Power Converter and Bidirectional Switch

Abstract

Provided is a bi-directional switch having: a first silicon carbide transistor; a first diode provided in series with the first silicon carbide transistor, the first diode having a lower on voltage at the rated current of the bi-directional switch than a built-in diode in the first silicon carbide transistor; a second silicon carbide transistor provided in parallel with the first diode; a second diode provided in series with the second silicon carbide transistor and provided in parallel with the first silicon carbide transistor, the second diode having a lower on voltage at the rated current of the bi-directional switch than a built-in diode in the second silicon carbide transistor; and a connection line for connecting a first contact point between the first silicon carbide transistor and the first diode and a second contact point between the second silicon carbide transistor and the second diode.

Description

[Technical Field]

[0001] The present invention relates to a power conversion device and a bidirectional switch. [Background technology]

[0002] BACKGROUND ART Power converters using bidirectional switches are known in the art (see, for example, Patent Documents 1 and 2). [Prior art document] [Patent documents] [Patent Document 1] Patent No. 4839943 [Patent Document 2] Japanese Patent No. 5999526 Problems to be solved

[0003] Preferably, the bidirectional switch has low loss.

[0004] To solve the above problems, a first aspect of the present invention provides a power conversion device including a plurality of bidirectional switches. Each bidirectional switch may have a first terminal and a second terminal. The bidirectional switch may have a first silicon carbide transistor provided between the first terminal and the second terminal. The bidirectional switch may have a first diode provided in series with the first silicon carbide transistor between the first terminal and the second terminal, the forward direction being from the first terminal to the second terminal, and the first diode having a lower on-voltage at the rated current of the bidirectional switch than the built-in diode of the first silicon carbide transistor. The bidirectional switch may have a second silicon carbide transistor provided in parallel with the first diode between the first terminal and the second terminal. The bidirectional switch may include a second diode provided in series with the second silicon carbide transistor between the first terminal and the second terminal and in parallel with the first silicon carbide transistor, the second diode having a forward direction from the second terminal to the first terminal and an on-voltage lower than that of the built-in diode of the second silicon carbide transistor at a rated current of the bidirectional switch. The bidirectional switch may include a connection line connecting a first connection point between the first silicon carbide transistor and the first diode and a second connection point between the second silicon carbide transistor and the second diode.

[0005] The first and second diodes may be silicon diodes having PN junctions.

[0006] The withstand voltage of the first diode may be higher than the withstand voltage of the first silicon carbide transistor.

[0007] The withstand voltage of the second diode may be higher than the withstand voltage of the second silicon carbide transistor.

[0008] The conduction start voltage of the first diode may be lower than the conduction start voltage of the built-in diode of the second silicon carbide transistor.

[0009] The conduction start voltage of the second diode may be lower than the conduction start voltage of the built-in diode of the first silicon carbide transistor.

[0010] The power conversion device may include a control unit that controls both the first silicon carbide transistor and the second silicon carbide transistor to be in the on state when the bidirectional switch is turned on.

[0011] When the bidirectional switch is turned on, the current flowing through the first diode may be smaller than the current flowing through the second silicon carbide transistor.

[0012] When the bidirectional switch is turned on, the current flowing through the first diode may be 10% or less of the current flowing through the second silicon carbide transistor.

[0013] The plurality of bidirectional switches may include a first bidirectional switch and a second bidirectional switch, and the control unit may transition a first silicon carbide transistor of the second bidirectional switch to an on state at a first timing when commutating a current from the first bidirectional switch to the second bidirectional switch.

[0014] The control unit may transition the first silicon carbide transistor of the first bidirectional switch to the off state at a second timing that is after the first timing.

[0015] The control unit may transition the second silicon carbide transistor of the second bidirectional switch to the on state at a third timing that is after the second timing.

[0016] The control unit may transition the second silicon carbide transistor of the first bidirectional switch to the off state at a fourth timing that is after the third timing.

[0017] In the first diode and the second diode, a carrier lifetime of holes when turned off may be 1 μs or more.

[0018] A second aspect of the present invention provides a bidirectional switch. The bidirectional switch may include a first terminal and a second terminal. The bidirectional switch may include a first silicon carbide transistor provided between the first terminal and the second terminal. The bidirectional switch may include a first diode provided in series with the first silicon carbide transistor between the first terminal and the second terminal, the forward direction being from the first terminal to the second terminal, and having a lower on-voltage at a rated current of the bidirectional switch than a built-in diode of the first silicon carbide transistor. The bidirectional switch may include a second silicon carbide transistor provided in parallel with the first diode between the first terminal and the second terminal. The bidirectional switch may include a second diode provided in series with the second silicon carbide transistor between the first terminal and the second terminal and in parallel with the first silicon carbide transistor, the forward direction being from the second terminal to the first terminal, and having a lower on-voltage at a rated current of the bidirectional switch than a built-in diode of the second silicon carbide transistor. The bidirectional switch may include a connection line connecting a first connection point between the first silicon carbide transistor and the first diode and a second connection point between the second silicon carbide transistor and the second diode. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram illustrating an example of a power conversion device 200 according to an embodiment of the present invention. [Figure 2] 1 is a diagram illustrating an example of a bidirectional switch 100. FIG. [Figure 3] 1A and 1B are diagrams illustrating an example of the operation of the bidirectional switch 100. [Figure 4] 1A and 1B are diagrams illustrating an example of the operation of the bidirectional switch 100. [Figure 5] FIG. 2 is a diagram showing a first bidirectional switch 100-a and a second bidirectional switch 100-b connected to a common output phase. [Figure 6] 10 is a diagram showing an example of waveforms of gate voltages Qa1, Qa2, Qb1, and Qb2. FIG. [Figure 7] This shows the state at time t0 in FIG. [Figure 8] This shows the state at time t1 in FIG. [Figure 9] This shows the state at time t2 in FIG. [Figure 10] This shows the state at time t3 in FIG. [Figure 11] This shows the state at time t4 in FIG. [Figure 12] 10A and 10B are diagrams illustrating another example of the operation of the bidirectional switch 100 in the ON state. [Figure 13] 10 is a diagram illustrating another configuration example of the bidirectional switch 100. FIG. [Figure 14] 3 is a diagram showing an example of the structure of a first diode 121. FIG. [Figure 15] An example of the carrier concentration distribution along the AA line in FIG. 14 is shown. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention. Furthermore, not all combinations of features described in the embodiments are necessarily essential to the solution of the invention. In this specification and drawings, elements having substantially the same function and configuration are designated by the same reference numerals to avoid repetitive description, and elements not directly related to the present invention are not shown. In addition, in a single drawing, elements having the same function and configuration may be designated by the same reference numeral, and the reference numerals may be omitted for other elements. In this specification, terms such as "same" or "equal" may also include cases where there is an error due to manufacturing variations, etc. Such an error is, for example, within 10%. Furthermore, the magnitude of current, etc. described in this specification is the magnitude at room temperature (25°C) unless otherwise defined.

[0021] FIG. 1 is a diagram illustrating an example of a power conversion device 200 according to an embodiment of the present invention. The power conversion device 200 generates output power from input power by switching one or more bidirectional switches 100. The power conversion device 200 may receive multi-phase power and output multi-phase power. The power conversion device 200 of this example receives three-phase (R, S, T) power and outputs three-phase (U, V, W) power. The power conversion device 200 of this example is a matrix converter having multiple bidirectional switches 100 that switch which output phase (U, V, W) wiring the wiring of each input phase (R, S, T) is connected to. However, the power conversion device 200 is not limited to a matrix converter.

[0022] The power conversion apparatus 200 includes one or more bidirectional switches 100 and a control unit 220. The power conversion apparatus 200 may include a filter 210. The control unit 220 controls the on / off state of each bidirectional switch 100. The filter 210 removes a predetermined frequency component from an input voltage or current or an output voltage or current. The filter 210 may be, for example, a low-pass filter that smooths the voltage or current.

[0023] Each bidirectional switch 100 switches whether or not input power to a first terminal 101 is passed to a second terminal 102. In this example, a bidirectional switch 100 is provided for each combination of input phase and output phase. For example, a power conversion device 200 with a three-phase input and three-phase output has 3×3=9 combinations of input phases and output phases. In this case, the power conversion device 200 may have nine bidirectional switches 100.

[0024] Fig. 2 is a diagram showing an example of a bidirectional switch 100. Although Fig. 2 shows a single bidirectional switch 100, each bidirectional switch 100 may have the structure shown in Fig. 2. The bidirectional switch 100 has a first terminal 101, a second terminal 102, a first silicon carbide transistor 111, a first diode 121, a second silicon carbide transistor 112, a second diode 122, and a connecting line 150. The first terminal 101 and the second terminal 102 correspond to the first terminal 101 and the second terminal 102 shown in Fig. 1.

[0025] A silicon carbide transistor is a transistor formed on a silicon carbide (SiC) substrate. The silicon carbide transistor in this example is a MOSFET formed on a SiC substrate.

[0026] The first silicon carbide transistor 111 is provided between the first terminal 101 and the second terminal 102. The first silicon carbide transistor 111 transitions between an on / off state in response to a control signal input from the control unit 220 to a control terminal G. The first silicon carbide transistor 111 in this example is an N-channel MOSFET having a drain terminal connected to the first terminal 101.

[0027] In this specification, when the arrangement of circuit elements is described, the arrangement on an electrical path is described. For example, providing the first silicon carbide transistor 111 between the first terminal 101 and the second terminal 102 means that the first silicon carbide transistor 111 is provided on the electrical path connecting the first terminal 101 and the second terminal 102. In this case, the position of the first silicon carbide transistor 111 in space does not have to be between the first terminal 101 and the second terminal 102.

[0028] The first diode 121 is provided in series with the first silicon carbide transistor 111 between the first terminal 101 and the second terminal 102. The first diode 121 is arranged such that the direction from the first terminal 101 to the second terminal 102 is the forward direction. The first silicon carbide transistor 111 and the first diode 121 are connected at a first connection point 131. In this example, the first silicon carbide transistor 111 is provided between the first terminal 101 and the first connection point 131, and the first diode 121 is provided between the first connection point 131 and the second terminal 102. In another example, the first silicon carbide transistor 111 and the first diode 121 may be arranged in an interchangeable manner.

[0029] The second silicon carbide transistor 112 is provided in parallel with the first diode 121 between the first terminal 101 and the second terminal 102. As shown in FIG. 2 , when the first diode 121 is connected to the second terminal 102, the second silicon carbide transistor 112 is connected to the second terminal 102 in parallel with the first diode 121. When the first diode 121 is connected to the first terminal 101, the second silicon carbide transistor 112 is connected to the first terminal 101 in parallel with the first diode 121. The second silicon carbide transistor 112 transitions between an on / off state in response to a control signal input from the control unit 220 to a control terminal G. The second silicon carbide transistor 112 in this example is an N-channel MOSFET having a drain terminal connected to the second terminal 102.

[0030] The second diode 122 is provided in series with the second silicon carbide transistor 112 between the first terminal 101 and the second terminal 102. The second diode 122 is also provided in parallel with the first silicon carbide transistor 111. The second diode 122 is arranged such that the direction from the second terminal 102 to the first terminal 101 is the forward direction. The second silicon carbide transistor 112 and the second diode 122 are connected at a second connection point 132. In this example, the second diode 122 is provided between the first terminal 101 and the second connection point 132, and the second silicon carbide transistor 112 is provided between the second connection point 132 and the second terminal 102. In another example, the second silicon carbide transistor 112 and the second diode 122 may be arranged in an interchangeable manner.

[0031] The connection line 150 is a wiring that connects the first connection point 131 and the second connection point 132. When both the first silicon carbide transistor 111 and the second silicon carbide transistor 112 are in the off state, no current flows between the first terminal 101 and the second terminal 102, regardless of the magnitude of the voltages at the first terminal 101 and the second terminal 102, and the bidirectional switch 100 is in the off state.

[0032] When the bidirectional switch 100 is turned on, the control unit 220 turns on the first silicon carbide transistor 111 and the second silicon carbide transistor 112 simultaneously or sequentially. As a result, regardless of the magnitude of the voltages at the first terminal 101 and the second terminal 102, a current flows between the first terminal 101 and the second terminal 102, and the bidirectional switch 100 is turned on.

[0033] For example, when the voltage of the first terminal 101 is higher than the voltage of the second terminal 102, turning the first silicon carbide transistor 111 on causes a current to flow between the first terminal 101 and the second terminal 102 through the first silicon carbide transistor 111 in a forward conducting state and the first diode 121. In this case, the second silicon carbide transistor 112 may also be controlled to an on state. This causes current to flow through the first diode 121 and the second silicon carbide transistor 112 in a reverse conducting state, thereby reducing the overall on-resistance of the bidirectional switch 100. Note that the forward conducting state of a transistor refers to a state in which a current flows from the drain terminal to the source terminal, and the reverse conducting state refers to a state in which a current flows from the source terminal to the drain terminal.

[0034] Furthermore, when the voltage of the second terminal 102 is higher than the voltage of the first terminal 101, turning the second silicon carbide transistor 112 on causes a current to flow between the second terminal 102 and the first terminal 101 through the second silicon carbide transistor 112 in a forward conducting state and the second diode 122. In this case, the first silicon carbide transistor 111 may also be controlled to an on state. This causes a current to flow through the second diode 122 and the first silicon carbide transistor 111 in a reverse conducting state, thereby reducing the overall on-resistance of the bidirectional switch 100.

[0035] By using silicon carbide transistors as the switching elements of the bidirectional switch 100, losses in the switching elements can be reduced. Furthermore, even when the gate of a silicon carbide transistor is in the off state, the built-in diode can be turned on in response to a reverse voltage. In other words, silicon carbide transistors do not have reverse blocking performance. In contrast, by providing a diode in anti-series with each silicon carbide transistor, it is possible to block current in both the forward and reverse voltage directions.

[0036] The first diode 121 has a lower on-voltage at the rated current of the bidirectional switch 100 than the built-in diode of the first silicon carbide transistor 111. The on-voltage of each diode is the forward voltage of the corresponding diode. The rated current of the bidirectional switch 100 is the rated current flowing between the first terminal 101 and the second terminal 102. The value of the rated current may be a specification value determined by the manufacturer or user of the bidirectional switch 100. The specification value of the rated current is the value of the current density per unit area of ​​the transistor or diode chip (A / cm 2 ) may be specified. In this case, the rated value of the current flowing between the first terminal 101 and the second terminal 102 may be set by multiplying the area of ​​the chip of the transistor or diode by the current density. Furthermore, the on-voltages of the first silicon carbide transistor 111 and the first diode 121 when a predetermined rated current flows may be compared. Furthermore, the respective on-voltages may be values ​​measured at room temperature (25°C). By reducing the on-voltage of the first diode 121, the loss in the first diode 121 can be reduced. At the rated current, the on-voltage of the first diode 121 is determined by the built-in diode At the rated current, the on-voltage of first diode 121 may be lower than the on-voltage of the built-in first silicon carbide transistor 111. diode The on-state voltage of the diode may be 0.9 times or less, 0.7 times or less, or 0.5 times or less of the on-state voltage of the diode. The on-state voltage of the diode can be adjusted by the presence or absence of lifetime killers (recombination centers such as crystal defects) in the semiconductor substrate, the impurity concentration in the semiconductor substrate, etc.

[0037] The second diode 122 has a lower on-voltage at the rated current of the bidirectional switch 100 than the built-in diode of the second silicon carbide transistor 112. By lowering the on-voltage of the second diode 122, it is possible to reduce the loss in the second diode 122. At the rated current, the on-voltage of the second diode 122 is lower than the built-in diode of the first silicon carbide transistor 111. diodeAt the rated current, the on-voltage of the second diode 122 may be lower than the on-voltage of the second silicon carbide transistor 112. diode It may be 0.9 times or less, 0.7 times or less, or 0.5 times or less of the on-state voltage of the transistor.

[0038] Furthermore, the withstand voltage of the first diode 121 may be higher than the withstand voltage of the first silicon carbide transistor 111. The withstand voltage of a diode may be the value of a reverse voltage at which a reverse current starts to flow. The withstand voltage of a transistor may be the value of a forward voltage at which a current starts to flow when the gate is in an off state. By designing the withstand voltage of the first diode 121 to be higher than the withstand voltage of the first silicon carbide transistor 111, the avalanche resistance of the series circuit of the first diode 121 and the first silicon carbide transistor 111 is determined by the first silicon carbide transistor 111. Therefore, it is not necessary to consider the avalanche resistance of the first diode 121, and it becomes easy to reduce the on-voltage (forward voltage) described above. The withstand voltage of a diode can be adjusted by the thickness of the semiconductor substrate used, the impurity concentration in the semiconductor substrate, etc.

[0039] Furthermore, the withstand voltage of the second diode 122 may be higher than the withstand voltage of the second silicon carbide transistor 112. By designing the withstand voltage of the second diode 122 to be higher than the withstand voltage of the second silicon carbide transistor 112, the avalanche resistance of the series circuit of the second diode 122 and the second silicon carbide transistor 112 is determined by the second silicon carbide transistor 112. Therefore, it is not necessary to consider the avalanche resistance of the second diode 122, and it becomes easy to reduce the on-voltage (forward voltage) described above.

[0040] In this example, the first diode 121 and the second diode 122 are silicon diodes having PN junctions. This allows each diode to be manufactured at low cost. The first diode 121 and the second diode 122 may be separate chips. The first silicon carbide transistor 111 and the second silicon carbide transistor 112 may be separate chips or may be provided on the same chip.

[0041] FIG. 3 is a diagram showing an example of the operation of the bidirectional switch 100. In each diagram, the on state of each transistor may be indicated as "on" and the off state as "off." This example shows a state in which the bidirectional switch 100 is in the middle of transitioning from the off state to the on state. In the power conversion device 200 shown in FIG. 1, one of the two bidirectional switches 100 connected to a common output phase is controlled from on to off, and the other is controlled from off to on. In this case, it is preferable to control the two transistors of the bidirectional switch 100 sequentially, rather than turning them on or off simultaneously.

[0042] When the bidirectional switch 100 is transitioned from the off state to the on state, first, the first silicon carbide transistor 111 is transitioned from the off state to the on state, and the second silicon carbide transistor 112 is in the off state.

[0043] 3, a current passing through the first silicon carbide transistor 111 can flow to the first diode 121 and the built-in diode of the second silicon carbide transistor 112. In each figure, the current path is sometimes schematically indicated by a dashed arrow. However, when a forward current flows through the built-in diode of the second silicon carbide transistor 112 in the off state, stacking faults (SF) may develop from basal plane dislocations (BPD) contained in the SiC crystal of the semiconductor substrate, resulting in an increase in on-state voltage.

[0044] The conduction start voltage of the first diode 121 may be lower than the conduction start voltage of the built-in diode of the second silicon carbide transistor 112. The conduction start voltage of each diode is a forward voltage at which a forward current starts to flow through the diode. As a result, in the state shown in FIG. 3 , a current flows through the first diode 121 before a current flows through the built-in diode of the second silicon carbide transistor 112. This makes it possible to suppress the progression of stacking faults in the second silicon carbide transistor 112. The conduction start voltage of the first diode 121 may be lower than the conduction start voltage of the built-in diode of the second silicon carbide transistor 112 by 0.2 V or more, 0.5 V or more, 1 V or more, or 2 V or more. Similarly, the conduction start voltage of the second diode 122 may be lower than the conduction start voltage of the built-in diode of the first silicon carbide transistor 111.

[0045] 4 is a diagram showing an operation example of the bidirectional switch 100. In this example, the bidirectional switch 100 is shown in an on state. In this example, both the first silicon carbide transistor 111 and the second silicon carbide transistor 112 are on. In this case, as shown in FIG. 4, a current flows through the first silicon carbide transistor 111, the connection line 150, and the second silicon carbide transistor 112.

[0046] As described above, a current from the first silicon carbide transistor 111 may also be diverted to the first diode 121. The current flowing to the first diode 121 may be smaller than the current flowing to the second silicon carbide transistor 112. The current flowing to the first diode 121 may be 10% or less, or 1% or less, of the current flowing to the second silicon carbide transistor 112. Alternatively, no current may flow to the first diode 121.

[0047] FIG. 5 is a diagram showing a first bidirectional switch 100-a and a second bidirectional switch 100-b connected to a common output phase. The first bidirectional switch 100-a and the second bidirectional switch 100-b have the same configuration as the bidirectional switch 100 shown in FIG. 2. The reference numerals of the respective members of the first bidirectional switch 100-a are appended with the suffix "-a", and the reference numerals of the respective members of the second bidirectional switch 100-b are appended with the suffix "-b". The second terminal 102-a of the first bidirectional switch 100-a and the second terminal 102-b of the second bidirectional switch 100-b are connected to a terminal 160. The terminal 160 is a terminal connected to one of the output phases.

[0048] Furthermore, the gate voltage applied to the first silicon carbide transistor 111-a of the first bidirectional switch 100-a is denoted by Qa1, and the gate voltage applied to the second silicon carbide transistor 112-a is denoted by Qa2. Similarly, the gate voltage applied to the first silicon carbide transistor 111-b of the second bidirectional switch 100-b is denoted by Qb1, and the gate voltage applied to the second silicon carbide transistor 112-b is denoted by Qb2. Each of these gate voltages may be generated by the control unit 220.

[0049] 6 is a diagram showing an example of the waveforms of the gate voltages Qa1, Qa2, Qb1, and Qb2. The control unit 220 of this example transitions the first bidirectional switch 100-a from the ON state to the OFF state, and transitions the bidirectional switch 100-b from the OFF state to the ON state. In other words, transition is made from a state in which a current flows between the first bidirectional switch 100-a and the terminal 160 to a state in which a current flows between the second bidirectional switch 100-b and the terminal 160. This type of control is referred to as commutating the current from the first bidirectional switch 100-a to the second bidirectional switch 100-b.

[0050] Fig. 7 shows the state at time t0 in Fig. 6. In Fig. 7 and subsequent figures, it is assumed that voltage Va at first terminal 101-a is higher than voltage Vb at first terminal 101-b. At time t0, gate voltages Qa1 and Qa2 are at H level, and gate voltages Qb1 and Qb2 are at L level. Each silicon carbide transistor is turned on when an H level gate voltage is applied, and turned off when an L level gate voltage is applied.

[0051] In this example, both the first silicon carbide transistor 111-a and the second silicon carbide transistor 112-a are in the ON state. This causes the first bidirectional switch 100-a to be in the ON state. Furthermore, the first silicon carbide transistor 111-b and the second silicon carbide transistor 112-b are in the OFF state. This causes the second bidirectional switch 100-b to be in the OFF state. Therefore, a current flows from the first terminal 101-a to the terminal 160 of the first bidirectional switch 100-a.

[0052] FIG. 8 shows the state at time t1 in FIG. 6. Time t1 is an example of a first timing. At time t1, the gate voltage Qb1 transitions from an L level to an H level. In this case, the first silicon carbide transistor 111-b transitions to the ON state. However, the voltage Vo at the terminal 160 is approximately equal to the voltage Va at the first terminal 101-a. Therefore, the voltage Vb at the first terminal 101-b is lower than the voltage Vo at the terminal 160. Therefore, even when the first silicon carbide transistor 111-b transitions to the ON state, no current flows from the first terminal 101-b to the second terminal 102-b.

[0053] FIG. 9 shows the state at time t2 in FIG. 6. Time t2 is an example of the second timing. At time t2, the gate voltage Qa1 transitions from H level to L level. In this case, the first silicon carbide transistor 111-a transitions to the OFF state. This makes it possible to cut off the current from the first terminal 101-a to the second terminal 102-a. Note that in the states shown in FIGS. 7 and 8, by reducing the current flowing through the first diode 121-a, the reverse recovery loss of the first diode 121-a can be reduced. Furthermore, by maintaining the second silicon carbide transistor 112-a in the ON state at time t2, it is possible to leave a path for current to return from the terminal 160 to the first terminal 101-a.

[0054] When the first silicon carbide transistor 111-a transitions to the off state, the voltage Vo at the terminal 160 is cut off from the voltage Va at the first terminal 101-a. As a result, in the second bidirectional switch 100-b, a forward voltage is applied to the first silicon carbide transistor 111-b and the first diode 121-b, causing a current to flow. In other words, by controlling the first silicon carbide transistor 111-a, the current cutoff of the first bidirectional switch 100-a and the conduction of the second bidirectional switch 100-b can be synchronized.

[0055] As described above, by making the conduction start voltage of the first diode 121-b smaller than the conduction start voltage of the built-in diode of the second silicon carbide transistor 112-b, it is possible to prevent current from flowing through the second silicon carbide transistor 112-b in the state of Fig. 9. This makes it possible to prevent the progression of stacking faults in the second silicon carbide transistor 112-b.

[0056] FIG. 10 shows the state at time t3 in FIG. 6. Time t3 is an example of a third timing. At time t3, the gate voltage Qb2 transitions from an L level to an H level. In this case, the second silicon carbide transistor 112-b transitions to the ON state. This allows most or all of the current from the first silicon carbide transistor 111-b to flow to the second silicon carbide transistor 112-b. In other words, the current flowing to the second diode 122-b can be suppressed. This makes it possible to suppress reverse recovery loss in the second diode 122-b when the second bidirectional switch 100-b transitions to the OFF state.

[0057] FIG. 11 shows the state at time t4 in FIG. 6 . Time t4 is an example of a fourth timing. At time t4, the gate voltage Qa2 transitions from an H level to an L level. In this case, the second silicon carbide transistor 112-a transitions to the OFF state. This also blocks the current path from the second terminal 102-a to the first terminal 101-a, turning the first bidirectional switch 100-a into the OFF state. If no current flows from the second terminal 102-a to the first terminal 101-a, the process at time t4 may be simultaneous with time t3 or may occur earlier than time t3. This state can be determined by monitoring the voltages of the terminals. For example, if the voltage Va is higher than both the voltages Vb and Vo, the process at time t4 may be simultaneous with time t3 or may occur earlier than time t3.

[0058] FIG. 12 is a diagram showing another example of the operation of the bidirectional switch 100 in the on state. The bidirectional switch 100 of this example is in a state where the element temperature is higher than in the example of FIG. 4. When the element temperature is high, the forward voltage of the first diode 121 decreases, and the current flowing through the first diode 121 may increase. This may increase the reverse recovery loss. On the other hand, when the element temperature is high, the on-resistance of the second silicon carbide transistor 112 increases. Therefore, if the current flowing through the second silicon carbide transistor 112 decreases, the on-loss decreases. This makes it possible to suppress the overall loss.

[0059] Fig. 13 is a diagram showing another configuration example of the bidirectional switch 100. In the bidirectional switch 100 of this example, the positions of the first diode 121 and the first silicon carbide transistor 111 are swapped, and the positions of the second diode 122 and the second silicon carbide transistor 112 are swapped, compared to the configuration shown in Fig. 2. The other structures are similar to the example of Fig. 2.

[0060] FIG. 14 is a diagram showing an example of the structure of the first diode 121. While FIG. 14 shows the first diode 121, the second diode 122 also has a similar structure. As described above, it is preferable that the on-state voltage (forward voltage) of the first diode 121 is small. On the other hand, for a diode element, local crystal defects may be formed by injecting charged particles such as helium into the semiconductor substrate to adjust the carrier lifetime of holes. This shortens the time during which a tail current flows during reverse recovery, thereby reducing reverse recovery loss. However, shortening the carrier lifetime of holes increases the on-state voltage. The first diode 121 of this example does not have local crystal defects for adjusting the carrier lifetime. This reduces the on-state voltage of the first diode 121.

[0061] The first diode 121 is a silicon diode having a PN junction. The first diode 121 of this example has a silicon semiconductor substrate 180, an anode electrode 161, and a cathode electrode 162. The semiconductor substrate 180 has an N-type drift region 166, a P-type anode region 164, and an N+-type cathode region 168. The anode region 164 is connected to the anode electrode 161, and the cathode region 168 is connected to the cathode electrode 162. The drift region 166 is disposed between the anode region 164 and the cathode region 168. The boundary between the drift region 166 and the anode region 164 is a PN junction.

[0062] The diode element may have a lifetime adjusting region 170 in the drift region 166 near the anode region 164. The lifetime adjusting region 170 is a region in which crystal defects are locally formed by irradiating with helium or the like. By forming crystal defects that combine with holes, the lifetime of the holes is shortened. The first diode 121 of this example does not have a lifetime adjusting region 170 in the drift region 166. For example, the first diode 121 does not have a helium concentration peak.

[0063] FIG. 15 shows an example of the carrier concentration distribution along line AA in FIG. 14. The carrier concentration distribution may be a distribution measured by, for example, the spreading resistance method (SR method). When the lifetime adjusting region 170 is provided, the carrier concentration distribution in the drift region 166 has local valleys (see the dashed line in FIG. 15). In contrast, the first diode 121 has an almost flat carrier concentration distribution in the depth direction of the drift region 166, as shown by the solid line in FIG. 15. "Almost flat" means, for example, that the fluctuation range of the carrier concentration is ±20% or less. This allows the on-resistance of the first diode 121 to be reduced.

[0064] The first diode 121 of this example does not have a lifetime adjusting region 170. Therefore, the carrier lifetime of holes when turned off is relatively long. When the first diode 121 is turned off, the average value of the carrier lifetime of holes present in the drift region 166 may be 1 μs or more, 2 μs or more, or even 3 μs or more. The same applies to the carrier lifetime of the second diode 122. Furthermore, the reverse recovery time when the first diode 121 is turned off from a state in which the above-mentioned rated current is flowing through it may be 1 μs or more, 2 μs or more, or even 3 μs or more. The same applies to the reverse recovery time of the second diode 122.

[0065] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention. [Explanation of symbols]

[0066] REFERENCE SIGNS LIST 100 bidirectional switch, 101 first terminal, 102 second terminal, 111 first silicon carbide transistor, 112 second silicon carbide transistor, 121 first diode, 122 second diode, 131 first connection point, 132 second connection point, 150 connection line, 160 terminal, 161 anode electrode, 162 cathode electrode, 164 anode region, 166 drift region, 168 cathode region, 170 lifetime adjustment region, 180 semiconductor substrate, 200 power conversion device, 210 filter, 220 control unit

Claims

1. Three input wirings for receiving three-phase input power; Three output wirings for outputting three-phase output power; at least nine bidirectional switches between the input wirings and the output wirings for switching which of the output wirings each of the input wirings is connected to; A power conversion device comprising: Each two-way switch is a first terminal and a second terminal; a first silicon carbide transistor provided between the first terminal and the second terminal; a first diode that is provided in series with the first silicon carbide transistor between the first terminal and the second terminal, the direction from the first terminal toward the second terminal being a forward direction, and the first diode having an on-voltage at a rated current of the bidirectional switch lower than that of a built-in diode of the first silicon carbide transistor; a second silicon carbide transistor provided in parallel with the first diode between the first terminal and the second terminal; and a second diode provided in series with the second silicon carbide transistor between the first terminal and the second terminal and in parallel with the first silicon carbide transistor, the direction from the second terminal to the first terminal being a forward direction, and the second diode having an on-voltage at a rated current of the bidirectional switch lower than that of a built-in diode of the second silicon carbide transistor; a connection line connecting a first connection point between the first silicon carbide transistor and the first diode and a second connection point between the second silicon carbide transistor and the second diode; a control unit that controls both the first silicon carbide transistor and the second silicon carbide transistor to be in an on state, The first diode and the second diode are silicon diodes having PN junctions. Power conversion device.

2. The withstand voltage of the first diode is higher than the withstand voltage of the first silicon carbide transistor. The power conversion device according to claim 1 .

3. The withstand voltage of the second diode is higher than the withstand voltage of the second silicon carbide transistor. The power conversion device according to claim 2 .

4. The conduction start voltage of the first diode is lower than the conduction start voltage of the built-in diode of the second silicon carbide transistor. The power conversion device according to any one of claims 1 to 3.

5. The conduction start voltage of the second diode is lower than the conduction start voltage of the built-in diode of the first silicon carbide transistor. The power conversion device according to claim 4.

6. When the bidirectional switch is turned on, the current flowing through the first diode is smaller than the current flowing through the second silicon carbide transistor. The power conversion device according to claim 1 .

7. When the bidirectional switch is turned on, the current flowing through the first diode is 10% or less of the current flowing through the second silicon carbide transistor. The power conversion device according to claim 6.

8. In the first diode and the second diode, a carrier lifetime of holes when turned off is 1 μs or more. The power conversion device according to any one of claims 1 to 7.

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