High voltage generator

Details High voltage generator High voltage generator

1999-09-07

2001-06-26

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Power supply from deflection circuit source

C363S021030

Reexamination Certificate

active

06252360

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a high voltage generator for CRT such as CRT monitor for computer and TV receiver of high resolution. The invention also relates to stabilization of output voltage of such high voltage generator.
BACKGROUND OF THE INVENTION
In the recent trend of higher picture quality and higher density of personal computer, Hi-Vision TV receiver, digital TV receiver and others, the deflecting frequency is widely used, and the frequency tends to be higher.
In these display devices using CRT, in particular, various proposals have been made relating to:
horizontal deflection,
high voltage generating circuit, and
control.
The basic operation of the deflecting circuit and high voltage generating circuit in the CRT display device is described below.
FIG. 10
is an explanatory diagram of conventional deflecting circuit and high voltage generating function.
As shown in
FIG. 10
, a horizontal deflecting circuit (not shown)
feeds a horizontal synchronizing pulse to a deflection control circuit
14
, and
drives a horizontal deflecting transistor
2
.
The deflecting circuit is composed of a deflecting yoke
5
, a damper diode
3
, and a capacitor
4
. The power source is supplied to the deflecting circuit through a deflecting transformer
8
.
The horizontal deflecting circuit is used to deflect an electron beam in the horizontal direction.
During scanning period, the horizontal deflecting circuit realizes horizontal deflection by
passing a sawtooth type current into the deflecting yoke
5
, and
returning from end point to start point of scanning during the blanking period.
The resonance pulse occurring in the blanking period is called a blanking pulse. A deflecting and high voltage integrated circuit for obtaining a high voltage output by adding a high voltage winding
61
to the deflecting transformer
8
is used in a general TV receiver.
However, the high voltage output is lowered by increase of electron beam current.
Accordingly, by generation of high voltage fluctuation, both amplitudes of horizontal deflection and vertical deflection fluctuate.
Further, this high voltage fluctuation causes image deformation.
As their solving means, the following two methods are known.
1) Deflecting and high voltage separated circuit system: The deflecting circuit and high voltage circuit are separately composed, and each circuit is controlled independently.
2) Deflecting and high voltage integrated circuit system. The high voltage is stabilized by minimizing effects of high voltage to the deflecting circuit.
Both systems require
stabilization of high voltage,
simple constitution,
efficient operation, and
operation at low oscillating voltage.
FIG. 11
, FIG.
12
and
FIG. 13
show the prior art in the deflecting and high voltage separated circuit. FIG.
11
and
FIG. 12
are explanatory diagrams of conventional high voltage generating circuit.
FIG. 13
shows an example of operating voltage and current waveform of the conventional high voltage generating circuit.
In
FIG. 11
, one end of a primary winding
32
of a high voltage generating transformer
69
is connected to the drain of a switching element
27
.
Other end of the primary winding
32
is connected to a step-down converter composed of a switching element
10
, a flywheel diode
11
, a smoothing capacitor
13
, and an inductor
12
.
By a signal from a deflecting control circuit
14
, a synchronizing triangular wave generating circuit
63
is synchronized at f [Hz], and issues a triangular wave for comparison. The secondary side output voltage of the high voltage generating transformer
69
is divided by detecting resistances
45
,
46
. The divided voltages are fed into an error amplifier
35
, and compared with a reference voltage
34
. The output amplified by the error amplifier
35
is fed into a pulse width modulation (PWM) comparator
64
together with the triangular wave for comparison. The output of the PWM comparator
64
is connected to the gate of a switching element
10
through a drive circuit
65
.
At the gate of the switching element
27
, a specific drive pulse is supplied from the deflecting control circuit
14
. At the primary side of the high voltage transformer
69
, a damping circuit
66
composed of inductor, capacitor and resistance is connected in series to the primary winding
32
. This damping circuit
66
is for suppressing the oscillating current due to resonance of the leakage inductance and distributed capacity.
The high voltage generating circuit of such circuit constitution is a step up/down DC/DC converter operating by two switching elements.
In this case,
the switching element
10
is switched by the PWM signal synchronized at f [Hz], and
the feedback system is composed so that the high voltage output may be uniform.
In this constitution, the object of stabilization of high voltage output is achieved.
However, the constitution in
FIG. 11
requires
two switching elements,
synchronizing triangular wave oscillating circuit
63
, and
PWM comparator
64
and the circuit is complicated.
The constitution in
FIG. 11
is also accompanied by
switching loss due to two switching elements, and
loss of damping circuit
66
and the efficiency is lowered.
The PWM operation by triangular wave can control in a wide range. However, it is complicated because it requires setting of maximum ON time and others.
An example of circuit using one switching element is shown in FIG.
12
.
In
FIG. 12
, one end of the primary winding
32
of the high voltage generating transformer
69
is connected to the drain of the switching element
27
. The deflecting circuit
62
issues a synchronizing signal of automatic frequency control (AFC) or the like to the synchronizing triangular wave oscillating circuit
63
. The oscillating circuit
63
issues a triangular wave for comparison. The detection of high voltage and error amplifier circuit are same as in FIG.
11
. Similarly, the output of the PWM comparator
64
drives the gate of the switching element
27
through the drive circuit
65
. The control operation for stabilization of high voltage is same as in FIG.
11
. However, the high voltage generating mode by switching is a discontinuous mode operation by a step-up converter.
The circuit in
FIG. 12
is simple, using only one switching element. However, when the switch
27
is turned off, the circuit in
FIG. 12
generates a flyback pulse. The high voltage winding supplies a high voltage in this OFF period (Toff). At this time, owing to the inductance of the primary winding
32
, distributed capacity of high voltage winding, and capacity of the switching element
27
, a violent oscillating voltage is generated as shown in FIG.
13
. As a result, when the switch
27
is turned on, a high peak current flows instantly if the drain voltage is at the peak of the oscillating voltage.
This phenomenon causes image noise or adverse effects on control operation (oscillation, malfunction). The circuit in
FIG. 12
has such defects.
Also in the circuit in
FIG. 12
,
the stress to the switching element
27
is large,
the loss increases,
the oscillating current when turning on the switch is large, and
image noise or malfunction may be caused.
To prevent these defects, the damping circuit
66
is connected to the primary circuit. However, its loss is as much as several watts.
Therefore, when the damping circuit
66
is connected to the primary circuit,
the efficiency is lowered,
large parts must be used, and it is less economical,
additional circuits are required, such as synchronizing triangular wave oscillating circuit
63
and PWM comparator
64
, and
setting of maximum ON time is complicated. They are same as explained in FIG.
11
.
FIG. 14
shows a prior art of a deflecting and high voltage integrated circuit. This is an explanatory diagram of a conventional deflecting high voltage generating circuit. In the case of this circuit, a first high voltage generating transformer
60
is installed at one leg of a U-shaped core
53
. In order to minimize the effects of high voltage fluctuations

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