Power factor correction
Why a charger cannot simply rectify AC and be done with it.
- Explain power factor correction at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
A simple rectifier and capacitor draws sharp current pulses near the peak of the AC waveform, which is bad for the grid and inefficient for the charger. Power factor correction fixes both.
In depth
A load that draws current out of phase with, or in a distorted shape from, the voltage waveform has a power factor below 1, wasting grid capacity even though it draws the same peak current.
Active PFC uses a boost converter stage, switched fast enough that its input current can be shaped to track the input voltage waveform almost exactly, drawing smooth, sinusoidal current in phase with the grid voltage. Only after this shaping does the isolated DC/DC stage take over.
Grid codes in most regions require chargers above a modest power threshold to include active PFC, both to protect the grid and to reduce harmonic pollution affecting other equipment.
Key takeaways
- Uncorrected rectification draws distorted, out-of-phase current that stresses the grid.
- Active PFC uses a boost stage to shape input current into a clean sine wave in phase with voltage.
- Most jurisdictions mandate PFC above a certain charger power level.
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