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An open source automatic transfer switch based on the Infineon CoolMOS™ C7 MOSFET.

I have created the world’s first open source automatic transfer switch (ATS), a device used by companies looking for the ultimate uptime for their data centers, like Facebook Google, and Amazon.

High uptime data centers have more than one power company providing power to them for redundancy (if one power company has a blackout or maintenance). An automatic transfer switch automatically swaps the devices connected to it between two power inputs.

I set out to create the world’s first open source (MIT Licensed) Automatic Transfer Switch (ATS) using Infineon’s new 600V CoolMOS™ C7 Gold superjunction MOSFETs. The low RDSon means that there is no need to parallel FETs to achieve the required current handling capacity (15+A), and the high breakdown voltage allows comfortable use with high peak voltages encountered in 120V/240V mains. In addition, the very low 1μA leakage current helps protect users in the event that they are servicing one of the utility inputs to the ATS while the other one is active.

How It Works
The system has 3 main building blocks: the switching cells, the voltage input sensing, and the supervisor microcontroller.

There are 3 switching cells per AC input, one for the Hot, one for the Neutral, and one for the Earth.

The Hot sides of the AC cannot just be tied together because they may be out of phase and may have different amplitudes, up to ±10% from nominal.

The Neutral and Earth connections cannot be tied together because their voltage at the outlet (with respect to panel ground) depends on the circuit load. If, for example, Utility 1 had the majority of the load, the additional current on the conductor would cause it to rise with respect to the electrical panel, potentially up to the limit defined by NEC 2008: 210.19, 5% of the nominal voltage (in the case of 240V a loaded Neutral or Ground could rise as much as 12V above the panel Ground). In the event that the Neutral or Ground were not switched voltages high enough to present a hazard to people servicing one of the AC inputs could be present.”

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