This unique modified sine wave inverter concept has been designed and invented by me. The entire unit along with the oscillator stage and the output stage can be easily built by any electronic enthusiast at home. The present designed is able to support 500 VA of output load.
Let's try to understand the circuit functioning in details:
The Oscillator Stage:
Looking at the circuit diagram above, we see a clever circuit design comprising both, the oscillator as well as the PWM optimization feature included.
Here, the gates N1 and N2 are wired up as an oscillator, which primarily generates perfectly uniform square wave pulses at its output. The frequency is set by adjusting values of the associated 100K and the 0.01 uF capacitor. In this design it is fixed at the rate of around 50 Hz. The values can be altered appropriately for getting a 60 Hz output.
The output from the oscillator is fed to the buffer stage consisting of four parallel and alternately arranged NOT gates. The buffers are used for sustaining perfect pulses and for avoiding degradation.
The output from the buffer is applied to the driver stages, where the two high-power darlington transistors take the responsibility of amplifying the received pulses, so that it can be finally fed to the output stage of this 500 VA inverter design.
Until this point the frequency is just an ordinary square wave. However the introduction of the IC 555 stage entirely changes the scenario.
The IC 555 and its associated components are configured as a simple PWM generator. The mark-space ratio of the PWM can be discretely adjusted with the help of the pot 100K.
The PWM output is integrated to the output of the oscillator stage via a diode. This arrangement makes sure that the generated square wave pulses are broken into pieces or chopped as per the setting of the PWM pulses.
This helps in reducing the total RMS value of the square wave pulses and optimize them as close as possible to a sine wave RMS value.
The pulses generated at the bases of the driver transistors are thus perfectly modified to resemble sine wave forms technically.
The Output Stage:
The output stage is quite straight forward in its design. The two winding of the transformer are configured to the two individual channels, consisting of banks of power transistors.
The power transistors at both the limbs are arranged in parallel to increase the overall current through the winding so as to produce the desired 500 watts of power.
However to restrict thermal runaway situations with the parallel connections, the transistors are connected with a low value, high wattage wire wound resistor at their emitters. This inhibits any single transistor from getting over loaded and fall into the above situation.
The bases of the assembly are integrated to the driver stage discussed in the previous section.
The battery is connected across the center tap and the ground of the transformer and also to the relevant points in the circuit.
Switching ON power immediately starts the inverter, providing rich modified sine wave AC at its output, ready to be used with any load upto 500 VA.
The component details are supplied in the diagram itself.
The above design can also be modified into a 500 watt PWM controlled mosfet sine wave inverter by replacing the driver transistors simply by a few mosfets. The design shown below would provide about 150 watts of power, for obtaining 500 watts, more number of mosfets may be required to be connected in parallel with the existing two mosfets.