The invention relates to a power supply circuit, and to a display apparatus comprising such a power supply circuit.

The power consumption of a plasma display apparatus (further referred to as PDP) is largely determined by the power consumption of the plasma module which comprises a plasma panel and the drive electronics. The plasma panel comprises a matrix of plasma cells associated with select electrodes and crossing address electrodes. In three electrode displays, the select electrodes comprise parallel arranged sustain electrodes and common electrodes. Usually, the plasma panel is subfield driven. A field of video information to be displayed is divided into a number of subfields. Each subfield comprises an erase phase during which the charge in the plasma cells gets a defined starting value, an addressing phase and a sustaining phase. During an addressing phase, lines of plasma cells (each line is associated with a sustain electrode and a common electrode) are selected one by one to store a charge in the plasma cells dependent on the data. A cell primed during the address period to produce light will do so during the sustain period wherein sustain pulses are supplied between the sustain electrodes and the common electrodes.

Especially, the power consumption during the sustaining phase of the plasma module is large.

Usually, a plasma display apparatus comprises at least two main power converters, one which mainly supplies power to the sustain driver and another one which supplies power to the other circuits (such as logic circuits). Also, other control voltage supplies may be required. For example, the maximum power to be supplied by the power converter for the sustain driver may be in the range from 350 to 800 Watts for large PDP displays of 42"and 50".

It is a drawback that the power converter which supplies power to the sustain driver has to supply a very high power. Consequently, a bulky and expensive transformer is required in this power converter.

It is an object of the invention to provide an arrangement of power converters wherein the highest loaded power converter becomes less bulky.

A first aspect of the invention provides a power supply circuit as defined in claiml. A second aspect of the invention provides a display apparatus comprising such a power supply circuit as claimed in claim 4. Advantageous embodiments are defined in the dependent claims.

The power supply circuit comprises a first power converter which supplies a first DC output voltage between a first output node and a second output node, and a second power converter which supplies a second DC output voltage between the second output node and a third output node. A first load is coupled between the first node and the third node, and a second load is coupled between the second node and the third node.

The second load is arranged to receive the second DC output voltage of the second power converter and thus the power required by the second load is completely delivered by the second power converter. The first load receives the series arranged first DC output voltage and second DC output voltage. Consequently, the power required by the first load is not only supplied by the first power converter, but also for a part by the second power supply. This decreases the power to be supplied by the first power converter. The transformer of the first power converter becomes less bulky and less costly.

In an embodiment as defined in claim 5, the display apparatus comprises a plasma display panel. The first load comprises the sustain driver, and the second load comprises the address driver. Now, the high load to be supplied by the first power converter decreases because part of it is delivered by the second power converter.

For example, if in accordance with the prior art power supply, the first load consumes 720 Watts (for example, 4A at 180V) and the second load consumes 120W (for example, 2A at 60V) the first power converter has to supply 720W and the second power converter has to supply 120W. In the power supply in accordance with the invention, the first load still consumes 720 Watts, 240W (4A at 60V) is supplied by the second power converter, and 480W (4A at 120V=180V-60V) by the first power converter. The total load of the second power converter is 360W (240W + 120W). Thus, the power supplied by the first power converter decreases considerably and allows the use of usually available and reasonable priced components in the first power converter. And, in particular, the lower power to be supplied allows the transformer to be less bulky. However, the power supplied by the second power converter increases, but still stays within a range in which the power converter can be

designed with reasonable priced components. The total power supply will be cheaper and less bulky.

It is an advantage to use the power converter which supplies power to the address drivers to supply part of the power required by the sustain drivers, because the power consumed by the address drivers decreases when the power consumed by the sustain driver is close to the maximum display load.

During the sustaining phase, the plasma cells (pixels) which have to generate light may vary between none and all. Consequently, variations of the power consumption as a function of the display load are very large. The display load is denoted as a percentage of a full-white image, thus, the average pixel gray value over the actual image divided by the maximum gray (is the full-white) value.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings.

In the drawings: Fig. 1 shows a block diagram of an embodiment of a power supply in accordance with the invention, Fig. 2 shows a circuit diagram of an embodiment of a power supply in accordance with the invention, and Fig. 3 shows a block diagram of a display apparatus with a power supply in accordance with the invention.

Fig. 1 shows a block diagram of an embodiment of a power supply in accordance with the invention. The power converter 1 receives an input voltage VI to supply an output voltage VO1 between the nodes N 1 and N2. The power converter 2 receives the input voltage VI to supply the output voltage V02 between the nodes N2 and N3. The load LI is coupled between the nodes N1 and N3. The load L2 is coupled between the nodes N2 and N3.

The input voltage may be a DC voltage, for example a rectified mains voltage.

The power converters 1 and 2 may be of any type, for example, a flyback converter or a resonance LLC converter. The power converters 1 and 2 may be of a different type. The power converters 1 and 2 preferably supply stabilized output voltages VOl and V02.

V02 is voltage across the load L2, thus the power required by L2 is completely supplied by the power converter 2. The voltage across the load LI is the addition of the output voltages V01 and V02. Thus the power consumed by the first load LI is for a part supplied by the power converter 1 and for a part by the power converter 2. With respect to the prior art, the power converter 1 has to supply less power, however, the power converter 2 has to supply more power.

It is especially important to lower the power to be supplied by the power converter 1 if the power converter 1 has to supply a very large power with respect to the power converter 2. A power converter which has to supply a high power is bulky and requires expensive components (transformer, switches, and rectifiers). It is very costly to design and manufacture such a high power converter if the components required are not available on the market and have to be designed specifically for this power converter. It is not a problem to increase the power of the relatively low loaded power converter 2. In total, the combination of the power supplies 1 and 2 will be cheaper and less bulky.

Especially in applications wherein a shallow design is important, for example in matrix display apparatuses, such as plasma displays, the transformers of the power converters 1 and 2 should be small.

Fig. 2 shows a circuit diagram of an embodiment of a power supply in accordance with the invention in which the power converters 1 and 2 are resonant LLC converters. Such resonant LLC converters are known from US-A-6,344, 979 which discloses a LLC series resonant DC to DC converter. This prior art LLC converter comprises a square- waveform generator, an LLC resonant network, a high frequency transformer, a rectifier circuit and an output filter.

The square-waveform generator is a half bridge inverter which contains two switches. Instead of a half bridge inverter, a full bridge inverter may be used. The LLC resonant network is coupled across one of the switches. The switches alternatively turn on and off. The LLC resonant circuit comprises a series arrangement of a series capacitor, a series inductor and a parallel inductor. The parallel inductor is arranged in parallel with a primary winding of a transformer. The series inductor can be implemented as an external component or as a leakage inductance of the transformer. The parallel inductor can be implemented as an external component or as the magnetizing inductance of the transformer.

The rectifier circuit is connected to a secondary winding of the transformer to supply a DC output voltage to the load. The rectifier circuit may comprise a center-tapped or a full-bridge rectifier. The output filter comprises a capacitor to filter out the high frequency ripple.

The power converter 1 comprises a series arrangement of two MOSFET switches Sl 1 and S 12 which receives the input voltage VI and which forms the square wave generator. The LLC resonant circuit comprises the series arrangement of the resonance capacitor CR1, the series inductor LSI and the primary winding LM11 of the transformer T 1.

The LLC resonant circuit is arranged in parallel with the switch S12. The transformer T1 further has a secondary winding LM12 which supplies the output voltage VO1 between the nodes N1 and N2 via the rectifier circuit RE1. The rectifier circuit RE1 may comprise a single diode but preferably is a full bridge or a center tapped rectifier. A smoothing capacitor Cl 1 is arranged between the nodes N1 and N2. The load L 11 is arranged between the node N1 and ground which is the node N3. The switches Sol 1 and S 12 are controlled by the control signals CS 11 and CS 12 to alternatively conduct with a duty cycle of about 50%.

The power converter 2 comprises a series arrangement of two MOSFET switches S21 and S22 which receives the input voltage VI and which forms the square wave generator. The LLC resonant circuit comprises the series arrangement of the resonance capacitor CR2, the series inductor LS2 and the primary winding LM21 of the transformer T2.

The LLC resonant circuit is arranged in parallel with the switch S22. The transformer T2 further has a secondary winding LM22 which supplies the output voltage V02 between the nodes N2 and N3 via the rectifier circuit RE2. The rectifier circuit RE2 preferably is a full bridge or a center tapped rectifier. A smoothing capacitor C22 is arranged between the nodes N2 and N3. The load L22 is arranged between the node N2 and ground. The switches S21 and S22 are controlled by the control signals CS21 and CS22 to alternatively conduct with a duty cycle of about 50%.

The control of the switches S11, 512, 521, S22 required to stabilize the output voltages VO1 and V02 is not relevant for the invention, and is therefore not discussed.

In the arrangement of Fig. 2, with respect to the prior art, the power supplied by the first power converter 1 decreases considerably and allows the use of usually available and reasonable priced components in the first power converter 1. And, in particular, the lower power to be supplied allows the transformer T1 to be less bulky. However, the power supplied by the second power converter 2 increases, but still stays within a range in which the power converter can be designed with reasonable priced components. The total power supply will be cheaper and less bulky.

If the value of the voltage V02 is fixed by the load L22 and the values of the voltages VO1 and V02 are too far apart or if the value of the voltage VO1 is smaller than the value of the voltage V02, it is possible to add an extra winding on the transformer T2 which

supplies a more suitable voltage V02. This extra winding need not supply power to an other load and thus can be selected optimally.

The type of power converters used is not relevant to the invention. The LLC converters are shown by way of example, and may have any other suitable arrangement. For example, in the second converter the resonance capacitor CR2 may be divided into two series arranged capacitors which series arrangement is arranged in parallel with the series arranged switches S21 and S22. the resonance inductor LS2 and the transformer T2 are arranged in series between a junction of the switches S21 and S22 and a junction of the series arranged capacitors. The same structure may be used in the first power converter 1. The advantage of splitting the capacitor CR2 is that the ripple current injected into the input voltage VI will be reduced.

Fig. 3 shows a block diagram of a display apparatus with a power supply in accordance with the invention. The display apparatus comprises a power converter 1, a power converter 2, a sustain driver 3, an address driver 4 and a plasma display panel 5.

Both the power converter 1 and the power converter 2 receive an input voltage VI which is the rectified mains voltage. The power converter 1 supplies the output voltage VO1 across the nodes N1 and N2, the power converter 2 supplies the output voltage V02 across the nodes N1 and N3 which is ground. A smoothing capacitor C1 is arranged between the nodes N1 and N2, and a smoothing capacitor C2 is arranged between the nodes N2 and N3.

The plasma display panel 5 has plasma cells or pixels (not shown) in which the address driver 4 writes data DA. Usually, the data DA is written line by line by selecting the lines of pixels successively. The sustain driver 3 supplies sustain pulses SU to the plasma display panel 5 to generate light in the plasma cells in which data DA is written indicating that the cell should emit light during the sustain phase.

The power converter 2 supplies the relatively low power required by the address driver 4 and part of the relatively high power required by the sustain driver 3. The power converter 1 supplies the majority of the power required by the sustain driver 3.

For example, if the output voltage VO1 is 180V and the output voltage V02 is 60V, the current drawn by the sustain driver 3 is 4A, and the current drawn by the address driver 4 is 2A, in accordance with the prior art power supply, the power converter 1 has to supply 720 Watts (4A at 180V) and the power converter 2 has to supply 120W (2A at 60V).

In the power supply in accordance with the invention, the sustain driver 3 still requires 720 Watts of which, 240W (4A at 60V) is supplied by the second power converter 2, and 480W

(4A at 120V=180V-60V) instead of 720W by the first power converter 1. The total load of the second power converter 2 is now 360W (240W + 120W) instead of 120W. The power of both power converters becomes much more equal, and is in a range which allows use of reasonably priced components. The very expensive and bulky components of the 720W power converter are prevented.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, the invention is not limited to use in plasma display apparatuses, but is useful in all situations wherein at least two separate power converters are used and one of the power converters has to supply a much higher power than the other power converter.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word"comprising"does not exclude the presence of elements or steps other than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.