JPH0516764Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a high voltage generation circuit using a flyback transformer.

<Prior Art> In recent years, cathode ray tube devices that serve both as television receivers and display devices for computers and the like have been used. Such devices usually use a flyback transformer as a high-voltage power source, but in order to increase the safety of the cathode ray tube device, the power circuit section and the video circuit section are separated in terms of alternating current.

For example, as shown in FIG.
is isolated from the drive circuit 5 by a drive transformer 4, and the primary coil side is isolated from the secondary coil side by a flyback transformer 6. In addition, 7 is an antenna, 8 is a video detection/intermediate frequency amplification circuit, 9 is a video amplification circuit, 1
0 is a synchronous separation circuit, 11 is a vertical deflection circuit, and 12 is a cathode ray tube. However, the flyback transformer has the role of supplying high voltage to the cathode ray tube 12, and fluctuations in the voltage appear as fluctuations in the screen, and if the voltage becomes abnormally high, there is concern that X-rays may be generated. If an accident such as a short circuit occurs in the high voltage coil, there is a risk of fire due to an increase in abnormal current. For this reason, a protection circuit as shown, for example, within the dotted line 13 in FIG. 5 is usually provided. This circuit consists of the high voltage coil 6 of the flyback transformer 6.
An abnormality in the current _IH flowing through b is detected and the low voltage coil 6
Comparison circuit 1 is a circuit that interrupts the current I _B flowing through a.
4 compares the current flowing through the low-voltage coil 6a and the high-voltage coil 6b, and when, for example, I _H no longer satisfies the relationship expressed as I _B = AI _H + B (where A and B are constants), the thyristor 15 This turns on the fuse 16, thereby preventing accidents such as fire.

<Problems to be Solved by the Invention> However, when such a protection circuit is provided, it is impossible to interrupt the low-voltage coil side circuit and the high-voltage coil side circuit in the flyback transformer in terms of alternating current.

<Means for solving the problems> The present invention is intended to solve the above-mentioned drawbacks.
Connect the primary coil of a current detection transformer whose primary coil and secondary coil are electrically isolated from each other to the high-voltage coil side of the flyback transformer, and connect at least a diode and a capacitor to both ends of the secondary coil. A rectifier circuit that rectifies positive voltage and a rectifier circuit that rectifies negative voltage that are connected in series are connected in parallel.
It is equipped with an AC cutoff circuit with a terminal between each diode and capacitor, and by using this circuit, protection is provided while cutting off AC between the low voltage coil side and the high voltage coil side of the flyback transformer. This enabled the installation of circuits.

<Function> When this AC cutoff circuit is used, when a flyback pulse current i _H flows through the primary coil, a pulse voltage proportional to the turns ratio is generated in the secondary coil.
It is rectified by the rectifier circuit, and a voltage difference between the two rectifier circuits appears at the output terminal.

The above-mentioned protection circuit operates using this DC voltage difference.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In FIG. 1, one end of a low voltage coil 6a of the flyback transformer 6 is connected to the collector of a horizontal output transistor 17, and the other end of the coil 6a is grounded via a DC power supply 18. Further, one end of the high voltage coil 6b of the flyback transformer is connected to the anode of the cathode ray tube 12 through a rectifier diode 19 to supply a high voltage obtained by rectifying the flyback pulse. Note that 20 is the capacitance of the cathode ray tube. The other end of the high voltage coil 6b is grounded through the primary coil 21a of the current detection transformer 21. A current detection resistor 22 is connected to both ends of the primary coil 21a as necessary. The secondary coil 21b of the current detection transformer 21 is electrically insulated from the primary coil 21a, and first and second rectifier circuits 23 and 24 are connected to both ends thereof. This rectifier circuit includes capacitors 23a and 24a.
The cathodes of diodes 23c and 24c are connected in series with a parallel circuit of resistors 23b and 24b. Terminals 25 and 26 are provided at the cathode ends of the diodes of the first and second rectifier circuits 23 and 24 to take out the rectified output. Note that a capacitor of a predetermined capacity may be connected between the diodes 23c and 24c as necessary. Further, a series circuit consisting of a damper diode 27, a resonant capacitor 28, a horizontal deflection coil 29, and an S-shaped distortion correction capacitor 30 is connected in parallel between the emitter and collector of the horizontal output transistor 17. Furthermore, although the above embodiment has been described in which the horizontal deflection circuit and the high voltage generation circuit are configured as one circuit, the horizontal deflection circuit and the high voltage generation circuit may be configured separately.

<Operation> When the horizontal drive pulse Vb is input to the horizontal output transistor 17, a sawtooth current _IB flows through the low voltage coil 6a of the flyback transformer 6. When this current flows, a flyback pulse _VH as shown in FIG. 2a is generated in the high voltage coil 6b of the flyback transformer, which is rectified by the diode 19 and then supplied to the anode of the cathode ray tube 12. In this case, looking at the current _iH flowing through the high-voltage coil 6b, the cathode ray tube 1
When the high voltage current I _H does not flow in 2, a sawtooth resonant current i _H as shown in FIG. 2b flows. That is,
The high voltage coil 6b has a distributed capacitance for high frequency current due to its structure, and a resonant current _iH flows through this distributed capacitance. Also, since this current i _H flows through the primary coil 21a and the resistor 22 of the current detection transformer 21, the voltage across the primary coil 21a is
V _c becomes V _c =R _s ×i _H. However, R _s is resistance 22
When the resistance value is , the angular frequency of the current is ω, and the inductance of the primary coil 21a is L ₁ , R _s <<
Let ωL be ₁ .

Note that when the brightness of the cathode ray tube is increased, the capacitor 2
Since the voltage across the 0 voltage drops, a resonant current _iH having a charging current component 27 flows through the high voltage coil 6b, as shown in FIG. 2c.

When a resonant current _iH flows through the current detection resistor 24 connected to both ends of the primary coil of the current detection transformer 21, a voltage corresponding to the resonant current is induced in the secondary coil 21b, which is electrically insulated from the primary coil side. do. The positive component of this voltage is rectified and smoothed in the rectifier circuit 24, and the negative component is rectified and smoothed in the rectifier circuit 23.
V _s appears. Specifically speaking, high voltage current is I _H
When =0, the positive component and negative component are symmetrical, as shown in FIG. 2b, so the differential voltage V _s becomes zero. Next, when the high voltage I _H flows, the resonant _current includes a charging current component 27 shown by diagonal lines in FIG. This voltage is a direct current voltage that changes in proportion to changes in the high voltage current. Note that capacitor 23
a and 24a are discharged with time constants R ₁ C ₁ and R ₂ C ₂ . However, C ₁ and C ₂ are the capacitances of the capacitors 23a and 24a, and R ₁ and R ₂ are the resistance values of the resistors 23b and 24b.

FIG. 3 shows a circuit in which the protection circuit 13 shown in FIG. 5 is connected using the AC cutoff circuit according to the present invention. The part surrounded by the dotted line 13 is a protection circuit part, the terminal 26 is grounded through a DC power supply 29, and the terminal 25 is connected to the base of a transistor 14a which is one input terminal of the comparator circuit 14. Transistor 1 which is the other input terminal of comparison circuit 14
The voltage across the _IB current detection circuit consisting of a resistor 30 and a capacitor 31 is input to the base of 4b.

When the flyback transformer is operating normally, the transistor 14b is turned on and almost no current flows through the series circuit of the resistors 32 and 33, so the gate voltage of the thyristor 15 becomes almost zero. In this state, between the current I _B flowing through the low voltage coil of the flyback transformer and the high voltage current I _H ,
An equilibrium is maintained that satisfies the formula I _B =AI _H +B.
At this time, for example, if a rare short occurs in the high voltage coil 6b, the horizontal deflection current I _B increases and the base potential of the transistor 14b decreases. As a result, the gate voltage of the thyristor 15 increases and the thyristor 15 becomes conductive, thereby causing the fuse 16 to melt. In other words, the operation of the horizontal deflection circuit is stopped and the occurrence of a fire or the like is prevented.

<Example 2> The current detection resistor 22 in the AC cutoff circuit can be eliminated. In this case, both ends of the primary coil 21a of the current detection transformer 21 have the formula V _c =L ₁
A voltage expressed as ×di _H /dt is generated. However, t is time.

As mentioned above, in this invention, an AC cutoff circuit is provided on the high voltage coil side of the flyback transformer. For example, a protection circuit or the like can be provided. Therefore, even if an accident such as a rare shot occurs in the high voltage coil, the high voltage generating circuit will not catch fire and safety will be maintained.

[Brief explanation of the drawing]

Figure 1 is a wiring diagram of the high voltage generation circuit according to the present invention.
Fig. 2 is a waveform diagram of the circuit shown in Fig. 1, Fig. 3 is a wiring diagram of a high voltage generation circuit showing an example of using the AC cutoff circuit according to the present invention, Fig. 4 is a circuit block diagram of a television receiver, and Fig. 4 is a circuit diagram of a television receiver. Figure 5 shows a wiring diagram of a conventional high voltage generation circuit. In the figure, 6 is a flyback transformer, 6a is a low voltage coil, 6b is a high voltage coil, 21 is a current detection transformer, 21a is a primary coil, 21b is a secondary coil, 22 is a current detection resistor, 23 and 24 are rectifier circuits, 23a, 24a are capacitors, 23b, 24
b is a resistor, and 23c and 24c are diodes.

Claims (1)

[Scope of utility model registration request] a flyback transformer having a low-voltage coil and a high-voltage coil; a current detection transformer in which a primary coil is connected to the low-voltage side of the high-voltage coil, and the primary coil and the secondary coil are insulated in terms of alternating current;
A first rectifier circuit that rectifies a positive voltage and a second rectifier circuit that rectifies a negative voltage are connected in parallel to both ends of the coil, and each rectifier circuit is connected to a series circuit of at least a capacitor and a diode. 1. A high voltage generation circuit characterized in that a terminal is provided between each capacitor and a diode.