Abstract:
There is provided a power supply for an electrical device, an electrical device including such a power supply and a method for supplying power to an electrical device. The power supply comprises a transformer having primary and secondary windings. The primary winding is connectable to a AC voltage supply and circuitry on the secondary side is arranged to provide a DC output voltage for the electrical device. The power supply also comprises a switch between the primary winding of the transformer and the AC supply, a rectifier for rectifying the AC voltage, and a current limiter. The switch is arranged to switch on at some point as the rectified AC voltage increases, once it has reached a non-zero value, thereby providing a current flow through the primary winding and hence through the secondary winding. The current limiter is arranged to limit the amount of current flowing through the primary winding. The switch is arranged to switch off before the rectified AC voltage starts to increase again.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a power supply for an electrical device and a method for supplying power to an electrical device. In particular, the invention relates to a power supply which has low power consumption.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is becoming increasingly important to conserve energy and reduce power losses and power supplies with low power consumption are becoming more and more important. Such power supplies find applications in many situations, for example as standby power supplies in electrical devices (e.g. in televisions, washing machines), within external power supplies for supplying power to detect whether the 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) or as standalone power supplies for electrical devices that require low power consumption, including low power external power supplies (e.g. a night light, plugged into the AC wall socket, to provide dim lighting).  
         [0003]     In a first known arrangement, a power supply (used in a number of applications) comprises a transformer, the primary winding of which is connected directly to an AC power supply, the secondary winding of which provides an output voltage for the electrical device. To have low power consumption for this arrangement, the current through the primary winding of the transformer (which is connected directiy to the AC supply ) must be small. In order to have a small current, the impedance of the primary winding, as seen by the AC supply, must be large. With a typical AC supply frequency of 50 or 60 Hz, to have a large impedance in the primary winding, a large inductance will be required. To obtain such a large inductance in the primary winding, more turns are required, which will make the transformer impractically large. Alternatively, to avoid a large transformer, the wire used for the turns can be made thinner, but this means higher resistance which, in turn, means more losses. Essentially, to have a very low power consumption in this arrangement, we need a perfect inductor of high inductance and this is just not feasible.  
         [0004]     A second known arrangement is known as a switching mode power supply (SMPS) and there are several different implementations of SMPS. Whilst SMPS provides advantages over the first arrangement, the rapid switching in that arrangement creates a large amount of noise. Also, SMPS are more complex in design and are more costly.  
         [0005]     Currently, standby power supplies, such as those described above, typically have a power consumption of several hundred milliwatts or even as high as several watts. However, a typical power requirement of control circuits to “wake up” a device from standby can be as low as only a few milliwatts. So there is a big mismatch between the actual power required for a device in standby mode and the power consumed.  
       SUMMARY OF THE INVENTION  
       [0006]     According to a first aspect of the invention there is provided a power supply for an electrical device, the power supply comprising: 
        a) a transformer comprising a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being connectable to an AC voltage supply and circuitry on the secondary side being arranged to provide a DC output voltage for the electrical device;     b) a switch between the primary winding of the transformer and the AC voltage supply;     c) a rectifier for rectifying the AC voltage; and     d) a current limiter; 
 
 wherein the switch is arranged to switch on at some point as the rectified AC voltage increases from zero to a maximum and once the rectified AC voltage has increased to a non-zero value, thereby providing a current flow through the primary winding and hence a current flow through the secondary winding, 
 
 wherein the current limiter is arranged to limit the amount of current flowing through the primary winding, and 
 
 wherein the switch is arranged to switch off before the rectified AC voltage starts to increase again. 
       
 
         [0011]     In this arrangement, there is no current drain through the transformer windings until the switch is switched on. Once the switch is switched on, current from the AC supply is transferred to the primary winding and this provides a big enough voltage drop to provide the DC output voltage. The current limiter limits the current flow through the primary winding, however, so that power consumption can be controlled. The non-zero value reached by the rectified AC voltage is preferably a significant proportion of the peak voltage and, even more preferably the peak voltage.  
         [0012]     The AC power supply is typically the mains power supply, for example at 110 VAC, 120 VAC, 230 VAC or 240 VAC at 50 or 60 Hz.  
         [0013]     In one preferred embodiment, the apparatus further comprises a switch timer for switching the switch on and off. The switch timer may be arranged to switch the switch on at some point as the rectified AC voltage increases from zero to a maximum. The switch timer may be arranged to switch the switch off before the rectified AC voltage starts to increase again.  
         [0014]     The switch timer may be an RC timer comprising a resistor and capacitor between a node, whose voltage matches the rectified AC voltage, and ground. In that case, the capacitor may be arranged to charge up as the rectified AC voltage increases from zero to a maximum. When the switch is switched on, the capacitor may be discharged, the energy stored in the capacitor being transferred to the primary winding.  
         [0015]     The switch is preferably arranged to switch on close to each peak of the rectified AC voltage. If the apparatus comprises a RC switch timer, the value of the resistor and capacitor may be chosen such that the switch switches on close to each peak of the rectified AC signal. This maximizes the current through the primary winding of the transformer, by providing a maximized voltage across it when the switch is switched on.  
         [0016]     In one embodiment, the current limiter comprises at least one charge storage device. In a preferred embodiment, the current limiter comprises two charge storage devices. The or each charge storage device may be a capacitor. The value of the capacitor or capacitors may be chosen appropriately to limit the current flow through the primary winding to a desired current level.  
         [0017]     The power supply may be arranged such that current stops flowing through the primary winding once the at least one charge storage device of the current limiter has substantially fully charged i.e. the switch may be arranged to switch off once the charge storage device or devices of the current limiter have substantially fully charged. If the arrangement includes a switch timer, the switch timer may be arranged to switch the switch off once the charge storage device or devices of the current limiter have fully charged and this preferably takes place at some point as the rectified AC signal decreases from its peak to zero. Once the switch is switched off, no current flows through the windings of the transformer so, as already mentioned, the amount of current flow through the windings can be controlled by setting the value of the charge storage device or devices of the current limiter appropriately.  
         [0018]     In one embodiment, the switch timer is operable with a switch timer reset, the switch timer reset being arranged to reset the switch timer after the switch has been switched off i.e. once current has stopped flowing through the transformer windings. Resetting the switch allows the switch to switch on again when the rectified AC signal is increasing from zero to its next maximum. If the switch timer is an RC timer, the switch timer may be reset once the capacitor has fully discharged, which may occur as the rectified AC voltage decreases from its maximum to zero.  
         [0019]     Thus, in one preferred embodiment, operation is as follows. As the rectified AC voltage increases from zero to a maximum, the capacitor of the RC switch timer is charging up and once it has charged a certain amount, which preferably coincides with the peak of the rectified AC voltage, the switch timer switches the switch on, thereby providing a current flow through the windings corresponding to in rush of current from the AC supply and, less importantly, discharge of the RC switch timer capacitor. Along with the current through the windings there is a current flow through the charge storage device or devices of the current limiter. As the current flows through the windings, the charge storage device or devices of the current limiter charge up and, once fully charged, the switch switches off, so that current stops flowing through the windings and power consumption is limited. Once the RC switch timer capacitor has fully discharged, by transferring its stored energy across the windings, the switch timer is reset. This takes place as the rectified AC signal falls from its maximum to zero once again so that the switch is ready to switch on as the rectified AC voltage increases once again.  
         [0020]     In one preferred embodiment, the switch is arranged to use positive feedback to effect fast switching from off to on. The fast switching from off to on means that the time for a voltage to drop across the switch is minimized and this reduces power loss in the switch itself.  
         [0021]     In that embodiment, the switch may comprise a first transistor and a second transistor, the collector of the first transistor being coupled to the base of the second transistor. In addition, the collector of the second transistor may be coupled to the base of the first transistor and this may be via a feedback capacitor. This arrangement may provide positive feedback because, as the voltage at the collector of the second transistor rises, the voltage at the base of the first transistor rises too, which further increases the voltage at the second transistor collector and so on.  
         [0022]     The power supply may further comprise a voltage limiter for preventing breakdown of the device at high voltages. The voltage limiter may comprise a charge storage device arranged to charge up as the rectified AC voltage increases from zero to a maximum. The charge storage device may be a voltage limiting capacitor.  
         [0023]     In one preferred embodiment of the invention, the rectifier is arranged to full-wave rectify the AC voltage. This means that the rectified AC voltage increases from zero to a peak twice in every cycle of the original AC signal.  
         [0024]     Although a preferred embodiment, where the rectifier full-wave rectifies the AC voltage, has been described, it is of course possible that the rectifier only half-wave rectifies the AC voltage. In that case, the rectified AC voltage will go from zero to a maximum only once in each AC cycle.  
         [0025]     The circuitry on the secondary side may provide the DC output voltage for the electrical device via a charge storage device (e.g. a capacitor) which charges up during each AC cycle. In that arrangement, the capacitor is preferably located between ground and an output node so that, as the capacitor charges up during each AC cycle, the voltage at the output node increases towards a steady state DC voltage .  
         [0026]     The power supply may further comprise circuitry for reducing electromagnetic emission caused by the switching of the switch between on and off modes. Electromagnetic emission (known as ringing) may be caused by the rapid on and off switching and this can be reduced by the use of appropriate circuitry. In one embodiment, the circuitry comprises a capacitor and a resistor appropriately positioned between the secondary winding and the output node.  
         [0027]     The power supply may further comprise a regulator for regulating the DC output voltage. This is useful when the load requires a particularly steady DC voltage supply. The regulator may be located between the output voltage and the switch and may be arranged to switch the switch off if the output voltage exceeds a selected threshold. The regulator may comprise a zener diode.  
         [0028]     According to the invention, there is also provided an electrical device comprising a power supply as described above.  
         [0029]     According to the first aspect of the invention, there is also provided a power supply for an electrical device, the power supply comprising: 
        a) a transformer comprising a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being connectable to an AC voltage supply and circuitry on the secondary side being arranged to provide a DC output voltage for the electrical device;     b) a switch between the primary winding of the transformer and the AC voltage supply;     c) a rectifier for full wave rectifying the AC voltage; and     d) a current limiter comprising a first capacitor and a second capacitor; 
 
 wherein the switch is arranged to switch on at some point as the rectified AC voltage increases from zero to a maximum in each half cycle of the rectified AC voltage and once the rectified AC voltage has reached a non-zero value, thereby providing a current flow through the primary winding and hence a current flow through the secondary winding, 
 
 wherein the current limiter is arranged to limit the amount of current flowing through the primary winding by stopping the current flowing through the primary winding when the first and second capacitors are fully charged, and 
 
 wherein the switch is arranged to switch off as the rectified AC voltage decreases from a maximum to zero in each half cycle of the rectified AC voltage. 
       
 
         [0034]     According to a second aspect of the invention, there is provided a method for supplying power to an electrical device, the method comprising the steps of: 
        a) providing a transformer having a primary winding and a secondary winding, the primary winding being connected to an AC voltage supply via a switch;     b) providing a rectifier for rectifying the AC voltage;     c) as the rectified AC voltage increases from zero to a maximum, once the rectified AC voltage has increased to a non zero value, switching on the switch to provide a current flow through the primary winding and hence a current flow through the secondary winding, the amount of current flowing through the primary winding being limited by a current limiter;     d) converting the current flow through the secondary winding to a DC output voltage for the electrical device; and     e) switching off the switch before the rectified AC voltage starts to increase again.        
 
         [0040]     In this method, there is no current drain through the transformer windings until the switch is switched on. Once the switch is switched on, current from the AC supply is transferred to the primary winding, which provides a big enough voltage drop to provide a DC output voltage for the electrical device. The current flowing through the primary winding is limited by the current limiter, however, so that the power consumption can be controlled.  
         [0041]     The AC power supply is typically the mains power supply, for example at 110 VAC, 120 VAC, 230 VAC or 240 VAC at 50 or 60 Hz.  
         [0042]     Step c) of switching on the switch may comprise a switch timer switching on the switch. Step e) of switching off the switch may comprise a switch timer switching off the switch. The switch timer may be arranged to switch the switch on at some point as the rectified AC voltage increases from zero to a maximum. The switch timer may be arranged to switch the switch off before the rectified AC voltage starts to increase again.  
         [0043]     The switch timer may be an RC timer comprising a resistor and capacitor between a node, whose voltage matches the rectified AC voltage, and ground. In that case, the capacitor may be arranged to charge up as the rectified AC voltage increases from zero to a maximum. When the switch is switched on, the capacitor may be discharged, the energy stored in the capacitor being transferred to the primary winding.  
         [0044]     Preferably, step c) of switching on the switch comprises switching on the switch close to each peak of the rectified AC voltage. If the step of switching on the switch comprises a RC switch timer switching on the switch, the value of the resistor and capacitor may be chosen such that the switch switches on at the peak of the rectified AC signal. This maximizes the current through the primary winding of the transformer by providing a maximized voltage across it when the switch is on.  
         [0045]     In one embodiment, the current limiter comprises at least one charge storage device. In a preferred arrangement, the current limiter comprises two charge storage devices. The or each charge storage device may be a capacitor.  
         [0046]     In one embodiment, step e) nf switching off the switch may comprises switching off the switch once the charge storage device or devices of the current limiter have substantially fully charged. If the arrangement includes a switch timer, the switch timer may be arranged to switch the switch off once the charge storage device or devices of the current limiter have fully charged. In that arrangement, once the switched is switched off, no current flows through the windings. Thus, the amount of current drain through the windings can be controlled by setting the size of the charge storage devices appropriately.  
         [0047]     The method may further comprise the step of charging up a charge storage device as the rectified AC voltage increases from zero to a maximum, the charge storage device acting as a voltage limiter for preventing breakdown of the device at high voltages. In that case, the energy stored in the charge storage device is transferred to the primary winding of the transformer when the switch is switched on.  
         [0048]     In one embodiment, the switch timer is operable with a switch timer reset for resetting the switch timer after the switch has been switched off. If the switch timer is an RC switch timer, the switch timer reset may be arranged to reset the switch once the capacitor of the RC timer has fully discharged. Resetting the switch allows the switch to switch on again as the rectified AC signal increases from zero to a maximum once again.  
         [0049]     In one preferred embodiment, the switch is arranged to use positive feedback to effect fast switching from off to on. The fast switching from off to on means that the time for a voltage to drop across the switch is minimized and this reduces power loss in the switch itself.  
         [0050]     In that embodiment, the switch may comprise a first transistor and a second transistor, the collector of the first transistor being coupled to the base of the second transistor. In addition, the collector of the second transistor may be coupled to the base of the first transistor and this may be via a feedback capacitor. This arrangement may provide positive feedback because, as the voltage at the collector of the second transistor rises, the voltage at the base of the first transistor rises too, which further increases the voltage at the second transistor collector and so on.  
         [0051]     Preferably, the rectifier is arranged to full-wave rectify the AC voltage. This means that the rectified AC voltage increases from zero to a peak twice in every cycle of the original AC signal.  
         [0052]     In one embodiment, step d) of converting the voltage peak in the secondary winding to a DC output voltage for the electrical device comprises charging up a capacitor during each AC cycle, the voltage across the capacitor being the DC output voltage. In that arrangement, the capacitor is preferably located between ground and an output node so that, as the capacitor charges up during each AC cycle, the voltage at the output node increases towards a steady state DC voltage.  
         [0053]     The method may further comprise the step of regulating the DC output voltage. This is useful when the load requires a particularly steady DC voltage supply. The regulator may be located between the output voltage and the switch and may be arranged to switch the switch off if the output voltage exceeds a selected threshold.  
         [0054]     In one embodiment, steps c), d) and e) of the method are repeated until a steady state DC output voltage for the electrical device is obtained.  
         [0055]     According to the second aspect of the invention, there is also provided a method for supplying power to an electrical device, the method comprising the steps of: 
        a) providing a transformer having a primary winding and a secondary winding, the primary winding being connected to a AC voltage supply via a switch;     b) providing a rectifier for full wave rectifying the AC voltage;     c) as the rectified AC voltage increases from zero to a maximum in each half cycle of the rectified AC voltage, once the rectified AC voltage has reached a non-zero value, switching on the switch to provide a current flow through the primary winding and hence a current flow through the secondary winding, the amount of current flowing through the primary winding being limited by a current limiter comprising two capacitors;     d) converting the current flow through the secondary winding to a DC output voltage for the electrical device; and     e) switching off the switch as the rectified AC voltage decreases from a maximum to zero in each half cycle of the rectified AC voltage.        
 
         [0061]     According to the second aspect of the invention, there is also provided a method for supplying power to an electrical device, the method comprising the steps of: 
        a) providing a transformer having a primary winding and a secondary winding, the primary winding being connected to a AC voltage supply via a switch;     b) providing a rectifier for rectifying the AC voltage;     c) performing the following steps at least once in each AC cycle: 
            i) as the rectified AC voltage increases from zero to a maximum, once the rectified AC voltage has increased to a non zero value, switching on the switch to provide a current flow through the primary winding and hence a current flow through the secondary winding, the amount of current flowing through the primary winding being limited by a current limiter;     ii) converting the current flow through the secondary winding to a DC output voltage for the electrical device by charging up an output charge storage device, the voltage across the charge storage device being the DC output voltage; and     iv) switching off the switch before the rectified AC voltage starts to increase again, 
 
 wherein, the output charge storage device steadily charges up such that, after a number of AC cycles, the output charge storage device is substantially fully charged so as to provide a steady state DC output voltage for the electrical device. 
   
               
 
         [0068]     According to a third aspect of the invention, there is provided an electrical device connectable to an AC voltage supply and operable in each of normal mode and standby mode, the electrical device comprising: 
        a main power supply for providing power during normal mode;     a control for switching the main power supply on and off; and     a standby power supply for providing power, during standby mode, to the control for switching on the main power supply, the standby power supply comprising:     a) a transformer comprising a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being connectable to an AC voltage supply and circuitry on the secondary side being arranged to provide a DC output voltage for the electrical device;     b) a switch between the primary winding of the transformer and the AC voltage supply;     c) a rectifier for rectifying the AC voltage; and     d) a current limiter; 
 
 wherein the switch is arranged to switch on at some point as the rectified AC voltage increases from zero to a maximum and once the rectified AC voltage has increased to a non-zero value, thereby providing a current flow through the primary winding and hence a current flow through the secondary winding, 
 
 wherein the current limiter is arranged to limit the amount of current flowing through the primary winding, and 
 
 wherein the switch is arranged to switch off before the rectified AC voltage starts to increase again. 
       
 
         [0076]     The main power supply supplies the power (to the device itself and also to the control) during normal mode whereas the standby power supply supplies the power during standby mode to the control so that the control has the necessary power to switch on the main power supply when the device goes from standby mode to normal mode.  
         [0077]     In the standby power supply, there is no current drain through the transformer windings until the switch is switched on. Once the switch is switched on, current from the AC supply is transferred to the primary winding and this provides a big enough voltage drop to provide the DC output voltage for the electrical device. The current limiter limits the current flow through the primary winding, so that the power consumption when in standby mode can be controlled.  
         [0078]     In a first embodiment, the control may be a receiver for receiving instructions to switch on and off the main power supply. This may be the case when the electrical device is a device which requires power during normal mode for its own operation and power in standby mode for enabling the control to switch back to normal mode from standby mode. Examples of this type of device are a washing machine, radio or microwave oven. The receiver may be a remote control receiver for receiving remote instructions to switch on and off the main power supply. This may be the case for an electrical appliance which is operable in normal mode and standby mode and can be switched between the two modes by the use of a remote control, for example a television, DVD player or a radio or another type of electrical device having normal and standby modes.  
         [0079]     In a second embodiment, the electrical device is an external power supply for an electrical appliance, the device operating in normal mode when the electrical appliance is electrically connected to the device and the device operating in standby mode when the electrical appliance is not electrically connected to the device. Some examples of such external power supplies are a charger for a telephone (either a mobile cellular telephone or a portable landline telephone) and an AC adaptor. Other types of external power supplies are also envisaged.  
         [0080]     If the electrical device is an external power supply, the control may be a sensor for sensing when the electrical appliance is electrically connected to the device. Thus, when the sensor senses that the electrical appliance is electrically connected to the device (e.g. when a telephone is placed in the cradle for charging), it may be arranged to switch on the main power supply using power provided by the standby power supply. When the sensor senses that the electrical appliance is no longer electrically connected (e.g. the telephone has been removed from the charging cradle), it may be arranged to switch off the main power supply, at which time, the power will be provided by the standby power supply.  
         [0081]     According to the third aspect of the invention, there is also provided an external power supply for an electrical appliance, the external power supply being connectable to an AC voltage supply and operable in normal mode when the electrical appliance is electrically connected to the external power supply and in standby mode when the electrical appliance is not electrically connected to the external power supply, the external power supply comprising: 
        a main power supply for providing power during normal mode;     a sensor for sensing when the electrical appliance is electrically connected to the external power supply and for switching the main power supply on and off; and     a standby power supply for providing power, during standby mode, to the sensor for switching on the main power supply, the standby power supply comprising:     a) a transformer comprising a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being connectable to an AC voltage supply and circuitry on the secondary side being arranged to provide a DC output voltage for the electrical device;     b) a switch between the primary winding of the transformer and the AC voltage supply;     c) a rectifier for rectifying the AC voltage; and     d) a current limiter; 
 
 wherein the switch is arranged to switch on at some point as the rectified AC voltage increases from zero to a maximum and once the rectified AC voltage has increased to a non-zero value, thereby providing a current flow through the primary winding and hence a current flow through the secondary winding, 
 
 wherein the current limiter is arranged to limit the amount of current flowing through the primary winding, and 
 
 wherein the switch is arranged to switch off before the rectified AC voltage starts to increase again. 
       
 
         [0089]     Some examples of electrical devices have been given but it will be understood by the skilled reader that the invention finds application with many different devices, not just those listed. In addition it should be understood that features which are described in relation to one aspect of the invention may also be applicable to another aspect of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0090]     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 the accompanying drawings, of which:  
         [0091]      FIG. 1  is a block diagram of a first embodiment of the invention;  
         [0092]      FIG. 2  shows a circuit implementation of the first embodiment of the invention shown in  FIG. 1 ;  
         [0093]      FIG. 3  is a graph of the voltage at node  200  of  FIG. 2  over time;  
         [0094]      FIG. 4   a  is a graph of the voltage across primary winding of transformer X 1  of  FIG. 2  over time;  
         [0095]      FIG. 4   b  is a magnified view of one cycle of  FIG. 4   a;    
         [0096]      FIG. 5   a  is a graph of the current drain through primary winding of transformer X 1  of  FIG. 2  over time;  
         [0097]      FIG. 5   b  is a magnified view of one cycle of  FIG. 5   a;    
         [0098]      FIG. 6   a  is a graph of the voltage across secondary winding of transformer X 1  of  FIG. 2  over time;  
         [0099]      FIG. 6   b  is a magnified view of one cycle of  FIG. 6   a;    
         [0100]      FIG. 7  is a graph of the voltage at output node  206  of  FIG. 2  over time;  
         [0101]      FIG. 8  is a block diagram of a second embodiment of the invention;  
         [0102]      FIG. 9  shows a first circuit implementation of the second embodiment of the invention shown in  FIG. 8 ;  
         [0103]      FIG. 10  shows a second circuit implementation of the second embodiment of the invention shown in  FIG. 8 ;  
         [0104]      FIG. 11  shows the standby power supply of the invention in use in a first application;  
         [0105]      FIG. 12  shows the first application as shown in  FIG. 11  including the second embodiment of the invention as shown in  FIG. 8 ; and  
         [0106]      FIG. 13  shows the standby power supply of the invention in use in a second application. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0107]      FIG. 1  is a block diagram of a first embodiment of the invention and  FIG. 2  shows a circuit implementation of that embodiment.  
         [0108]     Referring to  FIGS. 1 and 2 , the input is AC power supply V 1 . The AC power supply may be any AC voltage at any frequency e.g. 110 VAC, 120 VAC, 230 VAC or 240 VAC at 50 or 60 Hz. The AC power supply V 1  is connected to a current limiter  101  comprising two capacitors C 1  and C 2 . As will be described, the power consumption may be controlled by changing the value of those capacitors. The AC signal is then rectified by rectifier  103  formed by four diodes D 1 , D 2 , D 3  and D 4 . Note that the rectifier is a full-wave rectifier providing a DC output voltage with two maxima per AC cycle. Capacitor C 3  acts as voltage limiter  105  to limit the voltage at node  200  in order to prevent the breakdown of the device due to exceedingly high voltages. If the circuit elements have a high breakdown voltage i.e. above the maximum of the AC supply peak voltage, then capacitor C 3  may be omitted. Capacitor C 3  will be discussed further below.  
         [0109]     Arrangement  111  is a switch located between the AC power supply and the primary winding of transformer X 1  so that, when the switch is on, there is a current drain through the primary winding and when the switch is off, there is no current drain through the primary winding. Resistor R 1  and capacitor C 4  together form RC timer  107  that controls the timing of the switching of the switch  111 , as will be described below. In addition, resistor R 1  is chosen to be large and capacitor C 4  small so that there is minimal current drain to prevent loss. Diode D 5  acts as timer reset  109  for the RC timer  107  by providing a discharge path for capacitor C 4  when the AC signal at node  200  is low after the switch  111  is switched on.  
         [0110]     The switch  111  is formed by two transistors Q 1  and Q 2 , two resistors R 2  and R 3  and capacitor C 5  and is connected to transformer X 1 . Switch  111  is arranged to switch on very quickly through the use of positive feedback. The advantages of fast switching on are discussed below.  
         [0111]     At the secondary side of the transformer X 1 , diode D 6  acts as a rectifier and capacitor C 7  is a filter capacitor. Capacitor C 7  charges up, to provide a steady state DC voltage at output node  206  for the load R load .  
         [0112]     The operation of the arrangement of  FIG. 2  is as follows. During a first half cycle of operation, as the voltage of the rectified AC signal at node  200  rises, capacitors C 1  and C 2  are discharging (from the previous half cycle) and capacitor C 4  (and capacitor C 3  if present) is/are being charged up.  
         [0113]     When the voltage at node  202  (which is also the base voltage of transistor Q 1 ), is sufficiently high due to the charging up of capacitor C 4  (this happens near the peak of the AC signal), the base-emitter of transistor Q 1  is forward biased, causing transistor Q 1  to switch on. As Q 1  is switched on, the voltage at node  203 , (which is also the base voltage of transistor Q 2 ), drops. This causes transistor Q 2  to switch on resulting in a rapid draining of current through the primary side of transformer X 1  and through resistor R 3 , which means an increase in voltage at node  204 . This rise in voltage is translated back to node  202  via the feedback capacitor C 5 . This means that the voltage at node  202  and consequently the base-emitter voltage of transistor Q 1  rise even more rapidly, causing more current to conduct through collector-emitter of transistor Q 1 , leading to even more current drain through emitter-collector of transistor Q 2  and a further voltage increase at node  204 . That is, the arrangement provides a system of positive feedback which produces very fast switching on.  
         [0114]     The reason that fast switching is advantageous is in order to reduce losses in the switch itself. As the switch  111  is switched on, a current flows through the switch. Voltage present across the switch (in this case especially the emitter-collector voltage of transistor Q 2 ) will result in losses. Ideally the switch should switch on instantaneously so that the time to drop the voltage across the switch to ground is instantaneous. (The voltage across the switch is indicated at node  207 .) However, in practice, instantaneous switching is not possible but fast switching will shorten the time to drop the voltage across the switch which will, in turn, reduce losses. Thus, using positive feedback to increase the switching speed, reduces loss in the switch itself.  
         [0115]     As described, once the rectified signal voltage is at or near its peak, the switch  111  is switched on. This closes the circuit and causes a rapid rush of current through the primary side of the transformer X 1  and through C 1  and C 2 , which charges C 1  and C 2 . When this happens, the voltage at node  200  falls rapidly to ground because node  200  is shored to ground through the primary winding of the transformer X 1  when switch  111  is switched on and because the capacitors C 1  and C 2  in the AC input line act as impedances and there is a voltage drop across C 1  and C 2  when node  200  falls to ground. Once C 1  and C 2  are fully charged, the current stops flowing (i.e. the switch is effectively switched off). This limits the amount of current to the primary winding of the transformer in each cycle.  
         [0116]     While switch  111  is on, capacitors C 3  and C 4  are discharging through the primary winding of the transformer X 1 . Once capacitor C 4  has discharged, the RC timer  107  is reset and the RC timer  107  and switch  111  await the next peak from the rectified AC signal at node  200  in the next half cycle. During the next half cycle, as the rectified AC signal increases from zero to a maximum, capacitors C 1  and C 2 , which are now charged up, can discharge. As already mentioned, resistor R 1  is selected to be large so that negligible current is drained through it. Thus, all the current will be drained through the primary side of transformer X 1 , keeping losses to a minimum. It will be appreciated that the direction of the voltages across C 1  and C 2  alternates in each half cycle because of the direction of the original AC signal.  
         [0117]     The short pulse of current drain in the primary side of the transformer X 1  results in a corresponding pulse of current flow through the secondary side of transformer X 1 . At the secondary side of the transformer X 1 , diode D 6  acts as a rectifier and capacitor C 7  is a filter capacitor. At each half cycle of operation, there is a current pulse through the secondary side of the transformer X 1  and capacitor C 7  is charged up bit by bit due to those current pulses until a steady state DC voltage is reached at output node  206 . This DC voltage is provided to the load R load . (R load  may, for example, be a remote control receiver which requires power during standby mode.) The output node  206  provides the necessary output voltage. The value of capacitor C 7  is appropriately chosen to ensure the proper functioning of R load  at the required voltage.  
         [0118]     As already mentioned, once capacitors C 1  and C 2  re fully charged, current stops flowing through the windings. Thus, the value of capacitors C 1  and C 2  can be chosen to set the current flow through the windings to a desired level. This controls the amount of power consumed.  
         [0119]     Note that it is important, in this embodiment, that the diode D 5  is present as this enables the switch to reset each cycle. If the diode D 5  were not present, the switch  111  would never reset so the arrangement would not work, because, after switching on the first time, it would not switch off and the arrangement would simply work like the prior art arrangements with a constant current drain through the transformer windings and insufficient voltage across the windings to supply DC output voltage.  
         [0120]     Also note that it is preferable that the switch  111  switches on as close to the peak of the AC signal as possible. This creates a maximum voltage peak across the windings when switch  111  is switched on. If the switch  111  were to switch on at the start of the AC signal (i.e. when the AC voltage is at zero), the arrangement would not work as it would simply be as if the switch were not there, and there would be no sudden in rush of current from the AC supply and the capacitor C 4  (and C 3  if present) would not have time to charge up. That is, the switch must switch on once the rectified AC signal has increased a bit, and the switch preferably switches on close to the peak of the rectified AC signal as this maximizes the voltage peak.  
         [0121]     As already mentioned, C 3  acts as a voltage limiter and, in certain circumstances, may be omitted. However, if C 3  is present, it will charge up, along with C 4 , as the rectified AC signal increases to its peak. Thus when switch  111  is switched on, the energy stored in both C 4  and C 3  is transferred across to the transformer windings. In practice, the contribution from C 4  (and C 3  if present) to the voltage peak is minimal; the voltage peak is primarily provided by the in rush of current directly from the AC supply.  
         [0122]     Capacitor C 6  and resistor R 4  together form snubber circuit  117 . The function of snubber circuit  117  is to reduce the ringing due to the transience caused by the switching. This is added in practical applications to reduce electromagnetic emission from the circuit due to this ringing, but the arrangement will still operate without the snubber circuit  117 .  
         [0123]      FIGS. 3, 4   a ,  4   b ,  5   a ,  5   b ,  6   a ,  6   b  and  7  show various properties with respect to time at points on the circuitry of  FIG. 2 . The figures illustrate the processes taking place during each AC cycle as the voltage at the output node is rising to a steady state voltage.  
         [0124]      FIG. 3  is a graph of the voltage at node  200  over time. In each cycle, the voltage at node  200  rises to a peak. Then, when the switch  111  switches, resulting in current drain through primary side of transformer X 1 , the voltage at node  200  drops to ground. In this example, it can be seen that each cycle takes 10 ms i.e. the frequency of the rectified AC signal is 100 Hz so the AC power supply operates at 50 Hz.  
         [0125]      FIG. 4   a  is a graph of the voltage across primary winding of transformer X 1  over time. In each cycle, there is a voltage peak corresponding to the current drain through primary side of transformer X 1  as switch  111  is switched. The voltage peaks shown in  FIG. 4   a  are very spiky. Of course, the voltage peaks are not instantaneous and a magnified view of one cycle of  FIG. 4   a  is shown in  FIG. 4   b . Note that, with this arrangement, the voltage peaks are large (much larger than they would be with the prior art arrangements which have no switch between the transformer primary winding and the AC supply) so that the DC output voltage can be provided to the load.  
         [0126]      FIG. 5   a  is a graph of the current through primary winding of transformer X 1  over time. In each cycle, there is a spiky current drain as switch  111  switches. The current drain peaks correspond to the voltage peaks of  FIG. 4   a . Of course, the current drains are not instantaneous and a magnified view of one cycle of  FIG. 5   a  is shown in  FIG. 5   b . The length of time of the current drain through the primary winding is determined by the supply voltage, the inductance in the transformer winding and, if C 3  (voltage limiter) is present, by capacitance C 3 .  
         [0127]     With each current drain through primary winding of transformer X 1 , there is a corresponding current pulse through secondary winding of transformer X 1 .  FIG. 6   a  is a graph of the voltage across secondary winding of transformer X 1  over time. It can be seen that, in each cycle, there is a voltage peak corresponding to the current pulse.  FIG. 6   b  shows a magnified view of one cycle of  FIG. 6   a.    
         [0128]     As described, with each current pulse through secondary winding of transformer X 1 , the capacitor C 7  charges up a little bit i.e. the voltage at node  206  increases a little bit. That is, over several cycles, the capacitor C 7  charges up bit by bit and the voltage at node  206  increases bit by bit.  FIG. 7  is a graph of the voltage at node  206 . It can be seen that the voltage at output node  206  rises at each switching cycle and ultimately reaches a steady state DC voltage.  
         [0129]      FIG. 8  is a block diagram of a second embodiment of the invention and  FIG. 9  shows a first circuit implementation of that embodiment.  
         [0130]     It can be seen that the second embodiment illustrated in  FIG. 8  is the same as the first embodiment except for the addition of regulator  119 . That is, in summary, the arrangement includes AC power supply V 1  , current limiter  101  (effected by two capacitors C 1  and C 2 ), rectifier  103  (effected by four diodes D 1 , D 2 , D 3  and D 4 ), voltage limiter  105  (capacitor C 3 ), RC timer  107  (effected by resistor R 1  and capacitor C 4 ) and timer reset  109  (diode D 5 ) for switch  111  (effected by transistors Q 1  and Q 2 , resistors R 2  and R 3  and capacitor C 5 ), transformer X 1 , rectifier  113  (diode D 6 ), filter  115  (capacitor C 7 ) and optional snubber circuit  117  (effected by capacitor C 6  and resistor R 3 ).  
         [0131]     The arrangement additionally includes regulator  119 . The function of regulator  119  is to reduce the fluctuation of the DC voltage at the output (node  206 ) to the load. This is important for loads that require good power supply voltage regulation.  
         [0132]      FIG. 9  shows a first circuit implementation of the  FIG. 8  embodiment. In this implementation, the regulator  119  consists of a transistor Q 3 , a resistor R 6  and a zener diode D 7 . If the output voltage at node  206  (see  FIG. 7 ) becomes too high, the zener diode D 7  will break down. This will forward bias the base emitter of transistor Q 3 , causing transistor Q 3  to switch on. By switching on Q 3 , the charging of C 4  will be stopped as current is drained to ground through resistor R 1  and transistor Q 3 . In effect, the RC timer  107  and consequently switch  111  are switched off. This temporarily stops the transfer of energy from the primary side to the secondary side of the transformer X 1  so stops the capacitor C 7  charging, until the output voltage at node  206  drops to below the breakdown voltage of the zener diode D 7 .  
         [0133]      FIG. 10  shows a second circuit implementation of the  FIG. 8  embodiment. In this implementation, transistor Q 3  is replaced by optocoupler IC 1  in the regulator  119  and resistor R 6  and zener diode D 7  are appropriately connected. Using an optocoupler is advantageous because there is then no physical connection between the primary and secondary sides of the circuit. The optocoupler functions as a switch in the circuit just like transistor Q 3  in  FIG. 9 . When the voltage at output node  206  is sufficiently high such that zener diode D 7  breaks down, the light emitting diode (LED) within the optocoupler emits light and the phototransistor within it is turned on. This causes current to be drained to ground through resistor R 1  and the phototransistor of the optocoupler.  
         [0134]     As already mentioned, the optocoupler means that the primary and secondary sides of the circuit are not physically connected as the switching function is achieved using light. Using an optocoupler may be more acceptable for safety requirements (because there is such a high tension on the primary side of the circuit) as the two sides of the circuit are then isolated.  
         [0135]      FIG. 11  shows the power supply of the invention in use in a first application and  FIG. 12  shows that application including the second embodiment of the invention (as shown in  FIG. 8 ).  FIG. 11  and  12  show the power supply used as a standby power supply in an electrical appliance, for example a television or washing machine. The appliance is directly connected to an AC power supply for providing power for its own operation during normal use. ( FIG. 13  shows the power supply used in an external power supply, for example a mobile cellular telephone charger, and will be discussed below.)  
         [0136]      FIG. 11  shows an appliance  1101  which is connected to an AC power supply (e.g. mains supply). The appliance operates from a main power supply when in operational mode but is able to be switched from operational mode to standby mode and vice-versa. The appliance  1101  typically has a main power supply  1103  and some form of control. In this case, the control function is implemented using a remote control receiver  1105  in the appliance, which may have external control means (e.g. the remote control) and internal control means (e.g. automatic standby after some period of idle). The appliance also includes a power supply  1107  according to the invention for supplying power during standby mode and a control circuit  1109 .  
         [0137]     Operation of the arrangement will now be described in general terms. During normal operation of the appliance  1101 , the main power supply  1103  provides power to the remote control receiver  1105  and for other functions of the appliance. When an instruction is given to the remote control receiver  1105  to put the system into standby mode, main power supply  1103  can be shut down and the remote control receiver  1105  can control the main power supply  1103  to switch off via control circuit  1109 . The supply of power during standby mode to remote control receiver  1105  will then be taken over by standby power supply  1107  so that the remote control receiver  1105  can wait for the instruction to switch on the system. When an instruction is passed to switch on the system through remote control receiver  1105 , standby power supply  1107  can also provide the power to switch on the main power supply  1103  via control circuit  1109  and the remote control receiver  1105  can control the main power supply  1103  to switch on via control circuit  1109 .  
         [0138]      FIG. 12  shows the arrangement of  FIG. 11  using the power supply of  FIG. 8  (i.e. the second embodiment already described) as the standby power supply  1107  and operation in more specific terms will now be described.  
         [0139]      FIG. 12  shows an AC power supply connected to main power supply  1103  and standby power supply  1107 . The main power supply  1103  is connected to an output for the main functions of the appliance when in normal operation. The main power supply is also connected to control circuit  1109  which is connected to the output node  206  of standby power supply  1107  and to the remote control receiver  1105  which acts as the load for standby power supply  1107 . The main power supply  1103  also supplies power to the micro-controller within the remote control device.  
         [0140]     When the main power supply  1103  is on (i.e. during normal operation), the voltage supplied to the remote control receiver  1105  at output node  206  is set to be slightly higher than the breakdown voltage of the zener diode D 7  in regulator  119 . This will cause the base-emifter of transistor Q 3  in regulator  119  to be forward biased, causing transistor Q 3  to switch on. This means that current will be drained to ground via resistor R 1  and transistor Q 3  so that charging of capacitor C 4  is prevented. Thus, the RC timer  107  and consequently the switch  111  are both switched off. This means that, during normal operation when the main power supply  1103  is switched on, the standby power supply  1107  is switched off.  
         [0141]     When the main power supply  1103  is off (i.e. during standby operation), the voltage at output node  206  will drop below the breakdown voltage of zener diode D 7 . As transistor Q 3  is switched off, the RC timer  107  will be switched on and the switch  111  will activate i.e. proceed to switch on and off twice during each AC cycle in accordance with the RC timer  107  and timer reset  109 , so as to provide a pulsed current drain through the secondary winding and thereby steadily charge up the capacitor C 7 . This means that, when the main power supply  1103  is switched off, standby power supply  1107  will be switched on to provide power (DC voltage at output node  206 ) to the remote control receiver  1105  during standby mode. As already mentioned, the standby power supply  1107  may also provide power to switch on the main power supply  1103  via control circuit  1109  when there is an instruction to switch on the system from the remote control receiver  1105 .  
         [0142]      FIG. 13  shows the power supply of the invention in use in a second application.  FIG. 13  shows the power supply used within an external power supply. An external power supply is a device that takes input from the AC power supply and provides power supply, more commonly in the form of a DC voltage, to its load. An example of such an external power supply is a telephone charger.  
         [0143]      FIG. 13  shows an external power supply  1301  for providing external power, which is connected to an AC power supply (e.g. mains supply). During normal operation, the main power supply  1303  will provide the power to the load at the output. Sensor  1305 , which may be a current sensor, switches on the main power supply  1303  when a load is present (e.g. when a device to be charged is connected to the charger) and switches it off when the load is removed. During standby mode, the power is provided by the standby power supply  1307 .  
         [0144]     The arrangement works in a similar way to the arrangement described with reference to  FIGS. 11 and 12 . When the main power supply  1303  is switched on (i.e. during normal mode), the standby power supply  1307  is switched off.  
         [0145]     When the load is removed (i.e. during standby mode), the sensor switches off the main power supply  1303  and the standby power supply  1307  is switched on. The standby power supply  1307  provides the power for the sensor  1305  to switch on the main power supply  1303  from standby mode to normal mode when the load is connected to the external power supply  1301 .  
         [0146]     It can be seen from the above description that the invention provides a power supply with a low power consumption. The power supply may be used in many applications where low power consumption is important. Some examples are as standby power supplies in electrical devices (e.g. in televisions, washing machines, microwaves, stereos and other devices which are operable in normal mode and standby mode), within external power supplies for supplying power to detect whether the electrical device is connected or not and to switch on the main power supply (e.g. within a portable telephone charger) or as standalone power supplies for electrical devices that require low power consumption, including low power external power supplies (e.g. a night light, plugged into the AC wall socket, to provide dim lighting). Other examples of applications may also be envisaged.  
         [0147]     The power consumption of the described power supply can be very low and can certainly be as low as a few milliwatts, which, as already mentioned, is a typical power required to “wake up” a device from standby. This is in contrast to typical power consumption using conventional methods, which is commonly from several hundred milliwatts to several watts. The actual power supplied can be set according to requirements, by changing the values of the circuit components.