HK1118965B - Dc/ac inverter, method for dc-to-ac signal conversion, and display system - Google Patents

Description

DC/AC inverter, DC-AC signal conversion method and display system

Technical Field

The present invention relates to a direct current to alternating current (DC/AC) converter circuit, and more particularly to a DC/AC inverter for a Liquid Crystal Display (LCD) backlight.

Background

Liquid crystal display panels are widely used in various applications ranging from portable electronic devices to stationary units, such as video cameras, notebook computers, and industrial machines. The liquid crystal display panel itself does not emit light but needs to be illuminated with a light source. The most commonly used backlight source is a Cold Cathode Fluorescent Lamp (CCFL). However, the lighting and operation of CCFLs requires a high ac voltage. The lighting voltage is typically about 1000 volts and the operating voltage is about 500 volts. To generate such a high alternating voltage using a direct current power source (e.g., a rechargeable battery), a DC/AC inverter needs to be designed.

In addition to delivering the required AC high voltage signal, DC/AC inverters are increasingly demanding in terms of increased efficiency and reliability, reduced size, and reduced cost. In addition, the DC/AC inverter needs to provide a pure sinusoidal AC voltage signal in order to minimize rf electromagnetic scattering and provide optimal current-to-brightness conversion for the CCFL. Conversely, an irregular sine wave with a high crest factor (crest factor) will reduce the life of the CCFL tube. The crest factor refers to the ratio of the peak lamp current to the average lamp current.

With the division into operating frequencies, today's DC/AC inverters can be divided into two categories, namely fixed operating frequencies and variable operating frequencies. 5,619,402 discloses a fixed operating frequency DC/AC inverter, the contents of which are fully incorporated by reference into this application. The DC/AC inverter has a constant operating frequency regardless of the DC input signal and the load condition. Therefore, although such a DC/AC inverter has the advantages of high efficiency, reliability and low electromagnetic interference under the condition of relatively low DC input signal and relatively large load, the crest factor of the lamp current is high when the DC input signal is relatively high or the load is relatively small, which results in the shortened service life of the backlight lamp.

As shown in fig. 1, the "buck/Royer" circuit 100 is a typical DC/AC inverter with variable operating frequency. The circuit 100 is essentially comprised of a step-down buck regulator 110 and a self-tuned Royer oscillator 120 including a step-up transformer 121. The step-down buck regulator 110 provides an unrectified dc input signal V from the battery or possibly from the power supply line_dcTo a stable voltage within the nominal input range of the self-tuned Royer oscillator 120. The self-tuned Royer oscillator 120 is comprised of a step-up transformer 121, two power switches 123 and 125, a resonant capacitor 127, a base winding 129, a ballast capacitor 131, and a Pulse Width Modulation (PWM) controller 130. The operating frequency of the circuit 100 is set to correspond to a certain resonant frequency. The resonant frequency is determined by the resonant circuit consisting of step-up transformer 121, resonant capacitor 127, base winding 129, ballast capacitor 131, and the CCFL load. Therefore, the operating frequency will change dynamically as the CCFL load conditions change. This mode of operation is referred to as a variable operating (frequency) mode.

Us patent nos. 5,430,641, 5,619,402, 5,615,093 and 5,818,172 are some derivative topologies of variable operating frequency DC/AC inverters. Although the DC/AC inverter disclosed in the above patent has a good crest factor of the AC signal, the DC/AC inverter has the disadvantages of low conversion efficiency, two-stage power conversion, and high electromagnetic interference. In addition, high magnetic flux densities occur at the transformer in the DC/AC inverter when the load is large or short-circuited. Due to this high magnetic flux density, the transformer may saturate and components inside the DC/AC inverter may be damaged.

Thus, both conventional fixed and variable operating frequency DC/AC inverters have disadvantages.

Disclosure of Invention

One embodiment of the present invention provides a DC/AC inverter for driving a load. The DC/AC inverter includes a DC power source for providing a DC input voltage, a converter circuit coupled to the DC power source for converting the DC input voltage to an AC signal for driving a load, and a control circuit coupled to the converter circuit for setting a frequency of the AC signal. The control circuit may also operate the DC/AC inverter using a fixed frequency mode and a variable frequency mode.

Another embodiment of the present invention provides a method of converting a dc electrical signal to an ac electrical signal to drive a load. The method includes the steps of setting a predetermined threshold condition, operating in a fixed frequency mode wherein a fixed frequency corresponding to the frequency of the ac electrical signal is maintained at a constant frequency, operating in a variable frequency mode wherein the frequency corresponding to the ac electrical signal varies in accordance with a corresponding resonant frequency of a resonant circuit, and switching between the fixed frequency mode of operation and the variable frequency mode of operation as a function of the predetermined threshold condition.

Another embodiment of the present invention provides a display system. The display system includes a display device, a light source coupled to the display device for providing light to and emitting light from the display device, a processing unit coupled to the display device for generating data to be displayed on the display device, and a controller coupled to the light source for automatically selecting an optimal operating frequency from a constant frequency or a resonant frequency.

Drawings

The advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the following drawings.

Fig. 1 is a schematic diagram of a prior art "buck/Royer" circuit 100.

Fig. 2 is a block diagram of a DC/AC inverter according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a DC/AC inverter according to an embodiment of the present invention.

Fig. 4 is a graph of various waveforms for the DC/AC inverter shown in fig. 3 operating in a fixed frequency mode.

Fig. 5 is a graph of various waveforms for the DC/AC inverter shown in fig. 3 operating in a variable frequency mode.

Detailed Description

The examples set forth in the following detailed description are intended to illustrate the invention and are not intended to limit the scope of the invention to the examples. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

Fig. 2 is a block diagram of an exemplary DC/AC inverter 200 provided by an embodiment of the present invention. The DC/AC inverter 200 receives a DC input signal VIN from a DC power source 210 and provides a high voltage AC output signal VOUT to a load 230. The dc power supply 210 may be a battery, an adapter, or the like. The load 230 is typically one or more discharge lamps, such as CCFLs, used for backlighting the LCD panel. The DC/AC inverter 200 is mainly composed of a converter circuit 220 and a control circuit 250. The converter circuit 220 includes a plurality of switches and a transformer for converting a dc signal to an ac signal. The control circuit 250 is coupled to the converter circuit 220 for providing a dimming control signal. The dimming control signal controls the on-state of each switch in the converter circuit 220 to adjust the amplitude or frequency of the ac output signal VOUT, and thus the brightness of the load 230.

In addition, the control circuit 250 can automatically operate in a fixed frequency mode or a variable frequency mode according to the voltage value of the dc input signal and the load condition in order to obtain the optimum amplitude and frequency of the ac output signal. When the dc input signal voltage is relatively low or the load is relatively large, a ramp signal of a constant frequency may be provided to the control circuit 250, and the control circuit 250 operates in the fixed frequency mode. When the dc input signal voltage is relatively high or the load is relatively small, a ramp signal of variable frequency is provided to the control circuit 250, and the control circuit 250 operates in a variable frequency mode. In other words, a threshold may be preset according to the dc input voltage and the load condition, and when the threshold is reached, the switching between the fixed frequency mode and the variable frequency mode is performed.

The ramp signal may be provided by an oscillator 260 coupled to the control circuit 250. The oscillator 260 is also coupled to a detector 270 and an RC network 280, the detector 270 and the RC network 280 determining the values of the variable frequency and the fixed frequency, respectively. The detector 270 detects the resonant frequency of the resonant circuit 290, the resonant circuit 290 being comprised of a power supply drive including resonant elements in the converter circuit 220 and the load 230. When the dc input signal VIN is relatively high or the load is relatively small, the detector 270 generates a SYNC signal with a frequency equal to the resonant frequency and sends the SYNC signal to the oscillator 260, and the oscillator 260 generates a ramp signal according to the SYNC signal. In this case, the frequency of the ramp signal is a variable frequency, and the control circuit 250 operates in a variable frequency mode. The frequency of the ac output signal eventually varies according to the resonant frequency. When the dc input signal is relatively low or the load is relatively large, the frequency of the SYNC signal generated by the detector 270 will be a constant value, and the control circuit 250 will operate in a fixed frequency mode. In this case, the frequency of the ramp signal is constant, as determined by RC network 280, and control circuit 250 operates in a fixed frequency mode. The frequency of the ac output signal is ultimately set to a constant frequency determined by RC network 280.

In addition, to tightly control the brightness of load 230, DC/AC inverter 200 uses a closed loop design. To implement this closed loop design, a feedback circuit 240 is connected between the control circuit 250 and the load 230 for sensing the current flowing through the load 230. The feedback circuit 240 may also include a voltage feedback for circuit protection.

Fig. 3 is an exemplary schematic diagram 300 of a DC/AC inverter. Referring to fig. 3, a circuit 300 is comprised of a dc power supply 310, an inverter circuit 320, a load 330, a feedback circuit 340, a control circuit 350, an oscillator 360, a detector 370, and an RC network 380. The dc power supply 310 includes a battery 301 for providing a dc input signal VIN. The converter circuit 320 includes a plurality of switches 321, 323, 325, and 327, including a transformer 329 and capacitors 331 and 333. The plurality of switches may be comprised of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and positioned in diagonal switch pairs. As shown in fig. 3, the switches 321 and 323 are connected in series between the dc input signal VIN and ground, and a common node of the two switches is connected to one end of the transformer 329. Switches 325 and 327 are connected in series between the dc input signal VIN and ground, with a common node connected to the other end of transformer 329. The two diagonal switch pairs define alternate conduction paths between the dc input signal VIN and the transformer 329. The dc input signal VIN is converted into an intermediate ac signal by flowing through the alternately conducting paths. The intermediate ac signal is then boosted by transformer 329 to become the ac output signal VOUT.

As described above, the control circuit 350 can adjust the amplitude or frequency of the ac output signal VOUT by controlling the on-states of the switches. The conductive states of the plurality of switches are controlled by drive signals DRV1 through DRV4 from the control circuit 350. The control circuit 350 is also coupled to the oscillator 360 to receive the ramp signal and to the feedback circuit 340 to receive the feedback signal (FB). The control circuit 350 generates the driving signal according to the ramp signal and the feedback signal.

Referring to fig. 3, the control circuit 350 may be composed of a comparator 351, a Pulse Width Modulation (PWM) selector 353, Break Before Make (BBM) circuits 355 and 357, and driving circuits 359, 361, 363, and 365. The comparator 351 generates a PWM signal by comparing the ramp signal and the feedback signal. The PWM selector 353 generates two PWM signals, PWM1 and PWM2, from the PWM signal by defining different action edges (action edges), e.g., rising and falling edges. These two PWM signals are then sent to BBM circuits 355 and 357, respectively. In each BBM circuit, the input PWM signal is converted into two complementary signals. Each complementary signal is then converted to a drive signal by one of the drive circuits 359, 361, 363, and 365. As shown in fig. 3, the drive circuit 359 generates a drive signal DRV1 and transmits to the switch 321, the drive circuit 361 generates a drive signal DRV2 and transmits to the switch 323, the drive circuit 363 generates a drive signal DRV3 and transmits to the switch 325, and the drive circuit 365 generates a drive signal DRV4 and transmits to the switch 327. Due to the presence of the BBM circuit 355, the switches 321 and 323 can be guaranteed to be not on at the same time. Similarly, the switch circuits 325 and 327 can be guaranteed to not conduct simultaneously due to the presence of the BBM circuit 357. In addition, the control circuit 350 may operate in a fixed frequency mode or a variable frequency mode by controlling the ramp signal. Ultimately causing the DC/AC inverter to generate a sine wave of optimal amplitude and frequency and provide it to the load 330.

The ramp signal is controlled by oscillator 360, RC network 380, and detector 370 collectively. RC network 380 is formed by resistor 381 and capacitor 383, which are connected in series and between dc input signal VIN and ground.

The detector 370 includes an AND gate (AND gate)391, a comparator 393, a first edge flip-flop 395, a second edge flip-flop 397, AND an OR gate (OR gate) 399.

The non-inverting input of comparator 393 is connected to the common node of switches 321 and 323 through electrical line LX1, and the inverting input is connected to the common node of switches 325 and 327 through electrical line LX 2. The output of comparator 393 is connected to two edge flip-flops 395 and 397. The outputs of the edge flip-flops 395 and 397 are then provided as inputs to an or gate 399. Finally, or gate 399 outputs the SYNC signal described above. In actual operation, the detector tracks the resonant frequency determined by the resonant element and the load 330 within the converter circuit 320 by detecting the zero cross current (zero cross current) of the diagonal switch pair. The current for the diagonal switch pair can be indicated by wires LX1 and LX 2. If a zero crossing current is detected, the resonant frequency is tracked and the signal SYNC appears as a square wave with the resonant frequency, indicating that the variable frequency mode is selected. Or the constant magnitude of signal SYNC without tracking the resonant frequency indicates that a fixed frequency mode is selected.

In addition, the comparator detects the zero-crossing current only when both switches 323 and 327 are conductive, so that and gate 391 may be used to and the drive signals DRV2 and DRV4 and generate an enable signal to comparator 393. Based on the enable signal, it is ensured that zero-crossing current is detected when both switches 323 and 327 are turned on. In addition, as already mentioned above, edge flip-flops 395 and 397 are connected to the PWM selector 353 to define the action edges.

Oscillator 360 is connected to a common node 385 of resistor 381 and capacitor 383 in RC network 380, where a ramp signal is generated. The oscillator 360 is also connected to the output of an or gate 399 in the detector to receive the signal SYNC. As shown in fig. 3, the oscillator 360 may be composed of a first comparator 367, a second comparator 369, a flip-flop 371, an or gate 373, and a discharge switch 375. The first comparator 367 and the second comparator 369 form a voltage comparator. The non-inverting input of the first comparator 367 receives a low threshold voltage, e.g., 50 mV. The inverting input of the second comparator 369 receives a high threshold voltage VIN/N, where N may be an integer. An inverting input of the first comparator 367 and a non-inverting input of the second comparator 369 are connected to the node 385 for receiving the ramp signal. Based on the comparison of the ramp signal with the high voltage threshold and the low voltage threshold, a set Signal (ST) is sent from the output terminal of the first comparator 367 to the set (S) terminal of the flip-flop 371, and a Reset (RST) signal is sent from the output terminal of the second comparator 369 to the or gate 373. Meanwhile, or gate 373 receives the SYNC signal from detector 370. An output terminal of the or gate 373 is connected to a reset (R) terminal of the flip-flop 371. The output of flip-flop 371 is used to control the conductive state of switch 375. Switch 375 is connected between node 385 and ground. The ramp signal generated on node 385 has either the constant frequency or the variable frequency depending on the dc input signal VIN and the load conditions. The constant frequency is determined by the resistance of resistor 381 and the capacitance of capacitor 383 in RC network 380. The variable frequency is then equal to the resonant frequency tracked by the detector 370. The control circuit 350 determines the operation and the fixed frequency mode or the variable frequency mode according to the ramp signal.

Additionally, one skilled in the art will appreciate that feedback circuit 340 may use any circuit commonly used to sense current flowing through load 330. Since the feedback function is not closely related to the present invention, a detailed description of the feedback circuit 340 is omitted. It will also be appreciated by those skilled in the art that the converter circuit 320 may be configured in other well-known topologies besides the one presented herein, such as half-bridge, push-pull, etc., and that the control circuit 350 will drive the switches in the converter circuit 320 in a corresponding configuration. For various configurations, the present invention provides that the control circuit 350 can automatically operate in either a fixed frequency mode or a variable frequency mode.

Referring to fig. 4 and 5, the operation of the DC/AC inverter shown in fig. 3 will be described in detail. Assuming that a certain threshold of the dc input signal VIN is considered and the load condition is predetermined, when the dc input signal VIN does not exceed a preset voltage value or the load magnitude is greater than a preset value, the threshold is not reached, and the control circuit 350 operates in the fixed frequency mode. Fig. 4 shows various waveforms in a fixed frequency mode.

Referring to fig. 4, it can be seen that the operating frequency of control circuit 350, or the frequency of the PWM signal, is represented by waveform 415, which is determined by one discharge and charge cycle of the ramp signal, which is represented by waveform 413. When the ramp signal decreases to a low threshold voltage, i.e., 50mV, the set signal ST is triggered and the discharge switch 375 is opened. The discharge period of the ramp signal ends and the charge period begins. When the ramp signal rises to a high threshold voltage, i.e., VIN/N, the reset signal RST represented by waveform 407 is triggered and the discharge switch 375 is turned on. The charging period of the ramp signal ends and the discharging period begins. Those skilled in the art will appreciate that the charging and discharging speeds are determined by resistor 381 and capacitor 383 in RC network 380. Therefore, the operating frequency and the charging and discharging periods are determined by the preset high and low voltage thresholds, the resistance of resistor 381, and the capacitance of capacitor 383.

With constant frequency control, the diagonal switch pair will not have zero cross current at the same time, as shown by waveforms 403 and 405, respectively. Waveforms 403 and 405 are the voltages in wires LX1 and LX2, respectively. These voltages may indicate current flow through the diagonal switch pairs, so a zero voltage crossing of waveforms 403 and 405 indicates a zero current crossing of two diagonal switch pairs. As can be seen by waveforms 403 and 405, the zero crossing current does not occur simultaneously for both diagonal switch pairs. Without the synchronous zero crossing current, the signal SYNC will have a constant magnitude as shown by waveform 407. In addition, waveform 401 represents the current Ipri flowing through the base winding of transformer 329 shown in fig. 3. Waveform 411 represents the feedback signal provided by feedback circuit 340. The feedback signal crosses the ramp signal and determines the duty cycle of the PWM signal.

The control circuit 350 will remain in the fixed frequency mode until the threshold is reached. In other words, if the dc input signal VIN exceeds the predetermined voltage value or the load is less than the predetermined value, the resonant frequency can be tracked by detecting the synchronous zero crossing voltages on the lines LX1 and LX 2. Fig. 5 shows various waveforms in the variable frequency mode.

Referring to fig. 5, it can be seen that the operating frequency of the control circuit 350, i.e., the frequency of the PWM signal shown in waveform 515, is again determined by the charge and discharge cycles of the ramp signal, which is shown in waveform 513. Unlike waveform 413 shown in fig. 4, the ramp signal cannot be charged to reach the predetermined high threshold voltage VIN/N, so the operating frequency is no longer determined by the predetermined high and low voltage thresholds, the resistance of resistor 381, and the capacitance of capacitor 383.

In fact, the operating frequency and the charging and discharging period of the ramp signal at this time are determined by the frequency of the signal SYNC, which is shown as the waveform 507. The frequency of the signal SYNC further reflects the resonance frequency, and is obtained by detecting the zero-crossing voltage of the electric lines LX1 and LX 2. As shown by waveforms 503 and 505, a pulse in waveform 507 is triggered when the voltages on lines LX1 and LX2 cross zero in synchrony. Referring to fig. 3, this pulse will reset flip-flop 371, which in turn turns on discharge switch 375. Therefore, the charging period of the ramp signal is immediately ended, and the discharging period is started before the preset high threshold voltage VIN/N is reached. Thus, the operating frequency is determined by the resonant frequency in the variable frequency mode. Since the high threshold voltage VIN/N is not reached, the reset signal RST remains at a constant magnitude, as shown by waveform 509. In addition, a waveform 501 represents a current Ipri flowing through a base winding of the transformer 329 shown in fig. 3. Waveform 511 represents the feedback signal (FB) provided by feedback circuit 340. The feedback signal crosses the ramp signal and determines the duty cycle of the PWM signal.

In practice, the DC/AC inverter may be used in a display system. For example, the display system may further include a display screen, a light source disposed behind the display screen to provide backlight, and a processing circuit coupled to the display screen to generate data based on which the display screen displays an image. The DC/AC inverter will serve as a controller for the lighting and normal operation of the light source. The DC/AC inverter is connected to the light source for converting a DC input signal from an external DC power source into an AC output signal and supplying the AC output signal to the light source. Driven by the AC output signal, the light source is lit and emits light, which is emitted from the display screen. Depending on the condition of the DC input signal, the DC/AC inverter can be operated in either fixed frequency mode or variable frequency mode to achieve higher efficiency and better crest factor of the AC output signal. Good crest factors may extend the lifetime of the light source and are therefore more desirable.

When the DC input signal does not exceed a predetermined voltage value or the lamps used as the light source are more than a predetermined number, a preferable crest factor is guaranteed, and thus the DC/AC inverter operates in a fixed frequency mode to obtain a high efficiency. When the DC input voltage exceeds a predetermined voltage value or the number of lamps used as light sources is less than a predetermined number, the DC/AC inverter will operate in a variable frequency mode to ensure a better crest factor.

The embodiments described herein are merely typical embodiments of the invention, which are intended to be illustrative of the invention and not limiting. It will be apparent to those skilled in the art that numerous other embodiments are possible without departing substantially from the spirit and scope of the invention as defined by the appended claims. Accordingly, the above-described embodiments are intended to be illustrative of the present invention and not to limit the scope of the invention, which is defined by the appended claims and their legal equivalents, rather than by the description hereinbefore. The claims are intended to cover all such equivalents.

Claims (19)

1. A dc/ac inverter for driving a load, the dc/ac inverter comprising:

a DC power supply for supplying a DC input voltage;

a converter circuit connected to the dc power source, the converter circuit converting a dc input voltage to an ac signal for driving a load; and

a control circuit coupled to the converter circuit, the control circuit setting a frequency of the ac signal, wherein the control circuit operates the dc/ac inverter using either a constant frequency mode or a variable frequency mode.

2. The dc/ac inverter according to claim 1, further comprising:

a resistor connected to the control circuit;

a capacitor connected to the control circuit; and

and an oscillator connected to the control circuit, wherein the oscillator sets the frequency of the ac signal to a constant frequency according to the resistance value of the resistor and the capacitance value of the capacitor when operating in a constant frequency mode.

3. The dc/ac inverter according to claim 1, further comprising:

a resonant circuit connected to the control circuit, the resonant circuit having a resonant frequency, wherein the frequency of the ac signal is set in accordance with the resonant frequency when the control circuit operates the dc/ac inverter in the variable frequency mode.

4. A dc/ac inverter as claimed in claim 3, characterized in that the resonant circuit comprises a resonant element and a load in the converter circuit.

5. The dc/ac inverter according to claim 1, further comprising:

a detector is connected to the control circuit and determines whether a predetermined threshold condition has been reached, and when the predetermined threshold condition is reached, the control circuit switches from constant frequency mode operation to variable frequency mode operation.

6. The dc/ac inverter of claim 5, wherein the predetermined threshold condition comprises the dc input voltage reaching a specified voltage, the control circuit operating the dc/ac inverter in a variable frequency mode when the dc input voltage exceeds the specified voltage, and the control circuit operating the dc/ac inverter in a constant frequency mode when the dc input voltage does not exceed the specified voltage.

7. The dc/ac inverter of claim 5, wherein the predetermined threshold condition comprises a load condition, the load condition being a load size less than a predetermined value, the control circuit operating the dc/ac inverter in the variable frequency mode when the load condition is reached, and the control circuit operating the dc/ac inverter in the constant frequency mode when the load condition is not reached.

8. The dc/ac inverter of claim 1, wherein the control circuit operates in a constant frequency mode or in a variable frequency mode as a function of the dc input voltage and the load condition.

9. The dc/ac inverter as claimed in claim 1, wherein the load comprises a light source for a liquid crystal display backlight.

10. A method for converting a dc input voltage to an ac signal to drive a load, comprising:

setting a preset threshold condition;

operating in a fixed frequency mode, wherein the fixed frequency corresponds to the frequency of the alternating current signal and is maintained at a constant frequency;

the frequency converter works in a variable frequency mode, wherein the variable frequency corresponds to the frequency of an alternating current signal and is changed according to the resonant frequency corresponding to a resonant circuit; and

switching between fixed frequency mode operation and variable frequency mode operation as a function of a preset threshold condition.

11. The method of claim 10, further comprising:

when the frequency-variable amplifier operates in the fixed-frequency mode, the frequency of the alternating current signal is set to be a constant frequency, and the constant frequency is determined by a resistor and a capacitor.

12. The method of claim 10, further comprising:

when the frequency-variable circuit works in the frequency-variable mode, the frequency of the alternating current signal is changed to follow the resonant frequency of the resonant circuit.

13. The method of claim 10, wherein the predetermined threshold condition comprises the dc input voltage reaching a specified voltage.

14. The method of claim 10, wherein the predetermined threshold condition comprises a load condition, wherein the load condition is a load size less than a predetermined value.

15. The method of claim 10, wherein the load comprises a light source.

16. A display system, comprising:

a display device;

a light source coupled to the display device for providing light to and emitting light from the display device;

a processing unit coupled to the display device for generating data for display on the display device; and

a controller is coupled to the light source and automatically selects an optimal operating frequency between a constant frequency mode and a resonant mode.

17. The display system of claim 16, wherein the controller selects the constant frequency mode when the input voltage is lower than a predetermined voltage value and selects the constant frequency mode when the magnitude of the light source is greater than the predetermined value.

18. The display system of claim 16, wherein the controller selects the resonant mode when the input voltage exceeds a predetermined voltage value and selects the resonant mode when the magnitude of the light source is less than the predetermined value.

19. A display system as claimed in claim 16, characterized in that the resonance mode is such that the frequency of the ac output signal for driving the light source becomes synchronized with the resonance frequency of the resonant element inside the display system.