Please advise, what is the role of the transistor in the soft start circuit of a switching power supply?

kal9623287

Please advise, what is the role of the transistor in the soft start circuit of a switching power supply? [Copy link]

I would like to ask, this circuit is a soft-start circuit that is often seen in switching power supplies. What is the role of the transistor?

tagetage

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The old BJT process, such as the UCX84X series, does not have a soft start and requires external configuration. However, due to its low cost, it is still widely used in the industrial field.

Reason: When starting, the maximum duty cycle is running, which can easily cause overshoot or even overshoot;

Purpose: To control the duty cycle of PWM from small to large;

Principle: Soft start is achieved by clamping the Vcom pin and disabling the loop during startup.

When it starts, it runs at the maximum duty cycle, and the corresponding output voltage rises quickly, which is easy to cause overshoot or even overshoot. Now the common collector circuit is used to control the rise rate of COMP of UCX843, so as to control the duty cycle of PWM, so that the duty cycle increases from small to large until the voltage of COMP pin is no longer clamped. The circuit that adjusts COMP pin is called soft start circuit.

The dotted box in the figure is a soft start circuit. The transistor Q1 and the peripheral circuit form a common collector circuit. The characteristics of the common collector circuit are: the voltage gain is close to 1, so when the transistor is turned on, the voltage of the emitter of the transistor is approximately the voltage of the base.

When the switching power supply inputs a given voltage, the soft start circuit works, the input voltage charges the capacitor Css through the voltage divider resistor, and the base voltage of the transistor starts to rise from 0. The rising rate is determined by the charging capacitor Cs and the resistor Rss, and the corresponding emitter voltage of the transistor increases synchronously.

The voltage of the COMP pin of UCX843 is clamped, causing the duty cycle of the PWM wave to gradually increase. At this time, the loop PI regulator has no regulating effect.

When the voltage of the COMP pin reaches the maximum voltage, it is no longer clamped. When the base voltage of the transistor increases to the value of VBE+VCOMP, the transistor is turned off and the soft start circuit is completed.

However, the input voltage still charges the capacitor until the capacitor voltage reaches a stable value. At this time, the PI regulator of the loop starts to work, the COMP pin voltage begins to be controlled by the PI regulator, and the circuit enters normal operation.

maychang

What the second post said is completely correct. This is the soft start circuit of the UC384X series power control chip.

led2015

It is used to control the startup speed of the power supply according to the voltage change of the capacitor. By controlling the switching state of the element, the output voltage of the power supply can be smoothly adjusted.

kal9623287

tagetage posted on 2024-9-9 11:49 http://mp.weixin.qq.com/s?src=11×tamp=1725853579&ver=5495&signature=52sBsiIq2GvEbSOJrj- ...

The voltage of the COMP pin of UCX843 is clamped, which makes the duty cycle of the PWM wave gradually increase.

Adjusting the values of these two resistors and capacitors can adjust the startup time. Why does the duty cycle change accordingly?

Where is the duty cycle of the switch tube controlled?

kal9623287

led2015 Published on 2024-9-9 20:02 It is used to control the startup speed of the power supply according to the voltage change of the capacitor. By controlling the switching state of the component, the output voltage of the power supply can be smoothly adjusted...

Why is the startup speed of the power supply controlled according to the voltage change of the capacitor?

Who controls the duty cycle?

kal9623287

tagetage posted on 2024-9-9 11:49 http://mp.weixin.qq.com/s?src=11×tamp=1725853579&ver=5495&signature=52sBsiIq2GvEbSOJrj- ...

Also, how do you set the overshoot current threshold in the soft-start circuit of the UCX843 you mentioned?

tagetage

kal9623287 posted on 2024-9-10 15:10 Why is the startup speed of the power supply controlled according to the voltage change of the capacitor? Who controls the duty cycle?

Where is the duty cycle of the switch tube controlled? -----The chip controls it.

maychang

kal9623287 posted on 2024-9-10 15:10 Why is the startup speed of the power supply controlled according to the voltage change of the capacitor? Who controls the duty cycle?

[Who controls the duty cycle? ]

For the UC384X series power control chips, this problem is particularly troublesome and cannot be explained in one sentence.

This series of chips has two feedback loops, one is the current loop and the other is the voltage loop. The COMP pin is the output of the amplifier in the voltage loop. However, the duty cycle of PWM is not directly controlled by the COMP pin, but the output of the voltage loop controls the current loop, and the current loop controls the duty cycle of PWM.