I'm in the initial stages of designing a pure sine wave inverter. My design is as follows:
Wien Bridge Oscillator (5V at 50 Hz) -> Class D Amplifier -> Output transformer
My question is, is this a good design? Are there better designs out there? The inverter will utilize a 12V lead acid battery as its source and output 220V at 50 Hz. The Amplifier will amplifier the signal upto 12V and the output transformer will then step-up the voltage to 220V.
An alterate design that comes to mind is:
Wien Bridge Oscillator (5V at 50 Hz) -> Class D Amplifier (Amplify to 220V)
Since the battery is only 12V, by using a DC-DC Converter the DC source can be stepped upto 220V DC – which the amplifier than utilizes to amplify the sine wave signal to 220V.
How good are these designs? Will they work at all? If not, why so? And if they do, how good are they for a general purpose inverter?
Best Answer
You say you're not familiar with digital oscillators yet, but I think it's worth looking into. Using a microcontroller with PWM output you can input the signal directly in the amplifier's format. So you won't need an A/D conversion at the amplifier's input stage.
The code for the microcontroller is very simple. Basically it's like this: you need a sine lookup table, and a timer interrupt generating your sample clock. This can be 5kHz, for instance, then you get 100 samples per cycle, which is enough for an inverter application; the transformer will work as a low-pass filter. Every 200\$\mu\$s you take the next value from the table and set the PWM register with this value. The output will be pulse-width modulated following the sine pattern.
This means that your class-D amplifier doesn't need the input stage with the triangle wave generator + comparator (this would otherwise be you A/D converter), so it only consists of the driver stage for driving the output MOSFETs.
You get a crystal precision frequency virtually for free.
PS: a center-aligned PWM signal will give you a cleaner sine approximation, but many microcontrollers only can generate edge-aligned PWM. This Freescale controller is available in an SOIC-8 package and is an example of a microcontroller which can do center-aligned PWM.
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