EP1504459A1

EP1504459A1 - Single-unit magnetic coupler and switching power supply

Info

Publication number: EP1504459A1
Application number: EP03740672A
Authority: EP
Inventors: Christophe Thales Intellectual Property TAURAND; Philipe Thales Intellectual Property BOGDANIK
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2002-05-03
Filing date: 2003-04-25
Publication date: 2005-02-09
Also published as: US20050180177A1; FR2839381A1; US6975097B2; FR2839381B1; WO2003094181A1

Abstract

The invention concerns a single-piece magnetic coupler comprising a first inductance (Lp) consisting of a first winding of phase PHI and having the same number of turns N between the two ends of the first winding and, magnetically coupled to the first inductance (Lp), a second inductance (Ls) consisting of a second winding of similar phase PHI and having the same number of turns N between the two ends of the second winding, characterized in that the ends of the first and second windings are mutually connected by a connection including a capacitance (C1, C2) of equal value.

Description

UNIT MAGNETIC COUPLER AND POWER SUPPLY

CUTTING

The invention relates to a unitary magnetic coupler.

The invention also relates to a switching power supply and information transmission equipment using such a coupler.

The field of the invention is that of power supplies having the function of delivering DC voltages from an AC or DC network.

We consider more particularly low power supplies, typically less than 150 W.

We seek to minimize the size and weight of power supplies.

“Flyback” or “Forward” type power supplies are low-power switching power supplies, frequently used in particular because of their simplicity of control. The “Flyback” type power supply is particularly interesting because of its reduced size since it requires only one magnetic element to carry out the energy conversion.

It will be recalled that in switching power supplies, the DC voltage is cut by a switch operating by switching according to a so-called cutting frequency. We will now describe a type of power configuration

“Flyback” and a “Forward” type supply configuration: these are examples chosen from various known configurations.

A “flyback” type supply diagrammatically shown in FIG. 1a) is a switching power supply with energy accumulation.

It comprises a primary circuit P comprising connected in series, a voltage source V * _n> a switch M, for example a transistor OS and an inductance Lp consisting of a winding of Np turns, and a secondary circuit S comprising connected in series , an inductance Ls consisting of a winding of Ns turns, magnetically coupled to Lp, a capacitance C _out connected to a load represented here by a resistor R _cha rge and a rectifier D, for example a diode. For each of the windings of Lp and Ls, phase φ, corresponding to the direction of the winding, is identified by a circle. In the example presented, the first and second windings have the same phase. The transformer T designates the coupling circuit comprising the primary inductance Lp and the secondary inductance Ls.

The current flowing through the primary circuit is i _p , and the voltages across the primary circuit and the switch are V * _n and VM respectively; the current flowing through the secondary circuit is i _s , and the voltages across the secondary circuit and the diode are V _or t and V _{D respectively} .

In this “Flyback” type supply, the two windings are not traversed simultaneously by the current. The operation of this power supply, called inductive storage, is based on the realization of energy transfer cycles comprising a phase of magnetic energy accumulation in the inductive element of the primary circuit (in this case Lp), followed a phase of restitution of this energy to a secondary source through the secondary circuit.

We will now describe the different operating phases of this supply, in relation to Figures 1b) then 1c). We first recall a basic principle which underlies certain explanations below: it is impossible to impose a voltage discontinuity across a capacitor and a current discontinuity in an inductor.

When the switch M is closed (FIG. 1b), that is to say during T _o n, the energy is stored in the inductance Lp; the diode is blocked because the voltage VD at its terminals is negative and therefore the current i _s is zero.

When the switch M is open, that is to say during T _of cT ₀ n, T _dec being the _chopping period, the current i _p is zero (FIG. 1c), the continuity of the magnetic energy causes the transfer into the inductance Ls of the energy previously stored in the inductance Lp and the conduction of the diode D: D demagnetizes the transformer T. This phase ends if the current is canceled in the diode D or if l 'We reach the end of the cutting period.

FIGS. 1d) and 1e) the waveforms in continuous mode, according to which the current i _s does not cancel at the end of operation of the secondary diode D. It is furthermore assumed for simplicity that the current i _p instantly goes from its maximum value to 0.

The voltage V * _ _P across the inductance Lp, shown in Figure 1d) varies as a function of time between a maximum value equal to V * _n and a minimum value equal to -V _or t * Np / N _s .

The current i _p shown in FIG. 1e) varies as a function of time between a maximum value i _Ma χ and 0; the current i _s varies as a function of time between 0 and a maximum value equal to i-viax ^* Ns / N _p .

A “Forward” type supply diagrammatically represented in FIG. 2a) is a switching power supply with direct energy transfer.

It comprises a primary circuit P comprising connected in series, a voltage source V * _n , a switch M, for example a MOS transistor and an inductance Lp consisting of a winding of Np turns, and, mounted in parallel with the inductance Lp and the switch M, a demagnetization circuit of the transformer which can be a diode D _of m in series with an inductance L _e m _, magnetically coupled to Lp, consisting of a winding of N em turns. The diode Dd _θ m and the inductance Ldem can be replaced by other components. The secondary circuit S comprises in series an inductance

Ls consisting of a winding of Ns turns, magnetically coupled to Lp, a capacitance C _or t connected to a load represented here by a resistance

Rc _h arge, an inductor L, a first rectifier D1, for example a diode, and mounted in parallel with the inductor Ls and the rectifier D1, a second rectifier D2 which can also be a diode.

The phase φ of each of the windings of Lp, L _of m and Ls is identified by a circle. In the example presented, the windings of Lp and Ls have the same phase, contrary to the phase of the winding of L _of m-

As previously, the transformer T designates the coupling circuit comprising the primary inductance Lp, the secondary inductance Ls and the inductance L _d em-

The current flowing through the primary circuit is i _p and the voltages across the primary circuit and the switch are Vj _n and V respectively; the current passing through the secondary circuit is i _S) and the voltages at the terminals of the secondary circuit and of the diode D1 are respectively V _out and V _D * ι. In this case of a “Forward” type supply, the two windings operate simultaneously; there is a direct transfer of energy between the inductances Lp and Ls.

We will now describe the different operating phases of this power supply in relation to Figures 2b) then 2c) and finally 2d).

When the switch M is closed (Figure 2b), that is to say during T _o n, part of the energy is stored in the inductance Lp (this is a parasitic part therefore much less than the energy direct transfer) and the other part of the energy is directly transferred between the inductances Lp and Ls and the diode D1 starts to conduct; a current i _s flows in the secondary circuit; diodes D2 and D _e m are blocked because the voltages at their terminals are negative.

When the switch M is open (FIG. 2c), the diode D1 is blocked, and the diodes D2 and D _of m turn on. According to the basic principle mentioned above, the freewheeling diode D2 ensures the continuity of the current i _s in the inductance L and the diode Ddem ensures the continuity of the magnetic energy stored in the inductance Lp during the previous phase (ie during T _on ) by transferring this stored energy to V ^ for a time equal to T _on ^* Ndem / N _p : D em demagnetizes the transformer T.

At the end of demagnetization (Figure 2d), that is to say during T _d e _c -T _on * (1 + N _of m / N _p ), D _of m is blocked; D2 remains in conduction. This is the freewheeling phase.

FIGS. 2e) and 2f) represent the waveforms. For simplicity, it is assumed that the current i _p instantly passes from its maximum value to O.

The voltage V * _ _p across the inductance Lp, shown in Figure 2e) varies as a function of time between V ^* _n and -V _in ^* Np / Ndem-

The current i _p shown in FIG. 2f) varies as a function of time between a maximum value i _Maxp and 0; the current i _s varies as a function of time between a maximum value equal to ii iax _s and a minimum value i _m ι _n .

In the following, it is generally considered that a primary circuit P comprises at least one switch M mounted in series with a voltage source V ^* _n and a first inductance Lp, that a secondary circuit comprises at least one rectifier D mounted in series with a second inductance Ls and a capacitance C _out connected to a load, and the primary and secondary circuits are coupled by means of a coupling circuit comprising at least the primary inductances Lp and secondary Ls magnetically coupled together. 5 We wish to further reduce the size and weight of these power supplies.

To be able to use components of reduced size while obtaining the same possibilities of energy conversion in terms of available power at the output, it is then necessary to increase the frequency of

10 cutting which in particular has the disadvantages of increasing the losses in the transformer, the switching losses in the other components, which decreases the overall efficiency and therefore raises the temperature and decreases reliability.

High frequency imperfections in the transformer are

15 conventionally modeled by a leakage inductance Lf in series with the inductance Lp, as shown in FIG. 3a) for a “Flyback” type supply and FIG. 3b) for a “Forward” type supply.

In the case of a “Flyback” type supply operating in discontinuous mode, according to which the current i _{s is} canceled at the end of operation

20 of the secondary diode D, the voltage across the terminals of the switch M at the opening can be approximated by the following formula:

25 When opening, it is considered that the current decreases linearly from its maximum value to 0, in a time Tf _a ιι which is the closed / open switching time of the switch. We then have, at the opening of the switch M, Ton being the time during which the switch M is closed,

3 ^{* J} 0 ^υ V ^v „M = V ^v in + ^~ - -v _τ * N ^v ou, t + '- _τ * N ^v in * - -≈-

Νs L _p T _fall

It can be seen that the leakage inductance creates an overvoltage term at the terminals of the switch, of the form

and the power P _f implemented by the leakage inductance is

1 V- ² * T ² p _f - = _ * _--. - _ ---- _ * - * L _fj T _of c being the cutting period.

2 L _p T _dec

The energy stored in the leakage inductor is generally dissipated during the switching phases.

Furthermore, the switching losses at the opening of the switch are proportional to T _fa ιι.

By reducing the switching time T _fa ii, the switching losses are therefore reduced, but the overvoltage term at the terminals of the switch is increased.

With, for example, an opening time T _fa n 100 times lower than the closing time T _on , and a leakage inductance L _f of the order of 1% of L _p , an switching overvoltage is obtained equal to the supply voltage Vj _n . This has disastrous consequences on the voltage dimensioning of the switch M, in this case the transistor M which must be a transistor with a higher voltage range and therefore more expensive and less efficient.

In the case of a "Forward" type diet, other equations are obtained but their interpretation leads to the same observation.

Several types of circuit can be used to counter the effect of the f u ite i nd uctance.

Dissipative R, C, D circuits (that is to say comprising Resistance, Capacitance and Diode) are very effective in limiting the overvoltage but they dissipate all the energy contained in the leakage inductance, which reduces the overall efficiency. . An example of a “Flyback” type power supply using an R, C circuit,

D is shown in Figure 4. The capacitance C limits the overvoltage term at the opening of the switch M; resistor R discharges the voltage across C and thus dissipates the energy contained in the leakage inductance. Switching assistance circuits (CALC) are often used to reduce overvoltages at the terminals of switch M.

An example of a “Flyback” type supply using a poorly dissipative CALC circuit is shown in FIG. 5. As before, the capacity limits the overvoltages at the terminals of the switch M. To recover the energy stored in C, an oscillating assembly with L and C makes it possible to reverse the voltage at the terminals of C. In practice, the losses in the diodes D1, D2 and in the inductance L limit the share of energy recovered by the assembly; in addition it is also necessary to dampen the oscillations of the assembly L, C, which also degrades the yield.

Finally, such an assembly is more complex and therefore less reliable and the efficiency of the supply has increased only slightly.

An important object of the invention is therefore to propose a circuit making it possible to reduce in “Flyback” or “Forward” type power supplies, the overvoltages at the terminals of the switch M, the switching losses and the energy losses contained in the leakage inductance.

To achieve these goals, the invention provides a unitary magnetic coupler comprising a first inductance Lp consisting of a first phase winding φ and having a number N of turns between the two ends of the first winding and, magnetically coupled to the first inductance Lp , a second inductance 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, characterized in that the ends of the first and second windings are connected together by a connection comprising a capacity of the same value.

Such a coupler whose inductance of the primary circuit has the same number of turns as the inductance of the secondary circuit makes it possible to have the same voltage across the terminals of the primary and secondary windings of the same phase and therefore to use a capacitive link to counter the effect of the leakage inductance, without increasing the switching losses.

The invention also relates to a switching power supply comprising 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. "_-" - "

PCT / FR03 / 01319

The power supply can be of the “FLYBACK” or “FORWARD” type.

Preferably, the primary P and secondary S circuits are capable of generating across the terminals of each capacity of the unit magnetic coupler, a voltage not varying as a function of the switching frequency.

The invention also relates to information transmission equipment comprising at least one information transmission-reception device connected to a two-wire data bus, characterized in that it comprises a unitary coupler as described, capable of connect the information transmission / reception device to the two-wire data bus.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows, given by way of nonlimiting example and with reference to the appended drawings in which: FIGS. 1a) to 1e) already described show diagrammatically, respectively a “Flyback” type supply, its operating phases when the switch is closed and then open, and its waveforms, FIGS. 2a) to 2f) already described schematically, respectively represent a “Forward” type supply, its operating phases when the switch is closed and then open, then when the diode D _of is also open, and its waveforms, FIGS. 3a) and 3b) already described schematically represent, respectively, a “Flyback” type supply. and a “Forward” type supply, comprising a leakage inductance, FIG. 4 already described schematically represents a supply of “Flyback” type comprising a dissipative R, C, D circuit, FIG. 5 already described diagrammatically represents a “Flyback” type supply comprising a poorly dissipative CALC circuit, FIG. 6 diagrammatically represents a unit coupler according to the invention, the figures 7a) and 7b) schematically represent, respectively, a “Flyback” type supply and a “Forward” type supply, comprising a unitary coupler according to the invention, FIG. 8 schematically represents an example of an information transmission system comprising a unitary coupler according to the invention.

The circuit used to reduce in a supply, the overvoltages at the terminals of the switch M, the losses by switching and the losses of energy contained in the leakage inductance, is a unitary coupler.

A unitary coupler is a transformer in which the winding of the inductance Lp of the primary circuit has the same number of turns and the same phase as the winding of the inductance Ls of the secondary circuit.

This also makes it possible to have the same voltage across the primary inductors Lp and secondary inductors Ls and therefore, according to the basic principle mentioned above, to use a capacitive connection between these two inductors to counter the effect of the inductance of transformer leak.

Such a unitary coupler according to the invention is shown in FIG. 6. A first link (link 1) comprising a first capacitor C1 connects the ends of the windings of the inductors Lp and Ls; and a second link (link 2) comprising a second capacitor C2, of the same value as the first capacitor C1, connects the other ends of the inductors Lp and Ls.

This coupler allows coupling at relatively low frequencies (a few tens of kHz), up to a few tens of MHz while reducing losses.

Thus the coupling efficiency is increased despite the use of less bulky and therefore less expensive components.

The capacitor chosen for the capacitive connections preferably has very low parasitic inductance and series resistance. One can choose for example a multilayer ceramic capacitor.

This coupler is advantageously used in “Flyback” or “Forward” type power supplies as shown in FIGS. 7a) and 7b).

The capacitive links cancel the overvoltage at the terminals of the switch M when opening M. It is therefore not necessary to add circuits R, C, D or CALC, nor to oversize the switch M in tension.

In addition, the energy stored in the leakage inductance is directly transferred into the capacitive links which transfer this energy into the secondary circuit.

Among the various existing “Flyback” and “Forward” power supply configurations, some generate high common mode voltages at the switching frequency; the configurations minimizing the common mode voltages between the primary and secondary circuits are chosen so that the capacitive links can be used, that is to say the configurations allowing the voltage across the capacitors C1 (respectively C2) to vary not depending on the switching frequency.

A “Flyback” type supply not generating high common mode voltages at the switching frequency, and using a unit coupler according to the invention is shown in FIG. 7a).

It comprises a primary circuit P comprising connected in series, a voltage source V * _n ,, an inductance Lp and a switch M, and a secondary circuit S comprising connected in series, a capacitance C _or t, connected to a load shown here by a resistor Rcharge, a rectifier D and an inductance Ls.

The coupling circuit between the primary P and secondary S circuits comprises a unitary coupler according to the invention; the inductances Lp and Ls are therefore identical and connected by capacitive links of the same value. A “Flyback” type power supply model using a unit coupler according to the invention was produced; we obtained, for an input voltage V _in = 28 V DC and a power P = 50 W, a gain in efficiency of around 2 to 5% with a drop in overvoltages when a report is opened 4. A “Forward” type power supply that does not generate high common mode voltages at the switching frequency, and using a unit coupler according to the invention is shown in FIG. 7b).

It comprises a primary circuit P comprising connected in series, a voltage source V ^* _n ,, an inductance Lp and a switch M, and mounted in parallel with the inductance Lp and the switch M, means for demagnetize the transformer, such as for example those of FIG. 2a); it also includes a secondary circuit S comprising connected in series, a capacitance C _out , connected to a load represented here by a resistance Rcharge, an inductance L, a first rectifier D1 and an inductance Ls, and mounted in parallel with the rectifier D1 and inductance Ls, a rectifier D2.

The coupling circuit between the primary P and secondary S circuits comprises a unitary coupler according to the invention; the inductances Lp and Ls are therefore identical and connected by capacitive links of the same value.

The unitary coupler according to the invention is applicable in particular to power supplies in which a M transistor is used as switch M, and / or as rectifiers D1, D2 and or Ddem, uncontrolled rectifiers such as diodes, or even controlled rectifiers such only Mos.

The unitary coupler according to the invention is in particular applicable to converters with inductive storage as described in patents Nos. 2,729,471, 2,729,516 and 2,773,013.

The power supplies described allow better efficiency and lower overvoltages at the terminals of the switch without significantly increasing the complexity of the circuits, as in the case of circuits including R, C, D or CALC circuits.

The use of capacities enables high frequency coupling, which can exceed 100 MHz. The unitary coupler according to the invention can then be used to transmit information at high frequency: capacitive coupling allows rapid transmission of information which is relayed by magnetic coupling at frequencies of a few tens of KHz to several tens of MHz.

An example of an information transmission system is shown in FIG. 8. It comprises two pieces of equipment E1 and E2 linked together by a two-wire data bus B. Each of the pieces of equipment E1 and E2 comprises a device for transmitting and receiving information T / R connected to the data bus B by means of a unit coupler according to the invention and resistors R.

More generally, the coupler according to the invention can be applied to any device using a magnetic transformer.

Claims

1. Unit magnetic coupler comprising a first inductor (Lp) consisting of a first phase winding 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, characterized in that the ends of the first and second windings are connected together by a connection comprising a capacity (C1, C2) of the same value.

2. Switching power supply comprising a primary circuit (P) coupled to a secondary circuit (S) by means of a coupling circuit, characterized in that the coupling circuit comprises a unitary magnetic coupler according to the preceding claim.

3. Switching power supply according to the preceding claim, characterized in that the power supply is of the "FLYBACK" type.

4. Switching power supply according to the preceding claim, characterized in that the primary circuit (P) comprises at least one switch (M) mounted in series with a voltage source (Vj _n ) and with the first inductance (Lp), and the secondary circuit comprises at least one rectifier (D) connected in series with the second inductor (Ls) and a capacitor (C _or t) connected to a load (Rcharge) -

5. Switch mode power supply according to claim 2, characterized in that the power supply is of the "FORWARD" type.

6. Switching power supply according to the preceding claim, characterized in that the primary circuit (P) comprises at least one switch (M) mounted in series with the first inductor (Lp) and a voltage source (V _in ), and mounted in parallel with the switch (M) and the first inductor (Lp), a demagnetization circuit of the transformer magnetic (T) and the secondary circuit further comprises, connected in series with the second inductor (Ls), a capacitor (C _or t) connected to a load (Rc _h a _r g _e ), a third inductor (L), a first rectifier (D1), and, mounted in parallel with the second inductor (Ls) and the first rectifier (D1), a second rectifier D2.

7. Switching power supply according to any one of claims 2 to 6, characterized in that the primary (P) and secondary (S) circuits are capable of generating at the terminals of each capacitor (C1, C2) of the unitary magnetic coupler, a voltage not varying as a function of the switching frequency.

8. Information transmission equipment comprising at least one information transmission-reception device (T / R) connected to a two-wire data bus (B), characterized in that it comprises a unitary coupler according to the Claim 1, capable of connecting the information transmission-reception device (T / R) to the two-wire data bus (B).