The function and connection of optocoupler in switching power supply
Commonly used optocoupler models for feedback include TLP521, PC817, etc. Taking TLP521 as an example, this article introduces the characteristics of this type of optocoupler.
The primary side of TLP521 is equivalent to a light-emitting diode. The larger the primary current If, the stronger the light intensity, and the larger the current Ic of the secondary transistor. The ratio of the current Ic of the secondary transistor to the current If of the primary diode is called the current amplification factor of the optocoupler, which varies with temperature and is greatly affected by temperature. The optocoupler used for feedback utilizes the principle that "changes in the primary current will cause changes in the secondary current" to achieve feedback. Therefore, in situations where the ambient temperature changes dramatically, due to the large temperature drift of the amplification factor, feedback should be avoided as much as possible through optocouplers. In addition, when using such optocouplers, attention must be paid to designing peripheral parameters to operate within a relatively wide linear band. Otherwise, the sensitivity of the circuit to operating parameters is too strong, which is not conducive to the stable operation of the circuit.
Usually TL431 combined with TLP521 is chosen for feedback. At this point, the working principle of TL431 is equivalent to an internal voltage error amplifier with a reference of 2.5 V, so a compensation network needs to be connected between pin 1 and pin 3.
The first common method of optocoupler feedback is shown in Figure 1. In the figure, Vo is the output voltage and Vd is the supply voltage of the chip. Connect the COM signal to the error amplifier output pin of the chip, or connect the internal voltage error amplifier of the PWM chip (such as UC3525) to the in-phase amplifier form, and connect the COM signal to its corresponding in-phase terminal pin. Note that the ground on the left is the output voltage ground, and the ground on the right is the chip power supply voltage ground. The two are isolated by optocouplers.
The working principle of the shown connection method is as follows: when the output voltage increases, the voltage at pin 1 (equivalent to the reverse input terminal of the voltage error amplifier) of TL431 increases, the voltage at pin 3 (equivalent to the output terminal of the voltage error amplifier) decreases, the primary current If of optocoupler TLP521 increases, the output current Ic at the other end of optocoupler increases, the voltage drop on resistor R4 increases, the voltage at pin com decreases, the duty cycle decreases, and the output voltage decreases; On the contrary, when the output voltage decreases, the adjustment process is similar.
The second common connection method is shown in Figure 2. Unlike the first connection method, in this connection method, the fourth pin of the optocoupler is directly connected to the output terminal of the error amplifier of the chip, and the voltage error amplifier inside the chip must be connected in a form where the potential of the in-phase terminal is higher than that of the in-phase terminal. By utilizing a characteristic of the operational amplifier - when the output current of the operational amplifier exceeds its current output capacity, the output voltage value of the operational amplifier will decrease, and the larger the output current, the more the output voltage will decrease. Therefore, in the circuit using this connection method, it is necessary to connect the two input pins of the error amplifier of the PWM chip to a fixed potential, and the same direction terminal potential must be higher than the reverse direction terminal potential, so that the initial output voltage of the error amplifier is high.
