Patent Application
20080123373
The subject matter disclosed herein relates generally to power converter systems including semiconductor switches.
Photovoltaic (PV) cells generate direct current (DC) power with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy into AC energy. Typical PV inverters employ two stages for power processing with the first stage configured for providing a constant DC voltage and the second stage configured for converting the constant DC voltage to AC current. Often, the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system. The efficiency of the two-stage inverter is an important parameter affecting PV system performance and is a multiple of the individual stage efficiencies with each stage typically causing one-half of the system losses.
Thus it is desirable to increase the efficiency of each stage of the PV inverter. Typically, first stage boost converters include normally-off silicon MOSFET (metal oxide semiconductor field effect transistor) or IGBT (insulated gate bipolar transistor) switching devices.
In accordance with one embodiment, a system comprises an energy source configured for operating as a current limited source and a DC-to-DC converter configured to receive current from the energy source and comprising a normally-on switch.
In accordance with another embodiment, a power converter system comprises a DC-to-DC current fed converter comprising a normally-on switch configured for providing an adjusted DC voltage and a voltage fed inverter configured for converting the adjusted DC voltage into an AC current.
In accordance with another embodiment, a photovoltaic inverter comprises a DC-to-DC current fed boost converter comprising a normally-on switch, a diode, and an inductor and configured for providing a constant DC voltage from a photovoltaic energy source; and an inverter configured for converting the DC voltage into an AC current.
In accordance with another embodiment, a power converter system comprises a DC-to-DC current fed converter configured for providing an adjusted DC voltage and a current switched inverter configured for converting the adjusted DC voltage into an AC current, the current switched inverter comprising normally-on switches.
In accordance with another embodiment, a power converter system comprises a current switched inverter configured for converting a filtered voltage of a current limited energy source into an AC current, the current switched inverter comprising normally-on switches.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
A current limited source is a source that, when short circuited, naturally limits the current to levels within the working range of the system or slightly above the working range of the system but not so much above that equipment damage results. A current limited source has a specific maximum current value and typically exhibits a high impedance across its terminals. Often, current limited sources are additionally voltage limited. In one embodiment, energy source 12 comprises a photovoltaic energy source. Other types of energy sources may be used, however, with one example being a fuel cell.
DC-to-DC converter 14 typically comprises a current fed converter. A current fed converter, as used herein, means a converter that is fed by a current limited source. In another more specific embodiment, DC-to-DC converter 14 comprises a boost converter for maintaining a constant DC voltage level.
Switch 15 typically comprises a wide bandgap semiconductor material such as silicon carbide or gallium arsenide. Other potential switch materials include gallium nitride, diamonds, and carbon nanotubes. Silicon carbide (SiC) switching devices, for example, often have superior conduction and switching behaviors as compared with silicon switching devices and may therefore increase the efficiency of DC-to-DC converter 14.
Switch 15 comprises a normally-on switch of any appropriate type. One example of a normally-on switch is a junction field effect transistor (JFET). Some types of metal oxide semiconductor field effect transistors (MOSFETs), such as depletion mode MOSFETs, are also normally-on switches.
In a more specific example, a SiC JFET is used as switch 15 in a boost stage 14 of a photovoltaic inverter system 10. Devices with normally-on switching characteristics are not typically used in power electronic systems out of concern that the devices' terminals will short circuit in the event of a failure. However, because a photovoltaic source (such as a solar cell) is a current limited source, the normally-on characteristic of a SiC JFET is not a safety critical issue. If DC-to-DC converter 14 fails, the SiC JFET switch 15 will short circuit the photovoltaic energy source 12, but the current will only be a percentage above the normal operating current. Typically the short circuit current will be less than or equal to twenty or thirty percent higher than the normal operating current of the energy source. In a more specific embodiment, the short circuit current is less than or equal to ten percent higher than the normal operating current of the energy source. The photovoltaic energy source and associated cables and connectors (not shown) can carry the increased current without overheating, even during lengthy faults. This current limiting feature is a difference between photovoltaic and fuel cell sources as compared with more conventional DC sources such as batteries and generators. In the embodiment of
When a silicon carbide switch is used, it is convenient to also include a silicon carbide diode 22 in DC-to-DC converter 14. In one example, diode 22 comprises a schottky diode. A schottky diode is useful because it has almost no reverse recovery losses and thus results in reduced switching losses in the DC-to-DC converter. A SiC schottky diode has slightly higher conduction losses but lower net losses than a standard PN junction silicon diode, for example. Additionally, SiC devices can operate at higher temperatures than silicon devices. Although silicon carbide diodes are described herein for purposes of example, other materials may be used with one example including gallium nitride.
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While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
