Electrical – 1 A current source mosfet heats up


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I set up this circuit on a breadboad but it doesn't seen to work correctly.
I used 1 ohm(5%) for the sense resestor and set the voltage using a divider down to 1v but there're 2 issues:

1- the voltage across the resistor is about .8 V and goes down after a little bit I guess because the mosfet on resistance increases with temperature.

2- The mosfet heats up so quickly (it's heat-sinked) although it has a very low on resistance of .025 ohm (irfz44n).

I tried to put a small series resistance with the the gate of the mosfet so it prevent ringing but still the same problem. I even put 10 kOHM resistor to make a schmitt trigger but also didn't work.

I'm using a 12 V, 1 A limited voltage source and LM358 for the OpAmp.

Best Answer

low on resistance of .025 ohm (irfz44n).

The Rds(on) in the datasheet applies when the FET is fully switched (aka used in the saturation region).

This circuit uses the FET in the linear region, so this spec doesn't apply.

Anyway it's very easy to calculate the power consumed by the FET. The current is 1 A. The voltage is 12 V less a tiny amount for the sense resistor. So the power consumption must be very near 12 W.

With 62 C/W junction-to-ambient thermal resistance (per the IR/Infineon datasheet), that's more than enough let the smoke out of the part.