Better get that 'power factor' corrected!

Other Parts Discussed in Post: UCC28180

Deployment of smart meters is in full swing worldwide. Traditionally, consumers like me and you pay only for the kWh (kilowatt-hour) we consumed to power all the electrical equipment in our homes – from air conditioners to internet-enabled, big-screen HDTVs, etc. However, for all equipment without Power Factor Correction (PFC), the energy draw from the electrical outlet is, in fact, much higher and represented by the kVAh (kiloVoltAmp-hour). The cost of the difference is graciously borne by our friendly, neighborhood utility company.

smart meter

Smart meters can measure both the kWh we consume and kVAh that the utility company generates and delivers in the first place. Beware - these smart devices are in a position to expose our bad consuming habits. We’d better get that Power Factor corrected quick, lest the utility companies get smart and decide to come at us with vengeance for their pound of flesh.

One way to protect ourselves from this vengeance is using a power factor correction (pfc) controller. To illustrate my point, I’ll use TI’s UCC28180 PFC controller for the remainder of this post because it’s wide frequency range lends itself to a broad range of applications. Now, let’s talk PFC.

“Active” vs. “Passive” PFC control

Active” PFC control uses a switch mode power converter. “Passive” PFC control involves simply inserting passive electrical components, our good old inductors and capacitors, at the front end of electrical equipment. Although component count may be higher, there is huge savings in overall equipment cost, size, weight and even manufacturing costs by moving from passive PFC control to active PFC control.

A classic example is in a commercial multi-kW air-conditioner (A/C).  In this instance, the size and weight of the passive PFC inductor is so huge, you have to bolt it to the chassis and add a wire harness to connect it to rest of the electronics driving the main compressors and motors. Adopting an active PFC approach utilizing high-frequency switching, the size and weight of the inductor shrink manifold resulting in reduced magnetics cost. Furthermore, from a mechanical design perspective, the inductor can mount directly onto the main electronics board, thus reducing special assembly costs. Having the capability to program the exact switching frequency further provides the flexibility to optimize the design for size, cost, efficiency and specific choice of power switch (MOSFET, IGBT etc).

Total Harmonic Distortion

Total Harmonic Distortion (THD) is a much desired performance metric these days. Simply put, this parameter represents what fraction of the fundamental AC input line current harmonic (47-63Hz) is represented by all the rest of the harmonics combined. Measured as a percent of the fundamental harmonic, the goal is to keep this metric as low as 5-10%, especially when the equipment is consuming significant power, understood as 50% to 100% of nameplate power rating. 

Related post on THD in LED lighting: How to reduce total harmonic distortion to below 10%

In equipment powered by uninterruptable power supplies (UPS), the need for low THD may extend down to even 10-20% of nameplate power rating if the equipment tends to dwell at these load conditions for a prolonged period. This is due to the fact that UPS face a difficult task of delivering a well regulated AC output under a high THD load. A classic example of this situation is a datacenter server power supply running off a UPS, where the server would be idling at these ‘light’ load conditions for several hours during night time, when businesses and offices are closed.

UCC28180 Power Factor and THD (85-264VAC/360W design, 67kHz operation)

With the analog PFC controllers available in the industry today, low THD is achievable when a strong current sensing signal is supplied to the device. However, a strong current sense signal, especially at light loads, implies designing with big shunt resistors to measure the input AC current. However, bigger shunts dissipate more power (IRMS2R). Using available CCM PFC controllers, like TI’s UCC28180, as shown in figure 1, THD as low as 5% with shunt resistance that is 50% smaller is achieved.

Now – go forth and get that ‘power factor’ corrected!

For more on the UCC28180 - download datasheet, order samples, order EVM

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