Buck Converter Control Loop Design - Page 1
Forum section: electronics / projects

author: jackbob
author_rank: Regular Contributor
timestamp: April 03, 2019, 06:53:27 am
content: I am designing a buck switching converter, the project is for a class so please don't suggest a purpose-built controller IC. I am designing the controller using a type 3 compensation op-amp circuit and built the circuit in LTspice. The simulation behaved as expected however there was a very large overshoot on the converter output during startup. I tried tweaking as many parameters as I could while maintaining an acceptable gain and phase margin but could not get rid of the overshoot. I then thought of how the compensation circuit worked and thought I could add a small RC circuit to delay the reference voltage seen by the op-amp during startup. The small addition of the RC circuit delays the reference voltage very briefly as the capacitor charges up through the resistor during startup. Once the capacitor is charged, it behaves like an open circuit as if it is not there. This delay of the reference voltage prevents the op-amp compensator circuit from generating a large error during startup.I have included screenshots of both circuits before and after adding the RC delay boxed in red on the second circuit. I also included pictures of the startup transients for both converters to show the difference.My question is will adding this RC circuit to the reference voltage pin change the transfer function or stability of the circuit? Is there a possibility adding this could somehow cause the converter to enter an unstable state? It seems to operate just as expected but I want to convince myself this will not contribute to instability of the system.

author: jackbob
author_rank: Regular Contributor
timestamp: April 03, 2019, 06:57:52 am
content: Quote from: blueskull on April 03, 2019, 06:55:10 amIs that not what adding the RC circuit to the reference voltage does? It's not a ramp but the same idea, it delays the reference voltage. I'm pretty sure I did exactly what you just explained.

author: jackbob
author_rank: Regular Contributor
timestamp: April 03, 2019, 07:24:44 am
content: Quote from: blueskull on April 03, 2019, 07:00:37 amAs I mentioned this is for a school assignment and no maximum component current parameters were given. I realize if this were a real circuit, transient currents that high would likely not go well with components this small. I will play around with the delay to see if I can reduce the current further to be more realistic for components in a converter of this size.I am planning to possibly tweak this design slightly with common component values and actually build this circuit to analyze and learn from.Quote from: blueskull on April 03, 2019, 07:02:00 amI have to use a compensator and PWM for the assignment but I will look into this control topology you mentioned as it sounds very interesting. I plan to specialize in power electronics and I am always excited to learn more.

author: jackbob
author_rank: Regular Contributor
timestamp: April 03, 2019, 07:33:16 am
content: Quote from: blueskull on April 03, 2019, 07:00:37 amWithin a couple minutes I was able to reduce the transient current a significant amount by increasing the delay as you said. Thank you for your help and suggestions! I appreciate it.