Ballast scheme for operating multiple lamps

[0002] This invention relates generally to a ballast scheme for operating multiple lamps and, more particularly, to a ballast scheme for operating multiple fluorescent lamps having substantially the same current flowing through each lamp.

[0003] Conventional ballasts for powering multiple lamps, such as disclosed in U.S. Pat. No. 4,293,799, include a plurality of transformers for isolating the lamps from direct connection to a utility line. The primary windings of the transformers are connected in series. The secondary winding of each transformer is connected to a lamp. Substantially the same current flows through each lamp when the lamps have substantially the same impedance.

[0004] Such conventional ballasts, however, often do not sense lamp load conditions so as to achieve/maintain one or more desired lamp parameters. These parameters can include, but are not limited to, the level of illumination, power regulation, preheat, ignition stop/cutoff and/or capacitive mode protection.

[0005] It is therefore desirable to provide an improved ballast for operating multiple lamps in which substantially the same level of current flows through each lamp. The improved ballast should include a scheme for sensing lamp load conditions in order to achieve/maintain one or more desired lamp parameters. The scheme should avoid the use of expensive components and minimize the number of components required.

[0006] In accordance with a first aspect of the invention, a ballast for powering more than one lamp includes an inverter having an output; a resonant inductor and a transformer for each lamp. Each transformer has a primary winding and a secondary winding. The primary windings are serially coupled together and to the resonant inductor. The ballast further includes a single sensing element for sensing the flow of current through all lamps and a controller for controlling the switching operation of the inverter in response to the sensed lamp current.

[0007] Each of the lamps has substantially the same current flow through serial coupling of the primary windings and resonant inductor to each other. There is no need to provide separate chokes for each lamp in order to balance the current flow among the lamps. The single sensing element senses the flow of current through all lamps. More particularly, the controller in response to the sensed lamp current controls the switching operation of the inverter in order to achieve/maintain one or more desired lamp parameters. Through the use of a single sense element and a single controller, the ballast avoids the need for separate feedpaths for sensing individual lamp conditions and/or the need for separate controllers for controlling the individual lamp conditions. The ballast scheme therefore avoids the use of expensive components (e.g. controllers and chokes) and minimizes the number of components required in order to operate multiple lamps.

[0008] It is a feature of this first aspect of the invention that each transformer has a secondary winding connected to a corresponding lamp. The inverter operates at a switching frequency above a resonant frequency, the resonant frequency being based on the impedance of the resonant inductor and each transformer. The single sensing element is preferably connected between a secondary winding of one of the transformers and a reference potential (e.g. ground potential).

[0009] In accordance with a second aspect of the invention, a method of operating a ballast for powering more than one lamp includes the steps of supplying an AC signal from an inverter to a resonant inductor and a plurality of transformers; sensing through a single sensing element the flow of current through all lamps; and controlling the switching operation of the inverter in response to the sensed lamp current. The serial combination of the resonant inductor and primary windings of each transformer receive the AC signal. Each transformer is associated with a different lamp.

[0010] It is a feature of this second aspect of the invention that the inverter be operated at a switching frequency above a resonant frequency, the resonant frequency being based on the impedance of the resonant inductor and each transformer.

[0011] Accordingly, it is an object of the invention to provide an improved ballast for operating multiple lamps in which substantially the same current flows through each lamp.

[0012] It is another object of the invention to provide an improved ballast scheme for sensing lamp load conditions in order to achieve/maintain one or more desired lamp parameters.

[0013] It is a further object of the invention to provide an improved ballast scheme which avoids the use of expensive components and minimizes the number of components required.

[0014] Still other objects and advantages of the invention will, in part, be obvious and will, in part, be apparent from the specification.

[0015] The invention accordingly comprises several steps in the relation of one or more such steps with respect to each of the others, and a device embodying features of construction, combination of elements, and arrangements of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

[0016] For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings in which:

[0017] FIG. 1 is a schematic diagram of a ballast in accordance with the invention.

[0018] As shown in FIG. 1, a ballast 10 supplies a substantially DC signal from a DC source 13 to an inverter 16. Inverter 16 is shown in a half-bridge configuration but can alternatively be in a full bridge configuration. Inverter 16 includes a pair of switches 19 and 22 which typically are MOSFETs driven by a controller 25 and a pair of DC blocking capacitors 21. Switches 19 and 22 are connected in series (i.e. totem-pole configuration) across DC source 13. DC blocking capacitors 21 also are connected in series across DC source 13. Controller 25 produces a pair of driving signals at pins G1 and G2 for controlling the switching states of switches 19 and 22, respectively, that is, for controlling, in part, the switching frequency of inverter 16.

[0019] Inverter 16 produces an AC voltage at a pair of nodes 23 which serve as the output of inverter 16. Ballast 10 can power a plurality of lamps RLa, RLb and RLc. It is to be understood that the ballast configuration is designed to power any number of lamps and is not limited to the three lamps shown and described herein. Lamps RLa, RLb and RLc are coupled to the output of inverter 16 through the combination of a resonant inductor 28 and a plurality of transformers 34a, 34band 34c, respectively. Each transformer 34a, 34band 34chas a primary winding 37a, 37band 37ccoupled to a secondary winding 40a, 40band 40cand to an additional secondary winding 43a, 43b, and 43c, respectively. Primary windings 37a, 37band 37cand resonant inductor 28 are serially connected together. This serial combination is connected across the output of inverter 16. A balanced current (i.e. substantially the same current) flowing through each lamp RLa, RLb and RLc is achieved by serially connecting primary windings 37a, 37band 37ctogether.

[0020] Lamp RLa is connected between secondary winding 40aand a reference potential (e.g. ground potential). Lamp RLb is connected between secondary winding 40band the reference potential. Lamp RLc is similarly connected between secondary winding 40cand the reference potential. A junction 45 connects together secondary windings 40a, 40band 40c. Secondary windings 40a, 40band 40care effectively connected in parallel and serially coupled to the reference potential through a resistor 46.

[0021] A resonant circuit is formed through the impedance of resonant inductor 28 and transformers 34a, 34band 34c. The resonant capacitance of the resonant circuit is created by the parasitic capacitance of each transformer and is represented in FIG. 1 by resonant capacitors 43a, 43band 43cconnected in parallel with primary windings 37a, 37band 37c, respectively. Alternatively, one or more discrete capacitors can serve as the resonant capacitance or in combination with the parasitic capacitors. The resonant inductance is formed by resonant inductor 28 by itself or in combination with the leakage inductance (not shown) of one or more of the transformers 34a, 34band 34c.

[0022] Controller 25 is well known in the art and is disclosed in U.S. Pat. No. 5,742,134, the latter of which is incorporated herein by reference thereto. Controller 25 includes a plurality of pins including pins G1, G2, RIND, VL and LI1. Pins G1 and G2 produce the driving signals for controlling the switching states of switches 19 and 22, respectively. The RIND pin reflects the level of current flowing through a resonant inductor 28 and is connected to a junction 24 joining an output 23 of inverter 16 to resonant inductor 28. Inverter 16 also includes an output 25.

[0023] Pin LI1 in combination with a signal fed into another pin (not shown) of controller 25 reflects the current flowing through lamps RLa, RLb and RLc. More particularly, pin LI1 receives a sample of the current flowing through lamp RLb. Inasmuch as the currents through each of the lamps are substantially the same, the sample of the current flowing through lamp RLb which is fed into pin LI1 reflects the current flowing through each of the lamps. Pin LI1 is connected to a junction 45 joining resistor 46 to secondary winding 40b.

[0024] A voltage at a pin VL reflects the peak voltage applied to lamps RLa, RLb and RLc. A scaling resistor 31 is connected to pin VL for scaling down the voltage which would otherwise be applied to pin VL. The serial combination of additional secondary winding 43aand a diode 46ais connected between scaling resistor 31 and the reference potential (e.g. ground potential). The serial combination of additional secondary winding 43band a diode 46bis connected between scaling resistor 31 and the reference potential. The serial combination of additional secondary winding 43cand a diode 46cis connected between scaling resistor 31 and the reference potential. The voltages applied to the lamps RLa, RLb and RLc by secondary windings 40a, 40band 40care sampled by the additional secondary windings 43a, 43band 43c, rectified by diodes 46a, 46band 46cand fed into pin VL by scaling resistor 31.

[0025] Ballast 10 operates as follows: Inverter 16 converts the substantially DC voltage generated by DC source 13 into an AC voltage which is supplied across outputs 23 and 25. Controller 25 controls the AC voltage generated by inverter 16 so as to initially develop a sufficiently high voltage across lamps RLa, RLb and RLc to ignite the latter and thereafter to operate lamps RLa, RLb and RLc in a steady state mode. Lamps RLa, RLb and RLc each have substantially the same level of current flow through serial coupling of the primary windings and resonant inductor to each other. There is no need to provide separate chokes for each lamp in order to balance the current flow among the lamps. The current flowing through resistor 46 reflects/senses the flow of current through all lamps.

[0026] Controller 25 in response to the sensed lamp current controls the switching operation of inverter 16 in order to achieve/maintain one or more desired lamp parameters.

[0027] As can now be readily appreciated, through the use of a single sense element and a single controller, the ballast avoids the need for separate feedpaths for sensing individual lamp conditions and/or the need for separate controllers for controlling the individual lamp conditions. The ballast scheme therefore avoids the use of expensive components (e.g. controllers and chokes) and minimizes the number of components required in order to operate multiple lamps.

[0028] It will thus be seen that the objects set forth above and those made apparent from the preceding description are efficiently attained and since certain changes may be made in the above construction without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0029] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.