POWER SEMICONDUCTOR DEVICE FOR IGNITER

Abstract

A power semiconductor device for an igniter comprises: a semiconductor switching device causing a current to flow through a primary side of an ignition coil or shutting off the current flowing through the primary side of the ignition coil; an integrated circuit driving and controlling the semiconductor switching device; and a temperature sensing element sensing temperature of the semiconductor switching device, wherein the integrated circuit including an overheat protection circuit limiting a current through the semiconductor switching device to a value lower than a current through the semiconductor switching device during normal operation, when temperature sensed by the temperature sensing element is over predetermined temperature.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power semiconductor device for an igniter having an overheat protection function to protect a semiconductor switching device at an abnormally high temperature in an ignition system for an internal combustion engine.

2. Background Art

An ignition system for an internal combustion engine such as an automobile engine has, as components for generating a high voltage to be applied to an ignition plug, and a power semiconductor device incorporating an ignition coil (inductive load), a semiconductor switching device for driving the ignition coil and a circuit device (semiconductor integrated circuit) for controlling the semiconductor switching device. These components constitute a so-called igniter. The ignition system also has an engine control unit (ECU) including a computer. In many cases, an overheat protection function for protecting the semiconductor switching device in the event of occurrence of abnormal heat generation or the like during operation by sensing the abnormal heat generation and forcibly shutting off the current flowing through the semiconductor switching device is provided in the power semiconductor device (see, for example, Japanese Patent Laid-Open No. 8-338350).

Because the overheat protection function is an operation according to self-protection of the power semiconductor device, the timing of shutting off is performed independently of ignition signal timing performed by the ECU. There is, therefore, a possibility of ignition occurring at an inappropriate time in the ignition sequence as a result of a shutoff operation by the overheat protection function to cause a backfire or knocking in the engine.

As a measure against the problem, methods have been proposed for softly shutting off the current so as not to cause ignition at the time of shutting off, i.e., for preventing an unnecessary ignition operation by setting the speed of shutting off the current flowing through the primary side coil of the ignition coil low enough to avoid inducing arc discharge on the ignition plug (see, for example, Japanese Patent Laid-Open Nos. 2001-248529 and 2008-45514).

SUMMARY OF THE INVENTION

By the overheat protection function of the conventional power semiconductor device for igniters, the current flowing through the semiconductor switching device is softly shut off in the event of increase in temperature to an abnormally high level so as not to induce arc discharge on the ignition plug. However, the shutting operation by the overheat protection function is started upon sensing an abnormally high temperature and the shutoff state is thereafter maintained as long as the device temperature is above a predetermined constant level. There is, therefore, a problem that the engine is caused to enter a completely stopped state simultaneously with sensing overheat regardless of the control signal on the ECU side and is maintained in the stopped state. This cannot be said to be the best measure from the viewpoint of motor vehicle fail-safe control. For the purpose of preventing an erroneous operation, a hysteresis is ordinarily set in a comparator for overheat determination such that a recovery is not made unless a temperature lower than the temperature at which the shutoff is made is again reached. Therefore a considerably long time is taken to enable restarting of the engine.

Also, realization of a soft shutoff without inducing arc discharge in the ignition plug requires the provision of a circuit for producing a time constant of about 10 to 100 msec. Forming such a kind of circuit in the semiconductor integrated circuit entails a problem that the chip size is increased or the number of manufacturing steps is increased. Forming such a partial circuit outside the semiconductor integrated circuit also entails a problem that the manufacturing cost of the power semiconductor device is increased due to an increase in the number of component parts.

In view of the above-described problems, an object of the present invention is to provide a power semiconductor device for igniters which does not perform the operation to shut off the semiconductor switching device at any time other than the time at which the ignition signal is issued on the ECU side, and which is, therefore, capable of preventing ignition at any inappropriate time while protecting itself in the event of increase in temperature to an abnormally high level.

According to the present invention, a power semiconductor device for an igniter comprises: a semiconductor switching device causing a current to flow through a primary side of an ignition coil or shutting off the current flowing through the primary side of the ignition coil; an integrated circuit driving and controlling the semiconductor switching device; and a temperature sensing element sensing temperature of the semiconductor switching device, wherein the integrated circuit including an overheat protection circuit limiting a current through the semiconductor switching device to a value lower than a current through the semiconductor switching device during normal operation, when temperature sensed by the temperature sensing element is over predetermined temperature.

In the event of increase in temperature to an abnormally high level, the current through the semiconductor switching device is limited to a value lower than its value during the normal operation to reduce Joule loss in the semiconductor switching device, thereby protecting the semiconductor switching device. Basically, the complete shutoff operation is not performed at any time other than ECU ignition signal timing. Therefore, no erroneous ignition by inappropriate timing occurs and the need for a soft current shutoff circuit is eliminated. Further, the shutoff operation is not positively performed; only limiting of the current through the semiconductor switching device to a low level is performed. The engine is not stopped immediately after detection of overheat. Therefore, a time margin can be provided in which suitable steps are performed on the ECU side.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an ignition system according to a first embodiment of the present invention.

FIG. 2 is a timing chart for illustrating the operation of the ignition system according to the first embodiment of the present invention.

FIG. 3 shows the relationships between temperature and the reverse saturation current through the Schottky barrier diode used as a temperature sensing element according to first to fourth embodiments of the present invention.

FIG. 4 shows the relationships between temperature and a current limit value applied in a semiconductor switching device according to first to fourth embodiments of the present invention.

FIG. 5 is a circuit diagram showing an ignition system according to a second embodiment of the present invention.

FIG. 6 is a circuit diagram showing an ignition system according to a third embodiment of the present invention.

FIG. 7 is a circuit diagram showing an ignition system according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1 shows an embodiment of an ignition system according to the present invention. In the ignition system shown in FIG. 1, a power supply Vbat such as a battery is connected to one end of a primary coil 61 of an ignition coil 6, while an igniter power semiconductor device 5 is connected to the other end of the primary coil 61. The power supply Vbat is also connected to one end of a secondary coil 62, and an ignition plug 7 having one end grounded is connected to the other end of the secondary coil 62.

Furthermore, an ECU 1 outputs a control input signal for driving a semiconductor switching device 41 to the igniter power semiconductor device.

In this ignition system, the igniter power semiconductor device 5 has a semiconductor switching device 4 including an insulated gate bipolar transistor (IGBT) 41 for causing a current to flow through the primary coil 61 or shutting off the current flowing through the primary coil 61, and an integrated circuit 3 for driving and controlling the IGBT 41 according to the control signal from the ECU 1 and other operating conditions.

As the IGBT 41, which is a main component of the semiconductor switching device 4, an IGBT having, in addition to the ordinary electrode terminals, i.e., the collector, emitter and gate, a sense emitter for sensing the collector current Ic, through which a current proportional to (for example, about 1/1000 of) the collector current flows, is adopted. Also, a Zener diode 42 provided for protection against a surge voltage is connected between the collector and the gate in the reverse direction.

Further, a Schottky barrier diode 43 is provided on the same substrate as a temperature sensing element for sensing the temperature of the semiconductor switching device 4. The anode of the Schottky barrier diode 43 is connected to the emitter terminal of the IGBT 41, while the cathode of the Schottky barrier diode 43 is connected to the reference side of a current mirror circuit in the integrated circuit 3 described below.

The functions of the integrated circuit 3 and the ignition operation of the entire ignition system will now be described with reference to the timing chart of FIG. 2.

A high-level control input signal applied at time t1 from the ECU 1 to an input terminal of the integrated circuit 3 undergoes waveform shaping in a Schmitt trigger circuit 11 and thereafter turns off a first Pch MOS 12. A first current mirror circuit constituted by a second Pch MOS 17 and a third Pch MOS 18 then operates to cause an output current Ig2 to flow through a first resistor 23, thereby generating a gate drive voltage to the IGBT 41.

A reference-side current value Ig1 of the first current mirror circuit is equal to the result of subtraction of an output current value If2 of a current-limiting circuit described below and an output current 132 of an overheat protection circuit described below from an output current value Ib1 of a constant-current source 19. With respect to this reference-side current Ig1, a current Ig2 according to the mirror ratio of the first current mirror circuit is produced as an output current.

A collector current Ic such as shown in FIG. 2 flows through the primary coil 61 and the IGBT 41 according to a time constant determined by the inductance and the wiring resistance of the primary coil 61.

Next, a low-level control input signal is applied at time t2 from the ECU 1. The first Pch MOS 12 is thereby turned on to stop the first current mirror circuit. Charge accumulated on the gate of the IGBT 41 is discharged in an extremely short time through the first resistor 23. As a result, the IGBT 41 is shut off.

At this time, a high voltage of about 500 V is generated on the collector terminal of the IGBT 41 by the primary coil 61 in the direction to maintain the current that has been flowing. This voltage is boosted to 30 kV according to the winding ratio of the ignition coil 6 to generate arc discharge on the ignition plug 7 connected to the secondary coil 62.

Next, a case where the high-level control input signal is applied from the ECU 1 for a comparatively long energization time period at time t3 will be described.

By the application of the high-level control input signal from the ECU 1, the collector current Ic is gradually increased from time t3 in the way described above. However, a current limit value for inhibiting the collector current In from becoming equal to or higher than a predetermined constant value is set for the purpose of preventing melting of the winding of the ignition coil 6 and magnetic saturation of the transformer.

Limiting of the collector current Ic is realized by a mechanism described below. A sense current Ies from the IGBT 41 flows through a second resistor 24 in the integrated circuit 3 to generate a voltage across the second resistor 24 according to the collector current Ic of the IGBT 41. This voltage is compared with a voltage Vref1 of a first reference voltage source 22 by an amplifier 21. A V-I conversion circuit 20 outputs a current If1 according to the difference between the compared values. From this current If1, a second current mirror circuit constituted by a fourth Pch MOS 13 and a fifth Pch MOS 14 produces an output current according its mirror ratio. This output current is output as a current-limiting signal If2. The current-limiting signal If2 acts in the direction to reduce the current Ig2 from which the gate drive voltage to the IGBT 41 is generated. As a result, the gate voltage is reduced to inhibit the collector current Ic from increasing. That is, the entire system operates in a negative feedback manner with respect to the collector current Ic, thereby limiting the collector current Ic to a predetermined constant value.

When the collector current Ic becomes equal to the current limit value at time t4, the gate voltage to the IGBT 41 is lower and the IGBT 41 operates in pentode fashion. That is, while the collector current Ic is flowing, the collector voltage is not sufficiently reduced; Joule loss is being produced in the IGBT 41.

When the operation temperature is increased, the allowable power dissipation of the IGBT 41 is reduced. Therefore, an overheat protection function to limit Joule loss according to temperature is required for protection of the IGBT 41. The mechanism of the overheat protection function will be described below.

The cathode of the Schottky barrier diode 43 mounted on the semiconductor switching device 4 is connected to the reference side of a third current mirror circuit constituted by a sixth Pch MOS 15 and a seventh Pch MOS 16 in the integrated circuit 3. Also, the output current Is2 from the third current mirror circuit acts in the direction to reduce the current Ig2 from which the gate drive voltage to the IGBT 41 is generated, as in the case of the above-described current-limiting function.

The reverse saturation current Is through the Schottky barrier diode increases abruptly when the temperature exceeds about 170° C., as shown in the temperature characteristic graph in FIG. 3.

Thus, when the operating temperature exceeds about 170° C., the collector current Ic is reduced by reducing the gate drive voltage by means of the Schottky barrier diode 43 and the third current mirror circuit constituted by the sixth Pch MOS 15 and the seventh Pch MOS 16. The overheat protection function to limit Joule loss in the IGBT 41 is thus realized.

The above-described mechanism has the effect of reducing the current Ig2 from which the gate drive voltage is generated in common with the above-described current-limiting function. In other words, the above-described overheat protection function is a function to set the current limit value lower than it is during the normal operation when the operating temperature exceeds about 170° C., as shown in FIG. 4.

The overheat protection function in the first embodiment is not to positively shut off the IGBT 41 but only to reduce the collector current limit value for the IGBT 41. That is, no shutoff is made at any time other than the proper ECU 1 timing, and prevention of erroneous ignition with the ignition plug 7 is enabled without additionally providing a soft shutoff function.

If the operating temperature continues to increase, the current limit value continues to decrease, finally making impossible the supply of energy sufficient for causing arc discharge on the ignition plug 7. However, generally speaking, the operating speed of the ECU 1 is extremely high in comparison with rising of the operating temperature. There is, therefore, a sufficient time period from a start of overheat protection to a point in time at which a misfire actually occurs, and a sufficient time margin can be taken in which the ECU 1 can detect a misfire due to overheat protection and take suitable steps.

Second Embodiment

FIG. 5 shows a second embodiment of the igniter power semiconductor device according to the present invention. In the drawings, components equivalent in function to each other are indicated by the same reference characters. Description will not be redundantly made for them.

The second embodiment has a feature in that the Schottky barrier diode mounted on the semiconductor switching device 4 in the first embodiment is mounted in the integrated circuit 3. In the igniter power semiconductor device 5, the semiconductor switching device 4 and the integrated circuit 3 are disposed close to each other on a same conductor circuit board. Therefore the thermal coupling between these components is markedly good. For this reason, the same effect as that in the case where the temperature sensing element is mounted on the semiconductor switching device 4 can also be obtained.

It is more desirable that the Schottky barrier diode 25 in the integrated circuit 3 be mounted at a position close to the semiconductor switching device 4 in the layout, e.g., in the vicinity of the side of the integrated circuit 3 facing the switching device 4.

In the present embodiment, the connection lines and pads for the Schottky barrier diode 43 required in the first embodiment can be removed; the efficiency of layout patterning of the semiconductor switching device 4 is improved; and the components can be disposed with improved area efficiency. Therefore the igniter power semiconductor device 5 can be implemented in a reduced size at a low cost.

It is possible to positively utilize the state where the Schottky barrier diode 25 provided as a temperature sensing element is mounted in the integrated circuit 3. For example, a Schottky barrier diode may be used as the diode used to form the constant-current source 19 in place of the ordinary PN junction type to adjust a temperature characteristic of the constant-current source to that of the Schottky barrier diode 25 provided as a temperature sensing element.

It is possible to make steeper the current limit value reduction characteristic at the time of overheat protection as well as the temperature characteristic of the temperature sensing element by giving a temperature characteristic to the constant-current source 19. An extremely high degree of characteristic matching between the components can be achieved in the same integrated circuit 3. Therefore the temperature characteristics of the constant-current source 19 and the Schottky barrier diode 25 provided as a temperature sensing element can be matched to each other with high accuracy.

Third Embodiment

FIG. 6 shows a third embodiment of the igniter power semiconductor device according to the present invention. In the first and second embodiments, there is a possibility of failure to obtain the desired reduction characteristic with respect to the current limit value at the time of overheat protection due to manufacturing process variation in the reverse saturation current through the Schottky barrier diode. This variation may be adjusted through an external connection terminal to improve the yield of the product and to enable the adjustment of the current limit value attenuation sensitivity according to use of the product.

In the example of the circuit shown in FIG. 6, three temperature sensing element selecting circuits S1, S2, and S3 having different output current values are provided and selection between validity and invalidity of each temperature sensing element output from the outside of the igniter power semiconductor device 5 is enabled.

Schottky barrier diodes 25 are respectively incorporated in each of the temperature sensing element selecting circuits S1 to S3. The sizes of the diodes therein are binary-weighted (for example, if the size in S1 is 1, the size in S2 is 2 and the size in S3 is 4). Further, each temperature sensing element selecting circuit is validated/invalidated by turning on/off an eighth Pch MOS 26. To turn on/off the eighth Pch MOS 26, an external terminal is grounded or opened.

In this way, selection of the Schottkey barrier diode size from the eight sizes: 0 to 7 can be made through a combination of the temperature sensing elements in the circuits S1 to S3. Selection among the temperature sensing elements may be set from the outside of the igniter power semiconductor device 5 as in the present embodiment. If adjustment is performed only in the manufacturing process, the arrangement may be such that no external terminals are provided and selection from validity and invalidity of each temperature sensing element is made through the existence/nonexistence of wire bonding between pads from S1 to S3 provided on the integrated circuit 3 and a ground terminal.

Fourth Embodiment

FIG. 7 shows a fourth embodiment of the igniter power semiconductor device according to the present invention. As described in the first embodiment, when the overheat protection operation is started in response to an increase in operating temperature, it is desirable to perform suitable feedback steps such as notifying the ECU 1 of the situation, referring to table data of the control signal ON time and reducing the engine output.

In the present embodiment, an overheat protection operation condition output circuit 10 is provided in the integrated circuit 3. A ninth Pch MOS 40 is connected to the output side of the third current mirror circuit that detects the reverse saturation current Is1 through the Schotkky barrier diode 25 provided as a temperature sensing element, and an output current Is3 from the ninth Pch MOS 40 flows through a third resistor 43. The voltage generated across the third resistor 43 and a voltage Vref2 of a second reference voltage source 42 are compared by a comparator 41. An output from the comparator 41 is taken out of the igniter power semiconductor device 5 to be monitored by the ECU 1.

With the igniter power semiconductor device 5 in the fourth embodiment configured as described above, the ECU 1 can always grasp whether or not the overheat protection operation is presently being performed from the output of the comparator 41 and can therefore perform suitable feedback steps.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2009-278422, filed on Dec. 8, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.

Claims

1. A power semiconductor device for an igniter comprising:

a semiconductor switching device causing a current to flow through a primary side of an ignition coil or shutting off the current flowing through the primary side of the ignition coil;

an integrated circuit driving and controlling the semiconductor switching device; and

a temperature sensing element sensing temperature of the semiconductor switching device,

wherein the integrated circuit including an overheat protection circuit limiting a current through the semiconductor switching device to a value lower than a current through the semiconductor switching device during normal operation, when temperature sensed by the temperature sensing element is over predetermined temperature.

2. The power semiconductor device for an igniter according to claim 1, wherein the temperature sensing element and the semiconductor switching device are provided on a common substrate.

3. The power semiconductor device for an igniter according to claim 1, wherein the temperature sensing element and the integrated circuit are provided on a common substrate.

4. The power semiconductor device for an igniter according to claim 1, wherein the temperature sensing element includes a Schottky barrier diode.

5. The power semiconductor device for an igniter according to claim 1, wherein temperature characteristic of the temperature sensing element can be externally adjusted.

6. The power semiconductor device for an igniter according to claim 1, further comprising an overheat protection operation condition output circuit outputting a signal indicating that the overheat protection circuit is limiting the current through the semiconductor switching device.