Patent Publication Number: US-2022231535-A1

Title: Offline uninterruptible power source and control method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to the field of electronic circuits, in particular to an offline uninterruptible power source and a control method therefor. 
     BACKGROUND ART 
     An offline uninterruptible power source directly supplies power from a mains supply to a load when a mains voltage is within a normal range, and a rechargeable battery is in a charging state; when the mains voltage is abnormal or power failure is caused, an inverter is switched into a working state to convert a direct current in the rechargeable battery into a stable alternating current to be output; and when the mains voltage is lower or higher, a transformer connected to alternating-current input end is used for adjusting the mains voltage, and thus, an alternating-current output end has a stable output voltage. 
       FIG. 1  is a circuit diagram of an offline uninterruptible power source in the prior art. As shown in  FIG. 1 , the offline uninterruptible power source  1  includes an electromagnetic compatibility (EMC) filter  11  connected between an alternating-current input end  10  and an alternating-current output end  10 ′ of the EMC filter  11 ; a safety switch  12 , which is configured as a double-pole single-throw relay or configured to include a safety switch unit  121  and a safety switch unit  122 ; a first switch  131 , a second switch  132  and an output switch  14 , which are successively connected between the safety switch  12  and a terminal L of the alternating-current output end  10 ′; a transformer Tr 1 , wherein one end T 1  of a primary side of the transformer Tr 1  is connected between the safety switch  12  and a terminal N of the alternating-current output end  10 ′, the first switch  131  is operable such that the safety switch  12  is connected to either of the other end T 2  or a tap T 3  of the primary side of the transformer Tr 1 , and the second switch  132  is operable such that the output switch  14  is connected to either of the other end T 2  or the tap T 3  of the primary side of the transformer Tr 1 ; and a rechargeable battery  16 , a bidirectional converter  15  and an auxiliary power supply system  17 , wherein the rechargeable battery  16  is connected to a secondary side of the transformer Tr 1  by the bidirectional converter  15 , and an input end of the auxiliary power supply system  17  is connected to two ends of the rechargeable battery  16  and is used for supplying various required voltages to the offline uninterruptible power source  1 . In order to simplify the circuit diagram, a mains detection device for detecting a mains voltage, a battery detection device for detecting a charging state of the rechargeable battery  16  and a control device for controlling a working state of the bidirectional converter  15  and controlling on-off states of the safety switch  12 , the first switch  131 , the second switch  132  and the output switch  14  are not shown in  FIG. 1 . 
     When the mains voltage is higher, the offline uninterruptible power source  1  is controlled to be in an automatic voltage drop adjustment mode, wherein the safety switch  12  and the output switch  14  are controlled to be switched on, the first switch  131  is controlled to be connected to the terminal T 2  of the primary side of the transformer Tr 1 , the second switch  132  is controlled to be connected to the tap T 3  of the primary side of the transformer Tr 1 , and therefore, the alternating-current output end  10 ′ outputs a voltage-dropped alternating current. Meanwhile, the bidirectional converter  15  is controlled to convert an alternating current of the secondary side of the transformer Tr 1  into a direct current so as to charge the rechargeable battery  16  and supply the direct current to the auxiliary power supply system  17 . 
     When the mains voltage is lower, the offline uninterruptible power source  1  is controlled to be in an automatic voltage boosting adjustment mode, wherein the safety switch  12  and the output switch  14  are controlled to be switched on, the first switch  131  is controlled to be connected to the tap T 3  of the primary side of the transformer Tr 1 , the second switch  132  is controlled to be connected to the terminal T 2  of the primary side of the transformer Tr 1 , and therefore, the alternating-current output end  10 ′ outputs a voltage-boosted alternating current. Meanwhile, the bidirectional converter  15  is controlled to convert the alternating current of the secondary side of the transformer Tr 1  into the direct current so as to charge the rechargeable battery  16  and supply the direct current to the auxiliary power supply system  17 . 
     When the mains voltage is abnormal (for example, the voltage is overhigh) or power failure is caused, the offline uninterruptible power source  1  is controlled to be in a battery mode, wherein the safety switch  12  is controlled to be switched off, the output switch  14  is controlled to be switched on, the second switch  132  is controlled to be connected to the terminal T 2  of the primary side of the transformer Tr 1 , the bidirectional converter  15  is controlled to work so as to convert the direct current of the rechargeable battery  16  into an alternating current which is subjected to voltage transformation by the transformer Tr 1 , and then, the required alternating current is obtained on the alternating-current output end  10 ′. Meanwhile, the rechargeable battery  16  supplies the direct current to the auxiliary power supply system  17 . 
     When the mains voltage is within the normal range, the offline uninterruptible power source  1  is controlled to be in a normal mode. The safety switch  12  and the output switch  14  are controlled to be switched on, and the first switch  131  and the second switch  132  are controlled to be both connected to the terminal T 2  of the primary side of the transformer Tr 1 . Meanwhile, the bidirectional converter  15  is controlled to convert the alternating current of the secondary side of the transformer Tr 1  into the direct current so as to charge the rechargeable battery  16  and supply the direct current to the auxiliary power supply system  17 . 
       FIG. 2  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 1  in a normal mode after a rechargeable battery is fully charged. As shown in  FIG. 2 , a mains supply of the alternating-current input end  10  is transmitted to the alternating-current output end  10 ′ after passing through the EMC filter  11  as well as the safety switch  12 , the first switch  131 , the second switch  132  and the output switch  14  which are switched on. On one hand, the electric energy efficiency is reduced by contacts of the three switches between the safety switch  12  and the terminal L of the alternating-current output end  10 ′. On the other hand, the primary side of the transformer Tr 1  is electrically connected with the alternating-current output end  10 ′, and therefore, the transformer Tr 1  is higher in no-load loss. Moreover, the transformer Tr 1  works all the time, the bidirectional converter  15  is controlled to work all the time, and the rechargeable battery  16  is in a floating charging state all the time after being fully charged. When the rechargeable battery  16  is in the floating charging state all the time, a positive plate of the rechargeable battery  16  may be inactivated to generate a great deal of PbSO4 which is absorbed onto a negative plate. In this way, the rechargeable battery  16  will be reduced in activity and increased in internal resistance, and furthermore, the rechargeable battery  16  will be rapidly reduced in capacity and greatly shortened in life. 
     SUMMARY OF THE INVENTION 
     For solving the above-mentioned technical problems existing in the prior art, the present invention provides an offline uninterruptible power source including: 
     a safety switch connected between an alternating-current input end and an alternating-current output end; 
     a transformer, wherein one end of a primary side thereof is connected to one terminal of the alternating-current output end; 
     a first switch, a second switch and an output switch, which are successively connected between the safety switch and the other terminal of the alternating-current output end, wherein the first switch is operable such that the safety switch is connected to either of the other end or a tap of the primary side of the transformer, and the second switch is operable such that the output switch is connected to either of the other end or the tap of the primary side of the transformer; 
     a third switch, which is operable such that the safety switch is connected to either of the first switch or the other terminal of the alternating-current output end; and 
     a bidirectional converter, which is configured to controllably convert an alternating current of a secondary side of the transformer into a direct current so as to charge a rechargeable battery and convert the direct current of the rechargeable battery into an alternating current. 
     Preferably, the offline uninterruptible power source further includes: 
     a charger, wherein an input end thereof is connected to the alternating-current input end by the safety switch and is configured to controllably convert an alternating current of the alternating-current input end into a direct current; and 
     an auxiliary power supply system, wherein an input end thereof is connected to an output end of the charger. 
     Preferably, the third switch includes: 
     a common terminal connected with the safety switch; 
     a first switching terminal connected between the output switch and the other terminal of the alternating-current output end; and 
     a second switching terminal connected to the first switch. 
     Preferably, an output end of the charger is connected to two ends of the rechargeable battery. 
     Preferably, the offline uninterruptible power source further includes a diode, wherein an anode thereof is connected to a positive electrode of the rechargeable battery, and a cathode thereof is connected to a positive terminal on the output end of the charger. 
     Preferably, the output power of the charger is smaller than the maximum charging power of the rechargeable battery, and an output voltage of the charger is higher than voltages on two ends of the rechargeable battery. 
     Preferably, the offline uninterruptible power source further includes: 
     a mains detection device, configured to detect a mains voltage of the alternating-current input end; 
     a battery detection device, configured to detect a charging state of the rechargeable battery; and 
     a control device, configured to control working states of the charger and the bidirectional converter according to the mains voltage and the charging state of the rechargeable battery and control on-off states of the safety switch, the first switch, the second switch, the third switch and the output switch. 
     Preferably, when the mains voltage is higher than or equal to a first threshold voltage and is lower than a second threshold voltage, the control device controls the safety switch to be switched on, controls the third switch to be connected to the first switch, controls the first switch to be connected to the tap of the primary side of the transformer, controls the second switch to be connected to the other end of the primary side of the transformer, and controls the output switch to be switched on; wherein when the rechargeable battery is not fully charged, the control device controls the charger to stop working, and controls the bidirectional converter to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the control device controls the bidirectional converter to stop working, and controls the charger to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the mains voltage is higher than or equal to the second threshold voltage and is lower than a third threshold voltage, the control device controls the safety switch to be switched on; wherein when the rechargeable battery is not fully charged, the control device controls the third switch to be connected to the first switch, controls the first switch and the second switch to be connected to the other end of the primary side of the transformer, controls the output switch to be switched on, and controls the bidirectional converter to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the control device controls the third switch to be connected to the other terminal of the alternating-current output end, controls the output switch to be switched off, controls the bidirectional converter to stop working, and controls the charger to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the mains voltage is higher than or equal to the third threshold voltage and is lower than a fourth threshold voltage, the control device controls the safety switch to be switched on, controls the third switch to be connected to the first switch, controls the first switch to be connected to the other end of the primary side of the transformer, controls the second switch to be connected to the tap of the primary side of the transformer, and controls the output switch to be switched on; wherein when the rechargeable battery is not fully charged, the control device controls the charger to stop working, and controls the bidirectional converter to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the control device controls the bidirectional converter to stop working, and controls the charger to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the first threshold voltage is higher than the mains voltage or is higher than or equal to the fourth threshold voltage, the control device controls the safety switch to be switched off, controls the second switch to be connected to the other end of the primary side of the transformer, controls the output switch to be switched on, and controls the bidirectional converter to convert the direct current of the rechargeable battery into the alternating current. 
     The present invention further provides a control method for the above-mentioned offline uninterruptible power source. The control method includes the following steps: detecting a mains voltage of the alternating-current output end, detecting a charging state of the rechargeable battery, controlling working states of the charger and the bidirectional converter according to the mains voltage and the charging state of the rechargeable battery, and controlling on-off states of the safety switch, the first switch, the second switch, the third switch and the output switch. 
     Preferably, when the mains voltage is higher than or equal to a first threshold voltage and is lower than a second threshold voltage, the safety switch is controlled to be switched on, the third switch is controlled to be connected to the first switch, the first switch is controlled to be connected to the tap of the primary side of the transformer, the second switch is controlled to be connected to the other end of the primary side of the transformer, and the output switch is controlled to be switched on; wherein when the rechargeable battery is not fully charged, the charger is controlled to stop working, and the bidirectional converter is controlled to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the bidirectional converter is controlled to stop working, and the charger is controlled to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the mains voltage is higher than or equal to the second threshold voltage and is lower than a third threshold voltage, the safety switch is controlled to be switched on; wherein when the rechargeable battery is not fully charged, the third switch is controlled to be connected to the first switch, the first switch and the second switch are controlled to be connected to the other end of the primary side of the transformer, the output switch is controlled to be switched on, and the bidirectional converter is controlled to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the third switch is controlled to be connected to the other terminal of the alternating-current output end, the output switch is controlled to be switched off, the bidirectional converter is controlled to stop working, and the charger is controlled to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the mains voltage is higher than or equal to the third threshold voltage and is lower than a fourth threshold voltage, the safety switch is controlled to be switched on, the third switch is controlled to be connected to the first switch, the first switch is controlled to be connected to the other end of the primary side of the transformer, the second switch is controlled to be connected to the tap of the primary side of the transformer, and the output switch is controlled to be switched on; wherein when the rechargeable battery is not fully charged, the charger is controlled to stop working, and the bidirectional converter is controlled to convert the alternating current of the secondary side of the transformer into the direct current; and when the rechargeable battery is fully charged, the bidirectional converter is controlled to stop working, and the charger is controlled to convert the alternating current of the alternating-current input end into the direct current. 
     Preferably, when the first threshold voltage is higher than the mains voltage or is higher than or equal to the fourth threshold voltage, the safety switch is controlled to be switched off, the second switch is controlled to be connected to the other end of the primary side of the transformer, the output switch is controlled to be switched on, and the bidirectional converter is controlled to convert the direct current of the rechargeable battery into the alternating current. 
     The offline uninterruptible power source provided by the present invention has high electric energy transmission efficiency; a small charger with a lower input withstand voltage can be selected such that the cost of the charger is reduced; and the rechargeable battery can be prevented from being in the floating charging state, moreover, the life of the rechargeable battery can be prolonged, and the cost of the rechargeable battery can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be further described below with reference to the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of an offline uninterruptible power source in the prior art; 
         FIG. 2  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 1  in a normal mode after a rechargeable battery is fully charged; 
         FIG. 3  is a circuit diagram of an offline uninterruptible power source in accordance with a first embodiment of the present invention; 
         FIG. 4  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 3  in a normal mode after a rechargeable battery is fully charged; 
         FIG. 5  is a circuit diagram of an offline uninterruptible power source in accordance with a second embodiment of the present invention; and 
         FIG. 6  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 5  in a normal mode after a rechargeable battery is fully charged. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to make objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. 
       FIG. 3  is a circuit diagram of an offline uninterruptible power source in accordance with a first embodiment of the present invention. As shown in  FIG. 3 ,  FIG. 3  is substantially the same as  FIG. 1 , except that an offline uninterruptible power source  2  further includes a charger  28  and a third switch  29 . The third switch  29  includes a common terminal connected with a second safety switch unit  222  of a safety switch  22 , a first switching terminal S 1  electrically connected between an output switch  24  and a terminal L of an alternating-current output end  20 ′ and a second switching terminal S 2  connected with a common terminal of a first switch  231 . The third switch  29  is operable such that the safety switch  22  is connected to either of the first switch  231  or the terminal L of the alternating-current output end  20 ′, and the safety switch  22  is used for achieving a feedback protection function. An input end of the charger  28  is connected to an alternating-current input end  20  by the safety switch  22 , that is, a terminal of the input end of the charger  28  is connected between a first safety switch unit  221  and a terminal N of the alternating-current output end  20 ′, the other terminal of the input end is connected between the second safety switch unit  222  and the third switch  29 , and an output end of the charger  28  is connected to an input end of an auxiliary power supply system  27 . In order to simplify the circuit diagram, a mains detection device for detecting a mains voltage, a battery detection device for detecting a charging state of a rechargeable battery  26  and a control device for controlling working states of the charger  28  and a bidirectional converter  25  and controlling on-off states of the safety switch  22 , the first switch  231 , a second switch  232 , the third switch  29  and the output switch  24  are not shown in  FIG. 3 . 
     Working modes of the offline uninterruptible power source  2  will be introduced below according to different situations. 
     (1) When the mains voltage is within a normal range, the offline uninterruptible power source  2  is controlled to be in a normal mode. The safety switch  22  (that is, the first safety switch unit  221  and the second safety switch unit  222 ) is controlled to be switched on. If the rechargeable battery  26  is not fully charged, a common terminal of the third switch  29  is controlled to be connected with the second switching terminal S 2  (that is, the third switch  29  is enabled to be connected to the first switch  231 ), the first switch  231  and the second switch  232  are controlled to be both connected to a terminal T 22  of a primary side of a transformer Tr 2 , the output switch  24  is controlled to be switched on, and an alternating current of the alternating-current input end  20  will be transmitted to the alternating-current output end  20 ′. Meanwhile, the bidirectional converter  25  is controlled to convert an alternating current of a secondary side of the transformer Tr 2  into a direct current so as to rapidly charge the rechargeable battery  26  and supply the direct current to the auxiliary power supply system  27 . 
     If the rechargeable battery  26  is fully charged, at the moment, the third switch  29  is controlled in a manner that the common terminal of the third switch  29  is connected to the first switching terminal S 1  (that is, the third switch  29  is connected to the terminal L of the alternating-current output end  20 ′), the output switch  24  is controlled to be switched off, the bidirectional converter  25  is controlled to stop charging the rechargeable battery  26 , and meanwhile, the charger  28  is controlled to convert the alternating current of the alternating-current input end  20  into a direct current so as to charge the rechargeable battery  26  and supply the required direct current to the auxiliary power supply system  27 .  FIG. 4  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 3  in a normal mode after a rechargeable battery is fully charged. By comparing an equivalent circuit shown in  FIG. 4  with an equivalent circuit shown in  FIG. 2 , it can be known that: firstly, the primary side of the transformer Tr 2  is not connected to a circuit such that transformer no-load loss does not exist, and the bidirectional converter  25  stops working. Secondly, the safety switch  22  is directly connected to the terminal L of the alternating-current output end  20 ′ by the third switch  29  such that the loss of contacts of two switches is reduced. Thirdly, the third switch  29  is in a normally-switched-off state, the output switch  24  is in a normally-switched-on state, and therefore, the loss of electromagnetic coils (not shown in  FIG. 3 ) of the output switch  24  is reduced. Tested by an inventor under the same condition, it is found that the electric energy transmission efficiency of the equivalent circuit shown in  FIG. 2  is 95.62% and the electric energy transmission efficiency of the equivalent circuit shown in  FIG. 24  is increased to 98.75%, and therefore, the electric energy transmission efficiency is increased, and then, the certification standard of the Energy Star executed by USEPA and DOE is met. 
     (2) When the mains voltage is higher, the offline uninterruptible power source  2  is controlled to be in an automatic voltage drop adjustment mode, wherein the safety switch  22  is controlled to be switched on, the third switch  29  is controlled in a manner that the common terminal of the third switch  29  is connected to the second switching terminal S 2  (that is, the third switch  29  is connected to the first switch  231 ), the first switch  231  is controlled to be connected to the terminal T 22  of the primary side of the transformer Tr 2 , the second switch  232  is controlled to be connected to a tap T 23  of the primary side of the transformer Tr 2 , and the output switch  24  is controlled to be switched on. 
     Wherein if the rechargeable battery  26  has not been fully charged, the charger  28  stops working, the bidirectional converter  25  is controlled to convert the alternating current of the secondary side of the transformer Tr 2  into the direct current so as to rapidly charge the rechargeable battery  26  and supply the direct current to the auxiliary power supply system  27 . 
     If the rechargeable battery  26  has been fully charged, the bidirectional converter  25  stops working, the charger  28  is controlled to convert the alternating current of alternating-current input end  20  into the direct current so as to charge the rechargeable battery  26  and supply the required direct current to the auxiliary power supply system  27 . 
     (3) When the mains voltage is lower, the offline uninterruptible power source  2  is controlled to be in an automatic voltage boosting adjustment mode, wherein the safety switch  12  is controlled to be switched on, the third switch  29  is controlled in a manner that the common terminal of the third switch  29  is connected to the second switching terminal S 2  (that is, the second safety switch unit  222  is connected to the first switch  231 ), the first switch  231  is controlled to be connected to the tap T 23  of the primary side of the transformer Tr 2 , the second switch  232  is controlled to be connected to the terminal T 22  of the primary side of the transformer Tr 2 , and the output switch  24  is controlled to be switched on. 
     If the rechargeable battery  26  has not been fully charged, the charger  28  stops working, the bidirectional converter  25  is controlled to convert the alternating current of the secondary side of the transformer Tr 2  into the direct current so as to rapidly charge the rechargeable battery  26  and supply the direct current to the auxiliary power supply system  27 . 
     If the rechargeable battery  26  has been fully charged, the bidirectional converter  25  stops working, the charger  28  is controlled to convert the alternating current of alternating-current input end  20  into the direct current so as to charge the rechargeable battery  26  and supply the required direct current to the auxiliary power supply system  27 . 
     (4) When the mains voltage is abnormal (for example, the voltage is overhigh) or power failure is caused, the offline uninterruptible power source  2  is controlled to be in a battery mode, wherein the safety switch  22  is controlled to be switched off, the second switch  232  is controlled to be connected to the terminal T 22  of the primary side of the transformer Tr 2 , the output switch  24  is controlled to be switched on, meanwhile, the bidirectional converter  25  is controlled to convert the direct current of the rechargeable battery  26  into an alternating current which is subjected to voltage transformation by the transformer Tr 2 , and then, the required alternating current is achieved on the alternating-current output end  20 ′. Meanwhile, the rechargeable battery  26  supplies the direct current to the auxiliary power supply system  17 . 
     When the mains voltage is overhigh, the safety switch  22  is controlled to be switched off, the situation that the mains supply on the alternating-current input end  20  is transmitted to the input end of the charger  28  by the safety switch  22  in a switch-off state may not be achieved, and therefore, the charger  28  may select a small charger with a lower input withstand voltage such that the cost of the circuit is reduced. 
     In combination with the above-mentioned working mode, it can be known that after the rechargeable battery  26  is fully charged, the charger  28  is controlled to work so as to supply power to the auxiliary power supply system  27 . In the uninterruptible power source, the power of the auxiliary power supply system  27  is much lower than the maximum charging power of the rechargeable battery  26 , and therefore, a small-sized charger of which the power is adapted to the power of the auxiliary power supply system  27  may be selected, that is, the output power of the charger  28  is smaller than the maximum charging power of the rechargeable battery  26 , so that the cost of the circuit is reduced. 
     In the normal mode, when the rechargeable battery  26  is not fully charged, the first switch  231  and the second switch  232  are controlled to be both connected to the terminal T 22  of the primary side of the transformer Tr 2 , then, all windings of the primary side of the transformer Tr 2  are electrically connected to the alternating-current input end  20 , and thus, the transformer Tr 2  can be effectively prevented from being saturated. 
     In the battery mode, the second switch  232  is controlled to be connected to the terminal T 22  of the primary side of the transformer Tr 2 , then, all the windings of the primary side of the transformer Tr 2  are electrically connected to the alternating-current output end  20 ′, and thus, an alternating current voltage which is high as much as possible may be obtained on the alternating-current output end  20 ′ under the condition that a duty ratio is constant. Compared with the mode that the second switch  232  is controlled to be connected to the tap T 23  of the primary side of the transformer Tr 2 , this mode lies in that the rechargeable battery  26  with a lower output voltage may be selected, and thus, the cost of the rechargeable battery  26  is reduced. 
       FIG. 5  is a circuit diagram of an offline uninterruptible power source in accordance with a second embodiment of the present invention. A mains detection device, a battery detection device and a control device are not shown in  FIG. 5 . As shown in  FIG. 5 , an offline uninterruptible power source  3  is substantially the same as the offline uninterruptible power source  2  shown in  FIG. 2 , except that the offline uninterruptible power source  3  further includes a diode D 3  connected between a positive electrode of a rechargeable battery  36  and a positive terminal on an output end of a charger  38 , wherein an anode of the diode D 3  is connected to the positive electrode of the rechargeable battery  36 , and a cathode of the diode D 3  is connected to the positive terminal on the output end of the charger  38 . 
     Working modes of the offline uninterruptible power source  3  will be introduced below according to different situations. 
     (1) When the mains voltage is within a normal range, a control method for the offline uninterruptible power source  3  is the same as a control method for the offline uninterruptible power source  2 , the descriptions thereof are omitted herein.  FIG. 6  is an equivalent circuit diagram of the offline uninterruptible power source shown in  FIG. 5  in a normal mode after a rechargeable battery is fully charged. As shown in  FIG. 6 , the charger  38  is controlled to supply a required direct current to an auxiliary power supply system  37 . A charging voltage of the charger  38  is higher than voltages on two ends of the rechargeable battery  36 , the phenomenon that the rechargeable battery  36  supplies power to the auxiliary power supply system  37  via the diode D 3  may be avoided, and the charger  38  is incapable of further charging the rechargeable battery  36  in a fully-charged state due to a reverse cut-off function of the diode D 3 . Therefore, at the moment, the rechargeable battery  36  is in a sleep mode, and thus, the service life can be prolonged. 
     (2) When the mains voltage is higher, a control method for the offline uninterruptible power source  3  is the same as a control method for the offline uninterruptible power source  2 , the descriptions thereof are omitted herein. Similarly, the rechargeable battery  36  is in the sleep mode after being fully charged, and thus, the service life can be prolonged. 
     (3) When the mains voltage is lower, a control method for the offline uninterruptible power source  3  is the same as a control method for the offline uninterruptible power source  2 , the descriptions thereof are omitted herein. Similarly, the rechargeable battery  36  is in the sleep mode after being fully charged, and thus, the service life can be prolonged. 
     (4) When the mains voltage is abnormal or power failure is caused, a control method for the offline uninterruptible power source  3  is the same as a control method for the offline uninterruptible power source  2 , the descriptions thereof are omitted herein. 
     It can be known from the above that after the rechargeable battery  36  is fully charged, a bidirectional converter  35  stops charging the rechargeable battery  36 , and meanwhile, the charger  38  starts to work and supply the required direct current to the auxiliary power supply system  37 . Due to the reverse cut-off function of the diode D 3 , at the moment, the charger  38  is incapable of charging the rechargeable battery  36 . In addition, the output voltage of the charger  38  is enabled to be higher than the voltages on the two ends of the rechargeable battery  36 , at the moment, the phenomenon that the rechargeable battery  36  supplies power to the auxiliary power supply system  37  via the diode D 3  may be avoided. The rechargeable battery  36  is neither charged nor discharged, thereby being prevented from being in a floating charging state all the time. 
     When the rechargeable battery  36  is required to be charged, the bidirectional converter  35  works to rapidly charge the rechargeable battery  36 . After the rechargeable battery  36  is fully charged, the bidirectional converter  35  stops working, the rechargeable battery  36  is automatically in a sleep mode, and thus, the life of the rechargeable battery  36  is greatly prolonged. 
     In other control methods provided by the present invention, in a normal mode or an automatic voltage adjustment mode, when the rechargeable battery  36  is not fully charged, the charger  38  is controlled to work all the time so as to supply the required direct current to the auxiliary power supply system  37 . 
     In the above-mentioned embodiments of the present invention, the bidirectional converter may be implemented by adopting a single circuit module achieving the above-mentioned functions or implemented by adopting a combination of circuit modules. For example, a combination of a circuit module capable of converting a direct current into an alternating current and a circuit module capable of converting an alternating current into a direct current to charge a battery, such as an inverter, is selected. 
     The EMC filter is suitable for inhibiting noise and high harmonic waves of a power gird as well as noise and high-frequency harmonic waves generated by a switching power source, and therefore, the offline uninterruptible power source does not have to be provided with the EMC filter in an application occasion where the requirement for power utilization quality is not high. In other embodiments of the present invention, the EMC filter is connected to an alternating-current input end by a safety switch (that is, the safety switch is connected between the alternating-current input end and the EMC filter). 
     According to the control method provided by the present invention, the different working modes are selected based on the mains voltage on the alternating-current input end. For example, when the mains voltage is lower than a first threshold voltage, the offline uninterruptible power source is in a battery mode; when the mains voltage is higher than or equal to the first threshold voltage and is lower than a second threshold voltage, the offline uninterruptible power source is in an automatic voltage boosting adjustment mode; when the mains voltage is higher than or equal to the second threshold voltage and is lower than a third threshold voltage, the offline uninterruptible power source is in a normal mode; when the mains voltage is higher than or equal to the third threshold voltage and is lower than a fourth threshold voltage, the offline uninterruptible power source is in an automatic voltage drop adjustment mode; and when the mains voltage is higher than or equal to the fourth threshold voltage, the offline uninterruptible power source is in the battery mode. The control method provided by the present invention is not intended to limit specific numerical values of the first threshold voltage, the second threshold voltage, the third threshold voltage and the fourth threshold voltage, but is selected by a user according to a rated voltage of a mains supply, a voltage deviation range allowed by a load and a turn ratio of a transformer. For example, if a rated voltage value of the mains supply is 220 V, a voltage range allowed by the load is 200 V to 240 V, and a ratio of turns of the terminal T 1  and the tap T 3  of the primary side of the transformer Tr 1  to the turns of the terminal T 1  and the terminal T 2  is 5:6, the first threshold voltage may be selected as 167 V, the second threshold voltage may be selected as 200 V, the third threshold voltage may be selected as 240 V, and the fourth threshold voltage may be selected as 288 V. 
     Although the present invention has been described with reference to preferred embodiments, the present invention is not limited to the embodiments described herein. The present invention may further include various changes and variations without departing from the scope of the present invention.