Various aspects of home brew inverters

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Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 10:41pm 13 Apr 2018      

Poida, generating a PWM sine wave directly from a lookup table is fairly straightforward. What makes it much more complicated is continuously adjusting (regulating) the output voltage, and in some cases programming the output frequency up and down as well.

Its not difficult in theory to adjust the amplitude up and down, its just simple multiplication. The real difficulty is getting conventional software to do it fast enough in real time with fine enough time resolution for high frequency PWM.

The way I have done this myself long ago, was to use a dual port ram.

A basic microcontroller can do all the required number crunching, working as fast as it can go, and continuously modifies and updates a sine wave stored in a dual port ram, which is then used as a lookup table template for one complete mains cycle.

The bits stored in the ram can then be read out at a much higher speed from the second "output" port of the ram. This can produce high frequency PWM directly, using just a continuously clocked counter and a magnitude comparator. The counter clocking speed could be made variable to give a varying output sine wave frequency if that is a requirement.

The dual port ram effectively isolates the software side from the much faster hardware driven output side.

The microcontroller can chug along relatively slowly and continuously update the amplitude of the stored sine wave byte by byte completely asynchronously, and it can do that without any timing constraints. All sorts of goodies like PID control, and soft start can then readily be written into the software.

The EG chip is probably some kind of common commercial FPGA chip that works in a similar way to the above, in that the various internal functions are pipelined, with several things happening simultaneously and at different speeds.

Its the only way that I can see to get both the high speed and the very fine time resolution and include quite a few nice features all in a single low cost chip.
Cheers,  Tony.