The role, connection method and working principle of optocoupler in switching power supply
Several common connection methods and their working principles
Optocoupler models commonly used for feedback include TLP521, PC817, etc. Here we take TLP521 as an example to introduce the characteristics of this type of optocoupler.
The primary side of TLP521 is equivalent to a light-emitting diode. The greater the primary current If, the stronger the light intensity, and the greater the current Ic of the secondary transistor. The ratio of the secondary transistor current Ic to the primary diode current If is called the current amplification coefficient of the optocoupler. This coefficient changes with temperature and is greatly affected by temperature. The optocoupler used for feedback realizes feedback by using "changes in the primary side current will cause changes in the secondary side current". Therefore, in situations where the ambient temperature changes drastically, due to the large temperature drift of the amplification factor, it should be avoided as much as possible through an optocoupler. feedback. In addition, when using this type of optocoupler, attention must be paid to designing the peripheral parameters so that it can operate within a relatively wide linear band. Otherwise, the circuit will be too sensitive to the operating parameters, which is not conducive to the stable operation of the circuit.
Usually TL431 combined with TLP521 is chosen for feedback. At this time, the working principle of TL431 is equivalent to a voltage error amplifier with an internal reference of 2.5 V, so a compensation network must be connected between its pins 1 and 3.
The first common optocoupler feedback connection method, Vo is the output voltage, and Vd is the power supply voltage of the chip. The com signal is connected to the error amplifier output pin of the chip, or the internal voltage error amplifier of the PWM chip (such as UC3525) is connected to a non-inverting amplifier, and the com signal is connected to its corresponding non-inverting pin. Note that the ground on the left is the output voltage ground, and the ground on the right is the chip supply voltage ground. The two are isolated by an optocoupler.
When the output voltage increases, the voltage of pin 1 of TL431 (equivalent to the reverse input terminal of the voltage error amplifier) increases, the voltage of pin 3 (equivalent to the output pin of the voltage error amplifier) decreases, and the primary current If of the optocoupler TLP521 increases. Large, the output current Ic of the other end of the optocoupler increases, the voltage drop on the resistor R4 increases, the com pin voltage decreases, the duty cycle decreases, and the output voltage decreases; conversely, when the output voltage decreases, the adjustment process is similar .
The common second connection method is different from the first connection method in that the 4th pin of the optocoupler in this connection is directly connected to the error amplifier output end of the chip, and the voltage error amplifier inside the chip must be connected to the non-inverting end. The form in which the potential is higher than the potential of the inverting terminal uses a characteristic of the op amp - when the output current of the op amp is too large (exceeds the op amp's current output capability), the output voltage value of the op amp will decrease, and the greater the output current, The more the output voltage drops. Therefore, in a circuit using this connection method, the two input pins of the error amplifier of the PWM chip must be connected to a fixed potential, and the potential of the same direction end must be higher than the potential of the reverse end, so that the initial output voltage of the error amplifier is high.
