Patent Abstract:
A control circuit for an inductive load driver includes a control block activated by a trigger signal and an output coupled to the control terminal of a power element. The control circuit includes an auxiliary current generator capable of delivering a current that is added to the current provided by control block and the sum of these currents is provided to the control terminal of the power element. The auxiliary current generator enables the inductive load driver to operate normally even though the trigger voltage is less than an optimal voltage value.

Full Description:
FIELD OF THE INVENTION 
   The present invention relates to a control circuit for an inductive load driver suitable for use in electronic ignition applications and smart power devices. 
   BACKGROUND OF THE INVENTION 
   As it is well known in the art, in smart power devices a linear control circuit is generally capable of driving a power element and allowing the highest voltage and deliverable current to be provided and diagnostic functions to be performed. These smart power devices are powered in two different modes. In a first mode, the control circuit receives a trigger signal and is powered by a battery. In a second mode, the control signal receives a trigger signal, and is also powered by the trigger signal, and is not coupled to the battery. 
   In particular,  FIG. 1(   a ) shows a known solution. A control block  1  drives a first power element, in particular an IGBT transistor TR 1  of electronic device  2 . Control block  1  is powered by a supply voltage Vbat and activated by a trigger signal V TRIGGER . 
   The first power element TR 1  is inserted between a first voltage reference, in particular a supply voltage Vbat and a second voltage reference, for example a ground reference GND. The first power element TR 1  has a first conduction terminal, in particular a collector terminal C, coupled to the first voltage reference Vbat, a second conduction terminal, in particular an emitter terminal E, coupled to GND, as well as a control terminal, in particular a gate terminal G coupled to control block  1 . The first power element TR 1  is also coupled to an inductive load, for example a primary winding  3 A of a coil, coupled in turn to the supply voltage reference Vbat and to an igniter plug  3 B of the coil itself. 
   A second power element, in particular an IGBT sensing transistor TR 1   SENSE , and a sensing resistance Rs are coupled in series to each other and in parallel between the terminals C and E of the first power element TR 1 . 
   During conduction, the two power elements TR 1  and TR 1   SENSE  sink different fractions of the same conduction current I COIL . 
     FIG. 1(   b ) shows a second known solution.  FIG. 1(   b ) shows a control block  10 , which is changed with respect to the control block  1  shown in  FIG. 1(   a ). The supply voltage of block  10  is provided by the voltage at the high state of the trigger signal, as well as being used to activate the control block  10 . 
   For the smart power devices such as those shown in  FIGS. 1(   a ) and  1 ( b ), the lowest and the highest output current I COIL  and the lowest and the highest voltage of control signal V TRIGGER  are specified. It is necessary that the smart power device  2  shown in  FIGS. 1(   a ) and  1 ( b ) correctly operates even in the worst case operating situations. A worst case operating situation occurs for low battery voltages Vbat at extreme temperatures, when the trigger voltage V TRIGGER , at the high state, can be reduced. For example, the signal ground can be separated from the power ground, and the real voltage being applied between the gate G and emitter E terminals of the first power element TR 1  is further reduced by the voltage drop ΔV introduced by connection cables and connectors. 
     FIG. 2  shows the same control block  10  of  FIG. 1(   b ), wherein the voltage drop ΔV introduced by cables and connectors is shown. 
   For electronic ignition applications, in the automotive field, under “normal” operating conditions, it is traditionally required that a power element, in the case of  FIG. 2  the first power element TR 1 , is capable of delivering an output current I COIL  no lower than 17 amperes, with a trigger signal V TRIGGER  at the input of the block whose level is equal to 5 volts. Under worst case operating conditions, however, the voltage value of the trigger signal V TRIGGER  can be reduced down to 2.5 volts. 
   In the non-limiting case wherein the power element TR 1  is an IGBT transistor, the highest voltage applied on the gate terminal G is given by the trigger voltage V TRIGGER  minus the voltage drop ΔV introduced by the control block  10 . This voltage also determines the highest output current I COIL  deliverable by smart power device  2 . 
   In this situation, it is very difficult to meet the required specification concerning the minimum output current I COIL  of 17 amperes, with the reduced-voltage trigger signal V TRIGGER , unless an IGBT transistor with an oversized area is used. 
     FIGS. 3(   a ) and  3 ( b ) shows, in a series of graphs, signals related to simulations on the circuit of  FIG. 2  performed for the first power element TR 1 . In particular an IGBT transistor whose active area is equal to 10 mm 2 , driven by the control block  10  limiting the output current I COIL  to 20 amperes was used. 
     FIG. 3(   a ) relates to the case of the trigger voltage V TRIGGER  at the high state of 2.5 volts and  FIG. 3(   b ) shows the simulation results as the amplitude of the voltage varies. 
   When the trigger voltage V TRIGGER  is 2.5 volts [V(TRIGG — 1)], the output current I(COIL — 1) stays lower than 4 amperes, which is quite lower than the predetermined limitation current, since the actual voltage V(GATE — 1) calculated on the gate terminal G corresponds to about 2.3 volts. 
   When the trigger signal V TRIGGER  reaches a low state, the IGBT transistor TR 1  is disabled and the power accumulated in the primary winding  3 A of a coil transfers to the secondary winding generating a spark on the igniter plug  3 B. 
   When the current in the collector terminal C of the IGBT transistor TR 1  is too low, the accumulated power can be insufficient for generating the mixture combustion in the explosion chamber, with the subsequent misfiring phenomenon which is, as is well known, very damaging for the motor. 
   From the simulations of  FIG. 3(   b ) it can thus be appreciated how the current I COIL , to reach the limitation value of 20 amperes, requires a trigger signal V TRIGGER  whose amplitude is no lower than 4 volts. 
   To overcome the above-mentioned problems, the prior art suggests the use of a charge pump fed by a trigger signal V TRIGGER , capable of generating an output voltage being sufficiently high as to conveniently bias the gate terminal G of the IGBT transistor TR 1 , allowing thus the current I COIL  required by the application to be delivered therefrom. 
   Although this solves the problem, the solution has a serious drawback. The noise generated by the inner oscillator of the charge pump can cause electromagnetic noise making the device incompatible with emission regulations. 
   Moreover, this noise, which is also reflected on the voltage of collector terminal C, is transferred to the coil secondary winding, by means of the turn ratio, which can generate undesired overvoltages. 
   To address these problems, it would be thus necessary to further increase the circuit complexity of the control block  10  by inserting filters. 
   What is desired, therefore, is a control circuit for a smart power device having a reduced amplitude trigger signal, such that a sufficiently high current is produced to avoid ignition problems, but overcomes the problems associated with the prior art solutions described above. 
   SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, an auxiliary current generator capable of delivering an auxiliary current is added to the current output of the control circuit in order to bias, according to the provided specifications, the control terminal of an IGBT power element. 
   According to an embodiment of the present invention a control circuit for an inductive load driver includes a control block for receiving a trigger signal (V TRIGGER ) and an output coupled to a control terminal of a power element such as an IGBT transistor. An auxiliary current generator delivers a current that is added to the current output by the control block to supply a driving current I GATE  to the power element. 
   According to an embodiment of the present invention, the control block of the inductive load driver is not coupled to an external power supply, so that it can be assembled in simpler and cheaper packages like three pin packages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1   a  schematically shows a first control block, fed by a battery, driving a power element of an electronic device driving an inductive load, according to the prior art; 
       FIG. 1   b  schematically shows a second control block, fed by a trigger signal, driving the power element of the electronic device driving an inductive load, according to the prior art; 
       FIG. 2  schematically shows the second control block, fed by the trigger signal, driving the power element of the electronic device and wherein a voltage drop introduced by cables and connectors is also shown, according to the prior art; 
       FIGS. 3(   a ) and  3 ( b ) are timing diagrams that show the evolution of voltage and current waveforms obtained by simulating the operation of the electronic device of  FIG. 2 , according to the prior art; 
       FIG. 4  is a cross-sectional diagram of a portion of a semiconductor integrated circuit comprising a power element and bonding pad for the extraction of a collector voltage value; 
       FIG. 5  schematically shows a control circuit according to an embodiment of the present invention for a power element of an inductive load driver; 
       FIG. 6  shows a typical operating curve of a JFET structure for being integrated inside the control circuit, according to the present invention; and 
       FIG. 7  schematically shows a testing circuit for the optional evaluation of a particular configuration of the control circuit of  FIG. 5 , according to the present invention. 
       FIGS. 8(   a ) and  8 ( b ) show the result of the simulation of the circuit of  FIG. 7 . 
   

   DETAILED DESCRIPTION 
     FIG. 5  shows a control circuit  6  receiving at its input a trigger signal V TRIGGER  and a collector voltage value V L  and outputting a current value I GATE  driving a first power element, particularly an IGBT transistor TR 1  of an inductive driver circuit  9 . 
   In particular, control circuit  6  comprises a control block  10  receiving, as in the prior art, at its input the trigger signal V TRIGGER  and generating a first current I DRIV . 
   Control circuit  6  also comprises, according to the invention, a current generator block A receiving at its input voltage V L  and producing a second current, in particular an auxiliary current I AUX . 
   According to the invention, the input voltage V L  of the control circuit  6 , by means of the auxiliary current I AUX  generator block A, is detected at node L, which is the cathode terminal common to two intrinsic diodes D 1  and D 2  located in series between the collector and the emitter of the power element TR 1  of the inductive driver device  9 . 
   Voltage V L  is proportional to the collector voltage of power element TR 1  and it is produced in an area located in the physical structure of the transistor itself as is shown in detail in  FIG. 4 . 
   Referring again to  FIG. 5 , according to the present invention, the contributions of the currents—first I DRIV  and second I AUX —are added, by means of a logic OR gate  7 , and they output a current I GATE  to control the power element TR 1  of inductive load driver  9 . 
   Power element TR 1  is associated, as already described in the prior art, to a second power element, in particular an IGBT sensing transistor TR SENSE . 
     FIG. 4  shows the structure used to provide voltage V L  from the IGBT transistor TR 1  of inductive load driver  9 , which is coupled to block A, which in turn provides the auxiliary current I AUX .  FIG. 4  shows an integrated structure comprising the first power element TR 1  as well as the sensing transistor TR SENSE . A bonding pad  19  is also provided near the edge structure of power element TR 1  in order to provide the value of voltage V L . 
   The composite structure of the transistor TR 1  comprises, a collector layer  11 , a heavily doped P+ semiconductor layer  12 , and a heavily doped N+ semiconductor layer  13 . 
   On semiconductor layer  13  a weakly doped N− epitaxial layer  14  is formed including P-type wells  15 . Pairs of N-type active areas with electrodes  18  are formed in wells  15 . 
   Additional pad  19  is located on the edge of the transistor TR 1  structure. Using pad  19 , the voltage V L  of the transistor is provided, which is brought back to the input of the auxiliary current I AUX  generator block A. 
     FIG. 4  also shows diodes D 1  and D 2  coupled together with a common cathode coupled to node “L”. 
   According to an embodiment of the present invention, by using high voltage technology, such as VIPower® technology, the auxiliary current I AUX  generator block A can be realised with a J-FET transistor structure [(I=f(V)] integrated inside the control circuit  6 . This solution has the advantage of pinching at high voltage, as shown in the curve of  FIG. 6 . 
   Referring now to  FIG. 7 , a second possible optional solution to realise the auxiliary current I AUX  generator block A is the use of a third power element, in particular a JFET transistor TR 2  which can be integrated inside the same IGBT transistor TR 1  of inductive load driver  9 . According to the present invention, in this case, the control circuit  6  can be realised with low voltage technology. 
   As can be appreciated in  FIG. 7 , block A has been realised with a JFET transistor TR 2  monolithically integrated inside the IGBT transistor TR 1  of the inductive load driver  9 . 
   In the evaluation circuit of the effectiveness of this second solution shown in  FIG. 7 , the JFET transistor TR 2  has the control terminal coupled to the emitter E of transistor TR 1 , a first conduction terminal connected to the cathode node L being common to the two intrinsic diodes D 1  and D 2 , and a second conduction terminal coupled to the gate terminal G of the transistor TR 1  of the inductive load driver  9 . A diode D 3  is inserted between the second conduction terminal of the JFET transistor TR 2  and the gate terminal G of the transistor TR 1 , while a Zener diode Dz is coupled between the second conduction terminal of the JFET transistor TR 2  and GND. 
   An operational amplifier OP 1  is also provided, serving as current limiter, with an inverting input (−) coupled to a generator block  8  of reference voltage VREF, receiving at its input the trigger signal V TRIGGER , a non-inverting input (+) coupled to the emitter terminal E SENSE  of the sensing transistor TR 1   SENSE , and an output coupled to the gate terminal G of transistor TR 1 . 
   The voltage reference block  8  and the operational amplifier OP 1  serving as current limiter allow the limitation function of the highest current output from the first transistor TR 1  to be provided. 
   To implement the evaluation circuit of the effectiveness of this second solution shown in  FIG. 7 , it is necessary that control block  10  have the following features: it delivers the current I DRIV  when the input voltage V TRIGGER  is higher than the output voltage thereof, coinciding with the voltage Vg of the control terminal G of the transistor TR 1 ; and it does not absorb current, similarly to a diode. 
     FIGS. 8(   a ) and  8 ( b ) show the result of the simulation of the circuit of  FIG. 7 . In  FIG. 8(   a ) the voltage V(GATE — 1), which can be applied on the control terminal G of the transistor TR 1 , can overcome the trigger voltage V(TRIGG — 1) at the high state, which is the supply voltage of the control block, allowing the transistor TR 1  to operate with a high output current. 
   The control terminal G capacitance is charged in two following steps: in a first step the current I DRIV  charges the control terminal G of transistor TR 1 , until the gate voltage is lower than the V_TRIGGER voltage at the high state; and in a second step, when I DRIV  is zero, the control terminal G of transistor TR 1  is charged only by current I AUX  coming from the JFET transistor TR 2 . 
   The voltage on control terminal G of transistor TR 1  is at the end limited by operational amplifier OP 1  serving as current limiter, operating in adjustment, in order to have the predetermined coil current I(COIL — 1). 
   In  FIG. 8(   b ) the limitation current is always reached, as the amplitude of the trigger voltage V TRIGGER  varies. As the amplitude of the signal V TRIGGER  varies, the corresponding voltage V(GATE) of the control terminal G of the transistor TR 1  reaches a value which is sufficient to let the collector current reach the limitation value. 
   This type of solution allows the device to reach the nominal current even in the worst case, in order to always charge the coil in an optimal way, and to have in the turn-off step the convenient amount of energy available. 
   According to an embodiment of the present invention a control circuit is provided for power devices driven by input signals having, at a high logic state, a non-optimal voltage value. 
   While there have been described above the principles of the present invention in conjunction with a preferred embodiment thereof, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Technology Classification (CPC): 5