How does the current protection circuit of the MOS transistor work?

How does the current protection circuit of the MOS transistor work?

First, let's look at its circuit working principle.Initial state

When the circuit is powered on, due to the voltage division of R97 and R99, the base voltage of Q63 is pulled, and it is in the cut-off state. At this time, the gate voltage of the MOS transistor Q62 is close to the source voltage ( 2V), and since the gate-source voltage Vgs is less than the threshold voltage (because of the presence of the Zener diode D28, the voltage is clamped at about  12V), the MOS  transistor is in the cut-off state, and there is no output at the output OUT.

Conducting state

When a low level signal is applied to the base of Q63 (e.g. through an external control circuit), Q3 conducts, and its collector voltage is pulled low. At this time, the gate voltage of the MOS transistor Q62 is grounded through R98, the-source voltage Vgs is greater than the threshold voltage, the MOS transistor conducts, and the power supply supplies power to the load RL through the MOS transistor, and output OUT outputs a  24V voltage.

So, how does it protect the circuit?

Resistor R95 is the current sensing resistor, a resistance of 0.7 Ω. When the load current exceeds 1A or the output is short-circuited, the voltage drop across the sensing resistor R95 will exceed 0.7V. At this time, Q63 is in saturation, and the saturation voltage drop between its collector anditter is about 0.3V. Since the emitter and collector of the bipolar transistor are connected in parallel with the source and gate of the MOS , at time, the gate-source voltage of the MOS  transistor Q62 is clamped at about 0.3V, Vgs no longer meets the turn- condition, PMOS transistor Q62 is turned off, and the subsequent output is cut off, which can effectively protect the load and subsequent circuits