Abstract:
The instant disclosure relates to a circuit for comparing a voltage with a first threshold, in which said first threshold depends on a second threshold of opening at least one first normally closed breaker.

Description:
BACKGROUND 
       [0001]    The present disclosure generally relates to the conversion of electrical energy, and more specifically aims at the conversion of electrical energy in ambient energy harvesting generators or power supplies. The present disclosure also aims at a circuit for comparing a voltage with a threshold. 
       DISCUSSION OF THE RELATED ART 
       [0002]    To power electronic systems having a low power consumption, it has been provided to use generators capable of converting energy available in the system environment, for example, mechanical energy, into electrical energy. Generators where ambient mechanical energy is converted into electrical energy by a piezoelectric element are in particular known. To transform the electrical energy supplied by the piezoelectric element into electrical energy capable of being used by an electronic system, such generators comprise an electrical energy conversion device placed downstream of the piezoelectric element. The electrical energy conversion device may comprise a circuit for comparing a voltage with a threshold. 
         [0003]    Examples of electrical energy conversion devices are described, in particular, in French patent application published under number 2873242, previously filed by the applicant, and in article “Power Conversion and Integrated Circuit Architecture for High Voltage Piezoelectric Energy Harvesting” by Pierre Gasnier et al., describing prior works conducted by the applicant. 
       SUMMARY 
       [0004]    An embodiment provides a circuit for comparing a voltage with a threshold, comprising: a first inverter having first and second power supply nodes respectively coupled to first and second nodes of application of said voltage; and a first normally-on switch connecting an input of the first inverter to the first node of application of the voltage, a control gate of the first switch being connected to the second node of application of the voltage. 
         [0005]    According to an embodiment, the first power supply node of the first inverter is connected to the first node of application of the voltage via a voltage limiter. 
         [0006]    According to an embodiment, the voltage limiter comprises a second normally-on switch between the first node of application of the voltage and the first power supply node of the first inverter. 
         [0007]    According to an embodiment, the voltage limiter further comprises at least one third normally-on switch cascaded with the second switch. 
         [0008]    According to an embodiment, the voltage limiter comprises a diode having its anode on the side of the first node of application of the voltage and having its cathode on the side of the first power supply node of the first inverter. 
         [0009]    According to an embodiment, the comparison circuit comprises a second inverter in series with the first inverter. 
         [0010]    According to an embodiment, an output node of the comparison circuit is connected to an output of the second inverter. 
         [0011]    According to an embodiment, the first switch is a depletion MOS transistor. 
         [0012]    According to an embodiment, the input of the first inverter is connected to an output of the comparison circuit via a first resistor. 
         [0013]    According to an embodiment, the input of the first inverter is connected to the second node of application of the voltage via a second resistor. 
         [0014]    According to an embodiment, the input of the first inverter is connected to the first switch via a third resistor. 
         [0015]    Another embodiment provides an energy conversion circuit, comprising: a first element comprising an electrical energy converter; and a voltage comparison circuit of the above-mentioned type. 
         [0016]    According to an embodiment, the energy conversion circuit further comprises a second electrical energy storage element, capable of being powered by the first element. 
         [0017]    According to an embodiment, the energy conversion circuit further comprises a normally-on bypass switch placed between an input node and an output node of the first element. 
         [0018]    According to an embodiment, the comparison circuit is configured to compare the voltage across the second element with the threshold. 
         [0019]    According to an embodiment, the comparison circuit is configured to control the bypass switch according to the voltage across the second element. 
         [0020]    According to an embodiment, the comparison circuit is configured to control a normally-off switch connected to a power supply node of a circuit for controlling the electrical energy converter. 
         [0021]    According to an embodiment, the second element is configured to power a circuit for controlling the electrical energy converter, and a third electrical energy storage element is configured to power an external load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0023]      FIG. 1  schematically shows an embodiment of an electrical energy conversion device; 
           [0024]      FIG. 2  shows an embodiment of a circuit for comparing a voltage with a threshold; 
           [0025]      FIG. 3  is a simplified timing diagram illustrating the operation of the comparison circuit of  FIG. 2 ; 
           [0026]      FIG. 4  shows an alternative embodiment of a circuit for comparing a voltage with a threshold; 
           [0027]      FIG. 5  schematically shows an alternative embodiment of the electrical energy conversion device of  FIG. 1 ; and 
           [0028]      FIG. 6  schematically shows another embodiment of an electrical energy conversion device. 
       
    
    
       [0029]    For clarity, the same elements have been designated with the same reference numerals in the different drawings. 
       DETAILED DESCRIPTION 
       [0030]    Ambient energy (for example, mechanical energy) harvesting generators may be used in various fields, for example, transport, to power pressure, temperature, vibration sensors, etc., placed on a vehicle; industry, to power machine monitoring sensors; housing, to power wireless switches, pressure sensors, stress sensors, etc.; the medical field, to power assistance or monitoring equipment implanted in a patient&#39;s body; environment, to power sensors for monitoring meteorological or other phenomena; defense and space, to power appliance or border monitoring sensors; and consumer electronics, to totally or partially power portable devices such as phones, MP3 players, remote controls, etc. The use of ambient energy harvesting generators especially enables to form totally self-contained wireless sensors, which are thus easy to install and do not require repeated and expensive battery replacement operations. The use of ambient energy harvesting generators also enables to extend the lifetime of the batteries of certain sensors. 
         [0031]    In a mechanical energy harvesting generator, the harvested energy may originate from various sources, for example, vibrations, shocks, deformations, from a force exerted by a user, etc. The mechanoelectrical conversion element may be of piezoelectric type, or of another type, for example, of electrostatic, electromagnetic, magnetostrictive type, etc. 
         [0032]    Most of the time, the electrical energy supplied by the mechanoelectrical conversion element cannot be directly used to power an electronic system. Indeed, usual electronic systems are powered with D.C. voltages of a few volts, for example, in the range from 2 to 12 volts, while the mechanoelectrical conversion element generally delivers an A.C. or transient voltage of strong amplitude, for example, greater than 30 volts, and a very low current, for example, in the range from 10 nanoamperes to 100 microamperes, or less. 
         [0033]    To transform the electrical energy supplied by the mechanoelectrical conversion element into energy exploitable by an electronic system, a mechanical energy harvesting generator may comprise an electrical energy conversion device, for example comprising a voltage or current converter, or a current-to-voltage converter. 
         [0034]    The electrical energy conversion device may comprise a rectifying element, for example, a diode bridge, receiving the energy delivered by the mechanoelectrical conversion element and, at the output of the rectifying element, a capacitive storage element, for example, a capacitor, an accumulator, or a battery, delivering a D.C. voltage having a level adapted to the power supply of an electronic system. A problem is that the efficiency of such a conversion device is relatively low, particularly when there exists a significant voltage level difference between the input and the output of the rectifying element. 
         [0035]    To increase the conversion efficiency, an electrical energy conversion device comprising an active electrical energy converter, for example, a switched-mode converter, the mechanoelectrical element, and the storage element, may be provided. In operation, the switched-mode converter receives an amplitude signal (possibly rectified) depending on the quantity of mechanical energy received and on the characteristics of the mechanoelectrical conversion element, and delivers across the storage element a D.C. signal having a level adapted to powering an electronic system. Losses due to the signal level difference between the output of the conversion element and the storage element are thus decreased. A problem is due to the fact that a switched-mode converter requires being powered to be able to operate. In steady state, the electrical energy for supplying the switched-more converter may be sampled from the output storage element of the voltage converter. However, during a generator starting phase, if the storage element is discharged, a booster power supply is necessary. It may further be necessary to provide a circuit for comparing a voltage with a threshold to detect the end of the starting phase. A problem is that known voltage comparison circuits have a relatively significant electrical power consumption. Further, at the end of the starting phase, the switching between the booster power supply and the power supply by the storage element may raise difficulties. 
         [0036]    It would be desirable to be able to solve all or part of the problems of known electrical energy conversion devices. 
         [0037]    It would further be desirable to be able to have a circuit for comparing a voltage with a threshold, which overcomes all or part of the problems of known comparison circuits. 
         [0038]      FIG. 1  schematically shows an example of an embodiment of an electrical energy conversion device  100 , capable of converting an A.C. or transient electrical energy, for example, supplied by a mechanoelectrical conversion element (not shown) or any other fluctuating or intermittent energy source, into a D.C. voltage, for example, compatible with the powering of an electronic circuit (not shown). 
         [0039]    Device  100  comprises a rectifying element  101  comprising input nodes A and B, respectively connected or coupled to input nodes E 1  and E 2  of device  100  (capable of being connected to output nodes of an energy harvesting device), and output nodes C and D. In the shown example, rectifying element  101  comprises a diode  102  between nodes A and C, a diode  103  between nodes B and C, a diode  104  between nodes D and A, and a diode  105  between nodes D and B, the anodes of diodes  102 ,  103 ,  104 , and  105  being respectively on the side of node A, on the side of node B, on the side of node D, and on the side of node D. Device  100  further comprises a switched-mode converter  107  of D.C./D.C. type, comprising input nodes E and F, respectively connected to output nodes C and D of rectifying element  101 , and output nodes G and H, respectively connected to output nodes S 1  and S 2  of device  100 . Device  100  further comprises a storage element  109 , for example, a capacitor, an accumulator, or an electric battery, between nodes S 1  and S 2 . Element  109  may also have a filtering function. Device  100  further comprises an electronic circuit  111  for controlling transistors (not shown in  FIG. 1 ) of switched-mode converter  107 . Circuit  111  comprises high and low power supply nodes I and J respectively connected to nodes S 1  and S 2 . In the shown example, node J is directly connected to node S 2 , and node I is connected to node S 1  via a normally-off switch  113 , for example, a P-channel MOS transistor. 
         [0040]    According to an aspect, device  100  comprises a normally-on bypass switch  115  between an output node of rectifying element  101  and an output node of switched-mode converter  107 . Switch  115  may be a normally-on transistor or depletion transistor, that is, a transistor where a channel exists when no voltage is applied to its control node (for example, when the gate-source voltage is zero in the case of a MOS transistor). Switch  115  for example is a DMOS transistor, also called depletion MOS transistor. In the example of  FIG. 1 , conduction nodes of switch  115  are directly respectively connected to output node C of rectifying element  101  and to output node G of switched-mode converter  107 . As a variation, a diode (not shown) may be series-connected with switch  115 , between nodes C and G, for example, upstream of switch  115 , to only allow the flowing of current through switch  115  from node C to node G. 
         [0041]    Device  100  further comprises a circuit  117  for comparing a voltage with a threshold, capable of controlling switches  115  and  113  according to the voltage across storage element  109 . In the shown example, circuit  117  comprises input nodes of high potential M and of low potential N respectively connected to output nodes S 1  and S 2  of device  100 , and an output node O connected to the control gates of switches  115  and  113 . 
         [0042]    The operation of the electrical energy conversion device of  FIG. 1  will now be described. 
         [0043]    At the beginning of a starting phase, for example, when device  100  has not been used for a long period, storage element  109  is discharged, that is, the voltage between output nodes S 1  and S 2  is substantially zero. Since circuit  117  is not powered, no control signal is applied to switches  115  and  113 . Switch  115 , which is normally on, is thus in the conductive state, and switch  113 , which is normally off, is in the non-conductive state. Further, since circuit  111  for controlling switched-mode converter  107  is not powered, switched-mode converter  107  is inactive. 
         [0044]    When an A.C. or transient electrical signal, for example, output by a mechanoelectrical conversion element of a mechanical energy harvesting generator, is received on input nodes E 1  and E 2  of device  100 , this signal is rectified by element  101 , which requires no specific power supply (other than the input signal that it receives) to operate. Switch  115  being in the on state, it forms a conductive path for bypassing switched-mode converter  107 , and the rectified electrical signal output by element  101  is transferred onto output nodes S 1  and S 2  of device  100 . This signal charges storage element  109 . 
         [0045]    When the charge level of storage element  109  exceeds a threshold, this is detected by circuit  117 , which makes switch  115  turn off and switch  113  turn on. In this example, switch  115  is an N-channel depletion transistor (DMOS), and switch  113  is a P-channel enrichment transistor (MOS). The application, by circuit  117 , of a same low level control signal on the gates of transistors  115  and  113  thus enables to simultaneously control the turning-off of transistor  115  and the turning-on of transistor  113 . 
         [0046]    The turning-on of switch  113  causes the powering-on of control circuit  111  of switched-mode converter  107 , and thus the activation of converter  107 . The electrical energy necessary to control the transistors of switched-mode converter  107  is drawn from storage element  109  by circuit  111 . The turning-off of switch  115  interrupts the conductive path for bypassing switched mode converter  107 . The electric output signal of rectifying element  101  is thus no longer directly transferred across storage element  109 , but is transformed by switched-mode converter  107  and the output signal of converter  107  charges storage element  109 . 
         [0047]    An advantage of the embodiment of  FIG. 1  is that device  100  can start autonomously, even when storage element  109  is fully discharged (for example, after a long period without being used). Device  100  thus requires no booster power supply. 
         [0048]    Another advantage is that, in steady state, that is, after a starting phase during which storage element  109  is charged to a level sufficient to power switched-mode converter  107 , device  100  has a high conversion efficiency as compared with a device comprising no active electrical energy converter (that is, receiving a specific electrical supply energy, other than the input signal to be converted). 
         [0049]    As a variation, switches  113  and  115  may, instead of being simultaneously controlled by a same signal, as in the example of  FIG. 1 , be controlled via different signals. As an example, a control unit, not shown, for example, a microcontroller, may be provided between output O of circuit  117  and the control gates of switches  113  and  115  to control switch  113  independently from transistor  115 . Switch  113  may for example be made to turn on slightly before the turning-on of switch  115  to guarantee that switched-mode converter  107  is operational as soon as switch  115  has been turned off. In another alternative embodiment, switch  113  may be suppressed, that is, node I may be directly connected to node S 1 . 
         [0050]      FIG. 2  shows in further detail an embodiment of circuit  117  for detecting the threshold voltage of the device of  FIG. 1 , or circuit for comparing a voltage with a threshold. In this example, circuit  117  comprises three depletion MOS transistors (DMOS)  201 ,  203 , and  205 , and two CMOS inverters (or NMOS-PMOS pairs)  207  and  209 . The drains (D) of transistors  201 ,  203 , and  205  are connected to high-potential input node M of circuit  117 . The source (S) of transistor  201  is connected to low-potential input node N or ground node of circuit  117  via a resistor  202 , the source (S) of transistor  203  is connected to input node p of inverter  207 , and the source (S) of transistor  205  is connected to a high power supply node q of inverters  207  and  209 . Inverters  207  and  209  are series-connected, that is, output r of inverter  207  is connected to the input of inverter  209 . The output of inverter  209  is connected to output node O of circuit  117 . Node N is connected to a low power supply node s of inverters  207  and  209 . The gate of transistor  205  is connected to the source of transistor  201 . The gates of transistors  201  and  203  are connected to node N. 
         [0051]    The operation of circuit  117  of  FIG. 2  will now be described in relation with  FIGS. 1 ,  2 , and  3 . 
         [0052]      FIG. 3  is a timing diagram illustrating the time variation, during a starting phase of device  100  of  FIG. 1 , of voltages V 1  between nodes M and N of circuit  117 , V 2  between nodes p and N of circuit  117 , V 3  between nodes q and N of circuit  117 , and V 4  between nodes O and N of circuit  117 . 
         [0053]    In the example of  FIG. 2 , DMOS transistors  201 ,  203 , and  205  are N-channel transistors, that is, normally-on transistors which turn off when a source-gate voltage greater than a positive starting or turn-off threshold is applied. References V TH201 , V TH203 , and V TH205  will be used hereafter to designate the respective turn-off thresholds of transistors  201 ,  203 , and  205 . 
         [0054]    At a time t0 of beginning of a starting phase, voltage V 1  between input nodes M and N of circuit  117  is substantially zero (storage element  109  discharged). Voltages V 2 , V 3 , and V 4  are also substantially zero. 
         [0055]    At a time t1, when storage element  109  ( FIG. 1 ) starts charging, voltage V 1  increases. Transistors  203  and  205  being in the on state, voltages V 2  and V 3  follow the same variation as voltage V 1 . Output voltage V 4  of circuit  117  also increases. 
         [0056]    At a time t2, when voltage V 2  comes closer to turn-off threshold V TH203  of transistor  203 , transistor  203  tends to turn off. Transistor  203  then behaves as a voltage limiter and voltage V 2  substantially settles at value V TH203 . Voltage V 1  keeps on increasing along with the charge of capacitor  109 , and voltages V 3  and V 4  follow the same variation as voltage V 1 . 
         [0057]    At a time t3, when power supply voltage V 3  of inverters  207  and  209  exceeds a threshold equal to approximately twice saturation level V TH203  of input voltage V2 of the inverters, output r of inverter  207  switches from a low state to a high state. The output of inverter  209  then switches from a high state (voltage V 4  substantially equal to power supply voltage V 3  of the inverters) to a low state (voltage V 4  substantially zero), that is, output node O of circuit  117  is substantially taken to the potential of ground node N. Such a switching marks the end of the starting phase. In the electrical energy conversion device of  FIG. 1 , it causes the turning-off of switch  115  and the turning-on of switch  113 , and thus the activation of switched-mode converter  107 . 
         [0058]    At a time t4 little after time t3, voltage V 3  settles at a value substantially equal to V TH201 +V TH205 , and this, even if voltage V 1  starts increasing beyond this value. This enables to limit the power consumption of inverters  207  and  209 . 
         [0059]    After time t4, output voltage V 4  remains in the low state as long as input voltage V 1  remains higher than the switching threshold of circuit  117 , that is, approximately twice saturation level V TH203  of input voltage V 2  of the inverters in this example. If voltage V 1  falls below this threshold, output voltage V 4  of circuit  117  switches back to a high state. 
         [0060]    An advantage of circuit  117  of  FIG. 2  is that, when voltage V 1  reaches the switching threshold of circuit  117 , the state switching of output O of circuit  117  is particularly fast. When circuit  117  is used in electrical energy conversion device  100  of  FIG. 1 , this particularly enables to avoid an unwanted locking of device  100  in a configuration where switches  113  and  115  would be both partially on. 
         [0061]    Another advantage is that, after the starting phase, when device  100  operates in steady state, inverters  207  and  209  do not switch, and DMOS transistors  201 ,  203 , and  205  are in an almost totally off state, each DMOS transistor having its gate-source voltage substantially equal to the turn-off threshold of the transistor. As a result, the power consumption of circuit  117  in steady state is very low, for example, lower than 50 nano-amperes. 
         [0062]    Another advantage of circuit  117  is that it does not require, to operate, receiving a specific electrical power supply other than the voltage to be monitored between its inputs nodes M and N. 
         [0063]    Inverters  207  and  209  of circuit  117  for example are so-called simple inverters, that is, each comprising first and second complementary transistors in series between high and low power supply nodes of the inverter, the gates of the two transistors being interconnected. As a variation, inverters  207  and  209  may be so-called encapsulated inverters, that is, each comprising, in addition to the first and second transistors of a simple inverter, third and fourth transistors respectively between the high power supply node and the source of the first transistor, and between the low power supply node and the source of the second transistor, the gates of the third and fourth transistors being connected to the gates of the first and second transistors. As a variation, inverters  207  and  209  may be so-called encapsulated delayed inverters, that is, each comprising, in addition to the four transistors of an encapsulated inverter, a RC delay circuit between the gates of the first and second transistors, and the gates of the third and fourth transistors. The use of inverters of encapsulated or encapsulated-delayed type especially enables to limit the power consumption of circuit  117 , while avoiding for a conduction path to be created between the high and low power supply nodes of the inverter on switching of circuit  117 . 
         [0064]    The inventors have observed that circuit  117  of  FIG. 2  has a good performance when using, for DMOS transistor  201 , a component bearing reference BF992 having a 1.4-volt turn-off threshold, for DMOS transistors  203  and  205 , components bearing reference BF994 having a 1-volt turn-off threshold, for inverters  207  and  209 , assemblies of encapsulated type using components bearing reference MC14007, and for resistor  202 , a 500-MΩ resistor. The described embodiments are of course not limited to this specific case. 
         [0065]    Various variations of circuit  117  of  FIG. 2  may be provided, where such variations may possibly be combined. 
         [0066]    As a first variation, a capacitance may be added between ground node N of circuit  117  and each of the inputs and/or outputs of inverters  207  and  209 , to stabilize the input and/or output states of the inverters. 
         [0067]    As a second variation, a resistor may be added between ground node N of circuit  117  and each of the inputs and/or outputs of inverters  207  and  209 , to ease the switching of the inverters or their returning to the initial state in case of a decrease in voltage V 1 . 
         [0068]    In the example of  FIG. 2 , transistor  203  plays the role of a limiter of input voltage V 2  of inverter  207 , and the cascade of transistors  201  and  205  plays the role of a limiter of power supply voltage V 3  of inverters  207  and  209 . As a third variation, it may be provided to form each of these voltage limiters with a number of cascaded DMOS transistors different from the example of  FIG. 2 , which particularly enables to adjust the switching threshold of circuit  117 . For example, in the example of  FIG. 2 , transistors  201 ,  205  and resistor  202  may be replaced with a single DMOS transistor (having its drain, its source, and its gate respectively connected to nodes M, q, and N) having a turn-off threshold equal to V TH201 +V TH205 , or by a voltage limiter comprising a number of cascaded DMOS transistors greater than  2 , and transistor  203  may be replaced with a plurality of cascaded DMOS transistors. 
         [0069]    As a fourth variation, the limiter of voltage V 3 , formed in the example of  FIG. 2  by the cascade of transistors  201  and  205  and by resistor  202 , may be replaced with a simplified voltage limiter, comprising a first diode (not shown) having its anode connected to node M and having its cathode connected to node q. In this fourth variation, a second diode (not shown) may further be provided between node M and the drain (D) of transistor  203 . It should be noted that each of the first and second diodes may be replaced with an association of a plurality of diodes in series, according to the voltage drop which is desired to be obtained between node M and node q on the one hand, and between node M and the drain of transistor  203  on the other hand. 
         [0070]    It should further be noted that the limiter of voltage V 3 , formed in the example of  FIG. 2  by the cascade of transistors  201  and  205  and by resistor  202 , is optional. As a fifth variation, this voltage limiter may be suppressed, and node M may for example be directly connected to node q. 
         [0071]    As a sixth variation, DMOS transistors  201 ,  203 , and  205  of circuit  117  may be replaced with other types of normally-on switches having a similar operation, that is, tending to turn off when a control voltage exceeding a threshold is applied thereto, for example, JFET transistors. 
         [0072]    As a seventh variation, circuit  117  may comprise a number of CMOS inverters in series greater than  2 . This particularly enables to increase the circuit switching speed. As a variation, circuit  117  may comprise a single inverter (that is, inverter  209  may be suppressed, and output r of inverter  207  may be directly connected to output O of circuit  117 ). 
         [0073]      FIG. 4  shows another embodiment of a circuit  617  for comparing a voltage with a threshold. As an example, circuit  617  may be used to replace circuit  117  for comparing a voltage with a threshold in the electrical energy conversion circuit previously described in relation with  FIG. 1 , or in electrical energy conversion circuits of the type described hereafter in relation with  FIGS. 5 and 6 . 
         [0074]    In the example of  FIG. 4 , circuit  617  comprises the same elements as circuit  117  of  FIG. 2 , substantially arranged in the same way, and further comprises additional resistors. In the following, only the differences between circuits  617  and  117  will be detailed. In the shown example, circuit  617  comprises a resistor  621  between input node p of inverter  207  and the source node (S) of transistor  203  (instead of a direct connection in circuit  117  of  FIG. 2 ), a resistor  623  between node p and node N, and a resistor  625  between input node p of inverter  207  and output node O of the comparison circuit. A resistor  627  may optionally be provided between the source node (S) of transistor  203  and node N. Resistors  621 ,  623 , and  625  give circuit  617  hysteresis properties. Circuit  617  behaves as a comparison circuit of Schmitt trigger type with two switching thresholds, a high threshold VH and a low threshold VB (with VB&lt;VH). In other words, in operation, output O of circuit  617  switches to the low state when voltage V 1  between nodes M and N exceeds threshold VH, but only switches back to the high state when voltage V 1  falls below threshold VB. 
         [0075]    Circuit  617  is particularly advantageous in electrical energy conversion circuits of the type previously described in relation with  FIG. 1 , or of the type described hereafter in relation with  FIGS. 5 and 6 . Indeed, in such circuits, the switching of the voltage comparison circuit causes the activation of a switched-mode converter, and may cause a temporary decrease of voltage V 1  monitored by the comparison circuit. In the absence of hysteresis, such a voltage decrease may cause a new switching of the comparison circuit, causing the almost immediate deactivation of the switched-mode converter. Such a monitored voltage decrease phenomenon on switching of the comparison circuit can especially be observed in electrical energy conversion circuits with two storage elements, of the type described hereafter in relation with  FIG. 5 . 
         [0076]    It should be noted that circuit  617  of  FIG. 4  is compatible with the various above-mentioned alternative embodiments of circuit  117  of  FIG. 2 . In a preferred embodiment, a circuit for comparing a voltage with a threshold of the type shown in  FIG. 4 , but where the voltage limiter formed by transistors  201  and  205  and by resistor  202  is replaced with a first diode having its anode connected to node M and having its cathode connected to node q, and wherein a second diode is forward-connected between node M and drain (D) of transistor  203 , is provided. 
         [0077]      FIG. 5  schematically shows an alternative embodiment of the electrical energy conversion device of  FIG. 1 . Conversion device  300  of  FIG. 5  comprises the same elements as device  100  of  FIG. 1 , and further comprises, in addition to storage element  109  connected between nodes S 1  and S 2 , a second capacitive storage element  302 , for example, a capacitor, having a first electrode  302   a  connected to node G via a diode  304 , the anode of diode  304  being on the side of node G, and having a second electrode  302   b  connected to node H. In device  300 , switched-mode converter  107  and diode  304  are components of an element  306  having outputs nodes  306   a  and  306   b  respectively connected to electrodes  302   a  and  302   b  of storage element  302 . Device  300  further differs from device  100  of  FIG. 1  in that high power supply nodes M and I of circuits  111  and  117 , instead of being connected to node S 1 , as in the example of  FIG. 1 , are connected to output node  306   a  of element  306 , on the cathode side of diode  304  (via switch  113  for node I). Further, switch  115 , instead of directly connecting output C of rectifying element  101  to output node G of switched-mode converter  107  as in converter  100 , connects it to node  306   a.    
         [0078]    During a starting phase, as long as switched-mode converter  107  is inactive, only storage element  302  charges. When element  302  reaches a charge level sufficient to power switched-mode converter  107 , circuit  117  turns off switch  115  and turns on switch  113 , which causes the activation of switched-mode converter  107 . In steady state, storage elements  302  and  109  are both charged by the electrical output signal of switched-mode converter  107 . In other words, the alterative embodiment of  FIG. 5  separates the storage element used for the power supply of switched-mode converter  107  (element  302 ) from that used to power an external electronic system (element  109 ). A storage element  109  of greater capacitance than storage element  302  (for example, in the order of 1 μF for element  302  and in the range from 10 μF to 1 mF for element  109 ) may for example be provided. An advantage is that this enables to start the switched-mode converter faster, the charge speed of element  302  being greater than that of element  109 . 
         [0079]    As a variation, a number of storage elements greater than two may be provided, for example, to supply voltages of different levels in order to simultaneously power a plurality of different electronic systems. 
         [0080]    It should be noted that to form an electrical energy conversion device with two storage elements or more, other assemblies than that of  FIG. 5  may be provided. As an example, the switched-mode converter may comprise a transformer comprising, at the primary, a winding, and at the secondary, as many windings as there are storage elements in the conversion device, each winding of the secondary being electromagnetically coupled to the primary winding, and each winding being connected to one of the storage elements of the conversion device. Each storage element is thus mainly charged with the energy received by the secondary winding of the transformer which is associated thereto. As a variation, the switched-mode converter may comprise a transformer comprising, at the primary, a winding, and at the secondary, a single winding electromagnetically coupled to the primary winding, the energy received by the secondary winding being distributed between the various storage elements, for example, by means of switchings using MOS transistors and/or diodes. 
         [0081]      FIG. 6  schematically shows an example of another embodiment of an electrical energy conversion device  400 , capable of turning an A.C. or transient electrical signal (fluctuating signal), for example, supplied by a mechanoelectrical conversion element (not shown), into a D.C. signal, for example, compatible with the power supply of an electronic system (not shown). 
         [0082]    Device  400  comprises a rectifying element  101 , for example, a diode bridge, comprising input nodes A and B, respectively connected to input nodes E 1  and E 2  of the device, and output nodes C and D. Device  400  further comprises a switched-mode converter  407  comprising input nodes E and F, respectively connected to input nodes E 1  and E 2  of device  400 , and output nodes G and H, respectively connected to output nodes S 1  and S 2  of device  400 . Output node D of rectifying element  101  is directly connected to output node H of switched-mode converter  407 , and output node C of rectifying element  101  is connected to output node G of switched-mode converter  407  via a normally-on switch  115 . In other words, in the embodiment of  FIG. 6 , the switched-mode converter is placed in parallel with rectifying element  101 , between the input and the output of device  400 , rather than in series with the rectifying element as in the embodiments of  FIGS. 1 and 3 . Device  400  further comprises a storage element  109 , for example, a capacitor, between nodes S 1  and S 2 . Device  400  further comprises an electronic circuit  111  for controlling transistors of switched-mode converter  407 . Circuit  111  comprises high and low power supply nodes I and J respectively connected to nodes S 1  and S 2 . In the shown example, node J is directly connected to node S 2 , and node I is connected to node S 1  via a normally-off switch  113 . Device  400  further comprises a circuit  117  for comparing a voltage with a threshold, to control switches  115  and  113  according to the charge level of element  109 . In the shown example, circuit  117  comprises input nodes of high potential M and of low potential N respectively connected to output nodes S 1  and S 2 , and an output node O connected to the control gates of switches  115  and  113 . 
         [0083]    According to an aspect of the embodiment of  FIG. 6 , switched-mode converter  407  itself comprises a rectifier, so that, in steady state, when the switched-mode converter is active, rectifying element  101  no longer needs being used, and may be deactivated by the turning-off of switch  115 . 
         [0084]    In the example of  FIG. 6 , switched-mode converter  407  comprises a transformer comprising, at the primary, two windings  409  and  411  and, at the secondary, a winding  413  electromagnetically coupled to both winding  409  and winding  411 . On the primary side, switched-mode converter  407  comprises a first branch comprising, in series between nodes E and F, winding  409 , a diode  415  having its anode on the side of winding  409 , and a switched-mode transistor  417 , for example, an N-channel MOS transistor. The first branch further comprises, in antiparallel with transistor  417  and in anti-series with diode  415 , a free wheel diode  419  which may be the parasitic source/drain diode of transistor  417 . Switched-mode converter  407  further comprises, on the primary side, a second branch, parallel to the first branch, comprising, in series between nodes E and F, winding  411 , a diode  421  having its cathode on the side of winding  411 , and a switched-mode transistor  423 , for example, P-channel MOS transistor. The second branch further comprises, in antiparallel with transistor  423  and in anti-series with diode  421 , a free-wheel diode  425  which may be the parasitic source/drain diode of transistor  423 . On the secondary side, switched-mode converter  407  comprises, in series between nodes G and H, winding  413  and a diode  427  having its anode on the side of node H. 
         [0085]    Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
         [0086]    In particular, the embodiments described in the present disclosure are not limited to the specific example of starting circuit described in relation with  FIG. 2 . It will be within the abilities of those skilled in the art to form electrical energy conversion devices of the type described in the present disclosure, by replacing circuit  117  with another starting circuit capable of implementing the desired operation, for example, a circuit comprising an internal reference comparator having an output connected to the gates of switches  113  and  115 , the comparator being configured to switch from a high state to a low state when the voltage across a storage element of the device exceeds a threshold. 
         [0087]    Further, the electrical energy conversion devices described in the present disclosure may be used in systems other than mechanical energy harvesting generators, for example, thermoelectric generators, photovoltaic generators, transponders or radiofrequency devices, etc. More generally, the described electrical energy conversion devices may be used in any system requiring the transformation of an electrical input signal into an electrical signal of different level. The described conversion devices are particularly advantageous when there is a significant voltage level difference between the input and the output of the device, and/or when the electrical energy source at the input of the device is intermittent or fluctuating. 
         [0088]    It should further be noted that the rectifying element provided in the conversion devices of  FIGS. 1 ,  4 , and  5  is optional. Further, the forming of switched-mode converters  107  of the conversion devices of  FIGS. 1 and 4  has not been detailed in the present application. It should be noted that the embodiments of  FIGS. 1 and 4  are compatible with all usual switched-mode converters, and more generally with all active electrical energy converters, that is, comprising at least one transistor, and requiring a specific electrical power supply for the operation thereof. Further, the embodiment of  FIG. 6  is not limited to the specific described example of switched-mode converter  407 . More generally, the embodiment of  FIG. 6  is compatible with any active converter capable of implementing a rectification function. 
         [0089]    Further, circuits  117  and  617  described in relation with  FIGS. 2 and 4  may be used in other electrical energy conversion devices than those described in the present application. As an example, circuits  117  and  617  may be used to detect a voltage threshold in an electrical energy conversion device of the type described in above-mentioned patent application 2873242, or in above-mentioned article “Power Conversion and Integrated Circuit Architecture for High Voltage Piezoelectric Energy Harvesting”. 
         [0090]    Further, although circuits  117  and  617  are particularly advantageous for a use in devices for converting electrical energy originating from fluctuating or intermittent sources such as ambient energy harvesting devices, circuits  117  and  617  may also be used in other electrical energy conversion devices and, more generally, in any device requiring a circuit capable of monitoring a voltage and of switching a node between a first and a second state when the voltage to be monitored exceeds a threshold.