What is the purpose of adding a capacitor to the source and drain of a MOS tube?

Knight97538

What is the purpose of adding a capacitor to the source and drain of a MOS tube? [Copy link]

In many places, we see a capacitor added to the source and drain of a MOS tube. What is the function of this capacitor?

chunyang

I didn't see any capacitor between DS in the picture posted by the OP. G is the gate.

chunyang

Usually, as shown in the original poster's post, the capacitor between the gate and drain of the MOS tube cannot change suddenly because the voltage across its two ends cannot change suddenly, which will slow down the conduction process of the MOS tube when it is driven. This connection method has two purposes: one is to reduce the surge current that may be generated when the MOS tube is turned on, and the other is to suppress the spatial electromagnetic interference that may be induced by the gate loop of the MOS tube, causing the MOS tube to be mis-conducted.

chunyang

Specific working process:

Assuming that the level at the lower end of R212 is currently high, it is obvious that the MOS tube is in the cut-off state at this moment. At this time, the voltage at both ends of C183 is high on the left and low on the right. When the level at the lower end of R212 becomes low, the voltage at both ends of C183 cannot change suddenly and needs to be discharged through R212. Therefore, the gate potential of U13 can only be gradually reduced, and the conduction process of U13 is therefore slowed down. If there is a large-capacitance capacitive link in the load on the right side of U13, the slow conduction of U13 can reduce the surge current at the moment of U13 conduction.

Similarly, when the original poster's circuit is in a static state, that is, U13 is in a steady state of cutoff or conduction, if spatial electromagnetic interference is coupled in the U13G pole loop, if there is no C183, because the gate capacitance of the MOS tube is much smaller than C183, then the G pole potential may be changed, thereby affecting the current state of the MOS tube, especially when the G pole loop and other nodes in the circuit are at a higher impedance.

chunyang

If there is a large capacitive link in the load, surge suppression needs to be emphasized. The value of C183 is often large, and it needs to form a reasonable RC time constant with the resistance of R212 in order to achieve reasonable surge suppression parameters. If only the suppression of electromagnetic interference is considered, the value of C183 is relatively low. Whether C183 is needed and its specific value need to be comprehensively considered based on circuit characteristics, PCB design, working environment, etc.

Gen_X

It is the soft start function that can indeed reduce the surge.

忠自兵

Awesome, first time seeing you