Abstract:
A device for sensing an alternating current includes a circuit board on whose surface is arranged a planar conductor path in which a current to be measured flows. Applied on the opposite surface of the conductor path is a planar coil in which the change in the current to be measured induces a voltage. The device is advantageously used in a converter having a direct-current voltage link circuit.

Description:
BACKGROUND INFORMATION 
     Conventional devices consist of a current-carrying conductor around which a coil is arranged. An alternating current flowing in the conductor induces in the coil a voltage which is proportional to the change over time in the current. Suitable analysis of the induced voltage yields information concerning the current flowing in the conductor. Devices which perform this type of current measurement are available commercially as complete components, and can easily be integrated into electronic circuits. All that is needed to be provided for the purpose is one connection for the current to be measured, as well as a further connection to lead the measured signal out. Unsatisfactory aspects of such conventional current measurement components are their comparatively large space requirement which limits the miniaturization of circuits, and their high price due to their embodiment as a sealed subassembly. 
     One of the objects of the present invention is to provide a current measurement device which requires as little installation space as possible and is also economical. 
     SUMMARY OF THE INVENTION 
     According to the present invention, the sensing device is configured in planar fashion on the surfaces of a circuit board, and requires no installation depth. Production of the actual measurement arrangement is accomplished by configuring a conductive path structure, and is thus extraordinarily economical. One of the advantages of the sensing device according to the present invention is that additional actions for galvanic separation of the current-carrying conductor and the measurement arrangement are not necessary. Manufacture is thereby made simpler and more flexible. The configuration as a conductive path structure also offers the advantages of conductive paths in terms of thermal insensitivity, contact reliability, insensitivity to mechanical damage, and good reproducibility of the mechanical arrangement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a basic sensing principle for a sensing device according to the present invention. 
     FIG. 2 shows a cross section through the sensing device. 
     FIG. 3 shows a schematic diagram of a monitoring circuit. 
     FIG. 4 shows an arrangement utilizing the sensing device in a welding system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates the physical principle underlying the sensing device according to the present invention. Alternating currents I Z1 , I Z2 , which carry along with them respective magnetic fields Φ Z1 , Φ Z2  surrounding conductors  11 ,  12 , flow in opposite directions through the respective conductors. A conductor loop  10  is located beneath conductors  11 ,  12  in such a way that magnetic fields Φ Z1 , Φ Z2  penetrate conductor loop  10 . A change in magnetic fields Φ Z1 , Φ Z2  resulting from a change in currents I Z1 , I Z2  induces in conductor loop  10  a voltage U I  which is an indication of the alternating current flowing in conductors  11 ,  12 . 
     FIG. 2 shows a cross section through a sensing device utilizing the principle described above with respect to FIG.  1 . Two conductive paths (e.g., conductors)  11  and  12  are applied at a spacing D 1  from one another onto a first surface  17 , designated here as the upper side, of a circuit board  13 . Applied onto the opposite surface  18 , designated here as the lower side, of circuit board  13 , is a conductor loop  10 . In order to magnify the measured signal, it is wound several times around itself as indicated by turns  101 ,  102 . Outside diameter D 2  of conductor loops  10 ,  101 ,  102  is matched to spacing D 1  of conductive paths  11 ,  12  on upper side  17  of circuit board  13 . It is selected, as a function of spacing D 1 , in such a way that the turns of conductor loops  10 ,  101 ,  102  are reliably penetrated by the magnetic field surrounding conductive paths  11 ,  12 . 
     Voltage U i  induced in conductor loop  10  is, as shown in FIG. 3 which reproduces a conductor loop  10  in a plan view, conveyed for amplification to an amplifier  14 . The amplified measured signal U i  is conveyed to first input  151  of a comparator  15 . Connected to the second input  152  thereof is a reference voltage U ref  which corresponds to a maximum value for the current changes dI Z1 /dt, dI Z2 /dt occurring in conductors  11 ,  12 . There is thus present at output  16  a signal which provides information as to whether the current changes dI Z1 /dt, dI Z2 /dt occurring in conductors  11 ,  12  lie within a predefined normal range or exceed a limit value. 
     FIG. 4 shows an application of the sensing device described above in a welding system operating according to the frequency converter principle. It shows at its input side a rectifier unit  20  for transferring a three-phase input alternating current L 1 , L 2 , L 3  into a direct-current link circuit constituted by the two conductors  11 ,  12 . The direct-current voltage present in link circuit (e.g., two conductors)  11 ,  12  is converted in the downstream inverter  21  back into a high-frequency alternating-current voltage which is then conveyed to a welding transformer unit  22 , from which it is applied via welding electrodes  23  onto a workpiece  1  being processed. Four power semiconductor switches  211  through  214 , each with associated drivers  215  through  218 , are the core of inverter  21 . To control power semiconductor switches  211  through  214 , drivers  215  through  218  are connected to a welding controller  24 . Also connected to welding controller  24  is an inductive conductor loop  10 , as described with respect to FIGS. 2 and 3. Conductor loop  10  is located on the upper side of a circuit board  25  on whose lower side, opposite conductor loop  10 , conductors  11 ,  12  forming the link circuit are guided. A circuit as shown in FIG. 3 is located inside welding controller  24 . In this application, the purpose of sensing device  10 ,  11 ,  12 ,  24 ,  25  is to detect the occurrence of overcurrents and, if applicable, bring about deactivation of power semiconductor switches  211  through  214 . The overcurrents detected are, in particular, those caused by short circuits in the welding circuit. 
     The overcurrent monitoring system illustrated in FIG. 4 can be transferred to many other circuits. It is also suitable, in particular, for frequency converter-driven drive arrangements, to detect therein, in timely fashion, overcurrents that may be caused by link circuit short circuits or short circuits in the motor winding or the motor supply lead. 
     Induction loop  10  supplies a signal of the form dI/dt that is proportional to the change over time in the measured current. If what is desired is to know the value of the current itself, this can easily be implemented by incorporating an integration member into the circuit.