EP2715105A1

EP2715105A1 - Power supply for radiofrequency ignition with dual-stage amplifier

Info

Publication number: EP2715105A1
Application number: EP12731053.0A
Authority: EP
Inventors: André AGNERAY; Franck Deloraine
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 2011-05-25
Filing date: 2012-05-24
Publication date: 2014-04-09
Also published as: WO2012160317A1; FR2975863B1; FR2975863A1

Abstract

The invention relates to a power supply device for a radiofrequency ignition, said device being intended to supply a DC voltage at a predefined frequency to a series resonator and comprising a power supply source (3) able to deliver a DC power supply voltage (Valim), a DC-DC converter (4) amplifying said DC power supply voltage (Valim) and delivering on one output a DC voltage, known as the intermediate voltage (Vinter), the amplitude of which is higher than said DC power supply voltage, and a power switch controlled by a control signal for selectively applying said intermediate voltage (Vinter) to the series resonator at a control frequency equal to said predefined frequency. According to the invention, the DC-DC converter comprises two stages, a voltage raising stage (41) and a voltage regulating stage (42) so as to reduce the voltage drops at the output side of the converter when the ignition generates sparks.

Description

POWER SUPPLY FOR RADIOFREQUENCY IGNITION WITH AMPLIFIER

DOUBLE FLOOR

The present invention relates generally to systems for generating plasma between two electrodes of a spark plug, used in particular for controlled radiofrequency ignition of a gaseous mixture in combustion chambers of an internal combustion engine. The invention relates more particularly to the radiofrequency ignition supply device and finds its application mainly in the automotive field.

In plasma generation systems, so-called plasma generation circuits incorporating spark plugs are used to generate multi-filament discharges between their electrodes to initiate combustion of the mixture in the combustion chambers of the engine. The radiofrequency ignition and the multi-spark plug in question here are described in the following patent applications filed in the name of the applicant FR 2 859 830, FR 2 859 869 and FR 2 859 831.

Referring to Figure 1, such a plug coil is conventionally modeled by a series resonator 1, the resonant frequency f _c is greater than 1 MHz, and typically about 5 MHz. The resonator, arranged at the level of the spark plug, comprises in series a resistor R, an inductance L and a capacitance C. Ignition electrodes 10 and 12 of the coil-plug are connected to the terminals of the capacitor C. When the resonator is fed by a periodic high voltage (between 200V and 600V) at its resonant frequency f _c "(1 / (2 J (x C)), the amplitude across the capacitance C is amplified, making it possible to develop multi-filament discharges between spark plug electrodes over centimeter distances at high pressure and at peak voltages below 25 kV.

This radio frequency ignition application requires the use of a power supply capable of generate voltage pulses, typically of a duration of the order of 100 ns, which can reach amplitudes of the order of 600 V to 1 kV, at a frequency very close to the resonance frequency of the radio frequency resonator of the coil -candle. The greater the difference between the resonance frequency of the resonator and the operating frequency of the power supply, the higher the resonator overvoltage coefficient (ratio between the amplitude of its output voltage and its input voltage) is high.

FIG. 2 diagrammatically illustrates such a feed device referenced 2. FIG. 2 is further detailed in the patent application FR 2 859 869. The feed device comprises a pseudo-class E type output stage for transforming the DC voltage supplied by a power source, in appropriate periodic pulses. According to the embodiment of FIG. 2, the supply device 2 comprises a supply source 3 delivering a DC voltage V _a ii _m , for example 12 volts, a DC-DC converter 4 for amplifying the DC voltage V _a ii _m and deliver a voltage Vin _te r, called intermediate voltage, on its output. The power supply device also comprises a switching circuit 5 for selectively applying the intermediate voltage to the plasma generating resonator 1 intended to be connected at the output of the supply device. This switching circuit 5 essentially comprises a MOSFET transistor of power M, used as a switch, and a control amplifier 7 intended to generate a control signal VI of the transistor M. The transistor M is connected between the output of the DC-DC converter 4 and an output of the supply device connected to the resonator 1 of the coil-candle. The frequency of the control signal VI is taken substantially equal to the resonance frequency of the resonator 1. Thus, the intermediate voltage Vi _nter supplied by the DC-DC converter 4 is applied, via the switch M, to an output of the device connected to the series resonator 1 at the resonant frequency of the latter.

According to the embodiment illustrated in FIG. 2, the intermediate voltage Vi _nter is advantageously supplied to the transistor M via a parallel resonant circuit 6, comprising an inductance Lp in parallel with a capacitor Cp and connected between the output of the DC-DC converter 4 and the drain of the transistor M. The parallel resonant circuit 6, the control amplifier 7 and the switching circuit form a radio frequency amplifier.

Near its resonant frequency, the parallel resonator 6 transforms the supply voltage Vi _nter into an amplified voltage Va, corresponding to the supply voltage multiplied by the overvoltage coefficient of the parallel resonator. It is therefore the amplified supply voltage Va which is, in this embodiment, present on the output of the supply circuit at the drain of the transistor M. The switch M controls the excitation frequency of the parallel resonator at the frequency defined by the control signal VI. The resonance frequency of the parallel resonator 6 is preferably substantially equal to the frequency of the control voltage VI and therefore substantially equal to the resonance frequency of the series resonator 1, in order to obtain optimum amplification.

As can thus be seen, the intermediate voltage V _INTER and the control signal of the transistor M then constitute essential means for controlling the quality and duration of the spark produced between the electrodes of the coil-candle.

As previously described, the intermediate voltage

Vi _nter is produced from a DC voltage V _a i _im , usually the battery voltage, by a DC-DC converter 4. For reasons of cost, this converter generally comprises a single amplification stage, Boost type or flyback. The energy in the storage capacitor placed at the output of the amplification stage is used successively by each series resonator. The amplifier must therefore be able to recharge in a relatively short time which is a function of the number of series resonators to be fed successively, which has an impact on the sizing of the converter elements.

The control of the amplification stage conventionally comprises two phases:

a charging phase during which the storage capacitor is recharged to the desired voltage Vi _nter required for the formation of the next spark, and

a discharge phase or spark phase during which ignition control is generated and the energy present in the storage capacitor is consumed by the series resonator and the spark.

Before the discharge phase, the amplification stage must therefore make it possible to reach the desired maximum intermediate voltage even at the highest engine speed. Boost or Flyback type topologies generally make it possible to respect this first condition. Furthermore, it is also essential that, during the discharge phase, the fall of the intermediate voltage in the storage capacitor is low, for example less than 10 or 20 volts, in order to guarantee the generation of a long-lasting spark. in the combustion chamber and thus ensure optimum combustion of the fuel mixture in the combustion chamber. Topologies such as Boost or Flyback do not meet this second condition.

Also, the problem that seeks to solve the invention is to ensure optimal combustion on all engine operating points. For this, it is necessary that the power device is capable of providing up to 1 joule to the spark plug and that the voltage drop at the time of the discharge phase is low, typically less than 20 volts. It is also necessary that the operation of the feeding device can guarantee the generation of sparks of relatively long excitation duration, typically between 100ys and 500ys, or even 1ms.

Various solutions have been envisaged to achieve this objective. A first solution is to use a storage capacitor having a high value capacity to maintain the voltage Vi _nter at the desired value, typically 250 volts, during the discharge phase. For an intermediate voltage Vi _nter equal to 250 volts with a maximum voltage drop of 10 volts of this voltage during the discharge phase and a transfer of energy from 1 Joule, it is necessary to use a capacitor having a capacitance of 408 yF. The energy stored in the capacitor is then of the order of 12.75 joules during the charging phase. This solution has two disadvantages: it is very dangerous (because of the amount of energy stored) and very expensive.

A second solution could be to provide a regulated direct supply of the coil-candle from the 12V edge network of the vehicle. But the battery should then provide a current of several hundred amperes, which is unrealistic.

Also, to solve this problem, the invention proposes to use a two-stage DC-DC converter, namely a DC-DC converter comprising a first voltage booster stage provided with a first storage capacitor followed by a second a voltage regulator stage provided with a second storage capacitor, the storage capacitor of the step-up stage then constituting a reserve of energy arranged between the input of the DC-DC converter and the second storage capacitor of the amplifier during the discharge phases of the supply device, which reduces the voltage drop at the output of the DC-DC converter during the discharge phases.

For this purpose, the invention relates to a device for supplying a radiofrequency ignition, said device for supplying a DC voltage at a predetermined frequency to a series resonator and comprising

a power source capable of delivering a continuous supply voltage,

a DC-DC converter amplifying said DC supply voltage and delivering on an output a DC voltage, said intermediate voltage whose amplitude is greater than said DC supply voltage,

a power switch controlled by a control signal for selectively applying said intermediate voltage to the series resonator at a control frequency equal to said predefined frequency,

characterized in that the DC-DC converter comprises a step-up stage for generating, from the DC supply voltage, a so-called ballast voltage across a first storage capacitor and a voltage regulation stage for generating from said ballast voltage the intermediate voltage across a second storage capacitor, said ballast voltage being greater than the DC supply voltage.

The presence of this double stage greatly reduces the voltage drop across the second capacitor present at the output of the DC-DC converter during the discharge phases of the supply device. The first storage capacitor acts as a complementary energy reservoir when the intermediate voltage at the output of the supply device tends to fall.

According to a particular embodiment, the voltage booster stage is a Boost-type circuit for limiting the resistive losses in this stage.

According to the invention, the step-up stage is controlled to charge said first storage capacitor during a charging phase and to generate, at the terminals of said first storage capacitor, a ballast voltage. substantially equal to a predefined value at the end of said charging phase. The control of the voltage booster stage is preferably synchronized to the control signal of the power switch.

According to a particular embodiment, the predefined voltage ballast value is determined according to the rotational speed of the heat engine in which the radio frequency ignition is installed. This value is for example equal to 340 volts when the rotational speed of the engine is high.

According to an advantageous embodiment, the ballast voltage is greater than the intermediate voltage and the voltage regulation stage is a voltage step-down stage.

According to a particular embodiment, the voltage step-down stage is a Buck type circuit.

The voltage regulation stage is controlled to charge, during said charging phase, said second storage capacitor with energy stored in said first storage capacitor and to generate an intermediate voltage substantially equal to a predefined value across said second storage capacitor at the end of said charging phase. This predefined value varies according to the operating point of the motor is typically in a range from 50 to 200 V. The control of the voltage regulation stage is preferably synchronized to the control signal of the power switch.

Furthermore, the voltage regulation stage is also controlled to maintain, during a subsequent discharge phase to the charging phase, the intermediate voltage substantially equal to said preset value of intermediate voltage.

The invention also relates to a radiofrequency ignition device comprising a supply device as defined above and a plasma generation series resonator connected to the output of the supply device. The invention will be better understood, and other objects, details, features and advantages will become more clearly apparent from the following detailed explanatory description, with reference to the accompanying drawings, of which:

FIG. 1, already described, is a diagram of a series resonator modeling a plasma generation radiofrequency coil-candle,

FIG. 2, already described, is a diagram modeling a supply device, used for the control of the series resonator of the spark plug coil of FIG. 1,

Fig. 3 is a diagram of a DC-DC converter of the feeder of Fig. 2; and

FIG. 4 represents curves illustrating the voltage at the terminals of the two storage capacitors and the control signals of the switches of the DC-DC converter of FIG. 3.

As seen previously, to obtain a small voltage drop at the output of the supply device during the discharge phase, it is proposed to use a two-stage DC-DC converter, comprising a voltage booster stage and a regulation stage of the output voltage. Both floors will now be described in more detail. First stage of the DC-DC converter

Several voltage step stage structures well known to those skilled in the art are possible: a Boost structure, a Flyback structure, a forward structure, a capacitive half-bridge structure, a complete bridge structure. At the maximum engine rotation speed, typically 8200 rpm, and at maximum available energy, of the order of 1 Joule, the charging phase of the device power supply lasts a maximum of 3.7ms on a four-cylinder, four-stroke engine. Moreover, if we consider that the discharge phase lasts at most 500 ys, then, under these conditions which are extreme, the first stage works of time. The structure of this first

The voltage converter must therefore preferably minimize the resistive losses.

Also, according to a preferred embodiment illustrated in FIG. 3, the voltage booster stage referenced 41 is of the Boost type. Energy from the power supply 3 is temporarily accumulated in an induction coil 410 and then transferred via a diode 411 into a so-called ballast capacitor 412, across which a voltage V _ba n _ast is measured. Note _V i _im the voltage at the power supply terminal 3. A transistor, forming a switch 413, is used to control the energy storage phase in the coil and the transfer phase to the capacitor. The switch 413 is controlled by a control signal C1 of square shape. The operation of such a stage is well known to those skilled in the art. During the energy accumulation phase, the switch 413 is closed (on state), which causes an increase in current in the coil 410 and the storage of energy in the form of magnetic energy in the coil 410. this accumulation phase, the diode 411 is blocked and the capacitor is disconnected from the power source 3. When the switch is open, the coil 410 is in series with the power source 3 and its electromotive force s' then add to that of the power source. The current flowing through the coil also flows through diode 411 and capacitor 412. This results in energy transfer between coil 410 and capacitor 412.

This structure is the least expensive and one of the simplest to implement. Indeed, it has a reduced number of components. Induction coil 410 unique is used instead of for example a transformer in the case of a flyback structure. In addition, since the source of transistor 413 is connected to ground, the transistor is easier to control than if its source was floating.

For a voltage V n _ba _ast much higher than the input voltage _V i _im, the transistor conduction time 413 must be much greater than the non-conduction time for the charging phase. The charging time of the coil 410 is thus defined to be greater than the discharge time. As a result, the average current in the coil is therefore substantially equal to the average current supplied by the power source 3, which is the most favorable situation in terms of resistive losses. Second stage of the DC-DC converter

The second stage of the DC-DC converter has the main role of rapidly regulating the output voltage of the converter to the desired intermediate voltage Vi _{nter value} . It also has the function of transferring energy between two capacitors having high voltages between their terminals. Several structures are possible for this stage, either a voltage booster or a voltage booster.

In the case of a second step-up stage, the first step-up stage converts the voltage V _aum into a DC voltage V _ba n _ast and the second stage converts the voltage V _ba n _ast to a DC voltage across the terminals. a capacitor, denoted V _inter at the desired value, this voltage Vi _nte r being greater than V _ba n _ast . The first stage converts for example the voltage V _to ii _m = 12 volts into a voltage V _ba n _ast = 200 volts and the second stage converts the voltage V _ba n _ast to a voltage Vi _nte r = 250 volts.

In the case of a second step-down step, the first step-up stage converts the voltage V _to n _m into a DC voltage V _ba n _ast > Vi _nte r and the second stage converts the voltage V _ba n _ast to the Voltage Vi _nter - The first stage converts for example the voltage V _a i _im = 12 volts at a voltage V _ba n _ast = 340 volts and the second stage converts the voltage V _ba n _ast to a voltage

Vinter = 250 VOlTS.

This second case is a preferred embodiment. Indeed, at maximum energies supplied identical (for example 1 Joule), it is known to those skilled in the art that the peak current flowing in the coil of a Boost stage is greater than that flowing in the coil of a Buck stage. . A second loop-type buck step-down stage is therefore the solution which is the least expensive and which has the lowest resistive losses. This solution is that illustrated in FIG.

With reference to FIG. 3, the second stage is a loop-type buck step-down stage 42. This stage comprises a transistor, forming a switch 423, connected in series with a coil 420 and a capacitor 422. A diode 421 is connected in parallel with the series assembly formed of the coil 420 and the capacitor 422. The switch 423 is controlled by a control signal C2 of rectangular or square shape.

The operation of this stage is well known to those skilled in the art. When the switch 423 is closed (on state), the voltage across the coil 420 is equal to the difference between the voltage V _ba n _ast across the capacitor 412 and the voltage Vi _nter across the capacitor 422. The current circulating through the coil 420 thus increases linearly. The voltage across the diode 421 is negative, no current passes through it. When the switch 423 is open (off state), the diode 421 becomes conducting to ensure the continuity of the current flowing through the coil 420. The voltage across the coil is then -Vi _nte r and the current flowing through the reel decreases. Overall operation of the DC-DC converter

FIG. 4 represents curves illustrating the operation of the DC-DC converter of FIG. 3 in the context of a device for supplying a radiofrequency ignition.

A first curve represents the control signal VI of the switch M of the supply device 2. During the discharge (or spark) phases of the supply device, the switch M operates in switching mode (alternations of states open and closed). During the charging phases, the switch is open (locked state). A second curve represents the evolution of the voltage V _ba n _ast present at the output of the step-up stage 41 during the charging and discharging phases. A third curve represents the evolution of the voltage Vi _nte r present at the output of the voltage step-down stage 42. Finally, the fourth and fifth curves show the state of the transistors 413 and 423 during said charging and discharging phases. .

The control of the voltage booster stage 41 is relatively simple. The objective is to have at least before the discharge phase of the supply device a voltage V _ba ii _ast which is constant and strictly greater than the voltage Vi _nte r- The voltage V _ba n _ast is for example between 300 and 400 volts. This voltage V _ba n _ast is regulated by an integral proportional or proportional control loop well known to those skilled in the art, controlling the control signal C1 of the transistor 413. The transistor 413 operates in commutation during the charging phase of the feeding device. Its control signal C1 is synchronized to the control signal VI of the power transistor M. The signal C1 is a logic signal having a variable frequency and duty cycle. This frequency and this duty cycle are determined by the control loop and are a function of the inductance of the coil 410, the capacity of the capacitor 412 and the state of charge of the latter. During the discharge phase of the supply device, the transistor 413 is in a blocked state.

The control of the voltage step-down stage 42 is described below. During the charging phase of the device the capacitor 422 is charged and, during the discharge phase of the supply device, it charges and discharges.

At the beginning of each charging phase, the desired voltage Vi _nter across the capacitor 422 and the available charging time are known. At maximum engine rotation speed (8200 rpm) and maximum available energy (i _nte r = 250 volts), the charging phase lasts maximum 3.7 ms. However, to reduce the resistive losses, it is preferable to have the longest possible cooldown time while guaranteeing the obtaining of the voltage Vi _nter requested. The available charging time can be calculated from the rotational speed of the motor or the time between the two previous sparks. This available charging time can be calculated as follows:

Load = - δ = ΔΤ x δ

With: N = rotational speed of the engine;

ΔΤ = duration between 2 previous sparks;

δ = coefficient related to accelerations (δ <1).

During the charging phase of the supply device, the charge of the capacitor 422 is achieved by a suitable control of the transistor 422. By way of illustration, the simplest control is to generate a control signal C2 of constant frequency. According to a more advantageous embodiment, a control signal C2 controlled by frequency and duty cycle will be used to minimize the resistive losses in the down-converter stage during this charging phase. It is also conceivable to use a proportional or integral proportional control loop to regulate the voltage Vi _nte r during this charging phase.

During the discharge phase, the objective of the voltage step-down stage is to guarantee a constant voltage Vi _nter or failing to limit the voltage drop. The step-down stage continues to operate in switching mode. According to the invention, the energy consumed in priority during this phase is the energy available in the capacitor 412 (of the first stage) charged under the voltage V _ba n _ast . As this second stage operates as a voltage step-down, it is possible to draw energy from this capacitor as long as the voltage V _ba n _ast is greater than the voltage Vi _nte r desired.

The energy available in the capacitor 412 is then

1

^E ballast ^{~ ~ x C} ballast V ^v b ² allast - V ^v i ² nt er

2

with: C _ba ii _as t = capacity of capacitor 412;

10 V _ba n _ast = voltage across capacitor 412;

inter = voltage across the capacitor 422.

As an indication, for V _ba n _ast = 340V, V _int er = 250V and C _ba n _ast = 40 μ, we obtain E _ba n _ast = 1 J, which is the totality of the energy required for ignition in

15 extreme conditions.

The energy transfer is provided by controlling the transistor 423 with a control signal C2 variable duty cycle during this discharge phase.

The maximum duty cycle is limited by the

20 maximum current that can flow through the coil 420.

Also, according to an advantageous embodiment, the coil 420 is dimensioned taking into account this limit to ensure rapid control during the discharge phase. The coil 420 is then chosen such that:

with: iLmax = maximum current flowing in the coil 420;

and

loutmax ⁼ maximum current to be supplied at the output of 1 stage 42

It should be noted that, in this second stage, the capacitor 422 performs a dual filtering and storage function, that is to say it must have a low frequency impedance while having a high capacitance for storage. In this double topology

In the first stage, energy storage is mainly provided by capacitor 412. Capacitor capacity 422 and therefore reduce the cost of the DC-DC converter as a whole.

Although the invention has been described in connection with a particular embodiment, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims

A device (2) for supplying a radio frequency ignition, said supply device being adapted to supply a DC voltage at a predetermined frequency to a series resonator (1) and comprising

a power source (3) capable of delivering a DC supply voltage (V _a i _im ),

- a DC-DC converter (4) amplifying said DC supply voltage (V _a ii _m ) and delivering on an output a DC voltage, said intermediate voltage (Vi _nte r) whose amplitude is greater than said voltage of continuous feeding,

a power switch (M) controlled by a control signal (VI) for selectively applying said intermediate voltage (Vi _nte r) to the series resonator at a control frequency equal to said predefined frequency, characterized in that the continuous converter - Continuous comprises a voltage booster stage (41) for generating, from the DC supply voltage, a voltage (V _ba n _ast ) said ballast at the terminals of a first storage capacitor (412) and a stage voltage regulator (42) for generating from said ballast voltage the intermediate voltage across a second storage capacitor (422), said ballast voltage being greater than the DC supply voltage.

2. Device according to claim 1, characterized in that the voltage booster stage (41) is a Boost-type circuit.

3. Device according to claim 1 or 2, characterized in that the voltage booster stage (41) is controlled to charge said first storage capacitor (412) during a charging phase and generate at the terminals of said first storage capacitor a ballast voltage substantially equal to a predefined value at the end of said charging phase.

4. Device according to any one of claims 1 to 3, characterized in that the control of the voltage booster stage is synchronized to the control signal of the power switch.

5. Device according to claim 3 or 4, characterized in that said preset ballast voltage value is determined according to the rotational speed (N) of the heat engine in which is installed the radio frequency ignition.

6. Device according to any one of the preceding claims, characterized in that the ballast voltage (V _ba n _ast ) is greater than the intermediate voltage (i _n ter) and in that the voltage regulation stage (42) ) is a step-down step.

7. Device according to claim 6, characterized in that the voltage step stage is a Buck type mounting.

8. Device according to any one of the preceding claims, characterized in that the voltage regulation stage is controlled to charge, during said charging phase, said second storage capacitor with energy stored in said first capacitor of storage and generate, at the terminals of said second storage capacitor, an intermediate voltage substantially equal to a predefined value at the end of said charging phase.

9. Device according to claim 6, characterized in that the control of the voltage regulation stage is synchronized to the control signal of the power switch.

10. Device according to claim 8 or 9, characterized in that the voltage regulation stage is controlled to maintain, during a subsequent discharge phase to the charging phase, the intermediate voltage substantially equal to said preset value of intermediate voltage. .

A radio frequency ignition device comprising a power supply device (2) according to any one of the preceding claims and a plasma generating series resonator (1) connected to the output of the power supply device.