A sinewave inverter using class-D amplifier functions by converting a small sinewave input frequency into equivalent sine PWMs, which is finally processed by an H-bridge BJT driver for generating the mains sinewave AC output from a DC battery source.
What is Class-D Amplifier
The working principle of a class-D amplifier is actually simple yet extremely effective. An input analogue signal such as an audio signal or a sinusoidal waveform from an oscillator is chopped into equivalent PWMs also called SPWM.
These sine equivalent PWMs or SPWMs is fed to a power BJT stage, where these are amplified with high current, and applied to the primary of a step up transformer.
The transformer finally transforms the sine equivalent SPWM into 220V or 120V sine wave AC, whose waveform is exactly in accordance with the input sine wave signal from the oscillator.
Advantages of Class-D Inverter
The main advantage of a class-D inverter is its high efficiency (almost 100%) at a reasonably low cost.
Class-D amplifiers are easy to build and set up, which enables the user to produce efficient, high power sine wave inverters quickly without many technical hassles.
Since the BJTs have to work with PWMs, it allows them to be cooler and more efficient, and this in turn allows them to work with smaller heatsinks.
A Practical Design
A practical class-D inverter circuit design can be witnessed in the following diagram:
The working of the pwm class-D inverter is fairly simple. The sine wave signal is amplified by the op amp A1 stage to adequate levels for driving the electronic switches ES1---ES4.
The electronic switches ES1---ES4 open and close causing rectangular pulses to be generated across the bases of the transistors T1---T4 bridge alternately.
The PWM or the width of the pulses is modulated by the input sine signal resulting in a sine equivalent PWMs fed to the power transistors,and the transformer, ultimately producing the intended 220V or 120V sine-wave mains AC at the output of the transformer secondary.
The duty factor of a rectangular signal produced from the ES1---ES4 outputs is modulated by the amplitude of the amplified input sine wave signal, which causes an output switching SPWM signal proportional to the sine wave RMS. Thus the on-time of the output pulse is in accordance with the instantaneous amplitude of the input sine signal.
The switching period interval of the on-time and the off -time together determines the frequency which will be constant.
Consequently, a uniformly dimensioned rectangular signal (square wave) is created in the absence of an input signal.
As a way to achieve fairly good sine wave at the output of the transformer, the frequency of the rectangular wave from ES1 should be at the very least two times as high as the highest frequency in the input sine signal.
Electronic Switches as amplifiers
The standard working of the PWM amplifier is implemented by the 4 electronic switches made around ES1---ES4. Supposing that the input of the op amp input at the zero level, causes the capacitor C7 to charge via R8, until the voltage across C7 attains the level that is sufficient to switch ON ES1.
ES1 now closes and begins discharging C7 until its level drops below the switch ON level of ES1. ES1 now switches OFF initiating the C7 charging again, and the cycle rapidly turns ON/OFF at a rate of 50 kHz, as determined by the values of C7 and R8.
Now, if we consider the presence of a sine wave at the input of the op amp, it effectively causes a forced variation on the charge cycle of C7, causing the ES1 output PWM switching to get modulated as per the rise and fall sequence of the sine wave signal.
The output rectangular waves from the ES1 now produces SPWM whose duty factor now varies in accordance with the input sine signal.
This results in a sine wave equivalent SPWM to be alternately switched across the T1---T4 bridge, which in turn switches the transformer primary to generate the required AC mains from the secondary wires of the transformer.
Since the secondary AC voltage is created in accordance with the primary SPWM switching, the resultant AC is a perfectly equivalent sine wave AC of the input sine signal.
Sine wave Oscillator
As discussed above, the class-D inverter amplifier will need a sine wave signal input from a sine wave geneartor circuit.
The following image shows a very simple single transistor sine wave generator circuit which can be effectively integrated with the PWM inverter.
The frequency of the above sine wave generator is around 250 Hz, but we will need this to be around 50 Hz, which can be changed by altering the values of C1---C3, and R3, R4 appropriately.
Once, the frequency is set, the output of this circuit could be linked with the C1, C2 input of the inverter board.
PCB Design and Transformer Wiring
Transformer: 0-9V/220V current, will depend on the transistors wattage and battery Ah rating
The proposed class-D PWM inverter is a small 10 watt test sample prototype. The 10 watt low output is due to the use of low power transistor for T1---T4.
The power output can be easily upgraded to 100 watts by replacing the transistors with TIP147/TIP142 complementary pairs.
It can increased to even higher levels by using higher BUS DC line for the transistors, anywhere between 12V and 24V