FR2693074A1 - Power supply circuit for car headlight discharge bulb - has micro-programmable unidirectional quasi-resonant switch with high voltage transformer producing smoothed output voltage - Google Patents

Abstract

The supply circuit is connected to a car battery (1). The input is supplied to a transformer (3) in parallel with a quasi-resonant voltage switch (2). The switch (K1) is connected to a diode (D1) and a capacitor (C) and a command electrode (13) which are controlled by a pre-programmed microprocessor command circuit (10). The output current (11) in the discharge lamp circuit (9) is compared with the input current (14) in the microprocessor to smooth the current response, by adjusting the quasi-resonant voltage switch and main transformer switch (K2) operation. ADVANTAGE - Uses less primary current and avoids damaging transient phase current parasitics which can shorten component life.

Description

 The present invention relates to a supply device for a discharge lamp, intended to equip a vehicle headlight.

 It also relates to a vehicle headlight, comprising at least one discharge lamp, and which comprises a supply device according to the invention.

 In the prior art, devices for supplying a discharge lamp have already been proposed. In particular, it is known that, before the discharge lamp produces a luminous flux whose quality allows its application to the lighting of vehicles, especially when the discharge lamp is supplied from an unstable DC voltage source such as vehicle battery, it is necessary to use power devices with a lamp ignition circuit.

 In the priming phase of the supply of a discharge lamp, it is necessary to produce a high voltage under a low current so as to produce an electric arc between the electrodes of the lamp in the gas which it contains.

 It is sometimes necessary to repeat the ignition phase several times before obtaining a suitable electric arc. We then enter, once the successful ignition condition is detected in a maintenance phase, to maintain the on state of the discharge lamp.

 The present invention mainly relates to a new means of starting the lamp.

 In fact, in the prior art, power devices have already been proposed connected to the terminals of the battery on board the vehicle and which produce a high voltage necessary for supplying a discharge lamp.

 Such devices include a quasi-resonant voltage converter, and in particular a mono-directional converter.

 Such a converter has a primary side which receives a low continuous voltage and cut out according to a predetermined diagram using a quasi-resonant voltage switch.

 The converter also has a secondary side at the terminals of which one can collect a high impulse voltage and / or an alternating medium voltage of neighboring shape which make it possible, depending on the operating mode of the converter, either to obtain the ignition, or else to keep the lamp connected to the secondary of the aforementioned converter on.

 However, the devices of the prior art require, in particular during the ignition phase, large currents involved because of the transient phenomena exploited in this type of assembly. These currents are damaging to the components of the circuit and its lifetime.

 The present invention provides a remedy for this drawback of the prior art in that it makes it possible to use the currents flowing in the primary of the converter which are weaker than those involved in a priming by capacitor discharge, or by interruption. current in a non-linear impedance as is the case in the prior art.

 Indeed, the present invention relates to a supply device for a discharge lamp, intended to equip a vehicle headlight.

 The supply device is of the type comprising a quasi-resonant voltage converter, and mono-directional, and is characterized in that it comprises a lamp ignition circuit, provided with means for temporarily increasing the ratio converter, in order to take advantage of its natural overvoltage.

 According to another aspect of the invention, the means for increasing the transformation ratio of the converter comprises a step-up transformer.

According to another aspect of the invention, the means for transiently increasing the conversion ratio of the converter is connected to the output of the converter by means of connection and disconnection means, the variation of impedance of which is predetermined as a function of the lamp ignition characteristic. In particular, this connection and disconnection means is constituted either by a controllable switch, such as a power transistor admitting high voltages and low currents, or by an uncontrolled switch constituted for example by a series circuit consisting of two diodes
Zener mounted head to tail.

 The invention also relates to a vehicle headlamp, comprising at least one discharge lamp, which is characterized in that it comprises a supply device according to the invention.

Other characteristics and advantages of the present invention will be better understood with the aid of the description and the figures which are
- Figure 1: a diagram of a first embodiment of the priming part of a supply device according to the invention
- Figures 2 to 5: equivalent diagrams of various operating phases of the device of Figure 1
- Figure 6: four graphs a to b showing the currents and voltages during various operating phases of the device of Figure 1
- Figure 7: a diagram of a second embodiment of the device of the invention;
- Figures 8 and 9: equivalent diagrams of the device of Figure 7 in various operating phases;
FIG. 10: a graph representing the currents and the voltages during the various operating phases of the device in FIG. 7.

 In Figure 1, there is shown a first embodiment of the supply device according to the invention.

 In the diagram of Figure 1, there is shown in the supply device only the elements or components necessary for the ignition phase directly concerned by the invention.

 The power supply device is connected to a positive terminal of a battery 1, via a first terminal of the primary of a converter 2, 3.

 The negative terminal of battery 1 is connected to the other terminal of the primary of converter 2, 3.

 In the invention, the converter 2, 3 is a quasi-resonant voltage converter, and unidirectional, which comprises in particular a transformer 3 whose foot of the primary 4 is connected by a quasi-resonant voltage switch, referenced 2 in the drawing , to the negative or DC low voltage terminal of battery 1.

 The quasi-resonant switch in voltage 2 comprises a controllable switch K connected to a diode D1 and to a capacitor C, which can be connected according to various known diagrams.

 In the preferred embodiment, the three components K1, D1 and C are connected in parallel.

 The controllable element K1 is generally constituted by a switching transistor, the control electrode 13 of which is connected to the output of a pre-programmed control device 10.

 The control device contains a program which can be carried out in wired form or in micro-programmed form, which is executed as a function of control parameters, supplied at the input of the control device 10, and produces a certain number of commands to destination of the feeding device.

 According to the invention, the supply device comprises a priming part (shown in FIG. 1) and a holding part (not shown in FIG. 1) which operate successively. The holding part is composed of an additional winding at the secondary of the transformer 3, and of a conventional circuit for adaptation to the lit discharge lamp. The elements of the holding part, not directly concerned with the present invention, can be constituted by any known means, incorporated in the present invention.

 The converter 2, 3 comprises a secondary at the terminals of which the discharge lamp is conventionally placed.

 According to the invention, there is interposed between the discharge lamp 9 and the secondary 5 of the converter circuit 2, 3, a lamp ignition circuit provided with means for transiently increasing the transformation ratio of the converter 2, 3.

 When the converter 2, 3 comprises a pulse transformer 3, it has a transformation ratio m, and at the terminals of its secondary 5 there is a high voltage pulse and / or a high alternating voltage whose amplitude increases with the ratio of transformation m.

 To ensure the control of the device, the supply device is equipped with a means for measuring the current and / or the voltage passing through the discharge lamp 9, means referenced 11 in FIG. 1, and which is connected by a link 16 to a suitable input of the control device 10.

 The supply device may also include, in series with the primary of the converter 2, 3, a means of measuring the magnetizing current established in the converter, in particular by measuring the current passing through its primary 4, the measurement of the primary current equivalent to that magnetizing current as long as K2 is open.

 This measurement means is referenced 11 in FIG. 1 and is connected by a link 15 to a suitable input of the control device 10.

 According to the invention, there is interposed between the discharge lamp 9 and the secondary of the converter, a connection and disconnection means composed of a diode D2 and a controllable switch K2, and a second step-up transformer 6 consisting of a primary 7 and a secondary 8. In general, the means for transiently increasing the conversion ratio of the converter is connected to the output of the converter by means of connection and disconnection K2, D2; K2, the variation of impedance of which is predetermined as a function of the ignition characteristic of the lamp.

In the embodiment of FIG. 1, the connection and disconnection means comprises a switch that can be controlled like a power transistor, connected to a first terminal of the secondary of the converter, and the control electrode of which is connected to the output d '' a control device which produces control signals so that the switch
- is open outside the priming phase;
- either open at the start of the priming phase;
- either closed after the start of the ignition phase and when the magnetizing current seen from the converter primary is decreasing;
- either reopened when the current flowing through the converter primary is increasing again;
- And to repeat this priming cycle as long as a condition for successful priming is not detected by the control device 10.

 The controllable switch K2 is constituted by a power transistor, and in particular a transistor admitting high voltages and low currents.

 The control electrodes of switches K1 and K2 respectively 13 and 12 are connected to corresponding control signal outputs of the control device 10.

 We will now describe the various operating phases in priming of the device of FIG. 1.

 At the start of the first phase, the switch K1 is closed, and the switch K2 is open.

 In this configuration, the initial magnetizing current i passing through the device is negative.

 In Figure 2 there is shown an equivalent diagram of the assembly of Figure 1 in this first phase. The circuit consists of a loop connecting the positive terminal of the battery to a first terminal of an equivalent inductance Lt, the other terminal of which is connected to the negative terminal of the battery which has a voltage E.

The magnetizing current evolves linearly over time according to the equation
i (t) = i (0) + (E / Lt) xt
When the control device 10 emits a suitable signal, the switch K1 is open. At this instant, it has been measured that the magnetizing current i, using the measuring device 14 has become positive again.

 During phase 2, the switches K1 and K2 are both open, the magnetizing current is positive and the voltage across the terminals of the quasi-resonant switch 2 is Vc (0) = 0.

 In Figure 3, there is shown the equivalent diagram of the device, which is then constituted by a loop comprising a DC voltage source E constituted by the battery whose positive terminal is connected to a first terminal of an equivalent inductance Lt, of which the other terminal is connected to a first terminal of a capacitor C with a capacity equal to that of the equivalent capacitor constituted by the quasi-resonant switch 2 at the terminals of which there is a voltage Vt, and the other terminal of which is connected to the negative terminal of the battery.

 In this phase, the current is that coming from the discharge of the capacitor C at the terminals of which there is a voltage Vc.

We have the equations
E = Lv x (di / dt) + Vc and
i = C (dvc / dt)
The solution of these equations indicates that the voltage across the capacitor C is substantially sinusoidal at a frequency proportional to the square root of the product of the equivalent inductance Lt and the capacitance of the capacitor of the quasi-resonant switch 2.

 Phase 2 ends when the controllable switch K2 is closed.

 This closure is controlled by the control device 10 when it measures or detects that the magnetizing current seen from the primary of the converter, and measured by the measuring means 14, is decreasing.

In a preferred embodiment, to obtain optimal operation, closing the switch
K2 occurs when the voltage Vc across the capacitor goes through its maximum.

 At this instant, the current in the magnetizing current is canceled.

 During phase 3, the switch K1 is open, and the switch K2 is closed.

 In Figure 4, there is shown an equivalent diagram of the circuit of Figure 1 during this phase.

 The equivalent circuit comprises a DC voltage source E, constituted by the on-board battery, connected in series with the capacitor C of the quasi-resonant switch 2 and with a parallel circuit constituted by the equivalent inductances Lt, corresponding to the converter 3, and (L't / m2) corresponding to the equivalent inductance of the second transformer 6 reduced to the primary of the converter 3.

 The first inductance is crossed by a magnetizing current it, and the second inductance L't is crossed by a current mi't.

 During this phase, the voltage across the capacitor C decreases and the phase ends when this voltage is canceled.

 At this instant, the diode D1 of the quasi-resonant switch 2 becomes conductive.

 Phase 4 starts when the voltage across the capacitor Vc is canceled.

 In this state, the diode D1 is always conductive and the equivalent diagram of the device of FIG. 1 has been represented in FIG. 5.

 As the diode D1 is conductive, the equivalent circuit of Figure 5 corresponds to the replacement of the capacitor C by a short circuit.

 Phase 4 ends when the switch K2 is commanded to open. At this instant, an overvoltage appears at the terminals of K2. It is notably limited by the parasitic Drain-Source capacitor of the transistor which constitutes the switch K2. In the case where the controllable switch K2 is not constituted by a field effect transistor, it is then necessary to provide a capacitor in parallel with the switch K2 or any clipper (Transil, VDR, ...).

 The opening of the switch K2 is controlled by the control device 10 before the current i is canceled. Therefore, the spontaneous blocking of diodes D1 and D2 is avoided.

 At the end of phase 4, the magnetizing current is reversed. After opening the switch K2, the current in the magnetizing inductor is continuous.

 Phase 5, which ends a priming cycle, consists of the open state of the switch K2 and the fact that the diode D1 is conductive. In this case, the magnetizing current is negative and the equivalent diagram is then that of FIG. 2 during phase 1.

 Phase 5 ends when K1 closes, which must occur before current i is canceled.

 We then return to the initial conditions of phase 1.

 The control device 10 then detects using the detector 11 if the lamp 9 is in the condition of a successful ignition. In the positive case, the control device then enters the maintenance mode which ends the priming mode. In the negative case, a priming cycle is repeated.

 In Figure 6, there is shown the appearance of currents and voltages in the device of Figure 1 during the various phases of the priming cycle.

 Diagram (a) represents the evolution of the voltage across the terminals of the quasi-resonant switch 2.

 Diagram (b) represents the primary voltage of converter 3.

 Diagram (c) represents the evolution of the magnetizing current seen from the primary of transformer 6, over time.

 Diagram (d) represents the temporal succession of phases 1 to 5 of the priming cycle.

 During phase 1, the magnetizing current it (diagram c) is increasing. When entering phase 2, the magnetizing current it is used to charge the capacitor C of the quasi-resonant switch.

 The voltage Vc across the quasi-resonant switch is therefore increasing (diagram a).

 At the end of phase 2, the closing of the switch K2 applies the voltage generated by the switch 1, 2 to the primary 7 of the transformer 6.

 Thanks to the various transformation ratios, the two transformers, respectively 3 of the converter and 6 of the ignition means, it is understandable that it is therefore possible to produce a very high ignition voltage with the lamp.

 Phase 4 ends when the magnetizing current is negative and we again enter a phase of return to the initial being.

 In Figure 7, there is shown a diagram of a second embodiment according to the present invention.

 In FIG. 7, the elements similar to those of FIG. 1 have been represented by the same reference numbers, and there has been changed the connection and disconnection means which has been replaced by a single non-controlled switch K2. It is not necessary, thanks to the characteristics of this uncontrolled switch, to use a diode like the diode D2 in FIG. 1.

 In a first embodiment, the switch K2 is constituted by a non-linear resistor.

In another embodiment, the switch
K2 consists of two Zener diodes connected in head to tail series.

Generally, the uncontrolled switch K2 has
- a high impedance outside the priming phase;
- high impedance at the start of the ignition phase;
- a low impedance after the start of the ignition phase while the magnetizing current seen from the converter primary is decreasing;
- a high impedance after the magnetizing current seen from the primary of the converter becomes again increasing.

 When the uncontrolled switch K2 is constituted by Zener diodes, the Zener diodes are selected so that their threshold voltage is such that the voltage across the terminals of the quasi-resonant switch K1 is sufficient to ensure ignition of the lamp when the magnetizing current of the converter primary is canceled.

 In FIGS. 8 and 9, two equivalent diagrams have been shown during a priming cycle of the device in FIG. 7.

 When the ignition cycle begins, the discharge lamp behaves like an open circuit. Therefore, as in the case of Figure 1, the equivalent diagram takes into account the fact that the supply device then operates empty.

 At the start of phase 1, the quasi-resonant switch K1 is closed. In this case, the assembly of FIG. 7 behaves like the assembly of FIG. 1.

 During phase 2, the quasi-resonant switch K1 was opened and the Zener diodes are blocked.

In this case, phase 2 takes place like phase 2 of the assembly of FIG. 1. This phase ends when the magnetizing current seen from the primary of the converter is decreasing, and that the uncontrolled switch K2 has a low impedance.

In the case of Zener diodes, this phase ends when the voltage across the secondary of transformer 3 becomes higher than the threshold voltage
Vz Zener diodes. At this instant the Zener diodes become conductive.

 We then enter phase 3, the device of FIG. 7 having an equivalent diagram represented in FIG. 8.

 In the equivalent diagram of FIG. 8, the supply device is represented by two equivalent inductances Lt, L't / m2 in parallel, the inductance L't / m2 being in series with a source of direct voltage equal to Vz / m.

 At the terminals of the equivalent inductance L't / m2 is a voltage V't / m.

 The two inductors in parallel are connected to a first terminal of the capacitor C of the quasi-resonant switch K1, the other terminal of which is connected to the negative or lower voltage terminal of the battery.

The magnetizing current it crossing the inductance
Lt decreases rapidly, and the current it passing through the second inductor L't is negative throughout phase 3. Phase 3 ends when the voltage vc is canceled. At this instant, the diode D1 of the quasi-resonant switch 2 becomes conductive.

 We then enter phase 4 during which the Zener diodes are conducting. The equivalent circuit of the device in Figure 7 is shown in Figure 9. As the capacitor C is fully discharged, and the diode D1 is conducting, the equivalent circuit of Figure 9 corresponds to the equivalent circuit of Figure 8 in which replaced capacitor C with a short circuit.

 Phase 4 ends when the magnetizing current passing through the inductance L't is canceled. At this moment the Zener diodes are blocked.

 Phase 5 takes place when the diode D is conductive and the Zener diodes are blocked. In this case, the operation of phase 5 of the device in FIG. 1 is resumed.

 In FIG. 10, the appearance of the currents and voltages in the device of FIG. 7 is shown.

 Vertically, the sharing of the various phases 1 to 5 of the priming cycle has been shown. This priming cycle can be repeated naturally as long as the priming condition is not detected as successful by the power controller.

 The first curve represents the magnetizing current it. It increases linearly during phase 1 and then decreases in the form of a sinusoid arch during phases 2 and 3. Then it increases again linearly during phases 4 and 5.

 The second curve shown is that of the voltage vc across the capacitor C of the quasi-resonant switch K1. This voltage is formed by a sinusoid arch during phases 2 and 3.

 The third curve is constituted by the appearance of the voltage across the terminals of the second equivalent inductance L't, this voltage being seen from the primary of the converter 3 and being equal to Vz / m. Note that the average value of this voltage is zero (in steady state).

 Other modifications can be made with respect to the embodiments of FIGS. 1 and 7.

 On the other hand, the invention is advantageously combined with other devices for maintaining the voltage across the discharge lamp when it has been started thanks to the invention.

Claims (10)

 1) Supply device for a discharge lamp intended to equip a vehicle headlight, of the type comprising a converter (2,3) quasi-resonant in voltage, mono-directional, characterized in that it comprises a circuit (6 , K2, D2; 6, K2) for starting the lamp, provided with means for temporarily increasing the conversion ratio of the converter in order to take advantage of its natural overvoltage.  2) Device according to claim 1, characterized in that the means for increasing the conversion ratio of the converter comprises a step-up transformer (6).  3) Device according to claim 1 or 2, characterized in that the means for transiently increasing the conversion ratio of the converter is connected to the output of the converter by means of connection and disconnection (K2, D2; K2) whose variation impedance is predetermined based on the starting characteristic of the lamp.  4) Device according to claim 3, characterized in that the connection and disconnection means comprises a controllable switch (K2 ') like a power transistor, connected to a first terminal of the secondary of the converter, and including the control electrode is connected to the output of a control device (10) which produces control signals so that the switch (K2)  - is open outside the priming phase;  - either open at the start of the priming phase;  - either closed after the start of the ignition phase and when the magnetizing current seen from the converter primary is decreasing;  - either reopened when the magnetizing current seen from the primary of the converter is again increasing;  - And to repeat this priming cycle as long as a condition for successful priming is not detected by the control device (10).  5) Device according to claim 4, characterized in that the control device (10) produces a signal (12) so that the switch (K2) is closed when moreover the voltage across the switch (K1) is maximum and / or the magnetizing current (i) seen from the primary of the converter passes through a zero value.  6) Device according to claim 4, characterized in that the connection and disconnection means also comprises a spontaneous switch, such as a diode (D2), connected to a second terminal of the secondary of the converter which is blocked at least when the voltage at its limits are reversed in a predetermined direction.  7) Device according to claims 5 and 6, characterized in that the control device (10) produces a signal (12) so that the switch (K2) is closed before the magnetizing current is canceled when it is increasing in order to avoid spontaneous blocking of the diodes (D1, D2).  8) Device according to claim 3, characterized in that the connection and disconnection means comprises an uncontrolled switch (K2) such as a nonlinear resistor or a series circuit consisting of at least two Zener diodes head to tail, the characteristics are determined so that the uncontrolled switch (K2) has  - a high impedance outside the priming phase;  - high impedance at the start of the ignition phase;  - a low impedance after the start of the ignition phase, while the magnetizing current seen from the primary of the converter is decreasing;  - a high impedance after the magnetizing current seen from the primary of the converter becomes again increasing.  9) Device according to claim 8, characterized in that the converter comprises a transformer (3) whose foot of the primary (4) is connected by a quasi-resonant voltage switch (2) to a continuous low voltage and in that the uncontrolled switch (K2) has two diodes Identical head-to-tail zener whose threshold voltage (Vz) chosen so that the voltage across the quasi-resonant switch (2) is sufficient to ensure the ignition of the lamp (9) when the magnetizing current seen of the converter primary is canceled.  10) Vehicle headlamp comprising at least one discharge lamp, characterized in that it comprises a supply device according to one of the preceding claims.