Abstract:
A power management device comprises: an input for receiving a transient energy pulse; a first storage section and a second storage section for storing energy from the input; an output; a switching section for selectively connecting the input, first storage section, second storage section and output in at least first and second configurations, wherein in the first configuration the first and second storage sections are connected so as to distribute energy from the transient energy pulse between the first and second storage sections, in the second configuration the respective voltages across the first and second storage sections are combined additively to produce an output voltage at the output, whereby the output voltage after switching to the second configuration is greater than the output voltage before switching to the second configuration.

Description:
[0001]    This application claims the priority under 35 U.S.C. §119 of European patent application no. 10196871.7, filed on Dec. 23, 2010, the contents of which are incorporated by reference herein. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a power management device and method, and more particularly to a device and method for managing power from a transient power source. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is common for electronic circuits have a continuous energy supply, such as a steady dc voltage, during operation. However, in some cases it may be necessary to use a power source that is transitory or transient in nature, such as the energy pulse generated by a piezoelectric generator. Piezoelectric generators and other forms of energy harvesting device are finding use in a wide range of applications where it is undesirable or impossible to provide a wired power supply or where replacement of batteries is impractical. Energy harvesting is also beneficial for the environment, by reducing the need for production and disposal of batteries, which include toxic components, and by making use of renewable energy sources and “waste” energy. An example of an application using a piezoelectric generator is in powering a wireless switch. The switch may emit a radio signal when actuated, for example, to turn on or off a device configured to receive the signal. The switch may be arranged such that actuation strikes a piezoelectric crystal, generating a voltage pulse. This pulse charges a capacitor, which in turn powers a radio circuit that emits the signal, avoiding the need for a battery or other stored power source. 
         [0004]    Where power is supplied by transient energy pulse, it is known to use the energy pulse to charge a capacitor, and the energy stored on the capacitor may then be used to power an electronic circuit or system. An exemplary arrangement is shown in  FIG. 1 , which shows a voltage source  100  that generates a transient signal V in , that is passed to power management circuit  110 . Power management circuit  110  supplies a current I Load  to power an electronic system  120 . The power management circuit  110  is shown in more detail in  FIG. 2 , and includes a capacitor C 1  which acts as a storage element and is charged by the energy pulse received at V in . A diode  220  between the capacitor C and V in  prevents a current flowing from the capacitor to V in  when the voltage across C, V C , is greater than that at V in , such as immediately following the energy pulse. The voltage V C  is supplied to V out , which connects to the circuit or system to be powered (I Load ). 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with the present invention there is provided a power management device comprising: an input for receiving a transient energy pulse; a first storage section and a second storage section for storing energy from the input; an output; a switching section for selectively connecting the input, first storage section, second storage section and output in at least first and second configurations, wherein the first configuration the first and second storage sections are connected so as to distribute energy from the transient energy pulse between the first and second storage sections, in the second configuration the respective voltages across the first and second storage sections are combined additively to produce an output voltage at the output, whereby the output voltage after switching to the second configuration is greater that the output voltage before switching to the second configuration. 
         [0006]    The power management device may be such that the output voltage immediately after switching to the second configuration is greater than the output voltage immediately before switching to the second configuration. 
         [0007]    The switching section may be arranged to switch to the second configuration when the voltage across the output decays to a threshold voltage. 
         [0008]    The switching section may be switched based on output from at least one of: a voltage comparator, or a timer circuit. 
         [0009]    The power management device may further comprise a diode or rectifier between (i) the input and (ii) the first and second storage sections. 
         [0010]    The switching section may switch the input, first storage section, second storage section and output via an intermediate configuration when switching from the first configuration to the second configuration, and the intermediate configuration may isolate the second storage section from the first storage section. 
         [0011]    The switching section may be arranged to switch to a third configuration in which the first storage section is (i) disconnected from the second storage section and the output, and (ii) partially discharged, and the switching section may be arranged to switch from the first configuration to the third configuration, and then to the second configuration. 
         [0012]    The power management device may further comprise a third storage section switchably connectable to the first storage section, wherein in the third configuration the first storage section is connected to the third storage section so as to charge the third storage section by discharge of the first storage section. 
         [0013]    In the third configuration the first storage section and the third storage section may be connected in parallel. 
         [0014]    The power management device may further comprise a load circuit arranged to receive energy from the output, the load circuit having a maximum input voltage, wherein the first storage section is partially discharged in the third configuration, whereby the output voltage does not exceed the maximum input voltage when the power management device is switched to the second configuration by the switching section. 
         [0015]    The first storage section may include a first storage element switchably connected to a second storage element, and the switching section may be arranged to switch the second storage element from a state in which it is disconnected from the first storage element to a state in which it is connected to the first storage element in order to charge the second storage element before switching to the second configuration. 
         [0016]    The power management device may further comprise a load circuit arranged to receive energy from the output, the load circuit having a minimum operating voltage, wherein the switching section switches from the first configuration when the output voltage reaches or goes below a threshold, wherein the threshold is substantially equal to the minimum operating voltage. 
         [0017]    The power management device may be arranged such that the voltage produced at the output immediately before switching to the second configuration is non-zero. 
         [0018]    The invention also provides a power supply comprising: the power management device, and a transient power source arranged to provide the transient energy pulse to the power management device. 
         [0019]    The transient power source may be a piezoelectric generator. 
         [0020]    The invention further provides a power management device comprising: first and second capacitors arranged in parallel for storing energy received from an energy pulse supplied by a transient power source, and for providing the stored energy to an output; a switching section for switching the first and second storage sections into a series arrangement for providing the stored energy to the output. 
         [0021]    The switching section may switch the first and second capacitors to the series arrangement when it is determined that a predetermined time period has elapsed, or a voltage at the output is (i) less than a predetermined level, or (ii) less than or equal to a predetermined level. 
         [0022]    The switching section may be arranged to switch the first and second capacitors into a third configuration after the first configuration and before second configuration, and the third configuration may be arranged to at least partially discharge the first capacitor. 
         [0023]    In the third configuration the first capacitor may be connected in parallel with a third capacitor, so as to charge the third capacitor by the at least partial discharge of the first capacitor, and in the third configuration the second capacitor may be connected to the output so as to produce a voltage at the output. 
         [0024]    The invention also provides a method of supplying power, the method comprising: a step of receiving at an input an energy pulse from a transient power source; storing energy from the energy pulse by first and second energy storage sections arranged in a first configuration; producing, by the first and second storage sections, an output voltage at an output; switching the first and second energy storage sections to a second configuration, such that the output voltage is greater immediately after the switching than immediately before the switching. 
         [0025]    The method according may further comprise: after the storing and before the switching to the second configuration, switching to a third configuration in which the first energy storage section is isolated from the output and is partially discharged, and an output voltage is produced at the output by the second energy storage section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: 
           [0027]      FIG. 1  shows a prior art arrangement using a power management device. 
           [0028]      FIG. 2  is a circuit diagram of a prior art power management device. 
           [0029]      FIG. 3  is a simplified example of input and output voltages when the prior art device of  FIG. 2  is supplied with an energy pulse. 
           [0030]      FIG. 4  is a circuit diagram illustrating an embodiment of the invention. 
           [0031]      FIG. 5  illustrates example input and output voltages when the device of  FIG. 4  is supplied with an energy pulse, and also shows the states of the switches in  FIG. 4 . 
           [0032]      FIG. 6  is a circuit diagram illustrating another embodiment of the invention. 
           [0033]      FIG. 7  illustrates example input and output voltages when the device of  FIG. 6  is supplied with an energy pulse, and also shows the states of the switches in  FIG. 6 . 
           [0034]      FIG. 8  is a flow chart showing a method of operation according to the invention and usable with the arrangement of  FIG. 4 . 
           [0035]      FIG. 9  is a flow chart showing a method of operation according to the invention and useable with the arrangement of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0036]    The invention will now be illustrated by reference to non-limiting examples that are intended to describe rather than define the present invention. 
         [0037]      FIG. 3  shows a simplified example of the input and output voltages V in  and V out  in the arrangement of  FIG. 1 . An initial input energy pulse, which is triangular in this example, starts at time t 0  and peaks at time t 1 , charging the capacitor C and producing a potential at V out  equal to that at V in . The energy pulse at V in  then drops rapidly to 0V, but the diode  220  prevents current flowing from the capacitor C to V in . V out  decays as current flows from the capacitor C to the load circuit connected at V out . The decay of V out  is shown here as a linear decrease. In practice, the rate and form of the decay will depend on the nature of the circuit connected to V out . For example, if the circuit has a constant resistance, the decay would be exponential. 
         [0038]    In most cases, the circuit or system to be powered has a minimum operating voltage, below which it cannot operate. This is shown as V min  in  FIG. 3 . When V out  is below V min  (at time t 4  in  FIG. 3 ) the circuit to be powered ceases to operate. 
         [0039]    The inventor of the present invention has realized that the energy that remains stored on the capacitor when the output voltage decays below a minimum operating voltage of a load circuit is not used, and that greater efficiency can be achieved if the output voltage is converted to a higher potential when it reaches the minimum operating voltage. This increases the duration in which V out  is above the minimum operating voltage, permitting the circuit attached to V out  to operate for longer, and potentially complete more complex tasks. 
         [0040]    The present invention is directed to overcoming deficiencies in prior art power supplies for use with transient power sources, and increasing the energy utilized from the transient source. 
         [0041]      FIG. 4  shows a power management circuit  400  according to an embodiment of the invention. The power management circuit  400  of  FIG. 4  includes a first capacitor C 1 , linked to an input node V in  via a diode  420 . The circuit of  FIG. 4  also includes a second capacitor C 2  and first through third switches S 1 , S 2  and S 3 . The switches are arranged to selectively connect the first and second capacitors C 1 , C 2  in series and parallel arrangements. In particular, the switches are arranged so that when the first and second switches S 1  and S 2  are closed and the third switch S 3  is open, the first capacitor C 1  and the second capacitor C 2  are connected in parallel. Furthermore, when the first and second switches S 1  and S 2  are open and the third switch S 3  is closed, the first capacitor C 1  and the second capacitor C 2  are connected in series. In the present embodiment, the first switch S 1  is positioned between the positive plate of the first capacitor C 1  and the positive plate of the second capacitor C 2 , while S 2  is between the positive plate of the first capacitor C 1  and the negative plate of the second capacitor C 2 . The second switch S 2  is between the negative plate of C 2  and ground (earth). 
         [0042]    The circuit of  FIG. 4  is arranged to receive a positive voltage at V in  (i.e. a positive voltage component will pass the diode  420 , but as would be apparent to the skilled person, the circuit could be arranged with opposite polarities, that is reversing the diode  420  and receiving a negative voltage at V in  (i.e. such that a negative voltage component passes the diode  420 ). In this case, the positive and negative plates of the capacitors would be reversed relative to the above description. The capacitors themselves need not have an intrinsic polarity, and could be standard parallel plate capacitors, for example. 
         [0043]    The first through third switches S 1 , S 2  and S 3  form part of a switching section that additionally includes a controller  430  for the switches. The switches S 1 , S 2  and S 3  may be embodied by switching devices such as transistors, FETs or any other components that achieve the switching function described herein. The controller  430  for the switches may include a microprocessor, voltage comparator or other component(s) suitable for controlling the switches S 1 , S 2  and S 3  as described herein.  FIG. 4  illustrates the operational connection between the controller  430  and the switches S 1 , S 2  and S 3  by dashed lines leading to the switches. The controller  430  may also receive information on the voltages at V in  and/or V out , and this is shown by dashed lines leading to the controller  430 . The controller  430  could, alternatively or in addition to information on V in , receive information on the voltage on the other side of the diode  420  from V in . The controller may be embodied by any suitable component or components. For example, the controller may include an application-specific integrated circuit (ASIC). In some embodiments the controller may be arranged as a bistable or tri-stable circuit, and may include a voltage comparator, operational amplifier and/or timing circuit. 
         [0044]    The switching section is arranged so that, when an energy pulse is received via V in , the power management circuit  400  is in a first configuration, in which the first and second capacitors C 1  and C 2  are connected in parallel. In this configuration both capacitors are charged by the energy pulse. In the embodiment of  FIG. 4 , this corresponds to S 1  and S 2  being closed and S 3  being open. In the arrangement of  FIG. 4  C 1  and C 2  may have the same capacitance values, but this is not essential. Both capacitors C 1  and C 2  may be charged to the same voltage, but this need not be the case in all embodiments. As a result of the arrangement in  FIG. 4 , the sum of the peak voltages across each of the capacitors C 1  and C 2  is greater than the peak input voltage, although in the first configuration the voltages across C 1  and C 2  are not combined additively. 
         [0045]    The switching section is further arranged such that, after the energy pulse, the power management circuit  400  is switched to a second configuration, in which the first and second capacitors C 1  and C 2  are in series. Accordingly, the voltages across each of the capacitors C 1  and C 2  combine additively to produce an output voltage at V out  that is greater than the voltage across either of C 1  or C 2  individually. 
         [0046]    A circuit of system to be powered, herein referred to as a load circuit, is connected at note V out . 
         [0047]      FIG. 5  shows the variation of input and output voltages with time according to an exemplary embodiment of the arrangement of  FIG. 4 .  FIG. 5  also shows the states of the switches S 1 , S 2  and S 3 . “Closed”, ie. conducting, is shown as a high signal and “Open”, i.e. non-conducting, is shown as a low signal. 
         [0048]    As can be seen in  FIG. 5 , the power management circuit  400  is initially in the first configuration, such that the capacitors C 1  and C 2  are in parallel (S 1  and S 2  closed, S 3  open). At time t o  an energy pulse is received at V in , charging both of the capacitors C 1  and C 2 . The energy pulse may be from any transient power source, and may be from a piezoelectric generator, for example. The form of V in  is not particularly limited, other than being transient. Transient herein is used to describe a signal that is initially zero, then non-zero for a short period, and then zero again. The period in which V in  is non-zero is short relative to the period in which V in  is 0V, and is also short relative to the period of operation of the load circuit after V in  returns to 0V. As would be understood by the skilled person, a transient energy pulse is distinguished from a dc input voltage. The shape of V in  is not particularly limited. V in  is illustrated with a linear increase and decrease and a single peak, but other possibilities exist, and V in  can have any shape consistent with the above description of a transient signal. 
         [0049]    At time t 1 , the capacitors C 1  and C 2  have been charged to the voltage Vpeak and the energy pulse begins to decrease toward zero, as reflected by the input voltage V in  in  FIG. 5 . The charge (and energy) stored on the capacitors C 1  and C 2  decreases, schematically shown as a linear decrease (although the skilled person would appreciate that the decrease may take other forms), until the output voltage V out  reaches a threshold voltage V thresh . Where the load circuit has an associated minimum operating voltage V min , V thresh  is preferably equal to or slightly higher than V min . 
         [0050]    At t 2  the output voltage V out  reached V thresh , and the switching section switches the power management circuit  400  to the second configuration. In the second configuration, S 1  and S 2  are open and S 3  is closed, and the capacitors are in series. Thus, the voltages across the capacitors C 1 , C 2  are combined additively, and the output voltage increases. In the arrangement of  FIG. 4 , V out  is increased to 2×V thresh , the sum of V C1  and V C2 , the voltages across C 1  and C 2 , respectively. 
         [0051]    As a result of the switching section switching the power management circuit  400  to the second configuration, V out  is increased, remaining above V thresh , and also above V min . This extends the period of time in which V out  is greater thatn V min , permitting the load circuit to operate for a longer period of time than in the arrangement of  FIG. 2 . The load circuit can operate until V out  crosses (becomes smaller than) V min  at t 4 . On the other hand, if the power management circuit  400  remained in the first configuration and was not switched to the second configuration, the load circuit would be able to operate only until t 3  when V out  would have crossed V min . 
         [0052]    In the second configuration, V out  decreases more rapidly than in the first configuration, as the combined capacitance of C 1  and C 2  is switched from C 1 +C 2  in the first configuration to 1/C 1 +1/C 2 ) −1  in the second configuration. 
         [0053]    According to this arrangement, no power is drawn from V source  after the initial charging period, as V source  is transient. 
         [0054]    In some embodiments V thresh  could be set equal to or less than V min . Where the output voltage V out  drops below V min , the load circuit may cease to function, but would resume or restart functioning when the power management circuit  400  switched to the second configuration, assuming V out  then exceeds V min . Where it is acceptable or desirable for such resuming or restarting, V thresh  may be less than V min . 
         [0055]    The switching section may include a voltage comparator in order to determine when V out  reaches V thresh  and cause the power management circuit  400  to switch to the second configuration when V out  is less than V thresh  (or when V out  is equal to V thresh ). In an alternative embodiment, the switching section may include a timer. In this case, the energy pulse would start (or reset) the timer, and the switching section would cause the power management circuit to switch to the second configuration after a time period (approximating the period of time between t 0  and t 2 ) has elapsed. V out  at the end of this time period would define V thresh , and the time period may be selected such that V thresh  approximates a particular voltage, such as V min . The time period may be determined by the switching section, and may be a fixed time period. The time period may be variable, being determined by the switching section based on the value of Vpeak, for example. Other factors could be used to determine the time period. 
         [0056]    The switching section may be arranged to switch each of switches S 1 , S 2  and S 3  simultaneously. Alternatively, one of more of the switches S 1 , S 2 , S 3  can be switched separately. Where the switches are not switched simultaneously, they are preferably switched according to a predetermined sequence. In the embodiment of  FIG. 4 , a preferred sequence of switching is S 1  and S 2  opening simultaneously or in sequence, followed by S 3  closing as quickly as possible thereafter, or at least a short period thereafter. This sequence ensures that S 2  and S 3  are not closed at the same time, and so prevents the positive terminal of C 1  being connected to ground, which would allow charge from C 1  to flow to ground without passing through the load circuit. 
         [0057]      FIG. 5  shows V out  decaying to 0V after t 4  at the same rate as before t 4 . However, this is not necessarily the case, and V out  may remain constant, decay more slowly, or decay more rapidly. For example, the controller  430  may be arranged to disconnect (e.g. by a further switch that is not illustrated) the load circuit from V out  when it is determined that V out  is below V thresh  at t 4 . Assuming negligible leakage, this would result in V out  remaining constant at, or just below, V thresh  until another energy pulse is received at V in . Where leakage is not negligible, V out  would continue to decay, but more slowly than before t 4 . The switching section may be arranged to switch the power management circuit to the first configuration at or after t 4 , in readiness for a next energy pulse. In this case, V out  would be reduced abruptly (e.g. halved) when switching from the second to the first configuration. 
         [0058]      FIG. 8  illustrates a method  800  performed by an exemplary embodiment of the arrangement of  FIG. 4 . The method starts at step  805  and at step  810  the power management circuit  400  is in the first configuration. The energy pulse is received at step  815  and charges capacitors C 1  and C 2  at step  820 . Energy is supplied via V out  at step  825 , and C 1  and C 2  discharge accordingly. Steps  815 ,  820  and  825  may be performed simultaneously. At step  830  the switching section determines whether V out ≦V thresh . Alternatively, the switching section could determine whether V out ≦V thresh . If V out  is determined to be greater than V thresh , the method returns to step  825 . When V out  is determined to be less than or equal to V thresh , the method continues to step  835 , where the power management circuit  400  is switched to the second configuration, in which C 1  and C 2  are connected in series and the voltages across C 1  and C 2  combine additively, resulting in an increase in V out . At step  840  the power management circuit  400  continues to supply energy via V out , and C 1  and C 2  continue to discharge. At step  845  a determination is made as to whether the operation has completed. This could be based on, for example: (i) a time elapsed since receiving the energy pulse; (ii) whether V out  has decreased to or below V min , in which case the load circuit may be unable to continue to operate; or (iii) whether the load circuit has completed the functions it is required to perform and no longer needs energy. If it is determined that operation is not completed, the method returns to step  840 . If it is determined that operation is completed, the switching section returns the power management circuit  400  to the first configuration (step  850 ), in preparation for receiving a subsequent energy pulse. The method then ends at step  855 . The determination that operation has finished need not require an active decision-making element. Furthermore, the power management circuit  400  may be arranged to return to the first configuration when a next energy pulse is received, or between energy pulses. For example, the switches S 1 , S 2  and S 3  may be arranged to the default to the first configuration in the absence of a signal generated by the energy stored on C 1  and C 2 . In such cases, step  845  may be unnecessary or may be performed passively. 
         [0059]    Generally, dc-dc converter circuits, for converting an input dc voltage to an output dc voltage, are known, but these are intended for use with a continuous source of power, and work by continually drawing power from the input dc source. Thus, such converter circuits are not suitable for use when the power source is transient, and there is no energy available between the transient powering events, which may be a long time. Furthermore, dc-dc converters typically include a large number of components, and may draw a significant amount of energy compared with the energy available from a transient source. For these reasons, conventional dc-dc converter circuits may not be suitable for use with a transient power source. 
         [0060]      FIG. 6  shows another embodiment of the present invention. The embodiment of  FIG. 6  is similar to that of  FIG. 4 , with an additional capacitor, C 3  and an additional switch S 4 . The other components of  FIG. 6  are as described above in relation to the corresponding components of  FIG. 4 . 
         [0061]    Capacitor C 3  and switch S 4  are arranged in series with each other, and both are in parallel with capacitor C 1 . The controller  630  of the switching section is arranged to control switch S 4 , in addition to switches S 1 , S 2  and S 3 . In the first configuration S 4  is open, and so there is no connection between C 3  and either of C 1  and C 2 . In the second configuration S 4  is closed so that C 3  is in parallel with C 1  and each of C 1  and C 3  are in series with C 2 . 
         [0062]    The arrangement of  FIG. 6  is particularly advantageous when the load circuit has a maximum operating voltage, V max , which V out  must not exceed. In the power management device of  FIG. 4 , when V max  is less than 2×V thresh  (or the sum of voltages across C 1  and C 2 ) the output voltage V out  immediately after switching to the second configuration will exceed V max , possibly damaging the load circuit. The arrangement of  FIG. 6  can be used to avoid V out  exceeding V max . 
         [0063]      FIG. 7  shows the variation of input and output voltages with time according to an exemplary embodiment of the invention.  FIG. 5  also shows the states of the switches S 1 , S 2 , S 3  and S 4 . As in  FIG. 5 , a high signal shows as “Closed”, or conducting, state, and a low signal shows an “Open” or non-conducting state. 
         [0064]      FIG. 7  shows that initially the circuit is in the first configuration, with capacitors C 1  and C 2  in parallel (S 1  and S 2  closed, S 3  and S 4  open). At time t 0  an energy pulse is receive as V in , charging each of the first and second capacitors C 1  and C 2 . At time t 1 , the capacitors C 1  and C 2  have been charged to the peak voltage Vpeak and the energy pulse (V in ) begins to decrease to zero. In some arrangements, the capacitors will not necessarily be charged completely to Vpeak, and may be charged to a lower voltage, for example. The charge stored on the capacitors C 1  and C 2  decreases as the capacitors discharge through the load circuit via node V out . As in  FIG. 5 , the discharge is illustrated as linear, but may take other forms. Due to diode  620  providing isolation between V in  and V out  and the charge stored on the capacitors C 1 , C 2 , V out  decreases at a different rate (more slowly than) V in . 
         [0065]    At time t 2 , the output voltage V out  reaches the threshold V thresh , and the switching section switches the power management circuit to a third configuration. In the third configuration the first capacitor C 1  is disconnected from the second capacitor C 2 , and connected in parallel to the third capacitor C 3 . Capacitor C 3  is initially discharged, according to the current example, and so in the third configuration charge is transferred from the first capacitor C 1  to the third capacitor C 3 . In the third configuration, C 2  remains connected to V out , providing power to the load circuit via V out    
         [0066]    At time t 2 ′ the switching section switches the power management circuit to the second configuration, in which the first and third capacitors are in parallel with each other, and the second capacitor C 2  is in series with each of C 1  and C 3 . This causes the output voltage to increase to the sum of the voltages across the first and second capacitors C 1  and C 2 . If the interval between t 2  and t 2 ′ is sufficient to fully charge C 3 , the voltage across C 3  will equal the voltage across C 1 , but this is not essential. The interval between t 2  and t 2 ′ is not particularly limited, but typically would be chosen to be relatively short, being just long enough for C 1  to discharge into C 3 , such that V C1  and V C3  are generally equal. 
         [0067]    After t 2 ′, the output voltage V out  decreases. At t 4  V out  reaches V min , and V out  is then too low to power the load circuit. As described in relation to  FIG. 5 , various possibilitier exits for V out  after t 4 . For example, V out  may continue decreasing, remain at or just below V thresh , or may change abruptly. 
         [0068]      FIG. 9  illustrates a method  900  suitable for use with the embodiment of  FIG. 6 . Steps  905 ,  910 ,  915 ,  920 ,  925 ,  930 ,  935 ,  940 ,  945 ,  950  and  955  respectively correspond to steps  805 ,  810 ,  815 ,  820 ,  825 ,  830 ,  835 ,  840 ,  845   850  and  855 , described above in relation to  FIG. 8 .  FIG. 9  also includes steps  931 ,  932 ,  933  and  934 . In the method of  FIG. 9 , after it is determined in step  930  that the output voltage is less than or equal to the threshold voltage, the method proceeds to step  931 , in which the power management device  400  is switched to the third configuration. Energy is then supplied to V out  by C 2  (step  932 ) and C 2  discharges, although as noted above, the discharge of C 2  in this configuration may be negligible. In step  934  charge is transferred from C 1  to C 3 , charging C 3  with a corresponding discharge of C 1 . Steps  932  and  933  may occur simultaneously, depending on the relative timing of the switches S 1  and S 4 . At step  934 , it is determined whether the power management circuit  400  should be switched to the second configuration. This determination could be based on a predetermined time delay following the switch to the third configuration and/or could be based on the voltage across C 1 , for example. The determination could additionally or alternatively be based on V out . If it is determined that the power management circuit  400  should be switched to the second configuration, the method continues with step  935 . Otherwise, the method returns to step  932 . 
         [0069]    According to the embodiment of  FIG. 4 , when the power management circuit is switched to the second configuration, V out  increases to 2×V thresh . However, as noted above, the load circuit may have a maximum voltage that should not be exceeded, V max . Moreover, it is possible that 2×V thresh  is greater than V max . In such cases, the embodiment of  FIG. 6  is particularly advantageous, and can be used to prevent V out  exceeding V max , even if 2×V thresh  is greater than V max . 
         [0070]    In some embodiments according to the arrangement of  FIG. 6 , the capacitanceof the third capacitor C 3  and/or the interval t 2 -t 2 ′ can be chosen such that C 1  is discharged to a level where V C1 =V max −V thresh  between t 2  and t 2 ′. In this case, assuming that the discharge of C 2  is negligible between t 2  and t 2 ′, V out  will increase to V max . In some cases it will be desirable for the value of V out  to increase to just below V max  at time t 2 ′. More generally, the skilled person can select the capacitance of the third capacitor C 3  such that the voltage increases to a desired peak value at time t 2 ′. If the discharge of the second capacitor C 2  is not negligible between t 2  and t 2 ′, this can be taken into account based on actual or likely discharge rates through the load circuit. 
         [0071]    According to the arrangement of  FIG. 6 , no power is drawn from V source  after the initial charging period, as V source  is transient. 
         [0072]    In the arrangements of  FIG. 4  and  FIG. 6 , the controller  430 ,  630  for the switches may be power by V out  and could form part of the load circuit. In this case the load circuit would control switching of switches S 1 , S 2 , S 3  and S 4  (where present), while also performing the normal operations of the load circuit. 
         [0073]      FIG. 5  shows switches  51 , S 2  and S 3  switching at the same time, t 2 . However, as noted above, other possibilities exist, and the switches could be arranged to switch in sequence one or two at a time. When S 1  is arranged to open at time t 2 , before S 2  and S 3  are closed at time t 2 ′ (not shown in  FIG. 5 ), C 2  will discharge while the charge on C 1  remains the same (in the interval t 2 -t 2 ′). This means that between t 2  and t 2 ′ the voltage across C 2  will decrease while the voltage across C 1  remains constant. Accordingly, by varying the period between t 2  and t 2 ′, the peak voltage at t 2 ′ can be controlled. 
         [0074]      FIG. 7  shows S 1  and S 4  switched at the same time (t 2 ), and S 2  and S 3  switched together at time t 2 ′ after t 2 . However, S 4  could be closed before S 1  is opened, for example. The order and timing of switching in  FIGS. 5 and 7  is not particularly limited. 
         [0075]    In some arrangement, S 1  may be open when the voltage pulse is received. In this case, C 1  is charged by the energy pulse, and C 2  may be charged from C 1  after the energy pulse has passed by closing S 1 . In this case, the initial arrangement is different from the first configuration and the power management circuit  400 ;  600  is switched to the first configuration after the energy pulse has passed. 
         [0076]    The third capacitor C 3  and fourth switch S 4  in  FIG. 6  form an additional stage relative to the arrangement of  FIG. 4 . Further stages could be added. For example, a fourth capacitor and fifth switch could be added in series with each other and in parallel with C 1  and C 3 . 
         [0077]    As would be appreciated by the skilled person, certain simplifying assumptions have been made in the foregoing description, in the interests of providing a clear description the present invention. For example, in reality V source  is a real voltage or current source and may not behave as an idealized source, e.g. it may be finite impedance and/or limited energy. As previously noted, form of V in  is not particularly limited, other than being transient. Transient herein is used to describe a signal that is initially zero, then non-zero for a short period, and then zero again. The period in which V in  is non-zero is short relative to the period in which V in  is 0V, and is also short relative to the period of operation of the load circuit after V in  returns to 0V. As would be understood by the skilled person, a transient energy pulse is distinguished from a dc input voltage. The shape of V in  is not particularly limited. V in  is illustrated with a linear increase and decrease and a single peak, but other possibilities exist, and V in  can have any shape consistent with the above description of a transient signal. 
         [0078]    For simplicity, capacitors are shown having linear charging and discharging rates. However, the rate of charging and/or discharging may be non-linear. 
         [0079]    Herein, each of capacitors C 1  and C 2  is charged to the same peak voltage. However, this is not necessarily the case and may depend on the specific circuit arrangement. Similarly, description of the voltage increasing to 2×V thresh  depends on the circuit arrangement, and other possibilities exist. 
         [0080]    In practice the load circuit will not be an ideal current source, but a real impedance or circuit load. 
         [0081]    The above embodiments include a diode  420 ,  620 . However, any rectifying element could be used. In particular, if a full bridge rectifier is used, useful energy may still be obtained even if V in  becomes negative. In some embodiments, a rectification element may be unnecessary. For example, if the connection to the source if the energy pulse may be broken (e.g. by a switch) soon after Vpeak is reached. 
         [0082]    Switches S 1 , S 2 , S 3  and S 4  may be embodied by any suitable switching element, as would be clear to the skilled person. Transistors may be used, for example. The capacitance values of the capacitors C 1 , C 2  and C 3  could be appropriately selected by the skilled person, taking into account the voltage source and/or the load circuit, and are not particularly limited. The drawings use the circuit diagram symbol for fixed, non-polarized capacitors, but any suitable energy storage element could be used. 
         [0083]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0084]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. 
         [0085]    The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0086]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.