Fluorescent lamp ballast with electronic preheat circuit

This disclosure relates to ballasts for powering fluorescent lamps including compact fluorescent lamps (CFLs). This type of lamp includes cathodes (filaments) which are preferably preheated before ignition to extend the operational life of the lamp. The lamp cathodes are covered with emission mix to facilitate passage of electrons through the gas for production of light. Over time, the emission mix is sputtered off of the cathodes in normal operation, but a larger amount is sputtered off when the lamp is ignited with cold cathodes. When the emission mix becomes depleted, a higher voltage is required for the cathodes to emit electrons, a condition sometimes referred to as end-of-life (“EOL”). The higher voltage results in an increase in temperature which may overheat the lamp and in some cases crack the glass if the lamp is not replaced.

Conventional low cost CFL ballasts often use a positive temperature coefficient (PTC) thermistor to heat the lamp cathodes of the lamp prior to ignition (preheat). The PTC is coupled in parallel with a capacitor connected across the CFL, and initially conducts allowing preheating current to flow through the lamp cathodes. With continued conduction, the PTC device heats up and the PTC resistance increases, eventually triggering ignition of the gas in the lamp. The PTC, moreover, is typically situated in close proximity to the lamp to keep the PTC in the high-impedance condition during normal operation of the lamp. However, PTC devices are costly and occupy valuable space in the ballast. In addition, the PTC element never reaches infinite impedance and thus conducts some amount of current throughout operation of the ballast (even if some of the energy to keep the PTC device warm comes from lamp heating). Thus, the use of PTC devices for cathode preheating negatively impacts ballast efficiency. Furthermore, PTC preheating circuits need time to cool before reapplication of power to avoid cold-cathode ignition and the associated lamp degradation. Thus, a need remains for improved ballasts and techniques for preheating fluorescent lamp cathodes without using PTC components.

Ballast devices and filament preheating methods are provided in which a resonant impedance of a self-oscillating inverter is selectively adjusted to control the inverter frequency for preheating lamp cathodes via inverter output current during a preheating period after power is applied and to thereafter change the inverter frequency for lamp ignition.

A fluorescent lamp ballast is provided, having a rectifier or other DC power circuit to receive an AC input and to produce a DC output, and a frequency controlled inverter that converts the DC to provide an inverter output for powering one or more fluorescent lamps. The ballast also includes a preheating circuit that selectively modifies an impedance in the frequency control circuit to control the frequency of the inverter output to be in a first range during a preheating period following application of power to the DC power circuit to preheat at least one cathode of the lamp using power from the inverter output. The preheating circuit then controls the frequency of the inverter output to be in a different second range following ignition of the lamp. The ballast in some embodiments may include diodes individually coupled across lamp terminals associated with first and second cathodes of the lamp to block current flow from the inverter output and terminate oscillation of the inverter when the lamp is disconnected from the terminals, but primarily to reduce the power dissipation in the cathodes. Some embodiments of the preheating circuit modify an inverter capacitance to control the inverter output frequency, such as by providing an auxiliary capacitance, a switching device coupled between the auxiliary capacitance and the inverter capacitance, and a timer circuit to actuate the switching device to connect the auxiliary capacitance in parallel with the inverter capacitance a predetermined time following application powerup. In other embodiments, the preheating circuit modifies an inverter inductance to control the frequency of the inverter output, where the preheating circuit includes a switching device coupled across the inverter inductance and a timer circuit that actuates the switching device to shunt the inverter inductance a predetermined time following after power is applied to the DC power circuit.

A fluorescent lamp ballast is also provided, which includes a DC power circuit, an inverter to convert the DC output of the power circuit to produce an inverter output to power at least one fluorescent lamp, a preheating circuit operative to preheat the lamp cathodes, and first and second diodes individually coupled across lamp terminals associated with first and second cathodes of the lamp to block current flow from the inverter output and terminate oscillation of the inverter when the lamp is disconnected from the terminals.

A method is provided for operating one or more fluorescent lamps, including converting an AC input to produce a DC output, converting the DC output using an inverter to produce an inverter output to power at least one fluorescent lamp, and modifying at least one impedance to control an operating frequency of the inverter to be in a first range during a preheating period following application of power to the inverter to preheat at least one cathode of the lamp using power from the inverter output and to control the frequency of the inverter output to be in a different second range following ignition of the lamp. In certain embodiments, modifying the impedance includes selectively connecting an auxiliary capacitance in parallel with at least one capacitance of the inverter a predetermined time following application of power to the inverter. In other embodiments, selectively shunting at least one inductance of the inverter a predetermined time following application of power to the inverter.

One or more exemplary embodiments are set forth in the following detailed description and the drawings, in which:

FIG. 1 is a schematic diagram illustrating an exemplary fluorescent lamp ballast with an inverter output frequency controlled by a preheating circuit to provide filament heating via the inverter output during initial startup;

FIG. 2 is a graph illustrating the inverter output frequency controlled by the preheating circuit in the ballast of FIG. 1 for initial cathode preheating;

FIG. 3 is a schematic diagram illustrating a fluorescent lamp ballast embodiment with a preheat circuit operative to modify a capacitance of the inverter for preheating the lamp cathodes;

FIG. 4 is a schematic diagram illustrating another fluorescent lamp ballast embodiment in which the preheat circuit modifies an inductance of the inverter lamp cathode preheating; and

FIG. 5 is a schematic diagram illustrating another embodiment of a fluorescent lamp ballast with diodes coupled across lamp terminals to block current flow from the inverter output and to terminate inverter oscillation when the lamp is removed.

Referring now to the drawings, where like reference numerals are used to refer to like elements throughout, and where the various features are not necessarily drawn to scale, the present disclosure relates to ballasts and methods that may be used in connection with any type of fluorescent lamps and will be described in the context of certain embodiments used with compact fluorescent lamps (CFLs). Moreover, the described embodiments and shown in single-lamp applications, although multiple-lamp configurations are possible.

FIG. 1 shows a ballast 100 with a DC power circuit 110 that converts AC power at an input 104 to provide a DC output 112 to an inverter 120 . Any form of DC power circuit 110 may be employed, for example, a full or half-bridge passive rectifier, an active rectifier, or other circuitry that provides a DC output. The inverter 120 may be any switching-type DC-AC converter controlled by pulse width modulation, duty cycle control or other suitable switching control technique having suitable switching devices operated to generate an output 124 suitable for powering one or more fluorescent lamps 108 . The example of FIG. 1 is a self-oscillating inverter producing an output 124 to power a CFL 108 coupled to a ballast output 106 , and the inverter 120 includes a frequency control circuit 122 operative to control the frequency of the inverter output 124 . The inverter 120 drives a resonant circuit including an inductance T 1 a and capacitances C 6 and C 8 , and the CFL load is coupled with the output via terminals 108 a to which CFL filaments (hereinafter ‘cathodes’) are connected. The ballast output 106 includes the capacitor C 6 coupled between two opposing cathode terminals 108 a as well as diodes D 1 and D 2 individually coupled across lamp terminals 108 a associated with first and second cathodes of the lamp 108 . In operation before lamp ignition, the preheating current from the inverter 120 flows through one lamp cathode, the capacitor C 6 and then through the other cathode. Once the lamp 108 is ignited, arc current flows in the lamp 108 with the diodes D 1 and D 2 rectifying the voltage across the cathodes and reducing the power dissipated in the cathodes during steady-state. Moreover, if the lamp 108 is removed during ballast operation, the diodes D 1 and D 2 block current flow from the inverter output 124 and terminate the inverter oscillation to avoid potential oscillation run-away conditions.

Referring also to FIG. 2 , the ballast 100 of FIG. 1 includes a preheating circuit 250 operatively coupled with the inverter 120 to adapt the inverter frequency control circuit 122 by modification of one or more impedances therein. In this manner, the preheating circuit 250 performs inverter frequency control, which in turn controls the output current level of the inverter 120 . In particular, as shown in the graph 160 of FIG. 2 , the preheating circuit 250 operates to control the inverter frequency 162 in a first range (e.g., about 100 KHz in one example) during a preheating period T PH following application of power to the DC power circuit 110 (at t 0 in FIG. 2 ) to preheat the lamp cathode(s) using power from the inverter output 124 . In specific embodiments outlined below, the preheat time T pH from t 0 to t 1 is set by a timing circuit 252 in the preheating circuit 250 . Once the preheating period expires (t 1 in FIG. 2 ), the preheating circuit 250 lowers the frequency 162 of the inverter output 124 to a second range (e.g., 60 KHz in one example) to initiate lamp ignition and thereafter to control the lamp current to the desired level in normal operation.

FIG. 3 shows a detailed embodiment of a fluorescent lamp ballast 100 with a preheating circuit 250 operative to modify a resonant capacitance C 3 of the inverter for preheating the lamp cathodes via inverter output frequency control. An AC source 104 provides input power via a fuse F 1 to an input filter stage including inductor L 1 and capacitor C 1 to a full wave bridge rectifier DC power circuit 110 comprised of diodes D 3 -D 6 to provide a DC output to a self-oscillating inverter 120 . The inverter 120 in FIG. 3 includes upper and lower switching devices Q 1 and Q 2 , respectively, coupled in series between upper and lower DC bus rails 112 a and 112 b , and a capacitance C 2 is provided between the upper DC bus rail 112 a and a circuit ground at the lower DC rail 112 b . Any type or form and number of switching devices Q 1 and Q 2 may be used, where the exemplary switches Q 1 and Q 2 are NPN and PNP bipolar transistors, respectively. The switches Q 1 and Q 2 are alternatively switched to create a generally square-wave signal at an inverter output node 124 to excite a resonant circuit formed by the output transformer winding T 1 a and capacitances C 6 and C 8 to thereby drive a high frequency bus at the connection of diode D 1 and T 1 a . The switches Q 1 and Q 2 are alternately activated to provide a square wave having an amplitude of ½ the DC bus level at the common inverter output node 211 (e.g., half the DC bus voltage across the terminals 112 a and 112 b ), and this square wave inverter output excites the resonant circuit.

The inverter 120 includes a transformer T 1 with windings for output power sensing and control for self-oscillation with adjustable inverter operating frequency 162 , including a first winding T 1 a in series between the inverter output 124 and the high frequency bus, along with winding T 1 b in a switch drive control circuit including a frequency control circuit 122 formed by a capacitance C 3 and an inductor L 2 in series between the inverter output 124 and the base terminals of Q 1 and Q 2 . Capacitor C 4 is also connected between the switch base terminals and the inverter output 124 , a resistance R 2 is coupled between the positive bus terminal 112 a and the inverter output 124 , and a capacitance C 7 is coupled between the inverter output 124 and the negative bus terminal 112 b . In addition, resistance R 1 is coupled between the base terminals and the lower DC bus terminal 112 b to bias the base drives. In operation, the transformer winding T 1 a acts as a primary in the resonant circuit and the secondary winding T 1 b provides oscillatory actuation of the switches Q 1 and Q 2 according to the resonance of the resonant circuit, thereby providing a self-oscillating inverter 120 to drive the lamp 108 . AC power from the high frequency bus provides an AC output 106 used to drive one or more lamp loads 108 , where any number of lamps 108 can be coupled with the high frequency bus for different lighting applications.

The inverter 120 creates the square wave signal at the output 124 at an inverter frequency set by the impedances of the frequency control circuit 122 . In the preheating period T PH ( FIG. 2 above), the frequency is determined by the series LC combination of C 3 and L 2 . This frequency, being higher than the T 1 a , C 6 frequency, keeps the lamp voltage below the voltage required for ignition. This preheat frequency also reduces the voltage applied to the lamp 108 , thereby reducing the glow current prior to ignition, resulting in improved lamp life, particularly when the ballast 100 is subjected to rapid cycles.

The preheating circuit 250 in the example of FIG. 3 includes an auxiliary capacitance C 12 connected in a series circuit with a MOSFET switching device Q 3 across the inverter capacitance C 3 , such that when the switch Q 3 is conducting (ON), the capacitance of the frequency control circuit 122 is controlled by the sum of the capacitances C 3 +C 12 (e.g., 69 nF in the illustrated embodiment). Q 3 is initially OFF, and thus in the preheating period T PH following initial powerup of the ballast 100 , the capacitance of the frequency control circuit 122 is C 3 (e.g., 22 nF) and the inverter 120 is maintained in a first frequency range (e.g., about 100 KHz as shown in FIG. 2 in one example) to preheat the lamp cathodes using power from the inverter output 124 . The preheating circuit 250 includes a timer circuit 252 with resistors R 3 and R 4 and a timing capacitor C 11 , which actuate the switch Q 3 to connect the auxiliary capacitance C 12 in parallel with C 3 of the frequency control circuit 122 a predetermined time T PH following application of power to the DC power circuit 110 . Once power is applied to the ballast 100 , the timing capacitor C 11 charges through resistor R 4 and a diode D 7 to the point where the gate voltage of Q 3 exceeds the threshold Vt (t 1 in FIG. 2 ). Q 3 thus turns on, connecting C 12 in parallel with C 3 of the inverter 120 to set the frequency 162 of the inverter output 124 to be in a second range (e.g., about 60 KHz in the illustrated example), after which the lamp 108 ignites an normal operation begins. The values of the components C 11 and R 4 may be selected to provide any desired preheating period T PH for adequately preheating the lamp cathodes before lamp ignition.

FIG. 4 illustrates another exemplary ballast 100 having similar operation to the embodiment of FIG. 3 . In the example of FIG. 4 , however, the preheating circuit 250 controls the inverter frequency 162 by initially limiting the voltage to the inductor L 2 in the frequency control circuit 122 , thereby increasing the frequency of the inverter 120 and preheating the lamp cathode filaments via the resonant capacitor C 6 . As with the above embodiment of FIG. 3 , increasing the inverter frequency reduces the voltage applied to the lamp, thereby reducing the glow current prior to ignition, while preheating the cathodes using inverter output current without the use of a PTC device. In this embodiment, the inductance L 2 is selectively modified by the preheating circuit 250 to control the frequency 162 of the inverter output 124 . The preheating circuit 250 in FIG. 4 includes a switching device Q 4 coupled in series with a capacitor C 21 across the inductance L 2 , along with a timer circuit 252 operative to actuate the switching device Q 4 to shunt the inductance L 2 a predetermined time T PH after power is applied to the ballast 100 . The timing circuit 252 in this example includes a timing capacitor C 22 coupled in series with a charging diode D 7 and a resistor R 21 . Q 4 is initially conductive (ON) and capacitors C 21 and C 22 are discharged. As the inverter 120 begins to oscillate, C 22 is charged via D 7 and R 21 , while the gate voltage of Q 4 remains above its threshold voltage Vt, whereby Q 4 shunts the inductor L 2 with capacitor C 21 . This shunting maintains the voltage across L 2 low enough to drive the inverter frequency high (e.g., 100 KHz in this example). Once the voltage across C 22 is sufficient to reduce the C 3 gate voltage below Vt (e.g., at t 1 in FIG. 2 ), Q 4 turns OFF (non-conductive), causing the inverter frequency to fall to the second range (e.g., 60 KHz). This increases the lamp voltage to initiate lamp ignition and normal operation ensues.

Referring now to FIG. 5 , A ballast 100 is shown for operating one or more fluorescent lamps 108 , including a rectifier 110 operative to receive an AC input 104 and to produce a DC output 112 , and a self-oscillating inverter 120 that converts the DC output to produce an inverter output 124 to power one or more fluorescent lamps 108 , generally as described above in connection with FIGS. 3 and 4 . The embodiment of FIG. 5 includes a conventional PTC device coupled with the resonant capacitance C 6 and an additional capacitor C 7 for preheating the lamp cathodes. In addition, the ballast 100 provides first and second diodes D 1 , D 2 individually coupled across the lamp terminals 108 a associated with first and second cathodes of the lamp 108 to block current flow from the inverter output 124 and terminate oscillation of the inverter 120 when the lamp 108 is disconnected from the terminals 108 a . Prior to lamp ignition, with a cool PTC device, preheating current flows through one lamp cathode, the capacitor C 6 , the PTC device and then through the other cathode. The cool PTC is initially low impedance (e.g., 600 OHMs in one example) and thus conducts preheating current through the lamp cathodes. As this preheating current continues to flow, the PTC heats up and its resistance increases, eventually triggering ignition of the gas in the lamp 108 . Once the lamp 108 is ignited, arc current flows in the lamp with the diodes D 1 and D 2 rectifying the voltage across the cathodes. Moreover, if the lamp 108 is removed during ballast operation, the diodes D 1 and D 2 block current flow from the inverter output 124 and terminate the inverter oscillation to avoid potential oscillation run-away conditions.

The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure. In addition, although a particular feature of the disclosure may have been illustrated and/or described with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, references to singular components or items are intended, unless otherwise specified, to encompass two or more such components or items. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”. The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.