Abstract:
The invention pertains to a synchronous rectification device ( 10 ) of the H-bridge type supplying a coil ( 5 ) of a phase of a synchronous machine, comprising four switches ( 21, 31, 22, 32 ) disposed on the electrical links ( 11, 12 ) of this H-bridge and intended to be instructed by an electronic circuit ( 40 ), characterized in that each switch comprises at least one transistor (T 1 ) instructed by the electronic circuit ( 40 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/573,523, filed Mar. 23, 2006 now abandoned. U.S. patent application Ser. No. 10/573,523, filed Mar. 23, 2006 is an application filed under 35 USC 371 based on PCT Application No. PCT/FR04/02407 filed Sep. 24, 2004. PCT Application No. PCT/FR04/02407 filed Sep. 24, 2004 claims priority to French Application No. 03 11191 filed Sep. 24, 2003. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a synchronous rectification device of the H-bridge type supplying a synchronous electric machine. 
     The invention also relates to a polyphase synchronous electric machine devised to work in motor mode and in generator mode, for example a motor vehicle alterno-starter. 
     BACKGROUND 
     A polyphase synchronous electric machine, for example a variable-reluctance motor, comprises (see  FIG. 1 ) a rotor  1 , a permanent magnet or equivalent, integral with a spindle  2  controlled in angular rotation, and a stator  3  having a plurality of pair-wise opposite salient poles  4  corresponding to the phases of the machine. The ferromagnetic structure of the stator comprises coils  5  supplied with electric current to induce a magnetic field orienting the rotor. 
     The electric current supply to the coils, generally a battery  20 , whose output is filtered by a filter  30  delimited by a discontinuous closed line comprising a choke  350  and a capacitor  360 , is controlled separately in each coil by virtue of a synchronous rectification device comprising an H-bridge  10  situated between the battery and the earth so as to create a rotating field driving the rotor at the same so-called synchronism speed. 
     The H-bridge  10  delimited by a discontinuous closed line comprises two parallel electrical links  11  and  12  forming the vertical branches of an H, each link  11 ,  12  comprising two electronic switches  21 ,  31  and  22 ,  32 , separated by a midpoint  13  or  14 , the midpoints  13  and  14  of these links being connected together by the coil  5  of a phase of the machine. 
     In a contemporary embodiment of the H-bridge, generally adopted by reason of its simplicity, the switches may be transistors  23 ,  34 , and diodes  33 ,  24 , as in the example of  FIG. 1 , the diodes and the transistors being situated on a diagonal. 
     The transistors  23  and  34  are instructed by an electronic circuit  40  to allow the passage of the current through the coil  5  while the diodes  33  and  24  act as spontaneous switches making it possible to absorb the energy accumulated in the said coil. This manner of operation is termed asynchronous. 
     However the diodes, which exhibit significant leakage currents, bring about a not inconsiderable energy loss, even when stopped, and the overall efficiency of the machine may find itself limited to 85%. 
     Moreover, the switches are previously determined so as to always be instructed or always be “spontaneous”, and the energy losses in the H-bridge turn out to be unbalanced, which hinders the reliability of the machine. 
     An alterno-starter consists for example of a variable-reluctance synchronous machine comprising an H-bridge and able to work in generator and motor modes, depending on the synchronous and asynchronous instructions which are imposed successively on the switches  21 ,  22 ,  31 ,  32 . 
     Instructed as a generator, it transforms part of the mechanical energy available on the motor shaft into electrical energy to supply the electrical installation of the vehicle and recharge the battery through the diodes, hence in asynchronous mode. 
     Instructed as a motor, it transforms the electrical energy available on the battery, into mechanical energy either to start the engine of the vehicle, in which case it carries out the starter function, or to help it when cold, or to execute the so-called “stop and go” function of frequent stops and restarts, especially in town. 
     In the latter case, it is above all the transistors, instructed in synchronous mode, which transmit the energy from the battery to the coils of the stator, occasionally, and over short durations. 
     This synchronous machine has to be very reliable. 
     SUMMARY 
     To improve the overall energy efficiency and the reliability of the above machine, it is possible to seek to refine the H-bridges by dissociating the circuits which provide the power in motor mode from those which provide it in generator mode. Specifically, since they do not operate simultaneously, it is possible to separate them and to refine them in their essential functions in a specific manner. However this solution would be expensive. 
     The applicant has chosen a different more economical route and proposes a machine of the above type capable of catering for operation in motor mode and operation in generator mode by the same H-bridges while avoiding therein the energy losses due to the leakage currents of the diodes and improving the overall reliability thereof. 
     Accordingly, the invention relates firstly to a synchronous rectification device ( 10 ) of the H-bridge type supplying a coil ( 5 ) of a phase of a synchronous machine, comprising four switches ( 21 ,  31 ,  22 ,  32 ) disposed on the electrical links ( 11 ,  12 ) of this H-bridge and intended to be instructed by an electronic circuit ( 40 ), characterized in that each switch comprises at least one transistor (T 1 ) instructed as a diode or otherwise by the electronic circuit ( 40 ) depending on whether or not the intensity I of the current crossing the coil exceeds a predetermined threshold S. 
     By dispensing with the diodes, one avoids the loss of energy due to their leakage current, thereby contributing to a first improvement of the overall efficiency of the synchronous machine. 
     Preferably, each switch consists of a certain number of transistors in parallel, the said number being determined by the power to be drawn in the switch. 
     The optimisation of the number of transistors as a function of the power further improves the overall efficiency of the machine. In addition to an improvement in its reliability, by virtue of the redundancy of the transistors that is thus obtained by placing them in parallel, one even improves its security of operation. 
     In particular, the numbers of operational transistors in the switches may be chosen by the electronic circuit itself to improve the efficiency of the synchronous machine in the motor or generator modes of its operation. 
     Advantageously, the transistors are all identical, this being less expensive in respect of the industrial manufacture of the machine. 
     Advantageously again, the transistors are MOSs. 
     The invention also relates to a polyphase synchronous electric machine with per phase a coil supplied, by a DC supply, under the control of a synchronous rectification device of the above type comprising four switches instructed by an electronic circuit, machine characterized in that the electronic circuit is designed to instruct the four switches in distinct pairs, each pair consisting of two of the four switches, always chosen in series with the coil, all the pairs being alternately instructed either to supply the coil with forward or reverse current, or else to restore the energy accumulated therein. 
     By using a synchronous rectification device of the type seen above, the efficiency of the machine is improved to 95%. 
     By alternately reversing the roles of the pairs of switches, the functions of the said switches are made general-purpose and there is a balancing of the losses over time, thereby improving the reliability of the H-bridge, hence of the synchronous machine. 
     Preferably, a sensor of current being provided on the circuit of the coil, the electronic circuit is designed to instruct a pair of switches in synchronous mode if the current in the coil is greater in absolute value than a predetermined threshold, otherwise in asynchronous mode, the transistors concerned then intervening only through their internal diode, the other two switches being instructed to close. 
     Advantageously, the electronic circuit for instructing the switches is designed to offset the instructions for the said switches in time upon the change of mode to avoid the short-circuiting of the DC supply. 
     Again preferably, the synchronous machine comprises on its rotor a sensor of position of the rotor linked to the electronic circuit, and the electronic circuit is designed to instruct the polyphase synchronous machine as a function of the position of the rotor according to a motor mode or a generator mode, in accordance with a usage cue delivered by the engine processor of a motor vehicle. 
     The invention will be better understood with the aid of the following description of a preferred embodiment of the synchronous rectification device and of the synchronous electric machine equipped with the synchronous rectification device according to the invention, with reference to the appended drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a functional block diagram of an ordinary synchronous electric machine, 
         FIG. 2  is the same diagram for a synchronous electric machine equipped with its synchronous rectification device in accordance with the invention, 
         FIG. 3  is an electrical diagram of an electronic switch entering into the construction of the synchronous rectification device according to the invention, 
         FIG. 4  is a timing diagram of the operation of the synchronous rectification device according to the invention, 
         FIG. 5  is a timing diagram of the operation of the same device implemented in the synchronous electric machine according to the invention, and 
         FIG. 6  is a timing diagram of the instructing of the switches so as to avoid the short-circuiting of the DC supply. 
     
    
    
     DETAILED DESCRIPTION   
     With reference to  FIG. 2 , the polyphase synchronous electric machine comprises all the elements of the machine of  FIG. 1  seen above: a battery  20 , a filter  30 , a synchronous rectification device, here an H-bridge  10 , an electronic circuit  40 , here a programmable logic circuit, a motor  50  delimited by a discontinuous closed line with a rotor  1  rotating about a spindle  2  and a stator  3  comprising poles  4  and coils  5  whose terminals connect the midpoints  13  and  14  of the corresponding H-bridges  10 , only one of which is represented in  FIGS. 1 and 2 . 
     In addition, the machine comprises on each coil  5  a current sensor  45  delivering a cue regarding the current I flowing around the coil, indicating the value and the direction of the current, and also whether the current I is, in absolute value, smaller or larger than a predetermined threshold S. 
     Finally, the programmable logic circuit  40  receives a cue θ, delivered by an angular sensor,  42 , regarding the angular position of the rotor  1  on its spindle  2  and cues M originating from the engine processor of the vehicle (not represented). 
     On the basis of these cues, the programmable logic circuit  40  instructs the H-bridge  10  via electrical links  41  acting on the four switches  21 ,  22 ,  31 ,  32 . 
     The four switches are all structurally identical. A switch  21 ,  22 ,  31 ,  32 , comprises, with reference to  FIG. 3 , n identical transistors T 1 , . . . , Tn placed in parallel between their input  27 ,  28 ,  37 ,  38  and their output  25 ,  26 ,  35 ,  36 . It is for example possible to choose n from 3 to 5, or more, it being possible for the transistors of a switch to be instructed simultaneously through a single instruction  41  or separately through instructions  411  to  41   n , the number of transistors to be instructed being optimized according to the operating modes of the machine and the power to be transmitted. 
     MOSs may be chosen as transistors. 
     The instructions for the switches will now be explained. 
     With reference to  FIGS. 2 and 4 , during an alternation A 1 , the four switches  21 ,  31 ,  22 ,  32  of the H-bridge  10  being respectively designated by the customary labels MHS, DLS, DHS, MLS (as MOS High/Low Side and Diode High/Low Side, despite the absence of diodes and the general-purpose nature of the functions of the switches), are instructed according to N phases, here, in the example, N being equal to six successive phases φ1 to φ6: 
     φ1: MHS and MLS instructed to close, DHS and DLS instructed as a diode, 
     φ2: MHS and DHS instructed to close, MLS and DLS instructed as a diode, 
     φ3: MHS and MLS instructed to close, DHS and DLS instructed as a diode, 
     φ4: DLS and MLS instructed to close, MHS and DHS instructed as a diode, 
     φ5: MHS and MLS instructed to close, DHS and DLS instructed as a diode, 
     φ6: DHS and DLS instructed to close, MHS and MLS instructed as a diode. 
     The switches are said to be instructed as a diode if the transistors are instructed to simulate a diode behaviour, in the manner which will be explained later. 
     It may be seen that the four switches are instructed by the programmable logic circuit  40  in distinct pairs, each pair consisting of two switches in series with the coil  5 . 
     The first and last phases of the alternation A 1 , here φ1 and φ6, are themselves split into two subphases M 1  and M 2  corresponding to two different modes of operation. If the current I in the coil  5  is greater in absolute value than the threshold S, then the switches are instructed as diodes as has just been seen, in synchronous mode. 
     Otherwise, the threshold S not being reached by the current I in the coil, the switches DHS and DLS are no longer instructed, the transistors concerned intervening naturally only via their internal diode, in asynchronous mode. 
     The overstepping of the threshold S by the current I cannot be effected without a device explained hereinafter lest the supply be short-circuited (a phenomenon known as “cross-conduction”). 
     With reference to  FIG. 5 , during the next alternation A 2 , the four switches  21 ,  31 ,  22 ,  32  are instructed according to the N phases, here six phases φ1′ to φ6′, symmetric with the previous ones, successively: 
     φ1′: DHS and DLS instructed to close, MHS and MLS instructed as a diode, 
     φ2′: DHS and MHS instructed to close, DLS and MLS instructed as a diode, 
     φ3′: DLS and DHS instructed to close, MLS and MHS instructed as a diode, 
     φ4′: DLS and MLS instructed to close, MHS and DHS instructed as a diode, 
     φ5′: DHS and DLS instructed to close, MHS and MLS instructed as a diode, 
     φ6′: MHS and MLS instructed to close, DHS and DLS instructed as a diode. 
     In the same way as above, the first and last phases φ1′ and φ6′ of the alternation are split into two subphases M 1 ′ and M 2 ′ corresponding to the two modes of operation, synchronous and asynchronous, of the H-bridge. 
     The four switches  21 ,  22 ,  31 ,  32  are therefore instructed by the programmable logic circuit  40  in distinct pairs, each pair consisting of two of the four switches, still chosen in series with the coil  5 , and according to successive alternations A 1 , A 2  in the course of which the programmable logic circuit  40  alternately instructs all the pairs either to supply the coil with forward or reverse current, or else to restore the energy accumulated therein. 
     The programmable logic circuit  40  instructs the transistors “as diodes” in the following manner: on the basis of the cue regarding the value of the current I delivered by the sensor  45 , it instructs the disabling of the transistor only if this current is in a determined direction, the passing direction of the diode simulated, otherwise it instructs the transistor so as to close it, that is to say render it passing but at the same time establish a current I in the coil at a value which would be established in accordance with the characteristic of the diode simulated if it were to act by asynchronous rectification. 
     As two transistors of one and the same link cannot be rendered passing simultaneously without short-circuiting the supply  20 , in particular upon the change of mode during the above phases φ1, φ6; φ1′, φ6′, the programmable logic circuit  40  is designed to avoid this short-circuit known as cross-conduction by instructing the switches MHS and DLS concerned in a manner offset in time by a duration Δt, as shown in  FIG. 6 . 
     The duration Δt chosen is at least equal to the time to establish the logic levels in the transistors of these switches. 
     The cue θ emanating from the sensor of angular position  42  of the rotor and the cue M coming from the engine processor allow the programmable logic circuit  40  to instruct the polyphase synchronous machine
         on the one hand in motor mode by optimizing the number N of phases φi necessary as a function of the speed of the rotor  1  and by determining the number n of transistors to be rendered operational in each switch of each H-bridge  5  to optimize the efficiency as a function of the power to be transmitted, or   on the other hand in generator mode, likewise determining therein the number n, as a function of the recharging of the DC supply  20  to optimize this recharging.       

     Thus the numbers n of operational transistors in the links are chosen by the programmable logic circuit  40  to improve the efficiency of the synchronous machine in the two modes of operation, motor and generator.