MOSFET Current Limiter

Discussion in 'The Projects Forum' started by Rottenbind, Mar 25, 2011.

  1. Rottenbind

    Thread Starter New Member

    Mar 25, 2011
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    0
    Hello everyone. I've got a current limiter circuit that I'm working on here that "almost works". I've attached a schematic below.


    [​IMG]

    The circuit sends 50 VDC to an external load whose resistance varies. The incoming current first goes through a current sensor whose output signal goes to a microcontroller (not shown). The microcontroller evaluates the current measurement and adjusts an analog output to between 0 and 10 volts. This control voltage is connected to the bases of six MOSFET transistors to throttle the current that goes through them.

    When I make the microcontroller send different control voltages, the output current changes accordingly, so it would appear the the current is being limited like I am hoping. The problem is that I keep frying the MOSFETs. I'm putting 30 amps through the external load and the MOSFETS should be good for 52A / 1250W each. Since I've got six of them in parallel, I don't get why I'm frying them. I'm not an electrical engineer so I may be approaching this in a completely wrong way. Any suggestions?

    Thanks,

    Ron
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    They are good for 1250W each, IF (a big, fat IF) you can keep the temperature of the case at 25°F (77°F).

    You have not informed us what the low limit of resistance of your load is. However, let's just say that at worst case, your load only drops 10v across itself with 30A of current flow, for a resistance of 0.333... Ohms.
    That leaves 40v to drop across the MOSFETs.

    For the moment, let's imagine that it's a perfect world and all six MOSFETs have identical characteristics for threshold voltage, Rds(on), etc., so each MOSFET will need to pass 30/6 = 5 Amperes current. 40V x 5A = 200 Watts of power that you will have to carry away from EACH MOSFET package, for a total of 1,200 Watts power dissipation - and you'd need to keep the cases at 25°C. 1,200 Watts is more energy than a typical home microwave oven puts out.

    However, your MOSFETs will not be perfectly matched for threshold voltage, Rds(on), etc., so some of them will conduct more than the others. That means some of them will be dissipating more power than the others, and will need even more cooling. If they don't have enough cooling, you'll get a "domino effect", where first one MOSFET will burn open, causing the load to be re-distributed to the other five MOSFETs, then a 2nd will overheat and burn open, and suddenly the rest will fail in rapid succession.

    You might have to use water cooling to keep the MOSFETs from burning up, even after connecting them to heat sinks.
     
    Last edited: Mar 25, 2011
  3. Rottenbind

    Thread Starter New Member

    Mar 25, 2011
    4
    0
    I was afraid of that - that it's a heat dissipation issue. I was hoping that by using 6 MOSFETs in parallel that this wouldn't be much of an issue. Maybe I need 100 in parallel! :eek: Along with a big fan!

    Sounds like I'm approaching current limiting in the wrong way. In my application, the output load is an electric motor that, in normal use, is fine. But occasionally the output shaft is loaded up to the point of causing the motor to stall. When this happens, the motor draws huge amounts of current and fries itself. This is why I'd like to put some kind of current limiter circuit ahead of the motor.

    My first thought on the matter was to just use a simple resistor in series with the input and be done with it. But it would have to be rated for about 6KW or so! Plus, it would be a big-time power drainer.

    Another thought that I've had was to use a mechanical slip-clutch to prevent the stall condition. The problem is... there isn't any room to physically mount one.

    I've also ruled out just using a simple fuse. In this application, the circuit needs to be able to recover from over-current conditions quickly and on its own to get the motor back and running ASAP. Even those self-resetting fuses would take too long to allow the circuit to come back to life.

    So that's how I came to use MOSFETs... it seemed like I could throttle the current to the load as needed. But they get hot and burn themselves up... at least in the way that I'm implementing things.

    Any ideas? I feel kind of stuck at the moment.

    Ron
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You should also consider how fast your system can react, compared to how quickly the current can increase. If there is too much delay between the time when the current exceeds the intended limit value and the shutting down of the gate drive, the current might reach a much higher level before the FETs begin to turn off. This would increase the peak dissipation seen by the FETs.
     
  5. Rottenbind

    Thread Starter New Member

    Mar 25, 2011
    4
    0
    Good point, Adjuster. I was hoping that the FETs would act as a limiter on their own to some degree, even before any measurements and adjustments are made to the control voltage. At a constant control voltage to the gates, the FETs should only allow a certain amount of current to pass through. So if demand spikes suddenly by the motor, the FETs won't allow the high current to pass through. That's my thinking, anyway... not sure if it's true!

    Ron
     
  6. Rottenbind

    Thread Starter New Member

    Mar 25, 2011
    4
    0
    I did a SPICE simulation of my circuit and WOW! Those FETs are dissipating a lot of power. It's no wonder that I'm burning them out.

    [​IMG]

    In the graphs, as time goes on (left to right), the gate voltage is varied. The various plots represent output current to the load (green) and the power dissipated by one FET (blue), all at various output load resistances.

    It's pretty clear that the FETs are dissipating LOTS of power in my implementation, so that explains why they're burning out. Just not sure where to go from here at the moment.

    Ron
     
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