As we all know, TL431 in the switching power supply (SMPS) is the reference voltage feedback loop. The device combines a reference voltage and error amplifier open collector with simple operation and low cost advantages. Although the TL431 has long been widely used in the industry, but some designers will ignore the bias current, resulting in the final product inadvertently reduce performance.
TL431 simplified schematic shown in Figure 1, the figure includes the driver NPN transistor voltage reference and error amplifier, the power in the closed system, the part of the output voltage has been working with TL431's Vref (reference voltage) for comparison.

Equivalent circuit of Figure 1 TL431

Figure 2 SMPS simplified DC model (without considering the input fluctuations)
Simplified DC converter model shown in Figure 2, Vout and Vref by the transfer rate through the resistor divider effect by comparing the theoretical value of the output voltage Vref / α. However, the entire gain links and a variety of resistance will affect the output voltage, as follows, each of which represents a gain of both the Greek alphabet, RSOL said the open-loop output impedance.
Vout = (Vref-α × Vout) × β × G-RSOL × Vout / RL (1)
Vout = Vref × β × G / (1 + α × β × G + RSOL / RL) (2)
Static error = Vref/α- Vout = Vref × (RSOL + RL) / [α × (RSOL + α × β × G × RL + RL)] (3)
From (3) can be seen, increasing the value of the gain will help reduce the static error and increase the output voltage accuracy. Affected by the loop gain is another important parameter is the output impedance, the system's output impedance obtained using different calculation methods. Any generator can be reduced to its Thevenin equivalent, that is, a voltage source Vth (no-load measured Vout, main role in type 2 RSOL / RL = 0) with an output impedance of Rth of the series circuit. When the load resistor RL is set loop output impedance Rth, the output voltage Vout can be reduced to the Vth / 2, in order to calculate the output impedance Rth, can be expressed as RSCL. That Vth / 2 = Vout find RSCL, from (2) can be obtained:
Vref × β × G / (1 + α × β × G) / 2 = Vref × β × G / (1 + α × β × G + RSOL / Rth) (4)
RSCL = RSOL / (1 + α × β × G) (5)
By (5) can draw the following conclusions:
If the DC gain error amplifier is greater and DC is higher, RsCL close to zero;
Because of the feedback return path is compensated, so when the gain decreases with increasing frequency, RSCL begins to increase. Impedance module increases with frequency, indicating that the impedance is similar to the inductor;
When the gain is reduced to zero, the system output impedance of the impedance with no feedback, the same, RSOL. At this point, the system open-loop work.
Therefore, in order to reduce the static error, and reduce the dynamics of the converter output impedance, most SMPS design staff will remain in the design of the larger DC gain value. Here's DC gain provided by the TL431 can be used as shown in Figure 3, the pure integrator configuration to connect.

Figure 3 uses a traditional connection TL431 shunt regulator configuration
Suppose in Figure 3 Rbias does not exist. First calculate the divider network Rupp and Rlow, bridging the current Ib to be greater than the TL431 reference pin bias current 6.5A (max), to reduce bias caused by Rupp error. For the 12V output voltage, assuming that Ib = 1mA. Rlow imposed by the TL431 voltage 2.5V, and Rupp applied current 1mA, so you can calculate the Rlow to 2.5 / 1m = 2.5k, and Rupp is the sum of (12-2.5) / 1m = 9.5k. Further choose a smaller bias current to reduce standby power consumption under no-load conditions. Determine the current value of the bridge, you can calculate RS. RS must be able to provide enough current to optocoupler collector (or the feedback pin) is less than 1.2V, to start the no-load work under the skip. In the NCP1200, the pin 2 and the internal 5V reference voltage between a 8k pull-up resistor. If the feedback current is 475A, 2 Pull the pin to 1.2V (Vpin2 = 5-475 × 8k). Taking into account the optical coupler in poor circumstances, 50% of current transfer ratio (CTR), then the RS must be less than (Vout-2.5-1V) / 9508.94k, assumed to be 8.2k.
CTR of 150% of the poor, that it is smaller LED current is required, if the 8.2k resistor in series with the TL431, the following occurs:
1. Light load conditions: IFB = 475A, then IL = 475 / 1.5 = 316A
2. The load: VFB = 2.3V, IFB = 337.5A, then IL = 337.5 / 1.5 = 225A
3. Heavy load: VFB = 3V, IFB = 250A, then IL = 250/1.5 = 166A
In this case, TL431 bias current changes not only with the load current, but also changes with the change in CTR optocoupler. In addition, reducing the RS does not play any role, it should be LED by adjusting the internal current controller to adjust the right side of the feedback voltage. This design problem stems from the TL431 data sheets: must be inserted more than 1mA of bias current to the gain from the different specifications of the TL431 benefit. If you can not correct the bias TL431, will reduce the open-loop gain, resulting in increased, RSCL increases.
This problem can be offset by increasing the resistance Rbias, applying a bias current in the external solution. As the lack of current, it is necessary to calculate the resistance in poor circumstances, that is, the highest CTR heavy load conditions and the time value. Then IL = 166uA. Therefore, RS voltage on the 166 × 8.2k = 1.36V. Assuming LED's forward voltage drop is 1V, the cathode voltage of 12 -1.36-1 = 9.64V. Vout constant known as the 12V, 1mA current through Rbias be applied, Rbias = (12-9.64) / 1m = 2.36k, 2.2k or be a normalized value. Therefore, the minimum is applied in the TL431 current 1mA + 166uA = 1.16mA. With no load, IL = 316 uA, the cathode voltage of 12 - (8.2k × 316) -1 = 8.4V, therefore, the total bias current flowing through the TL431 is (12-8.4) / 2.2k = 1.63mA , together with the actual feedback current 316uA, the total bias current of 1.95mA, should be in safe current range.
Composed of power supply in the NCP1200 there were no bias resistor bias resistance and the experimental results shown in Figure 4. No bias components, the output impedance measurement is 57m; connection bias resistor (resistance to 3.3k), the output impedance down to 4m.

Figure 4 TL431 bias current is too low, performance will decline significantly
In short, through an external resistor bias on the TL431 correctly is very important. If you can not afford the extra 1mA output current budget (due to no-load standby power consumption to minimize the cost), you should use TLV431 (Vref = 1.24V) or NCP100 (Vref = 0.7V), because they only need the minimum bias 100A current and breakdown voltage is even smaller. In addition, 8.2k series resistor RS is extremely rare, because the resistance of the collector with pull-up resistor optocoupler can generate DC gain. If the resistance is about 1k or slightly larger than 1k, even close to the standard value.