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Use Opto-coupler in flyback power supply -Digital or Analog (linear mode)?

chaitanyab

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Hi,

I am reading lot of application notes about the design of TL431 and opto coupler circuit for flyback converter. However none of them tell me how the opto-coupler is used? as digital or analog (linear mode)? I have used opto-couplers for isolation and transmitting digital signals. Can any optocoupler transfer signal in analog? or is it digital and we need to filter the PWM to get the smooth DC for the feedback. I am bit confused here

Thanks
 

crutschow

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Yes, the feedback from the TL431 is usually linear through the opto-coupler.
If opto's are biased in the linear region they can transmit an analog signal.
But in the linear design you have to accommodate the typically large range in transfer gain of the opto.
 

chaitanyab

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Thanks.
How to bias the opto-coupler in the linear region? which graph to use? For instance one of the app note is suggesting to use PC817X ( 73758.pdf (farnell.com) )optocoupler. Now I can see the graph of transistor VCE vs Ic and it probably shows the linear region Figure 8 but how to keep it there as we can only control the Forward current through the photo diode.
Flyback.PNG
 

betwixt

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Thats the whole point. The current through the transistor is controlled by the LED and the LED current is controlled by the PSU output voltage. If the output voltage goes higher, the LED passes more current, the transistor conducts more, the 'FB' pin is pulled lower and that reduces the output voltage. That how the output voltage is stabilized.

Brian.
 

chaitanyab

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Hi Brian,

Thanks. So may be one naïve question why do we need TL431 as a transconductance amplifier? Can't the output voltage directly supply the current via simple resistor to the diode inside the opto-coupler?
 

betwixt

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Yes it can but a small difference in output voltage would only result in a small change in LED current.
Basically, what the TL431 does is lift the bottom of the LED up to a higher but stable voltage, still lower than the output by enough that the LED can operate. Now a small change in output voltage results in a much larger change in LED current and hence more feedback.

Brian.
 

treez

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some on/off controlled designs just have a zener going straight into the opto led...and regulate vout like that.....typically small converters...eg TNY range by power integrations sometimes do this.

But the TL431 has an internal error amplifier and so can control beautifully the vout, via the led......we need to use the opto because we need the sec isolated from the pri...and the TL431/OPTO enables to get an error amp controlled feedback, and yet still maintain the isolation.

If you dont want TL431/OPTO, then you can just have a normal error amplifier and controller on the secondary side, and drive your primary fets via isolation transformers from the secondary...if you want.
--- Updated ---

Attached, i have taken to indulge you in glorious simulation should you so wish.... its an LTspice simulation of an opto used in an offline smps. It doesnt use the TL431, but just an opamp based error amplifier which is pretty much the same thing. LTspice is free download.
Feast your mincers on this and i hope you enjoy!
 

Attachments

  • Flyback _opto_FORUM.zip
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Last edited:

crutschow

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why do we need TL431
To have an accurate voltage reference for the regulated output voltage.
Otherwise the voltage would depend upon any gain changes in the feedback signal through the opto.
It doesnt use the TL431, but just an opamp based error amplifier
The 2.5V reference voltage (V7) at the (+) op amp input serves the same purpose as the TL431 with fewer parts.
 

mtwieg

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TL431 is frequently used for this purpose because it combines the functions of a reference voltage, error amplifier, and transconductance amplifier in one part. Nothing magical about it.
 

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