When the power fails, the next hundred seconds belong to the circuit
Illustrative image: Unsplash contributor on Pexels
Storm Austen is forecast to bring disruptive weather to parts of the UK, with potential power cuts among the impacts identified by the Met Office. For a small electronic device, losing the incoming supply need not mean losing power at the same instant.
A little stored energy can preserve the interval between the two. It might be used to finish writing a record, hold a controller through a short interruption or send a final status message. The hard part is making the transition happen while the rest of the circuit still has enough voltage to work.
Texas Instruments’ PMP30693 reference design is a useful worked example. It charges supercapacitors from a low-voltage DC input and automatically transfers the load to a buck-boost converter when that input disappears. TI reports a regulated 3.7V output at 300mA for more than 100 seconds before the voltage drops.
That is about 1.11W at the load. Sustaining it for 100 seconds represents roughly 111 joules of delivered energy, calculated from the quoted voltage, current and duration. The energy initially stored must be higher because the converter and other parts of the circuit consume some of it.
Voltage falls as the capacitor empties
A capacitor’s stored energy is one half of its capacitance multiplied by voltage squared. The usable part is the difference between the energy at the starting voltage and the energy remaining when the supply can no longer operate:
E = ½ C (Vstart² − Vend²).
This is why the final voltage matters. A circuit that stops working while substantial charge remains in the capacitor cannot use all the energy suggested by its fully charged state. A buck-boost stage can extend the useful range by regulating as its input moves above and below the required output, within the converter’s own limits.
The load matters just as much. A radio can have a low average demand while drawing a much higher current during transmission. Capacitor resistance, connection resistance and the converter’s transient response can briefly pull the rail down before the total stored energy is anywhere near exhausted. The average-current calculation and the worst pulse therefore describe different tests.
Charging is part of the power budget too
An empty supercapacitor initially looks like a demanding load. If charging is uncontrolled, the circuit can take more current than the main supply can provide, causing a voltage dip while the device is supposed to be recovering.
PMP30693 includes adjustable charging limits and reverse blocking. Its active balancing also manages the division of voltage across the series-connected capacitors. A series stack raises the available working voltage, but unequal cell behaviour can otherwise place more voltage across one cell than intended.
TI’s test report examines charging, balancing, discharge time, output ripple and load transients. That is a more useful picture of a backup circuit than one hold-up-time number, because it includes the periods before and after the interruption as well as the discharge itself.
The reference board’s headline result is specific to a 3.7V, 300mA load. It is not a claim that the same hardware will keep a router or an entire control cabinet running for the same period. A different load changes both the required storage and the behaviour of the conversion stage.
For a sensor or small controller, a hundred seconds can contain several very different jobs: continued measurement, a transmission, a controlled shutdown or an attempt to reconnect. The circuit’s available energy is finite, and each of those activities spends it at a different rate.
