Method and circuit for pulsed fluorescent lamp operation

FIELD OF THE INVENTION

The invention relates to a method and circuit for pulsed operation of a fluorescent lamp, a ballast comprising such circuit and an assembly of such circuit and such fluorescent lamp.

BACKGROUND OF THE INVENTION

US 5 907 222 discloses a controller for driving a fluorescent lamp. The controller comprises an arc current regulator and a filament current regulator. When the lamp is operated in a pulsed or dimmed manner, heating of electrodes of the lamp is required to keep the lamp is an operational working area and to ensure an ignition of the lamp. Thereto, a separate filament current regulator is provided which provides for filament current to heat the electrodes of the lamp, and a separate arc current regulator to control an arc current as well as a voltage over the lamp.

Despite the advantages of such a controller, a rather complex circuitry will be required, due to the fact that in essence a double control is provided, which may increase circuit complexity, thereby possibly affecting one or more design parameters such as cost, volume, component count, etc. in an unfavorable way.

OBJECT OF THE INVENTION An object of the invention is to control heating of the fluorescent lamp electrodes as well as ignition and arc current of the lamp, by a simple control circuit.

SUMMARY OF THE INVENTION

The above object may be achieved by a method for pulsed operation of a fluorescent lamp, the method comprising: providing the lamp via a network having a frequency dependent transfer function, with a power signal;

controlling during a heating part of a pulse repetition time period a frequency of the power signal to be in a first frequency band to provide an electrode current to the lamp to heat an electrode of the lamp; and controlling during an operating part of the pulse repetition time period the frequency of the power signal to be in a second frequency band to ignite the lamp and to provide arc current and electrode current to the lamp.

Further, in an aspect of the invention, the above object may be achieved by a circuit for pulsed operation of a fluorescent lamp, the circuit comprising: a power signal generator for generating a power signal to power the lamp, - a network having a frequency dependent transfer function, the network being connected to the power signal generator and for providing the power signal to the lamp; and a lamp controller for driving the power signal generator, the lamp controller having a setting input for receiving a setting input signal and being arranged for controlling when the setting input is at a first level, the power signal generator to set a frequency of the power signal in a first frequency band to provide an electrode current to the lamp to heat an electrode of the lamp; and controlling when the setting input is at a second level, the power signal generator to set the frequency of the power signal in a second frequency band to ignite the lamp and to provide arc current and electrode current to the lamp. According to the invention, alternately, the lamp driving frequency will be set to be in a first frequency band and in a second frequency band. In the first frequency band, a voltage over the lamp will be relatively low, due to the frequency dependency of the resonance network. As a consequence of the low voltage, an arc in the lamp will not be maintained, and therefore, the lamp will turn off. Electrical current flowing via the network through the electrodes will only heat the electrodes. When the frequency is brought towards the second frequency band, the voltage over the lamp will increase, which will lead to an ignition of the lamp. The lamp will remain on during the second time period, and will be turned out at the end of the second time period when the frequency is altered, as at that moment in time the voltage over the lamp will decrease due to a change in frequency. Therefore, pulsed operation of the lamp and heating of the electrodes can be achieved with a single drive signal. Furthermore, the single drive circuit enables providing a single control circuit for control of the current in the two time periods. As an example, the network may comprise a resonant network such as an inductor/capacitor resonant network, the first frequency band being chosen to be outside a resonance frequency band of the resonant

network, while the second frequency band being chosen to be in the resonance frequency band. By the resonance, a significant increase of the electrical quantity (e.g. voltage) driving the lamp may be obtained, which allows a reliable ignition of the lamp when setting the second frequency band to be in the resonance frequency band. Other embodiments are possible too, as an example, the second frequency band may be below a resonance frequency band of such resonant network. In other embodiments, the network may comprise any suitable network having one or more components which show a frequency dependency, such as one of more capacitors, one of more inductors, transformers, etc.

In the circuit according to the invention, a setting input may be provided, a signal at the setting input to control the frequency of the power signal to be in the first or second frequency band. Thereby, the switching on and off of the lamp can be controlled by an external signal, which allows synchronization of the pulsed operation of the lamp with e.g. an external source.

In an embodiment, to control an heating current, and consequently to stabilize a temperature of the electrodes of the lamp, during the heating part of the pulse repetition period a reference source is set to a first output value, a signal representative of an electrode current flowing through the lamp is compared with the first output value of the reference source, and wherein the frequency of the power signal is adjusted based on the comparison between the signal representative of the electrode current and the first output value of the reference source.

In a embodiment, to control an arc current, during the operating part of the pulse repetition period the reference source is set to a second output value, a signal representative of the electrode current is subtracted from a signal representing a total electrical current flowing through the lamp, a result of the subtraction being compared with the second output value of the reference source, and wherein a frequency of the power signal is adjusted based on the comparison between the result of the subtraction and the second value of the reference source. Thus, by subtracting a signal representative of the electrode current from a signal representative of the total current trough the lamp, an indication of the arc current is obtained, which can be applied to control and/or monitor the arc current, and consequently the light intensity of the lamp.

By changing a ratio of the operating part and the heating part of the pulse repetition period; and changing the first output value of the reference source to adapt a desired electrode current to a changed duration of the heating part according to the changed

ratio, electrode heating may be compensated for a variation in duty cycle of the electrode heating part of the pulse cycle, to thereby e.g. keep a desired electrode temperature.

With the circuit according to the invention, similar preferred embodiments achieving same or similar effects as described above, may be provided. In a further preferred embodiment of the circuit according to the invention, the controller may be formed by a simple, cost effective circuit in that the controller comprises a pair of bipolar transistors, the respective bases of the bipolar transistors being electrically connected, the emitters of the bipolar transistors being electrically connected to the reference source and to the signal representing the total electric current flowing through the lamp respectively, a collector of one of the transistors being electrically connected to an integrator or the controller.

Furthermore, to allow the controller to adjust a frequency of the power signal, the power signal generator may comprise a controlled oscillator, a frequency of the oscillator being controllable by an electrical input quantity of the oscillator, an output of the integrator being electrically connected to the controlled oscillator for providing the electrical input quantity.

Still further, when the network comprises an electrode current supply capacitance connected between the electrodes of the lamp, the controller may comprise a capacitor, one terminal of which being connected to a driving side electrode of the lamp, the capacitor for providing a current there through which provides the signal representative of the heating current: the inventors have realized that an indication of the electrode heating current which also flows through the current supply electrode, can be obtained by connecting one terminal of a capacitor to the driving side electrode (i.e. the "hot" side of the lamp where a high voltage is provided by the network tot he lamp), and keeping the other terminal of the capacitor at a low voltage similar to a voltage at the other electrode of the lamp, a current through the capacitor providing an indication of the current through the electrodes of the lamp.

The invention further comprises a ballast for driving a fluorescent lamp, the ballast comprising a circuit according to the invention. Still further, the invention comprises an assembly of a fluorescent lamp and a circuit according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts an example of a graph of the driving of the fluorescent lamp according to an embodiment of the invention;

Fig. 2 depicts a block schematic view of a control circuit for controlling a fluorescent lamp according to an embodiment of the invention; and

Fig. 3 depicts a simplified circuit diagram of a control circuit according to an embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLES Fig. 1 depicts a graphical view of lamp driving frequency versus time. Along a horizontal axis, time is depicted, while along a vertical axis, a lamp drive power signal PS is depicted. As illustrated in fig. 1, the lamp is driven with an alternating current power signal, a frequency of which changes over time. It is noted that also an amplitude or a voltage over the lamp may change due to the change in frequency, e.g. due to a frequency dependency of a network via which the lamp is driven, as will be explained in more detail below. It is noted that in this example, the network comprises a resonant network, as will be outlined in more detail below. As is depicted in fig. 1, the lamp driving signal in this embodiment shows a first time period Tl wherein the lamp driving signal has a first frequency (more generally speaking: wherein the lamp driving signal is in a first frequency range or band), and a second time period T2 wherein the lamp driving signal has a second frequency (more generally speaking: wherein the lamp driving signal is in a second frequency range or band). In this example, in the first time period Tl, the lamp driving frequency is above a resonance frequency of the network, while in the second time period T2, the lamp driving frequency is in a resonance frequency band of the network. In the first period, a voltage over the lamp will be relatively low, due to the frequency dependency of the resonance network. As a consequence of the low voltage, an arc in the lamp will not be maintained, and therefore, the lamp will turn off. Electrical current flowing via the network through the electrodes will only heat the electrodes. When the frequency is lowered and brought towards the resonance frequency band of the lamp, the voltage over the lamp will increase, which will lead to an ignition of the lamp. The lamp will remain on during the second time period, and will be turned out at the end of the second time period when the frequency is increased, as at that moment in time the voltage over the lamp will decrease due to a change in frequency. Tl and T2 together form pulse repetition time period T, as indicated in fig. 1. In this document, the term resonance frequency band is to be understood as a frequency band within which the

network shows some extent of resonance, e.g. 5 % or more peaking in a suitable electrical transfer characteristic thereof. As depicted in fig. 3, the network may comprise a combination of an inductor and a capacitor. Alternatively, the network may comprise a combination of a transformer and a capacitor. Any other network may be applied too. In the example depicted in fig. 3, the network comprises a bypass capacitor Cr that is connected between the electrodes at opposite sides of the lamp. Heating current flowing through the electrodes is led by the bypass capacitor from a first of the electrodes to a second of the electrodes. The network in fig. 3 further comprises inductor L. Cs forms a coupling capacitor, connected in series with inductor L, to allow a single ended (i.e. single supply voltage) driving of the lamp. Fig. 2 shows a block schematic view of a circuit for driving fluorescent lamp

TL. Power supply 10 (e.g. a mains, a mains adapter, a battery or a rectified mains) supplies electrical energy to power supply circuit 20 which provides the power signal PS to the lamp TL via network 30. Power supply circuit 20 operates on the electrical power supplied to it by the power supply 10, to generate an alternating current lamp drive current at a lamp drive frequency. Thereto, the power supply circuit 20 may comprise a half bridge or other suitable switching element. Further, the power supply circuit 20 may comprise an oscillator to generate the frequency of the lamp drive current. The oscillator may comprise a controlled oscillator, which may be controlled by lamp controller 40. The lamp controller is in this example provided with a feedback signal FB representing a current through the lamp TL. Controller 40 generates a control signal to control power supply circuit 20, e.g. by providing a control signal (such as a voltage) to a controlled oscillator (such as a voltage-controlled oscillator) of the power supply circuit 20. The controller is further provided with a setting input S, the controller in response to a suitable signal at the setting input S and being provided with the feedback signal FB, driving the power supply circuit 20 to form a feedback control system operating the lamp. By supplying a suitable signal to the setting input, a frequency of the power signal provided to the lamp, can be altered, as the feedback loop will settle to a corresponding frequency in response to a corresponding signal at the setting input S. In some embodiments, the setting input S may be provided with an analogue signal to form a set-point signal, while in other embodiments, the setting input may form a switching input, to be provided with e.g. a digital or similar signal, to set an internal reference of the controller to a suitable value under control of the input signal at the setting input. In such embodiments, in internal reference, such as a reference voltage source, may be provided, an output value of which is controlled by the input signal at the setting input S, to e.g. have the reference voltage source change between different values, to thereby make the feedback control circuit settle at

different frequencies. In such an embodiment, the setting input may thus be provided with a signal having a first level during the first time period and a second level during the second time period, to thereby drive the lamp with a power signal having the frequency pattern as depicted in fig. 1. Fig. 3 shows a more detailed view of an embodiment of the circuit according to the invention. The power supply circuit as depicted in fig. 2 comprises voltage controlled oscillator VCO and a half bridge, which has been schematically indicated by Vhb. The network 30 comprises inductor L, capacitor Cr to bridge the electrodes, and coupling capacitor Cs, a function of which having been described above. A feedback signal is generated by series resistor R, which provides for a feedback voltage proportional to a total lamp current, i.e. the electrode current which is provided via the capacitor Cr, and the arc current in case that the lamp is on. The controller 40 in this example comprises a comparator formed by transistors Ql, Q2, an integrator formed by capacitor Ci and current source II. The reference signal is generated by voltage source V2, which is controlled by a signal to be provided at input S. During the heating part of the pulse repetition period, a signal at S is provided having a first value, and hence the reference source V2 provides a reference voltage having a corresponding value. The comparator formed by transistors Ql, Q2 now compares, the voltage of the reference source with the voltage over series resistor Rs ( or more precisely, with the peaks of the voltage alternating voltage over Rs), and outputs a current to the comparator formed by capacitor Ci and current source Ii. In a stationary state, A current provided by current source Ii equals a current sourced by transistor Ql. The voltage controlled oscillator is driven by the (output voltage of) the integrator, which via the feedback effect results in a frequency of oscillation which is above the resonance frequency band of the network, and consequently in a turning-off of the lamp. In this situation, the voltage over the series resistor Rs is thus proportional to the heating current. Therefore, a feedback control operation is now provided which controls the heating current: the comparator formed by Ql, Q2 compares the voltage over series resistor Rs with reference voltage V2, and drives the integrator and voltage controlled oscillator VCO to obtain a heating current which results in a voltage over resistor Rs which matches the momentary value of V2.

The operation of the circuit according to fig. 3 will now be described in the situation where the lamp is ignited and operated (i.e. turned on). In the second time period (as described with reference to fig. 1), the driving signal at the input S provides for a different value as compared to the first time period. As an example, the driving signal at

driving input S may have digital levels low and high, a signal having digital low level e.g. indicating the first time period and setting the voltage source V2 to the first reference voltage value, while a signal having a high level setting the voltage source V2 to a second reference voltage value. As a result of the second voltage value of the reference source, the comparator formed by Ql, Q2 will drive the integrator and therefore the voltage controlled oscillator VCO to alter (in this example: lower) the frequency of oscillation, and hence the frequency with which the lamp is driven via the network. The frequency having reached the resonance frequency band of the network, voltage over the lamp will increase resulting in an ignition thereof. In the absence of transistor Q3, capacitor Cv and resistor R3, total current through the lamp (i.e. a sum of the electrode current heating the electrodes of the lamp, and the arc current) will now be controlled by the circuit, causing the alternating current voltage across series resistor Rs to balance the output of the voltage source. Thus, a single control circuit controls both electrode heating current as well as arc current of the lamp.

A pulse repetition frequency of the lamp may be determined by a repetition frequency of the signal provided at input S, thereby allowing for a synchronization of the lamp pulses with an external source, which may be useful in many applications, e.g. an image repetition frequency of a display which is illuminated by the lamp.

To provide a controlled light output of the lamp, it is desirable to control in the second time period (i.e. when the lamp is ignited and on) the arc current instead of the sum of the arc current and the heating current. In an embodiment, this is provided by transistor Q3, capacitor Cv and resistor R3. In the second time period, transistor Q3 is switched to a nonconducting state by a suitable value of the signal at input S, which will provide an additional signal to the input of the comparator formed by Ql and Q2. As a terminal of capacitor Cv is connected to an high voltage side electrode of the lamp, a voltage across a series connection of capacitor Cv and resistor R3 will be largely correspond to a voltage across capacitor Cr which is connected between the lamp electrodes, and series resistor Rs. As the electrode heating current is guided through capacitor Cr, the voltage across capacitor Cr will be related to the heating current. A current through capacitor Cv may therefore form an indication of the heating current. In a practical implementation, a capacitance of capacitor Cv may be chosen to be lower than a capacitance of Cr resulting in a current through Cv which is lower than the heating current. To compensate, a value of resistor R2 which is connected in series between the reference voltage source and the emitter of Ql (the emitter of Ql forming an input of the comparator), may be chosen to satisfy the equation Cr * Rs = Vc * R2, to obtain a voltage across R2 which provides an indication of the electrode heating current. As the voltage which

provides the indication of the electrode heating current is in this embodiment added to the reference voltage source, it will in terms of the feedback control system be effectively subtracted from the voltage across series resistor Rs. Thus a voltage representing the electrode heating current will be subtracted from a voltage representing (an approximation of) the total lamp current, which provides for a controlling of a difference between these currents, being (an approximation of) the arc current. Thus, the control circuit as described with reference to fig. 3 now allows a controlling of the heating current in the first (i.e. the heating) time period and a controlling of the arc current in the second (i.e. the ignition and operation of the lamp) time period with a simple, single control system. The output value (e.g. output voltage) of the reference source may be made variable to enable establishing a relation between pulse width of the pulsed operation of the lamp, and electrode heating current: With lower pulse widths, the electrode heating current tends to diminish due to arc current. This can be counteracted by a corresponding change of the output value of the reference source, to increase the electrode current thereby keeping electrode temperature constant. Furthermore, when the lamp is on, the output value of the reference source may be used to set the arc current. Thereby, intensity of the lamp may thus be set by a change in a duty cycle of the heating / operating cycle, and/or a change in the reference source output value to change a desired arc current. An application where arc current may be changed may include scanning backlight systems with double pulse schemes where two pulses are used with different current magnitudes.