How it works
Input power is the input voltage times the input current. Output power is found the same way. The ratio of the two is the efficiency, and their difference is the loss.
Pin = Vin × IinPout = Vout × Ioutη = Pout / Pin × 100 (%)Ploss = Pin − Pout
Worked example
With 12 V at 1 A in and 5 V at 2 A out, the input power is 12 W and the output power is 10 W. The efficiency is 83.3 % and the loss is 2.00 W. That loss becomes heat in the components, which is the starting point for thermal design.
Design tips
- Efficiency varies with load. For battery products, the efficiency at light load and in standby has more effect on runtime.
- Multiplying the loss by the thermal resistance gives a rough estimate of the temperature rise, as in the LDO calculation above.
- Switching converter currents are pulsed. To get the true average, use a power analyzer or a probe with enough bandwidth, and keep the ground connection short.
- When comparing efficiencies, keep the input voltage, temperature and load conditions the same.
Limitations
- This is a calculation based on DC average values. With pulsating currents, the result depends on the measurement method.
- An efficiency above 100 % usually means a problem with the current probe direction, the voltage measurement point or the timing. Recheck the measurement setup.
- Whether the controller's own power consumption is included in the input power changes the efficiency interpretation.
Frequently asked questions
What values should I use when measuring efficiency?
Measure input voltage and current, and output voltage and current, as averages over the same time window. For currents with large ripple, a power analyzer or a probe with enough bandwidth gives more accurate results.
What if the efficiency is above 100 %?
That is physically impossible, so it is a measurement error. Check whether the current probe is reversed, whether the voltage is measured at the load side, and whether the input and output measurement times match.
Should I design by efficiency or by loss?
For thermal design, the loss in watts matters. For battery life, the input power and efficiency matter. At the same output, a higher efficiency means less loss, which lowers both the heat and the thermal resistance requirement.