Switch selection for a high power H-bridge

Discussion in 'General Electronics Chat' started by stube40, Feb 9, 2010.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
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    I'm trying to design an H-bridge that can control the output polarity of an electromagenetic coil with inductance of around 50mH and very low resistance. The source voltage is around 150V at 40A and hence the design and the switch selection aren't trivial.

    The timing constraints make things even tougher - the circuit needs to be able to switch within a few ms of the control signal trigger going high. Once switched, the circuit will stay in the same polarity for around 100ms or so. Hence, it's far more about response time and lag than it is about actual switch frequency.

    I've considered both solid state relays (IGBTs and MOSFETs) as well as electro-mechanical relays. The solid state stuff seems to have considerable power losses due to the voltage drop between emitter and collector. The electro-mechanical relays suffer from being too slow and/or occasionaly bouncing when driven hard (which is likely to blow my PSU)

    Does anyone have any suggestions for a good solid state solution that has low power losses, or an electromechanical solution that is fast enough and wont bounce?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Forget the electroemchanical relays. They are not suitable for fast and continuous switching. Use MOSFETs for fast switching and low losses.
     
  3. stube40

    Thread Starter Member

    Feb 3, 2010
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    I did a quick calculation that one FET solution would dissapate a good 72W at 40A.

    However, it got me thinking that I might be able to get this working with an electromechanical relay. As I mentioned, timing is very important as I want the switch to make the circuit to within +/- 1ms of a target time. However, I will get at 100ms advance notice of when this time is and hence I could pre-fire the switch to hit the precise time as long as the lag of the switch was consistent. If so, then this might bring electro-mechanical relays back into the picture. Maybe the lag varies though? Also, might I get problems with the contact bouncing? Or maybe electromechanical relays have high power loses too?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Post the datasheet of that FET and your calculations. Also, explain more precisily the switching pattern.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    At that current, you will have power dissipation in MOSFETs.

    However, you will have a great deal of arcing, bouncing and pitting with relay contacts.

    You will also have a good deal of trouble trying to maintain a consistent switch rate for the relay solution.
     
  6. stube40

    Thread Starter Member

    Feb 3, 2010
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    Hey Mik3,

    Its the Infineon IPW60R045CP CoolMOS I was referring to (PDF attached).

    It has an RdsOn of 45mOhm and I just did a P = I2R = 40 * 40 * 0.045 = 72W. Is this the correct way to compute the dissapation?

    The switcher is to be used as the main control of an experiment in which a moving electromagnetic coil has to be charged in one polarity at 150V / 40A and then charged in the opposite polarity when it reaches a certain position. At full speed, the polarity switches happen every 100ms and we can predict when they will be required as we monitor the position of the moving coil. The actual switching timing needs to be fairly tight else things start banging together and making very expensive-sounding breaking noises!

    a crude type of single piston that is formed by two permanent magnets and a moving coil electro magnet in the middle. At 300
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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  8. mik3

    Senior Member

    Feb 4, 2008
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    It is not the correct way because your current is not constant at 40 Amps. Integrate the v(t)*i(t) between t=0 and t=ton to find the energy when the MOSFET is on and then multiply by the switching frequency to find the total dissipated power.
     
  9. stube40

    Thread Starter Member

    Feb 3, 2010
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  10. stube40

    Thread Starter Member

    Feb 3, 2010
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    Got you!! I'll plug these values in to Excel and do the calculations, I should get a heap less usage
     
  11. stube40

    Thread Starter Member

    Feb 3, 2010
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    Took your advice and with a 7.5mOhm RdsOn at a maximum 40A, the integral came out at just a tab above 10W which is more acceptable.
     
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