App note: 1000VA Digital pure sine UPS

Here’s a reference design from Microchip for a 1000VA pure sine uninterpretable uninterruptible power supply. UPS usually use 3 circuits to function. A charger for battery, a inverter to convert DC battery power to mains AC, and a controller that switches from mains to battery when the power is interrupted. This design utilizes a single dsPIC digital signal controller to handle all three functions.

A flyback switch mode dc/dc converter charges and maintains battery voltage. The inverter requires 380V DC to create the 220V AC output, the 12V DC battery supply is bootsed to 380V using another DC/DC converter in push/pull configuration. The dsPIC monitors the battery charge and directly drives the MOSFETs in both DC/DC converters.

A full bridge inverter with an LC filter creates AC output, and a simple DPDT relay switches between battery power and mains. The time from mains failure to battery switchover is 10ms.

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  1. I detect a tinge of perpetual motion on the battery charging flowchart. I think the charger is supposed to get its AC from the EMI filter to relay line, not the output of the battery itself.

    Also I won’t say you “can’t buy” a relay fast enough to meet the 10ms spec, because you can. Its just that “most” would be lucky to be twice that fast under ideal conditions. I’m thinking like a Picker PTRH-T series datasheet is as close as you’re getting and thats still 15 ms just to actuate the relay alone. I suppose something that can only handle a KW probably has a lighter armature so maybe 10 ms for the whole system is possible after all…

    1. Vince, the design website the article points to includes all the theory- the inverter is a Line-Interactive UPS- the relays are not the primary mechanism to handle the power fail transfer. The flowchart is accurate. Take a look at the app note and the web-page. It includes the full schematic, BOM, and gerbers. The app note has a lot of basic theory- it is 86 pages, pretty dense with information.

    2. Actually it is accurate. In mains mode the inverter mode is off. There the DPDT relay routes the power to output and also is connected to the LC filter. The LC Filter filters the mains and is given to the full bridge. which if you recollect has body diodes on the Bridge mosfets which in turn rectify the 220/230V and present a 380V DC bus to the Flyback charger. My company is trying to implement this app note. Hence my interest.

  2. This has to be the most hilarious blog entry yet. The schematic must be missing at least one arrow in the flow chart, because there is no output from the Full-Bridge Inverter/Rectifier to the Flyback Switch Mode Charger. Also, “uninterpretable” pretty much sums up the graphic.

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