Method and apparatus for controlling minimum brightness of a fluorescent lamp

• US 6,654,268 B2
• Filed: 09/03/2002
• Issued: 11/25/2003
• Est. Priority Date: 06/22/2000
• Status: Expired due to Fees

• Current Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Original Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Inventors: Choi, Hwangsoo
• Primary Examiner(s): Nguyen, Matthew V.
• Application Number:
  US10/234,653
• Publication Number:
  US 20030058670A1
• Time in Patent Office: 448 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, 315/307, 315/308
• 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 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 duration determined by a first pulse generator circuit which counts cycles of current flowing through the fluorescent lamp with respective amplitudes above a preset threshold.

2. The power conversion circuit of claim 1, 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 first pulse generator circuit.

3. The power conversion circuit of claim 1, wherein the controller comprises:

a pulse width modulation circuit;

an oscillator circuit; and

a dimming control circuit.

4. The power conversion circuit of claim 3, wherein the dimming control circuit comprises:

a second pulse generator circuit configured to determine an initial duration for the active states;

the first pulse generator circuit configured to determine a second duration for the active states; and

a logic gate configured to output a signal to the pulse width modulation circuit with a duty cycle corresponding to a greater of the initial duration for the active states and the second duration for the active states.

5. The power conversion circuit of claim 4, wherein the logic gate is an OR-gate.

6. The power conversion circuit of claim 4, wherein the first pulse generator circuit comprises:

an amplifier configured to produce a pulse when a voltage representative of the current flowing through the fluorescent lamp transitions from below a reference voltage to above the reference voltage;

a counter coupled to the amplifier and configured to advance a count in response to the pulse; and

a comparator configured to compare a control value with a current value of the count and to provide an indication when the current value of the count equals or exceeds the control value.

7. The power conversion circuit of claim 6, wherein the control value is provided via control signals and is stored in a memory element of the first pulse generator circuit.

8. The power conversion circuit of claim 6, wherein the counter is an n-bits binary counter which resets periodically.

9. The power conversion circuit of claim 6, wherein the comparator is an n-bits digital comparator.

10. The power conversion circuit of claim 6, wherein the amplifier includes internal hysteresis.

11. A method of controlling a reduced 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 selected count in each burst.

12. A power conversion circuit for driving a fluorescent lamp at a selected reduced 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 reduced number of the cycles.