This invention relates to power inverters, and in particular, to power inverters controlled by a pulse-width modulator.
A power inverter converts DC to AC by passing a succession of variable amplitude DC pulses through a filter circuit. These DC pulses transport energy stored in a large capacitor, or other DC source, to a filter circuit.
The output of the filter circuit is effectively a moving average of the input pulses. When the duty cycle of the pulse train is low, a low voltage exists at the output because the time-average power carried by the pulse-train is low. Conversely, when the duty cycle is high, a high voltage exists at the output because the time-average power carried by the pulse train is high. By appropriately controlling the duty cycle of the pulse train, it is possible to generate a smoothly varying AC voltage at the output of the inverter.
An inverter of the type described above typically includes one or more switches to generate the pulse train. These switches are most often implemented as transistors or thyristors having a gate driven by a micro-controller. When the switch is in its open state, a high voltage exists across the switch terminals but no current flows, hence the switch consumes no power. When the switch is in its closed state, a large current flows, but across such a negligible voltage drop that for all practical purposes, the transistor switch consumes no power.
As a result of the finite switching times inherent in semiconductor devices, between the open and closed states of the switch there lies a transition interval during which the voltage and the current are both non-negligible. While operating in this transition interval, the transistor switch consumes significant power. This power, integrated over the transition interval is referred to as the xe2x80x9cswitching lossxe2x80x9d per cycle.
One way to reduce switching loss is to reduce the difference between the pre-switching voltage and the post-switching voltage. Intuitively, switching between 10,000 volts and 0 volts can be expected to take longer and to result in a higher switching loss than merely switching between 10,000 volts and 5,000 volts. This is the principle behind a multi-level inverter, in which the difference between the two voltages that the switch is to switch between is adaptively changed based on the desired output voltage of the inverter.
Another way to reduce switching loss is to place a resonant circuit between the switch and the inverter output. The resonant circuit is configured to have a resonant frequency that is much higher than the switching frequency. Changing the state of the switch triggers oscillation in the resonant circuit. Because of this oscillation, the voltage across the switch periodically crosses the zero voltage axis.
If the state of the switch is changed when the voltage across the switch is approximately zero, then the transition interval will occur at a time when the voltage across the switch is essentially zero. Each zero crossing thus provides a window of opportunity for changing the state of the switch without incurring significant switching losses. By having a resonant frequency that is much higher than the switching frequency, one can assure that there will be a nearby zero crossing for the controller to use when the time comes to change the state of the switch. This is the principle of a quasi-resonant inverter.
The invention reduces switching losses in a multi-level power inverter by placing a resonant capacitor in parallel with each switching element of the switching circuit of the inverter. This resonant capacitor interacts with a resonant inductor connected to a pole of the switching circuit to form a resonant circuit across the switching elements. As switching elements are turned on and off, different resonant capacitors are switched smoothly in and out of the resonant circuit.
A power inverter incorporating the principles of the invention includes a switching circuit for switching between any two voltage levels selected from N voltage levels, where N is an integer greater than or equal to three. The switching circuit includes at least N+1 switching elements and a corresponding plurality of resonant capacitors. Each of the resonant capacitors is connected in parallel with a corresponding one of the switching elements. As the switching elements turn on and off, different ones of the resonant capacitors interact with a resonant inductor connected to a pole of the switching circuit and with an output capacitor. This interaction results in the maintenance of a resonance condition as seen from the active switching elements.
In one aspect of the invention, a tri-level inverter includes a switching circuit with four switching elements, each having a first terminal and a second terminal across which a resonant capacitor is connected. The first terminal of the first switching element is available for connection to a positive DC voltage source. The second switching element has a first terminal that is connected to the second terminal of the first switching element and a second terminal connected to the pole of the switching circuit. The third switching element has a first terminal connected to the pole of the switching circuit and a second terminal connected to the first terminal of the fourth switching element. The second terminal of the fourth switching element is available for connection to a negative DC voltage source.
The resonant capacitors placed across each of the switching elements absorb energy that is normally dissipated in the switching elements. This energy is later sent back to the load. This significantly reduces switching losses in the switching element because the resonant capacitors can absorb the energy that would otherwise be dissipated.