Patent Publication Number: US-8975931-B2

Title: Circuit configuration and method for limiting current intensity and/or edge slope of electrical signals

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit configuration and a method for limiting the current intensity and/or the edge slope of electrical signals. 
     2. Description of the Related Art 
     Electrical switching elements, such as drive components, usually have a fixed, unchangeable edge slope, which may not be adapted to a desired application case in optimum fashion. For example, when standard components are used or standard assemblies, it may not always be possible to influence an internal drive current of the respective standard element. Even a specified short circuit limitation, to the extent that a switching element used has one, may not always be adapted optimally to a special, desired application case. 
     Too high an edge slope of a switching process or clock-pulsed signals may, among other things, exceed EMC emission boundary values (electromagnetic compatibility) and thus injure it. 
     Furthermore, too high a current limitation set or even a missing current limitation in a fault case, for instance, in a short circuit state, may bring about an overload of a component or an assembly, and potentially damage it thereby. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, a circuit configuration is provided for the limiting of current intensity and/or edge slope of electrical signals, having a voltage source and a switching element that is connected to the voltage source, and which is equipped for switching the voltage source, wherein the switching device further has a limiting unit, the limiting unit being functionally positioned between the switching element and the voltage source, and the limiting unit being equipped to limit the current intensity and/or an edge slope of an electrical signal in response to a switching process of the voltage source while using the switching element. 
     The circuit configuration according to the present invention may, on the one hand, provide a current limitation, and on the other hand, an edge limitation. The circuit configuration may particularly be implemented using few standard components, and may thus be implemented cost-effectively and having low space requirement, for instance, on a printed-circuit board. Known driver circuits may be broadened, especially without major systems interventions, by a circuit configuration for limiting the current intensity and/or the edge slope according to the present invention. This may be implemented particularly in that the internal drive signals of a driver circuit do not have to be influenced. 
     The limitation of short circuit current and an edge slope for a desired application case may be adjustable by the selection of suitable component values of the circuit configuration, easily parameterizable to the desired application case. A readily developed standard layout of an electronic circuit may be adaptable to a certain application case by the simple reassembling of the respective component values to the desired requirements. Thus, a desired short circuit current and a required edge slope, and consequently an EMC boundary value, is able to be set simply and flexibly by the selection of the component values. 
     By such a flexibility, for instance, in an EMC optimization to be carried out, an edge slope that is occurring may very rapidly and simply be adapted to a required edge slope. In particular, it may thus not be necessary extensively to redesign an already existing or developed switching component, in order to correspond to a changed EMC emission boundary value for a new application. Consequently, a lengthy, cost-encumbered and risk-encumbered reworking of a switching assembly may be omitted. 
     The circuit configuration may, however, also be able to be used itself as a driver stage, so that it is able to be actuated directly via a logic signal, in a space-saving and cost-saving manner. 
     In particular, the circuit configuration may provide a targeted effect on a steeply dropping signal edge at the low-side outputs, without smoothing away a rising signal edge. Furthermore, the circuit configuration, according to the present invention, may provide a small voltage drop in the switched-on state of the switching element, at the latter. 
     The circuit configuration of the present invention may generally be used together with a switching element, implemented, for instance, as an open collector output, an open drain output or a relay. In particular, it may provide a current intensity and/or an edge slope limitation for both slowly changing and for clocked signals. 
     It may be particularly advantageous to limit the current intensity and/or the edge slope of the output voltage of the voltage source. Based on the possibly large amplitudes that go along with this, a limiting unit acting on the voltage source may be particularly suitable for maintaining required EMC boundary values. 
     Moreover, between limiting element and voltage source, there may be situated a capacitive element and/or a diode element, which is positioned parallel to the switching element, the limiting element and the voltage source. 
     The edge slope may further be able to be set in an improved manner by such an element, and it may simultaneously be used as a protective circuit or a protective element for the limiting element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first exemplary specific embodiment of a circuit configuration for limiting current intensity and/or the edge slope of electrical signals, according to the present invention. 
         FIG. 2  shows a further exemplary embodiment of a circuit configuration for limiting current intensity and/or the edge slope of electrical signals, according to the present invention. 
         FIG. 3  shows a further exemplary embodiment of a circuit configuration for limiting an edge slope of electrical signals, according to the present invention. 
         FIG. 4  shows an exemplary signal curve of the circuit configuration according to  FIG. 1 . 
         FIG. 5  shows an exemplary specific embodiment of a method for limiting current intensity and/or the edge slope of electrical signals, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a first exemplary specific embodiment of a circuit configuration for limiting current intensity and/or the edge slope of electrical signals, according to the present invention. 
     Circuit configuration  100 , in this instance, is made, as an example, of an npn transistor  118 , whose base is connected to a fixed potential  108 . The connection may take place, for instance, via resistor  106  or via a voltage divider. The emitter of element  118  is connected to switching element  102  via a component  104  having a resistor. Switching element  102  is that element which, viewed overall with element  114  having a resistor and voltage source  116 , is to be broadened by the current limitation and/or the edge slope limitation while using limitation unit  120 . 
     Optionally, at the base of limiting element  118  of limiting unit  120  circuit configuration  100  may have provided to it a capacitive element  110   a,b . In this instance, capacitive element  110   a  may be connected between the base and the collector of transistor element  118 , and capacitive element  110   b  between the base and the emitter. The collector connection of transistor element  118  may represent the output of the circuit configuration, connected in  FIG. 1  in exemplary fashion via an element  114  having a resistor to voltage source  116 . 
     Furthermore, capacitive elements  112   a,b  may be present and situated at the circuit output. Capacitive element  112   b  may, for instance, be developed as a capacitor, and capacitive element  112   a  as an ESD diode. In the case in which an ESD diode is provided, it may, as a function of the embodiment of circuit configuration  100 , be equipped, at the same time, to protect the base emitter diode of transistor element  118 , for instance, in the case of an npn transistor  118 , from a breakdown in a case of polarity reversal. 
     The electrical resistance of element  114  may advantageously be clearly greater than that of the electrical resistor of element  104 . Thereby, in the case of switched-on switching element  102 , there comes about only a slight voltage at the circuit output. A ratio of 10:1 or greater than 10:1 is preferred. Consequently, in the case of the switched-on switching element, there comes about at the output a dropping voltage, without taking into account a low voltage drop over elements  118  and  102 , of less than 10% of the value of voltage source  116 . 
     As switching element  102 , one may use, for instance, a transistor element, a relay element or a switching contact. 
     The method of functioning of the edge slope limitation of the circuit configuration according to  FIG. 1  is as follows: Without limitation unit  120  which, in the final analysis, includes each element except switching element  102 , resistor element  114  as well as voltage source  116 , as a function of its respective embodiment and the possibly optional elements thus provided, there may come about, when switching element  102  is switched on, a steeply dropping voltage edge at the output of circuit configuration  100 , since this is relatively at low resistance, drawn directly via switching element  102  to low potential. The edge that is created in response to switching off switching element  102 , runs clearly flatter, because of the electrical resistor of resistance element  114  in connection with the capacitance present at the output. The output capacitance may be present in this case either as a component having a capacitance or only as a parasitic capacitance. 
     The aim of a limitation of the edge slope may thus be, in particular, especially to flatten off the falling edge of the output signal, but without smoothing away the rising edge too greatly, in the process, so that, in particular, the signal integrity may remain ensured. 
     The switching on of switching element  102  effects a drop in the emitter potential of limiting element  118 , so that limiting element  118  also switches on. Parasitic capacitances and perhaps present capacitive components  112   a,b  now discharge resistor element  104  and switching element  102  via limiting element  118 . This creates a voltage drop at resistor element  104  as well as switching element  102 . This voltage drop, in turn, leads to a reduction in the available base emitter voltage of limiting element  118 , and thus to a current feedback. From this, there results a pinch-off of limiting elements  118 , and thus a limitation of the discharge current. The falling edge at the output runs flatter through this limitation. 
     When switching element  102  is switched off, parasitic capacitances and optionally present capacitive components  112   a,b  are charged comparatively slowly via resistance element  114 . Such a charging current does not limit limiting unit  120 , however, so that the edge, which is anyhow already rising flatly at the output, is not additionally smoothed away. 
     In response to the use of an optional capacitive component  110   a,b , if necessary, the falling edge may additionally be smoothed away. 
     Element  110   a  may particularly represent an artificial increase in a Miller capacitance of limiting element  118 , whereby it switches more slowly. In response to the switching on of switching element  102 , limiting element  118  also begins to conduct, whereby the potential at its collector begins to drop off. Because of a capacitive coupling of the collector to the base, by capacitive element  110   a , this in turn acts counter to the progression of element  118 , so that the switching on of limiting element  118  slows down. The switching-on process is concluded only after capacitive element  110   a  is completely recharged. 
     However, the additional capacitance may also be recharged during switching off switching element  102 . The rising edge at the output may, thereby, in spite of it, be influenced only slightly since, in practice, relatively low capacitance values may already be sufficient for capacitive element  110   a.    
     The preferred, but not exclusive values or value ranges of the individual elements may be assumed to be as shown below: 
     voltage source  116 : 1 V to 50 V; 
     element  112   b : 10 pF to 100 nF; 
     element  112   a : Zenner voltage &lt;60 V; 
     element  118 ,  102 : to be selected corresponding to requested limiting current, typically 0.1 mA to 100 mA, at power outputs also &gt;1 A; 
     Element  104  is preferably selected so that, in the case of a limiting current, a voltage drop is present that is sufficient for the current feedback (approximately in the range of 0.5 V to 3 V). 
     Element  114 : by a factor of 10&gt;element  104 , so that in response to the switching of element  102 , a meaningful voltage difference occurs at the output; 
     Element  108 : Minimum value ca. 2× base-emitter voltage of element  118 , thus for a bipolar transistor at least ca. 1.2 V. Maximum value clearly smaller than element  116 , so that a limitation of the falling edge or the current in the short circuit case comes about. For example, the voltage of element  108  is approximately in the range of 1:5 to 1:20 smaller than the voltage of element  116 ; 
     Element  106  is used for limiting the base current of element  118 . One would typically select the base current to be ca. 10× to 200×&lt;than the limiting current of the circuit. The factor (10 to 200) depends on the current amplification of the selected transistor; 
     Elements  110   a ,  110   b : 10 pF to 22 nF. If the smoothing away of the rising edge is to be avoided, a value in the lower range will be used for element  110   a , as a general tendency, that is, generally a smaller value than for element  112   b.    
     Optional capacitive element  110   b  may also be used to slow down the switching-on process of limiting element  118 . As soon as switching element  102  switches on, thus being in the conductive state, the base of element  118  is drawn capacitively to ground, so that element  118  is blocking for now. Only by recharging element  110   b  via resistor element  106  does limiting element  118  begin to conduct. 
     The method of functioning of a current limitation of the circuit configuration according to  FIG. 1  is as follows: When element  102  is switched on, the current in the output line leads to a voltage drop at resistor element  104 . In regular operation, this voltage drop is so small, however, that the base-emitter voltage at limiting element  118  is sufficient to power up element  118  itself, for example, during use of a transistor, advantageously, however, not necessarily to saturation. 
     If the output line is now, at low resistance, connected to positive potential, for example, in a short circuit case, the current rises in the output line only to the extent until the voltage drop at element  104  having a resistor and switching element  102  is so great that the base-emitter voltage of limiting element  118  is no longer sufficient for fully powering up the transistor. The voltage drop over the collector-emitter path of limiting element  118  thus rises, and the current in the output line is limited. In this operating case, a sufficient power dissipation may be necessary, consequently a cooling of limiting element  118 . 
     The output voltage of voltage source  116  may particularly be accepted as the voltage of the series circuit of voltage source  116  and of resistor element  114 . 
     With further reference to  FIG. 2 , an additional exemplary embodiment is shown of a circuit configuration for limiting current intensity and/or edge slope of electrical signals, according to the present invention. 
     Alternatively to a series circuit of limiting unit and switching element  102 , as shown in  FIG. 1 , a parallel circuit of limiting element  118  and switching element  102  may also be implemented. In this instance, switching element  102  controls limiting element  118  at its base, and the output signal is thereby inverted. 
     The current amplification of limiting element  118  may optionally be used so that for switching element  102  a switching element may be used that has lower power. In this case, direct activation of limiting element  118  from an open drain output of a microcontroller may be possible. Thereby a special amplifier stage may be saved. 
     The method of functioning of the circuit configuration according to  FIG. 2  is analogous to the circuit configuration according to  FIG. 1 . 
     If a MOSFET is used as contact element  102 , its parasitic body diode may protect the base-emitter diode of limitation element  118  from a breakdown in response to polarity reversal of the circuit. In this case, however, the body diode has to have a sufficiently high current carrying capacity. 
     With further reference to  FIG. 3 , an exemplary embodiment is shown of a circuit configuration for limiting the edge slope of electrical signals, according to the present invention. 
     The circuit configuration according to  FIG. 3  may particularly be considered as a pure edge limitation. When switching element  102  is switched off, capacitive element  302  is charged to the potential of voltage source  116  via resistor element  106 , limitation element  118  and resistor element  114 . Now, when switching element  102  switches on, limitation element  118  is blocked. Capacitive element  302  now discharges via resistor element  300  and switching element  102 , so that the base potential of limitation element  118  drops off slowly. The latter begins to conduct, and the output voltage level follows the curve of the base voltage of limitation element  118 . As soon as switching element  102  switches off again, capacitive element  302  is charged via resistor element  106 , limitation element  118  and resistor element  114 , whereby the output potential rises to the value of voltage source  116  again. If an element  112   a,b  is present, the capacitance of capacitive element  302  may be selected to be small compared to the capacitance of element  112   a,b , whereby the rising edge of the circuit configuration is only slightly influenced. 
     At the moment of switching on element  102 , at the circuit output, particularly the behavior of a controlled edge may be implemented. 
     For the circuit configurations of  FIGS. 1 and 2 , a variable current limitation may continue to be implementable. To do this, limitation element  118  should be connected at the base side to a variable potential. The quantity of this potential may be able to be set by the current limitation. 
     The bipolar transistor for limitation element  118 , shown in exemplary fashion, may be executed in all circuit configurations of  FIGS. 1 to 3 , alternatively also by other transistor types, such as a MOSFET or a JFET, as operational amplifier or tube element. The connections should then be selected correspondingly to the component used. 
     In the event that in the circuit configurations of  FIGS. 1 to 2  a self-conducting n-channel FET is selected for limitation element  118 , its gate connection to ground potential may also take place, for example, so that no auxiliary voltage may be required. 
     If switching element  102  is missing in the circuit configuration according to  FIG. 2  or is replaced by a short circuit in the circuit configuration according to variant  1 , a pure current limitation circuit configuration comes about in the respective circuit configurations. 
     All circuit configurations, in deviation from those implementations shown in the figures, also to be understood schematically, may also be used wholly or partially in integrated form. 
       FIG. 4  shows an exemplary signal curve of the circuit configuration according to  FIG. 1 . 
     At time t 1 , in the circuit configuration according to  FIG. 1 , switching element  102  is switched off. Output voltage V (collector) thereby rises to the exemplary value of 16 V. At time t 2 , switching element  102  switches on. In case exclusively one switching element  102  and elements  114  and  116  are provided, switching takes place having a slope of edge  400 , essentially directly, in exemplary fashion. In the case of situating limiting unit  120 , falling edge  402  is limited in its slope, whereby it falls at a slower rate and is only established at point t 3  in its lower value, which is essentially determined by the ratio of the electrical resistances of resistor elements  114  and  104 . 
       FIG. 5  shows an exemplary specific embodiment of a method for limiting current intensity and/or edge slope of electrical signals, according to the present invention. 
     Method  500  for limiting current intensity and/or edge slope of electrical signals has the steps carrying out  502  of a switching process of the voltage source while using the switching element and limiting  504  a current intensity and/or an edge slope of an electrical signal, particularly of the output voltage, while using a limiting unit.