Need help reading datasheet

Discussion in 'General Electronics Chat' started by Gadjex, Apr 21, 2014.

  1. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    This is my first post and I hope I am not breaking any rules. If so please let me know.

    I am just getting into electronics and circuit design. I would like to power a 24V DC motor and I am not sure how much amps I can safely pull from the MOSFET I have.

    I have 4 I-Pak packages of IRFU7440PbF Power MOSFETs and I am having trouble figuring out what the safe max amperage is @ 24V in a H-bridge with the lower MOSFETs being driven by PWM.

    These will run close to 25% duty, have no heatsink and be at room temperature.

    Datasheet for IRFU7440PbF

    Thank you,

    Rob
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How will the mosfet be driven?
    You will need a gate voltage of at least 5 Volts and preferable 10 Volts.

    Bertus
     
  3. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    I am using a full H-Bridge driver (HIP4081A) that will use a charge-pump and bootstrap.
     
  4. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    I forgot to add the driver is driven with 12 volts.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When using a drive voltage of 12 Volts the fet will be driven fine.
    Keep in mind that the boost capacitor will have the correct value.

    What will be the properties of the used motor?
    What is the stall current?

    Bertus
     
  6. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    The motor is a linear actuator with 3.75 amps max load and 9 amps stalled. I will be controlling position with feedback sensor. 25% duty cycle is max.

    Thanks for the continued help.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The current will be no problem.
    Most power will be dissipated when the mosfet is turned on or off.
    That why it is importent to have a correct boost capacitor.
    The HIP datasheet will tell you how to calculate it.

    Bertus
     
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  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    A 24v motor driven with a 12v PSU and 25% duty cycle?

    12v at 25% duty will only apply about 3v to the 24v motor, so its speed and current consumption wil be quite low.

    You need to state what current your motor draws with a 3v DC PSU and your typical load applied to it. And test with a stall load too.

    That info will help to show what the current requirements of the FET h-bridge will be.

    It's possible the current will be low (a couple of amps average?) so you can can use a simple low power h-bridge IC instead of a messy discrete h-bridge with 4 power FETs and charge pumps and driver IC.

    Besides ease of assembly, the h-bridge IC will probably also contain other benefits like thermal shutdown, and inbuilt overcurrent limiting. :)
     
  9. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    It is a 24V motor at 3.75A max load 9A stalled. The H-bridge driver (HIP4081A) is running from 12V. 16V is max for the driver.

    By 25% duty I mean "duty cycle", oops. Not 25% PWM duty, but the amount of motor full on vs off.

    If you can recommend an H-bridge IC to accommodate this 24V, lets call it 4A motor, I will be more than happy.

    Edit: The motor will be powered with a 24V nominal input.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Your in good shape. Even stalled it's only .25 watts in the FETs. @ 110C/watt only a 25C rise. Be careful with the HIP to keep the FETs close. It is really fast.
     
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  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Once your PSU is 12v, the stall current will be about half ie 4.5A.

    Once you decide to go to 24v PSU the choices for hbridge ICs get very limited, or hard to solder. :(

    I would probably buy a $25 DC motor driver module and enjoy all the protective features;

    [​IMG]
    http://www.pololu.com/product/1451
    "This carrier board for ST’s VNH5019 motor driver IC operates from 5.5 to 24 V and can deliver a continuous 12 A (30 A peak). It works with 2.5 to 5 V logic levels, supports ultrasonic (up to 20 kHz) PWM"
     
  12. Gadjex

    Thread Starter New Member

    Apr 8, 2014
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    Unfortunately the IC on this board will not be useful in a 24V nominal application. Voltages will be over 24V at times. But good find either way.
     
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