Description
The buck-boost inverter uses a pair of coupled inductors, four switches and 2 isolation diodes to accomplish buck-boost operations, converting an input dc voltage of a wide variation range into a controlled ac output. The ac output can be controlled to have a low total harmonic distortion, and can be fed to a utility grid or to a standalone load.

The inverter must go through three modes to transfer power from the dc input to the ac output. Mode 1 involves storing magnetic energy into an inductor which is the primary side of the fly-back transformer. This is also known as the charging mode.

Mode 2 involves allowing current to flow into the load and Mode 3 releases the stored magnetic energy into the load via another inductor.

The inverter can be operated in a Discontinuous Conduction Mode (DCM) or in a Continuous Conduction Mode (CCM). By implementing Pulse-Width Modulation (PWM) strategies for the control of power semiconductor switches, a controlled ac output with low harmonic distortion is achieved.

The control strategies include sinusoidal PWM (SPWM), current hysteresis control and energy control. SPWM generates a sinusoidal modulation pulse based on a sinusoidal reference signal and a certain form of carrier wave. This modulated signal serves as the switching signal for the inverter. Demodulation is simply accomplished by a simple low pass filter at the output. In the current hysteresis control strategy, the output current is forced to follow a reference sinusoidal waveform. The output waveforms are also filtered with a simple L-C low pass filter. The energy control strategy is a digital implementation of the energy of conservation law. By dividing the output period into numerous intervals, the demand energy in each interval is computed based on the desired voltage and current waveforms. This energy needed is supplied from the source through the stored magnetic energy in an inductor by turning ON or OFF the power semiconductor switches.

Potential Applications
For the past decade, wind energy has been growing at a yearly rate of more that 30%. Other distributed power generators such as photovoltaic, small hydro, and fuel cells are also experiencing significant growth mainly because of the increase in energy demand and the environmental concerns of other energy sources. Many electric utilities are deregulating and allowing competition for the supply of electricity. With net-metering, users can sell their electricity at the same cost at which they are buying it. This invention has an extensive commercial potential with the applications of distributed power generators, particularly in residential applications, commercial buildings, industrial and government establishments where inverters are required to convert power from those variable energy sources into fixed voltage and fixed frequency ac power and fed into electric power systems. Other potential markets (some are sub-markets of the above) include a large market for recreational vehicles and boats, cottages, home generation, and communication stations.

State of development
Currently under final stage of proof of concept, funding through NSERC 121 Phase I, looking for prototype collaborators.

Additional Information
The principle investigator and contributor to the main idea of this project is Liuchen Chang, a professor in the Department of Electrical and Computer Engineering at UNB.