Abstract:
The invention relates to a method and an apparatus for regulating a current through an inductive load, which can be connected to a power supply, to a prescribed nominal current value, where the method comprises the following method steps:
       a) turning on and off the power supply in pulsed fashion, with the power supply being turned on when the current (I) flowing through the load (L) reaches a first limit value (I —   1 ), which is below the nominal current value (I_nominal) by a hysteresis value (H), and with the power supply being turned off when the current (I) reaches a second limit value (I —   2 ), which is above the nominal current value (I_nominal) by the hysteresis value (H),   b) determining a period duration between two successive turn-on or turn-off times,   c) comprising determined the period duration with a given period duration (Tnominal), and   d) changing the hysteresis value (H) on the basis of the comparison between the determined period duration and the prescribed period duration (Tnominal).

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and an apparatus for regulating a current flowing through an inductive load. 
   BACKGROUND 
     FIG. 1  shows a circuit arrangement having an inductive load, which is represented by an inductance L and an ohmic resistor R, and shows a current regulating arrangement for regulating a current IL through the load. This current regulating arrangement comprises a switch S which is connected in series with the load and which is used for connecting the load to a supply voltage V in pulsed fashion. With the switch closed, when approximately all of the supply voltage is present across the load, the current through the load rises in a sufficiently well known manner and falls when the switch S is opened, with the current through the load when the switch is opened being accepted by a freewheeling diode FWD which is connected in parallel with the load. 
   To determine the current drawn by the load, the load current is detected using a measuring resistor RS which is connected in series with the load and which is connected to an actuating circuit  2 . This actuating circuit uses an actuation signal  21  to actuate the switch S on the basis of the voltage Vrs across the current measuring resistor RS, and hence on the basis of the load current IL, with the aim of regulating the mean value of the load current to a prescribed value. 
   To regulate the mean value of the load current IL to a nominal value I_nominal using a current regulating arrangement as shown in  FIG. 1 , two concepts are fundamentally known which are explained below with reference to  FIGS. 2 and 3 . The two figures show, by way of example, the respective time profile for the load current IL and for the actuation signal  21  for the respective method. For the illustration in  FIGS. 2 and 3 , it is assumed, in order to assist understanding, that the inductive load is an ideal inductance, which means that a triangular current profile is obtained when the switch S is turned on and off in pulsed fashion. 
   In the method illustrated with reference to  FIG. 2 , the switch S is respectively turned on when the load current IL has dropped to a lower limit value I_ 1 , which is below the nominal value I_nominal by a firmly prescribed hysteresis value H, and turned off when the load current has risen to an upper limit value I_ 2 , which is above the nominal value I_nominal by the hysteresis value H. In the case of this method, the hysteresis in the load current IL is constant after the upper limit value I_ 2  has been reached for the first time, and corresponds to twice the hysteresis value H. 
   The gradient with which the load current IL rises when the switch S is turned on is dependent on the supply voltage V to which the load is connected in pulsed fashion. In this case, the gradient of the load current IL increases as the supply voltage increases, which results in a shortening of the turned-on period and hence in an increase in the switching frequency, which is shown using the right-hand part of the time profiles in  FIG. 2 , in which the time profile of the load current IL and of the actuation signal is shown for a relatively high supply voltage V. By contrast, the gradient of the current IL when the switch is opened is essentially independent of the supply voltage. 
   In the case of the method explained with reference to  FIG. 2 , the switching frequency is very highly dependent on the characteristics of the load, of the switch S, of the freewheeling diode FWD, of the temperature and particularly of the supply voltage V. Since the switching losses in the switch S are dependent on the switching frequency, the switch S needs to be proportioned, for safety reasons, such that it is able to take on the high power losses which arise at high switching frequencies, which increases the manufacturing costs for the switch. Furthermore, the frequency of electromagnetic radiated interference resulting from the switching processes likewise fluctuates over a wide range in the case of this method, which makes it more difficult to suppress the propagation of this radiated interference. 
   In the case of the method illustrated with reference to  FIG. 3 , the switch is turned on at the rate of a firmly prescribed frequency. To set the turned-on period for the switch, the mean value of the load current IL is determined over one or more previous actuation periods and is compared with the nominal value I_nominal. In this case, the turned-on period is adapted on the basis of the determined comparison result, that is to say is increased in comparison with the previous value if the determined mean value is below the nominal value I_nominal and is reduced in comparison with the previous value if the determined mean value is above the nominal value I_nominal. The turned-on period can be regulated digitally or in analog fashion, for example using a proportional integral controller (PI controller). 
   A drawback of this method is that the transient response is worse than in the method with constant hysteresis. Furthermore, suitable proportioning of the control loop in order to attain sufficient stability is difficult and complex. 
   SUMMARY 
   It is an aim of the present invention to provide a method for regulating the mean value of a current flowing through an inductive load in which the power loss resulting from switching processes is subject to only small fluctuations and which can be implemented easily and without stability problems, and to provide an apparatus for implementing a method of this type. 
   The method for regulating the mean value of a load current through an inductive load, which can be connected to a supply voltage, to a prescribed nominal current value involves the power supply being turned on and off in pulsed fashion, with the power supply being turned on when the current flowing through the load reaches a first limit value, which is below the nominal current value by a hysteresis value, and with the power supply being turned off when the current reaches a second limit value, which is above the nominal current value by the hysteresis value, determining a period duration between two successive turn-on or turn-off times, comparing the determined actual period duration with a nominal period duration, and changing the hysteresis value on the basis of the comparison between the actual period duration and the nominal period duration. 
   In the case of the inventive method, the controlled variable is the period duration which is determined and whose value is compared with the nominal period duration. The manipulated variable is the hysteresis value, which is changed on the basis of the comparison result in order to regulate the period duration to the value of the nominal period duration. This nominal value for the period duration is reached after a transient process, whose duration is dependent not only on the parameters of the control loop but also, inter alia, on the load, on the supply voltage and on the temperature. 
   In the text below “turned-on phase of the load” is to be understood to mean a period of time during which the switch is turned on and off in pulsed fashion in order to effect a current with a regulated mean value through the load. At the start of such a turned-on phase, when the current through the load is initially still zero, a fixed initial value for the hysteresis value is preferably prescribed which is subsequently changed on the basis of the comparison between the actual period duration and the nominal period duration in order to regulate the period duration to the value of the nominal period duration. Furthermore, it is also possible to store a hysteresis value at the end of an earlier turned-on phase and to use this stored value as initial value for a later turned-on phase. 
   To regulate the hysteresis value on the basis of the comparison between the actual period duration and the nominal period duration, it is possible to set the hysteresis value using an integrating control method (I control) or a proportional/integrating control method (PI control). The difference between the actual period duration and the nominal period duration determines the control error in this case. The amount by which the hysteresis value is changed after each comparison between the actual period duration and the nominal period duration can be dependent on the control error in this case. It is also possible for the hysteresis value, after each comparison, always to be incremented or decremented by a prescribed value on the basis of advantages of the control error, so long as the control error is greater than zero. 
   In addition, the hysteresis value for reaching the nominal period duration may also be determined analytically using the actual period duration, the nominal period duration and the instantaneous hysteresis value, provided that the load current rises in linear fashion when the switch is closed and falls in linear fashion when the switch is opened, that is to say provided that the current has a triangular profile. 
   The apparatus for regulating the mean value of a current through an inductive load to a prescribed nominal current value comprises a controllable power supply circuit to which an actuation signal is supplied and which provides the supply voltage as stipulated by the actuation signal, a current measuring arrangement for measuring the current flowing through the load and for providing a current measurement signal. The apparatus also comprises an actuating circuit to which a hysteresis value, the current measurement signal and the nominal current value are supplied and which provides the actuation signal such that the supply voltage is turned on when the current measurement signal reaches a first limit value, which is below the nominal value by the hysteresis value, and is turned off when the current measurement signal reaches a second limit value, which is above the nominal current value by the hysteresis value. The apparatus also comprises a time measuring arrangement for determining a period duration for an actuation period in the power supply circuit and for providing a period duration value which is dependent on the actuation period, and also an evaluation circuit to which a nominal period duration value, the determined actual period duration value and a previous hysteresis value determined during a previous period are supplied and which is designed to set the hysteresis value on the basis of the nominal period duration value, the determined actual period duration value and the previous hysteresis value. 
   The power supply circuit for providing the supply voltage comprises a voltage source and a switch, for example, with the switch being connected in series with the load between connection terminals on the voltage source and being actuated by the actuation signal. It goes without saying that any other switchable voltage sources can be used for supplying the load. 
   By way of example, the current measuring arrangement comprises a measuring resistor, which is connected in series with the load, and a measuring amplifier, which detects a voltage drop that is present across the measuring resistor and is brought about by the load current and which provides the current measurement signal on the basis of this voltage drop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is explained in more detail below using exemplary embodiments with reference to figures. 
       FIG. 1  shows a circuit arrangement with an inductive load and a current regulator based on the prior art. 
       FIG. 2  shows the time profile of the load current ( FIG. 2   a ) and of an actuation signal for the power supply to the load ( FIG. 2   b ) in the case of a method based on the prior art, in which a constant hysteresis for the load current profile is set. 
       FIG. 3  illustrates the time profile of the load current ( FIG. 3   a ) and of an actuation signal for the power supply to the load ( FIG. 3   b ) in the case of a method based on the prior art, in which the power supply is turned on at a fixed frequency. 
       FIG. 4  shows an exemplary embodiment of an inventive current regulator for regulating the mean value of a load current to a nominal value, which has an actuating circuit for generating an actuation signal for a voltage source and a regulator arrangement for setting a hysteresis value. 
       FIG. 5  shows a first exemplary embodiment of the actuating circuit. 
       FIG. 6  shows a second exemplary embodiment of the actuating circuit. 
       FIG. 7  illustrates the way in which the actuating circuit works. 
       FIG. 8  illustrates the profile of the load current and of the actuation signal when using a regulator arrangement with an integrating control response. 
       FIG. 9  illustrates the profile of the load current and of the actuation signal when using a regulator arrangement which calculates a hysteresis value analytically using a measured period duration value, a nominal period duration value and an instantaneous value for the hysteresis value. 
   

   Unless indicated otherwise, identical reference symbols in the figures denote identical parts having identical meanings. 
   DETAILED DESCRIPTION 
     FIG. 4  shows a circuit arrangement with a load RL and an inventive current regulating arrangement for regulating the mean value of a load current IL to a prescribed nominal value. The current regulating arrangement comprises a controllable voltage source with connection terminals K 1 , K 2  for connecting the load. In the exemplary embodiment shown, the controllable voltage source comprises a constant voltage source  50 , which provides a supply voltage V, and a switch S which is actuated as stipulated by an actuation signal S 40  and which connects the load to the voltage source  50  in the closed state and breaks a conductive connection between the voltage source and the load in the open state. The switch S is in the form of a MOS transistor, for example. 
   The current regulating arrangement also comprises a current measuring arrangement  10  with a current measuring resistor  11 , which is connected in series with the load R,L, and with a current measuring amplifier  12 , which taps off a measured voltage Vs that is present across the current measuring resistor  11  and results from the load current IL and which provides a current measurement signal S 12  which is dependent on the load current IL. The current measuring amplifier  12  comprises, in particular, a level shifter which ensures that the current measurement signal S 12  is always referenced to a reference-ground potential regardless of the switch position of the switch S. This level shifter arrangement is necessary because the potential at the node which is common to the current measuring resistor  11  and to the switch S fluctuates between reference-ground potential GND, with the switch S closed, and the supply voltage V, with the switch S open. 
   To provide the actuation signal S 40  for the switch S, the current regulating arrangement has an actuating circuit  40  to which the current measurement signal S 12 , a nominal current value Inominal and a hysteresis value H are supplied. The actuation signal S 40  which is available at the output of the actuating circuit  40  is preferably converted to a level suitable for actuating the switch S from a logic level by a driver circuit DRV. The actuating circuit  40  is designed to generate a pulsed actuation signal S 40  for the switch S on the basis of the current measurement signal S 12 , the nominal current value Inominal and the hysteresis value H, the switch S being closed during one period duration of this pulsed actuation signal S 40  for a respective prescribed period of time S, in order to apply the load to the supply voltage V, and then being opened by the start of the next actuation period. 
   In addition, the current regulating arrangement comprises a time measuring arrangement  20  to which the actuation signal S 40  is supplied and which provides a time measuring signal Tm that is dependent on a period duration of this actuation signal S 40  and preferably corresponds to the period duration of the actuation signal S 40 . However, the time measuring arrangement  20  may also be designed such that the time measurement value Tm corresponds to the turned-on period or to the turned-off period of the actuation signal S 40 , where “turned-on period” denotes the period of time during which the actuation signal S 40  is at a level which is suitable for turning on the switch S, and “turned-off period” denotes the period of time during which the actuation signal S 40  is at a level which is suitable for turning it off. 
   This time measurement value Tm is supplied to a regulator arrangement  30  to which, besides this time measurement value Tm, a nominal time value Tnominal is supplied which represents a nominal value for the time measurement value Tm. Depending on whether the time measurement value Tm corresponds to the period duration of the actuation signal S 40 , the turned-on period or the turned-off period of the actuation signal S 40 , this nominal value Tnominal is chosen such that it corresponds to a nominal value for the actuation period, for the turned-on period or for the turned-off period of the actuation signal S 40 . The actuation signal S 40  is regulated on the basis of a time measurement value using a hysteresis value H which the regulating arrangement  30  generates on the basis of a comparison between the time measurement value Tm and the nominal time value Tnominal and which is supplied to the actuating circuit  40 . The aim of regulating the actuation signal S 40  on the basis of the actuation period, the turned-on period or turned-off period is to set a prescribed period duration and hence a prescribed switching frequency for the switch S. 
   With the switch S closed, the load current IL rises, and the load current IL falls when the switch S is subsequently opened, when the load current IL is accepted by the freewheeling diode FWD. The current measurement signal, which is proportional to the load current, behaves accordingly. The actuation signal S 40  for the switch S is in this case generated, with reference to  FIG. 7 , such that, after closing, the switch S remains closed until the current measurement signal S 12  reaches an upper limit value S 12 _ 2 , which corresponds to the sum of a nominal value S 12 _nominal and a hysteresis value H, and, after opening, the switch S remains open until the current measurement signal S 12  has fallen to a lower limit value S 12 _ 1 , which corresponds to the nominal value S 12 _nominal minus the hysteresis value H. The current measuring resistor  11  and the current measuring amplifier  12  are preferably tuned to one another such that the current measurement signal S 12  is proportional to the load current IL. The load current which flows when the switch S is turned off is then correspondingly proportional to the upper limit value S 12 _ 2 , the load current which flows when the switch S is turned on is then proportional to the lower limit value S 12 _ 1 , and the hysteresis by which the load current fluctuates is proportional to twice the hysteresis value H′. The nominal value S 12 _nominal is suitably chosen, taking into account the proportionality factor introduced by the current measuring arrangement  10 , in order to regulate the mean value of the load current to a prescribed nominal value. 
     FIG. 5  shows a first exemplary implementation of the circuitry of the actuating circuit, which generates the actuation signal S 40  on the basis of the hysteresis value H, the nominal value S 12 _nominal and the current measurement signal S 12 . The circuit arrangement comprises a first and a second comparator  41 ,  42 , an RS-type flipflop  45  connected downstream of the comparators  41 ,  42 , and also a subtractor  43  and an adder  44 . The subtractor  43 , to which the nominal current value S 12 _nominal and the hysteresis value H are supplied, provides the lower limit value S 12 _ 1 , which is supplied to the positive input of the first comparator  41 . The second input of this first comparator  41  is supplied with the current measurement signal S 12 . The adder  44 , to which the nominal current value S 12 _nominal and the hysteresis value H are supplied, provides the upper limit value S 12 _ 2 , which is supplied to the negative input of the second comparator  42 . The positive input of this second comparator  42  is supplied with the current measurement signal S 12 . The flipflop  45  is set upon a rising edge at the output of the first comparator  41 , that is to say when the current measurement signal S 12  has dropped below the lower limit value S 12 _ 1 , in order to generate a high level for the actuation signal S 40  and to turn on the switch S. The flipflop is reset upon a rising edge of the output signal from the second comparator  42 , that is to say when the current measurement signal S 12  exceeds the value of the upper limit value S 12 _ 2 . 
   The time profiles of the signals which arise in the actuating circuit become clear from the time profiles in  FIG. 7 . In the illustration in  FIG. 7 , the switch S is closed by a high level from the actuation signal S 40  at a time t 0 , as a result of which the load current IL and the current measurement signal S 12  rise. In this case, the actuation signal S 40  remains at a high level until the current measurement signal reaches the second limit value S 12 _ 2 , whereupon the flipflop  45  is reset by means of the second comparator  42  in order to bring about a low level for the actuation signal S 40  and to open the switch S as a result. The switch remains open until the current measurement signal S 12  has dropped to the first limit value S 12 _ 1 , whereupon the flipflop  45  is set again by means of the first comparator  41  in order to generate a high level for the actuation signal S 40  and to close the switch S. 
     FIG. 6  shows a further exemplary embodiment of the actuating circuit  40 , which has merely a comparator  46 , a D-type flipflop  47  and an addition/subtraction circuit. In this case, the addition/subtraction circuit  48  is supplied with the nominal value S 12 _nominal and the hysteresis value H, and this circuit  48  adds the hysteresis value H to the nominal current value S 12 _nominal or subtracts the hysteresis value from this nominal current value S 12 _nominal as stipulated by the actuation signal S 40 . The output value from this addition/subtraction circuit is supplied to the positive input of the comparator  46 , whose negative input is supplied with the current measurement signal S 12 . The comparator output signal actuates the D input of the D-type flipflop  47 . The addition/subtraction circuit  48  is designed to add the hysteresis value H to the nominal value S 12 _nominal when the actuation signal S 40  is at a high level, in order to provide the upper limit value S 12 _ 2 . In this case, the actuation signal S 40  retains a high level until the current measurement signal S 12  has risen to this second limit value S 12 _ 2  and the actuation signal from the comparator  46  assumes a low level. The D-type flipflop  47  accepts the output signal from the comparator  46 , which means that the actuation signal S 40  assumes a low level. When the actuation signal S 40  is at a low level, the addition/subtraction circuit subtracts the hysteresis value H from the nominal value S 12 _nominal in order to provide the lower limit value S 12 _ 1 . The comparator output signal  46 , and hence the actuation signal S 40 , then remain at a low level until the current measurement signal S 12  has dropped to the first limit value S 12 _ 1 . 
   It is an aim of the present current regulating arrangement to achieve an at least approximately constant switching frequency for the switch S after a transient process. This is achieved in the current regulating arrangement by varying the hysteresis value H on the basis of a comparison between the time measurement value Tm and the nominal time value Tnominal, as explained below with reference to  FIG. 8 . 
     FIG. 8  illustrates a transient process for the current measurement signal S 12 , which is proportional to the load current IL, after a time t 1 . On account of the proportionality between the current measurement signal S 12  and the load current IL, the current measurement signal&#39;s time profile shown in  FIG. 8  corresponds in quality to the time profile of the load current IL. The values assumed by the load current at the different times in the time profile shown in  FIG. 8  are indicated in brackets. In this case, the upper limit value IL_ 2  of the load current corresponds to the upper limit value S 12 _ 2 , a lower limit value for the load current corresponds to the lower limit value S 12 _ 1 , and the load current&#39;s mean value I_nominal which is set corresponds to the nominal value S 12 _nominal. The time t 1  in  FIG. 8  denotes a time at which the switch S is closed after the inductive load has been fully demagnetized. In this context, the regulating arrangement  30  first of all supplies an initial value H 0  for the hysteresis value H to the actuating circuit  40 . The switch S remains closed at first until the current measurement signal S 12  reaches the upper limit value S 12 _ 2  for the first time at a time t 2 . From this time t 2  onward, the switch is opened whenever the current measurement signal S 12  reaches the upper limit value S 12 _ 2  and is then closed again whenever the current measurement value S 12  has dropped to the lower limit value S 12 _ 1 . The hysteresis value H, and hence the limit values S 12 _ 1 , S 12 _ 2 , which are situated symmetrically around the nominal current value S 12 _nominal are varied in order to set the switching frequency, as becomes clear, in particular, from the time profile of the actuation signal S 40  in  FIG. 8 . The aim is to regulate the period duration of the actuation signal S 40  to a nominal period duration value Tnominal, and hence to regulate the switching frequency to a nominal frequency Fnominal. To this end, the regulating arrangement  30  compares the time measurement value Tm and the nominal value Tnominal with one another in order to readjust the hysteresis value H if there is a discrepancy between the measured period duration value Tm and the nominal value Tnominal. This readjustment is illustrated schematically in  FIG. 8 . In the exemplary embodiment shown, the period duration is greater than the nominal period duration at first, which means that the actual period duration Tm is greater than the nominal period duration value by the value of a control error ΔT. The regulating arrangement  30  is designed to reduce the hysteresis value H in the event of such a control error, so as to reduce the hysteresis of the current measurement signal S 12 , and hence of the load current IL, and thereby to reduce the period duration until the period duration TM matches the nominal period duration. 
   To this end, the regulating arrangement  30  may be in the form of a PI controller, which generates the hysteresis value H on the basis of the nominal period duration value Tnominal and the actual period duration value Tm. Since the time measuring arrangement  20  provides a new period duration value Tm after each period duration of the actuation signal S 40 , the hysteresis value H is adapted over time in stages, in each case after a period duration. 
   While, when a PI controller is used, the change in the hysteresis value H is dependent on the difference between the period duration value Tm and the nominal period duration value Tnominal, it is also possible to design the controller  30  such that the hysteresis value is respectively incremented or decremented by firmly prescribed values, in each case depending on whether the actual period duration value Tm is greater than or less than the nominal period duration value Tnominal. 
   Assuming that the profile of the load current IL, and hence the profile of the current measurement signal S 12 , are at least approximately triangular in shape, it is also possible for the regulator arrangement  30  provided to be an arithmetic and logic unit which determines the hysteresis value H analytically from the actual period duration value Tm, the nominal period duration value Tnominal and an instantaneous value H1 for the hysteresis value in line with the following relationship:
 
 H=H 1 ·T nominal/ T 1   (1)
 
   In this context, H 1  denotes the instantaneous hysteresis value, T 1  denotes the period duration value T 1  associated with this instantaneous hysteresis value H 1 , and Tnominal denotes the nominal period duration value. 
   As  FIG. 9  shows, when an arithmetic and logic unit of this type is used as regulating arrangement  30  it is possible to achieve the nominal period duration of the actuation signal S 40 , or the nominal frequency, after just one actuation period of the actuation signal S 40 . 
   The inventive current regulator and the inventive current regulating method allow a load current IL through an inductive load to be regulated to a prescribed mean value I_nominal, at which a nominal frequency for the actuation signal is reached after a transient phase. Apart from the transient process, which is comparatively short and, in terms of electromagnetic radiation, thus presents a minor problem, the switching frequency of the switch is regulated to a prescribed value which is dependent on the nominal period duration value. This easily results in narrowband radiated electromagnetic interference, which can be suppressed by taking suitable measures.