The Batteryless UPS for the Raspberry Pi
The Juice4Halt module is a supercapacitor based energy storage. It contains two independent DC/DC converters. The first one is a bidirectional step-up/step-down converter working as the interface between the stable 5V supply rail and the supercapacitor. During charging the converter works in step-down mode and transports energy from the external power supply to the supercapacitor. In case of a power failure the load device (Raspberry Pi or another SBC) is supplied from the supercapacitor via DC/DC converter working in step-up mode.
The second DC/DC converter is a Front-end step-down converter. The only function is converting a high input voltage down to 5.1V for the 5V rail. It is necessary to use the Front-end converter in case of an external power supply.
Modes of operation
- charging the supercapacitor using the step-down DC/DC converter when external power supply voltage is available at the input
- the load device at the output (Raspberry Pi or another SBC) stays disconnected from 5V power rail until supercapacitor charging is completed
- the Raspberry Pi (or another SBC) is powered from the external power supply
- the supercapacitor is directly connected to the 5V output to ensure reliable operation without short-term power failures or brown-outs
Brownout (or short-term power failure)
- starts when voltage falls below 4.9V
- external power supply will be disconnected
- Raspberry Pi or another SBC is powered from supercapacitor
- ends when voltage falls below 4.75V, at this point the external power supply voltage is being checked:
- when external power supply voltage is present reconnect to power supply and charge the supercapacitor back otherwise enter the Shutdown mode
For more detail: The Batteryless UPS for the Raspberry Pi