about autoprotected mosfet

Discussion in 'General Electronics Chat' started by yougarage, Feb 24, 2012.

  1. yougarage

    Thread Starter New Member

    Nov 29, 2010
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    Hello!
    I'm using a mosfet from SGS-THOMSON, the VND5N07 to switch a lamp with pwm. I drive it with a 4 Amps mosfet driver, the MCP14E3. A 22 Ohm resistor is placed between the driver and the mosfet.
    This mosfet has the feature of being autoprotected, when the threshold temperature is reached, the gate is disconnected from the mosfet, and is connected to ground via a 100 Ohm resistor, so that the input voltage can be monitored for diagnostic purposes.
    Now, if the device go in protection mode, the driver will sink to ground all the 14 volts with which it's supplied via an equivalent resistance of 122 Ohm, for a current of about 115 milliamps. This means I would need a 2 watt resistor, right?
    So I thought to overcome this problem using a 1k resistor, instead of 22 Ohm, and to avoid switching speed losses, place a capacitor in parallel with the resistor.
    Do you think this could be a reliable solution?
    What value should I choose for the capacitor?
     
  2. #12

    Expert

    Nov 30, 2010
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    I think you are going to have a problem with gate capacitance. The MOSFET needs 18 nC of charge but the driver datasheet speaks of 2 to 6 nf. The capacitor is a bad idea.

    For the resistor, .115A ^2 x 22 ohms = .29 watts
    .29W x 2 = .58W so a 22 ohm, 1 watt resistor will work here.
    I don't see a problem with that.

    ps, it would be helpful if YOU posted the datasheets. Saves us helpers a lot of time.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    I don't see a problem with using a larger input resistor and a capacitor in parallel with that, if you make the cap large enough to supply the required gate charge current during switching.
     
  4. #12

    Expert

    Nov 30, 2010
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    crutschow is better at this than I am. Go with his answer.

    I was concerned that the driver might not like a capacitive load. I think I was wrong.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    My impression of the resistor was to damp the LC factor of the wire and prevent ringing. If you add a capacitance that would seem to counteract the damping effect, you might as well get rid of the resistor and capacitor.

    The LC factor I am referring to is the gate capacitance (which is equivalent to a capacitance to ground), and the inductance of the wire going to the gate. Together they form a resonant circuit. Adding a resistor damps any ringing. If you want to turn on the gate faster use a smaller resistor, and keep the wire lengths short.
     
  6. yougarage

    Thread Starter New Member

    Nov 29, 2010
    16
    0
    Hi Thanks for your replies and for posting the datasheet links.
    Next time I will post them :)
    As I wrote above, I'm not having problems during the normal operation of the mosfet.
    The problem arise when the device reaches the protection temperature.

    From the datasheet:
    During some tests, the device worked great with a 22 Ohm resistor, but when I led it to protection thresold, it stopped working.
    When the protection happens, the driver will continue supplying the gate, but the gate now is connected to ground via a 100 Ohm resistor, so the total resistance will be 122 Ohm.
    Considering the continuous current capability of the driver of 1 Amp and the voltage equals to 14 volts, the driver will sink to ground 14 volt via a 122 Ohm equivalent resistor.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You are correct. I forgot about the reason for the resistor in the first place. :p

    So to use a higher resistor value to minimize the power dissipation in the protection mode, you would need to add a capacitor in series with the small resistor, with both being in parallel with the larger resistor, to prevent the ringing.
     
  8. #12

    Expert

    Nov 30, 2010
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    So the problem arises when the protection circuit works properly because you let it get hot enough to go into protection mode, and that would require a 1 watt resistor if you use 22 ohms and the duty cycle is 100% high during protection mode.

    Are you repeating yourself because we have not figured out what you need?
     
  9. yougarage

    Thread Starter New Member

    Nov 29, 2010
    16
    0
    Thanks for your replies
    I will make some tests with the above solution!
     
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