Optocouplers - What data to they transmit.....

[Paul C]

Currently working on a DVD PSU, does not seem to complex a PSU but the low voltage & high voltage are linked by an optocoupler & it got me thinking, what data is this optocoupler passing between the 2 parts of the board? I know the basic function of an optocoupler & why they are used but I'm trying to understand its function in this PSU.

I might need some assistance in repairing this PSU so I'll post another thread in the correct section when I get some time to look at it

 

[KrisBlueNZ]

The optocoupler provides feedback that is necessary for voltage regulation.

When the power supply starts up, the output voltage starts to rise. As it approaches the set output voltage, a voltage monitor (often a TL431) in the output circuit starts to pass current through the LED in the optocoupler. This causes the optocoupler's phototransistor to conduct, which provides an indication to the power supply controller IC on the primary side, telling it to back off the amount of energy it is driving into the transformer. The higher the output voltage goes, the more the optocoupler is activated.

So the circuit reaches an equilibrium at the desired output voltage.

The gain from the output voltage error, through the optocoupler, and into the controller, is quite high, so a small error in output voltage causes a significant change in the control IC's output. This is needed to keep the regulation tight.

Optocouplers do not have a linear transfer characteristic, but as long as the transfer characteristic is monotonic, regulation is achieved.

 

[Paul C]

Excellent answer! Thanks Kris