This is a very common circuit in practical applications.

Does anyone know what the specific functions of the two resistors in the circuit are?

We answer the following two questions:

1. 

1. Why is a resistor (R1) connected in parallel with the MOS tube GS?

2. 

3. 

2. Why is a resistor connected in series with the G pole of the MOS tube? (R2)

4. 

Solution 1: Resistors in parallel between GS

Function: used to release the current of parasitic capacitance

Reason: It is related to the parasitic capacitance of the MOS tube. A voltage is applied to the DS terminal to charge Cgd and Cgs. The G pole voltage slowly increases until the MOS tube is turned on.

1. Bleeder resistor

It can prevent ESD static electricity, avoid being in a high-resistance state, causing the MOS tube to malfunction and damage the gate GS of the MOS tube;

2. Fixed bias

When the front-stage circuit is open, the voltage is applied to the DS terminal, charging Cgd, causing the G pole voltage to increase, which can ensure that the MOS tube is effectively turned off.

Resistor value: 5K-10K

If the resistance is too large, it will affect the shutdown speed of the MOS tube. If the resistance is too small, the driving current will increase.

Question 2: Why should a resistor be connected in series with the G pole of a MOS tube?

 The main reason is the parasitic capacitance, and the second is the suppression of oscillation .

Connect a resistor in series with the G pole to form an RC charging and discharging circuit with Ciss, which can effectively reduce the instantaneous current value and avoid damaging the MOS tube. (Ciss = Cgd+Cgs)

In addition, when the MOS tube is connected to the circuit, there will be parasitic inductance generated by the lead, which together with the parasitic capacitance will form an LC oscillation circuit. Connecting a resistor in series can effectively reduce the Q value of the oscillation circuit and avoid circuit failure.

Resistor value: The resistance is recommended to be within 100 ohms, which will reduce the turn-on and turn-off time of the MOS tube and increase the loss.

However, this resistance value is unstable . In the anti-reverse connection circuit, a larger resistance value is also acceptable, which can reduce the power loss during operation.