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I am having a slight confusion regarding the selection compensation pin of the UC3842 IC and the current mode control system given in the datasheet.

  1. Why is there an RC combination at the feedback of the error amplifer? Can I simply use a resistor at that pin?
  2. Secondly as far as I have seen, the optocoupler will allow a current to flow in the right side only if the voltage gets below the threshold. But this will only give either a high or a low voltage at that pin. It wount give the amount by which the voltage got decreased or the real reference as is done in the TL494 IC where the IC gets a propper reference from the voltage divider as shown below. But here from getting either a high or a low how can we figure the measure of change of pwm duty cycle??

enter image description here UC3842: enter image description here Kindly help me out in understanding these two concepts and the working of this IC with optocoupler. Considering I am using the voltage mode of the UC3842 IC. Here is a scehmatic of the changes made in the current and voltage mode control: enter image description here

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  • \$\begingroup\$ I believe you are already aware R+Cs are commonly used for controller compensation. The references go into some detail explaining their calculation. Could you be more specific what your exact point of confusion is? || The diagram you provide shows current mode UC3842; we are at a loss what "voltage mode" means here -- please provide a schematic to illustrate this. \$\endgroup\$
    – Tim Williams
    Commented Jan 28 at 17:04
  • \$\begingroup\$ @TimWilliams actually I am a bit confused regarding these compensations in controller and would be favorable if you could help me out in this. Secondly I have added the example schematic of using this IC in a voltage mode control. \$\endgroup\$
    – kam1212
    Commented Jan 28 at 17:33

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The TL494 PWM controller hosts two op-amps for controlling current and voltage. However, if the vast majority of CC-CV controllers (constant-current and constant-voltage) use ORed op-amps wired in an inverting configuration, the TL494, for an unknown reason, has its two op-amps configured in a non-inverting configuration which makes compensation an extremely unpleasant exercise. It is part of the response I gave on SE and I released a PDF you can download from my page. The \$RC\$ network you see on the schematic is linked to compensation of this converter but I suspect trial-and-error is behind this process.

Regarding the UC384x and an optocoupler, I recommend to stay away from the application circuit you reproduced. The most reliable way is to disable the internal op-amp by grounding its FB pin. You then connect the COMP pin to the 5-V VREF pin via a given pull-up resistance - say 10 kOhms - and you connect the optocoupler from COMP and the GND pin of the IC:

enter image description here

It important here to care about the PCB layout - sorry for the crapCAD schematic : ) the two copper traces from the optocoupler (FB and GND) go side-by-side and the emitter connects to the IC ground which is quiet: do not connect to a noisy point or the converter won't be stable. Same for the collector, go to the COMP pin and have the filter capacitor which introduces a wanted pole, wired very close to the IC. This is an industry-standard configuration that I have seen many times in ac-dc converters.

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  • \$\begingroup\$ the optocoupler circuit you provided makes sense but I wasnt able to understand the one in the application note as in my simulations I wasnt getting results with that. \$\endgroup\$
    – kam1212
    Commented Jan 28 at 17:29
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    \$\begingroup\$ Ok, if you want to check how to make the UC operate in VM - for experiments only - then check slide 137 of my APEC 2019 seminar. \$\endgroup\$
    – Verbal Kint
    Commented Jan 28 at 17:37
  • \$\begingroup\$ Sir do you have link where I can find all your lectures, seminar,books etc. I find your answers very usefull. Secondly why have you mentioned to use it only in experimentation? Cant I use it for my actual PCB design? \$\endgroup\$
    – kam1212
    Commented Jan 28 at 17:40
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    \$\begingroup\$ Of course, it's all free to download from my webpage. \$\endgroup\$
    – Verbal Kint
    Commented Jan 28 at 17:42

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