Abstract:
A UPS circuit, comprising a low frequency transformer (TX) and a battery (B), the low frequency transformer (TX) comprising a primary side winding, and a secondary side winding connected to the battery (B); the primary side winding is connected to a municipal power loop via a switching device (SW), and the switching device (SW) can operate in a chopping mode.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a UPS circuit and, more particularly, to a UPS circuit comprising a low frequency transformer. 
       BACKGROUND OF THE INVENTION 
       [0002]    A UPS (Uninterruptible Power System) is an alternating current power supply with an energy storage device, which provides uninterruptible power supply to a load using the energy storage device such as a battery during power failures. When the mains power supply is available, the UPS regulates the mains power supply and supplies it to the load. When the mains power supply is not available (power failure), the UPS supplies alternating currents to the load using the energy storage device, so as to keep the load working normally and prevent the software and hardware of the load from damage. 
         [0003]      FIG. 1  shows a basic circuit configuration of the UPS, which comprises an input end T in  receiving an AC power from the mains power supply and an output end T out  supplying the AC power to the load. There are an electro-magnetic interference filter EMI, switches SW 1 , SW 2 , SW 3  and SW 4 , and a low frequency transformer TX between the input end T in  and the output end T out . In the mains power supply mode, SW 3  and SW 4  are closed, and the AC power is delivered from the input end T in  to the output end T out . At the same time, the AC power is delivered from a secondary side of the low frequency transformer TX to a circuit module AD for realizing AC/DC conversion. The circuit module AD rectifies the AC power into a DC power and then charges a battery B. When the mains power supply is not available and the UPS works in the battery mode, the circuit module AD works in inverter mode, and converts the DC power provided by the battery B into the AC power and provides the AC power to the secondary side of the transformer TX. A primary side of the TX provides the AC power to the output end T out  of the UPS circuit. The switches SW 1  and SW 2  are used to regulate the mains power supply voltage when it is too high or too low. 
         [0004]    However, the efficiency requirements to the UPS have become increasingly higher with the proposition of environmental protection strengthening, in particular EPA Energy star. This type of UPS circuit has a big energy loss, which makes it difficult to meet the Energy star standard. 
         [0005]    In the mains power supply mode, the UPS has a big self-loss even in a fully charged state. The loss mainly comprises: 1) an overall circuit conduction energy loss and a mains power supply components loss; 2) a loss for keeping the whole control system and the switching components operating; 3) an excitation loss of the low frequency transformer. Wherein, the excitation loss accounts for over 60% of the total no-load loss, and thus is the main cause of the no-load loss. 
         [0006]    Core materials with lower magnetic losses may be used to reduce the excitation loss of the low frequency transformer. However, the cost is higher and the effect is limited. Another method to reduce the excitation loss is to disconnect the low frequency transformer from the mains power supply circuit in the mains power supply mode. In theory, in the mains power supply mode, if the battery B has been fully charged, the low frequency transformer does not need to power the battery B and disconnecting the low frequency transformer will not affect the UPS circuit. In practice, however, the battery has a self-discharge effect and the electrical energy stored therein will be drained slowly even if the battery does not power the load. In addition, some control circuits need to get a small amount of electrical energy from the battery. Consequently, the low frequency transformer is needed to charge the battery continuously. The method of disconnecting the low frequency transformer can completely eliminate the excitation loss of the low frequency transformer, but it is needed to get some energy from the mains power supply in consideration of the control circuits and the self-discharge of the battery. Therefore, an additional independent power supply is required to provide the energy, which causes a higher production cost. 
       SUMMARY OF INVENTION 
       [0007]    In view of the foregoing, an object of the present invention is to provide a UPS circuit for avoiding the drawbacks of the prior art. 
         [0008]    The present invention provides a UPS circuit comprising a low frequency transformer and a battery, wherein the low frequency transformer comprises a primary side winding and a secondary side winding connected to the battery, and wherein the primary side winding is connected to a mains power supply via a switch device (SW) which can operate in a chopping mode. 
         [0009]    Preferably, the secondary side winding can be connected to the battery via a circuit module for realizing AC/DC conversion. 
         [0010]    Preferably, the switch device further has a turn-on mode, and the switch device operates in the turn-on mode when the battery is being charged. 
         [0011]    Preferably, the switch device can be chosen to operate in the chopping mode after the battery is fully charged. 
         [0012]    Preferably, the switch device is turned on intermittently when it is in the chopping mode. 
         [0013]    Preferably, the switch device is turned on and turned off periodically when it is in the chopping mode. 
         [0014]    Preferably, a period with which the switch device is turned on and turned off is the same as a period of the voltage input into the UPS circuit. 
         [0015]    Preferably, the switch device is turned on and turned off aperiodically. 
         [0016]    Preferably, the secondary side winding comprises a first sub-winding and a second sub-winding, and the first sub-winding and the second sub-winding are connected to the battery via an inverter circuit module and a rectifier circuit module respectively. 
         [0017]    In the UPS circuit comprising a low frequency transformer of the present invention, a switch SW is configured between the primary side winding of the low frequency transformer and the mains power supply, and can be turned on intermittently. Therefore, the excitation loss of the low frequency transformer and thus the overall energy loss of the UPS circuit may be greatly reduced while meeting the power demands of the battery B and the control circuit. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]    The present invention will be further explained in combination with the embodiments with reference to the accompanying figures, wherein: 
           [0019]      FIG. 1  is a schematic diagram of a prior art UPS circuit; 
           [0020]      FIG. 2  is a schematic diagram of a UPS circuit according to one embodiment of the present invention; 
           [0021]      FIG. 3 - FIG. 5  show three waveforms of a voltage input into a low frequency transformer TX; and 
           [0022]      FIG. 6  is a schematic diagram of a UPS circuit according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0023]    In the following parts, the present invention will be described in greater details with reference to the embodiments and the accompanying drawings so as to make its objects, solutions and advantages clearer. It should be understood that the specific embodiments described herein only intend to interpret the present invention, without making any limitation thereto. 
         [0024]    The embodiment provides a UPS circuit, the structure of which is shown in  FIG. 2 , comprising an input end T in  receiving an AC power from a mains power supply and an output end T out  supplying the AC power to a load. There are an electro-magnetic interference filter EMI, switches SW 1 , SW 2 , SW 3  and SW 4 , and a low frequency transformer TX between the input end T in  and the output end T out . Wherein, the switch SW 3  is used to disconnect the low frequency transformer TX from the mains power supply when the UPS supplies power to the output end T out  with a battery B thereof. The switches SW 1  and SW 2  cooperate with each other to regulate the mains power supply voltage when the mains power supply voltage is too high or too low. 
         [0025]    When the mains power supply voltage is normal, contacts  1  and  2  of SW 1  are connected, and contacts  1  and  2  of SW 2  are connected too, as shown in  FIG. 2 . When the mains power supply voltage is too high, contacts  1  and  2  of SW 1  are connected, and contacts  1  and  3  of SW 2  are connected, so as to make buck regulation. When mains power supply voltage is too low, contacts  1  and  3  of SW 1  are connected, and contacts  1  and  2  of SW 2  are connected, so as to make boost regulation. The switch SW 4  is used as an output switch of the UPS for controlling whether the UPS should supply power to the load. A secondary side of the low frequency transformer TX is connected to the battery B via a circuit module AD for realizing AC/DC conversion. During charging, the circuit module AD for realizing AC/DC conversion rectifies an AC power output from the secondary side of the low frequency transformer TX into a DC power and then charges the battery B. When the power of the battery B is needed, the circuit module AD works in an inverter mode and converts the DC power from the battery B into the AC power. The AC power is then provided to the secondary side of the low frequency transformer TX and thus to the output end T out  of the UPS. A primary side of the low frequency transformer TX is connected to the mains power supply, namely the input end T in  of the UPS circuit, via a switch SW. 
         [0026]    The switch SW has a turn-on mode and a chopping mode. In the turn-on mode, the switch SW is always in a turn-on state. And in the chopping mode, the switch SW can continuously alternate between a turn-on state and a turn-off state, so as to control the waveform of the voltage input into the low frequency transformer TX to reduce the voltage-time product of the low frequency transformer TX.  FIGS. 3-5  show the waveforms of the voltage input into the low frequency transformer TX when the switch SW continuously and repeatedly alternates between the turn-on state and the turn-off state. The sine waves in  FIGS. 3-5  are waveforms of the voltage from the mains power supply to the low frequency transformer TX, wherein the solid line parts indicate the time periods during which the switch SW is turned on, and the dashed line parts indicate the time periods during which the switch SW is turned off. 
         [0027]    When the UPS is just started, the switch SW works in the turn-on mode. Now the mains power supply charges the battery B via the low frequency transformer TX and the circuit module AD while supplying power to the output end T out  via the input end T in . When the battery B is fully charged, it does not need to be continuously charged from the mains power supply. Now, the switch SW is configured to work in the chopping mode so as to continuously and repeatedly alternate between the turn-on state and the turn-off state. That is, the switch SW is intermittently turned on. By this way, the self-discharge loss of the battery B can be complemented and the power demand of the control circuit can be met, and in the mean time, the voltage-time product input into the low frequency transformer TX can be reduced to reduce the excitation loss thereof. 
         [0028]    When the mains power supply is too high or too low, and it is needed to make SW 1  and SW 2  cooperate with each other to regulate the mains power supply voltage, the switch SW is configured to work in the turn-on mode so as to make the primary side winding of the low frequency transformer realize boost and buck. 
         [0029]    In addition,  FIGS. 3-5  only show three exemplary waveforms of the voltage input into the low frequency transformer TX, which are not limitation. A person skilled in the art may alter the waveform of the voltage input into the low frequency transformer TX based on actual demands. For example, a person skilled in the art may set the turn-on time periods and turn-on frequency of the switch SW according to the self-discharge rate of the battery B and the power demand of the control circuit. 
         [0030]    Moreover, the UPS is usually powered by the mains power supply which is normally a sine wave, therefore the present invention is explained in  FIGS. 3-5  by using the sine waves as examples. It should be appreciated that the UPS according to the present invention also applies to the power supply with other waveforms. 
         [0031]    According to other embodiments of the present invention, the turn-on and turn-off of the switch SW can be periodic. That is, the turn-on time periods and turn-off time periods are alternative and constant. The turn-on and turn-off of the switch SW can also be aperiodic. That is, the turn-on time periods and turn-off time periods appear aperiodically. For the periodic turn-on and turn-off, the period for switching SW may be the same as or different from the period of the mains power supply sine wave. 
         [0032]    According to other embodiments of the present invention, the low frequency transformer is not limited to the type described above. Other types of low frequency transformers can also be adopted by the solution of the present invention to reduce the excitation loss, such as the low frequency transformer with two secondary side windings shown in  FIG. 6 . The low frequency transformer shown in  FIG. 6  comprises a first secondary side winding TX 1  and a second secondary side winding TX 2 . The secondary side windings TX 1  and TX 2  are connected to the battery B via an inverter circuit module DC-AC and a rectifier circuit module AC-DC respectively. During charging, the low frequency transformer charges the battery B via the secondary side winding TX 2  and the rectifier circuit module AC-DC. When needing the battery B to supply power, the inverter circuit module DC-AC converts the DC power from the battery B into the AC power and provides the AC power to the secondary side winding TX 1  and thus to the output end T out  of the UPS. 
         [0033]    In the UPS circuit of the present invention, the switch SW may be various types of switches which can realize an intermittent turn-on. A person skilled in the art may choose the switch based on actual demands. 
         [0034]    To sum up, the present invention provides a UPS circuit comprising a low frequency transformer, which is able to meet the power demands of the battery B and the control circuit and greatly reduce the excitation loss of the low frequency transformer by setting a switch SW between the primary side winding of the low frequency transformer and the mains power supply and making the switch SW turn on intermittently. 
         [0035]    Embodiments of the present invention have been described in terms of the preferred embodiments, but the present invention is not limited to the embodiments described above, and various amendments and changes may be made within the scope of the present invention.