Patent Application
20090153539
The present disclosure relates to a power supply circuit for a liquid crystal display (LCD).
An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Usually, the liquid crystal display device needs a power supply circuit to provide a working voltage.
Referring to
The first transistor 14 is a p-channel metal-oxide-semiconductor field-effect transistor (MOSFET). A gate electrode 141 of the first transistor 14 is connected to the five volt DC power supply 13 via the resistor 17. A source electrode 142 of the first transistor 14 is connected to the five volt DC power supply 13. A drain electrode 143 of the first transistor 14 is connected to the output terminal 12.
The second transistor 15 is a negative-positive-negative (NPN) bipolar transistor. A base electrode 151 of the second transistor 15 is connected to the control signal input terminal 11. An emitting electrode 152 of the second transistor 15 is connected to the gate electrode 141 of the first transistor 14. A collecting electrode 153 of the second transistor 15 is grounded.
A working principle of the power supply circuit 10 for the LCD is described as follows. When the LCD is connected up a commercial power, the five volt DC power supply 13 provides a five volt voltage to the source electrode 142 of the first transistor 14. When the LCD is powered on, an electric potential of the control signal input terminal 11 is a logic high electric potential. The second transistor 15 is switched on, and the gate electrode 141 of the first transistor 14 is grounded via the collecting electrode 153 and the emitting electrode 152 in turn. Therefore, the first transistor 14 is switched on, a five volt voltage of the five volt DC power supply 13 is provided to the output terminal 12 via the source electrode 142 and the drain electrode 143.
When the LCD is powered off, an electric potential of the control signal input terminal 11 is a logic low electric potential. The second transistor 15 is switched off. The five volt DC power supply 13 provides a voltage to the gate electrode 141 of the first transistor 14 via the resistor 17. Therefore, the first transistor 14 is switched off, and the five volt DC power supply 13 stops providing voltage for the output terminal 12.
When the first transistor 14 is switched on, and the five volt voltage is provided to the output terminal 12 via the activated first transistor 14, a rush current is generated at the moment that the first transistor 14 is switched on. The rush current may accelerate an aging process of electronic devices of the LCD. Thus a service life of the LCD is liable to be reduced.
Further, in case that the LCD is short-circuited, a high short circuit current passes through the first transistor 14. Thus, the first transistor 14 is liable to be destroyed. Thus the reliability of the power supply circuit 10 is low.
It is desired to provide a new power supply circuit used in an LCD which can overcome the above-described deficiencies.
In one exemplary embodiment, a power supply circuit includes an output terminal configured for proving electric power to a load circuit, a direct current (DC) power supply, a first resistor, a second resistor, a first switch and a second switch. The first switch includes a control electrode is grounded via the first resistor, a first current conducting electrode is connected to the DC power supply, and a second current conducting electrode is connected to the output terminal. The second switch includes a control electrode is connected to the output terminal, a first current conducting electrode is connected to the DC power supply, and a second current conducting electrode is connected to the control electrode of the first switch. The second resistor interconnects the first current conducting electrode of the first switch and the second current conducting electrode of the first switch.
Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
Reference will now be made to the drawings to describe preferred and exemplary embodiments of the present disclosure in detail.
Referring to
The first transistor 24 is a p-channel MOSFET. A gate electrode 241 of the first transistor 24 is grounded via the first resistor 27. A source electrode 242 of the first transistor 24 is connected to the five volt DC power supply 23. A drain electrode 243 of the first transistor 24 is connected to the output terminal 22, and is grounded via the filter capacitor 26. The second resistor 28 is interconnects the source electrode 242 and the drain electrode 243. The second resistor 28 can, for example, be a protective tube.
The second transistor 25 is a positive-negative-positive (PNP) bipolar transistor. A base electrode 251 of the second transistor 25 is connected to the output terminal 22. An emitting electrode 252 of the second transistor 25 is connected to the five volt DC power supply 23. A collecting electrode 253 of the second transistor 25 is connected to the gate electrode 241 of the first transistor 24.
A working principle of the power supply circuit 20 for the LCD is described as follows. When the LCD is connected up a commercial power, the five volt DC power supply 23 provides a five volt voltage to the source electrode 242 of the first transistor 24 and the emitting electrode 252 of the second transistor 25. Thus, a voltage difference between the emitting electrode 252 and the base electrode 251 is higher than threshold voltage of the second transistor 25. Thus the second transistor 25 is switched on, and the gate electrode 241 of the first transistor 24 is connected to the five volt DC power supply 23 via the collecting electrode 253 and the emitting electrode 252 in turn. Thus the first transistor 24 is switched off, and the filter capacitor 26 is charged by the five volt DC power supply 23 via the second resistor 28.
Along with the increase of charging time for the filter capacitor 26, a voltage of the output terminal 22 is increased gradually. Thus, a voltage difference between the emitting electrode 252 and the base electrode 251 is lower than the threshold voltage of the second transistor 25. Thus, the second transistor 25 is switched off, and the first transistor 24 is switched on. The five volt DC power supply 23 provides a voltage to the output terminal 22 via the source electrode 242 and the drain electrode in turn.
When an internal circuit (not shown) of the LCD is short-circuited, the second transistor 25 is switched on.
Because the filter capacitor 26 is charged by the five volt DC power supply 23 before the first transistor 24 is switched on, the voltage of the output terminal 22 is increased, and a voltage difference between the source electrode 242 and the drain electrode 243 of the first transistor 24 is decreased. Therefore, a rush current passed through the first transistor 24 is reduced at the moment that the first transistor 24 is switched on.
Furthermore, when an internal circuit of the LCD is short-circuited, the second transistor 25 is switched on. Thus, the first transistor 24 is switched off, and is protected from being destroyed. Therefore the reliability of the power supply circuit 20 is high.
Referring to
The third transistor 31 is an NPN bipolar transistor. A base electrode 311 of the third transistor 31 is connected to the control signal input terminal 305. An emitting electrode 312 of the third transistor 31 is grounded. A collecting electrode 313 of the third transistor 31 is connected to a gate electrode 341 of the first transistor 34 via a first transistor 37, and is connected to a five volt DC power supply 33 via the first resistor 37 and the third resistor 306 in turn.
When the LCD is powered on, an electric potential of the control signal input terminal 305 is a logic high electric potential. Thus, the third transistor 31 is switched on, and the gate electrode 341 of the first transistor 34 is grounded via the first resistor 37, the collecting electrode 313 and the emitting electrode 312 of the third transistor 31 in turn. Thus, the first transistor is switched on, the second transistor is switched off, and the five volt DC power supply 33 is provided to an output terminal 32 via the source electrode 342 and the drain electrode 343 of the first transistor 34.
When the LCD is powered off, an electric potential of the control signal input terminal 305 is a logic low electric potential. Thus, the third transistor 31 is switched off. The five volt DC power supply 33 provides a voltage to the gate electrode 341 of the first transistor 34 via the third resistor 306. Thus, the first transistor 34 is switched off, and the five volt DC power supply 33 stops providing voltage for the output terminal 32.
When an internal circuit of the LCD is short-circuited, the second transistor 35 is switched on. Thus, the first transistor 34 is switched off, and is protected from being destroyed.
Referring to
A working principle of the power supply circuit 40 for the LCD is described as follows. When the LCD is powered off, an electrical potential of a control signal input terminal 405 is a logic low electric potential. Thus, a third transistor is switched off. The five volt DC power supply 43 is connected to a gate electrode 441 of a first transistor 44 via a third resistor 406, and is connected to a gate electrode 451 of a fourth transistor 450 via the third resistor 406 and the fourth resistor 460 in turn. Thus, the first transistor 44 is switched off, and the fourth transistor 450 is switched on. The five volt DC power supply 43 is stops providing a voltage to the output terminal 42. At the same time, residual voltage of the LCD is quickly discharged through the fifth resistor 470.
Referring to
Referring to
In the above-described first embodiment, the five volt DC power supply of the first embodiment to the fifth embodiment can be changed according to a requirement. For example, the DC power supply of the power supply circuits of the first embodiment to the fifth embodiment provides a twelve volt DC voltage in case that the output terminals need higher voltages.
The first transistor 24 of the first embodiment can be a PNP bipolar transistor. A control electrode of the first transistor 24 is grounded via a first resistor. A first current conducting electrode of the first transistor 24 is connected to a five volt DC power supply. A second current conducting electrode of the first transistor 24 is connected to an output terminal, and is grounded via a filter capacitor.
The second transistor 25 of the first embodiment can be a p-channel MOSFET. A control electrode of the second transistor 25 is connected to an output terminal. A first current conducting electrode of the second transistor 25 is connected to a five volt DC power supply. A second current conducting electrode of the second transistor 25 is connected to a gate electrode of a first transistor.
The third transistor 31 of the second embodiment can be an n-channel MOSFET. A control electrode of the third transistor 31 is connected to a control signal input terminal. A first current conducting electrode of the third transistor 31 is grounded. A second current conducting electrode of the third transistor 31 is connected to a gate electrode of a first transistor via a first resistor, and is connected to a five volt DC power supply via the first resistor and a third resistor in turn.
The fourth transistor 450 of the third embodiment can be an NPN bipolar transistor. A control electrode of the fourth transistor 450 is connected to a gate electrode of a first transistor via a fourth resistor. A first current conducting electrode of the fourth transistor 450 is grounded. A second current conducting electrode of the fourth transistor 450 is connected, to an output terminal via a fifth resistor.
The power supply circuit of the fourth embodiment further includes a sixth resistor, a seventh resistor and a protective capacitor. The sixth resistor interconnects the five volt DC power supply and the control signal input terminal 505. The seventh resistor interconnects the control signal input terminal 505 and a base electrode of a third transistor. One terminal of the protective capacitor is grounded, and the other terminal is connected to the base electrode of the third transistor.
It is to be further understood that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of structures and functions associated with the embodiments, the disclosure is illustrative only, and changes may be made in detail (including in matters of arrangement of parts) within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200710124991.4 | Dec 2007 | DE | national |
