BALLAST FEEDBACK SCHEME

Abstract: A ballast feedback scheme including a single stage current feedback inverter. To avoid overboosting of the voltage impressed across a buffer capacitor only a portion of the high frequency lamp current is fedback to the buffer capacitor during both ignition and steady state operation of the lamp. Ignition of the lamp is well controlled.

Description: Ballast feedback scheme. This invention relates generally to a feedback scheme for an electronic ballast and, more particularly, to a scheme for minimizing overboost voltage conditions due to feedback. Conventional ballasts, such as disclosed in U.S. Patent No. 5,404,082, are smaller and less costly, in part, through elimination of a preconditioner stage. Such ballasts also include an output stage formed from an inverter and a resonant circuit and are commonly referred to as a single stage inverter. The preconditioner stage following lamp ignition (i.e. during steady state conditions) is often used to increase (i.e. boost) the ballast input voltage in conditioning the voltage applied to the inverter. In the absence of the preconditioner stage, boosting of the ballast input voltage is achieved by supplementing the ballast input voltage with a high frequency signal fedback from the resonant circuit. This high frequency signal represents the resonant inductor current. The ballast input voltage (typically rectified) and then boosted is applied to a buffer capacitor which serves as a DC source for the inverter. When the high frequency feedback signal is higher than necessary, an overboost voltage is impressed across the buffer capacitor. Under overboost voltage conditions, a very high stress is placed on various ballast components including the buffer capacitor and switches within the inverter. The buffer capacitor is typically of the electrolytic type. The inverter switches are typically power MOSFETs. Both the buffer capacitor and inverter switches can be damaged and fail under overboost voltage conditions. During lamp ignition, ballast schemes such as disclosed in U.S. Patent No. 4,511,823, also lead to overboost voltage conditions and are due, in part, to current flowing through the resonant inductor being fedback and charging the buffer capacitor. Complicated control circuitry is required to avoid such overboost resulting in a more costly and more difficult ballast to manufacture. It is therefore desirable to provide an improved ballast feedback scheme which reduces the high frequency feedback signal in order to avoid overboost voltage conditions during ignition and steady state operation. The scheme should be particularly applicable to an inverter having a single stage with current feedback and high power factor. In accordance with a first aspect of the invention, a ballast includes a rectifier, a buffer capacitor coupled across the output of the rectifier and a pair of switches serially connected together having a switch junction therebetween and coupled across the buffer capacitor. The ballast further includes a first serial combination of a resonant inductor and a resonant capacitor which together form a resonant circuit and are coupled between the switch junction and a reference node connecting the buffer capacitor and one of the pair of switches together. A second serial combination of a lamp load and a first reactor are connected in parallel with the resonant capacitor. The ballast also includes a second reactor for splitting current flowing through the lamp load between itself and the first reactor. A feedback path which includes the second additional reactor serves to boost the voltage across the buffer capacitor. The ballast avoids overboost voltage conditions by providing a feedback path which includes only a portion of the current flowing through the lamp load. In other words, the amount of current being fedback for boosting the voltage across the buffer capacitor is far less than conventional feedback schemes by splitting the current flowing through the lamp load between the first and second reactors. Furthermore and until the lamp is ignited, there is no feedback since there is no lamp current to be fedback, that is, there is no overboosting of the voltage across the buffer capacitor during ignition. Ignition of the lamp therefore can be well controlled. In contrast thereto, conventional feedback schemes can feedback far greater levels of current resulting in overboost voltage condition such as disclosed in U.S. Patent Nos. 5,404, 082 and 4,511,823. It is a feature of this first aspect of the invention that the level of current flowing through the lamp load is substantially different from the level of current flowing through the resonant inductor. The feedback path can be connected to a junction joining a pair of diodes which are coupled between the buffer capacitor and rectifier. The feedback path further can include a feedback capacitor. This feedback capacitor can be in parallel with one of the diodes. Accordingly, it is an object of the invention to provide an improved ballast feedback scheme which reduces the high frequency feedback signal in order to avoid overboost voltage conditions during ignition and steady state operation. It is another object of the invention to provide an improved feedback scheme which is particularly applicable to an inverter having a single stage with current feedback and high power factor. Still other objects and advantages of the invention will, in part, be obvious and will, in part, be apparent from the specification. For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings in which: FIG. 1 is an electrical schematic of a ballast in accordance with a first embodiment of the invention; and FIG. 2. is an electrical schematic of a ballast in accordance with a second embodiment of the invention. As shown in FIG. 1, a mains sinusoidal AC voltage source ACS is connected to a ballast 10. Source ACS is connected to a pair of input nodes Nl and N2 of ballast 10. A pair of windings LI and L2 are each connected at one end to input nodes Nl and N2, respectively. A serial combination of a pair of capacitors Cl and C2 is connected to the other ends of windings LI and L2, respectively. A junction Jl joining together capacitors Cl and C2 is connected to ground. Windings LI and L2 and capacitors Cl and C2 in combination form an electromagnetic filter (EMF). Harmonics generated by ballast 10 are removed by this filter and thereby prevented from being fed into source ACS. The AC voltage from source ACS is applied through the filter to a full bridge rectifier formed by a plurality of diodes Dl, D2, D3 and D4. The anode of diode Dl and the cathode of diode D2 are connected to a junction J2 joining capacitor Cl and winding LI together. The anode of diode D3 and the cathode of diode D4 are connected to a junction J3 joining capacitor C2 and winding L2 together. One end of a capacitor C3 is connected to a junction J8 joining together the cathodes of diodes Dl and D3. The other end of capacitor C3 and a junction J9 joining together the anodes of diodes D2 and D4 are grounded. Capacitor C3 serves to filter high frequency components generated by ballast 10. The full bridge rectifier rectifies the AC voltage which is applied to a serial combination of a buffer capacitor Cb and diodes D5 and D6. An electrolytic capacitor typically serves as buffer capacitor Cb. A serial combination of switches SI and S2, commonly referred to as totem- pole arrangement, are in parallel with buffer capacitor Cb. Switches SI and S2 typically are power MOSFETs with gates Gl and G2, respectively, and are turned on and off by a driver (not shown) connected to gates Gl and G2. A DC blocking capacitor C4 is connected to the junction J4. A junction J5 (i.e. serving as a grounded reference node) connect switch S2, buffer capacitor Cb, diode D6, a resonant capacitor Cr and a reactor CS 1 together. Capacitor C4 and switches SI and S2 together form an inverter of the half-bridge type. A serial combination of capacitor C4, a resonant inductor Lr and resonant capacitor Cr are connected in parallel across switch S2. Resonant inductor Lr and resonant capacitor Cr form a resonant circuit. The serial combination of a reactor CS 1 and a lamp LA, such as a fluorescent lamp, is connected in parallel across resonant capacitor Cr. Lamp LA can be of the pre-heat or rapid-start type. Reactor CSl can be either a capacitor (as shown in FIG. 1) or and an inductor. The resonant circuit is generally operated slightly above the resonant frequency of the resonant circuit once lamp LA is ignited and in a steady state mode of operation (i.e. in an inductive mode). The switching frequency of the inverter begins at a very high frequency (e.g. about 120K Hz) and is ramped downwardly toward the resonant frequency of the resonant circuit. Ignition of lamp LA occurs, for example, at about 70-80K Hz with steady state operation at about, for example, 45-50K Hz. Preferably, resonant inductor Lr is a primary winding of a transformer T. A pair of secondary windings SW1 and SW2 are connected across and for heating a pair of filaments FI and F2 of lamp LA, respectively. In an alternative embodiment of the invention, lamp LA can be of instant start-type whereby heating of filaments FI and F2 through windings SW1 and SW2 can be eliminated. A second reactor CS2 is connected between a junction J6 joining together reactor CSl and lamp LA and a junction J7 joining together diodes D5 and D6. Reactor CS2 can be either a capacitor (as shown in FIG. 1) or an inductor. A feedback capacitor Cf is connected in parallel with diode D6. Reactor CS2 and feedback capacitor Cf together form a feedback path along which a portion of the high frequency current flowing through lamp LA is supplied to buffer capacitor Cb for boosting of the capacitor Cb voltage. During operation of ballast 10, the rectified voltage supplied by the full bridge rectifier which is applied to buffer capacitor Cb is boosted by the high frequency current flowing through lamp LA along the feedback path. Ballast 10 avoids overboost voltage conditions by providing a feedback path which includes only a portion of the current flowing through lamp load LA. The amount of current being fedback for boosting the voltage across buffer capacitor Cb is far less than a conventional feedback scheme by splitting the current flowing through lamp load LA between the first reactor CSl and second reactor CS2. Furthermore and until lamp LA is ignited, there is no feedback since there is no lamp current to be fedback, that is, there is no overboosting of the voltage across buffer capacitor Cb during ignition. Ignition of lamp LA therefore can be well controlled. The values of reactors CSl and CS2 are chosen based on lamp current and lamp voltage conditions. A smaller portion of lamp LA current can be designed to be fedback to buffer capacitor Cb for high lamp current conditions by increasing the impedance of the feedback path (e.g. making the impedance of reactor CS2 higher than the impedance of reactor CSl). A larger portion of lamp LA current can be designed to be fedback to buffer capacitor Cb for high lamp voltage conditions, by decreasing the impedance of the feedback path (e.g. making the impedance of reactor CS 1 higher than the impedance of reactor CS2). Referring now to FIG. 2, in accordance with an alternative embodiment of the invention, a ballast 10' is constructed and operates in substantially the same manner as ballast 10. Those components which are the same in construction and operation within ballasts 10 and 10' have been identified by the same reference numerals/letters. Ballast 10' eliminates the need for feedback capacitor Cf by reflecting its affect on the feedback path through a change in the impedance of capacitors CSl and CS2 as denoted by capacitors CSl' and CS2', respectively.

Claims: CLAIMS: 1. A ballast (10, 10'), comprising: a rectifier (Dl, D2, D3, D4): a buffer capacitor (Cb) coupled across the output of the rectifier; a pair of switches (SI, S2) serially connected together having a switch junction (J4) therebetween and coupled across the buffer capacitor; a first serial combination of a resonant inductor (Lr) and a resonant capacitor (Cr) together forming a resonant circuit and coupled between the switch junction and a reference node (J5) connecting the buffer capacitor and one of the pair of switches (S2) together; a second serial combination of a lamp load (LA) and a first reactor (CSl, CSl') connected in parallel with the resonant capacitor; and a second reactor (CS2, CS2') for splitting current flowing through the lamp load between itself and the first reactor; wherein a feedback path which includes the second additional reactor serves to boost the voltage across the buffer capacitor. 2. The ballast (10, 10') of claim 1, wherein the level of current flowing through the lamp load is substantially different from the level of current flowing through the resonant inductor. 3. The ballast (10, 10') of claim 1, wherein the feedback path is connected to a junction joining a pair of diodes (D5, D6) which are coupled between the buffer capacitor and rectifier. 4. The ballast (10) of claim 1, wherein the feedback path further includes a feedback capacitor (Cf). 5. The ballast (10) of claim 3, wherein the feedback path further includes a feedback capacitor (Cf). 6. The ballast (10) of claim 5, wherein the feedback capacitor (Cf) is in parallel with one of the diodes (D6).

Inventors: HERNANDEZ ADAN F, JANCZAK JERZY, HERNANDEZ, ADAN, F., JANCZAK, JERZY
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H05B41
IPC codes: H05B41