What is a soft start circuitry?

[rockycheng]

Hello everyone,

I want to know the principle of soft start circuitry, which could be used in DC-DC converters and power amplifiers. Could anyone offer some papers or websites? Thank you very much!!


Best Regards
Rocky

 

[IanP]

Re: What is soft start?

Here is how you may describe the soft start in DC-DC converters (and not only):

In DC-DC converters the Soft Start may be used to program how slowly the converter turns on. In the most cases this is realized by an internal or external capacitor. The voltage on this capacitor clamps the error amplifier, causing the output voltage to ramp up.
At turn on, the capacitor is completely discharged. There is a charge interval during which the converter is completely off. This is known as the turn on delay interval. Then, the capacitor voltage traverses the active region. The capacitor voltage level in the active region depends on the load condition of the DC/DC Converter. If the converter is lightly loaded, the error amplifier voltage required to produce full regulated output is relatively low. Therefore, the transition through the active region is relatively fast. If the converter is fully loaded, the error amplifier voltage required to produce full regulated output is relatively high. Therefore, the transition through the active region is relatively slower.

To visually see how it works see picture below ..

Rgards,
IanP

 

[harrytrinh]

Re: What is soft start?

Hi,

The purpose of soft start is to prevent the output ( voltage or current) inrush!
For DC to DC converter, There are some ways to do it. For example with PWM control you could control duty cycle trhough control the output of the error amplyfier. If the duty cycle is not inrush then the output won't inrush too.
In order soft start the ouput of the error amplifier you could soft start refference signal or soft start directly the output of the error amplifier. Actually there are many way to do it. Here just is the basic idea. Good luck!

 

[rockycheng]

Re: What is soft start?

Thanks!!

How to soft start a Class-D power amplifier? Which node should I clamp at startup?