This invention pertains to a voltage inverter circuit usable with a DC power supply, and more particularly, to such an inverter circuit having improved efficiency of operation provided by discrete and Darlington transistors connected in parallel for selective operation, by generating a pulse width modulated signal representative of a first order linear approximation of the output RMS voltage as determined from a non-load side of a power transformer, by low-load pulse width control circuitry for substantially reducing the output power generated during near no-load conditions, by protection circuitry which reduces the operation of the circuit to a level non-destructive of the circuit components during excessive load conditions, or by suppression circuitry which feeds energy from voltage spikes back into the DC power supply. 
Voltage inverters typically invert a DC voltage (for example, the 12, 24 or 48 volts typically found in motor vehicles and other portable equipment) to an AC voltage, such as 117 volts RMS at 60 Hz.  Such inverters make it possible to provide AC power to equipment requiring alternating power from a DC power supply which is typically portable and isolated from an utility electric power distribution system. 
A wide variety of inverter circuits exist.  Some circuits are single ended and others are of a push-pull type having a center-tapped transformer.  In order to obtain alternating current on the secondary of the transformer it is necessary to drive current through a primary coil or coils alternately in reverse directions.  With the advent of transistors, electronic switching of the current through the coil has been achieved by using transistors as switches.  Typically for low power considerations individual or discrete transistors are used as the switching element.  For high power applications, Darlington-configuration transistors have been used. 
Also, in order to regulate the operation of the switches to provide a fairly continuous output load, the load is measured on the secondary coil with sensed changes being used to control the operation of the switches on the primary side of the transformer.  Otherwise, typically, the switches are controlled in order to maintain them relative to a reference source or voltage without monitoring the actual secondary voltage output. 
The U.S. patent to Williamson (U.S. Pat.  No.  3,564,393) discloses a single-sided inverter which measures what is termed a flyback voltage supplied to a capacitor as reflected on the primary coil.  This voltage is then compared to a reference voltage for generating a pulse width control signal. 
Other conventional inverters have complex feedback circuits which measure the RMS of the output voltage for generating a control signal.  Also, during very low-load conditions, circuits typically dissipate a significant amount of energy in the switching elements even though very little load is transmitted to the secondary coil for output.  Circuit protection portions of inverters conventionally disable the working circuit as soon as an excessive load is detected.  This does not allow for continued operation of the circuit while measures are taken to correct the overload condition. 
In the preferred embodiment of the instant invention, an inverter circuit is provided which has a pair of discrete transistors connected in parallel with a Darlington transistor with a current apportioning voltage divider connected between their respective bases.  It further includes a transistor switch controller which senses the voltage on the non-load side of the primary coil of a transformer for generating a pulse width modulated signal which is a first order linear approximation of voltage on the secondary or output coil.  A low-load sensing circuit reduces the pulse width of the control signal to the transistor switches to about 1/3 of its normal width.  High current protection is provided in a circuit which senses when the power through the switch exceeds a maximum level.  When it does, the control signal pulse is reduced to a very narrow pulse which is not of sufficient duration to harm the transistor switches.  This narrow pulsing continues until either the problem causing the overload condition is removed from the secondary coil or a disabling circuit prevents any further operation of the transistor switches.  Finally, a spike suppression circuit is provided which stores energy from voltage spikes caused by the switching operation and feeds them back to the power supply through a transistor. 
It can be seen that such a circuit provides for improved efficiency of operation by using discrete transistors as switches during low-load conditions and saving the Darlington transistors for the high load operation when they are more efficient.  Further, the low-load pulse width limiting circuit provides for continued operation of the system during near no-load conditions while maintaining an output at the desired voltage.  The circuit protection provided in this circuit ensures that the transistor switches will not operated beyond their power capabilities.  Further, it has the inherent advantage of being useful for starting motors when current required is typically higher than normal operating currents by starting the motors very slowly and bringing them up to speed.  This permits starting larger motors than otherwise would be possible.  Further, energy is saved by feeding back a substantial portion of the energy occurring in the inevitable switching spikes.  These and other objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.