EP1519638B1 - Method for operating a low pressure discharge lamp - Google Patents

Abstract

The operating method has the lamp electrodes (E1,E2) of the gas discharge lamp (LP) supplied with a heating current before ignition of the lamp via a transformer (P1,S1,S2) with a controlled switch (T3) connected in its primary circuit, with monitoring of the electrical resistance variation of at least one lamp electrode via a resistance element (R1) on the primary side of the transformer.

Description

The invention relates to a method for operating at least one low-pressure discharge lamp according to the preamble of patent claim 1.

I. State of the art

The publication WO 00/72640 A1 describes a circuit arrangement and a method for operating a low-pressure discharge lamp by means of a half-bridge inverter, wherein the lamp electrodes of the at least one low-pressure discharge lamp during a heating phase before igniting the gas discharge in the at least one low-pressure discharge lamp by means of a transformer whose primary-side current by means of a controllable switching means is clocked, are acted upon by a heating current and the change in the electrical resistance of at least one lamp electrode is monitored in order to recognize the connected to the operating device type of low-pressure discharge lamp. The change in the electrical resistance of the lamp electrode is monitored by means of an ohmic resistor, which is arranged on the secondary side of the transformer.

Publication WO 00/72642 A1 discloses an electronic ballast for operating at least one low-pressure discharge lamp on an inverter with a connected load circuit which contains the lamp and a series resonant circuit and an evaluation circuit which reacts to different operating states of the lamp and in the event of a defect or removal of the lamp Generates signals that are used to switch off the inverter.

11. Presentation of the invention

It is the object of the invention to provide a simplified method for detecting the type of low-pressure discharge lamp connected to the operating device.

This object is achieved by the features of claim 1. Particularly advantageous embodiments of the invention are described in the dependent claims.

The inventive method for operating at least one low-pressure discharge lamp by means of an inverter, wherein the lamp electrodes of the at least low-pressure discharge lamp during a heating phase before the ignition of the gas discharge in the at least one low-pressure discharge lamp by means of a transformer whose primary-side current is clocked by means of a controllable switching means, are acted upon by a heating current and the change in the electrical resistance of at least one lamp electrode is monitored, according to the invention is characterized in that the controllable switching means is switched in synchronism with a first inverter switching means and the change in the electrical resistance of the at least one lamp electrode by means of a resistive element arranged on the primary side of the transformer is determined by selecting the voltage drop across the resistive element at at least two different times during the heating phase is evaluated.

According to the method of the invention, the current through the primary winding of the transformer and not the heating current is evaluated on the secondary side of the transformer for detecting the lamp type during the preheating phase of the lamp electrodes. This can be dispensed with measuring arrangements in the secondary circuits of the transformer and the monitoring device can be simplified accordingly. In addition, the method according to the invention can advantageously be used for the operation of a plurality of low-pressure discharge lamps, since the multi-lamp operation requires no additional measuring devices. The increase in the electrical resistance of the lamp electrodes with increasing heating, regardless of the number of operated in the load circuit low-pressure discharge lamps according to the invention detected solely by means of a resistive element on the primary side of the transformer by the voltage drop across the resistor element is evaluated at least two different times during the heating phase ,

Preferably, the voltage drop across the resistive element is evaluated at a first time in a time window in the range of 10 ms to 50 ms after the beginning of the heating phase is arranged in order to evaluate the cold resistance of the lamp electrodes reliably. In addition, according to the invention, the voltage drop across the resistance element is evaluated at a second time, which is arranged at the end of the heating phase in order to reliably evaluate the heat resistance of the lamp electrodes. From the comparison of these two measurements, it can be determined whether the lamp electrodes were cold at the beginning of the heating phase or whether a replacement resistor was connected instead of the lamp. The lamp type can already be determined from the second measured value alone. According to the invention, a lamp type detection is carried out only when the absolute value of the difference between the two aforementioned measured values exceeds a predetermined size. In the other case, it is assumed that either instead of a low-pressure discharge lamp, a replacement resistor is connected to the operating device or the lamp electrodes were not sufficiently cooled at the beginning of the heating phase since the last lamp operation.

The evaluation of the voltage drop across the resistor element is advantageously carried out by means of a low-pass filter. The low-pass filter averages the voltage drop across the resistive element over a time interval that is long compared to the switching clock of the controllable switching means and the inverter, but short compared to the duration of the heating phase of the lamp electrodes. The duration of the heating phase before the ignition of the gas discharge in the lamp is preferably constant and is about 600 ms, while a switching cycle of the controllable switching means in the heating phase takes about 10 μs.

The energy stored in the primary winding of the transformer is dissipated in an advantageous manner during the turn-off of the controllable switching means by means of a second inverter switching means in order to prevent a voltage overload of the controllable switching means. The energy stored in the primary winding is preferably fed back into the link capacitor, which serves as a DC voltage source for the inverter in order to use it for the lamp operation can. - -

III. DESCRIPTION OF THE PREFERRED EMBODIMENT

Figur 1

Eine schematische Darstellung einer ersten Schaltungsanordnung zur Durchführung des erfindungsgemäßen Verfahrens

Figur 2

Den zeitlichen Verlauf des Spannungsabfalls an dem vom primärseitigen Strom des Transformators durchflossenen Widerstand nach Mittelung durch das Tiefpassfilter für einen ersten Betriebszustand

Figur 3

Den zeitlichen Verlauf des Spannungsabfalls an dem vom primärseitigen Strom des Transformators durchflossenen Widerstand nach Mittelung durch das Tiefpassfilter für einen zweiten Betriebszustand

Figur 4

Den zeitlichen Verlauf des Spannungsabfalls an dem vom primärseitigen Strom des Transformators durchflossenen Widerstand nach Mittelung durch das Tiefpassfilter für einen dritten Betriebszustand

Figur 5

Eine schematische Darstellung einer zweiten Schaltungsanordnung zur Durchführung des erfindungsgemäßen Verfahrens

The invention will be explained in more detail below with reference to a preferred embodiment. Show it:

FIG. 1

A schematic representation of a first circuit arrangement for carrying out the method according to the invention

FIG. 2

The time profile of the voltage drop across the resistor through which the primary current of the transformer passes after averaging by the low-pass filter for a first operating state

FIG. 3

The time profile of the voltage drop across the resistance flowing through the primary side current of the transformer after averaging by the low-pass filter for a second operating state

FIG. 4

The time profile of the voltage drop across the resistor through which the primary current of the transformer passes after averaging by the low-pass filter for a third operating state

FIG. 5

A schematic representation of a second circuit arrangement for carrying out the method according to the invention

The circuit arrangement shown in FIG. 1 is an electronic ballast for operating a low-pressure discharge lamp, in particular a fluorescent lamp.

This circuit arrangement has two field effect transistors T1. T2, which are arranged in the manner of a half-bridge inverter. Both field effect transistors receive their control signal from a microcontroller MC. Parallel to the DC voltage input of the half-bridge inverter T1, T2, a DC link capacitor C1 is arranged with a comparatively large capacity. The DC link capacitor C1 serves as a DC voltage source for the half-bridge inverter. At the DC link capacitor C1, a DC voltage of about 400 volts is provided, which is the AC line voltage by means of a mains voltage rectifier (not shown) and a boost converter (not shown) is generated. The DC link capacitor C1 is arranged parallel to the voltage output of the boost converter. Connected to the output M of the half-bridge inverter is a load circuit formed as a series resonant circuit, which consists essentially of the lamp inductor L1 and the ignition capacitor C2. Parallel to the ignition capacitor C2, the discharge path of the fluorescent lamp LP and the coupling capacitor C3 are connected, which is charged during operation of the lamp in the steady state of the half-bridge inverter to half the supply voltage of the half-bridge inverter. The lamp electrodes E1, E2 of the fluorescent lamp LP are formed as electrode filaments each having two electrical terminals. Parallel to the electrode coil E1, E2 is in each case a secondary winding S1, S2 of a transformer connected, which serves for inductive heating of the electrode coils E1, E2. The primary winding P1 of this transformer is connected in series with the switching path of a further field effect transistor T3, whose control electrode is also acted upon by the microcontroller MC with control signals, and a measuring resistor R1. The series circuit of the components P1, T3 and R1 is connected to the output M of the half-bridge inverter. A first terminal of the primary winding P1 is connected to the output M of the half-bridge inverter and to the lamp inductor L1, while the second terminal of the primary winding P1 is connected to the field effect transistor T3 and in the forward direction through a diode D1 with the high potential terminal (+). of the DC link capacitor C1 is connected. A first terminal of the measuring resistor R1 is connected to the ground potential (-), while the second terminal of the measuring resistor is connected to the field effect transistor T3 and via a low-pass filter R2, C4 to the voltage input A of the microcontroller MC.

By means of half-voltage supply of the half-bridge inverter charged coupling capacitor C3 and the alternating switching transistors T1, T2 of the half-bridge inverter, the load circuit L1, C2, LP acted upon in a known manner with a high-frequency AC voltage whose frequency is determined by the switching clock of the transistors T1, T2 and in the range of about 50 kHz up to 150 KHz. Before igniting the gas discharge in the fluorescent lamp LP whose lamp electrodes E1, E2 are inductively acted upon by means of the transformer P1, S1, S2 with a heating current. For this purpose, the transistor T3 is turned on and off by the microcontroller MC in synchronism with the transistor T1. During the turn-on of the transistors T1, T3 therefore flows through the primary winding P1 and the measuring resistor R1, a current. During the turn-off of the transistors T1, T3, the current flow through the measuring resistor R1 is interrupted. The stored in the magnetic field of the primary winding P1 energy is supplied during the turn-off of the transistors T1, T3 and the duty cycle of the transistor T2 via the diode D1 to the DC link capacitor C1. Due to the alternating switching transistors T1, T2 and the synchronous to the transistor T1 switching transistor T3 flows through the primary winding P1, a high-frequency current, which induces corresponding heating currents for the electrode coils E1, E2 in the secondary windings S1, S2. With the aid of the low-pass filter R2, C4, the voltage drop across the measuring resistor R1 is averaged over a time interval of a plurality of switching cycles of the transistor T3 and supplied to the voltage input A of the microcontroller MC. The input voltage at terminal A of the microcontroller MC is converted by means of an analog-to-digital converter into a digital signal and evaluated in the microcontroller MC.

The heating phase of the electrode filaments E1, E2 before the ignition of the gas discharge in the fluorescent lamp LP lasts approximately 600 ms. The microcontroller MC detects the voltage drop across the capacitor C4 of the low pass filter at two different times during the heating phase. The first detection of the voltage drop across the capacitor C4 by the microcontroller MC is performed about 30 ms after the start of the heating phase and the second detection at the end of the heating phase, that is, about 600 ms after the start of the heating phase. If the absolute value of the difference of the two voltage values exceeds a predetermined threshold value of, for example, 0.1 V, the voltage value detected at the end of the heating phase for detecting the lamp type of the fluorescent lamp LP is compared with a reference value stored in the microcontroller MC. If the threshold is not exceeded, there is no evaluation of the voltage drop at Capacitor C4 or on the measuring resistor R1. The time profile of the voltage drop at the measuring resistor R1 or at the capacitor C4 of the low-pass filter is correlated with the time profile of the electrical resistance of the electrode coils E1, E2 during the heating phase. The heat resistance of the electrode coils E1, E2, that is, their resistance at the end of the heating phase is different for different types of fluorescent lamps. Therefore, the heat resistance of the electrode filaments can be used for lamp type detection.

Figures 2 to 4 show the time course of the voltage drop across the current flowing through the primary side current of the transformer P1, S1, S2 resistor R1 after averaging by the low-pass filter R2, C4 for three different operating states of the circuit arrangement according to the preferred embodiment of the invention.

The time course shown in Figure 2 of the voltage drop across the capacitor C4 corresponds to the operation of the circuit arrangement with a fluorescent lamp LP whose electrode coils E1, E2 were cold at the beginning of the heating phase, that is, had room temperature. Therefore, the voltage drop across the capacitor C4 initially increases, reaching a maximum of 0.48 V after about 30 ms, and then steadily decreasing to assume a minimum of 0.22 V at the end of the heating phase after 600 ms. The maximum is correlated with the cold resistance of the electrode filaments E1, E2 and the minimum at the end of the heating phase is correlated with the heat resistance of the electrode filaments E1, E2. The electrical resistance of the existing of tungsten electrode coils E1, E2 is temperature-dependent, that is, it increases with increasing temperature.

FIG. 3 shows the time profile of the voltage drop across the capacitor C4 for the same circuit arrangement and the same fluorescent lamp LP. However, the electrode filaments E1, E2 were not fully cooled at the beginning of the heating phase, due to the last lamp operation. Therefore, the voltage curve shown in Figure 3 has a less pronounced maximum of only 0.27 V at about 30 ms and the minimum of the curve is also reached at the end of the heating phase, but is only 0.20 V.

The time profile of the voltage drop across the capacitor C4 shown in FIG. 4 corresponds to the operation of the above circuit arrangement with an ohmic equivalent resistance instead of the electrode filaments E1 or E2 of the fluorescent lamp LP. The voltage drop across the capacitor C4, apart from the rise during the first approximately 30 ms of the heating phase, is independent of time and is approximately 0.22 V.

The microcontroller MC detects the voltage drop across the capacitor C4 for the first time about 30 ms after the start of the heating phase and the second time about 600 ms after the beginning of the heating phase. If the absolute value of the difference of the two voltage values exceeds a predetermined threshold value of, for example, 0.1 V, the voltage value at the end of the heating phase is compared with a reference value stored in the microcontroller MC and used for lamp type detection. This case is given only in the voltage curve shown in Figure 2. In the other two cases, that is to say, in the case of the voltage profiles shown in FIGS. 3 and 4, no evaluation is carried out with regard to the lamp type detection. In these two cases, the data stored by the last lamp operation in the microcontroller MC are used for the operation of the circuit arrangement or the electronic control gear.

After completion of the preheating phase of the electrode coils E1. E2, the required ignition voltage for igniting the gas discharge in the fluorescent lamp LP is provided to the capacitor C2 by means of the resonance peaking method by reducing the switching frequency of the half-bridge inverter T1, T2 to be close to the resonance frequency of the series resonant circuit L1, C2. After the ignition of the gas discharge in the fluorescent lamp, a brightness control of the fluorescent lamp LP can be performed by varying the switching frequency of the half-bridge inverter T1, T2. During the dimming operation of the fluorescent lamp LP, its electrode filaments E1, E2 are acted upon by means of the transformer P1, S1, S2 and the transistor T3 with a heating current which flows in addition to the discharge current through the electrode filaments E1, E2. The heating current or the heating power is dependent on the brightness of the Fluorescent lamp set. At low brightness, that is, at high dimming of the fluorescent lamp LP high heat output is set. The heating power is adjusted by changing the pulse width of the transistor T3, in particular by changing the duty cycle of the transistor T3. The transistor T3 is turned on in synchronism with the transistor T1. The turn-on of the transistor T3 is at maximum heating power 100% of the turn-on of the transistor T1. With lower heating power, the turn-on of the transistor T3 is shorter than the turn-on of the transistor T1.

FIG. 5 shows a further circuit arrangement which is particularly well suited for the application of the method according to the invention. This circuit arrangement is largely identical to the circuit arrangement shown in FIG. Therefore, in the figures 1 and 5 identical components bear the same reference numerals. In contrast to the circuit arrangement shown in FIG. 1, the circuit arrangement shown in FIG. 5 has two additional diodes D2, D3 which are each connected in series with a secondary winding S1 or S2 and an electrode spiral E1 or E2. The arrangement of the diodes D2, D3 and the winding sense of the transformer windings P1, S1, S2 is coordinated so that the transformer, P1, S1, S2 with the diodes D2, D3 and the transistor T3 form a forward converter. During the conducting phase of the transistor T3, the current through the primary winding P1 in the secondary windings S1, S2 induces a heating current for the electrode filaments E1, E2. During the blocking phase of the transistor T3, the diodes D2, D3 are poled in the reverse direction, so that during the meantime no heating current can flow. The stored energy in the primary winding P1 is dissipated during the conducting phase of the transistor T2 via the diode D1 to the capacitor C1.

The invention is not limited to the embodiment described in more detail above. Instead of evaluating the voltage drop across the resistor R1 during the preheating phase of the electrodes E1, E2 only at the beginning and at the end of the preheating phase, the entire time profile of this voltage drop can be evaluated by means of the microcontroller MC or only the maximum of the voltage drop on the resistor R1 to the final value of this voltage drop at the end of the preheat phase are compared to allow detection of the lamp type of the low-pressure discharge lamp or fluorescent lamp LP.

Claims (5)

1. Method for operating at least one low-pressure discharge lamp by means of an inverter, in which the lamp electrodes (E1, E2) of the at least one low-pressure discharge lamp (LP) have a heating current applied to them during a heating phase prior to the ignition of the gas discharge in the at least one low-pressure discharge lamp (LP) by means of a transformer (P1, S1, S2), whose primary-side current is clocked by means of a controllable switching means (T3), and the change in the electrical resistance of at least one lamp electrode (E1, E2) is monitored by means of a resistive element (R1) which is arranged on the primary side of the transformer (P1, S1, S2) by the voltage drop across the resistive element (R1) being evaluated at at least two different points in time during the heating phase, characterized in that the controllable switching means (T3) is switched in synchrony with a first inverter switching means (T1), and the absolute difference between the voltage drop determined across the resistive element (R1) at a first point in time during the heating phase and the voltage drop determined across the resistive element (R1) at a second point in time which is arranged at the end of the heating phase is compared with a predetermined threshold value, and the voltage drop determined at the second point in time is evaluated for the purpose of identifying the type of lamp if the absolute difference exceeds the predetermined threshold value.
2. Method according to Claim 1, characterized in that the voltage drop across the resistive element (R1) is evaluated by means of a low-pass filter (R2, C4).
3. Method according to Claim 1, characterized in that the energy stored in the primary winding (P1) is dissipated during the switch-off time of the controllable switching means (T3) with the aid of a second inverter switching means (T2) and a diode circuit (D1).
4. Method according to Claim 1, characterized in that the first point in time, at which the voltage drop across the resistive element (R1) is evaluated, is arranged in a time window of 10 ms to 50 ms after the beginning of the heating phase.
5. Method according to Claim 1, characterized in that, once the gas discharge in the at least one low-pressure discharge lamp (LP) has been ignited, the voltage drop across the resistive element (R1) is evaluated for the purpose of regulating the heating power of the lamp electrodes (E1, E2), and the heating power is varied by varying the switch-on time of the controllable switching means (T3), the controllable switching means (T3) being switched on in synchrony with the first inverter switching means (T1), and its switch-on time being less than or equal to the switch-on time of the first inverter switching means (T1).

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