JPH04313790A - High voltage protecting circuit - Google Patents

Abstract

PURPOSE:To surely execute the protection against a leak of a high voltage in a display in which a steady state maximum current of a power source of a high voltage circuit is varied, such as a multi-scan display which can receive many kinds of signals. CONSTITUTION:A detection value E is derived by detecting the amplitude of a pulse P2 obtained in a tertiary winding of a flyback transformer 8 by a diode 16 and resistances 41, 42. A reference value ERL being a little lower than a value in a steady state of the detection value E is generated, and compared with the detected value E by a comparator 49. When a state that the detected value E is lower than the reference value ERL is generated, a transistor 55 is turned off by a low level output of the comparator 49 and a cathode voltage of a Zener diode 18 is boosted to a threshold or above. As a result, the potential of a high voltage protection input terminal 1x rises and a horizontal oscillating circuit 1 stops its oscillation.

Description

[Detailed description of the invention]

[Purpose of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high voltage protection circuit for a display using a CRT.

0002

2. Description of the Related Art Generally, in a display using a CRT, in order to display an image, it is necessary to apply a high voltage of about 20 to 30 KV to the anode of the CRT depending on its size. For this purpose, a high voltage generation circuit is required.

FIG. 2 shows an example of a high voltage generation circuit. In FIG. 2, 1 is a horizontal oscillation circuit. The output side of this horizontal oscillation circuit 1 is connected to the base of a drive transistor 3 via a resistor 2. The collector of the drive transistor 3 is connected to a control power source (not shown) through the primary winding of the drive transformer 4 and the resistor 5, and the emitter is grounded.One end of the secondary winding of the drive transformer 4 is output through the resistor 6. It is connected to the base of transistor 7, and the other end is grounded. The collector of the output transistor 7 is connected to the primary winding of the flyback transformer 8 and the high voltage stabilizing circuit 9.
It is connected to the power supply +B via the transistor 10, and its emitter is grounded. The high voltage stabilizing circuit 9 is connected between a transistor 10, an amplifier 11 whose inverting input is a voltage obtained by dividing the high voltage EH of the flyback transformer 8, and whose non-inverting input is a reference voltage, and the emitter of the transistor 10 and ground. It is composed of a capacitor 12 that is A choke coil 13 is connected in parallel to the primary winding of the flyback transformer 8. A diode 14 with the polarity shown is connected between the collector of the output transistor 7 and the ground.
and a capacitor 15 are connected in parallel. One end of the tertiary winding of the flyback transformer 8 is connected to a rectifying diode 16.
is connected to the cathode of the Zener diode 18 via the anode, cathode and resistor 17, and the other end is grounded. A capacitor 19 is connected between the cathode of the diode 16 and ground, and a resistor 20 is connected between the cathode of the Zener diode 18 and ground. The anode of the Zener diode 18 is connected to the oscillation stop input terminal 1x of the horizontal oscillation circuit 1, and is also grounded via a capacitor 21 and a resistor 22 in parallel.

In the circuit configured as described above, the horizontal oscillation circuit 1 outputs a rectangular wave in a horizontal period and drives the output transistor 7 via the drive transformer 4. This generates a flyback pulse P1, which is boosted and rectified by the flyback transformer 8 to obtain a high voltage EH, a focus voltage EH, and a screen voltage Es. The high voltage stabilizing circuit 9 then controls the input voltage (voltage on the emitter side of the transistor 10) to keep the high voltage constant.

[0005] When the high pressure increases, a large amount of X-rays are emitted from the CRT, which has an adverse effect on the human body, so protection is required. Generally, the horizontal oscillation circuit 1 has a terminal 1x that inputs a voltage obtained by rectifying the pulse P2 generated in the tertiary winding of the flyback transformer 8, and stops oscillation when the voltage at the terminal 1x rises, that is, when the high voltage rises. However, it has a function to stop the circuit operation and eliminate high voltage. This provides protection against high pressure rises and maintains safety.

However, recently another problem has arisen. Originally, the gaps in the flyback transformer 8 were filled with resin in order to insulate high voltage, so there should be no leakage or the like. However, depending on the usage environment, minute cracks may occur in the resin after long-term use. However, there is no problem as long as there is a sufficient distance between the flyback transformer 8 and the surrounding conductors, but recent displays have become more sophisticated and have more circuits, etc., so it is not always possible to provide a sufficient distance. Therefore, current leaks through the crack. Since the power supplied to the high voltage generation circuit is large, it can be supplied even if the power consumption in the leak path is large. For this reason, the amount of heat generated may also increase, and there is a risk that surrounding resin or attached dust may be scorched and smoke may be emitted, or in the worst case, it may catch fire.

Therefore, it is necessary to create and add a protection circuit against such leakage. Figure 3 shows an example of the protection circuit.
Shown below. In FIG. 3, 31 has one end connected to the power supply +B in FIG.
The other ends thereof are resistors connected to the collectors of the transistors 10 in FIG. 2, respectively. A transistor 32 is connected to one end of the resistor 31.
The emitter of the transistor 32 is connected to the horizontal oscillation circuit 1 of FIG.
is connected to the oscillation stop input terminal 1x. A resistor 35 is connected between the base of the transistor 32 and the other end of the resistor 31.
A capacitor 36 is connected between the emitter and base of the transistor 32. Resistor 33 and Resistor 3
A capacitor 37 is connected between the common connection point of 4 and the ground.

With the circuit configured as shown in FIG. 3, the current flowing in the high voltage circuit passes through the resistor 31 and causes a voltage drop. When a leak occurs, the power consumption on the secondary side of the flyback transformer 8 increases, and naturally the power supply current on the primary side also increases. Therefore, the voltage drop across the resistor 31 also increases. The voltage generated at this time is applied between the emitter and base of the transistor 32, and when the voltage drop becomes large, the transistor 32
is turned on, current flows through the terminal 1x through the resistors 33 and 34, and the voltage increases. As a result, the horizontal oscillation circuit 1 stops oscillating and the high voltage is stopped, thereby providing protection against leakage.

[0009]

[Problem to be Solved by the Invention] In a normal display, the steady maximum current of the primary side power supply current of the high voltage generating circuit is almost constant, so the circuit shown in FIG. 3 is effective, but in the case where the steady maximum current changes. cannot be used for Recently, displays that can receive two or more signals on a single display have been manufactured. In such displays, if the horizontal frequencies of the signals are different, the flyback pulse P
In order to keep the amplitude of EH constant and the voltage EH constant, it is necessary to change the power supply voltage approximately in proportion to the horizontal frequency. If the high voltage load is constant, the power consumption of the circuit is almost constant, so the power supply current is inversely proportional to the voltage.

In other words, the larger the ratio between the highest and lowest horizontal frequencies of the signals that can be received, the different the value of the steady maximum current of the power supply for each signal will be. Therefore, if you set a current value that applies leakage protection to a signal with a high horizontal frequency and a large steady-state maximum current, even if a leak occurs during a signal with a high horizontal frequency, the abnormal current will not reach the set current value. There is a risk that the protective operation will not be performed.

As described above, conventional protection circuits are effective for displays where the steady maximum current of the power supply is constant, but they are difficult to apply to displays where the steady maximum current changes if the amount of change is large. be.

The present invention has been made in view of the above points, and its purpose is to prevent high voltage leaks in displays where the steady maximum current of the power supply of the high voltage circuit changes, such as a multi-scan display that can receive a variety of signals. An object of the present invention is to provide a high voltage protection circuit that can provide reliable protection. [Structure of the invention]

[0013]

[Means for Solving the Problems] In the present invention, the occurrence of leakage is detected not by the power supply current, but by detecting the pulse height of the flyback pulse, and setting a voltage slightly lower than the steady value of the detected value as a reference value. The detected value is compared with a reference value, and when the detected value continues to be lower than the reference value, a signal is output, and that signal stops the operation of the high voltage generation circuit.

[0014]

[Operation] If the high voltage is constant, the amplitude of the steady flyback pulse will also be constant regardless of the signal being imaged, so the detected value will also be approximately constant regardless of the signal being imaged. When an abnormally high voltage load occurs, such as when a leak occurs, a large amount of energy is taken away from the flyback pulse, so the amplitude of the flyback pulse becomes small. Therefore, regardless of the type of signal being received, the detected value will be lower than normal and will be below the reference value, so the protection operation will work regardless of the type of signal being received.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, the same parts as in FIG. 2 are denoted by the same reference numerals, and the explanation thereof will be omitted. In FIG. 1, the amplitude of the flyback pulse P1 is detected by rectifying the pulse P2 generated in the tertiary winding with a diode 16. Then, an appropriate detected value E is obtained by resistors 41 and 42. Reference value ERL
, ERH stabilizes the power supply +B2 with a resistor 43 and a Zener diode 44, and the voltage V is stabilized with resistors 45, 46, 47.
Obtain the partial pressure with . Here, ERH is set to a value slightly higher than the steady value of the detected value E, and ERL is set to a lower value (for example, about ±20%). These reference values ERH and ERL are input to comparators 48 and 49 and compared. That is, the detected value E is input to the inverted input of the comparator 48 and the comparator 49
into the non-inverting input of The reference value ERH is set to comparator 4.
ERL is input to the non-inverting input of comparator 49, and ERL is input to the inverting input of comparator 49. The comparators 48 and 49 have open collector type outputs, and their outputs are connected in common and connected to the power supply V via a pull-up resistor 50. This causes the output to go low when either comparator output goes low. Therefore, when the detected value E becomes lower than the reference value ERL or higher than the reference value ERH, it becomes low. Therefore, when the outputs of the comparators 48 and 49 are low, it means that the detected value is not in a steady state. Capacitor 51 is resistor 52
Together, they form a low-pass filter that prevents the comparator output from going low due to temporary phenomena such as high-voltage discharge, and also prevents protection from being applied even if the comparator output goes low during the transition period when the power is turned on. The low-pass filter voltage is divided by resistors 53 and 54 and applied to the base of a transistor 55 whose emitter is grounded.

In the steady state, the outputs of the comparators 48 and 49 are at a high level, so a voltage is applied to the base of the transistor 55. Therefore, the transistor 55 is turned on and the collector voltage Vc becomes approximately zero V. On the other hand, if the comparator output continues to be low, that is, if the detected value E becomes higher than the reference value ERH or lower than the reference value ERL, the low-pass filter output decreases and the transistor 55 is activated for a predetermined period of time. Turns off after the elapsed time. Then, the capacitor 56 starts to be charged and the collector voltage Vc of the transistor 55 starts to rise. Correspondingly, the cathode voltage of the Zener diode 18 connected via the resistor 57 also rises, and when it rises to the threshold voltage, the Zener diode 18 becomes conductive and the potential of the high voltage protection input terminal 1X of the horizontal oscillation circuit 1 increases. Then, the horizontal oscillation circuit 1 stops oscillating. As a result, the output transistor 7 is turned off, the flyback pulse P1 disappears, and the high voltage EH
The abnormal condition will also be resolved.

In FIG. 1, when a leak occurs at high pressure, the flyback pulse P1 is
The conduction energy increases and the pulse height decreases. As a result, the amplitude of the pulse P2 becomes proportionally smaller, its rectified voltage also becomes lower, and the detected value E becomes lower. Therefore, the comparator 49 operates and the output becomes low, and the transistor 55
turns off. As a result, the terminal 1x of the horizontal oscillation circuit 1
The voltage increases, oscillation stops, and the high voltage drops, which stops leakage and prevents smoke and ignition. still,
In the above embodiment, the detected value E is 3 of the flyback transformer 8.
Although it is generated from the pulse P2 of the secondary winding, it is also possible to divide the rectified primary side flyback pulse P1, or use the rectified high voltage EH of the secondary side pulse. The effect of this can be obtained.

In addition, in the above embodiment, the high voltage is stopped by stopping the oscillation of the horizontal oscillation circuit 1, but the method is not limited to this, and the oscillation of the power supply is stopped by sending the voltage Vc to the power supply circuit and switching the power supply. Alternatively, the supply of input power to the high voltage circuit may be stopped, such as by turning off the power.

In the embodiment described above, a reference value ERH is provided so that even when the detected value E increases, that is, when X-rays increase due to a rise in the high pressure EH, the comparator 48 detects and protects the leak in the same way as when there is a leak. However, of course, if the conventional protection against X-rays works, there will be no problem in the protection operation even if the creation of this reference value ERH and the comparator 48 are omitted.

[0020]

As described above, according to the present invention, there is provided a detection circuit for detecting the amplitude of a flyback pulse, and a reference value ERL that is lower by a predetermined value than the detection value E detected by the detection circuit when the high voltage generation circuit is in a steady state. A comparison circuit that compares the detected value E with the detected value E, and an operation stop circuit that stops the operation of the high voltage generation circuit when the detected value E becomes lower than the reference value ERL are provided. Even if the display receives signals with different horizontal frequencies and the power supply current of the steady high voltage generation circuit is different, it is possible to reliably detect abnormalities due to leaks that occur in the high voltage, and to protect the high voltage. Can be hung.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional high voltage generation circuit.

FIG. 3 is a conventional detection circuit diagram for protection when a high voltage leak occurs.

[Explanation of symbols]

1...Horizontal oscillation circuit 1x...
Horizontal oscillation stop input terminal 3...Drive transistor 4...Drive transformer 7...Output transistor 8...Flyback transformer 9...High voltage stabilization circuit 16...Diode 18, 44...Zener diode 41, 42, 43, 45 ,46,47,52,53,5
4, 57...Resistor 48, 49...Comparator 51, 56, 6
0...Capacitor 55...Transistor.

Claims (1)

[Claims] 1. A high voltage protection circuit for a high voltage generation circuit of a display using a CRT, which includes a detection circuit that detects the amplitude of a flyback pulse, and a detection value E detected by the detection circuit when the high voltage generation circuit is in a steady state. a comparison circuit that compares the detected value E with a reference value ERL that is lower by a predetermined value;
A high voltage protection circuit comprising: an operation generating circuit that stops the operation of the high voltage generating circuit when the detected value E becomes lower than a reference value ERL.