# Relationship between switching frequency and controller's crossover frequency in buck SMPS

#### Electronic_Maniac

Joined Oct 26, 2017
253
Context: tuning the control loop of a buck SMPS. I specicifally have a TI TPS54260-based circuit in mind, but I think this applies to SMPS design in general.

Quoting from Type 2 compensation network :

Mr. Maniktala suggests that the loop gain transfer function of the switcher intersect
the 0 dB point at approximately 1/6th the switching frequency, with a slope of -1.

I have heard this before from another engineer, but did not get a chance to ask why. What is the reason the crossover frequency of the control network should be a fraction of the switching frequency? Why specifically around 1/5th or 1/6th?

Can someone explain me the reason for this in simple electrical terms ? I am a beginner. So please explain me in that level. Would be really grateful

#### SteveSh

Joined Nov 5, 2019
105
This is not my area of expertise, but I've sat through enough SMPS design reviews to have picked up enough knowledge to be dangerous Simplistically, you don't want the control loop to react to the switching pulses or switching ripple on the output. The control loop, the one that regulates the output voltage, is to compensate for input voltage or output load variations. That's why if you apply a step load change to the output, you'll see the output voltage drop (if the change is an increase in the load), then ramp back up in some fashion to the set point.

#### Electronic_Maniac

Joined Oct 26, 2017
253
This is not my area of expertise, but I've sat through enough SMPS design reviews to have picked up enough knowledge to be dangerous Simplistically, you don't want the control loop to react to the switching pulses or switching ripple on the output. The control loop, the one that regulates the output voltage, is to compensate for input voltage or output load variations. That's why if you apply a step load change to the output, you'll see the output voltage drop (if the change is an increase in the load), then ramp back up in some fashion to the set point.
Thank you. Could you please explain me regarding the switching and cross over frequency relation a bit detail

#### SteveSh

Joined Nov 5, 2019
105
If the switching frequency of your power is 400 KHz for example, you want your control loop to "close" at a frequency much less than that.

One our recent SMPS's had a switching frequency of ~630 KHz. The control loop's 0 dB crossover point was 6.5 KHz, with a phase margin of 30 deg.

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#### Electronic_Maniac

Joined Oct 26, 2017
253
If the switching frequency of your power is 400 KHz for example, you want your control loop to "close" at a frequency much less than that.

One our recent SMPS's had a switching frequency of ~630 KHz. The control loop's 0 dB crossover point was 6.5 KHz, with a phase margin of 30 deg.
Thank you. I am sorry to ask this question. But still, can you explain a little more clearly by what you meant by "you want your control loop to "close" at a frequency much less than that." . A little more explanation would really help me. I am having a tough time understand these concepts. Please.

#### SteveSh

Joined Nov 5, 2019
105
Do you have any understanding of feedback theory, control loops, what a pole and a zero is?