H-Bridge Design Issue

Discussion in 'General Electronics Chat' started by circc, Apr 18, 2011.

  1. circc

    Thread Starter New Member

    Apr 18, 2011
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    Hello everybody,

    I am building an H Bridge for a motor controller and I have encountered a problem, in which I havent found a solution online. Maybe is a stupid question, but I really dont know what to think.

    I use 2 PCH MOSFETs for the high side and 2 NCH MOSFETs for the low side and my power supply is about 24V. So the PCH's source is connected to 24V, and if I want this FET to conduct I have to give on gate a potential low enough, considering the Vth. But, the thing is that every FET that I found online can handle a gate to source voltage up to 20V, so the minimum of the input voltage (ie the gate voltage) has to be 4V. So far it is ok, but another problem is that using a FET with max Vgs=20V I have to use NCH FETs for the low side with a threshold voltage higher than 4V (so not to conduct when the PCH does) and searching on the internet I did not find any.

    Is it ease to find FETs with threshold voltage higher than 4-5 Volts?

    Also, I cannot get how to drive the bridge. I found many ICs, but I want to do it by myself with decrete components. Anyway, the main problem is how to manage the fact that FETs handle Vgs<20 and 2<Vth<4.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your problem is not with the threshold voltage, which is a level of voltage difference between the source and gate where the FET just starts to conduct. You H bridge drivers need gate to source differences of at least 10 volts to fully conduct. If that voltage is lower, the FET's are in the ohmic region, and wasting power as heat.

    Can you post the schematic?
     
  3. circc

    Thread Starter New Member

    Apr 18, 2011
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    0
    Maybe I have not comprehended the semiconductors' theory efficiently, but I thought that the threshold voltage is the voltage that I have to think about when the transistor functions like a switch. So I thought that if the gate to source voltage (Vgs) is higher than the threshold voltage (Vth) the FET conducts. Is this wrong? And you say that I need Vgs>10? So, the FET can function as a switch properly with Vgs>10? But, if I do not want a particular FET to conduct and I have Vgs = 5V the FET will conduct (even a small degree), that wont be a problem?

    This is the schematic, lacking of the safety diodes.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Use only N-channel MOSFETs and use a gate driver IC to drive them.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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  6. ifixit

    Distinguished Member

    Nov 20, 2008
    638
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    The gate thershold for the P-CH's is 2 to 4 volts less than +24 V (~+21V). The gate thershold for the N-CH's is 2 to 4 volts more than GND (~+3V). So if the input to the left, or right side is at say +12 V, then both left or right side drivers are on at the same time, which will short the supply to GND during the transition.

    Another problem is that because your drive signal needs to be 24V, there will be more than 20V on the gates, which exceeds the limits specified.

    How fast do you need to switch? The faster you need to switch... the more high tech your solution will be.

    Check out thoses links.

    Regards,
    Ifixit
     
  7. circc

    Thread Starter New Member

    Apr 18, 2011
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    Yes and for example if the input is +4V (because if it less the FET will be destroyed) the high side FET will conduct (Vgs=-20V) and the low side will conduct too because the threshold voltage is about 3 Volts. Thats my problem actually.


    You are right. And I have to build the bridge for a 24V supply, but I cannot find the right FETs. Most of them can handle Vgs<20V and 2<Vth<4. So what can I do?

    The PWM frequency will be set at 20kHz.

    I will check the links out and I will come back if I have more questions. Thank you for the moment, guys.
     
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