BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to a power module in which a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is incorporated, and to a three-phase inverter system including this power module. The present disclosure further relates to a method of testing a power module.

Description of the Background Art

A power module in which a power MOSFET and a freewheeling diode connected in anti-parallel with the power MOSFET are incorporated has been brought into practical use. In such a power module, a Schottky barrier diode characterized by a high-speed switching operation and a low forward voltage drop is often used as the freewheeling diode (see for example WO2010/004802).

One of failure modes of the MOSFET (also referred to as MOS transistor) is degradation of a body diode resulting from energization thereof. This failure occurs when forward current is continuously applied to a PN junction diode like a body diode, due to growth of a stacking fault in an epitaxial layer caused by electron-hole pair binding energy. Particularly in the case where a wide-bandgap semiconductor such as SiC (silicon carbide) and GaN (gallium nitride) and the like is used as a semiconductor material, the stacking fault is likely to occur and therefore this failure mode is likely to present a problem.

SUMMARY OF THE INVENTION

In the case where a Schottky barrier diode with a low forward voltage drop is used as the freewheeling diode, most of freewheeling current flows through the Schottky barrier diode. Thus, current flowing through the body diode of the MOSFET can be reduced. However, when the MOSFET performs a switching operation, counter-electromotive force which occurs in a parasitic inductance component at each of the collector terminal and the emitter terminal of the MOSFET as well as the cathode terminal and the anode terminal of the freewheeling diode causes instantaneous current to flow through the body diode of the MOSFET. Therefore, degradation of the body diode resulting from energization may still present a problem.

Usually, for a power module in which a MOSFET is incorporated, an energization test is conducted by applying forward current to the body diode for a predetermined time. In this way, low-quality chips are screened out (because degradation in characteristics of the low-quality chips is promoted). However, in the case where a Schottky barrier diode is used as the freewheeling diode, it is difficult to effectively conduct this screening. This is for the reason that most of current flows through the freewheeling diode, and therefore, the energization time has to be made considerably long.

The present disclosure takes the above-described problems into consideration, and a main object of the present disclosure is to provide a power module with a MOSFET body diode on which an energization test can be conducted efficiently, even when a Schottky barrier diode is used as the freewheeling diode.

According to a first aspect of the present disclosure, a power module includes: a casing; a first terminal, a second terminal, a third terminal, and a fourth terminal which are fixed to the casing and connectable to an outside; a first MOS (Metal Oxide Semiconductor) transistor; a second MOS transistor; a first Schottky barrier diode; and a second Schottky barrier diode. The first MOS transistor is contained in the casing, is connected between the first terminal and the second terminal, and has a forward direction from the second terminal to the first terminal. The second MOS transistor is contained in the casing, is connected between the second terminal and the third terminal, and has a forward direction from the third terminal to the second terminal. The first Schottky barrier diode is contained in the casing, has an anode connected to the first terminal, and has a cathode connected to the fourth terminal. The second Schottky barrier diode is contained in the casing, has a cathode connected to the third terminal, and has an anode connected to the fourth terminal. No other diode except for respective body diodes of the first and second MOS transistors is connected to the second terminal.

According to a second aspect of the present disclosure, a power module includes: a casing; a first terminal, a second terminal, a third terminal, a fourth terminal, and a fifth terminal which are fixed to the casing and connectable to an outside; a first MOS transistor; a second MOS transistor; a first Schottky barrier diode; and a second Schottky barrier diode. The first MOS transistor is contained in the casing, is connected between the first terminal and the second terminal, and has a forward direction from the second terminal to the first terminal. The second MOS transistor is contained in the casing, is connected between the second terminal and the third terminal, and has a forward direction from the third terminal to the second terminal. The first Schottky barrier diode is contained in the casing, has a cathode connected to the second terminal, and has an anode connected to the fourth terminal. The second Schottky barrier diode is contained in the casing, has an anode connected to the second terminal, and has a cathode connected to the fifth terminal.

According to a third aspect of the present disclosure, a power module includes: a casing; a first terminal, a second terminal, and a third terminal which are fixed to the casing and connectable to an outside; a first MOS transistor; a second MOS transistor; a first Schottky barrier diode; and a second Schottky barrier diode. The first MOS transistor is contained in the casing, is connected between the first terminal and the second terminal, and has a forward direction from the second terminal to the first terminal. The second MOS transistor is contained in the casing, is connected between the second terminal and the third terminal, and has a forward direction from the third terminal to the second terminal. The first Schottky barrier diode is contained in the casing, is parallel with the first MOS transistor, has an anode connected to the first terminal, and has a cathode connected to the second terminal. The second Schottky barrier diode is contained in the casing, is parallel with the second MOS transistor, has an anode connected to the second terminal, and has a cathode connected to the third terminal. A current value at a first crossing point is within a range of ±10% of a rated current of the power module, where the first crossing point is a crossing point of:

a current-voltage characteristic of the first MOS transistor when forward current is caused to flow in a body diode of the first MOS transistor; and

a current-voltage characteristic of the first Schottky barrier diode when forward current is caused to flow in the first Schottky barrier diode. Further, a current value at a second crossing point is within a range of ±10% of the rated current of the power module, where the second crossing point is a crossing point of:

a current-voltage characteristic of the second MOS transistor when forward current is caused to flow in a body diode of the second MOS transistor; and

a current-voltage characteristic of the second Schottky barrier diode when forward current is caused to flow in the second Schottky barrier diode.

The significant feature of the power module according to the first aspect is that two separate output terminals configured to output an AC voltage are provided, namely an output terminal (second terminal) between the first and second MOSFETs and an output terminal (fourth terminal) between the first and second freewheeling diodes. The significant feature of the power module according to the second aspect is that two separate positive-side terminals and two separate negative-side terminals configured to receive an input DC voltage are provided, namely negative-side terminals (first terminal and fourth terminal) respectively connected to the first MOSFET and the first freewheeling diode and positive-side terminals (third terminal and fifth terminal) respectively connected to the second MOSFET and the second freewheeling diode. Thus, when an energization test is conducted, forward current can be caused to flow in only the body diode of the MOSFET. Therefore, low-quality MOSFET chips can be screened out efficiently.

Further, regarding the power module according to the third aspect, current larger than the rated current is caused to flow in the forward direction of the body diode of the MOSFET when an energization test is conducted, to thereby allow more forward current to flow in the body diode than the current flowing in the freewheeling diode. Thus, low-quality MOSFET chips can be screened out efficiently.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

