Variable AC Lab Supply (inverter) design question - Page 1
Forum section: electronics / projects

author: essele
author_rank: Frequent Contributor
timestamp: March 11, 2023, 07:34:13 pm
content: Hi,I've been experimenting with an inverter based AC power supply with the intent of ultimately developing it as a lab supply. It's a pretty simple design with a microcontroller generating a PWM signal, driving two IR2110's in a full-bridge config, with the output then going through a step-up transformer, the secondary then having a cap to filter the output (same concept as the EGS002/EG8010 low-power option.)On the output I have a STPM34 (powered by an isolated dc-dc module) which monitors the voltage and current and works out power, power factor, along with a range of other metrics. I use an isolated SPI connection back to the microcontroller to look at the output voltage on a cycle-by-cycle basis and adjust the PWM to achieve (nearly) constant output regardless of changes in current etc.It works well, but it's really an experimental platform at this point, I have a few more ideas I want to test with different control loop algorithms etc.But ... I made a fairly major mistake with the board so it needs a respin. It seems to me that I have two options as far as microcontroller placement and isolation is concerned.The current approach has the microcontroller in the same domain as the IR2110's and MOSFETs then a whole series of isolated signals going backwards and forwards to the STPM34 (using CA-IS3721HG's.)But, I could stick the microcontroller in the secondary domain, and then just feed back the six signals to the IR2110's. This would reduce the number of signals needing isolation, probably increase my SPI speed, but would need a more chunky isolated power module, especially if I want to drive a screen etc. It also means I could probably generate the 16MHz clock for the STPM34 from my microcontroller and remove a crystal and caps.Is there any reason that this is a bad idea? The only thing I can think of is if ever I wanted some external connectivity then I would have to isolate that.Any help appreciated.Thanks,Lee.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: March 11, 2023, 09:12:44 pm
content: I would let the isolation needs drive the design. If I define the isolation requirements, this is the list:- Ui, buttons, etc to be galvanically isolated from mains- Secondary (assuming high voltage) to be isolated from mains and UI as well.- Host communication via USB.It sets the need for 3 isolated regions. Plenty of options to isolate SPI measurement and controller signals, speed should not be an issue. For 09wering the MCU control I’d do a small mains transformer, 5VA or so. Less trouble than finding the perfect DC/DC isolated converter and no concerns about the startup sequence.

author: essele
author_rank: Frequent Contributor
timestamp: April 07, 2023, 11:53:11 am
content: Thanks ... that was useful input. I have since discovered a TI reference design that uses one of their real time CPU's which looks really interesting.I'm slightly confused by the output section, specifically the handling of "earth". My understanding is that earth is usually bonded to neutral for some very good reasons. Yet in this schematic the filtering looks like it will cause the earth to sit halfway between N and L which seems wrong to me.The only thing that I can think of is that it effectively allows you to bond earth to either of the other conductors (effectively shorting one of the two caps in each pair), does that seem possible?in my use case I'll ultimately want to have some choices about how earth is handled, and check for leakage etc, so will have to figure out a more complex solution, but I'd like to understand this first.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 07, 2023, 08:03:57 pm
content: What is the url to the TI Ref design?

author: essele
author_rank: Frequent Contributor
timestamp: April 07, 2023, 09:28:45 pm
content: I can’t find the original TI link (on my phone), but this is the same doc and has a load of links in it…https://www.mouser.com/datasheet/2/405/tiduay6c-1092818.pdf

author: moffy
author_rank: Super Contributor
timestamp: April 08, 2023, 12:29:41 am
content: The bonded earth wasn't always around, at least in Australia growing up the earth and neutrals were kept separate (I still like the idea of ground only for fault currents), might be the case in the USA as well, I don't know though.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 08, 2023, 04:33:37 am
content: 2 fuses indicates the possible use of an unpolarised connection to mains (e.g European schuko) , or possibly 2 live conductors e.g. US 220V. So yes, L and N could be swapped, or could actually be L1 and L2. Anyway, that's one reason why the filter caps are the way they are. They won't override any neutral - earth bond at the consumer unit though.BTW, having 2 fuses is generally not recommended for countries where the mains connector polarity is guaranteed e.g. UK, just in case the fuse on the N side blows but the L side doesn't.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 09, 2023, 01:32:45 am
content: QuoteEarth’s role is protection. The power supply’s default setup should be protection too. Thus the output jack’s earth to be connected to the input’s earth.If you want to measure leakage, etc, it’s better to do it via a harness you plug between the power supply and the device, IMO.When I see a grounded outlet, I expect it to work the same way all the time. You might forget what workaround you built after a few years and bad things might happen. Or you sell it on eBay, or a coworker wants to use it not knowing the details.

author: essele
author_rank: Frequent Contributor
timestamp: April 09, 2023, 12:15:24 pm
content: Hi,Yes, I get the point about earth, however this is ultimately going to be a variable lab AC power supply, and having the choice (via a very obvious GUI) of isolated earth (with leakage measurement) and ‘connected to normal earth’ will be useful for different senarios.Lee.