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High side driver/Low side driver

  1. Jul 24, 2015 #1
    I have to work with control units at my new job and I am not sure what something being driver high side versus low side mean? let's says we have a motor connected to a microcontroller unit. if one end +ve of the motor is connected to the controller and the other end of the motor is connected to ground, does that mean it is driven low side? would there be a voltage limitation is such a situation as it the controller can only give small voltage max, like say max 5 volts or maybe 9?
     
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  3. Jul 24, 2015 #2

    meBigGuy

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    High side means there is a switch between the voltage source and the motor. Low side means there is a switch between the motor and it's ground.
     
  4. Jul 25, 2015 #3

    Hesch

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    I suggest that is has to do with the ouput stage for a pwm-signal:

    yNWTc.png

    The signal must be pushed/pulled as high and low as possible to prevent losses.
    Q1 and Q2 are IGBT-transistors and thus the Vbe must be at least 15V to turn the transistors fully on.
    As for Q2 there is no problem, but say you want to raise the output voltage to 598V, you must raise VB(Q1) to 613V, but you have only 600V at disposal. That's why the driver ( here FAN7390 ) has been made, so that the 600V can be stepped up:

    The capacitor CBOOT is charged with 15V when the output is low. When the output voltage is raised high ( to 398V ), pin 8 (VB) is raised to 613V, and this voltage is supplied to R1 through pin 7 (HO).

    So the drivers as for Q1 and Q2 are not built in the same way. Therefore they are named high side and low side driver
     
    Last edited: Jul 25, 2015
  5. Jul 25, 2015 #4
    I've seen the term used in audio speaker drivers with two transistors, one at each supply rail, driving a speaker.

    I've never seen it defined, but rather taken from context.
     
  6. Jul 25, 2015 #5

    meBigGuy

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    Good point. I was envisioning a different circuit. I think you can pretty much always say High side connects to power, low side to ground (or negative supply).
     
  7. Jul 26, 2015 #6

    Svein

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  8. Jul 27, 2015 #7

    meBigGuy

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    I don't understand how a single high side switch is any more complex than a single low side switch. High side is a PMOS with S connected to supply, and a driver pulling the G to ground to turn the PFET on. Low side switch is NMOS with S to ground and a driver driving the G high to turn it on.

    That's ignoring transient suppression, etc. Basically, either switch can be simple. Or either switch can be a "smart switch".

    The smart driver you linked to is a smart high side switch. There are smart low side switches also. http://pdf.datasheetcatalog.com/datasheet/siemens/BTS117.pdf

    Not sure why you referenced a smart switch for high side only.
     
  9. Jul 27, 2015 #8

    Svein

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    As I said, a low side driver is easy because the logic generally will interface directly to the driver (see figure).
    upload_2015-7-27_18-37-9.png
    As you remarked, a high side driver needs some sort of level translation between the logic and the power device.
     
  10. Jul 27, 2015 #9

    meBigGuy

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    Oh yeah --- 1 transistor if the supply voltage is above the logic voltage. That's really a lot more complex.</sarcasm off>
    images?q=tbn:ANd9GcRHJDi5nRFRBkowv9OKpOeFFyr77EvesfUsHqQd0GXYV5NoL6oj.png
     
  11. Jul 28, 2015 #10
    In the power circuits the actual gate driver is often isolated from the control side, by an opto for example, in this case getting the proper power to drive the gate of the High side switch (mosfet / igbt) is tricky and typically requires a bootstrap circuit since when the Top IGBT is "on" the gate is ~ at the +DC bus V. You will see this in Hesch's post #3 - but this is still using a separate 15V supply. So typically the High side is one that has some type of bootstrap capability, low side has a reference to the -DC bus. In larger power ( Est > 50A RMS) - a Dual driver module which includes a complete isolated power supply for the top and bottom is often used, as the larger IGBT prefer to be turned off to a negative voltage - this adds to cost of the driver significantly. Another function the High/Low side IC and Dual Driver will incorporate is Deadtime / shoot through protection - since this requires coordination of the gate signals for both the top and bottom switch.
    One reference

    If you will be working in Power Electronics - there is also PowerGuru.org
     
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