A single-phase grid connected transformerless photovoltaic (PV) inverter for
residential application is presented. The inverter is derived from a boost cascaded with
buck converter along with a line frequency unfolding circuit. Due to its novel operating
modes, high efficiency can be achieved because there is only one switch operating at high
frequency at a time, and the converter allows the use of power MOSFET and ultra-fast
reverse recovery diode. This dissertation begins with theoretical analysis and modeling of
this boost-buck converter based inverter. And the model indicates small boost inductance
will leads to increase the resonant pole frequency and decrease the peak of Q, which help
the system be controlled easier and more stable. Thus, interleaved multiple phases
structure is proposed to have small equivalent inductance, meanwhile the ripple can be
decreased, and the inductor size can be reduced as well. A two-phase interleaved inverter
is then designed accordingly.
The double-carrier modulation method is proposed based on the inverter’s
operation mode. The duty cycle for buck switch is always one if the inverter is running in
boost mode. And the duty cycle for boost switches are always zero if the inverter is
running in buck mode. Because of this, the carrier for boost mode is stacked on the top of
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the carrier for buck mode, as a result, there is no need to compare the input and output
voltage to decide which mode the inverter should operate in. And the inverter operates
smoothly between these two modes. Based on similar concept, three advanced
modulation methods are proposed. One of them can help further improve the efficiency,
and one of them can help increase the bandwidth and gain, and the last one takes the
advantage of both