I'm playing with some latching circuits :
- circuit 1 - more details here
- circuit 2 - more details here
Latching with these circuits works, but above a given input voltage ( > 30V for circuit 1, > 16V for circuit 2), I can latch ON but I cannot latch OFF.
For circuit 1, I've used : BC547B bipolar transistor, and FQP27P06 Mosfet P
For circuit 2, I've used : FQP27P06 Mosfet P and FQP30N06L Mosfet N
For both circuit, load is a DC-DC converter connected to a 5V microcontroller (drawing ~20mA), all components are put on a breadboard. Voltage is provided by a labo power supply. 10K resistors have been replaced by 15K.
Above a given voltage, both circuits cannot latch off. Here is a scope capture when I try to power off circuit 2 :
- blue plot : load voltage
- yellow plot : P mosfet gate
- green plot : N mosfet gate
here is a zoom for circuit 2 :
we can see P channel gate voltage slowly decreasing. When N channel closes, it decreased to Max Value - 1.15V
When voltage is reduced to 7V latching works :
How can we explain this behavior? How can I prevent it?