Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2002-06-18
2003-10-28
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S291000, C315S2090PZ, C315S307000, C310S318000, C310S311000, C310S314000
Reexamination Certificate
active
06639367
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention is related to the field of power supply circuits, and more particularly to power supply circuits using piezoelectric transformers to supply power to fluorescent lamps.
Recent advances in ceramics technology have yielded a new generation of so-called “piezoelectric transformers” (also referred to herein as “piezo transformers”) that are useful in certain applications. These devices, which are constructed using laminated thin layers of ceramic material, exploit a well-known phenomenon called the “piezoelectric effect” to provide AC voltage gain, in contrast to the magnetic field effects relied upon by conventional wound transformers. Like conventional transformers, piezo transformers are fairly rugged and can be used to obtain voltage gain in high-voltage applications. Additionally, due to their thin profile, piezo transformers can be used in applications where bulkier wire-wound transformers are impractical. For example, piezo transformers are used in power supplies that provide high-voltage power to fluorescent lamps used as backlights in portable computers. Due to their thin profiles, piezo transformers used in such applications do not adversely affect the desired sleekness of the portable computer enclosure.
Piezo transformers operate most efficiently when operated at frequencies at or near a multiple of a fundamental resonant frequency, which is a function of mechanical characteristics of the transformer such as material type, dimensions, etc. However, piezo transformers are high-impedance devices, and therefore their resonance characteristics as well as other characteristics are sensitive to the loading of the transformer output in operational circuits. Resonant frequency, voltage gain at the resonant frequency, and sharpness of the gain-versus-frequency curve all diminish with increased loading.
The diminishing of resonant frequency and gain with an increase in loading are purposely exploited when a piezo transformer is used to drive a fluorescent lamp. The frequency of the signal applied to the primary inputs of the piezo transformer is slowly swept from a frequency higher than the unloaded resonant frequency toward lower frequencies. As the resonant frequency is approached, the gain increases to the point that the transformer output voltage is sufficiently high to “strike”, or initiate conduction in, the lamp. Once the lamp begins conducting, it presents a much higher load to the transformer, causing the voltage gain and therefore the output voltage of the transformer to drop considerably. The conduction characteristics of the lamp are such that it continues to conduct current at the reduced voltage, so the circuit then enters a stable, lower-voltage operating condition. The intensity of the lamp is regulated by controlling the frequency of the AC drive supplied to the piezo transformer as a function of the lamp current.
The control circuits for piezo transformer based power supplies are often implemented using integrated circuits, which exhibit certain cost characteristics. For example, it is often desirable from a cost perspective to limit the number of input/output pins of an integrated circuit, and likewise limit the amount of active circuit area of an integrated circuit. However, such limitations may run counter to the need for certain functionality in the system in which the integrated circuit is used, such as the above-described regulation of lamp current by controlling operating frequency and the initial sweeping of the operating frequency to strike the lamp correctly. It would be useful to provide a controller integrated circuit for piezo transformer based power supplies that performs these functions while minimizing integrated circuit area and pin count so as to realize greater cost effectiveness.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a control circuit for a piezo transformer based power supply is disclosed that controls operating frequency both for startup as well as for steady state regulation in an improved manner.
The control circuit contains oscillator circuitry that establishes an operating frequency as a function of an oscillator control signal, and drive circuitry that generates a switching control signal at the operating frequency for a primary circuit of the piezo transformer. Sense circuitry generates a feedback signal indicative of an operational parameter of the power supply that is sensitive to frequency-dependent gain of the piezo transformer, such as the current in a cold cathode fluorescent lamp powered by the power supply.
Feedback circuitry varies the oscillator control signal in response to the feedback signal in a manner tending to regulate the operational parameter to a predetermined value. The feedback circuitry includes initialization circuitry that operates during an initialization phase to establish an initial value of the oscillator control signal corresponding to an initial operating frequency, and that thereafter permits the feedback circuitry to gradually drive the oscillator control signal to a normal operating value such that the operating frequency is swept from the initial operating frequency to a normal operating frequency. This operation can provide for the correct striking of a lamp, for example, by establishing a maximum operating frequency initially and then sweeping the operating frequency downward to the normal operating frequency, at which point regulation occurs.
In one embodiment, the feedback circuitry includes a feedback error amplifier whose inputs and outputs are connected to switching circuitry that effects the operation in the initialization phase and the normal operating phase. During the initialization phase, the oscillator control signal is connected to a predetermined voltage, and the error amplifier inputs are connected to predetermined voltages to condition the error amplifier output. Thereafter, one error amplifier input is connected to receive the feedback signal, and the error amplifier output is connected to generate the oscillator control signal. Ensuring operation results in the gradual sweeping of the operating frequency from the initial operating frequency to the normal operating frequency.
The initialization circuitry constitutes a small addition to the feedback circuitry that effects the correct initialization of the oscillator frequency. Prior controller circuits have required entirely separate initialization circuitry and integrated circuit package pins, with attendant cost drawbacks.
Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows.
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Gupta Vishal
Wells James Edward
Brady W. James
Swayze, Jr. W. Daniel
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