Short circuit current protection on FET

[vvenky88]

Dear Friends,
I am using the IRF9640 P-channel mosfet in my circuit.I want to protect my circuit against short circuit current protection of 3A. What should i do?
i have attached the schematic for your reference...






With thanks & regards,
venkat,

Moved to Analog circuit design. Post in the right category [alexan_e]

 

[FvM]

You'll need a current sense resistor and a latching overcurrent sense and shut-down circuit. Your circuit is missing a free-wheeling diode for the inductive load, by the way.

 

[vvenky88]

how to select the current sense resistor and how to use in the circuit? I am new to this...try to explain me...thanks...

You'll need a current sense resistor and a latching overcurrent sense and shut-down circuit. Your circuit is missing a free-wheeling diode for the inductive load, by the way.

 

[FvM]

A current sense resistor will be mostly place in the source line of the transistor. It should be low resistance to reduce the voltage drop and power dissipation, but must be large enough to get a detectable voltage for the overcurrent detector.

A simple short circuit protection can be formed by a current limiter, e.g. a single BJT transistor shorting the gate control signal, using Vbe voltage of 0.6V as threshold. But operating the transistor as current source involves high power dissipation, thus a current limiter must be complemented by a delayed shutdown circuit to protect the transistor.

In software controlled systems, a low part count solution can be a combination of a hardware current limiter, signalling the overcurrent state to the processor, and a software shutdown.

 

[vvenky88]

Ok. For example in my circuit i am using P-channel MOSFET IRF9640 which has maximum current of 11A. But i required only 3A to the external load(Relay). If the external load is taking more than 3A then i should drop the output. In this condition how to choose the current sense resistor? and how to connect in my circuit? thanks...

A current sense resistor will be mostly place in the source line of the transistor. It should be low resistance to reduce the voltage drop and power dissipation, but must be large enough to get a detectable voltage for the overcurrent detector.

A simple short circuit protection can be formed by a current limiter, e.g. a single BJT transistor shorting the gate control signal, using Vbe voltage of 0.6V as threshold. But operating the transistor as current source involves high power dissipation, thus a current limiter must be complemented by a delayed shutdown circuit to protect the transistor.

In software controlled systems, a low part count solution can be a combination of a hardware current limiter, signalling the overcurrent state to the processor, and a software shutdown.

 

[jdupz]

Maybe can use mosfet with maximum current of 3a and no need to use shutdown circuit. Using this method you can save some components. Your mosfet will only deliver maximum of 3a.

 

[jegues]

A simple short circuit protection can be formed by a current limiter, e.g. a single BJT transistor shorting the gate control signal, using Vbe voltage of 0.6V as threshold..

I'm just trying to follow here, did you mean shorting from base to collector?

I thought a gate control signal would be related to a MOSFET not a BJT?

 

[FvM]

I'm just trying to follow here, did you mean shorting from base to collector?

No.

I thought a gate control signal would be related to a MOSFET not a BJT?

Yes. The suggestion is to add an overcurrent limiting circuit to the MOSFET that uses a single small signa BJT and a current sense resistor.

Maybe can use mosfet with maximum current of 3a and no need to use shutdown circuit.

Unlikely. Assumed you'll find a suitable transistor, it must also be able to handle a power of 330 W. Study the datasheet SOA characteristics, you won't find any.

 

[jegues]

No.


Yes. The suggestion is to add an overcurrent limiting circuit to the MOSFET that uses a single small signa BJT and a current sense resistor.


Unlikely. Assumed you'll find a suitable transistor, it must also be able to handle a power of 330 W. Study the datasheet SOA characteristics, you won't find any.

Thank you for clarifying.

 

[FvM]

I was thinking of a circuit like below. The MOSFET must be at least able to handle the Imax*Ub peak power. The short must be terminated by the control circuit unless the transistor can endure the power continuously.