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MOS tube G pole series small resistance of what is the role, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

We often see that in the power supply circuit, the G pole of the power MOS tube is often connected in series with a small resistor, ranging from a few ohms to dozens of ohms.

As shown above, switching power supply, G series resistance R13

This resistor has two functions: to limit the current of the G pole and to suppress the oscillation.

Limiting G-pole current

The MOS tube is driven by voltage, and the G-level current is very small, but because of the existence of parasitic capacitance, when the MOS tube is turned on or off, because of the need to charge the capacitor, all the instantaneous current is relatively large. Especially in the switching power supply, the MOS tube is turned on and off frequently, so it is necessary to consider the impact of this.

As shown in the figure above, the parasitic capacitance of the MOS tube has three, Cgs,Cgd,Cds

Generally, in the MOS tube specification, the following three parameters are generally marked: Ciss,Coss,Crss, the relationship between them and parasitic capacitance is as follows:

Ciss=Cgs+Cgd

Coss=Cds+Cgd

Crss=Cgd

For example, take MOS FDS2582 as an example:

Ciss=1290pF, as you can see, this parasitic capacitance is considerable.

A simple estimate is made, assuming that Vgs=10V,dt=Tr (rise time) = 20ns, Ciss=1290pF, then the instantaneous current of the G pole when switching is I=Ciss*dVgs/dt = 0.6A.

A resistor is connected in series at the base to form a RC charge-discharge circuit with Ciss, which can reduce the instantaneous current value and avoid damaging the driver chip of the MOS transistor.

Because the increased resistance will slow down the turn-on and on-off time of the MOS tube and increase the loss, it can not be connected too much. This is why the resistance is several ohms or tens of ohms.

Suppression of oscillations

When the MOS transistor is connected to the circuit, there will also be parasitic inductance generated by the lead, which, together with the parasitic capacitor, forms the LC oscillation circuit. For the switch square waveform, there are many frequency components, so it is likely to be the same as or close to the resonant frequency to form a series resonant circuit.

A series of resistors can reduce the Q value of the oscillating circuit, so that the oscillation attenuates quickly and does not cause circuit failure.

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