# Patent

## Title
Cold start DC/DC converter

## Abstract

A DC/DC converter comprising a first charge pump circuit including first MOS transistors including first depletion MOS transistors, an oscillating circuit connected to the charge pump circuit only at the gates of some at least of the first MOS transistors, including the first depletion MOS transistors.

## Background
This application claims the priority benefit of French Patent application number 14/62060, filed on Dec. 8, 2014, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.
BACKGROUND The present disclosure relates to a DC/DC voltage converter.
DISCUSSION OF THE RELATED ART
A DC/DC voltage converter may be used in an electronic power recovery system. Such a system generally comprises a transducer capable of converting non-electric power, for example, thermal power, mechanical power, radioactive power, etc., into electric power, generally delivered under a low voltage, and a DC/DC converter capable of converting this low voltage into a higher voltage, for example, for power storage or electronic circuit power supply purposes.
It is generally desirable for the electronic power recovery system to be able to operate, and in particular to start, with no addition of external power other than the electric power collected by the transducer. Such a starting is called a cold start. In particular, it may be desirable for the DC/DC converter to be able to have a cold start even when the voltage supplied by the transducer is low, for example, lower than the threshold voltage of the semiconductor junctions, which is approximately 300 mV.
There exist cold start DC/DC voltage converters using electronic components, such as electric transformers or tunnel-effect diodes, which cannot be simply formed by conventional methods of manufacturing field-effect transistors of metal-oxide-semiconductor type, also called MOS transistors.
It would however be desirable to be able to form the DC/DC converter with conventional MOS transistor technologies, particularly to decrease the manufacturing cost thereof.
SUMMARY
An object of an embodiment is to overcome all or part of the disadvantages of previously-described cold start DC/DC voltage converters.
Another object of an embodiment is that the DC/DC converter can have a cold start while only being powered with a low voltage, particularly lower than 300 mV.
Another object of an embodiment is for the DC/DC converter to only comprise electronic components capable of being formed with conventional MOS transistor manufacturing technologies.
Thus, an embodiment provides a DC/DC converter comprising a first charge pump circuit comprising first MOS transistors having first depletion MOS transistors, an oscilla-ting circuit connected to the charge pump circuit only at the gates of some at least of the first MOS transistors, including the first depletion MOS transistors.
According to an embodiment, the oscillating circuit comprises a ring oscillator.
According to an embodiment, the first charge pump circuit comprises a first terminal intended to receive a first voltage, at least one capacitor, and at least one conduction path between the first terminal and an electrode of the capacitor only comprising one of the first depletion MOS transistors or some of said first depletion MOS transistors in series.
According to an embodiment, the oscillating circuit is powered with the first voltage.
According to an embodiment, the first charge pump circuit is intended to supply a second voltage and the oscilla-ting circuit is powered with the second voltage.
According to an embodiment, the first charge pump circuit further comprises a second terminal intended to supply the second voltage and the first charge pump circuit comprises at least one conduction path between the first terminal and the second terminal only comprising some of said first depletion MOS transistors in series.
According to an embodiment, the converter comprises a second charge pump circuit comprising second MOS transistors, including second depletion MOS transistors, the oscillating circuit being connected to the second charge pump circuit only at the gates of at least some of the second MOS transistors including the second depletion MOS transistors, the second charge pump circuit comprising a third terminal intended to receive the first voltage and a fourth terminal intended to supply a third voltage of a sign opposite to that of the first voltage, and the oscillating circuit is powered between the second voltage and the third voltage.
According to an embodiment, the converter comprises a third charge pump circuit only comprising third enhancement MOS transistors, the oscillating circuit being further connected to the third charge pump circuit only at the gates of some at least of the third enhancement MOS transistors, the third charge pump circuit comprising a fifth terminal intended to receive the first voltage and a sixth terminal intended to supply a fourth voltage of same sign as the first voltage.
According to an embodiment, the converter further comprises a switching circuit capable of successively powering the oscillating circuit with the second voltage and with the fourth voltage.
An embodiment also provides an electronic power recovery system comprising a transducer capable of supplying a voltage from a power source and a converter such as previously defined, the charge pump circuit receiving said voltage.
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.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows in the form of a block diagram an embodiment of cold start DC/DC converter;
FIG. 2 is a more detailed electric diagram of an embodiment of the converter of FIG. 1;
FIG. 3 is an electric diagram of a detail of an element of the converter of FIG. 2;
FIG. 4 shows a curve of the variation of the drain current of a depletion MOS transistor according to the gate-source voltage;
FIGS. 5 and 6 are electric diagrams of a portion of the converter of FIG. 2 at two successive steps of operation of the converter;
FIG. 7 shows curves of the variation of the transformation ratio of the converter according to the voltage received by the converter respectively for the converter of FIG. 2 and for a converter with enhancement transistors;
FIGS. 8, 9, and 10 are electric diagrams of other embodiments of a cold start DC/DC converter;
FIG. 11 shows curves of the variation of the transformation ratio of the converter according to the voltage received by the converter respectively for the converter of FIG. 10 and for a converter with enhancement transistors;
FIGS. 12 and 13 are electric diagrams of other embodiments of a cold start DC/DC converter;
FIG. 14 shows curves of the variation of the transformation ratio of the converter according to the voltage received by the converter respectively for the converter of FIG. 13 and for a converter with enhancement transistors;
FIGS. 15 and 16 are electric diagrams of other embodiments of a cold start DC/DC converter; and
FIG. 17 shows an embodiment of a power recovery system



DETAILED DESCRIPTION
The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and are detailed. In particular, the structure of a transducer is well known by those skilled in the art and has not been described in detail. Unless otherwise specified, expressions “approximately”, “substan-tially”, and “in the order of” mean to within 10%, preferably to within 5%. Further, a signal which alternates between a first constant state, for example, a low state, noted “0”, and a second constant state, for example, a high state, noted “1”, is called “binary signal”. The high and low states of binary signals of a same electronic circuit may be different. In particular, the binary signals may correspond to voltages or to currents which may not be perfectly constant in the high or low state. In the following description, the source and the drain of a MOS transistor are called “power terminals” of the MOS transistor. Further, in the present description, term “connected” is used to designate a direct electric connection, with no intermediate electronic component, for example, by means of a conductive track, and term “coupled” or term “linked” will be used to designate either a direct electric connection (then meaning “connected”) or a connection via one or a plurality of intermediate components (resistor, capacitor, etc.).
FIG. 1 shows in the form of a block diagram an embodiment of a converter 5 comprising:
a charge pump circuit 10 (Charge Pump) receiving a voltage Ve at an input node IN and supplying a voltage Vs to an output node OUT; and
an oscillating circuit 11 supplying at least one digital control signal φ to charge pump circuit 10.
Oscillating circuit 11 may comprise:
an oscillator 12 (Oscillator) supplying at least one clock signal clk; and
an interface circuit 14 (Buffer) receiving clock signal clk and supplying digital control signal φ.
In the present embodiment, oscillator 12 and interface circuit 14 are powered with input voltage Ve and are further connected to a source of a reference potential GND, for example, the ground, which is taken to be equal to 0 V in the following description.
Voltage Ve is a DC voltage, capable of yawing over time, supplied by an electronic circuit, not shown. It may be the voltage supplied by a transducer capable of transforming into electric power another form of power, for example, thermal power, mechanical power, or radiative power. Voltage Ve may be lower than 300 mV, for example, in the range from 100 mV to 200 mV. In the following description, voltages Ve and Vs are reference to ground GND.
Voltage Vs supplied by charge pump circuit 10 is equal to the product of voltage Ve and of a transformation ratio k, positive or negative, which depends on the structure of charge pump circuit 10. Terminal OUT may be connected to a dissipative load, for example, for the powering of an electronic device, and/or to a capacitive load, for example, for power storage.
Clock signal clk and control signal φ are binary signals. Control signal φ alternates between voltage Ve and 0 V.
Interface circuit 14 is capable of shaping clock signal clk to supply control signal φ in a form adapted to charge pump circuit 10. As a variation, it is possible for interface circuit 14 not to be present, and clock signal clk may then directly be used as a control signal.
According to an embodiment, charge pump circuit 10 comprises depletion MOS transistors having their gates controlled by control signal φ. A depletion transistor is a transistor for which a conductive channel is present between the source and the drain even when the gate is at 0 V. The threshold voltage of an N-channel depletion MOS transistor is negative.
Charge pump circuit 10 comprises at least one capacitor. The conduction path or the conduction paths between input node IN and one of the capacitor electrodes is only formed by the channels of depletion MOS transistors having their gates controlled by the oscillating circuit. Control signals φ supplied by interface circuit 14 are only supplied to MOS transistor gates. This enables to decrease the current to be supplied by interface circuit 14 and advantageously enables to ensure that the charge pump circuit operates during a cold start even if voltage Ve is low.
Preferably, at least one conduction path between input node IN and output node OUT is only formed by the channels of depletion MOS transistors having their gates controlled by the oscillating circuit. This advantageously enables, in a cold start, converter 10 to provide a voltage Vs different from 0 V as soon as voltage Ve is different from 0 V.
According to an embodiment, charge pump circuit 10 only comprises MOS transistors and at least one capacitor. According to an embodiment, all the MOS transistors of charge pump circuit 10 are depletion MOS transistors. According to another embodiment, charge pump circuit 10 comprises at least one depletion MOS transistor and enhancement MOS transistors.
FIG. 2 shows a more detailed embodiment of converter 5 of FIG. 1 where oscillator 12 is a ring oscillator, where charge pump circuit 10 is a voltage doubler and where interface circuit 14 comprises inverters.
In this embodiment, charge pump circuit 10 comprises four N-channel depletion MOS transistors T1, T2, T3, and T4 and one capacitor C1. A first power terminal of transistor T1 is connected to input node IN and the second power terminal of transistor T1 is connected to a first electrode of capacitor C1. The gate of transistor T1 receives a first control signal φ1. A first power terminal of transistor T2 is connected to input node IN and the second power terminal of transistor T2 is connected to the second electrode of capacitor C1. The gate of transistor T2 receives a second control signal φ2. A first power terminal of transistor T3 is connected to the second electrode of capacitor C1 and the second power terminal of transistor T3 is connected to ground GND. The gate of transistor T3 receives first control signal φ1. A first power terminal of transistor T4 is connected to the first electrode of capacitor C1 and the second power terminal of transistor T4 is connected to output node OUT. The gate of transistor T4 receives second control signal φ2. The bulk of each MOS transistor T1 to T4 is connected to ground GND.
Oscillator 12 comprises a succession of an odd number of series-connected inverters, the output of the last inverter of the succession of inverters being connected to the input of the first inverter of the succession of inverters. Three series-connected inverters INV1, INV2, INV3 are shown in FIG. 2, the output of inverter INV3 being connected to the input of inverter INV1. Inverter INV3 supplies clock signal clk. Oscillator 12 further comprises an inverter INV4 having its input connected to the output of inverter INV3, and supplying a clock signal clkb. Each clock signal clk, clkb is a binary signal. Signals clk and clkb are complementary. The period of signal clk particularly depends on power supply voltage Ve of oscillator 12 and on the number of series-connected inverters.
Interface circuit 14 comprises an inverter INV5 receiving signal clk and supplying clock signal φ1. Interface circuit 14 comprises an inverter INV6 receiving signal clkb and supplying control signal φ2. Each inverter INV1 to INV6 is connected to anode for supplying voltage Ve and is connected to ground GND.
FIG. 3 shows an embodiment of an inverter INV. Each inverter INV1 to INV6 may have the structure shown in FIG. 3. Inverter INV comprises an input node A and an output node B. Inverter INV comprises a MOS transistor T5, for example, a P-channel enhancement MOS transistor, having its source connected to a node for supplying voltage Ve, having its drain connected to node B, and having its gate connected to node A. The bulk of transistor T5 is connected to a node for supplying voltage Ve. Inverter INV further comprises a MOS transistor T6, for example, an N-channel enhancement MOS transistor, having its source connected to ground GND, having its drain connected to node B, and having its gate connected to node A The bulk of transistor T6 is connected to ground GND.
The operating principle of charge pump circuit 10 according to the embodiment shown in FIG. 2 will now be described in relation with FIGS. 4 to 6.
In nominal operation, interface circuit 14 supplies binary controls signals φ1 and φ2 which are complementary. In the present embodiment, each signal φ1 and φ2 corresponds to a voltage which substantially alternates between voltage Ve and 0 V. In steady state, charge pump circuit 10 supplies a voltage Vs which is proportional to Ve, transformation ratio k being theoretically equal to 2 and, in practice, slightly smaller than 2.
FIG. 4 is a curve CI of the variation of drain-source current Ids of an N-channel depletion MOS transistor according to gate-source voltage Vgs in ohmic state. Four bias points P1 to P4 have been shown on curve CI.
FIG. 5 illustrates the conduction state of each MOS transistor T1 to T4 when control signal φ1 is at Ve and control signal φ2 is at 0 V. Transistor T1 is substantially at bias point P2, transistors T3 and T4 are substantially at bias point P4, and transistor T3 is substantially at bias point P1. The current then preferably flows along dashed line 20. Capacitor C1 is then charged under voltage Ve.
FIG. 6 illustrates the conduction state of each MOS transistor T1 to T4 when control signal φ2 is at Ve and control signal φ1 is at 0 V. Transistor T2 is at bias point P1, transistors T1 and T3 are at bias point P4, and transistor T4 is at bias point P3. The current then preferably flows along dashed line 22, which results in a discharge of capacitor C1 towards output node OUT. Voltage Vs at output node OUT is maintained substantially at approximately k*Ve, k being close to 2 in steady state in the present embodiment.
FIG. 7 shows variation curves Ck1 and Ck2, according to voltage Ve, of transformation ratios k1 and k2 in steady state respectively of converter 5 shown in FIG. 2 and of a converter having the same structure as converter 5 with the difference that transistors T1 to T4 are replaced with N-channel enhancement MOS transistors. Curves Ck1 and Ck2 have been obtained for MOS transistors having a 0.18-μm channel length.
For curves Ck1 and Ck2, transformation ratio k1 and k2 of converter 5 decreases when voltage Ve decreases. However, the decrease is faster when enhancement MOS transistors are used. Indeed, transformation ratio k2 decreases below 1.75 as soon as voltage Ve decreases below 420 mV while transformation ratio k1 decreases below 1.75 only when voltage Ve decreases below 250 mV. The present embodiment thus enables to operate the DC/DC converter at an input voltage smaller than that which can be obtained with a converter having the same structure, formed of enhancement MOS transistors.
FIG. 8 shows another embodiment of a DC/DC converter 30. Converter 30 comprises all the elements of converter 5 with the difference that inverters INV1 to INV6 are supplied with voltage Vs instead of receiving voltage Ve.
In normal operation, voltage Vs is higher than voltage Ve. The power supply of oscillating circuit 11 of converter 30 is advantageously more efficient than that of converter 5. Further, the conductivity of MOS transistors T1 to T4 of converter 30 is increased when the gates thereof are taken to Vs in the on state with respect to MOS transistors T1 to T4 of converter 30 having their gates only taken to Ve in the on state.
When voltage Ve increases from the zero value, oscillator 12 of converter 30 cannot start as long as output voltage Vs is smaller than the start voltage of oscillator 12. The current then directly flows through transistors T1 and T4 of charge pump circuit 10 so that voltage Vs rises until it becomes substantially equal to voltage Ve. As soon as voltage Vs has reached the start voltage of oscillator 12, the operation of converter 30 carries on as previously described for converter 5 in relation with FIGS. 4 to 6.
Further, as soon as oscillator 12 has started, voltage Vs becomes greater than Ve, so that the oscillation frequency of oscillator 12 increases faster than if oscillator 12 was supplied with voltage Ve and the variation rate of the transformation ratio of charge pump circuit 10 is increased.
Generally, charge pump circuit 10 may correspond to any type of voltage step-up, step-down, or inverter charge pump circuit.
FIG. 9 shows, as an example, another embodiment of a converter 35 where charge pump circuit 10 is a voltage step-up circuit having a transformation ratio of approximately 4. Charge pump circuit 10 of converter 35 comprises transistors T1 to T4 and capacitor C1 previously described in relation with FIG. 2 and further comprises N-channel depletion MOS transistors T7 to T12 and capacitors C2 and C3.
The power terminal of transistor T4 which is not connected to capacitor C1 is connected to a first electrode of capacitor C2. A first power terminal of transistor T7 is connected to input node IN and the second power terminal of transistor T7 is connected to the second electrode of capacitor C2. The gate of transistor T7 receives first control signal φ1. A first power terminal of transistor T8 is connected to ground GND and the second power terminal of transistor T8 is connected to the second electrode of capacitor C2. The gate of transistor T8 receives second control signal φ2. A first power terminal of transistor T9 is connected to the first electrode of capacitor C2 and the second power terminal of transistor T9 is connected to a first electrode of capacitor C3. The gate of transistor T9 receives first control signal φ1. A first power terminal of transistor T10 is connected to input node IN and the second power terminal of transistor T10 is connected to the second electrode of capacitor C3. The gate of transistor T10 receives second control signal φ2. A first power terminal of transistor T11 is connected to ground GND and the second power terminal of transistor T11 is connected to the second electrode of capacitor C3. The gate of transistor T11 receives first control signal φ1. A first power terminal of transistor T12 is connected to the first electrode of capacitor C3 and the second power terminal of transistor T12 is connected to output node OUT. The gate of transistor T12 receives second control signal φ2.
FIG. 10 shows another embodiment of a converter 40. Converter 40 comprises all the elements of converter 35, shown in FIG. 9, with the difference that charge pump circuit 10 is a voltage inverter having a transformation ratio k of approximately −1 and that oscillator 12 and interface circuit 14 are connected to output node OUT supplying voltage Vs instead of being connected to ground GND.
Charge pump circuit 10 of converter 40 comprises four N-channel depletion MOS transistors T13, T14, T15, and T16 and one capacitor C4. A first power terminal of transistor T13 is connected to ground GND and the second power terminal of transistor T13 is connected to a first electrode of capacitor C4. The gate of transistor T13 receives second control signal φ2. A first power terminal of transistor T14 is connected to input node IN and the second power terminal of transistor T14 is connected to the second electrode of capacitor C4. The gate of transistor T14 receives second control signal φ2. A first power terminal of transistor T15 is connected to the second electrode of capacitor C4 and the second power terminal of transistor T15 is connected to ground GND. The gate of transistor T15 receives first control signal φ1. A first power terminal of transistor T16 is connected to the first electrode of capacitor C4 and the second power terminal of transistor T16 is connected to output node OUT. The gate of transistor T16 receives first control signal φ1. The bulk of each transistor T13, T14, T15, and T16 is connected to ground GND.
The fact for oscillator 12 and for interface circuit 14 to be biased between voltage −Ve and voltage Ve advantageously enables to improve the operation of converter 40. Indeed, in operation, transistors T13, T14, T15 and T16 are more blocked when their gates receive voltage −Ve rather than at ground GND. Further, in a cold start, as long as the oscillator has not started, voltage Vs remains substantially at 0 V. As soon as oscillator 12 has started, voltage Vs decreases below 0 V, so that the oscillation frequency of oscillator 12 increases faster than if oscillator 12 was connected to ground GND and the variation rate of the transformation ratio of charge pump circuit 10 is increased.
FIG. 11 shows variation curves Ck1′ and Ck2′, according to voltage Ve, of transformation ratios k1′. and k2′ respectively of converter 40 shown in FIG. 10 and of a converter having the same structure as converter 40 with the difference that transistors T13 to T16 are replaced with N-channel enhancement MOS transistors. Curves Ck1′. and Ck2′ have been obtained for MOS transistors having a 0.18-μm channel length.
For curves Ck1′ and Ck2′, transformation ratios k1′ and k2′, which are negative, decrease when voltage Ve increases. However, the decrease is faster when depletion MOS transistors are used. Indeed, transformation ratio k1′ decreases below −0.5 as soon as voltage Ve increases above 200 mV while transformation ratio k2′ decreases below −0.5 only when voltage Ve increases above 300 mV.
FIG. 12 illustrates, in the form of a block diagram, another embodiment of a converter 45. Converter 45 comprises two charge pump circuits 101 and 102. Charge pump circuit 101 is capable of supplying a positive voltage Vs1, of same sign as voltage Ve and charge pump circuit 102 is capable of supplying a voltage Vs2 of a sign opposite to that of voltage Ve. Oscillator 12 and interface circuit 14 are supplied with positive voltage Vs1 and negative voltage Vs2. Interface circuit 14 supplies control signals φ1 and φ2 to the two charge pump circuits 101 and 102. In normal operation, control signals φ1 and φ2 alternate between voltage Vs1, which may be greater than Ve, and voltage Vs2, which is negative. This enables to improve the conduction and the blocking of the MOS transistors of charge pump circuit 101 and 102. The transformation ratio of charge pump circuit 101 is thus increased and becomes closer to the theoretical maximum value. Further, as soon as oscillator 12 has started, voltage Vs1 is greater than voltage Ve and voltage Vs2 decreases below 0 V, so that the oscillation frequency of oscillator 12 increases faster than if oscillator 12 was connected to voltage Ve and to ground GND and the variation rate of the transformation ratio of charge pump circuit 10 is increased.
FIG. 13 shows, as an example, an embodiment of a converter 50, corresponding to a more detailed embodiment of converter 45 of FIG. 12, where charge pump circuit 101 has the structure of charge pump circuit 10 of converter 5, where charge pump circuit 102 has the structure of charge pump circuit 10 of converter 40, where oscillating circuit 11 has the structure of oscillating circuit 11 of converter 10 with the difference that oscillating circuit 11 is supplied with voltage Vs1 instead of voltage Ve and receives voltage Vs2 instead of being connected to ground GND. Interface circuit 14 supplies control signals φ1 and φ2 to the two charge pump circuits 101 and 102. In normal operation, control signals φ1 and φ2 alternate between voltage Vs1, which, in the present embodiment, is substantially equal to 2Ve, and voltage Vs2, which, in the present embodiment, is substantially equal to −Ve.
FIG. 14 is a drawing similar to FIG. 7 for converter 45 of FIG. 12 in the case where charge pump circuit 101 is a voltage step-up circuit having the structure of the charge pump circuit of converter 35 shown in FIG. 9. FIG. 14 shows variation curves Ck1′′ and Ck2′′, according to voltage Ve, of transformation ratios k1′′ and k2′′ of charge pump circuit 102 respectively of converter 45 and of a converter having the same structure as converter 45 with the difference that the transistors of charge pump circuits 101 and 102 are replaced with N-channel enhancement MOS transistors. Curves Ck1′′ and Ck2′′ have been obtained for MOS transistors having a 0.18-μm channel width.
For curves Ck1′′ and Ck2′′, transformation ratio k1′ and k2′′ decreases when voltage Ve decreases. However, the decrease is faster when enhancement MOS transistors are used. Indeed, transformation ratio k2′′ decreases below 2.75 as soon as voltage Ve decreases below 250 mV while transformation ratio k1′′ decreases below 2.75 only when voltage Ve decreases below 155 mV.
FIG. 15 shows another embodiment of a converter 55. Converter 55 comprises two charge pump circuits 101 and 102. Charge pump circuit 101 is connected to input node IN and supplies a voltage Vs1 to an output node OUT1. Charge pump circuit 101 may correspond to any of the charge pump circuits previously described in relation with FIG. 2, 8, 9, 10, or 13. Charge pump circuit 102 is connected to input node IN and supplies a voltage Vs2 to an output node OUT2. Charge pump circuit 102 corresponds to an amplifier formed with enhancement MOS transistors. As an example, charge pump circuit 102 may correspond to any of the charge pump circuits previously described in relation with FIG. 2, 8, 9, 10, or 13 by replacing the depletion MOS transistors with enhancement MOS transistors. In FIG. 15, the shown charge pump circuit 102 has the same structure as charge pump circuit 10 shown in FIG. 9 with the difference that depletion MOS transistors T1 to T4 and T7 to T12 have been respectively replaced with enhancement MOS transistors T1′ to T4′ and T7′ to T12′. Converter 55 comprises oscillator 12 and interface circuit 14 such as previously described. According to an embodiment, oscillator 12 and interface circuit 14 are connected to output node OUT1. Charge pump circuit 102 may have the same structure as charge pump circuit 101 with the difference that the depletion MOS transistors of charge pump circuit 101 are replaced with enhancement MOS transistors.
During a cold start at low voltage Ve, charge pump circuit 101, which supplies voltage Vs1, starts faster and under a lower voltage Ve than charge pump circuit 102. Charge pump circuit 102 only starts when voltage Vs1 is greater than the threshold voltage of the enhancement MOS transistors forming it. Indeed, control signals φ1 and φ2 of the gates of the enhancement MOS transistors of charge pump circuit 102 vary substantially from 0 V to Vs1. When charge pump circuit 102 operates normally, it may be advantageous to use voltage Vs2 supplied by charge pump circuit 102 since the power efficiency, equal to the ratio of the power supplied by the charge pump circuit to the power received by the charge pump circuit, of charge pump circuit 102 may be greater than the power efficiency of charge pump circuit 101.
According to an embodiment, charge pump circuit 101 is connected to input node IN by a switch SW1. Further, oscillator 12 and interface circuit 14 are connected to output node OUT1 by a switch SW2 and to output node OUT2 by a switch SW3. Switches SW1, SW2, and SW3 are controlled by a control unit, not shown, which may comprise a dedicated circuit. According to an embodiment, during a cold start with a low voltage Ve, switches SW1 and SW2 are on and switch SW3 is off. Oscillating circuit 11 is then powered with voltage Vs1. The control unit may be capable of comparing voltage Vs1 with a reference voltage, for example, equal to the sum of the threshold voltage of the enhancement MOS transistors of charge pump circuit 102 and of a security margin. When voltage Vs1 is greater than the reference voltage, the control unit may turn off switch SW1 to increase the power transfer to charge pump circuit 102. Further, switch SW2 may then be turned off and switch SW3 may be turned on so that circuits 12 and 14 are then powered with voltage Vs2.
FIG. 16 shows another embodiment of a charge pump circuit 10 of voltage doubler type. Charge pump circuit 10 shown in FIG. 16 comprises all the elements of charge pump circuit 10 previously described in relation with FIG. 2 with the difference that depletion MOS transistor T3 is replaced with an N-channel enhancement transistor T16. The bulk of transistor T16 is connected to ground GND. This enables to improve the power efficiency of the converter. Preferably, depletion MOS transistors T1 and T2 are not replaced with enhancement MOS transistors, to avoid degrading the starting performance of charge pump circuit 10.
An enhancement MOS transistor may be provided in parallel with a depletion MOS transistor for at least one of the depletion MOS transistors of the charge pump circuit. This enables to improve the power efficiency of the converter while keeping the advantages of the use of depletion MOS transistors. As an example, in FIG. 16, a P-channel enhancement MOS transistor T17 may be assembled in parallel with depletion MOS transistor T4, a first power terminal of transistor T17 being connected to the first electrode of capacitor C1, the second power terminal of transistor T17 being connected to output node OUT. The gate of transistor T17 receives control signal φ1. The bulk of transistor T17 may be connected to output node OUT. This enables to improve the power efficiency of the converter.
FIG. 17 shows an embodiment of a power recovery system 60 comprising a transducer 62 capable of supplying voltage Ve to a converter 64 which may correspond to any of previously-described converters 5, 30, 35, 40, 45, 50, 55. Transducer 62 may be capable of transforming into electric power another form of power, for example, thermal power, mechanical power, or radiative power.
Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. In particular, although, in the previously-described embodiments, oscillator 12 is a ring oscillator, it should be clear that oscillator 12 may have a different structure. As an example, the previously-described embodiments may be implemented with a relaxation oscillator.
Further, it should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. As an example, the embodiments previously described in relation with FIGS. 13 and 15 may be combined, and the converter may comprise two charge pump circuits used to supply the two voltages between which the oscillating circuit is supplied at the starting as shown in FIG. 13 and one charge pump circuit with enhancement MOS transistors, such as shown in FIG. 15, used to supply the output voltage of the converter once the converter has started.
Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

## Claims

1. A DC/DC converter comprising:
a first charge pump circuit comprising:
first MOS transistors including first depletion MOS transistors,
a first terminal intended to receive a first voltage,
a second terminal intended to supply a second voltage,
a least one capacitor,
at least one conduction path between the first terminal and the second terminal only comprising some of said first depletion MOS transistors in series, and
at least one conduction path between the first terminal and an electrode of the capacitor comprising only one of the first depletion MOS transistors or only some of said first depletion MOS transistors in series,

a second charge pump circuit comprising:
second MOS transistors including second depletion MOS transistors, and
a third terminal intended to receive the first voltage and a fourth terminal intended to supply a third voltage of a sign opposite to that of the first voltage, and wherein the oscillating circuit is powered between the second voltage and the third voltage, and

an oscillating circuit connected to:
the charge pump circuit only at the gates of at least some of the first MOS transistors including the first depletion MOS transistors,
the second charge pump circuit only at the gates of at least some of the second MOS transistors including the second depletion MOS transistors, wherein the oscillating circuit is supplied with the second voltage.

2. The converter of claim 1, wherein the oscillating circuit comprises a ring oscillator.

3. The converter of claim 1, wherein the oscillating circuit is powered with the first voltage.

4. The converter of claim 3, comprising a third charge pump circuit only comprising third enhancement MOS transistors, the oscillating circuit being further connected to the third charge pump circuit only at the gates of some at least of the third enhancement MOS transistors, the third charge pump circuit comprising a fifth terminal intended to receive the first voltage and a sixth terminal intended to supply a fourth voltage of same sign as the first voltage.

5. The converter of claim 4, further comprising a switching circuit capable of successively powering the oscillating circuit with the second voltage and with the fourth voltage.

6. An electronic power recovery system comprising a transducer capable of supplying a voltage from a power source and the converter of claim 1, the charge pump circuit receiving said voltage.

7. A DC/DC converter comprising:
a first charge pump circuit comprising:
first MOS transistors including first depletion MOS transistors,
a first terminal intended to receive a first voltage,
at least one capacitor, and
at least one conduction path between the first terminal and an electrode of the capacitor comprising only one of the first depletion MOS transistors or only some of said first depletion MOS transistors in series,

a third charge pump circuit only comprising third enhancement MOS transistors,
an oscillating circuit connected to:
the first charge pump circuit only at the gates of at least some of the first MOS transistors including the first depletion MOS transistors,
the third charge pump circuit only at the gates of some at least of the third enhancement MOS transistors, wherein:
the oscillating circuit is powered with the first voltage, and
the third charge pump circuit comprising a fifth terminal intended to receive the first voltage and a sixth terminal intended to supply a fourth voltage of same sign as the first voltage, and


a switching circuit capable of successively powering the oscillating circuit with a second voltage and with the fourth voltage.

8. The converter of claim 7, wherein the oscillating circuit comprises a ring oscillator.

9. The converter of claim 7, wherein the first charge pump circuit is intended to supply the second voltage and wherein the oscillating circuit is supplied with the second voltage.

10. The converter of claim 9, wherein the first charge pump circuit further comprises a second terminal intended to supply the second voltage and wherein the first charge pump circuit comprises at least one conduction path between the first terminal and the second terminal only comprising some of said first depletion MOS transistors in series.

11. The converter of claim 10, comprising a second charge pump circuit comprising second MOS transistors including second depletion MOS transistors, the oscillating circuit being connected to the second charge pump circuit only at the gates of at least some of the second MOS transistors including the second depletion MOS transistors, the second charge pump circuit comprising a third terminal intended to receive the first voltage and a fourth terminal intended to supply a third voltage of a sign opposite to that of the first voltage, and wherein the oscillating circuit is powered between the second voltage and the third voltage.

12. An electronic power recovery system comprising a transducer capable of supplying a voltage from a power source and the converter of claim 7, the charge pump circuit receiving said voltage.