As I understand it is hardly ever a good idea to use an op-amp as a comparator, since there are much beter comparators available to complete such a task.
Operational amplifiers are sometimes used as comparators when speed is not required or when precision or some other characteristic unique to an operational amplifier is required.
Most examples I came across, such as in the good old HP e3610 design, they seem to use an LF411 JFET op-amp to compare the measured voltage to a set voltage(which they achieve by changing the resistor divider
The LF411 is being used as an operational amplifier.
Basically the question is, why use an op-amp as the error amplifier when a comparator is supposedly much faster?
That is exactly the reason a comparator cannot be used.
The trick when negative feedback is used is that the gain needs to drop below unity before 360 degrees of phase lag is reached or the circuit becomes an oscillator. Operational amplifiers have frequency compensation to do exactly this while comparators often have the opposite in the form of hysteresis which provides positive feedback to increase gain at high frequencies.
The external negative feedback connection is 180 degrees. The frequency compensation for a typical operational amplifier adds another 90 degrees but reduces the gain at high frequencies so stability is maintained. If the power circuits add too much phase lag, then the control loop will oscillate but this can be solved by adding phase lead in the feedback network or lowering high frequency gain further.
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