Electric lamp and discharge devices: systems – Pulsating or a.c. supply – With power factor control device
Reexamination Certificate
2000-08-31
2001-10-09
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
With power factor control device
C315S307000, C315S291000
Reexamination Certificate
active
06300723
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for power factor control, for example, to a preconditioner of an electronic ballast and, more particularly, to a power factor controller of an electronic ballast preconditioner.
Power factor controllers control the ballast input current in response to the ballast input voltage in attempting to provide a ballast power factor of 1.0 (i.e. to make the ballast input look like a simple resistor). The ballast power factor is the ratio of the actual power of an alternating or pulsating current to the apparent power drawn by a ballast. When the input current relative to the input voltage contains phase displacement, harmonic distortion or both, the power factor will be less than 1.0. The amount of displacement between the voltage and current indicates the degree to which the load is reactive. Harmonic distortion, that is, the production of harmonic frequencies, arises from the nonlinear operation of the ballast in drawing power from a mains sinudoidal signal.
A conventional power factor controller often assumes that the input voltage to the ballast will be substantially sinusoidal. Non-linear loads (e.g. machines, motors) drawing power from the mains can temporarily distort the mains voltage resulting in a substantially nonsinusoidal input voltage. Control of the input current based on a non-sinsoidal input voltage can be difficult and can lead to power factor correction of far less than 1.0. It also can be difficult to maintain a total harmonic distortion (TBD) of less than 10% without adversely affecting the power factor.
Maintaining low THD while supplying a DC voltage to the ballast inverter with low ripple typically requires an undesirably large, expensive electrolytic capacitor. Such capacitors deteriorate with time (i.e. dry out) losing their capacitance and therefore their capability of maintaining low ripple. Unacceptable TED levels can result. The relatively large size of these electrolytic capacitors also hampers integrating the components of the ballast preconditioner onto an integrated circuit thereby limiting reduction in ballast manufacturing cost and size.
It is therefore desirable to provide an improved power factor controller which is unaffected by the temporary distortion of the mains voltage. The improved power factor controller should maintain a relatively high power factor (e.g. greater than 95%) without use of a relatively large electrolytic capacitor in order to maintain a low THD.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the invention, a ballast preconditioner includes a capacitor for producing a substantially DC voltage, a switch for controlling the amount of energy stored in the capacitor in response to a switching signal; and a power factor controller for producing the switching signal. The power factor controller includes a reference waveform generator having a look-up table for storing values of at least one waveform. The switching signal produced by the power factor controller is based on the at least one waveform accessed from the look-up table.
The power factor controller is unaffected by temporary distortion of a mains voltage caused by non-linear loads such as machines through storage of the waveform in the look-up table. The waveform stored is typically a half cycle of a sine wave.
It is a feature of the invention that the preconditioner includes a scaling circuit for adjusting the size of the waveform accessed from the look-up table. The scaling circuit is responsive to the voltage across the output of the preconditioner in adjusting the size of the waveform accessed from the look-up table.
The preconditioner is powered by an AC signal at a first frequency. The reference waveform generator generally further includes a phase lock loop having a pulse output signal at a second frequency. This second frequency is a multiple of the first frequency. The pulse output signal, which serves as a clock signal for reading the values of the at least one waveform stored in the look-up table, is a series of pulses (pulse train). The beginning of the pulse train is synchronized to a substantially zero voltage condition of the AC signal. Consequently, any phase displacement between the input ballast voltage and the signal for turning on the switch is substantially eliminated. The power factor controller can include a microcontroller having a read-only memory serving as the look-up table.
In accordance with another aspect of the invention, a method for operating a ballast preconditioner includes generating a waveform based on values stored in a look-up table, producing a switching signal based on the generated waveform, controlling the amount of energy stored in a capacitor in response to the switching signal, and producing a substantially DC voltage across the capacitor based on the stored energy.
It is a feature of this second aspect of the invention that the method further include adjusting the size of the waveform generated based on the voltage across an output of the preconditioner.
Accordingly, it is an object of the invention to provide an improved ballast preconditioner having a power factor controller which is unaffected by the temporary distortion of the mains voltage.
It is another object of the invention to provide an improved ballast preconditioner having a power factor controller which maintains a relatively high power factor without the use of a relatively large electrolytic capacitor in order to maintain a low THD.
Still other objects and advantages of the invention will, in part, be obvious and will, in part, be apparent from the specification.
The invention accordingly comprises several steps in the relation of one or more such steps with respect to each of the others, and a device embodying features of construction, combination of elements, and arrangements of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 5371443 (1994-12-01), Sun et al.
patent: 5619289 (1997-04-01), Katyl et al.
patent: 5650694 (1997-07-01), Jayaraman et al.
Giannopoulos Demetri J.
Wacyk Ihor T.
Wang Shenghong
Blocker Edward
Branzblau Bernard
Philips Electronics North America Corporation
Vo Tuyet T.
Wong Don
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