JP2760232B2 - High voltage circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high voltage circuit used for a display device using a cathode ray tube (CRT).

[0002]

2. Description of the Related Art Generally, in a display device using a CRT, the luminance of the CRT is increased, an electron beam becomes excessive, and a high voltage circuit is overloaded to prevent a high voltage generating transistor from being destroyed. In order to prevent the occurrence of the white peak collapse phenomenon on the screen due to the saturation of the transistor in the output stage inside the video amplification circuit,
It is well known that a beam limiter circuit is provided to regulate the CRT beam current value so that it does not exceed a specified value for reasons such as regulation of the maximum rated cathode current value (beam current value) related to the life of T. is there.

FIG. 3 is a circuit diagram showing an example of a conventional high-voltage circuit having a beam limiter circuit. In FIG. 3, a high voltage output NPN transistor (high voltage generating transistor)
Q1, damper diode D1, resonance capacitor C1,
The dummy yoke DY and the drive capacitor C2 constitute the voltage resonance type high voltage output circuit 1. The collector pulse Vcp generated by the high voltage output transistor Q1 is input to the input winding T1a of the flyback transformer T1 via the terminal P1. Here, the flyback transformer T1 is configured by a multi-singler system in which output high-voltage windings T1b, T1c, T1d and high-voltage rectifier diodes D2 to D4 are mixed. The pulse Vcp input to the input winding T1a is boosted by the flyback transformer T1, and a high voltage HV is obtained at the output (high voltage terminal P3) of the high voltage rectifier diode D2. Then, this high voltage HV is applied to the anode of the CRT 2.

The low voltage terminal P of the flyback transformer T1
An integration circuit (first current detection circuit) 4 includes a smoothing capacitor C4 having one end grounded and a beam current detection resistor R3 having one end connected to the DC power supply voltage input terminal P5.
Is provided. A detection voltage corresponding to the beam current value detected by the resistor R3 is supplied to the contrast / brightness control circuit 3 (negative feedback). The video amplification circuit 4 inputs a video signal to the cathode of the CRT 2, and the contrast / brightness control circuit 3 controls the video amplification circuit 4 to control the contrast and brightness of the video projected on the CRT 2. Note that a resistor R3 and a capacitor C3 for detecting the beam current value are used.
The integrating circuit 4 and the contrast / brightness control circuit 3 constitute a so-called static beam limiter circuit as described above.

Further, a series circuit of a high-voltage detecting divisional resistor R1 and R2 and a smoothing capacitor C3 are connected in parallel between the output of the high-voltage rectifier diode D2 and the ground.
The detection voltage obtained by dividing the high voltage HV by 1 and R2 is input to the control circuit 5. The output voltage of the control circuit 5 is input to the high-voltage regulator circuit 6, and the high-voltage regulator circuit 6 stabilizes the high voltage HV by supplying the output voltage to the input winding T1a of the flyback transformer T1 via the terminal P2.

Here, the voltage at the terminal P2, which is the output voltage of the high-voltage regulator circuit 6, is VP2, the period of the input horizontal synchronizing signal is TH, the resonance period is TR, the inductance of the dummy yoke DY is L1, and the input of the flyback transformer T1. Assuming that the inductance of the winding T1a is L2, the transistor Q
1, the collector pulse Vcp and the resonance period TR are respectively Vcp = VP2 {(π / 2). (TH / TR-1) +1} (1) TR = π {C1 / L1 / L2 / (L1 + L2) ｝ ^1/2 … (2) Further, assuming that the input / output winding ratio of the flyback transformer T1 is n, the high voltage HV is represented by HV = n ＝ VP2Ｖ (π / 2) ・ (TH / TR -1) (3)

In order to stabilize the high voltage HV, if the period TH of the horizontal synchronizing signal, the resonance period TR, and the input / output turn ratio n of the flyback transformer T1 are fixed, the high voltage HV can be changed. It can be seen that the output voltage VP2 of the high voltage regulator circuit 6 may be varied.

FIG. 4 is a circuit diagram showing a specific configuration of the contrast / brightness control circuit 3 in FIG. contrast·
The brightness control circuit 3 includes resistors R31 to R35, a capacitor C31, a diode D31, and NPN transistors Q31 and Q32. The cathode of the diode D31 is connected to the connection point between the resistor R34 and the resistor R35 (resistance voltage dividing circuit), and the anode of the diode D31 is negatively connected to the emitter of the pedestal clamp transistor Q31 via the resistor R32. . A resistor R31 is connected to the collector of the transistor Q31.
Is connected, and a pulse is supplied to its base. The emitter of the transistor Q31 is connected to the capacitor C31 and the base of the transistor Q32. The emitter of the transistor Q32 is grounded via the resistor R33.

In the contrast / brightness control circuit 3 configured as described above, the diode D31 is normally off, but the diode D31 is turned on when a video signal having a high white peak level is received (received), and the resistance voltage is divided. By clamping the emitter potential of the transistor Q31 to the output voltage of the circuits R34 and R35 (the voltage at the connection point of the resistors R34 and R35), the output stage transistor (not shown) in the video amplifier circuit 4 in FIG. The white peak level of the supplied video signal is reduced, and the beam current is limited to a specified value or less.

[0010]

FIG. 5 shows the relationship between the beam current Iboff when the beam limiter circuit is not operated and the beam current Ibon when the beam limiter circuit is operated in the above-mentioned conventional high-voltage circuit. In the static characteristics of the beam limiter circuit, there is no problem in the beam limiter off region indicated by the symbol and the beam limiter on region indicated by the symbol. However, if there is a sudden signal change or channel switching in the transition region indicated by, the beam current value increases → the detection voltage generated in the beam current detection resistor R3 decreases → the diode D31 turns on → the beam limiter circuit turns on →
It is easy to oscillate in a loop of decreasing the beam current value → increase of the detection voltage → turning off the diode D31 → turning off the beam limiter circuit → increase of the beam current value →... A phenomenon may occur or flicker oscillation may appear.

That is, the capacitor C4 shown in FIG.
Is about 1 μF, when the luminance changes from dark to bright over the entire screen, there is no sufficient effective current capacity to be supplied from the capacitor C4, the potential of the terminal P4 immediately drops, and the beam limiter circuit operates to increase the voltage. The bounding phenomenon on the screen hardly occurs regardless of the regulation performance of the circuit. However, when the change in the white peak power is large partially on the screen, the effective current capacity supplied from the capacitor C4 is sometimes sufficient or insufficient. When the effective current capacity supplied from the capacitor C4 is sufficient, the current is moderately smooth, so that the beam limiter circuit does not operate and the beam current can flow, and a sufficient white peak current can flow. If the regulation performance of the circuit is poor, a phenomenon occurs in which the screen size of the white peak portion on the screen increases. Conversely, when the effective current capacity supplied from the capacitor C4 is insufficient, the potential of the terminal P4 drops, so that the beam limiter circuit operates and flicker oscillation is likely to occur in the above-described oscillation loop.

Therefore, at least as a means for preventing oscillation without changing the DC potential of the output voltage of the resistance voltage dividing circuits R34 and R35 shown in FIG. 4, the capacitance value of the capacitor C4 is reduced to, for example, 0.1 μF. , The oscillation is surely eliminated, but the output voltage waveforms of the resistive voltage dividing circuits R34 and R35 always have a vertical cycle ripple voltage waveform with almost no beam current flowing during the vertical blanking period.
Has a large peak value of the ripple voltage as shown in FIG. Then, the detection of the beam current value by the resistor R3 is similar to the peak voltage detection, and the luminance characteristic does not become the desired characteristic shown by the solid line in FIG. It will not flow.

Conversely, the capacitance value of the capacitor C4 is set to, for example, 1
When the value is increased to 0 μF, the smoothing ability is sufficient even for a change in which the white peak current is large on the screen and the peak power is sharp, and the voltage waveform at the terminal P4 is as shown in FIG. Therefore, the beam limiter circuit does not operate, and flicker oscillation and the like can be suppressed.
However, since the capacitance value of the capacitor C4 is large, the capacitor C4 when the brightness changes from dark to bright against an average beam current change such that the brightness changes over the entire screen.
The effective current capacity supplied from 4 is sufficient, and sufficient current can be passed without operating the beam limiter circuit.
If the regulation performance of the high-voltage circuit is poor, the average high-voltage fluctuation increases, and a bounding phenomenon occurs on the screen.

It goes without saying that the higher the white peak current is, the brighter the white peak current can be projected in the range where the image quality is not saturated, but from the above, the capacitance value of the capacitor C4 can be increased if the regulation performance of the high voltage circuit is good. It can be said that a white peak current can also flow and flicker oscillation can be prevented. However, recently, in order to improve regulation performance, a series regulator system having an excellent response speed is exclusively used as the high-voltage regulator circuit 6 shown in FIG. A waveform shown by a broken line in FIG. 7A indicates a high-voltage regulation characteristic when the high-voltage regulator circuit 6 is not provided. FIG. 7B shows the static characteristics of the correction of the high-voltage regulator circuit 6. The high-voltage regulator circuit 6 corrects the waveform shown by the broken line in FIG. 7A to obtain the waveform shown by the solid line in FIG. In FIG. 7A, the beam current Ib is small and the high voltage HV is constant even without the high voltage regulator circuit 6, and the beam current Ib is large in the region and the high voltage HV is controlled by the high voltage regulator circuit 6. Is a region in which the output voltage VP2 of the high-voltage regulator circuit 6 has reached the corrected maximum effective voltage (the power supply voltage input to the terminal P6). The wider the correction voltage range is, the higher the high-voltage regulation with respect to the beam current is, and the more the power consumption increases.

In short, in a conventional high-voltage circuit,
(A) When the white peak power partially increases on the screen, flicker oscillation occurs. (A) When the capacitance value of the capacitor C4 is increased to prevent this (A), the average beam current increases. Has a bounding phenomenon, (C)
If the correction voltage range of the high-voltage regulator circuit 6 is widened in order to prevent (a), a vicious cycle of increasing the power consumption of the high-voltage regulator circuit 6 has been a problem. SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and a white peak current can be sufficiently supplied while suppressing a high voltage correction voltage range by a high voltage regulator circuit, and flicker oscillation and a bounding phenomenon occur. It is an object of the present invention to provide a high-voltage circuit that does not have any problem.

[0016]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems of the prior art, the present invention boosts an input pulse and rectifies the boosted pulse to apply a high voltage to an anode of a cathode ray tube. A flyback transformer to be applied, a high voltage regulator circuit for controlling the high voltage to be constant,
A control circuit for controlling the contrast or brightness of a video signal input to the cathode of the cathode ray tube; detecting a beam current flowing through the cathode; generating a detection voltage corresponding to the beam current; and negatively feeding back the control circuit A first current detection circuit configured to include a first smoothing capacitor, wherein a current change amount of the beam current per unit time is detected, and the current change amount is a predetermined constant value. It is an object of the present invention to provide a high voltage circuit characterized by providing a second current detection circuit for connecting a second smoothing capacitor in parallel with the first smoothing capacitor when the value exceeds the value.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A high voltage circuit according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing an embodiment of the high voltage circuit of the present invention, and FIG. 2 is a waveform diagram for explaining the high voltage circuit of the present invention. In FIG. 1, the same parts as those in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 1, what is newly provided according to the present invention is that resistors R4 to R6, capacitors C5 and C6, a diode D5, a Zener diode D surrounded by a dashed line.
6, a dynamic beam current detection circuit (second current detection circuit) 7 including transistors Q2 to Q5 for detecting a current change amount of the beam current per unit time and generating a voltage corresponding to the current change amount. It is. In the high-voltage circuit of the present invention, by providing this dynamic beam current detection circuit 7, it operates as a dynamic beam limiter circuit for detecting a current change (speed) of the beam current per unit time together with the above-described beam limiter circuit. It is to let. That is, in the high-voltage circuit of the present invention,
In addition to detecting the magnitude of the beam current, the amount of change in the beam current per unit time is also detected.

The terminal P4 is connected to one terminal of a capacitor C5, and the other terminal of the capacitor C5 is connected to a diode D8 whose cathode is grounded. The point A, which is the connection point between the capacitor C5 and the diode D8, has a PNP
The collector of the transistor Q4 is connected, and its base is connected to the NPN transistor Q5 whose emitter is grounded. The emitter of the transistor Q4 is connected to the DC power input terminal P5 via the resistor R6. The anode of the Zener diode D6 is connected to the point A, and the cathode of the diode D5 is connected to its cathode. The anode of the diode D5 is connected to the DC power input terminal P5 via the resistor R4. The anode of the diode D5 is connected to the base of an NPN transistor Q2 whose emitter is grounded, and the collector is connected to the base of the NPN transistor Q3 whose emitter is grounded. A capacitor C6 is connected between the collector of the transistor Q3 and the terminal P4, and a diode D7 whose anode is grounded is connected between the collector of the transistor Q3 and the capacitor C6. Also, the transistor Q
5 and the base of the transistor Q3 are connected.

By connecting the capacitor C5 and the charging path diode D8 of the capacitor C5 between the terminal P4 and the ground as shown in the figure, a voltage substantially equal to the voltage of the terminal P4 is applied to both ends of the capacitor C5. . Assuming that the voltage of the terminal P4 is V1 and the forward voltage of the diodes D5 and D8 is 0.6 [V], (V
1-0.6) A voltage of [V] is applied. The point A is clamped by the diode D8 to 0.6 [V], the transistor Q2 is turned on via the resistor R4, the diode D5 is turned off because no forward voltage is applied, and the transistor Q3 is turned off. The smoothing capacitor C6 provided in parallel with the capacitor C4 is off in terms of AC. Similarly, the transistor Q5 is off, and the discharging transistor Q4 is off. That is,
Since there is no discharge path in the capacitor C5, the charge remains in a state of being accumulated.

Then, the beam current suddenly changes from small to large, and the voltage V1 at the terminal P4 changes to the voltage V2 during the time T1.
, The capacitor C5 has no discharge path, so that no current flows and only the voltage changes, and the voltage at point A becomes V2-
(V1-O.6) The voltage drops toward the negative potential of [V].
When the Zener voltage Vz of the Zener diode D6 is 0 to [V
2- (V1-O.6)], the transistor Q2 is turned on when the voltage at the point A falls below -Vz.
Is inverted from the on state to the off state, and the transistor Q3 changes from the off state to the on state. Then, the diode D7
At the same time, the capacitor C6 is AC grounded, and the capacitor C6 is connected in parallel with the capacitor C4.

At the same time, the transistor Q5 is inverted from the off state to the on state, and the discharge path having the time constant TD between the resistor R6 and the capacitor C5 operates. The relationship between the discharge time constant TD and the current change per unit time of the terminal P4, that is, the voltage change per unit time (V2−V1) / T1, is A
The voltage drop at the point is determined.

As shown in FIG. 2A, when the time T1 is small (T1 << TD), the negative potential is large as shown by a and drops to approximately (V2-V1 + 0.6) [V]. , With a constant discharge time constant TD until the voltage at point A becomes -Vz. When the time T1 is medium (T1 <TD), the discharge does not drop to (V2−V1 + 0.6) [V] as shown by b, but discharge starts early, and the discharge at the point A becomes −Vz. I do. When the time T1 is large (T1> TD), it indicates a beam current change in which the dynamic beam limiter circuit does not operate as shown by c. The above is the case where the beam current changes from small to large and holds large. In the case of a moving image, the voltage immediately changes from large to medium or small, so the voltage of the terminal P4 also increases, Accordingly, the potential at point A also increases, so that the discharge time constant of the discharge curve shown in FIG.

FIG. 2B illustrates the waveform indicated by a in FIG. 2A taking the above into consideration. 2A shows the case of a in FIG. 2A, in which the beam current changes from small to large and remains large. The case indicated by a2 is the case where the beam current changes from small to large and to medium. The case indicated by a3 is a case where the beam current changes from small to large and small, and the discharge curve is steep. In FIG. Is clamped to -Vz [V], but FIG. 2B shows that the voltage at point A is clamped to 0.6 [V] due to the rise in the voltage of the terminal P4. .

That is, a signal change in which the beam current increase per unit time is large (such as a white peak change in a part of the screen) can be said to be a signal change in which the peak power is large, and exceeds a set threshold value (the above-mentioned zener voltage Vz). The capacitor C6 and the capacitor C6 are connected in parallel to the capacitor C4 to increase the capacitance, thereby preventing flicker oscillation and an increase in the screen size at the white peak portion. At this time, since the average amount of increase in the beam current is small, the bounding phenomenon does not occur even if the capacitance of the smoothing capacitor increases. Conversely, in a signal change in which the average beam current increase is large (when the luminance of the entire screen changes from dark to bright), the amount of change per unit time is small, so that the above-described threshold is not exceeded, and the capacitor C6 is used. There is no parallel connection to the capacitor C4.

As described above, according to the present invention, when the average beam current amount increases, the capacitance of the capacitor C4 (beam current smoothing capacitor) is kept small, and when the white peak power increases, the capacitor C4 (beam current smoothing capacitor) decreases. By increasing the capacity of the capacitor, the above-mentioned problems to be solved by the invention are described from (A) to (C).
Can break the vicious circle to Therefore, if the capacitor C4 is increased in order to suppress the flicker oscillation, a bounding phenomenon is likely to occur in proportion to the capacitance of the capacitor C4. Therefore, the voltage correction range of the high-voltage regulator circuit 6 must be increased in proportion thereto. That problem can be solved satisfactorily.

[0027]

As described in detail above, the high-voltage circuit of the present invention detects the amount of change in the beam current per unit time, and when the amount of change in the current exceeds a predetermined value, the first voltage is detected. The second in parallel with the first smoothing capacitor of the current detection circuit
Is provided, the capacity of the first smoothing capacitor is kept small when the average beam current increases, and the first current detecting circuit is connected only when the white peak power increases. The capacity of the smoothing capacitor can be increased. Therefore, the voltage correction range of the high-voltage regulator circuit (series regulator) can be reduced, so that a high-voltage circuit without increasing power consumption and free from flicker oscillation and bounding phenomenon can be obtained. Therefore, there is also a feature that the volume of the heat sink added to the semiconductor element of the high-voltage regulator circuit can be reduced, so that the cost and the area occupied by the substrate can be reduced. Further, the high-voltage circuit of the present invention is effective for use in a display device capable of projecting bright and glossy white.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a high-voltage circuit according to the present invention.

FIG. 2 is a waveform diagram for explaining a high-voltage circuit according to the present invention.

FIG. 3 is a circuit diagram showing a conventional high-voltage circuit.

FIG. 4 is a circuit diagram showing a specific configuration of a contrast / brightness control circuit 3 in FIG. 3 (or FIG. 1).

FIG. 5 is a diagram illustrating static characteristics of a beam limiter circuit.

FIG. 6 is a voltage waveform diagram of a terminal P4 in FIG. 3 (or FIG. 1).

FIG. 7 is a diagram showing static characteristics of the high voltage HV and static characteristics of the high voltage regulator circuit in FIG. 3 (or FIG. 1).

[Explanation of symbols]

REFERENCE SIGNS LIST 1 voltage resonance type high voltage output circuit 2 cathode ray tube 3 contrast / brightness control circuit 4 video amplifier circuit 5 control circuit 6 high voltage regulator circuit 7 dynamic beam current detection circuit (second current detection circuit) C4 smoothing capacitor (first smoothing capacitor) C5 capacitor (third capacitor) C6 smoothing capacitor (second smoothing capacitor) D8 charging path diode T1 flyback transformer

Claims (2)

(57) [Claims] 1. A flyback transformer for boosting an input pulse and applying a high voltage obtained by rectifying the boosted pulse to an anode of a cathode ray tube; and a high voltage regulator circuit for controlling the high voltage to be constant. A control circuit for controlling the contrast or brightness of a video signal input to the cathode of the cathode ray tube; detecting a beam current flowing through the cathode; generating a detection voltage corresponding to the beam current; A first feedback smoothing capacitor including a first smoothing capacitor
A current detection circuit for detecting a current change amount of the beam current per unit time, and when the current change amount exceeds a predetermined constant value, the current change amount is determined in parallel with the first smoothing capacitor. A high-voltage circuit comprising a second current detection circuit for connecting the second smoothing capacitor. 2. The second current detection circuit, comprising: a second smoothing capacitor; a third capacitor provided between an output terminal of the first current detection circuit and ground; and a charge path diode connected in series. And a circuit for connecting the second smoothing capacitor in parallel to the first smoothing capacitor when a voltage at a connection point between the third capacitor and the diode drops to a predetermined threshold. 3. The high voltage circuit according to claim 1, wherein the discharge path of the third capacitor is operated, and the discharge path of the third capacitor and the second smoothing capacitor are opened when returning to the predetermined threshold value again.