Abstract:
A hot-swap device preventing the crashing of a system which has two power sources when performing power swapping operation between the two power sources by continuously providing spare-power to the system during the switching period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a switching device. More specifically, it relates to a hot-swap device allowing on-line switching of the power source of a system without causing the system to crash. 
     2. Description of the Related Art 
     A relay is a popular switching devices applied to perform switching operation. However, the relay can not perform a switching operation without introducing a time delay because of its mechanism and structure. Therefore, electronic systems using relays for switching operation may crash due to the switching delay of the relays. For example, using a relay to switch one battery set to the other battery set supplying power to a notebook computer could make the notebook computer crash. 
     FIG. 1 schematically shows a conventional swapping circuit for switching two battery sets installed in a notebook computer. Referring to FIG. 1, one of the two batter sets  12  and  14  is selected by a relay  10  to supply power to a notebook computer  16 . For example, the battery set  12  first supplies power to the notebook computer  16 . When the battery set  12  is going to run out of its power and users want to replace the battery set  12  with a new one without shutting down the notebook computer  16 , the power supplied to the notebook computer  16  is swapped form the battery set  12  to the battery set  14  via the control of the relay  10 . Then users can take out the battery set  12  and install a new one. When the relay  10  performs the switching operation, the metal rod or slice  10 P (in FIG. 1) moves from the terminal A to the terminal B so that the notebook computer  16  obtains power from the battery set  14  instead of the battery set  12 . However, the metal rod or slice  10 P does not move from terminal A to terminal B instantly; some switching delay is inevitable. Therefore, the notebook computer  16  does not obtain steady power during the period when the metal rod or slice  10 P moves from terminal A to terminal B. Accordingly, the notebook computer may crash. Moreover, the more aged the relay  10 , the longer the switching delay is, and the possibility of the notebook computer crashing is increased. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a hot-swap device that prevents a system having two power sources from crashing when performing power swapping operation between the two power sources by continuously providing spare-power to the system during the switching period. 
     The present invention achieves the above-indicated objects by providing a hot-swap device comprising: a selection device for selecting a first power source or a second power source to feed power to the system; and a spare-power generating device for continuously feeding spare power to the system during the period when the selection device cuts the connection from the first power source to the system and before the selection device feeds the second power source to the system. 
    
    
     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 schematically shows a conventional swapping circuit for switching two battery sets installed in a notebook computer; and 
     FIG. 2 schematically illustrates a circuit diagram of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates an embodiment of a hot-swap device according to the present invention. In this embodiment, the hot-swap device is applied to switching two battery sets installed in a notebook computer system. 
     Referring to FIG. 2, the hot-swap device at least comprises a relay  20 , and a spare-power generating device  28 . The relay  20  serves as a selection device for coupling a first battery set  22  or a second battery set  24  to a notebook computer  26  according to the control of a selecting signal (not shown). The spare-power generating device  28  receives a first and second divided voltages (V 1  and V 2 ) of the second battery set  24 , and continuously outputs a first voltage (V s1 ) or a second voltage (V s2 ) to the output of the relay  20  in response to the level variation of the second battery set  24 . In this embodiment, the levels of the first and second voltages (V s1  and V s2 ) equal those of the first and second divided voltages (V 1  and V 2 ) respectively. 
     The spare-power generating device  28  comprises: a first comparator  28   a  for comparing the first divided voltage V 1  with a reference voltage V ref ; a second comparator  28   b  for comparing the second divided voltage V 2  with the reference voltage V ref ; a MOS transistor N 1  serving as a first switch with its input (drain) coupling to the first voltage (V s1 ); a MOS transistor N 2  serving as a second switch with its input (drain) coupling to the second voltage (V s2 ); and a logic-control device  28   c , receiving the outputs of the first and second comparators ( 28   a,    28   b ). 
     The reference voltage (V ref ) is set to the minimal voltage required by the notebook computer  26  for normal operation. In this embodiment, the logic-control device  28   c  is an AND gate with one input connecting to a NOT gate  28   d ; the diodes D 1  and D 2  connecting to the MOS transistors (N 1 , N 2 ) are used as isolation devices. 
     When the second divided voltage V 2  is greater than the reference voltage V ref , the second comparator  28   b  outputs a signal with high logic level “H”, thereby turning on the MOS transistor N 2 . Therefore, the second voltage V s2 , appears at terminal F. 
     When the second divided voltage V 2  is less than the reference voltage V ref  and the first divided voltage V 1  is greater than the reference voltage V ref , the second comparator  28   b  outputs a signal with low logic level “L” and the first comparator  28   a  outputs a signal with high logic level “H”; and the logic-control device  28   c  outputs a signal with high logic level “H”, thereby turning on the MOS transistor N 1 . Consequently, the first voltage V s1  appears at terminal E. 
     In this embodiment, the first battery set  22  comprises 9 battery units connected in series, and the voltage specification of the first battery set  22  is about 10.8 V. The second battery set  24  comprises 14 battery units connected in series, and the voltage specification of the second battery set  22  is about 16.8 V. The reference voltage V ref  is set to be less than 8.4 V. Furthermore, the voltage level of the second divided voltage V 2  equals those of 7 battery units connected in serial in the second battery set  24 ; and the voltage level of the first divided voltage V 1  equals that of 8 battery units connected in serial in the second battery set  24 . 
     If the second battery set  24  has sufficient power capacity (the voltage level of the second battery set  24  equals to 16.8 V), then the second divided voltage V 2  (8.4 V) is greater than the reference voltage V ref . Therefore, the second comparator  28   b  outputs a signal with high logic level “H”, and makes the MOS transistor N 2  turn on. Therefore, the second voltage V s2  appears at terminal F. 
     When the relay  20  switches the power source supplied to the notebook computer  26  from the first battery set  22  to the second battery set  24 , terminal G is in a floating state, and the voltage at terminal G will be less than the minimal voltage required by the notebook computer  26  for normal operation during the switching period (or delay). During the switching period (or delay), the second voltage V s2  at terminal F will appear at the terminal G through the diode D 2 , thereby providing the second voltage (V s2 =8.4 V) to the notebook computer  26 . When the second battery set  24  (with voltage level 16.8 V) is fed to the notebook computer  26  through the relay  20 , the diode D 2  is turned off and isolates the terminals F and G. 
     Consequently, because the notebook computer  26  is always coupled to the second voltage V s2  during the switching delay of the relay  20 , users can switch the power source supplied to the notebook computer  26  from the first battery set  22  to the second battery set  24  without shutting down the notebook computer and without crashing the notebook computer  26 . 
     Once the second battery set  24  does not have sufficient power capacity (the voltage level of the second battery set  24  is less than 16.8 V), the second divided voltage V 2  is less than the reference voltage (and 8.4 V), while the voltage level of the first divided voltage V 1  is less than 9.6 V but still greater than the reference voltage (and 8.4 V). Under this condition, the second comparator  28   b  outputs a signal with low logic level “L” and the first comparator  28   a  outputs a signal with high logic level “H”; the logic-control device  28   c  outputs a signal with high logic level “H”, thereby turning on the MOS transistor N 1 . Consequently, the first voltage V s1 , appears at terminal E. 
     Similarly, when the relay  20  switches the power source supplied to the notebook computer  26  from the first battery set  22  to the second battery set  24 , terminal G is in a floating state, and the voltage at terminal G will be less than the minimal voltage required by the notebook computer  26  for normal operation during the switching period (or delay). During the switching period (or delay), the first voltage V s1  at terminal E will appears at the terminal G through the diode D 1 , thereby providing the first voltage (9.6V&gt;V s1 &gt;8.4V) to the notebook computer  26 . When the second battery set  24  is fed to the notebook computer  26  through the relay  20 , the diode D 1  is turned off and isolates the terminals E and G. 
     Consequently, because the notebook computer  26  is always coupled to the first voltage V s1  during the switching delay of the relay  20 , users can switch the power source supplied to the notebook computer  26  from the first battery set  22  to the second battery set  24  without shutting down the notebook computer and without introducing any crash to the notebook computer  26 . 
     In addition, when the relay  20  switches the power source supplied to the notebook computer  26  from the second battery set  24  to the first battery set  22 , the notebook computer  26  will not crash because the notebook computer  26  is always coupled to the first voltage V s1  or the second voltage V s2  during the switching delay of the relay  20 . 
     The hot-swap device described of the present invention overcomes the switching delay problems of a relay. However, the application of the hot-swap device is not limited to this. The present invention can be applied to any kind of power switching device subject to switch delay problems to prevent a system crash. 
     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.