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Home > Technology List > Analysis of the optocoupler circuit of switching power supply

Analysis of the optocoupler circuit of switching power supply

Published time: 2018-08-30

Who is the optocoupler

Optocouplers, also known as opto-isolators,optical coupler or photoelectric coupler.

It is a device that transmits electrical signals by medium of light. The illuminator (infrared LED) and the photoreceptor (photosensitive semiconductor tube) are usually packaged in the same package

When the input terminal is powered,the light emitting diode emits light, and the phototransistor generates light current after receiving the light, which flows out from the output end, thereby achieving "electrical-optical-electrical" conversion. A typical application circuit is shown in Figure 1.

A typical application circuit diagram

Figure 1 A typical application circuit diagram

What are the characteristics of optocouplers

Optocouplers are widely used in daily life,for example:

1.The optocoupler can form various logic circuits. Since the anti-interference performance and isolation performance of the optocoupler are better than those of the transistor,it is more reliable to form the logic circuit formed.

2.In the switching circuit, it is often required to have good electrical isolation between the control circuit and the switch, which is difficult for a general electronic switch, but it is easy to implement with a photocoupler.

What are the advantages of optocouplers  Jotrin Electronics will tell you the advantages of optocoupler.

The main advantages of optocoupler are: 

1.signal one-way transmission

2.The input end and the output end fully achieve the complete electrical isolation between the front end and the load

3.The output signal has no influence on the input end,to reduce circuit interference, make circuit design easy, make the operation stably

4.non-contact, long service life and high transmission efficiency. 

Optocouplers are new components that is developed in the 1970s.Optocouplers are now widely used in electrical insulation, level shifting, interstage coupling, drive circuits, switching circuits, choppers, multivibrators, signal isolation, interstage isolation, Pulse amplifier circuit, digital instrumentation, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrumentation, communication equipment and computer interface. 

In the monolithic switching power supply, the optocoupler feedback circuit can be constructed by using a linear optical coupler, and the duty ratio is changed by adjusting the current of the control terminal to achieve the purpose of precision voltage regulation.

Typical optocoupler circuit-base on the analysis of TLP521,PC817

The Common used in feedback optocoupler models are TLP521, PC817,etc. Jotrin Electronics limited will take TLP521 as an example to introduce the characteristics of the optocouplers. 

The image shows the internal structure of the optocoupler and the pin diagram at below.

the internal structure of the optocoupler and the pin diagram

Figure 2 the internal structure of the optocoupler and the pin diagram.

The primary side of the TLP521 is equal 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 secondary triode current Ic to the primary diode current If is called the current amplification factor of the optocoupler, which varies with temperature and is greatly affected by temperature. 

The optocoupler that be used for feedback optocoupler uses the principle "primary current change will cause the secondary side current to change" Then achieve the effect of feedback. Therefore, in the case of severe changes in ambient temperature, since the temperature drift of the amplification factor is relatively large, it should be avoided to achieve the feedback through optocoupler. 

In addition, Jotrin Electronics limited reminds everyone that the use of such optocouplers must pay attention to the design of peripheral parameters, make it works in a relatively wide linear band, otherwise the circuit is too sensitive to operating parameters, which is not conducive to the stable operation of the circuit.

TL431 is usually selected for feedback in combination with TLP521. At this time, the working principle of TL431 is equivalent to a voltage error amplifier with internal reference of 2.5 V (the output voltage is compared with the error amplification, and then the sampling voltage is controlled by the photoelectric coupling to control the pulse width duty ratio to achieve the purpose of stabilizing the voltage) Therefore, between the 1st and 3rd feet, the compensation network should be connected.The Figure 2 shows the TL431 pinout and wiring diagram.

The common feedback optocoupler is the first connection

Figure 3 The first connection for feedback optocoupler 

The common feedback optocoupler is the first connection. Vo is the output voltage and Vd is the supply voltage of the chip. The com signal is connected to the error amplifier output pin of the chip. 

Note that the ground on the left is the output voltage ground, and the ground on the right is the power supply voltage of the chip. They are isolated by optocouplers. 

The operate principle of the connection shown in Figure 3 is as follows: When the output voltage rises, the voltage of pin 1 of the TL431 (corresponding to the inverting input of the voltage error amplifier rises), and the pin 3 (corresponds to the output pin of the voltage error amplifier) voltage drop, the primary current If of the optocoupler TLP521 increases, the output current Ic of the other end of the optocoupler increases, the voltage drop across the resistor R4 increases, the com pin voltage decreases, the duty cycle decreases, and the output voltage decreases. Conversely, when the output voltage is reduced, the adjustment process is similar.

In the form of higher potential than the inverting terminal, one of the characteristics of the op-amp is used. When the output current of the op amp is too large (beyond the op amp current output capability), the output voltage of the op amp will decrease, and the higher the output current, the more the output voltage will drop. Therefore, Jotrin Electronics limited said that the circuit of this connection method must connect the two input pins of the error amplifier of the PWM (Pulse Width Modulation) chip to a fixed potential, and the potential of the same direction should be higher than the reverse direction. The terminal potential makes the initial output voltage of the error amplifier high.

The working principle of the connection shown in Figure 3 is: when the output voltage rises, the primary current If increases, and the output current Ic increases. Since Ic has exceeded the current output capability of the voltage error amplifier, the com pin voltage drops. The duty ratio is reduced and the output voltage is decreased; conversely, when the output voltage is decreased, the adjustment process is similar.

The third common connection for feedback optocoupler is shown in Figure 4.Basically this connection is similar to the first one, the difference is that there is an additional resistor R6. The function of this resistor is that injectting an additional current into the TL431 to prevent the TL431 from working properly due to the injection current being too small. In fact, if the resistance value R3 is appropriately selected, the resistor R6 can be omitted. The adjustment process is basically consistent with the first connection.

The fourth common connection is shown in Figure 4. The connection method is similar to the second connection method. The difference is that a resistor R4 is connected between the com terminal and the photocoupler pin 4, and its function is consistent with R6 in the third connection method, and the working principle is similar to the second connection

The third common connection for feedback optocoupler

Figure 4 The third common connection for feedback optocoupler 

The feedback modes 1, 3 are applicable to any duty cycle (the ratio of the on-time to the cycle), and the feedback modes 2 and 4 are more suitable for use in a case where the duty ratio is relatively small.

The Summary of article

Jotrin Electronics summarize that the optocoupler of the switching power supply is mainly isolated, providing feedback signals and switching functions. The power supply of the optocoupler in the switching power supply circuit is supplied from the secondary voltage of the high-frequency transformer.When the output voltage is lower than the voltage of the Zener diode, the signal optocoupler is turned on, and the duty ratio is increased to increase the output voltage; Turning off the optocoupler reduces the duty cycle, causing the output voltage to decrease. When the secondary load of the high-frequency transformer is overloaded or the switch circuit is faulty, there is no optocoupler power supply. The optocoupler controls the switch circuit to not start, thus protecting the switch tube from being broken down and burned.

Related electronic components 
The introduction of TLP521
The TOSHIBA TLP521-1, -2 and -4 consist of a photo-transistor optically coupled to a gallium arsenide infrared emitting diode.
The TLP521-2 offers two isolated channels in an eight lead plastic DIP package, while the TLP521-4 provides four isolated channels in a sixteen plastic DIP package.
• Collector-emitter voltage: 55 V (min)
• Current transfer ratio: 50% (min)
Rank GB: 100% (min)
• Isolation voltage: 2500 Vrms (min)
• UL recognized: UL1577, file no. E67349
• c-UL recognized: CSA Component Acceptance Service No. 5A

Buy TLP521 online

The introduction of TL431

The TL431 is a controllable precision regulated source produced by Texas Instruments. Its output voltage can be arbitrarily set to any value from 2.5V to 36V with two resistors. The device's typical dynamic impedance is 0.2Ω, which is used in many applications to replace Zener diodes, such as digital voltmeters, op-amp circuits, adjustable voltage supplies, switching power supplies and more.


• Adjustable Voltage and Current Referencing respectively. 
• Secondary Side Regulation in Flyback SMPSs 
• Zener Replacement
• Voltage Monitoring 
• Comparator with Integrated Reference 

The TL431 datasheet and pdf 


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