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Motor duty cycle control using MOSFETs

Other Parts Discussed in Thread: CSD19536KCS, UCC27531

Hai,

      I am trying to drive a separately excited motor using  CSD19536KCS. My plan is to control , duty cycle of motor's armature  using this circuit. Input frequency of PWM is 2.2KHz. The above mentioned MOSFET support above 120 Amps. My requirement is continuous 75 amps and 24 volt.

   I already tested different MOSFET driver configuration and finally reached a circuit I attached herewith. 

Here I controlled up to 30 amps, 12 volt continuously using single MOSFET. Heat is developed here but its ok.

Over that heat is developed tremendously.   

When I am using 2 MOSFETs in parallel more noise is generated.

      In most of the applications  , there are more than one mosfets in parallel , even though the single one can handle the ratings.

What is the reason ? Is it for temperature handling?

  • Hello,

    The CSD19536KCS FET you use in your Application is overheating for two reasons:
    1. The Gate Drive with Opto-coupler is improper to use with a fast switching Mosfet like CSD19536KCS. The Mosfet is overheating due to excessive Power dissipation at turn OFF through R3 resistor 6.7k Ohm. Two FETs in parallel simply run in thermal runaway by using 6.7k Ohmturn OFF resistor .
    2. Drain Cap C2 dumps all the stored Charge on FET during turn ON event.

    To correct above issues, please consider the following recommendations:

    1. Use a TI driver IC specifically designed for Mosfet gate drive such as UCC27531. You can use a resistor to adjust the turn ON speed on OUTH output, but don't add any resistor between OUTL output to FET gate.
    2. You need a power resistor series with C2 cap to properly implement an RC snubber. Cap value in the 10nF...47nF should be enough.
    3. Snubber Cap C2 should be chosen as SMD type low inductance, otherwise through hole types are inefective due to additional inductance.

    Best regards.
  • Hai,
    Lucian Hriscu , thank you for your replay. Now Iam trying to do using the currently available components. Your suggestion will consider on second level.
  • Hai,

        I changed my design using another gate driver specific opto-coupler.

    It works fine but Iam using coupling capacitors across Drain and Source and it gets hot.

    working frequency --20KHz

    capacitor parallel to D-S   --2.2uf ,MPX-x2 and 105,400v  polyster box  in parallel

    A power diode used as fly wheel diode

    10k pull down to gate to source

    2.2R to gate

    I below attaching the gate pulse when MOSFET loaded

  • Hello,

    Understood. The circuit you're trying to implement is a Chopper designed to drive a DC motor.

    Since you fixed the gate drive circuit now, the Drain waveform you show above is incorrect for a Flyback design.

    When the FET is in OFF state ( gate drive is low), the drain voltage should be the Vin supply rail plus a diode drop above ( your clamp diode).

    The over-dampening of the plateau voltage  is happening due to Drain-to-Source excessive capacitance 2.2uF used without any series resistance.

    When you turn on the FET, all Energy stored on this 2.2uF Cap is discharge on to FETs channel causing excessive power Loss, hence the overheating.

    Please try to use an RCD clamp as attached below:

    Depending your frequency, cap C1 can be increased up to 1uF/100V. You need to choose resistor R1 in a power package able to take the power dissipation.

    Diode D1 should be 100V type, same rating as the Power FET.

    The clamp Diode across the Motor winding needs to remain unchanged.

    Best regards.

    Lucian

  • Hai,

        Thank you

     Some modification made as your suggestion and get a better wave form