Switch-Mode Power Supply – Why Use Parallel Diodes on Output

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:

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}\$.

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.)

An SMPS controller¹ IC may support multiple converter topologies. But a single IC almost never supports all common converter types.

There is a family of converters derived from boost converter: flyback, SEPIC, boost proper. A controller that can do boost, usually can do the rest of the boost family too. Usually, one controller can't do both buck family and boost family.

¹ SMPS controller IC drives external power switches. SMPS converter IC has power switches on board. Converter ICs tend to be less versatile than controllers.