Patent Publication Number: US-7710699-B2

Title: Current limitation in an inductance with a limit current adaptation

Description:
PRIORITY CLAIM 
   This application claims priority from French patent application No. 03/51072, filed Dec. 16, 2003, which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate to the field of circuits for protecting an inductance so that the current flowing through it does not exceed its saturation current, and relate to any system in which the current is desired to be limited in an inductance. An example application for embodiments of the present invention is the field of power converters (step-up or step-down) in which a switch controlling the current in an inductance is controlled by a train of pulses, for example, modulated in width (PWM), in frequency (FWM), etc. 
   2. Discussion of the Related Art 
     FIG. 1  very schematically shows a conventional example of a step-up converter. 
   Such a converter uses an inductance L in series with a diode D between a terminal  1  for applying a D.C. input voltage Vin and a terminal  2  for applying an output voltage Vout for a load  3  (Q). A capacitor Cout for storing voltage Vout is connected between terminal  2  and ground M. Further, a switch K (for example, a MOS transistor) connects the anode of diode D (unction point of the inductance and of the diode) to ground. Switch K is controlled by a width-modulated pulse train to control the output voltage based upon the needs of the load or a predetermined value. The operation of such a converter is known. When switch K is on, a current flows through inductance L from voltage source Vin (for example, a battery) while capacitor Cout supplies load  3 . When switch K turns off, the power stored in inductance L recharges capacitor Cout at the same time as it supplies the load. 
   To protect inductance L against a deterioration, it must be ascertained that the current flowing therethrough does not exceed its saturation current from which the inductance behaves as a wire. For this purpose, the turning-on of switch K is conventionally conditioned by a current threshold in the inductance or in the switch. 
     FIG. 2  shows a conventional example of an integrated circuit  10  for controlling a PWM converter integrating an inductance protection function. It shows, integrated to this circuit, switch K (in dotted lines) between two terminals  11  and  12  of circuit  10  respectively connected to the anode of diode D (here, a Zener diode) and to ground M. To control output voltage Vout, circuit  10  also comprises a terminal  13  of connection to terminal  2  of the converter. Circuit  10  is supplied from D.C. voltage Vin filtered by a capacitor Cin, terminal  1  being connected to a terminal  14  of circuit  10 . 
   In the example of  FIG. 2 , the converter is intended to supply light-emitting diodes  20  (LEDs) ensuring a screen backlighting function (for example, of a mobile phone). Circuit  10  then integrates, optionally, a second switch K for turning on/off the load formed of diodes  20  in series. Switch K′ shown in dotted lines in the form of a MOS transistor connects a terminal  15  of circuit  10  connected to load  20  to a terminal  16  of this same circuit connected to ground M, generally via a protection resistor Rp. Of course, switch K′ may be external to circuit  10 . 
   Circuit  10  comprises means not shown for measuring the current in inductance L. This current measurement is generally induced from the voltage across the inductance, measured at terminals  11  and  14  of the circuit. Another method consists of measuring (for example, by means of a shunt) the current in switch K. Circuit  10  compares the information linked to the current in inductance L with a reference value to force the turning-off of switch K in case said value is exceeded, independently from the PWM digital signal control point provided on a terminal  17 . 
   A first family of known circuits comprises a predetermined reference, internal to circuit  10 . A disadvantage of such a solution is that circuit  10  is then dedicated to an inductance L or, to stand several inductances of different values, requires setting of a minimum threshold while some inductances could stand higher currents. 
   A second solution consists of providing a resistor Rs connected to an additional terminal  18  of circuit  10  and to ground M to parameterize the limiting threshold of circuit  10 . Such a solution enables changing resistance Rs when the value of the limiting current is desired to be modified, for example, after an inductance change. A disadvantage of this solution is that it requires a terminal ( 18 ) of additional connection of integrated circuit  10  as well as an external resistor. 
     FIG. 3  illustrates the variation of the maximum tolerable or standable current (IL max ) according to value L of an inductance. The curve, the exact shape of which depends on the considered inductance family (materials, conductor diameter, power dissipation capacity, etc.) shows a decrease in the maximum standable current along with the increase in the inductance value. Accordingly, when a limiting threshold is desired to be set in a circuit  10 , be it internally or externally, so that it can stand several different inductances L, a relatively low threshold Imax has to be set to be able to protect all the inductances in the range. As a result, according to the inductance connected to circuit  10 , its capacities are not fully exploited. 
     FIGS. 4A and 4B  illustrate the operation of a circuit such as shown in  FIG. 2  for two different inductance values L 1  and L 2 . The left-hand portion of the timing diagrams of  FIGS. 4A and 4B  has been drawn for an inductance of relatively high value L 1  with respect to the value of a relatively low inductance L 2 , the operation of which is illustrated in the right-hand portion of the timing diagrams.  FIG. 4A  illustrates current IL in the inductance while  FIG. 4B  illustrates the OFF and ON periods of switch K It should however be noted that this applies to the case of a step-up converter such as illustrated in  FIG. 2 , the off and on periods of the switch being inverted in the case of a step-down converter. 
   To enable operation with inductances L 1  and L 2 , limiting current Imax set by circuit  10  is a function of the inductance of lower value L 1 . As illustrated in the right-hand portions of the timing diagrams, this results in inductance L 2  having its limiting current ILmax greater than current Imax is not fully exploited. Indeed, even admitting that the duty cycle of the turn-on pulses of switch K is set in the same way as for low-value inductance L 1 , the limiting function is activated as soon as current I exceeds value Imax, while the control of the converter with an output voltage control point Vout would have required a longer power storage period. 
   The need to have an integrated circuit for controlling a converter that can accept several different inductance values is more and more frequent. Indeed, the manufacturers of integrated control circuits  10  are generally distinct from inductance manufacturers which will assemble the inductance and circuit  10  in a converter. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention aims at improving the exploitation of the performances of an inductance while protecting it. 
   Another embodiment of the present invention especially aims at controlling the limiting current of a circuit for controlling an inductance with the value of said inductance. 
   A further embodiment of the present invention also aims at providing a solution unresponsive to possible variations of supply voltage Vin, especially when this voltage corresponds to a battery voltage. 
   Another embodiment of the present invention also aims at providing an integrated circuit for controlling a switch to be connected to an inductance, which has a current-limiting function therein, without requiring any external resistor for parameterizing the limiting current value. 
   One embodiment of the present invention provides a circuit for limiting the current in an inductance, comprising means for interrupting the power storage in the inductance at the end of a delay triggered by the current in the inductance. 
   According to an embodiment of the present invention, said delay is a function of the supply voltage of the inductance. 
   According to an embodiment of the present invention, the circuit comprises:
         means for comparing the current in the inductance with a first predetermined threshold value;   means for triggering a delay element when said first threshold is reached; and   means for interrupting the power storage in the inductance after a delay set by said delay element.       

   According to an embodiment of the present invention, said delay is proportional to the inverse of the inductance supply voltage. 
   According to an embodiment of the present invention, said first threshold is determined according to a family of inductances for which it is intended. 
   An embodiment of the present invention also provides a method for limiting the current in an inductance including interrupting the power storage in the inductance at the end of a delay triggered by the current in the inductance. 
   According to an embodiment of the present invention, said delay is a function of the inductance supply voltage. 
   According to an embodiment of the present invention, the method consists of:
         triggering a delay element upon occurrence of a current exceeding a first predetermined threshold in the inductance; and   interrupting the power storage in the inductance at the end of a determined delay.       

   According to an embodiment of the present invention, the delay is proportional to the inverse of the inductance supply voltage. 
   An embodiment of the present invention also provides a circuit for controlling a voltage converter comprising a cut-off switch, comprising a limiting circuit. 
   Features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 ,  2 ,  3 ,  4 A, and  4 B, previously described, are intended to show the state of the art and the problem to solve; 
       FIG. 5  very schematically illustrates in the form of blocks a method for controlling the limiting current of an inductance according to an embodiment of the present invention; 
       FIG. 6  illustrates the implementation of a method according to one embodiment of the present invention on an inductance value variation; 
       FIG. 7  illustrates the implementation of a method according to one embodiment of the present invention on a variation of the inductance supply voltage; 
       FIG. 8  shows, in a view to be compared with  FIG. 2 , an integrated circuit for controlling a step-up converter switch according to one embodiment of the present invention; 
       FIG. 9  shows a limiting circuit according to one embodiment of the present invention; and 
       FIGS. 10A ,  10 B, and  10 C illustrate, in timing diagrams, the operation of the circuit of  FIG. 9 . 
   

   DETAILED DESCRIPTION 
   The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the circuit for generating the control pulses of switch K with a control of the output voltage has not been detailed and is no object of the present invention, the present invention and its limiting circuit being implementable with any conventional control of the on periods of the switch. 
   A feature of an embodiment of the present invention is to compare the current in the inductance of a converter with a predetermined current threshold, and to trigger the delay when the current in the inductance exceeds this predetermined threshold, the limitation being then considered as being activated, at the end of the delay brought to the detection. 
   Another feature of embodiments of the present invention is that the delay is a function of supply voltage Vin of the inductance, more specifically, is proportional to the inverse of this voltage. 
     FIG. 5  very schematically shows in the form of blocks a functional embodiment of a limiting circuit according to an embodiment of the present invention. This circuit comprises a comparator  30  of current IL in the inductance (or in the switch) with respect to a threshold I 0 max. The result of comparator  30  is sent onto a delay element  31  (τ(1/V)) having its output conditioning state ON/OFF of the switch (not shown), delay element  31  preferably receiving voltage Vin to control the value of the delay with the inverse of this voltage. 
   Embodiments of the present invention take advantage of the fact that the current in the inductance is a function of the ratio between the voltage which is applied thereto and the value of this inductance (dl=(V/L)dt). Thus, embodiments of the present invention provide generating a delay proportional to the inverse of the voltage, and stopping the current in the inductance with a delay with respect to the exceeding of a predetermined threshold. This amounts to interrupting the power storage in the inductance at the end of a delay triggered according to the current in the inductance, this delay being preferably a function of the voltage supplying the inductance. 
     FIG. 6  illustrates the application of a method according to one embodiment of the present invention to a variation in the inductance value. This drawing shows the shape of current IL in the inductance along time for three decreasing values L 1 , L 2 , L 3 . 
   Current threshold I 0 max is set from a curve of the type shown in  FIG. 3 , determined empirically or from a sample of inductances to which the circuit must apply, and is of course chosen to be smaller than the saturation current of the inductance of maximum value that the circuit can stand. 
   If the current in the inductance reaches threshold I 0 max, this triggers the delay element setting delay τ and the switch is turned off at the end of this delay τ. In  FIG. 6 , voltage Vin is assumed to be constant so that delay τ applied to the different times of exceeding of value I 0 max is the same for the three inductance values L 1 , L 2 , L 3 . However, since the current slope depends on the value of this inductance (is inversely proportional to the inductance value), the application of the same delay to the different current slopes results in interrupting this current for increasing values as the inductance value decreases (IL 1 max, IL 2 max, IL 3 max). By selecting in adapted fashion value I 0 max and delay τ, the curve of the limiting current versus the inductance of the type illustrated in  FIG. 3  can approximately be followed. This curve is non-linear and, as a first approximation, varies in I 0 max k/L, where k is a constant 
     FIG. 7  illustrates the application of a method according to one embodiment of the present invention to an inductance of the same value but undergoing a supply voltage variation. In  FIG. 7 , the example of inductance value L 2  has been taken for three increasing voltages V 1 , V 2 , V 3 . Value I 0 max is the same as in  FIG. 6 . Since the inductance has the same value, the application of the method must lead to the application of an always identical threshold IL 2 max. This condition is fulfilled by here varying delay τ in a way inversely proportional to the supply voltage, to obtain three respectively decreasing delays τ 1 , τ 2 , τ 3  and thus ensure the compliance with threshold IL 2 max. 
     FIG. 8  shows, in a view to be compared with  FIG. 2 , a circuit  40  integrating a switch K (in dotted lines) for controlling an inductance L in a voltage step-up converter assembly. Same elements as in  FIG. 2  are found, but for the notable exception of resistor Rs which, according to this embodiment of the present invention, is no longer necessary. This embodiment of the present invention thus spares an external connection terminal of integrated circuit  40  as well as an external resistor. 
     FIG. 9  shows an embodiment of a limiting circuit integrated to circuit  40  of  FIG. 8 . This circuit is intended to provide the ON/OFF signal of switch K based on the sole detection of current in inductance L. Accordingly, the circuit of  FIG. 9  samples the detection information from terminals  14  and  11 . The voltage information sampled from terminal  11  is compared (comparator  42 ) with a first predetermined voltage reference Vref 0  (inverting input). Optionally, an amplifier  41  having its non-inverting input directly connected to terminal  11  and having its inverting input connected via a resistor R 41  to ground may be used upstream of comparator  42 . The inverting input is also looped back on the output of amplifier  41  by means of a resistor R 42 . Reference voltage Vref 0  is set according to the desired current I 0 max. 
   The output of comparator  42  crosses an inverter  43  before driving the control electrode (for example, the gate of a MOS transistor) of a switch  44  used to short-circuit a capacitor  45  forming the delay element. Capacitor  45  is supplied by a constant current source  46  drawing its power from terminal  14 , that is, directly from voltage Vin. The junction point of source  46  and of capacitor  45  is connected to the non-inverting input of a second comparator  43  having its inverting input receiving a second predetermined reference Vref 1  and having its output providing the ON/OFF signal. Reference voltage Vref 1  defines the variable duration of the delay of a method according to an embodiment of the present invention. 
     FIGS. 10A ,  10 B,  10 C illustrate, in the form of timing diagrams, the operation of the circuit of  FIG. 9 .  FIG. 10A  shows the shape of voltage V 41  at the output of amplifier  41  which forms an image of current IL in the inductance.  FIG. 10B  illustrates the shape of voltage V 45  across capacitor  45  and  FIG. 10C  illustrates the shape of the output voltage of amplifier V 43 , that is, of the ON/OFF signal. 
   When the current in the inductance increases, voltage V 41  increases proportionally. At a time t 1  where threshold Vref 0  is reached, the charge of capacitor  45  starts, after the opening of switch  44  by the switching of amplifier  42 . When this charge reaches level Vref 1  (time t 2 ), the output of amplifier  43  switches, which turns off the switch K and activates the limiting function. 
   Of course, embodiments of the present invention are likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the example implementation of the circuit discussed in relation with  FIG. 9  may be replaced with a circuit performing the same function, for example, using digital means. 
   Further, the selection of the thresholds to implement methods according to embodiments of the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove and on the inductance range for which the limiting circuit is desired to be intended. 
   Moreover, the interrupting of the power storage in the inductance is not necessarily obtained by interrupting the battery-inductance loop as discussed in relation with a step-up converter. According to the assembly, its supply may be interrupted with a switch placed between the power source and the inductance, a free wheel operation may be forced, a discharge may be forced, etc. 
   Finally, although embodiments of the present invention have been more specifically described in relation with a hardware analog implementation, its implementation may use digital and/or software means. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 
   Embodiments of the present invention may be applied to a variety of different types of electronic circuits, such as power converters like step-up and step-down converters. These electronic circuits may, in turn, be included in a variety of different types of electronic devices and systems, such as mobile or cellular phones, computer systems, and so on.