Patent Publication Number: US-7723929-B2

Title: Variable inductive power supply arrangement for cold cathode fluorescent lamps

Description:
This application claims the benefit of U.S. Provisional Application No. 60/867,296 filed Nov. 27, 2006, the entire contents and disclosure of which are hereby incorporated herein by reference. 

   This invention relates to a power supply arrangement for cold cathode fluorescent lamps (CCFLs). 
   BACKGROUND 
   It is well known to provide a liquid crystal display (LCD) device with a CCFL backlight. In order to provide a sufficient and uniform brightness over a large display area, such as may be necessary for example for a television display, the backlight typically comprises a plurality of CCFLs. For example, an LCD display device may comprise a plurality of CCFLs arranged horizontally parallel to and one above another behind an LCD so as to extend over substantially all of the area of the LCD, with reflectors behind the CCFLs, and a diffusion screen, polarizing filter, and colour filters between the CCFLs and the LCD. In such an arrangement there may be a substantial number of CCFLs, for example from 16 to 36 for a large display area. 
   A CCFL, like other discharge tubes, has a breakover or ignition voltage which must be exceeded in order to establish a discharge. A voltage-current characteristic of the CCFL is such that, after establishment of a discharge, voltage falls to a valley voltage with increasing current, and then increases with increasing current in a normal operating region in which the voltage may be appreciably less than the breakover voltage. For example, the breakover voltage of a CCFL supplied with a substantially sinusoidal AC voltage may be of the order of 1500 volts, and the operating voltage may be of the order of 700 to 800 volts, with an operating current, at this operating voltage, of the order of 5 to 10 mA. A slope of the voltage-current characteristic in the operating region is referred to as the impedance of the plasma or of the CCFL. 
   A result of this characteristic is that a plurality of CCFLs can not simply be connected together directly in parallel to a single power supply, because initiation of the discharge in one of the CCFLs would prevent a discharge being established in other CCFLs. A ballast resistor can be connected in series with each of the CCFLs to facilitate such a parallel connection to a single power supply, but this would result in an unacceptable power loss in the ballast resistors. Alternatively, a ballast capacitor can be connected in series with each of the CCFLs to facilitate such a parallel connection to a single power supply. 
   With either ballast resistors or capacitors, inevitable differences between the characteristics of the individual CCFLs result in different backlight levels over the area of the display. Such arrangements do not provide any individual control of the CCFLs, as may also be desired for example to provide different brightnesses in different areas of the display device. 
   It is known to provide an individual inverter (DC to AC power converter) for supplying AC power to each CCFL of a display device, and to provide a single DC to DC converter whose output voltage is supplied to all of the inverters. For example this DC to DC converter may have an output voltage in a range from 12 to 24 volts, and may also serve to supply DC power to other parts of apparatus, for example a television receiver, including the display device. A respective current sensing resistor can be connected in series with each CCFL to provide a feedback signal to the respective inverter, thereby to control the output of the inverter so that all of the CCFLs provide substantially the same backlight level. However, this involves the disadvantage that the display device requires as many separate inverters as it has CCFLs, which can be a significant number especially for large displays. 
   In such a known power supply arrangement, for example the single DC to DC converter may be supplied with power from an AC supply via a PFC (power factor correction) circuit which provides a DC output voltage of the order of, for example, 400 volts. It can be appreciated that the inverters can be supplied from the output of the PFC circuit instead of from the output of a DC to DC converter, but this does not avoid the disadvantage of requiring as many inverters as the display device has CCFLs. 
   Consequently, there is a need for an improved power supply arrangement for CCFLs, in particular for backlights for LCD display devices. 
   SUMMARY OF THE INVENTION 
   According to one aspect, this invention provides a power supply arrangement comprising: an AC (alternating current) supply line; a variable inductance for connection in series with a cold cathode fluorescent lamp (CCFL) for supplying power from the AC supply line to the lamp; and a controller for controlling the variable inductance to control current of the lamp. 
   The power supply arrangement can include a switch mode power converter for supplying power to the AC supply line. 
   The variable inductance preferably comprises a first inductor for connection in series with the CCFL, a second inductor inductively coupled to the first inductor, and an AC switch coupled to the second inductor and controlled by the controller. Advantageously the second inductor and the AC switch are electrically isolated from the first inductor. A capacitor can be coupled in parallel with the first inductor to form a resonant circuit which in one state of the AC switch has a resonant frequency at approximately a frequency of the AC on the AC supply line. 
   The power supply arrangement can include a current sensing element for providing to the controller a signal representing current of the CCFL. 
   The switch mode power converter can include a transformer having a secondary winding coupled to the AC supply line, the power supply arrangement including a capacitor coupled in parallel with the secondary winding to form a resonant circuit with a resonant frequency near but displaced from a frequency of the AC on the AC supply line. The transformer can have at least one other secondary winding providing another output of the switch mode power converter. 
   For supplying power from the AC supply line to each of a plurality of CCFLs, the power supply arrangement can comprise a plurality of variable inductances each for connection in series with a respective one of the CCFLs for supplying power thereto from the AC supply line, the controller being arranged to control each variable inductance to control current of the respective CCFL. In this case preferably each variable inductance can be as recited above, and the power supply arrangement can include, for each of the plurality of CCFLs, a current sensing element for providing to the controller a signal representing current of the respective CCFL. 
   Another aspect of the invention provides a power supply arrangement comprising: an AC (alternating current) supply line; a plurality of variable inductances each for connection in series with a respective one of a plurality of cold cathode fluorescent lamps (CCFLs) for supplying power from the AC supply line to the lamps; and a controller for controlling the variable inductances to control currents of the lamps. 
   The invention also provides a backlight for a display device, the backlight comprising a plurality of CCFLs and a power supply arrangement as recited above, each of the plurality of CCFLs being connected in series with a respective one of the variable inductances. 
   A further aspect of the invention provides a method of providing power to a plurality of cold cathode fluorescent lamps (CCFLs), comprising the steps of: producing an AC (alternating current) supply using a switch mode power converter; coupling the AC supply to each of the CCFLs via a respective one of a plurality of variable inductances; and controlling each of the variable inductances to control current of the respective CCFL. 
   Preferably, when each variable inductance comprises a first inductor coupled in series with the respective CCFL, a second inductor inductively coupled to and electrically isolated from the first inductor, and an AC switch coupled to the second inductor, the step of controlling each of the variable inductances comprises opening and closing the respective AC switch with a controlled duty cycle. The method preferably further includes the step of sensing current of the plurality of CCFLs, the duty cycle of each AC switch being controlled in dependence upon the sensed current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further understood from the following description by way of example with reference to the accompanying drawings, in which the same references are used in different figures to denote similar parts and in which: 
       FIG. 1  illustrates a known power supply arrangement for a plurality of CCFLs; 
       FIG. 2  illustrates another known power supply arrangement for a plurality of CCFLs; 
       FIG. 3  illustrates a known form of power converter; 
       FIG. 4  illustrates a power supply arrangement for a plurality of CCFLs in accordance with an embodiment of this invention; 
       FIG. 5  illustrates parts of a power supply arrangement for one or more CCFLs in accordance with another embodiment of this invention; 
       FIG. 6  is a graph illustrating as a function of frequency relative impedances of the variable inductance in the arrangement of  FIG. 5 ; and 
       FIG. 7  illustrates a backlight for a display device including a plurality of CCFLs. 
   

   DETAILED DESCRIPTION 
   Referring to the drawings, a known power supply arrangement for a plurality of CCFLs is illustrated in  FIG. 1 . As shown in  FIG. 1 , AC power is supplied to a power factor correction (PFC) circuit  10 , an output of which is supplied to a DC to DC converter  12 . An output of the DC to DC converter  12 , which may also be used to power other electronic circuits not shown, is connected to the inputs of a plurality of N inverters  14 , of which only two are shown. The PFC circuit  10  produces at its output a DC voltage for example of the order of 380 volts, and the DC to DC converter  12  produces at its output a DC voltage for example of the order of 12 or 24 volts. 
   Each inverter  14  produces an AC output voltage which is supplied to a respective one of N CCFLs  16 , each connected in series with a respective current sensing resistor  18 . A voltage dropped across each current sensing resistor  18  is fed back as a control signal to the respective inverter  14 , to maintain a desired current through the respective CCFL  16 . Accordingly, each inverter  14  can provide an AC output voltage that is sufficient initially to establish a discharge in the respective CCFL and thereafter to maintain current through the respective CCFL at a desired level, for example in a range of 5 to 10 mA. 
   As discussed above, such a known power supply arrangement requires as many inverters  14  as there are CCFLs  16 . This requirement can be avoided by another known power supply arrangement for example as shown in  FIG. 2 . 
   In the power supply arrangement of  FIG. 2  AC power is again supplied to a power factor correction (PFC) circuit  10 , an output of which is supplied to a switch mode power converter  20  which provides an AC output on an AC supply line  22 . The AC output is supplied to each of a plurality of CCFLs  16  via a respective one of a plurality of ballast capacitors  24 , with a return path via a single sensing resistor  18 . A voltage dropped across the resistor  18  represents a total current of all the CCFLs and is supplied to the power converter  20  as a feedback signal to control the AC output voltage of the power converter. 
   The power supply arrangement of  FIG. 2  avoids the need for as many inverters as there are CCFLs as in the arrangement of  FIG. 1 , but does not provide any individual control of the CCFLs  16 . Consequently, differences between characteristics of the individual CCFLs  16 , and between characteristics of the individual ballast capacitors  24 , result in variations of brightness of the CCFLs  16 , and hence different backlight levels over the area of a display device including the CCFLs. 
   The power converter  20  can have any desired form, but for example can be a resonant converter having an LLC (inductor-inductor-capacitor) topology as described further below, in order to facilitate zero voltage switching in known manner to reduce power losses and interference.  FIG. 3  illustrates by way of example a known form of power converter having an LLC topology. 
   As shown in  FIG. 3 , a switch mode power converter having an LLC topology comprises a half-bridge arrangement of two MOSFET switches  30  and  31  which are switched on and off in a complementary manner, via a gate drive circuit  32  including a delay element (D) providing a switching dead time, under the control of a control signal CK, to connect a junction  33  alternately to an input voltage Vin, for example constituted by the output voltage of a PFC circuit  10  as described above, and ground or zero volts. The junction  33  is coupled via a series connected capacitor  34  and inductor  35  to an AC output  36  of the power converter  20 . The output  36  is connected to ground via an inductor  37 . The inductors  35  and  37 , and the capacitor  34 , give rise to the ‘LLC’ designation of the power converter. 
   A power supply arrangement for a plurality of CCFLs, in accordance with an embodiment of this invention, is illustrated in  FIG. 4 . As in the power supply arrangement of  FIG. 2 , as shown in  FIG. 4  AC power is supplied to a power factor correction (PFC) circuit  10 , an output of which is supplied to a switch mode power converter  20  which provides an AC output to the AC supply line  22 . This AC output is supplied to each of the plurality of N CCFLs  16  in series with a respective one of N variable inductance units  40  and a respective one of N current sensors in this case constituted by current sensing resistors  18 . For simplicity and clarity  FIG. 4  shows only two of the CCFLs  16 , current sensing resistors  18 , and variable inductance units  40 . The power supply arrangement of  FIG. 4  also includes a controller or control unit  42  which, for each of the N CCFLs  16 , is supplied with a voltage dropped across the respective current sensing resistor  18  and controls the respective variable inductance unit  22  to maintain a desired average current through the respective CCFL  16 . 
   In the power supply arrangement of  FIG. 4 , the AC output voltage Vac of the power converter  20  on the AC supply line  22  is monitored by a voltage sensor  44  which provides feedback control to the power converter  20  to maintain a desired value of this AC supply voltage Vac. This desired voltage value can be varied for example to vary a brightness level of all of the CCFLs  16 . 
   Thus instead of the N inverters  14  required in the power supply arrangement of  FIG. 1 , the power supply arrangement of  FIG. 4  includes only one power converter  20  and a plurality of N variable inductance units  40 , as well as the control unit  42  for controlling the variable inductance units  40 . However, in contrast to the power supply arrangement of  FIG. 2 , it enables the currents of the CCFLs to be individually and independently sensed and controlled, so that the brightness of individual lamps can be adjusted accordingly. The control unit  42  can be conveniently provided in an ASIC (application specific integrated circuit), and each of the variable inductance units  40  can be provided for example as described below, so that these can be provided at relatively low cost especially for large values of N. 
   In an embodiment of the invention, each of the variable inductance units  40  conveniently can be switched between two different states, as further described below by way of example. Such switching can occur at a relatively low frequency compared to a switching frequency of the power converter  20 ; for example each state for each of the units  40  can correspond to a multiple of a period of the switching states of the power converter  20 . The switching frequency of the power converter  20  can be phase controlled and locked to that of the PFC circuit  10  to facilitate reduced interference. As shown by different ground symbols for the input and output sides of the power converter  20 , this can optionally provide isolation between its input and output. 
   Parts of a power supply arrangement, in accordance with another embodiment of this invention, for example using an LLC power converter topology, are shown in greater detail in  FIG. 5 , in which only one CCFL  16 , with its variable inductance unit  40  and sensing resistor  18 , is shown. Others can optionally be provided and connected in a similar manner as indicated by dashed lines. 
   As shown at the left side of  FIG. 5 , the power converter  20  comprises MOSFET switches  30  and  31  with a junction  33  connected to a capacitor  34  in a similar manner to that of  FIG. 3 .  FIG. 5  does not show any gate drive circuit for the MOSFETs  30  and  31 ; this can be provided in the same manner as the gate drive circuit  32  of  FIG. 3 , or in any other desired manner. In the power supply arrangement of  FIG. 5 , the inductors  35  and  37  of the power converter shown in  FIG. 3  are constituted by the leakage inductance and the magnetizing inductance, respectively, presented by a primary winding  50  of a transformer  51  connected between the capacitor  34  and ground. This provides the LLC topology of the power converter  20  in a particularly simple and convenient manner. However, the leakage and magnetizing inductances presented by the primary winding  50  of the transformer  51  can if desired be supplemented by additional inductances, coupled respectively in series and in parallel with the primary winding  50 , in the manner shown in  FIG. 3 . 
   Alternatively, the power converter  20  need not be a resonant converter, the MOSFET switches  30  and  31  still forming a half-bridge the junction  33  of which is coupled via the capacitor  34  to the transformer winding  50 , the capacitor  34  in this case having a relatively large capacitance and serving simply as a DC blocking capacitor. The power converter  20  can alternatively have any other desired form. 
   In the power supply arrangement of  FIG. 5 , a secondary winding  52  of the transformer  51  provides the AC output voltage on the AC supply line  22  for each CCFL  16  in the manner shown in  FIG. 4 . Each of one or more further secondary windings, represented by a secondary winding  53  in  FIG. 5 , provides a low voltage output of the power converter  20  to one or more rectifier and DC loads  54 , for example for supplying power to the control unit  42  (shown in  FIG. 4 ) and to other parts of apparatus, which may for example include a display device for which the CCFLs  16  provide a backlight. Feedback control (not shown in  FIG. 5 ) to the power converter  20  may be provided from the secondary circuits  53 ,  54 , instead of from a voltage sensor  44  responsive to the AC supply voltage Vac as shown in the power supply arrangement of  FIG. 4 . In this case the remainder of the arrangement of  FIG. 5  still permits separate control of the average current of the, or each, CCFL  16 . 
     FIG. 5  shows at its right side one CCFL  16 , in series with its respective variable inductance unit  40  and current sensing resistor  18 , coupled to the AC supply line  22 . One or more (possibly a substantial number of) other CCFLs  16 , each in series with its respective variable inductance unit  40  and current sensor, are not shown in  FIG. 5  but can be similarly connected in parallel as indicated by dashed lines. 
   As shown in  FIG. 5 , the variable inductance unit  40  comprises a first inductor  55  in series with the CCFL  16 , with a capacitor  56  in parallel with the inductor  55  to form a parallel resonant circuit. The capacitor  56  can be partly, or even entirely, constituted by capacitance of the inductor  55 . A second inductor  57 , for example having an inductance much smaller than that of the inductor  55 , is loosely inductively coupled to the first inductor  55 . A bidirectional or AC switch  58 , protected by series-connected oppositely poled zener diodes  59  connected in parallel with it, is connected between the ends of the second inductor  57  and is controlled by a control signal CS supplied from the control unit  42 . 
   A turns ratio of the inductors  55  and  57 , for example of the order of 10:1 to 100:1, can be selected so that the AC switch  58  is subject only to relatively low voltages, despite the high voltages that are required for starting and operating the CCFL  16 . This provides a substantial advantage, and also facilitates integration of the switch  58  into the control unit  42 . By way of example the AC switch  58 , represented in  FIG. 5  by a simple switch for simplicity, can comprise two MOSFETs connected in series with opposite polarity, or a MOSFET that is coupled to the second inductor  57  via a diode bridge, or via a full wave rectifier with a center tapped inductor  57 . 
   Alternatively, the inductors  55  and  57  could be replaced by a tapped inductor, with the switch  58  being connected in parallel with a tapped part of the overall inductor. However, in this case the AC switch  58  is not electrically isolated from the rest of the high voltage circuit including the CCFL  16 , so that control of the AC switch  58  is more difficult, for example requiring the use of a high speed optical coupler. The electrical isolation of the switch  58  from the first inductor  55 , provided as shown in  FIG. 5 , facilitates control of the AC switch  58 . 
   The parallel resonant circuit formed by the first inductor  55  and the capacitor  56  is designed to have a resonant frequency, when the switch  58  is open, that is approximately the same as the frequency of the AC output of the power converter  20  on the AC supply line  22 . For example, this may be of the order of 100 kHz. Consequently, with the switch  58  open this parallel resonant circuit provides a high impedance so that very little current flows via the CCFL  16 . When the switch  58  is closed, the effective inductance of this parallel resonant circuit is reduced, for example by a factor of the order of 20, due to leakage inductance and the inductor  57 . Consequently the parallel resonant circuit is tuned to a much higher resonant frequency, and presents a relatively low impedance at the frequency of the voltage Vac on the AC supply line  22 , facilitating current flow to the CCFL  16 . Controlling a duty cycle of the switch  58 , as shown for its control signal CS in  FIG. 5 , thus controls an average current of the CCFL  16 . 
   Although as described above the parallel circuit comprising the inductor  55  and the capacitor  56  has a high impedance at the AC supply frequency when the AC switch  58  is open and a low impedance at the AC supply frequency when the AC switch  58  is closed, alternatively a converse arrangement can be provided with this parallel circuit providing a high impedance at the AC supply frequency when the AC switch  58  is closed, and a low impedance at the AC supply frequency when the AC switch  58  is open. 
   As shown in  FIG. 5 , the current sensing resistor  18  drops a voltage dependent on the current of the CCFL  16  and supplies this to the control unit  42 , which produces the control signal CS to maintain a desired average current of the CCFL  16 . Any other desired form of sensing and feedback arrangement may alternatively be provided. For example, a voltage drop across the inductor  55 , in either or both states of the switch  58 , may instead be used to provide a feedback signal to the control unit  42 . To this end for example the capacitor  56  can be formed by two series-connected capacitances forming a capacitive voltage divider to provide a feedback signal at a reduced voltage. Alternatively, a current transformer (not shown) can be provided for monitoring current flowing via the CCFL  16  and for providing a feedback signal to the control unit  42 . 
   By way of example,  FIG. 6  illustrates, as a function of frequency, relative impedances of the variable inductance in the power supply arrangement of  FIG. 5 , as determined by simulation of one particular set of component values and characteristics. As shown in  FIG. 6 , a curve  62  represents the high impedance state with the switch  58  open. This curve has a peak at a frequency determined by the resonant circuit as discussed above, and the switching frequency of the power converter  20  can be selected to be at or near this peak. A curve  64  in  FIG. 6  represents the low impedance state with the switch  58  closed. 
   Consequently, it can be seen that switching between the two impedance states of the variable inductance unit  22  with a controlled duty cycle of the switch  58  enables an average current of the CCFL  16  to be controlled by the control unit  42 . The rate of this switching can be much lower than the switching frequency of the power converter  20 . 
   Especially for driving a plurality of CCFLs  16 , it is desirable for the power converter  20  to provide a relatively low output impedance. In the power supply arrangement of  FIG. 6 , this is facilitated by providing a capacitor  60  connected in parallel with the secondary winding of the transformer secondary winding  52 , thereby providing a resonant circuit with a resonant frequency that is near but displaced from the AC supply frequency on the AC supply line  22 . For example, for an AC supply frequency of 100 kHz, this resonant circuit may be tuned to a resonant frequency of the order of 80 kHz. 
   It will be appreciated that numerous parameters of the power supply arrangement, including the switching frequency of the power converter  20  and the characteristics of the transformer  50 , inductors  55  and  57 , and capacitors  56  and  60 , can be selected and/or varied to provide particular desired characteristics of operation of the CCFLs  16 . For example, in the case of an LLC power converter  20  which produces lower output voltage with increasing frequency, the switching frequency can be controlled to provide a desired output voltage from the secondary winding  52 . 
   The operating characteristics of the power supply arrangement are also affected by parasitic elements, not shown, such as inductance and capacitance of the CCFL  16 , which can also be taken into account in determining these operating characteristics. 
   Among the parameters affecting the operating characteristics of the power supply arrangement are the impedance of the CCFL  16 , which is appreciably lower in the normal operation of the CCFL than in an initial state of the CCFL before ignition, or start of a discharge. The differing impedances can be used to advantage in association with the other impedance or resonance characteristics of the power supply arrangement to achieve a higher voltage applied to the CCFL  16  for starting a discharge than is applied to the CCFL  16  when it is in its normal operating state. Alternatively, a sufficient voltage for starting a discharge can be applied to the CCFL  16  even in its normal operating state, the average current of the CCFL  16  being limited by the feedback control arrangement including the control unit  42 . 
   By way of example, the control unit  42  can provide a pulse density modulation control to provide a desired average current in the CCFL  16 . To this end, the control unit  42  can provide a table of different duty cycle sequences for such modulation, with entries in the table being used sequentially to provide similar DC or low frequency components, providing the desired average current control, with different AC spectral contents for reducing or spreading electro-magnetic interference. 
   For completeness,  FIG. 7  diagrammatically illustrates a backlight for a display device including a plurality of CCFLs. As shown in  FIG. 7 , an array of CCFLs  16  is positioned behind a liquid crystal display (LCD)  70 . A reflector screen  72  is positioned behind the CCFLs  16 , and a diffuser  74  and polarization and colour filters  76  are positioned between the CCFLs  16  and the LCD  70 , to provide a desired uniform back-lighting of the LCD  70  over its entire area, this being facilitated by individual control of average currents of the CCFLs  16  by the power supply arrangement as described above. 
   Although particular embodiments of the invention are described above, it should be appreciated that numerous changes, modifications, and variations can be made without departing from the scope of the invention as claimed. 
   For example, different types of inverter or power converter  20 , other types and/or arrangements of the variable inductance unit  22 , and sensing arrangements other than the current sensing resistors  18  may be provided. The control unit can alternatively provide open loop control of the variable inductance unit  22 , instead of closed loop control dependent upon a sensed parameter as described above. Although the power supply arrangements described above relate to supplying power to a plurality of CCFLs, and are particularly advantageous for increasingly large numbers of CCFLs, they can also be applied to powering only a single CCFL, or to powering one or more other types of discharge tube or other types of load.