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Question about Diode Sizing for DC Speed controller

Discussion in 'Electronic Design' started by chapmjw, Feb 8, 2007.

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  1. chapmjw

    chapmjw Guest

    I'm designing a speed controller for a mid size dc motor. I've got the
    Mosfet(s) selected and power generation worked out. The flyback diode
    I'm trying to size will be on the same heat sink as the mosfet.

    I need some help with the following:
    How can I choose the right size diode? (current rating)
    Power dissipated by the diode?

    Low side drive (mosfet sorce to ground)
    24 Volt
    40 amp max current
    Details for the motor or unknown at this time.

  2. Tom Bruhns

    Tom Bruhns Guest

    So...I suppose the "right" answer will depend not only on the motor
    but on the load on the motor as well. Assuming an electrical model
    with a moderate amount of inductance, whatever the inductor peak
    current is, is what the diode peak current will be, not accounting for
    high dv/dt and charge being swept out of the diode as it reverses.
    With an infinite inductance, the current is constant at whatever the
    motor draws, and the average diode current will just be that current
    times the percentage of time the diode is conducting. If you're
    supplying 24V to the motor, the diode conducts zero percent of the
    time, but if you are driving a heavy load at low speed, so the motor
    current is 40 amps when the motor voltage is 1 volt, then the diode
    will be conducting most of the time. Diode dissipation is just the
    average over one cycle of the instantaneous diode current times the
    instantaneous diode voltage drop. LTSpice can simulate this sort of
    stuff quite nicely for you. And you may find that you can save
    considerable power if you use a half-H-bridge, with a mosfet to the +
    side, too, though you need a very low on-resistance mosfet to make
    that work at 40 amps. Oh, and beware of conditions of regenerative
    braking, where the motor delivers power back to the controller.

  3. Phil Allison

    Phil Allison Guest

    "chapmjw" <

    ** So what does that 40 amp figure mean if you have no motor info ????

    Your Q is basically silly, since the answer depends on info YOU say you have
    not got and must have.

    How long is a piece of string ??

    .......... Phil
  4. chapmjw

    chapmjw Guest


    Thanks for the reply, that's the info I was looking for.

  5. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Huh ???

    At full PWM width setting, the flyback diode carries no current.

    At a low PWM settings, the diode carries nearly the same ( average) current
    as the motor - but this will be a small value as the effective DC voltage
    seen by the motor is low.

    Worst case dissipation for the diode is about 50 % duty cycle with the motor
    stalled or near so.

    All this assumes the PWM frequency is reasonably high - ie well above 50

    .......... Phil
  6. If the motor is run very near stall, at full current, the
    diode will carry almost all the motor current. I would use
    a diode essentially rated for full motor current, and heat
    sink it for that current. With a 24 volt supply, you can
    cut the diode losses almost in half by using a Schottky
    diode, but it will probably cost more than a fast silicon
    junction diode. Doal diodes are very easy to find, and
    because they are well coupled, thermally, you can parallel
    the two sections.

    Here is an example that costs less than $6 from Digikey,
    quantity 1:
  7. Tom Bruhns

    Tom Bruhns Guest

    Try that again, Phil. If it's locked-rotor, you get lots of current
    at low voltage. The motor doesn't mind, as long as you keep it cool:
    zero RPM means it's not turning its own fan, but nothing says it can't
    be cooled with a fan driven by a different motor.

    In the midst of playing in my spare time with a half-bridge to drive
    up to 20A to a motor at up to 100V (but also 20A at ~10V when driving
    a heavy load slowly),
  8. Phil Allison

    Phil Allison Guest

    "Tom Bruhns"
    "Phil Allison"

    ** Every word is correct.

    ** Hardy a precise statement.

    Ohms law applies to a stalled DC motor.

    ......... Phil
  9. Agreed. At near stall, the PWM duty cycle gets pretty low,
    yet the current gets really high. This is because the motor
    is producing little back EMF, so a small average voltage can
    drive a large current.
    A motor delivering full rated torque at low speed draws full
    rated current.
    Most drives exceed current limit and won't reach 50% duty
    cycle at stall.
  10. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Replacing the MISSING context line !!!!

    ** Then your words above are quite wrong.

    ** Nonsense.

    Low speed operation and stopped are not equivalent to " stalled ".

    ** More non specific language ....... dear, oh dear.

    ** The rain in Spain stays mainly on the plain ....

    ( You are not even on the same topic. )

    ** More complete irrelevances to the actual topic.

    " The rain in Spain stays mainly on the plain .... "

    The issue I raise is about clear expression and not confusing folk with
    wrong or ambiguous usage.

    Try reading my post again and do NOT disassemble it and post silly replies
    to its components.

    " At full PWM width setting, the flyback diode carries no current.
    At a low PWM settings, the diode carries nearly the same ( average) current
    as the motor - but this will be a small value as the effective DC voltage
    seen by the motor is low.
    Worst case dissipation for the diode is about 50 % duty cycle with the motor
    stalled or near so. "

    ........ Phil
  11. Robert Scott

    Robert Scott Guest

    What is really relevant is the relationship between the PWM frequency and the
    L/R time constant of the motor inductance and circuit resistance. As others
    have said, all other things being equal, the current is greatest for a stalled
    motor. But as you pointed out, that current doesn't travel through the flyback
    diode exact during the periods when the PWM is pulsed off. 50% duty cycle is
    probably close to the worst-case situation, but in certain situations you might
    make it even worse with a little different duty cycle.

    Robert Scott
    Ypsilanti, Michigan
  12. Tom Bruhns

    Tom Bruhns Guest

    In the case of the controller I'm playing with, the output is filtered
    DC to keep from spraying 100kHz all over the neighborhood. The
    inductor is 100uH; the input voltage is nominally 160VDC. Current
    limit is set at 20A. Locked rotor, the motor is nominally 0.4 ohms;
    it's rated for 90VDC operation at full output. For this case, clearly
    the diode--or rather the "other" mosfet in this case--sees highest
    average current when the output voltage is lowest, and the current is
    at the limit. That would normally be with locked rotor, about
    20A*(160-20*0.4)/160, but might be momentarily even worse when
    reversing the motor. At nominally 80V @ 20A output, the average
    current in each FET is about 20A*1/2. -- And yes, the application
    for this motor is such that it is asked to deliver full torque at a
    wide range of speeds. It's my whole reason for replacing an AC
    induction motor with the DC motor.

    Perhaps Phil is considering a controller with no current limit. I'd
    just as soon have both my controller and the motor last longer than it
    would if there were no current limit.

  13. Phil Allison

    Phil Allison Guest

    "Tom Bruhns"

    ** Probably be impossible to get 50% PWM duty cycle into a stalled motor
    when overcurrent limiting is included.

    Anyone know what "begging the question " means ???

    .......... Phil
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