I'm trying to implement a latching circuit for around 4.1V of voltage (LiPo battery). Below is the circuit:
The problem is that the circuit is always on as soon as the power is applied (so even before pressing the button).
I tried removing both transistors and can confirm that the MOSFET works as expected. No voltage on VSYS after turning on, but it appears after grounding the gate.
After adding only Q2 (2SC5663) back (Q1 is still de-soldered) VSYS has voltage when the power is applied, so it's always on. After I remove R4, the transistor is no longer on, and I can turn it on by pressing the U5 button. However, it also turns on even when I touch one of R4 pads with my finger (the one connected to the base of Q2). So it feels like Q2 is overly sensitive.
The original transistor in the schematic was BC547 but I needed an SMD transistor and found 2SC5663.
Why is this happening? Is there a way to fix this?