current-sensingmosfetnpnshort-circuit
I am using a circuit shown below.
it is designed to sense 1.5 A whenever the load takes current >= 1.5 A. Rsen senses the related voltage and activates the transistor.
Here I use a Rsen = 0.46 R(0.7V/1.5A).
Whenever 1.5 A flows the BC547 turns ON and the gate of MOSFET gets turned off (Vgs less than Vth).
But our problem is source of MOSFET provide a voltage and MOSFET getting over heated.
Can someone provide a solution for this?
For 1.5 A at 12 Volts, switched by 3.3 Volts, here is a MOSFET solution that would work well. The MOSFET suggested here is an IRLML2502 available from eBay and other sites for as little as $2.35 for 10 with free shipping.
simulate this circuit – Schematic created using CircuitLab
The IRLML2502 has a maximum on-resistance of 0.08 Ohms at 2.5 Volts gate voltage, and less as the gate voltage gets closer to 3.3 Volts. It can withstand 20 Volts Drain to Source, so it will work well with a 12 Volt supply. Drain-Source current rating is greater than 3 Amperes, providing over 100% margin of safety.
At 0.08 Ohms and 1.5 Amperes, the MOSFET will dissipate 180 milliWatt when fully on. Even allowing for the switching edges of the PWM, dissipation will not exceed 250 mW or so, hence no heat sink is required for this application.
Regarding the assumptions:
Some of your assumptions are correct. This answer provides one better way, and I am sure there are others.
It shouldnt oscillate if there was only a resistance from source to V- and the voltage between V+ and V- was constant DC.
The situation in practice is another. The instruments and the wires to V+ and V- are remarkable inductive and capacitive reactances. In addition I like to see the wire between G and D (where it is when compared to other wires - a possible feedback transformer). If it oscillates, somewhere is enough phase lag (=slowness) that prevents finding the balance. Fet itself has a remarkable capacitanse from G to D and S and the wires increase the effect.
The situation resembles the shower incident. A man in the shower never gets the water to the perfect temperature when he turns it hot, if it's too cold and turns it cold when it's too hot. The slowness is the flow delay.
Please, show us a photo of your circuit. The wire lengths and routes are interesting. The whole case is interesting and is perfect teaching material.
Try, if the oscillation stops when you connect a 10...100 nF capacitor with short wires between V+ and V- as near the fet as possible. Does the frequency vary if you bend and move the wires?
Best Answer
Your circuit is in fact a current regulator: it won't let the current exceed the threshold, but it will never cut off the load in case of over-current. And power MOSFETs make very poor linear regulators because they tend to go into thermal runaway and die.
If you want over-current protection, you need to latch the over-current signal either temporarily or until a reset. If you PWM comes from an MCU, you can route your over-current signal to its input pin and program the protection the way you want.
Another form of protection is to install a polyfuse in series with the load: polyfuses have the latch-up behaviour built-in. Getting back into a working state would require power-down and cooldown delay, so it's a less flexible solution.