Electrical – What’s wrong with this power latch design

circuit-designlatchmosfet

I've been trying to design a push-on, push-off power control circuit, and came up with this design:

schematic

simulate this circuit – Schematic created using CircuitLab

Theory of operation:
Pressing the button causes the gate of M1 to go high, forcing the gate of M1 to remain high through R2, latching M1 on. Q1 is turned on through R4. This causes the left hand side of the button to go low. Pressing the button again causes the gate of M1 to go low, turning off M1 and Q1, causing the left hand side of the button to go high in preparation for turning on again.

This doesn't work in practice. What have I missed?

Best Answer

It's bad design. You press the button and this initially activates the mosfet. That mosfet then turns the bjt on and before you have thought about removing your finger the voltage that was turning the mosfet on (via the pushbutton) has crashed to ground due to the bjt.

Then, a micro second later the mosfet is off and so is the bjt and the process repeats and becomes an oscillator while your finger is pressed.

When you eventually remove your finger the mosfet has a fifty fifty chance of being on. However this won't help because the gate will discharge to ground via the 100 kohm and the circuit turns off.

Note that you cannot expect the sources voltage to be able to drive the gate to hold that mosfet on. Gate has to be several volts above source to fully or partially turn on the mosfet.

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