"In the face of serious problems such as energy shortage and climate warming, human beings turn to find and use clean energy technologies for survival and development. Clean energy includes solar energy, wind energy, thermal energy, vibration energy, ocean energy, and other energy, such as human kinetic energy, biochemical energy and so on. With the development of science and technology, wireless sensor network technology has penetrated into all aspects of human production and life. Wireless communication network has gradually developed into the Internet of things that can communicate between anyone and objects at any time and anywhere. The scale of the network is expanding rapidly, but at the same time, the overall stability and sustainable development of the Internet of things are becoming more and more prominent. At the same time, in order to meet the needs of human life, more and more sensors need to be placed in inaccessible or harsh areas. The harsh environment in these areas determines that people cannot use chemical batteries to power wireless sensor nodes, because it is often impossible to replace chemical batteries in these areas. Because of these reasons, this paper thinks of using renewable energy (dynamic energy) to power wireless communication nodes to solve these problems.
This paper presents a system of micro thermoelectric generator for wireless sensor network. The system takes the micro thermoelectric generator as the energy source and the ultra-low power consumption energy management chip bq2504 of Texas Instruments as the DC-DC boost converter. It can collect energy from the energy source as low as 80mv, use the peripheral circuit to track and control the power point of the energy source, and store energy when necessary in combination with the energy buffer, Then, the effective collection and utilization of micro temperature difference energy are realized by mic841n dual voltage comparator and tps78001 ultra-low voltage differential linear regulator. The system realizes the function of wireless sensor network through efficient energy collection and effective energy management. It has become a real energy self supplying wireless sensor system. At the same time, it also meets the development requirements of green radio in China's communication industry.
1. Wireless sensor network node architecture model based on micro thermoelectric generator
In order to meet the requirements of wireless sensor network system supplied by micro thermoelectric generator, this paper designs the following system architecture of wireless sensor node transmitter, as shown in Figure 1 below.
Fig. 1 transmitter architecture of wireless sensor network node of micro thermoelectric generator
As can be seen from Figure 1, the transmitting end structure of wireless sensor network node powered by micro thermoelectric generator is composed of thermoelectric energy collector, boost circuit with MPPT function, energy buffer and system load (wireless sensor node). The thermoelectric energy collector is composed of thermoelectric conversion chips. The surface area and the number of layers of thermoelectric conversion chips can be determined according to the size of the actual application site and the amount of electric energy required, so as to meet different application environments.
Power management IC is mainly composed of power point tracking module (MPPT), power output interface, charger (DC-DC boost module) and energy buffer. The energy buffer circuit is composed of energy storage capacitor, comparator circuit and voltage regulator circuit. The load mainly includes processing the data collected by the sensor and transmitting it through the wireless transmitting module.
As can be seen from Figure 1, in the wireless sensor network node powered by micro thermoelectric generator, power management integrated circuit (PMIC) is an extremely important part. It contains many and important circuit functions, which is the key of the energy acquisition system of micro thermoelectric generator.
2. Design scheme of power supply energy management control circuit (PMIC)
2.1 overall design scheme of power supply energy management control circuit (PMIC)
In this paper, the power management control circuit mainly includes the following functions, power point tracking, DC-DC boost conversion and energy buffer.
As shown in Figure 2, the energy acquisition and tube circuit of the wireless sensor system based on the energy self supply of the micro thermoelectric generator is mainly composed of chips bq2504, mic841n, tps78001, energy storage capacitors and their corresponding peripheral circuits. The ultra-low voltage boost conversion and management chip bq2504, the low-power multifunctional voltage comparator mic841n and the linear regulated output chip tps78001 together constitute the multiple functions of temperature difference energy acquisition and management of wireless sensor network nodes supplied by micro temperature difference generators.
Fig. 2 Schematic diagram of system temperature difference energy acquisition and application circuit
In this paper, bq2504 power management chip mainly absorbs energy from thermal energy conversion module with ultra-low power consumption. Bq2504 is a 16 pin, 3mm * 3mm sub packaged high-efficiency energy management chip. The 16 pins are distributed counterclockwise in turn. This paper realizes the efficient management of micro energy by reasonably applying the corresponding functions of these pins. In addition, a significant advantage of the chip is that it has an ultra-low starting voltage, which makes it possible to extract energy from an energy source as low as 80mv during stable operation, and boost and convert the ultra-low voltage for storage and use in subsequent circuits. In this circuit, the power point tracking of collecting electric energy from ultra-low power energy source is realized with appropriate peripheral circuits, which plays a vital role in the micro temperature difference energy self supply system. At the same time, the circuit protection of overvoltage and undervoltage is set through the peripheral circuit to ensure the stable operation of the chip.
Mic841n is an ultra-low power dual voltage comparator with internal reference voltage. In this paper, the following linear regulator is driven by setting the upper and lower limits of its voltage comparison. Its working feature is to continuously detect the voltage on pin Vdd and compare it with the working voltage set on pins lth and HTH, so as to determine the output voltage (i.e. the output signal of pin out), and then control the working state of voltage regulator tps78001.
Tps78001 is an ultra-low power regulator produced by Ti. It can stabilize the output voltage of the circuit. By setting the resistance parameters of the corresponding peripheral circuit, the output can get a stable voltage, so that the wireless sensor transmitting node behind can be driven stably.
In order to better explain the circuit designed in Fig. 2, the chips and related electronic components contained in each module of the above circuit diagram, as well as the working mode and function are described in detail below.
The circuit in Figure 2 is the overall circuit diagram of the self power supply system of the micro thermoelectric generator. According to the function of the actual circuit, it can be divided into three circuits, which are replaced by circuits a, B and C.
Circuit a is a DC-DC boost converter circuit with MPPT function based on bq2504 chip and an energy storage circuit; Circuit B is a dual voltage comparator circuit based on mic841n chip; Circuit C is a voltage regulator output circuit based on tps78001 chip.
2.2 design of DC-DC boost conversion and energy storage circuit with MPPT function
As shown in Fig. 3, circuit a is mainly composed of power management chip bq2504 and its peripheral circuits.
Fig. 3 Schematic diagram of DC-DC boost circuit and energy storage circuit with MPPT function
First, connect the relevant electronic components of circuit a according to Fig. 3. TEG (thermoelectric generator) is a micro thermoelectric generator, which outputs the bare voltage of thermoelectric conversion. The main functions of circuit a are MPPT, DC-DC boost conversion and energy storage circuit. How to realize these three functions is described in detail below.
2.2.1 functional circuit design of power point tracking (MPPT)
Power point tracking (MPPT) is a technology that makes use of the electric energy generated by generator. In this paper, the output voltage of the temperature difference chip of the micro thermoelectric generator is adjusted through a certain electrical module, so as to realize the standardization of the output power of the thermoelectric generator. According to the known output characteristic curve of micro thermoelectric generator, the output power can be obtained when the output voltage is about 50% of the open circuit voltage. The technology of extracting power from TEG is mainly to dynamically change the switching frequency of DC / DC converter. According to this characteristic, this paper uses bq2504 to realize that the output voltage is half of the open circuit voltage by using the resistance proportional voltage division method, and then realizes the optimization of output power.
As shown in Figure 3, in order to realize the MPPT function, pin 2 (VIN_ DC) and pin 3 (VOC_ Samp) is connected to resistance oc2 and resistance OC1 respectively. Pin 2 is connected to pin 3 through oc2, and pin 3 is grounded through OC1, as shown in the circuit schematic diagram.
The resistance values of roc1 and roc2 are then determined as follows:
VIN_ DC is the voltage output terminal, which makes VOC through the voltage division of roc1 and roc2_ The voltage at samp is:
In addition, because the output voltage of TEG is approximately equal to the output power that can be obtained when the open circuit voltage is 50%, the value of roc1 / (roc1 + roc2) should be 1 / 2, so roc1 = roc2. In the actual circuit design, 10m Ω is selected as its resistance value in this paper, so roc1 = roc2 = 10m Ω.
Bq2504 chip samples VOC every 16S_ The voltage value of Samp can ensure that the output power point of micro thermoelectric generator can be accurately tracked in a short time when the output power of thermoelectric generator changes, so as to realize effective electric energy collection.
2.2.2 design of DC-DC ultra-low voltage boost function circuit
Another important function of bq2504 is to continuously draw energy from the voltage as low as 80mv during stable operation, which is very important for micro thermoelectric generator. The charging circuit of bq2504 is composed of DC-DC boost module integrated in the chip. The internal boost module adjusts the input voltage to the voltage required by the energy storage device of the chip through pulse frequency modulation. In order to protect the electric energy storage (energy storage capacitor) equipment with long life and high efficiency, this paper sets the undervoltage threshold (UV) and charging completion threshold (Vbat) for the charging circuit combined with bq2504_ OK), overvoltage threshold (OV), undervoltage and overvoltage threshold are set respectively to avoid excessive discharge and overcharge of energy storage equipment of energy storage capacitor and prolong the working life of energy storage capacitor as much as possible. VBAT_ The setting of OK is used to control the charging and discharging process, and then control the workflow of the whole circuit.
In this paper, combined with the actual situation of charging circuit, this paper sets Vbat_ OV=3.5V，VBAT_ UV=2.8V，VBAT_ OK=3V，VBAT_ OK_ HYST=3.2V.
Then determine the resistance value of the peripheral resistance according to the following formula:
In the circuit, Vbias is the internal reference voltage of chip bq2504, and its value is 1.240v. In the circuit design, it is agreed in this paper that ruv1 + ruv2 = 10m Ω, rov1 + rov2 = 10m Ω, rok1 + rok2 + rok3 = 10m Ω; Combined with equations (2), (3), (4), (5), this paper obtains:
RUV1=4.43M; RUV2=5.57M;
ROV1=5.31M; ROV2=4.69M;
ROK1=3.875M; ROK2=5.5M; ROK3=625K;
2.2.3 design of DC-DC ultra-low voltage boost function circuit
The energy buffer circuit designed in this paper is connected to an energy storage capacitor through a diode D1 at the output position of bq2504 chip. Through the application of energy storage capacitor, when the temperature difference energy is sufficient, the energy after DC-DC conversion can not only be used by wireless sensor nodes, but also the excess energy can be stored in the energy storage capacitor to save energy; When the power collected by the thermoelectric generator is insufficient, the energy storage capacitor can temporarily act as an energy source to ensure the effective operation of the subsequent wireless sensor nodes, and the reverse charging of the thermoelectric generator by the energy storage capacitor is avoided due to the existence of diode D1.
In practical application, selecting the resistance connection circuit according to these resistance values can realize the monitoring and protection of charge and discharge of energy storage capacitor and prolong the working life of energy storage capacitor.
2.3 circuit design of dual voltage comparator mic841n
In this paper, mic841n is used as the voltage comparator, which can detect the storage voltage of the energy storage capacitor and control the working state of the subsequent linear regulator. Figure 4 shows the working reference circuit of mic841n. This paper relies on the reference circuit to reasonably set device parameters such as peripheral resistance to realize its comparison control function.
First, as shown in Fig. 4, connect the circuit, and its Vin is terminated with the positive pole of the energy storage capacitor of the previous circuit; Vin terminal is connected to lth terminal through resistor R2; The lth terminal and the HTH terminal are connected through a resistor R3; HTH terminal resistance R4 and then grounding; Vout is connected to the en terminal of tps78001 chip.
Then determine the resistance value of the peripheral resistance of mic841n according to the following method:
According to the characteristics of mic841n chip, the low voltage threshold is:
For mic841n chip, VREF = 1.240v
Since this paper is to drive a wireless transmitting module, according to the working voltage range (2.4v-3.0v) of the wireless transmitting module used in this paper, VIN (LO) = 2.4V, VIN (LO) = 3.0V. Therefore, the resistance values of peripheral resistors R4, R2 and R3 can be determined in this paper. In this paper, R4 + R2 + R3 = 1m Ω is set in practical operation. Combined with formula (6) and formula (7), R4 = 484k Ω, R2 = 413k Ω, R3 = 103K Ω can be calculated in this paper.
Charging the energy storage capacitor with the energy collected by the micro thermoelectric generator is a process in which the voltage at both ends of the energy storage capacitor increases gradually, and its discharge process is a process in which the voltage at both ends of the capacitor decreases slowly. The voltage at VIN of the input dual voltage comparator mic841n is the voltage at both ends of the capacitor, so the output result of mic841n is shown in Fig. 5.
From the output results, it can be seen that only when the voltage of the capacitor is within a certain range can a high level be output, which can be used to control the interruption of the subsequent regulator module and make effective use of energy.
2.4 tps78001 is the design of discharge voltage stabilizing circuit for energy storage capacitor
In practical application, the voltage at both ends of the electric energy storage device such as energy storage capacitor will gradually decrease with the extension of discharge time. In the example studied in this paper, the energy collected by the micro thermoelectric generator is very limited, and the subsequent radio frequency transmitting module needs to work within a certain voltage range. If the energy storage capacitor is allowed to discharge freely, the radio frequency transmitting module will only work for a short time, and the voltage of the capacitor in other times is not enough for the radio frequency transmitting module, This power will be wasted
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