VARIABLE AMPLITUDE ELECTRONIC BALLAST

This is a continuation of provisional patent application Serial No.

60/937,420, filed June 27, 2007, the contents of which are incorporated by reference.

TECHNICAL FIELD The present invention relates generally to electronic ballasts for gas discharge lamps. More specifically, the present invention introduces means for generating all necessary electrical parameters for appropriate lamp operation by employing amplitude modulation rather then the most commonly facilitated method of frequency modulation, thereby achieving a more sufficient utilization of the input power, as well as a substantial decrease of inherent ballast losses, thus, providing higher energy- efficiency in an overall lighting application.

BACKGROUND ART

Ballasts are an integral component of the lighting industry and are either magnetic or electronic. Magnetic ballasts utilize components which are heavy and cumbersome. Electronic ballasts use electric circuits on a light-weight and reduced size circuit board. A ballast may be used to start a gas discharge lamp, and regulates electrical current used by the lamp. Buyers choose a specific ballast based on the input voltage, output wattage and the starting sequence (i.e., preheating, ignition and steady state operation) of the lamp. Until now, however, most electronic ballast manufacturers have followed a

short-sighted design approach, requiring a unique electronic ballast for every input voltage, output wattage and lamp type combination. As an example, a high output T5 lamp works at optimal efficiency generally with only one particular ballast. If the ballast and lamp are not compatible or matched, the operation of the lamp will not be efficient, thereby adversely affecting brightness, the life of the lamp, and the cost of operation. Additionally, each ballast has to be specifically wired for each lamp voltage input, such as 100V, 120V and so forth. Such wiring is accomplished at the manufacturer's factory or the end user is required to wire the ballast for each lamp depending on application. Therefore, a different ballast is required for each input voltage. These manufacturers often also sell dimmers, timers and controllers as separate, auxiliary components, to be used with their particular ballast design.

DISCLOSURE OF INVENTION

In keeping with one aspect of this invention, an electronic ballast has a rectifier for producing a substantially constant direct current (DC) output voltage from an alternating current (AC) power source, and a pair of pulse width modulators (PWM) operatively connected to a buck/boost converter. The buck/boost converter produces a variable DC bus voltage from the constant DC voltage output. A half-bridge resonant inverter produces an AC output from the variable DC bus voltage, and a gas discharge lamp is operatively connected to the inverter's AC output. A controller determines or is programmed with the operating parameters for the lamp, and varies the magnitude of the DC bus voltage to produce the ignition and running voltages required for the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. l is a block diagram of an embodiment of the present invention; FIG. 2 is a flowchart describing a preparation process for operating a gas discharge lamp using the apparatus of FIG. 1;

FIG. 3 is a flowchart describing a preheating operation used in the apparatus of FIG. 1;

FIG. 4 is a flowchart describing an ignition process used by the apparatus of FIG. 1 ;

FIG. 5 is a flowchart for normal operation of gas discharge lamps using the apparatus of FIG. 1.

BEST MODE OF CARRYING OUT THE INVENTION

As seen in FIG. 1, an electronic ballast 10 is provided for at least one gas discharge lamp 106. The lamp 106 can be any of various gas discharge lamps, including fluorescent lamps, and can have a variety of operational specifications. A typical lamp has a preheat power requirement (optional), an ignition voltage requirement, a steady state operating voltage requirement, and a constant operating current.

The ballast 10 has a rectifier that produces a direct current voltage output 202 from an alternating current power source 100. An output 200 of the alternating current power source 100 is filtered through an electromagnetic compatibility filter (EMC) 101. An output 201 of the EMC 101 is fed to the

rectifier 102 which has appropriate filtering (not shown) at its output 202, to produce a substantially constant DC voltage output.

Two pulse width modulators (PWM) in a microcontroller unit (MCU) 111 produce PWM outputs 302, 303. The outputs 302, 303 are operatively connected to a buck/boost converter 103 through drivers 108, 109. The drivers 108, 109 produce outputs 300, 301, respectively.

The preferred embodiment of the MCU 111 is a "PϊC16HV875" which is manufactured by Microchip Corporation, but the overall design is not limited to that particular device, since alternative MCU 's may be employed, such as MC68HC908LB8 from Freescale, or AT90PWMx made by ATMEL Corporation.

Among other important features, those MCU's provide an option to be flashed and reprogrammed while already implemented in a final target application. This feature is commonly referred to as "In-Circuit Serial Programming" (ICSP). Hence, an external programming device (e.g. Notebook or Palm-PC) may be connected to the MCU through a programming interface 112, in order to change certain parameters "on the fly" at any given time.

Such an external programming device might be also used as an in-circuit monitoring unit. It is therefore a relatively easy task to access data that are of interest from the MCU in real-time, while the overall application is in full operation. Thus, an option is provided to retrieve fault status information with regards to situations in which a particular lighting application is malfunctioning.

This diagnostic option is helpful, since it can be exactly determined if the ballast itself, or the peripheral electrical environment, constitutes the cause of the failure(s), and hence trouble- shooting is significantly eased.

The buck/boost converter 103 includes an inductor 103 a, a first FET or other suitable switching device 103b, a second FET or other suitable

switch 103c, a Schottky diode 103d, an inductor 103e and a capacitor 103f. The converter 103 also has a Schottky diode 103g and a capacitor 103h connected as shown. The output 300 can drive the gate of the FET 103b and the output 301 can drive the gate of the FET 103c. In order to boost the output voltage 202 above the DC output voltage 202, FET 103c is always on, and FET 103b is driven according to the output 300. Inductors 103 a and 103e work in conjunction with the diode 103g and output capacitor 103h to raise the voltage of the output 202 to a desired level at a DC bus output 203. The magnitude of the DC bus voltage 203 is varied as needed by adjusting the duty cycle of the PWM outputs 302, 303. To reduce the DC bus voltage (i.e., buck) below the DC output voltage 202, the FET 103b is always off, while FET 103c is controlled according to the output 301. Diode 103d is the active discharge diode in the buck operation, and capacitor 103f functions as the output capacitor in the buck operation. The DC bus voltage 203 is fed to a half-bridge resonant inverter

105, which produces an AC output 204 from the DC bus voltage 203. The frequency of the AC output 204 is determined by a fixed frequency oscillator 104a. A high side driver 104b and a low side driver 104c amplify the output of the oscillator 104a. The outputs of the drivers 104b, 104c are fed to FET switches (or alternative suitable switching devices) 105b, 105a, respectively. The switches 105a, 105b produce an output that is fed to a series resonant tank that includes an inductor 105c and a capacitor 105d. The natural resonant frequency of the tank is about the same as the fixed frequency of the oscillator 104a. The output 204 (i.e., the voltage across the capacitor 105d) is operatively connected to the lamp(s) 106. The MCU 111 controls operation of the ballast 10 through appropriate programming, which will be described later.

The MCU 111 obtains data regarding the specifications of the particular lamp 106 used with a ballast 110 from memory in the MCU 111. The data can be entered into the memory, and other software functions can be established and controlled, through the programming interface 112. A dimming interface 110 can be provided to the MCU 111 for dimming purposes, and a component power supply 107 is provided for operating some of the components in the ballast 10, as shown.

The MCU 111 provides an enable signal 304 for the oscillator 104a, and an oscillator disabling signal 305 that disables the oscillator 104a. The half bridge 105 in conjunction with additional circuitry (not shown) sends a feedback signal 308 to the MCU that reflects the lamp current, a dimming feedback signal 309 used for dimming purposes, and a fault feedback signal 310 that indicates a fault of some kind.

Operation of the ballast 10 through the MCU 111 is described in FIGS. 2-5. FIG. 2 describes the process executed by the MCU 111 when power is initially provided by the AC power source 100 (Sl 10). The MCU 111 and on-board hardware resources are initialized (S 120), and the MCU 111 retrieves the specifications for the lamp 106 from nonvolatile memory (S 130). The operating specifications can be pre-programmed into the memory by the manufacturer of the MCU or user, and include information such as operating voltage, ignition voltage and pre-heat voltage, if used. Steady state operating current is also stored in the memory.

If the operating specifications for the lamp (108) are available in the memory (S 140), the MCU 111 confirms that sufficient component voltage is available (S 150), and if so, the MCU 111 computes the specific DC bus voltage magnitudes for all required lamp operating points (S 160). The MCU 111 then identifies the specific operating modes to meet the DC bus requirements for

individual operating tasks (for example, preheating, ignition and steady state operation) (S 170), and saves all required data in nonvolatile memory (S 180).

Referring again to (S 140), if the data is unavailable for any reason, the MCU 111 can access remote data (S141). If remote access is available (S 142), the lamp specifications can be manually obtained from an appropriate source, such as a manufacturer's datasheet or website (S132). If the data is acquired through remote access, it is then stored in nonvolatile memory (S 131).

The data obtained at (S 132) can be entered manually in the nonvolatile memory 131, through the programming interface 112. If remote access is unavailable (S 142), the system waits until a remote connection is established, or the operating specifications are otherwise entered into the nonvolatile memory 131.

Referring again to (S 150), if the component voltage is not stabilized, the MCU 111 waits for the system to stabilize (S 151 ), by providing an under voltage lockout (UVLO). In any event, when the specifications for the lamp are stored in nonvolatile memory, the preparation process is finished, and the MCU 111 begins the next task, which is the pre-heating task described in FIG. 3, if the application requires pre-heating (programmed start), or the ignition task, if pre-heating is not needed (rapid start).

Referring now to FIG. 3, the MCU 111 retrieves the operating parameters for the filament pre-heat task (i.e., voltage and time requirements) from nonvolatile memory I l ia (S210). The MCU 111 then adjusts the DC bus voltage as required (S230). The oscillator 104a is activated to produce an appropriate pre-heating current in the lamp filaments (S240). If a fault condition is detected at (S250), the oscillator 104a is shut down and a fault counter is incremented (S251). If more than three faults have occurred (S260), the process starts over again (S270). If three faults have not occurred, the MCU 111 waits

for a predetermined time (e.g., two seconds) (S262) and reactivates the oscillator 104a again at (S240).

If no fault condition is detected at (S250), the MCU 111 determines whether the specified pre-heating time has elapsed (if pre-heating is required). If not, the MCU 111 waits until a predetermined time has elapsed. If so, the MCU 111 completes the pre-heating process described in FIG. 3, and begins the ignition process described in FIG. 4.

Referring now to FIG. 4, the MCU 111 clears the fault counter in

(S310), and retrieves the operating parameters for the ignition task from the nonvolatile memory I l ia (S320). The MCU 111 determines whether the buck/boost converter should operate in the boost mode or the buck mode (S330), and the DC bus voltage is adjusted to perform ignition (S340).

The oscillator 104a is activated at or near the natural resonant frequency of the serial LC tank (S350), and a determination is made as to whether any faults are present (S360). If so, the oscillator 104a is shut down, and the fault counter is incremented by one (S361). If the fault counter has not exceeded three faults (S371) the MCU 111 waits for a predetermined relaxation time (e.g., two seconds) (S372), and activates the oscillator 104a again (S350).

If fault conditions are not detected as (S360), the MCU 111 determines whether a predetermined ignition time has elapsed (S370). If not, the MCU 111 waits until the ignition time has elapsed, and if so, the MCU 111 completes the ignition task and begins the steady state operating task described in FIG. 5.

Referring now to FIG. 5, the fault counter is cleared (S410), and the operating parameters for normal steady state operation of the lamp are retrieved from nonvolatile memory I l ia (S420). The operating parameters for the buck/boost converter are also retrieved from memory (S430), and the

magnitude of the DC bus voltage is adjusted as required for the steady state operating mode of the lamp (S440).

The oscillator 104a is activated at or near the resonant frequency of the series LC tank (S450), and the MCU 111 determines whether any fault conditions are present (S460). If so, the oscillator 104a is shut down(S461) and the operation flow returns instantly to the start position (S464). If no faults are detected, the lamp is in the steady state operating state, and will continue to run until power is disrupted (S470). Fault monitoring is continuous. The MCU 111 also constantly considers whether the dimming level is changed (S480). If so, the DC bus voltage is adjusted accordingly. While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.