GB2273001A

GB2273001A - High-voltage transformer and lighting circuit

Info

Publication number: GB2273001A
Application number: GB9324300A
Authority: GB
Inventors: Tadamasa Fujimura; Masakazu Ushijima; Shinzo Murase; Hirokazu Matsui
Original assignee: MEITAKU SYST KK; Meitaku System Co Ltd
Current assignee: MEITAKU SYST KK; Meitaku System Co Ltd
Priority date: 1992-11-25
Filing date: 1993-11-25
Publication date: 1994-06-01
Also published as: GB9324300D0; JPH06163160A; FR2698480A1; DE4340077A1; JP3138761B2

Abstract

Primary and secondary windings or coils (PW3, PW4 and SW2) forming a high-voltage transformer (T2) are wound on a rod form of core or bobbin (6) one or both ends of which being located in a holder (7, 8) which is provided with input and output terminals of the primary or secondary coil. A C-shaped holder member (see fig. 2) may be used to hold both ends of the rod form of core or bobbin (6) and both ends of a discharge lamp. The above arrangements may be used in a lighting circuit convertor system for a discharge lamp. The above system may be employed for backlighting a liquid crystal panel. <IMAGE>

Description

j 2273001

TITLE OF THE INVENTION

HIGH-VOLTAGE TRANSFORMER AND LIGHTING CIRCUIT USING HIGH-VOLTAGE TRANSFORMER BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-voltage transformer and lighting circuit using high-voltage transformer putting a light to discharge lamps such as fluorescent lamps, particularly relates to a high-voltage transformer and lighting circuit using high-voltage transformer suitable for use on a portable type of small liquid crystal display devices (hereinafter referred to as LCD).

2. Prior Art

One of conventional lighting circuits widely used for putting on discharge lamps such as fluorescent lamps is a lighting tube-choke circuit. Except a starting circuit (e.g., a lighting tube) using a commercial AC power source, such a lighting circuit is basically made up of three circuits, an in-phase circuit with the resistor connected in series with a discharge lamp, a lagging circuit connected in series with an inductance, and a phase circuit connected in series with a capacitor.

However,, a lighting circuit or transformer for lighting up a fluorescent lamp used as an LCD backlight used on portable TV sets or portable electronic equipment such as computers - 1 ordinarily uses as the power source primary batteries such as dry batteries or chargeable secondary batteries. In this case, a DC voltage + Vcc is switched to light up a fluorescent lamp FL, as shown in FIG. 6.

In the conventional arrangement shown in FIG. 61 a DC voltage +Vcc is supplied from a primary or secondary power source to the terminal TE.

For instance, one-end of a choke coil CH of 200 MF is connected to this terminal TE, and the other end of the choke coil CH is connected to the center tap CT of the first primary winding PW1 of a high-voltage transformer Tl.

The terminal TE is connected to the bases of first and second transistors Q1 and Q2 via resistors R1 and R2, and to the second primary winding-PW2 of the high-voltage transformer T1.

The first and second transistors Ql and Q2 are grounded at the emitters, with the collectors connected to both terminal windings of the first primary winding PWI of the high-voltage transformer T1. A capacitor Cl is connected in parallel between said both terminal windings.

The secondary winding SWl of the high-voltage transformer Tj is connected to the cathodic and anodic filament of a fluorescent lamp FL via a ballast capacitor C2 (of about 33 PF to 100 PF).

The above arrangement is a sort of DC-DC converter, in which the first transistor Ql is biased through the second primary winding PW2 of the high-voltage transformer T2 to Put on the first transistor Q1, and the DC current +Vcc applied to the center tap CT induces a current il, thereby inducing a given voltage in the secondary winding SW1 of the high-voltage transformer T1.

Upon the second transistor Q2 put on, a current i2 is induced by the DC voltage +Vcc applied to the center tap CT, so that a given voltage can be induced in the secondary winding SW1 of the high-voltage transformer T1.

For the high-voltage transformer used with the abovementioned conventional lighting circuit an EI type core, an UI type core, a pot type core, an EE type core or the like may be used. Referring typically to the high-voltage transformer shown in FIG. 7A, the core 1, if reduced in size, has a height H of about 20 mm and a diameter of about 25 mm. Another typical transformer, shown in FIG. 7B, includes a core 1 of large size having a height H of about 20 mm, a width W of about 25 mm and length L of about 30 mm.

It is possible to make the fluorescent lamp FL smaller, e.g., substantially similar in size to the picture frame of a portable LCD and have a diameter of about 7 to 8 mm. However, a ballast capacitor C2 needed for the secondary winding SW1 of the high-voltage transformer Tj is required to withstand a high voltage of about 2 KV and so make the high-voltage transformer large and inexpensive.

When the above high-voltage transformer Tj and capacitor C2 are built on a printed substrate, the substrate is made very large,, as expressed by a width of about 20 mm, a length of about 100m and a height of about 20 mm. However, much difficulty is involved in making the overall arrangement comprising a high-voltage transformer and a lighting circuit compact.

An object of the invention is to provide a high-voltage transformer that is free from the above-mentioned problems, in other words, that is made small, and to provide a lighting circuit which can dispense with any ballast capacitor which is otherwise responsible for size increases.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a highvoltage transformer including primary and secondary coils wound on a rod form of core or bobbin, and a holder member that is disposed at one or both ends of said rod form of core or bobbin to hold said rod form of core or bobbin, said holder member being provided with the input and output terminals of said primary or secondary coil.

According to another aspect of the invenion, there is provided a highvoltage transformer including primary and secondary coils wound on a rod form of core or bobbin, a Cshaped holder member for holding both ends of said rod form of core or bobbin, and a discharge lamp bridging legs of said Cshaped holder member.

According to the third aspect of the invention, there is provided a lighting circuit designed to convert a DC voltage from a DC source to a high-frequency signal to light up a discharge lamp, which includes a driving means that is driven by said high-frequency signal, a high-voltage transformer built up of primary and secondary coils wound on a rod form of core or bobbin connected to the output end of said driving means, and a discharge lamp connected directly to the secondary coil of said high-voltage transformer.

According to the first aspect of the invention, it is possible to achieve a high-voltage transformer so small-enough that it can be located along the side edge of a light-guiding plate of an LCD.

According to the first aspect of the invention, it is again possible to achieve a high-voltage transformer so smallenough that it can be located along the side edge of a lightguiding plate of an L=.

In the lighting circuit according to the third aspect of the invention, the driving circuit for driving the highvoltage transformer of a rod structure is connected on the primary side of the high-voltage transformer, so that it can be excited by a high-frequency signal from a high-frequency oscillator via a switching circuit, whereby the discharge lamp can be connected directly to the secondary winding of the highvoltage transformer. Thus, this high-voltage transformer can function as a leakage flux type of transformer which can dispense with any high withstand voltage capacitor to be connected directly to the discharge lamp and enables-the lighting circuit to be built up of ICs. and so can be made compact.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained, by way of example but not by way of limitation, with reference to the accompanyIng drawings. in which:

FIG. 1 represents one high-voltage transformer according to the invention and the lighting circuit using it.

FIG. 2 represents another high-voltage transformer according to the invention and the lighting circuit using it.

FIG. 3 is a schematic of one embodiment of the highvoltage transformer according to the invention.

FIG. 4 is a schematic of one embodiment of the lighting circuit with the high-voltage transformer of the invention built in it.

FIG. 5 is an exploded perspective of one liquid crystal TV set in which the lighting circuit using the high-voltage transformer according to the invention is incorporated.

FIG. 6 is a circuit diagram of a lighting circuit with a conventional high-voltage transformer incorporated in it.

FIG. 7 is a perspective view of a conventional highvoltage transformer.

DETAILED DESCRIPTION OF THE INVENTION

Prior to explaining the high-voltage transformer of the invention with reference to FIGS. 1 to 3. the general structure of the lighting circuit of the invention will be explained with reference to FIG. 4.

In FIG. 4, a high-frequency oscillator 3 oscillates a high-frequency signal of a rectangular wave at about 110 KHZ.

Signals from the oscillator 3 are fed to a first "not" circuit IN, and a second "not" circuit IN 2 connected in series with a third "not" circuit IN 3.

The output of the first "not" circuit IN, is fed to the base of a first switching transistor Q3 via a base resistor R3. and the output of the third "not" circuit IN3 is fed to the base of a second switching transistor Q4 via a base resistor R4. The normal waveform portion of the oscillated signal is fed to the base of the second switching transistor Q 4, because the second and third "not" circuits IN2 and IN3 are cascade-connected thereto, and the inverted waveform portion of the oscillated signal is fed to the base of the first switching transistor Q3.

Each emitter of the first and second switching transistors Q3 and Q4 is grounded, with each collector connected to the bases of first and second driving transistors Q5 and Q6. The collector of the first switching transistor Q3 is connected to the collector of the second driving transistor Q6 via a resistor R6. while the collector of the second switching transistor Q4 is connected to the collector of the first driving transistor Q 5 via a resistor R 5 Each emitter of the first and second driving transistors Q 5 and Q6 is grounded, with each collector connected to the cathodes of diodes CD 1 and CD 2. The anodes of the diodes CD 1 and CD2 are grounded. Each collector is connected to one terminal of the primary windings PW 3 and PW 4 of a highvoltage transformer T2 to be described later, and the other - 7 ends of the primary windings PW and PW are connected in series with each other at a point, from which the center tap CT 1 extends. The center tap CT, is supplied with a DC voltage +Vcc = + 6 V.

The high-voltage transformer T2 includes a core 2 made up of a core bar of a manganese type ferrite. Primary and secondary windings PW 3 1 PW4 and SW 2 are wound on this core bar 2 through a given insulating material. A fluorescent lamp FL 1 is directly connected in series with the secondary winding SW2. while the primary and secondary windings are connected one to another such that the secondary winding SW2 is grounded at one end and connected at the other end to one electrode of the fluorescent lamp FL1, while one end of the other secondary winding is connected to the other electrode of the fluorescent lamp FL1.

The fluorescent lamp FL serving as a discharge lamp is a small fluorescent lamp for backlighting an LCD, which, for instance, has a firing voltage of about 700 V, a discharge voltage of 360 V and a tube current of about 7 mA.

The high-voltage transformer T2 used with the above mentioned circuit may be constructed in such a bobbin form as shown in FIG. 3. The bobbin 6, shown in FIG. 3, is in a substantially cylindrical configuration and made up of synthetic resin such as diazoterephthalate, polycarbonate or polyester or glass. Both ends of the the bobbin 6 serve as holder members 7 and 8 and are provided with pins 9 and 10 for connection with the terminals of the primary windings PW3,, PW4 and the secondary winding SW 2 The bobbin 6 is provided on the outer surface with a plurality of collars 11, between which the primary and secondary windings PW31 PW4 and SW2 are wound on the bobbin 6. In this embodiment, the number of turns of the primary windings PW3 and PW4 of a 0.2 mm wire is 20 and the number of turns of the secondary winding SW2 of a 0.05 mm wire 3000. Note that the bobbin 6 has a diameter of 3.5 mm. Also note that the total length L of the bobbin 6 is 30 mm; the maximum diameter Do of the coiled bobbin 6 is 5 mm; and a rod form of ferrite core 2 of 1.5 mm x 30 mm, is fixedly inserted through the central cylindrical hole in the bobbin 6.

The high-voltage transformer T 2 of the structure mentioned above can provide a leakage transformer so simple in structure that it is likely to give rise to flux leakage because the magnetic coupling is rough for the reason that the primary windings PW 3 and PW4 and the secondary winding SW2 are wound on both ends of the rod bobbin 6. A conventional leakage transformer is too increased in size to be used as a high-voltage transformer for backlighting a portable display device in particular. The reason is that when the primary and secondary windings are wound on an ET type core, a longer magnetic path must be provided so as to make the coupling rough.

The use of a leakage transformer as a high-voltage transformer results in an increase in the secondary current with a corresponding increase in flux leakage, giving rise to a drop of the terminal voltage of the secondary voltage and so reducing a current increase. Thus the secondary current can be kept constant in a given load range, dispensing with the large capacitor C for resisting high voltage connected to the secondary side. This effect makes a great deal of contribution to making a lighting circuit compact.

How the lighting circuit shown in FIG. 4 works will now be explained at great length.

The high-frequency signal of 110 KHZ oscillated from high-frequency oscillator 3 is inverted by the output of the first "not" circuit IN 1, and fed to the base of the first switching transistor Q 3 via the base resistor PV thereby biasing that base.

Likewise, the high-frequency signal appearing at the output of the third "not" circuit IN 3 via the second and third "not" circuits IN2 and IN 3 is fed to the base of the second switching transistor Q4 via the base resistor R 4 to bias that base.

Thus, the first and second switching transistors Q 3 and 94 are put on and off depending on the positive and negative pulses of the high- frequency signal.

With the first switching transistor Q 3 put on, the first driving transistor Q 5 is so driven that current i 2 from the source voltage +Vcc can pass through the center tap CT 1 of the primary winding of the highvoltage transformer T 2 and flow through the primary winding PW 3 and the collector and emitter of the driving transistor Q6, - With the second switching transistor Q 4 put on, the second driving transistor Q6 is so driven that current i. from the source voltage +Vcc can pass through the center tap CT 1 of the primary winding of the high-voltage transformer T 2 and then flow through the primary winding PW 4 and the collector and emitter of the driving transistor Q 6' Note that the diodes CD 1 and CD 2 absorb surge.

As an exciting switching voltage corresponding to the frequency of the high-frequency signal is applied to the primary windings PW 3 and PW 4 of such a high-voltage transformer T 2, a secondary voltage is induced on the secondary winding SW 2 side that is roughly coupled to the primary windings PW 3 and PW4' Although the fluorescent lamp FL, is connected directly to the secondary winding SW2. the secondary current can remain contact due to a large leakage inductance, so that the fluorescent lamp FL, can be driven directly without recourse to any capacitor.

In the high-voltage transformer T 2 of the arrangement mentioned above, the primary and secondary windings are wound on the rod form of bobbin 6 provided on both its ends with the holder members 7 and 8 forming a terminal portion. In what follows, another arrangement of the highvoltage transformer T 21 which can again be effectively used according to the invention, will be explained at great length with reference to FIGS. I and 2.

FIGS. 1A to 1E represent an arrangement in which the above lighting circuit is made integral with the high-voltage transformer. FIG. 1A-C are plan, front and right side views of that arrangement, respectively; FIG. 1D is a sectional view taken along the line C-C' of FIG. 1B; and FIG. 1E is a sectonal view taken along the line A-A'or B-B' of FIG. 1B. Throughout FIGS. 1A-E,, the same parts as in FIG. 3 are represented by the same numerals.

A pin 10 that is substantially similar in shape to that shown in FIG. 3 is provided at the right end of a bobbin 6, and a cylindrical holder member 8 is fitted over the bobbin 6, having an array of grooves 12 from which wires are withdrawn. Note, of course, that the holder member 8 can receive a core 2.

The bobbin 6 is similar in construction to that explained with reference to FIG. 3, except that it is divided by five collars 11. Primary and secondary windings are wound on these divided parts. More exactly, the holder member 8 includes two grooves 12 from which the wires are withdrawn, as illustrated in FIG. 1E, and is disposed at the right end of the bobbin 6. As shown in FIG. 1D that is a sectional view taken along the line C-C' of FIG. 1B. the collar 11A is provided on its upper portion with six grooves 12 from which wires are withdrawn.

A holder member 13 of synthetic resin or other material, that has highenough insulating properties, is fitted over the left end of the bobbin 6. As can be seen from FIG. 1B and FIG. 1E that is a sectional view taken along the line A-A' or B-B' thereof, the holder 13 is built up of a columnar portion 13A including in its upper site six grooves 12 from which wires are withdrawn. This portion 13A is additionally provided with a cavity 13B having a diameter substantially equal to that of the core 2. A portion of the columnar portion 13A, except the grooves 12, is made flat to form a terminal block 13C, on which pins 14 are provided to form terminals for connection to the wires of the windings of the high-voltage transformer. This terminal block 13C is provided through it with a female screw 15 that is in threaded engagement with an adjusting screw of the core 2.

The primary windings or coils PW3 and PW4 Of the highvoltage transformer T2 are wound on the bobbin 6 between the holder 13 and the first collar 11A. and the given number of turns of wire is provided on the bobbin 6 between each of the remaining collars 11 and the holder 8. thereby forming the secondary winding or coil SW2. Typically, this high-voltage transforme has a total length Ll of about 37 mm, and a maximum diameter D of 5 mm.

Then, a printed board 18A of about 6 mm. in width, about 50 mm in length and about 1mm in thickness is patterned on the holder 13 of the highvoltage transformer T2, and connected to an IC 18B forming a lighting circuit 18 through the pins 14 provided on the terminal block 13C. Thus an L2 of about 40 mm in length is achieved and the high-voltage transformer and the lighting circuit thereof is of 5 mm. or less in width and 100 mm, or less in length. Note that 18D stands for a mount.

An example of the compact liquid crystal TV receiver using such a highvoltage transformer T2 and an integrated lighting circuit is shown in FIG. 5. FIG. 5 represents the TV receiver in an assembled state. Reference numeral 22 stands for a rear frame of the TV receiver, which is made integral with a cover 16 to form a portable TV receiver frame. Reference numeral 17 represents batteries which are used as the power source for driving the TV receiver and drive a backlighting fluorescent lamp FL 1 as well as an integrated lighting circuit 18 and the high-voltage transformer T. The TV driving circuit is integrated on a printed board (not shown) and received in the rear frame 22.

Reference numeral 19 stands for an LCD panel which is fitted in a window 20 in the cover 16. A light-guiding plate 21 is disposed on the rear surface of the panel 19. The light-guiding plate 21 is provided on its upper end with a backlighting fluorescent lamp FL, and at its lower end with the high-voltage transformer T 2 and integrated lighting circuit 18, both shown in FIG. 1, as an integral piece. Thus the overall assembly is about 5 mm x about 77 mm in length.

According to this example, the high-voltage transformer and the lighting circuit can be reduced from the conventional size of 20 mm (width) x 100 mm (length) x 20 (height) to a size of 5 mm x 77 mm (length) and so can be made integral with the light-guiding plate 21.

Referring then to FIG.2, there is shown another structure for mounting the high-voltage transformer T 2 used on the lighting circuit according to this example. As illustrated, the high-voltage transformer T2 and backlighting fluorescent lamp F11 is bridged over both legs 24 and 24 of a substantially C-shaped holder member 22, and an IC 18B forming 14 - 1 the lighting circuit is located on the bottom of the holder member 22. Both the legs 24A and 24B are provided with grooves 23A and 23B, respectively, in which the light-guiding plate 21 is inserted. As can be seen from FIG. 5r the fluorescent lamp FL1. high-voltage transformer and lighting circuit are located below the light-guiding plate 21.

While the invention has been described with reference to some preferred embodiments, it is understood that many modifications may be made to material, shape, structure, size, arrangement, application, and addition without departing from the purpose of the invention mentioned above.

- 15

Claims

1. A high-voltage transformer including primary and secondary coils wound on a rod form of core or bobbin, and a holder member that is disposed at one or both ends of said rod form of core or bobbin to hold said rod form of core or bobbin, said holder member being provided with the input and output terminals of said primary or secondary coil.

2. A high-voltage transformer including primary and secondary coils wound on a rod form of core or bobbin, a Cshaped holder member for holding both ends of said rod form of core or bobbin, and a discharge lamp bridging legs of said Cshaped holder member.

3. A high-voltage transformer as claimed in Claim 1 or 2 wherein said high voltage transformer is located on the side edge of a backlighting light-guide plate of a liquid crystal panel.

4. A lighting circuit designed to convert a DC voltage from a DC source to a high-frequency signal to light up a discharge lamp including a driving means that is driven by said high-frequency signal, a high-voltage transformer built up of primary and secondary coils wound on a rod form of - 16 core or bobbin connected to the output end of said driving means, and a discharge lamp connected directly to the secondary coil of said highvoltage transformer.

5. A lighting circuit as claimed in Claim 4, in which a holder member for holding a high-voltagetransformer including said lighting circuit is located on the side edge of a backlighting light-guide plate of a liquid crystal panel.

6. A high-voltage transformer substantially as hereinbefore decribed and as illustrated in theaccompanying drawings.

7. A lighting circuit substantially as hereinbefore described and as illustrated in the accompanying drawings.