Generator suitable for powering a dental curing light

The generator for a piezoelectric motor is also suitable for powering a high power LED for a dental polymerisation lamp via a rectifier, and comprises two transformers each including a primary winding and a secondary winding and four switches controlled by an ultrasonic reference oscillator, two switches being arranged to alternately connect the secondary windings of the two transformers to the piezoelectric load, and the other two switches being arranged to alternately connect the two primary windings to a voltage supply so that during the positive alternation, the primary winding of one of the transformers is charged with energy whereas the secondary winding of the other transformer is discharged into the piezoelectric load, and so that during the negative alternation, the secondary winding of the first transformer discharges the energy thereof whereas the primary winding of the first transformer is charged.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Phase Application in the United States of International Patent Application PCT/EP2011/065869 filed Sep. 13, 2011, which claims priority on European Patent Application No. 10177186.3 of Oct. 16, 2010. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention concerns a generator for powering a piezoelectric transducer and which is also suitable for powering a high power LED for a dental polymerisation lamp via a rectifier. It further concerns a device comprising the aforementioned generator and a dental polymerisation lamp including a rectifier.

PRIOR ART

In dentistry, photosensitive composites are commonly used, particularly to fill dental caries and other holes in teeth. To achieve this, a dental composite liquid or paste is first of all placed on or in a tooth, and the composite is then irradiated by a lamp so that it polymerises and hardens. To this end, there are known dental polymerisation lamps, which include LEDs (light emitting diodes) for producing the luminous energy necessary for polymerisation. Some LED manufacturers also propose high power LEDs emitting a coloured light called “dental blue light” whose spectrum is limited to a relatively narrow band around the wavelength of460nm. One advantage of these “dental blue light” LEDs is that they produce relatively little parasitic heat and are thus suitable for insertion straight into the patient's mouth.

To operate, the LEDs of a dental polymerisation lamp must be connected to an electrical power supply typically supplying a continuous voltage of around 10 to 20 volts and around fifteen watts of power. However, it will be clear that these values are only given by way of example and that the supply voltage depends, amongst other things, on the number of LEDs contained in the lamp, and on the way in which the LEDs are interconnected.

In dentistry, ultrasonic dental equipment can also be used. This equipment is used by dentists for descaling or more generally for removing any deposits on the surface of teeth. Usually, ultrasonic dental equipment takes the form of a handpiece in which an ultrasonic insert, forming the actual vibrating tool, is mounted. A piezoelectric transducer is also arranged in the handpiece to produce an ultrasonic vibration and to communicate the vibration to the insert.

In order to work, the piezoelectric transducer of the ultrasonic equipment must be connected to an electric power supply supplying an alternating voltage at ultrasonic frequency and up to several hundred volts. In a known manner, the handpiece may be connected to the electric power supply by a flexible lead. Moreover, a connector may be arranged at the junction between the handpiece and the lead, to enable the handpiece to be detached, in particular to be cleaned or sterilised.

The Mectron S.R.L. company has recently started selling a versatile electric ultrasonic generator which can be connected to two types of handpieces operating in a completely different manner: both handpieces used as ultrasonic equipment and handpieces used as dental polymerisation lamps. One advantage of having this type of versatile electric generator is that it is no longer necessary to have two separate electric power supplies, one for the ultrasonic tool and the other for the polymerisation lamp.

The use of a single versatile generator instead of two gives rise to some difficulties. Indeed, in order to work, the ultrasonic equipment requires an alternating voltage of several tens of kHz and several hundred volts. Conversely, the dental polymerisation lamp only normally requires a continuous voltage of around ten volts to operate. To overcome this difficulty and to power the polymerisation lamp with a generator provided for an ultrasonic apparatus, one possible solution is to arrange a circuit in the lamp for lowering the voltage and converting the alternating voltage into continuous voltage.

This solution also gives rise to certain difficulties. In fact, most generators for a piezoelectric transducer behave as described in FR Patent No. 2,391,001.FIG. 1annexed hereto, taken from this prior art document, is a diagram showing the delivered power P according to the transducer impedance Z, respectively in the case of minimum power (curve I), intermediate power (curve II), and maximum power (curve III). Referring to the intermediate power curve II, it is seen that while the transducer impedance Z remains less than threshold Zb, the power P delivered to the transducer increases proportionally to the impedance. If the impedance exceeds the reference threshold, the constant current generator is blocked and the voltage generator is unblocked. From this point on, the delivered power decreases according to a hyperbolic law as the impedance increases. It is clear thus that the usual power generators for piezoelectric transducers have the drawback of only supplying maximum power for a very precise transducer impedance value (as evidenced by the generally triangular shape of curves I and II ofFIG. 1A). It is therefore clear that, to power a polymerisation lamp, it is not sufficient simply to lower and rectify the voltage. It is also necessary to check that the impedance of the load, formed by the lamp and the circuit for lowering the voltage, is adapted to the generator.

It is therefore an object of the present invention to supply an electric ultrasonic generator able to power a dental polymerisation lamp wherein the delivered power does not depend substantially on the load impedance, and another object of the present invention is to supply an electric ultrasonic generator able to power a dental polymerisation lamp, which does not require the insertion of a circuit to lower the voltage between the generator and the lamp.

SUMMARY OF THE INVENTION

The present invention achieves this object by providing a generator conforming to the annexed claim1.

DETAILED DESCRIPTION OF ONE EMBODIMENT

FIG. 2is an electric diagram of a particular embodiment of the generator of the invention, the output of which is connected to a high power LED5of a dental polymerisation lamp7. Lamp7is also provided with a rectifier9. The generator for powering a dental polymerisation lamp7includes two transformers11A and11B each comprising a primary winding L1and a secondary winding L2. Each of the two secondary windings L2is connected by one of the terminals thereof to one of the two terminals of lamp7, with a diode (13or15) also being inserted between each secondary winding and the lamp. The other terminal of each of the secondary windings L2is connected to earth.

Each of the primary windings L1of transformers11A and11B is series connected with a switch (19A or19B), between the terminals of a power supply. In the present example, the power supply is formed by a voltage source referenced17, one terminal of which is connected to each of primary windings L1and the other terminal is connected to earth. Switches19A and19B, like the other switches mentioned in this description, are electrically controlled switches which may be implemented in the form of transistors. These switches will be referred to simply as “switches” below. In addition to being connected to the secondary windings L2of the two transformers, the two terminals of lamp7are also connected to earth via a diode23and a switch21B, respectively a diode25and a switch21A. In other words, lamp7is series connected with diode13, diode25and switch21A between the terminals of secondary winding L2of transformer11A, and lamp7is also series connected with diode15, diode23and switch21B, between the terminals of secondary winding L2of transformer11B.

Switches19A,21B are arranged to be controlled by a first periodic control signal, termed here a “direct” signal, whereas switches19B and21A are arranged to be controlled by a second periodic control signal which is phase shifted by a semi-period relative to the first periodic signal, and which is termed here the “inverse” signal. In the present example, a means (not shown) controls the duration of the periodic pulses forming the first and second control signals. This pulse width modulation (PWM) preferably acts on both signals, so that the periodic pulses of the two control signals both have the same duration.

From the following description, those skilled in the art will understand that the power supplied by the generator depends on the pulse duration, and that the PWM means thus controls the power supplied by the generator. It should be specified, however, that the present invention is not limited to a generator whose power is controlled by PWM. Indeed, according to another embodiment, the power supplied by the generator could for example be set once and for all. Alternatively, it is also possible to control the power supplied by the generator by adjusting the voltage supplied by power supply17, or by varying the frequency of the first and second periodic control signals. As regards this latter possibility, it is important to note that, unlike a piezoelectric transducer, a dental polymerisation lamp does not form a resonant circuit, but only a resistive circuit (in other words, the characteristics of the polymerisation lamp connected to a generator do not determine the frequency at which the generator has to operate).

Switches19A and21B are closed throughout the duration of the “direct” signal pulses. Throughout the duration of the “inverse” signal pulses, it is switches19B and21A which are closed. During an “inverse” signal pulse, the circuit formed by the secondary winding L2of transformer11A, lamp7, diodes13and25and switch21A is closed and the energy stored in transformer11A is transferred to the load. Simultaneously, switch19B is closed and primary winding L1of transformer11B is directly connected to voltage source17. The current through the primary winding produces an increase in magnetic flux. Energy is therefore stored in the magnetic circuit. During a “direct” signal pulse the reverse is true. Secondary winding L2of transformer11B gives back its energy by discharging into the circuit including lamp7, diodes15and23and switch21B, whereas the current through primary winding L1of transformer11A causes energy to be stored in its magnetic circuit. The graph ofFIG. 3Ashows the behaviour of the current and the voltage in windings L1, L2and L3of one of the two transformers11A or11B in an example case where the duration of a pulse is exactly equal to a semi-period. It is seen that current IL1in the primary winding of the transformer regularly increases for an alternation before dropping back to zero and remaining there for the duration of the next alternation. The secondary winding takes over at the transition between two alternations. It is seen that a current IL2, of decreasing intensity, flows through secondary winding. In the example illustrated, the current IL2flows until the stored energy has completely dissipated. The variations of intensity in current IL2are accompanied by corresponding variations in voltage UL2between the terminals of the secondary winding.

It is clear that the fact of having two transformers11A and11B and connecting lamp7alternately to one and then the other transformer results in an alternating supply voltage being supplied to the lamp. Moreover, those skilled in the art will appreciate that, in short, the function of switches19A,19B,21A,21B is to control transformers11A and11B so that they operate in flyback mode.

It may happen that the load impedance is insufficient to dissipate all of the energy stored in the transformer. This situation is illustrated by the graph inFIG. 3C. Referring to this graph, it can be seen that, when the load impedance is particularly low, current IL2and voltage UL2do not have time to drop back to zero before the end of an alternation. It can also be seen that the energy not dissipated in the secondary winding is in the primary winding at the start of the next alternation. This non-dissipated energy is responsible for the non-zero intensity of current IL1in primary winding L1at the start of the alternation (FIG. 3C). It will thus be clear that, below a certain threshold, the smaller the impedance, the greater the intensity of current IL1will be in the primary winding.

FIG. 2also shows two measuring circuits39A and39B. These measuring circuits are each arranged to measure the current in the primary winding (L1) and one of the two transformers11A and11B. Basically, the current measured by the measuring circuits is a function of the impedance of the load connected to the generator. This measurement may thus be used to regulate the generator. In particular, any trapezoidal current shape (IL1,FIG. 3C) is a sign that part of the energy has not been dissipated and remains in the transformer from the preceding alternation. Moreover, the current measurement can detect, for example, any short-circuits or a resonant frequency, or can also automatically determine the type of load connected to the generator (resonant circuit or resistive circuit).

FIG. 2also shows that transformer11A and11B each include a tertiary winding L3. Winding L3of transformer11A is series connected with a diode31and a resistor35, between voltage source17and earth. Likewise, winding L3of transformer11B is series connected with diode33and a resistor37, between voltage source17and earth. As will be seen in more detail below, the function of windings L3is to limit the maximum voltage supplied at the output of secondary winding L2.

The speed with which the current intensity in L2decreases when the energy stored in one of the transformers is transferred to the load naturally depends on the impedance associated with the load. The higher the impedance, the more quickly the current intensity decreases, and the higher the voltage between the terminals of the secondary winding will be. TheFIG. 3Bgraph shows the behaviour of the generator ofFIG. 2in a situation where the impedance of the load connected to the generator is particularly high.FIG. 3Bshows that the intensity of current IL2decreases substantially more quickly than inFIG. 3A. Moreover, voltage UL2at the start of an alternation is also considerably higher than in the case ofFIG. 3A. It will be clear that if, for one reason or another, the impedance of lamp7becomes very large (because of a burnt out component in the circuit, for example), the output voltage UL2is liable to increase to the point of damaging the generator. This is the reason why, in the present example, the two transformers11A and11B each include a third winding L3which is inductively coupled to the primary and secondary windings L1and L2.

Referring again toFIG. 2, it is seen that diodes31and33are connected to windings L3by their cathode and connected to earth by their anode. Since the other terminal of each winding L3is connected to the positive terminal of voltage source17, the diodes are normally subject to a negative voltage UL3. In these conditions, diodes31and33prevent the current from passing through. However, if the voltage induced in L3exceeds the continuous supply voltage, the voltage UL3remaining across the diodes becomes temporarily positive, and a current IL3can start to flow in L3. This transitory current IL3has the effect of limiting voltage UL2at the terminals of winding L2. The presence of winding L3thus allows to limit voltage UL2at a value which is determined by selecting the ratio between the induction values L2and L3.

FIG. 1Bis a graph including a first curve, which shows the behaviour of the power supplied by the generator which has just been described according to the piezoelectric transducer impedance. The graph also includes a second curve, which shows the behaviour of the power of a prior art generator for a piezoelectric transducer such as that described in the aforementioned FR Patent No. 2,391,001.FIG. 1Bshows that the first curve includes a first increasing portion, a second constant portion and finally a third decreasing portion. The second portion occupies all of the central part of the graph and thus corresponds to medium impedance values. In this range, the power supplied by the generator according to the invention is substantially constant, and the behaviour of the generator corresponds to that shown by the graph inFIG. 3A. The first portion of the curve is for impedance values which are insufficient to dissipate all of the energy stored in the transformers prior to the end of an alternation. This first portion of the curve is for a range in which the behaviour of the generator corresponds to that shown in the graph inFIG. 3C. Within this range, the power supplied is reduced in proportion to the impedance. The third portion of the curve is for the highest impedances. The generator behaviour in this area corresponds to that described by the graph inFIG. 3B. In this area, the voltage between the terminals of secondary winding L2is limited by winding L3and the current thus decreases progressively as the impedance increases.

The first curve ofFIG. 1Bthus demonstrates that the power supplied at the output by a generator according to the invention is substantially constant for a large load impedance value range. When the generator powers a piezoelectric transducer, this feature of the invention enables the transducer to be supplied with constant power independently of any fluctuations in the mechanical load to which the piezoelectric transducer is subject. Moreover, when an LED of a dental polymerisation lamp (whose impedance is much smaller) is substituted for the piezoelectric transducer, the power supplied by the generator is not affected.