HK1004463A - Circuit arrangement - Google Patents

Description

Circuit arrangement

The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, which arrangement comprises a Buck converter comprising switching means, inductive means and rectifying means together, which converter is connected to an input for connection to a power supply and to an output for connection to a lamp arrangement comprising lamp means for supplying a current through the switching means to the lamp, which switching means are periodically switched so as to be alternately in a conducting and a non-conducting state, the Buck converter operating in a self-oscillating mode when the lamp corresponds to a stable lamp operating state.

ｃA circuit arrangement of the type described in the preceding paragraph is known from EP- ｃA-4001931 ═ US 5068572. The known circuit arrangement is particularly suitable for forming part of a projection TV (television) apparatus for igniting and operating a high-pressure discharge lamp.

Buck-type switching mode power supplies are also known by names such as: down-conversion converter, buck converter, inductor coupled buck converter, direct buck converter.

Although in general in Buck converters the input and output are directly electrically connected to each other, it is equally possible to use a circuit which is isolated between the input and output, for example in the form of a transformer.

In the known circuit arrangement, the Buck converter operates in self-excited operation, characterized in that: the switching means is switched from a non-conductive to a conductive state at the moment when the current through the inductive means becomes zero, so that the switching takes place immediately.

It is possible with the known circuit arrangement to supply a constant power supply of a wide range of currents and voltages to the connected lamp, so that the lamp produces a high and stable luminous flux. The self-oscillation mode is characterized in that: low conversion losses in the periodic conversion of the conversion means, in particular in the current-voltage range in which the lamp can operate stably. Preferably, during lamp operation, the down-converter is selected such that the frequency of the transition from the non-conducting to the conducting state is above the frequency reached by human hearing. This also means that the inductive means can be chosen to be comparatively small.

In the known circuit arrangement, a transition from a conducting to a non-conducting state takes place when a signal proportional to the current through the inductive means becomes equal to an independently set control signal. It is possible to control the power supplied to the lamp by controlling the current through the output of the circuit arrangement, for example in dependence on the voltage across the output. Although the power dissipation used in the load (lamp) can be controlled, the power supply control can be realized with a comparatively simple construction with the known circuit arrangement, the known circuit arrangement has a number of disadvantages.

It has been found that in the known circuit arrangement, in certain cases, the frequency with which the conversion means are caused to convert can be considered to be very high or very low. Particularly in the initial phase, i.e. in the lamp state before the steady operating state of the lamp, high and low switching frequencies occur.

The result of the higher switching frequency is a considerable increase in switching losses, which even results in the switching device becoming ineffective. The occurrence of lower switching frequencies leads to noise pollution problems, since the switching frequencies may enter the range of human hearing.

It is therefore an object of the present invention to provide a circuit arrangement which largely obviates the above-described disadvantage. According to the invention, a circuit arrangement of the type described in the preceding paragraph is for this purpose characterized in that the circuit arrangement has means for operating the Buck converter in forced oscillation mode in dependence on the lamp state.

The use of forced switching provides the possibility to operate the down-converter in dependence of the lamp conditions, i.e. in dependence of the voltage at the output. It is thus avoided that the switching means operate at a very low and/or a very high switching frequency.

Preferably, the apparatus provided by the invention provides discontinuous mode operation of the Buck converter when the lamp state corresponds to a state prior to occurrence of an arc discharge in the lamp. Preferably, the apparatus provides the Buck converter with continuous mode operation when the lamp conditions correspond to a condition in which an arc discharge has been established after a breakdown in the lamp, prior to a stable lamp operating condition.

Some further explanations are as follows: the concept of "forced oscillation mode" in connection with Buck converters refers to those modes in which the switching of the switching means from a conducting to a non-conducting state is not triggered by the current through the inductive means becoming zero.

Two types of forced oscillation are: continuous mode, discontinuous mode.

In the continuous mode, the current is continuously passed through the inductive means, as is also the case during the transition of the switching means from the non-conductive to the conductive state. When the circuit arrangement operates in a continuous mode, the ripple in the current through the load connected to the output is relatively small, and therefore a relatively strong current can be supplied to the load. In this case, since the switching means switches with a strong current from the non-conducting to the conducting state, the corresponding switching losses are also high.

When the circuit arrangement operates in a discontinuous manner, the current through the inductive means becomes zero, but the switching process of the switching means from the non-conductive state to the conductive state is not triggered thereby. Although it is possible to keep the current through the inductive means to zero for a later period of time until the switching means are conductive, the actual construction of the circuit arrangement often comprises a secondary circuit which forms part of the inductive means and which acts as a tuning circuit for the following period of time. After the said next period has elapsed, it is possible to make the transition with low transition losses by appropriately selecting the instant of transition to the on state. When the circuit arrangement operates in a discontinuous manner, the ripple of the current in the connected load branch is relatively long and the voltage across the load branch is also relatively high.

Ignition and operation in high-pressure discharge lamps can be distinguished between the following lamp states:

a non-igniting lamp that is extinguished;

followed by a breakdown of the lamp after the glow discharge, and a transition from the glow discharge to the arc discharge,

starting the lamp;

stable lamp operation.

In the extinguished, non-ignited state of the lamp, no conduction takes place. The voltage across the lamp is equal to the external voltage applied to the lamp.

When a high voltage pulse, called ignition pulse, is generated across the lamp, a breakdown occurs in the lamp, whereby a conduction in the form of a glow will occur in the lamp, followed by an arc discharge giving a sufficient current to make the lamp conductive due to the breakdown, causing the discharge voltage (lamp voltage) voltage and thus the voltage across the lamp to drop sharply to a few volts.

The starting of the lamp is such a lamp state: in this state, the arc discharge caused by ignition progresses to a stable lamp operating state. At the start of the run-up, with a strong current through the lamp, the arc voltage gradually decreases. To prevent overloading of the lamp, it is required under certain conditions to limit the current to a maximum value, whereupon the power to the lamp is controlled and only allowed to rise slowly.

In a stable operating state, the lamp has a stable lamp voltage which corresponds to the lamp power consumption and the thermal equilibrium in the lamp.

In the extinguished, non-ignited state of the lamp, it is advantageous for a fast and reliable ignition of the connected lamp to be achieved with a higher voltage at the output of the circuit arrangement. Since in this state the lamp is non-conductive, the occurrence of high current ripples at the output is not a great disadvantage. Thus, the operation of the circuit arrangement is very suitable for this lamp state in a non-continuous manner.

During lamp starting it is important to supply the lamp with a relatively strong current in order to achieve a stable operating state of the lamp quickly and reliably. The operation of the circuit arrangement in the continuous mode is at least well suited for this initial state at start-up. The high conversion losses occurring in the continuous mode do not constitute a large negative factor, since the starting of the high-pressure discharge lamp takes place only for a limited short period of time.

The circuit arrangement of the invention is particularly suitable for projection Television (TV) apparatus. The circuit arrangement is also very advantageously used in a headlight system of a motor vehicle. The two last applications relate to the ignition and operation of compact high-pressure discharge lamps, in which case in the steady-state operating state a high constant luminous flux and a high ignition voltage are provided. After breakdown, especially in automotive headlight systems, it is of utmost importance that the light must be emitted very quickly, so that a stable operating state is reached from start-up in a very short period of time.

In projection Television (TV) applications, the maximum current through the inductive means is limited in a circuit arrangement operating in a continuous mode in order to prevent overloading of the lamp.

When the circuit arrangement operates in a continuous mode or in a discontinuous mode, it is possible to control the switching means from the non-conductive to the conductive state, for example by means of a control signal generated on the basis of detecting a voltage across the lamp, a current through the lamp, or a combination of both.

In a practical circuit arrangement, the control signal for switching the device from the non-conductive to the conductive state is generated in dependence on the timing signal when the circuit arrangement is operated in a continuous or discontinuous manner. The time signal is preferably related to the time interval elapsed since the instant the switching means enters the non-conducting state. This has the advantage that a considerable simplification of the construction of the circuit arrangement is possible. The difference in the value of the steady operation of the individual lamps is found to be of no importance. After the operation of the circuit arrangement has been switched from the continuous mode to the self-oscillating mode, the starting of the lamp continues until a steady state is reached. Preferably, the circuit arrangement is operable in a self-oscillating mode while performing constant power control.

During operation in the free-running mode, in the known circuit arrangement, the control signal for switching from the non-conducting to the conducting state is implemented by: after the current in the inductive means has become zero, the polarity is changed and an auxiliary voltage is generated across the inductive means, which auxiliary voltage is used as a voltage source for generating a control signal for switching the conversion means into the conductive state. This constitutes a reliable and well-performing structure in a practically implemented circuit arrangement, but this is only one of many possible implementations from practice and literature. In many cases, the current through the inductive means increases again substantially instantaneously and without a polarity change when the conversion means enters the conductive state.

The above-mentioned change of polarity of the current through the inductive means is only a short instant, which is practically not taken into account in the following, whereas after the current has become zero, the gradient of the current through the inductive means is regarded as an immediate increase of the current through the inductive means due to the switching means being switched to the on-state.

The above and other aspects of the invention will be explained in more detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an embodiment of a circuit arrangement of the present invention;

FIG. 2 illustrates current as a function of time in portions of a circuit arrangement implementation during self-oscillation mode operation;

FIG. 3 shows current as a function of time in several parts of an embodiment of a circuit arrangement during operation in a continuous mode; and

fig. 4 shows the current as a function of time for parts of an embodiment of the circuit arrangement during operation in discontinuous mode.

Fig. 1 is a schematic diagram of a circuit arrangement for igniting and operating a high-pressure discharge lamp 7, which circuit arrangement is provided with switching means 1, inductive means 2 and rectifying means 4, which together form a Buck converter, which circuit arrangement is connected to an input 5 for connection to a power supply and to an output 6, which output 6 comprises means 8 for the lamp 7 for supplying current to the lamp, wherein the Buck converter operates in a self-oscillating mode when the lamp is in a corresponding state in which the lamp is stable in operation, the switching means periodically switching alternately between a conducting and a non-conducting state. The Buck converter is also provided with a capacitive buffer 3.

The circuit arrangement is provided with a control circuit 10 for controlling the switching device 1, which control circuit generates a control signal for switching the switching device alternately in a conducting and a non-conducting state.

The means 8 comprise a lamp and in many practical cases a converter is included for causing a current of periodically changing polarity to flow through the lamp during operation of the lamp. The device 8 usually also comprises an ignition circuit for generating voltage pulses for igniting the lamp. Both the converter and the ignition circuit generally form part of the circuit arrangement of the invention. The converter may be omitted when the lamp is operated at dc voltage.

The circuit arrangement is provided with means 11 for operating the Buck converter discontinuously in the state of the lamp before an arc discharge occurs in the lamp.

The means 12 operate the BUCK converter in a continuous mode when the lamp shape corresponds to a state in which an arc discharge has formed after a breakdown in the lamp, but before a steady operating state of the lamp.

The means 11 and 12 thus together form a means for operating the Buck converter in forced oscillation in dependence on the lamp state and are connected to the control circuit 10 for this purpose.

Parameter 20 represents the electrical connection between the sensing terminal of the current flowing through the inductive device 2 and the control circuit 10, sensing, in particular detecting, that the current flowing through the inductive device becomes zero. This coupling is used to initiate the process of switching the switching means from a non-conductive to a conductive state.

The control circuit 10 comprises means (not shown) for generating a control signal for switching the device from a conducting to a non-conducting state in a known manner. In the described embodiment, a constant power control is added to the power supplied to the output 6 when the circuit arrangement is operating in self-oscillating mode.

In an embodiment of ｃA practical implementation of the described circuit arrangement the control signal for switching the switching means from the non-conductive to the conductive state is generated in ｃA manner similar to the circuit arrangement of EP- ｃA-0401931, in which the known circuit arrangement operates in ｃA self-oscillating manner.

The means 11 and 12 in the practical implementation of the circuit arrangement are formed such that each means generates a time signal related to the time interval that has elapsed since the switching means was brought into the non-conducting state, the means 11 being designed such that they cannot be switched into the conducting state until after a minimum time interval. The means 12 are designed such that the switching means are switched to the conducting state only after a maximum time interval.

In the description of the practical implementation, the minimum time interval provided by the means 11 is 5 μ s. The maximum time interval achieved by the device 12 is 36 mus.

Fig. 2, 3 and 4 show the current I through the inductive means in an embodiment of a practical implementation of the described circuit arrangement_LAnd a current I through the conversion means_sThey are both as a function of time. Fig. 2 corresponds to the self-oscillating mode, fig. 3 corresponds to the continuous mode, and fig. 4 corresponds to the discontinuous mode.

The actual circuit arrangement implemented is suitable for connection to a 220V, 50Hz power supply. For this purpose, the circuit arrangement is provided with a section (known per se not shown) between the input terminal arrangement and the conversion means for converting an alternating voltage connected to the input terminal into a direct voltage adapted to the operation of the conversion means.

The described practical circuit arrangement is suitable for UHP-type high-pressure metal halide lamps manufactured by philips. The lamp has a nominal power of 100W for a nominal lamp voltage of 85V and a lamp current of 1.2A. An IRF840 MOSFET type, manufactured by international rectifier, is used as the conversion means 1. The inductive means 2 consist of a transformer with a ferrite core with 100 turns of a primary winding and 30 turns of a secondary winding, forming part of the control circuit 10 and as a voltage source for generating a control signal for switching the switching means to the on-state. The capacitive buffer means 3 has a capacitance of 0.82 muf. The rectifying device 4 is composed of a 13YV29F500 diode manufactured by philips. The Buck converter used in the self-oscillation mode can then provide a constant power in the range of 50 to 110V to the output.

In the extinguished, non-ignited lamp state, the current device operates in a discontinuous manner. The voltage across the output terminal is 160V. An ignition circuit (not shown) generates an ignition pulse of 20KV, which in turn ignites the lamp.

In the lamp state, in which the arc discharge in the lamp has reached, the voltage across the lamp drops to 15V, whereupon the voltage across the output also drops to this level. In this way, the circuit arrangement changes abruptly from discontinuous to continuous mode. A current limited to a maximum of 2A is then applied to the lamp. During lamp starting, the lamp voltage gradually rises and the current through the lamp gradually decreases. This indicates that the current through the inductive means drops very quickly during non-conduction of the conversion means.

The operation of the circuit arrangement is switched from the continuous mode to the self-oscillating mode when the switching means is in the non-conducting state and when the current through the inductive means becomes zero within 36 microseconds, the circuit arrangement being provided with means for controlling the power at the summing terminal to a constant level.

In the described practical implementation, this corresponds to a voltage of 50V across the lamp and across the output. Subsequently, lamp starting continues until the lamp voltage has reached a steady level, so that the lamp operates in steady state. Constant power control brings the lamp to a steady state operation at a nominal lamp voltage of 85V.

The time for which the switching means are in the non-conducting state when the lamp is operating steadily at nominal lamp power is approximately 18 microseconds.

Claims (5)

1. A circuit arrangement for igniting and operating a high-pressure discharge lamp has

A Buck converter comprising switching means, inductive means and rectifying means, the converter 5 being connected at an input for connection to a power converter and at an output for connection to means comprising the lamp for supplying the lamp with a current which is periodically switched alternately into the switching means being conductive and non-conductive. The Buck converter operates in a self-oscillating mode when the lamp is in a state corresponding to a steady lamp operation, characterized in that the circuit arrangement has means for operating the Buck converter in a forced oscillating mode in dependence on the lamp state.

2. The circuit arrangement as claimed in claim 1, wherein the circuit arrangement has means for operating the Buck converter in a discontinuous manner in the lamp state before an arc discharge occurs in the lamp.

3. A circuit arrangement as claimed in claim 1 or 2, characterized in that the circuit arrangement has means for operating the Buck converter in a continuous mode when the lamp state corresponds to a state in which an arc discharge has been formed in the lamp after breakdown and before a steady operating state of the lamp.

4. A circuit arrangement as claimed in claim 1, 2 or 3, characterized in that the circuit arrangement is adapted to constitute a high-pressure discharge lamp for ignition and operation of a part of a projection TV apparatus.

5. A circuit arrangement as claimed in claim 1, 2 or 3, characterized in that the circuit arrangement forms part of a headlight system of a motor vehicle.