Switching power supply optocoupler role and connection and working principle
1 Several common connection methods and their working principle
Commonly used in the feedback photocoupler model TLP521, PC817, etc.. Here to TLP521 as an example, to introduce the characteristics of such photocoupler.
TLP521 primary side is equivalent to a light-emitting diode, the greater the primary side current If, the stronger the light intensity, the greater the current Ic of the secondary triode. The ratio of the vice-side transistor current Ic to the original diode current If is called the current amplification coefficient of the optocoupler, and the coefficient varies with temperature and is greatly affected by temperature. The optocoupler used for feedback uses "the change of the primary current will lead to the change of the secondary current" to realise the feedback, therefore, in the occasion where the ambient temperature changes drastically, due to the temperature drift of the amplification coefficient, the feedback should not be realised by the optocoupler as much as possible. In addition, the use of such optocouplers must pay attention to the design of peripheral parameters, so that it works in a relatively wide linear band, otherwise the circuit is too sensitive to the operating parameters, is not conducive to the stable operation of the circuit.
Usually choose TL431 combined with TLP521 for feedback. In this case, the operating principle of the TL431 is equivalent to a voltage error amplifier with an internal reference of 2.5 V, so the compensation network should be connected between its pin 1 and pin 3.
Common optocoupler feedback 1st connection, as shown in Figure 1. In the figure, Vo is the output voltage, Vd is the power supply voltage of the chip. com signal is connected to the output pin of the error amplifier of the chip, or the internal voltage error amplifier of the PWM chip (e.g., UC3525) is connected to an in-phase amplifier, and the com signal is connected to its corresponding in-phase pin. Note that the left side of the ground for the output voltage ground, the right side of the ground for the chip power supply voltage ground, the two are isolated by optocoupler.
The working principle is as follows: when the output voltage rises, the voltage of TL431 pin 1 (equivalent to the inverse input of the voltage error amplifier) rises, and the voltage of pin 3 (equivalent to the output of the voltage error amplifier) falls, the primary current If of the optocoupler TLP521 increases, and the output current Ic of the other end of the optocoupler increases, the voltage drop on the resistor R4 increases, and the voltage of the com pin falls, and the duty cycle decreases, and the output voltage decreases; conversely, the com signal is connected to its corresponding in-phase pin. The output voltage decreases; conversely, when the output voltage decreases, the regulation process is similar.
Common second connection, and the first connection is different, the connection of the optocoupler pin 4 directly to the chip's error amplifier output, and the chip's internal voltage error amplifier must be connected to the in-phase end of the potential higher than the inverted end of the potential of the form, the use of op-amps a characteristic - when the output current of the op-amp is too large (more than the current output capacity of the op-amp), the op-amp's output current is too large (more than the current output capacity of the op-amp), the op-amp's output current is too large. When the op-amp output current is too large (more than the op-amp current output capacity), the output voltage of the op-amp will drop, the larger the output current, the more the output voltage drops. Therefore, the use of this connection circuit, be sure to PWM chip error amplifier's two input pins to a fixed potential, and must be higher than the inverted end of the potential, so that the initial output voltage of the error amplifier is high.
