3. Theory behind the self-starting DC-DC converter

Figure 3 shows the circuit diagram of the proposed self-starting DC-DC converter designed for boosting low level voltages from the TEG, denoted in the Figure 3 as Vin. The input capacitance of the circuit, Cin, represents the capacitance of the p-n junctions of the TEG. Based upon a standard boost converter topology and a blocking oscillator, the key aspect of the design is the use of a counter-wound secondary winding in conjunction with a normally-on device which is used to provide the self-oscillatory behaviour, thus eliminating the need for an external gate drive [38]. The elimination of a separate gate drive is crucial for the successful commissioning of a silicon carbide energy harvesting system designed for use with low voltage DC sources such as solar cells and thermogenerators. The voltages provided by these sources are magnitudes smaller than the voltages required to provide the gate drive requirements for a conventional converter topology; therefore a self-oscillating design becomes the only viable option.

The simplicity of the design is also important when considering the capability of high temperature components. At present, commercially available SiC components are limited to discrete power devices and do not include the control circuitry required for complex designs. The commercial drive behind silicon carbide electronic devices to date has been focussed on the power electronics market, where the ability of silicon carbide to operate at high frequencies and with low power losses has been utilised for the realisation of highly efficient circuits that offer significant space saving over conventional systems. Here it is demonstrated that the ability of silicon carbide components to operate at these high temperatures can be harnessed for the production of a stepup converter with the ability to power circuits within the high temperature environment itself, with minimum component count, thus reducing the overall cost of a high temperature energy harvesting system.

winding of the coupled inductor increases exponentially with time reducing the voltage across the primary winding, inducing a voltage on the secondary winding of the transformer due to the change in the primary current. Capacitor C1 is charged to a negative voltage with respect to the circuit ground. When the bias across the capacitor exceeds the pinch off potential of the JFET, the on state resistance of the JFET increases, so that the current through the FET drops to zero. This reduction in current flowing through the JFET results in the decrease in the current flow through the primary winding of the transformer. When the current in the primary winding becomes zero, the voltage on the secondary winding reaches zero as well and C1 is discharged through the resistor R1 to ground. Once the bias across the capacitor falls below the pinch off potential of the JFET, the on state resistance of the JFET reduces significantly, current flows through the primary winding of the inductor and the circuit operation repeats. The switching frequency of the converter is determined primarily by the gate-source capacitance

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of the SiC JFET and the inductance of the primary side of the drive transformer.

without the inclusion of the external RC components.

Figure 4. The self-starting step-up converter with the paralleled RC.

As shown by the circuit topology in Figure 5 the RC circuit connected to the JFET gate can be removed and the secondary winding directly connected to ground. The stray capacitance of the primary winding is sufficient to enable the self-oscillation occur and the converter operates

When the normally-on silicon carbide JFET conducts, current begins to flow through the primary winding of the transformer and the channel of the JFET to ground, this induces a negative bias in the secondary winding. As the current flowing through the primary winding increases, the negative voltage on the secondary winding increases in magnitude and the channel of the JFET is progressively pushed toward pinch off. Once the magnitude of the voltage on the secondary winding reaches the threshold voltage of the JFET, the JFET becomes non-conducting. This causes the magnetic field contained in the ferrite core of the transformer to collapse and the voltage in the primary winding increases as is observed in a standard boost converter topology. Whilst the JFET is non-conducting, power is transferred to the output

#### 3.1. Principle of operation

As shown in Figure 4, the self-oscillating converter circuit uses a depletion mode JFET and a Schottky diode in a boost configuration, wherein coupled inductors are used to feed the gatesource of the SiC based switching device and act to initialise the oscillations. The operation of the circuit is as follows; at start-up and as the input voltage rises, the SiC JFET as a normally-on device conducts and the current flows through the inductor. The current in the primary

Figure 3. The proposed self-starting boost converter.

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Figure 4. The self-starting step-up converter with the paralleled RC.

3. Theory behind the self-starting DC-DC converter

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

harvesting system.

3.1. Principle of operation

Figure 3. The proposed self-starting boost converter.

Figure 3 shows the circuit diagram of the proposed self-starting DC-DC converter designed for boosting low level voltages from the TEG, denoted in the Figure 3 as Vin. The input capacitance of the circuit, Cin, represents the capacitance of the p-n junctions of the TEG. Based upon a standard boost converter topology and a blocking oscillator, the key aspect of the design is the use of a counter-wound secondary winding in conjunction with a normally-on device which is used to provide the self-oscillatory behaviour, thus eliminating the need for an external gate drive [38]. The elimination of a separate gate drive is crucial for the successful commissioning of a silicon carbide energy harvesting system designed for use with low voltage DC sources such as solar cells and thermogenerators. The voltages provided by these sources are magnitudes smaller than the voltages required to provide the gate drive requirements for a conventional

converter topology; therefore a self-oscillating design becomes the only viable option.

The simplicity of the design is also important when considering the capability of high temperature components. At present, commercially available SiC components are limited to discrete power devices and do not include the control circuitry required for complex designs. The commercial drive behind silicon carbide electronic devices to date has been focussed on the power electronics market, where the ability of silicon carbide to operate at high frequencies and with low power losses has been utilised for the realisation of highly efficient circuits that offer significant space saving over conventional systems. Here it is demonstrated that the ability of silicon carbide components to operate at these high temperatures can be harnessed for the production of a stepup converter with the ability to power circuits within the high temperature environment itself, with minimum component count, thus reducing the overall cost of a high temperature energy

As shown in Figure 4, the self-oscillating converter circuit uses a depletion mode JFET and a Schottky diode in a boost configuration, wherein coupled inductors are used to feed the gatesource of the SiC based switching device and act to initialise the oscillations. The operation of the circuit is as follows; at start-up and as the input voltage rises, the SiC JFET as a normally-on device conducts and the current flows through the inductor. The current in the primary winding of the coupled inductor increases exponentially with time reducing the voltage across the primary winding, inducing a voltage on the secondary winding of the transformer due to the change in the primary current. Capacitor C1 is charged to a negative voltage with respect to the circuit ground. When the bias across the capacitor exceeds the pinch off potential of the JFET, the on state resistance of the JFET increases, so that the current through the FET drops to zero. This reduction in current flowing through the JFET results in the decrease in the current flow through the primary winding of the transformer. When the current in the primary winding becomes zero, the voltage on the secondary winding reaches zero as well and C1 is discharged through the resistor R1 to ground. Once the bias across the capacitor falls below the pinch off potential of the JFET, the on state resistance of the JFET reduces significantly, current flows through the primary winding of the inductor and the circuit operation repeats. The switching frequency of the converter is determined primarily by the gate-source capacitance of the SiC JFET and the inductance of the primary side of the drive transformer.

As shown by the circuit topology in Figure 5 the RC circuit connected to the JFET gate can be removed and the secondary winding directly connected to ground. The stray capacitance of the primary winding is sufficient to enable the self-oscillation occur and the converter operates without the inclusion of the external RC components.

When the normally-on silicon carbide JFET conducts, current begins to flow through the primary winding of the transformer and the channel of the JFET to ground, this induces a negative bias in the secondary winding. As the current flowing through the primary winding increases, the negative voltage on the secondary winding increases in magnitude and the channel of the JFET is progressively pushed toward pinch off. Once the magnitude of the voltage on the secondary winding reaches the threshold voltage of the JFET, the JFET becomes non-conducting. This causes the magnetic field contained in the ferrite core of the transformer to collapse and the voltage in the primary winding increases as is observed in a standard boost converter topology. Whilst the JFET is non-conducting, power is transferred to the output 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 transistor becomes conducting again to complete the switching cycle.

4. The self-starting DC-DC converter

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

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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].

in Figure 7 show the I-V characteristics of the JFET used in the circuit.

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

4.1. Experimental results

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 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.
