Embodiments of the invention are described in more detail below on the basis of the drawings. Further characteristics and advantages of the invention follow from this.
FIG. 1 shows a circuit diagram of a conventional transformer
FIG. 2 shows a schematic circuit diagram of a current balancing circuit to distribute a current between a plurality of lamps.
FIG. 3 shows the circuit diagram of the transformer according to FIG. 1 depicting the main inductance.
FIG. 4 shows the transformer according to FIG. 3 having a capacitor according to the invention to compensate the main inductance.
FIG. 5 shows an embodiment of a current balancing circuit having the modified transformers according to the invention, wherein the capacitor is connected in parallel to the primary winding.
FIG. 6 shows an embodiment of a current balancing circuit having the modified transformers according to the invention, wherein the capacitors are connected in parallel to the secondary winding.
FIGS. 1 to 3 have already been described in detail in the introductory section of the description. Please refer to the relevant passages in the text.
FIG. 4 shows a modified transformer 10 according to the invention comprising a primary winding 12, a secondary winding 14 and the main inductance 16. According to the invention, a capacitor is connected in parallel to the main inductance 16, i.e. to the primary winding 12, the capacitor giving rise to a reactive current IC that flows in an opposite direction to the reactive current IL of the main inductance. In this case, the capacitor, together with the main inductance of the transformer, forms a high-impedance network that operates in or almost at parallel resonance. The capacitance of the capacitor must be so dimensioned that the reactive current IC equals the reactive current IL at the relevant operating frequency of the transformer. By these means, the overall reactive current can be considerably reduced, typically to the value of the inductance tolerance (20%). Consequently, the reactive current can be reduced to one fifth. According to the quadratic dependence cited above, this means a reduction in current tolerance to 1/25 of the current tolerance without compensation.
Capacitance is calculated as described below using the equation for parallel resonance:
Here, L is the main impedance of the transformer (on the capacitor side), fop the operating frequency of the transformer.
FIG. 5 shows a circuit for balancing the current that is similar to the circuit in FIG. 2 comprising a plurality of balancing transformers 10a, 10b, . . . , 10n, which distribute the current of a high voltage source 24 uniformly over a plurality of lamps 20a, 22a, 20b, 22b, . . . , 20n, 22n. According to the invention, appropriate balancing capacitors 18a, 18b, . . . , 18n that compensate the influence of the primary inductance in the transformers 10a, 10b, . . . , 10n are connected in parallel to the primary windings of the transformers 10a, 10b, . . . , 10n. In the secondary circuit that is formed by the secondary windings of the transformers 10a, 10b and 10n connected in series, a precision resistor 26 can be provided whose voltage drop may be used to measure the current in the secondary circuit. This can be used, for example, to detect the failure of a lamp since the current in the secondary circuit would be altered by such a failure.
FIG. 6 shows an embodiment of a current balancing circuit to distribute a current between a plurality of lamps 20a, 22a, 20b, 22b, . . . , 20n, 22n that is modified with respect to FIG. 5. In contrast to the circuit according to FIG. 5, here the capacitors 18a, 18b, . . . , 18n are connected on the secondary side of the transformers in parallel to the secondary windings. In principle, it is of no consequence to the invention whether the balancing capacitor is provided on the primary side or on the secondary side of the transformer. Employing the capacitors on the secondary side of the transformers can, however, be advantageous if different numbers of windings are used for the primary windings and the secondary windings. If the number of windings in the secondary windings are made less than the number of primary windings, the transfer rate and the voltage on the secondary windings is also reduced. This makes it possible to use capacitors having lower electric strength. However, the necessary capacitance value then increases with the square of the transfer rate of the transformer. Depending on the application, optimum pricing between a larger capacitance value and lower electric strength of the capacitors has to be determined.
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Identifaction Reference List
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10
Transformer (10a, 10b, . . . , 10n)
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12
Primary winding
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14
Secondary winding
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16
Main inductance
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18
Capacitor (primary capacitance)
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20
Lamp (20a, 20b, . . . , 20n)
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22
Lamp (22a, 22b, . . . , 22n)
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24
AC voltage source
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26
Precision resistor
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Patent Application
20080048495
