EP0054445B1 - Electronic induction heating arrangememt - Google Patents

Abstract

1. Electronic induction heating device comprising an oscillating circuit made up of a capacitor and an inductance coil which performs the function of a primary winding, the secondary winding being constituted by the charge to be heated, this oscillating circuit being connected to the supply circuit via two electronic switches associated with a control circuit, a resistor and a capacitor being mounted in parallel with each of these switches, characterized in that the two electronic switches - for example two thyristors (7) - are associated with a control oscillator (10) which causes them to operate at the resonance frequency of the oscillating circuit (2-3) so as to open and close at the same time, these switches being designed to perform the function of impedances of very high value at the time of opening.

Description

The present invention relates to electronic induction heating devices.

The power components of such devices must meet relatively strict criteria of intensity, speed and voltage, which makes them uncommon and expensive.

• a) suffisamment rapides en commutation pour pouvoir fermer et ouvrir un circuit oscillant à des fréquences de l'ordre de 20 à 40 khz,
• b) capables de supporter des intensités élevées, de l'ordre de 40 ampères en régime permanent (pour une puissance de 2 kw induite),
• c) susceptibles de supporter, à leurs bornes, des tensions très élevées de l'ordre de 1000 volts compte tenu qu'un circuit résonnant pour un appareil de cuisson à usage ménager développe des tensions de l'ordre de 800 à 900 volts pour un foyer de 2 kw alimenté par un réseau de 220 volts.

Indeed, to meet the needs, for example in the case of an induction cooking appliance for household use, these components must be:

• a) fast enough in switching to be able to close and open an oscillating circuit at frequencies of the order of 20 to 40 kHz,
• b) capable of withstanding high intensities, of the order of 40 amperes in steady state (for an induced power of 2 kw),
• c) capable of withstanding very high voltages at their terminals of the order of 1000 volts, given that a resonant circuit for a household cooking appliance develops voltages of the order of 800 to 900 volts for a 2 kw fireplace powered by a 220 volt network.

These three conditions are difficult to meet together.

This is the reason why current devices are not always satisfactory.

Thus FR-A-2 430 679 describes a device intended for electric induction cooking formed by an oscillating circuit, consisting of capacitors and a choke which acts as an inductor, inductor to which the ferromagnetic charge to be heated is coupled . The oscillating circuit is connected to the supply circuit by two thyristors subject to a control circuit, this circuit making the thyristors operate successively.

A resistor and a capacitor are mounted in parallel with each thyristor to limit the variations in voltages across their terminals.

The supply circuit acts as a DC voltage source, obtained by rectifying the mains voltage.

As for DE-A-2 634 667, it describes a device intended to supply energy to the inductor of an induction furnace, this energy being conveyed by an alternating current obtained by an inverter. This inverter is connected to a DC power supply.

Since it is easier to find components on the market that meet two of the three conditions defined above than to meet these three conditions together, the aim of the present invention is to eliminate one of these three conditions, in this case that relating to the fact of being able to withstand a repetitive maximum voltage in the blocked state and this, so as to produce an induction heating device, the design of which is such that it has very high reliability, despite the performance limits of electronic components currently on the market.

To this end, the subject of the invention is an electronic induction heating device, comprising an oscillating circuit comprising a capacitor and a choke which acts as primary winding, the secondary winding being constituted by the load to be heated, this circuit oscillating being connected to the supply circuit by two electronic switches subject to a control circuit, a resistor and a capacitor being mounted in parallel with each of these switches, characterized in that the two electronic switches - for example two thyristors - are subject to a control oscillator making them operate at the resonant frequency of the oscillating circuit to open and close at the same time, these switches playing the role of impedances of very high value when they are opened.

Thus, when these two switches close, the oscillating circuit is connected directly to the two supply lines and, thereby, loaded with energy.

On the other hand, when the two switches are open, the oscillating circuit, loaded with energy (and comparable in this phase to a generator), will find at its terminals two open switches, and consequently, oscillate freely. This will have the effect that this circuit will tend to center, or find its equilibrium, between two impedances of great value.

Moreover, the features and advantages of the present device will appear more clearly during the following description, given with reference to the accompanying drawings.

• La figure 1 est un schéma général d'une forme préférentielle de réalisation du dispositif selon l'invention;
• la figure 2 représente les courbes de variation des tensions aux bornes des deux interrupteurs électroniques, ainsi que la courbe de variation de l'intensité dans la self du circuit oscillant;
• la figure 3 est un schéma général d'une autre forme de réalisation du dispositif selon l'invention.

On these:

• Figure 1 is a general diagram of a preferred embodiment of the device according to the invention;
• FIG. 2 represents the curves of variation of the voltages at the terminals of the two electronic switches, as well as the curve of variation of the intensity in the inductor of the oscillating circuit;
• Figure 3 is a general diagram of another embodiment of the device according to the invention.

In the example illustrated in FIG. 1, the oscillating circuit 1 of the corresponding device is constituted by a circuit known under the name "circuit-plug".

This includes a plane inductor 2 mounted in parallel with a capacitor 3, the values of which are such that the circuit resonates at the desired frequency, for example 25 K.hertz in the aforementioned case of a household appliance. The inductor 2 then acts as the primary winding, the secondary being constituted by the charge to be heated, in the corresponding appliance, in this case a cooking utensil (saucepan, frying pan, etc.).

In accordance with the essential characteristic of the device according to the invention, the oscillating circuit 1 is connected to the two lines 5 and 6 of the power circuit mentation by means of two electronic switches, which are therefore arranged on the other side of this oscillating circuit. Note that in the example shown in Figure 1, the oscillating circuit 1 is supplied by a direct current source, this current coming from the rectification of two half-waves of the mains voltage. It is however possible that the power supply to the oscillating circuit is provided by a DC voltage source.

In the example shown in Figure 1, each of the electronic switches is constituted by a thyristor 7 with which is mounted in anti-parallel a diode 8 whose direction of conduction is therefore opposite.

The trigger of each of the thyristors 7 is connected to the secondary of a transformer 9, the primary of which is supplied by a control oscillator 10. The arrangement of the control circuit thus formed is rel that the thyristors operate at the resonant frequency of the oscillating circuit 1 by opening and closing at the same time. The oscillator 10 of the control circuit is of the astable type, and it is followed by one, or even several, transistor (s) making it possible to obtain compatible control pulses in duration, in polarity and in power to correctly control the power switches.

However, the arrangement of such a circuit is not complete, as presented below. In fact, it must be taken into account that the power switches do not defuse when the pulse on the control electrode drops to zero volts. To achieve this goal, it is necessary to reverse the polarity at its terminals (anode-cathode).

The plug circuit is perfectly adequate to fulfill this additional role. Indeed, the overvoltage phenomena of such circuits are well known.

However, there is a difficulty due to the low inductive value used (34 gH) in the present example. Indeed, this low value causes the thyristors to open in 2.5 _li seconds, which is not enough to properly charge the oscillating circuit, while maintaining the envisaged working frequency.

However, an increase in the value of the inductor 2 cannot be envisaged, since its mechanical dimensions would quickly become unusable and, moreover, it would be difficult to find containers that can be paired with the inductor. It is therefore unnecessary to list all other reasons.

This is why, in order to maintain a resonant frequency of 25 to 30 K.Hz and a conduction time of the switches of the order of 10 µseconds, it is included, in the line which supplies the resonant circuit, a small self 13 of 3.5 µHenri in the example. This inductor can be formed in different ways, for example on air, on a nucleus of different shapes, or more simply by passing the power wire of the oscillating circuit through a toroid of good value.

This small choke has the effect of slowing down or rather of delaying the action of the heating choke 2, which results in a conduction time of the tyristors of the order of 10 u. seconds in the example.

• A est la phase de conduction (interrupteur fermé)
• B est la phase de conduction des diodes C est la phase d'oscillation libre

All this will appear more clearly by breaking down the different phases of conduction and free oscillation of the circuit, which appear on the voltage variation curves at the terminals of two electronic switches. These are shown in Figure 2, in which:

• A is the conduction phase (switch closed)
• B is the conduction phase of the diodes C is the free oscillation phase

1. Closing phase of the two switches

In this phase, the oscillating circuit is charged with energy through the two power tyristors 7. The plug circuit is then normally connected directly to lines 5 and 6 of the supply circuit.

2. Opening phase of the two switches

It is in this phase that the originality of the device according to the invention manifests itself.

Indeed, the overvoltages of the plug circuit occurring 10 µs after the closing of the tyristors have the effect of reverse biasing the tyristors, which are blocked. The voltages higher than the polarity inversion voltage of the tyristors are passed through the diodes 8 and the plug circuit leaves in free oscillation.

From this moment, the oscillating circuit (comparable in this phase to an energy generator) will find at its terminals the open switches 7. The voltage across these terminals is indirectly linked to the internal parameters of the switches.

The exact centering of the voltages is directly linked to the internal parameters of the switches.

Also, to avoid sorting the components, in order to pair them, it is advantageous to mount in parallel with each tyristor, a small value capacitor 11 and a resistor 12. This makes it possible to properly balance the voltages at the terminals of each tyristor 7 .

Consequently, each switch supports at its terminals half of the voltage developed by the oscillating circuit, ie 300 volts in the example.

It should be noted that the voltage developed by this type of oscillating circuit is lower than in other types of oscillating circuits. In this case, this voltage is of the order of 600 to 700 volts instead of 800 to 900 volts.

This fact is due to the low inductive value 34 µHenri, which combines the functions of primary winding and means of discharge. Now, this low value allows the energy stored in capacity 3 to flow faster than in other types of circuits, resulting in lower developed voltages.

• 1. En conservant des tyristors de mêmes caractéristiques que sur un montage à un interrupteur (tenue en tension 1000 V), ce nouveau système offreune marge de sécurité pour une tension de 1400 V dans l'exemple.
• 2. La marge de sécurité de tension peut être réduite en augmentant la tension d'alimentation si l'on désire augmenter la puissance.
• 3. Il est possible, en gardant une marge raisonnable de sécurité de tension (600 V par exemple) d'utiliser des tyristors moins performants en tension.
• 4. Il est évidemment toujours possible, pour augmenter la puissance, de multiplier le nombre de paires de tyristors par montage.

On the other hand, the fact that the tyristors only see the half-voltage present across the oscillating circuit, offers multiplex advantages, the main ones of which are:

• 1. By keeping tyristors with the same characteristics as on a switch assembly (voltage resistance 1000 V), this new system offers a safety margin for a voltage of 1400 V in the example.
• 2. The voltage safety margin can be reduced by increasing the supply voltage if you want to increase the power.
• 3. It is possible, while keeping a reasonable margin of voltage safety (600 V for example) to use tyristors less efficient in voltage.
• 4. It is obviously always possible, to increase the power, to multiply the number of pairs of tyristors per assembly.

The particular design of the device according to the invention therefore greatly multiplies the safety coefficients, since the voltages at the terminals of the switches are divided by two.

It should also be noted that this design has the advantage of eliminating the usual drawbacks of electronic induction heating devices, including the risks of thermal runaway of the active components (power switch). In this case, one switch effectively protects the other, knowing that the components are never perfectly identical, and moreover, never exposed to the same temperatures.

On the other hand, the leakage currents are reduced by the mounting of two switches, which offers a good guarantee against thermal runaway.

However, as already indicated, the main advantage of the device according to the invention lies in the fact that, for the same voltage of the oscillating circuit, the voltage safety margin of the electronic switches is considerably increased.

However, it goes without saying that the device according to the invention is not limited to the single preferred example described above. Thus, the oscillating circuit of this device can be produced differently. On the other hand, the power tyristors could be replaced by power transistors, with some touch-ups inherent in the equipment used.

Furthermore, FIG. 3 represents the diagram of an embodiment in which the tyristors 7 of the previous example are replaced by transistors 7a.

Furthermore, it goes without saying that the present device is not limited to the production of cooking appliances for household use, since it can be the subject of various applications.

Claims (2)

1. Electronic induction heating device comprising an oscillating circuit made up of a capacitor and an inductance coil which performs the function of a primary winding, the secondary winding being constituted by the charge to be heated, this oscillating circuit being connected to the supply circuit via two electronic switches associated with a control circuit, a resistor and a capacitor being mounted in parallel with each of these switches, characterized in that the two electronic switches - for example two thyristors (7) - are associated with a control oscillator (10) which causes them to operate at the resonance frequency of the oscillating circuit (2-3) so as to open and close at the same time, these switches being designed to perform the function of impedances of very high value at the time of opening.

2. Device in accordance with claim 1, characterized in that provision is made in one of the supply lines of the oscillating circuit for an inductance coil (13) having the function of delaying the action of the inductance coil (2) of the oscillating circuit.

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