Semiconductor module

A PFC module includes: a diode bridge having first and second diodes in the upper arm, and third and fourth diodes in the lower arm; and first and second switching elements for power factor correction. The first and second diodes are Schottky barrier diodes formed by using a wide bandgap semiconductor. The third and fourth diodes, and the first and second switching elements are diodes and switching elements respectively formed by using silicon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a semiconductor module with a power factor correction (PFC) circuit.

2. Description of the Background Art

Use of a PiN (p-intrinsic-n) diode formed by using silicon (Si) in a semiconductor module for power control (power module) conventionally produces the storage effect of minority carriers to generate a recovery current. This is one of factors for loss increase of the power module. Such loss is reduced significantly by a Schottky barrier diode. So, in many cases, a Schottky barrier diode (SBD) formed by using Si is used in the conventional power module.

A semiconductor element using a wide bandgap semiconductor is regarded as a promising element to function as a next-generation switching element capable of realizing high breakdown voltage, low loss, and high resistance to heat. Patent literatures 1 to 5 listed below each disclose an example in which a semiconductor device formed by using silicon carbide (SiC) (SiC device) that is a typical wide bandgap semiconductor is applied in a power module.Patent Literature 1: Japanese Patent Application Laid-Open No. 10-93015 (1998)Patent Literature 2: Japanese Patent Application Laid-Open No. 11-233712 (1999)Patent Literature 3: Japanese Patent Application Laid-Open No. 11-510000 (1999)Patent Literature 4: Japanese Patent Application Laid-Open No. 2003-18862Patent Literature 5: Japanese Patent Application Laid-Open No. 2006-149195

A PFC module including a rectifying diode bridge and a power factor correction (PFC) circuit is known as one of power modules with diodes. A diode bridge of a conventional PFC module is constructed of Schottky barrier diodes made of Si. Meanwhile, voltage drop in a forward direction (forward voltage (VF)) of the Schottky barrier diode made of Si increases considerably if a breakdown voltage is several hundred volts or higher. So, if used in a power module for high-voltage control, the Schottky barrier diode made of Si may increase loss of the power module more seriously than a PiN diode.

In contrast, a Schottky barrier diode formed by using SiC makes it possible to maintain a forward voltage at a low level even if a breakdown voltage is several hundred volts or higher. So, a power module with low loss and excellent recovery characteristics is achieved by applying a Schottky barrier diode made of SiC in the power module. However, an SiC device entails higher cost than an Si device. So, manufacturing cost is increased if a power module is formed as an SiC device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a low-loss and low-cost PFC module with excellent recovery characteristics.

A semiconductor module of the present invention includes a diode bridge having first and second diodes with cathodes connected to each other, and third and fourth diodes with anodes connected to each other. The first and third diodes are connected in series, and the second and fourth diodes are connected in series. A first switching element is connected to a connection node between the first and third diodes. The second switching element is connected to a connection node between the second and fourth diodes. The first and second diodes are Schottky barrier diodes formed by using a wide bandgap semiconductor. The third and fourth diodes, and the first and second switching elements are diodes and switching elements respectively formed by using silicon.

The first and second diodes in the upper arm of the diode bridge function not only as rectifying elements but also as free-wheeling diodes while the first and second switching elements are in operation for PFC. To be specific, the first and second switching elements operate at high frequencies during PFC control, so recovery loss generated during the free-wheeling operation of the first and second diodes contributes largely to the loss of the entire PFC module. The first and second diodes formed as SiC devices with low loss and excellent recovery characteristics allow significant reduction of the loss of the PFC module. Further, not all the power chips but only the first and second diodes are formed as SiC devices, so that cost increase is suppressed. Also, the characteristics of the low-loss SiC devices that allow size reduction contribute to the size reduction of the PFC module.

EMBODIMENT FOR CARRYING OUT THE INVENTION

First Preferred Embodiment

FIG. 1is a circuit diagram of a PFC module as a semiconductor module of the present invention. As shown inFIG. 1, the PFC module includes a diode bridge with the first to fourth diodes D1R, D1S, D2Rand D2S, first and second switching elements SWRand SWSfor power factor correction, and a driver IC100for controlling the first and second switching elements SWRand SWS.

The diode bridge is constructed of the first and second diodes D1Rand D1Sin the upper arm with cathodes connected to each other, and the third and fourth diodes D2Rand D2Sin the lower arm with anodes connected to each other. The first and third diodes D1Rand D2Rare connected in series, and the second and fourth diodes D1Sand D2Sare connected in series. A terminal R connecting the anode of the first diode D1Rand the cathode of the third diode D2R, and a terminal S connecting the anode of the second diode D1Sand the cathode of the fourth diode D2Sfunction as input terminals from which AC power is supplied.

A terminal P connecting the cathodes of the first and second diodes D1Rand D1Sis an output terminal of a higher potential side. The first switching element SWRis placed between a connection node (terminal R) between the first and third diodes D1Rand D2R, and a terminal N as an output terminal of a lower potential side. The second switching element SWSis placed between a connection node (terminal S) between the second and fourth diodes D1Sand D2S, and the terminal N.

An external control IC (not shown) is connected to a terminal N2connecting the anodes of the third and fourth diodes D2Rand D2S. The control IC supplies a control signal to a terminal VIN of the driver IC100in response to a signal of the terminal N2to control the first and second switching elements SWRand SWS, thereby making the first and second switching elements SWRand SWSperform a predetermined operation for power factor correction. InFIG. 1, a terminal VDis a power supply terminal of the driver IC100, and a terminal GND is a ground terminal.

The structure of a conventional PFC module is described next that is given as a Comparative Example of the present invention.FIG. 2includes a plan view and a sectional view showing the structure of the conventional PFC module. The conventional PFC module includes first to fourth diodes D1R, D1S, D2Rand D2Sthat are Schottky barrier diodes made of Si, and first and second switching elements SWRand SWSthat are for example IGBTs formed by using Si. As the first to fourth diodes D1R, D1S, D2Rand D2S, PiN diodes are used if a breakdown voltage is several hundred volts or higher and Schottky barrier diodes are used if a breakdown voltage is several hundred volts or lower, as described above. That is, the first to fourth diodes D1R, D1S, D2Rand D2S, and the first and second switching elements SWRand SWSare all Si devices.

In the PFC module shown inFIG. 2, chips of the first and second diodes D1Rand D1Sare placed on a lead frame104(first lead frame) functioning as a terminal P. Chips of the third diode D2Rand the first switching element SWRare placed on a lead frame102(second lead frame) functioning as a terminal R. Further, chips of the fourth diode D2Sand the second switching element SWSare placed on a lead frame103(third lead frame) functioning as a terminal S. A driver IC100is placed on a certain lead frame functioning as control terminals (terminals VIN, VD, GND and the like) used to control the driver IC100.

InFIG. 2, the lead frames101and105function as terminals N and N2respectively. Further, the chips and the lead frames are connected to each other through wire lines110.

The aforementioned constituent elements are held with a molding resin forming a casing200of the PFC module. The lead frames101to104project in part as external connection terminals from one side surface of the casing200. The lead frame105projects from a different side surface of the casing200together with the control terminals (terminals VIN, VD, GND and the like) of the driver IC100. The PFC module ofFIG. 2is provided with through holes121and122for attachment that penetrate through the casing200.

As shown in the sectional view on the right side ofFIG. 2, in the conventional PFC module, the lead frames101to104are bent downward inside the casing200, and parts of the lower surfaces of the lead frames101to104at which the lead frames101to104hold the chips placed thereon are exposed at the bottom surface of the casing200. An insulating sheet120of high heat conductivity is affixed to the exposed parts of the lead frames101to104. This structure makes it possible to dissipate heat easily from the first to fourth diodes D1R, D1S, D2Rand D2S, and the first and second switching elements SWRand SWSto outside.

The structure of a PFC module of a first preferred embodiment of the present invention is described next by referring toFIG. 3. InFIG. 3, elements having the same functions as those of the corresponding elements ofFIG. 2are identified by the same reference numerals.

The first preferred embodiment includes first to fourth diodes D1R, D1S, D2Rand D2S, and first and second switching elements SWRand SWSthat are provided as chips for power control (power chips). The first and second diodes D1Rand D1Sin the upper arm of a diode bridge are Schottky barrier diodes formed as SiC devices. Like in the conventional example, regarding the other chips, the third and fourth diodes D2Rand D2Sare Schottky barrier diodes formed as Si devices, and the first and second switching elements SWRand SWSare IGBTs for example formed as Si devices. As the third and fourth diodes D2Rand D2S, PiN diodes are used if a breakdown voltage is several hundred volts or higher and Schottky barrier diodes are used if a breakdown voltage is several hundred volts or lower.

The first and second diodes D1Rand D1Sin the upper arm of the diode bridge function not only as rectifying elements but also as free-wheeling diodes while the first and second switching elements SWRand SWSare in operation for PFC. To be specific, the first and second switching elements SWRand SWSoperate at high frequencies during PFC control, so recovery loss generated during the free-wheeling operation of the first and second diodes D1Rand D1Scontributes largely to the loss of the entire PFC module. The first and second diodes D1Rand D1Sformed as SiC devices with low loss and excellent recovery characteristics allow significant reduction of the loss of the PFC module. Further, not all the power chips but only the first and second diodes D1Rand D1Sare formed as SiC devices, so that cost increase is suppressed.

As shown in the sectional view ofFIG. 3, a lead frame104holding the first and second diodes D1Rand D1Sthereon is not bent downward (in a direction perpendicular to the surface of the lead frame104on which the first and second diodes D1Rand D1Sare placed) inside the casing200. In this case, degradation of the heat dissipating characteristics of the first and second diodes D1Rand D1Smay be thought of. However, such degradation will not occur as the SiC devices have excellent resistance to heat. Generation of heat is also suppressed as the losses of the first and second diodes D1Rand D1Sare reduced compared to large losses thereof generated in the conventional example. This makes the insulating sheet120shown inFIG. 2unnecessary, leading to cost reduction.

Second Preferred Embodiment

FIG. 4shows the structure of a PFC module of a second preferred embodiment. In the second preferred embodiment, first and second diodes D1Rand D1Sformed as SiC devices are arranged on the outer side than third and fourth diodes D2Rand D2S, and first and second switching elements SWRand SWS. More specifically, the first and second diodes D1Rand D1Sare placed near a side surface of a casing200from which lead frames101to104project. Like in the first preferred embodiment, a first lead frame holding the first and second diodes D1Rand D1Sthereon is not bent downward (in a direction perpendicular to the surface of the first lead frame on which the first and second diodes D1Rand D1Sare placed) inside the casing200.

This structure can shorten wire lines110for connecting the first and second diodes D1Rand D1S, and the lead frames102and103. The length of the wire lines110affects the durability of the PFC module to withstand a surge current. Shortening the wire lines110enhances the durability to withstand a surge current. If the PFC module has a rating of 600 V and 20 A (rms), for example, the length of the wire lines110for connecting the first and second diodes D1Rand D1S, and the lead frames102and104can be shortened to approximately half the length of the conventional example (shortened to 5 mm if the length of the conventional example is 10 mm).

Third Preferred Embodiment

FIG. 5shows the structure of a PFC module of a third preferred embodiment. Like in the second preferred embodiment, first and second diodes D1Rand D1Sformed as SiC devices are arranged on the outer side than third and fourth diodes D2Rand D2S, and first and second switching elements SWRand SWS(near a side surface of a casing200from which lead frames101to104project).

An SiC device allows size reduction of a chip as a result of its low resistance. As an example, a module having a rating of 600 V and 20 A (rms) can reduce the chip sizes of the first and second diodes D1Rand D1Sby about half. So, a central portion of the casing200becomes empty space if the first and second diodes D1Rand D1Sare placed on the outer side. In the third preferred embodiment, a through hole122is formed in this space. To be specific, at least one of the first and second diodes D1Rand D1Sis arranged between the through hole122and the aforementioned side surface of the casing200in plan view (in the plan view ofFIG. 5, the through hole122, and the first and second diodes D1Rand D1Sare aligned vertically).

The third preferred embodiment allows the through hole122to be placed on the inner side of the casing200, thereby realizing reduction of the package size of the PFC module. If the module has a rating of 600 V and 20 A (rms), for example, the length of the package (length of the casing200) can be shorter by about 4 mm than those of the first and second preferred embodiments (FIGS. 3 and 4). Dotted lines ofFIG. 5indicate the shapes of the casing200and the through hole122of the first and second preferred embodiments.

As described above, the present invention makes effective use of the characteristic features of an SiC device including high breakdown voltage, low loss, and high resistance to heat. These are not only the features of an SiC device, but also the features of semiconductor devices using other wide bandgap semiconductors. So, the aforementioned effects are also achieved if the first and second diodes D1Rand D1Sare Schottky barrier diode formed by using a wide bandgap semiconductor except SiC.