Christophe Basso is an Application Engineering Director at ON Semiconductor in Toulouse, France, where he leads an application team dedicated to developing new offline controllers specifications. He has originated numerous integrated circuits among which the NCP120X series has set new standards for low standby power converters. Read more about Christophe at the end of this article.

QUESTION: Why is it Important to Plot a Power Stage Small-Signal Response?

ANSWER: This is the first question you must ask if you are serious about compensating a power supply. Too often, I have seen engineers building a prototype and throwing arbitrarily-selected component values at the error amplifier, hoping it would let the power supply at least stabilize after start up. Then, by tweaking compensating components values on-the-fly as the output undergoes a transient step, the power supply is more or less stabilized by taming undershoots and ringing portions. A few prototypes later, the design is validated for pilot run and here we go for mass production!

This is a scenario that I have seen many times while visiting power supply designers as an application engineer for ON Semi. Even if trials and errors must absolutely be banned when it comes down to loop control, I cannot blame these gentlemen for their method. The reason is simple, 99% percent of an engineer’s time is spent on safety tests, making sure the converter dies peacefully, without smoke – sometimes without noise! – when resistance R236 is open or short circuited or when the controller pin 1 is shorted to pin 2 or even worse, to any of the other pins, including high voltage ones! Believe me, testing and solving for safety is an extremely long and tedious exercise, furthermore if extreme cost and time pressure exists. If you overlook important parts of the design (safety limits, stability margins and so on…) no wonder the telephone rings a few months later, asking the design engineer to urgently fly to the remote factory as most of power supplies do not pass the simple start-up sequence: the overshoot trips the Over Voltage Protection (OVP) circuit and the converter safely latches off. The money the company believed it has saved by cutting the development time, instantaneously vanishes if a factory enters a line-down situation or worse, if a product re-call is necessary. In short, do NOT neglect stability design by thinking that a simple 0.1-µF capacitor across the TL431 will do the job. Spend the necessary time on it, read some of the reference books and you will quickly realize how new tools can make the stabilization process quite simple at the end.

The power stage response is the first thing you need to stabilize your converter. This is how your converter responds to an ac stimulus applied to its control pin while operating in various conditions (light load, full load, high or low line and so on). Without it, there is nothing you can do besides trial and errors as already described. I can see several ways to obtain this transfer function: