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The switched mode power supply controller IC contains a feedback loop. The output voltage goes into a voltage divider resistor circuit and the output from this voltage divider goes into the controller IC again.

I have three questions that have a very specific answer.

  1. Feedback signal has amplitude but also a frequency due to noise or other factors. What do we do about frequency response of this feedback path? Does it require special analysis and care?
  2. Besides white noise and the actual switching activity of the SMPS system, what else can introduce AC component into the feedback signal?
  3. Must the resistor value be in kohm, or it could be ohm or mega ohm as well?
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    \$\begingroup\$ The switched mode power supply controller IC is involved within the main voltage regulation feedback loop. It doesn't always wholly contain the loop you mention in your question but, it can contain (other) wholly internal loops that serve the main functionality. \$\endgroup\$
    – Andy aka
    Commented Apr 10, 2023 at 13:40

2 Answers 2

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Feedback signal has amplitude but also a frequency due to noise or other factors. What do we do about frequency response of this feedback path? Does it require special analysis and care?

Yes, of course it does for several reasons. The open loop gain and phase must meet certain gain and phase criteria in order for the loop to be stable. The response is often rolled off somewhat above the crossover frequency in order to reduce noise as well.

Besides white noise and the actual switching activity of the SMPS system, what else can introduce AC component into the feedback signal?

Load transients. Coupled EMI.

Must the resistor value be in kohm, or it could be ohm or mega ohm as well?

There's no general requirement on the magnitude of the feedback resistors, but there are system constraints and often constraints for the specific controller used. For example, some controllers have upper limits on feedback resistance because of leakage currents or noise performance. Sometimes system constraints have lower limits to avoid excessive current drain from the source. So you wouldn't want to use 1 ohm resistors for a 20 V output for example.

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  • \$\begingroup\$ Is there a specific name for the analysis of the frequency analysis of the SMPS feedback loops? The youtube videos related to SMPS on PCBs does not go into this topic, it only mentioned it once. \$\endgroup\$
    – gyuunyuu
    Commented Apr 10, 2023 at 19:31
  • \$\begingroup\$ @Quantum0xE7 It's just the application of control theory to the specific case of SMPS. Chris Basso, who often posts here has one example presentation on it here: cbasso.pagesperso-orange.fr/Downloads/PPTs/… and you can also find a lot of good info on his site. \$\endgroup\$
    – John D
    Commented Apr 10, 2023 at 19:37
  • \$\begingroup\$ OK, last thing, is there any reason why would one want to use anything other than a buck converter? Buck seems to be the most common SMPS. \$\endgroup\$
    – gyuunyuu
    Commented Apr 10, 2023 at 22:05
  • \$\begingroup\$ @Quantum0xE7 If you want your output to be higher in voltage than the input you woul d use a boost converter. If you want your output to be somewhere between your min and max input range you would use a buck-boost converter. If you want your output to be negative you would use an inverting buck-boost converter. These are also possible with isolated topologies. There are also power factor correction circuits. SMPS topologies are a complex subject, so some study would be necessary to understand the options. \$\endgroup\$
    – John D
    Commented Apr 10, 2023 at 22:18
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In the switched mode power supply (SMPS) controller, the feedback loop is important for regulating the output voltage. The frequency response of the feedback path needs to be carefully analyzed and taken care of to ensure stable operation. Other than white noise and switching activity, external factors like electromagnetic interference (EMI) can introduce AC components into the feedback signal. The resistor value in the voltage divider can vary and doesn't have to be limited to kohms—it can be in ohms or even megaohms based on the design requirements.

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  • \$\begingroup\$ With linear regulators we just put a couple of capacitors around them and its done, yes? But with SMPS we need to do a whole analysis since they use a feedback path, yes? \$\endgroup\$
    – gyuunyuu
    Commented Jun 13, 2023 at 9:39

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