A supply circuit for a gas-discharge lamp, particularly for motor vehicles

Abstract: The circuit comprises a dc/dc converter (2) and an H-shaped bridge circuit (HB) which is connected to the converter (2) by means of a filter capacitor (C F ) and has two output terminals (O1, O2) between which the lamp (L) is connected. The bridge circuit (HB) comprises four controlled switches (M1-M4) each of which is disposed between an input terminal (I1, I2) and an output terminal (O1, O2) and which can be driven in alternate pairs to form, with the lamp (L), two paths or diagonals of the bridge (HB). A starting transformer (TT) has its secondary winding (W2) connected in series with the lamp (L) between the output terminals (O1, O2) of the bridge circuit (HB). A starter circuit (IC; H; C; SC) is connected to the primary winding of the transformer (TT). A control unit (ECU) connected to the control inputs of the switches (M1-M4) of the bridge circuit (HB) is arranged to drive the switches (M1-M4) in a manner such that the switches which are conductive when the current-flow starts in the lamp (L) belong to the path or diagonal of the bridge circuit (HB) for which the currents flowing in the lamp (L) as a result of the flow of a pulsed current in the primary winding of the starter transformer (TT) and as a result of the discharge of the filter capacitor (C F ), respectively, circulate in the same direction.

Description: The present invention relates to a supply circuit for a gas-discharge lamp particularly for motor vehicles. More specifically, the invention relates to a circuit comprising: a dc/dc converter the input of which can be connected to a direct-current voltage supply, an H-shaped bridge circuit with first and second input terminals connected to the output of the dc/dc converter by means of a filter capacitor, and with two output terminals between which the lamp is connectable, the bridge circuit comprising four controlled switches, each of which is disposed between an input terminal and an output terminal of the bridge, and which can be driven in alternate pairs to form, with the lamp, two paths or diagonals of the bridge circuit which can be connected selectively to the filter capacitor, a transformer for starting the current-flow in the lamp, including a primary winding and a secondary winding, the latter being connectable in series with the lamp between the output terminals of the bridge circuit, a starter circuit, connected to the primary winding of the transformer for causing a pulsed current to flow in the primary winding to start the current-flow in the lamp, and control-circuit means connected to the control inputs of the switches of the bridge circuit. Such a circuit is disclosed in EP 595 414 and EP 411 617. In known circuits of the type defined above, before current starts to flow in the lamp, the control-circuit means drive the switches of the two diagonals of the bridge circuit so that they conduct alternately at a predetermined frequency. The current-flow in the lamp may thus start randomly whilst one or other diagonal of the bridge circuit is conductive. When the current starts to flow in the lamp, the sign or direction of the current in the lamp may or may not be reversed, according to which diagonal of the bridge circuit is conducting. If the sign or direction of current in the lamp is reversed, there is a danger that the lamp will go out as a result of the zero-crossing of the current. An object of the present invention is to provide a supply circuit for a gas-discharge lamp of the type defined above which avoids the risk of the lamp going out. Such an object is already known from EP 595 414. This object is achieved, according to the invention, by a supply circuit of the type specified above, the main characteristic of which lies in the fact that the control-circuit means are arranged to drive the bridge circuit in a manner such that, the switches which are conductive when current starts to flow in the lamp belong to the path or diagonal of the bridge for which the currents flowing in the lamp as a result of the flow of the pulsed current in the primary winding of the starter transformer and as a result of the discharge of the filter capacitor, respectively, circulate in the same direction. Further objects, characteristics and advantages of the invention will become clear from the detailed description which follows purely by way of non-limiting example, with reference to the appended drawings, in which Figures 1 to 3 show circuit diagrams, partially in block form, of possible embodiments of the circuit according to the invention. In Figure 1, a gas-discharge lamp, indicated L, is connected to a supply circuit according to the invention, generally indicated 1. The circuit comprises a dc/dc converter 2 of known type, the input of which can be connected to a direct-current voltage supply B (for example, the battery of a motor vehicle) by means of a control switch 3. The output of the converter 2 is connected to a capacitor C F . Typically, this capacitor has the function of filtering the voltage which the converter 2 applies between the input terminals I1 and I2 of an H-shaped bridge circuit, generally indicated HB. The bridge circuit has two output terminals O1, O2, between which the lamp L is connected in series with the secondary winding W2 of a starting transformer TT. The transformer is typically a pulse transformer (a Tesla transformer). The bridge circuit HB comprises four electronic switches M1-M4 constituted, for example, by MOSFET transistors. Each of these switches is connected in a respective branch b1-b4 of the bridge circuit HB, between an input terminal I1, I2 and an output terminal O1, O2. The control inputs of the switches are connected to corresponding outputs of an electronic control unit ECU which is arranged to drive them so that they conduct in alternate pairs (M1, M2 and M3, M4, respectively) to form (with the lamp L and the winding W2) two paths or diagonals of the bridge which can be connected selectively to the capacitor C F and thus to the output of the converter 2. The starting transformer TT has its primary winding W1 (which, typically, is formed by a few turns) connected in series with a discharger SC. This discharger may have a striking potential, for example, of 1000 V ± 15%, that is, for example, between 850 and 1150 V. A starting capacitor C is connected in parallel with the series formed by the winding W1 and the discharger SC. This capacitor is connected to the output of a starter circuit IC of known type for charging the capacitor to a voltage high enough to cause a discharge in the discharger SC in order to start the current-flow in the lamp L. When this discharge takes place in operation, the capacitor C is discharged rapidly into the primary winding W1 of the transformer TT causing an overvoltage to appear between the terminals of the secondary winding W2. The control unit ECU is arranged to cause the switches of a specific diagonal to be closed (that is, conductive) in the bridge circuit HB when the capacitor C is discharged into the winding W1 of the transformer TT so that the current flowing in the lamp L as a result of the overvoltage created in the secondary winding W2 and the current flowing in the lamp as a result of the discharge of the filter capacitor C F circulate in the same direction. With reference to Figure 1, if the plate of the capacitor C which is at the higher potential is that indicated by the + sign in the drawing and if the coupling between the windings of the transformer TT is that corresponding to the conventional dots in the drawing, the diagonal of the bridge circuit HB which satisfies the aforesaid condition (the aforementioned components of the current in the lamp L having the same direction) is that including the electronic switches M1 and M2. With reference to Figure 1, the unit ECU is therefore arranged to cause the switches M1 and M2 to be conductive when current starts to flow in the lamp, whilst the switches M3 and M4 are cut off. After current has started to flow in the lamp L, the control unit ECU drives the two pairs of switches M3, M4 and M1, M2, respectively, in an on-off manner, so that they are conductive alternately thus making the two diagonals of the bridge circuit conductive alternately, in known manner. Figure 2 shows a particular embodiment of a circuit according to the invention. In this drawing, parts and elements already described have again been given the same reference numerals. In the circuit of Figure 2, a direct-current high-voltage supply, indicated H, can supply, for example, a voltage of 800 V. The supply may be constituted, for example, by an additional output of the dc/dc converter 2. The supply H has one terminal connected to the input terminal I1 of the bridge circuit HB and its other terminal connected to a plate of the starting capacitor C. The other plate of the capacitor C is connected to the output terminal O2 of the bridge. The control unit ECU is connected to the filter capacitor C F (in a manner not shown) so as to be able to detect the voltage which develops at the terminals thereof after the closure of the control switch 3. This unit is also arranged to pilot the starting of the current-flow in the lamp L in the following manner. The unit ECU first causes the switch M3 to close or become conductive. In this condition, the starting capacitor C is connected to the supply H and tends to be charged to the supply voltage V H . Conveniently, the voltage V H has a value below the minimum threshold of the breakdown voltage of the discharger SC. If, as stated above, the breakdown voltage of the discharger is 1000 V ± 15%, the voltage V H is conveniently, for example 800 V. As soon as the voltage V CF in the capacitor C F reaches a predetermined value (for example, 360 V), the ECU then causes the switch M3 to open and the switch M2 to close. In this situation, the starting capacitor C tends to be charged to a voltage equal to the sum of V CF and V H . With the numerical data indicated above by way of example, at this stage, the capacitor C tends to be thus charged to a voltage of 800 + 360 = 1160 V, and hence to a voltage greater than the maximum value of the breakdown voltage of the discharger SC. A discharge is therefore certain to occur in the discharger SC whilst the voltage in the capacitor C is increasing towards this value, and this discharge takes place when the voltage in the capacitor C F is certain to have reached the minimum value indicated above. Simultaneously with the closure of M2 described above, the ECU also closes the switch M1. In the case of Figure 2, the diagonal of the bridge circuit HB for which current starts to flow in the lamp is therefore that which includes the switches M1 and M2. The circuit described above with reference to the drawing thus ensures that current starts to flow in the lamp at a moment at which the voltage in the output capacitor C F of the converter 2 is sure to have reached an adequate value. (In fact, it is important that the voltage in C F reaches an adequate value since, after starting, it is actually the discharge current of this capacitor which flows in the lamp L; if the capacitor C F were insufficiently charged, the lamp L might go out immediately after starting). Whilst the voltage in the capacitor C F is brought to this value, the voltage in the starting capacitor C is brought to a value which is quite high but is nevertheless sure to be lower than the minimum threshold of the breakdown voltage of the discharger SC. Figure 3 shows a variant of the circuit of Figure 2. In the circuit of Figure 3, the direct-current high-voltage supply H is constituted by a voltage multiplier the input terminals of which are connected to the terminals of the capacitor C F . The voltage multiplier has one output terminal connected to the output O1 of the bridge circuit HB and the other terminal connected to a plate of the starting capacitor C. The other plate of the capacitor is again connected to the output terminal O2. In operation, a voltage kV CF , where k is greater than 1, for example 3, is generated between the output terminals of the voltage multiplier H. The control unit ECU of the circuit according to Figure 3 pilots the starting of the current-flow in the lamp L in the following manner. The unit ECU first closes the switches M3 and M4. In this condition, the starter capacitor C is charged to a voltage of (k-1)V CF . If V CF is less than or equal to, for example, 360 V and k = 3, at this stage, the starting capacitor C is thus charged to a voltage equal to or less than 720 V, and hence to a voltage below the minimum threshold of the breakdown voltage of the discharger SC. As soon as the voltage V CF in the capacitor C F reaches a predetermined value (for example, 360 V), the unit ECU cuts off the switches M3 and M4 and closes the switches M1 and M2. In this condition, the voltage in the starting capacitor C tends to be brought to the value (k+1)V CF and hence to a value equal to or greater than 1440 V. The discharge in the discharger SC and the start of conduction in the lamp L are therefore sure to take place whilst the voltage in the capacitor C is rising at this stage. The circuit of Figure 3 can thus also start the current-flow in the lamp L whilst a predetermined diagonal of the bridge circuit HB is conductive (in this case, that including the switches M1 and M2), and surely after the voltage in the capacitor C F has reached a predetermined level. In the circuit of Figure 3, the direct-current voltage supply H could, on the other hand, be constituted by an additional output of the converter 2, as stated above with reference to Figure 2. Moreover, in the circuit of Figure 2, the supply H could be formed as described above in connection with Figure 3, that is, by a multiplier (or, more generally, a voltage raiser) connected to the capacitor C F . In general, the flow of current in a gas-discharge lamp may be critical at the stage immediately following the starting of conduction in the lamp, in the sense that, if the switching of the current (brought about by the unit ECU which pilots the diagonals M1, M2 and M3, M4 of the bridge circuit HB so that they are conductive alternately) takes place prematurely, the lamp might go out. To avoid this potential problem, the control unit ECU of a circuit according to the invention is conveniently arranged so that the switches of the diagonal of the bridge circuit HB which was conductive when conduction started are kept closed immediately after current starts to flow in the lamp L for a predetermined period of time longer than the period corresponding to the switching frequency of the current in the lamp in normal running conditions. This period of time for which the diagonal of the bridge circuit with which the current-flow was started in the lamp is kept conductive is conveniently long enough for the current-flow in the lamp to reach a non-critical condition. In gas-discharge lamps, the current intensity decreases appreciably immediately after conduction starts (the so-called "take-over" and heating or "warm-up" stages of the lamp) and, in normal running conditions, settles at a low value. If, as stated above, the diagonal of the bridge circuit HB in which conduction was started is kept conductive for a certain period of time after starting, the electrodes of the lamp may be quite stressed since, for this period of time, there is a flow of ions which always leaves the same electrode and always strikes the same other electrode . Moreover, at this stage, as just mentioned, the intensity of the current flowing in the lamp is quite high. Conveniently, in order to compensate for the initial stressing of the lamp L, the unit ECU may be arranged in a manner such that, after the initial prolonged closure of the diagonal for which the current-flow in the lamp starts, the two diagonals of the bridge are piloted alternately in an on-off manner with conduction times which are longer, on average, for the diagonal of the bridge HB which was cut off or open upon starting. This can be achieved by the unit ECU by an alteration of the frequency and/or the duty cycle for driving the switches of the two diagonals of the bridge circuit such that the average value of the current flowing in the lamp L in one conduction direction tends, over a predetermined period of time, to equal the average value of the current flowing in the lamp in the other direction. This control procedure can advantageously compensate for or balance the initial stressing of the lamp. This compensation can be carried out by the control unit ECU either in a non-adaptive manner, based on predetermined procedures, or in an adaptive manner achieved, for example, with voltage and/or current sensors connected to the unit ECU for detecting any asymmetrical operation of the lamp.

Claims: A supply circuit for a gas-discharge lamp (L) comprising: a dc/dc converter (2) the input of which can be connected to a direct-current voltage supply (B), an H-shaped bridge circuit (HB) with first and second input terminals (I1, I2) connected to the output of the converter (2) parallel to a filter capacitor (C F ), and with two output terminals (O1, O2) between which the lamp (L) is connectable, and comprising four controlled switches (M1-M4) each of which is disposed between an input terminal (I1, I2) and an output terminal (O1, O2) of the bridge circuit (HB) and which can be driven in alternate pairs to form, with the lamp (L), two paths or diagonals of the bridge (HB) which can be connected selectively to the capacitor (C F ), a transformer (TT) for starting the current-flow in the lamp (L), including a primary winding (W1) and a secondary winding (W2), the latter being connectable in series with said lamp (L) between the output terminals (O1, O2) of the bridge circuit (HB), a starter circuit (IC; H; C; SC) connected to the primary winding of the transformer (TT) for causing a pulsed current to flow in the primary winding (W1) to start the current-flow in the lamp (L), and control-circuit means (ECU) connected to the control inputs of the switches (M1-M4) of the bridge circuit (HB), characterized in that the control-circuit means (ECU) are arranged to pilot the bridge circuit (HB) in a manner such that the switches which are conductive when current starts to flow in the lamp (L) belong to the path or diagonal of the bridge circuit (HB) for which the currents flowing in the lamp (L) as a result of the flow of the pulsed current in the primary winding of the starter transformer (TT) and as a result of the discharge of the filter capacitor (C F ), respectively, circulate in the same direction. A circuit according to Claim 1, in which the control-circuit means (ECU) are connected to the filter capacitor (C F ) and are arranged to detect the voltage (V CF ) at its terminals, and in which the starter circuit comprises a discharger (SC) in series with the primary winding (W1) of the transformer (TT), a starting capacitor (C) connected in parallel with the series formed by the discharger (SC) and the primary winding (W1), and voltage-generator means (H) for charging the capacitor (C) to a voltage high enough to bring about a discharge in the discharger (SC), the breakdown voltage of the discharger (SC) being between predetermined minimum and maximum values, the circuit being characterized in that the starter circuit (H, C, SC) is connected to the bridge circuit (HB) and the arrangement is such that, before the current starts to flow in the lamp, the starting capacitor (C) can be charged to a first voltage value below the minimum value of the breakdown voltage of the discharger (SC) as a result of the selective closure of at least one switch of a first diagonal of the bridge circuit (HB) and the subsequent selective closure of at least one switch of the other diagonal of the bridge circuit (HB), and is then charged to a second value which is higher than the first and high enough to bring about a discharge in the discharger (SC), the control-circuit means (ECU) being arranged so that, before the current starts to flow in the lamp, they bring about the selective closure of the switches of the other diagonal of the bridge circuit (HB) only after the voltage in the filter capacitor (C F ) has reached a predetermined value. A circuit according to Claim 2, characterized in that the voltage-generator means comprise a direct-current high-voltage supply (H) connected in series with the starting capacitor (C) between an input terminal (I1) and an output terminal (O2) of the bridge circuit (HB) and in that the control-circuit means (ECU) are arranged for selectively causing the conduction of the switches (M3, M2) of the bridge circuit (HB) connected to the output terminal (O2) before the current starts to flow in the lamp. A circuit according to Claim 2, characterized in that the voltage-generator means comprise a direct-current high-voltage supply (H) connected in series with the starting capacitor (C) between the output terminals (O1, O2) of the bridge circuit (HB), and the control circuit means (ECU) are arranged for selectively causing the conduction of the switches (M3, M4; M1, M2) of one diagonal and of the other diagonal of the bridge circuit (HB), respectively, before the current starts to flow in the lamp (L). A circuit according to Claim 3 or Claim 4, characterized in that the direct-current high voltage supply is derived from the converter (2). A circuit according to Claim 4, characterized in that the direct-current high-voltage supply (H) comprises a voltage-raiser circuit having its input terminals connected to the filter capacitor (C F ) and its output terminals connected to the starting capacitor (C) and to an output terminal (O1) of the bridge circuit (HB). A circuit according to any one of the preceding claims, in which the control-circuit means (ECU) are arranged, in normal running conditions after the current has started to flow in the lamp, to pilot the switches (M1, M2; M3, M4) of the two diagonals of the bridge circuit (HB) alternately in an on-off manner at a predetermined switching frequency, the circuit being characterized in that the control-circuit means (ECU) are arranged so that the switches of the diagonal of the bridge circuit (HB) which was conductive when conduction started continue to be kept conductive immediately after the current starts to flow in the lamp for a predetermined period of time longer than the period corresponding to the switching frequency. A circuit according to Claim 7, characterized in that the control-circuit means (ECU) are arranged to drive the switches of one diagonal and of the other diagonal of the bridge circuit (HB) alternately, after the current starts to flow in the lamp (L), in a manner such that the average value of the current flowing in the lamp (L) in one direction over a predetermined time interval tends substantially to equal the average value of the current flowing in the lamp (L) in the other direction. A circuit according to Claim 8, characterized in that the control-circuit means (ECU) are arranged to regulate the average value of the current flowing in the lamp (L) in one direction and in the other direction according to predetermined procedures, in a non-adaptive manner. A circuit according to Claim 8, characterized in that the control-circuit means (ECU) are arranged to regulate the average value of the current flowing in the lamp (L) in one direction and in the other direction according to lamp voltage and/or current conditions, in an adaptive manner.

Inventors: FRATTA ANTONINO, NEPOTE ANDREA, FRATTA, ANTONINO, NEPOTE, ANDREA
Applicants: MAGNETI MARELLI SPA, MAGNETI MARELLI S.p.A.
Application year: 1995
Main classification: H05B41
IPC codes: H05B41