Abstract:
An electronic ballast for fluorescent lighting includes an inverter circuit having an output circuit coupled to a pair of lamp terminals. A protection circuit is coupled to one of the lamp terminals. The protection circuit includes a differential voltage sensing circuit that is functional to sense the lamp voltage pulses as sudden changes in voltage across a DC blocking capacitor and, in response, to provide a positive AC voltage pulse. A pulse accumulation circuit is coupled to the differential voltage sensing circuit. The pulse accumulation circuit is responsive to the positive AC voltage pulses from the differential voltage sensing circuit to accumulate the positive AC voltage pulses into the ballast shutdown signal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application(s) which is/are hereby incorporated by reference: U.S. Provisional Patent Application No. 61/221,512, filed Jun. 29, 2009. 
    
    
     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. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic ballasts for powering gas discharge lamps. 
     More particularly, this invention pertains to circuits and methods using in an electronic ballast for detecting a lamp end of life condition and/or a short-circuit fault condition at the ballast output. 
     For safety and equipment reliability purposes, electronic ballasts used in fluorescent lighting must include protection circuitry for lamp end of life (EOL) conditions. This need is particularly significant for T5 or smaller lamps. Preferably, the EOL protection circuit will shut down the ballast when the lamp reaches an EOL condition. 
     A typical class D inverter topology for an electronic ballast is shown in  FIG. 1 . A DC rail voltage V_rail is conventionally outputted by a voltage source such as a power factor correction (PFC) section (not shown) or a rectifier circuit (not shown). The rail voltage V_rail is converted by a half-bridge inverter into a high frequency AC voltage. In the embodiment of  FIG. 1 , switching elements Q 1  and Q 2  are MOSFETs that are driven by an IC driver circuit. Capacitor C_dc_blocking is a DC blocking capacitor which prevents DC current from going through the resonant inverter output circuit defined by resonant inductor T_resonant and resonant capacitor C_resonant. A gas discharge lamp (Lamp) is connected across the resonant capacitor C_resonant. The resonant circuit provides proper lamp starting and steady state voltages for the Lamp. Capacitor C_lamp_block is also a DC blocking capacitor to prevent any DC current from passing through the lamp in the output. 
     When a fluorescent lamp reaches its end of life, the lamp voltage typically pulses asymmetrically and the lamp may exhibit visible flickering. The asymmetric pulse will generate a DC voltage offset across the lamp. 
     What is needed is a lamp EOL protection circuit for an electronic ballast that can exploit the existence of the asymmetric lamp voltage pulses to sense that the lamp is in an end of life condition and then initiate appropriate actions to protect the ballast. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the electronic ballast of the present invention includes an inverter circuit having an output circuit coupled to a pair of lamp terminals. A protection circuit is coupled to one of the lamp terminals. The protection circuit is configured to detect lamp voltage pulses that occur at the lamp terminal when a lamp coupled to the lamp terminals reaches an end of life condition. The protection circuit may accumulate the lamp voltage pulses into a ballast shut down signal that is usable by the ballast to initiate shut down of the ballast when the accumulated ballast shut down signal reaches a predetermined shutdown level. 
     In another aspect, the electronic ballast may have a DC blocking capacitor connected between the lamp terminal and circuit ground. In this embodiment the protection circuit may include a differential voltage sensing circuit coupled to the DC blocking capacitor. The differential voltage sensing circuit may be configured to sense the lamp voltage pulses as sudden changes in voltage across the DC blocking capacitor and, in response, to provide a positive AC voltage pulse. 
     In yet another aspect, the protection circuit of the present invention may include a pulse accumulation circuit coupled to the differential voltage sensing circuit. The pulse accumulation circuit may be responsive to the positive AC voltage pulses from the differential voltage sensing circuit to accumulate the positive AC voltage pulses into the ballast shutdown signal. 
     In a further aspect, the electronic ballast of the present invention may respond to a short circuit fault at the lamp terminals by generating an abnormally high AC voltage at the lamp terminals. In one embodiment, the pulse accumulation circuit may be configured such that during the short circuit fault, a capacitor will be continuously charged until the ballast shutdown signal reaches a predetermined shutdown level. 
     In still another aspect, the electronic ballast of the present invention may include a pulse accumulation circuit that is configured to rapidly discharge a first capacitor after a shutdown of the ballast so that charging of a second capacitor is inhibited. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional electronic ballast circuit. 
         FIG. 2  is a schematic diagram of one embodiment of electronic ballast with a lamp EOL detection and protection circuit in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, IGFET, 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. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge or various other forms of power conversion or inversion as known to one of skill in the art. 
     The term “controller” as used herein may refer to 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, designed to perform functions as further defined herein. 
     Referring generally to  FIG. 2 , one embodiment of an electronic ballast  10  with a lamp EOL detection output short protection circuit  20  may be described. Where the ballast of  FIGS. 1 and 2  share common elements and features, similar elements and features are given the same reference numerals and redundant description thereof is be omitted below. 
     In the protection circuit  20 , a first end of a capacitor C 2  is coupled to a node between one lamp terminal and capacitor C_lamp_block. The second end of capacitor C 2  is connected to a first end of resistor R 1 . The second end of resistor R 1  is connected to circuit ground. Capacitor C 2  and resistor R 1  form a differential voltage sensing circuit which senses either a sudden change in DC voltage across capacitor C_lamp_block or a large change in AC voltage across the Lamp. Thus, capacitor C 2  may also be referred to as a sensing circuit capacitor and resistor R 1  may be referred to as a sensing circuit resistor. 
     The cathode of a diode D 31  is connected to the junction of capacitor C 2  and resistor R 1 . The anode of diode D 31  is connected to circuit ground. Diode D 31  may be a Zener diode that is configured to clamp the voltage across resistor R 1  during initial lamp start-up. 
     The cathode of a first pulse accumulation circuit diode D 32  may be connected to the junction of capacitor C 2 , resistor R 1 , and cathode of diode D 31 . First diode D 32  may be a zener diode that senses high positive voltage pulses across resistor R 1 . A first pulse accumulation circuit capacitor C 4  may be connected between the anode of diode D 32  and circuit ground. The reverse breakdown voltage of diode D 32  may be chosen such that during normal steady-state operation of the lamp and ballast, the voltage across first capacitor C 4  is a negative AC voltage. A first pulse accumulation circuit resistor R 3  may be connected in parallel with first capacitor C 4  to provide a discharge path for first capacitor C 4 . The anode of a second pulse accumulation circuit diode D 33  may be connected to the junction of the anode of first diode D 32 , first capacitor C 4  and first resistor R 3 . A second pulse accumulation circuit capacitor C 5  may be connected between the cathode of second diode D 33  and circuit ground. Second diode D 33  and second capacitor C 5  may form an accumulation rectifying circuit that collects and accumulates positive voltage pulses across first capacitor C 4  and provides a steady positive voltage signal that may be used as a pulse detection signal. A second pulse accumulation circuit resistor R 2  may be connected in parallel with second capacitor C 5 . Thus, the arrangement of first diode D 32 , first capacitor C 4 , first resistor R 3 , second diode D 33 , second capacitor C 5  and a second resistor R 2  may be described as positive pulse accumulation circuit or simply, a pulse accumulation circuit. 
     The pulse detection signal from the pulse accumulation circuit may be used as a ballast shutdown signal  25  to shut down or disable operation of the ballast  10 . Use of a shut down signal to disable or shut down an electronic ballast is well known in the art. In one embodiment, the ballast shutdown signal  25  may be coupled to an analog or digital shutdown input on driver IC  30 . In response to receiving the ballast shutdown signal  25  at a predetermined shutdown level, the driver IC  30  terminates gate drive signals to the inverter switching elements Q 1  and Q 2 . 
     The method of operation of the electronic ballast  10  and protection circuit  20  of  FIG. 2  may now be described. During normal operation of the ballast inverter, the voltage cross sensing resistor R 1  will be small magnitude AC voltage. The reverse breakdown voltage of first diode D 32  may be selected to be significantly larger than the positive peak voltage of the normal, small magnitude AC voltage across sensing resistor R 1 . Therefore, there will be no positive voltage pulses across first capacitor C 4  under normal operating conditions of the Lamp. 
     Whenever the Lamp reaches an EOL condition, the lamp voltage will begin to pulse. This pulse will generate a sudden DC offset voltage across the Lamp and across blocking capacitor C_lamp_blocking. The differential voltage sensing circuit (capacitor C 2  and resistor R 1 ) will sense this sudden DC voltage change and transfer it as a large AC voltage pulse across sensing resistor R 1 . The large AC voltage pulse then quickly charges first capacitor C 4  through first diode D 32 , if the peak voltage of the pulse is larger than the breakdown voltage of first diode D 32 . If the lamp voltage pulses are continuous, second capacitor C 5  will be charged through second diode D 33  to a predetermined ballast shutdown signal level, which can be set to initiate shutdown of ballast  10  such as by causing driver IC  30  to terminate gate drive signals to the inverter switching elements Q 1  and Q 2 . 
     After the ballast  10  is shut down, the voltage across sensing resistor R 1  will immediately drop to zero because there is no AC signal across the Lamp. First capacitor C 4  will then be quickly discharged through first diode D 32  and sensing resistor R 1 . Accordingly, the charge remaining in capacitor C 4  will not maintain charging of capacitor C 5  after the inverter  10  is shutdown. This fast voltage reset will insure reliable lamp starting. Thus the sensing circuit resistor R 1  and first capacitor C 4  in the pulse accumulation circuit may be configured to rapidly discharge the first capacitor C 4  after a shutdown of the ballast so that further charging of the second capacitor C 5  is inhibited. 
     The protection circuit  20  may also provide protection of the ballast  10  if there is a short circuit fault at the output of the inverter. For example, when the inverter output is shorted there will be a large magnitude AC voltage across capacitor C_lamp_blocking and sensing resistor R 1 . This large AC voltage will continuously charge capacitors C 4  and C 5  until the voltage across capacitor C 5  reaches the preset level for inverter shutdown. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful electronic ballast with pulse detection circuit for lamp end of life and output short protection, 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.