Power Factor Correction ICs

Most electronic systems use ac-dc switchmode power converters that draw current from the powerline in a non-sinusoidal fashion. This results in current and voltage distortions that can create problems with other equipment connected to the powerline.

Power factor describes the power relationships on an ac powerline . Current and voltage distortions occur with a reactive load, which has a real and a reactive power component. The vector sum of these two power components is the apparent power to the load. The phase angle between the real power and reactive power is the power factor angle. With a resistive load, the reactive power is zero and the apparent power equals the real power and the power factor is unity, or 100%. If the load is reactive, the power factor is lower (less than 100%).

For a nonlinear load with a distorted current waveform, the current consists of fundamental line frequency and various multiples. These harmonic currents do not contribute directly to the useful power dissipated in the load, but rather adds to the reactive power to create a higher value of apparent power. Total harmonic distortion, THD, is a common way of specifying and measuring the amount of distortion present on a waveform. Note that THD can be higher than 100%.

Most commonly used techniques for power system electronics incorporate a power factor correction (PFC) circuit ahead of the other electronics on the assembly. An example would be the PFC correction circuitry on the front-end of an off-line ac-dc power converter. In addition, most systems that employ an active PFC utilize feedback circuitry along with switchmode converters to synthesize input current waveforms consistent with high power factor.

An ac-dc power supply usually consists of a bridge rectifier followed by a input capacitor and the power stage. The capacitor reduces the ripple on the voltage waveform into the dc converter stage. The result is that large current pulses are drawn from the line over very short periods of time. This produces a spectrum of harmonic signals that may interfere with other equipment.

The impact of the added circuitry is:

  • Additional cost and complexity for the power converter
  • Lower power converter reliability
  • Slightly lower efficiency

Active PFC circuits are based on switchmode converter techniques and are designed to compensate for distortion as well as displacement on the input current waveform. They tend to be significantly more complex, but this complexity is becoming more manageable with the availability of specialized control ICs for implementing active PFC. Active PFC operates at frequencies higher than the line frequency so that compensation of both distortion and displacement can occur within the timeframe of each line frequency cycle, resulting in corrected power factors of up to 0.99.

The boost topology is the most popular PFC implementation. Almost all present day boost PFC converters utilize a standard controller chip for the purposes of ease of design, reduced circuit complexity and cost savings. These ICs greatly simplify the process of achieving a reliable high-performance circuit. In order for the converter to achieve power factor correction over the entire range of input line voltages, the converter in the PFC circuit must be designed so that the output voltage is greater than the peak of the input line voltage.

The active boost circuit corrects for deficiencies in both displacement and distortion. Its duty cycle is longest when the instantaneous value of the ac is near zero and shortest during peaks of each half cycle.

This PFC topology allows automatic range switching on the ac input at essentially no extra cost. Since this universal input function is now a requirement on the majority of power converters to allow for operation in all countries without any manual settings