Power module

A power module includes an IGBT; a MOSFET connected in parallel with the IGBT; a lead frame having a first frame portion on which the IGBT is mounted and a second frame portion on which the MOSFET is mounted, and having a step by which the first frame portion is located at a first height and the second frame portion is located at a second height larger than the first height; and an insulation sheet for a heat sink which is disposed on an underside of only the first frame portion of the lead frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power module such as a transfer-mold type IPM (Intelligent Power Module).

2. Description of the Background Art

In a power module used for an inverter, with a conventional configuration in which an IGBT (Insulated Gate Bipolar Transistor) and an FWD (Free Wheeling Diode) are connected in parallel with each other, it is difficult to reduce losses in the low current range due to the characteristics of the IGBT.

To improve losses in the low current range, it is considered to use a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) instead of an IGBT. However, with the MOSFET, there is a problem that the allowable current decreases, since the on-voltage in the high temperature/high current range increases.

To solve such problem, there is considered a configuration in which an IGBT with a low saturation voltage in the large current range is connected in parallel with a MOSFET with a low saturation voltage in the small current range (see, for example, Japanese Patent Application Laid-Open No. 04-354156 (1992)).

However, the configuration described in Japanese Patent Application Laid-Open No. 04-354156 (1992) is lacking in the viewpoint of adjustment of loss sharing between the IGBT and the MOSFET. Hence, there is a problem that the cost-performance of a power module cannot be optimized by the above-described adjustment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a power module capable of improving cost-performance by adjusting loss sharing between an IGBT and a MOSFET.

A power module according to the present invention includes an IGBT and a MOSFET connected in parallel with the IGBT. Furthermore, the power module includes a lead frame having a first frame portion on which the IGBT is mounted and a second frame portion on which the MOSFET is mounted, and having a step by which the first frame portion is located at a first height and the second frame portion is located at a second height larger than the first height. Furthermore, the power module includes an insulation sheet for a heat sink which is disposed on an underside of only the first frame portion of the lead frame.

According to the present invention, since the current-carrying capability of the MOSFET is smaller than that of the IGBT upon passage of high current, by increasing the loss burden on the IGBT side and reducing the loss burden on the MOSFET side, it becomes unnecessary for the MOSFET to have high heat sink performance. Therefore, an insulation sheet is disposed on the underside of only the first frame portion which is a location where the IGBT requiring high heat sink performance is mounted, and an insulation sheet does not need to be disposed at a location of the lead frame where the MOSFET is mounted. Thus, the sheet size of the insulation sheet can be reduced. With the above, the manufacturing cost of the power module can be reduced.

In the lead frame, the step by which the first frame portion is located at the first height and the second frame portion is located at the second height larger than the first height is formed, and accordingly, the distance from a heat sink surface which is the side where the IGBT is placed to the MOSFET can be increased, enabling to secure predetermined insulation performance of the MOSFET. In addition, since the current-carrying capability of the MOSFET is smaller than that of the IGBT upon passage of high current, the chip size of the MOSFET can be reduced. Therefore, the manufacturing cost of the power module can be further reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Preferred Embodiment

A first preferred embodiment of the present invention will be described below using the drawings.FIG. 1is a cross-sectional view of a power module1according to the first preferred embodiment of the present invention, andFIG. 2is a circuit diagram of the power module1. As shown inFIG. 1, the power module1includes an IGBT2, a MOSFET3, a drive circuit5, lead frames10and20, an insulation sheet30for a heat sink, and a mold resin6.

The lead frame10has an inner lead15which is electrically connected to the IGBT2and the MOSFET3; and an outer lead16connected to the inner lead15. The inner lead15has a first frame portion11located in a predetermined height position (first height); and a second frame portion12located in a height position (second height) larger than the height position of the first frame portion11. The second frame portion12and the first frame portion11are formed in this order from the side of the outer lead16. A step13is formed between the first frame portion11and the second frame portion12. The IGBT2is mounted on the first frame portion11, and the MOSFET3is mounted on the second frame portion12.

The insulation sheet30is disposed on the underside of only the first frame portion11. Here, since the power module1is used in a state of being placed on a conductive heat sink (not shown), the insulation sheet30is disposed for the purpose of insulating the lead frame10from the heat sink.

By increasing the distance from a heat sink surface of the power module1, i.e., a placement surface of the power module1that comes into contact with the heat sink, to the MOSFET3, the MOSFET3can secure predetermined insulation performance with respect to the heat sink surface. Hence, disposition of the insulation sheet30on the underside of the second frame portion12can be omitted.

The drive circuit5is a circuit for driving the IGBT2and the MOSFET3. The drive circuit5is mounted on a third frame portion21of the lead frame20. The lead frame20has an inner lead25which is electrically connected to the drive circuit5; and an outer lead26connected to the inner lead25. The third frame portion21is formed in the inner lead25and is formed in a height position larger than the height position of the first frame portion11. The IGBT2, the MOSFET3, the drive circuit5, the insulation sheet30, and the inner leads15and25of the lead frames10and20are sealed with the mold resin6.

As shown inFIG. 2, the IGBT2and the MOSFET3are connected in parallel with each other. More specifically, a collector of the IGBT2is connected to a drain of the MOSFET3, and an emitter of the IGBT2is connected to a source of the MOSFET3. A gate of the IGBT2and a gate of the MOSFET3are connected to an output terminal of the drive circuit5. Note that a diode4is connected in anti-parallel with the MOSFET3, and is an internal parasitic diode4.

Here, the current-carrying capability of the MOSFET3is smaller than that of the IGBT2upon passage of high current such as when the maximum rated current of the module flows, e.g., upon heavy load drive. Therefore, on the side of the MOSFET3, the current flowing upon passage of high current is suppressed and thus transient losses are reduced.

The threshold voltage of the MOSFET3is set to be higher than that of the IGBT2so that all currents flow through the side of the IGBT2in a transient state upon switching.

In a configuration in which the IGBT2and the MOSFET3are used in parallel as a switching device, generally, a sequence in which the IGBT2is always turned off first and then the MOSFET3is turned off is adopted which is implemented by setting the threshold voltage of the MOSFET3to be lower.

The advantageous effect obtained in this case is that tail current is suppressed and thus turn-off losses can be reduced. However, in a transient state, all currents (IGBT current+MOSFET current) always flow through the MOSFET3, resulting in an increase in the temperature of the MOSFET3.

In contrast to this, in the present preferred embodiment, by setting the threshold voltage of the MOSFET3to be higher than that of the IGBT2, the current flowing through the MOSFET3upon switching is suppressed, by which an increase in the temperature of the MOSFET3is suppressed. Here, the threshold voltages of the IGBT2and the MOSFET3are set according to the amount of impurity for channel implantation upon manufacturing.

Next, the circuit operation of the power module1will be described. In the case of going into a turn-on state by a control signal which is outputted from the output terminal of the drive circuit5changing from a low potential (“L”) to a high potential (“H”), when a gate voltage is provided to the IGBT2and the MOSFET3, since the threshold voltage of the IGBT2is lower, the IGBT2is turned on first, and an IGBT current starts to flow.

In the meantime, when the gate voltage reaches the threshold voltage of the MOSFET3, the MOSFET3is turned on, and a MOSFET current starts to flow. At the point in time when the MOSFET3is turned on, since a predetermined period of time has elapsed since the IGBT2has been turned on, the IGBT2is in a steady state. Thus, almost all currents have flown through the IGBT2and almost no current flows through the MOSFET3.

As such, by setting the threshold voltage of the MOSFET3to be higher than that of the IGBT2, the current flowing through the MOSFET3upon turn-on can be suppressed. Therefore, an increase in the temperature of the MOSFET3can be suppressed.

In the case of going into a turn-off state by the control signal changing from “H” to “L”, when the gate voltage provided to the IGBT2and the MOSFET3starts to drop, since the threshold voltage of the MSOFET3is higher, the MOSFET3is turned off first and the MOSFET current starts to drop. Thereafter, the gate voltage decreases, by which the IGBT current starts to drop and becomes lower than the threshold voltage of the IGBT2. Therefore, the IGBT2is turned off and the IGBT current stops flowing. By thus setting the threshold voltage of the MOSFET3to be higher than that of the IGBT2, the MOSFET3is turned off first upon turn-off. Thus, all currents flow through the IGBT2being in an on state at that point in time, and no current flows through the MOSFET3. Therefore, an increase in the temperature of the MOSFET3can be suppressed.

Next, advantageous effects brought about by the power module1according to the first preferred embodiment will be described by comparing the power module1with a power module100according to a comparative example.FIG. 5is a cross-sectional view of the power module100according to the comparative example. Note that, in the comparative example, the same components as those of the power module1are denoted by the same reference characters and description thereof is omitted.

In the power module100according to the comparative example, a lead frame10has a first frame portion11and a second frame portion12, and an IGBT2and a MOSFET3are mounted on the first frame portion11. Since the IGBT2and the MOSFET3are mounted on the first frame portion11, the distance from a heat sink surface to the IGBT2and the MOSFET3is reduced. Hence, in order to secure predetermined insulation performance of the MOSFET3with respect to the heat sink surface, there is a need to dispose an insulation sheet30not only underneath the IGBT2but also underneath the MOSFET3.

In contrast to this, in the power module1according to the first preferred embodiment, the second frame portion12is formed in a height position larger than the first frame portion11, and the IGBT2is mounted on the first frame portion11and the MOSFET3is mounted on the second frame portion12. Hence, the distance from the heat sink surface to the MOSFET3increases, enabling to secure the predetermined insulation performance of the MOSFET3with respect to the heat sink surface. Therefore, the insulation sheet30needs to be disposed on the underside of only the first frame portion11and does not need to be disposed on the underside of the second frame portion12.

As described above, in the power module1according to the first preferred embodiment, since the current-carrying capability of the MOSFET3upon passage of high current is smaller than that of the IGBT2, the loss burden on the side of the IGBT2can be increased and the loss burden on the side of the MOSFET3can be reduced, eliminating the need for the MOSFET3to have high heat sink performance. Therefore, the insulation sheet30is not disposed at a location of the lead frame10where the MOSFET3is mounted, and the insulation sheet30is disposed on the underside of only the first frame portion11which is a location where the IGBT2requiring high heat sink performance is mounted. Thus, the chip size of the MOSFET3can be reduced. In addition to this, since the sheet size of the insulation sheet30can also be reduced, the manufacturing cost of the power module1can be reduced.

In addition, in the lead frame10, the step13by which the first frame portion11is located at the first height and the second frame portion12is located at the second height larger than the first height is formed, and accordingly, the distance from the heat sink surface to the MOSFET3can be increased, enabling to secure the predetermined insulation performance of the MOSFET3.

In addition, since the current-carrying capability of the MOSFET3upon passage of high current is smaller than that of the IGBT2, the chip size of the MOSFET3can be further reduced. Therefore, the manufacturing cost of the power module1can be further reduced.

In addition, since the on-threshold voltage of the MOSFET3is higher than that of the IGBT2, even in a transient state upon overload, a large current can be prevented from flowing through the MOSFET3. Therefore, switching transient losses in the MOSFET3are reduced and an increase in the temperature of the MOSFET3is suppressed, enabling to improve the long-term reliability of the power module1. By improving the long-term reliability of the power module1, long-term use is possible, leading to a reduction in the amount of energy consumption.

Note that instead of setting the threshold voltage of the MOSFET3to be higher than that of the IGBT2, the drive circuit5may individually output control signals to the IGBT2and the MOSFET3to individually drive the IGBT2and the MOSFET3. In this case, by the drive circuit5driving the IGBT2and the MOSFET3such that the IGBT2and the MOSFET3are turned on in this order and the MOSFET3and the IGBT2are turned off in this order, the same advantageous effects as those obtained when the threshold voltage of the MOSFET3is set to be higher than that of the IGBT2are obtained. Here, a configuration in which the threshold voltage of the MOSFET3is set to be higher than that of the IGBT2and a configuration in which the drive circuit5individually drives the IGBT2and the MOSFET3are not essential and may be omitted.

In addition, as a MOSFET, a SiC-MOSFET formed on a silicon carbide (SiC) substrate may be adopted. Since the SiC-MOSFET has a lower on-threshold voltage compared to the Si-MOSFET, in the case of, in particular, turn-off, the SiC-MOSFET is turned off at a lower temperature than that for when the Si-MOSFET is adopted, resulting in low losses. Thus, an increase in the temperature of the MOSFET can be further suppressed, enabling to further improve the long-term reliability of the power module1.

Second Preferred Embodiment

Next, a power module1A according to a second preferred embodiment will be described.FIG. 3is a cross-sectional view of the power module1A according to the second preferred embodiment of the present invention. Note that, in the second preferred embodiment, the same components as those described in the first preferred embodiment are denoted by the same reference characters and description thereof is omitted.

In a lead frame10, a first frame portion11and a second frame portion12are formed in this order from the side of an outer lead16, and a third frame portion21on which a drive circuit5is mounted is located in a position adjacent to the second frame portion12. The third frame portion21is formed in a height position (third height) larger than the height position of the first frame portion11. For example, the height position of the third frame portion21is the same as the height position of the second frame portion12. Hence, the wiring length of a wire31between the drive circuit5and a power chip (an IGBT2and a MOSFET3) in the second preferred embodiment is shorter than the wiring length of a wire31between the drive circuit5and a power chip in the first preferred embodiment shown inFIG. 1.

As described above, the power module1A according to the second preferred embodiment further includes another lead frame20having the third frame portion21on which the drive circuit5is mounted, and the third frame portion21is formed at the third height larger than a first height, and the third frame portion21is adjacent to the second frame portion12of the first frame portion11and the second frame portion12. Thus, the wiring length of the wire31between the drive circuit5and the power chip can be reduced. Therefore, wire sweep caused by a mold resin6can be prevented, enabling to achieve an improvement in the quality of the product. As such, achievement of an improvement in the quality of the product also leads to an improvement in yield.

Third Preferred Embodiment

Next, a power module1B according to a third preferred embodiment will be described.FIG. 4shows the power module1B according to the third preferred embodiment of the present invention. Note that, in the third preferred embodiment, the same components as those described in the first and second preferred embodiments are denoted by the same reference characters and description thereof is omitted.

In a lead frame10, a step17which is different than the step13is further formed between a first frame portion11or a second frame portion12, and an outer lead16. Specifically, the step17is formed between the second frame portion12and the outer lead16, and the height position of the second frame portion12is a bit lower compared to the case of the first preferred embodiment. Hence, the distance from an IGBT2to a MOSFET3is a bit shorter compared to the case of the first preferred embodiment, and thus, thermal resistance decreases compared to the case of the first preferred embodiment.

As described above, in the power module1B according to the third preferred embodiment, in the lead frame10, the other step17which is different than the step13is further formed between the first frame portion11or the second frame portion12, and the outer lead16. Thus, while required insulation properties are secured, the distance from the IGBT2to the MOSFET3becomes a bit shorter compared to the case in which the step17is not provided, by which the thermal resistance of the MOSFET3can be reduced. Therefore, an increase in the temperature of the MOSFET3can be suppressed, which in turn enables to improve the long-term reliability of the power module1B.

Note that the preferred embodiments may be freely combined or may be appropriately modified or omitted without departing from the spirit and scope of the present invention.