Optocoupler is also called optoisolator, optocoupler or optocoupler. It is a device that uses light as the medium to transmit electrical signals. Usually, the light emitting device (infrared light emitting diode LED) and the light receiver (photosensitive semiconductor tube) are packaged in the same shell. When an electric signal is applied at the input end, the light-emitting diode emits light, and after the light is received by the photosensitive triode, the photocurrent is generated and flows out from the output end, thus realizing the “electric optical electric conversion”. Typical application circuit is shown in Figure 1-1.
The main advantages of optocoupler are: one-way transmission of signal, complete electrical isolation of front end and load between input end and output end, no influence of output signal on input end, reduction of circuit interference, simplification of circuit design, stable operation, contactless, long service life and high transmission efficiency. Optical coupler is a new device developed in 1970s. It has been widely used in electrical insulation, level conversion, interstage coupling, drive circuit, switch circuit, chopper, multivibrator, signal isolation, interstage isolation, pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid-state relay (SSR), instrument, communication equipment and microcomputer interface In the mouth. In the single-chip switching power supply, the linear optocoupler can be used to form the optocoupler feedback circuit, and the duty cycle can be changed by adjusting the control terminal current to achieve the purpose of precise voltage stabilization.
Figure 1_ 1 optocoupler switch control DC motor input circuit diagram
2 typical circuit of Optocoupler
Tlp521 and PC817 are commonly used for feedback. Taking tlp521 as an example, this paper introduces the characteristics of this kind of optocoupler. Figure 2-1 shows the internal structure diagram and pin diagram of the optocoupler.
The primary side of tlp521 is equivalent to a light-emitting diode. The higher the primary side current if is, the stronger the light intensity is, and the higher the secondary side transistor current IC is. The ratio of the secondary side transistor current IC to the primary side diode current if is called the current amplification coefficient of the optocoupler, which varies with temperature and is greatly affected by temperature. The optocoupler used for feedback is to realize feedback by using “the change of primary side current will lead to the change of secondary side current”. Therefore, when the ambient temperature changes violently, because the temperature drift of amplification coefficient is relatively large, the optocoupler should not realize feedback as far as possible. In addition, the use of this kind of optocoupler must pay attention to the design of peripheral parameters to make it work in 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.
TL431 and tlp521 are usually selected 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 (the output voltage is used for error amplification and comparison, and then the sampling voltage is fed back through the photoelectric coupler to control the pulse width duty cycle, so as to achieve the purpose of stabilizing the voltage), so a compensation network is connected between its 1 pin and 3 pin.
Figure 2-1 tlp521 internal structure and pin diagram
TL431 is a controllable precision voltage stabilizing source produced by Texas Instruments. The real object is shown in Figure 2-3. Its output voltage can be set to any value from 2.5V to 36V with two resistors. The typical dynamic impedance of the device is 0.2 Ω. It is used to replace the voltage stabilizing diode in many applications, such as digital voltmeter, operational amplifier circuit, adjustable voltage power supply, switching power supply, etc. Figure 2-2 shows the TL431 pin arrangement and wiring diagram.
Figure 2-2 TL431 pin arrangement and wiring
Common optocoupler feedback first connection method. VO is the output voltage and VD is the power supply voltage of the chip. The com signal is connected to the output pin of the error amplifier of the chip. 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 optocoupler. The working principle of the connection method shown in Figure 2-3 is as follows: when the output voltage rises, the voltage of pin 1 (equivalent to the reverse input of the voltage error amplifier) of TL431 rises, and pin 3 (equivalent to the output pin of the voltage error amplifier) When the voltage drops, the primary current if of tlp521 increases, the output current IC of the other end of tlp521 increases, the voltage drop of R4 increases, the voltage of COM pin decreases, the duty cycle decreases, and the output voltage decreases. On the contrary, when the output voltage decreases, the regulation process is similar.
Figure 2-3 the first and second connection methods of optocoupler feedback
The difference between the second connection and the first connection is that 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 the form of the in-phase terminal potential higher than the reverse terminal potential. By using one of the characteristics of the op amp, when the output current of the op amp is too large (exceeding the output capacity of the op amp current), the output voltage of the op amp will decrease The larger the output current is, the more the output voltage will drop. Therefore, the two input pins of the error amplifier of the PWM chip must be connected to a fixed potential, and the same terminal potential must be higher than the reverse terminal potential, so that the initial output voltage of the error amplifier is high. The working principle of the connection method shown in Figure 2-3 is: when the output voltage increases, the primary side current if increases, and the output current IC increases. Because IC has exceeded the current output capacity of the voltage error amplifier, the com pin voltage decreases, the duty cycle decreases, and the output voltage decreases; on the contrary, when the output voltage decreases, the regulation process is similar.
The third common connection method is shown in Figure 2-4. Similar to the first one, the difference is that a resistor R6 is added. The function of this resistor is to inject an additional current into TL431, so as to prevent TL431 from working normally due to too small injection current. In fact, if the resistance value R3 is properly selected, the resistance R6 can be omitted. The regulation process is basically the same as that of 1 connection.
The common fourth connection method is shown in Figure 2-4. This connection method is similar to the second one. The difference is that a resistor R4 is connected between the com terminal and the fourth pin of the optocoupler. Its function is the same as that of R6 in the third connection method. Its working principle is basically the same as that of connection method 2.
Figure 2-4 the third and fourth connection methods of optocoupler feedback
Feedback modes 1 and 3 are suitable for any duty cycle (the ratio of turn-on time to cycle), while feedback modes 2 and 4 are suitable for small duty cycle.
The optocoupler of switching power supply is mainly used to isolate, provide feedback signal and switch. In the switching power supply circuit, the power supply of the optocoupler is provided from the secondary voltage of the high-frequency transformer. When the output voltage is lower than the voltage of the regulator tube, the signal optocoupler is connected, and the duty cycle is increased to increase the output voltage; otherwise, the optocoupler is turned off to reduce the duty cycle to decrease the output voltage. Once the secondary load of the high-frequency transformer is overloaded or the switch circuit is faulty, there is no optocoupler power supply, and the optocoupler controls the switch circuit not to vibrate, so as to protect the switch tube from breakdown and burning.
Figure 3-1 circuit design of LED power driver chip tmg0165
The optocoupler is usually used with TL431. Figure 3-1 shows the circuit design of LED power driver chip (switching power supply chip) tmg0165. Two resistors are sampled in series to tl431r end for comparison with internal comparator. Then according to the compared signal, the resistance of tl431k terminal (anode connected to optocoupler) to ground is controlled, and the brightness of LED inside optocoupler is controlled. (inside the optocoupler, there is a light-emitting diode on one side and a photosensitive triode on the other). Controlling the resistance of the CE end of the other end of the triode changes the current of the LED power driver chip (switching power chip) tmg0165 detection pin (1 pin: voltage feedback pin, which adjusts the duty ratio by connecting the optocoupler to the ground). According to the size of the current, LED power driver chip (switching power chip) tmg0165 will automatically adjust the duty cycle of the output signal to achieve the purpose of voltage stabilization.