EP2208401B1 - Electronic ballast and method for operating at least one first and second discharge lamp - Google Patents

Abstract

An electronic ballast for operating at least two discharge lamps is provided, which may include a starting detection apparatus for detecting whether one of the discharge lamps has been started; wherein a drive circuit includes an input, which is coupled to a starting detection apparatus, wherein a threshold value entry apparatus is designed to enter a second threshold value for a setpoint value of a total current through at least two inductances, wherein the second threshold value has a smaller magnitude than a first threshold value; and wherein the drive circuit is designed to drive a first electronic switch and a second electronic switch taking into consideration the first threshold value as setpoint value prior to detection of the starting of a discharge lamp by the starting detection apparatus, and taking into consideration the second threshold value as setpoint value.

Description

Technical area

The present invention relates to an electronic ballast for operating at least a first and a second discharge lamp with a bridge circuit comprising at least a first and a second electronic switch, wherein the input of the bridge circuit is coupled to a DC supply voltage, wherein the output of the bridge circuit with a first terminal for a first discharge lamp and a second terminal for a second discharge lamp, wherein the first and second terminals are connected in parallel with respect to the output of the bridge circuit, at least one first and one second ignitors, one in series with the other Discharge lamp arranged inductance and arranged parallel to the respective discharge lamp capacitance comprises, a current measuring device which is designed to provide at its output a signal with the actual value of the total current through the least ns first and second inductance is correlated, and a drive circuit. The latter has at least a first and a second output for driving at least the first and the second electronic switch, a first input, which is coupled to the output of the current measuring device, a Schwellwertvorgabevorrichtung for specifying at least a first threshold value for the total current through at least the first and the second inductance, wherein the drive circuit designed is to control at least the first and the second electronic switch, taking into account the first SChwellwerts as a setpoint. It also relates to a method for operating at least a first and a second discharge lamp on such an electronic ballast.

State of the art

The present invention relates in particular to the problem that high and low pressure discharge lamps for ignition require high voltages. These voltages are generated by electronic ballasts using a series resonant circuit. In two- or multi-lamp electronic ballasts often two or more parallel load circuits are used. In the case of two-lamp electronic ballasts, by means of which the following invention is described by way of example, therefore, the bridge circuit must be designed for twice the current as in the operation of a single discharge lamp. Usually, both load circuits do not have exactly the same resonant frequency. This can cause one discharge lamp to fire earlier than the other. If this happens, the current in the load circuit with the ignited discharge lamp becomes significantly smaller. However, an ignition controller provided in the electronic ballast still allows twice the current compared to the ignition of a discharge lamp, this current then being largely due to the unloaded, ignited load circuit. flows. As a result, significantly higher ignition voltages are generated in this open circuit than desired.

An approach to get this problem largely under control, is known from the EP 1 337 133 A2 , This application relates to an operating circuit for a low-pressure gas discharge lamp, in which a digital control is designed so that it triggers shutdowns of a Sicherheitsabschalteinrichtung for excessively large currents by gradually lowering the operating frequency in the ignition, then again to increase the operating frequency in a new attempt at ignition again. In this way, it is attempted to achieve an overall ignition by repeated pulsed ignitions to shutdowns or by a continuous ignition at a minimum frequency at which no shutdown occurs.

The problem with this solution, however, is that there only the power is controlled and a shutdown is triggered when this power is too high. Thereafter, the ignition is repeated. If this system is used in two-lamp electronic ballasts, the above-mentioned problems arise. To prevent this, the load circuits and in particular the dielectric strength of the components, such as chokes, capacitors, diodes, have been selected correspondingly high. This is associated with high costs and therefore undesirable.

Presentation of the invention

The present invention is therefore the object of developing an aforementioned electronic ballast or an aforementioned method such that a dimensioning of the components involved for lower voltages is possible.

This object is achieved by an electronic ballast having the features of claim 1 and by a method having the features of claim 8.

The present invention is based on the finding that this object can be achieved if the electronic ballast further comprises an ignition detection device for detecting whether one of the discharge lamps has ignited. In this case, the drive circuit comprises a second input which is coupled to the ignition detection device, wherein the threshold value setting device is further designed to specify a second threshold value for the setpoint value of the total current through at least the first and the second inductance, wherein the second threshold value is smaller in magnitude than the first one threshold. Furthermore, the drive circuit is configured to drive at least the first and second switches before detecting the ignition of a discharge lamp by the ignition detection device as the target value after detecting the ignition of a discharge lamp by the ignition detection device in consideration of the second threshold value.

As a result of this measure, a lower ignition voltage arises at the remaining undamped load circuit, in which the lamp has not yet ignited, than in the case of the procedure according to the prior art. As a result, undesirably high ignition voltages are avoided, the components involved can be designed for lower voltages. This is true not only for the components of the resonant circuit, but also for other elements, for example the elements that are used for spiral monitoring.

A preferred embodiment is characterized in that the drive circuit further comprises a comparator which is designed to compare the actual value of the total current through at least the first and the second inductance with the respective desired value of this current. As drive circuit, which includes a comparator, for example, the microcontroller AT90PWM2 the company Atmel can be used. By using a comparator, compliance with the setpoint can be monitored very closely. This makes it possible to realize a precise, cost-effective dimensioning of the components involved.

Preferably, the ignition detection device comprises a power measurement device configured to determine a magnitude correlated with the power output at the at least first and second ports. In contrast to the ignition detection by evaluation of the current through the bridge circuit, as it is for example in the EP 1 337 133 A2 the measurement of power provides a more reliable indication of whether one of the involved lamps has ignited or not.

Further preferably, the drive circuit is designed, the actual value of the total current by varying the frequency of the signal, with which at least the first and the second electronic. Switches are controlled to regulate. In this case, the drive circuit is furthermore preferably designed to control the frequency of the signal with which at least the first and the second electronic switches are actuated be further lowered after detecting the ignition of a discharge lamp. This procedure is described in the applications < WO 2008/037290 A1 and EP1871147 A1 ,

The drive circuit of an electronic ballast according to the invention is in particular designed so that a changeover from the first to the second threshold value in a period of time which is a maximum of 1 ms, preferably a maximum of 200 μs, can be made. The wording "taking into account the first or second threshold" takes into account the fact that in practice a slight exceeding of the respective threshold value may occur, since only the current through the lower electronic switch when the bridge circuit is implemented by a half bridge is evaluated for the current measurement becomes. In particular, it must also be taken into consideration that a reset of the drive by the drive circuit can be realized very quickly when the threshold value is reached, but in practice it nevertheless involves a certain finite time span, so that after determining that the threshold value has been reached, it is still necessary Further, slight increase in current is possible before the drive circuit for a new ignition or ignition of the second discharge lamp to a predetermined value, in particular frequency value, is reset. Nevertheless, with regard to a fairly low expected overrun the threshold value the interpretation of the components involved quite accurate and therefore cost-effective.

In a preferred embodiment, the second threshold value is at most 80%, preferably at most 75% of the first threshold value. As a result, on the one hand unacceptably high currents are avoided, on the other hand enables the fastest possible ignition of the not yet fired load circuit.

Further advantageous embodiments will become apparent from the dependent claims.

The preferred embodiments presented with respect to an electronic ballast according to the invention and the advantages thereof also apply, as far as applicable, to a method according to the invention.

Short description of the drawing (s)

In the following, an embodiment of an electronic ballast according to the invention will be described in more detail with reference to the accompanying drawings. This shows a schematic representation of an embodiment of an electronic ballast according to the invention.

Preferred embodiment of the invention

Fig. 1 shows a schematic representation of the structure of an electronic ballast according to the invention for operating a first La1 and a second discharge lamp La2. On the input side, the ballast comprises a first E1 and a second terminal E2 for connection to a mains voltage U _N. This is followed by a block 10, which includes a rectifier and components for EMC protection. This is followed by a boost converter 12, which comprises an inductance L1, a switch S1, a diode D1 and a capacitor C1. At its output, the so-called intermediate _circuit voltage U _{zw is provided} on the one hand to a voltage divider R1, R2, on the other hand to the two switches S2, S3 of a half-bridge. The signal picked up at the resistor R2 is supplied to a unit 14 of a microcontroller 16, the unit 14 serving for PFC (Power Factor Correction). For this purpose, in particular the switch S1 of the boost converter 12 is actuated in a manner known to the person skilled in the art with regard to the magnitude of the voltage U _zw sensed by the resistor R2.

The switches S2, S3 of the half-bridge are controlled via a drive generator 18 of the microcontroller 16. The half-bridge center HB is coupled via a coupling capacitor C2 to a first and a second load circuit. The first load circuit comprises the already mentioned lamp La1 and a series resonant circuit comprising an inductance L2 and a capacitor C3. The second load circuit comprises the lamp La2 and a series resonant circuit comprising an inductor L3 and a capacitor C4.

A shunt resistor R3 is used to measure the current through the lower switch S3 of the half-bridge. For this purpose, the voltage drop across the resistor R3 is transmitted via an ohmic resistor R4 and a capacitor C4, which serve to averaging, to a device 20 for power measurement. It is used to determine the performance in addition to the current, the known intermediate _{circuit voltage} U _zw taken into account.

The device 20 for power measurement is coupled on the output side to a control circuit 22, which fulfills two functions. It comprises a threshold value setting device which predefines a threshold value for the set value of the total current through the first L2 and the second inductance L3. This threshold value varies with regard to whether neither of the two lamps La1, La2 has yet ignited or whether one of the two lamps La1, La2 has already ignited. In the event that one of the lamps La1, La2 has already ignited, the threshold value predefined by the control circuit 22 is significantly smaller than in the case in which neither of the two lamps La1, La2 has yet fired.

The respective setpoint value specified threshold value is compared with the current actual value in a comparator 24 of the microcontroller 22.

The control circuit 22 also serves to predefine the drive generator 18 with a frequency with which the switches S2, S3 of the bridge circuit are to be controlled. The control circuit 22 is in particular designed to further lower the frequency of the signal with which the switches S2, S3 are actuated, after detection of the ignition of one of the discharge lamps La1, La2.

For operation: The drive generator 18 controls the switches S2, S3 starting with a start frequency. This is gradually lowered, wherein continuously compared via the comparator 24 of the measured via the shunt resistor R3 current to a first threshold becomes. If this threshold value is reached, the ignition process is aborted and started again at the starting frequency. If, however, the lowering of the frequency leads to the ignition of one of the two discharge lamps La1, La2, this is detected by the device 20 for power measurement, whereupon the control circuit 22 provides the comparator 24 with an absolute smaller threshold value. If a further lowering of the frequency with which the drive generator 18 applies the switches S2, S3 to ignite the second discharge lamp, the ignition process is ended. If, however, the lowering of the frequency does not lead to the ignition of the second discharge lamp, the frequency is lowered until the comparator 24 determines that the second, smaller magnitude threshold has been reached.

If the second discharge lamp does not then ignite within a time of preferably 100 ms, the electronic ballast switches off. A network reset can be used to start a new ignition attempt.

Claims (8)

1. Electronic ballast for operating at least a first discharge lamp (La1) and a second discharge lamp (La2) with

   - a bridge circuit (S2, S3), which comprises at least a first electronic switch (S2) and a second electronic switch (S3), wherein the input at the bridge circuit (S2, S3) is coupled to a DC supply voltage (U_ZW), wherein the output of the bridge circuit (S2, S3) is coupled to a first terminal for a first discharge lamp (La1) and a second terminal for a second discharge lamp (La2), wherein the first terminal and the second terminal are connected in parallel with respect to the output (HB) of the bridge circuit (S2, S3);

   - at least a first starting apparatus (L2, C3) and a second starting apparatus (L3, C4), which each comprise an inductance (L2; L3), which is arranged in series with the respective discharge lamp (La1; La2), and a capacitance (C3; C4), which is arranged in parallel with the respective discharge lamp (La1; La2);

   - a current measuring apparatus (R3), which is designed to provide a signal at its output, said signal being correlated with the actual value of the total current through the at least first inductance (L2) and second inductance (L3); and

   - a drive circuit (18, 22) with

   - at least a first output and a second output for driving at least the first electronic switch (S2) and the second electronic switch (S3),

   - a first input, which is coupled to the output of the current measuring apparatus (R3);

   - a threshold value entry apparatus for entering at least a first threshold value for the setpoint value of the total current through at least the first inductance (L2) and the second inductance (L3);

   - wherein the drive circuit (18, 22) is designed to drive at least the first electronic switch (S2) and the second electronic switch (S3) taking into consideration the first threshold value as setpoint value;

   characterized
   in that the electronic ballast furthermore comprises:

   - a starting detection apparatus (20) for detecting whether one of the discharge lamps (La1; La2) has been started;
   wherein the drive circuit (18, 22) comprises a second input, which is coupled to the starting detection apparatus (20),
   wherein the threshold value entry apparatus is furthermore designed to enter a second threshold value for the setpoint value of the total current through at least the first inductance (L2) and the second inductance (L3),
   wherein the second threshold value has a smaller magnitude than the first threshold value; and
   wherein the drive circuit (18, 22) is designed to drive at least the first electronic switch and the second electronic switch taking into consideration the first threshold value as setpoint value prior to detection of the starting of a discharge lamp (La1; La2) by the starting detection apparatus (20), and taking into consideration the second threshold value as setpoint value after detection of the starting of a discharge lamp (La1; La2) by the starting detection apparatus (20).
2. Electronic ballast according to Claim 1,
   characterized
   in that the drive circuit (18, 22, 24) furthermore comprises a comparator (24), which is designed to compare the actual value of the total current through at least the first inductance (L1) and the second inductance (L2) with the respective setpoint value of this current.
3. Electronic ballast according to either of Claims 1 and 2,
   characterized
   in that the starting detection apparatus (20) comprises an apparatus (20) for power measurement, which is designed to determine a variable which has been correlated with the power output at the at least first terminal (E1) and second terminal (E2).
4. Electronic ballast according to one of the preceding claims,
   characterized
   in that the drive circuit (18, 22, 24) is designed to regulate the actual value of the total current by varying the frequency of the signal, with which at least the first electronic switch (S2) and the second electronic switch (S3) are driven.
5. Electronic ballast according to Claim 4,
   characterized
   in that the drive circuit is designed to further reduce the frequency of the signal, with which at least the first electronic switch (S2) and the second electronic switch (S3) are driven, after detection of the starting of a discharge lamp (La1; La2).
6. Electronic ballast according to one of the preceding claims,
   characterized
   in that the drive circuit (18, 22, 24) is designed to perform a switchover from the first to the second threshold value over a period of time which is at most 1 ms, preferably at most 200 µs .
7. Electronic ballast according to one of the preceding claims,
   characterized
   in that the second threshold value is at most 80%, preferably at most 75%, of the first threshold value.
8. Method for operating at least a first discharge lamp (La1) and a second discharge lamp (La2) using an electronic ballast with a bridge circuit (S2, S3), which comprises at least a first electronic switch (S2) and a second electronic switch (S3), wherein the input of the bridge circuit (S2, S3) is coupled to a DC supply voltage (U_ZW), wherein the output of the bridge circuit (S2, S3) is coupled to a first terminal (E1) for a first discharge lamp (La1) and to a second terminal (E2) for a second discharge lamp (La2), wherein the first terminal (E1) and the second terminal (E2) are connected in parallel with respect to the output of the bridge circuit (S2, S3); at least a first starting apparatus (L2, C3) and a second starting apparatus (L3, C4), which each comprise an inductance, which is arranged in series with the respective discharge lamp (La1; La2), and a capacitance, which is arranged in parallel with the respective discharge lamp (La1; La2); a current measuring apparatus (R3), which is designed to provide a signal at its output, said signal having been correlated with the actual value of the total current through the at least first inductance (L1) and second inductance (L2); and a drive circuit (18, 22, 24) with at least a first output and a second output for driving at least the first electronic switch (S2) and the second electronic switch (S3), a first input, which is coupled to the output of the current measuring apparatus (R3); a threshold value entry apparatus for entering at least a first threshold value for the setpoint value of the total current through at least the first inductance (L1) and the second inductance (L2); wherein the drive circuit (18, 22, 24) is designed to drive at least the first electronic switch (S2) and the second electronic switch (S3) taking into consideration the first threshold value as setpoint value;
   characterized by the following steps:

   a) beginning the driving of at least the first electronic switch and the second electronic switch taking into consideration the first threshold value as setpoint value;

   b) continuous detection to ascertain whether one of the discharge lamps (La1; La2) has started; and

   b) after detection of the starting of a discharge lamp (La1; La2): driving at least the first electronic switch and the second electronic switch taking into consideration a second threshold value as setpoint value, wherein the second threshold value has a smaller magnitude than the first threshold value.

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