Friday, June 13, 2008

Harvesting More Energy without Building More Power Plants

An AC-DC switchmode power supply without Power Factor Correction (PFC) can draw approximately 950 Watts from a typical 115VAC wall socket protected by a 15A circuit breaker before exceeding the UL mandated limit of 12A. A simple load like a toaster can draw almost 1400 Watts. The difference between the two is due to the higher Power Factor (PF) of the toaster, which presents a resistive load to the power line. If we correct the Power Factor of the switchmode supply it can then draw about as much power as the toaster, allowing it to provide more output power to its load from the same 115VAC/15A wall socket.

What is Power Factor Correction (PFC)?
Power Factor (PF) is technically the ratio of real power consumed to the apparent power (Volts-RMS x Amps-RMS), and is expressed as a decimal fraction between 0 and 1. PF is traditionally known as the phase difference between sinusoidal voltage and current waveforms. When the AC load is partly capacitive or inductive, the current waveform is out of phase with the voltage (Fig. 1, the dotted line is the current waveform). This requires additional AC current to be generated that isn't consumed by the load, creating wasted I2R (wattage) losses in the power lines.

An electric motor is inductive, especially when it is starting. The current waveform lags behind the voltage waveform, dropping the PF to well below 1 (similar to Fig. 1). This is why many motors have “starting” capacitors installed to counteract the inductance, and therefore correct the PF during motor startup.

Figure 1. Voltage and current waveforms are sinusoidal but out-of-phase; PF <1.

A simple resistive load has the highest PF of 1. An AC voltage across the resistor causes an AC current which is identical to and in-phase with the voltage waveform (Fig. 2).

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase; PF=1.

A switchmode power supply when viewed as an AC load is neither capacitive nor inductive, but non-linear. A switchmode supply conducts current in short pulses or spikes that are in-phase with the line voltage (Fig. 3). The product of “Volts-RMS x Amps-RMS” is considerable higher than the real power consumed, and thus the PF is much less than 1, typically around 0.65 or less.

Figure 3. Voltage waveform is sinusoidal, current waveform is non-sinusoidal but in-phase; PF<1.

Improving the Power Factor
Low Power Factors can be improved via Power Factor Correction (PFC) circuits. The types used for switchmode power supplies “smooth out” the pulsating AC current, lowering its RMS value, improving the PF and reducing the chances of a circuit breaker tripping. There are two basic types of PFC: Active and Passive. Active PFC is more effective, a bit more expensive, generally integrated into the switchmode power supply, and can achieve a PF of about 0.98 or better. Passive PFC is less expensive and typically corrects the PF to about 0.85.

Harvesting Additional Output Power
To determine just how much more power is available from the AC line and a power supply with PFC, the user needs to understand the following equation, which defines the amount of output power (Pout) available from a switchmode supply:

Pout = VL-RMS x IL-RMS x PF x Eff

For example, UL limits a system's line current to 80% of the circuit breaker's rating. For a typical 15A breaker, 12A is the maximum allowed, and the best-case power available is therefore 120VAC x 12A = 1440 Watts. Referring to the above equation, here are two examples of supplies with different power factors:

  • A switch-mode supply with 0.65 PF and 85% efficiency can only deliver (120 x 12 x 0.65 x 0.85) = 796 Watts (Pout).
  • However, if the power factor is corrected to 0.98, the same power supply can now deliver (120 x 12 x 0.98 x 0.85) = 1200 Watts (Pout), a 51% increase.

From the examples above, it can be seen that by employing power supply’s with PFC, more output power can be delivered to the OEM’s end-product, without the need to increase the AC power wire sizes, increase the circuit breaker’s rating or draw more current from the power plants. Thus, PFC has a significant effect on our environment relative to reducing the pollutants coming from electric power plants.

Meeting International Regulations
Since switchmode power supplies without PFC tend to draw the AC line current in a non-linear fashion, many unwanted harmonic currents are generated and reflected back on the AC power lines. These reflected harmonic currents are “pollutants” to the power grid that have a negative affect on other devices connected to the same power lines. These unwanted harmonic currents can range in frequency from the 100 Hz on up to over 2,000 Hz, and have a direct relationship to the Power Factor of switchmode power supplies.

An important reason to have PFC within your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. These regulations were put in place to maximize the available power generated each day by electric power plants located worldwide. The intent was to make the most of the power we have today without expanding our carbon footprint. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies. Power supplies with PFC circuits that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or better.

As previously mentioned, "Power Line Harmonics" are created whenever the line current is not a pure sinewave, as is the case with a switchmode power supply's input, which tend to have “pulsed” currents (Fig 3). Measuring power line harmonics is a mathematical means to describe a complex waveform's Power Factor by resolving it into a fundamental frequency and its many harmonics.

The harmonic currents do not contribute to the output load power, but cause unwanted heating in the wall socket, wiring, circuit breaker, and distribution transformers, resulting in wasted energy. When personal computers first hit the mainstream market, their power supplies lacked PFC. As a result, circuit breakers that seemed to be sized correctly for the load were tripping for no apparent reason. After investigation, it was determined that the poor power factor of the PC’s power supplies was the culprit.

Today new “green initiatives” are dictating that personal computers include power supplies which must have Power Factor Correction (PFC) and high efficiencies. PFC significantly reduces harmonics, resulting in almost a pure “fundamental” current frequency that will be in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial attenuation of harmonic currents. The vast majorities of AC-DC power supplies manufactured by Lambda Americas employ active PFC, are in accordance with EN61000-3-2 and provide typical power factors in the range of 0.97 to 0.99.

Lambda’s RTW Series of switchmode power supplies range in output power from 50 to 300-Watts. All units include PFC and have typical Power Factors of 0.98 (meets EN61000-3-2). Efficiencies reach up to 89%.

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