Abstract:
A capacitive power supply circuit, comprising a power storage element between two output terminals for providing a rectified output voltage; in series between a first input terminal for applying an A.C. voltage and a first of the output terminals, at least one capacitor and a first diode; a switch controllable by application of a signal on a triggering terminal; and means for controlling said switch to the on state when the output voltage is in a predetermined range of values.

Description:
PRIORITY CLAIM 
   This application claims priority from French patent application No. 04/51836, filed Aug. 11, 2004, which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate to the field of capacitive power supply circuits which are intended to provide a rectified smoothed output voltage across a power storage element (capacitor) from an alternating current (A.C.) supply voltage. 
   2. Discussion of the Related Art 
   Capacitive power supply circuits belong to the different solutions for supplying power to a load from an A.C. supply voltage originating, for example, from the electric supply mains (220 volts or 110 volts). 
   Capacitive power supplies are especially preferred to magnetic transformer or high frequency power supplies (switched-mode power supplies) for small powers (output currents of approximately some ten milliamperes) for reasons of cost or when the stand-by consumption is desired to be minimized. 
     FIG. 1  shows a conventional example of a capacitive power supply circuit This circuit essentially comprises a first capacitor C having a D.C. output voltage Vout provided between its terminals  1  and  2 . This voltage is obtained from an A.C. supply voltage Vac (for example, the electric distribution mains voltage) applied between two input terminals  3  and  4  of the capacitive power supply circuit terminals  2  and  4  being connected. Between terminals  3  and  1  are connected, in series, a resistor R 1 , a capacitor C 1 , and a diode D 1  having its cathode directly connected to terminal  1  (positive electrode of output capacitor C). Diode D 1  forms a halfwave element for rectifying voltage Vac to charge capacitor C. The value of output voltage Vout is set by a zener diode DZ 1  connecting the anode of diode D 1  to ground  2  (the cathode of diode DZ 1  being directly connected to the anode of diode D 1 ). The function of resistor R 1  is to limit the current surge on circuit power-on. This resistor is sometimes omitted. The function of capacitor C 1 , which is a high-voltage A.C. transistor (several hundreds of volts), is to limit the current provided to the load. Diode DZ 1  is used for the regulation while capacitor C, which is a low-voltage capacitor (a few tens of volts at most), is used as a power sink. Diode D 1  is used to prevent the discharge of capacitor C into the A.C. power supply. Generally, resistor R 1  is a normalized resistor and the most currently used output voltage is an output voltage of from a few volts to a few tens of volts. 
   The output current of the power supply circuit essentially is a function of the value of capacitor C 1  (and of voltage Vac, frequency and amplitude). Accordingly, capacitor C 1  is selected according to the load to be supplied. 
   As long as voltage Vout has not reached the threshold voltage of diode DZ 1  (neglecting the voltage drop in diode D 1 ), diode DZ 1  is blocked, enabling charge of the capacitor in positive halfwaves of voltage Vac. As soon as voltage Vout reaches value DZ 1 , the zener diode starts to avalanche, interrupting the charge of capacitor C. 
   Such a capacitive power supply has the advantage of an easy implementation as compared to other magnetic transformer or high-frequency solutions. 
   However, for output currents greater than some ten milliamperes, it generates significant losses when the system is at stand-by, that is, when no power is sampled by the load connected to terminals  1  and  2 . 
   The significant losses during system stand-by result in that, in practice, the capacitive power supply circuits are limited to applications of supply of a current on the order of some ten milliamperes. 
   To solve this problem, an A.C. power supply circuit using a controllable switching element has already been provided. 
     FIG. 2  shows an example of such a circuit. 
   As compared to the assembly of  FIG. 1 , zener diode DZ 1  is replaced with a cathode-gate thyristor Th forming a controllable switch. Thyristor Th connects the anode of diode D 1  to ground (common terminals  2  and  4 ), the cathode of thyristor Th being grounded. The gate of thyristor Th is connected by a zener diode DZ 2  to output terminal  1 , the anode of diode DZ 2  being connected to the gate of thyristor Th. Finally, a diode D 2  is connected in antiparallel with thyristor Th, its anode being connected to the terminal  4  while its cathode is connected to the anode of diode D 1 . Functionally, thyristor Th is intended to be on when capacitor C needs not be charged (voltage Vout greater than the threshold voltage of diode DZ 2 ) and to be off when a charge of capacitor C is required. 
     FIGS. 3A ,  3 B, and  3 C illustrate the operation of the circuit of  FIG. 2  and show, respectively, examples of shapes of voltage Vac, on periods (ON) of thyristor Th, and on periods (ON) of diode D 1  for the charge of capacitor C. As long as voltage Vac is, in a positive halfwave with the orientations of the drawings, smaller than threshold voltage VZ 2  of diode DZ 2 , thyristor Th is off and diode D 1  is on. From the time (time t 1 ) when voltage VZ 2  is reached by voltage Vac, thyristor Th turns on, which forbids continuing the charge of capacitor C. The load connected to terminals  1  and  2  is then supplied by the discharge of capacitor C and diode D 1  is off. During positive halfwaves, diode D 2  is off. 
   Towards the end of the positive halfwave (time t 2 ), when voltage Vac falls below threshold VZ 2 , thyristor Th blocks. However, since voltage Vout is then in principle greater than voltage Vac, diode D 1  remains off. Diode D 1  is however likely to turn back on by the turning back off of thyristor Th between times t 1  and t 2  if the load has consumed all the power stored in capacitor C. This hypothetical case is however rather unusual since capacitor C is sized according to the load that it must supply. 
   From the beginning of the negative halfwave (time t 3 ) and until the next zero crossing (time t 0 ) towards the positive halfwave, diode D 2  is forward biased and forbids Conduction of thyristor Th. 
   The losses in the circuit of  FIG. 2  result, in positive halfwaves, from the current in thyristor Th and, in negative halfwaves, from the current in diode D 2 . 
   The losses in negative halfwaves in diode D 2  approximately correspond to the losses in diode DZ 1  ( FIG. 1 ). However, in positive halfwaves, the losses in thyristor Th are much smaller than those in diode DZ 1  of  FIG. 1 . Indeed, the current in stand-by periods, that is, when voltage Vout remains greater than threshold voltage REF since the load does not consume, is a D.C. current in thyristor Th, and thus under a voltage on the order of one volt while, in the case of  FIG. 1 , it is the avalanche voltage of the zener diode (10 volts, or even more). 
   For a same admissible system stand-by loss level, the circuit of  FIG. 2  enables supplying loads with much greater currents (typically of several tends of milliamperes under a few tens of volts) without for all this increasing the stand-by consumption. 
   However, a disadvantage of the circuit of  FIG. 3  is that it poses problems of electromagnetic compatibility and requires use of a mains filter (not shown) upstream of the system. 
   A circuit such as described in relation with  FIG. 3  is described, for example, in U.S. Pat. No. 5,796,599. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention aims at providing a capacitive power supply circuit which generates no electromagnetic disturbance and avoids use of a mains filter. 
   Another aspect of the present invention also aims at providing a solution preserving reduced stand-by losses. 
   A further aspect of the present invention aims at providing a power supply circuit of simple structure as compared to magnetic transformer or high-frequency power supplies. 
   Another aspect of the present invention aims at providing an entirely-integrable solution. 
   According to a further aspect of the present invention, a capacitive power supply circuit includes:
         a power storage element between two terminals of provision of a rectified output voltage;   in series between a first input terminal of application of an A.C. voltage and a first output terminal of provision of said rectified voltage, at least one capacitor and a first diode;   a switch controllable by application of a signal on a triggering terminal; and   means for controlling said switch to the on state when the output voltage is in a predetermined range of values.       

   According to an embodiment of the present invention, said control means are formed of a hysteresis comparator. 
   According to an embodiment of the present invention, a second diode connects a first electrode of the capacitor, connected to the first diode, directly to a second output terminal. 
   According to an embodiment of the present invention, said switch is formed of a MOS transistor connecting the first electrode of the capacitor to the second output terminal, said control means being formed of a comparator having an inverting input receiving a voltage proportional to the output voltage and having a non-inverting input connected by a first zener diode to the first output terminal and by a resistor to the second output voltage. 
   According to an embodiment of the present invention, a second zener diode of value smaller than the first zener diode connects the output of the comparator to its non-inverting input. 
   According to an embodiment of the present invention, said second diode is formed by the parasitic diode of the MOS transistor. 
   According to an embodiment of the present invention, a surge current limiting resistor connects the first input terminal to a second electrode of the capacitor. 
   According to an embodiment of the present invention, said switch is formed of a triac connecting a second input terminal of application of the A.C. voltage to the second output terminal, said control means being formed of a comparator of the output voltage with at least one threshold voltage associated with a circuit for detecting the zero crossing of the A.C. supply voltage to control said triac. 
   According to an embodiment of the present invention, the storage element is a capacitor. 
   The foregoing features, and advantages of the present invention 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 , previously described, shows the diagram of a first conventional example of a capacitive power supply circuit; 
       FIG. 2 , previously described, shows a second conventional example of a capacitive power supply circuit; 
       FIGS. 3A ,  3 B, and  3 C illustrate, in timing diagrams, the operation of the circuit of  FIG. 2 ; 
       FIG. 4  shows a capacitive power supply circuit according to an embodiment of the present invention; 
       FIGS. 5A ,  5 B, and  5 C illustrate, in timing diagrams, the operation of the circuit of  FIG. 5 ; and 
       FIG. 6  shows, in the form of blocks, a variation of a capacitive power supply circuit according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   Same elements have been designated with same reference numerals in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the A.C. power sources usable by a power supply circuit according to the present invention have not been detailed, said sources being compatible with any conventional A.C. power supply. Similarly, the loads likely to be supplied by a capacitive power supply circuit of the present invention have not been detailed, the present invention being compatible with any conventional load, provided for the power supply circuit to be adapted to the current likely to be surged by this load. 
     FIG. 4  shows a preferred embodiment of a capacitive power supply circuit according to an embodiment of the present invention. 
   As previously described, such a circuit is intended to receive an A.C. voltage Vac applied between two input terminals  3  and  4  and to provide, across terminals  1  and  2  of a capacitor C forming a power storage element, a D.C. rectified voltage Vout. A resistor R 1  (optional), a capacitor C 1 , and a diode D 1  are series-connected between terminals  3  and  1 . The sizing of capacitor C 1  is performed, as previously described, according to the output current for the load (not shown) supplied by the circuit. The circuit of this embodiment of the present invention uses a controllable switch between the anode of diode D 1  and ground. This switch is intended to be on when capacitor C needs not be charged (voltage Vout greater than a reference voltage) and to be off when a charge of capacitor C is required. 
   The controllable switch is, for example, formed of a MOS transistor M having its drain connected to the anode of diode D 1  (and thus a first electrode of capacitor C 1 ) and having its source connected to terminals  2  and  4 . The gate of transistor M is connected, by a resistor R 2 , to the output of a comparator  5  (for example, an operational amplifier) having its respective supply terminals directly connected to output terminals  1  and  2  (that is, downstream of diode D 1 ). Inverting input (−) of comparator  5  receives a voltage representative of voltage Vout by being connected to the midpoint of a resistive dividing bridge formed of two resistors R 4  and R 5  in series between terminals  1  and  2 . The non-inverting input (+) of comparator  5  receives a reference voltage by being connected to the junction point of a zener diode DZ 3  with a resistor R 3  between terminals  1  and  2 . The non-inverting input (+) is further connected to the output of comparator  5  by a diode D 3  in anti-series with a zener diode DZ 4 , the cathode of diode D 3  being connected to the non-inverting input of comparator  5  while the cathode of zener diode DZ 4  is connected to its output. A capacitor C 2  connects the gate of the MOS transistor to ground  2  and forms with resistor R 2  an RC cell to stabilize the switchings of the MOS transistor. Capacitor C 2  is formed of the stray gate-source capacitance of transistor M or of an additional capacitor. Finally, a diode D 2 , preferentially formed of the parasitic diode of the MOS transistor, connects the anode of diode D 1  to ground  2 . 
     FIGS. 5A ,  5 B, and  5 C illustrate, in timing diagrams respectively showing examples of shapes of voltage Vac, of voltage Vout, and of the on periods (ON) of MOS transistor M, the operation of the circuit of  FIG. 4 . 
   During all the negative halfwaves of voltage Vac, diode D 2  is forward biased and forbids the conduction of diode D 1 . Losses are then limited to the current in resistor R 1  under the voltage (smaller than 1 volt) of forward-biased diode D 2 . 
   During positive halfwaves of voltage Vac, the conduction of transistor M is conditioned by comparator  5 , and thus by the amplitude of voltage Vout. As long as voltage Vout is smaller than voltage DZ 3 , the non-inverting input (+) is grounded by resistor R 3  and the output of comparator  5  is also grounded, and transistor M is off. As soon as voltage Vout reaches the threshold of diode DZ 3  (typically on the order of 15 volts) corresponding to the desired output voltage, the output of comparator  5  switches and becomes equal to the level of voltage Vout (due to the supply of comparator  5  from this voltage Vout). Zener diode DZ 4  is selected to have a value smaller than diode DZ 3  to set a hysteresis for the comparator operation. As soon as comparator  5  switches (time t 4 ), diode DZ 4  starts an avalanche, which prevents the conduction of diode DZ 3 . As soon as voltage Vout becomes smaller than voltage VZ 4  again, diode DZ 4  turns off and, since voltage Vout is smaller than voltage VZ 3 , comparator  5  switches to turn off transistor M. This operation is illustrated in  FIG. 5B  by the two respectively high and low thresholds VZ 3  and VZ 4  between which the charge and discharge periods of capacitor C vary. The possible excursion of output voltage Vout is thus set by the two thresholds VZ 3  and VZ 4  of diodes DZ 3  and DZ 4 . 
   The number of halfwaves of voltage Vac for which the capacitor C is considered as being sufficiently charged (between time t 4  where threshold VZ 3  is reached and a time t 5  when voltage Vout becomes smaller than threshold VZ 4  again) depends on the load connected downstream of capacitor C. The same occurs for the number of halfwaves between times t 5  and t 4  for which the charge of capacitor C is performed, on each positive halfwave of voltage Vac. 
   When the system is at stand-by, that is, no current is sampled by the load connected between terminals  1  and  2  and voltage Vout remains greater than threshold VZ 4 , the leakage current corresponds to that in MOS transistor M in the on state. 
   An advantage of this embodiment of the present invention is that, due to the controlled switching of transistor M and to the fact that the noise is averaged by the absence of a switching on each halfwave of voltage Vac, the circuit generates no disturbance on the A.C. power supply. This embodiment of the present invention thus avoids use of an upstream mains filter to comply with electromagnetic compatibility standards. 
   Another advantage of this embodiment of the present invention is that it enables regulating output voltage Vout, and accordingly, enables optimizing the sizing of capacitor C according to the application and, especially, using in practice capacitors of smaller size. 
     FIG. 6  schematically shows in the form of blocks a variation of the capacitive power supply circuit according to an embodiment of the present invention. The switch here is formed of a triac T connecting terminals  4  and  2 , and a capacitor C 1  in series with a diode D 1  connecting terminals  3  and  1 . As in the embodiment of  FIG. 4 , a diode D 2  connects the electrode of capacitor C 1  which is opposite to terminal  1 , directly to terminal  2 . 
   According to this variation, a control circuit  10  of the type described in relation with  FIG. 4 , that is, comparing output voltage Vout with a predetermined range, to which a conventional function for detecting the zero crossing of the A.C. voltage (ZVS) is added. Circuit  5 ′ controls the turning-on of triac T, which forbids the capacitor charge when it is off. It is thus an inverse control with respect to the controls described in relation with the preceding drawings. The use of a switch (triac T) likely to be turned on in the vicinity of the zero crossings of the A.C. supply voltage avoids use of a resistor R 1  to limit current surges on powering-on. 
   The variation of  FIG. 6  is more specifically intended for high powers. 
   Of course, embodiments of the present invention are likely to have various alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, the sizing of the different components and especially of the switch triggering threshold voltages of various embodiments of the present invention are within the abilities of those skilled in the art based on the functional indications given hereabove and on the voltage and power of the concerned application. 
   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. 
   The capacitive power supplies of  FIGS. 4 and 6  may be utilized in a variety of different types of electronic devices, particularly those requiring relatively low power outputs and stand-by consumption such as cellular phones, personal digital assistants, and personal music players like MP3 players.