WO2007000684A1

WO2007000684A1 - Lamp driving device

Info

Publication number: WO2007000684A1
Application number: PCT/IB2006/051980
Authority: WO
Inventors: Ulrich Boeke
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2005-06-27
Filing date: 2006-06-20
Publication date: 2007-01-04
Also published as: TW200708200A

Abstract

The present invention relates to a lamp driver with a load-resonant inverter (4) , which can be driven to deliver a desired power output level at desired frequency. The frequency can be chosen such, that interference with a display system can be avoided when the lamp driver is used in the backlighting system of this system. The lamp driver comprises means (6) for determining a parameter corresponding to the switching frequency or the output current of the inverter, a DC/DC converter (3) delivering an intermediate DC voltage to supply the inverter, and means for controlling the output of the DC/DC converter, such that the desired power level is delivered at the desired frequency.

Description

Lamp driving device

FIELD OF THE INVENTION

The present invention relates to a lamp driving device comprising a DC/AC inverter, being arranged to receive a DC voltage and to deliver an AC voltage with a controllable power level and at a desired frequency to a lamp load.

BACKGROUND OF THE INVENTION

Such a device is disclosed e.g. in US, 6259615, Bl, where fluorescent lamps are fed by a full-bridge inverter. The phase shift between the two half-bridge branches of the full-bridge converter is used to control the output power of the inverter. Thanks to the use of this phase shift as a control parameter, the switching frequency can be kept at the desired level. Thereby, undesirable "beat" effects can be avoided when the fluorescent lamps are used in a backlighting arrangement in connection e.g. with an LCD panel.

A disadvantage with this approach is that the full-bridge inverter configuration is rather complex. In normal designs, full-bridge inverters are considered at power levels exceeding 500 W. Therefore it is less desirable to use a full-bridge configuration in an LCD backlighting arrangement, that requires no more than 5- 10 W.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a less complex and thus less expensive solution that still allows e.g. a fluorescent lamp to be fed by an inverter at a desired frequency.

This object is achieved by means of device as defined in claim 1. More specifically, the device then comprises: a DC/DC converter generating said DC voltage as an intermediary voltage from an input voltage, means for extracting a parameter corresponding to one of the switching frequency or the output current of the

DC/AC inverter, and means for controlling the DC/DC converter, based on said parameter, to deliver an intermediate voltage, such that the desired power level is delivered at the desired frequency. By using the intermediate voltage, fed to the inverter, as a control parameter, the inverter switching frequency is allowed to be fixed. Thus a less complex half-bridge arrangement may be used.

The DC/DC converter may be a flyback converter, or, as an alternative, a dual- output converter.

The parameter preferably corresponds to the inverter output current, and the inverter switching frequency is then clamped to the desired frequency.

In a preferred embodiment, the device comprises switching means, allowing the switching frequency to deviate from the desired value during startup. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram, illustrating an embodiment of the invention. Fig. 2 illustrates schematically a load resonant inverter arrangement, where the switching frequency is manipulated.

Fig. 3 illustrates a controllable flyback converter. Fig. 4 illustrates a controllable dual output converter.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 is a block diagram, illustrating an embodiment of the invention. A fluorescent lamp 5, e.g. used in an LCD backlighting arrangement, is driven by a lamp driving device comprising a DC/DC converter 3 and a DC/AC inverter 4. The DC/DC converter 3 receives an input voltage V_IN via a rectifier 2 from a mains voltage 1 , or from a battery arrangement (not shown). The DC/DC converter produces an intermediary voltage V_DC, that may be e.g. 12 V (a typical value when the circuitry is used in the backlighting arrangement of a laptop computer). The intermediary voltage is fed to the DC/AC inverter 4. A lamp current controller 6 compares a measured lamp current signal k with a reference signal i_ref and generates a control signal V_CTRL that controls the output of the DC/DC converter 3, such that the inverter output current is kept at a desired value..

A control block 7 controls the switching frequency f_sw of the inverter 4. During normal operation (i.e. after start up) this frequency is clamped to a desired switching frequency f_syn_ch, that in this embodiment is delivered from a display block 8. During start-up however, it may be advantageous to allow the switching frequency to deviate from this value. A start-up procedure is now described. When the display, in which the lamp driver is used, is switched on, the display block 8 switches the inverter 4 on by setting an inverter input (on/off) high. During start-up, i.e. until the lamp(s) has reached its operating temperature, the inverter switching frequency f_sw is not controlled by the control block 7. Instead, the inverter 4 is allowed to oscillate freely at a switching frequency f_sw determined by load conditions and a feedback network as is well known per se. As soon as start-up is completed, the inverter 4 sets a control block input (on/off) high to acknowledge that the lamp(s) has now reached its normal operating temperature. Then, the control block 7 is activated, and begins to clamp the inverter switching frequency f_sw to the desired value (=f_synch).

In a conventional half-bridge arrangement, the inverter input voltage V_DC would be fixed, and a varying delivered power would result in a varying inverter switching frequency. In accordance with the embodiment of the invention, the inverter switching frequency can however be kept constant and bound to the desired frequency of f_syn_ch, although the amount of power delivered to the lamp is changed. By keeping the inverter switching frequency at the desired value, electromagnetic interference with other parts of e.g. a display device may be avoided. A general idea with the present invention is to provide a lamp driving arrangement where the switching frequency of the inverter may be kept at a desired value, i.e. where the switching frequency f_sw is not used to control the power level of the inverter. Instead the intermediate DC voltage is used as a control parameter. As illustrated above, this can be achieved by extracting the inverter output current and controlling this current by changing the intermediate voltage, while the switching frequency and phase is clamped to an external reference signal f_synch.

As an alternative, the switching frequency f_sw of the inverter may be extracted and compared with the reference signal f_syn_ch. This may be done e.g. using a PLL circuit (Phase Locked Loop) that provides a DC output corresponding to the frequency difference between f_sw and f_syn_ch. This DC output may be used to control the DC/DC converter 3 to deliver a DC voltage that keeps f_sw synchronized with f_synch within a wide inverter power output range.

Fig. 2 illustrates schematically an example of a load resonant inverter arrangement, where the switching frequency is manipulated. In this example the inverter is based on an integrated circuit 13 "UBA2070 CCFL (Cold Cathode Fluorescent Lamp) ballast driver IC" produced by Philips Semiconductors. This circuit comprises an internal VCO (Voltage Controlled Oscillator) arrangement 15. The integrated circuit 13 drives first and second series coupled MOSFET switches 17a, 17b by alternatingly activating the same, such that the connection point between the switches 17a, 17b alternates between earth potential and a driving voltage V_DD- This connection point is connected to an LC circuit formed by a capacitor 19 and a first transformer winding 21. A load circuit comprises a second winding 23, attached to the same transformer, and the lamp 25. More lamps may be connected in parallel with this lamp 25, each lamp then having its own ballast impedance, typically a capacitor in series with the lamp. The integrated circuit thus drives the load circuit in order to light the lamp 25. A feedback network generates a feedback signal. Based on this signal, the inverter is made to oscillate when the inverter is allowed to operate freely.

In this embodiment of the invention, the inverter switching frequency should however, during normal operation, be clamped to a reference signal f_syn_ch. This can be realized in the following way with a control block 7. In the above indicated integrated circuit 13, the internal VCO 15 is accessible at a connection (no. 3), where a capacitor 29 is to be attached in order to determine a property of the inverter. During conventional operation, this capacitor is charged and discharged by means of an internal current source in the integrated circuit. In this embodiment of the invention the control block comprises first and second series coupled MOSFET switches 26, 27, each being controlled by an AND gate 28a, 28b, respectively. One 26 of the series coupled MOSFET switches is connected to a supply voltage V_DD and the other 27 to earth. One input of each AND gate is connected to the on/off input of the control block, such that, if this input is low, both AND gate outputs are low and the gates are blocking (always low). In this state the inverter switching frequency is allowed to vary.

When the on/off input of the control block 7 is high however, the inputted reference signal f_syn_ch, that is a block wave signal, propagates through the AND gates 28a, 28b, one (28b) of which has a negated input, such that their output signals are complementary. The AND gates 28a, 28b drive the gates of the MOSFET switches 26, 27. The first and second switches are thus activated in a complementary fashion, such that the connection point between the switches alternates between being connected to earth and a driving voltage V_DD- This connection point is connected to the above mentioned VCO terminal. Thus the capacitor 29 can be charged and discharged by the control block 7, forcing the inverter to switch at a desired frequency. The internal resistance of the control block should be low enough, such that it dominates over the effect of the internal current source in the integrated circuit 13.

In this way, the inverter can be manipulated to operate at a desired frequency. Needless to say, the skilled person may find many other ways of achieving this. Moreover, other inverter driving circuits than the one mentioned above can be manipulated in a similar way.

If the lamp driver is used in the backlighting arrangement of a display device, the inverter switching frequency may be controlled to a value that is preferred with regard to EMI (electromagnetic interference) issues in relation to other parts of the display device. If e.g. the display device is a TFT-LCD it has a number of characteristic operating frequencies, e.g. the picture frame rate of typically 60 Hz, the horizontal sampling frequency (number of horizontal lines * picture frame rate) e.g. 1080*60 Hz=64,8 kHz, and the pixel data clock frequency in the MHz range. By choosing a suitable inverter switching frequency in relation to these frequencies, and letting the display block 8 in Fig. 1 provide a block wave signal with this frequency, display properties may be improved, since visually disturbing EMI effects may be reduced.

Fig. 3 illustrates a controllable DC/DC converter in the form of a flyback converter, which is of course well known per se. A control circuit 31 is capable of activating a MOSFET switch 33. When this is done an input voltage Vj_n, e.g. from a rectifier circuitry, is connected to earth via the primary winding of a transformer 35. Then a current begins to ramp through the primary winding, until the control circuit 31 switches the MOSFET 33 off. When this happens, energy, stored in the transformer during the build-up phase, induces a current through the secondary winding of the transformer 35, which is connected via a rectifying diode 37 to an output capacitor 39, providing the converter output voltage, in this case 12 V. Through a voltage divider arrangement, comprising a first and a second resistor 41, 43, an opto-coupler element 45 receives a fraction of the output voltage, and generates an output current if, that is inputted to the control circuit 31. Thus a feedback arrangement is accomplished that allows the provision of an accurate output voltage, even if the load varies. The transformer 35 and the opto-coupler element 45 provides mains insulation.

In order to modify the flyback-converter in accordance with an embodiment of the invention, a third resistor 47 and a series coupled additional transistor 49 is added in parallel with the voltage divider arrangement. In response to a control voltage V_CTRL from a comparator 6 (as illustrated in Fig. 1) the added components may change the relation between the feedback current if and the actual output voltage, such that an output voltage e.g. resulting in a suitable inverter output current may be achieved.

Fig. 4 illustrates a controllable dual output converter, used in a similar manner as the flyback-converter in Fig. 3. The dual output converter is disclosed per se in EP

1275047, A2, and is capable of providing two individually controllable output voltages V_a, V_b. This DC/DC converter may thus be used to supply both a signal processing module 51 of a display, receiving the first output voltage V_a, and a backlighting arrangement 53 of the same display, receiving the second output voltage V_b. The converter comprises a control circuit 55 being capable of driving two

MOSFET switches 57a, 57b, such that a primary circuit 59, comprising a transformer primary winding may receive voltages with two different polarities. Each output branch comprises an output circuit 61, 63, respectively. The output circuits 61, 63 are each connected to a secondary winding of the transformer but at different polarities. Each branch further comprises a feedback circuit 65, 67, respectively. The feedback circuit of the lamp driver branch may be provided with an additional resistor and an additional transistor, as described in connection with Fig. 3, so that the voltage V_b can be controlled in such a way that e.g. the inverter output current is kept at a desired value, as described earlier. The display signal processing module 51 may thus provide the lamp driver 53 with a desired synchronizing frequency signal f_syn_ch, and the lamp driver may generate a control signal VcTRL in correspondence the desired output current.

Of course other DC/DC topologies are possible in this context, such as e.g. boost converters.

In summary, the invention relates to a lamp driver with a load resonant inverter, which can be driven to deliver a desired power output level at desired frequency. The frequency can be chosen such, that interference with a display system can be avoided when the lamp driver is used in the backlighting system of this system. The lamp driver comprises means for determining a parameter corresponding to the switching frequency or the output current of the inverter, a DC/DC converter delivering an intermediate DC voltage to supply the inverter, and means for controlling the output of the DC/DC converter, such that the desired inverter switching frequency at the desired power level is achieved.

The invention is not restricted to the described embodiment. It can be altered in different ways within the scope of the appended claims.

Claims

CLAIMS:

1. A lamp driving device comprising a DC/AC inverter (4), being arranged to receive a DC voltage (V_DC) and to deliver an AC voltage with a controllable power level and at a desired frequency (f_syn_c) to a lamp load (5; 25), wherein the device comprises:

-a DC/DC converter (3) generating said DC voltage as an intermediary voltage from an input voltage (Vj₁₁),

-means for extracting a parameter corresponding to one of the switching frequency or the output current of the DC/AC inverter, and

-means (6) for controlling the DC/DC converter, based on said parameter, to deliver an intermediate voltage, such that the desired power level is delivered at the desired frequency.

2. A lamp driving device according to claim 1, wherein the DC/DC converter is a flyback converter.

3. A lamp driving device according to claim 1, wherein the DC/DC converter is a dual-output converter.

4. A lamp driving device according to claim 1, wherein the parameter corresponds to the inverter output current (k), and the inverter switching frequency is clamped to the desired frequency.

5. A lamp driving device according to claim 1, comprising switching means (28a, 28b) for allowing the switching frequency to deviate from the desired value during startup.

6. A display system, comprising a display panel and a backlighting system for providing illumination to the display panel, said backlighting system including a lamp driving device according to any preceding claim.