HK1118667B - Power supply circuit for liquid crystal display backlight and method thereof - Google Patents

Description

Power supply circuit for liquid crystal display backlight and method thereof

Technical Field

The present invention relates to a power circuit, and more particularly, to a power circuit for a Liquid Crystal Display (LCD) backlight.

Background

Liquid crystal displays are electrically controlled light valves that use a white "backlight," which may be a Light Emitting Diode (LED) or a Cold Cathode Fluorescent Lamp (CCFL), to illuminate a colored screen. At present, CCFLs are becoming more and more popular in backlight applications because of their highest efficiency. However, the lighting and operation of CCFLs requires a high Alternating Current (AC) voltage. Generally, the lighting voltage is 2 to 3 times higher than the operating voltage, and the longer lamp lighting voltage reaches 1000 volts. To generate such a high AC voltage using a DC power source, such as a rechargeable battery and a DC power supply, various DC/AC (DC/AC) inverters of CCFL driving architecture, such as Royer (self-oscillation), half-bridge, full-bridge, push-pull, are adopted. In addition, dimming control techniques have been developed to adjust the brightness of CCFLs. Pulse Width Modulation (PWM) dimming techniques, in particular, help to provide uniformity in display brightness and provide a wider selection of brightness that is soon becoming a viable option.

However, in PWM dimming, the inverter is actually turned on and off at the PWM frequency, so that a large ripple current is generated on the inverter power supply line. In addition, the above-mentioned CCFL driving scheme is generally used to drive one CCFL. The interest in large-size LCD displays, such as LCD televisions and computer monitors, has increased in recent years, making multiple CCFL backlights a desirable feature.

Fig. 1 is a block diagram of a prior art circuit 100. The circuit 100 is comprised of a DC power supply 110, a plurality of DC/AC inverters 120A-120N, a plurality of CCFL loads 130A-130N, and a controller 140. Each of DC/AC inverters 120A-120N converts a direct current voltage from direct current power source 110 to an alternating current voltage. Each CCFL in the CCFL loads 130A-130N uses one of the plurality of DC/AC inverters described above individually. The controller 140 provides a synchronous PWM dimming signal to the DC/AC inverter for controlling the conversion of DC to AC. Due to the synchronization of the PWM dimming signals, the current ripple on the power bus 150 connecting the DC power source and the plurality of DC/AC inverters 120A-120N is large.

Due to the potentially large enough current ripple input to the DC/AC inverter, the other devices are rendered useless. In addition, current ripple is also a major source of electromagnetic interference (EMI). Therefore, the system designer takes care of the current ripple on the power bus 150. Generally, designers place input inductors and large capacitors in the power supply to reduce current ripple on the power supply line 150. However, this method is effective only for high frequency current ripples and is ineffective for low frequency current ripples of several hundred hertz. That is, low frequency PWM dimming may complicate the design requirements of the dc power supply and create unnecessary visible noise on the LCD panel.

Fig. 2 is a block diagram of another prior art circuit 200 for driving multiple CCFLs. For the sake of brevity, fig. 2 and fig. 1 are omitted where they are repeated, and only the improvements will be described in detail herein. The circuit 200 includes a plurality of controllers 210A-210N that provide a series of phase shifted dimming signals PWM1-PWMN to the DC/AC inverters 120A-120N, respectively. Each DC/AC inverter is controlled by a phase-shifted dimming signal with a phase difference of 360 DEG/N from the adjacent DC/AC inverter, where N represents the total number of DC/AC inverters. Thanks to this series of phase shifted PWM dimming signals PWM1-PWMN, the current ripple on the power bus 150 is effectively reduced to one-nth of the current ripple in fig. 1.

In addition, those skilled in the art will appreciate that LEDs may be used in place of CCFLs for backlighting, and that the DC/AC inverter shown in FIGS. 1 and 2 would accordingly need to be replaced with a DC/DC converter for powering the LEDs.

Fig. 3 is a simulation diagram of the circuit shown in fig. 1 and 2. Curve (a) in fig. 3 represents the simulated current ripple according to the circuit 100 shown in fig. 1, and curve (B) represents the simulated current ripple according to the circuit 200 shown in fig. 2. The number of DC/AC inverters and CCFLs for circuit 100 and circuit 200 are each specified herein as 6. Referring to the curve (a), it can be seen that when the dc voltage is 24 v and the maximum input power during full brightness is about 100 w, the maximum and minimum difference of the currents is about 4 a when the dimming duty is about 50%. Referring to the curve (B), it can be seen that when the dc voltage is 24 v and the maximum input power during the full-on period is about 100 w, the dimming ratios of the respective dimming signals PWM1-PWM6 are about 50%, and the phase differences between the adjacent dimming signals are equal, the maximum and minimum difference in current is about 0.7 a. The current ripple in circuit 200 is approximately 1/6 for circuit 100.

Although the circuit shown in fig. 2 can reduce the current ripple, the number of controllers is greatly increased. In addition, each CCFL load in fig. 1 and 2 is powered by a single DC/AC inverter, resulting in a large number of components, high overall cost and large circuit size.

Disclosure of Invention

The invention provides a power supply with small current ripple and low cost. The power supply includes a power bus, a boost converter, a buck converter, and a controller. The power bus supplies power to the load. The boost converter and the buck converter are connected to a power bus for storing energy from a power line and releasing energy to a load, respectively. A controller is coupled to the boost converter and the buck converter for alternately operating the boost converter and the buck converter in response to a PWM signal.

Description of the 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 power supply circuit for an LCD backlight according to the prior art.

Fig. 2 is a block diagram of another prior art power supply circuit for an LCD backlight.

Fig. 3 is a simulation of fig. 1 and 2.

Fig. 4 is a block diagram of a power circuit according to an embodiment of the invention.

Fig. 5 is a timing diagram of the circuit shown in fig. 4.

Fig. 6 is a schematic diagram of the bi-directional power circuit shown in fig. 4.

Fig. 7 is a timing diagram of the bi-directional power supply circuit shown in fig. 6.

Fig. 8 is a timing diagram of the input current of the power circuit shown in fig. 4.

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. 4 is a block diagram of a power supply circuit 400 according to an embodiment of the invention. The power circuit 400 includes a dc power supply 110, a bi-directional power supply (BPS)410, and a controller 420 power line 150 connected to the power supply 110 and the BPS 410. The dc power source 110 may provide a dc voltage Vin and an input current to the power line 150. The BPS410, controlled by the controller 420, may reduce current ripple on the power line 150 before the current is delivered to the DC/AC inverter. The BPS410 is connected to the power line 150 and includes a boost converter 411, a buck converter 413, and a capacitor 415. The controller 420 is coupled to the BPS410 and controls the boost converter 411 and the buck converter 413 according to a dimming signal, which may be a PWM signal. The controller 420 is also coupled to the DC/AC inverter 120A for adjusting the power delivered to the plurality of loads (CCFLs 130A-130N) based on the PWM dimming signal. In practice, the PWM dimming signal may be provided from an external device or generated internally by the controller 420. Meanwhile, the controller 420 also receives a feedback signal from the BPS410 to ensure that the BPS410 operates in the critical current mode, and also receives a current feedback signal from the CCFLs for accurately controlling the brightness of the CCFL.

Those skilled in the art will appreciate that the DC/AC inverter 120A may use various topologies, such as Royer, full bridge, half bridge, and push-pull. Also, when the plurality of loads are LEDs, DC/AC inverter 120A may be replaced with DC/DC converters of various topologies, such as SEPIC, buck-boost, and buck configurations. In addition, with the power supply circuit 400, one DC/AC inverter is sufficient to drive multiple CCFLs in parallel. Similarly, one DC/DC converter is sufficient to drive multiple LEDs in parallel.

Fig. 5 is a timing diagram of the power circuit 400 shown in fig. 4. As shown in fig. 5, the PWM dimming signal has two states of ON and OFF. When the PWM dimming signal is in the ON state, the boost converter 411 is enabled and the buck converter 413 is disabled. When the PWM dimming signal is in the OFF state, the boost converter 411 is disabled and the buck converter 413 is enabled. Referring to FIG. 4, assume that the input current on power bus 150 is I when fully bright_pAs will be understood by those skilled in the art, the input current I_pIs provided by the DC power supply 110 and remains constant because the total output power of the DC/AC inverter 120A is constant during full brightness. However, during PWM dimming, DC power supply 110 provides powerThe input current to the bus 150 will have a significant current ripple, and thus the BPS410 is used to reduce the current ripple on the power bus 150. During the ON period of the PWM dimming signal, the power bus 150 will transmit an average input current I_bFor the boost converter 411, during the OFF period of the PWM dimming signal, the buck converter 413 will deliver an average input current I_oTo power bus 150 and ultimately to DC/AC inverter 120A. In general, during PWM dimming, the power bus 150 will deliver a current Ii to the DC/AC inverter 120A that includes current from the BPS410 and the DC power source 110. The current ripple on the power bus 150 is greatly reduced due to the constant current from the BPS 410.

In terms of energy conversion, during the ON period of the PWM dimming signal, the enabled boost converter 411 converts the dc voltage Vin ON the power bus 150 into a higher voltage Vs applied across the capacitor 415. The energy stored in the capacitor 415 can be derived from equation 1),

where E is defined as the energy stored in the capacitor 415, Cs is defined as the capacitance value of the capacitor 415, D is defined as the duty cycle of the BPS410, and vs (D) is a function of the variable D. During the OFF period of the PWM dimming signal, the energy stored in the capacitor 415 is discharged to the DC/AC inverter 120A through the enabled buck converter 413. At the same time, the energy transferred from the DC power source 110 is also received by the DC/AC inverter 120A. Since the total energy transferred to the DC/AC inverter 120A is the energy from the DC power source 110 and the stored energy (in the capacitor), the current ripple on the power bus 150 is significantly reduced by virtue of the stored energy. Moreover, to minimize current ripple on the power bus 150, it is critical to balance the energy flowing into and out of the BPS 410. In other words, the energy stored by the capacitor 415 when the PWM dimming signal is in the ON state should be substantially equal to the energy released to the DC/AC inverter 120A when the PWM dimming signal is in the OFF state. To achieve this, it is optimal for the BPS410 to operate in the critical current mode between the continuous current mode and the discontinuous current mode in each dimming cycle of the PWM dimming signal.

Fig. 6 is a schematic diagram of the BPS410 shown in fig. 4. The BPS410 includes transistors 601 and 603, rectifiers 605 and 607, inductor 609, auxiliary winding 611, resistors 615, 617 and 619, and capacitor 415. Transistors 601 and 603 are typically power MOSFETs and rectifiers 605 and 607 may be schottky diodes. Terminal 1 of transistor 601 receives a drive signal DRV1 from controller 420, terminal 2 is connected to the cathode of rectifier 607, and terminal 3 is connected to the anode of rectifier 607. Similarly, the transistor 603 is connected to the rectifier 605 in a similar manner. Further, a terminal 3 of the transistor 601 is grounded via a resistor 617, and a terminal 2 of the transistor 603 is grounded via a capacitor 415. Inductor 609 has one end connected to power bus 150 via resistor 615 and the other end connected to terminal 2 of transistor 601 and terminal 3 of transistor 603. In addition, the auxiliary winding and inductor 609 are placed in parallel to form a transformer, and an induced voltage is generated across the auxiliary winding 611. The auxiliary winding 611 is also connected in series with a resistor 619, and the resistor 619 can limit the current flowing from the auxiliary winding to the controller 420 to a safe range.

During the ON period of the PWM dimming signal, the BPS410 operates as a boost converter, consisting of the transistor 601, the rectifier 605, the inductor 609, and the capacitor 415. During the OFF period of the PWM dimming signal, the BPS410 operates as a buck converter, consisting of the transistor 603, the rectifier 607, the inductor 609, and the capacitor 415. When the BPS410 operates as a boost converter, it is ensured to operate in the critical current mode by the feedback signals CS and ZCD. When the BPS410 operates as a buck converter, operation in the critical current mode is ensured by the feedback signals CSH and ZCD. The feedback signals CS and CSH are detected by resistors 617 and 615, respectively. The feedback signal ZCD is provided by the auxiliary winding 611.

During the ON period of the PWM dimming signal, the transistor 601 is alternately turned ON and off by a driving signal DRV1 provided by the controller 420. When the transistor 610 is turned on, the rectifier 605 is reverse biased and the current in the inductor 609 rises linearly to a peak I_LPA. This represents a certain amount of energy stored in the inductor 609. When the transistor 601 is turned off, the energy stored in the inductor 609 and the energy on the power bus 150 are transferred to the capacitor 415 to charge the capacitor, and the voltage across the capacitor is charged to a value higher than the dc voltage Vin through the rectifier 605. At this time, the BPS410 operates as a boost converter, the relationship between the voltage Vs across the capacitor 415 and the dc voltage Vin can be obtained by equation 2),

where the operating frequency D of the BPS410 is equal to the switching duty cycle of the transistor 601.

In addition, the critical current mode is realized by controlling the switching timing of the transistor 601 according to the feedback signals CS and ZCD during the ON state of the PWM dimming signal. Feedback signal CS indicates the inductor current I_LWhether or not the peak value I is reached_LPA. When the inductor current reaches the peak current I_LPAThe controller 420 will respond to the feedback signal CS and turn off the transistor 601. Feedback signal ZCD indicates the inductor current I_LWhether or not 0 is reached. If the inductive current I_LReaching 0, the controller 420 responds to the feedback signal ZCD and turns on the transistor 601.

During the OFF state of the PWM dimming signal, the transistor 603 is alternately switched on and OFF by a driving signal DRV2 provided by the controller 420. When transistor 603 is turned on, rectifier 607 is reverse biased and the energy stored in capacitor 415 is released to inductor 609 and DC/AC inverter 120A shown in fig. 4. When transistor 603 is off, inductor current flows through rectifier 607 and transfers some of the energy stored in inductor 609 to DC/AC inverter 120A shown in fig. 4. At this time, the BPS410 operates as a buck converter, the relationship between the voltage Vs across the capacitor 415 and the dc voltage Vin can be obtained by equation 3),

where the operating frequency of the BPS410 is equal to the switching duty cycle of the transistor 603.

In addition, the critical current mode is realized by controlling the switching timing of the transistor 603 according to the feedback signals CSH and ZCD during the OFF state of the PWM dimming signal. Feedback signal CSH indicates the inductor current I_LWhether or not the peak value I is reached_LPB. When the inductor current reaches the peak current I_LPBThe controller 420 will respond to the feedback signal CSH and turn off the transistor 603. Feedback signal ZCD indicates the inductor current I_LWhether or not 0 is reached. If the inductive current I_LReaching 0, the controller 420 responds to the feedback signal ZCD and turns on the transistor 603.

Fig. 7 is a timing diagram of the BPS410 shown in fig. 5. Curve (a) represents a single cycle in which the ON and OFF states of the PWM dimming signal are the same in length. The time that PWM is in ON state is defined as T_AThe time when PWM is OFF is defined as T_BThe PWM dimming period is defined as T_SIs obviously T_SIs equal to T_AAnd the sum of TB. Curve (B) represents inductor current I when BPS410 is operating as a boost converter during the PWM ON state_LThe waveform of (2). In critical current mode, peak current I_LPASpecific average input current I_B2 times larger, as can be derived from equation 4) below,

where Ip is the constant input current during the fully dark period. Referring to equation 4), T in one PWM dimming period can be determined_ATime period, current I_LPAConstant, and T when the duty ratio of the PWM dimming signal changes_BIs in direct proportion. Curve (C) is T_BInductor current I when time interval BPS410 is used as a buck converter_LThe waveform of (2). In critical current mode, peak current I_LPBSpecific average output current I_O2 times larger, as can be derived from equation 5) below,

with reference to equation 5), the peak current I can be determined_LPBT in one PWM dimming period_BIs constant in time interval and is equal to T when the duty ratio of the PWM dimming signal is changed_AIs in direct proportion. From the viewpoint of energy flow, the following equation 6 can be obtained),

wherein E_inEnergy, E, flowing into the BPS410 for a TA period_outIs defined as T_BThe time period flows the energy of the BPS 410. When the duty ratios of the PWM dimming signals are different, the PWM dimming signals are respectively according to T_BAnd T_AAdjusting the peak current I_LPAAnd I_LPBBalance can be easily maintained. On the one hand, peak current I_LPAAnd I_LPBThe switching timing of transistors 601 and 603, respectively, may be determined as previously described. On the other hand, the switching timings of the transistors 601 and 603 can adjust the peak current I respectively_LPAAnd I_LPB。

Curve (D) represents T_AThe state of the transistor 601 during the period. As shown, the transistor 601 is alternately turned on and off by the drive signal DRV 1. The time for which the transistor 601 is turned on is defined as T_ONThe time for which the transistor 601 is turned off is defined as T_OFF。T_ONAnd T_OFFAs can be derived from equation 7) and equation 8) below,

where L is defined as the inductance of the inductor 609. Referring to equation 7), it can be determined when the duty ratio of the PWM dimming signal is set to a first preset value, for example, T_B/T_SWhen, T_ONIs constant and is equal to the peak current I_LPAIs in direct proportion. Referring to equation 8), it can be determined when T_AT when the voltage Vs across the capacitor 415 changes within a time period_OFFAre variables.

Curve (E) represents T_BThe state of the transistor 603 in the period. As shown, the transistor 603 is alternately turned on and off by the drive signal DRV 2. T of transistor 603_ONAnd T_OFFThe time can be derived from the following equations 9) and 10) respectively,

with reference to equation 9) at T_BT when the voltage Vs across the capacitor 415 changes during the period_ONAre variables. Referring to equation 10), it may be determined that T is set when the duty ratio of the PWM dimming signal is set to the second preset value_OFFConstant and corresponding to the peak current I_LPBIs in direct proportion. Usually when the first preset value is T_B/T_SWhen the second preset value is T_A/T_S。

Curve (F) is a waveform diagram of the voltage Vs across the capacitor 415, T_AThe time period is obtained according to equation 2), T_BThe time period is obtained according to equation 3). At T_ADuring this period, the duty cycle D of the BPS410 is equal to the switching duty cycle of the transistor 601, gradually increasing as shown by the curve (D). At T_BDuring this period, the duty cycle D of the BPS410 is equal to the switching duty cycle of the transistor 603, and gradually increases as shown by the curve (E). Therefore, as shown by curve (F), the voltage Vs depends on the operating frequency D, at T_AThe time period gradually increases from the initial minimum value Vmin to the maximum value Vmax at T_BThe period gradually decreases until it returns to the minimum value Vmin.

Curve (G) represents the operating frequency of the BPS 410. At T_APeriod of time, T_onThe time period is kept constant while T_offThe period of time gradually decreases. Can be determined at T_AThe operating frequency of the period BPS410 increases. Similarly, it can be determined at T_BThe operating frequency of the period BPS410 decreases. Thus, as shown by curve (G), the operating frequency of the BPS410 is at T during the PWM dimming period_AThe time period being from a minimum value F_minUp to a maximum value F_maxAt T_BTime period is reduced back to F_min。

Fig. 8 is a timing diagram of the input current of the power bus 150. The input current is defined as I_INAccording to equation 4) and equation 5) are plotted on the vertical axis against time on the horizontal axis. In PWM dimming, the duty cycle of the PWM signal is set to an exemplary 70%. According to equation 4), T_AThe average input current transmitted from the power bus 150 to the BPS410 over a period of time is 30% I_pIs the peak current I_LPAHalf of that. Average input current I_INAbsorbed by the BPS410, the squares (a) marked with diagonal lines from left to right represent the energy stored by the BPS 410. At T_BDuring the time period, the input current delivered by power bus 150 to DC/AC inverter 120A is equal to the current from DC power source 110 plus the output current I from BPS410_o. Finally, the average input current of DC/AC inverter 120A during PWM dimmingI_INEqual to the input current I during full dark_P. According to equation 5), the output current I_oEqual to the peak current I_LPBHalf of that. The blocks (B) marked with diagonal lines from right to left represent the energy released from the BPS410 to the DC/AC inverter 120A. Since the input energy and the output energy of the BPS410 are identical and the areas of the blocks (A) and (B) are equal, the output current I_oEqual to 70% I_p. Finally, during PWM dimming, the current delivered from the DC power supply to the DC/AC inverter is kept constant at 30% I_p。

Thus, to maintain the balance of the energy flowing into the BPS410, the voltage Vs across the capacitor 415 is not adjusted by the controller 420 during PWM dimming. Since the BPS410 does not absorb this energy as a load during operation of the boost converter, excessive voltage breakdown of the capacitor 415 and transistors 601 and 603 may occur. The voltage Vs can be derived from the following equation 11),

from equation 11), it can be determined that increasing Cs can prevent the voltage Vs from being at T_AA dangerously high pressure is reached before the end of the period.

Those skilled in the art will appreciate that the BPS410 may also operate as a buck converter during the ON state of the PWM dimming signal and as a boost converter during the OFF state of the PMW dimming signal by configuration without departing from the inventive spirit of the present invention.

In operation, the display system may include a display screen, a plurality of backlights for illuminating the display screen, and a power source for lighting and operating the backlights. The power circuit may include a DC power source, a DC/AC inverter and a power line coupled between the DC power source and the DC/AC inverter. The DC/AC inverter converts a DC power supply V from a DC power supply_inAnd converting into alternating voltage required by the backlight source. However, there may be large current ripples on the power bus that affect the performance of the display system. Just to effectively reduce the current ripple on the power bus, we use BPS.

The BPS is connected to the power line and may include a boost converter, a buck converter, and a capacitor, wherein the boost converter and the buck converter operate alternately in response to a dimming signal, which may be a PWM dimming signal. For example, when the PWM dimming signal is in the ON state, the boost converter is enabled and the buck converter is disabled. Thus, energy transferred from the dc power source to the power line will flow into the BPS and be stored in the capacitor through the enabled boost converter. When the PWM dimming signal is in the OFF state, the energy stored by the capacitor in the BPS will be released back to the power line and eventually received by the DC/AC inverter. Meanwhile, the DC/AC inverter also directly receives power from the direct current power source during the OFF state of the PWM dimming signal. Thanks to the energy released from the BPS, the proportion of energy received directly from the dc power supply is relatively low and the current ripple on the power supply line is significantly reduced. In addition, to effectively reduce current ripple, the BPS should be energy balanced, i.e., the energy flowing into the BPS should be exactly equal to the energy flowing out of the BPS. To maintain energy balance, the BPS are usually operated in the critical current mode.

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.

Claims (20)

1. A power supply, comprising:

a power bus for providing voltage to a load;

a boost converter connected to said power bus, said boost converter converting an input voltage to a higher output voltage;

a capacitor connected to said boost converter, said higher output voltage being stored across said capacitor;

a buck converter connected to said capacitor, said higher output voltage stored across said capacitor being reduced and provided to said power bus;

a controller coupled to the boost converter and the buck converter, wherein the boost converter and the buck converter operate alternately according to a Pulse Width Modulation (PWM) signal to balance an input energy and an output energy of the power supply to minimize ripple on a power bus.

2. The power supply of claim 1, wherein the boost converter is enabled and the buck converter is disabled when the PWM signal is in the ON state.

3. The power supply of claim 1, wherein the boost converter is disabled and the buck converter is enabled when said PWM signal is in an OFF state.

4. The power supply of claim 1, wherein the PWM signal corresponds to a dimming signal and the load corresponds to a light source.

5. The power supply of claim 1, wherein the power bus and controller are connected to an inverter or converter.

6. A bi-directional power supply, comprising:

a first transistor connected to the power supply line for boosting a first direct current voltage to a second direct current voltage;

a second transistor connected to a power supply line for stepping down the second dc voltage to the first dc voltage;

a capacitor connected to the first transistor and the second transistor for storing energy when the first transistor is turned on and for supplying energy when the second transistor is turned on;

a plurality of asynchronous rectifiers coupled to the first and second transistors, wherein the first and second transistors are controlled according to a control signal, the first transistor being alternately turned on and off during a state of the control signal, the second transistor remaining off; during another state of the control signal, the second transistor is alternately turned on and off and the first transistor remains off to balance the energy flowing into and out of the bi-directional power supply to reduce ripple current on the power supply line.

7. The bi-directional power supply of claim 6, further comprising:

a first current sense resistor connected to the first transistor;

and the first current detection resistor and the second current detection resistor provide feedback signals for controlling the connection and disconnection of the first transistor and the second transistor.

8. The bi-directional power supply of claim 6, further comprising an inductor coupled between the power supply line and the first transistor for operating the bi-directional power supply in a critical current mode between the continuous current mode and the discontinuous current mode.

9. The bi-directional power supply of claim 8, wherein the inductor forms part of a transformer that includes an auxiliary winding that provides a feedback signal for controlling the turning on and off of the first transistor and the second transistor.

10. The bi-directional power supply of claim 6, wherein said control signal comprises a pulse width modulated signal.

11. The bi-directional power supply of claim 6, wherein said control signal comprises a dimming signal.

12. A method of powering a load, comprising:

boosting the input voltage on the power line to a larger voltage;

applying the larger voltage to a capacitor, and storing energy in the capacitor;

releasing energy by discharging the capacitor;

reducing the voltage of the two ends of the capacitor and adding the reduced voltage to the power line;

controlling said step-up, charge, discharge and step-down in accordance with a PWM dimming signal, said step-up and charge being enabled during a state of the PWM dimming signal such that said step-down and discharge are disabled; during another state of the PWM dimming signal, the buck and discharge are enabled, disabling the boost and charge to balance the energy stored in the capacitor with the energy discharged from the capacitor, minimizing inrush current on the power supply line supplying the load.

13. The method of claim 12, further comprising:

enabling the boost and charging the capacitor when the PWM dimming signal is in an ON state;

when the PWM dimming signal is in an ON state, the voltage reduction and the capacitor discharge are disabled.

14. The method of claim 12, further comprising:

when the PWM dimming signal is in an OFF state, the boosting and the capacitor charging are disabled;

enabling the buck and discharging the capacitor when the PWM dimming signal is in the OFF state.

15. The method of claim 12, wherein the load corresponds to a light source.

16. A system, comprising:

a display device;

a power supply having a power bus connected to said display device, said power bus supplying power to said display device;

a DC/DC boost converter connected to the power bus;

a DC/DC buck converter connected to the power bus;

a capacitor connected to the boost converter and the buck converter, wherein the boost converter stores energy from the power bus in the capacitor when the boost converter is enabled, and the buck converter releases energy stored in the capacitor to the power bus when the buck converter is enabled;

and a controller coupled to the boost converter and the buck converter, the controller causing the boost converter and the buck converter to operate alternately in response to a PWM dimming signal.

17. The system of claim 16, further comprising:

an inverter connected to the power bus;

at least one light source connected to the inverter.

18. The system of claim 16, wherein the boost converter includes a first power MOSFET transistor and a first rectifier connected in series with the first transistor, and the buck converter includes a second power MOSFET transistor and a second rectifier connected in series with the second transistor.

19. The system of claim 18 further comprising a first current sense resistor and a second current sense resistor, the current sense resistors providing feedback signals for controlling the boost converter and the buck converter.

20. The system of claim 19, further comprising a transformer coupled to the power bus, the transformer comprising:

an inductor connected between the power bus and the first power MOSFET transistor for operating in a critical current mode between a continuous current mode and an interrupted current mode;

an auxiliary winding providing a feedback signal for controlling the turning on and off of the first power MOSFET transistor and the second power MOSFET transistor.