Flyback stability increased by simply increasing upper divider Resistor.

[cupoftea]

Hi,
The attached (LTspice and PNG) shows how you can get good offline flyback feedback with a TL431. -Better than using an
opamp. [Opamp circuit is the top circuit.] (ie, less components with TL431 circuit, as can be seen). This is due to the so-called "fast lane" feedback path from the Flyback Vout and going through
the opto.
This cannot so easily be arranged with an opamp setup.

Anyway, the great thing about the TL431 based feedback circuit as shown, is that if in current mode, then we can increase the gain
and phase margins by simply increasing the (resistance of the) upper feedback divider resistor (and then of course, adjusting the lower
feedback divider resistor to suit the Vout setting).

Why, when stable feedback can so easily be achieved, with so few components, do people protest against the TL431, and strangely give favour to
opamp based feedback circuits as in the top circuit?

 

[D.A.(Tony)Stewart]

maybe They don't sink as much as they would want to, but enough to cause trouble.
High variability between vendors.

 

[dick_freebird]

Increasing the feedback divider impedance will
pull in a secondary feedback pole frequency, with
the TL431 input capacitance. This might look like
a compensating zero in the bigger picture, pushing
out the unity gain frequency?