control systems and communications consulting

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
  3. Fred Bloggs

    Fred Bloggs Guest

    There's nothing wrong with the circuit, something else is blowing the
    FET. There must be a transient occurring on one or more of the three FET
    terminals that is causing the damage. The gate terminal is most
    susceptible and this is handled by placing a zener in parallel with it.
  4. default

    default Guest

    mosfets are very sensitive to gate drive and spurious signals / static
    electricity etc.. A small transient in the gate lead can eat a mosfet
    where a BJT will shrug it off. You are doing something that kills the
    transistor - bread boarding? Keep the gate lead short to minimize
    inductance. Protect the gate and source/drain if there is an
    inductance in the load. Use a scope and watch the transients.

    Look at the data sheet for the 7000 - particularly look at the safe
    operating area and you see how fragile they can be.

    A good source for the use of mosfets:
  5. Andrew

    Andrew Guest

    I hope I don't get a double post here, but it has been a bit and I'm
    not seeing my message pop up, so I'm thinking I forgot to hit reply!
    If it double posts, please forgive me.


    I was thinking the same thing, if it isn't ESD, it must be some kind
    of noise or transients on the transistor leads, which is why I was
    looking for a way to protect the transistor.

    I should mention that the input is being driven by a servo motor
    driver which I did not design or have the schematic for, or really
    know much about the design. I am just intercepting the signal and
    using it to drive my circuit.

    Is it common practice to use a zener as you mentioned, Fred? I'm not
    sure exactly how you meant to hook it up? Did you mean with the
    cathode at the gate and anode at ground? Or cathode at the input and
    anode at the gate? Is doing it this way superior to using a small
    resistor in between the input and gate as Tim suggested?

    After protecting the gate with a zener or resistor, I should still
    probably look at protecting the drain further with either a small
    resistor in line with the cap or a small resistor between the cap and
    10k resistor, right?

    I think you are right Tim, I could use a small resistor such as 200
    ohms. The Ids current capacity of the transistor is 200mA, and even a
    200 ohm resistor with the levels of voltage I'm expecting wouldn't be
    close to this limit.

    Default - the design is actually on a PCB right now. There is a bit
    of wire coming from the servo driver to the input of the board (maybe
    6 inches of 24awg wire (standard servo wire)) and due to space
    constraints, there is about 3 inches of 12mil trace on the PCB from
    the input to the gate of the transistor. I hadn't considered the
    inductance problem that longer traces could pose.
  6. Andrew

    Andrew Guest

    Sorry, I made a mistake. On the PCB the trace from input to gate is
    12mil wide and only about 0.5 inches total length. I was thinking of
    a different portion of the circuit, sorry for the confusion.
  7. default

    default Guest

    I'd want to know more about that . . . Servo motor or something like
    an RC servo? This thing has a motor connected to it? Intercepting
    makes it sound like it does. If that's the case, you need look no
    further, that is probably the cause.

    Motors are notorious for inductive transients - even the brushless
    ones can be pretty bad. If the driver is applying PWM or switching
    from nothing to all out, the wonder is it works at all.

    Gate resistors help but it all depends on the energy and persistence
    of the transient. The mosfet gate is a small capacitor (with an
    equally fragile dielectric). The resistor in series with the signal
    to the gate uses the gate capacity to make a low pass filter.
    Hopefully, the transient will develop across the resistor and not
    punch through the gate insulator.

    In the case of the 2N7000 it is only 20 picofarads - so that isn't
    much of a low pass filter.

    Talk of motors and intercepting signals (one presumes that the signal
    you are intercepting also has wires running about) would lead me to
    think you will need something a little more robust than a series gate

    If you need speed (2N7000 has to follow the signal with minimum phase
    shift) something like a bipolar might be better suited or if speed
    isn't too important a simple RC low pass filter might be better.
    Zeners are good but they have to be fast too - There are
    circuit/transient protectors that specify switching times and those
    may be the better choice.
  8. Andrew

    Andrew Guest

    Oh sorry, I guess my wording was misleading. Yes it is a servo motor
    driver that is intended to drive an RC servo motor, but instead, it is
    driving my circuit. There is no servo motor.
  9. default

    default Guest

    Well that doesn't sound too bad. RC servo is something like a one
    millisecond pulse (if my memory is working) that is pulse position
    modulated. 0-4.5 or 5 volt pulse. That shouldn't be a problem and I
    wouldn't think 6" of wire would be either.

    If you have a scope, look at the signal on the 10X probe range. And
    look at the output (drain) too for ringing.
  10. Nobody

    Nobody Guest

    The RF modulation is PPM, but the signal between the receiver and
    servo is PWM, with a 1500us pulse corresponding to neutral. The pulse
    frequency isn't significant.
  11. default

    default Guest

    You are right. It has been awhile since I messed with them. Pulse
    frequency is around 80 HZ

    To control the servo, you command it to a certain angle, measured from
    0 degrees. You send it a series of pulses. The ON time of the pulse
    indicates the angle to turn to; 1ms = 0 degrees, 2ms=max degrees(about
    120) and anything in between gives a proportional output angle. 1.5ms
    is generally considered to be the "center". The 1~2ms limit is
    manufacturers' recommendations; you can usually use a wider range
    around 1.5ms for grater throw. I did not do this, but you can use
    pulses of less than 1ms, and more than 2ms for an output angle of
    180degrees or more. The limiting factor is the feedback pot and the
    mechanical limits built in the servo.

    Years ago I built a little joystick pan and tilt from a pair of servos
    to aim a tiny camera. I used a quad op amp to generate the pulses for
    both servos.
  12. Andrew

    Andrew Guest

    Yeah, you are right about servos and they way they are controlled.
    The entire circuit I have is basically taking the servo output and
    creating a digital signal from it that throws a relay, so commanding
    one direction throws the relay one way, and commanding the other
    direction throws the relay the other way. This all works great -
    until the transistor blows. I've posted the entire portion related to
    the transistor, it is basically just inverting the input and changing
    it to 0-12V instead of 0-4.5V.

    I'm basically just looking for reasons why the transistor is blowing
    and I think you guys are right that there is some sort of transient
    noise. I'm going to add a resistor between battery and the 470pF cap,
    but I'm not sure if I should add a small resistor between input and
    gate, or if I should hook up a zener, or both? I'm also not sure how
    the zener would connect, I'm new enough at this sort of thing
    (transients, noise protection, etc) that I don't know how the zener
    helps in a case like this. I'm assuming it would be cathode at the
    gate and anode at the ground, and would limit the input voltage to a
    "max" of whatever the zener is rated to? So I would use a zener of
    like 4V? (The input signal is 0-4.5V). Is that the idea?
  13. Andrew

    Andrew Guest

    I was reviewing my older schematics and PCB designs, and I just
    realized something.

    I originally had a 2 layer PCB version of this circuit that worked and
    went through rigorous testing great! Unfortunately it was not a great
    (PCB) design and was noisy. I had filtering caps here and there that
    made it work, but I worried that this noisy design was going to cause
    issues later, so I switch to a 4 layer board which used a power and
    ground plane. When I switched to this *better* board, I was able to
    remove some filtering caps as it was a much better design and was
    significantly less noisy.

    One of the filtering caps that I removed was a 10nF cap that went from
    the gate to the source, to filter the input signal.

    Was this filtering capacitor also inadvertently "protecting" the
    transistor and that is why that design never failed, but now, the new,
    less noisy design that does not have this capacitor is more
    susceptible to noise/transients/spikes that could kill the transistor?

    Or does the removal of this capacitor theoretically have nothing to do
    with it because it was not "protecting" the transistor in any way?

    I'm guessing that it has alot to do with it, and the capacitor was
    protecting the gate input from transients and/or spikes.

    What do you guys think?
  14. Nobody

    Nobody Guest

    You do have a flyback diode on the relay coil, right?
  15. Andrew

    Andrew Guest

    No, I don't have a flyback diode on the relay, but it is in a
    completely different portion of the circuit that I haven't had any
    trouble with yet?

    I didn't understand the idea of a flyback diode at the time I first
    designed this circuit, and I do have a little better understanding of
    it now, and how to use it, I just don't fully understand what the
    voltage spikes can do if they are not controlled with a flyback
    diode. It seems to me that the spike would just be across the relay
    coil for a short period of time? I suppose that this voltage spike
    could kill the transistor (another 2N7000)? It hasn't happened yet on
    any of the prototype boards, but I suppose it is a real possibility.

    I think I will add one at any rate. Does it have to be a schottky in
    the case of a relay, or will any old diode suffice?

    Any any thoughts on the 10nF capacitor from gate to source possibly
    protecting the transistor?
  16. Nobody

    Nobody Guest

    The relay coil isn't connected to the transistor that's blowing?
    You cannot instantaneously change the current through a coil. If the coil
    is in series with a transistor, and you turn the transistor off, the
    current will keep flowing through the transistor anyhow. The effect of
    "turning off" the transistor is that the voltage drop across it will
    increase to whatever figure is necessary to keep the current flowing.

    E.g. if you have 1A flowing through the coil and transistor, and you
    instantaneously increase the collector-emitter (or drain-source) impedance
    to 1MOhm, you would get 1MV across the transistor. In practice, you can't
    get "instantaneous" turn-off, but you can still generate very high
    voltages; you could easily get 1KV from a relay coil if the switching is
    fast enough.
    Recovery time won't matter here. It only applies if you switch the relay
    off then switch it on again before the coil current has dropped to zero
    (which is likely to be in the millisecond range unless you have a really
    large coil).
    It will typically extend the turn-off time, potentially allowing the
    inductor's current to drop to zero before the transistor's impedance gets
    too high.
  17. default

    default Guest

    I like gate transient as the cause but . . .

    Looking at the characteristic curves for a 2N7000, you might just be
    blowing it because you aren't turning it on with enough voltage.
    Don't know what your load resistor is, but with 4.5 volts on the gate,
    your drain current has to be relatively low to avoid cooking the

    Look at the chart that shows "On-Resistance Variation with Gate
    Voltage and Drain Current" then calculate the load current you are
    using and compare that to the absolute maximum values. Little buggers
    are only good for 200 milliamps and 400 milliwatts

    The gate voltage has to be high enough to turn on the transistor fully
    or you end up generating heat in the linear range. The gate threshold
    for a 2N7000 (point where it begins conducting not where it is fully
    saturated) is three volts - you only have four point five - that might
    not be enough with a batch of marginal spec transistors.
  18. Andrew

    Andrew Guest

    The relay coil isn't connected to the transistor that's blowing?

    Nope. There is a relay with a 2N7000 in a different portion of the
    circuit. I haven't had any trouble with it, but I'm going to add the
    flyback diode for good measure.

    The circuit in question is the one described in the first post. The
    cliff notes, though, are that the circuit works great for a while, but
    after a varying amount of time, for an unknown reason, the transistor
    will blow. I'm guessing it is transients or noise of some kind. The
    exact same circuit, except WITH a 10nF cap from gate to source, was
    tested rigorously in four separate PCBs for many many hours in hot/
    cold temperatures and had zero problems.

    Since the time varies, I'm wondering if I was just lucky with those 4
    boards that they worked so great for so long, or if it is possible
    that the 10nF cap from gate to source was acting to block transients
    that otherwise would have killed the transistor?

    And thanks for the additional help on flyback diodes and the
    description of what can happen!

    Just one question on that matter..
    This is only for a short period of time though? Or will current flow
    through the transistor indefinitely (voltage across the transistor)?
  19. Nobody

    Nobody Guest

    No; that would give you free energy.

    The current will drop at a rate proportional to the voltage across the
    coil (minus the voltage drop for the coil's resistance) and inversely
    proportional to the coil's inductance. dI/dt = V/L.
  20. Andrew

    Andrew Guest

    Look at the chart that shows "On-Resistance Variation with Gate
    My transistor is a fairchild (if it matters) and from what I can tell,
    with 4.5V Vgs, I don't have to worry about on resistance until ~
    400mA. The load resistor for the circuit that is blowing the
    transistor is 10k when the transistor is on. With a max voltage of
    around 14V, that is 1.4mA Id, way way way under the worry zone as far
    as Rds on, right?

    Can anyone comment one way or the other on whether a 10nF cap from
    gate to source should have any real effect on protecting the
    transistor? I keep thinking yes, but it seems you guys don't agree?
    Wouldn't it limit potentially damaging spikes to the gate?
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