Control circuit for an inductive load driver

[0001] The present invention relates to a control circuit for an inductive load driver suitable for use in electronic ignition applications and smart power devices.

[0002] 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.

[0003] In particular, FIG. 1(a) shows a known solution. A control block 1 drives a first power element, in particular an IGBT transistor TR1 of electronic device 2. Control block 1 is powered by a supply voltage Vbat and activated by a trigger signal V_TRIGGER.

[0004] The first power element TR1 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 TR1 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 TR1 is also coupled to an inductive load, for example a primary winding 3A of a coil, coupled in turn to the supply voltage reference Vbat and to an igniter plug 3B of the coil itself.

[0005] A second power element, in particular an IGBT sensing transistor TR1_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 TR1.

[0006] During conduction, the two power elements TR1 and TR1_SENSE sink different fractions of the same conduction current I_COIL.

[0007] 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.

[0008] 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 TR1 is further reduced by the voltage drop ΔV introduced by connection cables and connectors.

[0009] FIG. 2 shows the same control block 10 of FIG. 1(b), wherein the voltage drop ΔV introduced by cables and connectors is shown.

[0010] 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 TR1, 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.

[0011] In the non-limiting case wherein the power element TR1 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.

[0012] 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.

[0013] 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 TR1. In particular an IGBT transistor whose active area is equal to 10 mm², driven by the control block 10 limiting the output current I_COIL to 20 amperes was used.

[0014] 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.

[0015] 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.

[0016] When the trigger signal V_TRIGGER reaches a low state, the IGBT transistor TR1 is disabled and the power accumulated in the primary winding 3A of a coil transfers to the secondary winding generating a spark on the igniter plug 3B.

[0017] When the current in the collector terminal C of the IGBT transistor TR1 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.

[0018] 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.

[0019] 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 TR1, allowing thus the current I_COIL required by the application to be delivered therefrom.

[0020] 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.

[0021] 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.

[0022] To address these problems, it would be thus necessary to further increase the circuit complexity of the control block 10 by inserting filters.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0037] 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 TR1 of an inductive driver circuit 9.

[0038] 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.

[0039] 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.

[0040] 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 D1 and D2 located in series between the collector and the emitter of the power element TR1 of the inductive driver device 9.

[0041] Voltage V_L is proportional to the collector voltage of power element TR1 and it is produced in an area located in the physical structure of the transistor itself as is shown in detail in FIG. 4.

[0042] 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 TR1 of inductive load driver 9.

[0043] Power element TR1 is associated, as already described in the prior art, to a second power element, in particular an IGBT sensing transistor TR_SENSE.

[0044] FIG. 4 shows the structure used to provide voltage V_L from the IGBT transistor TR1 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 TR1 as well as the sensing transistor TR_SENSE. A bonding pad 19 is also provided near the edge structure of power element TR1 in order to provide the value of voltage V_L.

[0045] The composite structure of the transistor TR1 comprises, a collector layer 11, a heavily doped P+ semiconductor layer 12, and a heavily doped N+ semiconductor layer 13.

[0046] 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.

[0047] Additional pad 19 is located on the edge of the transistor TR1 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.

[0048] FIG. 4 also shows diodes D1 and D2 coupled together with a common cathode coupled to node “L”.

[0049] 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.

[0050] 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 TR2 which can be integrated inside the same IGBT transistor TR1 of inductive load driver 9. According to the present invention, in this case, the control circuit 6 can be realised with low voltage technology.

[0051] As can be appreciated in FIG. 7, block A has been realised with a JFET transistor TR2 monolithically integrated inside the IGBT transistor TR1 of the inductive load driver 9.

[0052] In the evaluation circuit of the effectiveness of this second solution shown in FIG. 7, the JFET transistor TR2 has the control terminal coupled to the emitter E of transistor TR1, a first conduction terminal connected to the cathode node L being common to the two intrinsic diodes D1 and D2, and a second conduction terminal coupled to the gate terminal G of the transistor TR1 of the inductive load driver 9. A diode D3 is inserted between the second conduction terminal of the JFET transistor TR2 and the gate terminal G of the transistor TR1, while a Zener diode Dz is coupled between the second conduction terminal of the JFET transistor TR2 and GND.

[0053] An operational amplifier OP1 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 TR1_SENSE, and an output coupled to the gate terminal G of transistor TR1.

[0054] The voltage reference block 8 and the operational amplifier OP1 serving as current limiter allow the limitation function of the highest current output from the first transistor TR1 to be provided.

[0055] 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 TR1; and it does not absorb current, similarly to a diode.

[0056] 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 TR1, can overcome the trigger voltage V(TRIGG_—1) at the high state, which is the supply voltage of the control block, allowing the transistor TR1 to operate with a high output current.

[0057] 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 TR1, 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 TR1 is charged only by current I_AUX coming from the JFET transistor TR2.

[0058] The voltage on control terminal G of transistor TR1 is at the end limited by operational amplifier OP1 serving as current limiter, operating in adjustment, in order to have the predetermined coil current I(COIL_—1).

[0059] 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 TR1 reaches a value which is sufficient to let the collector current reach the limitation value.

[0060] 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.

[0061] 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.

[0062] 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.