Circuit Protection

Thread Starter

Andu2508

Joined Aug 21, 2021
5
Hello.

I'm a novice in electronics.
My circuit includes various ICs and a buck converter. I found that I should use protections at the entrance to the circuit. One of them is this, but I do not understand the role of the divider and how I should calculate the values of the two resistors ( R102 & R101).

If other information is needed, I attach it below. Thank you
1629538966009.png
 

LesJones

Joined Jan 8, 2017
3,506
R101 and R102 are NOT a potential divider. R101 is to limit the current when the voltage between source and ground exceeds 10 volts. (The voltage when the zener diode stars to conduct.) I think R102 is intended to stop any oscillation when the gate is driven from a low impedance source. I don't see that it is required in this case. I have no idea of the purpose of this part of a circuit without the full schematic.

Les.
 

LesJones

Joined Jan 8, 2017
3,506
I assume it is the battery positive connected to the top left. If so the body diode of the mosfet will conduct but the mosfet itself will do nothing as it's polarity is wrong. (source and drain reversed.)

Les.
 

Ian0

Joined Aug 7, 2020
3,463
DMG2301L is a P-channel device, but it is drawn as an N-channel.
The zener is to prevent gate over-votage.
The circuit you have shown isn't the buck-regulator. I think it is the reverse-polarity protection.
(IF it were the buck regulator where's the schottky diode, and the inductor is the wrong side of the capacitor)
 

crutschow

Joined Mar 14, 2008
27,894
DMG2301L is a P-channel device, but it is drawn as an N-channel.
No, it's drawn properly.
Pin 2 is the source and pin 3 is the drain.

The circuit shown in post #1 is for overvoltage protection (D101) and reverse bias protection (P-MOSFET Q100).
The P-MOSFET is connected so that it turns on for a plus input voltage (MOSFETS are bidirectional when on) and blocks a minus input voltage.

EDIT: For your interest, below is the LTspice simulation showing how the P-MOSFET conducts in the forward (plus voltage) direction and blocks the reverse:
The small step at the bottom of the load current (green trace) is where you see the forward voltage of the MOSFET body diode as it conducts before the MOSFET starts to turn on at its Vgs(th) value.


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