Figure 1-1 is a general current-mode control converter. This type of non-isolated converters integrate the EA (error amplifier) and Vref as parts of feedback control loop. And Type-2 amplifier network provides optimal compensation in the loop.

On the basis of the composition of TL431, Figure 1-2 shows how it can be used as a standard EA.

The pullup resistor (Rbias) value should be chosen to meet the minimum cathode current requirement, this is to provide sufficient bias current to TL431 under all circuit conditions.

TL431 has been used widely in isolated power supply applications and it provided design benefits of reducing the cost of feedback loop, taking up little board space within the performance tradeoff from the standard error amplifier and a precision reference.

Further detail from Figure 1-3, we now understand TL431 is powered from output of converter. And the amount of current flowing through the Rbias and the optocoupler diode to the cathode of TL431 are driving the gain and operation of the circuit. Figure 1-4 shows typical TL431 in conjunction with optocoupler for isolated feedback control.

As described, the bias current at both primary and secondary side are controlling the gain and frequency response of the circuit. However, it is very complicated analysis and even latest controllers have built the primary side bias, including current source and resistors, inside. Hence, we have options to follow design procedures for bias circuit of secondary side feedback.