A Circuit Question about LM2903

[GUOJIMIAO]

Hello, amazing people!
Where is my circuit problem?
Please see my circuit shown below

When overcurrent, the comparator LM2903 cuts off the mos tube Q1. If the G pole of Q2 is disconnected, the voltage at the upper end of R4 V_TEST can reach 5V. Theoretically, Q2 should be completely stopped, and the output of Q2 is 0V and flows through The current of the load at the output is also zero. But when the Q2 G pole is connected, when the overcurrent is detected, the voltage at the upper end of R4 V_TEST is more than 2V, the voltage at the output terminal of Q2 is reduced to more than 1V, and the current is still large. When a voltage of 5V is applied to the G pole of Q2 alone, Q2 can be cut off. So, what is the problem with this circuit?
Good Listening!

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[KlausST]

Hi,

There are a lot of circuit issues....

But it seems you did not think the operation to the end.

As expected:
* overcurrent
* comparator becomes activated
* Q1 goes OFF
* Q2 goes OFF
but then:
* current = zero
* no overcurrent situation aynmore
* comparator becomes deactivated
* Q1 goes ON
* Q2 goes ON
* Current increases
and all this repeats infinitely. starting with [overcurrent]

So I guess the circuit is oscillating. Check with a scope.

Klaus

 

[FvM]

Possible operation modes of overcurrent protection are:
- electronic fuse, permanent cut-off with or without delay. Needs a latch circuit and reset feature.
- current limiting. Needs a compensated error amplifier instead of a comparator. The power transistor must be rated for the maximal occuring power dissipation, e.g. 5 W for the presented 5V/1A current limiter