Power supply apparatus and display apparatus

Abstract

A liquid crystal display apparatus is capable of reducing a power loss incurred by a power-supply section in a process of generating a power-supply voltage for driving a backlight section. In the configuration of the power-supply section, a main power-supply circuit and an inverter circuit (or a DC-DC converter) are connected to an input-voltage generation unit in parallel to each other, which is used for rectifying and smoothing the commercial alternative current power. The main power-supply circuit and the inverter circuit (or the DC-DC converter) each include an isolation transformer including a primary-side winding not isolated from the commercial alternative current power and a secondary winding provided on the secondary side. The direct current input voltage is supplied to the primary side to be subjected to a power conversion process to generate an output voltage on the secondary side of the isolation transformer. Thus, the number of power conversion process stages for supplying power to the backlight section connected to the inverter circuit (or the DC-DC converter) is reduced by 1. As a result, the power loss incurred by the power-supply section can be reduced to a value smaller than that of the conventional one.

Description

On the other hand, the input power of the configuration according to the embodiment shown in FIG. 1 is expressed as follows:
(1/η1)P1+(1/η2)P2
where the quantities of notation η1 denoting the power conversion efficiency of the main power-supply circuit 2 and notation η2 denoting the power conversion efficiency of the inverter circuit are the same as those in the configuration shown in FIG. 6.

That is to say, in the configuration including an inverter circuit provided at the stage behind the main power-supply circuit as shown in FIG. 7, the power conversion efficiency along a path for obtaining the alternative current voltage at the output of the inverter circuit is a product of the power conversion efficiency of the main power-supply circuit and the power conversion efficiency of the inverter circuit. Thus, the power conversion efficiency along the path decreases substantially due to the multiplication.

In the case of the embodiment, on the other hand, the power conversion efficiency along the path for obtaining the alternative current voltage is dependent only on the inverter circuit. Thus, the power conversion efficiency can be maintained at a value higher than that of the conventional power-supply circuit shown in FIG. 7. That is to say, the power loss is small in comparison with the conventional power-supply circuit.

Since the power conversion efficiency along a path for obtaining the alternative current voltage used for driving the backlight section can be maintained at a value higher than that of the conventional power-supply circuit, the power loss caused by an increased load power P2 of the backlight section due to an increased size of the display screen for example can be suppressed to a quantity smaller than that of the conventional power-supply apparatus.

That is to say, in this case, the difference in input power between the configuration shown in FIG. 7 and the power-supply apparatus 10 according to the embodiment is expressed as follows:
(1/η1η2−1)P2

As is obvious from the above expression, the greater the load power P2 of the backlight section, the larger the difference in input power between the conventional configuration and the power-supply apparatus 10 according to the embodiment.

It is thus clear that, with the power-supply apparatus 10 according to the embodiment, the larger the size of the screen display and, hence, the larger the power consumption of the inverter circuit 4, the greater the power-loss reduction effect as compared with the conventional configuration.

In addition, as described above, since an alternative current voltage for driving the backlight section 5 can be obtained not through the main power-supply circuit 2, it is no longer necessary to keep up with a rising power of the main power-supply circuit 2 even if the size of the display screen is increased. Thus, the amount of heat dissipated in the main power-supply circuit 2 due to an increased display screen in size does not rise. As a result, it is no longer necessary to allocate a sufficient space as a countermeasure to cope with dissipated heat as is the case with the conventional power-supply apparatus. Accordingly, the downsizing of the display apparatus is possible.

In addition, it is no longer necessary to provide a cooling fan for coping with dissipated heat. It is thus possible to get rid of the operation sound of the fan as a source of discomfort suffered by the user.

On top of that, since the main power-supply circuit 2 no longer needs to supply power to the backlight section 5, power-supply specifications of the main power-supply circuit 2 need to be dependent on only conditions of the load 3. Thus, the design of the main power-supply circuit 2 can be standardized with ease.

In the case of the conventional configuration shown in FIG. 7, on the other hand, the power-supply specifications of the main power-supply circuit 2 are dependent also on, among others, the type of the backlight section 105 (the type of the display panel). This is because the power-supply specifications of the main power-supply circuit 2 are dependent on specifications of the inverter circuit and the specifications of the inverter circuit need to be modified in accordance with the type of the backlight section 105 (the type of the display panel). Thus, the design of the main power-supply circuit 2 can not be standardized with ease.

In the conventional configuration, the main power-supply circuit and the inverter circuit cannot be connected to form a parallel circuit as is the case with the embodiment described above due to the following reasons.

In the field of the conventional liquid crystal display apparatus, display apparatus having a small screen with a size in the range 15 to 17 inches are the majority. Thus, the power consumption of the inverter circuit is relatively small. Accordingly, in the case of the conventional configuration, the power loss incurred in a process of generating an alternative current voltage for driving the backlight section can be suppressed to a comparatively low level. For these reasons, the conventional configuration including an unisolated inverter inputting power from the main power-supply circuit is rather preferred to offer merits due to the fact that that this configuration does not raise problems of an increasing cost and a rising circuit space.

The idea of the present invention has been adopted because, in the first place, the screen of the liquid crystal display apparatus has been increasing in recent years, causing the power consumption of the backlight section to rise.

That is to say, in recent years, display apparatus typically having a screen size in the 40-inch class have been becoming popular. However, some of the display apparatus with a screen size in the 40-inch class have a backlight section inverter with a power consumption of about 200 W. If a backlight section inverter has a large power consumption as described above, the magnitude of a power loss incurred in every power conversion process also increases to a relatively large value if the display apparatus employing the backlight section inverter is designed into the conventional configuration. Thus, the conventional configuration raises a large number of problems.

The idea of the present invention is adopted as a technology for solving the problems. By applying the present invention to liquid crystal display apparatus as described above, a power-loss reduction effect can be obtained and, the larger the size of the display screen, the greater the power-loss reduction effect. Thus, in keeping up with future environment changes such as increases in display-screen size, the importance of the present invention is considered to increase.

Next, the configurations of power-supply apparatus each serving as a modified version of the first embodiment are explained.

FIG. 4 is a block diagram showing another simplified configuration of the power-supply apparatus 11 according to the first embodiment.

In the power-supply apparatus 11, a PFC (Power Factor Correction) converter circuit 7 is employed as a substitute for the rectification/smoothing section 1 shown in FIG. 1. That is to say, as one of countermeasures for eliminating power-supply harmonic distortions, for example, a converter for improving the power factor is provided at a stage preceding the main power-supply circuit. Thus, the power-supply apparatus 11 includes the PFC converter circuit 7 at a stage in front of the main power-supply circuit 2 as well as in front of the inverter circuit 4.

A typical configuration of the PFC converter circuit 7 is shown in FIG. 5.

The PFC converter circuit 7 shown in the figure is a voltage-boosting converter adopting the PWM control method. The PFC converter circuit 7 operates at a power factor approaching 1 so as to stabilize the direct current input voltage Ei.

First of all, as shown in the figure, the alternative current input voltage VAC generated by the commercial alternative current power-supply AC is supplied to the input terminals of a bridge rectification circuit Di employed in the PFC converter circuit 7. An output capacitor Co is connected between the plus and minus lines of the bridge rectification circuit Di in parallel to the bridge. A rectified output generated by the bridge rectification circuit Di is supplied to the output capacitor Co. Thus, a direct current input voltage Ei is obtained between the terminals of the output capacitor Co as shown in the figure.

The direct current input voltage Ei is supplied to the main power-supply circuit 2 and the inverter circuit 4 as shown in FIG. 4.

The configuration for improving the power factor as shown in the figure includes an inductor L, a high-speed recovery diode D and a switching circuit Q3.

The inductor L and the high-speed recovery diode D are connected as a series circuit between the plus output terminal of the bridge rectification circuit Di and the plus terminal of the output capacitor Co.

A MOS-FET is selected as the switching device Q3. As shown in the figure, the switching device Q3 is connected between the minus line of the bridge rectification circuit Di and a point of connection between the inductor L and the high-speed recovery diode D.

The switching device Q3 is driven by a driving control circuit not shown in the figure.

The driving control circuit typically executes PWM control based on the alternative current input voltage VAC and differentials of the direct current input voltage Ei to change the on duration of the switching device Q3. The on duration of the switching device Q3 is referred to as the duty of the switching device Q3. As a result of the control, the waveform of an alternative current input current flowing to the bridge rectification circuit Di matches the waveform of the alternative current input voltage VAC. That is to say, the power factor is improved in order to approach 1.

In addition, in this case, the duty (or the on duration) of the switching device Q3 changes also in accordance with differentials of the direct current input voltage Ei. Thus, variations in direct current input voltage Ei are also suppressed. That is to say, the direct current input voltage Ei is stabilized thereby.

Also in this modified version of the power-supply apparatus 11, the inverter circuit 4 generates an alternative current voltage for driving the backlight section not through the main power-supply circuit 2. Thus, the power loss incurred in a process to generate an alternative current voltage for driving the backlight section can be reduced to a quantity smaller than that of the conventional configuration. That is to say, in this case, in comparison with the conventional configuration shown in FIG. 7, the power loss is small even if a circuit equivalent to the PFC converter circuit 7 is employed in the conventional configuration.

In addition, in the case of the configuration employing the PFC converter circuit 7, the direct current input voltage Ei supplied to the main power-supply circuit 2 and the inverter circuit 4 is stabilized. Thus, the inverter circuit 4 can be designed on the assumption that a stable direct current input voltage is supplied to the inverter circuit 4. As a result, since the design of the inverter circuit 4 is simpler, the inverter circuit 4 is very advantageous from the practical point of view if its combination with the configuration for improving the power factor is to be taken into consideration.

FIG. 6 is a circuit diagram showing a simplified configuration of a power-supply apparatus 12 according to a second embodiment of the present invention. It is to be noted that sections shown in FIG. 6 as sections identical with their respective counterparts shown in FIG. 1 are denoted by the same reference numerals as the counterparts, and their explanations are not repeated.

The power-supply apparatus 12 shown in FIG. 6 is also used as a power-supply section of a liquid crystal display apparatus 21. That is to say, the power-supply apparatus 12 supplies driving power to the load 3 and a backlight section 15 as shown in the figure.

In addition, in this case, the backlight section 15 of the liquid crystal display apparatus 21 employs LEDs. The power-supply apparatus 12 supplies direct current driving currents to the backlight section 15.

A configuration for supplying direct current driving currents to the backlight section 15 includes a plurality of DC-DC converters 9a, 9b and 9c.

In this case, the backlight section 15 has a plurality of series circuits each including a predetermined plurality of LEDs connected to each other in series. As the series circuits for supplying a direct current to the series circuits including a predetermined plurality of LEDs receptively, a plurality of the DC-DC converters 9a, 9b and 9c are provided.

As shown in the figure, a direct current input voltage generated by the rectification/smoothing section 1 is supplied to the primary sides of the DC-DC converters 9a, 9b and 9c, and the primary sides of the DC-DC converters 9a, 9b and 9c are not isolated from the commercial alternative current power-supply AC. That is to say, much like the inverter circuit 4 employed in the configuration shown in FIG. 1, the DC-DC converters 9a, 9b and 9c are connected to the rectification/smoothing section 1 in parallel to the main power-supply circuit 2.

In addition, almost in the same way as the configuration of the main power-supply circuit 2, each of the DC-DC converters 9a, 9b and 9c includes an isolation transformer for isolating the commercial alternative current power-supply side and the load side from each other. On the primary side of the isolation transformer, a switching device and a driving circuit for driving the switching device are provided and, on the secondary side of the isolation transformer, a rectification/smoothing circuit is provided to form the configuration of a switching converter. That is to say, the direct current input voltage supplied to the primary side is subjected to a DC-DC power conversion process to generate another direct current voltage on the secondary side.

In addition, each of the DC-DC converters 9a, 9b and 9c also includes a control system for stabilizing a direct current to be supplied to the series circuit composed of a predetermined plurality of LEDs. Such a current stabilization control system typically has a detection circuit 4d, 4e and 4f for detecting the level of a current flowing through the series circuit of the LEDs and a feedback circuit 4g, 4h and 4i for feeding back a voltage detected by the detection circuit to the primary side through the isolation provided by the isolation transformer. In this configuration, in accordance with the voltage detected by the detection circuit and supplied by the feedback circuit, the switching frequency of a driving signal supplied by the driving circuit to the switching device is changed under control executed by the driving circuit.

Also in the configuration of the power-supply apparatus 12 according to the second embodiment described above, the power conversion means for generating a power-supply voltage for driving the backlight section employed in the liquid crystal display apparatus is not provided at a stage behind the main power-supply circuit 2, but provided in parallel to the main power-supply circuit 2 at a stage behind the rectification/smoothing section 1. That is to say, also in the configuration of the power-supply apparatus 12 according to the second embodiment, the power-supply voltage for driving the backlight section can be obtained by carrying out a power conversion process only once in each of the DC-DC converters 9a, 9b and 9c. Thus, the power loss incurred by the power-supply apparatus can be reduced to a value smaller than that of the conventional configuration shown in FIG. 8.

In addition, also in the power-supply apparatus 12 according to the second embodiment, a direct current voltage for driving the backlight section 15 can be obtained not through the main power-supply circuit 2. Thus, it is no longer necessary to cope with an increased power consumed by the main power-supply circuit 2 as a power caused by an increased size of the display apparatus.

On top of that, the DC-DC converters 9a, 9b and 9c are connected to the rectification/smoothing section 1 in parallel to the main power-supply circuit 2 as described above. Thus, also in the power-supply apparatus 12 according to the second embodiment, the larger the power consumption for driving the backlight section, the greater the obtained power-loss reduction effect in comparison with the configuration shown in FIG. 8 as the conventional configuration.

In addition, also in this case, the main power-supply circuit 2 no longer needs to supply power to the backlight section 15. Thus, the power-supply specifications of the main power-supply circuit 2 need to rely only on conditions of the load 3.

It is to be noted that, in the second embodiment, a plurality of DC-DC converters 9 connected in parallel is provided. This is because, if a plurality of LEDs connected to each other to form a series circuit is to be driven by only one DC-DC converter 9, the size of the DC-DC converter 9 will increase as is the case with the configuration shown in FIG. 8 as a configuration in which, if a plurality of LEDs connected to each other to form a series circuit is to be driven by only one chopper regulator 109, the size of the chopper regulator 109 will increase.

In addition, particularly in this case, a plurality of DC-DC converters 9 is connected to form a parallel circuit. Thus, in comparison with a configuration employing only one DC-DC converter 9, for each of the DC-DC converters 9, the core size and endurance voltage of the isolation transformer can be reduced to result in a compact device. As a result, the increase of the total size of the DC-DC converters 9a, 9b and 9c can be made infinitesimal.

On top of that, in the same way as the modified versions described earlier by referring to FIGS. 4 and 5, the power-supply apparatus 12 according to the second embodiment may employ a PFC converter circuit 7 as a substitute for the rectification/smoothing section 1 even though the power-supply apparatus 12 employing a PFC converter circuit 7 is not shown in any of the figures.

The second embodiment implements a typical configuration in which a plurality of LEDs is provided to form series connection circuits each associated with a DC-DC converter 9. In place of such a configuration, for series circuits each including a plurality of LEDs, only one DC-DC converter can also be connected to the rectification/smoothing section 1 in parallel to the main power-supply circuit 2. In this case, a plurality of isolation transformers each associated with one series circuit is provided in parallel inside the DC-DC converter 9 to form a plurality of DC-voltage generation systems. On the secondary side of each of the isolation transformers in the direct current-voltage generation systems, a direct current voltage is generated for the series circuit associated with the isolation transformer.

As another alternative, only one DC-DC converter can also be connected to the rectification/smoothing section 1 in parallel to the main power-supply circuit 2, but a plurality of isolation transformers each associated with one series circuit is provided to form a series circuit inside the DC-DC converter 9 to form a plurality of direct current-voltage generation systems on the secondary side.

In the case of a DC-DC converter 9 including a plurality of isolation transformers as described above, on the secondary side of each of the transformers, the level of a current flowing through a series circuit of LEDs is typically detected and, in accordance with the result of the detection, the voltage generated on the secondary side is stabilized. If such a configuration is adopted, much like a plurality of DC-DC converters 9 forming a parallel circuit, a direct current flowing through every series circuit of LEDs can be stabilized.

It is to be noted that, in accordance with the embodiments explained so far, the power-supply apparatus provided by the present invention functions as a power-supply section of a liquid crystal display apparatus. In these embodiments, the inverter circuit 4 or the DC-DC converter 9 generates respectively an alternative current voltage or direct current voltage for driving the backlight section. However, the present invention can also be applied to a wide range of configurations in which, for example, a second power conversion means generates an alternative current or direct current power-supply voltage for driving a load other than the backlight section.

In addition, in the embodiments, a transformer employed in the inverter circuit 4 or the DC-DC converter 9 can be an electromagnetic transformer or a piezo-electric transformer.

Claims

1. A display apparatus having a backlight section and a load other than said backlight section, said display apparatus comprising:

an input-voltage generation section for generating a direct current input voltage from an alternating current;

a first power conversion section for receiving said direct current input voltage, and for carrying out a DC-DC power conversion process on the direct current input voltage to generate a direct current power-supply voltage to be supplied to said load;

a second power conversion section connected in parallel with said first power conversion section, and including:

a primary side for receiving said direct current input voltage, and including:

a pair of series connected switches for switching the direct current input voltage to generate an alternating current input voltage being supplied to a primary winding of at least one transformer, and

a driving circuit for driving said pair of series connected switches,

a secondary side, isolated from said primary side by said at least one transformer, for generating, from an alternating current output voltage supplied by a secondary winding of the at least one transformer, a power-supply voltage to be supplied to a plurality of parallel connected backlights of said backlight section,

a voltage detection circuit connected in series with only one of the plurality of parallel connected backlights for detecting a voltage supplied to that backlight, and

a feedback section for receiving the detected voltage from said voltage detection circuit, for rectifying the detected voltage, and for supplying the rectified voltage to said driving circuit of said primary side of said second power conversion section,

said driving circuit controlling the quantity of light emitted by the plurality of parallel connected backlights to a constant value based on the rectified voltage; and

a display section for displaying a picture using said backlight section.

2. A display apparatus according to claim 1, wherein the at least one transformer includes a plurality of transformers each associated with a respective portion of the plurality of parallel connected backlights of said backlight section.

3. A display apparatus according to claim 1, wherein a plurality of parallel connected fluorescent tubes is are employed as the plurality of parallel connected backlights of said backlight section, and said secondary side of said second power conversion section generates the power-supply voltage to be supplied to each of said parallel connected fluorescent tubes.

4. A display apparatus according to claim 1, wherein said input-voltage generation section includes a rectification/smoothing circuit having a plurality of diodes for rectifying the alternating current, and a capacitor for smoothing a rectified output of said plurality of diodes, and said input-voltage generation section generates said direct current input voltage as a voltage appearing between terminals of said capacitor.

5. A display apparatus according to claim 1, wherein said input-voltage generation section includes a power-factor improvement converter for generating a stabilized direct current output voltage as the direct current input voltage.

6. A display apparatus according to claim 1, wherein said feedback section isolates the primary and secondary sides of the second power conversion section from each other.

7. A display apparatus having a backlight section and a load other than the backlight section, the display apparatus comprising:

an input-voltage generation section which generates a direct current input voltage from an alternating current;

a first power conversion section which receives the direct current input voltage, and carries out a DC-DC power conversion process on the direct current input voltage to generate a direct current power-supply voltage to be supplied to the load;

a second power conversion section connected in parallel with the first power conversion section, and including:

a primary side which receives the direct current input voltage, and including:

a pair of series connected switches which switches the direct current input voltage to generate an alternating current input voltage being supplied to a primary winding of at least one transformer, and

a driving circuit which drives the pair of series connected switches,

a secondary side, isolated from the primary side by the at least one transformer, which generates, from an alternating current output voltage supplied by a secondary winding of the at least one transformer, a power-supply voltage to be supplied to a plurality of parallel connected backlights of the backlight section,

a voltage detection circuit connected in series with only one of the plurality of parallel connected backlights for detecting a voltage supplied to that backlight, and

a feedback section which receives the detected voltage from the voltage detection circuit, rectifies the detected voltage, and supplies the rectified voltage to the driving circuit of the primary side of the second power conversion section,

the driving circuit controlling the quantity of light emitted by the plurality of parallel connected backlights to a constant value based on the rectified voltage; and

a display section which displays a picture using the backlight section.

8. A display apparatus according to claim 7, wherein the at least one transformer includes a plurality of transformers each associated with a respective portion of the plurality of parallel connected backlights of the backlight section.

9. A display apparatus according to claim 7, wherein a plurality of parallel connected fluorescent tubes is employed as the plurality of parallel connected backlights of the backlight section.

10. A display apparatus according to claim 7, wherein the input-voltage generation section includes a rectification/smoothing circuit having a plurality of diodes which rectify the alternating current, and a capacitor which smoothes smooths a rectified output of the plurality of diodes, and the input-voltage generation section generates the direct current input voltage as a voltage appearing between terminals of the capacitor.

11. A display apparatus according to claim 7, wherein the input-voltage generation section includes a power-factor improvement converter which generates a stabilized direct current output voltage as the direct current input voltage.

12. A display apparatus according to claim 7, wherein the feedback section isolates the primary and secondary sides of the second power conversion section from each other.

13. A display apparatus having a backlight section and a load other than the backlight section, the display apparatus comprising:

an input-voltage generation section for generating a direct current input voltage from an alternating current;

a first power conversion section for generating a direct current power-supply voltage to be supplied to the load;

a second power conversion section connected in parallel with the first power conversion section, and including:

a primary side for receiving the direct current input voltage, and including:

a pair of series connected switches for switching the direct current input voltage to generate an alternating current input voltage being supplied to a primary winding of at least one transformer,

a driving circuit for driving the pair of series connected switches,

a secondary side, isolated from the primary side by the at least one transformer, for generating, from an alternating current output voltage supplied by a secondary winding of the at least one transformer, a power-supply voltage to be supplied to a plurality of parallel connected backlights of the backlight section,

a voltage detection circuit connected in series with one of the plurality of parallel connected backlights for detecting a voltage supplied to that backlight,

a feedback section for receiving the detected voltage from the voltage detection circuit, for adjusting the detected voltage to generate an adjusted optical signal voltage, and for supplying the adjusted optical signal voltage to the driving circuit of the primary side of the second power conversion section, and

the driving circuit controlling the quantity of light emitted by the plurality of parallel connected backlights to a constant value based on the adjusted optical signal voltage; and

a display section for displaying a picture using the backlight section.

14. A display apparatus according to claim 13, wherein the at least one transformer is a plurality of transformers each associated with a respective portion of the plurality of parallel connected backlights of the backlight section.

15. A display apparatus according to claim 14, wherein the plurality of transformers are parallel connected with respect to one another.

16. A display apparatus according to claim 13, wherein the at least one transformer includes a plurality of secondary windings each associated with a respective portion of the plurality of parallel connected backlights of the backlight section.

17. A display apparatus according to claim 13, wherein a plurality of parallel connected fluorescent tubes are employed as the plurality of parallel connected backlights of the backlight section, and the secondary side of the second power conversion section generates the power-supply voltage to be supplied to each of the parallel connected fluorescent tubes.

18. A display apparatus according to claim 13, wherein the input-voltage generation section includes a power-factor improvement converter for generating a stabilized direct current output voltage as the direct current input voltage.

19. A display apparatus according to claim 13, wherein a junction located between the pair of series connected switches is connected to a first terminal of the primary winding of the at least one transformer, and a terminal of the pair of series connected switches is connected through a capacitor to a second terminal of the primary winding of the at least one transformer.

20. A display apparatus according to claim 13, wherein the junction point between the switches is connected to the primary winding, and wherein one terminal of the switches is electrically connected to the primary winding by a capacitor.

21. A power-supply apparatus for generating a direct current power-supply voltage having a backlight section and a load other than the backlight section, the power-supply apparatus comprising:

an input-voltage generation section for generating a direct current input voltage from an alternating current;

a first power conversion section for generating a direct current power-supply voltage to be supplied to the load;

a second power conversion section connected in parallel with the first power conversion section, and including:

a first power-supply side for receiving the direct current input voltage, and including:

a pair of series connected switches for switching the direct current input voltage to generate an alternating current input voltage being supplied to a first winding of at least one transformer,

a driving circuit for driving the pair of series connected switches,

a second power-supply side, for generating, from an alternating current output voltage supplied by a second winding of the at least one transformer, a power-supply voltage to be supplied to a plurality of parallel connected the backlight section,

a voltage detection circuit connected in series with one of the plurality of parallel connected the backlight section for detecting a voltage supplied to the backlight section,

a feedback section for receiving the detected voltage from the voltage detection circuit, for adjusting the detected voltage to generate an adjusted optical signal voltage, and for supplying the adjusted optical signal voltage to the driving circuit of the first power-supply side of the second power conversion section, and

the driving circuit controlling the quantity of light emitted by the plurality of parallel connected backlights to a constant value based on the adjusted optical signal voltage; and

the backlight section driving a light source as the load.

22. A power-supply apparatus of claim 21, wherein the at least one transformer is a plurality of parallel connected transformers.