Input power refers to the electrical power supplied to the power supply unit from the AC mains. In AC input systems, it is essential to distinguish between Apparent Power and Active Power due to the power factor.
- Apparent Power (VA): The total power flowing into the power supply, calculated as the product of the RMS input voltage and RMS input current. This value is critical for selecting the appropriate input wiring, circuit breakers, and facility power capacity.
- Active Power (W): Also known as "real power," this is the power actually consumed by the unit to generate DC output and dissipate internal heat.
In many switching power supplies, the input current waveform becomes non-sinusoidal (distorted) due to internal rectification and smoothing circuits. Consequently, the Apparent Power (VA) is typically higher than the Active Power (W). The ratio of Active Power to Apparent Power is defined as the Power Factor.
Note: When selecting an AC power source or checking facility requirements, always refer to the Apparent Power (VA) or the maximum input current rating.
Apparent Power (VA) = Input Voltage (Vrms) × Input Current (Irms)
Active power =
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