Abstract:
The invention relates to a unitary magnetic coupler including a first inductor (Lp) consisting of a first winding of phase φ and having a number N of turns between the two ends of the first winding and, magnetically coupled to the first inductor (Lp), a second inductor (Ls) consisting of a second winding of the same phase φ and having the same number N of turns between the two ends of the second winding, where the ends of the first and second windings of the unitary magnetic coupler are interconnected using links consisting of capacitors (C 1,  C 2 ) of equal value.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a unitary magnetic coupler.  
         [0002]     Other subjects of the invention are a switch mode power supply and data transmission equipment employing such a coupler.  
         [0003]     The field of the invention is that of power supplies designed to deliver direct-current from an alternating-current (AC) or direct-current (DC) power distribution network.  
       BACKGROUND OF THE INVENTION  
       [0004]     Specifically, power supplies operating at low power levels, typically less than 150 W, will be considered.  
         [0005]     One aim is to minimize the size and weight of the power supplies.  
         [0006]     “Flyback” or “forward” power supplies are low- power switch mode power supplies employed frequently, particularly because they are simple to control. The flyback design is a very interesting case because of its reduced size arising from the fact that it needs only one magnetic element to achieve the power conversion.  
         [0007]     It will be recalled that in switch mode power supplies the DC voltage is chopped by a switch that is switching on and off at a frequency called the switching frequency.  
         [0008]     A flyback power supply configuration and a forward power supply configuration will now be described; these are examples chosen from various known configurations.  
         [0009]     A flyback power supply, a circuit diagram of which is shown in  FIG. 1   a ), is an energy storage switch mode power supply.  
         [0010]     It comprises a primary circuit P consisting of, in series, a voltage source V in , a switch M, for example a MOS transistor and an inductor Lp made up of a winding of Np turns, and a secondary circuit S consisting of, in series, an inductor Ls made up of a winding of Ns turns, magnetically coupled to Lp, a capacitor C out  connected to a load represented here by a resistor R load  and a rectifier D, for example a diode.  
         [0011]     For each of the windings of Lp and Ls, the phase φ, corresponding to the direction of the winding, is identified by a circle. In the example shown, the first and second windings have the same phase.  
         [0012]     The coupling circuit consisting of the primary inductor Lp and the secondary inductor Ls is denoted by the transformer T.  
         [0013]     The current flowing through the primary circuit is i p , and the voltages across the terminals of the primary circuit and across the switch are V in  and V M  respectively. The current flowing through the secondary circuit i s  is, and the voltages across the terminals of the secondary circuit and across the diode are V out  and V D  respectively.  
         [0014]     In this “flyback” power supply design, current does not flow through both windings simultaneously. The operation of this power supply, called an “inductive storage” supply, is based on energy transfer cycles made up of a magnetic energy storage phase in the inductive element of the primary circuit (in this case Lp), followed by a phase for transferring this stored energy to a secondary source via the secondary circuit.  
         [0015]     The various operating phases of this power supply will now be described, with reference to  FIGS. 1   b ) and  1   c ).  
         [0016]     Let us first recall a basic principle that underlies some of the explanations to follow: it is impossible to force a voltage discontinuity across the terminals of a capacitor and a current discontinuity in an inductor.  
         [0017]     When the switch M is closed ( FIG. 1   b ), i.e. during T on , the energy is stored in the inductor Lp; the diode does not conduct since the voltage V D  across its terminals is negative and therefore the current i s  is zero.  
         [0018]     When the switch M is open, i.e. during T swt -T on , where T swt  is the switching period, the current i p  is zero ( FIG. 1   c ). The continuity of the magnetic energy leads to the transfer of the energy stored previously in the inductor Lp to the inductor Ls and also results in the diode D switching to its conducting state: D demagnetizes the transformer T. This phase ends if the current in the diode D falls to zero or if the end of the switching period is reached.  
         [0019]      FIGS. 1   d ) and  1   e ) show the waveforms in continuous mode, in which the current i s  does not fall to zero at the end of the conducting phase of the secondary-circuit diode D. To simplify the description, it is assumed that the current i p  changes instantaneously from its maximum value to zero.  
         [0020]     The voltage V Lp  across the terminals of the inductor Lp, represented in  FIG. 1   d ), varies as a function of time between a maximum value of V in  and a minimum value of −V out ×N p /N s .  
         [0021]     The current i p , represented in  FIG. 1   e ), varies as a function of time between a maximum value of i Max  and zero; the current i s  varies as a function of time between zero and a maximum value of i Max ×N s /N p .  
         [0022]     A forward power supply, a circuit diagram of which is shown in  FIG. 2   a ), is a switch mode power supply that directly transfers energy.  
         [0023]     It comprises a primary circuit P consisting of, in series, a voltage source V in , a switch M, for example a MOS transistor and an inductor Lp made up of a winding of N p  turns, and, in parallel with the inductor Lp and the switch M, a demagnetizing circuit for demagnetizing the transformer which circuit may be a diode D dem  placed in series with an inductor L dem , magnetically coupled to Lp, made up of a winding of N dem  turns. The diode D dem  and the inductor L dem  may be replaced by other components.  
         [0024]     The secondary circuit S consists of, in series, an inductor Ls made up of a winding of Ns turns, magnetically coupled to Lp, a capacitor C out  connected to a load represented here by a resistor R load , an inductor L, a first rectifier D 1 , for example a diode, and, in parallel with the inductor Ls and the rectifier D 1 , a second rectifier D 2  which may also be a diode.  
         [0025]     The phase φ of each of the windings of Lp, L dem  and Ls is identified by a circle. In the example given, the windings of Lp and Ls have the same phase, opposite to that of the winding of L dem .  
         [0026]     As in the previous case, the coupling circuit consisting of the primary inductor Lp, the secondary inductor Ls and the inductor L dem  is denoted by the transformer T.  
         [0027]     The current flowing through the primary circuit is i p , and the voltages across the terminals of the primary circuit and across the switch are V in  and V M  respectively. The current flowing through the secondary circuit is i s , and the voltages across the terminals of the secondary circuit and across the diode D 1  are V out  and V D1  respectively.  
         [0028]     In this “forward” power supply design, both windings operate simultaneously; there is a direct transfer of energy between the inductors Lp and Ls.  
         [0029]     The various operating phases of this power supply will now be described, with reference to  FIGS. 2   b ),  2   c ) and  2   d ).  
         [0030]     When the switch M is closed ( FIG. 2   b ), i.e. during T on , some of the energy is stored in the inductor Lp (this energy is a “parasitic” quantity and therefore much less than the energy of the direct transfer) and the remaining energy is directly transferred between the inductors Lp and Ls, and the diode D 1  conducts; a current i s  flows in the secondary circuit; the diodes D 2  and D dem  become nonconducting since the voltages across their terminals are negative.  
         [0031]     When the switch M is open ( FIG. 2   c ), the diode D 1  becomes nonconducting, and the diodes D 2  and D dem  switch to the conducting state. In accordance with the basic principle stated earlier, the diode D 2 , called a freewheeling diode, provides continuity of the current is in the inductor L and the diode D dem  provides continuity of the magnetic energy stored in the inductor Lp during the previous phase (i.e. during T on ) by transferring this stored energy to V in  over a time given by T on ×N dem /N p : D dem  demagnetizes the transformer T.  
         [0032]     At the end of the demagnetizing phase ( FIG. 2   d ), i.e. during T swt −T on ×(1+N dem /N p ) D dem  becomes nonconducting; D 2  remains conducting. This is the freewheeling phase.  
         [0033]      FIGS. 2   e ) and  2   f ) show the waveforms. To simplify the description, it is assumed that the current i p  changes instantaneously from its maximum value to zero.  
         [0034]     The voltage V Lp  across the terminals of the inductor Lp, represented in  FIG. 2   e ), varies as a function of time between V in  and −V in ×N p /N dem .  
         [0035]     The current i p , represented in  FIG. 2   f ), varies as a function of time between a maximum value of i Maxp  and zero; the current is varies as a function of time between a maximum value of i Maxs  and a minimum value of i min .  
         [0036]     From now on, it will be generally assumed that a primary circuit P includes at least one switch M placed in series with a voltage source V in  and a first inductor Lp, that a secondary circuit includes at least one rectifier D placed in series with a second inductor Ls and a capacitor C out  connected to a load, and that the primary and secondary circuits are coupled by a coupling circuit including at least the primary inductor Lp and the secondary inductor Ls magnetically coupled to each other.  
         [0037]     One aim is to further reduce the size and weight of these power supplies.  
         [0038]     In order to be able to use small components while achieving the same energy conversion possibilities in terms of the power available at the output, the switching frequency must be increased. This has the drawbacks of increasing losses in the transformer and switch-related losses in the other components, which in turn reduces the overall efficiency and therefore raises the temperature and reduces reliability.  
         [0039]     High-frequency imperfections in the transformer are conventionally modeled by a leakage inductance Lf in series with the inductor Lp, as shown in  FIG. 3   a ) for a flyback power supply and in  FIG. 3   b ) for a forward power supply.  
         [0040]     In the case of a flyback power supply operating in discontinuous mode, in which the current is falls to zero at the end of the conducting phase of the secondary-circuit diode D, the voltage across the terminals of the switch M when it opens can be given approximately by the following formula:  
         V   M     =       V   in     +       Np   Ns     ×     V   out       +       L   f     ×     ⅆ     ⅆ   t       ⁢     I   p             
 
         [0041]     When the switch opens, it is assumed that the current decreases linearly from its maximum value to zero over a time Tfall which is the closed/open switching time of the switch. Therefore, upon opening of the switch M, and with Ton being the time over which the switch M is closed:  
         V   M     =       V   in     +       Np   Ns     ×     V   out       +         L   f       L   p       ×     V   in     ×       T   on       T   fall               
 
         [0042]     Hence, the leakage inductance results in a term representing an overvoltage across the terminals of the switch, in the form:  
             L   f       L   p       ×     V   in     ×       T   on       T   fall         ,       
 
 and the power Pf due to the leakage inductance is:  
           P   f     =       1   2     ×         V   in   2     ×     T   on   2         L   p   2       ×     1     T   swt       ×     L   f         ,       
 
 where T swt  is the switching period. 
 
         [0043]     The energy stored in the leakage inductance is in general dissipated during the switching phases.  
         [0044]     Furthermore, the switching-related losses upon opening of the switch are proportional to Tfall.  
         [0045]     Therefore, reducing the switching time Tfall reduces the switching-related losses but increases the term representing the overvoltage across the terminals of the switch.  
         [0046]     For example, an opening time Tfall 100 times lower than the closure time Ton, and a leakage inductance Lf of about 1% of Lp, results in an overvoltage upon the switching action equal to the power supply voltage Vin. The consequences of this would be disastrous as regards the voltage dimensioning of the switch M, in this case the transistor M which must be a high voltage range transistor and therefore more expensive and less effective.  
         [0047]     In the case of a forward power supply, other equations are derived but the same observations are made on interpreting them.  
         [0048]     There are several types of circuits for countering the effect of the leakage inductance.  
         [0049]     Dissipative RCD (i.e. Resistor, Capacitor, Diode) circuits are very effective in limiting overvoltages but they dissipate all the energy stored in the leakage inductance resulting in a reduction in overall efficiency.  
         [0050]      FIG. 4  shows an example of a flyback power supply employing an RCD circuit. The capacitor C limits the term representing the overvoltage upon opening of the switch M; the resistor R discharges the voltage across the terminals of C and thus dissipates the energy stored in the leakage inductance.  
         [0051]     Snubber circuits are often employed to reduce the overvoltages across the terminals of the switch M.  
         [0052]      FIG. 5  shows an example of a flyback power supply employing a snubber circuit that dissipates very little energy. As in the previous case, the capacitor limits the overvoltages across the terminals of the switch M. To recover the energy stored in C, an oscillating circuit based on L and C inverts the voltage across the terminals of C. In practice, losses in diodes D 1  and D 2  and in the inductor L limit the portion of energy recovered by the circuit. Furthermore, the oscillations of the LC circuit must be damped, which also reduces the efficiency.  
         [0053]     Lastly, such a circuit is more complex and therefore less reliable, and the efficiency of the power supply would have improved only slightly.  
       SUMMARY OF THE INVENTION  
       [0054]     One important aim of the invention is therefore to propose a circuit for reducing, in flyback or forward power supplies, overvoltages across the terminals of the switch M, switching-related losses and losses of energy stored in the leakage inductance.  
         [0055]     To achieve these aims, the invention proposes a unitary magnetic coupler including a first inductor Lp consisting of a first winding of phase φ and having a number N of turns between the two ends of the first winding and, magnetically coupled to the first inductor Lp, a second inductor Ls consisting of a second winding of the same phase φ and having the same number N of turns between the two ends of the second winding, which unitary magnetic coupler is characterized in that the ends of the first and second windings are interconnected using links consisting of capacitors of equal value.  
         [0056]     This type of coupler, in which the inductor of the primary circuit has the same number of turns as the inductor of the secondary circuit, enables the same voltage to exist across the terminals of the primary and secondary windings of the same phase and therefore a capacitive link can be used to counter the effect of the leakage inductance without increasing switching-related losses.  
         [0057]     Another subject of the invention is a switch mode power supply having a primary circuit P coupled to a secondary circuit S by means of a magnetic coupling circuit, characterized in that the magnetic coupling circuit is a unitary magnetic coupler as described above.  
         [0058]     The power supply may be a flyback type or forward type.  
         [0059]     As a preference, the primary circuit P and the secondary circuit S are able to generate at the terminals of each capacitor of the unitary magnetic coupler a voltage that does not change as a function of the switching frequency.  
         [0060]     The invention also relates to data transmission equipment including at least one data transmit-receive device connected to a two-wire data bus, characterized in that it includes a unitary coupler as described above, able to connect the data transmit-receive device to the two-wire data bus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0061]     Other features and advantages of the invention will become apparent on reading the following detailed description, given by way of nonlimiting example and with reference to the accompanying drawings in which:  
         [0062]      FIGS. 1   a ) to  1   e ) described earlier schematically show, respectively, a flyback power supply, its operating phases when the switch is closed and then open, and its waveforms;  
         [0063]      FIGS. 2   a ) to  2   f ) described earlier schematically show, respectively, a forward power supply, its operating phases when the switch is closed and then open, then when the diode D dem  is also open, and its waveforms;  
         [0064]      FIGS. 3   a ) and  3   b ) described earlier are circuit diagrams of, respectively, a flyback power supply and a forward power supply, with leakage inductance;  
         [0065]      FIG. 4  described earlier is a circuit diagram of a flyback power supply with a dissipative RCD circuit;  
         [0066]      FIG. 5  described earlier is a circuit diagram of a flyback power supply with a snubber circuit that dissipates very little energy;  
         [0067]      FIG. 6  is a circuit diagram of a unitary coupler according to the invention;  
         [0068]      FIGS. 7   a ) and  7   b ) are circuit diagrams of, respectively, a flyback power supply and a forward power supply, with a unitary coupler according to the invention;  
         [0069]      FIG. 8  is a circuit diagram of an example data transmission system with a unitary coupler according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0070]     The circuit used to reduce, in a power supply, overvoltages across the terminals of the switch M, switching-related losses and losses of energy stored in the leakage inductance is a unitary coupler.  
         [0071]     A unitary coupler is a transformer in which the winding of the inductor Lp of the primary circuit has the same number of turns and the same phase as the winding of the inductor Ls of the secondary circuit.  
         [0072]     This also results in the same voltage existing across the terminals of the primary inductor Lp and the secondary inductor Ls and therefore, in accordance with the basic principle stated earlier, a capacitive link can be used between these two inductors to counter the effect of the transformer&#39;s leakage inductance.  
         [0073]     This type of unitary coupler according to the invention is shown  FIG. 6 . A first link (link 1) consisting of a first capacitor C 1  connects the ends of the windings of the inductors Lp and Ls and a second link (link 2) consisting of a second capacitor C 2 , of the same value as the first capacitor C 1 , connects the other ends of the inductors Lp and Ls.  
         [0074]     This type of coupler achieves coupling at frequencies ranging from relatively low frequencies (of some tens of kHz) to frequencies of some tens of MHz, at the same time reducing losses.  
         [0075]     Thus coupling efficiency is increased despite the use of smaller and therefore less expensive components.  
         [0076]     The capacitor chosen for the capacitive links has, as a preference, very low parasitic series resistance and inductance. For example, a multilayer ceramic capacitor may be used.  
         [0077]     This coupler is advantageously used in flyback or forward power supplies as shown in  FIGS. 7   a ) and  7   b ).  
         [0078]     The capacitive links cancel out the overvoltage across the terminals of the switch M as M opens. Therefore, RCD or snubber circuits need not be added and the switch M need not be overdimensioned in terms of voltage.  
         [0079]     Furthermore, the energy stored in the leakage inductance is transferred directly to the capacitive links that transfer this energy to the secondary circuit.  
         [0080]     Among the various existing flyback and forward power supply configurations, some generate high common mode voltages at the switching frequency. Configurations that minimize the common mode voltages between the primary and secondary circuits are chosen so that the capacitive links can be used, i.e. configurations for which the voltage across the terminals of capacitor C 1  (respectively C 2 ) does not vary as a function of the switching frequency.  
         [0081]     A flyback power supply that does not generate high common mode voltages at the switching frequency, and that employs a unitary coupler according to the invention is shown in  FIG. 7   a ).  
         [0082]     It comprises a primary circuit P consisting of, in series, a voltage source V in , an inductor Lp and a switch M, and a secondary circuit S consisting of, in series, a capacitor C out  connected to a load represented here by a resistor R load , a rectifier D and an inductor Ls.  
         [0083]     The coupling circuit between the primary circuit P and the secondary circuit S comprises a unitary coupler according to the invention; the inductors Lp and Ls are therefore identical and connected by capacitive links of equal value.  
         [0084]     An experimental flyback power supply employing a unitary coupler according to the invention has been produced. For an input voltage V in =28 V DC and a power P=50 W, an efficiency gain of about 2 to 5% was achieved with a lowering of overvoltages upon switch-opening by a ratio of 4.  
         [0085]     A forward power supply that does not generate high common mode voltages at the switching frequency, and that employs a unitary coupler according to the invention is shown in  FIG. 7   b ).  
         [0086]     It comprises a primary circuit P consisting of, in series, a voltage source V in , an inductor Lp and a switch M, and, in parallel with the inductor Lp and the switch M, means for demagnetizing the transformer, for example as per  FIG. 2   a ). It also comprises a secondary circuit S consisting of, in series, a capacitor C out  connected to a load represented here by a resistor R load , an inductor L, a first rectifier D 1  and an inductor Ls, and, in parallel with the rectifier D 1  and the inductor Ls, a rectifier D 2 .  
         [0087]     The coupling circuit between the primary circuit P and the secondary circuit S comprises a unitary coupler according to the invention; the inductors Lp and Ls are therefore identical and connected by capacitive links of equal value.  
         [0088]     The unitary coupler according to the invention can be applied in particular to power supplies in which a MOS transistor is used for the switch M, and/or in which uncontrolled rectifiers such as diodes, or even controlled rectifiers such as MOS transistors, are used for the rectifiers D 1 , D 2  and/or D dem .  
         [0089]     The unitary coupler according to the invention can in particular be applied to inductive storage converters such as the ones described in U.S. Pat. Nos. 2,729,471, 2,729,516 and 2,773,013.  
         [0090]     The power supplies described achieve improved efficiency and a lowering of the overvoltages across the terminals of the switch, without a significant increase in the complexity of the circuits as would be the case for circuits that include RCD or snubber circuits.  
         [0091]     Using capacitors means that high-frequency coupling can be achieved, at frequencies beyond 100 MHz. The unitary coupler according to the invention may hence be used to transmit data at high frequency: the capacitive coupling means that a high-speed data transmission is possible, which is relayed by the magnetic coupling at frequencies ranging from some tens of kHz to several tens of MHz.  
         [0092]     An example data transmission system is shown in  FIG. 8 . It is made up of two modules E 1  and E 2  interconnected via a two-wire data bus B. Each module E 1  and E 2  has a data transmit-receive device T/R connected to the data bus B by means of a unitary coupler according to the invention and resistors R.  
         [0093]     More generally, the coupler according to the invention may be applied to any device employing a magnetic transformer.