Silicon carbide based DC-DC converters for deployment in hostile environments

Abstract

The development of power modules for deployment in hostile environments, where the elevated ambient temperatures demand high temperature capability of the entire converter system, requires innovative power electronic circuits to meet stringent requirements in terms of efficiency, power-density and reliability. To simultaneously meet these conflicting requirements in extreme environment applications is quite challenging. To realise these power modules, the relevant control circuitry also needs to operate at elevated temperatures. The recent advances in silicon carbide devices has allowed the realisation of not just high frequency, high efficiency power converters, but also the power electronic converters that can operate at elevated temperatures, beyond those possible using conventional silicon-based technology. High power-density power converters are key components for power supply systems in applications where space and weight are critical parameters. The demand for higher power density requires the use of high-frequency DC-DC converters to overcome the increase in size and power losses due to the use of transformers. The increase in converter switching frequency reduces the size of passive components whilst increasing the electromagnetic interference (EMI) emissions. A performance comparison of SiC MOSFETs and JFETs in a high-power DC-DC converter to form part of a single phase PV inverter system is presented. The drive design requirements for optimum performance in the energy conversion system are also detailed. The converter was tested under continuous conduction mode at frequencies up to 250 kHz. The converter power efficiency, switch power loss and temperature measurements are then compared with the ultra-high speed CoolMOS switches and SiC diodes. The high voltage, high frequency and high temperature operation capability of the SiC DUTs are also demonstrated. The all SiC converters showed more stable efficiencies of 95.5% and 96% for the switching frequency range for the SiC MOSFET and JFET, respectively. A comparison of radiated noise showed the highest noise signature for the SiC JFET and lowest for the SiC MOSFET. The negative gate voltage requirement of the SiC MOSFET introduces up to 6 dBμV increase in radiated noise, due to the induced current in the high frequency resonant stray loop in the gate drive negative power plane. ii A gate driver is an essential part of any power electronic circuitry to control the switching of the power semiconductor devices. The desire to place the gate driver physically close to the power switches in the converter, leads to the necessity of a temperature resilient PWM generator to control the power electronics module. At elevated temperatures, the ability to control electrical systems will be a key enabler for future technology enhancements. Here an SiC/SOI-based PWM gate driver is proposed and designed using a current source technique to accomplish variable duty-cycle PWM generation. The ring oscillator and constant current source stages use low power normally-on, epitaxial SiC-JFETs fabricated at Newcastle University. The amplification and control stages use enhancement-mode signal SOI MOSFETs. Both SOI MOSFETs will be replaced by future high current SiC-JFETs with only minor modification to the clamp-stage circuit design. In the proposed design, the duty cycle can be varied from 10% to 90%. The PWM generator is then evaluated in a 200 kHz step-up converter which results in a 91% efficiency at 81% duty cycle. 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. 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. The first demonstration of a novel self-starting DC-DC converter is reported, to supply power to a wireless sensor node for deployment in high temperature environments. Utilising SiC devices a novel boost converter topology has been realised which is suitable for boosting a low voltage to a level sufficient to power a sensor node at temperatures up to 300 °C. The converter operates in the boundary between continuous and discontinuous mode of operation and has a VCR of 3 at 300 °C. 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.