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I am designing a system that is always on. It is powered via POE and it is doing some task that is not mission critical.

The system has an embedded system inside that runs Linux. The storage etc are all Emmc type flash storage.

The power system can generate an interrupt that signals power is no longer available. The software engineering team says they need a few seconds (2-3) to properly shutdown system.

I have a few questions.

  1. Should I put additional energy storage to cater for this 2-3 seconds. Our assessment is that the likelihood of system being damaged is low, most likely scenario is corruption of disks.
  2. What is the best way to achieve proper shutdown using some energy storage device. If I don’t want to use battery what are the options ?
  3. Can a pure capacitor based system could work ? How can I design such a system.
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    \$\begingroup\$ 400 volts stored in a 1,000uF capacitor is this much energy: 0.5 * C * VV, or 400*400/2,000 = 160,000/2,000 = 80 wattseconds \$\endgroup\$ – analogsystemsrf Jul 30 at 6:16
  • \$\begingroup\$ there are many UPS systems made from enthusiast for raspbberry pi, beagleboard, etc.. but I don't know if they really work as they should. \$\endgroup\$ – Marko Buršič Jul 30 at 8:30
  • \$\begingroup\$ If you don't want batteries, then you're essentially stuch with capacitors. POE works at a nominal 48V, which is more than supercapacitors can handle, so you'd require a bank of them. It would be possible, but probably much more bulky than a rechargeable battery. \$\endgroup\$ – Simon B Jul 30 at 9:18
  • \$\begingroup\$ In avionics, hold up is often achieved using these capacitors: cde.com/capacitors/aluminum-electrolytic/flatpack \$\endgroup\$ – Peter Smith Jul 30 at 13:52
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An UPS made of supercaps and switches.

  • first charge the supercap with constant current, the system is not powered yet
  • when the cap reaches a threshold voltage turn on the power switch MOSFET
  • when the power loss is detected issue a system shutdown, the caps shall provide enough energy
  • after few seconds disconnect power to the system, wait for few second more. go to first step

similar implementation:

https://www.hackster.io/Itverkx/safe-shutdown-and-auto-bootup-for-raspberry-pi-2edb5b

You can read a manual of commercial available UPS: http://linuxgizmos.com/raspberry-pi-supercapacitor-micro-ups-seeks-funding/

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Assuming that PoE sends nominally 48v, and assuming this is bucked down to 5v for your system, you already have most of what you need.

A capacitor will store energy, but you can only get the energy out if you allow the capacitor voltage to fall. Energy goes as voltage squared, so letting the voltage drop to 50% of what it was will allow you to extract 75% of the energy from the capacitor. This means you don't need a huge input voltage swing capability on your buck converter to make best use of the stored energy.

diagram to follow when I can get the schematic editor to save for me

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  • \$\begingroup\$ 50000 uF 63V cap across 48V PoE line should be enought. But shutdown process should start as soon as voltage falls below 40V \$\endgroup\$ – Volodymyr Kalinyak Jul 30 at 9:42

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