# Loop compensation of a Buck Converter

I have a buck converter. TPS54260

Vin = 7V-20V Vout = 5V Iload=600mA.

I realise that there is a similar question in this forum but I was not able to get clarity.

I want to understand what do you mean by loop compensation and how to select my output capacitors based on this loop compensation . I gone to youtube videos and seminars for TI. But I am not able to understand it clearly.

Can someone help me understand the the concept of Loop compensation and how to choose my output capacitor value?

Thank you

• To understand loop compensation you first should study control theory and understand how to analyze feedback systems for stability. Then, study power converter control topologies. This converter is a current mode converter so there are 2 loops, and inner current loop and an outer voltage loop. Finally, study making a linearized model of a power converter via state-space averaging or the PWM switch model. When you have all those things you can understand how to compensate a DC-DC converter. It's too broad a subject to answer here. Jul 21, 2019 at 17:13
• During study of all those things if you have specific questions you can post them here and likely get an answer. Until then to get your converter going make use of the guide in the datasheet and/or TI's online Webench tool. Jul 21, 2019 at 17:13

The loop compensation of the buck converter depends on many factors as you can imagine. Below are a few simple statements to help you narrow down your search and find the answers you are looking for:

1. control mode: it corresponds to the way you control the converter. There are many control schemes but the most popular are fixed-frequency voltage- or current-mode-control types.

2. voltage-mode control (VM): the switching frequency $$\F_{SW}\$$ is fixed and the loop adjusts the duty ratio $$\D\$$ via a dedicated pulse width modulator (PWM). This is also known as direct duty ratio control.

3. current-mode control (CM): the switching frequency $$\F_{SW}\$$ is also fixed but the loop adjusts the inductor peak current, indirectly adjusting the duty ratio. Please note that $$\D\$$ is the same for a given operating point whether you operate the converter in VM or CM.

4. operating mode: a switching converter like a buck associates two energy-storing elements. Looking at the inductor current $$\i_L(t)\$$ tells you how the converter operates. If the inductor current does not return to zero during a switching cycle, the converter operates in continuous conduction mode or CCM. If the inductor current goes to zero and remains there for a given duration (deadtime), the converter operates in discontinuous conduction mode or DCM. Usually, a converter operates in CCM at high output current and transitions to DCM as the load is getting lighter.

5. loop response: this is the point of interest here. Before attempting to control a given switching converter and regardless of its operating mode or scheme, you must have on hand its control-to-output transfer function. That is to say, how a small variation (or perturbation) applied to the control variable ($$\D\$$ for instance) propagates through the power stage and creates an output response. A Bode plot will instruct you how the amplitude of the perturbation is affected (magnitude plot in dB) and how its phase is also impacted (phase plot in degrees).

5a. VM: in VM, the buck transfer function is that of a second-order circuit affected by a resonant frequency and a quality factor. You must use a type 3 compensator in your compensation strategy (1 pole at the origin, 2 poles and 2 zeroes). When the buck transitions to DCM, the response becomes that of a 1st order circuit. Your compensation strategy must ensure a seamless transition between modes.

5b. CM: in CM, the power stage response is that of a 3rd-order circuit, A dominant pole in low-frequency and two high-frequency poles located at half the switching frequency. These two subharmonic poles must be damped with an artificial ramp for stability purposes. In light-load conditions, the CM converter remains a 1st order (the subharmonic poles are gone) and this makes the CM converter an easy-to-stabilize converter with a type 2 compensator (1 pole at the origin, 1 pole and 1 zero).

If you want to know more about the compensation of the buck converter, please have a look at this APEC 2019 seminar or study this book for an introduction to switching converters and their small-signal responses. Good luck with your design!

• thank you for the answer
– user220456
Aug 8, 2019 at 11:47