Control Engineering Basics
The output-voltage feedback loop of a buck DC-DC converter with a switching frequency of 500 kHz is being designed. Which of the following is the usual guideline for choosing the gain crossover frequency?
- AAt least ten times the switching frequency (5 MHz), so that it responds quickly to sudden load changes
- BEqual to the switching frequency, 500 kHz, so that the loop is synchronized with the switching and suppresses the output-voltage ripple best
- CWell below the switching frequency, for example at or below about 1/10 to 1/5 of it (50 to 100 kHz)
Show answer and explanation
Answer: C. Well below the switching frequency, for example at or below about 1/10 to 1/5 of it (50 to 100 kHz)
A switching converter can change its duty cycle only once per period, so it is a system that contains a kind of sampling. Bringing the gain crossover frequency close to the switching frequency loses phase margin through this delay and the ripple, so the usual guideline is about 1/10 to 1/5 of the switching frequency or lower, 50 to 100 kHz or lower in this example. Within that range, the higher the crossover frequency, the smaller the output-voltage excursion on a sudden load change. The output LC filter has a second-order resonance with a phase lag of up to 180°, so phase compensation is used to secure the phase margin.
- A. Above the switching frequency the duty cycle cannot be changed, so there is no control.
- B. At the switching frequency the ripple component enters the loop, and stable control is not possible.
- C. Correct. This avoids the delay due to sampling and the effect of the ripple.