JP4351122B2 - High frequency current lighting device - Google Patents

Description

  The present invention relates to a high-frequency current lighting device that lights a plurality of cold-cathode tubes simultaneously. This high-frequency current lighting device is used as a lighting device for a backlight for a liquid crystal display.

A cold cathode fluorescent lamp is often used as a light source of a light guide plate of a liquid crystal display. This cold cathode tube is a thin fluorescent tube having a diameter of several millimeters.
The light emission principle of a cold cathode tube is basically the same as that of an ordinary fluorescent tube (hot cathode tube). However, since the electrode does not have a filament, the structure is simple and the electrode can be made small.
The electrical characteristics are that the cathode fall voltage is high, the positive column (light emitting part) is thin, and the gas pressure is high, so that the discharge voltage is very high (300 to 700 V) compared to the hot cathode tube. The discharge current is usually about 5-7 mA.
"Cold cathode tube and hot cold cathode tube" [searched on June 20, 2004] Internet URL <http://tlm.co.jp/web/gijyutu/ccfl.html>

In order to prevent uneven illumination in a display device such as a liquid crystal display, it is necessary to unify the brightness of each cold cathode tube. Therefore, it is necessary to pass an equal current to each cold cathode tube.
However, since the cold cathode tubes have negative resistance characteristics, it is difficult to flow the same current when a plurality of cold cathode tubes are connected in parallel. In order to allow the same current to flow in each cold cathode tube, a current-sharing circuit for flowing an equal current must be added to the lighting device. For this reason, it costs and the whole lighting device becomes large.

If an inverter circuit is used for each cold-cathode tube, the current can be adjusted. However, the cost is increased and the entire lighting device is enlarged.
In addition, it is known that stray capacitance exists between the cold cathode tube and the reflector or between the cold cathode tubes, but in the case of parallel connection, the stray capacitance is effective in parallel connection. As a result, there is a problem that the efficiency of the entire lighting device is deteriorated.

The lighting device is also problematic in that high frequency noise generated when the switch of the inverter circuit is turned on and off gives electromagnetic interference to peripheral electronic devices.
Therefore, an object of the present invention is to provide an efficient high-frequency lighting device that maintains a uniform current flowing through each cold cathode tube, reduces the influence of the stray capacitance, and is efficient.
It is another object of the present invention to provide a high-frequency current lighting device that maintains a uniform current flowing through each cold-cathode tube, is thin and small, and generates less noise.

The high-frequency current lighting device of the present invention is connected to a high-frequency power supply circuit, a plurality of transformers in which primary side coils are connected in series to the high-frequency power supply circuit, and a secondary side coil of each transformer, respectively. A plurality of cold-cathode tubes, and the high-frequency power supply circuit has two switching elements operating at a high frequency, the two switching elements are driven in opposite phases to each other, and the two switching elements are both There is a period Toff in which both are off, and the period Toff is a resonance half period τ of a resonance circuit formed by a parallel parasitic capacitance C of the switching element and a leakage inductance L1 of primary coils of the plurality of transformers. It is determined to be any value from 0.8 times to 1.3 times (Claim 1). According to this high-frequency current lighting device, it can be considered that a plurality of cold-cathode tubes are equivalently connected in series to the high-frequency power supply circuit, so that an equal current flows to each cold-cathode tube. Can do.

The high-frequency power supply circuit has two switching elements that operate at a high frequency, and the two switching elements are driven in opposite phases to each other, and there is a period Toff in which both the two switching elements are off. Therefore , the harmonic noise generated at the time of switching can be reduced and the occurrence of electromagnetic interference can be prevented.
Furthermore, the high-frequency current lighting device of the present invention has two switching elements each divided into two groups operating at high frequencies, for a total of four switching elements, and the two switching elements in each group are driven in mutually opposite phases. There is a period Toff in which all of the four switching elements are off, and the period Toff is formed by a parallel parasitic capacitance C of the switching elements and a leakage inductance L1 of primary coils of the plurality of transformers. It is determined to be any value between 0.8 and 1.3 times the resonance half period τ of the resonance circuit . Also with this configuration, harmonic noise generated during switching can be reduced, and the occurrence of electromagnetic interference can be prevented.

A relationship between a period Toff in which all the switching elements are turned off, and a resonance half cycle τ of a series resonance circuit formed by the switching elements and primary coils of the plurality of transformers connected in series with each other. , Relational expression 0.8τ ≦ Toff ≦ 1.3τ
The that meet. Runode have this relation is satisfied, a series resonance circuit, about the same period as the current flowing to the plurality of cold-cathode tubes is reciprocated, it is possible to switch the switching element. Therefore, the current flowing through the plurality of cold cathode tubes can be switched smoothly, noise generated when switching the switching element can be further reduced, and the occurrence of electromagnetic interference can be more effectively prevented.

Resonance half period of the series resonance circuit includes a parallel parasitic capacitance C of the previous SL switching elements are created by using the leakage inductance L1 of the primary coil of the plurality of transformers.
If a primary coil of each transformer that drives the plurality of CCFLs and a primary coil of a transformer that supplies power to other loads are connected in series to the high-frequency power circuit (Claim 5) Power can be simultaneously supplied from a single high-frequency power supply circuit to a plurality of loads including the CCFL CCFL.

  In this case, if a parallel control regulator is connected to the secondary coil of the transformer that supplies power to the other load, it is possible to cope with loads having different drive voltages.

  According to the present invention, a plurality of cold cathode fluorescent lamps can be lit with uniform brightness. Moreover, the influence of the increase in stray capacitance associated with the plurality of cold cathode tubes can be reduced, and a high-efficiency high-frequency current lighting device can be realized.

FIG. 1 is a circuit diagram of a high-frequency current lighting device according to the present invention. In this high-frequency current lighting device, a circuit in which primary coils of a plurality of step-up transformers K1, K2,... Kn (generally referred to as “step-up transformer K”) are connected in series with each other is used for DC cutting. The high frequency power supply 1 is connected through the series capacitor C1.
Cold cathode fluorescent lamps CCFL are connected to the secondary coils of the step-up transformers K1, K2,.

The series capacitor C1 absorbs voltage fluctuation applied to the high-frequency current lighting device, and is connected between the leftmost step-up transformer K1 and the high-frequency power source 1 in FIG. However, the installation position of the series capacitor C1 is not limited to this, and may be any position in a circuit in which a plurality of step-up transformers K1, K2,... Kn are connected in series.
The sum of the primary side voltages of the step-up transformers K1, K2,... Kn is expressed as “load voltage Vk”. The current flowing through the primary side of the step-up transformer K is denoted as “load current Ik”.

Each step-up transformer K has a leakage inductance. In FIG. 1, the leakage inductance is collected at one place and indicated by L1. For example, if one transformer is required for a 30 W cold cathode tube, the leakage inductance L is about 5 μH, so if there are N cold cathode tubes (N is a natural number), N Double (5NμH).
Further, each cold cathode tube CCFL has a parasitic capacitance. Each parasitic capacitance is indicated by CL.

  The high frequency power supply 1 employs a partial resonance half bridge drive type high frequency power supply circuit. The high-frequency power source 1 includes a DC power source V1 (which may be a battery or may be obtained by rectifying a commercial power source), and two separately-excited oscillators 2 and 3 that oscillate rectangular wave voltages VG2 and VG3 having opposite phases. And MOSFET type switching elements SW2 and SW3 which are turned on and off based on rectangular wave voltages VG2 and VG3 generated from the separately excited oscillators 2 and 3, respectively. The oscillation frequency of the separately excited oscillators 2 and 3 is written as f2. f2 is usually selected from about 100 kHz to 200 kHz.

The drain-source voltages of the switching elements SW2 and SW3 are denoted as V2 and V3, respectively.
Parasitic capacitances exist in parallel between the drains and sources of the switching elements SW2 and SW3, respectively, and these are designated as parallel parasitic capacitances C. The parallel parasitic capacitance C usually has a value of about 100 pF to 4000 pF.

The resonance frequency of the series resonance circuit formed by the parallel parasitic capacitance C and the leakage inductance L1 is written as f1. The resonance frequency f1 is represented by 1 / 2π√ (CL1).
FIG. 2 is an operation waveform diagram of the high-frequency current lighting device of FIG. FIG. 2A illustrates the on / off periods of the switching elements SW2 and SW3 that operate based on the rectangular wave voltages VG2 and VG3 generated from the separately excited oscillators 2 and 3, respectively. FIG. 2B illustrates the drain-source voltages V2 and V3 of the switching elements SW2 and SW3. FIG. 2C illustrates a waveform of the load voltage Vk. FIG. 2D is a diagram for illustrating the g resonance half cycle τ. FIG. 2E is a waveform diagram of the load current Ik.

The rectangular wave voltages VG2 and VG3 generated from the separately excited oscillators 2 and 3 are opposite in phase to each other and both have a period of voltage 0. In both cases, the period of voltage 0 is indicated by “Toff” in FIG.
The length of the off period “Toff” of both switching elements SW2 and SW3 is equal to or slightly shorter than the resonance half cycle τ of the series resonance circuit formed by the parallel parasitic capacitance C and the leakage inductance L1. It is set to a slightly long value. The resonance half cycle τ of the series resonance circuit formed by the parallel parasitic capacitance C and the leakage inductance L1 is equal to 1 / 2f1 when expressed by the resonance frequency f1. Since the resonance frequency f1 is represented by 1 / 2π√ (CL1) as described above, the resonance half cycle τ is represented by π√ (CL1). Accordingly, the value of Toff is set to a value equal to or near π√ (CL1). The “near value” is, for example, preferably 0.5 to 2 times, more preferably 0.8 to 1.3 times. In the case of 0.8 times to 1.3 times, Toff is
0.8π√ (CL1) ≦ Toff ≦ 1.3π√ (CL1) (1)
It is set to satisfy.

Next, the operation of the high-frequency current lighting device of FIG. 1 will be described with reference to FIGS.
During the ON period of the switching element SW2, the current from the DC power source flows into the load, and the voltage Vk substantially maintains the power source voltage V1.
When the on period of the switching element SW2 ends, the switching element SW3 is not immediately turned on, but there is an off period “Toff” of both the switching elements SW2 and SW3. During this period “Toff”, the load current Ik begins to flow in the reverse direction through the series resonant circuit formed by the parallel parasitic capacitance C and the leakage inductance L1. The load current Ik that has started to flow in the reverse direction begins to change its direction at the time when the above-described resonance half cycle τ elapses. Therefore, by setting the period “Toff” to a value that is the same as or close to the resonance half cycle τ as described above, the switching element SW3 is turned on when the load current Ik that has started to flow in the reverse direction finishes flowing. can do.

If the switching element SW3 is turned on, the load current Ik can flow into the negative line of the power source V1 through the switching element SW3. For this reason, the load voltage Vk can maintain the negative value (-V1) of the DC power supply voltage.
As described above, by providing the off-period “Toff” of both switching elements SW2 and SW3, resonance occurs in the load current during this “Toff” period, and the negative value from the positive value V1 of the load voltage Vk is reduced. The transition to the value (-V1) can be performed smoothly. That is, it is possible to avoid a rapid change in the high-frequency current during this voltage transition. Therefore, generation of electromagnetic waves can be prevented and electromagnetic interference given to surrounding devices can be reduced.

  Note that the waveform of the load voltage Vk itself is delayed in time compared to the waveforms of the drain-source voltages V2 and V3 of the switching elements SW2 and SW3. This delay time is represented by “s” in FIG. The reason why the waveform of the load voltage Vk is delayed by the time s is that the load current is used for charging and discharging the parallel parasitic capacitance C and the leakage inductance L1. The length of the delay time s is about half of the resonance half period τ.

When the load voltage Vk exceeds the lighting threshold value of the cold cathode tube CCFL, the cold cathode tube CCFL is turned on, and when the load voltage Vk falls below the turn-off threshold value of the cold cathode tube CCFL, the cold cathode tube CCFL is turned off. Since the cold cathode tube CCFL is lit and extinguished, there is a hysteresis, so that the extinguishing threshold is naturally lower than the lighting threshold.
The period during which the load current Ik flows is a period corresponding to a “Toff” period and a delay time s following the “Toff” period, as shown in FIG.

In the above operation, the load current Ik flowing through the CCFL CCFL can be adjusted by changing the switching frequency f2.
In the circuit of FIG. 1, the capacitance of the series capacitor C1 is preferably set sufficiently large so that it hardly contributes to resonance determined by the parallel parasitic capacitance C of the switching element and the leakage inductance L1 of the step-up transformer K. That is, if the series resonance frequency of the circuit loop determined by the series capacitor C1 and the leakage inductance L1 is written as f3,
f3 <f2 (2)
To be.

With the circuit configuration of the embodiment of the present invention described above, currents of the same magnitude can be supplied to each cold cathode tube CCFL connected to the secondary side of the transformer. Therefore, the emission luminance of each cold cathode tube CCFL can be made uniform, and the display uniformity of the liquid crystal display can be maintained.
Further, for the resonance determined by the parallel parasitic capacitance C of the switching element, by using the leakage inductance L1 of the step-up transformer K, it is not necessary to attach a resonance coil or a resonance capacitor. Therefore, a lightweight and small high-frequency current lighting device can be realized.

Next, an application example of the high-frequency current lighting device of the present invention described above will be described.
A video / electronic device such as a liquid crystal television has a plurality of loads such as a cold cathode tube CCFL drive unit, a liquid crystal drive unit, a video signal processing unit, and an operation interface unit, and usually has different operating voltages.
Dividing into the cold cathode tube CCFL drive part and other parts, such as the liquid crystal drive part, video signal processing part, and operation interface part, and providing independent power supplies, the number of types of necessary power supplies becomes large, and as a result As a result, the total power supply efficiency is lowered, and it is necessary to prepare a power supply circuit for each necessary voltage, which causes problems such as a complicated circuit configuration and an increase in cost.

Therefore, collective power supply is realized by preparing a power supply that can be regarded as a single current source for all loads of video / electronic devices that require different power supply voltages.
FIG. 3 shows that the primary side coils of the step-up transformer K of a plurality of cold cathode fluorescent lamps CCFL are connected in series with each other, and the primary side coils of the step-up transformer are connected in series with each other to supply power to other loads. The circuit is shown.

In the figure, an AC input power source such as 100 V or 200 V is connected to the rectifier circuit 4 through a fuse or the like. In the rectifier circuit 4, this AC input power supply is once converted to DC. The power source converted into direct current is converted into a high frequency power source by the high frequency power circuit 1 of the partial resonance half bridge drive type, and is fed to the primary side of a plurality of step-up transformers connected in series.
Connected to the secondary side of each step-up transformer is a CCFL and a circuit portion that requires other constant voltages V1, V2, V3,..., Vn (collectively referred to as “V”). Has been.

For the circuit portion that requires the constant voltage V, parallel control regulators S1, S2,..., Sn (currently referred to as “parallel control regulator S” for current-voltage conversion) are provided on the secondary side of the step-up transformer. And the respective loads are connected to the output terminals of the parallel control regulator S.
An example of the circuit configuration of the parallel control regulator S is shown in FIG. Since the circuit configuration of these parallel control regulators S itself is known from Japanese Patent Laid-Open Nos. 4-46565 and 2003-333857, it will be briefly described here.

The parallel control regulator S includes a combination of a parallel type zero-cross switch SWs, a rectifier circuit 5, and a smoothing capacitor Cs, and connects the voltage detection circuit Ds and a load to the smoothing capacitor Cs. The smoothing capacitor C can be a capacitor having polarity. The switching of the zero cross switch SWs is performed by a voltage detection circuit Ds.
The voltage of the smoothing capacitor C, that is, the voltage applied to the load is V, and the voltage applied to the rectifier circuit 5 is Vi.

  The voltage detection circuit Ds has hysteresis, and when the voltage V applied to the load becomes higher than the predetermined value EU, the zero-cross switch SWs is closed (ON) at the first zero-crossing time point of the voltage Vi, and is applied to the load. If the voltage V becomes lower than the predetermined value EL (EL <EU), the zero cross switch SWs is opened (turned off) at the first zero cross point of the voltage Vi, which continues from that point.

With this control, the voltage V applied to the load can be maintained between the predetermined values EL and EU. The specific set values of the predetermined values EL and EU are determined according to the voltage V required for each load.
With the circuit configurations of FIGS. 3 and 4 described above, a power supply having a predetermined voltage can be supplied to circuit portions other than the CCFL CCFL.

  Although the embodiments of the present invention have been described above, the embodiments of the present invention are not limited to the above-described embodiments. For example, instead of the plurality of step-up transformers K, piezoelectric transformers may be used, and these piezoelectric transformers may be connected to the cold cathode tubes CCFL, respectively. The high-frequency power source 1 includes MOSFET type switching elements SW2 and SW3 that are turned on and off based on rectangular wave voltages having opposite phases, but as shown in FIG. 5, four switching elements SW2, SW3, and SW2 are provided. ', SW3', the switching elements SW2 and SW3 are driven in opposite phases, the switching elements SW2 'and SW3' are driven in opposite phases, and the switching elements SW2 and SW2 'are driven in opposite phases. Even in the all-bridge drive type high frequency power supply configured as described above, the present invention can be applied by providing a period Toff in which all the four switching elements are off. In addition, various changes can be made within the scope of the present invention.

It is a circuit diagram of the high frequency current lighting device of the present invention using a half bridge drive type high frequency power supply. It is an operation | movement waveform diagram of each part in the high frequency current lighting device of FIG. In the circuit diagram, the primary side coils of the step-up transformer K of the plurality of CCFLs CCFL are connected in series to each other, and the primary side coils of the step-up transformer are connected in series to supply power to other loads. is there. It is a figure which shows the circuit structural example of the parallel control regulator S connected to the secondary side of a step-up transformer. It is a circuit diagram of the high frequency current lighting device of the present invention using a full bridge drive type high frequency power supply.

Explanation of symbols

1 High-frequency power supply 2, 3, 6, 7 Separately excited oscillator 4 Rectifier circuit 5 Rectifier circuit C Parallel parasitic capacitance
CCFL Cold cathode tubes K1, K2,... Kn Boost transformer L1 Leakage inductance Lk Load currents S1, S2, ..., Sn Parallel control regulators SW2, SW3, SW2 ', SW3' Switching element Vk Load voltage

Claims (6)

A high frequency power supply circuit;
A plurality of transformers in which primary coils are connected to each other in series with respect to the high-frequency power circuit;
A plurality of cold-cathode tubes each connected to the secondary coil of each transformer ,
The high-frequency power circuit has two switching elements that operate at a high frequency,
The two switching elements are driven in opposite phases to each other, and there is a period (Toff) in which both the two switching elements are off,
The period (Toff) is 0 of the resonance half cycle (τ) of the resonance circuit formed by the parallel parasitic capacitance (C) of the switching element and the leakage inductance (L1) of the primary coil of the plurality of transformers. .High frequency current lighting device characterized in that it is determined to be any value from 8 times to 1.3 times . The high-frequency current lighting device according to claim 1, wherein the period (Toff) is substantially equal to a value of the resonance half cycle (τ). A high frequency power supply circuit;
A plurality of transformers in which primary coils are connected to each other in series with respect to the high-frequency power circuit;
A plurality of cold-cathode tubes each connected to the secondary coil of each transformer,
The high-frequency power supply circuit has two switching elements that are divided into two sets that operate at high frequencies, for a total of four switching elements,
The two switching elements in each set are driven in opposite phases to each other, and there is a period (Toff) in which all the four switching elements are off ,
The period (Toff) is 0 of the resonance half cycle (τ) of the resonance circuit formed by the parallel parasitic capacitance (C) of the switching element and the leakage inductance (L1) of the primary coil of the plurality of transformers. .High frequency current lighting device characterized in that it is determined to be any value from 8 times to 1.3 times . The high-frequency current lighting device according to claim 3, wherein the period (Toff) is substantially equal to a value of the resonance half cycle (τ). A primary coil of each transformer that drives the plurality of cold cathode tubes and a primary coil of a transformer that supplies power to other loads are connected in series to the high-frequency power circuit. The high-frequency current lighting device according to any one of claims 1 to 4.   6. The high-frequency current lighting device according to claim 5, wherein a parallel control regulator is connected to a secondary coil of a transformer that supplies power to the other load.

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