EP1465465B1 - Electronic ballast with a full-bridge circuit - Google Patents

Abstract

The device has a full bridge circuit supplied with a d.c. voltage (UBUS) with the lamp (LA) connected as the load for the bridge and a control circuit (T1, T2) alternately switching one diagonal voltage on and switching the other off. Both diagonals of the full bridge circuit have a regulated constant current source (OP1, OP2, S2, S4) for regulating the lamp current. Independent claims are also included for the following: a method of controlling the brightness of a gas discharge lamp.

Description

The present invention relates to an electronic ballast having a full bridge circuit for controlling the performance and the brightness of a gas discharge lamp or a method for controlling the brightness of a gas discharge lamp.

Electronic ballasts with full bridge circuits are preferably used for operating high-pressure gas discharge lamps, but are also used for low-pressure discharge lamps or fluorescent tubes. The use of a full bridge circuit offers the possibility of operating the lamps with a - possibly reversed polarity - low-frequency direct current, whereby the generation of interfering alternating electromagnetic fields can be reduced. Further, in this case, the influence of the lamp wiring on the operation resulting from the high-frequency line impedances is negligible. Ballasts with full bridge circuits are for example in the DE 44 01 630 A1 or the AT 392 384 B described.

The basic principle of a full bridge circuit is in Fig. 6 and will be briefly explained below. The full-bridge circuit is formed by four controllable switches S1 to S4, which in the present example are field-effect transistors, wherein the two first switches S1 and S2 form a first half-bridge and the two switches S3 and S4 form a second half-bridge. As a load of the full-bridge circuit, a series resonant circuit consisting of an inductance L and a capacitor C is arranged in its diagonal branch, ie the series connection of the inductance L and the capacitor C connects the common node between the two switches S1 and S2 of the first half-bridge to the common node between the two switches S3 and S4 of the second half-bridge. Parallel to the capacitor C, the gas discharge lamp LA is arranged. The input of the full bridge circuit is supplied with a DC voltage U _BUS , the output of the full bridge circuit is connected via a resistor R to ground.

The driving of the four switches S1 to S4 is effected by two driver circuits T1 and T2, which in turn are transmitted from a control circuit 6, the corresponding control commands for driving the switches S 1 to S4. The four switches S 1 to S 4 are generally activated in the following way: First, the switches S 1 and S 4 forming a first bridge diagonal are activated in a first phase, while the two switches S 3 and S 2 forming the second bridge diagonal are opened. In this first phase, a current flows from the input of the full-bridge circuit via the first Switch, consisting of the series resonant circuit and the gas discharge lamp LA load circuit and the switch S4. In this case, one of the two switches, for example, the switch S1 is permanently closed, while the switch S4 is clocked high-frequency. At a constant switching frequency of the switch S4, the power supplied to the lamp LA is increased or reduced by changing the duty cycle. In a second phase then the switches S1 and S4 of the first bridge diagonal are opened, while now the switches S3 and S2 of the second bridge diagonal are activated in an analogous manner, ie the switch S3 is permanently closed, while the switch S2 with one of the desired power corresponding Duty cycle clocked high frequency. The change between the two bridge diagonals causes the direction of the current through the lamp LA to change permanently, thereby avoiding mercury deposits on an electrode and increasing the life of the lamp. The control of the full-bridge circuit is taken over by the control circuit 6, which, on the one hand, supplies the setpoint value I _SOLL corresponding to the desired lamp _brightness and, on the other hand, the voltage _dropped across the shunt resistor R via the input line 7 as the actual value. According to the comparison result between the actual value and setpoint, the control circuit 6 generates control commands which are supplied via the lines 8 ₁ to 8 ₄ the two driver circuits T1 and T2, which in turn convert the control commands into corresponding signals for driving the gates of the four field effect transistors S 1 to S4 ,

The clocked switch of the active bridge diagonal is opened and closed with a frequency of approx. 20 to 50 kHz. Due to this high frequency clocking, parasitic currents flow across the lamp line capacitances, which make precise control of the lamp brightness impossible, especially at very low dimming levels, with the result that at very low dimming levels an undesirable, noticeable flickering of the lamp brightness occurs to the eye.

Another circuit for driving is from the EP 0 633 711 A1 known. This circuit in turn has a full bridge circuit to which the lamp is switched to load. However, the control of the lamp brightness is now not effected by a variation of the drive frequency, but by controlling the current supplied to the full-bridge circuit. For this purpose, a controllable constant current source in the form of a transistor is arranged at the input of the full bridge circuit, which is controlled by a control circuit in a suitable manner. In this case, the switching elements of the full-bridge circuit are only activated in such a way that a regular change in direction of the current flow takes place.

In this known circuit arrangement, although the current supplied to the full-bridge circuit can be regulated down to very low values, since the constant-current source is arranged at the input of the full-bridge circuit, the accuracy in regulating the lamp current itself is limited. Furthermore, in this known variant, relatively high leakage currents can occur at the transistor used as a constant current source.

Wood 'High Frequency Discharge Lamp Ballasts using Power Mosfets, IGBT's and High Voltage Monolithic Drivers' PCI Proceedings, June 1989, pp. 307-324, XP000775812 describes the advantageous use of MOSFETs for electronic ballasts.

US-A-4,346,332 describes an adjustable frequency AC voltage source for operating a gas discharge lamp.

EP-A-0 473 157 describes the reversal of the lamp current for a low-pressure gas discharge lamp at regular intervals.

It is therefore an object of the present invention to provide an electronic ballast with a full bridge circuit, which allows dimming of the gas discharge lamp over a very wide range. Furthermore, flicker phenomena should be avoided as far as possible at very low dimming values and power losses.

The object is achieved by a method for controlling the brightness of a gas discharge lamp according to claim 1 and by an electronic ballast having the features of claim 5.

The electronic ballast according to the invention has a full bridge circuit fed with a direct current voltage, the gas discharge lamp being part of a resonant circuit connected as a load of the full bridge circuit. A control circuit alternately turns on one bridge diagonal of the full bridge circuit and the other bridge diagonal on. According to the invention it is proposed that the two bridge diagonals each have a controllable constant current source for controlling the lamp current. In a first operating mode, which is used at low lamp brightness, then the regulation of the lamp current by the two controllable constant current sources of the bridge diagonal, wherein the inductance of the load circuit is not effective in this case due to the direct current, but only the ohmic DC resistance. In a second operating mode, however, at high lamp brightness, the control of the power supplied to the lamp is carried out in a known manner by a change in the duty cycle at a constant high frequency.

According to the solution according to the invention, high-frequency clocking of a switch is thus dispensed with at low lamp brightness during the turn-on time of a bridge diagonal. Instead, during the turn-on time of a bridge diagonal, the lamp is operated with a regulated DC current, thereby avoiding the problem of parasitic currents due to the high-frequency switching operations. This ensures that even with very low brightness values can be controlled very accurately to a constant lamp current and thus flicker of the lamp is suppressed. The low frequency switching between the two Brückendiagonalen is maintained, however, and preferably takes place with a frequency of more than 100 Hz, ie with a frequency above the detection threshold of the human eye, in particular with a frequency between 700 Hz and 2000 Hz. In addition, there is the possibility of a lamp operation at very low To forego brightness switching between the two bridge diagonals, since the mercury migration caused by the small lamp current is minimal and is compensated by the natural diffusion occurring in the lamp plasma. At high lamp brightness, however, the regulation of the lamp power takes place in a known manner, which has the advantage that leakage currents that would otherwise occur at the controllable constant current sources at high lamp power can be avoided.

According to the method according to the invention, two operating modes are used to control the brightness of a gas discharge lamp, wherein the gas discharge lamp is operated in the first operating mode at low lamp brightness with a regulated DC voltage and in a second operating mode at high lamp brightness with a direct current corresponding to the duty cycle with superimposed ripple current.

• Fig. 1 ein Ausführungsbeispiel einer Vollbrückenschaltung, die nicht Gegenstand der Ansprüche ist;
• Fig. 2 ein Blockschaltbild eines ersten Vorschaltgerätes, bei dem die in Fig. 1 dargestellte Vollbrückenschaltung zur Anwendung kommt;
• Fig. 3 ein Blockschaltbild eines zweiten Vorschaltgeräts, bei dem die in Fig. 1 dargestellte Vollbrückenschaltung zur Anwendung kommt;
• Fig. 4 ein Ausführungsbeispiel einer erfindungsgemäßen Vollbrückenschaltung;
• Fig. 5 ein Blockschaltbild eines elektronischen Vorschaltgerätes, bei dem die in Fig. 4 dargestellte Vollbrückenschaltung zur Anwendung kommt; und
• Fig. 6 eine bekannte Vollbrückenschaltung.

In the following the invention will be explained in more detail with reference to the accompanying drawings. Show it:

• Fig. 1 an embodiment of a full bridge circuit, which is not the subject of the claims;
• Fig. 2 a block diagram of a first ballast, in which the in Fig. 1 shown full bridge circuit is used;
• Fig. 3 a block diagram of a second ballast, in which the in Fig. 1 shown full bridge circuit is used;
• Fig. 4 an embodiment of a full bridge circuit according to the invention;
• Fig. 5 a block diagram of an electronic ballast, in which the in Fig. 4 shown full bridge circuit is used; and
• Fig. 6 a known full bridge circuit.

The arrangement of the four field effect transistors S1 to S4 of in Fig. 1 shown full bridge is identical to the known arrangement Fig. 6 , Again, a DC voltage U _{BUS is} applied to the input of the full-bridge circuit, the output of the full-bridge circuit forms a shunt resistor R connected to ground. However, only the gas discharge lamp LA is connected as load, the elements of a resonant circuit are no longer present in the first exemplary embodiment , Switching between the two bridge diagonals is again effected by the two driver circuits T1 and T2, which drive the four field-effect transistors S1 to S4 in a suitable manner. However, the regulation of the lamp brightness is no longer effected by a corresponding switching on and off of the switches S1 to S4 by the driver circuits T1 and T2, but by driving the arranged in the bridge diagonal field effect transistors S2 and S4 as controllable constant current sources. For this purpose, these two field-effect transistors S2, S4 are each operated by an operational amplifier OP1 or OP2 in their modulation range. They thus form a resistor which is connected in series with the lamp LA and defines in this way an operating point for the lamp LA.

The controllable constant current sources, or the two transistor precision current sources, are therefore formed by the two lower field effect transistors S2 and S4 of the two half bridges and the two operational amplifiers OP1 and OP2, each controlling the respective field effect transistors S2 and S4. The current flowing through the respective field effect transistor S2 or S4 is supplied to the operational amplifier OP1, OP2 as an actual value via a feedback line 9 ₁ or 9 ₂ , the second input signal forms a setpoint value I _SOLL corresponding to the desired lamp brightness, which for example corresponds to the two operational amplifiers OP1, OP2 can be supplied by a dimming circuit or the like. The two operational amplifiers OP1 and OP2 act as regulators which set the current flowing through the two field effect transistors S2 and S4, respectively, to a value corresponding to the desired value I _SOLL .

The two driver circuits T1 and T2, the control commands required for switching between the two bridge diagonals are fed in the usual way by a (not shown) control circuit. Here too, a low-frequency change takes place between the two bridge diagonals in order to reduce the mercury migration in the lamp LA resulting in a one-sided DC operation.

Since the regulation of the lamp current and thus the lamp brightness is carried out by the two controllable constant current sources, can be dispensed with the use of a current-limiting inductance. However, in order to keep the power losses at the two field effect transistors S2 and S4 of the two controllable constant current sources as low as possible, the voltage dropping across them should be relatively low. At the same time, however, it should have a certain minimum value in order to ensure that the two field-effect transistors S2 and S4 are operated in their linear region so as to enable effective regulation of the current.

This can be achieved in that the full bridge circuit, a DC voltage U _{BUS is} supplied, which is only slightly higher than the voltage falling across the gas discharge lamp LA, since the excess of the DC voltage U _BUS inevitably drops at the two transistors S2 and S4. For this reason, the ballast further comprises a control circuit 1, which is supplied via the two input lines 10 ₁ and 10 _2, the voltage drop across the field effect transistor S2 or S4 of the active bridge diagonal voltage as an actual value. This actual value is compared with a setpoint I _FETsoll , which corresponds to the value that allows a particularly effective current control. On the basis of this comparison, the control circuit 1 generates a control signal, which is used to control the DC voltage U _BUS .

This is in Fig. 2 shown, which shows the block diagram of a ballast. The input of the ballast forms a connected to an AC voltage source rectifier circuit 11, for example, a full-bridge rectifier, which supplies a rectified AC voltage U ₀ to a first switching regulator 3. This first switching regulator 3 is formed by a step-up converter, which generates a high intermediate circuit voltage U _Z , which is supplied to a second switching regulator 4. This second switching regulator 4 is a buck converter, which reduces the high intermediate circuit voltage U _Z to the required lower value for the DC voltage U _BUS . The reference numeral 2 is the in Fig. 1 illustrated full bridge circuit referred to.

As in Fig. 2 1, controls the control circuit 1 to the buck converter 4, in such a way that this generates a DC voltage U _BUS , which as planned is just above the lamp voltage LA, so that the falling across the two transistors S2 and S4 Voltage corresponds to the setpoint U _FETsoll . Alternatively, it would also be possible to measure the voltage drop across the gas discharge lamp LA and to generate based on this value, a control signal for driving the buck converter.

Another option is in Fig. 3 shown. Here, the smoothing circuit for generating the DC voltage U _{BUS is} not generated by two series-connected switching regulator, but by a buck-boost converter 5, in which the functions of in Fig. 2 Switching regulators 3 and 4 are combined in a circuit. This integration is possible because the demands on the control speed of the smoothing circuit are relatively low and thus is not to fear the generation of harmonics at the input of the ballast due to rapid changes in frequency and / or duty cycle.

The regulation of the lamp current through the two controllable constant current sources in addition to the suppression of flickering also means that when switching the lamp LA at low lamp brightness no flash can occur because the current limited due to the two controllable constant current sources from the beginning to the desired value is. Thus, the lamp LA is ignited at a current which has the lowest possible value for the initiation of the ignition process. To provide the ignition voltage required for this purpose, the buck converter 4 or the buck-boost converter is driven in such a way that it provides a maximum output voltage which is sufficient for the ignition. Another possibility is to use an ignition coil. With the electronic ballast according to the invention, it is possible to dim and ignite the gas discharge lamp to 1/1000 of its maximum brightness, without causing a Flackererscheinung or a switch-on occurs. A further advantage is that the lamp wiring has no influence on the dimming operation. This is because, as is still switched with a low frequency, but is dispensed with the high-frequency clocking of switches and thus by this "quasi-DC" no influence of the wiring impedances. The low frequency reverse pole frequency, i. the change between the two bridge diagonals should be at least slightly above the frequency that is still perceived by the eye, ie at least above 100 Hz. Particularly advantageously, a frequency between 700 Hz and 2000 Hz is selected.

An embodiment of the full bridge circuit according to the invention is in Fig. 4 shown. This differs, on the one hand, in that the gas discharge lamp LA is now in turn part of an existing of an inductance L and a capacitor C resonant circuit, which is connected as a load of the full bridge circuit, and the other in that on the in Fig. 1 described regulator 1 for controlling the DC voltage U _{BUS is} omitted. In this case, the full bridge circuit 2 is supplied with a constant DC voltage U _BUS , as shown schematically in FIG Fig. 5 is shown. This in this FIG. 5 illustrated electronic ballast now has the rectifier circuit, a boost converter 3 and the full bridge circuit 2.

As well as in Fig. 1 are at the in Fig. 4 illustrated full bridge circuit, the two of the operational amplifiers OP1 and OP2 and the associated field effect transistors S2 and S4 existing controllable constant current sources provided. Due to the constant DC voltage U _BUS , however, there is now the danger that at high lamp currents, ie at high brightness, the power loss resulting from the two transistors S2 and S4 increases to an impermissible level.

To avoid this, therefore, in the in Fig. 4 illustrated embodiment, depending on the lamp brightness to be achieved between two different operating modes, wherein in the range of low lamp brightness, the driving of the lamp LA in the same way as in Fig. 1 takes place, ie during the switch-on of one of the two bridge diagonals of the lamp is controlled by the corresponding controllable constant current source direct current. Due to the low currents at these brightness levels, the power losses occurring at the two transistors S2 and S4 play only a minor role, so that the waiver of the regulation of the DC voltage U _BUS can be accepted.

In a lamp operation with high brightness, however, the function of the two controllable constant current sources is suppressed and the four transistors S1 to S4 are as well as in the Fig. 6 triggered known methods shown. That is, with a relatively low frequency is changed between the two bridge diagonals, being clocked at high frequency during the turn-on of a bridge diagonal of one of the two transistors, so that the lamp with a direct current, which is superimposed on a high-frequency ripple current is operated. In order to achieve a brightness control in this mode, a drive with variable duty cycle is necessary, the inductance L forms in this mode, the current-limiting impedance in series with the lamp. In this second operating mode, the control circuit 6 is again responsible for the control of the lamp brightness and transmits via the lines 8 ₁ to 8 ₄ the corresponding control commands to the driver circuits T1 and T2, which accordingly drive the four transistors S1 to S4.

In the case of the high brightness values of the second operating mode, the line capacitances and line inductances do not play despite the high switching frequency Role because they are negligible relative to the lamp current and therefore do not interfere with the control operations. The danger of the appearance of flickering is not given at these high brightness. At low brightness values, in turn, there is the ideal ignition behavior due to the current regulation, with which the occurrence of light flashes is suppressed. Again, dimming up to 1/1000 of the maximum lamp brightness is possible.

The inventive concept is thus characterized by the fact that a lamp operation is realized, with which a dimming over a very wide range of brightness is made possible. In addition, there is the possibility to start the lamp even at very low brightness levels, without causing uncomfortable perceived flashes of light.

Claims (8)

1. A method for the control of the brightness of a gas discharge lamp (LA), which is a component of a resonant circuit (L, C) connected as a load of a full bridge circuit, wherein in each case one bridge diagonal is switched on and the other bridge diagonal of the full bridge is switched off, alternatingly,
   characterised in that
   the regulation of the power of the gas discharge lamp (LA) is effected during the switch-on time of one bridge diagonal
   in a first operational mode at low lamp brightness by means of constantly regulated direct current through the gas discharge lamp, and
   in a second operational mode at high lamp brightness by setting the duty ratio of a high-frequency current portion through the gas discharge lamp by clocking a switch of the full bridge circuit at high frequency.
2. A method according to claim 1,
   characterised in that
   the change-over between the two bridge diagonals is effected with a frequency of more than 100 Hz.
3. A method according to claim 2,
   characterised in that
   the change-over between the two bridge diagonals is effected with a frequency between 700 Hz and 2000 Hz.
4. A method according to one of claims 1 to 3,
   characterised in that
   in the case of a lamp operation at low brightness, only one single bridge diagonal is switched on.
5. An electronic ballast for the control of the operating behaviour and the brightness of a gas discharge lamp (LA), which is a component of a resonant circuit (L, C) connected as a load of a full bridge circuit, wherein a control circuit in each case switches on one bridge diagonal and switches off the other bridge diagonal of the full bridge, alternatingly,
   characterised in that
   the control circuit is designed to execute the regulation of the power of the gas discharge lamp (LA) during the switch-on time of one bridge diagonal
   in a first operational mode at low lamp brightness by regulating a direct current by a constant current source of the switched-on bridge diagonal or by transistors in their dynamic range, and
   in a second operational mode at high lamp brightness by setting the duty ratio of a high-frequency current portion through the gas discharge lamp by clocking a switch of the full bridge circuit at high frequency.
6. An electronic ballast according to claim 5,
   characterised in that
   the change-over between the two bridge diagonals carried out by the control circuit (T1, T2) is effected with a frequency of more than 100 Hz.
7. An electronic ballast according to claim 6,
   characterised in that
   the change-over between the two bridge diagonals carried out by the control circuit (T1, T2) is effected with a frequency between 700 Hz and 2000 Hz.
8. An electronic ballast according to one of claims 5 to 7,
   characterised in that
   in the case of a lamp operation at low brightness, only one single bridge diagonal is switched on.

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