0 Introduction In recent years, with the rapid development of power supply technology, switching power supplies are moving towards miniaturization, high frequency, and modularity. High-efficiency switching power supplies are being used more and more widely. Single-ended flyback converters, with their simple circuitry and ability to efficiently provide DC output, are particularly suitable for designing low-power switching power supplies. This paper briefly introduces the UC3842 current-mode pulse width modulator produced by Unirdex, describes its application in single-ended flyback switching power supplies, and provides a detailed analysis of the power supply circuit. The low-power switching power supply designed using the methods described in this paper has been applied to the GKS-9000 distributed control system independently developed by the Industrial Control Branch of NARI Technology Co., Ltd., and is operating well, with all indicators meeting the requirements of the actual project. 1. Basic Principle of Flyback Switching Power Supply The single-ended flyback switching power supply employs a highly stable dual-loop feedback control system (output DC voltage isolation sampling feedback external loop and primary coil magnetization peak current sampling feedback internal loop). This allows for rapid adjustment of the pulse duty cycle via the power supply's PWM (Pulse Width Modulator), effectively regulating the output voltage and primary coil magnetization peak current of the previous cycle within each cycle to achieve stable output voltage. The most significant characteristic of this feedback control circuit is its faster dynamic response speed when input voltage and load current change significantly, automatic load current limiting, and simple compensation circuitry. The flyback circuit is suitable for low-power switching power supplies, and its schematic diagram is shown in Figure 1. The following analysis examines the operation of the current-type PWM under ideal no-load conditions. Compared to voltage-type PWM, current-type PWM adds an inductor current feedback loop. In the diagram: A1 is an error amplifier; A2 is a current sensing comparator; U2 is an RS flip-flop; Uf is the feedback sample of the output voltage Uo. This feedback sample and the reference voltage Uref generate an error signal Ue through the error amplifier A1 (this signal is also the comparison clamping voltage of A2). Assuming the field-effect transistor Q1 is turned on, the inductor current iL increases linearly with a slope Ui/L, where L is the primary inductance of T1. The inductor current is sampled on the non-inductive resistor R1, u1 = R1iL. This sampled voltage is sent to the current sensing comparator A2 and compared with Ue from the error amplifier. When u1 > Ue, A2 outputs a high level, which is sent to the reset terminal of the RS flip-flop U2. The two-input NOR gate U1 then outputs a low level and turns off Q1. When the clock output is high, the NOR gate U1 always outputs a low level, blocking the PWM. Simultaneously with the oscillator output clock decreasing, both inputs of the NOR gate U1 are low, thus turning on Q1. Therefore, as can be seen from the above analysis, the rising edge of the current-type PWM signal is determined by the falling edge of the oscillator clock signal, while the falling edge of the PWM is jointly determined by the trap signal of the inductor current and the error signal from the error amplifier. Its operating timing is shown in Figure 2. A single-ended flyback switching power supply is characterized by the periodic on and off of the main switching transistor. When the transistor is on, energy is continuously stored in the primary winding of the transformer; when the transistor is off, the transformer supplies the load with the inductor energy stored in the primary winding through the rectifier diodes until the next pulse arrives, starting a new cycle. The pulse transformer in the switching power supply plays a very important role: firstly, it realizes the conversion of electric field to magnetic field to electric field energy, providing a stable DC voltage to the load; secondly, it can perform the function of a transformer, outputting multiple different DC voltage values ​​through the primary winding and multiple secondary windings of the pulse transformer, providing DC power to different circuit units; thirdly, it can achieve the electrical isolation function of a traditional power transformer, isolating hot ground from cold ground, avoiding electric shock accidents, and ensuring the safety of the user end. 2. Flyback Switching Power Supply Design The most crucial aspect of switching power supply design is the feedback loop design, as its quality directly determines the accuracy and stability of the power supply. We have already discussed how single-ended flyback switching power supplies use dual-loop feedback. The following section will introduce some issues to consider when designing two types of feedback loops for switching power supplies using the current-mode PWM chip UC3842. 2.1 Output DC Voltage Isolation Sampling Feedback External Loop The UC3842 is a high-performance, fixed-frequency, current-mode pulse-width integrated control chip designed specifically for offline DC-DC converter circuits. Its main advantages include a voltage regulation rate of 0.01%, an operating frequency up to 500 kHz, a startup current of less than 1 mA, and fewer external components. It is suitable for small switching power supplies ranging from 20 W to 80 W. Its operating temperature is 0℃ to 70℃, with a maximum input voltage of 30 V and a maximum output current of 1 A. It can drive bipolar power transistors and MOSFETs. The UC3842 is packaged in a DIP-8 form. Its internal block diagram and pin functions can be found in the relevant datasheet. A typical application circuit for the UC3842 is shown in Figure 3. The circuit works as follows: a DC voltage is applied to Rin, stepped down, and then applied to pin 7 of the UC3842, providing a startup voltage greater than 16V. Once the chip starts up, the feedback winding provides the voltage needed to maintain normal operation. When the output voltage increases, the feedback voltage generated on the feedback winding of the single-ended flyback transformer T1 also increases. This voltage is divided by a large voltage divider network formed by R1 and R3 and then sent to pin 2 of the UC3842. After comparison with the reference voltage, the voltage is amplified by an error amplifier, reducing the duty cycle of the drive pulse at pin 6 of the UC3842, thereby lowering the output voltage and stabilizing it. This circuit is simple in structure, easy to route, and low in cost. However, the sampling voltage of the UC3842 is not taken from the output terminal, resulting in low output voltage regulation accuracy, making it suitable only for applications with small loads. To overcome these problems, the feedback circuit can be improved by using an optocoupler and a voltage reference for feedback control, which can greatly improve the stability and accuracy of the switching power supply. When using this method for feedback control, sampling is required from the output terminal of the secondary winding, as shown in Figure 4. The voltage sampling and feedback circuit consists of an optocoupler PS2701, a TL431, and an RC network. R5 and C5 in the figure are used for frequency compensation of the TL431 and are indispensable. The sampling voltage is obtained by adjusting the voltage divider network composed of R6 and R7. This sampling voltage is compared with the 2.5V reference voltage provided by the three-terminal adjustable voltage regulator TL431. When the output voltage is normal, the sampling voltage is equal to the 2.5V voltage reference provided by the TL431. Therefore, the K-terminal potential of the TL431 remains unchanged, thus the current flowing through the optocoupler U3 diode remains unchanged, and consequently the current flowing through the optocoupler CE also remains unchanged. The feedback potential Uf at pin 2 of the UC3842 remains unchanged, so the duty cycle of the output drive at pin 6 remains unchanged, and the output voltage stabilizes at the set value. When the output 5V voltage increases for some reason, the sampled output voltage value obtained from the voltage divider network will increase accordingly. This causes the potential of the K terminal of the TL431 to decrease, increasing the current flowing through the optocoupler diode, and consequently increasing the current flowing through the CE circuit. Consequently, the potential of pin 2 of the UC3842 increases. From the internal schematic of the UC3842, we can see that the output voltage Ue of the error amplifier A1 decreases, meaning the clamping voltage of the current detection comparator decreases. Therefore, as shown in Figure 2, the duty cycle of the output drive at pin 6 of the UC3842 decreases, thus reducing the output voltage. This completes the feedback regulation process. 2.2 Primary Coil Magnetizing Peak Current Sampling Feedback Internal Loop The internal loop feedback of the primary coil magnetizing peak current sampling is also a crucial element in the switching power supply design. If the internal loop feedback design does not meet the circuit requirements, the switching power supply will not function properly. When designing the internal feedback loop, a ground-referenced sampling resistor Rs (see R1 in Figures 1 and 4 and R8 in Figure 3) needs to be connected in series with the switching transistor to convert the primary coil current into a voltage signal. This voltage is monitored by the current sensing comparator A2 and compared with the output level from the error amplifier A1. Under normal operating conditions, the peak inductor current is controlled by the voltage on pin 1, where: When the power supply output is overloaded or the output sampling is lost, abnormal operating conditions will occur. Under these conditions, the threshold of the current comparator is internally clamped to 1.0 V, and the maximum peak current of the primary coil of the switching power supply is the maximum current flowing through the primary coil of the transformer during short-circuit protection: Where: IP is the primary coil inductor current; Pout is the designed output power of the switching power supply; Vin is the input voltage of the switching power supply; D is the duty cycle of the PWM output signal; N is the power supply efficiency. Based on equations (2) and (3), we can deduce that: The power of the current sampling resistor can be further calculated based on the calculated Rs resistance value. After selecting the current sampling resistor, it needs to be sent to the current comparator of the UC3842 through an L-shaped RC low-pass filter network. The upper cutoff frequency of the L-shaped RC low-pass filter network is: From the logarithmic amplitude-frequency characteristic of the low-pass filter, we know that when the input signal frequency is lower than fh, the output signal is almost identical to the input signal; when the input signal frequency is higher than fh, the output signal will be significantly attenuated. The signal frequency on the Rs sampling resistor can be measured using an oscilloscope. Therefore, when selecting the RC parameters of the low-pass filter, it is necessary to ensure that the normal sampling voltage on the Rs resistor is not attenuated by the filter. When designing a switching power supply, if the RC parameters are not properly selected, causing the upper cutoff frequency fh of the filter to be too small, the normal Rs sampling signal will be attenuated. Thus, when the load increases, the PWM cannot increase the duty cycle of the control pulse, and the transformer will whistle due to excessive load. To solve this problem, the value of the filter capacitor C is reduced, thereby increasing fh, allowing the normal Rs sampling signal to pass through the filter. When the load increases, the switching power supply can stabilize the voltage well, and the transformer's whistling phenomenon does not occur. 3. Conclusion The design of switching power supplies is a highly practical subject. The method presented in this paper is only for reference. Many practical problems need to be continuously summarized and improved in practice. Only through practice can the design be continuously perfected.