motor driver assistance

Discussion in 'The Projects Forum' started by cl10Greg, Jun 7, 2013.

  1. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
    Hello everyone,

    I am working on a lsd circuit for an electric scooter (24v 500w design). I am using two parallel nmos transistors controlled by an arduino. The arduino sends the signal to a ucc27424 to drive the gates. The ucc is connected to a 12v supply and the input pwm is tied together for inputs and both outputs are tied together. I have a few questions regarding the driver though that I was hoping I can get help with.

    Does the driver need a gate resistor? I think it is good general practice but I am not sure if it is needed and if it would cost additional heat from the switching speed on/off limitation. The pwm signal is 1khz.

    Can I use a 100% duty cycle or is it advised to use lower to prevent excessive heat?

    Do I neex a pull down resistor since it is a push pull driver? From a safety standpoint it is a good idea to make sure the fets are off if the driver fails but again this would effect switching times and result in more heat.

    I did run a single fet test with a gate resistor and pull down. There was a little noise but the more concerning part was rhe fet heat over a short amount of time. it heated up to not touchable after about 20 seconds. It could have been from the switching but I also had one instead of two in parallel. I have since had these questions and I am using two bigger fets.

    Will heatsinks work alone or should I use forced air?

    Should I use a direct pot reading to pwm transfer or is there usually a ramp up procedure to prevent motor and driver heat (from the pwm nit being strong enough to turn the weighted motor)?

    Any help is appreciated. Sorry if fhe English is bad, typed from a phone.

  2. wayneh


    Sep 9, 2010
    IMHO, it's unlikely you need a driver at that low frequency. You didn't mention which MOSFETs you are using.

    Again it depends on the particular MOSFETs you are paralleling. They should be specified to run safely at full on. A MOSFET generates heat while it is in between fully on and fully off, and that's why you want to use a driver to provide fast switching. But I still think it may not be necessary at only 1kHz.
    I think a pull-down is always a good idea for the reason you mention. The power issue is moot, since the resistor could be, say, a 5kΩ.

    I think the proper MOSFETs driven properly will not need forced air, but definitely a heat sink.
    I'm no expert on the motor behavior, but I don't believe you need a ramp. Does the motor see direct PWM, or is the power filtered to DC? You need to be sure the motor is OK with PWM if that's what it will be seeing. Probably is.