Input current is the electrical current a power supply draws from its source. For AC-input power supplies, this value is typically specified in root mean square (RMS).
Many AC-input power supplies without a Power Factor Correction (PFC) circuit use a capacitive input rectifier. This circuit draws current in short pulses only near the peaks of the AC voltage, resulting in a non-sinusoidal input current waveform as shown in the figure.
The approximate RMS input current can be calculated using the following formula, which includes the Power Factor (PF):
Input Current (RMS) = Output Power / (Input Voltage (RMS) × Efficiency × Power Factor)
For power supplies without a PFC circuit, the power factor is significantly less than 1 (typically 0.5-0.7). For supplies with an active PFC circuit, the power factor is close to 1.0.
In contrast, a power supply equipped with a Power Factor Correction (PFC) circuit is designed to draw an input current that is sinusoidal and in phase with the input voltage. This results in a high power factor (close to 1.0) and a more efficient use of power from the AC line.
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