Patent Application
20100182810
The present invention relates to an alternating-current power supply device that converts a direct-current voltage into an alternating-current voltage through a transformer and that supplies the converted alternating-current voltage to a load. Especially, the present invention relate to a technique for supplying an alternating-current voltage to a fluorescent lamp serving as a load to thereby light the fluorescent lamp.
An alternating-current power supply device converts a direct-current voltage into an alternating-current voltage through a transformer, and allows load to be driven with the alternating-current voltage. A fluorescent-lamp lighting device is known as an example of a device in which a load is connected to the alternating power supply device. The fluorescent-lamp lighting device uses an alternating-current voltage to light a cold cathode fluorescent lamp serving as a load.
In general, a cold cathode fluorescent lamp (CCFL) is lighted when the alternating-current power supply device applies thereto a voltage of several hundreds V to a thousand and several hundreds V with a frequency of several tens of kHz. Meanwhile, there is a fluorescent tube called an external electrode fluorescent lamp (EEFL). The external electrode fluorescent lamp is different from the cold cathode fluorescent lamp in its electrode structure, but is hardly different in other points, having the same light emitting principle as the cold cathode fluorescent lamp. For this reason, the alternating-current power supply device for lighting the external electrode fluorescent lamp and the alternating-current power supply device for lighting the cold cathode fluorescent lamp are the same in principle. Accordingly, the alternating-current power supply device is described below using the cold cathode fluorescent lamp (called simply a fluorescent lamp below).
The fluorescent lamp and the alternating-current power supply device are used for liquid crystal televisions, liquid crystal monitors, illuminating devices, liquid crystal display devices, billboards and the like. Characteristics required for the alternating-current power supply device are that: (a) the frequency of the alternating-current voltage is about 50 kHz, and (b) a voltage applied to the fluorescent lamp is an alternating-current voltage having a peak-to-peak symmetrical waveform.
Regarding (a), the frequency of a voltage applied to a fluorescent lamp is generally about 10 kHz to 100 kHz. The frequency is determined by a user, considering various characteristics of the fluorescent lamp, such as luminance characteristics, efficiency characteristics, and luminance characteristics of when the fluorescent lamp is incorporated in a set. The alternating-current power supply device is driven with the determined frequency or a frequency close thereto. Accordingly, the frequency oftentimes cannot beset or changed according to the convenience of the alternating-current power supply device. Since the liquid crystal televisions, liquid crystal monitors, illuminating devices, and the like are used with on the order of 50 kHz, an alternating-current power supply device using 50 kHz is used below.
Regarding (b), in general, a voltage applied to the fluorescent lamp needs to be an alternating-current voltage having a peak-to-peak symmetrical waveform. The fluorescent lamp is a glass tube in which mercury, a noble gas, or the like are sealed. The fluorescent lamp lights up even when a direct-current voltage is applied thereto. However, the mercury inside concentrates on one side of the fluorescent lamp, gradually causing a difference in luminance between both ends of the fluorescent lamp. The life of the fluorescent lamp is thus shortened drastically. This is why an alternating-current voltage is applied to the fluorescent lamp. Nevertheless, even with an alternating-current voltage, the mercury might possibly be distributed in an unbalanced manner if the voltage waveform has different forms on the positive side and on the negative side. Therefore, it is required to apply an alternating-current voltage having a peak-to-peak symmetrical waveform. A sine wave and a trapezoidal wave are ideal. In practice, many systems apply a sine-wave voltage.
These fluorescent-lamp lighting devices use two or more switching elements to obtain a peak-to-peak symmetrical waveform. According to the number of switching elements, the drive circuit for the switching elements increases, such as a high-side driver, a low-side driver, and an insulation element. Consequently, a component cost, a manufacturing cost, and an implementation area also increase. Naturally, the component costs for the switching elements also increase.
For example, Patent Document 1 is known as a conventional technique.
Patent Document 1: JP-A 8-162280
As described, since more than two switching elements are needed, a component implementation area, a component cost, and a manufacturing cost increase.
An objective of the present invention is to provide an alternating-current power supply device that accomplishes a cost reduction by decreasing the number of switching elements.
To address the above object, the first invention includes: a direct-current power supply; a first transformer having a primary winding and a secondary winding; a first switching element connected to the direct-current power supply through the primary winding of the first transformer; an output circuit that receives a voltage generated at the secondary winding of the first transformer and outputs an alternating-current voltage; a control circuit that turns the first switching element on and off using a drive signal a cycle of which is a total period of a first period and a second period; and a reset circuit that resets the first transformer in the second period, wherein the control circuit generates the drive signal so that a total of on-periods of the first switching element is longer than a total of off-periods thereof in the first period, and generates the drive signal so that a total of the off-periods of the first switching element is longer than a total of the on-periods thereof in the second period, so that a negative side wave and a positive side wave of the alternating-current voltage wave are almost formed to symmetry.
The second invention is characterized in that in an alternating-current power supply device according to the first invention the drive signal is a pulse signal, and number of pulses in one cycle in the drive signal is 1 or more and is fixed.
The third invention is characterized in that in the alternating-current power supply device according to the second invention, the control circuit includes: a first oscillator that generates an oscillation signal having a first frequency; a second oscillator that generates an oscillation signal having a second frequency different from the first frequency of the first oscillator; and a logic circuit that ANDs the oscillation signal of the first oscillator and the oscillation signal of the second oscillator, and the pulse signal is an output signal of the logic circuit.
The fourth invention is characterized in that the alternating-current power supply device according to the second invention including at least one of a voltage detection circuit that detects an output voltage from the output circuit and a current detection circuit that detects an output current from the output circuit, wherein the control circuit includes a pulse-width modulation circuit that modulates a pulse width of the pulse signal, based on an output signal from the at least one of the voltage detection circuit and the current detection circuit.
The fifth invention is characterized in that the alternating-current power supply device according to the third invention including at least one of a voltage detection circuit that detects an output voltage from the output circuit and a current detection circuit that detects an output current from the output circuit, wherein the control circuit includes a pulse-width modulation circuit that modulates a pulse width of the pulse signal, based on an output signal from the at least one of the voltage detection circuit and the current detection circuit.
The sixth invention is characterized in that in the alternating-current power supply device according to the first invention, the first transformer further includes a reset winding that magnetically couples with the primary winding, and the reset circuit is connected in parallel to the direct-current power supply, and is a circuit in which the reset winding and a diode are connected in series to each other.
The seventh invention is characterized in that in the alternating-current power supply device according to the first invention, the reset circuit is connected in parallel to the primary winding of the first transformer, and is a circuit in which a parallel circuit of a resistance and a capacitor is connected in series to a diode.
The eighth invention is characterized in that in the alternating-current power supply device according to the first invention, the reset circuit is connected in parallel to the primary winding of the first transformer, and is a circuit in which a capacitor and a second switching element are connected in series to each other.
The ninth invention is characterized in that in the alternating-current power supply device according to the first invention, the output circuit is connected in parallel to the secondary winding of the first transformer, is a circuit in which a first reactor and a first capacitor are connected in series to each other, and outputs the alternating-current voltage from the first capacitor.
The tenth invention is characterized in that in the alternating-current power supply device according to the first invention, the output circuit is a circuit in which a second reactor and a primary winding of a second transformer are connected in series with respect to the secondary winding of the first transformer, and a secondary winding of the second transformer and a second capacitor are connected in parallel to each other, and the output circuit outputs the alternating-current voltage from the second capacitor.
The eleventh invention is characterized in that in the alternating-current power supply device according to the ninth invention, the first reactor is formed of a leakage inductance of the first transformer.
The twelfth invention is characterized in that in the alternating-current power supply device according to the tenth invention, the second reactor is formed of a leakage inductance of the second transformer.
The 13th invention is characterized in that in the alternating-current power supply device according to the tenth invention, the second reactor is formed of a leakage inductance of the first transformer and a leakage inductance of the second transformer.
With reference to the drawings, embodiments of an alternating-current power supply device of the present invention are described in detail below. The following embodiments are described, taking a case where the alternating-current power supply device of the present invention is applied to a fluorescent-lamp lighting device. This fluorescent-lamp lighting device is configured by connecting a fluorescent lamp as a load, to the alternating-current power supply device of the present invention.
The load is a fluorescent lamp in the following examples; however, it should be noted that the load does not necessarily have to be the fluorescent lamp, and the alternating-current power supply device of the present invention may be applied to other types of load.
One end of a reset winding P1a is connected to the primary winding P1 of the transformer T1. The primary winding P1 of the transformer T1 is magnetically coupled to the reset winding P1a. The other end (the side denoted by •) of the reset winding P1a of the transformer T1 is connected to the cathode of a diode D1. The anode of the diode D1 is connected to the negative terminal of the DC power supply Vin. The reset winding P1a of the transformer T1 and the diode D1 form a reset circuit 1.
A series circuit of a reactor L1 (first reactor) and a capacitor C1 (first capacitor C1) is connected to both ends of a secondary winding S1 of the transformer T1. The reactor L1 and the capacitor C1 form an output circuit 2 that receives a voltage generated at the secondary winding S1 of the transformer T1 and then outputs an alternating-current voltage to output terminals OP1, OP2. A leakage inductance of the transformer T1 may be used as the reactor L1. The capacitor C1 is connected at both ends to a series circuit of a capacitor Ca and a fluorescent lamp 7a, and to a series circuit of a capacitor Cb and a fluorescent lamp 7b.
Here, the duty ratio is an on-duty ratio of a pulse signal. Specifically, in one cycle of a pulse signal, the duty ratio is 100*pulse-on period/(pulse-on period)+pulse-off period), and is expressed in percentage.
As
During an off-period of the switching element SW1, a reset current 12 flows to the reset winding P1a of the transformer T1 by way of P1→Vin→D1→P1a. Accordingly, at the time when the switching element SW1 is turned off, the reset winding P1a resets an exciting energy of the transformer T1. In addition, during this reset period, a negative voltage is generated at the secondary winding S1 of the transformer T1.
In this way, an alternating-current voltage V(S1) having a rectangular wave is generated at the secondary winding S1 of the transformer T1. Then, an alternating-current voltage V(C1) having a sine wave is obtained after the filtering actions by the reactor L1 and the capacitor C1. The alternating-current voltage V(C1) is a voltage across the capacitor C1.
As shown in
However, the alternating-current voltage V(C1) cannot be controlled if the duty ratio is fixed at 50%. To control the intensity of a fluorescent lamp, a voltage applied and a current flowing to the fluorescent lamp need to be controlled. To do so, the duty ratio of the switching element SW1 needs to be controlled. This may lead to decrease in the duty ratio, depending on the conditions, and thus may make it impossible to output a positive and negative symmetrical sine wave. The reason why a positive and negative symmetrical sine wave cannot be obtained is that the period in which the voltage V(S1) of the secondary winding S1 of the transformer T1 is positive is short with respect to one cycle.
In this respect, in Embodiment 1, a control circuit 10 is provided to turn the switching element SW1 on and off using a drive signal one cycle of which is the total period of a first period and a second period. The control circuit 10 generates the drive signal so that, in the first period, the total of on-periods of the switching element SW1 may be longer than the total of off-periods thereof, and generates the drive signal so that, in the second period, the total of the off-periods of the switching element SW1 may be longer than the total of the on-periods. Thereby, a negative side wave and a positive side wave of an alternating-current voltage wave are almost formed to symmetry.
a) shows waveforms of the respective components obtained when the duty ratio of a drive signal is large;
Moreover, for example, when the period in which the pulse signals exist is the period TM1 and the period in which the pulse signals do not exist is the period TM2 as shown in
Next, the duty ratio of a pulse signal is considered. In
In other words, to drive the switching element SW1 with a duty ratio exceeding 50% is to operate the switching element SW1 without resetting the exciting energy of the primary winding P1 of the transformer T1. This operation causes a voltage at the capacitor C1. In addition, in the period TM2, the duty ratio does not have to be zero as long as it is 50% or lower.
Although two pulses are inserted in the period TM1 in Embodiment 1, similar effects can be obtained even with three or more pulses.
As described, according to Embodiment 1, a single switching element SW1 is used. Moreover, the control circuit 10 generates a drive signal (pulse signal) so that, in the period TM1, the total of on-periods of the switching element SW1 may be longer than the total of the off-periods, and generates a drive signal so that, in the second period, the total of the off-periods of the switching element SW1 maybe longer than the total of the on-periods. Consequently, the output circuit is allowed to form an alternating-current voltage having the waveform of a positive and negative symmetrical sine wave. Thereby, the number of switching elements can be reduced.
According to such configuration of Embodiment 2, when the switching element SW1 is off, an exciting energy of the transformer T1 is accumulated at the capacitor C4 via the diode D2 and is consumed by the resistance R1. Accordingly, the reset circuit 1a can reset the exciting energy induced to the primary winding P1 of the transformer T1. Thereby, effects similar to those of Embodiment 1 can be obtained.
The other configurations shown in
According to such configuration of Embodiment 3, when the switching element SW1 is on, a current flows by way of Vin→SW1→Cri→P1→Vin, and the energy is accumulated at the current resonance capacitor Cri and the primary winding P1 of the transformer T1a. When the switching element SW1 is off and the switching element SW2 is on, a current flows by way of P1→Cri→SW2→P1. Accordingly, the reset circuit 1b can reset the exciting energy of the transformer T1.
Thereby, effects similar to those of Embodiment 1 can be obtained by such configuration of Embodiment 3.
The first oscillator 11 generates a voltage (oscillation signal) V11 of for example 200 kHz (first frequency) having a rectangular wave. The second oscillator 12 generates a voltage (oscillation signal) V12 of for example 50 kHz (second frequency) having a rectangular wave. The AND circuit 13 (logic circuit) generates a drive signal for the switching element SW1 by ANDing the rectangular-wave voltage V11 of 200 kHz of the first oscillator 11 and the rectangular-wave voltage V12 of 50 kHz of the second oscillator 12. The drive circuit 14 drives the switching element SW1 using the drive signal V13 from the AND circuit 13.
The period TM1 and the period TM2 are determined by the duty ratio of the oscillation signal from the second oscillator 12. It is generally desirable to set the duty ratio of the oscillation signal from the second oscillator 12 to about 50%. For this reason, the switching element SW1 is oscillated intermittently by a signal of 50 kHz having a duty ratio of about 50%. In addition, the alternating-current voltage V(C1) can be controlled by changing the duty ratio of the oscillation signal from the first oscillator 11.
In
Accordingly, it is effective to synchronize the signal of the first oscillator 11 with the signal of the second oscillator 12. Here, to synchronize the signals is to keep constant the number of pulses (for example, 2 pulses) of a drive signal for the switching element SW1 in one cycle of an alternative voltage (the one cycle is the total period of the period TM1 and the period TM2, e.g., a period of 50 kHz). The fluctuations in an alternating-current voltage can be suppressed since the number of pulses of a drive signal for the switching element SW1 in one cycle of an alternating-current voltage is constant.
Synchronized two signals can be generated easily with a frequency divider circuit, a frequency multiplier circuit, or the like using, for example, a flip-flop, a timer, a counter, or the like.
In the example shown in
In the example shown in
In the examples shown in
Moreover, as
Synchronization of a drive signal for the switch SW1 with the frequency of an alternating-current voltage is only what should be accomplished here, and synchronization between the oscillator outputs is merely an example.
Here, for example, if the transformer T1 performs insulation in the system shown in
To address this, in Embodiment 5 shown in
In addition, the transformer T2, the reactor L2, and the capacitor C2 form an output circuit 2a that receives a voltage generated at the secondary winding S2 of the transformer T2 and then outputs an alternating-current voltage to output terminals OP1, OP2.
According to such configuration of Embodiment 5, the transformer T1 performs insulation required by the various safety standards, and the transformer T2 performs boosting. Accordingly, the above problems can be avoided. Moreover, since the transformer T1 generates a lower voltage having a rectangular wave, the transformer T1 can loose conditions of the various safety standards. Being a booster at the secondary side, the transformer T2 only has to perform so-called functional insulation.
In addition, as the reactor L2 shown in
Alternatively, a leakage inductance of the transformer T1 and a leakage inductance of the transformer T2 may be used as the reactor L2 shown in
A fluorescent-lamp lighting device stably lights a fluorescent lamp by detecting a current flowing into the fluorescent lamp and controlling the detected current to set to a predetermined value. As such a method, a method for detecting a current flowing into a fluorescent lamp is frequently used.
However, the current to the fluorescent lamp cannot always be detected because of application constraints, structural constraints, or the like. In this case, current control can be performed also by detecting other electricity amount.
In Concrete Example 1 of Embodiment 6 shown in
In Concrete Example 2 of Embodiment 6 shown in
A primary winding P3 of the transformer T3 and a reactor L3 are respectively connected to both ends of the secondary winding S1 of the transformer T1. A capacitor C3 and a series circuit of the fluorescent lamp 7b and the current detection circuit 19b are connected to both ends of a secondary winding S3 of the transformer T3.
A duty-ratio adjustment circuit 20b adjusts the duty ratio of a pulse signal of the switching element SW1, based on a signal from the first oscillator 11, a signal from the ¼ frequency divider circuit 18 serving as the second oscillator, a detected current from a current detection circuit 19a, and a detected current from the current detection circuit 19b. The photocoupler PC1 flows a current which is in accordance with an output from the duty-ratio adjustment circuit 20b. The drive circuit 14 drives the switching element SW1 on and off, using an output signal from the photocoupler PC1, namely, a pulse signal the duty ratio of which has been adjusted.
In this way, multiple fluorescent lamps 7a, 7b can be lighted using multiple boosting transformers T2, T3.
In Concrete Example 1 of Embodiment 7, there are two fluorescent lamps. However, more fluorescent lamps can be lighted at the same time by increasing the number of transformers.
Moreover, as illustrated in Concrete Example 2 of Embodiment 7 shown in
According to the present invention, a control circuit uses a single switching element to generate drive signals in the following manner. Specifically, the control circuit generates a drive signal so that, in the first period, the total of on-periods of a first switching element may be longer than the total of the off-periods thereof, and generates a drive signal so that, in the second period, the total of off-periods of the first switching element may be longer than the total of the on-periods thereof. Consequently, a negative side wave and a positive side wave of an alternating-current voltage wave almost can be formed to symmetry in an output circuit. Accordingly, the number of switching elements can be reduced.
The present invention is applicable to a power supply device such as a DC-AC converter.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-216108 | Aug 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP08/63546 | 7/29/2008 | WO | 00 | 12/15/2009 |
