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What is the reason for using optocouplers in switch mode power supplies?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.16
With the development of the electronic industry and the increasing intelligence of life, the demand for switch mode power supplies is increasing. As an essential power source in modern electronic technology, switch mode power supply has many advantages such as high efficiency, small size, adjustable voltage and output negative pressure. Do you know the reason for using optocouplers in switch mode power supplies?


The optocoupler inside the switching power supply


Reasons for using optocouplers in switch mode power supplies:

The main advantages of optocouplers are: unidirectional signal transmission, complete electrical isolation between the input and output terminals, no impact of the output signal on the input terminal, strong anti-interference ability, stable operation, contactless, long service life, and high transmission efficiency. Optocouplers are a new type of device developed in the 1970s and have been widely used in electrical insulation, level conversion, inter stage coupling, drive circuits, switch circuits, choppers, multi harmonic oscillators, signal isolation, inter stage isolation, pulse amplification circuits, digital instruments, long-distance signal transmission, pulse amplification, solid-state relays (SSRs), instruments, communication equipment, and microcomputer interfaces. In a single-chip switching power supply, a linear optocoupler can be used to form an optocoupler feedback circuit, which can adjust the control terminal current to change the duty cycle and achieve precise voltage regulation.

The main function of optocouplers in switching power supplies is to isolate, provide feedback signals, and act as switches. The power supply of the optocoupler in the switching power supply circuit is provided by the secondary voltage of the high-frequency transformer. When the output voltage is lower than the voltage of the voltage regulator, it is connected to the signal optocoupler, increasing the duty cycle to increase the output voltage; On the contrary, turning off the optocoupler reduces the duty cycle, resulting in a decrease in output voltage. If the secondary load of the high-frequency transformer is overloaded or there is a fault in the switch circuit, there will be no optocoupler power supply provided, and the optocoupler will control the switch circuit to not vibrate, thereby protecting the switch tube from breakdown and burning. Usually, optocouplers are used together with TL431. Two resistors are connected in series and sampled to the 431R terminal for comparison with an internal comparator. Then, based on the compared signal, control the resistance of the 431K terminal (anode connected to the optocoupler) to ground, and achieve the brightness control of the internal light-emitting diode of the optocoupler. The intensity of light emitted is measured by one side of the optocoupler, which is a light-emitting diode and one side is a photosensitive transistor. By controlling the resistance at the CE end of the other transistor, the LED power driver chip will automatically adjust the duty cycle of the output signal to achieve voltage stabilization.

The 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 edge of TLP521 is equivalent to a light-emitting diode. The larger the primary edge current If, the stronger the light intensity, and the greater 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 coefficient 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 the optocoupler. In addition, when using such optocouplers, attention must be paid to designing peripheral parameters to operate within a relatively wide linear band, otherwise the circuit's sensitivity to operating parameters is too strong, which is not conducive to the stable operation of the circuit.

The above is the reason for using optocouplers in switch mode power supplies. I hope this article is helpful to everyone.
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