DESCRIPTION

POWER CONTROL AND MAINS ISOLATION ARRANGEMENT

The present invention relates to power control and mains isolation arrangements, in particular for switch-mode power supplies. The present invention is particularly suited to, but not limited to, driving a backlight unit comprising fluorescent lamps, for example a backlight unit for a liquid crystal display.

Mains isolation is required in many switch-mode power supply applications. Power transformer elements convert energy from a switch-mode controlled non-isolated primary side to a mains isolated secondary side. The power transformer elements act as a mains isolation element. Additionally, further mains isolation elements are required in switch-mode power supplies. These further mains isolation crossings are placed either on the boundary between the secondary (mains isolated) side power conversion controller and primary (non-mains isolated) side power switch elements (power transistor switch action signals are transferred across the mains isolation boundary), or in the feedback path, between the secondary side and a primary (non-mains isolated) side power conversion controller.

One type of such a power supply is a power supply for a liquid crystal display (LCD) that includes a high voltage backlight inverter for driving the fluorescent lamps of a backlight unit. An example is as described in WO/02103 665 A1 .

In some types of backlight inverters, (lamp) control functionality is positioned on the primary side of the inverter transformer. In other types of backlight inverters, (lamp) control functionality is positioned on the secondary side of the inverter transformer. The former types require a relatively large number of isolation crossings. The latter types require less isolation crossings than the former type, however the required crossings need to comprise relatively high performance and consequently relatively expensive mains

isolation elements due to the requirement to transfer power transistor switching actions, including highly dynamic power transistor switching actions, across the mains isolation crossing.

The present inventor has realised it would be desirable to provide a switch-mode power supply arrangement with mains isolation in which the number of isolation crossing is reduced and/or in which the performance requirement for the isolation element of any isolation crossing is reduced. In a first aspect, the present invention provides a switch mode power control and mains isolation arrangement, comprising: a power transistor switch arrangement; one or more power transformers whose primary side/s is/are coupled to the power transistor switch arrangement and whose secondary side/s is/are adapted for coupling to a load; a pre-controller adapted for coupling to the load; a mains isolation element with an input on the isolated side coupled to the pre-controller; a power transistor drive modulator with an input coupled to an output on the non-isolated side of the mains isolation element, the power transistor drive modulator further comprising an output coupled to the power transistor switch arrangement; wherein the pre-controller is adapted for determining a modulator control signal and for outputting the modulator control signal to the input of the mains isolation element; the mains isolation element is adapted for forwarding the modulator control signal to the power transistor drive modulator; and the power transistor drive modulator is adapted for determining power transistor switch action signals dependent upon the modulator control signal and for outputting the power transistor switch action signals to the power transistor switch arrangement; the modulator control signal being lower bandwidth than the power transistor switch action signals.

The power transistor switch arrangement may be adapted for feeding back power structure protections to the power transistor drive modulator; and the power transistor drive modulator may be adapted for determining the

power transistor switch action signals dependent upon the modulator control signal and the power structure protections.

The power transistor switch arrangement may comprise a power transistor driver structure and a power transistor structure; and the power transistor drive modulator and the power transistor driver structure may be provided as an integrated module.

The mains isolation element may be an opto-coupler element.

The pre-controller may be a micro-controller element.

The switch mode power control and mains isolation arrangement may further comprise the load.

The load may be a backlight unit of a liquid crystal display.

The switch mode power control and mains isolation arrangement may further comprise one or more further mains isolation elements each with an input on the isolated side coupled to the pre-controller; wherein: the power transistor drive modulator may comprise one or more further inputs each coupled to an output on the non-isolated side of a respective one of the one or more further mains isolation element; the pre-controller may further be adapted for determining one or more further modulator control signals and for outputting each one or more further modulator control signal to the input of a respective one of the one or more further mains isolation elements; the one or more further mains isolation elements may be adapted for forwarding its respective further modulator control signal to the power transistor drive modulator; and the power transistor drive modulator may be further adapted for further determining the power transistor switch action signals dependent upon the one or more further modulator control signals; the one or more further modulator control signals may each be lower bandwidth than the power transistor switch action signals.

There may be only one further mains isolation element providing a total of two mains isolation elements, the modulator control signal of the first mains isolation element being a frequency control signal and the modulator control signal of the second mains isolation element being a phase shift control signal.

In a further aspect, the present invention provides a method of operating a switch mode power control and mains isolation arrangement comprising a power transistor switch arrangement, one or more power transformers whose primary side/s is/are coupled to the power transistor switch arrangement, and a load coupled to the secondary side/s of the one or more power transformers; the method comprising: a pre-controller on the mains isolated side of the switch mode power control and mains isolation arrangement determining a modulator control signal; feeding the modulator control signal via a mains isolation element to a power transistor drive modulator on the non-isolated side of the switch mode power control and mains isolation arrangement; the power transistor drive modulator determining power transistor switch action signals dependent upon the modulator control signal; and forwarding the power transistor switch action signals to the power transistor switch arrangement; the modulator control signal being lower bandwidth than the power transistor switch action signals.

The power transistor switch arrangement may feed back power structure protections to the power transistor drive modulator; and the step of the power transistor drive modulator determining the power transistor switch action signals dependent upon the modulator control signal may further comprise the power transistor drive modulator determining the power transistor switch action signals dependent upon the power structure protections.

The power transistor switch arrangement may comprise a power transistor driver structure and a power transistor structure; and the power transistor drive modulator and the power transistor driver structure may be provided as an integrated module.

The mains isolation element may be an opto-coupler element.

The pre-controller may be a micro-controller element.

The load may be a backlight unit of a liquid crystal display.

The method of operating a switch mode power control and mains isolation arrangement may further comprise: the pre-controller determining one or more further modulator control signals; feeding the one or more further modulator control signals via respective one or more further mains isolation

elements to the power transistor drive modulator; and the power transistor drive modulator determining the power transistor switch action signals further dependent upon the one or more further modulator control signals; the one or more further modulator control signals each being lower bandwidth than the power transistor switch action signals.

There may be only one further mains isolation element providing a total of two mains isolation elements, the modulator control signal of the first mains isolation element being a frequency control signal and the modulator control signal of the second mains isolation element being a phase shift control signal. In a further aspect, the present invention provides a switch mode power control and mains isolation arrangement, comprising: a power transistor switch arrangement; a power transformer; a load e.g. a backlight unit; a pre-controller; a mains isolation element; and a power transistor drive modulator; wherein the pre-controller determines and forwards via the mains isolation element a modulator control signal to the power transistor drive modulator which determines power transistor switch action signals dependent upon the modulator control signal and forwards the power transistor switch action signals to the power transistor switch arrangement; the modulator control signal being lower bandwidth than the power transistor switch action signals. Power structure protections may be fed back to the power transistor drive modulator which then may determine the power transistor switch action signals dependent upon the modulator control signal and the power structure protections.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a prior art power control and mains isolation arrangement; Figure 2 is a schematic block diagram showing certain further details of a backlight unit;

Figure 3 shows a power control and mains isolation arrangement including a controller as envisaged by the present inventor;

Figure 4 is a schematic illustration of a power control and mains isolation arrangement according to a first embodiment of the invention; and Figure 5 is a schematic illustration of a power control and mains isolation arrangement according to a second main embodiment of the invention.

The embodiments of the present invention can most clearly be explained by first describing an example of a prior art arrangement in more detail with reference to Figure 1 , which is a schematic illustration of a prior art power control and mains isolation arrangement 1.

The prior art power control and mains isolation arrangement 1 comprises the following elements: a power transistor driver structure 2, a power transistor structure 4, a non-mains isolated high voltage bus 6, a power transformer 8 with a primary side 10 and a secondary side 12, a backlight unit 14, a controller 16, a first mains isolation element 18, and (optionally) a second mains isolation element 20. The power transistor driver structure 2 and the power transistor structure 4 are each connected to the non-isolated high voltage bus 6. A first output of the power transistor driver structure 2 is coupled to an input of the power transistor structure 4, for forwarding driver outputs 22. The power output from the power transistor structure 4 is coupled to the primary coil 10 of the power transformer 8. The secondary coil 12 of the power transformer 10 is coupled to the backlight unit 14. A feedback output from the backlight unit 14 is coupled to the controller 16 for forwarding backlight feedback information 24 to the controller 16.

An input of the controller 16 is arranged for receiving control signals (such as enable and dimming signals) 26 from a small signal and control board inside the LCD-TV in which the backlight will be incorporated.

The output of the controller 16 is coupled to an input on the isolated side of the first mains isolation element 18 for forwarding power transistor switch action signals 28 from the controller 16 to the first mains isolation element 18. An output on the non-isolated side of the first mains isolation element 18 is coupled to the power transistor driver structure 2 for forwarding the power transistor switch action signals 28 from the first mains isolation element 18 to the power transistor driver structure 2. In other words, the output of the controller 16 is coupled to the power transistor driver structure 2 via the first mains isolation element 18, for forwarding power transistor switch action signals 28 from the controller 16 to the power transistor driver structure 2. Further mains isolation elements can be included. For example, for a full bridge structure, typically one mains isolation element is provided for each of the two half bridges forming the full bridge structure.

A further output from the power transistor driver structure 2 is coupled to an input on the non-isolated side of the second mains isolation element 20 for forwarding (optional) power structure protections 30 from the power transistor driver structure 2 to the second mains isolation element 20. An output on the isolated side of the second mains isolation element 20 is coupled to an input of the controller 16 for forwarding the power structure protections 30 from the second mains isolation element 20 to the controller 16. In other words, the further output of the power transistor driver structure 2 is coupled to the controller 16 via the second mains isolation element 20, for forwarding power structure protections 30 from the further output of the power transistor driver structure 2 to the controller 16. Also shown in Figure 1 is a representation of the effective "mains isolation boundary" 32 for the prior art power control and mains isolation arrangement 1. The mains isolation boundary 32 of the prior art power control and mains isolation arrangement 1 has three isolation crossings, namely the power transformer 8, the first mains isolation element 18 and the second mains isolation element 20. The following elements are on the non-isolated side of the mains isolation boundary 32: the power transistor driver structure 2, the power transistor structure 4, the non-isolated high voltage bus 6, and the

primary coil 10 of the power transformer 8. The following elements are on the isolated side of the mains isolation boundary 32: the secondary coil 12 of the power transformer 8, the backlight unit 14, and the controller 16.

In operation a first feedback loop is implemented, as follows. Power output from the power transistor structure 4 is fed to the power transformer 8. The resulting output power from the power transformer 8 is provided to the backlight unit 14 for powering operation of the backlight unit 14. Backlight unit feedback information 24 derived or output from operation of the backlight unit 14 is fed to the controller 16. The controller 16 determines the power transistor switch action signals 28 based upon the received backlight unit feedback information 24 (and on the received control signals 26 from the LCD-TV or LCD-Monitor set-level small signal and control board). The power transistor switch action signals 28 are fed to the power transistor driver structure 2. The power transistor driver structure 2 drives the power transistor structure 4. The operation of the power transistor driver structure 2, and hence the form of the power output from the power transistor structure 4, is dependent upon the power transistor switch action signals 28 determined and output by the controller 16.

A further feedback loop, i.e. a power structure protections feedback loop, is also implemented in the prior art arrangement of Figure 1 , as follows. Power structure protections 30 derived by or from the power transistor driver structure 2 are fed to the controller 16, which takes account of the received power structure protections 30 when determining the power transistor switch action signals 28. In other prior art examples, the power structure protections are additionally or alternatively derived by or from the power transistor structure 4.

Figure 2 is a schematic block diagram showing certain further details of the backlight unit 14 of this embodiment. The backlight unit 14 comprises an (optional) current balancing module and/or (optional) ballasting module 34, connected to a plurality of fluorescent lamps 36.

The present invention is based in part on the insight of the present inventor that the internal arrangement and functionality of the controller 16 can

be arranged in such a way as to allow re-arrangement of some of that functionality from the isolated side to the non-isolated side of a power control and mains isolation arrangement such as that shown in Figure 1 , with resulting advantages that will be explained in more detail later below. In particular, the present inventor has envisaged dividing the functionality of the controller 16 between two functional modules, namely a pre-controller and a power transistor drive modulator. Figure 3 shows a power control and mains isolation arrangement 41 including a controller 46 as envisaged by the present inventor. The controller 46 would serve the same role in the power control and mains isolation arrangement 41 of Figure 3 as the controller 16 serves in the power control and mains isolation arrangement 1 of Figure 1. Other elements of the power control and mains isolation arrangement 41 of Figure 3 are the same as, and operate the same as, corresponding elements in the power control and mains isolation arrangement 1 of Figure 1 , and are indicated by the same reference numerals as in Figure 1.

In more detail, referring to Figure 3, the controller 46 comprises a pre- controller 47 and a power transistor drive modulator 48. A first input of the pre- controller 47 is coupled to the backlight unit 14 for receiving the backlight unit feedback information 24. A second input of the pre-controller 47 is arranged to receive the control signals 26 from a LCD small signal board. An output of the pre-controller 47 is coupled to a first input of the power transistor drive modulator 48 for forwarding a modulator control signal 49 to the power transistor drive modulator 48. A second input of the power transistor drive modulator 48 is coupled to the second mains isolation element 20 for receiving the power structure protections 30. An output of the power transistor drive modulator 48 is coupled to the first mains isolation element 18 for forwarding the power transistor switch action signals 28 to the first mains isolation element 18. The present inventor has divided the overall functionality of the controller 46 between the pre-controller 47 and the power transistor drive modulator 48, with the pre-controller determining a modulator control signal 49

from the received inputs of the backlight unit feedback information 24 and the control signals 26, and forwarding the modulator control signal 49 to the power transistor drive modulator which processes the modulator control signal 49 to determine and output the final power transistor switch action signals 28. Additionally, since in this example power structure protections 30 are also processed, the power transistor drive modulator 48 combines processing of the received power structure protections 30 with the processing of the modulator control signal 49 when determining the final power transistor switch action signals 28 to be output. Note the present inventor has envisaged that such processing of the power structure protections 30 is performed separately from the processing of the backlight unit feedback information 24 and the control signals 26, and in particular can be processed by the power transistor drive modulator 48 and not the pre-controller 47. Further details of the difference between the modulator control signal 49 and the power transistor switch action signals 28 will be described in more detail below with reference to Figure 4.

Figure 4 is a schematic illustration of a power control and mains isolation arrangement 101 according to a first embodiment of the invention. The power control and mains isolation arrangement 101 comprises the following elements: a power transistor drive modulator 148, a power transistor driver structure 102, a power transistor structure 104, a non-isolated high voltage bus 106, a power transformer 108 with a primary coil 110 and a secondary coil 112, a backlight unit 114, a pre-controller 147, and a mains isolation element 118. The power transistor driver structure 102 and the power transistor structure 104 are each connected to the non-isolated high voltage bus 106. A first output of the power transistor driver structure 102 is coupled to an input of the power transistor structure 104, for forwarding driver outputs 122. Together the power transistor driver structure 102 and the power transistor structure 104 provide an example of a power transistor switch arrangement of a switch mode power supply. The power output from the power transistor structure 104 is coupled to the primary coil 110 of the power transformer 108. The secondary

coil 112 of the power transformer 110 is coupled to the backlight unit 114. The power transistor structure 104 and the transformer 108 convert the direct current (DC) input power, supplied by the non-isolated high voltage bus 106, to alternating current (AC) lamp drive current that is fed to the backlight unit 114 for powering the fluorescent lamps of the backlight unit 114. A feedback output from the backlight unit 114 is coupled to an input of the pre-controller 147 for forwarding backlight unit feedback information 124 (such as backlight current and optionally backlight/lamp voltage and backlight/lamp-status/error-data) to the pre-controller 147. A further input of the pre-controller 147 is arranged for receiving control signals 126 from an LCD-TV/monitor set-level small signal and control board.

The output of the pre-controller 147 is coupled to an input on the isolated side of the mains isolation element 118 for forwarding a modular control signal 149 from the pre-controller 147 to the mains isolation element 118. An output on the non-isolated side of the mains isolation element 118 is coupled to an input of the power transistor drive modulator 148 for forwarding the modulator control signal 149 from the mains isolation element 118 to the power transistor drive modulator 148. In other words, the output of the pre- controller 147 is coupled to the power transistor drive modulator 148 via the mains isolation element 118, for forwarding the modulator control signal 149 from the pre-controller 147 to the power transistor drive modulator 148.

An output of the power transistor drive modulator 148 is coupled to an input of the power transistor driver structure 102 for forwarding power transistor switch action signals 128 to the power transistor driver structure 102. A further output from the power transistor driver structure 102 is coupled to a further input of the power transistor drive modulator 148 for feeding back power structure protections 130 from the power transistor driver structure 102 to the power transistor drive modulator 148.

Also shown in Figure 4 is a representation of the effective "mains isolation boundary" 132 for the power control and mains isolation arrangement 101. The mains isolation boundary 132 has two isolation crossings, namely the power transformer 108, and the mains isolation element 118. The following

elements are on the non-isolated side of the mains isolation boundary 132: the power transistor drive modulator 148, the power transistor driver structure 102, the power transistor structure 104, the non-isolated high voltage bus 106, and the primary coil 110 of the power transformer 108. The following elements are on the isolated side of the mains isolation boundary 132: the secondary coil 112 of the power transformer 108, the backlight unit 114, and the pre-controller 147.

In operation a first feedback loop is implemented, as follows. Power output from the power transistor structure 104 is fed to the power transformer 108. The resulting output power from the power transformer 108 is provided to the backlight unit 114 for powering operation of the backlight unit 114. Backlight unit feedback information 124 derived or output from operation of the backlight unit 114 is fed to the pre-controller 147. The pre-controller 147 determines the modulator control signal 149 based upon the received backlight unit feedback information 124 (and on the received control signals 126 from the LCD small signal board). The modulator control signal 149 is fed to the power transistor drive modulator 148. The power transistor drive modulator 148 determines the power transistor switch action signals 128 based upon the received modulator control signal 149 (and in this example also based upon the received power structure protections 130 received from a further feedback loop as will be described in the next paragraph below). The power transistor switch action signals 128 are fed to the power transistor driver structure 102. The power transistor driver structure 102 drives the power transistor structure 104. The operation of the power transistor driver structure 102, and hence the form of the power output from the power transistor structure 104, is dependent upon the power transistor switch action signals 128 determined and output by the power transistor drive modulator 148.

A further feedback loop, i.e. a power structure protections feedback loop, is also implemented in conjunction with the first feedback loop described in the preceding paragraph, as follows. Power structure protections 130 derived by or from the power transistor driver structure 102 (in other embodiments the power structure protections 130 may be additionally or

alternatively derived by or from the power transistor structure 104) are fed to the power transistor drive modulator 148, which takes account of the received power structure protections 130 when determining the power transistor switch action signals 128. The backlight unit 114 is the same as the backlight unit 14 of the Figure

1 arrangement, and, as shown in Figure 2, comprises an optional ballasting and/or balancing module 34, connected to a plurality of fluorescent lamps 36.

In this example, the power transistor drive modulator 148 and the power transistor driver structure 102 together form an integrated module, namely a drive modulator and power transistor driver device 150.

The modulator control signal 149 is a relatively low bandwidth signal that merely provides modulation information, i.e. instructional information or transfer information, e.g. logical instructions, for the power transistor drive modulator 148 to follow or otherwise process when the power transistor drive modulator 148 determines and outputs the power transistor switch action signals 128. The modulator control signal 149 may be either a pure modulator control signal 149 that carries out feeding forward, or transferring the proper control value to the transistor drive modulator, or may be 'a complex' of the modulator control signal including 'Command Information' for the power transistor drive modulator 148. Command information is information that will command the power transistor drive modulator 148 to operate in a specific mode and that is modulated into the modulator control signal 149. In particular, the modulator control signal 149 has significantly less bandwidth than the power transistor switch action signals 128, the power switch action signals 128 being actual switching signals. For example, the power switch action signals 128 may comprise power transistor drive signals that are square wave signals having steep edges and therefore containing high harmonic contents required to precisely define the timing of the power transistor switch actions. The transistor drive modulators output will provide the actual power transistor drive signals, whose timing is modulated based upon information provided on the input of the transistor drive modulator. For example, when the control loop is 'in (static) regulation' and no disturbances are introduced on the system, the

input of the transistor drive modulator is fully static and contains a value belonging to the switch action modulation fitting the regulated system under the given regulated situation. Meanwhile the power transistor drive modulator outputs power switches control signals with the high dynamics required for accurate transistor switch action timing fitting the static regulation situation. This is a reason why the bandwidth of the modulator control signal 149 is very much lower than the transistor switch action signals 128. The bandwidth of the mains isolation element carrying modulator control signal 149 is determined by the required control loop bandwidth, rather than by the required harmonic content and timing accuracy required for the transistor drive signals. This results in lower system cost and a more easy system design implementation.

Consequently, the mains isolation element 118 can be one that requires a lower bandwidth (i.e. frequency range) than that required for the mains isolation element 18 of the prior art power control and mains isolation arrangement 1 of Figure 1. Compared with prior art arrangements making use of a primary-side controller element the mains isolation element 118 can have a less accurate transfer function than that required for the mains isolation element of the prior art. In other words, an overall less stringent performance (e.g. speed/bandwidth and accuracy) is required for the mains isolation element 118 in the power control and mains isolation arrangement 101 compared to that required for the mains isolation element 18 in the prior art power control and mains isolation arrangement 1 and for mains isolation elements in prior art arrangements making use of a completely primary-side controller arrangement. In this embodiment, the mains isolation element 118 is a low accuracy, low speed, low cost opto-coupler element.

Thus in the preceding paragraph one advantage of the power control and mains isolation arrangement 101 is described, namely the use of a simpler mains isolation element. A further advantage of the power control and mains isolation arrangement 101 , which applies to those embodiments which include power structure protections feedback, will now described in the following paragraph.

In the power control and mains isolation arrangement 101 , the power structure protections 130, which are produced on the non-isolated side of the mains isolation boundary 132, are fed back to the power transistor drive modulator 148, which is also on the non-isolated side of the mains isolation boundary 132. Consequently, there is no requirement for a further mains isolation element for the power structure protections 130 to be fed through. This is in contrast to the prior art power control and mains isolation arrangement 1 of Figure 1 which requires the second mains isolation element 20 in order to feed the power structure protections 30 from the power transistor driver structure 2 on the non-isolated side of the mains isolation boundary 32 to the controller 16 on the isolated side of the mains isolation boundary 32. Furthermore, since the feed route of the power structure protections 130 is simpler than in the prior art, the power structure protections can be arranged to provide a quicker response which can be used for an emergency protection role, and which may consequently be considered as a reflex protection. This allows the pre-controller (147) to be implemented by means of a slower (non- real-time) control element, such as a simple micro controller.

Thus, compared to the prior art, the power control and mains isolation arrangement 1 of Figure 1 requires a lower number of mains isolation elements and also lower performance mains isolation element(s), and reflex protection can be provided, typically without any requirements on mains isolation elements, or high speed requirements of the pre-controller element.

Further advantages of the present invention will be apparent from the following further discussion of aspects of the invention. The invention alleviates or overcomes the disadvantage of requiring

(multiple), high dynamic (high frequency), and/or accurate mains isolation crossing elements in a switch-mode architecture that includes a primary-side, non-mains isolated power switch structure and a secondary side, mains isolated, controller element, such as, but not restricted to the use in, a fluorescent lamp controller/driver architecture.

Contrary to the conventional approach of having mains isolation in the feedback path of a primary side controller/driver, or having mains isolation in

the power transistor actuation path of a secondary side controller/driver, that requires high dynamic signals to be transferred across the mains isolation boundary to a primary-side gate driver device, according to the aspects of the present invention the control information for the 'power transistor drive modulator' is communicated across the mains isolation boundary. This signal has significantly lower bandwidth than the actual power transistor drive signals and while it is part of the forward path of the control loop (and not part of the feedback loop), the transfer function of the mains isolation crossing element is not a critical part of the control loop, as it will only influence the loop gain of the control loop. As a consequence, by way of example, a low cost, low bandwidth single opto-coupler is sufficient for this approach.

Because the power transistor drive modulator (which can be, but is not limited to, a frequency control for a half-bridge structure, or a phase shift control for a full-bridge drive structure) is on the primary side, protection circuitry influencing the power transistor drive modulator can be implemented as a emergency/reflex function of the primary side modulator/driver part of the control architecture. The implementation of such protection does not require the explicit interaction of the secondary side controller and therefore doesn't require additional mains isolation crossings. The secondary side controller can be arranged to recognize reflex protection behaviour of the primary side drive modulator/power transistor driver element, through the feedback-path on the load/output of the controlled system, which by way of example in the above embodiment is the backlight unit.

Thus the above described embodiment provides a power controller structure with a secondary-side (mains isolated) pre-controller and a primary- side (non-mains isolated) drive-modulator, power driver, power transistor structure, isolated by means of a mains isolation element that carries the drive modulator control signal, rather than the high dynamic switch action information for the power transistors themselves. Advantageously, the arrangement can rely on a single, low accuracy, low cost opto-coupler element for complete mains isolation (the power transformer is also mains isolated, but this is standard). Furthermore, in appropriate applications, reflex protection can

operate autonomously on the primary side power driver modulator without being commanded by the controller function on the secondary side.

Thus, in the power conversion arrangement of the above described embodiment, the primary side of the power conversion transformer(s) is(are) driven by a switch-mode power structure (for example, single transistor element, full-bridge, half-bridge, or push-pull power topology). The switch action signals of the primary side power structure are controlled by a control element that adapts the power structure's switch action signals (for example, switch frequency, switch duty cycle, phase shift, or combination of the before- mentioned), such as to realize (a) well controlled/regulated value(s) of the (secondary-side) output property (properties). The controlled output property (properties) can be, for example, output voltage, output power, output current, or a combination of any one or more of these properties, or any other appropriate properties depending upon the application. Conventionally, the control element is a concentrated/integrated building block, which is positioned as a whole either on the primary (non-mains isolated) side, or the secondary (mains isolated) side. Depending on the location/position of the concentrated/integrated control element (primary, or secondary side), either the path between the (isolated/secondary-side) controller output and (non-isolated/primary side) power topology, or the feedback path, from (isolated/secondary-side) controlled output and the input of the (non-isolated/primary-side) concentrated/integrated control element needs to cross a mains isolation barrier.

Conventionally, when the (isolated/secondary-side) control element output path towards the (none isolated/primary-side) power structures needs to be isolated, the isolation elements are required to have a high bandwidth to transfer the switch action signals defined by the control element to the power structure. The high bandwidth requirement of this isolation boundary requires relatively expensive high bandwidth isolation elements (like isolation transformers, or ultra high speed opto-couplers) to be used.

Conventionally, when the feedback path from (isolated/secondary-side) output (controlled property) needs to be isolated, the isolation boundary needs

to accurately transfer the measured output property across the isolation boundary, while any deviation will result in reduced controller accuracy.

However, the above described embodiment provides a new implementation of the mains isolation boundary in a feedback controlled, transformer based, mains-isolated switch-mode power conversion system, which may typically be implemented without compromising any performance parameter of the system, and furthermore may provide a cost saving. The present invention neither places the mains isolation between the output of the control element and the power switch structure, nor places the mains isolation element in the feedback path between the controlled output/load and the feedback input of the control element. Instead, in effect the conventionally concentrated control element is split into two separate elements: a pre- controller that has a low-bandwidth output (equal to the required bandwidth of the control loop, rather than a bandwidth required to facilitate full harmonic content of the switch action signals in the power structure) and a power transistor drive modulator which is in effect a 'switch action modulator' part that translates the low bandwidth signal of the 'pre-controller' to the actual switch action control signals for the power structure. It is in-between these two elements that a low cost, low bandwidth mains isolation element is placed. And because of the fact that this mains isolation element is in the control system's forward path, the accuracy and/or linearity of the transfer function of the applied mains isolation element is not critical for the quality of the controller.

In the above described embodiment, the power control and mains isolation arrangement is for driving a fluorescent backlight unit of an LCD display. Further details that derive from this particular implementation are described in the next paragraph, although it is to be appreciated that in other embodiments other implementations, other details will apply.

The lamps in a fluorescent backlight unit use an AC drive, which is generated by a DC to AC backlight inverter. The backlight inverter comprises a transformer whose secondary side is connected to the fluorescent lamp system and generates the AC lamp drive current, a power structure for driving the primary side of the inverter transformer, and a fluorescent lamp controller

that defines the power structure switch action signals, such that the lamp system is driven at the intended operating conditions. The backlight inverter control functions (e.g. current, voltage etc.) are 'inverter autonomous' control properties, though typically a backlight system does not operate fully autonomously. The operation of a backlight system is typically 'commanded' by electronics in the display system. The small signal and control board in for example an LCD (TV, or Monitor) system, as present in the above described embodiment, 'steers' (in a feedback-less command-type of action) the backlight to be: on/off and commands the backlight unit (inverter/lamp system) to run at a certain brightness (dimming of the backlight between, for example, 100%=max power and generally 10% of the light power). The "small signal board" is not part of the backlight unit, but is the part of an LCD display system that contains the signal (audio/video) processing electronics. The 'interaction' between small signal board and backlight unit (lamp inverter system) is the control signals representing command actions of the small signal board defining whether the lamp system should be on, or off and at what intensity/brightness the backlight should operate. As the LCD display system electronics are located at the mains isolated side, a primary side concentrated controller requires the command signals coming for the display system electronics to cross a mains isolation boundary, adding to the number of mains isolation crossings. When using mains isolated pre-controller, this disadvantage is overcome, while the use of the primary side switch action modulator relaxes the requirements on the mains isolation towards the switch elements without the requirement of high-speed mains isolation crossings required for concentrated secondary side controller elements. In this way, the 'split controller' approach (pre-controller, plus switch action modulator) reduces the number and nature (performance requirements) of the mains isolation elements, e.g. to a minimum cost level. Thus the present invention provides an arrangement in which the conventionally concentrated/integrated controller is split into two parts and separated by a mains isolation element. The isolated side of the controller generates a low-bandwidth modulation signal, and the non-isolated side of the controller generates a higher-bandwidth switching

signal for the relevant structure, e.g. half-bridge structure, according to the modulation signal.

Since the isolation boundary is crossed by the modulation signal instead of the switching signal, a far cheaper opto-coupler may be used, as the bandwidth of the modulation signal is significantly lower than the bandwidth (the harmonic content) of the (e.g. square wave) switching signal(s).

The backlight unit 114 constitutes, in the above described embodiment, the mains isolated load/output of the power control and mains isolation arrangement 101. The invention may be applied in other embodiments to applications other than LCD backlighting, i.e. with other items with different types of load/output. In the above described embodiment, the power transistor drive modulator may control, for example, frequency control for a half-bridge structure, or phase shift control for a full-bridge structure. In view of this, the pre-controller (e.g. 147) can be implemented as a universal control element that could be used in combination with transistor drive modulators utilizing completely different switch topologies.

In further embodiments, there may be an error status feedback from the pre-controller element, e.g. an inverter controller, back to the display system, indicating an error on the backlight system. In the above described embodiment, pre-controller can be a low speed controller that can be implemented using a relatively low cost micro controller element, in comparison to the prior art where e.g.gf a dedicated ASSP (Application Specific Standard Product) is typically used, thereby allowing wide flexibility in implementation of customer specific requirements for the control behaviour.

In the above described embodiment, the power transistor drive modulator 148 and the power transistor driver structure 102 together form an integrated module, namely the drive modulator and driver device 150. However, this need not be the case, and in other embodiments the power transistor drive modulator 148 and the power transistor driver structure 102 may be implemented as separate items.

In the above described embodiment, power structure protections are implemented, and are able to provide, due to the topology of the power control and mains isolation arrangement 101 of the embodiment, reflex protection. The power structure protections may be any appropriate power structure protections depending on the application, for example they may be related to one or more of over-current and over-voltage protections.

In other embodiments, according to the application where the invention is being employed, power structure protections may not be used. Although in such implementations the advantage of the above described embodiment of not requiring a mains isolation element will not be applicable, nevertheless the present invention will still be advantageous due to the lessening of the performance requirement for the remaining mains isolation element(s) as described earlier above.

In the above described embodiment, the power transistor driver structure 102 and the power transistor structure 104 together provide an example of a power transistor switch arrangement of a switch mode power supply. In other embodiments, other examples of a power transistor switch arrangement of a switch mode power supply may be employed.

In the above described embodiment, the control information processed by the pre-controller is the backlight unit feedback information and the control signals from the LCD small signal board, both arising due to the particular application (LCD with backlight) in which the above described embodiment is implemented. However, it will be appreciated that in other implementations, i.e. other embodiments, other control information will be processed rather than the particular examples of backlight unit feedback information and control signals from an LCD small signal board.

In the above described embodiment, the load (i.e. the backlight unit) is described as being part of the power control and mains isolation arrangement 101. However, it will be appreciated that the remainder of the arrangement without the load included as such represents a power control and mains isolation arrangement that is able to be coupled to a load, and hence in itself

the power control and mains isolation arrangement without the load represents an embodiment of the present invention.

In the above described embodiment, the arrangement includes a single power transistor structure and a single transformer, and correspondingly the pre-controller has a single load feedback information input and a single output. However, in other embodiments, the arrangement may include multiple power transistor structures (optionally having respective individual driver structures) and/or multiple transformers, e.g. a transformer arrangement comprising multiple transformers, e.g. for scanning type fluorescent backlights. In such cases, multiple pre-controllers each with a single load feedback information input and a single output may be used, or alternatively one or more pre- controllers with multiple load feedback information inputs and a single output may be used.

In other embodiments (for example the second main embodiment described later below with reference to Figure 5), the arrangement may (per pre-controller element) include multiple (n) pre-controller outputs and (per drive modulator element) multiple (n) drive modulator inputs to control multiple (n) parameters of the switch actions generated by the drive modulator, coupled by means of multiple (n) mains isolation elements. The various switch action parameters of the drive modulator may for example be: switch frequency, duty- cycle or phase shift, enabling the independent control of multiple parameters. Each switch action parameter is controlled in a corresponding manner as the control of a single switch action parameter. The use of multiple pre-controller outputs/drive modulator inputs can be pursued when the control information cannot be combined through one single mains isolation element.

Figure 5 is a schematic illustration of a power control and mains isolation arrangement 201 according to a second main embodiment of the invention. The power control and mains isolation arrangement 201 comprises the same elements as, and is the same as, the arrangement described with reference to Figure 4, except where stated in the following. Also, the same reference numerals are used to indicate the same elements.

In this embodiment, in addition to the previously described mains isolation element 118 (for convenience referred to hereinafter in this description of the second embodiment as the first mains isolation element 118), the power control and mains isolation arrangement 201 comprises a further mains isolation element 158 (for convenience referred to hereinafter in this description of the second embodiment as the second mains isolation element 118).

In this embodiment, the following arrangement comprised by the first embodiment is again present. An output, (i.e. a first output) of the pre- controller 147 is coupled to an input on the isolated side of the first mains isolation element 118 for forwarding a modular control signal 149 (for convenience referred to hereinafter in this description of the second embodiment as the first modular control signal 149) from the pre-controller 147 to the first mains isolation element 118. An output on the non-isolated side of the first mains isolation element 118 is coupled to an input (i.e. a first input) of the power transistor drive modulator 148 for forwarding the first modulator control signal 149 from the first mains isolation element 118 to the power transistor drive modulator 148. In other words, the first output of the pre- controller 147 is coupled to the power transistor drive modulator 148 via the first mains isolation element 118, for forwarding the first modulator control signal 149 from the pre-controller 147 to the power transistor drive modulator 148.

In this embodiment, additionally the following additional arrangement is also comprised by the power control and mains isolation arrangement 201. A second output of the pre-controller 147 is coupled to an input on the isolated side of the second mains isolation element 158 for forwarding a modular control signal 159 (for convenience referred to hereinafter in this description of the second embodiment as the second modular control signal 159) from the pre-controller 147 to the second mains isolation element 158. An output on the non-isolated side of the second mains isolation element 158 is coupled to a second input of the power transistor drive modulator 148 for forwarding the second modulator control signal 159 from the second mains isolation element

158 to the power transistor drive modulator 148. In other words, the second output of the pre-controller 147 is coupled to the power transistor drive modulator 148 via the second mains isolation element 158, for forwarding the second modulator control signal 159 from the pre-controller 147 to the power transistor drive modulator 148. As described earlier above with reference to Figure 4 in the case of the first modulator control signal 149, the second modulator control signal 159 is a relatively low bandwidth signal that merely provides modulation information, i.e. instructional information or transfer information, e.g. logical instructions, for the power transistor drive modulator 148 to follow or otherwise process when the power transistor drive modulator 148 determines and outputs the power transistor switch action signals 128.

In this embodiment, there are two control parameters that it is desired to manage individually. One of the control parameters is switch frequency, the other is phase shift. In this embodiment the phase shift control also has a pulse width modulation on top of its magnitude information, hence a separate path is desired to manage the control of the operating frequency. Hence in this embodiment there are two separate modular control signal routes provided, and in this embodiment the first modulation control signal 149 is a frequency control signal, and the second modulation control signal 159 is a phase shift control signal.

However, it will be appreciated that in other embodiments, the plural modulation control signals may be other than for frequency and/or phase shift. It will also be appreciated that in other embodiments more than two modular control signal routes may be provided, each with a respective mains isolation element, with one route being provided for each control parameter it is desired to manage individually.

It will also be appreciated that the various options and possibilities described earlier above with reference to the first embodiment are also applicable to the above described second main embodiment.