Electronic Design

What’s The Difference Between Reactive Power Factor And AC-DC Supply Power Factor?

Why ac/dc conversion generates harmonics and how that relates to reactive power factor

Two related phenomena, both called “power factor,” cause problems on electrical power lines. In each case, the ac current being drawn by the load is not exactly synchronized with the ac voltage on the line. Though the consequences and solutions differ in both instances, parallels exist between their situations.

Two Ways In Which Power Factor Is Less Than Unity
Within the realm of power distribution, power factor is typically understood in terms of current in a reactive load lead, or lagging the phase of the line voltage (PF = cos φ, where φ is the phase difference, and “real” power = v  x  i  x  cos φ). In this scenario, a problem arises for the electric company. Regardless of its phase relationship to the voltage, all of those current accounts for real I2R losses along the transmission line.

For simple reactive power factor, power-factor correction (PFC) was (and still is) dealt with in terms of adding capacitive reactance at points in the power-distribution system. Correction takes the form of a bank of capacitors or a “synchronous condenser” (an unloaded synchronous motor).

More broadly, any line-powered apparatus that uses ac-dc power conversion also may require PFC. They can range in scale from battery chargers for portable devices to big-screen TVs.

In this case, power factor is best understood in terms of the power-line fundamental frequency’s series of harmonics. They waste power in the form of heat. However, the higher-frequency harmonics are more insidious due to skin effect, which limits them to the surface of conductors increasing the “R” value in the I2R equation. They also behave differently in the cores of transformers designed for lower operating frequencies.

From a safety standpoint, these harmonics, notably the third, can and do result in three-phase imbalance, with current flowing in the ground conductor in a “wye” (Y) configuration. Usually, the wye ground conductor isn’t sized to carry significant current.

What’s the origin of these power-line fundamental harmonics? Generally, they’re produced in the first stage of any ac-dc converter attached to the line—the familiar full-bridge rectifier and filter transformer. In the steady state, the load and everything downstream of the bulk capacitance draws current from the capacitor during part of each cycle and from the line during the balance of the time. This creates a spiky current waveform on the power line, including the power-line fundamental and all of its odd harmonics (Fig. 1).

1. Power-line-frequency current harmonics in ac-dc switching power supplies form when the load draws current from the power line during the time line voltage is higher than the voltage on the filter capacitor. The net effect is a load current that’s out of phase with the line voltage. It contains frequency components that exhibit skin effect on power lines, causing conduction losses, and excites eddy currents in power-company transformers that produce further losses.

Cumulatively, they represent an out-of-phase current at the fundamental frequency. In fact, one broad definition of power factor is:

where THD is total harmonic distortion.

North American PFC Requirements
North American PFC requirements for electronic devices that connect to the ac mains are less tightly regulated than in other parts of the world. The U.S. Department of Energy’s Energy Star Computer Specification includes “80 PLUS” power-supply requirements for desktop computers, servers, and laptops.

A U.S./Canadian electric utility-funded rebate program, 80 PLUS subsidizes the extra cost of computer power supplies that achieve 80% or higher efficiency at low, mid-range, and peak outputs, relative to the power rating on the nameplate. In addition, subsidized supplies must exhibit a power factor of at least 0.9, based on Equation 1. Some years ago, the 80 PLUS program was expanded to recognize higher-efficiency power supplies.

Regulating PF Outside North America
In the European Union, standard IEC/EN61000-3-2, “Electromagnetic compatibility (EMC) - Part 3-2, Limits, Limits for harmonic current emissions (equipment input current ≤16 A per phase)” sets current limits up to the 39th harmonic for equipment with maximum power-supply specs from 75 to 600 W. Its “Class D” requirements (the strictest) apply to personal computers, computer monitors, and TV receivers. (Classes A, B, and C cover appliances, power tools and lighting.)

In Japan, IEC61000-3-2 is referenced as JIC-C-61000-3-2. The standard is now mandatory in Australia, and China has incorporated it in its China Compulsory Certificate (CCC) mark. Under the U.S. Energy Independence and Security Act (EISA), efforts are being made to harmonize American standards with IEC61000-3-2.

IEC61000-3-2 Limits
Under IEC 61000-3-2, the limits for Class D harmonic currents are laid down in terms of milliamperes per watt of power consumed (see the table). These are based on the 230-V single-phase and 230/400-V three-phase power that’s common outside of North America. Consequently, the current limits must be adjusted for North American 120/240-V mains voltages.

Achieving Unity Power Factor
As noted, power-factor problems arise in ac-input switch-mode power supplies because they only draw current from the line when the ac-supply voltage waveform rises above the dc voltage on bulk storage (filter) capacitor(s). This non-symmetrical current draw introduces ac-line-voltage harmonics on the line.

The basic active PFC concept is fairly simple (Fig. 2). A control circuit switches a MOSFET to draw current through an inductor to fill in the gaps that would otherwise represent harmonics.

2. Power factor correction (PFC) in ac-dc supplies consists of using a control circuit to switch a MOSFET. It draws current through an inductor to fill in the gaps that would otherwise represent harmonics. When the PFC operates in “critical conduction” or “transition” mode (a), the average inductor current stays relatively low because the peak current is allowed to fall essentially to zero amperes. When in “continuous conduction” mode (b), the average current runs higher. Transition mode is easier to achieve; continuous mode results in power factors closer to unity.

A PFC controller can be designed to operate in either critical conduction mode (also called transition mode) or continuous conduction mode. The difference lies in the MOSFET’s switching speed, which in turn determines whether the inductor current (and the energy in the inductor) approaches zero or remains relatively high.

The terms “critical” and “transition” reflect the fact that each time the current approaches 0 A, the inductor reaches a point where its energy approaches zero. Transition mode operation can achieve power factors of 0.9. However, it’s limited to power levels of 600 W and below. It’s economical, though, because relatively few components are used.

The circuit topology for continuous conduction mode is similar, but ripple current reaches a much lower peak-to-peak amplitude. Also, it doesn’t go to zero. The inductor always has current flowing through it and all of its energy isn’t dumped at each pulse-width modulation (PWM) cycle—hence, the term “continuous.”

In this case, the average current produces a higher-quality composite of the ac line current, enabling power factors that are even closer to unity than 0.9. This is particularly important at higher power levels, since the higher currents magnify radiated and conducted electromagnetic interference (EMI) levels that critical conduction mode would have difficulty meeting.

References

1. IEC/EN61000-3-2, Electromagnetic compatibility (EMC) – Part 3-2 – Limits, Limits for harmonic current emissions (equipment input current ≤16 A per phase), http://webstore.iec.ch/webstore/webstore.nsf/Artnum_PK/35281 (there is a charge for downloading)
2. Factor PFC Into Your Power-Supply Design,” Sam Davis
3. PFC Control Technique Maximizes Light Load Efficiency,” Edward Ong
4. PFC—A Little Old-School Knowledge, Part I,” Paul Schimel

TAGS: Components