Abstract:
There is provided a power supply for an electrical device operable in active mode and in standby mode. The power supply comprises a transformer having a primary winding on the primary side and a secondary winding on the secondary side. The primary winding is connectable to an AC voltage supply and is arranged to comprise N turns when the electrical device is in active mode and more than N turns when the electrical device is in standby mode. Circuitry on the secondary side is arranged to provide an output voltage for the electrical device during active mode.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a power supply for an electrical device operable in active mode and in standby mode. In particular, the invention relates to a power supply for an electrical device, operable in active mode and in standby mode, which has very low power consumption during standby mode. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many electrical devices have two modes of operation: an active mode in which a load is connected to the output, and a standby mode in which no load (actually a very small load) is connected at the output. In active mode, the power supplied should be sufficient for the device to perform its usual functions and, in standby mode, minimal power should be expended: in most cases, just enough for the device to be switched back into active mode when necessary. 
         [0003]    It is becoming increasingly important to conserve energy and reduce power losses and power supplies which have minimal power consumption during standby mode are becoming more and more desired. Such power supplies find applications in many situations, for example as standby power supplies in electrical devices (e.g. in televisions, washing machines) or within external power supplies for supplying power to detect whether an electrical device is connected or not and to switch on the main power supply (e.g. within a portable telephone charger where the telephone is placed in a cradle for charging). 
         [0004]    Note that, throughout this specification, the terms “no-load mode” and “standby mode” are used interchangeably. Although, strictly speaking, the output load during standby is not zero, the load is extremely small and can be approximated to zero for all practical purposes. 
         [0005]      FIG. 1  shows one conventional arrangement of a transformer used in a power supply for a device which has a standby mode, for stepping down the voltage from the AC supply. The transformer  101  comprises a primary winding  101   a  and secondary windings  101   b ,  101   c  and  101   d . The primary winding  101   a  is connected to an AC supply  103  via a switch  105 . When switch  105  is closed, power is supplied to the transformer  101 . Secondary winding  101   b  provides the supply voltage to standby circuit  107  and secondary windings  101   c  and  101   d  provide the supply voltage to the main device  109  (i.e. the output load) via switches  111  and  113  respectively. The main device  109  may be put into standby mode by opening switches  111  and  113  so that the supply voltage is no longer supplied to the main device  109 . Those switches  111  and  113  may be operated by standby circuit  107  directly, by remote control or under some other form of control (e.g. automatic standby after a given period of time). 
         [0006]    The design of a transformer such as transformer  101  in  FIG. 1  is based on the power requirement of the device when in active mode. This may vary if the device is arranged to perform a number of different tasks each requiring a different power input. Once the maximum power requirement of the device during active mode has been determined, the transformer is then designed to deliver that maximum power (for at least some of the time) most economically (e.g. using the smallest possible amount of material) and with the smallest rise in temperature. 
         [0007]    Referring once again to  FIG. 1 , when the device is put into standby mode by a user, standby circuit  107  cuts off the power supply to the main device  109  by opening switches  111  and  113 . During standby mode, standby circuit only requires a small amount of power: in most cases, just enough to be able to switch the device back into active mode. Hence, during standby mode, the transformer  101  is actually much larger than required which means that its operation is rather inefficient. In that case, most of the power loss is due to the no-load losses from the primary winding of the transformer itself. These no-load losses consist mainly of core losses, which include hysteresis losses and eddy-current losses in the magnetic core, and copper losses due to the current flowing through the copper wire of the winding, which has a finite resistance. These three types of losses will be discussed further below. 
         [0008]      FIG. 2  shows an inductor coil having N turns, on a magnetic coil with an applied voltage. This is a close approximation to the primary winding of a transformer (such as transformer  101  in  FIG. 1 ) when in no-load mode. 
         [0009]    According to Faraday&#39;s law, the voltage is proportional to the rate of change of the magnetic flux: 
         [0000]    
       
         
           
             
               
                 
                   v 
                   = 
                   
                     N 
                      
                     
                       
                          
                         φ 
                       
                       
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where v is the applied voltage, N is the number of turns in the primary winding and φ is the total magnetic flux through the winding. 
         [0010]    If we assume a sinusoidal input voltage having frequency w i.e. one of the form ν=√{square root over (2)}V cos ωt, substituting this into equation (1) gives us: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       N 
                        
                       
                         
                            
                           φ 
                         
                         
                            
                           t 
                         
                       
                     
                     = 
                     
                       
                         2 
                       
                        
                       V 
                        
                       
                           
                       
                        
                       cos 
                        
                       
                           
                       
                        
                       ω 
                        
                       
                           
                       
                        
                       t 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     φ 
                     = 
                     
                       
                         
                           1 
                           N 
                         
                          
                         
                           ∫ 
                           
                             
                               2 
                             
                              
                             V 
                              
                             
                                 
                             
                              
                             cos 
                              
                             
                                 
                             
                              
                             ω 
                              
                             
                                 
                             
                              
                             t 
                           
                         
                       
                       = 
                       
                         
                           
                             
                               2 
                             
                              
                             V 
                           
                           
                             N 
                              
                             
                                 
                             
                              
                             ω 
                           
                         
                          
                         sin 
                          
                         
                             
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0011]    If we assume a uniform flux distribution, the magnetic flux density B is given by: 
         [0000]    
       
         
           
             
               
                 
                   B 
                   = 
                   
                     φ 
                     A 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where A is the cross sectional area of the core. 
         [0012]    Substituting equation (3) into equation (2) gives us: 
         [0000]    
       
         
           
             AB 
             = 
             
               
                 
                   
                     2 
                   
                    
                   V 
                 
                 
                   N 
                    
                   
                       
                   
                    
                   ω 
                 
               
                
               sin 
                
               
                   
               
                
               ω 
                
               
                   
               
                
               t 
             
           
         
       
     
         [0013]    The maximum flux density B max  is given when sin ωt=1. This gives: 
         [0000]    
       
         
           
             
               
                 
                   
                     B 
                     max 
                   
                   = 
                   
                     
                       
                         2 
                       
                        
                       V 
                     
                     
                       AN 
                        
                       
                           
                       
                        
                       ω 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0014]    The three types of losses, discussed above (hysteresis losses, eddy-current losses and copper losses) are given by equations (5), (6) and (7) below. 
         [0015]    The hysteresis loss P h  is given by: 
         [0000]        P   h   =K   h   f ( B   max ) α   (5) 
         [0000]    where f is the excitation frequency, α is the Steinmetz exponent which will depend on the particular properties of the material used for the core (usually taken to be between 1.6 and 2.0) and K h  is another constant also dependent on the particular properties of the core material. 
         [0016]    The eddy current loss P e  is given by: 
         [0000]        P   e   =K   e   f   2 ( B   max  ) 2   (6) 
         [0000]    where f is the excitation frequency and K e  is a constant dependent on the particular properties of the core material. 
         [0017]    The copper loss P Cu  is given by: 
         [0000]      P Cu =I RMS   2 R  (7) 
         [0000]    where I RMS  is the root-mean-squared current through the winding and R is the effective impedance of the winding. 
         [0018]    From equation (5), we see that, for the hysteresis loss: 
         [0000]    
       
         
           
             
               P 
               h 
             
             ∝ 
             
               
                 ( 
                 
                   B 
                   max 
                 
                 ) 
               
               α 
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               P 
               h 
             
             ∝ 
             
               1 
               
                 N 
                 α 
               
             
           
         
       
     
         [0019]    From equation (6), we see that for the eddy-current loss: 
         [0000]    
       
         
           
             
               P 
               e 
             
             ∝ 
             
               
                 ( 
                 
                   B 
                   max 
                 
                 ) 
               
               2 
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               P 
               e 
             
             ∝ 
             
               1 
               
                 N 
                 2 
               
             
           
         
       
     
         [0020]    That is, the core losses P h  and P e  increase as B max  is increased but decrease as the number of turns N is increased. 
         [0021]    The reader may assume from the above that the transformer should necessarily be designed with as many turns as possible in the windings in order to decrease B max  as far as possible and hence reduce the core losses. However, this is not the case because the design of a transformer is based on the power requirement of the device when in active mode and the aim of the transformer is to deliver the required power most economically. 
       SUMMARY OF THE INVENTION 
       [0022]    According to a first aspect of the invention there is provided a power supply for an electrical device operable in active mode and in standby mode, said power supply comprising: a transformer comprising a primary winding on the primary side and a secondary winding on the secondary side, wherein said primary winding is connectable to an AC voltage supply and is arranged to comprise N turns when said electrical device is in active mode and more than N turns when said electrical device is in standby mode, and wherein circuitry on said secondary side is arranged to provide an output voltage for said electrical device during active mode. 
         [0023]    Because the number of turns on the primary side increases during standby mode, this decreases losses during standby mode. 
         [0024]    In a first embodiment, the power supply further comprises a second primary winding on said primary side, said two primary windings being arranged to be connected in parallel when said electrical device is in active mode and in series when said electrical device is in standby mode. 
         [0025]    When the two primary windings are connected in series, the total number of turns on the primary side is larger than when the two primary windings are connected in parallel. Thus, during active mode, the first primary winding comprises N turns but, during standby mode, when the second primary winding is connected in series with the first primary winding, the first primary winding effectively comprises more than N turns. Thus, the number of turns on the primary side is greater during standby mode than in active mode. This increase in the number of turns reduces power losses during standby mode. Of course, it is possible for there to be more than two windings (any number e.g. three, four, five and so on) on the secondary side, in which case, they should be connected in such a way that there are more total turns when the device is in standby mode than when the device is in active mode. 
         [0026]    Preferably, said two primary windings are connectable to said AC voltage supply via a switch circuit, said switch circuit having a first configuration in which said two primary windings are connected in parallel and a second configuration in which said two primary windings are connected in series. 
         [0027]    In the first embodiment, said secondary side may comprise a toggle circuit for switching the electrical device from active mode to standby mode and from standby mode to active mode. In that arrangement, said two primary windings may be connectable to the AC voltage supply via a switch circuit, said switch circuit having a first configuration in which said two primary windings are connected in parallel and a second configuration in which said two primary windings are connected in series, and said toggle circuit may be arranged to toggle said switch circuit from said first configuration to said second configuration when said electrical device switches from active mode to standby mode and to toggle said switch circuit from said second configuration to said first configuration when said electrical device switches from standby mode to active mode. 
         [0028]    Thus, when a user switches the electrical device from active mode to standby mode, the toggle circuit switches the switch circuit from the first configuration (in which the two primary windings are connected in parallel) to the second configuration (in which the two primary windings are connected in series). When a user switches the electrical device from standby mode back to active mode, the toggle circuit switches the switch circuit from the second configuration to the first configuration. 
         [0029]    The toggle circuit may be connected to the switch circuit. Alternatively the control of the switch circuit by the standby circuit may be by means of remote control. 
         [0030]    In a second embodiment, the power supply further comprises a second primary winding on said primary side, only the first primary winding being connected when said electrical device is in active mode and the first and second primary windings being connected in series when said electrical device is in standby mode. 
         [0031]    When the two primary windings are connected in series, the total number of turns on the primary side is larger than when only one of the two primary windings is connected. Thus, during active mode, the primary winding comprises N turns but, during standby mode, when the second primary winding is connected in series with the first primary winding, the primary winding effectively comprises more than N turns. This increase in the number of turns reduces power losses during standby mode. Of course, it is possible for there to be more than two windings on the secondary wide, in which case, they should be connected in such a way that there are more total turns when the device is in standby mode than when the device is in active mode. 
         [0032]    Preferably, said two primary windings are connectable to said AC voltage supply via a switch circuit, said switch circuit having a first configuration in which only the first primary winding is connected and a second configuration in which the first and second primary windings are connected in series. 
         [0033]    In the second embodiment, said secondary side may comprise a toggle circuit for switching the electrical device from active mode to standby mode and from standby mode to active mode. In that arrangement, said two primary windings may be connectable to the AC voltage supply via a switch circuit, said switch circuit having a first configuration in which only the first primary winding is connected and a second configuration in which the first and second primary windings are connected in series, and said toggle circuit may be arranged to toggle said switch circuit from said first configuration to said second configuration when said electrical device switches from active mode to standby mode and to toggle said switch circuit from said second configuration to said first configuration when said electrical device switches from standby mode to active mode. 
         [0034]    In one embodiment, said secondary side includes a first secondary winding and a second secondary winding. In that embodiment, said first secondary winding may be connected to a toggle circuit, for toggling said electrical device from active mode to standby mode and from standby mode to active mode, and said second secondary winding may be arranged to provide said output voltage for said electrical device. 
         [0035]    Said second secondary winding may be connected to said electrical device via a switch and said switch may be operable by said toggle circuit. 
         [0036]    In one embodiment, the secondary side also includes a third winding, the first winding being connected to the toggle circuit and the second and third windings both being arranged to provide the output voltage for the electrical device. 
         [0037]    In one embodiment, when a user switches the device from active mode to standby mode, the toggle circuit opens the switch between the second secondary winding and the electrical device and switches the switch circuit on the primary side from the first configuration (in which the two primary windings are connected in parallel or in which only one of the two primary windings is connected) to the second configuration (in which the two primary windings are connected in series). When a user switches the device back to active mode, the toggle circuit switches the switch circuit on the primary side from the second configuration (in which the two primary windings are connected in series) to the first configuration (in which the two primary windings are connected in parallel or in which only one of the primary windings is connected) and closes the switch between the second secondary winding and the electrical device. 
         [0038]    According to a second aspect of the invention, there is provided a transformer for a power supply for an electrical device operable in active mode and in standby mode, said transformer comprising: a primary winding on the primary side, said primary winding being connectable to an AC voltage supply and being arranged to comprise N turns when said electrical device is in active mode and more than N turns when said electrical device is in standby mode; and a secondary winding on said secondary side. 
         [0039]    Because the number of turns on the primary side increases during standby mode, this decreases losses during standby mode. 
         [0040]    In a first embodiment, the transformer further comprises a second primary winding on said primary side, said two primary windings being arranged to be connected in parallel when said electrical device is in active mode and in series when said electrical device is in standby mode. 
         [0041]    When the two primary windings are connected in series, the total number of turns on the primary side is larger than when the two primary windings are connected in parallel. Thus, during active mode, the first primary winding comprises N turns but, during standby mode, when the second primary winding is connected in series with the first primary winding, the first primary winding effectively comprises more than N turns. Thus, the number of turns on the primary side is greater during standby mode than in active mode. This increase in the number of turns reduces power losses during standby mode. 
         [0042]    Preferably, said two primary windings are connectable to said AC voltage supply via a switch circuit, said switch circuit having a first configuration in which said two primary windings are connected in parallel and a second configuration in which said two primary windings are connected in series. 
         [0043]    The transformer may further comprise a toggle circuit on said secondary side, for toggling said electrical device from active mode to standby mode and from standby mode to active mode. 
         [0044]    In that case, said two primary windings are preferably connectable to said AC voltage supply via a switch circuit, said switch circuit having a first configuration in which said two primary windings are connected in parallel and a second configuration in which said two primary windings are connected in series, and said toggle circuit is preferably arranged to toggle said switch circuit from said first configuration to said second configuration when the electrical device switches from active mode to standby mode and to toggle the switch circuit from said second configuration to said first configuration when said electrical device switches from standby mode to active mode. 
         [0045]    Thus, when a user switches the electrical device from active mode to standby mode, the toggle circuit switches the switch circuit from the first configuration (in which the two primary windings are connected in parallel) to the second configuration (in which the two primary windings are connected in series). When a user switches the electrical device from standby mode back to active mode, the standby circuit switches the switch circuit from the second configuration to the first configuration. 
         [0046]    The standby circuit may be physically connected to the switch circuit. Alternatively the control of the switch circuit by the standby circuit may be by means of remote control. 
         [0047]    In a second embodiment, the transformer further comprises a second primary winding on said primary side, only the first primary winding being connected when said electrical device is in active mode and the first and second primary windings being connected in series when said electrical device is in standby mode. 
         [0048]    When the two primary windings are connected in series, the total number of turns on the primary side is larger than when only one of the two primary windings is connected. Thus, the number of turns on the primary side is greater during standby mode than in active mode. This increase in the number of turns reduces power losses during standby mode. Of course, it is possible for there to be more than two windings on the secondary wide, in which case, they should be connected in such a way that there are more total turns when the device is in standby mode than when the device is in active mode. 
         [0049]    Preferably, said two primary windings are connectable to said AC voltage supply via a switch circuit, said switch circuit having a first configuration in which only the first primary winding is connected and a second configuration in which the first and second primary windings are connected in series. 
         [0050]    In the second embodiment, said secondary side comprises a toggle circuit for switching the electrical device from active mode to standby mode and from standby mode to active mode. In that arrangement, said two primary windings may be connectable to the AC voltage supply via a switch circuit, said switch circuit having a first configuration in which only the first primary winding is connected and a second configuration in which the first and second primary windings are connected in series, and said toggle circuit may be arranged to toggle said switch circuit from said first configuration to said second configuration when said electrical device switches from active mode to standby mode and to toggle said switch circuit from said second configuration to said first configuration when said electrical device switches from standby mode to active mode. 
         [0051]    According to a third aspect of the invention, there is provided a method for switching an electrical device from active mode to standby mode, the method comprising the steps of: 
         [0052]    providing a power supply for the electrical device, the power supply comprising a transformer having two primary windings on the primary side and a secondary winding on the secondary side, each primary winding being connectable to an AC voltage supply via a switch circuit and the secondary winding providing an output voltage for the electrical device during active mode; 
         [0053]    disconnecting the secondary winding from the electrical device; and 
         [0054]    switching the switch circuit on the primary side from a first position in which the two primary windings are connected in parallel to a second position in which the two primary windings are connected in series. 
         [0055]    According to the third aspect of the invention, there is provided a method for switching an electrical device from active mode to standby mode, the method comprising the steps of: 
         [0056]    providing a power supply for the electrical device, the power supply comprising a transformer having two primary windings on the primary side and a secondary winding on the secondary side, each primary winding being connectable to an AC voltage supply via a switch circuit and the secondary winding providing an output voltage for the electrical device during active mode; 
         [0057]    disconnecting the secondary winding from the electrical device; and 
         [0058]    switching the switch circuit on the primary side from a first position in which only one of the two primary windings is connected to a second position in which the two primary windings are connected in series. 
         [0059]    According to a fourth aspect of the invention, there is provided a method for switching an electrical device from standby mode to active mode, the method comprising the steps of: 
         [0060]    providing a power supply for the electrical device, the power supply comprising a transformer having two primary windings on the primary side and a secondary winding on the secondary side, each primary winding being connectable to an AC voltage supply via a: switch circuit and the secondary winding providing an output voltage for the electrical device during active mode; 
         [0061]    switching the switch circuit on the primary side from a second position in which the two primary windings are connected in series to a first position in which the two primary windings are connected in parallel; and 
         [0062]    connecting the secondary winding to the electrical device. 
         [0063]    According to a fourth aspect of the invention, there is provided a method for switching an electrical device from standby mode to active mode, the method comprising the steps of: 
         [0064]    providing a power supply for the electrical device, the power supply comprising a transformer having two primary windings on the primary side and a secondary winding on the secondary side, each primary winding being connectable to an AC voltage supply via a switch circuit and the secondary winding providing an output voltage for the electrical device during active mode; 
         [0065]    switching the switch circuit on the primary side from a second position in which the two primary windings are connected in series to a first position in which only one of the two primary windings is connected; and 
         [0066]    connecting the secondary winding to the electrical device. 
         [0067]    Features described in relation to one aspect of the invention may also be applicable to another aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0068]    A known arrangement has already been described with reference to  FIGS. 1 and 2  of the accompanying drawings, of which: 
           [0069]      FIG. 1  shows a conventional power supply for a device operable in active and standby modes; and 
           [0070]      FIG. 2  shows a close approximation to a primary winding of a transformer during standby mode. 
           [0071]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with  FIGS. 3 ,  4   a ,  4   b  and  5  of the accompanying drawings, of which: 
           [0072]      FIG. 3  shows a power supply for a device operable in active and standby modes according to a first embodiment of the invention; 
           [0073]      FIG. 4   a  shows an equivalent circuit to the primary side of  FIG. 3  when switches S 1  and S 2  are at position A; 
           [0074]      FIG. 4   b  shows an equivalent circuit to the primary side of  FIG. 3  when switches S 1  and S 2  are at position B; and 
           [0075]      FIG. 5  shows a power supply for a device operable in active and standby modes according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0076]      FIG. 3  shows a transformer arrangement for a device according to a first embodiment of the invention. As in  FIG. 1 , transformer  301  comprises a primary side and a secondary side. 
         [0077]    The secondary side of the transformer  301  is much the same as that of the conventional arrangement shown in  FIG. 1 . The transformer comprises secondary windings  301   c ,  301   d  and  301   e . Secondary winding  301   c  provides the supply voltage to standby circuit  307 , via voltage regulator  306 , and secondary windings  301   d  and  301   e  provide the supply voltage to the main device  309  (i.e. the output load) via switches  311  and  313  respectively. The device may be put into standby mode by opening at least one of switches  311  and  313  so that the supply voltage is no longer supplied to or, in the case when only one switch is opened, adequate for the main device  309 . It is most preferable that all switches connected to the main device  309  are open during standby mode so that the main device  309  does not draw any power, and so that the main device  309  is not damaged by a voltage drop in the secondary winding  301   c . Those switches  311  and  313  may be operated by standby circuit  307  directly, by remote control or under some other form of control (e.g. automatic standby after a certain time period of inactivity). 
         [0078]    The primary side of transformer  301  is rather different from, conventional arrangements, however. The transformer  301  comprises two primary windings  301   a  and  301   b . Primary windings  301   a  and  301   b  are connected to AC supply  303  via switch  305 , the nature of the connection depending on the position of switches S 1  and S 2  in switch circuit  315 . Switch circuit  315  is controlled by standby circuit  307  on the secondary side. Standby circuit  307  acts as a toggle switch to activate active and standby modes. If switches S 1  and S 2  are both at position A, the arrangement is equivalent to the arrangement shown in  FIG. 4   a  i.e. windings  301   a  and  301   b  are in parallel. On the other hand, if switches S 1  and S 2  are both at position B, the arrangement is equivalent to the arrangement shown in  FIG. 4   b  i.e. windings  301   a  and  301   b  are in series. 
         [0079]    Operation of the  FIG. 3  arrangement will now be described. During active mode (i.e. normal operation), standby circuit  307  switches S 1  and S 2  to position A, configuring the windings  301   a  and  301   b  in parallel to provide power to the device  309 . When the main device  309  is put into standby mode, switches  311  and  313  are opened and then switches S 1  and S 2  are switched to position B. Thus, the two primary windings  301   a  and  301   b  are now in series. This is equivalent to doubling the number of turns (N) in the primary winding. Referring to equation (4), since the AC frequency, the AC voltage and the core cross-section remain constant, this increase in N results in a decrease in the maximum flux density B max . 
         [0080]    Thus, during standby mode, we see a decrease in B max . Referring to equation (5), which relates to hysteresis losses and equation ( 6 ), which relates to eddy-current losses, we see that, with this decrease in B max , the core losses are decreased. Of course, with an increase in the number of turns N, we also see an increase in the impedance of the winding R. Referring to equation (7), which relates to copper losses, this might result in an increased copper loss. However, the increased impedance R also results in an associated decrease in current flowing through the winding. Referring to equation (7), we see that this results in a decrease in copper loss. Since the copper loss is proportional to R but proportional to the square of the current, the overall result is a decrease in copper loss. 
         [0081]    Thus, with the arrangement of  FIG. 3 , the core losses and the copper losses in the primary winding during standby mode are all decreased. 
         [0082]    When the device is “woken up” from standby mode and returned to active mode, switches S 1  and S 2  are switched to position A and then switches  311  and  313  are closed. This returns the windings  301   a  and  301   b  to their parallel configuration which supplies normal power to the device  309 . 
         [0083]    In the above description, when switching from active mode to standby mode, the switches  311  and  313  on the secondary side are opened first and afterwards the switches S 1  and S 2  on the primary side are switched to position B. Similarly, when switching from standby mode to active mode, the switches S 1  and S 2  on the primary side are first switched to position A and afterwards the switches  31  and  313  are closed. However, the timing of the switching is not critical. When switching from active mode to standby mode, as the number of turns in the primary winding increases (e.g. doubles), the voltages in the secondary windings decreases (e.g. halved). Since the voltages are decreased, so are lower than the voltages during normal operation in active mode, there is not normally any danger of overload to the main device  309  so it is not necessary to open the secondary side switches before switching the primary side switches. When switching from standby mode to active mode, the voltage regulator alleviates any voltage fluctuation problems by preventing any voltage surge from damaging components. 
         [0084]    In the above embodiment, voltage regulator  306  is included before the standby circuit  307 . Such a voltage regulator would usually be required to maintain a constant voltage supply to the standby circuit during both standby and normal operation irrespective of the actual voltage across the secondary windings, but, although preferred, is not always strictly necessary. The voltage regulator  306  may be removed from the standby circuit if the standby circuit is rated to handle a wide voltage band. 
         [0085]      FIG. 5  shows a transformer arrangement for a device according to an alternative embodiment of the invention. As in  FIGS. 1 and 3 , transformer  401  comprises a primary side and a secondary side. The secondary side of the transformer  401  is identical to the arrangement shown in  FIG. 3 . That is, the transformer  401  comprises secondary windings  401   c ,  401   d  and  401   e . Secondary winding  401   c  provides the supply voltage to standby circuit  407  via voltage regulator  406  and secondary windings  401   d  and  401   e  provide the supply voltage to the main device  409  via switches  411  and  413  respectively. 
         [0086]    However, the primary side of transformer  401  differs from the primary side of transformer  301  of  FIG. 3 . The transformer  401  comprises two primary windings  401   a  and  401   b . Primary windings  401   a  and  401   b  are connected to AC supply  403  via switch  405 , the nature of the connection depending on the position of switch S 3  in switch circuit  415 . Switch circuit  415  is controlled by standby circuit  407  on the secondary side. If switch S 3  is at position D, current only flows in winding  401   b . When switch S 3  is at position C, current flows in both windings  401   a  and  401   b  in a series arrangement. Even with this difference in the primary side of transformer  401  compared to the primary side of transformer  301 , the two circuits still function identically, where the core losses and the copper losses in the primary winding during standby mode are all decreased. There is one less switch prone to failure in this arrangement compared to that of the arrangement in  FIG. 3 . 
         [0087]    It should be noted that, in the embodiment of  FIG. 5 , the wire used in winding  401   b  cannot be thinner because the thickness of the wire is derived from a current rating of the transformer  401 . As such, winding  401   a  has to be added to winding  401   b . Hence, the size of transformer  401  is increased. 
         [0088]    In the  FIG. 5  arrangement, only one of the primary windings is connected during active mode, whereas both primary windings are connected during standby mode. In  FIG. 5 , this is achieved by switching the switch S 3  between positions C and D. Note, however, that the arrangement of  FIG. 3  could be used to achieve the same result. For example, if switch S 1  is at position A and switch S 2  is at position B, winding  301   a  will be connected and winding  301   b  will not be connected. Similarly, if switch S 1  is at position B and Switch S 2  is at position A, winding  301   b  will be connected and winding  301   a  will not be connected. 
         [0089]    Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.