True Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor

Jul 1, 2010 12:00 PM
Dr. Slobodan Cuk, President, TESLAco, Irvine, CA

REFERENCES

1. Slobodan Cuk, “Modelling, Analysis and Design of Switching Converters”, PhD thesis, November 1976, California Institute of Technology, Pasadena, California, USA.

2. Dragan Maksimovic, “Synthesis of PWM and Quasi-Resonant DC-to-DC Power Converters”, PhD thesis, January 12, 1989, California Institute of Technology, Pasadena, California, USA.

3. Vatche Vorperian, “Resonant Converters”, PhD thesis, California Institute of Technology, Pasadena, California.

4. Slobodan Cuk, R.D. Middlebrook, “Advances in Switched-Mode Power Conversion”, Vol. 1, II, and III, TESLAco 1981 and 1983.

5. Stephen Freeland, “ I. A Unified Analysis of Converters with Resonant Switches II. Input -Current Shaping for Single Phase Ac-Dc Power Converters”, PhD thesis, October 20, 1987, California Institute of Technology, Pasadena, California, USA.

WHAT IS POWER FACTOR (PF) AND WHY LOW PF IS BAD

Here only a brief summary of the Power Factor and its significance is given as the more detailed account can be found elsewhere [2].

When the load resistor R is placed directly across the utility line such as shown in Figure 2a, the sinusoidal line current iAC drawn is directly proportional to and in phase with the sinusoidal line voltage vAC as illustrated in Figure 2b so:

iAC (t) = vAC ( t) /R (1)

For the most efficient and distortion-free operation of the utility line voltage, all loads attached to the utility line should behave as an effective resistance R often called “emulation” resistance. However, many loads, such as electronics equipment, computers, and other appliances operate from the dc voltage requiring an ac-dc conversion. The simplest Single-stage ac-dc converter is the four-diode bridge rectifier shown in Figure 3a, with large capacitor C across the load resistor R placed now on output dc side. This, in effect, nonlinear load draws the line current only during the peak of the sinusoidal line voltage resulting in a “peaky” input line current shown in Figure 3b. This “peaky” line current generates many current harmonics comparable in magnitude to the fundamental harmonic current at line frequency such as illustrated in Figure 3c. However, only the harmonic current at the same frequency as the line frequency and in phase with the line voltage (in this case fundamental harmonic at line frequency) as in Figure 2b contributes to the average power, P, delivered to the load. All other harmonics at multiple of the line frequency only increase the current demanded from the utility that is increasing losses, without contributing any actual (active) power to the load.

Power Factor is then a simple number, which describes the deviation from the ideally desired condition of ac line current being proportional to ac line voltage and is defined as:

Power Factor (PF) = P / Vrms Irms (2)

Where

P = Average load power expressed in units of watts (real power)

Arms = RMS (root-mean-square) voltage

Rims = RMS (root-mean-square) current

Vrms Irms = Apparent power expressed in volt-amperes (VA) instead in watts (W).

For the load resistor R directly across the ac line voltage as in Figure 2a:

PF = 1 (3)

R is given in terms of rms quantities as:

R = Vrms /Irms (4)

Therefore, the ideal conditions for the utility line is to have a unity power factor as in Equation (3), which is identical to the statement that the line current is proportional to the line voltage as expressed in the time domain in Equation (1).

Note that the bad power factor of 0.5 for the case of a nonlinear load of Figure 3a results also in large harmonic current content as shown in Figure 3c, displaying the magnitude of higher order harmonics (third, fifth, etc.) currents normalized with respect to the magnitude of the fundamental harmonic at line frequency. The direct consequence is that the higher order harmonics will simply increase the line current magnitude without contributing to the active power, P, to the load.

Thus, for the case displayed in Figure 3a, the only variable left in power factor definition of Equation (2) is the line current. I, since line voltage, V, is fixed by utility generators to 120V. Clearly, the higher the current, I, drawn by the utility line for the given average power delivered to the load, the lower the power factor (PF). The ac-dc converter in Figure 3a operating from 120V AC line voltage and delivering 600W of power to the load while drawing 10A of the line current has a PF = 0.5. However, the resistive load with power factor of 1.0 of Figure 2a drawing 600W from utility 120V line draws only 5A from the line. Clearly, the utility line suffers from the low power factor of the loads in two fundamental ways:

It must provide higher generating capability to support increased line current demand in case of poor load power factor, yet it charges the customer only for the average power in Watts delivered and not volt-amperes (VA) generated.

The generated high harmonics currents placed on the utility line cause major disturbance to other users of the same utility line. This is a reason why the regulations define the maximum harmonic current content allowed for a given power level.

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