Recycling DC?

[OpenCircuit]

I have been tracing this board (ATX Switching Power Supply) trying to understand operation in segments. This one section (below) has me wondering what is going on- ok many of the sections, but this one in particular. I realize it might be difficult to explain not knowing what is beyond the two diodes and the two rectifiers. It looks like DC is flowing back into the rectifier bridge. If so, why? If, wrong then what is the purpose?

My wildest explanation is overflow of DC voltage flowing back into the AC circuit to be recycled raising the efficiency rating, reducing heat and or avoiding DC overload...? Reality, I simply do not know.

[tatus1969]

sorry, but I don't get the point of what you mean. Maybe you can show the circuit elements that you refer from a picture of the actual PSU.

At least that I can say: there is no such thing as energy flowing back into the AC part. Typical PSUs have the following parts:
- input filtering
- AC -> DC (rectifier plus capacitor)
- optionally active PFC, which is a boost converter sitting between rectifier and capacitor
- high frequency DC -> AC
- transformer
- AC -> DC (rectifier, inductor [in most cases, playing an additional role], capacitor)
- output filtering
plus control circuits.

[sleemanj]

I think what you are saying is that before the input bridge rectifier you are seeing a diode attached to each of the live and neutral going somewhere?

If I had to make an uneducated guess, and assuming that your description is correct, I'd say for detecting the line frequency or some such.

Notice that in your thought bubble you have the component and the symbol reversed with respect to each other, the line/band marks the cathode of the diode.

[sleemanj]

I think what you are saying is that before the input bridge rectifier you are seeing a diode attached to each of the live and neutral going somewhere?

If I had to make an uneducated guess, and assuming that your description is correct, I'd say for detecting the line frequency or some such.

Notice that in your thought bubble you have the component and the symbol reversed with respect to each other, the line/band marks the cathode of the diode.

[sleemanj]

I think what you are saying is that before the input bridge rectifier you are seeing a diode attached to each of the live and neutral going somewhere?

If I had to make an uneducated guess, and assuming that your description is correct, I'd say for detecting the line frequency or some such.

Notice that in your thought bubble you have the component and the symbol reversed with respect to each other, the line/band marks the cathode of the diode.

[OpenCircuit]

Ignoring my thought bubble. Sleemanj, would the signal be traveling left through the diodes (S1G; sorry not SG1) or right back into AC current? I have confirmed the bands are on the left.

P.S. Thanks for the responses.

[OpenCircuit]

My $0.02: they are diodes to bring line voltage signal to PFC controller. PFC controller needs to know mains voltage in order to control input current, it needs to sample mains voltage at some point.

Perfect, thanks!