Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-03-15
2002-05-14
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06388901
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power supply and, more particularly, to a power supply suitable for display devices.
2. Description of the Related Art
Conventionally, various types of discharge-type display devices have been developed such as plasma display panels (hereinafter referred to as PDPS). It is necessary to supply at least two types of voltages to the discharge-type display device. The two types of voltages are a drive voltage of several tens to several ten thousands of volts for turning on the display elements and a control voltage of several volts employed as the power supply voltage for a control circuit of the display elements.
In the conventional power supply which outputs such different voltages at the same time, when the power supply output is shut down, the drive voltage is first disabled and then the control voltage is disabled in sequence. This sequence is required for the following reason. If the control voltage is first disabled prior to the drive voltage at the time of shutting down the power supply output, a high voltage would be applied to control and display ICs. This would present problems of causing the ICs to be damaged or unnecessary images to appear on the display device.
In Japanese Patent Laid-Open Publication No.Hei 4-91624, disclosed is a conventional power supply which outputs such different voltages at the same time (a first conventional example).
FIG. 1
is a circuit block diagram illustrating the configuration of the power supply of the first prior art. A power supply
109
shown in
FIG. 1
includes a rectifier circuit
102
connected to an external commercial power source
100
; switches
104
a
,
104
b
disposed between the commercial power source
100
and the rectifier circuit
102
; a capacitor
105
connected to the output of the rectifier circuit
102
; and a DC/DC converter
101
, connected to the commercial power source
100
via the rectifier circuit
102
, for generating output voltages of +5V and +35V. The power supply
109
also includes an output terminal
106
a
, connected to the output of the DC/DC converter
101
, for providing an output voltage of +5V; an output terminal
106
b
, connected to the output of the DC/DC converter
101
, for providing an output voltage of +35V; an optical coupling isolator
103
for outputting a signal in accordance with the input voltage of the DC/DC converter
101
; and a switching circuit
107
connected to the output of the optical coupling isolator
103
and including a photo MOS relay
108
which is opened or closed in accordance with a signal outputted from the optical coupling isolator
103
. The switching circuit
107
is connected between the DC/DC converter
101
and the output terminal
106
b
to open or close the connection between the DC/DC converter
101
and the output terminal
106
b.
In the power supply of the first prior art, the optical coupling isolator
103
detects the input voltage of the DC/DC converter
101
. Shutting down the power supply output from the commercial power source
100
will cause the optical coupling isolator
103
to output a signal to the photo MOS relay
108
. This will cause the photo MOS relay
108
to be opened forcedly, the switching circuit
107
to be opened, and the voltage of 35V to be shut down which needs to be disabled quickly. This provides the aforementioned sequence of disabling first the drive voltage and then disabling the control voltage.
In Japanese Patent Laid-Open Publication No.Hei 7-104711, disclosed is a power supply for an LCD (Liquid Crystal Display) (second prior art).
FIG. 2
is a circuit block diagram illustrating the configuration of the LCD incorporating the power supply of the second prior art. The LCD shown in
FIG. 2
includes batteries
211
, a DC/DC converter
214
for converting the output voltage of the batteries
211
to a control voltage Vcc, and a power switching transistor
212
connected between the batteries
211
and the DC/DC converter
214
. The LCD also includes a power control IC
213
connected to the base of the power switching transistor
212
via a resistor R
204
; a timer
215
connected to the power control IC
213
; an inverter
216
connected to the timer
215
; and an AND gate
217
, the input of which is connected to the inverter
216
and a delay device
201
A and the output of which is connected to the base of an LCD power front-stage transistor
219
via a resistor R
205
. The LCD also includes an LCD power rear-stage transistor
220
, the collector of which is connected to the power switching transistor
212
and the base of which is connected to the collector of the LCD power front-stage transistor
219
via a resistor R
206
; a device logic portion
201
E to which the control voltage Vcc is supplied from the DC/DC converter
214
: and a DC/DC converter
218
for converting the output voltage of the batteries
211
to a drive voltage Vee. The LCD further includes an LCD panel portion
201
F to which the drive voltage Vee is supplied from the DC/DC converter
218
; a delay device
201
A which is connected between the output terminal of the DC/DC converter
214
and the ground and which includes a resistor and a capacitor; and an FET
201
D driven by the delay device
201
A. The device logic portion
201
E is the control circuit of the LCD panel portion
201
F. In addition, the LCD has a capacitor C
201
connected to the DC/DC converter
218
. Furthermore, the LCD is provided with a resistor R
208
connected to the higher potential side of the FET
201
D in series therewith. Still furthermore, a resistor R
207
is connected between the collector of the LCD power front-stage transistor
219
and the FET
201
D.
In the power supply of the second prior art, when the power from the batteries
211
is shut down, the delay device
201
A causes the FET
201
D to conduct. This allows the drive voltage Vee to be disabled forcedly prior to the control voltage Vcc, thereby providing the aforementioned sequencing.
However, each of the aforementioned techniques present the problems described below. The power supply of the first prior art has a problem of requiring the employment of the optical coupling isolator
103
and the photo MOS relay
108
, thereby leading to an increase in cost of the power supply.
On the other hand, the power supply of the second prior art is a power circuit for an LCD display device and therefore supplies a drive voltage of several tens of volts. In contrast, a drive voltage of several hundreds to several ten thousands of volts is required for discharge-type display devices such as CRTs (Cathode Ray Tubes) or PDPs. In the power supply of the second prior art, such high voltages would be produced to cause the following problems.
FIG. 3
is a graph illustrating the time dependency of the output voltage of the power supply of the second prior art. As shown in
FIG. 3
, the drive voltage Vee and the control voltage Vcc increase together after the power output has been turned on and decrease together after the power has been turned off. At this time, the power supply of the second prior art allows the control voltage Vcc to be disabled in a short period of time. On the contrary, as shown in
FIG. 2
, the power supply has the capacitor C
201
of a large-capacity connected to the output terminal of the DC/DC converter
218
. This will not allow the power accumulated in the capacitor C
201
to be discharged quickly, causing the drive voltage Vee to be sustained at a high voltage for a long period of time. This causes the device logic portion
201
E to stop the operation thereof first, whereas a high voltage is kept being applied to the LCD panel portion
201
F. This causes the drive voltage Vee of a high voltage to be applied to the drive circuit of the inactivated LCD panel portion
201
F and to control circuits such as the gate circuit of the control IC of the device logic portion
201
E. When a high voltage is generated in the power supply of the second prior art to drive a discharge-type display devi
Hayes & Soloway P.C.
Nguyen Matthew
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