How to use MOS tube to achieve slow start? Why is slow start needed?

There are two ways to implement a slow start circuit. One is to select a chip with a slow start, and the other is to use a MOS tube to implement a slow start circuit.

 Soft start circuit with chip

Today we will look at the second method (using MOS tubes) to see how MOS tubes achieve slow start-up.

 This circuit is composed of D1, R1, C1 and Q10.

①D1 is a voltage-stabilizing diode to prevent excessive input voltage from damaging the P-MOS tube;

②The function of C1 and R2 is to realize the anti-jitter delay function;

③The function of R1 is to provide a fast discharge circuit for C1, which requires that the voltage divider value between R1 and R2 must be greater than the MOS conduction voltage.

The switch tube is PMOS, and the conduction condition is Ug ＜ Us. It is simply assumed that Ug=Us is cut off.

In this circuit, resistors R1, R2 and C1 form a voltage-dividing RC time constant circuit, and C1 is connected in parallel between the GS poles of Q10.

Circuit principle:

When the 12V power supply is turned on, C1 is not charged, Vgs=0, and MOS is not turned on.

After a while, 12V charges C1 through R2. When the voltage of C1 reaches Vth, MOS starts to conduct. At this stage, the function of delayed power-on is completed.

After the MOS tube starts to conduct, Vgs continues to increase (finally around -10V), the tube opens quickly, and the output voltage of the slow start gradually increases until it is basically consistent with the input voltage. In this process, the output voltage does not jump to the highest value instantly, so the interference of the impact current is greatly reduced.

It should be noted that:

1. The time constant circuit of the slow start must ensure that after the capacitor is charged, its voltage cannot be greater than its Vgs maximum voltage range.

2. The delay time of slow start should not be too long

Under what circumstances would a slow start circuit be used?

The first situation is that during the hot plugging process, after the mechanical contact of the two connectors, the contacts bounce instantly, causing power supply instability and oscillation.

The second situation is that there are filters or large electrolytic capacitors in the circuit, which generate large pulse currents when powered on, causing damage to the chip.