Patent Grant
8564216
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
This application claims benefit of the following patent application(s) which is/are hereby incorporated by reference: N/A
The present invention relates generally to electronic ballasts for powering fluorescent lamps. More particularly, the present invention relates to ballast circuitry for detecting asymmetric end-of-life conditions in a fluorescent lamp and disabling the ballast accordingly.
In the field of electronic ballasts, an asymmetric end-of-life (AEOL) condition is associated with the operating behavior of a fluorescent lamp as it approaches the end of its life cycle. In T5 fluorescent lamps, for example, a very important end-of-life lamp characteristic is electrode degradation. With electrode degradation, the electrode operates like a large rectifying resistor. Thus, in current-controlled ballasts, the ballast will supply more power to maintain a nominal current. This extra power is dissipated in the degraded electrode. With enough power dissipation, the electrode will become hot and cause the lamp glass or lamp holder to melt or otherwise become physically unstable. This causes a dangerous situation, and accordingly AEOL protection circuitry has been developed and is known in the art to detect an asymmetric voltage disparity across lamps in contrast to the sinusoidal voltage which normally appears, and to proactively shut down the electronic ballast.
It is important to design AEOL protection circuitry to be robust and various such circuits as are conventionally known in the art are consequently more complex or less cost effective than is otherwise desirable.
In accordance with one aspect of the present invention, an AEOL protection circuit and method are provided for detecting an AEOL condition in fluorescent lamps and disabling an associated electronic ballast.
In an aspect of certain embodiments of the present invention, an AEOL condition may be detected for various lamp types without requiring changes to circuit component parameters.
In another aspect, an AEOL condition may be detected without need for a differential voltage measurement across the one or more lamps with a ground fault capacitor present.
In another aspect, the sensitivity of the AEOL protection circuit may be easily adjusted, using a relatively simple and cost-effective circuit topology.
An embodiment of an electronic ballast in accordance with the present invention includes an inverter with first and second switching elements arranged in a half-bridge configuration. An inverter driver provides drive signals to turn on and off the first and second switching elements and generate an inverter output voltage. A resonant tank circuit is coupled to a node between the first and second switching elements, and a DC-blocking capacitor is coupled between the resonant tank circuit and a first lamp connection node. A grounding circuit includes a ground fault capacitor and a first resistor coupled in parallel between a second lamp connection node and ground, and drains the second lamp connection node to ground. A lamp voltage detection circuit is coupled between the first lamp connection node and ground and measures an output voltage across the first and second lamp connection nodes from a signal at the first lamp connection node.
An embodiment of a protection circuit for an electronic ballast in accordance with the present invention includes a grounding circuit with a capacitor and a first resistor coupled in parallel between a low-side lamp connection terminal and ground. The grounding circuit is effective to drain the low-side lamp connection terminal to ground. A voltage divider circuit includes second and third resistors coupled in series between a high-side lamp connection terminal and ground. A controller includes an input terminal coupled to a node between the second and third resistors and is effective to shut down the ballast based on a voltage-divided signal at the node being greater than a first predetermined threshold value or less than a second predetermined threshold value.
An embodiment of a method of operating an electronic ballast in accordance with the present invention includes a first step of controlling an inverter having first and second switching elements to generate a voltage across first and second lamp connection terminals. The second lamp connection terminal is drained substantially to power ground. A signal is measured at the first lamp connection terminal which is representative of a voltage across the first and second lamp connection terminals. The measured signal at the first lamp connection terminal is compared by a controller to a range defined by predetermined first and second threshold values. The controller then disables the inverter in response to a detected asymmetric end-of-life condition wherein the measured signal is outside of the defined range.
Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may.
The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices.
The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function.
The term “signal” means at least one current, voltage, charge, temperature, data or other signal.
The terms “switching element” and “switch” may be used interchangeably and may unless otherwise stated refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa.
Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated.
Referring generally to
Referring first to
A ballast 10 as represented in
An asymmetric end-of-life (AEOL) protection circuit 12 for the electronic ballast 10 of
Based on the signal at the first lamp connection node 14, an AEOL signal (V—AEOL) may be provided to a controller 24 having an input terminal coupled to the lamp voltage detection circuit 20. In various embodiments the controller 24 may be effective to disable the inverter 22 based on a comparison of the AEOL signal from the lamp voltage detection circuit 20 to a first predetermined threshold value and to a second predetermined threshold value. The controller 24 as represented in
The controller 24 and driver circuit 26 as referred to herein may each include at least a general microprocessor, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a microcontroller, a field programmable gate array, or various alternative blocks of discrete circuitry as known in the art, configured and/or programmed as is known in the art to implement and perform the functions as further defined herein. In alternative embodiments, the controller 24 and driver 26 as represented in
Referring now to
Referring now to
As a first or preliminary step (102) a lamp may be connected to first and second lamp terminals 14, 18 associated with the electronic ballast, after which time an inverter driver 26 may be enabled (step 104) to drive inverter switching elements Q1, Q2 on and off to generate an output voltage V_lamp.
The grounding circuit 16 during operation of the lamp drains the DC voltage across the lamp to near zero (step 106). The resistor R7 of the grounding circuit 16 may be chosen to be substantially greater in impedance than the ground fault capacitor C4 at operating frequencies, vet substantially lower in impedance than the DC impedance to ground at node 14 wherein the second lamp connection node 18 is substantially near power ground. Therefore, instead of needing to measure a differential voltage across the lamp, only a single voltage needs to be measured at the first lamp connection node 14.
The lamp voltage detection circuit 20 (as represented for example in
This voltage-divided signal is then DC biased by the DC biasing circuit 32 including the DC bias voltage source V_bias and the resistors R5, R6 (step 110). The voltage source V_bias may be of a sufficiently low voltage type that the DC biasing circuit 32 provides a DC bias that may be fed into a desired controller (e.g., a microcontroller unit as conventionally known in the art). In various embodiments a sub-step to step 110 may include setting the values of resistors R5, R6 of the DC biasing circuit 32 accordingly to change the sensitivity of asymmetric detection by the AEOL protection circuit 12 generally. The DC-biased signal may then be filtered by a filtering capacitor C3 before it is then fed to the controller 24 as VAEOL (step 112).
The controller 24 may be configured at this stage to compare the filtered signal VAEOL to first and second threshold values which define a range associated with detection of AEOL conditions (step 114). During a normal operating (non-AEOL) condition, the filtered signal VAEOL fed into the controller 24 remains at a voltage level set by the DC biasing circuit 32.
During an AEOL condition the filtered signal VAEOL will increase if there is a positive asymmetric disparity, and decrease if there is a negative asymmetric disparity across the lamp connection nodes 14, 18 or otherwise stated across the lamp.
In an embodiment, the controller 24 may compare the received signal VAEOL to first and second predetermined absolute positive values, wherein a received signal greater than the first predetermined threshold value or less than the second predetermined threshold value is indicative of a detected AEOL condition on the lamp. In another embodiment, the controller 24 may compare a variation of the received signal VAEOL from a predetermined DC bias applied by the DC biasing circuit against a predetermined upper and lower offset threshold value, wherein a variation greater the predetermined upper offset value or less than the predetermined lower offset value is indicative of a detected AEOL condition on the lamp.
In embodiments of the present invention where the output signal VAEOL from the lamp voltage detection circuit has been DC biased to a predetermined voltage, the DC bias may thereby not be dependent on the voltage V_lamp used to operate the lamp itself. This allows the AEOL protection circuit to work with various types of lamps without the need to adjust circuit components or component values accordingly.
If an AEOL condition is detected by the controller based on the comparison (i.e., “yes” in response to the query in step 116), the controller proceeds to supply a shutdown signal to the inverter driver circuit 26, whereby the ballast is disabled (step 118). In certain embodiments the inverter driver circuit 26 may disable the ballast by terminating switching operations in the inverter, but alternative methods of performing the disabling step may include, for example, raising the switching frequency in the inverter well above the normal operating frequency such that a relatively low output power is provided by the inverter. The process by which the ballast is disabled in accordance with step 118 is therefore outside of the scope of the present invention, as various processes are well known in the art having an equivalent function.
If an AEOL condition is not detected by the controller based on the comparison (i.e., “no” in response to the query in step 116), the process merely returns to step 108 or an equivalent segment of the aforementioned method 100 and continues operating the lamp and monitoring the lamp voltage for an AEOL condition.
The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Asymmetric End-of-Life Protection Circuit for Fluorescent Lamp Ballasts,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.
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