Electronic – switch mode power supply diodes

Either can work correctly if designed properly. If you have a dumb rectifier supply feeding a 7805, then all the rectifier part needs to do is guarantee the minimum input voltage to the 7805 is met.

The problem is that such a power supply only charges up the input cap at the line cycle peaks, then the 7805 will drain it between the peaks. This means the cap needs to be big enough to still supply the minimum 7805 input voltage at the worst case current drain for the maximum time between the peaks.

The advantage of a full wave rectifier is that both the positive and negative peaks are used. This means the cap is charged up twice as often. Since the maximum time since the last peak is less, the cap can be less to support the same maximum current draw. The downside of a full wave rectifier is that it takes 4 diodes instead of 1, and one more diode drop of voltage is lost. Diodes are cheap and small, so most of the time a full wave rectifier makes more sense. Another way to make a full wave rectifier is with a center tapped transformer secondary. The center is connected to ground and there is one diode from each end to the raw positive supply. This full wave rectifies with only one diode drop in the path, but requires a heavier and more expensive transformer.

A advantage of a half wave rectifier is that one side of the AC input can be directly connected to the same ground as the DC output. That doesn't matter when the AC input is a transformer secondary, but it can be a issue if the AC is already ground-referenced.

The "catch" diode on the primary side of a transformer-coupled SMPS serves the same purpose it does on any other inductive load such as a relay. Its reverse voltage rating needs to be at least as high as the voltage across the primary when the switch is on. Its (peak) forward current rating needs to be at least as high as the peak primary current at the moment the switch turns off. The average forward current and the total power dissipation will depend on the switching frequency and the value of the leakage inductance of the transformer.

Note that this assumes that we're talking about a "forward" converter, in which the transformer itself is not intended to store significant energy. In a "flyback" converter, you do not put a catch diode directly across the primary.