I bought a UPS, it has output of 220v AC, the problem is the output is square-wave, which causes distortion on TV/LCD, is there circuit that can take this Square-Wave as input, and output Sin-Wave?
Electronic – Converting Inverter Output from Square Wave to Sin Wave
filterinverterups
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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.
image from this document
Yes, at this input voltage a center tapped primary with low side switches on the ends seems like the way to go. For feedback, I'd look at opto-isolators first. Put something on the output side that can detect whether the voltage is above or below the regulation threshold and drive the opto accordingly. On the primary side, a simple control method is to simply stop the oscillations when the opto signal indicates the output is high. If you can live with some ripple, this should be effective, simple, and efficient.
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Best Answer
You can use a 2nd order low pass filter made from an inductor (in series with the AC) and a capacitor to neutral. You will also need a resistor: -
simulate this circuit – Schematic created using CircuitLab
1 Henry and 10uF form a 2nd order low pass filter that will attenuate the higher order frequencies from the sq wave and make the output signal more sinusoidal in nature. It might work fine or it still might give your TV problems if it is particularly sensitive to harmonics.
The resistor is needed to keep the Q of the circuit low and prevent high ringing voltages.
Q = \$\dfrac{1}{R}\sqrt{\dfrac{L}{C}}\$ = 3.16 using the values above.
Q needs to be about unity so I'd advise to make C bigger and L proportionally smaller but be aware that the value of resistance is in series with the TV and whatever current the TV takes, it flows through the resistor. For instance, 110 watts needs 0.5 amps from 220 V and this means the I^2R losses in the resistor are 25 watts - choose big wirewound resistors.