WO2008113667A1

WO2008113667A1 - High frequency solid state heating generator protected against over-intensity

Info

Publication number: WO2008113667A1
Application number: PCT/EP2008/052436
Authority: WO
Inventors: Bernard Darges
Original assignee: Thales
Priority date: 2007-03-09
Filing date: 2008-02-28
Publication date: 2008-09-25
Also published as: FR2913544B1; FR2913544A1

Abstract

The invention relates to a high-frequency heating generator that comprises a solid state amplifier (10, 30) with a plurality of n transistor modules (Ml, M2,... Mi,....M(n-1) Mn), with i ranging from 1 to n, the whole set of modules providing a high frequency current I at a voltage U to a resonant circuit (12) electromagnetically coupled to a part to be heated (16). Each of n modules provides a high-frequency potential (u1, u2,...ui,...un) between two high-frequency output terminals A and B of the module, the potential at the terminal A being higher than the potential at the terminal B. The modules are connected in series with a load, the terminal A of a module Mi being connected to the terminal B of a preceding module (Mi-1) and the two free ends of the set of serially-connected modules being connected in series to the resonant circuit. Control means (40) are configured to deactivate one or more module of the amplifier in order not to exceed a minimal current in the resonant circuit.

Description

HEATING GENERATOR HIGH-FREQUENCY SOLID CONDITION PROTECTS AGAINST OVERCURRENT

The invention relates to solid state high frequency industrial generators. These generators can be used in particular in heating devices by electromagnetic induction, dielectric loss or plasma.

The heating generators feed a resonant circuit at high frequency electromagnetically coupled to a room to be heated.

The electromagnetic induction heating consists of producing a conductive part heating by the circulation of currents induced by a magnetic field. This means makes it possible to heat the part in its mass without direct contact with the source of energy. The part to be heated (or induced) is surrounded by at least one current circulation loop (or inductor).

The working frequencies of the generator are between a few tens of kilohertz and a few megahertz. Electromagnetic induction heating is widely used in industry and science. In industry, it is used in particular in metallurgy to refine metals, to thermally treat metal parts or to produce welded tubes continuously.

Heating by dielectric losses consists of producing a heating of an insulating part by causing losses in its mass, from an alternating electric field. The part to be heated is a poor insulator. It is placed between two conductive plates powered by an alternative source. A capacitor is created whose dielectric is the part to be heated. The generators used generally have higher working frequencies than those of electromagnetic induction heating generators. They can be between a few tens of megahertz and some gigahertz. This heating mode is used in the wood industry for drying or gluing, in the textile industry or in the manufacture or shaping of plastics. Plasma heating involves ionizing a gaseous medium to transform it into plasma. The kinetic energy of the electrons is transformed into heat. There is a considerable rise in temperature. The part to be heated is placed in the plasma. The transformation of the gaseous medium into plasma is obtained by the emission of an antenna. The working frequencies of the generator are between 1 megahertz and a few tens of megahertz. The heating mode is used in many industrial applications such as melting refractory products, chemical synthesis etc. . .

The charge of the heating generator is a resonant circuit electromagnetically coupled to the part to be heated and equivalent to either a circuit R, L, C parallel, or to a circuit R, L, C series. The overvoltage coefficient Q of the resonant circuit is high and the detuning is easy in the vicinity of the resonance frequency.

The charging impedance of the heating generator is essentially variable because it depends on the characteristics of the elements constituting the resonant circuit and in particular on the material of the part to be heated, their dimensions and their relative position. This position is important especially if the part to be heated is moving, for example if it is an induction heated sheet metal plate that is rolled and welded continuously.

In solid-state heating generators, a power amplifier connected to the resonant circuit must be able to provide high frequency power that can vary from a few hundred kilowatts to several megawatts. A single elementary transistor module can not provide such power. A set of elementary transistor modules are connected in parallel to provide the total power required for heating.

FIG. 1 shows a state-of-the-art heating generator comprising such modules coupled directly in parallel.

The generator of FIG. 1 comprises a power stage 10 having n elementary modules M1, M2, ... M i, .... M (n-1), Mn with transistors coupled directly in parallel on an output S of the 'amplifier.

The power stage 10 is connected by its output S of high frequency power to a load 12 comprising an inductor 14 for the The inductor 14 is a coil coupled to the part 16. A compensation capacitor 20 in parallel with the coil produces a resonant circuit at the induction frequency.

An RF generator 24 supplies a high frequency voltage to the input of the amplifier supplying the high frequency inputs of the n modules of the power stage. A power supply AL 26, voltage Vcc, supplies all n transistor modules of the amplifier.

In the heating generators, the RF generator 24 is a voltage controlled oscillator (OCT) whose frequency can be controlled to adjust the phase shift between the high frequency current I in the load and the voltage U at its terminals. This phase shift must be controlled according to load impedance variations in order to maintain good performance and the reliability of the solid state power stage.

In state-of-the-art generators having a solid state power stage, the high frequency global current I supplied by the power stage is related to the load impedance 12 which, as has been specified, can vary in large proportions depending on the nature of the part 1 6 and its movement in the inductor 14. A too low impedance brought to the output S of the amplifier stage can cause an overcurrent of the high frequency current I supplied by the amplifier and the destruction of the transistors of the modules. In any case, the power must not be interrupted when the heating generator is in operation. In state-of-the-art solid-state heating generators, power modules are coupled directly and in parallel. During an overcurrent at the high frequency output S of the stage, the only possibility of protecting the transistor modules is to reduce the power supply voltage Vcc of the power supply AL 26, for example by servocontrol of this circuit. supply voltage Vcc at the value of current I.

The major disadvantage of this solution is the reduced reaction rate of such a servocontrol, of the order of several tens of milliseconds, insufficient speed to effectively protect the transistors. Another major disadvantage is the need to use an adjustable power supply including thyristors and therefore cost Student. In addition, the use of active power elements to control the supply voltage Vcc under load decreases the overall efficiency of the heating generator.

In order to improve the performance and reliability of the solid state generators, the invention proposes a high frequency heating generator comprising a solid state amplifier having a plurality of n transistor modules, M1, M2, ... Mi ,. ... M (n-1) Mn, with i between 1 and n, all the modules providing a high frequency current I under a voltage U to a resonant circuit electromagnetically coupled to a workpiece to be heated.

Each of the n modules provides a high frequency potential (u1, u2, ... ui, ... a) between two high frequency output terminals A and B of the module, the potential of the terminal A being greater than the potential of the terminal B. The said modules are connected in series with the load, a terminal A of a module Mi being connected to a terminal B of a previous module M (i-1), the two free terminals of all the connected modules in series, being connected to the resonant circuit.

Advantageously, the high frequency generator, according to the invention, comprises means for controlling the modules configured to deactivate one or more modules of the amplifier so as not to exceed a maximum current Imax of the current I in the resonant circuit, the heating generator behaves, when the maximum current threshold Imax is reached, as a current generator.

In one embodiment, the control means comprise a computer CPU having a first memory zone comprising a program for determining the modules to be taken out of service, a second memory zone for storing instructions such as the maximum current Imax of the current I in the resonant circuit. not to be exceeded at the output of the amplifier. In another embodiment, the control means receives current I measurement information in the resonant circuit and provides module disable control signals.

In another embodiment, the control means are configured to put into service (nk) modules necessary for the supply of current I in the resonant circuit, out of the total number n of the amplifier modules, where k is the out of service modules, the generator behaves as a voltage generator U for a current less than the maximum current Imax.

In another embodiment, the control means are configured to change the k selected modules out of service, out of the total number n of the modules of the amplifier, after a predefined period of time T.

In another embodiment, the control means are configured to move the k modules out of service along an amplification line materialized by the first M1 and the last Mn amplifier module of the amplification stage.

In another embodiment, the amplifier is powered by an unregulated power supply comprising a transformer, at least one rectifier bridge connected to the secondary of the transformer and a filter circuit at the output of the rectifier bridge.

A main purpose of the solid state heating generator, according to the invention, is to obtain a very high reliability of operation of the generator and thus obtain high availability and low maintenance cost.

The invention will be better understood using exemplary embodiments with reference to the indexed figures in which,

- Figure 1, already described, shows a block diagram of a heating generator of the state of the art; - Figure 2 shows a block diagram of an embodiment of a heating generator according to the invention.

FIG. 2 represents a block diagram of an exemplary embodiment of a solid state heating generator, according to the invention. The heating generator of FIG. 4 comprises:

a transistor power amplifier 30 having an input E driven by an input signal Ve generated by a high frequency RF generator 32 and an output S supplying the resonant circuit 12 (or generator load) (see FIG. heating the room 16,

a DC voltage supply Vcc of the power stage 30.

The amplifier 30 comprises, to provide the resonant circuit 12 with the total heating power required, a set of n transistor amplifier modules, M1, M2, ... Mi, .... M (n-1) Mn, with i between 1 and n, coupled in series.

In Figure 2 a module Mi is shown partially showing the output stage of the module which includes an output RF transformer Si.

The output transformer Si of each of the modules comprises a primary winding Ep and a secondary winding E electromagnetically coupled.

A switching structure Ti, of the MOS type, is connected across the primary winding Ep of the output transformer Si.

The structure Ti can be controlled by MC control means 40 to be put in a saturated state and bypass the primary winding Ep of the transformer Si.

When the primary winding Ep of the transformer is short-circuited by the structure Ti, the module Mi provides no power output.

Secondary winding ES has two output terminals, one A terminal and one B terminal. When the module is in operation, the potential of terminal A is considered to be greater than the potential of terminal B.

The control means 40 are configured to switch the switching structures T1, T2, ... Ti, Tn of the n elementary modules, either to be in a saturated state, (ui = o), either to be in operation and to supply the potential ui between the terminals A and B. For this purpose, the control means comprise a CPU unit 42 having a first memory zone Mm1 comprising a program for determining the modules to be put into service or out of service and a second memory area Mm2 to record setpoints such as the nominal operating current I _n and the maximum current Imax not to be exceeded at the output of the amplifier.

In normal operation, (or in service) the elementary module Mi provides an output potential ui between the terminals A and B of the transformer Si.

According to a main characteristic of the invention, all the modules of the power stage 30 are connected in series with the resonant circuit 12 of heating, a terminal A of a module Mi being connected to a terminal B of a previous module M (i-1), the two free terminals of all the modules connected in series, being connected to the resonant circuit 12.

In this configuration, the current I in the resonant circuit 12 passes through all the secondary windings Es in series of all the modules, M1, M2, ... Mi, .... M (n-1) Mn.

The voltage U at the terminals of the load is the vector sum of the elementary voltages u1, u2,... Ui, .... a, across the n modules in series.

We will subsequently describe the operation of the heating generator according to the invention.

In this embodiment, the current I supplied by the power stage must not exceed a maximum value Imax (current in the setpoint

Mm2 of the calculator UC). Let Z _{n be} the nominal impedance of the load at a given instant, the nominal current I _n in the load is expressed by the relation: l _n = U _n / Z _n

U _n being the corresponding nominal voltage. A number of modules are put into operation by the computer CPU by acting on the control means 40 of the modules to generate by the amplifier, the necessary current I _n of heating.

Assuming, in an example case, that the voltages generated by the elementary modules are identical and in phase, let u be this voltage: u = u1 = u2 = ... ui, ... = one, the number of modules Nm to put into service will be:

N _n = U _n / UN _n being, for example, the integer greater than the nearest value of U _n / u.

The current in the constant impedance load Z _n will be a function of the number of modules put into service. The computer is configured to adjust the number of modules to be put into service to obtain the rated current I desired.

The generator behaves as a voltage generator U for a current lower than the maximum setpoint current Imax.

The control means are configured, when the impedance Z of the load (resonant circuit) 12 varies, so that the current I at the output of the amplifier does not exceed the maximum current value Imax setpoint. The heating generator then behaves as a current generator. For this purpose the CPU 42 disables the number of modules necessary to lower the voltage U to a value U _m such that the maximum current Imax is not exceeded.

The heating generator comprises a current transformer CT in series with the load 12 providing the instantaneous value Im of the current I in the load. The current information Im is applied to an input Ec of the computer CPU which determines the number of modules to be short-circuited or put into service.

In a particular embodiment given by way of example, the generator is a generator for welding parts. The generator of a high frequency power output of 15Kw powered a load under a voltage U of 400V. In this exemplary embodiment, the number of transistor modules is n = 10 of 15KW of high frequency power each, coupled in series, according to the invention, the transformers Si having a transformation ratio of 1/1. The current I in the load will be I = 375A when all modules are active. For 150Kw of power in the load, this corresponds to a load impedance Z of 1.06 ohm. (Z = 400V / 375A).

Let's fix a maximum Imax current of 400A not to be exceeded. This Imax value will be the threshold to remove (or disable) a number k of amplifier modules.

During welding, the impedance of the load can decrease by 50%, ie Z = 0.5 ohm. Since the current limit is 400A (operation as a current generator), the total voltage U _m will therefore be U _m = 400A.0.5 = 200V. Knowing that the elementary voltage ui of the modules is 40V per module (ie U = 40V.10 = 400v), to have 200V, it will be necessary to leave in service 5 modules, with 5 modules out of service, k = 5.

In summary, the output voltage U will vary in steps of 40V in this case. It goes without saying that if we want a finer definition, it can be set up "lower tension weights" (or a lower value ui). This can be achieved either by transformers that have a number of windings different from all transformers, or by using a variable supply on a single module.

The removal or commissioning of the modules is by shorting the primary winding Ep of the module or modules in question by the heating generator itself. In normal operation, it generates its RF voltage (in phase with all others).

In another embodiment of the high frequency generator according to the invention, a change of the modules in service can be realized in order to distribute the heat on all n modules of the amplifier.

For example, where n is the total number of modules and (nk) the number of modules in service providing the desired power to the load (where k is the number of out-of-service modules not providing power, the unused k modules can be rendered assets to replace modules active for a certain time T which become inactive during the same time to maintain the same total current I output.

For this purpose the control means 40 of the heating generator are configured to carry out commissioning of (nk) modules necessary for supplying the rated current I to the load, out of the total number n of the amplifier modules, k being out of service modules. The k modules chosen are changed by the CPU unit 42 after a predefined period of time T.

The modules k out of service can be moved along a line of amplification materialized by the first 1 and the last n amplifier of the amplification stage.

For example, in the previously described practical case of amplifiers having n = 10 modules in total of which (nk) = 8 active modules in nominal operation, every T = 0.5 second, the unused modules (k = 2 modules) can be made active. Visually the non-active modules will move along the amplification line.

The advantage of such a generator according to the invention resides in its instantaneous adaptation to fast (real) part load variations which can destroy the transistor modules by overcurrent. The switching of the modules is performed by the control means in a few tens of microseconds.

Another advantage of the heating generator according to the invention lies in the use of an unregulated supply voltage supply Vcc 36 having a transformer 50, a rectifier bridge 52 connected to the secondary of the transformer 50 and a filter circuit 54. , much more reliable power supply than the regulated power supplies necessary for the operation of the generators of the state of the art. In a variant of the unregulated voltage supply Vcc, the suppression of the transformer 50 may be possible by directly connecting the rectifier bridge to the distribution network. The removal of the transformer 50 brings an additional cost reduction. In this case of direct connection to the network, it will be necessary to take precautions of use with respect to the galvanic isolation for the safety of the personnel.

The current variations I in the load due to possible variations of unregulated supply voltage Vcc are also compensated by the switching action of the modules by the computer CPU.

Such an invention is particularly applicable to the field of high frequency industrial (HFI) where reliability is required. A solid state power stage of an HFI generator may comprise several hundred transistor modules, hence the importance of making modules reliable.

Claims

A high frequency heating generator having a solid state amplifier (10, 30) having a plurality of n transistor modules, M1, M2, ... Mi, .... M (n-1) Mn, with i between 1 and n, all the modules providing a high frequency current I under a voltage U to a resonant circuit (12) electromagnetically coupled to a part to be heated (16), each of the n modules providing a high frequency potential (u1 , u2, ... ui, ... a) between two terminals A and B high frequency output of the module, the potential of the terminal A being greater than the potential of the terminal B, said modules are connected in series with the charge, a terminal A of a module Mi being connected to a terminal B of a previous module M (i-1), the two free terminals of all the modules connected in series, being connected to the resonant circuit, characterized in it comprises control means (40) configured modules for disabling one or more the modules of the amplifier so as not to exceed a maximum current Imax of the current I in the resonant circuit, the heating generator behaving, when the maximum current threshold Imax is reached, as a current generator.

2. High frequency generator according to claim 1, characterized in that the control means (40) comprise a CPU (42) having a first memory zone (Mm1) comprising a program for determining the modules to be decommissioned, a second zone of memory (Mm2) for recording setpoints such as a maximum current Imax of the current I in the resonant circuit (12) not to be exceeded at the output of the amplifier.

3. high frequency generator according to one of claims 1 or 3, characterized in that the control means (40) receive a measurement of the current I information in the resonant circuit and provide control signals for decommissioning modules .

4. High frequency generator according to one of claims 1 to 3, characterized in that the control means (40) are configured to commissioning (nk) modules necessary for the supply of the current I in the resonant circuit, out of the total number n of the amplifier modules, k being the out of service modules, the generator behaving like a voltage generator U for one current below the maximum current Imax.

5. High frequency generator according to one of claims 1 to 4, characterized in that the control means (40) are configured to change the k selected modules out of service, among the total number n of the modules of the amplifier, after a predefined time period T.

6. high frequency generator according to one of claims 1 to 5, characterized in that the control means are configured to move the k modules out of service along an amplification line materialized by the first M1 and the last Mn amplifier module of the amplification stage.

7. High frequency generator according to one of claims 1 to 6, characterized in that the module comprises an RF output stage having an output transformer Si having a primary winding Ep and a secondary winding E electromagnetically coupled, a Ti structure of switching, of the MOS type, connected to the terminals of the primary winding Ep of the output transformer Si, which can be set by the control means MC (40), in a saturated state and short-circuiting the primary winding Ep of the transformer Yes.

8. High frequency generator according to claim 7, characterized in that the secondary winding Es comprises the two output terminals of the module, the terminal A and the terminal B.

9. High frequency generator according to one of claims 1 to 8, characterized in that the amplifier is powered by an unregulated supply (36) of supply voltage Vcc comprising a transformer (50), at least one bridge rectifier (52) connected to the secondary of the transformer (50) and a filter circuit (54).