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What I have: 5VDC and / or 12VDC, a PIC micro

What I want: Floating 9VAC 50Hz, able to output 1A.

Googling details for this kind of inverter seemed to be a tricky case. There are a few DIY-pages that describe the innards of an inverter doing 5VDC -> 220VAC using a PIC, SPWM, and an H-bridge using four MOSFET with drivers. The common denominator for many of these is that they hook a low-pass filter to the MOSFET-outputs, and then to the load. But, applying the same theory, but for 9V, would mean not getting a floating 9VAC (18Vp-p) but an output with a 18Vp-p with a 9VDC offset. I do not understand how that would work.

Another solution for the output stage, instead of an LC-filter, is to add a transformer and let the MOSFET output feed the primary. However, I do not find any transformer that can work with sine pulsating 12V input and give 9VAC output. Digikey parameter search gave nothing but a 24-9-transformer at best. For proper 50Hz sinus replication, I would need a reference clock for the drivers. This can either be generated with an analog oscillator (using a couple of LM358), or using software controlled PWM (using variable duty cycles) with a PIC micro. Either way is fine for me, as this controls the output frequency.

What I'm still puzzled about is that I fail to understand how the output amplitude is controlled. If I use a transformer, then bombing it with pulsating 5V to let it transform it up (to whatever voltage) isn't the challenge, but if I don't use a transformer; what part / block in my circuit will be the one deciding the amplitude? Higher PWM-frequency, higher output amplitude?

In the case of not using a transformer, but an LP-filter instead, is there any way to get the output to "float" without DC-offsets? I know capacitors block DC, but I cannot see how that would work given that there are no negative reference voltages.

As you can see, I have quite a vague starting point for this experiment. I know what output I want, but I'm having a hard time finding what parts / topics I need to learn to reach my goal.

Question: Where do I start? What should I dive into first?

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  • \$\begingroup\$ Your MCU generating a 50 Hz DAC sine wave as reference to a simple class B amplifier (not headphone but small speakers) feeding a transformer? \$\endgroup\$
    – winny
    Commented Feb 1, 2019 at 15:25

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You can first transform your whatever value DC supply to a reasonable level in an isolated fashion. This could be 15-18V because 9 V AC would have around 13V peak voltage, so you want a little bit of headroom after that. This isolated DC-DC conversion can be achieved with multiple topologies. Search for isolated dc-dc. Flyback can be a starting point for that. There are many ICs and application notes that would allow you to achieve a pretty decent design in a reasonably short time. Then use a standard full-bridge+LC filter to generate the sine wave you want with the help of PIC and SPWM generation. The design complexity would be determined by your needs. If you want a low THD, stable sine wave then you would have to implement a closed-loop control, etc. However the basic principles and methods that apply to standalone 220V inverters would also apply in your case as well, with much easier voltage levels to work.

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  • \$\begingroup\$ Seems like a MAX1522 running in Continuous Conduction + Flyback will work for my case. Atleast in theory. Thanks for the tip. \$\endgroup\$
    – bos
    Commented Feb 7, 2019 at 9:43
  • \$\begingroup\$ There are tons of options. You can also look at LT8300/1/2/3 with varying power levels. The nice thing about this IC is that it implements feedback with just one resistor. \$\endgroup\$
    – felvan
    Commented Feb 7, 2019 at 12:36

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