Abstract:
According to the invention, a blocking oscillator type converter circuit functions with a transformator that only comprises a primary winding and a secondary winding. Said transformator does not comprise a return coupling winding for the blocking oscillator. The control voltage of the oscillator is derived from the primary voltage of the transformator during the free-wheeling phase. The invention also relates to a voltage monitoring circuit that works independently from the oscillator, the output voltage of the oscillator being repressed when the voltage on the output of the blocking oscillator is too high. A flow monitoring circuit functions independently from the oscillator and the voltage monitoring circuit and suppresses the impulses for the power transistor when the power on the output side exceeds a predetermined measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is the national phase of PCT/EP2008/009641, filed Nov. 14, 2008, which claims the benefit of German Patent Application No. 102007058614.2, filed Dec. 4, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to electrical power management, and more particularly to an electrical power supply with output flow monitoring. 
       BACKGROUND OF THE INVENTION 
       [0003]    In it increasingly important in electrical systems increase safety by providing additional redundancy. This is especially true in areas that pose a risk of explosion. In these areas, dangerous operating situations should be avoided as much as possible. 
         [0004]    With respect to the risk of explosions, dangerous conditions can occur when electrical currents and/or voltages that can generate an ignition spark appear on the lines leading into a dangerous zone. In addition, at appropriate power levels, there is also the risk that components can reach surface temperatures that can ignite an ignitable mixture. Both risks are increasing due to the increase in electrical loads, e.g., modern bus systems, that draw large levels of current. 
         [0005]    With this in mind, attention is placed on the power-supply devices and arrangements, so as to guarantee that, in addition to normally provided controls, there are mechanisms in place to rule out dangerous conditions even when an error occurs in the standard control loops. From the prior art, power-supply devices are known that operate according to the principle of a blocking-oscillator-type converter principle or a feed forward converter. These circuits use transformers with windings featuring galvanic separation and are used to increase or decrease the output voltage relative to the input voltage. These converter circuits contain reaction-coupling mechanisms in order to control the current and/or voltage on the secondary side. 
         [0006]    The converter circuits contain, on the primary side, at least one controlled semiconductor switch in series with the primary winding of the transformer, in order to generate the corresponding alternating current. 
         [0007]    Within this context, the problem of the invention is to create a power-supply arrangement with galvanic isolation, wherein this arrangement features increased safety. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0008]    According to one aspect of the invention, this problem is solved with a power-supply arrangement with the features of Claim  1 . According to the invention, a power-supply arrangement with galvanic isolation is created that is suitable for limiting the current on the secondary side. In this regard, a circuit variant is used in which the current flowing on the secondary side is detected indirectly by the relationships on the primary side. In the discharging process, the energy containing inductance is proportional to the current for a constant supply voltage as long as the saturation limit is not reached. Furthermore, by neglecting the control inductance, the current on the primary side is proportional to the current on the secondary side, so that, in the switched-off state, the voltage on the primary side is proportional to the current on the secondary side. 
         [0009]    The novel power-supply arrangement with galvanic isolation, in particular, in the explosion-proof construction, has a power-supply input. A series circuit made from the primary winding of a transformer and a controlled semiconductor lies parallel to the power-supply input. The primary winding of the transformer is galvanically isolated from the secondary winding. 
         [0010]    A controllable control circuit that delivers a periodic control signal for the semiconductor switch is allocated to the semiconductor switch, such that the output voltage or the current through this circuit is constant. A current limiting circuit is provided parallel to the semiconductor switch, and contains a capacitor lying parallel to the semiconductor switch. The capacitor likewise lies in series to the primary winding. A voltage monitoring circuit lies on the input side, in order to generate an output signal that acts on the switch state of the semiconductor switch such that the semiconductor switch is transitioned into the blocking state when the voltage on the capacitor exceeds a predetermined limiting value. 
         [0011]    In this regard, use is made of the fact that, after turning off the semiconductor switch, a free-running current flows that is, essentially, a mapping of the relationships on the secondary side. 
         [0012]    Thus, the current on the secondary side can be limited without additional current sensors, whose sensor signal is transmitted in a galvanically isolated way to the primary side of the transformer, being implemented on the secondary side. 
         [0013]    The measurement of the current on the secondary side is thus performed indirectly by means of a voltage measurement, which has the result that the arrangement features very low-power operation. Problems of temperature increase or internal resistance are thus avoided. 
         [0014]    According to the application, the number of turns of the secondary winding can be greater than or less than the number of turns of the primary winding. 
         [0015]    The controlled semiconductor switch can be formed by a MOSFET, for example, by a dual-gate MOSFET, so that the control is decoupled with high impedance by the current limiting from the control input for clocking the MOSFET. 
         [0016]    The controllable control circuit for generating the required clock signal can be formed by an astable multivibrator. The pulse cycle of the astable multivibrator is varied in the sense of a constant-current or constant-voltage regulator. This pulse cycle regulation can be realized at a constant clock frequency or with the help of a variable clock frequency, when the switched-on period is fixed. 
         [0017]    In one aspect of the invention, the current limiting circuit can also contain a low-pass filter in the voltage monitoring region, wherein this low-pass filter smoothes the signal applied at the series capacitor to the primary winding, in order to create a corresponding control signal for the voltage monitoring circuit. 
         [0018]    In a further aspect of the invention, the current monitoring circuit can contain a transistor, as well as a self-locking semiconductor switch element, for example, a thyristor that is controlled by means of the transistor. The operating section of the thyristor lies parallel to a control section of the control track semiconductor switch. 
         [0019]    In an aspect of the invention, it is advantageous if the arrangement works according to the principle of a blocking-oscillator-type converter. A half-wave rectifier circuit with a filter capacitor may also be connected to the secondary winding in one aspect. 
         [0020]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The FIGURE is a block circuit diagram of a circuit arrangement that is provided for powering electrical loads in an area at risk of explosions. 
       
    
    
       [0022]    While the invention is susceptible of various modifications and alternative constructions, a certain illustrative embodiment thereof has been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    The following description of the figures explains aspects to enable a better understanding the invention. A person skilled in the art can take additional details that are not described from the circuit diagram in a usual way, wherein this supplements the description of the FIGURE in this respect. It is clear that a series of modifications to the circuit are possible. 
         [0024]    A person skilled in the art can easily perform the exact dimensioning of the individual components with reference to the given functional explanation. Moreover, the circuit diagram is simplified and shows only elements that are required for understanding the invention. 
         [0025]    The sole FIGURE of the drawing shows the circuit diagram of the circuit arrangement according to the invention. The FIGURE shows the block circuit diagram of a circuit arrangement that is provided for powering electrical loads in an area at risk of explosions. Examples for such applications are bus systems. Here, a relatively high current flows for a relatively low voltage. For safety reasons, it must be ensured that the current may not assume impermissibly high values on the power-supply line led into the area at risk of explosions, even if a load fails and would draw a current that is too high. On the other hand, the devices also may not be put at the risk of too high a feed voltage being generated due to a loss of a load on the secondary side. 
         [0026]    Normally, these boundary conditions are managed by the internal control system. In the case of the circuit arrangement as shown in the FIGURE, additional redundancy is also provided that intervenes if the normal control should fail. 
         [0027]    The circuit arrangement has two input terminals  1  and  2  that together form a power-supply input  3 . The electrical energy is output on output terminals  4  and  5  that together represent a power-supply output  6 . The power-supply input  3  is isolated galvanically from the power-supply output  6  by means of a transformer  7  that includes a primary winding  8  and a secondary winding  9 . The required alternating voltage or the required alternating current through the primary winding  8  is generated with the help of a MOSFET  11 . No free-running diode is connected in parallel to the primary winding  8 . 
         [0028]    A redundant voltage limiting circuit  12 , as well as a current limiting circuit  13 , is connected to the primary winding  8 . The redundancy explained above is attained with both circuit arrangements. In addition, an oscillator  14  is present. 
         [0029]    The primary winding  8  and the MOSFET  11  form a series circuit that is connected in parallel to the inputs  1  and  2 . Here, the source lies on the input terminal  2  and the drain is connected to the primary winding. The MOSFET  11  provides a gate  15 . By means of a protective resistor  16 , the gate  15  lies on the oscillator  14  in the form of an astable, regulated multivibrator with variable pulse cycle. The secondary winding  9  whose sense of winding is designated with dots opposite the primary winding  8  is connected to a half-wave rectifier circuit. This comprises a rectifier diode  17  that connects one end of the secondary winding  9  to a filter capacitor  18  whose other connection leads back to the secondary winding  9 . 
         [0030]    The filter capacitor  18  thus lies parallel to the two output terminals  4  and  5  to which the load is connected. As can be seen, the rectifier arrangement involves a half-wave rectifier arrangement. The MOSFET  11  is operated so that, as a whole, a blocking-oscillator-type converter circuit is produced, i.e., the energy transfer within the transformer  7  occurs during the power-off phase of the MOSFET  11 . 
         [0031]    The circuit arrangement operates in a controlled way in that the pulse cycle with which the MOSFET  11  is controlled is re-adjusted accordingly. The voltage monitoring circuit  12  has the purpose of ensuring that the output voltage of the secondary winding  9  is forcibly limited independent of the normally active control. The voltage limiting circuit  12  contains a storage capacitor  19  that is connected in parallel to the primary winding  8  via a decoupling diode  21 . The arrangement is made so that the anode of the diode  21  is connected to the drain of the MOSFET  11 , while the capacitor  19  is connected to the terminal  1  with its end facing away from the diode  21 . 
         [0032]    A series circuit made from two ohmic resistors  22  and  23  lies parallel to the capacitor  19 . In addition, a bipolar PNP transistor  24  and also a bipolar NPN transistor  25  belong to the voltage limiting circuit  12 . With its emitter, the transistor  24  contacts the cathode of the diode  21 , while the collector is connected via a resistor  26  to the circuit ground  2  and a Z-diode  27  is connected to the base of the transistor  25 . With its main section terminals, a shunt regulator  28  lies between the base of the transistor  24  and the hot end of the primary winding  8  or the input terminal  1 . The control terminal of the shunt regulator  28  is connected to the node between the two ohmic resistors  22  and  23 . In this way, the two resistors  22  and  23  define the switching threshold above which the shunt regulator  28  becomes conductive. 
         [0033]    With its collector, the transistor  25  is finally connected to the gate  15 , while the emitter lies on the source of the MOSFET  11  or the input terminal  2 . The current limiting device  13  comprises a capacitor  28  that lies parallel to the power section of the MOSFET  11 , i.e., the capacitor  29  lies, on one end, on the source and, on the other end, on the drain. Furthermore, an RC element consisting of a resistor  30  and a capacitor  31  is connected in parallel to the capacitor  28 . 
         [0034]    The control section of a bipolar PNP transistor  32  whose emitter is connected to the drain of the MOSFET  11  lies parallel to the resistor  30 . A thyristor  33  whose control terminal is connected to the collector of the transistor  32  is controlled by means of the collector of the transistor  32 . The anode of the thyristor  33  lies on the gate  15  and the cathode is connected to the source of the MOSFET  11 . The thyristor  33  thus lies parallel to the power section of the MOSFET  11 . 
         [0035]    The circuit described in this respect works as follows. In normal operation, the control circuit for the MOSFET  11  ensures that this is charged with a square pulse by means of which the MOSFET  11  is periodically switched on and off via the gate  14 . In the conductive state, the MOSFET  11  generates a current through the primary winding  8 . In the power-off state, the magnetic energy stored in the primary winding  8  during the conductive phase of the MOSFET  11  is transmitted to the secondary winding  9 , so that the connected load is supplied with current. 
         [0036]    By varying the pulse cycle of the pulse led into the gate  15 , it is ensured that the voltage across the filter capacitor  18  is held constant for a given or variable load on the output terminal  6 . During the free-running phase, i.e., when the MOSFET  11  is switched off, the current through the primary winding  8  and the voltage appearing there reflect the current and voltage situation on the secondary winding  9 . 
         [0037]    In normal operation, during the free-running phase a voltage is produced on the primary winding  8  and this voltage results in that the filter capacitor  19  that acts as an integrator is charged by means of the diode  21 . The voltage produced on the capacitor  19  is stepped down by means of the divider resistors  22 ,  23  to a corresponding value, like that given from the following function description. In normal operation, the voltage is sufficiently high so that the shunt regulator  28  becomes conductive and thus generates a base current for the transistor  24 . The transistor  24  controlled in this way generates a voltage drop on the resistor  26  and this voltage drop is a mapping of the voltage on the secondary side of the transformer  7 . In normal operation, the Z-diode  27  remains blocked. Only if the voltage should become too high due to interference on the secondary side and interference of the oscillator  14  and if the switching threshold of the Z-diode  27  is exceeded, then a base current for the transistor  25  is also generated. The transistor  25  is turned on and the gate  15  of the MOSFET  11  short-circuits to ground. The MOSFET  11  is thus blocked. The blocking state remains until the voltage on the capacitor  19  together with the voltage on the input terminals  1 ,  2  reaches a value that is less than the threshold value of the Z-diode  27 . 
         [0038]    As can be seen from the drawing, the switching on of the transistor  24  is independent of the input voltage on the terminals  1 ,  2 . The conductive state of the transistor  24  is regulated exclusively by means of the voltage on the integration capacitor  19  connected to the shunt regulator  28 . However, the input voltage is incorporated into the control voltage for the transistor  25 . 
         [0039]    If no control voltage for the oscillator  14  is to be derived from the current that the transistor  24  delivers, the Z-diode  27  can be eliminated and the divider resistors  22 ,  23  are dimensioned so that the shunt regulator  28  opens only when the threshold for the permissible voltage on the primary winding  9  is exceeded, which corresponds to a corresponding overvoltage on the secondary winding  8  in the free-running state. 
         [0040]    After a specified time that is ultimately given from the time constant that the capacitor  19  produces in connection with the two resistors  22 ,  23 , as well as the collector current of the transistor  24 , the voltage drops below the threshold voltage of the Z-diode  27  and the transistor  25  transitions into the blocking state, so that clock pulses can be led into the MOSFET  11  again. The blocking oscillator resumes its function. If the overvoltage should appear again, the just explained interplay is repeated. 
         [0041]    For correspondingly smaller time constants of the control side of the transistor  24 , this intervention can also disappear within a pulse with which the MOSFET  11  is switched to the conductive state. In this case, the astable multivibrator  14  with which the input signal for the MOSFET  11  is generated can have a constant frequency and a constant pulse cycle. The pulse length is then ultimately set by the voltage limiting circuit  12 , in that the incoming pulse is periodically suppressed by means of the voltage limiting circuit  12 . 
         [0042]    As already indicated above, during the blocking phase, the profile of the voltage on the primary winding represents a measure for the current flowing on the secondary side. During the switch-on phase of the current through the primary winding  8 , magnetic energy is fed into the transformer  7 . The fed energy is proportional to the inductance of the primary winding and the time period over which the MOSFET  11  is turned on. 
         [0043]    During the switch-off phase, the energy is transmitted into the secondary coil, wherein the current that the secondary coil outputs to the load exhibits a characteristic time profile during the free-running time in connection with the capacitor  28  lying in series. The larger the current is, the larger the end-of-charging voltage becomes on the capacitor  28  before the MOSFET  11  transitions back into the conductive state. If an excess current drain appears, the voltage on the capacitor  28  will become so large that it is larger than the base-emitter voltage of the transistor  32  plus the voltage on the control section of the thyristor  33 . Therefore, the transistor  32  will conduct and thus also the thyristor  33 . Because the thyristor  33  lies parallel to the control section of the field-effect transistor  11 , the gate  15  here short-circuits to the source and switches off the transistor  11 . In this way, the possibility is given to the MOSFET  11  of switching on the current through the primary winding  8  again. 
         [0044]    The resistor  29  works, incidentally, in connection with the capacitor  31  as a low-pass filter, in order to form an average value of the profile of the voltage appearing on the capacitor  28 , because this is immediately and abruptly discharged when the MOSFET  11  is switched on. The oscillator  14  works as an astable multivibrator that is voltage-controlled. As an active assembly, it contains a differential amplifier  35  with an inverting input and a non-inverting input. The non-inverting input is connected by means of a reaction coupling resistor  36  to the output of the differential amplifier  35 , in order to generate Schmitt trigger response. 
         [0045]    A negative feedback resistor  37  connects the output to the inverting input that is further connected, incidentally, by means of a filter capacitor  38  to the circuit ground corresponding to the terminal  2 . The output of the differential amplifier  35  is connected to the gate  15  of the MOSFET  11 . The oscillator  14  receives the control voltage in that the non-inverting input of the differential amplifier  35  is connected via a resistor  39  to the collector of the transistor  24 . 
         [0046]    The voltage on the resistor  39 , where it is connected to the collector of the transistor  24 , thus corresponds to the sum of the voltages applied to the input  3  plus the voltage on the capacitor  19 . The function of such an astable multivibrator  14  is known to those of skill in the art and therefore will be explained only as a whole. In the oscillating state, when the differential amplifier  35  is turned on, the capacitor  38  is charged via the resistor  37 . As soon as this voltage above the control voltage is applied to the non-inverting input, the differential amplifier  35  is turned off, which produces Schmitt trigger characteristics with the help of the resistor  36 . When the differential amplifier  35  is turned off, the capacitor  38  is discharged from the output of the differential amplifier  35  via the resistor  37 . The input-output voltage switches on again as soon as the voltage on the capacitor  38  has dropped far enough. This series if events repeats itself periodically. 
         [0047]    Because the integration element made from the resistor  37  and the capacitor  38  is constant, the pulse cycle changes according to the input voltage applied to the non-inverting input. It will be appreciated that the oscillator  14  that controls the power semiconductor, in order to periodically turn on and off the primary current into the transformer  7 , is driven with the help of an oscillator  14  that likewise receives its control voltage from the primary winding  8 . Thus a third winding on the transformer  7  can be eliminated, in order to control the blocking oscillator by means of such a reaction coupling winding, as is sufficiently well known from the prior art. In the case of the blocking oscillator according to the invention, the control voltage is derived directly from the voltage. 
         [0048]    The circuit explained herein according to the invention contains, as a whole, three units in the form of the voltage monitoring device  12 , the current monitoring device  13 , and the astable multivibrator  14 . These units are independent of each other in so far as the circuit could be constructed only with the oscillator  14  without the current monitoring device  13  and without the voltage monitoring device  12 . 
         [0049]    Alternatively, the oscillator  14  may be replaced by an oscillator that works with a reaction coupling winding on the transformer  7 . On such an oscillator circuit, the voltage monitoring circuit  12  and/or the current monitoring circuit  13  can be used independently of each other. The determination as to which of these circuits to use may be made based on the desired degree of safety. 
         [0050]    A blocking-oscillator-type converter circuit works with a transformer that has only one primary winding and one secondary winding. The transformer contains no reaction coupling winding for the blocking oscillator. 
         [0051]    The control voltage for the oscillator is derived from the primary voltage of the transformer, that is, during the free-running phase. Furthermore, a voltage monitoring circuit is provided that works independent of the oscillator, suppressing the output voltage of the oscillator when the voltage on the output of the blocking-oscillator-type converter becomes too large. 
         [0052]    A current monitoring circuit works independent of the oscillator and the voltage monitoring circuit and suppresses the pulses for the power transistor when the current on the output side exceeds a specified amount. The monitoring circuits work practically without power, so that no special cooling measures are required. 
         [0053]    It will be appreciated that the foregoing description provides useful examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for the features of interest, but not to exclude such from the scope of the disclosure entirely unless otherwise specifically indicated. 
         [0054]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. For example, the illustrated calibration steps may optionally be executed in reverse order, and other alternative orders and steps may be practicable where logically appropriate without departing from the described principles.