Abstract:
The invention relates to an electronic power supply device, particularly a switched-mode power supply, for supplying power to a low-voltage load protected by a protective device, and to a method therefor. The invention also relates to a device for protecting a low-voltage load against an excess current and to an auxiliary power supply device for use with such a protection device. The conceptual core of the invention can be seen in providing measures which ensure that after a fault has been detected, for example a short circuit at the output, a current is supplied for a short period, for example 15 ms, which is of such a magnitude that a protective device can be reliably and quickly tripped. The period for this is selected in such a manner that electronic components, connected loads and feedlines are not damaged and destroyed.

Description:
FIELD OF THE INVENTION 
   The invention relates to an electronic power supply device, particularly a switched-mode power supply, for supplying power to a low-voltage load protected by a protective device, and to a method therefor. The invention also relates to a device for protecting a low-voltage load against an over-current and to an auxiliary power supply device for use with such a protection device. 
   BACKGROUND OF THE INVENTION 
   In industrial plants, low-voltage loads such as, e.g. controllers, amplifiers and the like, are supplied with a direct voltage of preferably 24 Volts, which is harmless to persons. Suitable power supply devices which provide such a direct voltage can supply output currents of 20 A and more. At such high currents, protection devices such as, e.g. fuses or circuit breakers, must be connected in series with the respective loads in order to protect these and, in particular, the feedlines, against thermal overload and short-circuit currents. In order to be able to trip circuit breakers reliably magnetically in the event of an electrical fault, for example a short circuit, tripping currents are required which amount to about 7.5-times the nominal current specified with regard to the power supply device. The tripping characteristic of circuit breakers is the result of their time/current tripping characteristic such as, for example, the class-B characteristic. With the usual dimensioning, such high tripping currents for circuit breakers can be supplied in the event of a short circuit by traditional 50 Hz transformers used as power supply devices. 
   Due to high electrical losses and the great weight, such 50 Hz transformers are more and more frequently replaced by electronic power supply devices such as, e.g. switched-mode power supplies and transformer power supplies clocked at high frequency in industrial power supplies. However, electronic power supply devices usually is limit the output current very rapidly when an electrical fault occurs, that is to say within between 10 and 100 μsec, to 1.1- to 1.5-times the value of the nominal current in order to protect loads and feedlines against thermal overloads and short-circuit currents. Although electronic power supply devices are on the market which can also generate an output current of up to 2.5-times the nominal current for a short time, that is to say up to 4 seconds, these low currents are not adequate for reliably and safely tripping circuit breakers magnetically. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is based on the object of providing an electronic power supply device, a protection device, an auxiliary power supply device and a method for supplying power to a low-voltage load, protected by a protective device, which enable protective devices, particularly electromagnetic circuit breakers, to be rapidly tripped even when electronic power supply devices are used. 
   The conceptual core of the invention can be seen in providing measures which ensure that after a fault has been detected, for example a short circuit at the output, a current is supplied for a short period, for example 15 ms, which is of such a magnitude that a protective device can be reliably and quickly tripped. The period for this is selected in such a manner that electronic components, connected loads and feedlines are not damaged and destroyed. 
   Accordingly, an electrical power supply device, is particularly a switched-mode power supply or a transformer current device clocked at high frequency is provided for supplying power to a low-voltage load. The electronic power supply device comprises a transformer, a device for detecting an electrical incident or fault such as, for example, a short circuit, and a device allocated to the detecting device for limiting the output current of the device to a first predetermined value, i.e. approximately to 1.1- to 2-times the nominal current of the power supply device. Furthermore, a device is provided which, when responding to the detection of a fault, sets the output current to a second predetermined value which is greater than the first predetermined value for a predetermined period, in such a manner that a protective device allocated to the electrical power supply device can be reliably tripped. The limiting device is constructed in such a manner that it limits the output current to the first predetermined value after the predetermined time has elapsed. 
   The protective or protection device is connected externally to the electronic power supply device in a manner known per se and is thus connected in series with the low-voltage load. To be able to implement a compact electronic power supply device for supplying power to a protected low-voltage load, the protective device can also be arranged in the electronic power supply device. 
   The protective device is preferably a circuit breaker which can be electromagnetically tripped. 
   To achieve safe and reliable tripping of the protective device in the event of a fault, the current setting or adjusting device supplies an output current which is approximately between 5- to 10-times the nominal current of the power supply device. As a rule, the limiting device supplies a first predetermined current value which is approximately between 1.1- to 1.5-times the nominal current of the electronic power supply device. 
   To prevent the low-voltage load and the feedlines from being overloaded in the event of a fault, for example of a short circuit, the current setting device supplies an increased output current, for instance, for 5 to 15 ms. The increased output current can also flow for a shorter or longer time. 
   Considered as a circuit, the current setting device providing the increased output current, together with the limiting device, can form a current control device. In this case, the solution to the above-mentioned technical problem can also be seen in that the current control device used normally in an electronic power supply device has a two-stage current limiting characteristic. The current control device then ensures that when an electrical fault occurs, an increased output current of, for example, 7-times the nominal current initially flows for a short time and then the “normal” limited output current with about 1.1-times the nominal current flows. 
   The detecting device for detecting an electrical fault preferably comprises a first detector for detecting a drop in the output voltage below a threshold value and/or a second detector for detecting a drop in the input voltage below a threshold value. This makes it possible to detect a short circuit because, as is known, the voltage drops is when a short circuit occurs. 
   The above-mentioned technical problem is also solved by a device for protecting a low-voltage load against an excess current. Accordingly, the protection device exhibits a main power supply device which has a transformer, a device for detecting an electrical fault and a device allocated to the detecting device for limiting the output current when responding to a detected fault. Furthermore, the main power supply device is associated with an auxiliary power supply device which can be switched in and which exhibits a device for detecting an electrical fault, particularly the drop in the output voltage of the main power supply device below a threshold value, and a device for providing a predetermined current for an adjustable time. Furthermore, a protective device which can be electrically connected to the auxiliary power supply device is provided, the current provided by the auxiliary power supply device being dimensioned in such a manner that when a fault is detected, the protective device is reliably tripped. 
   According to a particular embodiment, the auxiliary power supply device is implemented in the main power supply device or is connected externally to the main power supply device. 
   The auxiliary power supply device can be any energy store but preferably at least a capacitive and/or inductive energy store. 
   In order to automatically connect the auxiliary power supply device to the main power supply device when a fault occurs, the auxiliary power supply device exhibits a switching device which is activated when responding to a detected fault, particularly to the drop in the output voltage of the main power supply device below a threshold value. 
   An advantageous development provides that the auxiliary power supply device exhibits a device for charging the capacitive and/or inductive energy store. The charging device is constructed in such a manner that it charges the energy store at the times at which no fault has occurred. 
   The main power supply device is preferably a switched-mode power supply. 
   The detecting device for detecting an electrical fault preferably comprises a first detector for detecting a drop in the output voltage below a threshold value and/or a second detector for detecting a drop in the input voltage below a threshold value. This makes it possible to detect a short circuit. 
   The above-mentioned technical problem is also solved by means of an auxiliary power supply device for use with a protection device. The auxiliary power supply device comprises a device for detecting a drop in the output voltage of a main power supply device below a threshold value and a device which, when responding to a detected drop in the output voltage, supplies a predetermined current for an adjustable time so that a protective device can be reliably tripped. 
   Accordingly, a load protected by a protective device is supplied with a low voltage with the aid of an electronic power supply device, particularly a switched-mode power supply. According to the method, the input and/or output voltage of the power supply device is monitored in order to detect a drop in the input and/or output voltage below a threshold value. If a drop in the input and/or output voltage below the threshold value is detected, a current is provided for a predetermined time, the magnitude of which is dimensioned in such a manner that the protective device can be reliably tripped. After the predetermined time has elapsed, the current is limited to a lower value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the text which follows, the invention will be explained in greater detail by means of a number of exemplary embodiments, in conjunction with the attached drawings, in which: 
       FIG. 1  shows a power supply system for protectively supplying a low-voltage load with power according to a first embodiment, 
       FIG. 2  shows a two-step current limiting characteristic for the current controller shown in  FIG. 1 , 
       FIG. 3  shows an alternative power supply system for feeding a low-voltage load in a protected manner, 
       FIG. 4  shows a capacitive auxiliary power supply device which is used in a power supply system according to  FIG. 3 , and 
       FIG. 5  shows an inductive auxiliary power supply device which is used in a power supply system according to  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an exemplary power supply system comprising an electronic power supply device  10 , at the output terminals  20  and  21  of which a low-voltage load is connected which is symbolically represented by a resistor  30 . The resistance of the feedline to the low-voltage load  30  is taken into consideration by the resistor  40 . Furthermore, a protective device, an electromagnetic circuit breaker  50  in the present example, is connected in series with the low-voltage load  30 . The electronic power supply device  10  provides, for example, a direct voltage u a  of 24 V at the output terminals  20 ,  21 . Although only one low-voltage load  30  is connected to the electronic power supply device  10  in  FIG. 1 , a number of loads can naturally be connected preferably in parallel to the electronic power supply device  10 . A separate circuit breaker can then be allocated to each load. 
   The electronic power supply device  10  can be a switched-mode power supply which is supplied via a line voltage U N . The power supply device  10  conventionally contains a line transformer  60  associated with the input terminals  70  and  71 , a rectifier circuit  70  and a circuit breaker  80 . Furthermore, a current controller  90  is provided which can be modified in accordance with the invention. The current controller  90  is normally used for limiting the output current i a  to 1.1- to 1.5-times the nominal current of the power supply device  10  in the event of a fault, for example a short circuit at the output. This limited output current is called i kmin  in conformance with the current/voltage output characteristic of the modified current controller  90 , shown in  FIG. 2 . 
   The circuit breaker  50  is used for protecting the feedline, represented by the resistor  40 , and the low-voltage load  30  against thermal overload or short circuit currents. A problem of conventional electronic power supply devices consists in that, in the event of a fault, for example in the event of a short circuit, the output voltage u a  of the power supply device  10  can collapse so that the limited output current i kmin  normally supplied by the current controller  90  is not sufficient for magnetically tripping the circuit breaker  50 . 
   It is thus the aim of the invention to modify a conventional power supply device in such a manner that, in the event of a fault, especially in the event of a short circuit at the output, it can supply an output current which is about 5- to 7-times the nominal current for a conformance with the current/voltage output characteristic of the modified current controller  90 , shown in  FIG. 2 . Furthermore, the power supply device  10  must be constructed in such a manner that the excessive output current i kmax  may only flow so long that the feedline  40  and the low-voltage load  30  are not damaged. 
   For this purpose, according to a preferred embodiment, the current controller  90  is modified in such a manner that it has a 2-step characteristic which has the variation shown in  FIG. 2 . For this purpose, the power supply device  10  is implemented in such a manner that, in the event of a fault, particularly of a short circuit, it holds the output voltage at the operating voltage u a  for a short time—for instance 5 to 15 ms—in order to be able to provide the current i kmax . This output current i kmax  is required for being able to trip the circuit breaker  50  reliably and quickly. The time during which the increased output current i kmax  flows can be adjusted by corresponding RC elements. This time can also be selected to be shorter or longer depending on the dimensioning of the circuit breaker  30 , the low-voltage load and the feedlines. After this period has elapsed, the current limiter  90  enters into its usual protective mechanism and limits the output current i a  to the output current i kmin  which, as mentioned, approximately corresponds to 1.1- to 1.5-times the nominal current. This functionality is shown diagrammatically by the function block  95  in  FIG. 1 . 
   To implement the characteristic shown in  FIG. 2 , a large variety of circuit modifications with regard to the electronic power supply device  10  and especially the current controller  90  are conceivable. The two-step current/voltage output characteristic can be achieved, for example, by widening the analogue controlled system of a conventional current controller by additional operational amplifiers for limiting the output current to the value i kmax  and by integrating further delay elements which ensure that the limiting of the output current is reduced again to the usual value i kmin  in the event of a fault. Since electronic power supply devices also operate with microprocessor control today, the current/voltage output characteristics shown in  FIG. 2  can also be implemented by suitable software. In this arrangement, the nominal signal for the maximum output current i kmax  is generated, as a rule, by a microprocessor. 
   The low-voltage load  30  and the connected lines are not thermally overloaded by the current pulse of magnitude lasting only a few milliseconds. The components of the power supply device  10  such as, for example, the switch  80  and the rectifier diodes of the rectifier circuit  70 , are also selected in such a manner that a short-time increase in current does not entail any significant thermal loading of the components and cabling. 
   If the current controller  90  is controlled periodically, however, the components can be overloaded by the excessive short-circuit current i kmax  due to the periodic loading of the components. The periodic trigger times for the current controller  90  can be correspondingly restricted or controlled, for example, by a microprocessor, which is not shown. 
   In the present example, a fault case, for example a short circuit at the output, is detected by means of a detector  100  connected to the output terminals  20  and  21 . The detector  100  is implemented in such a manner that it can monitor the output voltage u a  of the power supply device  10  and can inform the current controller  90  when the output voltage u a  has fallen below a predetermined threshold value. The input voltage u N  of the power supply device  10 , present at the input terminals  70  and  71 , is preferably also monitored by means of a detector  105 . The detector  105  is constructed, for example, in such a manner that it can detect when the input voltage u N  drops below a threshold value. As soon as the detector  105  detects that it has dropped below the threshold value, the current controller  90  is activated. 
   In the text which follows, the operation of the electronic power supply device  10 , shown in  FIG. 1 , is explained in greater detail in conjunction with  FIG. 2 . 
   Let it be assumed that the voltage detector  105  has detected a short circuit in the power supply at the input of the power supply device  10  since the input voltage u N  has dropped below the threshold value set. The detector  105  thereupon activates the current controller  90  which provides the output current i kmax  in accordance with its current/voltage output characteristic. The output current i kmax  is provided for about 5 to 15 ms and ensures that the electromagnetic circuit breaker  50  is reliably tripped as a result of which the low-voltage load  30  is disconnected from the power supply device  10 . Following this, the normal control operation of the current controller  90  starts which limits the output current to the value i kmin . 
     FIG. 3  quite generally shows an alternative system for supplying power to a low-voltage load  30 , identical reference symbols being used for components which correspond to the components shown in  FIG. 1 . Accordingly, a conventional electronic power supply device  110  is again connected with its input to a line voltage u N . At the output terminals  20  and  21  of the electronic power supply device  110 , the circuit breaker  50  and a low-voltage load  30  are connected in series. The feedline to the low-voltage load is symbolically shown by the feedline resistance  40 . The alternative system shown in  FIG. 3  is again intended for achieving the aim that, in the event of an electrical fault, particularly of a short circuit, the circuit breaker  50  can be tripped quickly and reliably. It should first be mentioned that the electronic power supply device  110 , known per se, exhibits a line transformer, a rectifier circuit, a circuit breaker and a corresponding current controller as current limiter, as already shown in  FIG. 1 . Furthermore, detectors similar to the detectors  100  and  105  shown in  FIG. 1  can be provided in the electronic power supply device which detects electrical faults, for example short circuits. To be able to ensure a reliable magnetic tripping of the circuit breaker  50  in the event of an electrical fault, an auxiliary power supply device  120  is connected in parallel with the output terminals  20  and  21  of the electronic power supply device  110 . Shown diagrammatically, the auxiliary power supply device  120  contains an energy storage device  121  which can have one or more capacitors and/or inductances, a charging device  122  for charging the energy storing device  121  and a switching device  123  which, when an electrical fault is detected, discharges the store  121  in order to be able to generate for a short time a summation current i s  of such magnitude that the circuit breaker  50  is tripped. The summation current i s  is preferably 5- to 7-times the nominal current of the power supply device  110 . 
     FIG. 4  shows a technical implementation of the auxiliary power supply device  120  shown in  FIG. 3 . Accordingly, the auxiliary power supply device  120  exhibits a diode  130  as switching device. A capacitor  132  is connected in parallel with the input terminals via a switch  131 . A coil  134  is connected in parallel with the capacitor  132  as energy store via a further switch  133 . The coil  134  can be discharged via the diode  130  by means of a switch  135 . In the text which follows, the operation of the arrangement shown in  FIG. 3  is explained in greater detail in conjunction with the auxiliary power supply is device  120  shown in  FIG. 4 . 
   In normal operation, the switch  135  is closed. The switch  131  is closed and opened in mutual interaction with the switch  132  so that initially the capacitor  132  is charged up via the output voltage u a  of the power supply device  110  and is then discharged into the coil  134  by closing the switch  133 . The closed switch  135  ensures that the coil current only flows in the auxiliary power supply device  120 . A detector  136  connected between the input terminals of the auxiliary power supply device  120  monitors the output voltage u a  of the power supply device  110 . As soon as the detector  136  detects that the output voltage of the power supply device  110  has dropped below a threshold value, the detector  136  initiates the opening of the switch  135  so that the coil  134  can be demagnetized as a result of which the stored energy can flow off to the circuit breaker  30  as auxiliary current i h . In response to the voltage drop at the output terminals  20  and  21 , the current controller  90  of the power supply device  110  limits the output current i a  to 1.1- to 1.5-times the nominal current in a manner known per se. Due to the auxiliary power supply device  120 , however, a summation current i s , which is formed by the limited output current i kmin  and the auxiliary current i h  of the auxiliary power supply device  120 , is provided for a short time. The summation current i s  is sufficient for reliably tripping the circuit breaker  30 . After about 15 ms, the switch  135  of the auxiliary power supply device  120  is closed again and the coil  134  is correspondingly charged up. If necessary, the auxiliary power supply device  120  can be cyclically switched in. 
   The auxiliary power supply device  120  shown in  FIG. 3  can also be implemented by a capacitive energy store. For this purpose,  FIG. 5  shows a corresponding circuit arrangement. Accordingly, a capacitor  140  is connected to the output terminals  20  and  21  of the power supply device  110  via a diode  141 . The feedline to the capacitor  140  is shown symbolically by a feedline resistor  142 . A charging device  143  is used for correspondingly charging the capacitor  140 . To be able to provide the currents required for reliably tripping the circuit breaker  50 , capacitors having a high capacitance, for example with a capacitance of about 350 F are used. The diode  141  is used as controlled switch which ensures that the capacitor  140  is discharged as soon as the output voltage u a  of the power supply device  110  becomes less than the voltage present across the capacitor  140 . The diode  141 , the cathode of which is connected to the output terminal  20  of the power supply device  110  and the anode of which is connected to the capacitor  140 , thus ensures that when a short circuit is detected at the output of the power supply device  110 , the capacitor  140  is automatically connected to the power supply device  110 . At the same time, the diode  141  prevents the capacitor  140  from being discharged in normal operation. It is important to point out that the two circuit variants shown in  FIGS. 4 and 5  of the auxiliary power supply device  120  shown in  FIG. 3  are constructed in such a manner that, after an electrical fault has been detected, they are connected to the power supply device  110  for a short time, that is to say, for instance for 5 to 15 ms, in order to produce a summation current i s  which, for instance, is 5- to 7-times the nominal current of the power supply device  110  in order to ensure that the circuit breaker  50  reliably trips in the event of a fault.