PhD ThesisThe 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.
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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.EPSRC and BAE
SYSTEMS through the Dorothy Hodgkin Postgraduate Awar