2. Power sources and energy options for wireless sensor nodes

#### 2.1. Power sources for wireless sensor nodes

more chemically stable allowing electronic circuits made from this material to be deployed in

The development of silicon carbide (SiC) technology has become increasingly rapid in recent years with significant improvements in wafer and epitaxy growth technology of 8-inch and beyond [4]. By offering scalable and cost-effectively materials processing, SiC devices have become part of the mainstream in power electronic applications [5, 6] and a number of conferences are running dedicated sessions to the deployment of this technology from niche to mainstream. Further advantages in terms of small, signal level devices [7], sensors [8] and CMOS circuitry [9] have resulted in significant improvements in capabilities, however these are still only available at research level, where a number of groups are active. During recent years, SiC has emerged as a promising material for electronics where the underlying rationale for the continued investment in SiC technology is the excellent material properties. SiC is the only technologically relevant semiconductor that has a stable thermal oxide (SiO2) [10] and can exist in a large number of polytypes – different crystal structures built and derived from the high chemical stability of the SiC sub-unit organised into different stacking sequences. All forms of SiC are considered a wide bandgap material since the electronic bandgaps of the different polytypes range from 2.4 to 3.3 eV. The bandgap of 4H SiC is 3.23 eV at room temperature (compared to 1.12 eV for silicon) and this dramatically reduces the intrinsic carrier concentration in comparison to semiconductors such as silicon or gallium arsenide and this allows devices to theoretically operate at temperatures up to 1000C [11]. Electrical properties of SiC tailored toward devices also has a high saturation electron velocity, 2 107 cm<sup>s</sup>

thermal conductivity in excess of copper at room temperature and a critical electric field that is almost an order of magnitude higher than that of silicon. Fulfilling technical parameters and standards, SiC has been exploited in the high performance power MOSFETs and diodes that are commercially available, but also have the potential to realise high performance, high

For these high temperature environments are incompatible with standard battery technologies, and so, energy harvesting is a suitable technology when remote monitoring of these extreme environments is performed through the use of wireless sensor nodes (WSNs) [12–14]. There is now a variety of energy harvesting devices available which are capable of producing sufficient energy from the ambient surroundings to intermittently power a WSN [15–17]. Energy harvesting devices often produce voltages which are unusable directly by electronic loads and so require power management circuits to convert the electrical output to a level which is usable by monitoring electronics and sensors. Therefore a DC-DC step-up converter that can handle low input voltages is required [18, 19]. The required gate-drive circuitry for these converters need to be placed next to the switches to minimise system complexity, however, the successful operation of the gate drivers, especially with no heat sink in hostile environments will increase the power density for DC-DC converter modules. The advantages of SiC based power devices include high current densities, faster switching speeds and high temperature capabilities. To fully utilise the benefits of SiC devices in DC-DC converters used in harsh environments, the gate drive design requires special attention. To match the high temperature capabilities of SiC devices, the gate drivers also need to be capable of operation at

frequency oscillators, amplifiers and different topologies power inverters.

these elevated temperatures [20, 21].

1 , a

environments where conventional silicon based electronics cannot function.

4 Advanced Electronic Circuits - Principles, Architectures and Applications on Emerging Technologies

Wireless sensor networks have become a very popular enabling technology and have already entered the market place in a number of sectors. The majority of these platforms are powered by limited-life batteries. Hence alternative power sources are being continuously investigated and employed [26].

The rapid reduction in the size and power consumption of electronic components has helped speed up the research on communication nodes and wireless sensors. As the size of these WSNs decreases, their use becomes more widespread in the automobile industry, industrial environments and aerospace industry. However, their respective power supply has become a major issue, because the size reduction in CMOS electronics has significantly outpaced the energy density improvements in batteries, which are the most commonly used power sources. Consequently, the power supply is the limiting factor on both the size and lifetime of the sensor node. Energy reservoir power sources such as micro-scale batteries, micro-fuel cells, ultra-capacitors are characterised by their energy density and can be used to power WSNs but at the cost of increased size and reduced lifetime. Power scavenging sources are an alternative power source. Unlike energy reservoirs, power scavenging sources are characterised by their power density; the energy provided from these sources depends on the amount of time each source is in operation [26–29]. One of the popular power scavenging sources is via temperature gradients [30, 31]. Energy can be scavenged from the environment using the temperature variations that naturally occur. The maximum power-conversion efficiency from a temperature difference, the Carnot efficiency is given below in Eq. (1), where the temperature is in Kelvin:

$$\eta = \frac{\left(T\_{\text{high}} - T\_{\text{low}}\right)}{T\_{\text{high}}} \tag{1}$$

Assuming a room temperature of 27�C and for a source 5�C above room temperature, the maximum efficiency is 1.64% and for a source 10�C above room temperature is maximum efficiency is 3.22%. At low temperature differences and small scales, conduction will dominate and convection and radiation can be neglected. The heat flow through conduction is given by Eq. (2), where L is the length of material that the heat is flowing through and k is the thermal conductivity of the material used:

$$q'=k\frac{(\Delta T)}{L}\tag{2}$$

In this work a standard off-the-shelf TEG manufactured by Marlow (product number TG 12-801 L) was characterised in terms of the electrical output as a function of temperature difference between the two surfaces. A ceramic hotplate was used to provide a controlled temperature heat source whilst a thermocouple embedded into the base of a heat sink and fan provided the cooler side thus creating a measurable temperature difference across the device. The higher the temperature difference, the greater the output voltage becomes at any given current. Figure 2 shows the output power of the thermoelectric generator as a function of the output voltage. The solid line intersects with the waveforms where the maximum output power is at the optimum output voltages. The optimum voltage for the TEG is generally below 1.3 V and needs to be boosted in order to enable

Self-Oscillatory DC-DC Converter Circuits for Energy Harvesting in Extreme Environments

http://dx.doi.org/10.5772/intechopen.72718

7

In addition to the Voltage Conversion Ratio (VCR) requirements to supply a SiC sensor circuit, operating in a very high temperature environment (up to 300C) demands a high temperature step-up DC-DC converter. In addition to the power stage of the converter, the gate-drive circuitry is also required to operate at elevated temperatures. To eliminate the need for a high temperature gate driver and also to reduce the size of the power management circuitry, a self-starting DC-DC converter is desired [34]. Here, a self-starting DC-DC converter was designed to boost the low DC output voltage of a thermoelectric generator to a level sufficient to run a SiC sensor circuit for wireless monitoring of inhospitable environments [35–37]. These environments may be subject to high temperatures in the case of exhaust gas monitoring in turbine engines or oven environments, they may also be subject to radiation in the nuclear industry whether they are used in power generation or waste monitoring. The proposed DC-DC converter does not need an auxiliary power supply to drive the normally-on JFET. The converter self-starts and does not suffer from a start-up shoot through. The requirement of self-oscillation needs a depletion mode

device (e.g. normally-on JFET) as there will be no current flowing at start otherwise.

Figure 2. Output power as a function of voltage for the thermoelectric generator for various temperature differences.

the operation of the circuit for remote sensor applications.

2.3. The need for a high temperature self-starting DC-DC converter

Assuming a length of 1 cm and a temperature difference of 10�C, the heat flow (power) for silicon with a thermal conductivity of 140 W/mK is 14 W/cm<sup>2</sup> . Assuming that Carnot efficiency could be achieved, the output power would be 451 mW/cm<sup>2</sup> which is significantly higher than comparable power sources. In practice, the efficiencies for this type of energy harvester are well below the Carnot efficiency. One of the most common ways to convert the generated power from temperature differences to electricity is by using thermoelectric generators.

#### 2.2. Thermoelectric generators

Driving a wireless sensor node from ambient is attractive as it eliminates the need for wires or batteries. Despite the clear advantages of energy harvesting, these systems require a suitable power management strategy to convert the low voltage levels to a level usable by the wireless sensor systems. Many WSNs monitor physical quantities, which change slowly and therefore the measurements can be taken and transmitted less frequently. This means lower operating duty cycle and therefore many wireless sensor systems consume very low average power, hence they are suitable candidates for energy harvesting power sources.

Thermoelectric generators (TEGs) are energy harvesting devices capable of producing large amounts of current at low voltages from a thermal gradient across the device; through a phenomenon known as the Seebeck effect [32]. Modern TEG's are constructed out of p-n junctions of different semiconductor materials depending on their operational requirements, but commonly bismuth telluride (Bi2Te3). The mechanical construction of a typical TEG is shown in Figure 1.

Figure 1. Construction of a thermoelectric generator [33].

In this work a standard off-the-shelf TEG manufactured by Marlow (product number TG 12-801 L) was characterised in terms of the electrical output as a function of temperature difference between the two surfaces. A ceramic hotplate was used to provide a controlled temperature heat source whilst a thermocouple embedded into the base of a heat sink and fan provided the cooler side thus creating a measurable temperature difference across the device. The higher the temperature difference, the greater the output voltage becomes at any given current. Figure 2 shows the output power of the thermoelectric generator as a function of the output voltage. The solid line intersects with the waveforms where the maximum output power is at the optimum output voltages. The optimum voltage for the TEG is generally below 1.3 V and needs to be boosted in order to enable the operation of the circuit for remote sensor applications.

#### 2.3. The need for a high temperature self-starting DC-DC converter

and convection and radiation can be neglected. The heat flow through conduction is given by Eq. (2), where L is the length of material that the heat is flowing through and k is the thermal

6 Advanced Electronic Circuits - Principles, Architectures and Applications on Emerging Technologies

ð Þ ΔT

<sup>L</sup> (2)

. Assuming that Carnot efficiency

q<sup>0</sup> ¼ k

silicon with a thermal conductivity of 140 W/mK is 14 W/cm<sup>2</sup>

hence they are suitable candidates for energy harvesting power sources.

Assuming a length of 1 cm and a temperature difference of 10�C, the heat flow (power) for

could be achieved, the output power would be 451 mW/cm<sup>2</sup> which is significantly higher than comparable power sources. In practice, the efficiencies for this type of energy harvester are well below the Carnot efficiency. One of the most common ways to convert the generated power from temperature differences to electricity is by using thermoelectric generators.

Driving a wireless sensor node from ambient is attractive as it eliminates the need for wires or batteries. Despite the clear advantages of energy harvesting, these systems require a suitable power management strategy to convert the low voltage levels to a level usable by the wireless sensor systems. Many WSNs monitor physical quantities, which change slowly and therefore the measurements can be taken and transmitted less frequently. This means lower operating duty cycle and therefore many wireless sensor systems consume very low average power,

Thermoelectric generators (TEGs) are energy harvesting devices capable of producing large amounts of current at low voltages from a thermal gradient across the device; through a phenomenon known as the Seebeck effect [32]. Modern TEG's are constructed out of p-n junctions of different semiconductor materials depending on their operational requirements, but commonly bismuth telluride (Bi2Te3). The mechanical construction of a typical TEG is shown in Figure 1.

conductivity of the material used:

2.2. Thermoelectric generators

Figure 1. Construction of a thermoelectric generator [33].

In addition to the Voltage Conversion Ratio (VCR) requirements to supply a SiC sensor circuit, operating in a very high temperature environment (up to 300C) demands a high temperature step-up DC-DC converter. In addition to the power stage of the converter, the gate-drive circuitry is also required to operate at elevated temperatures. To eliminate the need for a high temperature gate driver and also to reduce the size of the power management circuitry, a self-starting DC-DC converter is desired [34]. Here, a self-starting DC-DC converter was designed to boost the low DC output voltage of a thermoelectric generator to a level sufficient to run a SiC sensor circuit for wireless monitoring of inhospitable environments [35–37]. These environments may be subject to high temperatures in the case of exhaust gas monitoring in turbine engines or oven environments, they may also be subject to radiation in the nuclear industry whether they are used in power generation or waste monitoring. The proposed DC-DC converter does not need an auxiliary power supply to drive the normally-on JFET. The converter self-starts and does not suffer from a start-up shoot through. The requirement of self-oscillation needs a depletion mode device (e.g. normally-on JFET) as there will be no current flowing at start otherwise.

Figure 2. Output power as a function of voltage for the thermoelectric generator for various temperature differences.
