[SOLVED] Optocoupler selection for feedback for flyback converter

[francis29]

How to choose the optocoupler for feed back for a flyback converter. Is it necessary to use TL431 with optocoupler (As i have seen in almost all flyback power supply)

 

[KlausST]

Hi,

usually the SMPS IC datasheet gives this information.

What SMPS IC and .. manaufacturer do you refer to?

Klaus

 

[francis29]

I am thinking of using UC3842

 

[KlausST]

Hi,

the datasheet gives recommendations what optocoupler to use according loop stabilisation.

But the datasheet can´t give you informations about safety rules. They depend on country. And they depend on safety levels, voltages, environment (temperature, humidity, air pressure...) ...
--> here you need to go through the safety ruels of your country.

Klaus

 

[francis29]

Input voltage range has anything to with the optocoupler selection??

 

[smijesh]

Read the working principle of optocoupler you can learn yourself!!!!

In SMPS feed back circuit , IC feed back pins have some threshold voltage (Say 2.2V). you should match output voltage to this voltage, if any increase in output voltage will reflect on the IC feed back pin then IC will control the output voltage by changing the pulse width. output side of the optocouoler required a voltage level it can be easily archived by TL431 otherwise Zener diode will work.

it is not bright, i wrote intentionally

 

[francis29]

okey
My question is why we have to connect the TL341 to cathode of optocoupler (input) and connect the reference pin of TL431 to a voltage divider of 2.5Volts

 

[schmitt trigger]

Why? Because optocouplers have a very wide CTR ratio, which additionally with operating time it ages (changes).

Therefore you require a local feedback loop to stabilize the optocoupler's transfer function. This is done simply and inexpensively with a TL431, that is the reason you see it used EVERYWHERE.

Now, even then the plain vanilla optocouplers may provide difficulty to stabilize. Therefore many semiconductor companies produce optocouplers which are optimized for PSU feedback have tighter CTR and better electrical isolation.

I've used both Vishay and Toshiba for that purpose, but I don't have part numbers handy right now.

 

[treez]

My question is why we have to connect the TL341 to cathode of optocoupler (input) and connect the reference pin of TL431 to a voltage divider of 2.5Volts

Thin of the tl431 as an opamp error amplifier………I appreciate that the feedback goes back to the non inverting input of the tl431, …but remember that there is an inversion stage at the output (cathode) of the tl431, so it is actually negative feedback , just as in the case of a opamp error amplifier beign used in the feedback loop of an smps.

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The attached is an ltspice simulation so that you can understand how an error amplifier works….the only difference in your smps one is that the error amp sets the level of the peak current into the pwm comparator, and so the duty cycle is set.

The opto just means you pull current through the opto and affect the voltage level into the pwm comparator like that