EP0978221B1 - Circuitry for dimming a fluorescent lamp - Google Patents

Description

The invention relates to a circuit arrangement for dimmable Operation of a fluorescent lamp, in particular for use in Motor vehicles as instrument lighting. From a standing start Corresponding circuit arrangements are known in the art, where the fluorescent lamp is operated at an operating frequency becomes. By switching the operating frequency on and off with one device and thus the lamp with one Dimming frequency, which is above the human's visual frequency Eye lies, that is achieved for the human eye The impression arises that the fluorescent lamp is of different brightness, depending on the pulse width of the dimming frequency. Such a circuit arrangement is known from JP-A-06333695. To the lamp current by the fluorescent lamp, it is necessary to either to provide an additional controller or to use an elaborately stabilized resonant circuit. The object of the invention is therefore to provide a simple structure Specify circuitry for dimming a fluorescent lamp.

This object is achieved by a device according to claim 1.

By designing the circuit with a push-pull converter to generate the operating frequency is easy built functional implementation of oscillator and controller reached.

A particularly simple push-pull converter is replaced by one Resonant circuit consisting of a capacitance and an inductor, which is connected to a first pole of the supply voltage. The oscillating circuit is alternately direct or via two switches via a third switch with the second pole of the supply voltage connectable. The two switches are each with one connection the connections of the capacitance and / or the inductance are connected. The With this circuit, fluorescent lamps can either be parallel to the inductance and / or capacitance can be arranged or via a transformer are supplied with the operating frequency, the primary winding of the Transformer advantageously forms the inductance of the resonant circuit.

The use of electronic switches such. B. transistors or Field effect transistors represent an inexpensive solution for the switches.

With a circuit arrangement according to claim 5 is a circuit arrangement realized with few components.

The circuit arrangement according to claim 6 gives a particularly effective Regulation of the lamp current, which is nevertheless simple and with few Components is built.

The positive feedback device in the form of a coil on the same bobbin how the inductance is applied can be done easily and simultaneously with the inductor.

The fact that the lamp current setpoint depends on the temperature the fluorescent lamp or the environment is specified, a Minimum brightness reached even at low temperatures.

Circuit arrangements according to the invention of particularly simple construction are given in claims 9 and 12. Especially when used a microprocessor for the control device, the possibly even for other tasks, for example in an instrument cluster a motor vehicle is present and the invention Brightness control used for instrument lighting the circuit can be implemented with little component effort. Naturally can the circuit with a separate microprocessor or be realized by means of switching gates.

Through a circuit arrangement in which the operating frequency of the Fluorescent lamp corresponds approximately to the resonance frequency of the resonant circuit, you get an almost sinusoidal operating frequency with little Harmonics. This reduces interference from the circuit can go out and thus increases the electromagnetic compatibility the circuit.

By simultaneously switching the two on and then off The switch before or at the beginning of the pulse sequence pause can be in the resonant circuit contained current short-circuited and so an afterglow of the Fluorescent lamp can be prevented safely. By inserting a Ballast choke between a pole of the supply voltage and the The circuit can additionally stabilize the oscillating circuit and are kept sinusoidal.

Figur 1:

eine erste Schaltung mit einem Gegentaktwandler,

Figur 2:

einzelne Verläufe von Zustandsgrößen der Schaltung aus Figur 1,

Figur 3:

eine zweite Schaltung mit einem Gegentaktwandler,

Figur 4:

einzelne Verläufe von Zustandsgrößen der Schaltung aus Figur 3,

Figur 5:

eine dritte Schaltung mit einem Gegentaktwandler,

Figur 6:

einzelne Verläufe der Zustandsgrößen der Schaltung aus Figur 5.

The invention is described in more detail below with reference to the figures for three possible configurations. Show it:

Figure 1:

a first circuit with a push-pull converter,

Figure 2:

individual courses of state variables of the circuit from FIG. 1,

Figure 3:

a second circuit with a push-pull converter,

Figure 4:

individual profiles of state variables of the circuit from FIG. 3,

Figure 5:

a third circuit with a push-pull converter,

Figure 6:

individual courses of the state variables of the circuit from FIG. 5.

The push-pull converter from FIG. 1 has an oscillating circuit consisting of the capacitor C and the coil L, which is connected directly to the positive supply voltage and can be connected via the transistors S1, S2 alternately to the ground potential via the series reactor Lv and the transistor S3. The following description assumes that the transistor S3 is turned on, that is to say that the transistors S1, S2 are connected to the second pole of the supply voltage. The voltage is also coupled through the coil L1, which is wound on the same coil former as the coil L, and the alternating voltage that occurs alternately blocks the transistors S1, S2 with the oscillation frequency of the resonant circuit. The operating point of the two transistors S1, S2 is set via the resistor R. The resonant circuit oscillates with its resonance frequency ω = 1: LxC , where L is the inductance of the coil and C is the capacitance of the capacitor C. The resonant circuit transmits its energy via the transformer, which is formed from the coils L, L1 and L2, to the lamp circuit which, in addition to the coil L2, also has the fluorescent lamp KL, the impedance Z and a shunt SH. The voltage is tapped between the fluorescent lamp KL and the shunt SH and fed to the rectifier G. The rectified voltage U1 is present at the minus input of the comparator K. At the positive input of the comparator there is a sawtooth voltage U2 with the frequency f3 = 1: T3, the course of which is shown in FIG. 2b. Depending on the lamp current IL and the voltage resulting from the shunt SH, the square wave voltage U3 of the frequency f3 at the output of the comparator K1 is changed in its pulse width W3.

The higher the lamp current, the shorter the pulse width W3 Square-wave voltage. In Figure 2 c is the on and off time shown Immediate impulses.

If the lamp current IL becomes larger, the pulse width W3 becomes shorter, with a smaller one Electricity correspondingly longer. The current setpoint can be set by the level of the triangular voltage can be set in Figure 2b. The output voltage U3 of the comparator K1 is connected to an input of the AND gate A led while to the second input of the AND gate A the dimming frequency f2 is set with the voltage curve U4 (Figure 2a). The dimming frequency f2 is rectangular and its pulse width W2 also changeable. The pulse width W2 of the dimming frequency f2 is determined the duty cycle of the push-pull converter and thus the fluorescent lamp KL, as will be described in more detail later. The Pulse width W2 of the dimming frequency f2 is e.g. either automatically depending the ambient brightness or manually depending on the desired Brightness of the fluorescent lamp KL adjustable.

The voltage U5 is present at the output of the AND gate A: It points during the pulse width W2 of the dimming frequency f2 switching pulses of the pulse width W3 with the switching frequency f3. Thus, the transistor S3 is turned on the switching pulses are controlled with the pulse width W3. With the first pulse with the pulse width W3 during a pulse width W2 Dimming frequency f2, the transistor S3 is turned on. During this The current IB can flow from the supply voltage source + UB into the time Resonant circuit flow. The resonant circuit begins at its resonance frequency to swing. If after the first pulse the pulse width W3 at time t2 transistor S3 blocks, the resonant circuit oscillates further and the current stored in the resonant circuit flows through the series choke Lv and the diode D connected as a freewheeling diode in the Resonant circuit back, but decreases accordingly.

With the next switching pulse of pulse width W3 at time t3 transistor S3 switches through again: current can flow out of the Supply voltage source + UB flow into the resonance circuit and the Current IB increases during the duty cycle.

The current fluctuates during the pulse width W2 of the dimming frequency f2 around its mean IM (Figure 2 f). With increasing or decreasing Pulse width W3 is increased or current IB reduced and the lamp current IL via the transformer. If the pulse width W2 of the dimming frequency f2 has ended and the last pulse the pulse sequence of frequency f3 at transistor t4 at time t4 has transistor S3 during the pause time P of the frequency f2 locked. The resonant circuit vibrates due to its load by the lamp KL and own losses, which become currents IB and IL back to 0 and the fluorescent lamp goes out. With the beginning of the next Pulse of the dimming frequency f2, it begins to light up again previously described. Since the dimming frequency f2 is above human The fluorescent lamp appears to the human eye depending on the pulse width W2 differently bright.

The fluorescent lamp KL can with a sufficiently large supply voltage also in the primary circuit e.g. arranged parallel to the capacitor C. are so that the secondary coil L2 can be dispensed with. Furthermore, the voltage can also be set via a shunt in the primary circuit can be tapped for the rectifier G.

The circuit from FIG. 3 also has an oscillating circuit from the capacitor C and the coil L on that with the positive supply voltage is connected and alternately via transistors S4, S5 is connectable to ground potential. The control device SE is via a control line SL1, SL2 with the base of the transistors S4, S5 connected.

Via the control lines SL1, SL2, the transistors S4, S5 during the pulse width W2, the dimming frequency f2 (FIG. 4a) with the pulse sequences controlled with the switching frequency f3 alternately, the duration T5 of the individual contiguous pulses for a transistor S4, S5 half of the oscillation period T1 of the resonant circuit is (Figure 4b, c). The resonant circuit thus receives the frequency f1 = 1: T1 (Figure 4d).

If the frequency f1 is equal to the resonance frequency of the resonant circuit is, the resonant circuit oscillates almost sinusoidally, so that only small disturbing harmonics occur. Therefore it is also advantageous if the oscillation period T of the resonance frequency is an even multiple the oscillation period T3 is the switching frequency f3.

In Figure 4, the oscillation period T1 of the resonant circuit corresponds to four times the oscillation period T3 of the individual pulses. Through the pulse width W3 of the individual pulses becomes the average current IM in the primary circuit and thus also the lamp current IL in the secondary circuit.

Provided you have both transistors at the end of the dimming pulse at time t5 S4, S5 controlled simultaneously (Figure 2b, c), that in the resonant circuit existing current short-circuited so that it quickly reaches a zero point decreases and so the fluorescent lamp KL without uncontrolled afterglow off.

The dimming frequency f2 is only present internally in the control device SE. Your pulse width W2 determines the duty cycle of the resonant circuit and so that the duty cycle of the fluorescent lamp KL. The one shown in Figure 3 Circuit corresponds to a control. Individuals can do this Pulse width values W2 of the dimming frequency f2 for various desired ones Brightnesses and / or operating temperatures in the storage devices be stored, which are directly present in the memory device SE or which the control device SE can access.

It is also possible to set up a control system by e.g. the Currents IB or IL are measured and the lamp current accordingly is settled.

FIG. 5 shows a fluorescent lamp L, which is connected to a high-voltage capacitor Z with the secondary circuit L2 of a transformer. The transformer is energized in its primary circuit L by two push-pull switching MOSFET transistors S6 and S7, which are controlled by a control device SL, the primary circuit L of the transformer being connected to the operating voltage U _B at the same time. Each gate G of the transistors S6, S7 is connected to the control device SL. The drain D of each transistor S6, S7 leads to the primary winding L of the transformer, the sources S of the MOSFET transistors S6, S7 leading together to a shunt resistor R1 which is connected to ground.

The control device SL processes a voltage drop across the shunt resistor R1 as an input signal. The voltage drop is fed to the inverting input of a comparator K, at the non-inverting input of which there is a reference voltage U _REF with a constant value. The output of the comparator K is connected to the control device SL.

Signal 1

Ansteuersignal am MOSFET-Transistor S6

Signal 2

Ansteuersignal am MOSFET-Transistor S7

Signal 3

Strom durch die Kaltkathodenleuchtstofflampe KL

Signal 4

Spannung über der Kaltkathodenleuchtstofflampe KL

The function of the control of the cold cathode fluorescent lamp KL is explained below with reference to FIGS. 6a and b. Here are plotted over time

Signal 1

Control signal at the MOSFET transistor S6

Signal 2

Control signal at the MOSFET transistor S7

Signal 3

Current through the cold cathode fluorescent lamp KL

Signal 4

Voltage across the cold cathode fluorescent lamp KL

The two MOSFET transistors S6, S7 are successively each triggered once with a pulse 1. This is the resonant circuit, consisting of the secondary coil L2, the high-voltage capacitor Z and the fluorescent lamp KL triggered. The resonant circuit reverberates an e-function (see signal 4, point 2). The gas in the cold cathode fluorescent lamp KL can ionize and organize itself during this time. After a certain time after toasting the first time Resonant circuit, e.g. B. after 80 microseconds, the transistors S6, S7 continuously controlled alternately (signal 1 and 2, point 4). The Cold cathode fluorescent lamp KL emits light immediately from this point in time (as signal 4 can be seen in point 3).

The control device SL controls the MOSFET transistors S6, S7 in a pulsed manner on (Figure 6a, signal 1 and 2). The one through the MOSFET transistors S6, S7 current flowing is considered voltage drop across the Shunt resistance R1 measured and evaluated by comparator K2, which depends on whether the measured voltage is the reference value exceeds or not, emits a low or high signal. The output signal of the comparator K2 becomes logical in the control device SL linked with signal 1. This causes the MOSFET transistors S6, S7 during control by the control logic in the Clock of the output signal of the comparator K controlled or blocked become.

After the desired number of control pulses, the two MOSFET transistors S6, S7 driven simultaneously, as in FIG. 6b, Signal 1 and 2 emerges at time 5. The resonant circuit L2, Z, KL the energy is suddenly withdrawn and the light emission of the Cold cathode fluorescent lamp breaks off immediately.

This device has the advantage that the flicker-free operation of the fluorescent lamp L only through the special control of the MOSFET transistors S6, S7 is reached. On extensive control circuits, as usual, you can do without.

Claims (19)

1. Circuit arrangement for the dimmable operation of a fluorescent lamp at a specific operating frequency (f1), having an apparatus for switching the operating frequency (f1) on and off at a dimming frequency (f2), the pulse width (W2) of the dimming frequency (f2) being variable, and the dimming frequency (f2) being less than the operating frequency (f1), characterized in that the fluorescent lamp current during the duration of the pulse width (W2) of the dimming frequency (f2) is adjustable by the supply voltage being switched on and off at a switching frequency (f3) with a variable pulse width (W3), the switching frequency (f3) being greater than the operating frequency (f1).
2. Circuit arrangement according to Claim 1, characterized in that it has a push-pull converter for generating the operating frequency (f1).
3. Circuit arrangement according to Claim 2,
   characterized in that the push-pull converter has a resonant circuit with a capacitance element and an inductance element (L),
   in that the resonant circuit is connected to a first pole of the supply voltage,
   in that the resonant circuit can be connected alternately via a first and second switch (S1, S2, S4, S5), which are each connected to a terminal of the inductance element (L) and/or of the capacitance element (C), to the second pole of the supply voltage directly or via a third switch (S3),
   in that the first and second switches (S1, S2, S4, S5) are connected, in each case by a power terminal, in each case to a terminal of the inductance element (L) and/or of the capacitance element (C),
   in that the fluorescent lamp (KL) is arranged in parallel with the inductance element (L) and capacitance element (C) or can be supplied with the operating frequency (f1) via a transformer, a primary winding of the transformer advantageously forming the inductance element (L) of the resonant circuit.
4. Circuit arrangement according to Claim 3, characterized in that the switches (S1, S2, S3, S4, S5) are electronic switches.
5. Circuit arrangement according to Claim 3 or 4, characterized in that the first and second switches (S1, S2) can be connected, by their respective power terminal, via the third switch (S3) to the second pole of the supply voltage,
   in that the third switch (S3) is connected to the first pole of the supply voltage via a current valve (D),
   in that the current valve (D) serves as a freewheeling diode when the third switch is not in the on state,
   in that the control terminals of the first and second switches (S1, S2) are connected to a positive feedback device,
   in that the third switch can be actuated by pulse trains in which the individual pulses have a switching period (T3 = 1 : f3) and are enabled for the duration of the pulse widths (W2) of the dimming frequency (f2).
6. Circuit arrangement according to Claim 5, characterized in that the output of an AND element (U) is connected to the control terminal of the third switch, to one input of which AND element the dimming frequency (f2) is applied and to the other input of which AND element the output of a comparator (K) is applied, to the positive input of which comparator a sawtooth- or triangular-waveform signal at the switching frequency (f3) is applied and to the negative input of which comparator a signal is applied which corresponds to the actual or assumed lamp current (IL) or to the primary current (IB).
7. Circuit arrangement according to Claim 5 or 6, characterized in that the positive feedback device comprises a coil (L1), which is wound onto the same coil former as the inductance element (L) and whose respective terminals are connected to the control terminals of the first and second switch (S1, S2), respectively.
8. Circuit arrangement according to one of the preceding claims, characterized in that the lamp-current desired value (IL) is predetermined as a function of the temperature of the fluorescent lamp or of the surroundings.
9. Circuit arrangement according to one of Claims 1 to 3, characterized in that the first and second switches (S4, S5) are connected, in each case by a power terminal, to the second pole of the supply voltage, in that the first and second switches (S4, S5) are connected by their respective control terminal to a control device (SE), in that the control device (SE) alternately drives the switches (S4, S5) with pulse trains whose individual pulses have the switching period (T3), the duration (T5) of the individual consecutive pulses for a switch (S4, S5) amounting to half the oscillation duration (T1) of the resonant circuit.
10. Circuit arrangement according to Claim 9, characterized in that the switching frequency (f3) is an even multiple of the operating frequency (f1).
11. Circuit arrangement according to Claim 9 or 10, characterized in that the operating frequency (f1) approximately corresponds to the resonant frequency of the resonant circuit.
12. Circuit arrangement according to Claim 1 or 2, characterized in that two power switches (S6, S7) are arranged in the primary circuit of a transformer, and can be driven in a push-pull manner by a control device (SL), in that a shunt resistor (R1) is arranged between the switches (S6, S7) and earth, the voltage drop of which resistor is used for the purpose of current regulation.
13. Circuit arrangement according to Claim 12, characterized in that the voltage drop is passed via a comparator (K2) to the control device (SL), the voltage drop being applied to a first input of the comparator (K2), to whose second input a reference voltage is passed.
14. Circuit arrangement according to Claim 12 or 13, characterized in that the switches (S6, S7) are MOSFET transistors whose drains (D) are connected to the primary circuit (L) of the transformer and whose gates (G) are connected to the control logic unit (SL), the sources (S) of the two transistors (S6, S7) being connected both to the shunt resistor (R1) and to the comparator (K2).
15. Circuit arrangement according to one of Claims 9 to 14, characterized in that, before or at the beginning of the pulse train intermission, the control device (SE, SL) simultaneously switches the two switches (S4, S5, S6, S7) on and then off again.
16. Circuit arrangement according to one of Claims 9 to 11 or 15, characterized in that the control device (SE) regulates the pulse widths (W3) of the switching frequency (f3) as a function of the lamp current.
17. Circuit arrangement according to one of Claims 9 to 11 or 15 to 16, characterized in that the control device (SE) controls the pulse widths (W3) of the switching frequency (f3) on the basis of the temperature of the lamp or of the surroundings.
18. Circuit arrangement according to one of Claims 9 to 11 or 15 to 17, characterized in that the control device controls the pulse width (W2) of the dimming frequency (f2) as a function of the ambient brightness or a desired value transmitter.
19. Circuit arrangement according to one of Claims 9 to 11 or 15 to 18, characterized in that a series inductor (Lv) is arranged between one pole of the supply voltage and the resonant circuit.

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