Electrical – Latching circuits – problem above input voltage


I'm playing with some latching circuits :

  • circuit 1 – more details here

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  • circuit 2 – more details here

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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 Voltage is provided by a labo power supply. 10K resistors have been replaced by 15K.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

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here is a zoom for circuit 2 :

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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 :

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How can we explain this behavior? How can I prevent it?

Best Answer

Circuit 1: VGS max is rated as 25 volts for Q2. At 30 volts, your gate source junction is breaking down. Circuit 2: When you push the button to latch off, P channel voltage is momentarily pulled up to [power input - power input * 15K/(100K+15K)] as @sstobe says. At 16 volts, this is about 2 volts, so you never turn the MOSFET off (VGS(Th) can be 2 volts from data sheet). You have to pull the gate voltage up to within less than this value.