JPH08275020A - Crt display device and overvoltage protection method therefor - Google Patents

Abstract

PURPOSE: To prevent destruction of a switching element due to in-tube discharge by providing an overvoltage protection means in a high voltage generating circuit of the CRT display device. CONSTITUTION: A resistor 12 is connected to one terminal of a secondary side 23b of a flyback transformer and a short-circuit current (discharge current of a capacitor Ca) due to in-tube discharge is detected by a voltage drop across the resistor 12. When a voltage Vd of a detection terminal 28 is decreased and transistors(TRs) Q11, Q12 of a suppression circuit 26 are turned on, the voltage level of its output terminal 29 reaches 0V resulting that an input voltage Vc of a reference voltage input terminal 21a of a high voltage control circuit 21 is rapidly dropped (to nearly 0.7V) and an output VE of a high voltage power supply 22 having been increased is suppression control and is going to be decreased. Thus, the increase in a flyback pulse voltage is suppressed to prevent destruction of a switching TR Q10. An anode voltage Va is gradually restored by a circuit time constant of a holding circuit 27.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CRT display device, and more particularly to an overvoltage protection system for a high voltage generating circuit.

[0002]

2. Description of the Related Art FIG. 2 is a raster scan type CRT.
1 shows a schematic configuration of a display device. The high voltage anode voltage generated in the high voltage circuit 2 is applied to the anode electrode 6 of the CRT 5. The electron beam emitted from the electron gun toward the phosphor screen of the anode potential in the CRT 5 has its trajectory bent by the deflection circuit 3, and periodically scans the tube surface from the upper left to the lower right. The video signal is amplified by the video amplifier 4, and the amount of electron beam is increased or decreased to produce light and dark on the screen.

The screen size of the CRT 5 is proportional to the deflection current of the deflection circuit 3 and semi-proportional to the square root of the anode voltage.
Therefore, the screen size changes when the anode voltage changes. In order to prevent the fluctuation of the screen, the high voltage circuit 2 is controlled so that the anode voltage is kept constant.

The high voltage circuit 2 comprises a high voltage generation circuit 20, a high voltage control circuit 21, and a high voltage circuit power supply 22. As the power supply 22 for the high voltage circuit, there are a series dropper and a switching regulator, but generally, a switching regulator with less power loss is often used.

The high voltage generating circuit 20 comprises a flyback transformer (FBT) 23 and a switching circuit 24. The voltage supplied from the high voltage circuit power supply 22 to the primary side of the FBT 23 is supplied to the switching circuit 24 from the low voltage side of the primary side coil of the FBT 23. Switching circuit 24
Is switched on and off by a horizontal synchronizing signal (horizontal deflection timing) by a switching transistor.
The flyback pulse is generated by controlling the charge and discharge of the resonance circuit by the capacitor and the inductance of the voltage supplied from the. This flyback pulse is applied to the primary side of the FBT 23 and boosted to generate a high-voltage pulse on the secondary side, which is rectified to obtain an anode voltage.

The high voltage control circuit 21 compares the anode detection voltage obtained by attenuating the anode voltage with the reference voltage and feeds back the voltage difference to the high voltage circuit power supply 22. When the beam current increases (that is, the load increases), the high-voltage anode voltage generally decreases, but the high-voltage control circuit 21 detects this decrease and increases the output of the high-voltage circuit power supply 22.
The decrease in the anode voltage is compensated, and the anode voltage is controlled to be constant. The anode voltage is proportional to the output voltage of the high-voltage circuit power supply 22 except for the voltage drop due to the beam current, and is substantially proportional to the horizontal deflection period. For this reason, the power supply output voltage proportional to the horizontal deflection frequency is supplied to keep the anode voltage constant.

In the CRT display device having such a structure, if a distributed capacitance due to impurities or the like exists inside the CRT, in-tube discharge occurs from the anode to the cathode of the CRT. The occurrence of the in-tube discharge causes a sudden surge current or voltage in each circuit of the CRT display device, and causes destruction of semiconductor parts and the like.

As a conventional countermeasure against the discharge in the tube, Japanese Patent Laid-Open No.
39678 (citing example 1) and JP-A-4-322294 (citing example 2) are known. The thing of the reference example 1 is
A surge detector is provided on the high voltage cable that connects the high voltage circuit and the CRT, and the level of the horizontal oscillation circuit is lowered or momentarily stopped according to the detected value, and the output of the high voltage rectifier circuit is temporarily reduced to eliminate the discharge inside the tube. Is to measure. In the reference example 2, a surge current that is suddenly generated in the FBT of the power supply section of the horizontal deflection circuit due to in-tube discharge is detected, and an overcurrent protection circuit that cuts off the operation of the power supply section is provided to provide the horizontal deflection circuit. Fuseless for protection and power supply.

[0009]

As in recent years, when the horizontal deflection frequency corresponding to one display device becomes wider, the output of the power supply 22 for the high voltage circuit also follows the same in order to keep the anode voltage constant. Then, it is necessary to change it in a wide range. In particular, in order to deal with higher frequencies, the capacitance of the resonance capacitor of the switching circuit 24 is reduced, and the retrace time for erasing the bright line is shortened. Along with this, the flyback pulse becomes higher, and the collector peak voltage of the switching transistor is 8
It has reached about 0%.

When the high voltage (anode) capacitor in parallel with the anode is discharged by the discharge in the tube, a larger current than usual is required on the primary side of the FBT for recharging. As a result, the output voltage of the power supply 22 for the high voltage circuit, that is, the collector peak voltage of the switching transistor is temporarily increased, and continues until the charging of the high voltage capacitor is completed. For this reason, the switching transistor having a small collector withstand voltage may be destroyed by this overvoltage.

However, in the above-mentioned reference example 1, the level of the horizontal oscillation circuit is lowered or momentarily stopped after detecting the overcharge due to the discharge in the tube, and the output of the high-voltage rectifier circuit is temporarily reduced, so that the overvoltage is reduced. It takes time from detection to suppression thereof, and in the case of instantaneous transient protection such as discharge in a tube, effective operation cannot be performed, and the switching transistor of the high voltage generation circuit may be destroyed during this period. Further, in the second reference example, only the protection measure of the horizontal deflection circuit is taken, and the overvoltage protection of the high voltage generation circuit is not considered.

[0012] As described above, in the conventional countermeasures against discharge in the tube of the CRT display device, the overvoltage generated in the high voltage generation circuit is not sufficiently taken into consideration, and the switching element having no margin for the overvoltage is a weak point.
Further, as a measure against overvoltage of the high voltage generation circuit, it is necessary to suppress or restore the high voltage output in consideration of the recharging time of the anode capacitor, but no consideration is given to this point.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a CRT display device which overcomes the problems of the prior art and which is provided with a high voltage generating circuit due to overvoltage, and in particular, a protection means for preventing the switching element from being destroyed.

It is an object of the present invention to provide a method for protecting an overvoltage of a CRT display device in order to protect a high voltage generating circuit from a discharge in a tube.

[0015]

SUMMARY OF THE INVENTION An object of the present invention is to provide a flyback transformer, a high-voltage rectifier circuit connecting an anode capacitor in parallel to the secondary side of the flyback transformer, and a power supply voltage applied to the primary side of the flyback transformer. In an overvoltage protection method for a CRT display device comprising a switching circuit for switching, monitoring an anode current flowing to a secondary side of the flyback transformer and usually having a value close to 0,
When the anode current is detected to increase above a predetermined level,
This is achieved by controlling the power supply voltage applied to the primary side in a downward direction and holding the suppression control for a predetermined time even after the anode current is restored.

Further, it is achieved by controlling the power supply voltage in a rising direction at a slow speed after the lapse of the predetermined time to normally restore the anode voltage applied to the anode electrode of the CRT from the high voltage rectification circuit. Further, the predetermined time is achieved by setting it in correspondence with the charging time of the anode capacitor.

Another object of the present invention is to provide a high-voltage rectifier circuit having a flyback transformer and an anode capacitor connected in parallel to the secondary side thereof, and a switching circuit for switching a power supply voltage applied to the primary side of the flyback transformer. In a raster scan type CRT display device including: detection means for detecting an abnormal increase in the anode current flowing to the secondary side of the flyback transformer;
This is achieved by providing overvoltage protection means including suppression means for lowering the power supply voltage applied to the primary side when an abnormal increase in anode current is detected.

Further, the overvoltage protection means is provided with a recovery means for recovering a normal value over a first predetermined period after reducing the power supply voltage applied to the primary side. Further, the overvoltage protection means is provided with a holding means for continuing to decrease the power supply voltage for a second predetermined period from the time of detecting an abnormal increase in the anode current or from the time of operation of the suppressing means.

[0019]

When the in-tube discharge occurs in the CRT, a closed loop is formed from the flyback transformer via the CRT and the ground, and the charge stored in the flyback capacitor is discharged, and usually a very small CRT anode current abnormally increases. . At the same time, the output of the high-voltage power supply applied to the primary side of the flyback transformer suddenly increases in an attempt to recover from the sudden drop in the anode voltage, and this overvoltage may destroy the switching element of the high-voltage generating circuit.

According to the configuration of the present invention, the abnormally increased anode current is promptly detected by the detection means provided in the closed loop, and the output of the high-voltage power supply circuit applied to the primary side of the flyback transformer is suppressed and controlled. To lower.
This suppression control continues to be suppressed until the charge of the anode capacitor is almost restored, and thereafter gradually returns to normal with a sufficient time constant.

As a result, the overvoltage applied to the switch element is suppressed, and the anode voltage is restored slowly so as to avoid the occurrence of the overvoltage due to the influence of the charging of the anode capacitor. As a result, the switching element of the high voltage generating circuit is prevented from being destroyed and the normal operation of the CRT can be safely and automatically restored.

[0022]

1 shows a high voltage circuit of a CRT display device according to an embodiment of the present invention. The same elements as those in FIG. 2 are designated by the same reference numerals. A new overcurrent detection circuit 25,
Overvoltage protection means including a high voltage output suppression circuit 26 and a holding circuit 27 is provided.

In the high voltage control circuit 21, a reference voltage V _c0 (2-3 v) for setting a predetermined anode voltage Va is applied to an input terminal (reference voltage input terminal) 21a, and an input terminal (anode voltage feedback terminal) is provided. The voltage Va ′ obtained by attenuating the anode voltage is fed back to 21b, and the output voltage V _E of the high-voltage circuit power supply 22 is adjusted so that the difference between them is zero.
To adjust.

The primary side 23a of the flyback transformer 23
A switching circuit 24 including a switching transistor Q10, a diode D10, a resonance capacitor C10, a dummy inductance L10, and a capacitor C11 is connected to the low voltage side of the. The switching transistor Q10 is turned on / off by the horizontal synchronizing signal.

A voltage is supplied from the high-voltage circuit power source 22 to the primary side 23a of the flyback transformer 23, and the voltage is
It is applied to the capacitor C11. When the transistor Q10 is on, the voltage of the capacitor C11 causes a current to flow through the transistor Q10 through the inductance L10. When the transistor Q10 is turned off, the electromagnetic energy stored in the inductance L10 charges the capacitor C10 to generate a flyback pulse. The flyback pulse generated in the capacitor C10 returns the current to the capacitor C11 through the inductance L10. When the voltage of the capacitor C10 becomes zero, the current flows through the diode D10 and charges the capacitor C11. When the electromagnetic energy of the inductance L10 becomes zero, the voltage of the capacitor C11 again causes a current to flow through the inductance L10 to the transistor Q10 (by this time, the transistor Q10 has been turned on again).
In this way, the flyback pulse generated in the collector of the switching transistor Q10 is boosted by the flyback transformer 23, and a high voltage pulse is generated on the secondary side 23b thereof. This is rectified by the rectifier 23c and the anode capacitor Ca to obtain the anode voltage Va.

The overcurrent detection circuit 25 includes resistors R12 and R1.
3, the negative side of the diode D11 and the secondary side 23b of the flyback transformer 23 are connected to form a detection end 28. The detection voltage Vd at the detection end 28 is Vd = R12.
As V _AA / (R12 + R13), usually diode D
It is set to be higher than the positive electrode side of 11. The threshold value V _{th that} has the same potential as the positive electrode side of the diode D11
Is the forward voltage drop of D11 Vf, the base of Q11
V _th = V _BB −V _BE −Vf, where V _BE is the voltage between the emitters.
Becomes

The suppression circuit 26 is composed of a control transistor Q11 which receives the voltage of the detection terminal 28 as an input, and a transistor Q12 of the next stage. That is, when the detected voltage Vd falls below the threshold value V _th , the transistor Q12 of the next stage is also turned on following the operation of the transistor Q11 of the previous stage, and the collector voltage Vc ′ thereof becomes 0. The collector voltage Vc ′ is normally higher than the reference voltage V _c0 by V _CC (strictly speaking, the capacitor C1
3 charging voltage), and its output point 29 is connected to the reference voltage input terminal 21a of the high voltage control circuit 21 via the backflow prevention diode D12. Transistor Q1
When the collector voltage Vc 'becomes 0v when 2 is turned on, the input terminal 21a is short-circuited to the ground via the diode D12 and the transistor Q12, and therefore the voltage V
c≈Vc ′, and the input terminal voltage Vc is lowered from V _c0 to near 0 (strictly speaking, the sum of the forward voltage drop of D12 and the forward voltage drop between the emitter and collector of Q12 is about 0.7 v). ). As a result, the output voltage V _E of the high-voltage circuit power supply 22 is suppressed and sharply lowered, and the collector peak voltage of the high-voltage generating circuit is also lowered.

The holding circuit 27 has a resistor R1 from the power source V _CC.
5, a series circuit of a resistor R16 and a capacitor C13 charged via the output terminal 29. The holding circuit 27 is charged according to the circuit time constant when the transistor Q12 turns from ON to OFF again. Along with this, the voltage Vc at the output terminal 29 is recovered, but the suppressing operation of the output voltage V _E is maintained until the voltage Vc becomes equal to or higher than the reference voltage V _c0 . The circuit time constant of the holding circuit 27 corresponds to the charging time of the anode capacitor Ca. Further, the resistance of the resistor R16 is sufficiently lower than that of the resistor R15. This holding circuit controls the output voltage V _{E of} the high-voltage circuit power supply 22 once suppressed.
Is slowly recovered to prevent the collector peak voltage from becoming an overvoltage due to the charging current of the anode capacitor Ca.

FIG. 3 is an explanatory view of a closed loop serving as a path of the short-circuit current Is when a discharge in the tube occurs in the CRT 5, FIG.
Are (a) the output voltage V _E of the high-voltage circuit power supply, (b) the anode voltage Va, and (c) the detection end voltage V during discharge in the tube.
FIG. 6D is a time waveform diagram showing a voltage change of the reference voltage input terminal voltage Vc in FIG.

At time t ₁ , when the discharge in the tube occurs, the CRT
Since the anode 5a and the ground are short-circuited, the electric charge stored in the anode capacitor Ca is discharged, and the anode voltage Va temporarily decreases as t ₁ → t ₃ in FIG. 4B. At this time, a short loop current I _s flowing out from the anode 5a of the CRT passes through the resistor R12 from the ground and returns to one end of the secondary side 23b of the flyback transformer, forming a closed loop. The resistor R12 of the overcurrent detection circuit 25 is arranged in this closed loop.

When the short circuit current I _s flows through the resistor R12, the detection end voltage Vd of the detection end 28 becomes t ₁ ⇒ in FIG.
It decreases as t ₃ . That is, an abnormal increase in the anode current is caused by a voltage drop (ΔVd = R12 ·
I _s ). However, before time t ₂ , the voltage Vc at the reference voltage input terminal 21a maintains the reference voltage V _C0 , while the anode voltage Va ′ at the anode voltage feedback terminal 21b continues to decrease (Va ′ <V _C0 ). The output V _E of the high-voltage circuit power supply is t ₁ ⇒ t _{3 in} FIG.
Rises like.

At time t ₂ , the detection end voltage Vd becomes equal to or lower than the threshold value V _th , the suppressing circuit 26 operates, and the voltage Vc at the reference voltage input end 21a decreases to around 0 v, and Va ′ >>
Since it becomes Vc, the output V _E of the power supply is suppressed and starts to fall.

The time t ₂ when Vd ≦ V _th is determined by the set value of V _th , but it is usually set shortly before the time t ₃ when the discharge of the anode capacitor Ca ends. The discharge time (t ₃ −t ₁ ) of the anode capacitor is determined by the capacitance of the anode capacitor Ca and the time constant τ ₁ of the circuit of the parallel combined resistance (R12 // R13) of the resistors R12 and R13. Represented.

[0034]

[Formula 1] t ₃ −t ₁ = −τ ₁ · ln (V _th · (R12 + R
13) / (V _AA · R12)) where τ ₁ = Ca · (R12 // R13).

An example of the circuit constants in this embodiment is as follows: Ca = 3000 pF, R12 = 12 kΩ, R13 =
5.2 kΩ, V _AA = 24v, V _BB = 5v, V _BE = 0.7
v, Vf = 0.7v, and the discharge time t ₃ −t ₁ is about 17
μs.

When the transistor Q12 of the suppression circuit 26 is turned on and the collector voltage Vc 'becomes 0, the capacitor C
The electric charge stored in 13 is discharged, and the reference voltage input point 2
The voltage Vc of 1a becomes about 0.7v. As a result, the output V _E of the high voltage power supply circuit 22 rapidly decreases. When the suppression according to the present embodiment is not performed, the output V _E continues to increase even after the time t ₂ as shown by the dotted line in FIG. 4A, and eventually the switching transistor Q10 is destroyed. However, as in the present embodiment, the output V _E is suppressed, the flyback pulse voltage is reduced accordingly, and the voltage applied to the collector of the transistor Q10 is also reduced. Therefore, it is necessary to prevent the switching transistor from being destroyed. You can

When time t ₃ is reached, the high voltage power supply circuit 22
A charging current flows to the anode capacitor Ca through the flyback transformer 23, and the anode voltage Va starts to rise in the charging time according to the time constant τ ₁ . Further, the detection end voltage Vd rises due to the voltage drop of the resistor R12 due to the charging current in the direction opposite to the short circuit current I _S, and becomes equal to or higher than the threshold value V _th at time t ₄ . This period of t _{2 to} t ₄ (determined by the threshold value V _th , but usually sufficiently shorter than the charging time t ₃ −t ₁ ),
The reference voltage input terminal 21a remains at 0.7v, and the power supply voltage V _E continues to decrease.

At time t ₄ , when the detection end voltage Vd ≧ the threshold value V _th , the suppression circuit 26 turns off the transistors Q11 and Q12 and stops the operation. However, the collector voltage Vc ′ of Q12 recovers at the charging time t ₆ determined by the time constant τ ₂ of the holding circuit. This time t ₆ is calculated as shown in Equation 2.

[0039]

## EQU2 ## t ₆ −t ₄ = −τ ₂ · ln (1- (V _C0 −0.
7) / V _CC ) Here, τ ₂ = C13 · (R15 + R16).

An example of the circuit constants in this embodiment is C13 = 22 μF, R15 = 82 kΩ, R16 = 3.
90Ω, V _CC = 5v, and time t ₆ -t ₄ is about 1 s. That is, after the occurrence of the discharge in the tube, the suppression of the high-voltage power supply output is started in a short time of ten and a few microseconds, and then the power supply voltage and the anode voltage are automatically restored in about 1 second, and the normal image display is performed again. Has been realized.

As shown in FIG. 4, the detection end voltage Vd is restored to the normal value at the time t ₅ when the charging of the anode capacitor Ca is almost completed, but the reference voltage Vc is restored sufficiently longer than Vd. Is gradually recovering. That is, the power supply voltage V _E is kept suppressed until the time t _{5 at} which the charging is completed, and then slowly restored.

This is because when the recovery of the anode voltage Va and the charging of the anode capacitor Ca are overlapped with each other, the anode voltage Va sharply rises as shown by the dotted line in the same figure (b), which causes a rapid rise of the primary side of the flyback transformer. This is for avoiding.

According to this embodiment, the suppression of the high-voltage power supply voltage is started immediately after the detection of the discharge in the tube to prevent the switching transistor of the high-voltage generation circuit from being destroyed. Furthermore, by holding the suppression for a certain period of time and gradually (slowly) recovering, safe automatic recovery is also possible.

[0044]

As described above, according to the present invention, it is possible to suppress the overvoltage due to the discharge in the tube of the CRT and to automatically restore it, so that it is possible to prevent the breakdown of the switching element of the high voltage generating circuit of the CRT display device.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a high voltage circuit of a CRT display device according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a CRT display device to which the present invention is applied.

FIG. 3 is an explanatory diagram showing a path of a discharge current flowing during in-tube discharge.

FIG. 4 is a time waveform diagram for explaining the suppressing operation according to the present embodiment and showing a voltage change in each part of the high voltage circuit before and after the discharge in the tube.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Power supply circuit, 2 ... High voltage circuit, 3 ... Deflection circuit, 4 ... Video amplifier, 5 ... CRT, 5a ... Anode electrode, 20 ... High voltage generation circuit, 21 ... High voltage control circuit, 21a ... Reference voltage input terminal, 21b ... Anode voltage feedback end, 22 ...
Power supply for high voltage circuit, 23 ... Flyback transformer (FB
T), 23a ... FBT primary side, 23b ... FBT secondary side,
23c ... Rectifier, 24 ... Switching circuit, 25 ... Overcurrent detection circuit, 26 ... Suppression circuit, 27 ... Holding circuit, 28 ...
Detection end, 29 ... Output end.

Claims (8)

[Claims] 1. A CRT display device comprising: a flyback transformer, a high-voltage rectifier circuit that connects an anode capacitor in parallel to the secondary side of the flyback transformer, and a switching circuit that switches a power supply voltage applied to the primary side of the flyback transformer. In the overvoltage protection method, an anode current that flows to the secondary side of the flyback transformer and is usually close to 0 is monitored, and when the anode current increases above a predetermined level and is detected, it is applied to the primary side. A method for overvoltage protection of a CRT display device, characterized in that the power supply voltage is controlled so as to decrease in a downward direction, and the suppression control is held for a predetermined time even after the anode current is restored. 2. The anode voltage applied to the anode electrode of the CRT from the high-voltage rectifier circuit is normally restored after the predetermined time has elapsed by controlling the power supply voltage in a rising direction at a slow speed. An overvoltage protection method for a CRT display device, comprising: 3. The overvoltage protection method for a CRT display device according to claim 1, wherein the predetermined time is set corresponding to a charging time of the anode capacitor. 4. The overvoltage protection method for a CRT display device according to claim 1, 2 or 3, wherein the increase of the anode current more than a predetermined value is generated by a discharge in a tube of the CRT. 5. A raster scan system comprising a flyback transformer, a high-voltage rectifier circuit that connects an anode capacitor in parallel to the secondary side of the flyback transformer, and a switching circuit that switches a power supply voltage applied to the primary side of the flyback transformer. In a CRT display device, a detection unit that detects an abnormal increase in the anode current flowing to the secondary side of the flyback transformer, and a control that reduces the power supply voltage applied to the primary side when the abnormal increase in the anode current is detected. A CRT display device comprising overvoltage protection means including means. 6. The overvoltage protection means according to claim 5, further comprising a recovery means for recovering a normal value over a first predetermined period after reducing the power supply voltage applied to the primary side. CRT display device. 7. The holding means according to claim 5, wherein the overvoltage protection means includes a holding means that continues to decrease the power supply voltage for a second predetermined period from a time when the abnormal increase in the anode current is detected or a time when the suppression means operates. A CRT display device comprising. 8. The overvoltage protection means according to claim 7, wherein after the second predetermined period ends, the overvoltage protection means restores the normal value over the first predetermined period according to claim 6. CRT display device.