Method and apparatus for controlling minimum brightness of a flourescent lamp

• US 6,469,922 B2
• Filed: 09/04/2001
• Issued: 10/22/2002
• Est. Priority Date: 06/22/2000
• Status: Expired due to Fees

• Current Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Original Assignee: Linfinity Microelectronics, Inc. (Microchip Technology Incorporated)
• Inventors: Choi, Hwangsoo
• Primary Examiner(s): NGUYEN, MATTHEW VAN
• Application Number:
  US09/946,856
• Publication Number:
  US 20020031002A1
• Time in Patent Office: 413 Days
• Field of Search: 363/16, 363/24, 363/25, 363/133, 363/134, 315/209.R, 315/219, 315/221, 315/223, 315/224, 315/227.R, 315/240, 315/DIG.4
• US Class Current: 363/134
• CPC Class Codes: H02M 7/53846 Control circuits
• CPC Class Codes: H02M 7/53862 using transistor type converters
• CPC Class Codes: H05B 41/2824 using control circuits for the switching element H05B41/2822 takes precedence
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence
• CPC Class Codes: Y10S 315/04 Dimming circuit for fluorescent lamps

Abstract

An efficient power conversion circuit for driving a fluorescent lamp uses a minimum pulse generator circuit to control the minimum on-time of a time modulated signal to increase the dimming range of the fluorescent lamp operating over a wide range of temperature and supply voltage. A minimum number of lamp current cycles with respective amplitudes above a preset threshold is typically required to avoid flickering or shimmering during minimum brightness. The minimum pulse generator circuit counts the lamp current cycles and adjusts the on-time accordingly to guarantee the minimum number of cycles with respective amplitudes above a preset threshold under all operating conditions.

Claims

1. A method of controlling the minimum brightness of a fluorescent lamp comprising the acts of:

supplying a sinusoidal voltage to the fluorescent lamp in periodic bursts;

generating a count for each cycle in a sinusoidal current flowing through the fluorescent lamp in response to the sinusoidal voltage, wherein amplitudes of the respective cycles exceed a preset threshold; and

adjusting durations of the bursts to achieve a minimum count in each burst.

2. A power conversion circuit for driving a fluorescent lamp at minimum brightness comprising:

means for providing a sinusoidal voltage to the fluorescent lamp during an on-time;

means for counting cycles in a sinusoidal current flowing through the fluorescent lamp in response to the sinusoidal voltage, wherein amplitudes of the respective cycles exceed a preset threshold; and

means for adjusting the on-time to achieve a minimum number of the cycles.

3. A method of controlling brightness of a fluorescent lamp comprising the acts of:

supplying a sinusoidal voltage to the fluorescent lamp in periodic bursts;

counting sinusoidal lamp current cycles with respective amplitudes above a preset threshold; and

adjusting duty cycles of the periodic bursts to achieve a desired count for each burst.

4. A power conversion circuit for driving a fluorescent lamp, the circuit comprising a controller configured to generate signals with active states and inactive states, wherein durations of the respective active states are equal to or greater than a minimum duration determined by a minimum pulse generator circuit which counts cycles of current flowing through the fluorescent lamp with respective amplitudes above a preset threshold.

5. The power conversion circuit of claim 4, wherein at least one control signal is provided to the controller indicating a control value for comparison with a value representing the cycles counted by the minimum pulse generator circuit.

6. The power conversion circuit of claim 4, wherein the controller comprises:

a pulse width modulation circuit;

an oscillator circuit; and

a dimming control circuit.