High or low side solenoid driver - advantages/disadvantages of each?

Discussion in 'The Projects Forum' started by madindehead, Jan 14, 2012.

  1. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Hey,

    I have a question for you all. I require a driving circuit for a solenoid (taking about 60 amps from a 12V LiPo battery).

    Now, I plan on using two FETs arranged in a half bridge config. Both will be N-type. Now, what is best to drive this solenoid: high side or low side.

    I understand that driving N-type on the high side is harder, but chips are available, so it's really not that hard anymore.

    I'm assuming that having high side drive might be better for such a high current application, and that it's safer to leave the solenoid connected to ground when it's not on (with a flyback diode parallel to the solenoid for the back EMF spike).

    I've been trying to find out more information about it, as I need to justify my choice of high or low side drive. I also know that high side drive is like sourcing current, and low side is sinking it.

    Any advice would be great thanks :)
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Just about all computer or uC circuits in production use a low side switch. Motor controls with a H-bridge are the exception. This is due to the easy of the low side mosfet and the need of keeping the distance from the gate drive short. The shorter the better on mosfet gate drives, less problems with the switching. Also a low side switch allows using a high side "bus" to power many loads from the bus if needed.

    A cars engine and other controls is a good example of this.
     
  3. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Well, when it comes to driving it, there aren't any problems with driving from a high side switch, since I am using a gate driver (well, half bridge driver with two MOSFETs). The high side to switch, and the low side to pull the MOSFET to ground when it isn't being switched.

    Thing is, since it's easy enough to do (both low and high side) is there any added benefit from using either? I don't understand your answer very well shortbus.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Why do you need two FETs?

    The advantage of switching low side is that there are more drivers available, meaning you will probably have less problems to find a suitable one for the FET you are using and they should also be less expensive.
    Since you a re not switching high frequency you may not even need a driver IC but a simple transistor driver for the MOSFET.

    What would be the advantage of having the solenoid connected to gnd when not connected in your opinion?
     
  5. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Well, I was thinking, it would be safer to have the solenoid connected between two ground pins when not being used, instead of floating and connected to 12V.

    I would have one FET at the top, between 12V and the solenoid, with the solenoid connected to the source of that FET and then the other end to ground (well, one of the coils, since there are 2 coils in this solenoid). The bottom FET is connected at its drain to the end of the solenoid (same connection as the source on other FET) and then connected to ground. Top FET is on: coil is driven. Bottom FET is on: coil is off, and tied to ground at both ends. Is it not safer than having the coil floating at 12V (also, since each coil has to be independent, and we need to be sure both coils won't try to activate at once.

    I will try and upload a diagram soon
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    yes please do this.

    I don't know about the two coil part but when you say "safer" you mean safer for what? For the FETs, for the user, for the solenoid?

    You only have to make sure that no component can be damaged by the induced voltage when turning the current through the coil off.

    A schematic and an explanation of why do you think it would be safer will help.
     
  7. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Safer for the user, as if the coil is disconnected, it won't have any voltage sitting in it (if we make sure the solenoid is properly turned off prior to disconnection).

    Also, it's connected to a LiPo battery, so I don't really want the coil to be connected to it, when it doesn't need to be. Does that make sense?

    [​IMG]

    This is my idea.

    Q3 is on, and the solenoid is connected to 12V and ground. Q4 is off at this time. When Q3 is off, Q4 is on, and the solenoid is connected to ground (with the flyback diode handling the back EMF). Is this going to cause problems with the back EMF or not? There are two of these, so at any time, both could be off, and potentially connected to 12V is I used a low side driver.

    I need to justify why I choose what I choose also. I just can't find any benefits currently of either high-side or low-side drive, as all the arguments regarding ease of driving low side don't really apply here (I need MOSFET drivers either way, they have a big gate capacitance). Since they are taking a lot of current from the battery (probably around 60-80 amps) would the high side not be better for providing that current? Just what I read about high-side drive acting like current sourcing, and low-side like sinking.
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    If Q4 will be closed when Q3 opens then it will be in parallel with the flyback diode, as well as the intrinsic diode that exist in the n-channel MOSFET.

    I don't see an advantage in using a high side switch over a low side. They are in series with the solenoid anyway. I also don't see a necessity for two FETs.

    Why would it be unsafe to handle 12V? Unless the user could accidentaly short circuit the battery ( with a tool for example) because the solenoid terminals are exposed, there would be no safety issue. At least not from an electrical standpoint.

    At 80 Amps you will either need an isolated gate driver IC or a very well thought out layout. High transient are going to be generated.
     
  9. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Well, the device is going to be handled by people who aren't electronic engineers eventually, and we're dealing with a LiPo battery, so something pretty dangerous if not treated properly.

    Yes, I will be using gate driver for the MOSFETs. As I said, the second FET is more so that the solenoid isn't simply left floating after it is disconnected. It's firing in very short bursts.

    Like I said, I mainly wanted to know if there is an advantage to using either high or low side in this instance. Thanks for the help though.
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Connecting one side of a component to one side of a voltage source means the component terminals are not floating. Even the terminal that is now open is not floating it will have the same voltage level as the other terminal (in the case of your solenoid.)
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Got to ask this:) What type of solenoid takes 60 -80 amps ?
     
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