Abstract:
A invention relates to a transformer having a primary coil and a secondary coil, with a ferromagnetic core inserted in a coil former. The alternating current resistance of the primary coil ( 3 ) drops on reaching a triggering temperature (TC) which lies above the operating temperature but below the softening temperature (TW) of the coil former ( 6 ) and/or insulation of coil winding ( 3, 4 ). The coil core ( 5 ) may consist at least partially of a material with a magnetic permeability which drops when the triggering temperature (TC) is exceeded.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a transformer having a primary coil and a secondary coil, with a ferromagnetic core inserted in a coil former. 
     Transformers of this type are known in the art and serve the purpose of transforming alternating current. The field of application of such transformers covers AC/DC or DC/DC converters and power supply units in particular. In such applications, it is of considerable importance to isolate the input circuit and output circuit galvanically from each other. The isolation of the galvanic regions in most cases consists of plastics material. This plastics material is capable of withstanding certain thermal loads. In the event of a fault, however, the temperature of the transformer may rise to a value at which the coil former or the winding insulation melts. The short-circuit then occurring between the primary winding and the secondary winding destroys the galvanic isolation. 
     To avoid such destruction of the galvanic isolation, it has been proposed in the prior art to monitor the temperature of the transformer housings by thermocouples. A common precautionary measure is also to introduce a fuse into the primary circuit. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of developing a transformer of the generic type to improve the electrical isolation protection. 
     According to the invention the alternating current resistance of the primary coil decreases on reaching a triggering temperature which lies above the operating temperature of the transformer but below the softening temperature of the coil former and/or insulation of the coil winding. For this purpose, the coil core preferably consists at least partially of a material with a magnetic permeability which drops when the triggering temperature is exceeded. For this purpose, the core preferably consists of a ferrite. This ferrite core is to have at least one region in which the Curie temperature is lower than the softening temperature of the coil core and/or coil insulation. If the temperature of the coil core rises below the Curie temperature, the permeability of the coil core remains virtually constant or increases slightly. On reaching the Curie temperature, the relative permeability of the material drops abruptly to the value  1 . This abrupt drop takes place over only a few degrees. The coil core preferably comprises two core parts. In this case it is adequate if one of the two core parts consists of a material in which the relative permeability changes abruptly at the temperature mentioned above. It is adequate if only a subregion of the core has these properties. The Curie temperature of the region or of the entire core preferably lies between 120 and 220° C. The invention also relates to a power supply unit or a voltage converter with a transformer in which the alternating current resistance of the primary coil decreases on reaching a triggering temperature which lies above the operating temperature, this triggering temperature lying below the softening temperature of the coil former and/or insulation of the coil winding. In this case, a fuse is connected in the primary circuit, the triggering current of the said fuse being lower than the current flowing through the primary coil when the triggering temperature is exceeded. In a preferred embodiment, the voltage converter is a DC/DC converter, in which the alternating current is generated by a switching IC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained below with reference to the attached drawings, in which: 
     FIG. 1 shows a first exemplary embodiment of the invention in schematic representation; 
     FIG. 2 shows a transformer; 
     FIG. 3 shows a coil core; 
     FIG. 4 shows a second exemplary embodiment of the invention; and 
     FIG. 5 shows a diagram of the inductance of a transformer in relation to the temperature. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The circuit arrangement represented in FIG. 1 comprises a transformer  1  with primary winding  3  and secondary winding  4 . The two windings  3 ,  4  are located, as represented in FIG. 2, on a common coil former  6 , in which a coil core  5  is fitted. The coil core  5  comprises two individual E-shaped coil cores  5 ′,  5 ″. The two coil cores  5 ′,  5 ″ have been pushed into the coil former  6  towards each other in such a way that an air gap  8  is defined between the cross-pieces of the E. The said air gap is not necessary. In the primary circuit, in which the current I flows, there is a fuse  2 . The primary winding  3  is under a voltage U 1 . At the secondary winding  4 , a secondary voltage A can be tapped. 
     The transformer represented in FIG. 2 has two secondary windings  4 ,  4 ′. 
     The coil core, which is represented in FIG. 3, has in each of its two core halves  5 ′,  5 ″ a region B in which the material has a relative magnetic permeability which is significantly greater than 1 at the operating temperature TA of the transformer. The Curie temperature TC of the regions B is chosen such that it is lower than the softening temperature TW of coil former  6  and of the insulation of the primary or secondary winding. In a preferred exemplary embodiment (not represented), the core  5  or one of the two core halves  5 ′,  5 ″ is produced from a material of which the Curie temperature TC lies between the operating temperature TA and the softening temperature TW of the transformer  1 . The Curie temperature TC preferably lies approximately around 130° C. This temperature lies distinctly below that temperature at which the plastics material of the coil former  6  softens. This temperature is, for example, around 220° C. 
     In practice, for cost reasons on the one hand and assembly reasons on the other hand, the entire coil core represented in FIG. 3, and not just a subregion B, will be produced from the material in question. 
     The exemplary embodiment represented in FIG. 4 concerns an AC/(DC)/DC converter. On the input side, the converter forms a rectifier GL with a damping capacitor C 1 . An alternating voltage U 1  may be connected to the rectifier GL. This voltage is rectified by the rectifier GL and damped in a known way by means of the capacitor C 1 . A direct voltage U 2  may also be applied directly to the capacitor C 1 . 
     The primary circuit is located at the capacitor C 1 . In this primary circuit, in which the primary winding  3  of a transformer is located, there are a fuse  2 , a low-impedance current-limiting resistor R 1  and a switching IC IC 1 . The switching IC IC 1  supplies an alternating voltage, the frequency of which may be fixed or else may be selectable. The amplitude or the pulse width of the alternating voltage supplied by the IC is determined by a control circuit  7 . Between the two terminals of the primary winding  3  there are also located a diode D 1  and a Zener diode  21 . 
     In the secondary circuit of the secondary winding  4  there are located a diode D 2  and a fuse S 1 . The secondary direct voltage, present at the diode D 2 , is tapped via a resistor R 3  and a Zener diode Z 2  in series with the latter and is fed to an optocoupler OC. The optocoupler OC is connected to the control input of the switching IC IC 1 . The alternating voltage supplied by the IC is regulated via the optocoupler OC in a known way, by means of a pulse width control or an amplitude control, so that the direct voltage U 2  on the primary side remains constant. 
     In the event of a fault in which, for example, the IC is destroyed and a direct current flows through the primary winding  3 , the primary circuit current I increases until it exceeds the triggering current of the fuse  2 . The fuse  2  then blows. The circuit is in a safe state. 
     In the event of a fault in which either, in the case of the exemplary embodiment according to FIG. 1, the alternating voltage U 1  assumes an excessively high frequency or voltage or, in the case of the exemplary embodiment according to FIG. 4, the integrated circuit IC 1  oscillates in an uncontrolled manner, the voltage present at the primary circuit coil  3  assumes high values. This leads to the temperature of the transformer  1  rising due to the associated higher losses. The temperature then rises above the normal operating temperature TA and reaches the Curie temperature TC of the material of the coil core  5 . On exceeding the Curie temperature, the relative permeability of the material (ferrite) drops to virtually  1 . This means that the core no longer contributes to the concentration of the magnetic flux. The consequence of the abrupt drop in relative permeability resulting from the temperature increase is a significant reduction in the inductance of the windings. Consequently, the primary winding opposes the alternating current present at it with a reduced resistance. Not only the imaginary part but also the real part of the resistance of the coil is significantly reduced. This means a simultaneous increase in the current flowing through the fuse  2 . This current increases beyond the triggering current, so that the fuse blows. 
     The abrupt dropping of the permeability on reaching a temperature of approximately 130° C. also influences the transfer behaviour between the primary side and the secondary side. The power transfer from the primary side to the secondary side is reduced considerably. 
     The serious increase in magnetic resistance, by a factor of 200 to 1000, when the Curie temperature TC is exceeded, reduces the magnetic flux considerably, so that the induced voltage drops on the secondary side and a power limit also occurs there. 
     The variation in the inductance of the primary coil of a transformer according to the invention is represented schematically in FIG.  5 . In this figure, TA indicates the operating temperature. TC denotes the Curie temperature and TW denotes the melting temperature of the coil housing or winding insulation.