hithesh123
Full Member level 6
I am trying to design a discrete current limit circuit. So far I have come up with 3 ckts. I can't decide which one to go with -
1. Classic 2 NPN transistor current limiter
2. PNP + NPN + op amp current limiter
3. Mosfet current limiter
Ckt 1 is simple, but the drop across BJT is high compared to other 2 ckts. The drop across the sense resistor also has to be 0.7v at Ilim. So some power is wasted here. I haven't found a good design reference for implementing foldback limiting.
The problem with (ckt3) Mosfet is the load is connected to high side. I am not sure if this is a problem, say in a battery charging ckt like a cell phone charger.
Advantage being low Rds ON and Mosfet (IRF540) can dissipate insane amount of power - 130W. Even if the ckt is shorted, no problem!
For the Mosfet ckt shown in fig, the current is controlled by the zener at the gate. Gate voltage is limited to 3.9v, which corresponds to about 1A.
I can replace the zener with an op amp and add a current sense resistor for better operation.
Ckt2 has low drop across the PNP pass transistor but component count is higher. Problem I see here is output getting shorted. No foldback limiting.
1. Classic 2 NPN transistor current limiter
2. PNP + NPN + op amp current limiter
3. Mosfet current limiter
Ckt 1 is simple, but the drop across BJT is high compared to other 2 ckts. The drop across the sense resistor also has to be 0.7v at Ilim. So some power is wasted here. I haven't found a good design reference for implementing foldback limiting.
The problem with (ckt3) Mosfet is the load is connected to high side. I am not sure if this is a problem, say in a battery charging ckt like a cell phone charger.
Advantage being low Rds ON and Mosfet (IRF540) can dissipate insane amount of power - 130W. Even if the ckt is shorted, no problem!
For the Mosfet ckt shown in fig, the current is controlled by the zener at the gate. Gate voltage is limited to 3.9v, which corresponds to about 1A.
I can replace the zener with an op amp and add a current sense resistor for better operation.
Ckt2 has low drop across the PNP pass transistor but component count is higher. Problem I see here is output getting shorted. No foldback limiting.