In the application of power supply equipment, the PMOS tube is often used as a switch, when the capacitive load of the load is large, the PMOS tube will cause the pre-supply voltage to drop at the moment of opening. Why is that?
The voltage drop of the pre-amp power supply at the moment of opening is caused by the switching speed being too fast, and the charging current at the moment when the MOS tube is turned on. Solve this problemThe point isto reduce the charging speed of large capacitors.
For example, in the post-power supply of some circuits, many circuits are in use at the same time, and many filter capacitors will be connected, resulting in a large total capacitance. When the power supply is suddenly turned on, these capacitors need to be charged, and the capacitance is larger.
Due to the large amount of current required at the beginning, the preamp was too late to provide it, which eventually led to the power supply being pulled down.
So, how can we reduce the capacitance of the bulk to avoid this?
We focus on three main measures (but not limited to):
OneSeries resistance
A series resistor between PMOS and the capacitor, on the capacitor, or before the PMOS, essentially reduces the charging current of the pre-power supply to the capacitor.
As for the series resistor, which we have mentioned many times before, it reduces the charging speed of the capacitor and the corresponding load capacity. However, the series resistors are preceded by PMOSBe carefulThe size of the series resistance is determined by the average value of the maximum load current and the maximum current of PMOS.
IIIncrease the power supply capacitance
Parallel capacitance between gate and source, or between gate and drain. It can reduce the turn-on speed of PMOS, which is to indirectly control the charging current of the capacitor by controlling the channel width of PMOS.
In addition, you can also choose to increase the appropriate size capacitor at the input of the pre-power supply to buffer and suppress the fluctuation of the power supply and reduce the amplitude of the power supply drop.
IIIAdd a power supply voltage regulator or follower
In the face of a sudden increase in load, the voltage regulator can ensure the stability of the voltage output, and the follower can automatically adjust the output voltage through negative feedback.
Okay, that's all for this issue, follow me, and we'll see you next time!
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