Method and apparatus for controlling minimum brightness of a fluorescent lamp
First Claim
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 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, said controller comprises:
- a pulse width modulation circuit;
an oscillator circuit; and
a dimming control circuit, wherein the dimming control circuit comprises;
a pulse generator circuit configured to determine an initial duration for the active states;
a minimum pulse generator circuit configured to determine a minimum 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 duty cycle and the minimum duration duty cycle.
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Abstract
All 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.
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Citations
7 Claims
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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 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, said controller comprises:
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a pulse width modulation circuit;
an oscillator circuit; and
a dimming control circuit, wherein the dimming control circuit comprises;
a pulse generator circuit configured to determine an initial duration for the active states;
a minimum pulse generator circuit configured to determine a minimum 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 duty cycle and the minimum duration duty cycle. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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 provide indication when the current value of the count equals or exceeds the control value.
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4. The power conversion circuit of claim 3, wherein the control value is provided via control signals and stored in a memory element of the minimum pulse generator circuit.
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5. The power conversion circuit of claim 3, wherein the counter is an n-bits binary counter which resets periodically.
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6. The power conversion circuit of claim 3, wherein the comparator is an n-bits digital comparator.
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7. The power conversion circuit of claim 3, wherein the amplifier includes internal hysteresis.
Specification