Abstract:
A power circuit of an electronic device which can reduce power consumption. Power is saved by turning off the voltage booster in the power-saving mode. A controller is used to generate a control signal indicating whether or not the electronic device is in a power-saving mode. A switch, coupled between the voltage booster and its power supply, is used for disconnecting the power supply from the voltage booster when the control signal indicates that the electronic device is in the power-saving mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a power-saving circuit for an electronic device. More specifically, the present invention relates to a power-saving circuit applied to the power supply circuit of a monitor. When the monitor is in a power-saving mode, this power-saving circuit can automatically turn off the power supply fed to the voltage booster of the monitor for reducing the power consumption of the monitor. 
     2. Description of the Related Art 
     In order to decrease wasted power, most computers in the current market can automatically detect usage/non-usage and enter a power-saving mode if required. Generally speaking, the power-saving mode can be further divided into several finer sub-modes. For the sake of clarity, however, only two modes, the normal mode and the power-saving mode, are discussed. 
     When a computer is set to enter the power-saving mode, the monitor of the computer is also notified. While in the power-saving mode, the monitor only provides power supply to essential circuitry that is used to continuously receive instructions from the computer and to quickly recover the normal mode when receiving the corresponding instruction. 
     Referring to FIG. 1 (Prior Art), an ordinary computer system includes a computer mainframe  20 , a monitor  28  and other accessories. Computer mainframe  20  can detect the usage condition and decide, according to the detection result, whether or not the computer system enters the power-saving mode. When deciding to enter the power-saving mode, computer mainframe  20  does not only enter into power-saving mode itself, but also notifies monitor  28  to enter the power-saving mode. 
     The modern monitor usually includes a voltage booster at the entry point of the external power supply for dealing with different voltage specifications. The voltage booster is mainly comprised of booster controllers, triac devices and other related components. In addition, the voltage booster can detect the alternative current (AC) voltage supplied by the external power supply and selectively boost the AC voltage according to the detection result. For example, suppose that the internal circuit of a monitor is designed for 200˜220 VAC. When detecting that the external power supply is 200˜220 VAC, the voltage booster can directly connect the external power supply to the internal circuit. When detecting that the external power supply is 100˜110 VAC, the voltage booster can boost the external power supply and then connect it to the internal circuit. Using the boosting scheme, this monitor can be used in various environments. 
     FIG. 2 (Prior Art) is a circuit diagram of the power supply circuit applied in the conventional monitor. As shown in FIG. 2, the power supply circuit includes a voltage booster  320 , a first rectifier  310 , a second rectifier  300  and a first power-saving circuit  370 . For the sake of clarity, FIG. 2 only illustrates a partial circuit that provides 15 VDC as a second rectifier  300  located in the secondary winding of transformer T 1 . In reality, there are several different circuits in the secondary winding for providing different voltages, such as −15V and 5V. IC 1  represents a booster controller chip, which is used to detect the amplitude of the external power supply and decide whether the triac chip IC 4  should perform the boosting operation. 
     Suppose that the monitor in FIG. 2 is designed for 200˜220 VAC. The booster controller chip IC 1  can be implemented by AVS1AC or AVS1BC developed by SGS-Thomson and the triac chip IC 4  can be implemented by AVS08CB developed by SGS-Thomson. Nodes AC 1  and AC 2  are connected to two terminals of the external AC power supply, respectively. When detecting that the external power supply is 200˜220 VAC, the booster controller chip IC 1  disables the triac chip IC 4 , and nodes A 1  and A 2  of the triac chip IC 4  are open-circuited. Therefore, the voltage on node A is about 200×{square root over (2)}˜220×{square root over (2)}. On the other hand, when detecting that the external power supply is 100˜110 VAC, the booster controller chip IC 1  enables the triac chip IC 4 , and nodes A 1  and A 2  of the triac chip IC 4  are close-circuited. Therefore, the voltage on node A can be boosted from the original voltage 100×{square root over (2)}˜110×{square root over (2)} to 200×{square root over (2)}˜220×{square root over (2)}. Using such scheme, the potential of node A can be maintained in 200×{square root over (2)}˜220×{square root over (2)} in different operating environments. Accordingly, the monitor can be normally operated. 
     A mode-detection chip IC 3  is used to receive instructions indicating the current mode from computer mainframe  20 . When receiving an instruction indicating the normal mode, the mode-detection chip IC 3  outputs a logic high signal to turn on transistor Q 3  and transistor Q 4 . Then second rectifier  300  can provide power to the internal circuit of the monitor  28 . When receiving an instruction indicating the power-saving mode, the mode-detection chip IC 3  outputs a logic low signal to turn off transistor Q 3  and transistor Q 4 . Then the second rectifier  300  stops providing power to the internal circuit of the monitor  28 . Therefore, the conventional monitor saves power by stopping the power supply connected to the secondary windings of the transformer T 1 . However, since the voltage booster  320  is directly connected to the external AC power supply, it is inevitable that the voltage booster  320  still consumes a certain amount of power. In fact, the power consumption of the voltage booster  320  is quite large. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a power-saving circuit applied to an electronic device, which can turn off the voltage booster in the power-saving mode, thereby reducing power consumption. 
     The present invention achieves the above-indicated objects by providing a power circuit for an electronic device. It comprises a power terminal for connecting to an external power source of the electronic device, a voltage booster that is connected to the power terminal and powered by a power supply for selectively boosting the received external power source, a controller for generating a control signal indicating whether or not the electronic device is in power-saving mode, and a switch for disconnecting the power supply from the voltage booster when the control signal indicates that the electronic device is in power-saving mode. The controller and the switch can be used as a power-saving circuit for controlling the power supply of the voltage booster. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 (Prior Art) is a diagram of an ordinary computer system; 
     FIG. 2 (Prior Art) is a circuit diagram of the power circuit used in the conventional monitor; 
     FIG. 3 is a circuit diagram of the power circuit used in the first embodiment of the present invention; and 
     FIG. 4 is a circuit diagram of the power circuit used in the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 3 is a circuit diagram of the power circuit used in the first embodiment. Compared with the circuit shown in FIG. 2, the power circuit in FIG. 3 further includes a second power-saving circuit  330 . The second power-saving circuit  330  has two transistors (Q 1  and Q 2 ), four resistors (R 3 ˜R 6 ) and a zener diode Z 1 . In this embodiment, the breakdown voltage of the zener diode Z 1  is about 15V. The function of the second power-saving circuit  330  is described as follows. 
     When receiving an instruction indicating the normal mode from computer mainframe  20 , the mode-detection chip IC 3  outputs a logic high signal to turn on transistor Q 3  and transistor Q 4 . Then second rectifier  300  provides power to the internal circuit of monitor  28 . In practice, the voltage on node B in the primary winding of the transformer T 1  is about 18V when the secondary winding of the transformer T 1  properly provides power. Since the voltage of node B, which is about 18V, is larger than the breakdown voltage of the zener diode Z 1 , transistors Q 1  and Q 2  are turned on in this case. Therefore, the external power supply provides power to the booster controller chip IC 1  via nodes AC 1  and AC 2 , and monitor  28  operates normally. 
     When receiving an instruction indicating the power-saving mode from computer mainframe  20 , the mode-detection chip IC 3  outputs a logic low signal to turn off transistor Q 3  and transistor Q 4 . Then second rectifier  300  stops providing power to the internal circuit of monitor  28 . Since there is a load reduction in the secondary winding of transformer T 1 , the voltage of node B at the primary winding is also reduced to 13V in practice. Since the voltage on the node B, which is about 13V, is smaller than the breakdown voltage of the zener diode Z 1 , transistors Q 1  and Q 2  are turned off in this case. Therefore, the external power supply is not coupled to the booster controller chip IC 1  via the nodes AC 1  and AC 2 . Accordingly, the booster controller chip IC 1  ceases to operate when in power-saving mode. 
     In this embodiment, the power-saving efficiency is improved since the booster controller chip IC 1  does not waste power in the power-saving mode. It is noted that the breakdown voltage of the zener diode Z 1  is a critical parameter in this embodiment. It is necessary that the zener diode Z 1  be turned on in the normal mode and off in the power-saving mode. 
     Second Embodiment 
     FIG. 4 is a circuit diagram of the power circuit used in the second embodiment. The power circuit shown in FIG. 4 is similar to that in FIG. 3, except that the zener diode Z 1  is replaced by a photo coupler  350 . The photo coupler  350  includes a photo emitter L 1  and a photo receiver Q 5 . Photo emitter L 1  is connected to the output terminal of the mode-detection chip IC 3 . Photo receiver Q 5  is powered by a reference voltage VREF (about 5V) of the rectifier chip IC 2 . The operation of the second power-saving circuit  340  is described as follows. 
     When receiving an instruction indicating the normal mode from computer mainframe  20 , the mode-detection chip IC 3  outputs a logic high signal to turn on transistor Q 3  and transistor Q 4 . Then second rectifier  300  provides power to the internal circuit of monitor  28 . In addition, the logic high signal is also transmitted to photo receiver Q 5  via the coupling operation of photo coupler  350 . Since the photo receiver Q 5  is turned on, transistors Q 1  and Q 2  are turned on in this case. Therefore, the external power supply provides power to the booster controller chip IC 1  via nodes AC 1  and AC 2 , and monitor  28  operates normally. 
     When receiving an instruction indicating the power-saving mode from computer mainframe  20 , the mode-detection chip IC 3  outputs a logic low signal to turn off transistor Q 3  and transistor Q 4 . Then the second rectifier  300  stops providing power to the internal circuit of monitor  28 . Since this logic low signal cannot turn on the photo emitter L 1 , photo receiver Q 5  and transistors Q 1  and Q 2  are turned off in this case. Therefore, the external power supply is not coupled to the booster controller chip IC 1  via nodes AC 1  and AC 2 . Accordingly, the booster controller chip IC 1  ceases to operate when in power-saving mode. 
     The advantage of the present invention is the stopping of the operation of the voltage booster when in power-saving mode. Therefore, the power-saving efficiency of the present invention is better than that of the prior art. In addition, the power-saving circuit of the present invention can be applied not only in the cathode ray tube (CRT) monitor, but also in many electronic devices that include voltage boosters and can be operated in power-saving mode, such as liquid crystal display (LCD) monitors, projectors and digital televisions. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.