LAMP DRIVING CIRCUIT

FIELD OF THE INVENTION This invention relates to lighting technology, in particular, to a lamp driving circuit for an electronic ballast.

BACKGROUND OF THE INVENTION

When an electronic ballast is used to drive more than one lamp, two types of circuit connections, i.e. series lamp circuit and parallel lamp circuit, are usually employed to connect the lamps. The parallel lamp circuits are widely used together with dimmable ballasts, with which the lamps may be dimmed. The main defects of parallel lamp circuits are that greater amounts of lamp currents pass through lamp inductors and an extra equalizer transformer is needed in the circuit. In practice, when a parallel lamp circuit is used for a dimmable ballast of more than one lamp, these defects may limit the application of the parallel lamp circuits.

A hybrid series-parallel lamp circuit is proposed to replace a purely parallel lamp circuit. Figure 1 shows a schematic view of the circuit structure of a hybrid series-parallel lamp circuit for driving four lamps, where lamp 1 and lamp 2 are connected in series into a branch circuit, lamp 3 and lamp 4 are connected in series into another branch circuit, and these two branch circuits are connected in parallel. T2_l and T2_2 are two windings of an equalizer transformer T2. An inductor Lr is connected to a high-frequency half-bridge circuit of an electronic ballast, which is not a critical part of the present invention and thus, not shown in the figures. Those two serially connected lamps are electrically asymmetric with respect to ground, The high frequency voltages of lamp 2 and lamp 4 drop at node 1 and node 2, respectively, forming hot points with respect to ground. Parasitic capacitive current Ipa passes parasitic capacitor Cpa, forming an AC path, as shown by dotted lines in figure 2, causing the amount of current passing lamp 1 and lamp 3 to be greater than that passing lamp 2 and lamp 4. From the point of view of vision, lamp 1 and lamp 3 are brighter than lamp 2 and lamp 4, in particular, when they are dimmed to below normal level, the difference in brightness will become more and more obvious, and finally, reach an extent that is unacceptable to users.

Another drawback of such a hybrid series-parallel lamp circuit is that, as shown in

figure 3, when only three lamps are connected in the circuit, and account being taken of the fact that the voltages of the two branch circuits need to be balanced by the equalizer transformer T2, half of the lamp voltage in each branch circuit will drop on T2, causing the cold points in the circuit, i.e. the locations of node 3 and node 4 in figure 3, to be transformed to hot points and carry the high-frequency voltage of T2 and consequently, the parasitic capacitive current flows to ground. The current IT2_1 shown in figure 3 is equal to the vector sum of the lamp current Ila2 and the parasitic current Ipa, while the current IT2_2 is equal to the vector sum of the lamp current Ila3 and the parasitic current Ipa. Since there is a phase difference of 180° between the voltages of node 3 and node 4, the currents passing the two windings of the T2 are different. Although the current IT2_1 and IT2_2 are made as equal as possible through the equalizer transformer T2, the result of this balancing is not desirable, as T2 does not know the real current of the lamp, resulting eventually in different lamp currents Ila2 and Ha. Furthermore, also due to the existence of the parasitic current Ipa, the lamp current Hal is different from the lamp current Ila2; this is similar to the case of four lamps of figure 2.

SUMMARY OF THE INVENTION

The objective of this invention is to provide a lamp driving circuit, which eliminates the imbalance through adding an inverse transformer, for driving a number of lamps. This object is achieved by a lamp driving circuit according to the present invention for driving more than one lamp, wherein the more than one lamps are located on at least two branch circuits connected in parallel, the lamp driving circuit comprising: a first transformer comprising at least two windings, which are connected in series to the at least two branch circuits connected in parallel; and an inverse means connected to the at least two branch circuits connected in parallel, for eliminating the AC voltage of the lamps with respect to ground to balance the current of each lamp.

According to another solution of a lamp driving circuit according to the present invention, the inverse means comprises a second transformer, the second transformer comprising a first winding connected in parallel to the at least two branch circuits connected in parallel and a second winding connected in series to the at least two branch circuits connected in parallel.

Compared with the prior art, the lamp driving circuit of the present invention eliminates the parasitic current in the series branch circuits by using the inverse transformer, and keeps the current passing through each lamp in the series branch circuits consistent and consequently, eliminates the imbalance of the lamps. Other objectives and effects of the present invention will be clearer and easier to understand through the description with reference to the accompanying drawings and the description of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in more detail with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a hybrid series-parallel lamp circuit of a prior art for driving four lamps;

Figure 2 shows the hybrid series-parallel lamp circuit of Figure 1 and a schematic view of some parasitic circuits in the hybrid series-parallel lamp circuit;

Figure 3 shows a hybrid series-parallel lamp circuit of a prior art for driving three lamps, and a schematic view of the circuit structure of a number of parasitic circuits of the hybrid series-parallel lamp circuit of the prior art;

Figure 4 shows a schematic view of an embodiment of a hybrid series-parallel lamp circuit for driving four lamps according to this invention;

Figure 5 shows a schematic view of an embodiment of a hybrid series-parallel lamp circuit for driving three lamps according to the present invention;

Figure 6 shows a general structure of a hybrid series-parallel lamp circuit for driving four lamps according to the present invention; and Figure 7 shows a general structure of a hybrid series-parallel lamp circuit for driving three lamps according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the driving circuit of the present invention is shown in figure 4. Compared with the prior arts, besides the equalizer transformer T2, an inverse transformer Tl is introduced in this embodiment. The voltage of each winding of Tl is identical to that of a lamp, but there is a phase difference of 180°. The inverse effect brought about by the inverse

transformer Tl may eliminate the AC voltage of the nodes 1 and 2 with respect to ground, i.e. make the AC voltage at the nodes 1 and 2 zero. As a result, two sets of series lamps are electrically balanced, and there is no parasitic current flowing from node 1 to the node 2, thus causing the current Hal (Ia2) passing lamp 1 and lamp 2 to be identical, while the current Ila3 (Ia4) passing lamp 3 and lamp 4 is identical as well.

When only three lamps are connected to the circuit, the inverse transformer Tl may further be simplified to comprise only two windings, the detailed circuit structure of which is shown in figure 5. Similarly to the case of four lamps, the inverse transformer Tl may eliminate the AC voltage of node 1 with respect to ground, meanwhile, the winding Tl_2 of Tl may replace the equalizer transformer T2 to compensate for the imbalanced lamp voltage in the two branch circuits. Under ideal conditions, when the impedance values of three lamps are identical, the voltage of T2 is zero. As nodes 5 and 6 have identical AC voltages with respect to ground, T2 is connected between the inductor Lr and nodes 5 and 6 to be able to obtain an appropriate balancing effect. The circuit shown in figure 5 may likewise make sure that identical currents pass through each lamp.

In practice, the inverse transformer Tl and the equalizer transformer T2 may be connected in many other ways depending upon the various needs. Figure 6 shows some possibilities of connection of T2 when four lamps are connected, wherein D1-D4 and El, E2 represent different possibilities of circuit connections, A1-A4 represent the locations of different windings of the inverse transformer Tl, and Bl -B 8 represent the locations of different windings of the equalizer transformer T2, and various combinations of such elements may form various lamp driving circuits.

Assuming that the turns ratio of Tl in the circuit shown in figure 4 is 1:1:1, due to the flux balancing condition, two times the current of the lamps will pass the winding Tl_l, while four times the current of the lamps will pass the Lr. In the case of figure 5, one time the current of the lamps will pass Tl_l, while three times the current of the lamps will pass the Lr. In the circuits shown in figures 2 and 3, only two times the current of the lamps will pass the Lr. If it is desired to avoid that more than two times the current of the lamps passes the Lr, an additional winding may be added to Tl. A4 shown in figure 6 may be the location of such an additional winding in the circuit. The turns ratio of the inverse transformer Tl and the way of connection of the winding A4 may be designed such that the flux on the winding A4 may counteract the flux on the windings A2 and A3, thus, only magnetizing current is left in the

winding Al, making the current passing Lr become two times the lamp current, so that it is identical to the current passing Lr in the prior art.

The inverse transformer Tl may be further optimized, for example, circuit connection El may be chosen to combine the windings A2 and A3 in order to save a winding, however, this has the drawback that the loss in the winding will somewhat increase. Table 1 below gives some schemes for circuit connections of the inverse transformer Tl, which may be selected in practice.

Table 1

For the equalizer transformer T2, different locations may be selected for its windings, as shown in Table 2 below, and when the connection location of B3+B4 is employed, i.e. the two windings are each located in the middle of two series lamps, which are the cold points in the circuit, the balancing effect is optimal. However, there is the drawback that the number of the connecting terminals of the lamps is increased. If other locations are selected, although the number of the connecting terminals of the lamps is reduced, symmetrical lamp wiring is required to obtain an appropriate balancing effect, since these locations are all hot points in the circuit.

Table 2

As blocking capacitors, C1-C3 in the circuit are not strictly limited with respect to their locations.

Figure 6 shows a general structure of a hybrid series-parallel lamp circuit for driving four lamps according to the present invention. With the same principle, when the present invention is used to drive only three lamps, a general structure is shown in figure 7. Similar to figure 6, different locations may be selected for the windings of the inverse transformer Tl and the equalizer transformer T2 in figure 7. Table 3 and Table 4 below give respectively some schemes for winding connections of the inverse transformer Tl and the equalizer transformer T2. The scheme of figure 7 will not be explained further, as it is similar to that of figure 6.

Table 3

Table 4

It may be known from the embodiments above that an additional inverse transformer is used in the lamp driving circuit of the present invention to eliminate the parasitic current in the series branch circuit, which makes the current passing each lamp of each series branch circuit consistent and consequently eliminates the imbalance of each lamp.

It should be noted that the embodiments above are exemplary and not to be construed as limiting the present invention, and within the scope of the appended claims, the skilled in the art should understand that various modifications may be made to the lamp driving circuit disclosed in the present invention without departing from the contents of the present invention. Therefore, the scope of the present invention should be defined by the appended claims. Furthermore, any reference number in the claims should not be construed as limiting the scope of the claims.