1. Field of the Invention
The present invention relates to the power factor which loads present to AC power-lines generally and more specifically to the reduction of the harmonic currents generated on an AC power-line by a DC power supply.
2. Description of the Related Art
The power factor, PF, that a load presents to an AC power-line has long been of concern. Typically, DC power supplies employ a bridge rectifier, a filter capacitor, and sometimes a filter choke. The input of the rectifier is coupled (by a fuse, switch, etc.) across an AC power-line. The output of the rectifier is either coupled by a series choke across the capacitor (choke input filter) or, absent the choke, directly connected across the capacitor (capacitor-input filter) to develop a DC output potential across the capacitor.
With the choke (input filter), DC power supplies draw from the AC power-line a current the waveform of which approximates a square wave (when the inductance of the choke is much greater than what is commonly referred to as the "critical" inductance). Absent the choke, the waveform more approximates a series of pulses each of which is synchronized with a corresponding peak of the AC power-line potential. In either case, the current drawn from the AC power-line includes harmonic components, one for each of the odd harmonics of the AC power-line frequency.
DC power supplies do not conform to all of the old power factor conventions. (For example, it makes little sense to define the power factor a DC power supply presents to an AC power-line as the cosine of the phase angle between the voltage developed across the input of the DC power supply and the current flowing into it.) However, DC power supplies do present many of the same problems. DC power supplies, like other loads having a relatively low power factor, draw from the AC power-line a current the rms level of which is disproportionately high in relation to the current that should be drawn for the power consumed. [In other words, they do conform to the definition which states that the power factor of a load (in this case, a DC power supply) is given by the ratio of the actual power consumed to the apparent power.] Capacitor-input filter type DC power supplies present to an AC power line a power factor, PF, of approximately 58% and generate on the line harmonic currents (the total harmonic distortion, THD) the level of which is approximately 160%.
A relatively high AC power-line rms current is of concern in that the AC power-generating facilities and AC power-transmission facilities (lines and transformers) must be sized to accommodate the current. Further, generation and transmission losses are primarily resistive losses which, therefore, increase as the square of the level of the rms AC power-line current. It is important to note that even relatively small loads (DC power supplies) may be of concern. Although a small personal computer, for example, may not draw the level of the current drawn by a large smoke stack scrubber, if the DC power supply of the computer has a relatively low power factor, the current drawn by the DC power supply may be of such a level as to limit what may also be plugged into a single AC power-line wall outlet.
Of further concern is the current harmonic distortion. The harmonic components can cause destructive heating of various parts of the AC power distribution system. The "triplen" harmonics (third, ninth, fifteenth, etc.) will also add in the neutral of a three phase system and could possibly overheat the neutral wire. Additionally, these harmonic components can cause distortion of the AC voltage. The amount of the voltage distortion depends on the (non-zero) source impedance of the AC line and on the other connected loads. This voltage distortion could possibly interfere with the proper operation of other connected loads.
In U.S. Pat. No. 4,222,096 of D. Capewell and U.S. Pat. No. 4,369,490 of F. Blum, a circuit is disclosed which includes a capacitor connected in parallel with the input of the bridge rectifier of a (capacitor-input-type) DC power supply and an inducator connected between the AC power-line and the input of the rectifier to couple the DC power supply to the AC power-line. The Blum patent indicates on column 5, lines 23-29, that without the above-mentioned circuit, the DC power supply was found to present a power factor of 65% to the AC power-line. Also, it was found that without the above-mentioned circuit, the level of the third harmonic current is 88%, the level of the fifth harmonic current was 65%, and the level of the seventh harmonic current was 38% of the level of the fundamental current. In one example, with the above-mentioned circuit, the DC power supply was found to present a power factor of 94% to the AC power-line. Also, with the above-mentioned circuit, the level of the third harmonic current was 20%, the level of the fifth harmonic current was 6% and the level of the seventh harmonic current was 2% of the level of the fundamental current.