Patent Publication Number: US-2007101176-A1

Title: Power switch circuit and power supply system using the same

Description:
1. Field of the Invention  
      The invention relates to power supply systems, and particularly to a power supply system with a power switch circuit.  
      2. Description of Related Art  
      Nowadays, central office terminals (COTs), such as asymmetrical digital subscriber loops (ADSLs), used in network communications require continuous power supply systems to ensure reliable operation. Therefore, most of the COTs have a main power supply and a backup power supply. When the main power supply becomes abnormal, the backup power supply starts up to provide power to the COTs.  
       FIG. 4  is a block diagram of an application environment of a conventional power switch circuit  40 . A direct current (DC) power source  30  is a main power supply, and an alternating current (AC) power source  10  and an adaptor  20  constitute a backup power supply. When the main power supply operates normally, the DC power source  30  outputs a DC signal Vout 1  to a COT  50  via a diode D 2 . When the main power supply operates abnormally, the power switch circuit  40  switches from the main power supply to the backup power supply. Therefore, the adaptor  20  converts an AC signal received from the AC power source  10  to another DC signal Vout 2  to be transmitted to the COT  50  via a diode D 1 . The diodes D 1  and D 2  protect current of the power switch circuit  40  and the COT  50  from flowing back to the adaptor  20  and the DC power source  30 .  
       FIG. 5  is a block diagram of the conventional power switch circuit  40 . The power switch circuit  40  includes a voltage divider circuit  41 , a reference voltage circuit  42 , a compare circuit  43 , and a switch circuit  44 . The voltage divider circuit  41  divides the DC signal Vout 1  output from the DC power source  30 , and generates a divided voltage to the compare circuit  43 . The reference voltage circuit  42  generates a reference voltage to the compare circuit  43 . The reference voltage is the minimum voltage of the COT  50  for normal operation. The compare circuit  43  compares the reference voltage with the divided voltage. If the divided voltage is greater than the reference voltage, the switch circuit  44  remains off. No signal is output to the adaptor  20 . Therefore, the COT  50  is powered by the DC power source  30 . If the divided voltage is less than the reference voltage, the switch circuit  44  is switched on. Therefore, the COT  50  is powered by the backup power supply.  
      The conventional power switch circuit  40  has only a preset reference voltage, for example, 35V. When the DC signal Vout 1  is less than 35V, the power switch circuit  40  switches from the main power supply to the backup power supply. If the DC power source  30  outputs a fluctuating DC signal Vout 1 , for example, the DC signal Vout 1  fluctuates between 34V and 36V, the power switch circuit  40  correspondingly switches between the main power supply and the backup power supply. As a result, the COT  50  has unstable power supply, thereby shortening the lifetime of the adaptor  20 .  
     SUMMARY OF INVENTION  
      An exemplary embodiment of the invention provides a power switch circuit for switching from one power source to another. The power switch circuit includes a voltage divider circuit, a first reference voltage circuit, a second reference voltage circuit, a first compare circuit, a second compare circuit, a synthesizing circuit, and a switch circuit. The voltage divider circuit generates a divided voltage according to a received signal. The first reference voltage circuit and the second reference voltage circuit respectively generate a first reference voltage and a second reference voltage. The first compare circuit, connected to the voltage divider circuit and the first reference voltage circuit, compares the first reference voltage with the divided voltage and generates a first comparison result. The second compare circuit, connected to the voltage divider circuit and the second reference circuit, compares the second reference voltage with the divided voltage and generates a second comparison result. The synthesizing circuit, connected to the first compare circuit and the second compare circuit, synthesizes the first comparison result and the second comparison result, and generates a synthesized signal. The switch circuit is connected to the synthesizing circuit, and switches on/off according to the synthesized signal.  
      Another exemplary embodiment of the invention provides a power supply system for supplying power to a central office terminal (COT). The power supply system includes a direct current (DC) power source, an alternating current (AC) power source, an adaptor, and a power switch circuit. The DC power source provides a power supply to the COT. The adaptor is connected between the AC power source and the COT, for converting a received AC signal to another DC signal to be transmitted to the COT. The power switch circuit is connected between the DC power source and the adaptor, for switching from a power source to another power source. The power switch circuit includes a voltage divider circuit, a first reference voltage circuit, a second reference voltage circuit, a first compare circuit, a second compare circuit, a synthesizing circuit, and a switch circuit. The voltage divider circuit generates a divided voltage according to a received signal. The first reference voltage circuit and the second reference voltage circuit respectively generate a first reference voltage and a second reference voltage. The first compare circuit, connected to the voltage divider circuit and the first reference voltage circuit, compares the first reference voltage with the divided voltage and generates a first comparison result. The second compare circuit, connected to the voltage divider circuit and the second reference circuit, compares the second reference voltage with the divided voltage and generates a second comparison result. The synthesizing circuit, connected to the first compare circuit and the second compare circuit, synthesizes the first compared result and the second compared result and generates a synthesized signal. The switch circuit is connected to the synthesizing circuit, and switches on/off according to the synthesized signal.  
      Other advantages and novel features will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a block diagram of an application environment of a power supply system of an exemplary embodiment of the present invention, the power supply including a power switch circuit;  
       FIG. 2  is a block diagram of the power switch circuit shown in  FIG. 1 ;  
       FIG. 3  is a circuit diagram illustrating details of the power switch circuit shown in  FIG. 1 ;  
       FIG. 4  is a block diagram of an application environment of a conventional power switch circuit; and  
       FIG. 5  is a block diagram of the conventional power switch circuit shown in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  is a block diagram of an application environment of a power supply system of an exemplary embodiment of the present invention. The power supply system includes an alternating current (AC) power source  100 , an adaptor  200 , a direct current (DC) power source  300 , a power switch circuit  400 , and a central office terminal (COT)  500 .  
      In the exemplary embodiment, a main power supply is the DC power source  300 , and a backup power supply includes the AC power source  100  and the adaptor  200 . When the main power supply operates normally, the DC power source  300  outputs a DC signal Vout 10  to the COT  500  via a diode D 20 . When the main power supply operates abnormally, the power switch circuit  400  switches from the main power supply to the backup power supply. Then, the adaptor  200  converts an AC signal received from the AC power source  100  to another DC signal Vout 20  transmitted to the COT  500  via a diode D 10 .  
      In the exemplary embodiment, a normal working voltage of the COT  500 , for example, is 48V, and the minimum working voltage of the COT  500 , for example, is 35V. That is, when a DC signal output from the DC power source  300  is less than 35V, the power switch circuit  400  switches from a main power supply to a backup power supply.  
       FIG. 2  is a block diagram of the power switch circuit  400  of an exemplary embodiment of the invention. The power switch circuit  400  includes a voltage divider circuit  401 , a first reference voltage circuit  402 , a second reference voltage circuit  403 , a first compare circuit  404 , a second compare circuit  405 , a synthesizing circuit  406 , and a switch circuit  407 .  
      The voltage divider circuit  401  generates a divided voltage according to a received DC power signal Vout 10 . The first reference voltage circuit  402  and the second reference voltage circuit  403  generate a first reference voltage and a second reference voltage, respectively. The first compare circuit  404  compares the divided voltage with the first reference voltage, and outputs a first comparison result to the synthesizing circuit  406 . The second compare circuit  405  compares the divided voltage with the second reference voltage, and outputs a second comparison result to the synthesizing circuit  406 .  
      The synthesizing circuit  406  synthesizes the first compared result and the second compared result and generates a synthesized signal. The switch circuit  407  is switched on/off according to the synthesized signal. Therefore, the power switch circuit  400  can switch from the main power supply to the backup power supply. In the exemplary embodiment, the first reference voltage is 6V, and the second reference voltage is 5V.  
       FIG. 3  is a circuit diagram illustrating details of the power switch circuit  400  as shown in  FIG. 1 . The first compare circuit  404  includes a first comparator A 1  having a first input end, a second input end and an output end A. In the exemplary embodiment, the first input end of the first comparator A 1  is positive, and is electrically connected to the first reference voltage circuit  402 . The second input end of the first comparator A 1  is negative, and is electrically connected to the voltage divider circuit  401 .  
      The second compare circuit  405  includes a second comparator A 2  having a first input end, a second input end, and an output end B. In the exemplary embodiment, the first input end of the second comparator A 2  is positive, and is electrically connected to the voltage divider circuit  401 . The second input end of the second comparator A 2  is negative, and is electrically connected to the second reference voltage circuit  403 .  
      The synthesizing circuit  406  includes a first NAND gate N 1  and a second NAND gate N 2 , which respectively include a first input end, a second input end, and an output end. The first input end of the first NAND gate N 1  is connected to the output end A of the first comparator A 1 , for receiving the first comparison result of the first compare circuit  404 . The second input end of the first NAND gate N 1  is connected to the output end Qn+1′ of the second NAND gate N 2 . The first input end of the second NAND gate N 2  is connected to the output end B of the second comparator A 2 , for receiving the second comparison result of the second compare circuit  405 . The second input end of the second NAND gate N 2  is connected to the output end Qn+1 of the first NAND gate N 1 .  
      The switch circuit  407  includes a resistor R and a switch component M 1 . The switch component M 1  has an input end, a first output end, and a second output end. In the exemplary embodiment, the switch component M 1  is a metallic oxide semiconductor field effect transistor (MOSFET). The input end of the MOSFET M 1  is a gate. The first output end of the MOSFET M 1  is a drain. The second output end of the MOSFET M 1  is a source. The gate of the MOSFET M 1  is connected to the output end Qn+1 of the first NAND gate N 1 . The drain of the MOSFET M 1  is connected to a power source Vcc via the resistor R, and the source of the MOSFET M 1  is grounded. In addition, the drain of the MOSFET M 1  outputs a signal Vc to the adaptor  200 .  
      In the exemplary embodiment, the divided voltage is one seventh of DC power signal Vout 10 . The first NAND gate N 1  and the second NAND gate N 2  of the synthesizing circuit  406  operates based on a following truth table:  
      When the DC power signal output from the DC power source  300  is 48V, the divided voltage of the voltage divider circuit  401  is greater than the first reference voltage 6V and the second reference voltage 5V. Therefore, the output end A of the first comparator A 1  outputs a logic low level 0, and the output end B of the second comparator A 2  outputs a logic high level 1. According to the above truth table, the output end Qn+1 of the first NAND gate N 1  outputs a logic high level 1, and the output end Qn+1′ of the second NAND gate N 2  outputs a logic low level 0. As a result, the MOSFET M 1  switches on, and a voltage signal output from the drain of the MOSFET M 1  is 0 such that no signal is transmitted to the adaptor  200 . Therefore, the COT  500  is powered by the main power supply, not by the backup power supply.  
      When the DC power signal output from the DC power source  300  drops from 48V to a value below 42V, for example, when the DC voltage output is 38V, the divided voltage of the voltage divider circuit  401  is less than the first reference voltage 6V, but greater than the second reference voltage 5V. Therefore, the output end A of the first comparator A 1  outputs a logic high level 1, and the output end B of the second comparator A 2  also outputs a logic high level 1. According to the above truth table, a logic level output from the output end Qn+1 of the first NAND gate N 1  is a logic high level 1. A logic level output from the output end Qn+1′ of the second NAND gate N 2  is a low voltage level 1. As a result, the MOSFET M 1  switches on, and a voltage output from the drain of the MOSFET M 1  is 0. Therefore, the COT  500  is powered by the main power supply, not by the backup power supply.  
      When the DC power signal output from the DC power source  300  drops from 42V to a value below 35V, for example, when the DC voltage output is 32V, which is the minimum working voltage of the COT  500 . The divided voltage of the voltage divider circuit  401  is less than the first reference voltage 6V and the second reference voltage 5V. Therefore, the output end A of the first comparator A 1  outputs a logic high level 1, and the output end B of the second comparator A 2  outputs a logic low level 0. According to the above truth table, the output end Qn+1 of the first NAND gate N 1  outputs a logic low level 0, and the output end Qn+1′ of the second NAND gate N 2  outputs a logic high level 1. As a result, the MOSFET M 1  switches off, and a voltage output from the drain of the MOSFET M 1  is Vcc. Therefore, the COT  500  is powered by the backup power supply.  
      When the DC power signal outputted from the DC power source  300  rises from the 32V to 38V, the divided voltage of the voltage divider circuit  401  is less than the first reference voltage 6V, but greater than the second reference voltage 5V. Therefore, the output end A of the first comparator A 1  outputs a logic high level 1, and the output end B of the second comparator A 2  also outputs a logic high level 1. According to the above truth table, a logic level output from the output end Qn+1 of the first NAND gate N 1  is a logic low level 0. A logic level output from the output end Qn+1′ of the second NAND gate N 2  is a logic high level 1. As a result, the MOSFET M 1  switches off, and a voltage output from the drain of the MOSFET M 1  is Vcc. Therefore, the COT  500  is powered by the backup power supply.  
      When the DC voltage output from the DC power source  300  is rises from 38V to a value above 42V, for example, 48V, the divided voltage of the voltage divider circuit  401  is greater than the first reference voltage 6V and the second reference voltage 5V. Therefore, the output end A of the first comparator A 1  outputs a logic low voltage level 0, and the output end B of the second comparator A 2  outputs a logic high voltage level 1. According to the above truth table, the output end Qn+1 of the first NAND gate N 1  outputs a logic high level 1, and the output end Qn+1′ of the second NAND gate N 2  outputs a logic low level 1. As a result, the MOSFET M 1  switches on, and a voltage output from the drain of the MOSFET M 1  is 0. Therefore, the COT  500  is powered by the main power supply, not by the backup power supply.  
      In the exemplary embodiment, the DC voltage output from the DC power source  300  is divided into three ranges by the power switch circuit  400 . The first voltage range is when the DC voltage is greater than 42V. The second voltage range is when the DC voltage is between 35V and 42V. The third voltage range is when the DC voltage is less than 35V. The second voltage range is a redundancy range of the power switch circuit  400 . That is, when the DC voltage output from the DC power source  300  drops from the second voltage range to the third voltage range, the COT  500  is powered by the backup power supply. When the DC voltage output from the DC power source  300  rises from the third voltage range to the second voltage range, the COT  500  is also powered by the backup power supply, not by the main power supply.  
      In the present invention, the power switch circuit  400  not only ensures stability of a circuit, but also ensures operational reliability of the COT  500 .  
      While embodiments and methods of the present invention have been described above, it should be understood that they have been presented by way of example only and not by way of limitation. Thus the breadth and scope of the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.