2021 Electric Competition | Question C-Three-Port DC-DC Converter

Question analysis part

A three-port DC-DC converter is designed and produced, and its structural block diagram is shown in Figure 1. The converter has two working modes: Mode I, which simulates photovoltaic cells supplying power to the load and charging the battery pack at the same time (IB≥0); Mode II, which simulates photovoltaic cells and battery packs supplying power to the load at the same time (IB≤0). According to the simulated illumination (size of US) and load conditions, the converter can work in mode I or mode II, and can realize automatic conversion of the working mode. Under various circumstances, the output voltage UO should be guaranteed to be stable at 30V.

basic requirements

(1) Under the conditions of US=50V, IO=1.2A, the converter works in mode I, UO=30V±0.1V, IB≥0.1A.

(2) IO=1.2A, US increases from 45V to 55V, voltage adjustment rate SU ≤ 0.5%.

(3) US=50V, IO is reduced from 1.2A to 0.6A, and the load regulation rate SI ≤ 0.5%.

(4) Under the conditions of US=50V and IO=1.2A, the converter efficiency is ≥90%.

play part

(1) IO=1.2A, US is reduced from 55V to 25V, requirements: the converter can automatically switch from mode I to mode II; achieve maximum power point tracking in the full range of US, deviation ≤0.1V; voltage regulation rate SU ≤ 0.1%.

(2) Under the conditions of US=35V, IO=1.2A, the converter works in mode II, UO=30V±0.1V, and the efficiency is ≥95%.

(3) US=35V, IO is reduced from 1.2A to 0.6A, the converter can automatically switch from mode II to mode I, and the load regulation rate SI ≤ 0.1%.

(4) Others.

3.Description

(1) In Figure 1, the DC regulated power supply, diode D, and resistor RS constitute a simulated photovoltaic cell. It is recommended to use a DC stabilized power supply with an output voltage of not less than 60V (can be obtained by connecting two circuits in series) and a rated current of not less than 3A. Pay attention to safety during use to avoid electric shock injuries. During the test, the DC stabilized power supply is provided by the competition area; diodes D. When selecting resistor RS, attention should be paid to current, power and other indicators. If necessary, a heat dissipation device should be installed, and care should be taken to avoid burns.

(2) The battery pack in Figure 1 consists of 4 18650 lithium-ion batteries with a capacity of 2000~3000mAh connected in series. It is required to use batteries with built-in management functions (or built-in protection boards). The battery pack does not need to be encapsulated in the work. It must be carried to the testing site by itself during testing. Batteries are not allowed to be replaced during the testing process.

(3) Participating teams should carefully read the technical information of the battery used, be able to correctly estimate or detect the state of charge of the battery, and set the initial state of the battery reasonably before the test to ensure that the battery can be charged and discharged normally during the test.

(4) Test points should be set up reasonably during production. Please refer to the figure below for details.

Question analysis

When we got question C, our group was very vague about the overall structure of question C. We once wanted to use three boost circuits to complete the design of the main circuit. Later, after studying photovoltaic cells, we found that it is very difficult to stabilize the load voltage through photovoltaic cells. , and then we optimized the entire system by stabilizing the output voltage of the battery pack, replacing the BOOST circuit with a bidirectional DC-DC circuit, so that the output terminal could reach a stable voltage of 30v, and then adjust the voltage of Ui. Automatic switching between mode one and mode two is achieved through changes in power.

system introduction

The bidirectional half-bridge circuit is used as the main topology, and STM32G071 is used as the control core to coordinate each module to achieve the task requirements. The circuit is divided into main circuit topology module, control module, PWM control signal drive module, auxiliary power module, and voltage and current sampling module. The main circuit topology module uses a dual half-bridge circuit; the control module uses the PWM output port of STM32G071 to generate PWM signals, collects voltage and current signals through ADS1115, and performs proportional and integral adjustment by the program to perform reliable closed-loop control. Judging from the measurement results, it is completed Understand the basic requirements of the topic and some of the requirements for performance.

1. 1 system solution

STM32G071 is used as the main control of the system, outputs PWM signals, and controls IR2104 to form a half-bridge chopper Boost circuit to achieve maximum power point tracking of the input port. At the same time, STM32G071 controls IR2104 to form a bidirectional half-bridge chopper Buck circuit to stabilize the output voltage. The 30v required by the voltage system changes the Buck circuit duty cycle according to Uo, allowing energy to flow freely in both directions from the output port and energy storage port. The PI proportional integral adjustment is made to the PWM signal according to the feedback signal, thereby achieving the output deviation ΔU1=|U1-0.5×Us|≤0.1v. The picture is a block diagram of the program system.

1.2.2 Voltage and current detection solution

Using INA282 special current detection chip. INA282 is a current shunt analog output current sense amplifier from Texas Instruments. Its voltage gain is 50 times and its common mode rejection ratio is high. The common mode range of INA282 is -14v to 80v. The current detection circuit using INA282 is simple and has low static power consumption.

2. Circuit and Programming

2.1 Functional circuit design

2.1.1 Controller module circuit design

The system uses STM32 F 071 as the main control, designs the baseboard circuit, and adds PMOS anti-reverse connection protection. When the power supply is reversed, the voltage between the gate and source of the PMOS is 0v, and the load loop is disconnected to protect the safety of the circuit. The PWM output port uses a 6N137 photoelectric isolation chip, which has a good isolation effect on the PWM output electrical signal and prevents the downstream half-bridge circuit from affecting the system main control.

Actual picture as shown

The microcontroller outputs the PWM signal to the half-bridge circuit, and the voltage data collected by the AD637 is read through the IIC.

2.1.2 DC current sampling circuit

The current sampling circuit uses TI's IN282 chip, and the sampling resistor uses a 20 milliohm resistor to convert the current signal into a voltage signal, which is read out through AD637. The IN282 circuit is shown in the figure.

Actual picture as shown

2.1.3 AD conversion circuit

In order to improve the accuracy of the system, the AD conversion circuit inside the STM32 microcontroller is not used, and the ADS1115 analog-to-digital conversion chip is used .  A DS1115 is a low-power 16 - bit analog-to-digital converter in an ultra-small leadless package. The ADS1115 device integrates a low-drift voltage reference and oscillator. Measure voltage Uo, voltage Ui and input current Ii. The ADS1115 circuit is shown in the figure.

2.1.4 Auxiliary power supply

The system auxiliary power supply uses TPS54360 . TPS54360 generates 12 V voltage and 5 V voltage to supply power to the AD conversion circuit, microcontroller and bidirectional DCDC circuit to meet the system's power requirements. TPS 54360 is a high-efficiency power conversion chip TPS54360 produced by Texas Instruments in the United States. It has a switching frequency of 100KHz to 2.5MHz. At the same time, because the voltage regulator adopts current mode control, the power supply current when the circuit is unloaded is reduced to 146uA, and the shutdown power supply current is reduced to 2uA, which also greatly improves the power conversion efficiency.

Actual picture as shown

2.1.5 Half - bridge circuit

The half-bridge circuit is shown in Figure 7. IR2104 is used as the half-bridge driver chip, and the field effect transistor is HY4008 , so a Schottky diode 1N5819 is added . The half-bridge driver IR2104S can obtain sufficient gate voltage through bootstrapping .

Actual picture as shown

2.3 Methods to improve efficiency

During the debugging process, the most difficult requirement we encountered was the second requirement of the play part, which requires the system efficiency to be greater than 95% . In order to fulfill the second requirement of the play part, we have adopted a variety of methods to improve system efficiency, which are summarized as follows:

1 . Adopt low power consumption microcontroller

At the beginning of the competition, the STM32 F 334 microcontroller was used, and later the low-power microcontroller STM32G071 was used. After using the STM32G071 microcontroller, the system loss was reduced by 1W.

2. Using low internal resistance power tube

Our signal uses HY4008 . HY4008 is a field effect transistor produced by Descendant Semiconductor, an excellent domestic semiconductor manufacturer. When it is turned on, its internal resistance is 2.9 milliohms and its withstand voltage is 80 V;

3 . Adopt high-efficiency auxiliary power module

The auxiliary power module uses TPS 54360. The power supply current when the circuit is unloaded is reduced to 146uA, and the shutdown power supply current is reduced to 2uA, which improves the power conversion efficiency.

4 ． Use chips with low power consumption

Use low-power chips to design circuits. For example, the AD conversion chip uses ADS1115 . ADS1115 is a low-power 16 -bit analog-to-digital converter .

  5 ． Reduce power consumption of microcontroller

   On the premise of meeting the system requirements, the clock frequency is reduced, and at the same time, the system power consumption is further reduced through the internal power management of STM32 G 071

6 ． Use low internal resistance inductor

When winding the inductor, use copper wire with low internal resistance to reduce the internal resistance of the inductor and reduce system losses.

2.3 Programming

The program design block diagram is shown in the figure below. The microcontroller measures the output voltage Uo, input voltage Ui and input current Ii, controls the Boost circuit to perform MPPT maximum power tracking, and adjusts the duty cycle of the voltage stabilizing circuit to form a closed loop for proportional and integral control to make the output The voltage Uo is stable at 30V.

The program flow chart of the system is shown in the figure.

The principle of MPPT program is as shown in the figure

This program uses the perturbation and observation method. When the output voltage stabilizes at 30V, the maximum power point tracking (MPPT) slightly changes the reference value of Ui by modifying the switching frequency of the half-bridge. Ui will follow this value. After it stabilizes, if the power increases, it will continue to move in that direction. Change the Ui reference value, otherwise fall back. Repeatedly. See Appendix 2 for the complete procedure.

Test Results

Maximum power point tracking is inaccurate and slow.

Complete the rest of the requirements