Phase controlled dimming system with active filter for...

Details Phase controlled dimming system with active filter for... Phase controlled dimming system with active filter for...

2000-09-18

2002-04-30

Philogene, Haissa (Department: 2821)

Electric lamp and discharge devices: systems

Regulation of the control current and/or potential applied...

C315S291000, C315S307000, C315SDIG004

Reexamination Certificate

active

06380692

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to electronic systems that employ a phase control technique to control the amount of power delivered from an AC source/AC line to a load, such as a lighting load. The present invention specifically relates to a lighting control system such as a dimming panel or a wall mounted dimmer switch, that employs a phase control technique to control the dimming level of a lighting load by altering the conduction angle of a thyristor that is in series with the load.
BACKGROUND OF THE INVENTION
The present invention is described herein in the context of a dimming system for a lighting load, but is not limited thereto. The present invention has applicability in any AC phase control system where it is desired to minimize undesired variations in the power delivered to a phase controlled load caused by a noisy or unstable AC source, especially at low levels of delivered power.
Most lighting control systems that have a dimming capability employ a thyristor in series with the AC lighting load to effect the dimming function. Dimming is performed by altering the conduction angle of the thyristor, usually by delivering a trigger signal to a gate of the thyristor such that the timing of the trigger signal varies with the selected dimming level. In a typical forward phase control system, generation of the trigger signal is synchronized with the AC line voltage (the fundamental frequency waveform of which is sometimes referred to herein as “the AC fundamental” such that, some time after a zero crossing of the AC line voltage is detected, the trigger signal is generated, the gate of the thyristor is energized, and the thyristor conducts for the remainder of the AC half cycle. During the time interval between the detection of the zero crossing and the generation of the trigger signal, the thyristor is non-conducting (during which time no power is delivered from the AC source to the load), and usually this time interval is altered in response to adjustment of a dimming knob or slider by a user, or in response to changes in a dimming signal level. Altering this time interval thus alters the conduction angle of the thyristor, and hence alters the RMS power delivered to the load. See commonly assigned U.S. Pat. No. 5,430,356 entitled “Programmable Lighting Control System With Normalized Dimming For Different Light Sources”, the entirety of which is incorporated herein by reference.
At low levels of delivered power (i.e., conduction beginning at phase angles greater than about 135° for each first half cycle, and greater than about 315° for each second half cycle, of the AC fundamental), even a small variation in the conduction angle usually represents a relatively large variation in the percentage of the total delivered RMS power. At these low power levels, any variation of the conduction angle, whether between AC cycles or over periods of time, can be manifested as annoying and unacceptable intensity changes, including visible flickering of the light source. Since the conduction angle is dependent on the detection of the zero crossing, it is crucial that zero cross detection be accurate and reliable. AC line conditions are rarely ideal, and less than ideal conditions can cause inaccuracy in the detection of zero crossings, with consequent intensity variations and/or flickering, as well as other problems, especially at low levels of delivered power.
The prior art has recognized that one condition that can cause intensity variations and/or flickering is intermittent and/or periodic electrical noise on the AC line. For example, voltage “spikes” can be imposed on the AC line by the switching on and off of heavy equipment such as large motor loads. See FIG.
1
. Electrical noise on the AC line can be incorrectly interpreted by the dimming circuitry as zero crossings of the AC fundamental, and these false interpretations can lead to premature and/or erratic conduction of the thyristor. The prior art has also recognized that another condition that can cue intensity variations and/or flickering is distortion of the AC waveform, which can be caused by the mere presence of other loads on the line. Distortion may be characterized by a “bumpy” or “wavy” AC waveform, i.e., one that is not a smooth sinusoid. See FIG.
2
. This “bumpiness” can also move relative to the AC fundamental, i.e., it is not synchronized to it. Distortion can also cause false zero crossing detection. One common prior art solution to the problem of detecting actual zero crossings in a noisy and/or distorted AC line is to employ a phase locked loop (PLL) to generate a signal internal to the dimming system having a frequency that is intended to track that of the AC fundamental. In this system, the internal signal is a new signal generated by the PLL that is intended to replicate the AC fundamental. The zero crossings of the internal signal are detected, and since it is relatively free of noise and distortion, zero crossing detection is relatively straightforward.
The prior art has also recognized (separately from the problems of noise and distortion) that frequency variations can occur in the AC line. A common prior art solution to the problem of detecting zero crossings in an AC line having unstable frequency is to sample the AC line during a small “sampling window” (e.g., 500 microsec. wide) at periodic intervals. In this type of system, known as “window detection”, a sample timer is set to open the sample window just before the next zero crossing of the AC line is expected e.g., for a 60 Hz line, the sample window is opened at 8.33 msec. intervals. During the time that the sample window is open, the AC line is monitored for a zero crossing; the AC line is not monitored for zero crossings between sample windows. Any zero crossing that is detected after the sample window has been opened can be taken as the actual zero crossing of the AC line, and the sample timer is reset. In a prior art system that has made and sold by the assignee hereof as the Grafik Eye 4000 Series dimming panel, the last zero cross detection is used as the actual zero cross crossing of the AC line. The window detection method can detect zero crossings in an AC line of unstable frequency provided that the change in period is not so substantial that the actual zero crossing falls outside of the sampling window.
The prior art, therefore, has attempted to detect zero crossings by either operating upon a separately generated signal that is intended to replicate both the phase and frequency of the fundamental of the AC line, or by operating upon the AC line itself.
Another condition that can cause intensity variations and/or flickering is changes in the RMS voltage of the AC line. Changes in the RMS voltage of the AC line can be caused by the presence of harmonics of the AC fundamental on the AC line; the presence of these harmonics changes the shape of the AC line voltage waveform from a pure sinusoid to, e.g., a generally sinusoidal waveform having flattened peaks, rather than round peaks. See FIG.
3
. Changes in the RMS voltage of the AC line will cause intensity changes in the lighting load because such changes result in variations of the total power delivered to the load, irrespective of when the zero crossings occur.
The prior art has failed to recognize that the conditions of noise/distortion on the AC line, on the one hand, and frequency variation of the AC line, on the other hand, may be simultaneously and/or alternatively present The prior art has also failed to recognize that, in addition to the presence of these conditions, the condition of changing RMS voltage may also be simultaneously and/or alternatively present. To make matters worse, all of these conditions may be variably and intermittently present on the AC line, and these and other line conditions may be constantly changing. A condition that is present at one moment may be gone or replaced by another at the next moment; one combination of conditions may exist at one moment and be replaced by another combination at the next moment; and/or all or no

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