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

DOI: 10.5772/intechopen.72718

Ming-Hung Weng, Daniel Brennan, Nick Wright and Alton Horsfall Ming-Hung Weng, Daniel Brennan,

Harvesting in Extreme Environments

Additional information is available at the end of the chapter Nick Wright and Alton Horsfall

http://dx.doi.org/10.5772/intechopen.72718 Additional information is available at the end of the chapter

#### Abstract

A novel self-starting converter circuit technology is described for energy harvesting and powering wireless sensor nodes, constructed from silicon carbide devices and proprietary high temperature passives for deployment in hostile environments. After a brief review of the advantages using Silicon Carbide (SiC) over other semiconductors in extreme environments, the chapter will describe the advantages and principles when designing circuitry and architectures using SiC for power electronics. The practical results from a novel self-starting DC-DC converter are reported, which is designed to supply power to a WSN for deployment in high temperature environments. The converter operates in the boundary between continuous and discontinuous mode of operation and has a Voltage Conversion Ratio (VCR) of 3 at 300C. This topology is able to self-start and so requires no external control circuitry, making it ideal for energy harvesting applications, where the energy supply may be intermittent. Experimental results for the self-starting converter operating from room temperature up to 300C are presented. The converter output voltage, switching frequency, total power loss and efficiency were presented at temperatures up to 300C.

Keywords: wide band gap semiconductors, silicon carbide, SiC, energy harvesting, wireless sensor networks, high temperature circuit, switching frequency, MOSFETs, JFETs, DC-DC power converters, field effect transistor switches

#### 1. Introduction

In recent years there has been increasing demand to investigate and monitor ever more hostile environments including those containing high temperatures and/or extreme radiation flux [1–3]. Silicon carbide (SiC) boasts a much higher band gap than conventional silicon and is therefore

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

more chemically stable allowing electronic circuits made from this material to be deployed in environments where conventional silicon based electronics cannot function.

SiC based switches such as SiC JFETs are capable of tolerating these elevated temperatures, however, various other components such as passives, magnetics or amplifiers will make this task rather challenging. From a system point of view, the gate drive requirements of normally-on SiC JFETs are a significant challenge. The issue with the start-up process in addition to the differences in the gate voltage requirements make them less desirable for power designers [22, 23]. However, the specific on-resistance of normally-off (enhancement-mode) JFETs is almost 15% higher than their normally-on counterparts [24]. Therefore in a circuit where on state losses are expected to be the dominant power losses, the normally-on (depletion mode) JFETs are better alternatives. Another disadvantage of normally-off SiC JFETs is that in order to keep the device in the on state, the gate-source junction must be forward biased [25]. This implies that similar to SiC

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

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

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based BJTs, there is a considerable drive current requirement, which undesirable.

2. Power sources and energy options for wireless sensor nodes

efficiency is given below in Eq. (1), where the temperature is in Kelvin:

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

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

> <sup>η</sup> <sup>¼</sup> Thigh � Tlow Thigh

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

(1)

2.1. Power sources for wireless sensor nodes

and employed [26].

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> 1 , a 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 frequency oscillators, amplifiers and different topologies power inverters.

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 these elevated temperatures [20, 21].

SiC based switches such as SiC JFETs are capable of tolerating these elevated temperatures, however, various other components such as passives, magnetics or amplifiers will make this task rather challenging. From a system point of view, the gate drive requirements of normally-on SiC JFETs are a significant challenge. The issue with the start-up process in addition to the differences in the gate voltage requirements make them less desirable for power designers [22, 23]. However, the specific on-resistance of normally-off (enhancement-mode) JFETs is almost 15% higher than their normally-on counterparts [24]. Therefore in a circuit where on state losses are expected to be the dominant power losses, the normally-on (depletion mode) JFETs are better alternatives. Another disadvantage of normally-off SiC JFETs is that in order to keep the device in the on state, the gate-source junction must be forward biased [25]. This implies that similar to SiC based BJTs, there is a considerable drive current requirement, which undesirable.
