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I am looking at datasheets for ICs like FAN4800 or NCP1337. Some of these are PFC boost converters and others are for other types of SMPS topologies.

My confusion is that I can't tell if these ICs are actually supposed to be regulating the output voltage. There is never any mention of PID or compensation techniques to use with these ICs. I am hoping that its all included internally, but I see no hints of a PID system when I look at the internal block diagrams of these parts. Is this correct to assume or do I need to design an external compensation circuit?

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Most controllers require you to implement external loop compensation. This makes sense, since most controllers can be used with a large number of topologies, and a near-infinite number of input power/output power combinations.

Consider the FAN4800 that you mentioned:

enter image description here

On the PFC side, \$V_{FB}\$ is the voltage feedback input, and you can see its external compensation on the \$V_{EAO}\$ pin. Also, \$I_{SENSE}\$ is the current feedback input, and you can see its external compensation between \$V_{REF}\$ and \$I_{EAO}\$.

enter image description here

On the PWM side, \$V_{DC}\$ is the voltage feedback, and the compensation is on the secondary side around the TL431A 'error amplifier'.

There are some rudimentary controllers out there with internal compensation, but you end up extremely limited in terms of powertrain (i.e. fixed frequency buck converters with a specific range of inductor and capacitor values that you are permitted to use.)

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