OF AN ELECTRONIC DIMMING BALLAST

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Patent Application No. 12/858,662, filed

August 18, 2010, entitled "METHOD OF CONTROLLING AN OPERATING FREQUENCY OF AN ELECTRONIC DIMMING BALLAST."

BACKGROUND OF THE INVENTION Field of the Invention

[0002] The present invention relates to an electronic dimming ballast, and more particularly, to a method of determining an approximation of a resonant frequency of a resonant tank circuit of an electronic dimming ballast, and adjusting an operating frequency of the ballast in response to the approximation of the resonant frequency.

Description of the Related Art

[0003] Prior art electronic ballasts for fluorescent lamps typically comprise a "front-end" circuit and a "back-end" circuit. The front-end circuit often includes a rectifier for receiving an alternating-current (AC) mains line voltage and producing a rectified voltage V _RE C _T , and a boost converter for receiving the rectified voltage V _RE C _T and generating a direct-current (DC) bus voltage V _B US across a bus capacitor. The boost converter is an active circuit for boosting the magnitude of the DC bus voltage above the peak of the line voltage and for improving the total harmonic distortion (THD) and the power factor of the input current to the ballast. The back-end circuit typically includes a switching inverter circuit for converting the DC bus voltage V _B US to a high-frequency inverter output voltage V _IN V (e.g., a square -wave voltage), and a resonant tank circuit for generating a sinusoidal voltage VS _IN from the inverter output voltage V _IN V and coupling the sinusoidal voltage VS _IN to the lamp electrodes. The amount of power delivered to the lamp may be adjusted by controlling a duty cycle DC _IN V of the inverter output voltage V _IN V to thus control the intensity of the lamp from a low-end intensity L _LE to a high-end intensity L _RE - An operating frequency ¾p of the inverter output voltage V _IN V may be held constant for much of the dimming range of the lamp between the low-end intensity L _LE to the high-end intensity L _HE -

[0004] In order for the resonant tank circuit to provide an appropriate amount of output impedance to the lamp, such that the lamp intensity is stable and does not flicker when controlled to the low-end intensity L _LE , the operating frequency fop of the inverter output voltage V _IN V is typically controlled to a low-end frequency f _LE that is slightly above a resonant frequency f _RE s of the resonant tank circuit at the low-end intensity L _LE - However, if the operating frequency fop of the inverter output voltage V _IN V is controlled too close to the resonant frequency f _RE s, the reverse recovery of diodes in the inverter circuit may cause noise and increased temperatures in the inverter circuit. Therefore, there is a frequency window above the resonant frequency f _RE s in which the operating frequency ¾ρ of the inverter output voltage V _IN V must be controlled when the lamp is at the low-end intensity L _LE - Since the resonant frequency f _RE s is dependent upon the tolerances of the components of the resonant tank circuit, the components of the resonant tank circuit as well as the value of the low-end frequency f _LE must be carefully chosen to ensure that the operating frequency fop of the inverter output voltage V _IN V is within the frequency window when the lamp is at the low-end intensity L _LE - Accordingly, there is a need for an electronic dimming ballast that is able to more accurately control the operating frequency fop of the inverter output voltage V _IN V with respect to the resonant frequency f _RE s when the lamp intensity is controlled near the low-end intensity L _LE -

SUMMARY OF THE INVENTION

[0005] According to an embodiment of the present invention, an electronic ballast for driving a gas discharge lamp comprises an inverter circuit, a resonant tank circuit, and a control circuit operable to determine an approximation of a resonant frequency of the resonant tank circuit and to control the inverter circuit in response to the approximation of the resonant frequency. The inverter circuit converts a DC bus voltage to a high-frequency output voltage having an operating frequency and an operating duty cycle. The resonant tank circuit couples the high-frequency output voltage to the lamp to generate a lamp current through the lamp and a lamp voltage across the lamp. The control circuit is coupled to the inverter circuit for controlling the operating frequency and the operating duty cycle of the high-frequency output voltage, so as to adjust the intensity of the lamp to a target intensity. The control circuit is operable to control the operating frequency of the high-frequency output voltage in response to the approximation of the resonant frequency and the target intensity of the lamp.

[0006] According to another embodiment of the present invention, a method of determining an approximation of a resonant frequency of a resonant tank circuit of an electronic ballast for driving a gas discharge lamp comprises: (1) providing a high-frequency output voltage having an operating frequency and an operating duty cycle to the resonant tank circuit; (2) the resonant tank circuit coupling the high-frequency output voltage to the lamp to generate a lamp current through the lamp and a lamp voltage across the lamp; (3) adjusting the operating frequency of the high- frequency output voltage from a frequency above the resonant frequency of the resonant tank circuit down towards the resonant frequency; (4) measuring the magnitude of the lamp voltage; and (5) storing the present value of the operating frequency of the high-frequency output voltage as the resonant frequency when the magnitude of the lamp voltage reaches a maximum value.

[0007] In addition, a method of driving a gas discharge lamp in an electronic dimming ballast having a resonant tank circuit characterized by a resonant frequency is described herein. The method comprises: (1) providing a high-frequency output voltage having an operating frequency and an operating duty cycle to the resonant tank circuit; (2) the resonant tank circuit coupling the high-frequency output voltage to the lamp to generate a lamp current through the lamp and a lamp voltage across the lamp; (3) controlling the operating duty cycle of the high-frequency output voltage, so as to adjust the intensity of the lamp to a target intensity; (4) determining an

approximation of the resonant frequency of the resonant tank circuit; and (5) automatically adjusting the operating frequency of the high-frequency output voltage in response to the approximation of the resonant frequency and the target intensity of the lamp.

[0008] Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention will now be described in greater detail in the following detailed description with reference to the drawings in which:

[0010] Fig. 1 is a simplified block diagram of an electronic dimming ballast for driving a fluorescent lamp according to an embodiment of the present invention;

[0011] Fig. 2 shows example timing diagrams of the magnitude of a lamp voltage developed across the lamp and an operating frequency of an inverter circuit of the ballast of Fig. 1 while attempting to strike the lamp;

[0012] Fig. 3 shows example waveforms of the magnitude of the lamp voltage and the operating frequency of the inverter circuit of the ballast of Fig. 1 during a resonant frequency detection procedure according to the embodiment of the present invention;

[0013] Fig. 4 is a simplified flowchart of the lamp strike procedure executed by a microprocessor of the ballast of Fig. 1 when the ballast receives a command to turn the lamp on;

[0014] Fig. 5 is a simplified flowchart of the resonant frequency detection procedure executed by the microprocessor of the ballast of Fig. 1 according to the embodiment of the present invention; and

[0015] Fig. 6 is a simplified flowchart of a target intensity adjustment procedure executed by the microprocessor of the ballast of Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION [0016] The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.

[0017] Fig. 1 is a simplified block diagram of an electronic dimming ballast 100 according to an embodiment of the present invention. The ballast 100 comprises a hot terminal H and a neutral terminal N that are adapted to be coupled to an alternating-current (AC) power source (not shown) for receiving an AC mains line voltage VAC- The ballast 100 is adapted to be coupled between the AC power source and a gas discharge lamp (e.g., a fluorescent lamp 105), such that the ballast is operable to control the amount of power delivered to the lamp and thus the intensity of the lamp. The ballast 100 comprises an RFI (radio frequency interference) filter and rectifier circuit 1 10 for minimizing the noise provided on the AC mains, and producing a rectified voltage V _RE C _T from the AC mains line voltage VAC- The ballast 100 further comprises a boost converter 120 for generating a direct-current (DC) bus voltage V _B us across a bus capacitor C _B us- The DC bus voltage V _B us typically has a magnitude (e.g., 465 V) that is greater than the peak magnitude V _PK of the AC mains line voltage VAC (e.g., 170 V). The boost converter 120 also operates as a power-factor

correction (PFC) circuit for improving the power factor of the ballast 100. The ballast 100 also includes an inverter circuit 130 for converting the DC bus voltage V _B US to a high-frequency inverter output voltage V _IN (e.g., a square-wave voltage), and a resonant tank circuit 140 for coupling the high-frequency inverter output voltage generated by the inverter circuit to filaments of the lamp 105.

[0018] The ballast 100 further comprises a control circuit, e.g., a microprocessor 150, which is coupled to the inverter circuit 130 for turning the lamp 105 on and off and adjusting the intensity of the lamp 105 to a target intensity L _TAR G _ET between a low-end (i.e., minimum) intensity L _LE (e.g., 1 %) and a high-end (i.e., maximum) intensity L _HE (e.g., 100%). The microprocessor 150 may alternatively be implemented as a microcontroller, a programmable logic device (PLD), an application specific integrated circuit (ASIC), or any suitable type of controller or control circuit. The microprocessor 150 provides a drive control signal V _DRI V _E to the inverter circuit 130 and may control one or both of two operational parameters of the inverter circuit (e.g., an operating frequency ¾p and an operating duty cycle DC _IN V) to control the magnitudes of a lamp voltage V _L generated across the lamp 105 and a lamp current I _I conducted through the lamp. The

microprocessor 150 receives a lamp current feedback signal V _FB - _IL , which is generated by a lamp current measurement circuit 152 and is representative of the magnitude of the lamp current I _I . The microprocessor 150 also receives a lamp voltage feedback signal V _FB -V _L , which is generated by a lamp voltage measurement circuit 154 and is representative of the magnitude of the lamp voltage V _L .

[0019] The ballast 100 also comprises a memory 156, which is coupled to the

microprocessor 150 for storing the target intensity L _T A _R G _ET and other operational characteristics of the ballast. The memory 156 may be implemented as an external integrated circuit (IC) or as an internal circuit of the microprocessor 150. A power supply 158 receives the bus voltage V _B US and generates a DC supply voltage Vcc (e.g., approximately five volts) for powering the

microprocessor 150, the memory 156, and other low- voltage circuitry of the ballast 100.

[0020] The ballast 100 may comprise a phase-control circuit 160 for receiving a

phase-control voltage V _P C (e.g., a forward or reverse phase-control signal) from a standard phase-control dimmer (not shown). The microprocessor 150 is coupled to the phase-control circuit 160, such that the microprocessor is operable to determine the target intensity L _TAR G _ET for the lamp 105 from the phase-control voltage Vpc. The ballast 100 may also comprise a communication circuit 162, which is coupled to the microprocessor 150 and allows the ballast to communicate (i.e., transmit and receive digital messages) with the other control devices on a communication link (not shown), e.g., a wired communication link or a wireless communication link, such as a

radio-frequency (RF) or an infrared (IR) communication link. Examples of ballasts having communication circuits are described in greater detail in commonly-assigned U.S. Patent

No. 7,489,090, issued February 10, 2009, entitled ELECTRONIC BALLAST HAVING ADAPTIVE FREQUENCY SHIFTING; U.S. Patent No. 7,528,554, issued May 5, 2009, entitled ELECTRONIC BALLAST HAVING A BOOST CONVERTER WITH AN IMPROVED RANGE OF OUTPUT POWER; and U.S. Patent Application No. 1 1/787,934, filed April 18, 2007, entitled

COMMUNICATION CIRCUIT FOR A DIGITAL ELECTRONIC DIMMING BALLAST, the entire disclosures of which are hereby incorporated by reference. [0021] The inverter circuit 130 comprises first and second series-connected switching devices (e.g., FETs Q132, Q134) and an inverter control circuit 136, which controls the FETs in response to the drive control signal V _DRI V _E from the microprocessor 150. The inverter control circuit 136 may comprise, for example, an integrated circuit (IC), such as part number NCP5111, manufactured by On Semiconductor. The inverter control circuit 136 may control the FETs Q132, Q134 using a d(l-d) complementary switching scheme, in which the first FET Q132 has a duty cycle of d (i.e., equal to the duty cycle DC _IN V) and the second FET Q134 has a duty cycle of 1-d, such that only one FET is conducting at a time. When the first FET Q 132 is conductive, the output of the inverter circuit 130 is pulled up towards the bus voltage V _B US- When the second FET Q 134 is conductive, the output of the inverter circuit 130 is pulled down towards circuit common. The magnitude of the lamp current I _I conducted through the lamp 105 is controlled by adjusting the operating frequency ¾ρ and/or the duty cycle DCO _P of the high-frequency inverter output voltage V _IN V generated by the inverter circuit 130.

[0022] The resonant tank circuit 140 comprises a resonant inductor LI 42 adapted to be coupled in series between the inverter circuit 130 and the lamp 105, and a resonant capacitor CI 44 adapted to be coupled in parallel with the lamp. For example, the inductor LI 42 may have an inductance Li ₄₂ of approximately 13.4 mH, while the resonant capacitor CI 44 may have a capacitance Ci ₄₄ of approximately 1.2 nF. The resonant tank circuit 140 is characterized by a resonant frequency fRES, i.e.,

such that the resonant frequency fpj¾ may be, for example, approximately 250 kHz. According to an embodiment of the present invention, the microprocessor 150 is operable to determine an

approximation of the resonant frequency fRES of the resonant tank circuit 140 (e.g., measure the resonant frequency), and use the approximation of the resonant frequency fRES during normal operation of the ballast 100, as will be described in greater detail below. In other words, the microprocessor 150 is operable to calibrate the resonant frequency fRES of the resonant tank circuit 140 in order to determine a more accurate value of the resonant frequency fRES that is not dependent upon the worst case tolerances of the components of the resonant tank circuit. [0023] When the microprocessor 150 receives a command to turn the lamp 105 on, the microprocessor 150 first preheats filaments of the lamp 105 and then attempts to strike the lamp during a lamp strike procedure 200, which will be described in greater detail below with reference to Fig. 4. The resonant tank circuit 140 may comprise a plurality of filament windings (not shown) that are magnetically coupled to the resonant inductor LI 42 for generating filament voltages for heating the filaments of the lamp 105 prior to striking the lamp. An example of a ballast having a circuit for heating the filaments of a fluorescent lamp is described in greater detail in U.S. Patent

No. 7,586,268, issued September 8, 2009, titled APPARATUS AND METHOD FOR

CONTROLLING THE FILAMENT VOLTAGE IN AN ELECTRONIC DIMMING BALLAST, the entire disclosure of which is hereby incorporated by reference.

[0024] Fig. 2 shows example timing diagrams of the magnitude of the lamp voltage V _L and the operating frequency ¾ρ of the inverter circuit 130 during the lamp strike procedure 200. After receiving a command to strike the lamp 105 (i.e., at time ti in Fig. 2), the microprocessor 150 first preheats the filaments of the lamp for a preheat time period T _PREHE A _T by controlling the operating frequency ¾p of the inverter circuit 130 to a preheat frequency fpREHEAT, e.g., approximately

130 kHz (which causes the lamp voltage V _L A _MP to be controlled to a preheat voltage V _L _ _PRE )- After the preheat time period T _PREHE A _T (i.e., at time t ₂ in Fig. 2) the microprocessor 150 sweeps the operating frequency ¾p of the inverter circuit 130 down from the preheat frequency fpREHEAT towards the resonant frequency f _RE s of the resonant tank circuit 140, such that the magnitude of the lamp voltage V _L increases until the lamp 105 strikes (i.e., at time t ₃ in Fig. 2). When the lamp 105 strikes, the magnitude of the lamp voltage V _L decreases and the magnitude of the lamp current I _L increases, and, as a result, the microprocessor 150 is able to detect the lamp strike in response to the lamp voltage feedback signal V _FB -V _L and the lamp current feedback signal V _FB - _IL -

[0025] According to the embodiment of the present invention, the microprocessor 150 is operable to execute a resonant frequency detection procedure 300 to determine an approximation of the resonant frequency f _RE s of the resonant tank circuit 140 prior to preheating the filaments and attempting to strike the lamp 105. Fig. 3 shows example waveforms of the magnitude of the lamp voltage V _L and the operating frequency ¾p of the inverter circuit 130 during the resonant frequency detection procedure 300. During the resonant frequency detection procedure 300, the microprocessor 150 controls the duty cycle DQ _N V of the inverter output voltage V _IN V to a minimum duty cycle DC _MIN (e.g., approximately 3%), such the lamp 105 will not be illuminated during the resonant frequency detection procedure 300. The microprocessor 150 then sweeps the operating frequency ¾ρ of the inverter circuit 130 from an initial operating frequency ¾ _Ν ιτ down towards the resonant frequency f _RE s, and monitors the magnitude of the lamp voltage V _L (using the lamp voltage feedback signal V _FB -V _L ). For example, the initial operating frequency ¾ _NIT may be equal to the preheat frequency fp _REHEAT , i.e., approximately 130 kHz). The magnitude of the lamp voltage V _L will reach a maximum value V _L - _M A _X when the operating frequency ¾ρ of the inverter circuit 130 is at the resonant frequency f _RE s (as shown at time t ₀ in Fig. 3). Accordingly, the microprocessor 150 stores the value of the operating frequency fop (when the magnitude of the lamp voltage V _L reaches the maximum value V _L - _M A _X ) as the resonant frequency fREs in the memory 156.

[0026] The microprocessor 150 may be operable to determine the approximation of the resonant frequency f _RE s in response to receiving a digital message via the communication

circuit 162, for example, during manufacturing of the ballast. In addition, the microprocessor 150 may be operable to execute the resonant frequency detection procedure 300 to determine the approximation of the resonant frequency f _RE s each time the lamp 105 is turned on. Alternatively, the microprocessor 150 could be operable to periodically determine the approximation of the resonant frequency f _RE s when the lamp 105 is off, or to determine the approximation of the resonant frequency f _RE s immediately after the lamp is turned off, for example, each time the lamp is turned off.

[0027] When the target intensity L _T A _R G _ET of the lamp 105 is at or near the low-end intensity L _LE , the microprocessor 150 controls the operating frequency ¾p to be close to the resonant frequency f _RE s to provide an appropriate ballasting impedance for stable lamp operation, but not so close to the resonant frequency that excessive noise and heat are generated in the inverter

circuit 130. Specifically, when the target intensity L _TAR G _ET is less than or equal to a threshold intensity L _TH (e.g., approximately 50%), the operating frequency ¾p is controlled to a low-end operating frequency f _LE . For example, the low-end operating frequency ¾, _Ε may be equal to approximately the approximation of the resonant frequency f _RE s (from the resonant frequency detection procedure 300) plus an offset frequency foFFSET (e.g., approximately two kHz). When the target intensity L _T A _R G _ET is greater than the threshold intensity L _TH , the operating frequency fop may be adjusted in response to the target intensity L _T A _R G _ET of the lamp 105 (e.g., to decrease the operating frequency ¾p as the target intensity L _T A _R G _ET increases according to a predetermined relationship). In addition, the microprocessor 150 may control the operating frequency ¾ρ in response to the approximation of the resonant frequency f _RE s when the target intensity L _T A _R G _ET is greater than the threshold intensity L _TH -

[0028] Fig. 4 is a simplified flowchart of the lamp strike procedure 200 that is executed by the microprocessor 150 when the ballast 100 receives a command to turn the lamp 105 on. Before preheating the filaments and attempting to strike the lamp 105, the microprocessor 150 first determines the approximation of the resonant frequency f _RE s by executing the resonant frequency detection procedure 300, which will be described in greater detail below with reference to Fig. 4. After executing the resonant frequency detection procedure 300, the microprocessor 150 controls the operating frequency ¾p of the inverter circuit 130 to the preheat frequency fpREHEAT at step 210, and waits for the length of the preheat time period T _PREHE A _T at step 212. After preheating the filaments for the preheat time period T _PREHE A _T , the microprocessor 150 attempts to strike the lamp 105.

Specifically, the microprocessor 150 starts a strike timeout timer at step 214 and decreases the operating frequency ¾ρ by a predetermined frequency value Af ₀ p (e.g., approximately 150 Hz) at step 216. The microprocessor 150 continues to decrease the operating frequency fop by the predetermined frequency value Δ¾ρ at step 216 until the lamp strikes at step 218 or the strike timeout timer expires at step 220. When the strike timeout timer expires at step 220, the

microprocessor 150 preheats the filaments and tries to strike the lamp 105 once again at steps 210- 220. When the lamp 105 has been struck at step 218, the microprocessor 150 adjusts the duty cycle DC _IN V of the inverter output voltage V _IN V of the inverter circuit 130 (i.e., via the drive control signal V _DRI V _E ) in response to the target intensity L _T A _R G _ET of the lamp at step 222, before the lamp strike procedure 200 exits.

[0029] Fig. 5 is a simplified flowchart of the resonant frequency detection procedure 300 that is executed by the microprocessor 150 prior to preheating the filaments and attempting to strike the lamp 105 during the lamp strike procedure 200 of Fig. 4. The microprocessor 150 first initializes the maximum lamp voltage value V _L _ _M A _X to an initial lamp voltage value V _L - _INIT (e.g., approximately 150 volts) at step 310, and controls the duty cycle DQ _N V of the inverter output voltage V _IN V to the minimum duty cycle DC _MIN at step 312, such the lamp 105 will not be illuminated during the resonant frequency detection procedure 300. The microprocessor 150 then controls the operating frequency to the initial operating frequency ¾ _Ν ιτ at step 314, decreases the operating frequency ¾ρ by the predetermined frequency value Δ¾ρ at step 315, and measures the magnitude of the lamp voltage V _L using the lamp voltage feedback signal V _FB -V _L at step 316. If the measured magnitude of the lamp voltage V _L from step 316 is less than the initial lamp voltage value V _L - _INIT at step 318, the microprocessor 150 once again decreases the operating frequency ¾ρ by the predetermined frequency value Δ¾ρ at step 315 and measures the resulting magnitude of the lamp voltage V _L at step 316.

[0030] When the measured magnitude of the lamp voltage V _L is greater than the initial lamp voltage value V _L _ _INIT at step 318, the microprocessor 150 then determines if the measured magnitude of the lamp voltage V _L is greater than or equal to the maximum lamp voltage value V _L - _M A _X at step 320. If so, the microprocessor 150 updates the maximum lamp voltage value V _L - _M A _X to be equal to the measured magnitude of the lamp voltage V _L at step 322, and sets a temporary resonant frequency fRES- _TEMP equal to the present value of the operating frequency ¾ρ at step 324, before decreasing the operating frequency ¾ρ by the predetermined frequency value Af ₀ p once again at step 315. If the measured magnitude of the lamp voltage V _L has fallen below the maximum lamp voltage value V _L - _M A _X at step 320, but is still greater than a minimum lamp voltage value V _L - _MI N (e.g., approximately 50 volts) at step 326, the microprocessor 150 continues to decrease the operating frequency ¾ρ at step 315 and compares the measured magnitude of the lamp voltage V _L to the maximum lamp voltage value V _L - _M A _X at step 320. When the measured magnitude of the lamp voltage V _L drops below the minimum lamp voltage value V _L - _MIN at step 326, the microprocessor 150 sets the resonant frequency fREs equal to the temporary resonant frequency f _RE S- _TEMP at step 328, and the resonant frequency detection procedure 300 exits.

[0031] Fig. 6 is a simplified flowchart of a target intensity adjustment procedure 400, which is executed by the microprocessor 150 in response to changes to the target intensity L _T A _R G _ET at step 410. If the target intensity L _T A _R G _ET is less than or equal to the threshold intensity L _TH (i.e., approximately 50%) at step 412, the microprocessor 150 controls the operating frequency ¾ρ to the low-end operating frequency Ϊ^Ε (i.e., the approximation of the resonant frequency f _RE s plus the offset frequency foFFSEi") at step 414. The microprocessor 150 then controls the duty cycle DQ _N V of the inverter output voltage V _IN V of the inverter circuit 130 in response to the target intensity L _T A _R G _ET at step 416, and the target intensity adjustment procedure 400 exits. If the target intensity L _TAR G _ET is greater than the threshold intensity L _TH at step 412, the microprocessor 150 adjusts the operating frequency ¾p in response to the target intensity L _T A _R G _ET at step 418, and controls the duty cycle DC _I NV of the inverter output voltage V _I NV in response to the target intensity L _T A _R G _ET at step 416, before the target intensity adjustment procedure 400 exits.

[0032] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.