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How critical is optocoupler's CTR parameter in SMPS feedback?

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chupocro:
Hi all,

I'll have to replace the a burnt optocoupler in computer monitor's SMPS but I can find only PC817 (without X) while the original was EL817X. According to the datasheet the difference is in CTR range:


--- Code: ---817     CTR  50 - 600    <-- I have
817A    CTR  80 - 160
817B    CTR 130 - 260
817C    CTR 200 - 400
817D    CTR 300 - 600
817X    CTR 100 - 200    <-- original
817Y    CTR 150 - 300
--- End code ---

I couldn't find the schematics of the monitor (AOC Q3279VWFD8) but I've found a schematics of another AOC monitor which is very similar. Here are the pictures of the parts that I removed because they were burnt and the schematics where I marked the path of 330V DC which went through the OZ531TG1N controller that was probably the first to be shorted.

I am waiting for NCP1271A controller to replace the original OZ531TG1N and SMK0965 transistors and I'd like to know if replacing the 817X optocoupler with 817 (without X) could cause some trouble.

moffy:
If you measure the CTR of your non 'X' version and it falls within the 'X' version's range then your problem is solved. The CTR of the optocoupler affects the loop gain and therefore stability of the control loop, it sounds like good design practice to go for the limited CTR range of the 'X' version.

wraper:
It should work fine. And you can measure actual CTR for a peace of mind if you want.

chupocro:
I was thinking about measuring the CTR but wasn't sure if the operating point while measuring should be matched with the conditions of the circuit.

I saw this simple straightforward method of measuring the CTR:

https://en.everlight.com/wp-content/uploads/2021/07/General-Purpose-Photo-Coupler-CTR-Measurement-and-Calculation-Ver1.0EN.pdf

and a quite complicated one:

https://www.edn.com/measure-an-optocouplers-ctr/

but I am not sure what exactly would be the reason for using the second method over the first one.

I have many 817 optocouplers and I'll measure them all to select the one with the CTR which is the nearest to the center of the X range.

PCB.Wiz:

--- Quote from: chupocro on March 01, 2024, 01:04:28 am ---I was thinking about measuring the CTR but wasn't sure if the operating point while measuring should be matched with the conditions of the circuit.

--- End quote ---
If you are comparing with the 817X, you need to match the 817X test conditions.

The opto is used in amplified mode, within a high gain difference loop, so the CTR has only a minor affect on the regulation.
If it's easy to select one, then doing so gives you more confidence to look elsewhere is there are still issues.

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