Analysis and Design of TL431 Feedback Loop

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Analysis and Design of TL431 Feedback Loop [Copy link]

TL431 (as shown in Figure 1) is one of the most commonly used three-terminal adjustable current reference sources. It has good thermal stability and high cost performance. It is widely used in op amp circuits, comparator circuits, ADC reference sources, adjustable voltage power supplies, switching power supplies, etc. It is particularly common in isolated switching power supply circuits. TL431 is often used as an op amp with a linear optocoupler to complete the compensation of the voltage loop. As shown in Figure 2 (L in the figure is a small inductor added to reduce the output voltage ripple). e=4]

Picture 1

[ b][img]https://e2echina.ti.com/resized-image/__size/1230x0/__key/communityserver-blogs-components-weblogfil Figure 2

Briefly introduce the principle of the closed-loop feedback loop of the flyback power supply: Linear optocouplers are only suitable for transmitting low-frequency signals and will produce large transmission errors during the transmission process. In order to eliminate the transmission error of the optocoupler, the error amplifier designed by TL431 is placed on the input side of the optocoupler. Once the output voltage is too high, the reference pin voltage of TL431 increases, which is equivalent to the voltage increase of the reverse input terminal of the op amp. The cathode of TL431 is equivalent to the output terminal of the op amp, and its voltage will decrease. The current flowing through the linear optocoupler diode increases, and the current of the linear optocoupler transistor increases at the same time. The RFB voltage drop increases, and the Vref voltage remains unchanged, so the comp voltage decreases, and the voltage of the reverse input terminal of the internal comparator of UC3843 decreases, the duty cycle decreases, and the output voltage decreases, thereby achieving the regulation of the entire feedback voltage. Among them, R3 is used to set the current through the linear optocoupler diode, R5 is used to increase the current flowing through TL431, R1 and R2 are output voltage sampling resistors, and R4, C1, and C2 form an output voltage compensator. Here, PC817 is used as an example of linear optocoupler. According to the data sheet of PC817, when CTR=120% and If<5mA, the output characteristic of the optocoupler is relatively flat, which will lead to too large a transfer function gain. However, according to the connection method in the figure, the Vce voltage is obtained by dividing the RFB voltage, so there is no problem even if If<5mA. However, If is generally designed to be around 10mA. In addition, TL431 requires IKA>1mA. Since If is already set to 10mA, R5 is ignored here.

Another common connection method is to connect comp directly to the output of the internal op amp of the chip without a pull-up resistor, and use the characteristic that the output voltage drops when the output current of the op amp is too large to achieve closed-loop regulation. If the steady-state duty cycle of the converter is large and the Ic current is small, when Ic<5mA, Ic will be very sensitive to the change of Ib and may become unstable. Therefore, this connection method is only suitable for occasions with a small steady-state duty cycle.

TL431 and UC384X are common combinations of flyback circuits in switching power supply applications. At present, many low-voltage applications have also begun to use boost controllers to build flyback circuits, such as LM5022, LM3481, etc. At present, TI also has some new TL431 products, such as TL431A, TL431B, ATL431, etc., which have better accuracy than the traditional TL431.

In the design circuit, you can use the principle of this article to select the peripheral parameters of TL431 to realize the voltage closed loop.