Opto-Isolator as a feedback element in an Isolated Flyback Converter

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pradhan.rachit

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Hello all.
I am working on a microcontroller based flyback converter. Since the flyback converter is an isolated DC-DC Converter, it is using an opto-isolator as a feedback element.

Attached with this post is an image from a Texas Instruments Application Report of a TL494 based multiple output flyback converter.




I want to use the same method for the feedback in the microcontroller based converter, but my question is.

"How is a voltage feedback signal given via an opto-isolator?" 8-O
 

Every time the 5v3 output reaches ~5.0v the voltage divider formed by R1/R2 makes U2 conduct, and consequently the opto input start conducting. At output side of the opto, the saturated conduction of transistor is felt as a "digital" information after error comparators inside TL494, meaning that the switching device Q2 must stop immediately.
 

Looking at the opto spec, the CTR is 50% as an emitter follower gives a linear response Iin*50% = Vout/Re.

in thus example the opto is monitoring the 5v main output with 4k series and 1.2k parallel across the IR input diode which has a Vf of 1.2V. Thus the equivalent circuit is a voltage divider with a 1.2V diode offset load.

Thus 5V output supplies current based on voltage difference and equivalent drop.
(5-1.2V)/4k= 0.95mA per 5V. This is an approximation. Since Re is also 4k , the emitter voltage is 0.5*0.95mA* 4k=1.9V out
 

Not explicitely stated in the question, but I presume you are looking for an analog isolator with tightly tolerated transfer ratio. Depending on the tolerance and linearity requirements, linear optocouplers with feedback like IL300 or analog isolation amplifiers should be considered. Highest performance isolation amplifiers have internal A/D + D/A conversion. Using an A/D on the secondary with digital isolation is also a usual solution for a digitally controlled converter.
 

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