【Zhejiang University of Science and Technology Electrical Competition】LED Flash Power Supply (Topic H)

## This project won the second prize in the 5th Zhejiang Sci-Tech University College Student Electronic Design Competition

#### Team number: ZSTU

#### Team members: Xiang Yikun, Guo Meiqian, Weng Xiangyu

#### Instructor: Jin Hai

## "School-level Electronic Design Competition" Design summary report

title: LED flash power supply

Design and make an LED flash power supply. The core of the power supply is a DC-DC current-stabilized power converter,
which converts the battery's electrical energy into a constant current output to drive a high-brightness white LED.
The power supply has two modes: continuous output and pulsating output, and has output voltage limiting protection and alarm functions.

Design idea: This system design can boost the input voltage of the battery, and after boosting, it enters the mode selection circuit, and the mode selection is controlled by the main control system. After selecting the mode, it enters the constant current control circuit, and the current passing through the constant current is finally input to the load end. The load end is connected to the sampling circuit. When the load changes and the voltage exceeds the limited amplitude, the sampling circuit samples and sends it to the control protection circuit and issues an alarm at the same time.

Overall plan: The core part of this system is the DC/DC converter. The input power is boosted by the DC/DC converter and then enters the mode selection module. The user controls the main control circuit to switch different output modes through buttons. After selection, the current is output through the constant current control circuit to achieve constant current output, and finally the output is connected to the load. The other input power is transformed as an auxiliary power supply to power the entire design circuit. The main control circuit is composed of a main control chip to achieve software control of the hardware circuit.
![1.png] According to the design requirements, we need to convert the input voltage of 3.0V-3.6V into an output voltage of 10V, so we need to boost the low voltage input to a high voltage output. There are two ways to boost DC voltage, one is non-isolated DC/DC, and the other is isolated DC/AC/DC.

Solution 1: Use non-isolated DC/DC. The advantages of non-isolated DC/DC converters are high efficiency, large output current, and low static current. Non-isolated circuits can be divided into two basic forms: series switching converters and parallel switching converters according to different circuit forms. The series switching converter is a step-down DC/DC converter, and the parallel switching converter is a step-up DC/DC converter.

Option 2: Use isolated DCAC/DC. The isolated DCAC/DC converter is mainly composed of an inverter, a high-frequency transformer and a rectifier. The basic working principle is that the input voltage is converted into a higher-frequency AC voltage by the inverter, and the high-frequency AC voltage is converted into the required AC voltage by the high-frequency transformer, and finally the DC voltage is obtained by rectification.

In terms of circuit design, the main control end of Option 1 is a switch tube, the peripheral circuit requires fewer components, and the design is simple and easy to implement. The circuit design of Option 2 consists of an inverter circuit, a transformer circuit, a filter circuit, etc., and the design is more complex and difficult to implement. In Option 1, there is always a common point between the input and output of the circuit. Option 2 uses a high-frequency transformer to isolate this common point. The circuit designed using Option 2 will be safer, but because our output DC voltage is within the safe voltage range, it will not cause harm to the human body. So we choose option 1 as the DC/DC converter solution.
The constant control circuit adopts the concept of constant current source. The essence of constant current source is to use devices to feedback the current and dynamically adjust the power supply state of the equipment, so that the current tends to be constant. As long as the current can be obtained, feedback can be effectively formed to establish a constant current source.
Option 1: Use a constant current diode. The advantage is that the circuit structure is simple, and the disadvantage is that the constant current characteristic of the constant current diode is not very good, the current specification is relatively small, and it cannot meet the design accuracy requirements.
Option 2: Use two transistors of the same model and use the relatively stable be voltage of the transistor as a reference. This constant current source is simple and easy, but even the be voltage of transistors of the same model has individual differences, so it is not suitable for precision constant current requirements.
Option 3: Connect a sampling resistor in series in the output circuit, and the resistance value is very small and will not affect the output of the circuit. The voltage at both ends of the sampling resistor is sampled, and the sampled voltage is amplified by the INA282 chip and fed back to the single-chip microcomputer. The single-chip microcomputer compares the sampled voltage signal with the reference value, and stabilizes the output voltage by continuously changing the duty cycle of the output PWM wave, thereby stabilizing the output current.
Scheme 1 and Scheme 2 are strictly limited by the selected components and cannot meet the accuracy requirements. Scheme 3 uses the current stabilization circuit designed with an op amp, which can well realize the current stabilization function by utilizing the feedback adjustment function of the op amp. Therefore, we use Scheme 3 as the scheme for the current stabilization control circuit of this design.
   ![3.png] Because the main control chip uses the STM2103 single-chip microcomputer with built-in integrated ADC, sampling can be realized directly. Therefore, the resistor voltage division method is adopted, and the voltage division resistor is connected in parallel to the output circuit to feed back the sampled voltage to the single-chip microcomputer. When the single-chip microcomputer detects that the output voltage is greater than 10.5V, it gives a buzzer signal alarm
program software flow chart:
![5.png]