Every power tool from a mobile charger to a printer, any end equipment powered from the AC grid represents a complex load where the input current is not always in phase with the instantaneous line voltage. The end equipment consumes both real power as well as reactive power from the grid. The ratio between the real(usable power measured in watts) and the total real-plus-reactive power is known as the power factor.
The single biggest reason power quality needs to be addressed are the regional requirements stipulated by efforts of the manufacturers, suppliers, and other concerned governing bodies entities worldwide.
A power factor correction(PFC) circuit intentionally shapes the input current to be in phase with the instantaneous line voltage and minimizes the total apparent power consumed. Power factor correction helps to optimize the input current within off-line power supplies so that these can enhance the real power from the available mains input. While this becomes extremely advantageous to the utility companies, a PFC circuit also provides advantages to the end applications. For example, a low power factor system will need a thicker cable to handle the larger current.
A PFC is needed for almost all AC/DC supplies over 75W of output power.
Understanding the problem
The first plot here essentially places a large capacitor(smoothing capacitor) across the input diode bridge to convert the sinusoidal input voltage to a DC input voltage for the DC-DC converter. This capacitor tends to get replenished only at the peak of the line in narrow slugs of current getting pulled from the line. These current slugs have enormous distortion. Current flows only during periods when the input AC voltage is higher than the charging voltage of the smoothing capacitor.
How is the power factor correction done?
Depending on the capacity of the power supply a large reactor is required. There are three PFC techniques:
1. Passive (static) PFC using a reactor; -Commonly used for low-capacity power supplies.
2. Switching (active) PFC that controls a current at high frequency using a switching device. Switching (active) PFC using MOSFETs as switching devices is commonly used for switched-mode power supplies requiring high efficiency and small size. A boost chopper is a basic topology of a switching PFC circuit.
3. Partial-switching PFC that switches on and off a switching device to control the current a few times per mains cycle and whose applications are restricted. Partial-switching PFC is widely used in combination with a voltage doubler rectifier for the power supplies of 100-VAC inverter air conditioners and other home appliances.
In addition, there are three current conduction modes:
- Critical Conduction Mode (CrCM): That turns on switching devices at a zero current
- Continuous Conduction Mode(CCM): That continuously passes current to a switching reactor.
- Discontinuous Conduction Mode (DCM): That passes current with an interval.
To reduce the size and improve the efficiency of power supplies, switching PFC circuits in various topologies have appeared, including interleaved and bridgeless PFC. Depending on the power supply capacity, various circuit topologies are used such as PFC using parallel switching devices, interleaved PFC, and bridgeless PFC that provides even higher efficiency.
Comparison of the three current conduction modes:
Final Words: Thank you very much for following till the end. If you want to know more about PFC application circuits I intend to create a new article covering the PFC application circuits. Follow me for updates
