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Engineers with years of experience share switching power supply design methods
In this paper, a DC/DC converter based on a Boost topology is designed and implemented. The system employs the UC3842 integrated circuit as the primary control unit, while an Atmega128 microcontroller is used to enhance the stability of the output voltage. A dual-loop feedback mechanism—incorporating both voltage and current loops—is introduced to ensure precise regulation of the output voltage. Additionally, the system integrates analog PWM technology, online protection mechanisms, and user interaction features. Experimental results demonstrate that all system performance metrics meet or exceed the design specifications.
With the rapid development of power electronics, power supply devices have become essential in various industrial and domestic applications. The stability of voltage and current, voltage regulation rate, load regulation rate, and overall efficiency directly impact the performance and safety of electronic equipment. Therefore, optimizing these parameters has become a critical consideration in the design of power supply units. This article presents a reliable switching power supply system that addresses these challenges effectively.
**1. Program Argumentation**
**1.1 Selection of DC-DC Converter Scheme**
The secondary winding of the isolation transformer provides an output voltage of 18 ± 3 V. After full-wave rectification and filtering, the DC voltage reaches approximately 18–26 V. The required output voltage range for the switching power supply is stable between 30 and 36 V. Both single-ended flyback and Boost topologies can satisfy this requirement. However, due to the difficulty in manufacturing and adjusting the pulse transformer in a flyback configuration, and the significant influence of material selection on system efficiency, the Boost topology is chosen as the main circuit. The main circuit diagram is shown in Figure 1.
Figure 1: Main circuit of the power converter
**1.2 Control Scheme Selection**
Several control strategies can be applied to a Boost converter, including direct control by a microcontroller or analog control circuits. While digital control offers flexibility, it may introduce delays and instability, affecting the accuracy of voltage regulation. Analog control, using operational amplifiers, allows for a more robust voltage-current double-loop structure, improving regulation accuracy. However, it involves complex circuitry and lower reliability. To simplify the design and improve reliability, the UC3842 integrated control chip is employed. The control circuit diagram is illustrated in Figure 2.
Figure 2: Main circuit and UC3842 control circuit
To address potential instability when the duty cycle exceeds 0.5, slope compensation or reduced loop gain can be used. Since the system already includes a microcontroller for monitoring and display, the microcontroller can be utilized to correct the control system, enhancing voltage accuracy without compromising 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 in the power devices, iron and copper losses in inductors, and control circuit losses. Among these, switching losses are the most significant. To reduce these losses, fast-switching MOSFETs with low on-resistance are selected. Ultra-fast recovery diodes are also used for the Boost diode. Inductive components are made from ferrite cores with minimal iron loss, and thicker enameled wires are used to minimize copper losses. The control circuit’s power is supplied via two methods: the main control chip (UC3842) is powered directly from the main circuit’s rectified and filtered output, while the microcontroller and its peripherals are powered by a separate auxiliary switching power supply.
**2. Circuit Design and Parameter Calculation**
**2.1 Selection of Main Circuit Components and Parameter Calculations**
**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 circuit operates in continuous conduction mode (CCM). Ignoring circuit losses, the duty cycle can be calculated as:
$$ D = \frac{V_{out}}{V_{in} + V_{out}} $$
At the lowest input voltage (18 V) and highest output voltage (36 V), the maximum duty cycle is 0.5. At the highest input voltage (26 V) and lowest output voltage (30 V), the minimum duty cycle is 0.13. The inductor value is determined based on the critical continuous condition at the minimum duty cycle, resulting in an actual inductance of 500 μH.
**2.1.2 Main Switch Selection**
The maximum drain-source voltage of the main switch is 36 V. Considering overload conditions, the maximum drain current is calculated as:
$$ I_{DS} = \frac{I_{out} \times (1 + D)}{D} $$
Taking into account voltage and current spikes, the switch must withstand at least 90 V and 12 A. The IRF3710 MOSFET is used, 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 average forward current of the diode is calculated as:
$$ I_{F(AV)} = \frac{I_{out} \times (1 - D)}{2} $$
Considering the variability in duty cycle, the FR607 diode is selected, rated for a peak current of 5.75 A.
**2.1.4 Output Filter Capacitor Selection**
The ripple voltage is kept below 200 mV. With a maximum load current of 3 A, the filter capacitor is calculated as:
$$ C_f = \frac{I_{out} \times T}{\Delta V} $$
A practical choice is a 1000 μF / 50 V electrolytic capacitor.
**2.2 Control Circuit Design and Analysis**
The control circuit uses the UC3842 to regulate the output voltage. The sampled voltage is compared to an internal reference, and the error signal is fed into the current comparator. A PI regulator is implemented between pins 1 and 2 of the UC3842.
**2.2.1 Switching Frequency Design**
The switching frequency is set using the timing resistor and capacitor of the UC3842's sawtooth generator:
$$ f_s = \frac{1}{R_t \times C_t} $$
With $ R_t = 16 k\Omega $ and $ C_t = 9.4 nF $, the switching frequency is approximately 11 kHz.
**2.2.2 Voltage Feedback Resistor Design**
The feedback network is designed to maintain a 2.5 V reference voltage at the output. With a maximum output of 36 V, resistors $ R_1 = 35 k\Omega $ and $ R_2 = 8.5 k\Omega $ are selected.
**2.2.3 Voltage Regulator Design**
The voltage error amplifier uses a proportional-integral (PI) structure with $ K_p = 10 $ and $ K_i = 1/3000 $.
**2.2.4 Current Sampling Resistor**
The current sampling resistor is chosen such that the voltage at pin 3 of the UC3842 remains below 1 V. Three 0.33 Ω resistors are connected in parallel.
**2.3 Protection Circuit Design and Calculation**
To enable 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. When an overcurrent is detected, the microcontroller either limits the output current or blocks it entirely. Once the fault is cleared, the system resumes normal operation.
**2.4 Human-Machine Interface Design**
The interface includes a PS2 keyboard for input and a 240×128 LCD display with Chinese characters for output. The PS2 keyboard communicates with the microcontroller via serial protocol, while the LCD uses a parallel data bus for communication.
**2.5 Auxiliary Power Supply Design**
An auxiliary switching power supply is built to provide power to the control circuit. It supplies +5 V / 100 mA and ±12 V / 100 mA.
**2.6 Efficiency Analysis and Calculation**
The total system efficiency is calculated by summing the losses in the control circuit, main switch, and diode. The control circuit consumes about 0.1 W, the main switch has a loss of 0.08 W, and the diode has a loss of 1.14 W. The total efficiency is approximately 88%.
**3. Software Flow Chart**
The software flow chart of the switching power supply system is shown in Figure 4.
Figure 4: Software flow chart
**4. System Testing**
**4.1 Test Instruments**
The following instruments were used: EE1410 function generator, TEK 1002B digital oscilloscope, UT88B multimeter, and others.
**4.2 Test Plan and Data**
Various tests were conducted, including adjustable output voltage, voltage regulation, load regulation, efficiency, and ripple voltage. The results showed that the system met or exceeded the design requirements.
**5. Conclusion**
Comprehensive testing confirmed that the system meets or exceeds all design specifications. Although the actual efficiency is slightly lower than the theoretical calculation due to unaccounted losses, the system performs well. Future improvements could involve implementing synchronous rectification to further increase efficiency. Overall, this design represents a reliable and efficient solution for DC/DC conversion.