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Why do so many feedback circuits rely on a optocoupler driver instead of using an isolated amplifier to directly transmit the feedback voltage to the switch controller?

Optocoupler Driver

Isolated Amplifier

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  • \$\begingroup\$ transmitting a digital signal is a lot more reliable than transmitting an analogue signal \$\endgroup\$ – JonRB Mar 29 '18 at 20:39
  • \$\begingroup\$ both circuits are anaglogue. \$\endgroup\$ – Jasen Mar 29 '18 at 20:39
  • \$\begingroup\$ @Jasen could you please explain how the optocoupler driver feedback circuit is analog? \$\endgroup\$ – A.S. Mar 29 '18 at 20:53
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    \$\begingroup\$ explained here. electronics.stackexchange.com/questions/365045/… \$\endgroup\$ – Jasen Mar 29 '18 at 20:55
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Isolation amplifiers can have a high cost. Analog Devices AD202KN is $15, good to 400 HZ at best, and the AD210KN which samples at 500 KHZ is about $30 each. Compare that to the $1.00 USD for many opto-couplers.

Also the isolation amplifiers have a 'conversion' time, as they are transformer coupled. Some INA series use capacitive coupling but the delay to integrate the signal back to analog is still there.

If you need a low cost and very fast responding isolated loop a fast opto-coupler cannot be beat. They can be a thousand times faster then a isolation amp, because they do not have to convert the signal to PWM, then intergrate the signal to convert it back to analog.

The response time of a fast opto-coupler is in the nS range, while a isolation amp may take several uS to 100 uS.

Isolation amps are good for measuring accuracy over a wide dynamic range, with at least 1,500 volts isolation, so you can measure line voltage at 600 VAC with no problem, and measure the current as well.

What isolation amps are NOT good for is servo-loops. They greatly increase the time it takes to correct for a change in voltage, current or load. It can make the circuit 'ring' as it settles down or worse yet the circuit may oscillate because the feedback is too slow.

Also isolation amps often need a modest low-pass filter at the output to remove any conversion noise, well up in the 100 KHZ to 500 KHZ range. Some need buffers as they may not have a substantial drive current. The AD202KN and AD210KN can only source or sink about 1 mA.

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  • \$\begingroup\$ The PS2801 is $0.25 on a reel, and other PC817 clones can be had for $0.10. \$\endgroup\$ – CL. Mar 30 '18 at 7:11
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basically it's cheaper to just use the single output optocoupler, it has a region where it's reasionably linear.

I if there is enough gain before before the opto it will find the linear region and the exact position of the linear region will only result in a small offset in the output.

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  • \$\begingroup\$ So although usually more expensive, an isolated amplifier will result in a slightly better output regulation? \$\endgroup\$ – A.S. Mar 29 '18 at 20:46
  • \$\begingroup\$ maybe better by a microvolt, but because it'd be slower it might result in more ripple. \$\endgroup\$ – Jasen Mar 29 '18 at 20:51
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Both are linear analog (emitter follower vs OA feedback) isolated optos but the IL300 is a poor choice for 3 reasons ;

  • cost reasons IL300=$5 in volume vs LTI4430=$2 and high BW opto ($0.21) 700kHz
  • worse response time good opto choices offer higher BW than IL300 ~100k~300kHz
  • low (unity) gain adds stability challenges for regulator to improve transient error. LTI adjust gain from 0 vs gain from a Vref so it is not as good.
    • no need for LMV431 Adj zener ref to boost gain or "431" ref type which is included in LTI4430 adds inverting 6x gain to Opto out which improves tracking error of output voltage by almost same amount.
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