Michihiko Nagao

Power flow of photovoltaic system using buck-boost PWM power inverter

This paper presents a buck-boost PWM power inverter and its application for residential photovoltaic power systems. The PWM power inverter is realized by combining two sets of high frequency buck-boost choppers and by making them operate in the discontinuous conduction mode. A PV power system with the power inverter has the following advantages: (1) the power generated by the photovoltaic array can be transferred to the load and the utility line under any array voltage; (2) isolation between the photovoltaic array and the utility line is performed by a small high frequency reactor operating as energy storage element; (3) unity power factor operation is provided; (4) there is no need of a reactor for interconnection with the utility line; and (5) so the system configuration is very simple. The input-output characteristics of the system are analyzed. As a result, the ripple component of the array current and the power flow between the interface and the utility line are derived and verified experimentally.

A novel one-stage forward-type power-factor-correction circuit

A novel power-factor-correction (PFC) circuit is proposed which is constructed with one stage by using a single forward converter. This PFC circuit has functions of high power factor and suppression of the output voltage ripple. The high power factor is accomplished by making the PFC circuit operate in the discontinuous conduction mode and the suppression of the output voltage ripple by using the exciting energy of the transformer in the forward converter.

Inductor commutation soft-switched PWM inverter driven by frequency modulated PWM signal

This paper presents a novel method using frequency modulation (FM) to improve the efficiency of inductor commutation soft-switched PWM inverter (ICSI). With this method, the circulating current for soft switching is effectively suppressed without sensing any current or voltage in the inverter. The efficiency rises about 10-40% compared with ICSI.

A high efficiency high power density AC-DC converter with holdup time extension capability

This paper proposes a high efficiency high power density AC/DC converter with holdup time extension capability. The proposed AC/DC converter is composed of a proposed two-phase PFC circuit and a ZVS/ZCS current fed isolated converter. During brief interruption of input power, the proposed PFC circuit operates as a boost regulator in order to keep a constant output voltage. Bulk storage capacitors in the PFC circuit are almost discharged in the boost operation. As a result, the storage capacitor volume can be greatly reduced and low-withstand-voltage elements can be used for the rectifier elements in the secondary side of the isolated converter. A bridgeless PFC circuit with the proposed technique is presented as a family circuit. The proposed AC/DC converter is verified by a simulation and an experiment. A 300W/12V AC/DC converter prototype achieves 93% efficiency at full load.

Two phase PFC circuit to keep constant output voltage even under momentary power outages

One of the important functions of AC/DC converters is to supply a stable output DC voltage to electronic equipment even if momentary power outages occur. During the momentary power outage, the power is supplied from the storage capacitor of a PFC circuit to load through an isolated converter. Therefore very large capacitors should be necessary to compensate during interruption of AC power. Furthermore, because the storage capacitor voltage decreases according to discharge of the storage energy, the isolated converter should be necessary to design corresponding to a wide input voltage range. This causes increasing the power loss and volume of the isolated converter. This paper proposes a two-phase PFC circuit for keeping a constant output-voltage even under momentary power outages. When the momentary power outage occurs, the proposed PFC circuit operates as a boost regulator in order to keep the constant output voltage. This circuit is simple topology in which a diode and a mode change switch are added to a conventional two-phase PFC circuit. Effectiveness of the proposed PFC circuit is tested by a simulation and an experiment.

A hybrid converter for improving efficiency at light load region

In order to reduce power consumption of electronic equipment in stand-by mode, idle-mode and sleep-mode, a simple efficiency improvement technique for switching regulator in light load region is proposed. During the light load, small switching elements in a MOSFET driver circuit are used instead of switching elements in a main regulator circuit to reduce switching and driving losses. Of course, during the load heavier than light load (normal load), the MOSFET driver drives the switching elements in the main regulator circuit. The efficiency of a 1.8V/7A prototype buck converter is improved from 60% to 85% by using the proposed technique.

High Frequency Transistor Inverter with CT

The commutation mechanism of high-frequency transistor inverters with a current transformer was studied. The current transformer eliminated the need for dead time in the commutation time, as in the usual procedure. It was also found that storage time can be reduced by connecting an external base resistance and that the storage time can be used for effective switching. However there is a trade off here with such parameters as the oscillation conditions and fall time.