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High Efficiency Sine Wave Inverter - Part 1
The goal of this series of pages is to explain and ultimately build a high efficiency pure sine wave inverter (DC to AC converter) from scratch. I have been working on this project for about 6 months and so far the inverter is basically working.
This inverter is not of the type that requires a transformer to step up the voltage. It is basically a monster high power audio amplifier designed to run appliances. For this reason its output is not like most commercial sine inverters. The great majority of inverters out there can generate a very blocky and noisy sine wave. This inverter is generating a nice and smooth high fidelity sine wave just as an audio amplifier would.
The main working blocks of the inverter are illustrated in the diagram. The first block is the power source. For this project I decided to go high voltage so the battery bank is 120VDC nominal. I decided to go high voltage because of efficiency reasons. Using this voltage will allow us to output the same power with very little current thus preventing MOSFETs from dissipating too much energy as heat.
Another reason for choosing a high voltage power source is because to make a perfect output sine wave we need to have a DC voltage who's amplitude is equal to the output peak to peak voltage of the sine wave.
As you might have figured out by now, it takes 175+175 volts input to create a 120V RMS sine wave. Using 120V as input is closer to 350V thus lowering the power losses associated with stepping up the voltage. If the same project would have been done with a standard system voltage of 12V, 24V or 48V the losses would have been unacceptable for the scope of this project.
The next building block is the device that will step up the voltage from 120V nominal to a regulated 360VDC. An inductive boost converter was chosen as they are known to be able to operate at around 97% efficiency. More detailed information about this block will be given later.
Now Ill discuss the main part of the project. The next block is the inverter. Now that we have our required input voltage for the desired output voltage all that is left to do is to convert it to an AC wave. As we all know this could be done easily by operating the MOSFETs in the linear region. I am not doing that because operating the MOSFETs in the linear region will give extremely low efficiencies.
To solve this problem I encoded the 60Hz sine wave into a high frequency square wave. This way the MOSFETs are being switched by regular square waves. The only difference is that the square wave carries the information of the sine wave encoded as a series of changing pulse widths.
This 'trick' is achieved by applying a generated sine wave of the same frequency as you want your output sine wave to be into the PWM comparator of your PWM chip. I did not used a standard technology PWM chip. what I did instead was to use op amps (tl084) to make my own PWM chip combined with a 555 timer as a ramp generator. It was done like that because it gives me more freedom to change every aspect of the circuit easily and consciously. I have nothing against using a readily made PWM chip like the tl494. It should work as well but I have not done any testing with it to be sure.
The last part of the basic system is a simple low pass filter. The purpose of this filter is to filter out the high frequency that carries the information about our precious sine wave. After the filter all that is left is the information that we inputed (the pure and clean sine wave).
In the next part of this article I will have posted schematics describing each block in detail. The power source block (batteries) will not be discussed any further. Thank you for reading this and any comments are welcomed.
WARNING: This information, all pictures shown here and all schematics are copyrighted material. The owner of this material, Argenis Bilbao (myself) prohibit the use of this information for anything other than personal use or educational purposes. The use of this information for commercial purposes without my authorization is a violation to copyrighted material and will be prosecuted by law.