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DC-DC converters (Buck and Boost) are referred to in the literature as direct converters, since power transfer from input to output occurs directly, without passing through intermediate energy storage elements. Rectification and DC–DC conversion circuits are capable of efficiently converting energy from energy harvesters for low-power systems. In this development, the focus is to characterize DC–DC converters for bioelectrochemical generators that will be arranged in an appropriate electrical configuration in terms of connection, size, and area. The objective of the project is the development of step-up voltage converters, the execution of bench tests, and the performance of field tests to supply energy to smart buoys.
Boost converters are known as step-up converters because they increase the input voltage level by storing energy in an inductor placed in series with the input. The buck converter is called a step-down converter because its output voltage is a fraction of the input voltage.
DC-DC converters can be defined as systems composed of power semiconductors operating as switches and passive elements, typically inductors and capacitors, whose function is to control the flow of power from an input source to an output source. These converters may be either isolated or non-isolated.
Isolated converters use coupled inductors; examples of these topologies include flyback, forward, and push-pull converters. Non-isolated converters, on the other hand, are based on structures that do not use galvanic isolation. Some topologies of this type include buck, boost, buck-boost, Ćuk, SEPIC, and Zeta.
In this project, a DC-DC converter will be used to step up the input voltage generated by the bioeletrochemical generators and convert it into usable voltage levels. Figure 1 and 2.
Figure 1:
Figure 2:
RESEARCH OBJECTIVES
Design DC–DC converters for bioelectrochemical generators with output voltages of 3.3 V and 5 V, suitable for powering IoT devices.
Specific objectives:
a) Bench-test the bioelectrochemical generators.
b) Develop the printed circuit board design.
c) Bench-test the DC–DC converter boards.
d) Design and build converters to be embedded in signaling and monitoring buoys.
e) Field-test the signaling and monitoring buoys.
Electrical design
For the execution of the project, commercial boards listed in Table 1 were identified. Figures 3, 4, 5, 6 and 7 present the models that may be used to power IoT devices and are capable of boosting the voltage while maintaining the output current up to the specified limit.
Figure3: TPS61021EVM-729
Figure 4: TPS61021EVM-723
Figure 5: LTC3105
Figure 6: TPS120EVM-179
Figure 7: ISL9111
The step-up board design can be implemented using the selected ICs. For output voltages of 3.3 V and 3.9 V, the TPS61201A IC was selected, with the output voltage configuration achieved by choosing two resistors with the values listed below:
R1 = 316 kΩ and R2 = 100 kΩ, thus Vout = 3.3 V;
R1 = 390 kΩ and R2 = 100 kΩ, thus Vout = 3.9 V.
The electrical schematic used is shown in Figure 6. The board layout is shown in Figure 8, and the 3D rendering of the board is shown in Figure 9.
Figure 8 – Electrical schematic used for configuring the output voltage
Figure 9 – Printed circuit board design.
Figure 10 – 3D model of the printed circuit board.
The design of the step-up board with output voltages of 3.0 V and 5.0 V used the ISL9111EH30Z and ISL9111EH50Z ICs. In these two models, feedback resistors are not required.
The electrical schematic used is shown in Figure 11. The board layout is shown in Figure 10, and the 3D rendering of the board is shown in Figure 12.
Figure 11 – Electrical schematic using the ISL9111 IC.
Figure 12 – Printed circuit board design for the ISL9111 IC.
Figure 13 – 3D model of the printed circuit board for the ISL9111 IC.
BENCH TESTING
For the bench tests, the initial tests were carried out with the generator boards connected to resistors and LEDs.
DC–DC converters are versatile devices that make it possible to step voltages up and/or down. Since the purpose of this work is voltage boosting, the converters used followed a boost topology. It should be noted that connecting the generator board to a converter circuit is very different from using a purely resistive load. Converters include energy storage elements such as inductors and capacitors, and also use a diode and a transistor in their basic configuration. In addition, they require a signal generation circuit and a driver to control the switching element, which makes the correct selection of the converters essential.
It is also important to emphasize that boost converters, once built, can increase the output voltage; however, the output power will always be equal to the input power minus the losses in the converter and driver. Therefore, it is not possible to increase current or power through the DC–DC conversion process. Figure 14 shows an explanatory diagram of how input and output power are related. The ICs selected for the project have efficiencies above 90%.
Figure 14 – Power diagram.
The initial tests were performed using the TPS61021 board, which has the following characteristics:
a) Input voltage range: 0.5 V to 4.4 V;
b) 0.9 V minimum start-up voltage;
c) Output voltage range: 1.8 V to 4.0 V;
d) 91% efficiency at VIN = 2.4 V, VOUT = 3.3 V, and IOUT = 1.5 A;
e) IOUT > 1.5 A with VOUT = 3.3 V when VIN > 1.8 V;
f) Typical operating current of 17 µA.
Figure 15 – Board with the TPS6121A IC under testing.
For the board with fixed output voltages of 3.0 V and 5.0 V using the ISL9111 IC, the boards were fabricated on a PCB prototyping machine. Figures 16 and 17 shows the finished board.
Figure 16 – Printed circuit board for the ISL9111 IC
Figure 17 – Soldered printed circuit board for the ISL9111 IC
Figure 18 – Printed circuit board for the ISL9111 IC under testing.
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