Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2003-04-09
2004-11-02
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C361S091700, C345S211000
Reexamination Certificate
active
06813172
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply circuit for a video display device, and more particularly, to a power supply circuit capable of improving transient state characteristics of a TVS (Transient Voltage Suppressor) diode that is used in an inner snubber circuit unit. The present application is based on Korean Patent Application No. 2002-34977 filed on Jun. 21, 2002, which is incorporated herein by reference.
2. Description of the Prior Art
A video display device such as a television requires direct current (DC) power for driving inner components. For example, during a scanning with an up-and-down and left-to-right sweep of an electron beam projected from an electron beam gun, the DC power is used to deflect the electron beam projected from the electron beam gun. In order to obtain the DC power, a power supply device is used to smooth and rectify an alternating current (AC) received from an external source.
FIG. 1
is a circuit diagram showing a power supply device for a conventional video display device.
As shown in the circuit diagram, a power supply circuit comprises a rectifier circuit unit
10
, a switching circuit unit
20
, a power transformer
30
, an output circuit unit
40
, and a snubber circuit unit
50
.
The rectifier circuit unit
10
rectifies an AC power externally inputted by using diodes D
1
~
4
for a rectification and a capacitor C
1
. The voltage rectified by the rectifier circuit unit
10
is supplied to a primary coil of the power transformer
30
. The power transformer
30
induces a voltage at a secondary coil by using an interaction occurring between the primary and secondary coils. At this time, the switching circuit unit
20
switches on/off the flow of the current along the primary coil of the power transformer
30
, thereby controlling the voltage to be induced at the secondary coil of the power transformer
30
. The output circuit unit
40
rectifies and smoothes the voltage induced at the secondary coil of the power transformer
30
to obtain a DC power.
The snubber circuit unit
50
consists of a residual current device (RCD) snubber circuit and first and second TVS (Transient Voltage Suppressor) diodes TVS
1
and TVS
2
serially connected to each other. When the switching circuit unit
20
switches on/off the current of the primary coil of the power transformer
30
, the snubber circuit unit
50
absorbs and removes a transient voltage in a reverse direction that is generated at the power transformer
30
. The first and second TVS diodes TVS
1
and TVS
2
are elements for protecting a weak circuit in an electrically transient state that is caused due to the electrostatic discharge, the inductive load switching or the induced lightning.
However, the first and second TVS diodes TVS
1
and TVS
2
used in the snubber circuit unit
50
have different levels of inner capacitance because a semiconductor fabrication does not allow a complete uniformity of the products. Due to the different levels of capacitance, the voltages supplied to the respective first and second TVS diodes TVS
1
and TVS
2
are unequal.
FIG. 2
shows waveforms of the voltages supplied to the first and second TVS diodes TVS
1
and TVS
2
.
Referring to
FIG. 2
, a waveform A represents total voltages that are supplied to both ends of the first and second TVS diodes TVS
1
and TVS
2
serially connected to each other, with the maximum value of 259V. A waveform Ref
1
represents a voltage that is supplied to both ends of the second TVS diode TVS
2
, with the maximum value of 213V. A waveform Ref
2
represents a voltage that is supplied to the first TVS diode TVS
1
, with the maximum value of 48.5.
As shown in
FIG. 2
, there is a difference between levels of the voltages that are supplied to the first TVS diode TVS
1
and the second TVS diode TVS
2
. This difference is caused by the different inner capacitances as described above.
When the voltages are not equally supplied to the first and second TVS diodes TVS
1
and TVS
2
serially connected to each other, one of the first and second TVS diodes TVS
1
and TVS
2
may be supplied with a transient voltage exceeding a rated voltage. Such a transient voltage exceeding the rated voltage causes a burnt phenomenon such that the TVS diode is damaged and has its lifespan reduced. Accordingly, abnormality of the snubber circuit unit
50
is incurred and thus the total power supply circuit cannot be protected from the transient voltage.
SUMMARY OF THE INVENTION
The present invention has been developed in order to solve the above problems in the related art. Accordingly, an object of the present invention is to provide a power supply circuit for a video display device capable of performing a stable operation by improving transient state characteristics of respective TVS diodes used in a snubber circuit unit.
The foregoing object is realized by providing a power supply circuit for a video display device comprising: a power transformer for inducing a voltage with respect to an input voltage by using an interaction occurring between a primary coil and a secondary coil; a switching circuit unit for controlling the voltage to be induced at the secondary coil of the power transformer by switching on/off a current flowing along the primary coil of the power transformer; first and second TVS diodes serially connected to each other, for absorbing a transient voltage in a reverse direction that is supplied through the primary coil of the power transformer; first and second resistances parallel connected to the respective first and second TVS diodes; a capacitor parallel connected to both ends of the first and second TVS diodes connected to each other and being charged with the transient voltage in the reverse direction that is supplied through the primary coil of the power transformer; and a diode for forming a passage of current in one direction when the capacitor is charged. It is preferred that the first and second resistances have the same resistance values.
Preferably, the power supply circuit further comprises a rectifier circuit unit for rectifying an input AC power and supplying the rectified power to the primary coil of the power transformer, and an output circuit unit for rectifying and smoothing the voltage induced at the secondary coil of the power transformer.
It is preferred that the switching circuit unit comprises: a field effect transistor for switching on/off the current flowing along the primary coil of the power transformer; and a controller for controlling an on/off switching operation of the field effect transistor.
Also, it is preferred that a bead-core is connected between the field effect transistor and the primary coil of the power transformer, for removing noise.
REFERENCES:
patent: 4550359 (1985-10-01), West
patent: 6301132 (2001-10-01), Vandelac
patent: 6606259 (2003-08-01), Cohen
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