Inverter for producing a true sine wave

#The invention relates to electrical power inverters, and in particular to power inverters for converting a direct current signal into an alternating current signal.

#Power inverters are known which convert direct current into alternating current. For example, Vector Products, Inc. manufactures and distributes a self-contained power inverter under the trade name MAXX POWER INVERTER and the VECTOR POWER ON BOARD series. These power inverters are connected to a 12-volt battery, for example through a cigarette lighter socket of a vehicle, for converting 12-volt direct current energy into 110-volt alternating current. Other known power inverters are disclosed, for example, in U.S. Pat. No. 5,901,056 to Hung, which is incorporated herein by reference. Such power inverters can be used to provide power to devices needing an AC power supply when only a DC power supply is available, for example, when travelling in a car or on a camping trip.

#There are essentially two types of power inverters that convert a DC input into an AC output. A first type of power inverter produces what is referred to as a modified sine wave. This modified sine wave is really more of a modified square wave than a modified sine wave. The other type of power inverter is a true sine wave inverter that converts a standard 12-volt DC signal into a true sinusoidal 110-volt alternating signal at a frequency of 60 Hertz. The most prevalent way to perform the power conversion in a true sine wave inverter is to take the input 12-volt DC signal and convert it by high frequency switching of a transformer into a stepped-up AC signal. The stepped-up AC signal is then converted back again to a DC signal. Field-effect transistors (FETs), or other transistors are then used to create the sine wave output from the DC signal. The transistors for creating the sine wave are operated in a linear mode.

#FIG. 1 is a circuit schematic in partial block circuit form showing the true sine wave inverter according to the invention; and

#FIG. 2 is a signal diagram used to explain the operation of the circuit shown in FIG. 1.

#A method and apparatus for converting a DC input signal into a true sine wave AC output signal is provided. In exemplary embodiments, a current is switched through a primary winding of a transformer using FETs or other switching means. The FETs are driven by a high frequency signal. The high frequency signal driving the FETs on the primary side of the transformer is pulse width modulated in accordance with the desired frequency of the output signal. The pulse width modulation is varied based upon a comparison of an output of the transformer with a reference signal having the desired frequency. The structure of this embodiment eliminates the heat-generating FETs or other transistors on the output side of the transformer in the known designs as discussed above. Accordingly, a simplified, cost-effective power inverter is provided.

#According to an embodiment of the invention, control of the pulse width modulation (PWM) of the signal driving the FETs is achieved with a feed back loop. An output signal of the power inverter is compared to a reference signal having the desired output frequency. The reference signal, typically a 60 Hertz reference signal, is provided to one input of a means for comparing, such as a comparator, differential amplifier or other comparable device. Another input of the means for comparing is provided with the feedback output signal of the power inverter. The means for comparing produces an error signal corresponding to the difference of the two input signals. The output of the means for comparing is provided to a means for pulse width modulation (PWM), for example, a commercially available PWM integrated circuit. The PWM integrated circuit produces an output wave having a duty cycle that is a function of the error signal from the means for comparing. The output of the PWM integrated circuit is provided to a means for switching, for example, an electronic switch, FET or other type of transistor connected for switching a transformer having a DC input at a center tap of the transformer primary winding. A stepped-up voltage, high frequency signal is produced across the secondary winding of the transformer. A filter such as a low pass filter is coupled to the secondary winding for passing a DC signal having an amplitude that varies at a rate that tracks the frequency of the reference signal.

#The differential amplifier or other means for comparing may be built into the PWM controller circuit that produces the high frequency signal for driving the switching FETs. A closed loop feedback circuit is formed in which the output of the transformer is modulated to look like the reference signal which, in this case, is a 60 Hertz sine wave. That is, the output of the transformer is caused to vary between the 0-volts and the maximum stepped up voltage at the frequency rate of the 60 Hertz reference sine wave, thereby producing a true sine wave output.