Start with the voltage the load can use
A capacitor can still hold energy after its voltage has fallen too low for a converter to operate. Size it for the energy released between the initial voltage and that lower operating limit, rather than assuming it can discharge to zero.
Use the converter’s specified minimum input under the intended load. Allow for dropout, wiring loss, and any undervoltage threshold. The initial voltage should reflect the lowest charged voltage you expect before an interruption, not just the supply’s nominal setting.
Calculate the energy budget
For a constant-power load, multiply delivered power by interruption time in seconds. Divide by converter efficiency, entered as a fraction in the equation, to find the energy the capacitor must supply. Combining this with the capacitor energy difference gives the ideal capacitance.
The calculator accepts milliseconds and an efficiency percentage, so enter 100 and 90 for 100 ms and 90%. It handles those conversions before evaluating the equation.
Example: 10 W for 100 ms
Suppose a converter must deliver 10 W while its input falls from 12 V to 9 V over a 100 ms interruption. At a constant 90% efficiency, the capacitor supplies about 1.111 J. The voltage-squared difference is 63 V², giving an ideal capacitance of 0.0352734 F, or 35.27 mF.
That is the energy requirement, not a recommendation to buy the next nominal part value. For comparison, an ideal 47 mF capacitor in the same voltage window would provide about 133.2 ms. Actual hold-up time can be shorter.
| Quantity | Value |
|---|---|
| Load power | 10 W |
| Hold-up time | 100 ms |
| Initial / minimum voltage | 12 V / 9 V |
| Efficiency | 90% |
| Ideal capacitance | 35.27 mF |
Check the part and the power path
Use the minimum effective capacitance after tolerance, temperature, aging, and DC bias. Check voltage rating and the manufacturer’s ripple-current and ESR data. A useful nominal capacitance does not establish that the part can deliver the required current.
As capacitor voltage falls, a constant-power converter draws more input current. ESR and interconnect drops can therefore end the useful discharge earlier than this energy-only model predicts. Efficiency may also change along the discharge.
Finally, check charging current, source isolation, startup, and recovery after repeated interruptions. Those circuits are outside the calculator. Confirm the complete design against converter and capacitor data, then measure hold-up under the expected conditions.
References & further reading
Equations describe the stated model. Worked values are rounded for reading; the calculator’s download retains its calculated precision. Read about our checks and their limits.