Control circuit for an inductive load driver

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.

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 block1drives a first power element, in particular an IGBT transistor TR1of electronic device2. Control block1is powered by a supply voltage Vbat and activated by a trigger signal VTRIGGER.

The first power element TR1is 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 TR1has 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 block1. The first power element TR1is also coupled to an inductive load, for example a primary winding3A of a coil, coupled in turn to the supply voltage reference Vbat and to an igniter plug3B of the coil itself.

A second power element, in particular an IGBT sensing transistor TR1SENSE, 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.

During conduction, the two power elements TR1and TR1SENSEsink different fractions of the same conduction current ICOIL.

FIG. 1(b) shows a second known solution.FIG. 1(b) shows a control block10, which is changed with respect to the control block1shown inFIG. 1(a). The supply voltage of block10is provided by the voltage at the high state of the trigger signal, as well as being used to activate the control block10.

For the smart power devices such as those shown inFIGS. 1(a) and1(b), the lowest and the highest output current ICOILand the lowest and the highest voltage of control signal VTRIGGERare specified. It is necessary that the smart power device2shown inFIGS. 1(a) and1(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 VTRIGGER, 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 TR1is further reduced by the voltage drop ΔV introduced by connection cables and connectors.

FIG. 2shows the same control block10ofFIG. 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 ofFIG. 2the first power element TR1, is capable of delivering an output current ICOILno lower than 17 amperes, with a trigger signal VTRIGGERat 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 VTRIGGERcan be reduced down to 2.5 volts.

In the non-limiting case wherein the power element TR1is an IGBT transistor, the highest voltage applied on the gate terminal G is given by the trigger voltage VTRIGGERminus the voltage drop ΔV introduced by the control block10. This voltage also determines the highest output current ICOILdeliverable by smart power device2.

In this situation, it is very difficult to meet the required specification concerning the minimum output current ICOILof 17 amperes, with the reduced-voltage trigger signal VTRIGGER, unless an IGBT transistor with an oversized area is used.

FIGS. 3(a) and3(b) shows, in a series of graphs, signals related to simulations on the circuit ofFIG. 2performed for the first power element TR1. In particular an IGBT transistor whose active area is equal to 10 mm2, driven by the control block10limiting the output current ICOILto 20 amperes was used.

FIG. 3(a) relates to the case of the trigger voltage VTRIGGERat the high state of 2.5 volts andFIG. 3(b) shows the simulation results as the amplitude of the voltage varies.

When the trigger voltage VTRIGGERis 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 VTRIGGERreaches a low state, the IGBT transistor TR1is disabled and the power accumulated in the primary winding3A of a coil transfers to the secondary winding generating a spark on the igniter plug3B.

When the current in the collector terminal C of the IGBT transistor TR1is 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 ofFIG. 3(b) it can thus be appreciated how the current ICOIL, to reach the limitation value of 20 amperes, requires a trigger signal VTRIGGERwhose 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 VTRIGGER, 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 ICOILrequired 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 block10by 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 (VTRIGGER) 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 IGATEto 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.

DETAILED DESCRIPTION

FIG. 5shows a control circuit6receiving at its input a trigger signal VTRIGGERand a collector voltage value VLand outputting a current value IGATEdriving a first power element, particularly an IGBT transistor TR1of an inductive driver circuit9.

In particular, control circuit6comprises a control block10receiving, as in the prior art, at its input the trigger signal VTRIGGERand generating a first current IDRIV.

Control circuit6also comprises, according to the invention, a current generator block A receiving at its input voltage VLand producing a second current, in particular an auxiliary current IAUX.

According to the invention, the input voltage VLof the control circuit6, by means of the auxiliary current IAUXgenerator block A, is detected at node L, which is the cathode terminal common to two intrinsic diodes D1and D2located in series between the collector and the emitter of the power element TR1of the inductive driver device9.

Voltage VLis proportional to the collector voltage of power element TR1and it is produced in an area located in the physical structure of the transistor itself as is shown in detail inFIG. 4.

Referring again toFIG. 5, according to the present invention, the contributions of the currents—first IDRIVand second IAUX—are added, by means of a logic OR gate7, and they output a current IGATEto control the power element TR1of inductive load driver9.

Power element TR1is associated, as already described in the prior art, to a second power element, in particular an IGBT sensing transistor TRSENSE.

FIG. 4shows the structure used to provide voltage VLfrom the IGBT transistor TR1of inductive load driver9, which is coupled to block A, which in turn provides the auxiliary current IAUX.FIG. 4shows an integrated structure comprising the first power element TR1as well as the sensing transistor TRSENSE. A bonding pad19is also provided near the edge structure of power element TR1in order to provide the value of voltage VL.

The composite structure of the transistor TR1comprises, a collector layer11, a heavily doped P+ semiconductor layer12, and a heavily doped N+ semiconductor layer13.

On semiconductor layer13a weakly doped N− epitaxial layer14is formed including P-type wells15. Pairs of N-type active areas with electrodes18are formed in wells15.

Additional pad19is located on the edge of the transistor TR1structure. Using pad19, the voltage VLof the transistor is provided, which is brought back to the input of the auxiliary current IAUXgenerator block A.

FIG. 4also shows diodes D1and D2coupled 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 IAUXgenerator block A can be realised with a J-FET transistor structure [(I=f(V)] integrated inside the control circuit6. This solution has the advantage of pinching at high voltage, as shown in the curve ofFIG. 6.

Referring now toFIG. 7, a second possible optional solution to realise the auxiliary current IAUXgenerator block A is the use of a third power element, in particular a JFET transistor TR2which can be integrated inside the same IGBT transistor TR1of inductive load driver9. According to the present invention, in this case, the control circuit6can be realised with low voltage technology.

As can be appreciated inFIG. 7, block A has been realised with a JFET transistor TR2monolithically integrated inside the IGBT transistor TR1of the inductive load driver9.

In the evaluation circuit of the effectiveness of this second solution shown inFIG. 7, the JFET transistor TR2has 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 D1and D2, and a second conduction terminal coupled to the gate terminal G of the transistor TR1of the inductive load driver9. A diode D3is inserted between the second conduction terminal of the JFET transistor TR2and the gate terminal G of the transistor TR1, while a Zener diode Dz is coupled between the second conduction terminal of the JFET transistor TR2and GND.

An operational amplifier OP1is also provided, serving as current limiter, with an inverting input (−) coupled to a generator block8of reference voltage VREF, receiving at its input the trigger signal VTRIGGER, a non-inverting input (+) coupled to the emitter terminal ESENSEof the sensing transistor TR1SENSE, and an output coupled to the gate terminal G of transistor TR1.

The voltage reference block8and the operational amplifier OP1serving as current limiter allow the limitation function of the highest current output from the first transistor TR1to be provided.

To implement the evaluation circuit of the effectiveness of this second solution shown inFIG. 7, it is necessary that control block10have the following features: it delivers the current IDRIVwhen the input voltage VTRIGGERis 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.

FIGS. 8(a) and8(b) show the result of the simulation of the circuit ofFIG. 7. InFIG. 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 TR1to operate with a high output current.

The control terminal G capacitance is charged in two following steps: in a first step the current IDRIVcharges 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 IDRIVis zero, the control terminal G of transistor TR1is charged only by current IAUXcoming from the JFET transistor TR2.

The voltage on control terminal G of transistor TR1is at the end limited by operational amplifier OP1serving as current limiter, operating in adjustment, in order to have the predetermined coil current I(COIL—1).

InFIG. 8(b) the limitation current is always reached, as the amplitude of the trigger voltage VTRIGGERvaries. As the amplitude of the signal VTRIGGERvaries, the corresponding voltage V(GATE) of the control terminal G of the transistor TR1reaches 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.