I. Basic Parameters of Optical Couplers
An optocoupler's internal structure consists of three basic parts: a light-emitting diode (LED), a light-transmitting insulating layer, and a phototransistor. The LED emits light, which penetrates the insulating layer to the phototransistor, thus achieving current transmission and isolation.
The main parameters of an optocoupler are:
1. Current Transfer Ratio (CTR): The minimum ratio of the current of the LED to the current of the phototransistor.
2. Insulation withstand voltage (transparent insulation layer): refers to the ability of the optocoupler to protect related circuits and itself from physical damage caused by high voltage.
3. LED drive current IF: By using high-efficiency LEDs and high-gain receiver amplifiers, the drive current IF can be reduced. At the same time, a smaller IF current can reduce the power consumption of the system and reduce the attenuation of the LED, thus providing long-term system reliability.
4. Common-mode rejection ratio (VCM): This refers to the maximum rate of rise and fall of the common-mode voltage per microsecond that the optocoupler can tolerate. This parameter is crucial in industrial motor applications. For example, the starting or braking process of a motor can introduce significant common-mode noise.
II. Application of Optocouplers in Switching Power Supplies
The characteristics of optocouplers are: they have unidirectional signal transmission, thereby achieving electrical isolation between the input and output terminals, that is, the output signal has no effect on the input terminal. They have the advantages of strong anti-interference ability, stable operating characteristics, high reliability, and high transmission efficiency, and are usually used in the control circuit of switching power supplies.
The typical application principle of optocouplers in switching power supplies is as follows: the error signal is sampled from the output terminal, and then the signal is transmitted to the PWM controller of the input IC through conversion and isolation. By adjusting the PWM duty cycle, high-precision regulated output is achieved.
The combination of optocoupler and TL431 forms the simplest switching power supply control circuit (feedback circuit) to achieve regulated output, as shown in Figure 2. Vs is the sampling signal provided to the inverting input of the TL431 error amplifier after the output voltage Vo is divided. This sampling signal Vs is converted into a current signal IF through optocoupler diode, TL431, and resistor R1, and then transmitted to the output of optocoupler to form an error signal Vea. This error signal Vea is compared with the triangular wave Vt of the PWM controller to obtain a rectangular pulse (PWM signal Vb with a certain duty cycle). Then, the on and off times of the power stage devices are adjusted to achieve stable output.
III. Feedback Loop Formed by Optocoupler and TL431
The stability of the feedback loop is crucial for switching power supplies. Without sufficient phase and gain margins, the dynamic characteristics of the power supply will deteriorate or directly cause output oscillations, leading to product damage or a shortened lifespan.
When designing the control circuit of a switching power supply, the stability of the feedback loop must be fully considered to give it negative feedback characteristics: in order for the product to remain stable even under the worst conditions, a phase margin of at least 45° is theoretically required.
The TL431 is a commonly used reference and error amplifier device for secondary feedback in switching power supplies. Different power supply methods significantly affect its transfer function. During the research and development phase, engineers typically use loop testing equipment to adjust loop stability, shorten product development cycles, and improve product stability and reliability.
IV. Key Points for Selecting Optical Couplers
The main applications of optocouplers in switching power supplies are to provide electrical isolation between the primary input and secondary output, and to form a feedback control loop when combined with a TL431. Therefore, the following principles must be followed when designing the circuit:
1. Select an optocoupler that conforms to relevant domestic and international standards for isolation breakdown voltage, based on the isolation withstand voltage between the product's input and output.
2. The ideal range for the current transfer ratio (CTR) is 50% to 200%. This is because when the CTR is too low, the LED in the optocoupler requires a larger operating current, which increases the optocoupler's power consumption; when the CTR is too high, it may affect normal output during circuit startup or sudden load changes.
3. Prioritize linear optocouplers because their CTR values have good linear adjustment within a certain range.
