Abstract:
A circuit for controlling a switch in series with a capacitive element. A circuit may include a bidirectional switch and a diode in parallel with first and second conduction terminals of the switch. The switch may be configured to control a capacitive element adapted to be coupled to an A.C. voltage. The switch includes first and second conduction terminals configured to conduct a same current when the switch is activated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application Ser. No. 11/57231, filed on Aug. 8, 2011, and the priority benefit of French patent application Ser. No. 10/61118, filed on Dec. 23, 2010, which applications are hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     1. Technical Field 
     Embodiments relate to the forming of control circuits for capacitive elements powered with an A.C. power supply voltage. Embodiments more specifically relate to a capacitive power supply voltage for an electronic device having at least two operating modes requiring different supply powers. 
     2. Discussion of the Related Art 
     Capacitive elements, which are generally controlled from an A.C. power supply source, are used in electronic motors, electronic lamps, or mere capacitors. For the two first mentioned load types, the capacitive element is generally placed at the secondary of a diode bridge. As to A.C. capacitors, they are series-connected with the A.C. voltage source, often including a series resistor. Such capacitors are often used for capacitive power supply circuits, which are one of the different solutions for supplying power to a load from an A.C. power supply voltage originating, for example, from the mains (220 volts or 110 volts). The power is generally provided across a low-voltage capacitor (for example, from a few volts to a few tens of volts). 
     Many electronic devices require at least two operating modes requiring different supply powers. For example, many electronic devices have a stand-by mode, with a decreased power consumption with respect to an active mode. It is then desired to avoid useless power consumption by the power supply circuit during periods when the supplied device is not active. 
     It would be desirable to have a capacitive power supply circuit of simple design, with at least two operating modes capable of providing different supply powers. 
     More generally, it would be desirable to improve the control of a switch in series with an A.C. powered capacitive element. 
     SUMMARY 
     Thus, an embodiment forms an A.C. switch enabling to control an element of capacitive nature, requiring a single control, that is, a switch which is only controlled one halfwave out of two. 
     Another embodiment provides a capacitive power supply circuit of simple design, having at least two operating modes capable of providing different supply powers. 
     Thus, an embodiment provides a circuit comprising: 
     a switch for controlling a capacitive element adapted to be coupled to an A.C. voltage; and 
     a diode in parallel with the switch. 
     According to an embodiment, a transistor is provided between a control terminal of the first switch and a first terminal of application of the A.C. voltage. 
     According to an embodiment, said capacitive element is intended to supply power to a capacitive load. 
     According to an embodiment, the switch comprises at least one thyristor. 
     According to an embodiment, the switch comprises two thyristors in antiparallel forming a triac. 
     According to an embodiment, the circuit further comprises an element for controlling the switch. 
     According to an embodiment, the capacitive element is arranged between a second terminal of application of the A.C. voltage and the switch. 
     According to an embodiment, the switch is arranged between a second terminal of application of the A.C. voltage and said capacitive element. 
     The present invention also provides a capacitive power supply circuit comprising: 
     a first branch capable of providing a first power level; 
     a second branch parallel to the first branch, capable of providing a second power level; and 
     a control circuit such as hereabove, the capacitive element and the switch being comprised in the second branch. 
     According to an embodiment, the first and second branches connect a second terminal of application of the A.C. voltage to a first terminal of provision of a D.C. voltage, and the first terminal of application of the A.C. voltage is connected to a second terminal of provision of the D.C. voltage. 
     According to an embodiment, said control element connects the base of the transistor to one of the first and second terminals of provision of the D.C. voltage. 
     The foregoing and other objects, 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  is an electric diagram of an embodiment of a positive capacitive power supply circuit with two power modes; 
         FIG. 2  is an electric diagram of an embodiment of a negative capacitive power supply circuit with two power modes; and 
         FIG. 3  is an electric diagram of another embodiment of a negative capacitive power supply circuit with two power modes. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings. Further, only those elements which are useful to the understanding of the present description have been shown and will be described. In particular, the destination of the power supply voltages generated by the described circuits has not been detailed, the described embodiments being compatible with usual applications of such power supply voltages. 
     The embodiments will be described in relation with an example of capacitive power supply. They more specifically apply to any control of a switch in series with a capacitive element powered by an A.C. voltage source. 
       FIG. 1  is an electric diagram of an embodiment of a positive capacitive power supply circuit with two power modes. 
     The circuit comprises two input terminals  11  and  12  intended to receive an A.C. power supply voltage Vac, for example, the mains voltage, and two output terminals  13  and  14  intended to provide a D.C. power supply voltage Vcc. Output terminal  14  forms the ground and is one with input terminal  12 . In this example, terminal  13  corresponds to a terminal for providing the positive power supply voltage. 
     A resistor R 1 , in series with a capacitor C 1  and a diode D 1  having its anode on the side of capacitor C 1 , connect terminal  11  to terminal  13 . Terminals  13  and  14  are connected by a capacitor C 3  across which D.C. voltage Vcc is provided. 
     Diode D 1  forms an element of halfwave rectification of voltage Vac to charge capacitor C 3 . Capacitor C 1  has the function of setting the current in the capacitive power supply (in stand-by mode) and capacitor C 3  has the function of storing and smoothing power supply voltage Vcc. The value of output voltage Vcc is set by a zener diode DZ grounding the anode of diode D 1 , the anode of diode DZ being on the ground side. The function of resistor R 1  is to limit the current surge, to protect diode DZ and output capacitor C 3  against a possible abrupt current peak on powering-on of the circuit. 
     The operation of a capacitive power supply is known per se. During growth phases of voltage Vac, a current flows through resistor R 1 , capacitor C 1 , and diode D 1  to load capacitor C 3 . As long as voltage Vcc has not reached the threshold voltage of diode DZ (to within the voltage drop in diode D 1 ), diode DZ is non-conductive, thus enabling capacitor C 3  to charge. As soon as voltage Vcc reaches the threshold voltage of diode DZ (for example, from a few volts to a few tens of volts), the zener diode starts an avalanche, thus limiting the charge voltage of capacitor C 3 . During negative halfwaves of voltage Vac, the current flows through diode DZ, capacitor C 1 , and resistor R 1 , and diode D 1  prevents the discharge of capacitor C 3  into the A.C. power supply. 
     Resistor R 1  and capacitor C 1  define a first capacitive power supply branch B 1  capable of providing a first power level. 
     In parallel with the first branch, a second capacitive power supply branch B 2  comprising a limiting resistor R 2  in series with a capacitor C 2  and a triac TR, connects terminal  11  to the anode of diode D 1  (cathode of diode DZ). A first conduction terminal A 1  of the triac on the side of gate G of the triac is placed on the anode side of diode D 1 , and second conduction terminal A 2  of the triac is on the side of capacitor C 2 . Capacitive power supply B 2  is capable of providing a second power level. Triac TR forms a switch for activating and deactivating this second branch. Branch B 2  further comprises a diode D 2  connected parallel to triac TR, the cathode of diode D 2  being on the side of terminal A 2 . This connection of diode D 2  with triac TR simplifies the control of the A.C. switch. Indeed, it is then only necessary to control the triac in a single halfwave, the other halfwave being automatically conductive due to diode D 2 , as soon as capacitor C 2  has been charged to a value different from the negative peak of the mains voltage. This simplification enables, for the case of the capacitive power supply application, to form a specific control circuit with elements  16  and  18 . 
     In a first operating mode, triac TR is maintained off. 
     During positive halfwaves of voltage Vac, diode D 2  being non-conductive, the charge current of capacitor C 3  only runs through first capacitive power supply branch B 1 , that is, resistor R 1  and capacitor C 1 . This corresponds to a first power level of the capacitive power supply. On the side of branch B 2 , capacitor C 2  remains at the value reached at the end of the previous halfwave. 
     During decreasing halfwaves of voltage Vac, the current flows through branch B 1 . In branch B 2 , diode D 2  remains blocked since capacitor C 2  remains charged to the negative peak value of voltage Vac, to within the recharge current, due to the discharge of capacitor C 2  into its internal resistor or into a possible external resistor in parallel with capacitor C 2 . 
     The A.C. switch, formed of triac TR and of diode D 2 , is thus effectively off during this first operating mode, both on increasing and decreasing halfwaves of voltage Vac. 
     In a second operating mode, triac TR is turned on for each increasing halfwave of voltage Vac, by application of an adapted turn-on signal on its gate. 
     During increasing halfwaves of voltage Vac, the charge current of capacitor C 3  flows through both branches B 1  and B 2 . This corresponds to a second power level of the capacitive power supply, greater than the first level. On the side of branch B 2 , capacitor C 2  charges to the positive peak value of voltage Vac. 
     During decreasing halfwaves of voltage Vac, triac TR is off, but the voltage across diode D 2  becomes positive (since voltage Vac becomes smaller than the voltage across capacitor C 2 ), diode D 2  becomes conductive, and the current flows through both branches B 1  and B 2 . Capacitor C 2  then charges to the negative peak value of voltage Vac. Without diode D 2 , the triac being off, capacitor C 2  would not discharge and would remain at the positive peak value of voltage Vac. At the next positive halfwave, it would then no longer be possible to have a positive current flow through branch B 2 , and thus to turn on the triac. This would result in a blocking of branch B 2 . Diode D 2  is then in charge of enabling the discharge of capacitor C 2  on each negative halfwave of voltage Vac when triac TR has been previously turned on. 
     Triac TR forms a switch enabling to switch between two power modes. The respective values of capacitors C 1  and C 2  are selected according to the powers required for the device to be powered. The value of capacitor C 2  will generally be selected to be greater than that of capacitor C 1  since the power required in active mode (high power) is generally greater than twice the power required in stand-by mode. 
     In the embodiment of  FIG. 1 , the switch, although formed by a triac which is a bidirectional component, is made functionally unidirectional, that is, it is only turned on when a positive current flows from capacitor C 2  to diode DZ (diode D 2  being reverse biased). The flowing of a positive current from diode DZ to capacitor C 2  (negative halfwaves) is ensured by diode D 2 , which enables to simplify the triac control circuit. Indeed, in the absence of diode D 2 , triac TR would have to be turned on for each decreasing halfwave of voltage Vac. This would require a complex circuit for controlling the triac since, during negative halfwaves of voltage Vac, the potential difference between terminal A 1  and terminal  12  is equal to the forward voltage drop of diode DZ (on the order of 0.6 V), which does not enable to draw a gate current. In particular, it would then be necessary to sample power from capacitor C 3  to be able to turn on the triac. In an alternative embodiment, triac TR may be replaced with any other one-way switch capable of being turned on in quadrant Q 2 , that is, by drawing a positive current from terminal A 1  to gate G while a positive voltage is applied between terminal A 2  and terminal A 1 . 
     In the example of  FIG. 1 , the triac control circuit comprises, between gate G and the ground, a resistor R 3  in series with an NPN-type bipolar transistor  16  having its emitter on the ground side. The base of bipolar transistor  16  is connected to terminal  13  by a MOS transistor  18 , possibly associated with a resistor in series with transistor  18  and the base of transistor  16 . The gate of transistor  18  receives a control signal CMD originating from any circuit capable of indicating a need for a switching from the first operating mode to the second operating mode and conversely. 
     In the example of  FIG. 1 , signal CMD is referenced to terminal  14 . It for example is a signal originating from the electronic device powered with voltage Vcc, or a circuit capable of automatically detecting load variations across capacitor C 3 . 
     Transistor  16  may be replaced with any switch having a control reference on the side of terminal  14 , for example, a MOS transistor. 
     To turn on the triac during a positive halfwave of voltage Vac, MOS transistor  18  is made conductive. Terminal  13  being at a voltage greater than the ground (and in practice greater than one volt, and thus greater than the base-emitter voltage drop of transistor  16  plus the on-state voltage drop of transistor  18 ), this causes the turning-on of transistor  16 , and the flowing of a positive current between terminal A 1  and the ground, through gate G, resistor R 3 , and transistor  16 . This gate current triggers the turning-on of the triac, which then remains on until the current that it conducts disappears. This corresponds to the high-power operating mode. On the contrary, if, during an increasing halfwave of voltage Vac, MOS transistor  18  is maintained on, bipolar transistor  16  and triac TR remain off. This corresponds to the low-power operation. 
       FIG. 2  is an electric diagram of an embodiment of a negative capacitive power supply circuit with two power modes. The circuit of  FIG. 2  operates similarly to the circuit of  FIG. 1 , with the difference that terminal  13  this time corresponds to a terminal of provision of a negative power supply voltage (with respect to ground  14 ). 
     As compared with the circuit of  FIG. 1 , a diode D 1 ′ replaces diode D 1 , the anode of diode D 1 ′ being on the side of terminal  13  and the cathode of diode D 1 ′ being on the side of conduction terminal A 1  of triac TR. Capacitor C 3  is then only charged during negative halfwaves of input voltage Vac. Zener diode DZ is replaced with a zener diode DZ′ having its anode on the cathode side of diode D 1 ′. Diode DZ′ sets output voltage Vcc. Diode D 2  is replaced with a diode D 2 ′ connected parallel to triac TR, the cathode of diode D 2  being on the side of terminal A 1 . 
     In the first operating mode, triac TR is maintained off. 
     During negative halfwaves of voltage Vac, diode D 2 ′ is non-conducting and the (negative) charge current of capacitor C 3  only runs through first capacitive power supply branch B 1 , that is, resistor R 1  and capacitor C 1 . On the side of branch B 2 , capacitor C 2  remains at its initial value. This corresponds to a first power level of the capacitive power supply. 
     During increasing halfwaves of voltage Vac, the current flows through branch B 1 . In branch B 2 , diode D 2  remains blocked since capacitor C 2  remains charged to the positive peak value of voltage Vac, to within the recharge current, due to the discharge of capacitor C 2  into its internal resistor or into a possible external resistor in parallel with capacitor C 2 . 
     The A.C. switch, formed of triac TR and of diode D 2 , is thus effectively off during this first operating mode, both on increasing and decreasing halfwaves of voltage Vac. 
     In a second operating mode, triac TR is turned on for each negative halfwave of voltage Vac, by application of an adapted signal on its gate G. 
     During decreasing halfwaves of voltage Vac, the charge current of capacitor C 3  flows through capacitive power supply branches B 1  and B 2 . This corresponds to a second power level of the capacitive power supply, greater than the first level. On the side of branch B 2 , capacitor C 2  charges to the negative peak value of voltage Vac. 
     During increasing halfwaves of voltage Vac, triac TR is off but, diode D 2 ′ being conductive, the current flows through both branches B 1  and B 2  of the capacitive power supply. Capacitor C 2  then charges to the positive peak value of voltage Vac. Without diode D 2 ′, the triac being off, capacitor C 2  would remain at the negative peak value of voltage Vac. At the next negative halfwave, it would then no longer be possible to have a current flow through branch B 2 , and thus to turn on the triac. This would result in branch B 2  being non-conductive. Diode D 2 ′ is thus in charge of enabling the discharge of capacitor C 2  on each increasing halfwave of voltage Vac when the triac has been previously turned on. 
     As in the embodiment of  FIG. 1 , the switch formed by triac TR is made functionally unidirectional, that is, it is only turned on when a negative current flows from capacitor C 2  to diode DZ′ (diode D 2 ′ being reverse biased). The flowing of a positive current from capacitor C 2  to diode DZ′ (positive halfwaves) is ensured by diode D 2 ′, as soon as the voltage of capacitor C 2  has been reversed by the previous conduction of triac TR, which enables simplifying the triac control circuit. In an alternative embodiment, triac TR may be replaced with any other one-way switch capable of being turned on in quadrant Q 4 , that is, by injecting a positive current from gate G to terminal A 1  while a negative voltage is applied between terminal A 2  and terminal A 1 . 
     In the example of  FIG. 2 , the triac control circuit comprises, between gate G of the triac and the ground, a resistor R 3  in series with a PNP-type bipolar transistor  16 ′ having its emitter on the ground side. The base of bipolar transistor  16 ′ is connected to terminal  13  by a MOS transistor  18 , possibly connected with a resistor in series with transistor  18  and the base of transistor  16 . The gate of transistor  18  receives a control signal CMD originating from any circuit capable of indicating a need for switching from the first operating mode to the second operating mode and conversely. In the example of  FIG. 2 , signal CMD is referenced to terminal  13 . Transistor  16 ′ may be replaced with any switch having a control reference on the side of terminal  14 , for example, a MOS transistor. 
     To turn on the triac during a negative halfwave of voltage Vac, MOS transistor  18  is made conductive. Terminal  13  being at a smaller voltage than terminal  12  causes the turning-on of bipolar transistor  16 ′, and the flowing of a negative current between terminal A 1  and the ground, through gate G, resistor R 3 , and transistor  16 ′. This gate current triggers the turning-on of the triac, which then remains on until the current that it conducts disappears. This corresponds to the high-power operating mode. On the contrary, if, during a decreasing halfwave of voltage Vac, MOS transistor  18  is maintained off, bipolar transistor  16 ′ and triac TR remain off. This corresponds to the low-power operation. 
       FIG. 3  is an electric diagram of another embodiment of a negative capacitive power supply circuit with two power modes. It should be noted that all that will be described in relation with  FIG. 3  transposes to a circuit for providing a positive power supply voltage. 
     In this example, triac TR is replaced with a thyristor TH with a cathode gate G′, diode D 2 ′ being connected in antiparallel with thyristor TH. Further, thyristor TH and diode D 2 ′ are placed upstream of capacitor C 2  (that is, between terminal  11  and capacitor C 2 ), rather than downstream of capacitor C 2  (that is, between capacitor C 2  and diode D 1 ′) as in the embodiment of  FIG. 2 . In the shown example, thyristor TH and diode D 2 ′ are connected between terminal  11  and resistor R 2 , the anode of thyristor TH being on the side of resistor R 2 . 
     In the first operating mode, thyristor TH is maintained off. 
     During decreasing halfwaves of voltage Vac, diode D 2 ′ being blocked, the (negative) charge current of capacitor C 3  only runs through first capacitive power supply branch B 1 , that is, resistor R 1  and capacitor C 1 . This corresponds to a first power level of the capacitive power supply. 
     During increasing halfwaves of voltage Vac, the current flows through branch B 1 . In branch B 2 , diode D 2 ′ remains non-conducting since capacitor C 2  remains charged to the positive peak value of voltage Vac, to within the recharge current, due to the discharge of capacitor C 2  into its internal resistor or if an external resistor is placed in parallel across capacitor C 2 . The A.C. switch, formed of TH and D 2 ′, is thus effectively off during this first operating mode, both on increasing and decreasing halfwaves of voltage Vac. 
     In the second operating mode, thyristor TH is turned on for each negative halfwave of voltage Vac, by application of an adapted signal on its gate G′. 
     During decreasing halfwaves of voltage Vac, the charge current of capacitor C 3  flows through both branches B 1  and B 2  of the capacitive power supply. This corresponds to a second power level of the capacitive power supply, greater than the first level. On the side of branch B 2 , capacitor C 2  charges to the negative peak value of voltage Vac. 
     During increasing halfwaves of voltage Vac, diode D 2 ′ being conductive, the current flows through both branches B 1  and B 2  of the capacitive power supply, which enables to discharge capacitor C 2 . 
     In the example of  FIG. 3 , the control circuit of thyristor TH comprises, between gate G′ and the ground, a diode D 3  in series with a resistor R 3 ′ and a PNP-type bipolar transistor  16 ″. The cathode of diode D 3  is on the gate side of thyristor TH, and the emitter of transistor  16 ″ is on the ground side. The base of transistor  16 ″ is connected to terminal  13  by a MOS transistor  18 , possibly associated with a series resistor. The gate of transistor  18  receives a control signal CMD originating from any circuit capable of indicating a need for a switching from the first operating mode to the second operating mode and conversely. In the example of  FIG. 3 , signal CMD is referenced to terminal  13 . Transistor  16 ″ may be replaced with any switch having a control reference on the side of terminal  14 , for example, a MOS transistor. 
     To turn on the thyristor during a negative halfwave of voltage Vac, MOS transistor  18  is made conductive. Terminal  13  being at a smaller voltage than terminal  12 , this causes the turning-on of bipolar transistor  16 ″, and the flowing of a current between the ground and the cathode terminal of thyristor TH, through transistor  16 ″, resistor R 3 ′, diode D 3  and gate G′. This gate current triggers the turning-on of the thyristor, which then remains on until the end of the negative halfwave of voltage Vac. This corresponds to the high-power operating mode. On the contrary, if, during a negative halfwave of voltage Vac, MOS transistor  18  is maintained off, bipolar transistor  16 ″ and thyristor TH remain off. This corresponds to the low-power operation. 
     It should be noted that in the embodiment of  FIG. 3 , diode D 3 , resistor R 3 ′, and transistor  16 ″ are located upstream of resistor R 1 . They should thus be selected to be capable of withstanding voltage Vac. High-voltage diode D 3  is especially used to protect the PN junction between gate G and the cathode of thyristor TH. 
     It should be noted that in the embodiment described in relation with  FIG. 3 , thyristor TH may be replaced with a triac. 
     An advantage of the discussed circuits is that they enable to control an element of capacitive type on an A.C. voltage with a halfwave control. 
     Another advantage is that the A.C. switch may be controlled while its control reference is different from the reference of the control circuit delivering signal CMD. 
     In an example of application to a power supply, an advantage of capacitive power supply circuits of the type described in relation with  FIGS. 1 to 3  is that they have two operating modes capable of providing different supply powers, while having a simple design. 
     Another advantage is that the current for starting the thyristors or triacs used as switches for activating or deactivating second branch B 2  of the capacitive power supply mainly originates from A.C. power voltage source Vac, and not from output capacitor C 3 . This enables avoiding unnecessary consumption of the power stored in capacitor C 3 . 
     Specific embodiments have been described. Various alterations, modifications and improvements will readily occur to those skilled in the art. 
     In particular, embodiments in which the switch for activating and deactivating second branch B 2  of the capacitive power supply comprises a thyristor, or two thyristors in antiparallel forming a triac, has been described hereabove. It will be within the abilities of those skilled in the art to use any other adapted switch. 
     Further, it will be within the abilities of those skilled in the art to replace zener diodes DZ and DZ′ with any other device capable of limiting the charge voltage of capacitor C 3 , for example, a properly controlled MOS transistor. 
     Moreover, MOS transistor  18  may be replaced with any circuit (for example, logic), providing a high level ( FIG. 1 ) or a low level ( FIGS. 2 and 3 ). 
     Further, capacitive power supply circuits with two power modes have been described as an example. It will be within the abilities of those skilled in the art to adapt the described operation to form capacitive power supply circuits having more than two distinct power modes. 
     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.