Phase controlled dimming system with active filter for preventing flickering and undesired intensity changes

• US 6,380,692 B1
• Filed: 09/18/2000
• Issued: 04/30/2002
• Est. Priority Date: 10/02/1997
• Status: Expired due to Term

• Current Assignee: Lutron Technology Company LLC
• Original Assignee: Lutron Electronics Company Incorporated (Lutron Electronics)
• Inventors: Moseley, Robin C., Hausman, Donald F., Newman, Robert C. Jr.
• Primary Examiner(s): Philogene, Haissa
• Application Number:
  US09/664,232
• Time in Patent Office: 589 Days
• Field of Search: 315/194, 315/199, 315/291, 315/276, 315/360, 315/362, 315/DIG.4, 315/307, 315/316, 323/217, 323/234, 323/235, 323/237, 323/300, 323/318, 323/319, 323/320, 323/322
• US Class Current: 315/194
• CPC Class Codes: H02M 5/2573 with control circuit
• CPC Class Codes: H05B 39/08 by shifting phase of trigger voltage applied to gas-filled controlling tubes also in controlled semiconductor devices in converters H02M5/00; with regulation G05F1/44
• CPC Class Codes: Y10S 315/04 Dimming circuit for fluorescent lamps

Abstract

A phase controlled dimming system having an active filter for receiving the AC line voltage waveform, and for recovering the AC fundamental waveform therefrom is disclosed. The recovered AC fundamental waveform is supplied to a zero crossing detector that provides zero crossing indications of the AC fundamental waveform. The recovered AC fundamental is substantially free of noise or distortion, and of frequency components greater than at least second order harmonics, that may be present on the AC line voltage waveform, and that might otherwise result in faulty or incorrect zero crossing detection. An output of the zero crossing detector is supplied to a microprocessor that controls the gating of a controllably conductive device interposed between the controlled load and the AC source based on the zero crossing information. The filter may take an analog or digital (software) form.

Claims

1. A load control system for reliably controlling the power applied to a load from an alternating current (AC) power source wherein the power may contain undesired noise, distortion, or frequency variation, the system comprising:

(a) a Bessel filter including an input to receive the power and an output to provide a filtered AC signal, the Bessel filter adapted to attenuate frequency components of the power that are substantially equal to third order harmonics and greater of an AC fundamental frequency;

(b) a zero crossing detector coupled to the output of the Bessel filter to provide a zero crossing indication signal;

(c) a microprocessor coupled to the output of the zero crossing detector, the microprocessor responsive to the zero crossing indication signal and to a signal indicative of a desired power level to provide a control signal; and

(d) a controllably conductive device including a control lead coupled to an output of the microprocessor and responsive to the control signal, an input coupleable to the AC power source, and an output coupleable to the load.

2. The load control system of claim 1 wherein said Bessel filter comprises:

(a) a first stage including a first differential amplifier having a first non-inverting input and a first inverting input, the first non-inverting input resistively coupled to the Bessel filter input, a first amplifier output, a first capacitor connected between the first amplifier output and the first non-inverting input, a second capacitor connected between the first non-inverting input and a circuit common, and a first resistor divider network connected between the first amplifier output and the first inverting input; and

(b) a second stage including a second differential amplifier having a second non-inverting input and a second inverting input, the second non-inverting input resistively coupled to the first amplifier output, a second amplifier output coupled to the Bessel filter output, a third capacitor connected between the second amplifier output and the second non-inverting input, a fourth capacitor connected between the second non-inverting input and the circuit common, and a second resistor divider network connected between the second amplifier output and the second inverting input.

3. The load control system of claim 1 wherein said zero crossing detector comprises:

(a) a differential amplifier having an inverting input, a non-inverting input, and an output, the inverting input coupled to the input of the zero crossing detector, and the output of the differential amplifier connected to the output of the zero crossing detector

(b) a resistor connected between the inverting and non-inverting inputs; and

(c) a capacitor connected between the non-inverting input and a circuit common.

4. The load control system of claim 1 wherein the Bessel filter provides a substantially linear phase delay of less than one-half of a period of the AC fundamental frequency.

5. The load control system of claim 1 wherein said controllably conductive device comprises a triac.

6. The load control system of claim 1 further comprising:

a low pass filter having an input coupled to the power source and an output coupled to the microprocessor, the low pass filter having a cutoff frequency adapted to substantially attenuate noise present in the power and to have substantially less phase delay than the Bessel filter;

the microprocessor detecting zero crossings of the output from the low pass filter and comparing the detected low pass filter zero crossings to the filtered AC signal zero crossings to determine the phase delay of the Bessel filter, the microprocessor adjusting the timing of the control signal to compensate for the phase delay of the Bessel filter.

7. A load control system for controlling power delivered from an AC line to a load, wherein the AC line has a fundamental component with a fundamental frequency, comprising:

(a) a filter coupled to the AC line so as to receive all components and noise present in, and any distortions of, the AC line, the filter having low pass circuitry that;

(i) provides a substantially linear phase delay of less than one-half of a period of the fundamental frequency;

(ii) attenuates frequency components of the AC line that are substantially equal to third order harmonics and greater of the fundamental frequency; and

,

(iii) provides a filtered signal substantially free of noise and distortion present on the AC line;

(b) a zero crossing detector coupled to the filter to receive the filtered signal and provide an indication of zero crossings thereof;

(c) a microprocessor receiving an indication of a desired power level to be delivered to the load and receiving the zero crossing indications and being responsive thereto to provide control signals; and

(d) a controllably conductive device receiving the control signals and, in response thereto, controllably applying the AC line to the load in response to the control signals.

8. A load control system according to claim 7 wherein said low pass circuitry comprises a Bessel filter.

9. A load control system according to claim 7 wherein said controllably conductive device comprises a triac.

10. A load control system according to claim 7 wherein said zero crossing detector comprises:

(a) a differential amplifier having an inverting input coupled to the filter, an non-inverting input, and an output coupled to the microprocessor;

(b) a resistor coupled between the inverting and non-inverting inputs; and

(c) a capacitor coupled between the non-inverting input and an electrical ground.