Detecting Short Circuit condition across FET load

[Lumina]

I am in the beginning stages of designing a circuit that will protect a low side FET when it sees a short circuited load. Currently I am contemplating using this FET and this driver . The FET is being driven with a PWM frequency of approx. 15 KHz . My load is some LED lights at 24 Volts and approx. 20 Amps . I would like to protect the FET from anything above about 25 Amps. I am considering using a .01 ohm sense resistor on the high side along with this current sense IC TSC101 followed by a comparator that would trigger an interrupt pin on my microcontroller which would then shut down PWM in the case of an overcurrent condition Does this seem like an ok solution? .I am also considering using a Hall effect sensor as the current sensing element. I like the simplicity of this circuit a user posted in another thread. Is this a better direction? I have also seen discussions on using Desaturation but not many examples using FETs, mostly IGBTs. I am however intrigued by this driver **broken link removed** which has a low Desat threshold of 250 mV as most drivers are up around 7 volts. While Desat detection looks like an elegant solution , I am concerned that going that route will lead me down a path of complications. I would really appreciate people's thoughts on the best way to detect a short circuited load so I can protect my FET in the case of a failure. I am also looking at putting a TVS from the Drain to Source to protect from large di/dt voltages in the case of a shorted load. Thanks a lot for your insights ;-)

 

[FvM]

Desaturation detection doesn't work for low voltage MOSFET. It works fairly for high voltage MOSFET, although they miss the clear current limiting feature of IGBTs. Finding a suitable threshold voltage corresponding to current values below a destructive level is not easy.

As you already mentioned, detection overcurrent based on Vds,on is very difficult for low voltage MOSFET and isn't supported by any driver IC.

Overcurrent detection and shut-off is well possible for inductive load circuits with limited dI/dt. For hard shorts with almost unlimited dI/dt it's hard to achieve.

MOSFET are self-protecting against overvoltage by it's avalanche capability. Check the avalanche energy specification of your MOSFET if an external TVS clamp makes sense at all.

 

[nikhilmahasvar]

Normally people use Diode mirror for over current protection in inductive load.

 

[FvM]

Diode mirror? Doesn't sound like a commonly understood technical term.

 

[Lumina]

Thanks so much for the help.

Desaturation detection doesn't work for low voltage MOSFET. It works fairly for high voltage MOSFET, although they miss the clear current limiting feature of IGBTs. Finding a suitable threshold voltage corresponding to current values below a destructive level is not easy.

As you already mentioned, detection overcurrent based on Vds,on is very difficult for low voltage MOSFET and isn't supported by any driver IC.

Thanks, so I will go for current detection using a resistor. In my initial post I suggested using a high side current sensing IC . Is there any benefit to measuring current on the high side vs between Source and Ground other than I guess it will raise my FET VGS turn on voltage? I am hoping to just power the driver from 5 volts since this FET has a low Gate-Source threshold voltage so putting a .01 ohm sense resistor on the low side shouldn't be too significant when it comes to turning it on.

@FvM you had posted this circuit before. Just wondering about using this circuit instead of high side sensing, though in this case I would need to amplify the voltage across the resistor with an Op Amp to turn on the BJT.

MOSFET are self-protecting against overvoltage by it's avalanche capability. Check the avalanche energy specification of your MOSFET if an external TVS clamp makes sense at all.

Thanks I will look into this. I am suggesting the use of a TVS because I have in the past seen FETs blown when a short is applied at the end of a long cable run even with current limited power supplies which is my situation. I was hoping with a clamping voltage below that of the FETs voltage rating a TVS could help in that situation.