Gallium Nitride Transistors in Extreme Temperatures

Exposure of electronic circuits to extreme temperatures are encountered in the well intervention industry. To perform maintenance on pipes, a robotic tool can be loweredin to the well environment where ambient temperatures can reach up to 200 ◦C. Component stress factors and losses under these temperatures must be kept minimal toextend expected life time as far as possible. Wide-Bandgap (WBG) have shown operational capability at 200 ◦C and higher. Where the majority of published research work on Silicon Carbide (SiC) for high temperature in high power converter, topics on Gallium Nitride (GaN) for lower power aim to gain more coverage. In this PhD thesis, the efect of temperature in GaN devices is studied. After device characterization up to 225 ◦C, a switched power stage circuit was investigated in idle operation. Two dead time strategies were employed: a Long Dead Time (LDT) and Short Dead Time (SDT) were compared on idle power dissipation. The diference showed that precise timing in the switched power stage is of high importance towards losses and therefore converter reliability. A synchronous buck converter was implemented to study the efect of load and temperature towards the switch node voltage behaviour. The converter was specifed for an input voltage of 50 V and output voltage of 5 V at 1 A. Again, two dead time strategies were deployed and it was proven that LDT aid converter efciency with lower efciency variation in increasing temperature. At maximum output power of 5W in ambient temperature of 175 ◦C, SDT reached an efciency of 78.7%, where LDT reached 81.9%. Further optimizations could be made with an Adaptive Dead Time (ADT) circuit, adjusting the dead time by sensing the inductor current and the operating temperature. Implementation of the circuit did not result in a higher efciency, falling 0.4% short, but improved the predictability of the converter operation over varying load and temperature conditions. The ADT circuit was further investigated using dynamic loads where reduction up to 9.1% in device operating temperature at 25 ◦C was achieved. A class D amplifer was implemented using GaN-FETs and was optimized for high temperature operation at 150 ◦C to function as a transmitter for a Power Line Communication (PLC) system. The amplifer was then compared with a class AB amplifer stage on signal integrity and capability to operate in extreme conditions. A maximum bandwidth of 150 kHz was established by models and measurements of an envisioned communication channel. The results showed that the class AB signal integrity was superior over the class D topology by 30 dB in THD measuremen at 150 ◦C. As the class D efciency was optimized for efciency performance at 150 ◦C, the power stage showed an efciency of 90.3% at 1.34W, resulting in low component stress and power dissipation. The conclusion can be made that GaN-FETs show operational capabilities for both high and extreme temperature deployment. Characterization was successful up to 225 ◦C and power converter applications have been demonstrated at 150 ◦C, 175 ◦C and 200 ◦C