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Engineers with years of experience share switching power supply design methods
In this paper, a DC/DC converter based on the Boost topology is designed and implemented. The system utilizes the UC3842 integrated circuit as the core control unit, while the Atmega128 microcontroller is used to enhance output voltage stability. A voltage-current feedback loop is incorporated into the UC3842, along with an output voltage averaging loop, to ensure stable regulation of the DC/DC converter. Additionally, the system integrates analog PWM technology, online protection mechanisms, and user interaction features. Experimental results demonstrate that the system's performance meets or exceeds the design specifications.
With the rapid development of power electronics, power supply devices have become essential in various fields of industry and daily life. The stability of voltage and current, the voltage regulation rate, load regulation rate, and overall efficiency of converters significantly impact the performance and safety of electrical and communication equipment. Therefore, improving these parameters has become a key consideration in power supply design. This paper presents a reliable switching power supply system that addresses these challenges effectively.
**1. Program Argumentation**
**1.1 DC-DC Converter Scheme Selection**
The secondary winding of the isolation transformer outputs 18 ± 3 V, and after full-wave rectification and filtering, the DC voltage ranges from approximately 18 to 26 V. The required output voltage of the switching power supply must be stabilized between 30 and 36 V. Both single-ended flyback and direct Boost topologies can meet this requirement. However, due to the complexity of manufacturing and adjusting the pulse transformer in a single-ended flyback configuration, as well as the significant influence of material selection on system efficiency, the Boost topology is chosen for this design. The main circuit is illustrated in Figure 1.
[Figure 1: Power converter main circuit]
**1.2 Control Scheme Selection**
Various control strategies can be applied to Boost converters, such as digital control via a microcontroller or analog control using operational amplifiers. While digital control offers flexibility, it may introduce delays that reduce low-frequency gain and affect output voltage accuracy. Analog control, on the other hand, allows for a more robust voltage-current double-loop structure, enhancing stability. However, it involves complex circuitry and lower reliability.
To simplify the control design and improve reliability, the UC3842 chip, typically used in flyback converters, is adapted for use in the Boost topology. This approach reduces the complexity of the control circuit. The UC3842-based control circuit is shown in Figure 2.
[Figure 2: Main circuit and UC3842 control circuit diagram]
Despite its advantages, the UC3842-based Boost converter may exhibit instability when the duty cycle exceeds 0.5. To address this, either the control loop's low-frequency gain must be reduced or slope compensation is applied. Since the system already includes an MCU for monitoring and display functions, the MCU can be used to correct the control loop, thereby improving voltage regulation without sacrificing system stability. The control scheme is depicted in Figure 3.
[Figure 3: Control scheme]
**1.3 Methods to Improve Efficiency and Implementation**
Key factors affecting system efficiency include switching losses of the power devices, iron and copper losses in inductors, and control circuit losses. Among these, switching losses are the most critical. To mitigate this, fast-switching MOSFETs with low on-resistance are selected, along with ultra-fast recovery diodes. Inductors are made from ferrite cores with minimal iron loss, and thicker enameled wires are used to reduce copper losses.
The control circuit’s power supply is divided into two parts: the UC3842 is directly powered by the main circuit’s rectified and filtered output, while the microcontroller and peripheral circuits are supplied by a separate small switching power supply. This approach minimizes power consumption and enhances overall efficiency.
**2. Circuit Design and Parameter Calculation**
**2.1 Main Circuit Device Selection and Parameter Calculation**
**2.1.1 Inductor Calculation**
The main circuit parameters are as follows: input voltage after rectification and filtering is 18–26 V, output voltage is adjustable between 30–36 V, maximum output current is 2 A, and the switching frequency is 10 kHz. The Boost converter operates in continuous conduction mode (CCM).
Ignoring circuit losses, the PWM duty cycle is calculated based on the output voltage expression:
$$ D = \frac{V_{out} - V_{in}}{V_{out}} $$
At minimum input voltage (18 V) and maximum output voltage (36 V), $ D_{max} = 0.5 $. At maximum input voltage (26 V) and minimum output voltage (30 V), $ D_{min} = 0.13 $.
The inductor current ripple is set to 30% of the average current at half load. Using the critical conduction condition, the inductance value is determined to be 500 μH.
**2.1.2 Main Switch Selection**
The maximum drain-source voltage of the switch is 36 V. Considering overload conditions, the peak current is calculated as 12 A. The IRF3710 MOSFET is selected, which has a maximum drain-source voltage of 100 V, a maximum drain current of 57 A, and an on-state resistance of 25 mΩ.
**2.1.3 Fast Recovery Diode Selection**
The diode is selected based on the average on-state current. A FR607 diode is used, rated for 5.75 A, to handle the maximum peak current during operation.
**2.1.4 Output Filter Capacitor Selection**
The output voltage ripple is limited to less than 200 mV. With a maximum load current of 3 A, a 1000 μF / 50 V electrolytic capacitor is used as the filter capacitor.
**2.2 Control Circuit Design and Analysis**
The UC3842-based control circuit includes a voltage error amplifier, a current comparator, and a PI regulator. The output voltage is sampled through resistors R1 and R2 and compared with the internal reference voltage. The error voltage is then used to generate a PWM signal, which controls the main switch.
**2.2.1 Switching Frequency Design**
The switching frequency is determined by the timing resistor and capacitor connected to the UC3842’s oscillator. With Rt = 16 kΩ and Ct = 9.4 nF, the switching frequency is set to 11 kHz.
**2.2.2 Voltage Feedback Sampling Resistor**
The feedback network is designed based on the principle of non-steady state error. With a reference voltage of 2.5 V and a maximum output voltage of 36 V, R1 = 35 kΩ and R2 = 8.5 kΩ are selected.
**2.2.3 Voltage Regulator Design**
The voltage error amplifier is a proportional-integral (PI) controller with Kp = 10 and Ki = 1/3000.
**2.2.4 Current Sampling Resistor Rs**
The current sampling resistor is set so that the signal at pin 3 of the UC3842 does not exceed 1 V. Three 0.33 Ω resistors are connected in parallel to achieve this.
**2.3 Protection Circuit Design and Calculation**
To implement online overcurrent protection, a Buck circuit is added between the Boost output and the load. The Buck switch is controlled by the microcontroller, and a Hall current sensor is used to monitor the output current. In case of overcurrent, the system enters either current-limit or blocking mode, depending on the threshold. After fault removal, the system automatically resumes normal operation.
**2.4 Human Interface Design**
The system includes a PS2 keyboard for input and a 240×128 LCD display for output. The PS2 interface provides a simple and efficient way to handle multiple keys, while the LCD display supports standard Chinese fonts and communicates with the microcontroller via a parallel bus.
**2.5 Auxiliary Power Supply Design**
A small switching power supply is built to provide power to the control section. It supplies +5 V/100 mA and ±12 V/100 mA.
**2.6 Efficiency Analysis and Calculation**
The power consumption of the control circuit components is measured and listed in Table 1. The main switch and diode losses are calculated, with the total system efficiency estimated to be around 85%.
**3. Software Flow Chart**
The software flow of the switching power supply system is illustrated in Figure 4, showing the sequence of operations from initialization to real-time monitoring and protection.
[Figure 4: Software flow chart]
**4. System Test**
**4.1 Test Instruments**
The following instruments were used: EE1410 function generator, TEK 1002B oscilloscope, UT88B multimeter, and others.
**4.2 Test Plan and Data**
Various tests were conducted, including output voltage adjustability, voltage regulation, load regulation, efficiency, ripple voltage, and overcurrent protection. All test results met or exceeded the design requirements, confirming the system's reliability and performance.
**5. Conclusion**
Comprehensive testing shows that the system’s performance meets or exceeds the design goals. Although the actual efficiency is slightly lower than theoretical calculations due to unaccounted losses, the system remains highly efficient. Future improvements could involve implementing synchronous rectification to further increase efficiency. Overall, this design represents a practical and effective solution for DC/DC conversion applications.