JPH0575884A - X-ray protection circuit - Google Patents

Abstract

PURPOSE:To realize the detection of excess current and the detection of excess voltage with a few circuit scale and to suppress X-ray radiation amount to specified amount and less. CONSTITUTION:When excess current exceeds VBE of a first detection transistor Qc, this is turned on, a third control transistor Qd is turned on by the voltage at that time and the supply of a horizontal drive signal is stopped. As the result, the oscillation of a flyback transformer 12 is stopped and X-ray is stopped to be radiated to specified amount and more by excess current. From excess current, a CRT and the flyback transformer 12 are protected. As a second detection transistor Qe is turned on at the time of excess voltage, the first detection transistor Qc is also turned on and a first control transistor Qa is compulsorily turned off. Thus, X-ray is not radiated to specified amount and more by excess voltage since the oscillation of the flyback transformer 12 is stopped. A second control transistor Qb can be protected also from excess voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for use in a high voltage generating circuit of a television receiver and the like.
Wire) protection circuit.

[0002]

2. Description of the Related Art X-rays are radiated to the outside from a picture tube (CRT) provided in a television receiver by an electron beam striking a fluorescent screen of the picture tube. Since X-rays have a great influence on the human body, it is regulated that the radiation dose of X-rays does not exceed a specified amount.

When the voltage (high voltage HV) or current applied to the CRT exceeds a specified value, the amount of the X-rays generated exceeds the specified amount. Therefore, conventionally, an overcurrent detection circuit or an overvoltage detection circuit as an X-ray protection circuit is provided. The overcurrent detection circuit and the overvoltage detection circuit function as a protection circuit for preventing the X-rays from being radiated more than a specified amount, and also protect the switching transistor connected to the CRT or the flyback transformer from the overcurrent or overvoltage. It also has the function of preventing the CRT and flyback transformer from burning.

FIG. 3 shows a conventional example of an overcurrent detection type X-ray protection circuit, in which 12 is a flyback transformer, and a horizontal drive signal is supplied from a terminal 14 to a primary coil 12a side thereof via a horizontal drive transformer 16. To be done. A horizontal drive signal of a horizontal cycle is supplied to the terminal 14, which turns on and off the first control transistor Qa to drive the horizontal drive transformer 16. The output of the horizontal drive transformer 16 is further supplied to the second control transistor (switching transistor) Qb, and the primary coil 12a of the flyback transformer 12 is switching-controlled by the drive current according to the horizontal period.

As a result, a predetermined flyback pulse is output to the side of the secondary coil 12b of the flyback transformer 12, whereby a predetermined high voltage HV is obtained at the output stage of the rectifier circuit 18. As is well known, the high voltage HV is CR
It is applied to the anode of T (not shown). Rectifier circuit 18
As shown in the figure, is composed of a resistor and a capacitor. The collector voltage of the second control transistor Qb is used as a drive voltage for a horizontal deflection coil (not shown) as is well known. Reference numeral 24 indicates a connection terminal of the horizontal deflection coil.

Primary coil 1 of flyback transformer 12
A resistor Rs is connected in series with the 2a side to detect an overcurrent state. That is, when the overcurrent flows and the voltage across the detection resistor Rs becomes equal to or higher than VBE of the overcurrent detection transistor Qc, the overcurrent detection transistor Qc is turned on by this, and the current loop formed by this on is provided. Voltage dividing resistor R2 for voltage dividing circuit
When the voltage Vb across R3, especially the resistor R3, becomes equal to or higher than VBE of the third control transistor Qd, the third control transistor Qd is turned on and the first control transistor Qa is forcibly turned off. As a result, the horizontal drive signal is not supplied to the flyback transformer 12, so that the oscillation operation thereof is stopped and the high voltage HV which is the output of the rectifier circuit 18 decreases. By such control operation, CR
Excessive amount of X-ray emission due to excessive current flowing in T can be prevented, and burnout of CRT due to overcurrent can be prevented.

FIG. 4 shows a conventional example of an overvoltage detection type X-ray protection circuit. In this example, the flyback transformer 12
A coil (overvoltage detection coil) 12c for overvoltage detection is provided in the. The detection pulse generated in the overvoltage detection coil 12c is rectified and smoothed by the rectification circuit 21, the voltage is divided by the pair of resistors 22 and 23, and the divided voltage Va is supplied to the overvoltage detection transistor Qe. Compared to Vref.

When the divided voltage Va exceeds the reference voltage Vref, the overvoltage detection transistor Qe is turned on and the third control transistor Qd is also turned on, which is different from the case of the overcurrent type shown in FIG. Similarly, the supply of the horizontal drive signal is cut off. As a result, the rise in high voltage is suppressed and the radiation dose of X-rays does not exceed the specified amount. Since the high voltage does not rise abnormally, the CRT and the flyback transformer 12 can also be protected.

[0009]

As described above, in the related art, since the over-current or over-voltage is detected to regulate the X-ray dose, when either one of the means is installed in the television receiver. , The protection measures from X-ray is insufficient. That is, since both the overvoltage and the overcurrent are not observed at the same time, one can be satisfied, but the other cannot be satisfied. In order to satisfy both of them, both the circuit of FIG. 3 and the circuit of FIG. 4 must be mounted, so that the circuit scale increases and the purpose of cost reduction cannot be utilized.

Therefore, the present invention has solved such a conventional problem, and it is possible to suppress the X-ray to an appropriate value or less for both overcurrent and overvoltage without increasing the circuit scale. It is the one.

[0011]

In order to solve the above-mentioned problems, in the present invention, an overcurrent detection resistor is connected to the primary side of a flyback transformer, and an overcurrent is present across the detection resistor. A first detection transistor for detection is connected, a second detection transistor for overvoltage detection is connected in series with the overcurrent detection resistor, and the second detection transistor is connected to the flyback transformer. A voltage related to the screen voltage obtained by the screen voltage generating circuit provided on the primary coil side is applied, and when an overcurrent or an overvoltage is detected, the first or second detection transistor is controlled to cause the flyback. The horizontal drive signal is not supplied to the primary side of the transformer, and the X-ray generated by overcurrent or overvoltage is specified. Is characterized in that so as not radiated over.

[0012]

Operation: Secondary coil 12b of flyback transformer 12
When an excessive current flows to the side, an excessive current also flows to the primary coil 12a side accordingly. This current is detected by the resistor Rs for overcurrent detection. Since the first detection transistor Qc is turned on by the excessive current,
As a result, the third control transistor Qd is immediately turned on, and the supply of the horizontal drive signal to the flyback transformer 12 is stopped. With this, the amount of X-ray radiation due to overcurrent does not exceed the specified amount. CRT due to overcurrent
And the flyback transformer is also protected.

When an excessive voltage is applied to the terminal 13 side, the terminal 3
When the screen voltage obtained in 1 becomes abnormally high, the second detection transistor Qe is turned on. When the second detection transistor Qe is turned on, the first detection transistor Qc is turned on. Therefore, the third control transistor Qd is turned on in this case as well, so that the oscillation operation of the flyback transformer 12 is stopped as described above. As a result, the amount of X-ray radiation does not exceed the specified amount due to overvoltage. Of course, this control operation also protects the second control transistor Qb.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an example of an X-ray protection circuit according to the present invention will be described in detail with reference to FIGS. The features of both the overcurrent detection type and the overvoltage detection type are utilized in the configuration of the present invention.

FIG. 1 shows an example of a main part of the present invention. The same components as those in FIGS. 3 and 4 are designated by the same reference numerals and the description thereof will be omitted. In the present invention, as shown in FIG. 1, a resistor Rs for detecting an overcurrent is connected in series with a primary coil 12a of a flyback transformer 12, and a detection transistor is provided at both ends thereof, a PNP type first detection in this example. The emitter and base of the transistor Qc are connected. A pair of voltage dividing resistors R are provided on the collector side of the first detection transistor Qc.
2, R3 are connected, and the divided voltage Vb obtained at the midpoint of the connection is supplied to the third control transistor Qd.

The divided voltage Vb may not be directly supplied to the third control transistor Qd, but may be supplied via a voltage comparator (not shown).

A terminal 24 to which a horizontal deflection coil is connected is led out to the collector side of the second control transistor Qb. A rectifying / smoothing circuit 30 is further connected to this collector, and a predetermined voltage obtained at its output terminal 31 is used as a screen voltage for a screen electrode of a CRT (not shown).
Is supplied to. Reference numeral 21 is a capacitor for resonance with respect to the horizontal deflection coil.

In the present invention, the screen voltage is divided by the voltage dividing resistors 26 and 27, and the divided voltage Va is applied to the overvoltage detecting transistor, that is, the second detecting transistor Qe. The second detection transistor Qe is connected to the base of the first detection transistor Qc via a base resistor R4.

The circuit configuration until the flyback transformer 12 is controlled to the safe side by detecting overcurrent or overvoltage is the same as the conventional configuration.

Now, in this configuration, the CRT is supplied with an appropriate current during normal operation or at the input terminal 13.
A proper drive voltage is applied to the flyback transformer 1
When 2 is oscillating (normal operation), the X-ray protection circuit 10 does not operate because only the X-ray dose within the specified amount is emitted from the CRT. Of course, the X-ray protection circuit 10 has an overcurrent and overvoltage protection function, but the X-ray protection circuit 10 does not operate because it is neither overcurrent nor overvoltage.

On the other hand, at the time of abnormality such as overload, an overcurrent flows through the CRT, and correspondingly an overcurrent also flows through the primary coil 12a of the flyback transformer 12. When this exceeds the reference value, the voltage across the detection resistor Rs exceeds VBE of the first detection transistor Qc, so that the first detection transistor Qc is turned on and the voltage dividing resistor R2. A predetermined current flows through R3. As a result, when the voltage (divided voltage) Va across the resistor R3 exceeds VBE of the third control transistor Qd, the third control transistor Qd is turned on.

As a result, the first control transistor Q
Since a is forcibly turned off, the horizontal drive signal supplied to the terminal 14 is not transmitted to the flyback transformer 12 side. That is, the second control transistor Qb
Is turned off and the oscillating operation of the flyback transformer 12 is stopped, whereby the amount of X-ray radiation due to overcurrent is suppressed to a prescribed amount or less. At the same time, burnout of the CRT and flyback transformer is prevented.

On the other hand, when an abnormally high drive voltage is applied to the input terminal 13 as compared with the normal state, the collector voltage of the second control transistor Qb changes from the state shown in FIG. 2A to a pulse shown in FIG. This collector voltage is the rectifying and smoothing circuit 3
Since it is smoothed by 0, the screen voltage at the terminal 31 becomes higher than that at the normal time, and this is divided and the divided voltage Vb is applied to the base of the second detection transistor Qe, so that the screen voltage exceeds a predetermined value. Then, the second detection transistor Qe is turned on. When the voltage Vb across the resistor R3 for voltage division exceeds a predetermined value accordingly, the third control transistor Qd is controlled and the supply of the horizontal drive signal is cut off as described above.

As a result, the flyback transformer 12
As a result of the oscillating operation being stopped, the X-ray radiation amount does not increase abnormally due to the overvoltage. Since an excessive voltage is not applied to the second control transistor Qb for a long time, there is no possibility that the second control transistor Qb is destroyed by the overvoltage.

[0025]

As described above, according to the present invention, the existing configuration is skillfully utilized to detect both the overcurrent and the overvoltage with a small number of elements so that the X-ray radiation amount is suppressed to the specified amount or less. It is the one.

According to this, the overvoltage detection system can be constructed by using the existing rectifying / smoothing circuit, and a part of the overcurrent detection system can also be used. Therefore, when the configuration of FIGS. 3 and 4 is mounted as it is. The number of circuit elements can be significantly reduced.
It is not necessary to wind an overvoltage detection coil around the flyback transformer.

As a result, the present invention has a feature that the cost can be drastically reduced. Therefore, the present invention is extremely suitable for being applied to an X-ray protection circuit such as a high voltage generation circuit of a television receiver as described above.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of an X-ray protection circuit according to the present invention.

FIG. 2 is a waveform diagram for explaining the operation.

FIG. 3 is a system diagram showing an example of a conventional overcurrent detection type X-ray protection circuit.

FIG. 4 is a system diagram showing an example of a conventional overvoltage detection type X-ray protection circuit.

[Explanation of symbols]

 12 Flyback transformer 16 Horizontal drive transformer 30 Rectifying and smoothing circuit Qa First control transistor Qb Second control transistor Qc First detection transistor Qd Third control transistor Qe Second detection transistor Rs Overcurrent detection resistor R2 , R3 voltage divider resistor

Claims (1)

[Claims] 1. A resistor for detecting an overcurrent is connected to the primary side of a flyback transformer, and a first detection transistor for detecting an overcurrent is connected to both ends of the detecting resistor. A second detection transistor for detecting overvoltage is connected in series with the resistor, and the second detection transistor is connected to a screen voltage obtained by a screen voltage generation circuit provided on the primary coil side of the flyback transformer. When a related voltage is applied and an overcurrent or overvoltage is detected, the first or second detection transistor is controlled to prevent the horizontal drive signal from being supplied to the primary side of the flyback transformer, thereby preventing overcurrent. Alternatively, the X-ray protection circuit is characterized in that the X-rays generated by overvoltage are not emitted more than a specified amount. .