Understanding Power Factor and Input Current Harmonic in Switched Mode Power Supplies


The proliferation of AC-DC switched mode power supplies has resulted in strict limits being imposed on AC input current power factor and harmonic content. The electronic system designer must take these into account in developing hardware requirements. A sound understanding of typical power factor correction circuit performance and dynamics as a function of line and load will aid in this task.


The electrical supply industry has placed requirements on the power factor of electrical equipment for many years. Historically, these requirements were developed around powered equipment consisting of resistive and reactive (inductive or capacitive) loads, which will present varying phase angles between the sinusoidal voltage applied to the load and the current flowing in it. With a purely resistive load the current and voltage are in phase so the real power consumed is just the product of Voltage and Current. However, with reactive elements there will be a phase shift between the current and voltage. For a pure capacitive load the current will lead the voltage by 90 degrees and for a pure inductive load the current will lag the voltage by 90 degrees. With a mixture of resistive and reactive loads the phase angle will be somewhere between +90 and -90 degrees, either leading or lagging. Figure 1 presents a typical reactive load current.

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