WO2014118451A1

WO2014118451A1 - Plasma-generating device with reduction of the overvoltage at the terminals of the communication transistor, and corresponding control method

Info

Publication number: WO2014118451A1
Application number: PCT/FR2014/050040
Authority: WO
Inventors: André AGNERAY
Original assignee: Renault S.A.S
Priority date: 2013-01-29
Filing date: 2014-01-10
Publication date: 2014-08-07
Also published as: FR3001601B1; FR3001601A1

Abstract

The invention relates to a plasma-generating device comprising a plasma-generating spark plug containing a capacitive inductive resonator (RS2) that can produce a high voltage, generating means (MGN) designed to deliver successive trains of command pulses at a control frequency, and a voltage generator (GENI) comprising a commutation transistor (M1) having a command electrode connected to the outlet of the generating means and an outlet that can deliver a train of voltage pulses to the resonator in response to the train of command pulses received on the command electrode of the commutation transistor at a frequency equal to the resonance frequency of the resonator. On the end part (PFi) of each train (TRi), the generating means are designed to deliver the command pulses at a control frequency different from the resonance frequency of the resonator.

Description

Plasma generating device with reduction of the overvoltage across the switching transistor, and corresponding control method

The invention relates to the generation of plasma, and more particularly to the suppression of overvoltages at the terminals of the switching transistor of the high voltage generator supplying the plasma generator.

The invention applies advantageously but not exclusively to the generation of plasma in a gas, and in particular the plasma generation used for controlled radiofrequency ignition for an internal combustion engine by the electrodes of a candle.

The principle of the resonant candle resides in the use of a resonant coil-capacitor assembly, the latter consisting of two electrodes (one of which has the base) separated by a dielectric material, such as ceramic. This high frequency resonator is generally called "coil-candle". This resonator is powered by a voltage generator capable of delivering a voltage pulse train to the resonator in response to a high frequency control pulse train received on the control electrode of the voltage pulse generator switching transistor. .

More precisely, this pulse generator comprises, besides the switching transistor controlled by successive trains of control pulses at a driving frequency, a coil-capacitor assembly powered by a medium voltage.

The combination of this pulse voltage generator and the series resonator ("coil-candle") is commonly called by those skilled in the art "pseudo-class E amplifier".

As indicated above, the control of the switching transistor is effected by control pulse trains applied to the gate of the switching transistor. When the transistor is conducting, a current flows in the coil of the voltage generator while when the transistor is off the current charges on the one hand the capacitor of the voltage generator and on the other hand feeds the coil of the series resonator. The drain potential of the transistor is raised. This high voltage periodically supplies the series resonator (coil-spark plug) which, thanks to a large overvoltage factor, creates the high voltage capable of triggering sparks.

In steady state, a large amount of reactive energy is stored in the inductive coil and capacitor of the series resonator. And, at the end of each pulse train, the excitation of the series resonator is stopped abruptly. The reactive energy is then transferred into the capacitor of the coil - capacitor assembly connected in parallel with the voltage generator, causing a momentary overvoltage between the drain - source terminals of the switching transistor. This overvoltage determines the maximum voltage at the drain that must support the transistor.

It is therefore particularly useful to reduce this surge.

A solution to limit this overvoltage has been described in French Patent Application No. 2,893,989 and provides for the addition of a Zener diode across the switching transistor. However, such a solution leads to additional costs.

An object of the invention is to propose a radically different solution for limiting the overvoltage at the terminals of the switching transistor.

According to one aspect of the invention, there is provided a method for controlling the switching transistor of a voltage generator for supplying voltage to a plasma generating plug containing a capacitive inductive resonator, the method comprising a delivery of successive trains. of control pulses at a driving frequency on the control electrode of the switching transistor. According to a general characteristic of this aspect, the control frequency of the resonance frequency of the resonator (coil-plug) is detuned on the final part of each train.

Although it is possible to detune these frequencies by decreasing the driving frequency with respect to the resonance frequency of the resonator, it is preferable to increase the control frequency with respect to the resonance frequency of the resonator. In fact, a greater reduction in the overvoltage is obtained.

According to one embodiment, said driving frequency is increased on the final part of each train by decreasing the duration during which the switching transistor is conducting. Indeed, it is chosen to maintain constant the duration during which the switching transistor is blocked and, by reducing the duration during which the transistor is conducting, the maximum energy available is reduced in the inductor of the parallel resonator of the voltage generator. transferable to the inductive-capacitive resonator (coil-candle).

The person skilled in the art will be able to adjust the frequency offset of the piloting frequency depending in particular on a compromise between the desired result and the undesirable effects of this frequency mismatch.

Thus, as an indication, on the final part of each train, it is possible to shift the driving frequency by a value between 0.5% and 5% of the resonance frequency of the resonator. Indeed, a frequency offset of less than 0.5% leads to a non-significant decrease in the overvoltage while a frequency offset greater than 5% induces undesired additional modulations leading to voltage peaks.

Moreover, the skilled person will also adjust the duration of the final part of each train during which the frequency mismatch is performed depending in particular on the quality of the desired spark at the spark plug.

As an indication, this final part of each train may comprise a number of pulses between Q / 4 and Q, where Q is the overvoltage coefficient of the resonator. According to another aspect, there is provided a plasma generation device, comprising a plasma generation plug containing a capacitive inductive resonator capable of producing a high voltage, generation means configured to deliver successive trains of control pulses to a driving frequency, and a voltage generator comprising a switching transistor whose control electrode is connected to the output of the generating means and an output capable of delivering a voltage pulse train to the resonator in response to the train of control pulses received on the control electrode of the switching transistor.

According to a general characteristic of this other aspect, on the final part of each train, the generating means are configured to deliver the control pulses at a control frequency different from the resonance frequency of the resonator.

According to one embodiment, on the final part of each train the generating means are configured to deliver the control pulses at a driving frequency higher than the resonance frequency of the resonator.

According to one embodiment, the switching transistor is turned on during a closing time and blocked during an opening time, the sum of these two durations being equal to the driving period of the pulses, and the generating means are configured to modify said driving frequency on the final part of each train by decreasing the closing time.

According to one embodiment, on the final part of each train, the generating means are configured to change the driving frequency by a value between 0.5% and 5% of the resonance frequency of the resonator.

Other advantages and characteristics of the invention will appear on examining the detailed description of embodiments and implementation, in no way limiting, and the accompanying drawings in which: Figures 1 and 2 schematically illustrate an embodiment of a plasma generating device according to the invention, and

FIG. 3 schematically illustrates an example of a pulse train controlling the switching transistor of the plasma generating device. In FIG. 1, the reference DIS designates a p lasma generation device comprising a BR resonant plasma generation plug and a GENI periodic pulse voltage generator.

The structure of a resonant candle BR is conventional and known per se by those skilled in the art. Thus, embodiments of such a candle are described, for example, in European Patent Applications Nos. 1,505,594 and 1,540,408.

More precisely, as illustrated diagrammatically in FIG. 1, the resonant plug BR comprises a series resonator RS2, comprising an inductive coil L2 and a capacitor C2 which, in the example described here, is constituted by the base assembly 1 -ceramic 2- central electrode 3. The base, forming the other electrode, is connected to ground.

This high voltage resonator RS2 is also called coil-candle.

This resonator RS2 is supplied with voltage by the periodic generator of pulses GENI.

As illustrated in FIG. 2, this voltage pulse generator GENI comprises a coil assembly L 1 and capacitor C 1 connected in parallel, as well as a switching transistor M 1, formed here of a P-channel power MOSFET, controlled on its gate G by successive trains TRi of control pulses generated by generation means MGN.

In the context of an application to a controlled ignition for internal combustion engine, the successive trains TRi are successively delivered as a function of the engine times. The generation means MGN may comprise conventional and known means of structure, configured to generate the modulated pulses according to a pulse width modulation (PWM: Puise With Modulation).

The natural frequency of the L2 series resonator C2 has a high resonance frequency, for example greater than 1 MHz, typically 5 MHz, with an overvoltage factor of, for example, between 40 and 200, typically 100.

The GENI pulse generator is powered by a voltage source VMT delivering a medium voltage, typically less than 1000 volts.

Moreover, this voltage source VMT preferably has a limited power, so that the energy applied between the electrodes of the spark plug is also limited, for example to 300 mJ by ignition and for safety reasons. To generate DC voltages greater than 12 volts in an automotive application, a 12 volts to Y vo lts converter can be used, where Y is the voltage supplied by the VMT power supply. It is thus possible to generate the desired DC voltage level from a battery voltage.

The stability of the DC voltage generated is not a priori a decisive criterion, it may for example be provided to use, for its qualities of robustness and simplicity, a switching power supply to supply the generator GENI.

When the transistor M 1 is conducting, a current flows in the coil L 1. When the transistor M 1 is off, the current on the one hand charges the capacitor C 1 and supplies the coil L 2 on the other hand.

The drain potential of the transistor M 1 'rises.

This high voltage, typically of the order of 500 volts, feeds the series resonator RS2, which, thanks to a large overvoltage factor, creates the high voltage HT capable of triggering the sparks.

In steady state, a large reactive energy is stored in the coil L2 and in the capacitor C2. At the end of the train pulses, the excitation is stopped abruptly. This reactive energy is then transferred into the capacitor C 1, causing a momentary overvoltage at the drain of the transistor M 1. This overvoltage, if it is too important, can be destructive for the transistor.

It is therefore advantageous to limit the value of this overvoltage.

In this respect, it is provided, as illustrated in FIG. 3, to detune at the end of the train, and more precisely on the final part PFi of each train TRi, the driving frequency of the control pulses of the transistor M 1, the resonance frequency of the series L2 C2 resonator.

More precisely, during the first part of the train TRi, the pulses IMP are delivered with a first driving frequency Fex l corresponding to a first driving period Tex l.

During this period Tex I, the transistor is successively passing for a duration Ton I and locked for a duration Toffl.

The control frequency Fex 1 is typically equal to the resonant frequency of the series L2 resonator C2 (coil-plug).

Then, on the final part PFi of the train TRi, this driving frequency is modified, here increased, to take a value Fex2 different from the resonance frequency of the series resonator L2 C2, here a higher value.

In this respect, a solution for modifying this driving frequency consists in leaving the duration Toff1 constant and acting on the duration Ton during which the transistor M 1 is conducting.

Thus, in FIG. 3, it can be seen that on the final part PFi, this duration Ton2 is reduced, which makes it possible to reduce the control period Tex2 and therefore to increase the control frequency.

This solution consisting in keeping the duration Toff1 of the transistor constant and decreasing the duration Ton is preferable because the reduction of this duration Ton reduces the maximum energy in the coil L 1 of the set L 1 C 1, the maximum energy available which can be transferred to series resonator L2 C2, and thus reduced, more effectively, the overvoltage at the drain of the transistor M l when the train TRi is completed.

The frequency offset on the PFi portion between the pulse control frequency and the resonant frequency of the resonator L2 C2 (coil-plug) may be between 0.5% and 5% of the resonant frequency of this coil-coil resonator. spark plug L2 C2.A typical value of frequency offset is 1%.

Thus, if the resonance frequency of the spark plug L2 C2 is equal to 5 MHz, the frequency offset can be 25 kHz.

Moreover, the duration of the final part PFi can be expressed in a number of pulses before the instant TF of cessation of the train. This number of pulses can typically be between Q / 4 and Q, where Q is the overvoltage coefficient of the resonator. Typically, the overvoltage coefficient of resonator L2 C2 (coil-plug) is equal to 100. Consequently, the number of pulses in the final part, or the number of periods, can typically be in this case between 25 and 100.

As an indication, for a frequency shift typically of

25 Khz, a portion PFI of 25 periods, corresponding to a duration of the order of 5 microseconds, allows to have a reduction of about 50% of the overvoltage.

Claims

1. A plasma generating device, comprising a plasma generating plug containing a capacitive inductive resonator (RS2) capable of producing a high voltage, generating means (MGN) configured to deliver successive trains of control pulses at a frequency of control, and a voltage generator (GENI) comprising a switching transistor (M l) whose control electrode is connected to the output of the generating means and an output capable of delivering a voltage pulse train to the resonator. response to the control pulse train received on the control electrode of the switching transistor, characterized in that on the final portion (PFi) of each train (TRi), the generating means is configured to output the control pulses. at a control frequency different from the resonance frequency of the resonator.

2. Device according to claim 1, wherein on the final part of each train the generating means (MGN) are configured to deliver the control pulses at a driving frequency higher than the resonance frequency of the resonator.

3. Device according to claim 1 or 2, wherein the switching transistor is conducting during a closing time and locked during an opening time, the sum of these two durations being equal to the driving period of the pulses, and the Generating means (MGN) are configured to modify said driving frequency on the final part of each train by decreasing the closing time (Ton2).

4. Device according to one of the preceding claims, wherein on the final part of each train, the generating means (MGN) are configured to change the piloting frequency by a value between 0.5% and 5% of the resonance frequency of the resonator.

5. Device according to one of the preceding claims, wherein said final part (PFc) of each train comprises a number of pulses between Q / 4 and Q, Q being the coefficient of overvoltage of the resonator (RS2).

IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 841-21-21 Boosting of a plasma generator spark plug containing a capacitive inductive resonator are described. control frequency on the control electrode of the switching transistor (M l), characterized in that on the final part (PFi) of each train, the control frequency of the resonance frequency of the resonator is detuned.

7. The method of claim 6, wherein on the final part (PFi) of each train increases the control frequency with respect to the resonance frequency of the resonator.

8. A method according to claim 6 or 7, wherein said piloting frequency is increased on the final part of each train by decreasing the duration (Ton2) during which the switching transistor is conducting.

9. Method according to one of claims 6 to 8, wherein on the final part (PFi) of each train, shifts the driving frequency by a value between 0.5% and 5% of the resonance frequency. of the resonator.

10. Method according to one of claims 6 to 9, wherein said final part (PFi) of each train comprises a number of pulses between Q / 4 and Q, Q being the overvoltage coefficient of the resonator.