1 System Solution Demonstration
1.1 Overall Solution Description
This question requires two working modes, simulating photovoltaic cells to power the load and charge the battery pack; and simulating photovoltaic cells and battery packs to power the load together. The input voltage of the photovoltaic cell is between 25-55V. According to the voltage division calculation of the load and internal resistance, it is sometimes lower than 30V, and the question requires maximum power tracking, so according to the range, it is between 12.5-27.5. However, the output voltage at the load end is required to be stable at 30V, and the input needs to be boosted at this time. The second mode in the question requires photovoltaic cells and battery packs to power the load together, so the battery pack should be connected in parallel to the circuit. The question requires that the battery pack should be charged and discharged, so the system uses bidirectional DC-DC for control.
Figure 1 System Structure Diagram
1.2 Comparison and Selection of Storage and Discharge Control
Solution 1: Full-bridge bidirectional DC-DC circuit: Bidirectional full-bridge DC-DC is a commonly used Buck-Boost topology, but using four MOS tubes will result in large losses.
Solution 2: Half-bridge bidirectional DC-DC: Use a boost half-bridge conversion circuit to control the size of the boost output voltage and the bidirectional flow of current by controlling the duty cycle of the upper and lower tubes of the half-bridge. Using two MOS tubes, the loss is relatively small.
Solution 3: Buck module and BUCK circuit: Through the buck module, the voltage at both ends of the battery is always higher than the battery, and the battery is continuously charged. At the same time, the BUCK circuit stabilizes the voltage at 30V. The disadvantage is that it can only complete the basic part, but not the full play part.
Solution analysis: The maximum voltage on the battery side is 14.8V and must be stabilized at 30V. Considering that the bidirectional DC-DC will stabilize the bus at 30V, the buck working state can be ignored. The battery voltage is <16.8V, and the bidirectional DCDC always works in the boost state. In summary, Solution 2 is selected.
2 Circuit and program design
2.1 Main circuit and device selection
1. Overall circuit design
diagram 2 System circuit design block diagram
2. Bidirectional DC design
Figure 3 Bidirectional DC circuit
2.2 Control program design
The design scheme uses two control boards, one for boost and MPPT control; the other for bidirectional DC-DC control. The control process is as follows:
Figure 4 Program flow chart
3 Theoretical analysis and calculation
3.1 Calculation of main circuit main component parameters
The inductance of this circuit satisfies the volt-second balance in steady state, that is, the inductor current rise value is equal to the inductor current drop value.
When D is 50% and Ts is 0.2us, the inductance is calculated to be 2.5uH. When D is 50% and
Ts is 0.2us, the capacitance is calculated to be 10uF.
3.2 Control method and parameter calculation
1. Buck-boost MPPT control method and parameter calculation
According to the requirements of the topic, the converter output voltage Uo is stable at 30V, and the maximum power point tracking is achieved at 25~55V. According to the maximum power transfer theorem, ignoring the interference of harmonic factors, when the transmission power in the circuit is the largest, there is approximately:
where is the equivalent resistance of the circuit, when the circuit is working normally,. Therefore, when the full range maximum power point is tracked, the range is about (25~55)/2=(12.5~27.5)V. It can be found that this range is significantly less than 30V, so the main DC-DC can achieve the goal by using Boost.
For safety, it is necessary to set upper and lower thresholds for the Boost. According to the actual situation of the circuit we designed, the relationship between the duty cycle β and the input and output voltage is
known to be (12.5~27.5)V, 30V, and the range of β is about
0.3 and 0.93, respectively.
3.3 Methods to improve efficiency
1. All tube drive auxiliary power and CPU power supply are powered by switching power supply.
2. Increase the switching frequency and use low internal resistance MOS tubes. The bidirectional DC-DC only needs the boost part, so the corresponding tube drive is removed to further reduce the loss.
3. All filter capacitors use low ESR capacitors.
4. Try to reduce the CPU main frequency and turn off irrelevant peripheral clocks.
5. The freewheeling diode uses a low voltage difference Schottky diode.
4 Test plan and test results
4.1 Test plan and test conditions
1. Test plan
First check whether the line connection has open circuit, short circuit and cold soldering. After confirming that it is correct, connect the multimeter to each test point. First debug the bidirectional DC-DC part, calibrate it, and then increase and reduce the load to achieve constant output while charging and discharging the battery.
When debugging the input boost DC-DC part, calibrate it, further debug the working process under different loads, and observe the results of PID adjustment output during MPPT tracking.
System joint debugging, according to the requirements of the topic, connect the system, adjust from 55V to 25V, adjust the load to 25Ω and 50Ω, record it, and then calculate and analyze it.
2. The test uses instruments and equipment
DC regulated power supply and 7 digital multimeters to measure separately.
4.2 Test Results
1. Basic Requirements of the Question The test results are as follows
1. Basic Requirement 1: Under the conditions of =50V, =1.2A, the output port voltage and the energy storage port current Record
the output port voltage and the energy storage port current
Serial No.
1
50.0
1.187
30.06
0.95
2
50.0
1.200
30.04
1.01
3
50.0
1.200
30.04
1.01
2. Basic Requirement 2: Under the conditions of =1.2A, the voltage regulation rate increases from 45V to 55V, and the voltage regulation rate is recorded.
Serial
No.
1
1.2
30.06
30.05
0.03%
2
1.18
30.06
30.04
0.06%
3
1.2
30.07
30.03
0.09%
4. Basic Requirement 3: Under the conditions of =50V, From 1.2A to 0.6A, the load regulation rate records
the load regulation rate
number
1
50
30.04
30.06
0.06%
2
50
30.04
30.05
0.03%
3
50
30.03
30.05
0.06%
4. Basic requirement 4: Under the conditions of =50V, =1.2A, record the converter efficiency
Converter efficiency
No.
1
50.00
1.200
30.02
2.51
25.03
1.53
15.57
95.26%
2
50.00
1.202
30.00
2.50
25.01
1.50
15.60
95.10%
3
50.01
1.201
30.01
2.51
25.02
1.52
15.60
95.14%
5. Part 1: =1.2A, reduced from 55V to 25V.
Voltage Regulation No.1
1.2
30.06 30.03 0.09 % 2 1.18 30.04 30.02 0.06% 3 1.2 30.03 29.99 0.12%
Deviation No.1 2 3 4 55.00 50.00 45.00 35.00 27.53 25.01 22.5 17.51 00.03 00.01 00.00 00.01 6. Performance Part 2: Under the conditions of =35V, =1.2A, and efficiency record converter efficiency No.1 35.0 1.202 30.02 1.75 17.51 0.48 14.89 95.49 % 2 35.0 1.200 30.01 1.76 17.50 0.47 14.80 95.40% 7. Performance part 3: =35V, reduced from 1.2A to 0.6A, load regulation rate record load regulation rate sequence number 1 35 29.99 29.98 0.03% 2 35 30.97 30.97 0.0% 3 35 30.97 30.98 0.03% 4.3 Test result analysis When using a DC power supply, the input current remains at 1.2A, the input voltage drops from 55V to 25V, the output voltage is stable at 30V0.1V, and the energy storage port current changes from inflow to outflow, realizing automatic switching between modes I and II. The converter efficiency is above 95% on average, and can achieve maximum power point tracking with a deviation of no more than 0.05V. Under the given conditions, the system's load regulation and voltage regulation meet the requirements of the question. 5 Other systems are also designed with key functions to calibrate the system's voltage, current and other parameters. The system's final output voltage can not only be stabilized at 30V, but its stable output voltage can also be changed by key adjustment.
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