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I am designing a 600W halfbridge converter with a voltage mode controller(TL494). I am using a type 3 compensator with TL431 as shown in the image below:enter image description here

where:

enter image description here

The capacitor C2 constitutes the collector emitter parasitic capacitance of the optocoupler plus the calculated capacitance for placing a pole with Rpullup. My question is, how can I cancel out the pole caused by Rpullup and the parasitic capacitance of the optocoupler so as not to limit my crossover frequency? so that the pole that I can place at the transistor side of the optocoupler comes from Rpullup and the calculated capacitor C2. I intend to use PC817 optocoupler for this design.

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  • \$\begingroup\$ Is this question strictly about transfer functions? Because the issue is best avoided by not needing to deal with it at all -- namely with a current-mode control, which gives far wider phase margin, at higher loop cutoff frequency. \$\endgroup\$
    – Tim Williams
    Commented Jan 8, 2023 at 14:06
  • \$\begingroup\$ @Tim Williams yes it is about transfer functions. I only have access to a voltage mode controller for building a prototype \$\endgroup\$
    – p_karis
    Commented Jan 8, 2023 at 14:10
  • \$\begingroup\$ TL494 can be used current mode -- it's just an uncommitted error amp inside, nothing's preventing you from feeding back current to it! \$\endgroup\$
    – Tim Williams
    Commented Jan 8, 2023 at 18:16
  • \$\begingroup\$ @Tim Williams Do you mean the peak pulse current limiting application? \$\endgroup\$
    – p_karis
    Commented Jan 8, 2023 at 18:22
  • \$\begingroup\$ Consider the case where output filter inductor current is sensed by a Hall effect sensor (assuming isolation such that TL494 is primary referred, gate drive is bootstrap half bridge driver I suppose, and secondary is isolated), and the error amp closes the loop on this current. The current setpoint is then obtained from the optoisolator, and the TL431 controls it to regulate output voltage. No peak pulse current limiting is necessary because the current setpoint is defined, by design, when opto is in cutoff. \$\endgroup\$
    – Tim Williams
    Commented Jan 8, 2023 at 18:27

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