MOSFET switching

Discussion in 'The Projects Forum' started by SAnton, Oct 22, 2008.

  1. SAnton

    Thread Starter New Member

    Oct 19, 2008
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    0
    Hello! I`ve got some problems in my design. I have Totem-pole switching circuit topology, upper MOSFET (high-side) I open by the driver applying 15V to the gate, but how can I open the lower MOSFET (low-side)? As I know we need gate to source potential, but there is negative polarity power supply on low-side transistor source (not GND). Please help with the solution.
     
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The FET will go into full conduction if you raise the gate at least 10 volts above the source. It will be off if the gate is at the source potential.
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    If your supply voltage is more than 10 Volts but no more than 20 volts then connect the gate of the lower side mos to ground and it will work. However, if you have a different supply voltage with the one i mentioned you have to use a gate driver.
    Also, for the upper mos to be fully turned on its gate voltage has to be 10 volts greater than the voltage across the motor.

    Which MOS are you using?
     
  4. SAnton

    Thread Starter New Member

    Oct 19, 2008
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    0
    MOSFET - IRF630N, gate potential +15V. I need to turn low-side and high-side MOSFETs in strict order, for examle, high-side working - stop (deadtime) - low-side working.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    That is correct.
     
  6. SAnton

    Thread Starter New Member

    Oct 19, 2008
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    To speak precisely, I want to design such scheme. Finally, I need to get ~1-2A pulse across the resistor, and each half-cycle the current direction must be changed. But it is not easy as I hoped. See schematics.
     
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  7. mik3

    Senior Member

    Feb 4, 2008
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    From the datasheet, you need 10 Volts to fully turn on the mos. For the lower mos you can easily drive it with a transistor and a a voltage divider as to use only 10 volts from the power supply (more than 20 volts across the gate-source and the device is gone!). Thus, you have to apply 15 volts on the gate with respect to ground.
    Now, for the upper mos to work properly you have to apply a gate voltage with respect to ground of 35 volts. This is not practical because you will need another power supply or a special circuit called mos gate driver. Also, by applying a voltage greater than 20 volts to the gate with respect to ground (for your case) you may destroy the mos because the source voltage wont rise immediately to 25 volts (so you will have a Vgs of more than 20 volts) because the mos needs some time to respond to the input signal and pass the maximum current through it. I suggest you to replace the upper mos with a p-type mos to simplify your drive circuit to a simple transistor and a voltage divider as the lower mos.
    In addition, you need a fast transient suppressor or varistor across the motor with a voltage of about 50 volts to suppress the back EMF of the motor when you switch it off because it will destroy your mos.

    If you want you can use a single supply and use an H-bridge driver like this:

    http://www.robotroom.com/BipolarHBridge.html

    It uses BJT's but you can replace them with mos.
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    From the datasheet, you need 10 Volts to fully turn on the mos. For the lower mos you can easily drive it with a transistor and a a voltage divider as to use only 10 volts from the power supply (more than 20 volts across the gate-source and the device is gone!). Thus, you have to apply 15 volts on the gate with respect to ground.
    Now, for the upper mos to work properly you have to apply a gate voltage with respect to ground of 35 volts. This is not practical because you will need another power supply or a special circuit called mos gate driver. Also, by applying a voltage greater than 20 volts to the gate with respect to ground (for your case) you may destroy the mos because the source voltage wont rise immediately to 25 volts (so you will have a Vgs of more than 20 volts) because the mos needs some time to respond to the input signal and pass the maximum current through it. I suggest you to replace the upper mos with a p-type mos to simplify your drive circuit to a simple transistor and a voltage divider as the lower mos.
    In addition, you need a fast transient suppressor or varistor across the motor with a voltage of about 50 volts to suppress the back EMF of the motor when you switch it off because it will destroy your mos.

    If you want you can use a single supply and use an H-bridge driver like this:

    http://www.robotroom.com/BipolarHBridge.html

    It uses BJT's but you can replace them with mos.
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    From the datasheet, you need 10 Volts to fully turn on the mos. For the lower mos you can easily drive it with a transistor and a a voltage divider as to use only 10 volts from the power supply (more than 20 volts across the gate-source and the device is gone!). Thus, you have to apply 15 volts on the gate with respect to ground.
    Now, for the upper mos to work properly you have to apply a gate voltage with respect to ground of 35 volts. This is not practical because you will need another power supply or a special circuit called mos gate driver. Also, by applying a voltage greater than 20 volts to the gate with respect to ground (for your case) you may destroy the mos because the source voltage wont rise immediately to 25 volts (so you will have a Vgs of more than 20 volts) because the mos needs some time to respond to the input signal and pass the maximum current through it. I suggest you to replace the upper mos with a p-type mos to simplify your drive circuit to a simple transistor and a voltage divider as the lower mos.
    If you want you can use a single power supply and drive the motor with an H-bridge driver.
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    What you are proposing is a half H-bridge. If you can float the resistor (instead if grounding one end), you can use a full H-bridge and get by with a single 25V supply. It also eases the level-shifting requirements. There are myriad variations on the H-bridge. If you think you can use one, I would start with a search of this site, and a Google search.
     
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