4. The self-starting DC-DC converter

#### 4.1. Experimental results

through the silicon carbide Schottky diode at a higher voltage level. The voltage induced on the secondary winding then drops due to reduced current flow in the primary and the JFET

During operation the converter operates at the boundary between Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM), the operating point often described as critical conduction mode. When the JFET is non-conducting, the voltage induced on the secondary winding decreases due to reduced current flow in the primary. Therefore the JFET becomes fully conducting again when the inductor current has reached zero, which results in a zero voltage on the gate of the normally-on JFET. The schematic in Figure 6 illustrates the current waveform in the primary inductor for a converter operating in critical conduction mode. During the on time (denoted by DT) the energy stored in the inductor increases and during the off-state (the remainder of the waveform) the inductor fully discharges. The end of the switching period coincides with the point at which the current through the inductor and hence the energy stored in the inductor falls to zero. In Figure 6, the average current Iavg is half the peak current, Ipeak. This peak current is determined by the rate of change of current through the primary inductor during

transistor becomes conducting again to complete the switching cycle.

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

the charging and discharging portions of the waveform.

Figure 5. The self-starting step-up converter without the paralleled RC.

Figure 6. The critical mode of operation, boundary between CCM and DCM.

The SiC JFET and SiC Schottky diode were packaged in high temperature ceramic packages and the circuit was tested in a temperature controlled oven. Both the Schottky diode and JFET were fabricated using process techniques developed at Newcastle University [39–41]. The data in Figure 7 show the I-V characteristics of the JFET used in the circuit.

Of note is the pinch off potential of the device, which is approximately 6.5 V, which is found to be optimal. FETs with a more negative pinch potential require a higher rate of change of current through the primary inductor in order to pinch off the channel – making them unsuitable for use in energy harvesting environments. A pinch off potential closer to zero offers advantages in terms of requiring a lower rate of change of current through the primary inductor. However, when the gate voltage is zero, the on state resistance of the channel is higher [42] and this reduces the efficiency of the converter. As shown by the circuit schematic in Figure 8, a 100 kΩ resistor was used as the load to mimic a wireless sensor node designed for high temperature energy harvesting applications. Typically due to the low power levels available in a circuit powered by energy scavenging, sensor nodes are designed to intermittently use the power generated, thus reducing the time averaged power draw, as there is insufficient power generated to continuously run a sensor node. This is specifically relevant where the node is based on the wireless transmission of data to a remote access node [43].

Figure 7. I-V characteristics of the SiC JFET used in the converter at 25C.

The data in Figure 9 show the voltage waveforms of the JFET during room temperature operation with an input voltage of 1 V. From Figure 9 it can be observed that the gate voltage of the FET (denoted by Vgs) reaches the pinch off potential, setting the current thorough the FET to zero, before rapidly becoming positive, resulting in a significant reduction in the on state resistance. The voltage across the FET channel, denoted by Vds, increases as the gate voltage becomes more negative, reaching a value of 5.5 V when the channel is fully pinched off. This enhancement in the voltage across the FET in comparison to the input voltage arises from the energy stored in the inductor and is the fundamental principle behind the operation of a boost converter. The voltage across the JFET reduces to zero when the gate voltage is positive, showing that the JEFT is conducting.

between 1.3 and 2.5 V over the full 300C range, demonstrating boost capabilities of up to 4.5 times the input voltage. At higher temperatures, the output voltage drops due to the increased SiC diode forward voltage drop, increased JFET on-resistance and increased copper loss in the inductor windings. The effect of these is also apparent in the data shown in Figure 12. At input voltages below 1.2 V, the converter does not self-start at high temperatures. At input voltages of 1, 1.1 and 1.2 V the converter failed to self-start at temperatures above 150, 200 and 250C, respectively. This is thought to be related to the reduction of the mutual inductance between the primary and secondary windings of the inductor caused by the magnetic properties of the ferrite being adversely affected. Therefore the voltage induced in the secondary as a result of the rate of change of current in the primary winding is lower, resulting in the gate voltage of the JFET being insufficient to reach the pinch off potential. In this situation the current through the JFET does not reduce to zero and the energy stored in the inductor is not transferred to the load as the voltage is

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

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

13

lower than the turn on voltage of the diode and the converter does not operate.

Figure 10. Output voltage as a function of temperature.

Figure 11. Switching frequency as a function of temperature.

The data in Figure 10 show the variation in converter output voltage as a function of temperature for a range of input voltages that are relevant to energy harvesting from a thermoelectric generator. It can be seen that the boost converter can successfully operate at input voltages

Figure 8. Schematic of the SiC self-starting DC-DC converter.

Figure 9. Voltage waveforms of the self-starting boost converter.

between 1.3 and 2.5 V over the full 300C range, demonstrating boost capabilities of up to 4.5 times the input voltage. At higher temperatures, the output voltage drops due to the increased SiC diode forward voltage drop, increased JFET on-resistance and increased copper loss in the inductor windings. The effect of these is also apparent in the data shown in Figure 12. At input voltages below 1.2 V, the converter does not self-start at high temperatures. At input voltages of 1, 1.1 and 1.2 V the converter failed to self-start at temperatures above 150, 200 and 250C, respectively. This is thought to be related to the reduction of the mutual inductance between the primary and secondary windings of the inductor caused by the magnetic properties of the ferrite being adversely affected. Therefore the voltage induced in the secondary as a result of the rate of change of current in the primary winding is lower, resulting in the gate voltage of the JFET being insufficient to reach the pinch off potential. In this situation the current through the JFET does not reduce to zero and the energy stored in the inductor is not transferred to the load as the voltage is lower than the turn on voltage of the diode and the converter does not operate.

Figure 10. Output voltage as a function of temperature.

The data in Figure 9 show the voltage waveforms of the JFET during room temperature operation with an input voltage of 1 V. From Figure 9 it can be observed that the gate voltage of the FET (denoted by Vgs) reaches the pinch off potential, setting the current thorough the FET to zero, before rapidly becoming positive, resulting in a significant reduction in the on state resistance. The voltage across the FET channel, denoted by Vds, increases as the gate voltage becomes more negative, reaching a value of 5.5 V when the channel is fully pinched off. This enhancement in the voltage across the FET in comparison to the input voltage arises from the energy stored in the inductor and is the fundamental principle behind the operation of a boost converter. The voltage across the JFET reduces to zero when the gate voltage is

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

The data in Figure 10 show the variation in converter output voltage as a function of temperature for a range of input voltages that are relevant to energy harvesting from a thermoelectric generator. It can be seen that the boost converter can successfully operate at input voltages

positive, showing that the JEFT is conducting.

Figure 8. Schematic of the SiC self-starting DC-DC converter.

Figure 9. Voltage waveforms of the self-starting boost converter.

Figure 11. Switching frequency as a function of temperature.

Figure 12. Power losses as a function of temperature.

In order to enable operation at lower input voltages, the turns ratio between the primary and secondary coils of the transformer can be increased, however this is limited by the physical size of the ferrite core used in this work. Based on the data shown in Figure 8, it can be seen that this converter is capable of boosting the energy harvested from a thermogenerator with a temperature difference of 40C. Energy harvesting from lower temperature differences is possible using two thermogenerators connected electrically in series but thermally in parallel.

across the SiC JFET during switching. The current ripple in the inductor also increases, resulting in higher conduction losses in the inductor. As the output voltage changes with temperature, the output power transferred to the constant resistive load also changes with

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

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

15

Figure 13. Efficiency of boost converter as a function of temperature.

As can be seen from the data in Figure 12, the increasing temperature results a reduction in the overall power losses; which can be explained by the reduced current in the resistive load due to the reduced output voltage. The reduction in the output voltage with temperature results in a reduced drain-source voltage for the JFET, resulting in the JFET operating in the linear region (as can be seen from the data in Figure 7), resulting in the SiC JFET acting more like a resistor at temperatures above 275C and so the power losses start to increase with temperature. This can be seen in Figure 12 for the data relating to input voltages of 1 and 1.1 V, where a significant increase in power loss occurs at 150 and 200C respectively. As the output power from the converter is low, the losses in the circuit will play a significant role in the overall efficiency of

To assess the converter performance at higher output currents, a 10 kΩ resistor was connected as the load and the converter performance was characterised for a range of input voltages at different temperatures. The converter output voltage as a function of temperature for a range of input voltages is shown by the data in Figure 14. As can be seen from the data, the output voltage of the converter decreases as the temperature increases, in a manner similar to that for the 100 kΩ load, as shown in Figure 10. At higher output current levels, the overall conduction and switching losses increase in the converter, resulting in a drop of the output voltage. As the load current has increased (10 kΩ), the output voltage of the converter is lower when com-

Similarly to the operation of the converter with the 100 kΩ output resistor, at input voltages below 1.3 V, the converter is not capable of self-starting at high temperatures. At input voltages of 1, 1.1, 1.2 and 1.3 V the converter did not self-start at temperatures above 125, 175, 200 and 250C,

pared to the case with a 100 kΩ load, due to the increased conduction losses.

temperature.

the circuit.

The results in Figure 11 show the effect of temperature on the operating frequency of the circuit. As described previously, the frequency of operation is determined by the rate of change of current in the primary winding of the transformer (which can be described in terms of the inductance of the winding) and the input voltage from the thermogenerator. This is directly related to change in the material properties of the ferrite core with temperature. As the temperature increases, the permeability and saturation magnetisation of the ferrite core reduces, resulting in the shift in the magnitude of the inductance of the primary winding. Therefore increasing the ambient temperature from 25 to 300C, results in the switching frequency of the converter decreasing from 183 to 161 kHz and from 143 to 107 kHz at input voltages of 2.5 and 1.3 V, respectively. As can be seen from the data in Figure 11, the switching frequency also increases with input voltage when increasing the supply from 1 to 2.5 V. This increase is directly linked to the rate of change of current in the primary winding.

The overall power losses and efficiency of the converter circuit as a function of temperature is shown in Figures 12 and 13, respectively. As can be seen from the data, the efficiency is lower at higher temperatures. This reduction is to be expected, as the resistance of the JFET channel increases due to the reduction in electron mobility [44] and the parasitic resistance of the inductor winding increasing. The diode voltage drop of the SiC Schottky diode also increases with temperature [45], however the effect is minor in comparison to the changes in the JFET and inductor. Hence, the overall power losses in the circuit increase. Increasing the input voltage results in higher power losses in the converter, due to the increased reverse voltage

Figure 13. Efficiency of boost converter as a function of temperature.

In order to enable operation at lower input voltages, the turns ratio between the primary and secondary coils of the transformer can be increased, however this is limited by the physical size of the ferrite core used in this work. Based on the data shown in Figure 8, it can be seen that this converter is capable of boosting the energy harvested from a thermogenerator with a temperature difference of 40C. Energy harvesting from lower temperature differences is possible using two thermogenerators connected electrically in series but thermally in parallel. The results in Figure 11 show the effect of temperature on the operating frequency of the circuit. As described previously, the frequency of operation is determined by the rate of change of current in the primary winding of the transformer (which can be described in terms of the inductance of the winding) and the input voltage from the thermogenerator. This is directly related to change in the material properties of the ferrite core with temperature. As the temperature increases, the permeability and saturation magnetisation of the ferrite core reduces, resulting in the shift in the magnitude of the inductance of the primary winding. Therefore increasing the ambient temperature from 25 to 300C, results in the switching frequency of the converter decreasing from 183 to 161 kHz and from 143 to 107 kHz at input voltages of 2.5 and 1.3 V, respectively. As can be seen from the data in Figure 11, the switching frequency also increases with input voltage when increasing the supply from 1 to 2.5 V. This

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

Figure 12. Power losses as a function of temperature.

increase is directly linked to the rate of change of current in the primary winding.

The overall power losses and efficiency of the converter circuit as a function of temperature is shown in Figures 12 and 13, respectively. As can be seen from the data, the efficiency is lower at higher temperatures. This reduction is to be expected, as the resistance of the JFET channel increases due to the reduction in electron mobility [44] and the parasitic resistance of the inductor winding increasing. The diode voltage drop of the SiC Schottky diode also increases with temperature [45], however the effect is minor in comparison to the changes in the JFET and inductor. Hence, the overall power losses in the circuit increase. Increasing the input voltage results in higher power losses in the converter, due to the increased reverse voltage across the SiC JFET during switching. The current ripple in the inductor also increases, resulting in higher conduction losses in the inductor. As the output voltage changes with temperature, the output power transferred to the constant resistive load also changes with temperature.

As can be seen from the data in Figure 12, the increasing temperature results a reduction in the overall power losses; which can be explained by the reduced current in the resistive load due to the reduced output voltage. The reduction in the output voltage with temperature results in a reduced drain-source voltage for the JFET, resulting in the JFET operating in the linear region (as can be seen from the data in Figure 7), resulting in the SiC JFET acting more like a resistor at temperatures above 275C and so the power losses start to increase with temperature. This can be seen in Figure 12 for the data relating to input voltages of 1 and 1.1 V, where a significant increase in power loss occurs at 150 and 200C respectively. As the output power from the converter is low, the losses in the circuit will play a significant role in the overall efficiency of the circuit.

To assess the converter performance at higher output currents, a 10 kΩ resistor was connected as the load and the converter performance was characterised for a range of input voltages at different temperatures. The converter output voltage as a function of temperature for a range of input voltages is shown by the data in Figure 14. As can be seen from the data, the output voltage of the converter decreases as the temperature increases, in a manner similar to that for the 100 kΩ load, as shown in Figure 10. At higher output current levels, the overall conduction and switching losses increase in the converter, resulting in a drop of the output voltage. As the load current has increased (10 kΩ), the output voltage of the converter is lower when compared to the case with a 100 kΩ load, due to the increased conduction losses.

Similarly to the operation of the converter with the 100 kΩ output resistor, at input voltages below 1.3 V, the converter is not capable of self-starting at high temperatures. At input voltages of 1, 1.1, 1.2 and 1.3 V the converter did not self-start at temperatures above 125, 175, 200 and 250C, respectively. At a higher output current, the effective voltage across the primary inductor is lower; due to the increased voltage drop across the parasitic resistance in the inductor, resulting in a lower induced voltage in the secondary winding. As described for the 100 kΩ load above, if this gate voltage on the JFET, which is equal to the voltage induced in the secondary winding does not reach the pinch off potential of the device, the converter will not oscillate. Hence for the higher output current (10 kΩ load) the converter is capable of self-starting operation at lower temperatures in comparison to the case with a 100 kΩ load resistor.

converter supplying a 100 kΩ load. As described for the 100 kΩ converter, at higher temperatures the increased resistance of both the JFET channel and the windings within the transformer itself result in a reduction in efficiency. The SiC diode voltage drop also increases with temperature, so the overall conduction loss in the circuit increases, resulting in a decrease in the system efficiency. However, it can be seen from the data that the decrease in efficiency with increasing temperature is not as significant for the high output power circuit. As the converter output power is significantly higher, the power losses in the circuit will play a less significant

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

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

17

A novel self-starting converter technology has been described, which is suitable for powering wireless sensor nodes by means of energy harvesting from a thermal gradient. The converter was constructed from silicon carbide devices and proprietary high temperature passives to enable deployment in hostile environments, such as those found in aerospace, oil and gas and nuclear applications. The self-oscillating nature of the circuit along with high temperature capability result in reduced component count and hence a more reliable approach for powering SiC based WSNs for hostile environments. The self-oscillating nature of the circuit along with high temperature capability result in reduced component count and hence a more reliable approach for powering SiC based WSNs for hostile environments. The operation principle of the self-starting converter was detailed for two configurations which reflect low and high output current. The effect of input voltage and primary inductance on the converter operation and switching frequency was correlated with the characteristics of the components used in the circuit manufacture and the operating conditions for the circuit. Experimental measurements on a converter showed that whilst the performance of the circuit is influenced by the ambient temperature, it is possible to boost the voltage from a thermoelectric generator

, Daniel Brennan1,2, Nick Wright1 and Alton Horsfall<sup>1</sup>

[1] Issa F et al. Radiation silicon carbide detectors based on ion implantation of boron. IEEE

\*

role in the overall efficiency of the converter hence the converter efficiency is higher.

to a level that is suitable for the operation of high temperature circuits.

1 School of Engineering, Newcastle University, Newcastle upon Tyne, UK

\*Address all correspondence to: alton.horsfall@ncl.ac.uk

Transactions on Nuclear Science. 2014;61:2105-2111

5. Conclusions

Author details

Ming-Hung Weng<sup>1</sup>

References

2 IsoCom Limited, Washington, UK

The converter overall efficiency as a function of temperature is shown by the data in Figure 15. As can be seen from the data, the efficiency of the converter is approximately twice that of the

Figure 14. Output voltage as a function of temperature.

Figure 15. Efficiency of boost converter as a function of temperature.

converter supplying a 100 kΩ load. As described for the 100 kΩ converter, at higher temperatures the increased resistance of both the JFET channel and the windings within the transformer itself result in a reduction in efficiency. The SiC diode voltage drop also increases with temperature, so the overall conduction loss in the circuit increases, resulting in a decrease in the system efficiency. However, it can be seen from the data that the decrease in efficiency with increasing temperature is not as significant for the high output power circuit. As the converter output power is significantly higher, the power losses in the circuit will play a less significant role in the overall efficiency of the converter hence the converter efficiency is higher.
