The latest Raspberry Pavilion 4B and other low-cost board computers are more common in the headless embedded design of monitoring and control applications. Develop applications running on Linux issues and integrate wireless connections on the development board, which opens up new ways to develop and provide innovative applications.
Although mainstream 5V mobile phone chargers can be relatively easily powered for such development boards, the energy collected from the environment is also increasingly concerned. This provides greater flexibility for system designers, which can be placed in place where the development board is not powered by the power cord. These development boards will not be able to use a charging battery without an external power supply, but the problem of this power supply can be solved by solar cells.
Powering with solar cells for embedded single board computers, which is increasingly feasible for systems that do not require screen. As the device power consumption on the development board decreases, the efficiency of solar cells and power management chips has improved, and the solar cell can be used directly to supply power to the development board, and the battery subsystem is trickle. This allows the battery to charge a single board computer and communication link for several months or for several years.
The Raspberry Pientel 4B is the key part of this trend, which integrates Wi-Fi and Bluetooth features on the development board equipped with a 1.2 GHz quad-core ARM® Cortex®-A53 processor. This avoids higher power consumption using wireless adapters through USB ports. The maximum operating current specified by the development board is 2.4 A, which can support peripherals of the USB port.
The development board is 31mA in standby mode, and the power consumption is raised to 580 mA at the processor and memory load. Another current load is a SMSCLAN9514 USB controller that consumes 74 mA when hanging mode. The 594 mA of the Ethernet connection is unlikely to be related, since power can be powered by an Ethernet cable.
The power consumption of the wireless connection depends on the set duty cycle, it should be set after the motherboard is started to avoid excessive peak current requirements.
This determines the startup power requirements of about 700 mA to 900 mA and about 150 mA idle power (need to be obtained from the energy collection source).
The required power can be provided by a series of solar panels, such as Mikroe-651 of Mikroelextronika. These solar panels provide 4 V output at 100 mA, allowing a solar panel having a size of 70 × 65 mm to provide start current. Alternatively, the 150 x 37mm AM AM-5902 of Panasonic can also provide up to 60 mA, require three solar panels to maintain idle power requirements.
Two of the solar panels will provide idle power, which indicates a standby rechargeable battery and a power management subsystem. Thus, when collecting data or sending this data to the gateway, these solar panels can be used to perform trickle charging for the battery to support the peak electricity consumption of the development board.
The rechargeable battery subsystem can be managed by certain devices, such as the BQ25504 of Texas Instruments. This device is used to charge the battery and ensure that the battery does not discharge when the power supply of solar cells is lowered, and the fluctuation source such as energy collection devices can be managed.
In order to provide a 5 V voltage required for a single board computer, two solar panels can be connected in parallel, and then connected to a rechargeable battery to provide the desired current.
In addition to the battery, a switch mode boost or buck converter and battery charger are required. Converters ensure that all energy generated by the solar panel can be acquired by the battery, which connects the inductor and power to allow the inductor to accumulate current, and store energy in the inductor. In the second cycle, the change in the current path causes the inductor to transmit the accumulated energy to the load. The load voltage may be higher or lower than the voltage of the inductor power source.
However, if the inductor is directly connected to the solar panel, the efficiency is low, and a capacitor is also adopted. By monitoring the voltage in the capacitor, the switch mode converter can activate when the solar panel output reaches peaks. When the output voltage is not sufficient to turn on the converter, the capacitor can also acquire energy from the solar cell, and then collect and store all energy.
This means that when the capacitor has sufficient electricity, the converter will work with burst mode to achieve fast charging. However, because the next energy burst is not indicated, it is difficult to end fast charging.
One way is to use another comparator to monitor the output voltage, disabled when the voltage reaches the maximum value, and the switch is enabled when the voltage drops below the predetermined level.
The BQ25504 is intended to use high-efficiency boost converters and chargers to effectively obtain and manage the output of solar cells. The device is started by simply obtaining a DC-DC boost converter / charger from the solar cell to get the micro-wattage / charger, and then works and effectively extracts the power.
In the typical circuit shown in Figure 3, the solar panel is connected to BQ25504 and the battery subsystem, and the collecting current is powered by the development board. BQ25504 uses battery monitoring output, which can be connected to the general IO pin of the Raspberry 4b development board. BQ25504 is mounted on the evaluation board shown in Figure 4 to provide links between solar cells and batteries.
When the boost converter outputs VSTOR reaches 1.8 V, the main rises converter can more efficiently obtain electrical energy from the solar cell when power is powered. It starts at a typical value of a VIN_DC as low as 330 mV, when VStor reaches 1.8 V, can continue to collect energy until Vin_DC falls to about 120 mV. The integrated PFM buck converter is also powered by VSTOR. If the input is available, it can provide a current of up to 100 mA from the VOUT pin.
One key element in the converter is to track the maximum power point (MPP) of the solar cell. This MPP changes with the amount of light and temperature on the solar panel, and implements a programmable maximum power point tracking (MPPT) sampling network to optimize electrical energy transmission of the device. By disabling the boost converter 256 MS, the BQ25504 performs periodic sampling of the open input input per 16 seconds and stores the programmed MPP ratio of the OC voltage on the external reference capacitor (C2) of VREF_SAMP. Typically, when the solar cell load is about 80% of the output voltage, the battery is in its MPP, and when the battery is lower than the maximum voltage (VBAT_OV) of the user, the boost charge is applied to the solar cell until Vin_DC reaches MPP Voltage. Thereafter, the boost charger adjusts the input voltage of the converter until the output reaches VBAT_OV to ensure that maximum power is transferred to the battery. These power will be used to provide the power required for the development board.
in conclusion
To connect the 5 V board computer such as a wireless connection to a solar cell, you need to use an intermediate battery and power management subsystem to provide the required stabilization current. Use BQ25504 and other devices to provide maximum power point tracking to ensure that optimizes the battery charging and provides a control line that returns the development board. Thereby, the development board can be used in a region where the power is not available, and the data can be returned to the network.
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