JP5531490B2 - Power converter - Google Patents

Description

    The present invention relates to a power conversion device including a voltage doubler rectifier circuit and an inverter circuit having a switching element.

    Conventionally, as disclosed in Patent Document 1, for example, a converter circuit that is connected to an AC power source and performs voltage doubler rectification, and an inverter circuit that has a plurality of switching elements and converts the output current of the converter circuit to AC The power converter provided with these is known.

JP 2001-259281 A

    By the way, although the power converter provided with the converter circuit which carries out voltage doubler rectification like the said patent document 1 is generally used for 100V type power supply systems, it is applicable also to a 200V type power supply system, such as an inverter circuit etc. There is a demand to share elements. However, there are the following problems.

    In such an inverter circuit of a power conversion device that performs voltage rectification, as in the case of the above-mentioned Patent Document 1, an IGBT is often used as a switching element. Here, consider the case where such a power converter is used in a 200V power supply system. In this case, the input current of the inverter circuit is halved by voltage doubler rectification, but if an element such as IGBT is used as the switching element, the conduction loss of the inverter circuit is not reduced so much. That is, since the IGBT is an element having an on-voltage drop, even if the current is lowered, the on-resistance is not reduced so much and the conduction loss is reduced so much (for example, only about half). On the other hand, since the input voltage of the inverter circuit is doubled by voltage doubler rectification, for example, if the switching element is thickened to increase the breakdown voltage of the switching element such as IGBT, the on-resistance of the switching element is doubled and the conduction loss is also increased. Doubled. As a result, there is a problem that the conduction loss of the switching element is hardly reduced despite the double voltage rectification.

    The present invention has been made in view of such a point, and an object of the present invention is to provide a low-loss and high-withstand-voltage power conversion device that includes a circuit for voltage doubler rectification with respect to a 200 V AC power supply. There is.

    According to a first aspect of the present invention, there is provided a converter circuit (20) connected to a 200V AC power source (1) and configured to double voltage rectify a current from the AC power source (1), and a plurality of unipolar elements. A switching element (31), and an inverter circuit (30) that converts the DC voltage rectified by the converter circuit (20) into alternating current by the on / off operation of the switching element (31). is there.

    In the first invention, the power supply voltage (200V) is doubled by the converter circuit (20) (400V) and output to the inverter circuit (30). Further, the power source current is reduced to half of the current by the converter circuit (20) and output to the inverter circuit (30). Here, a unipolar element having no on-voltage drop is used for the switching element (31) of the inverter circuit (30). That is, a unipolar element having no on-voltage drop is used for the switching element (31), not an element having an on-voltage drop like the IGBT. Therefore, when the input current is halved (1/2) in the inverter circuit (30), the on-resistance of the switching element (31) is reduced, and the conduction loss of the switching element (31) is 1/4 (= (1 / 2) × (1/2)). On the other hand, since a voltage twice as high as the power supply voltage is input to the inverter circuit (30), for example, if the switching element (31) is made thick in order to increase the breakdown voltage of the switching element (31), the switching element (31) The on-resistance of the capacitor is doubled and the conduction loss is also doubled. Therefore, in the inverter circuit (30) of the present invention, the conduction loss is 1/2 (= (1/4) × 2). Examples of unipolar elements include MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), JFETs (Junction FETs), and HFETs (Heterostructure FETs).

A first aspect of the present invention is at least one of the upper Symbol switching element (31) consists of a unipolar device used a wide band-gap semiconductor.

In the first aspect of the invention, at least one of the switching elements (31) is formed of a wide band gap semiconductor unipolar element such as SiC (silicon carbide) or GaN (gallium nitride) .

The first invention, the upper Symbol inverter circuit (30) is composed of upper and lower arms each having a said switching element (31), one of the switching elements of the upper arm and the lower arm (31) It is a normally-on type.

    In the first aspect of the invention, the normally-on type switching element (31) having a smaller on-resistance than the normally-off type is used. Note that normally-on elements include JFETs and HFETs.

In the first invention, the upper arm or the lower arm of the inverter circuit (30) has the normally-on type switching element (31).

A second invention is the synchronous rectification of the inverter circuit (30) according to the first invention, wherein the switching element (31) is turned on at a predetermined timing when a reverse current flows in the switching element (31). The control part (40) which performs is provided.

In the second aspect of the invention, for example, when a reverse current flows through the switching element (31), the switching element (31) is in an off state, and a predetermined time (for example, FIG. After the elapse of the dead time period (3), the switching element (31) is turned on. Then, a reverse current flows through the switching element (31). By performing such synchronous rectification, a reverse current flows through the switching element (31) without separately providing a free-wheeling diode for the switching element (31).

The third invention is used in the air conditioner in the first invention.

In the said 3rd invention, it uses as a power converter device of the motor for driving the compressor of the refrigerant circuit provided in an air conditioner.

In a fourth aspect of the present invention based on the third aspect of the present invention, an effective current value of the switching element (31) in a heating intermediate load condition, which is a condition for producing a half of the heating rated capacity of the air conditioner ( The relationship between I _rms ) and on-resistance (R _on ) is such that I _rms <0.9 / R _on .

In the fourth aspect of the invention, if the switching element (31) is selected and synchronous rectification is performed, it is possible to operate at an efficiency equal to or higher than that in the heating intermediate load condition without separately providing a reflux diode.

As described above, by the present invention lever, 200V system power conversion apparatus connected alternating current power source (1) of the (10), constitute a converter circuit (20) as a voltage doubler rectifier circuit, and the inverter Each switching element (31) of the circuit (30) is composed of a unipolar element. Therefore, by doubling the breakdown voltage of the switching element (31), the conduction loss of the switching element (31) is doubled. On the other hand, when the input current is halved, the conduction loss of the switching element (31) is reduced. Becomes 1/4. Therefore, as a whole, the conduction loss of the switching element (31) can be halved. Accordingly, it is possible to provide a low-loss and high-withstand-voltage power converter (10) while providing a circuit for voltage doubler rectification with respect to the 200V AC power supply (1).

According to the first aspect of the present invention, since at least one of the switching elements (31) is formed of a unipolar element using a wide band gap semiconductor, the power conversion device capable of operating at high speed and high temperature with lower loss ( 10) can be provided.

Further, according to the first aspect of the invention, since at least one of the switching elements (31) uses a normally-on type element, it is possible to provide a power converter (10) with lower loss.

According to the second aspect of the invention, the synchronous rectification is performed to turn on the switching element (31) at a predetermined timing when the reverse current flows through the switching element (31). More specifically, in the present invention, at the timing when the reverse current flows through the switching element (31), the switching element (31) is turned off, and a predetermined time has passed after the reverse current flows through the switching element (31). After a lapse (at a predetermined timing), synchronous rectification is performed to turn on the switching element (31). Such a synchronous rectification allows a reverse current to flow inside the switching element (31). That is, a reverse current can be passed through the switching element (31) without separately providing a free-wheeling diode such as SiC-SBD. Therefore, since the free-wheeling diode becomes unnecessary, it is possible to reduce the size and the cost. Further, the conduction loss can be reduced as compared with the case where the reverse current flows through the return diode. As a result, it is possible to provide a small-sized and low-loss power conversion device (10).

According to the fourth invention, the relationship between the effective current value (I _rms ) and the on-resistance (R _on ) of the switching element (31) under the heating intermediate load condition of the air conditioner is I _rms <0.9 / R _{If the} switching element is selected to be on and synchronous rectification is performed, efficiency equal to or higher can be achieved without separately providing a freewheeling diode under heating intermediate load conditions, and both cost reduction and high efficiency can be achieved. It becomes possible.

FIG. 1 is a circuit diagram illustrating a configuration of a power conversion device according to an embodiment. FIG. 2 is a diagram for explaining the operation of synchronous rectification according to the embodiment. FIG. 3 is a signal waveform diagram of a control unit that performs synchronous rectification according to the embodiment. FIG. 4 is a circuit diagram illustrating an operation during synchronous rectification according to the embodiment. FIG. 5 is a diagram showing an outline of the voltage-current characteristics of SiC-MOSFET, SiC-MOSFET parasitic diode, and SiC-SBD.

    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that the following embodiments and modifications are essentially preferable examples, and are not intended to limit the scope of the present invention, its application, or its use.

    As shown in FIG. 1, the power converter device (10) of this embodiment is provided with the converter circuit (20), the inverter circuit (30), and the control part (40).

    The converter circuit (20) is connected to an AC power source (1) which is a single-phase commercial power source, and doubles voltage rectifies the AC power source (1). The AC power source (1) is a 200V power source. The inverter circuit (30) converts the direct current rectified by the voltage doubler by the converter circuit (20) into a three-phase alternating current and supplies it to the motor (2). This motor (2) drives the compressor provided in the refrigerant circuit of an air conditioner.

    The converter circuit (20) includes a reactor (21), four diodes (22), two capacitors (23, 24), and a smoothing capacitor (25). The four diodes (22) form a bridge-connected diode bridge circuit and are connected to the AC power source (1). The two capacitors (23, 24) are connected in series with each other and connected to the output side of the diode bridge circuit. A smoothing capacitor (25) is connected to the output side of the two capacitors (23, 24), and an inverter circuit (30) is connected to the output side. The reactor (21) is provided between the AC power source (1) and the diode bridge circuit. Thus, the converter circuit (20) is configured to perform voltage doubler rectification.

    The inverter circuit (30) has six switching elements (31), which are three-phase bridge-connected. In this inverter circuit (30), the upper side in FIG. 1 constitutes the upper arm, and the lower side in FIG. 1 constitutes the lower arm. The switching element (31) is configured by a unipolar element (here, SiC-MOSFET) using a wide band gap semiconductor. A unipolar element is an element having no on-voltage drop (on-voltage drop). This SiC-MOSFET (31) has a built-in parasitic diode (32). That is, the parasitic diode (32) is formed inside the SiC-MOSFET (31), not an external diode. In the inverter circuit (30) of the present embodiment, the parasitic diode (32) of the SiC-MOSFET (31) is used as the freewheeling diode.

    The control unit (40) is configured to perform synchronous rectification by switching control (on / off control) of each switching element (31) of the inverter circuit (30). This synchronous rectification operation will be described later.

    In this power converter (10), the power supply voltage (200V) of the AC power supply (1) is doubled (400V) by the converter circuit (20) and output to the inverter circuit (30). Further, the power source current of the AC power source (1) is half of the current by the converter circuit (20) and is output to the inverter circuit (30). Here, since the SiC-MOSFET (31) of the inverter circuit (30) is a unipolar element with no on-voltage drop, the input current is halved (1/2) in the inverter circuit (30). The on-resistance of 31) is reduced, and the conduction loss of the SiC-MOSFET (31) is 1/4 (= (1/2) × (1/2)). On the other hand, twice the power supply voltage is input to the inverter circuit (30). However, since the SiC-MOSFET (31) is a high breakdown voltage element, it can cope with, for example, without increasing the thickness of the element itself. obtain. Therefore, it is not necessary to increase the on-resistance of the SiC-MOSFET (31). Therefore, in the inverter circuit (30) of the present embodiment, the conduction loss can be reduced to ¼ compared to the case where the conventional power converter that performs voltage doubler rectification is used for the 200V system power supply. Accordingly, it is possible to provide a low-loss and high-withstand-voltage power converter (10) while providing a circuit for voltage doubler rectification with respect to the 200V AC power supply (1). Moreover, in this embodiment, since it is not necessary to raise a proof pressure by thickening a switching element (31) etc., size reduction of a power converter device (10) is attained.

    In addition, the switching element (31) of the present embodiment has the same effect even when configured as a normally-on type unipolar element by JFET or HFET. In this case, the parasitic diode (32) is omitted from the switching element (31). A normally-on element has a lower on-resistance than a normally-off element. Therefore, the conduction loss in the inverter circuit (30) can be further reduced, and the power converter (10) with lower loss can be provided.

    Further, in the inverter circuit (30) of the present embodiment, one switching element (31) of the upper arm and the lower arm may be configured by a normally-on type unipolar element. In that case, an overcurrent or overvoltage abnormality can be prevented regardless of the state of the drive circuit of the switching element (31).

    The controller (40) performs switching rectification by performing switching control as shown in FIG. First, an output voltage command is transmitted to the control unit (40). The output voltage command is a signal of the required output voltage of the motor (2) with respect to the load of the air conditioner, and is transmitted from the control unit of the air conditioner. The control unit (40) switches each switching element (31) of the inverter circuit (30) so that the output voltage of the inverter circuit (30) becomes the received necessary output voltage. Specifically, the control unit (40) turns each of the upper arm SiC-MOSFET (31) and the lower arm SiC-MOSFET (31) on and off at a predetermined timing. That is, the control unit (40) turns on the upper arm SiC-MOSFET (31) and the lower arm SiC-MOSFET (31) alternately. Here, the SiC-MOSFET (31) of the lower arm is not turned on immediately after the period (period A) during which the SiC-MOSFET (31) of the upper arm is turned on, but is turned on after a predetermined dead time period. The Similarly, the SiC-MOSFET (31) of the upper arm is not turned on immediately after the period (period B) during which the SiC-MOSFET (31) of the lower arm is on, but is turned on after a predetermined dead time period. The This dead time period is a period for turning off all the SiC-MOSFETs (31) of the inverter circuit (30) in order to avoid a so-called short circuit.

    Synchronous rectification is performed in the inverter circuit (30) by the switching control of the control unit (40) described above. For example, when the current i (see FIG. 2) is positive (> 0), the lower-arm SiC-MOSFET (31) is synchronously rectified as shown in FIG. First, the SiC-MOSFET (31) of the upper arm is in an ON state, and a forward current flows through the SiC-MOSFET (31) (state shown in FIG. 4A). Next, the SiC-MOSFET (130) of the upper arm is turned off, and the SiC-MOSFET (31) of the lower arm is turned off at the timing when the reverse current flows, and the reverse current is applied to the lower arm. It flows through the parasitic diode (32) of the SiC-MOSFET (31) (the state shown in FIG. 4B). That is, the reverse current flows through the parasitic diode (32) during the dead time period. When the dead time period elapses, the lower arm SiC-MOSFET (31) is turned on, and a reverse current flows through the SiC-MOSFET (31) instead of the parasitic diode (32) (FIG. 4 (c)). ) State.) Due to the synchronous rectification of the lower-arm SiC-MOSFET (31), the output voltage v0 (i> 0) (v0 refer to FIG. 2) is as shown in FIG. On the other hand, when the current i (see FIG. 2) is negative (<0), the upper arm SiC-MOSFET (31) is synchronously rectified in the same manner as described above (as shown in FIG. 4). Due to the synchronous rectification of the SiC-MOSFET (31) of the upper arm, the output voltage v0 (i <0) (v0 refer to FIG. 2) becomes as shown in FIG. As can be seen, in any synchronous rectification, a voltage drop occurs due to conduction loss of the parasitic diode (32) in the dead time period in which the reverse current flows through the parasitic diode (32). However, after the dead time has elapsed, the SiC-MOSFET (31) is turned on so that a reverse current flows through the SiC-MOSFET (31), so that a voltage drop can be avoided. That is, in the inverter circuit (30), by performing synchronous rectification, a reverse current hardly flows through the parasitic diode (32) and flows into the SiC-MOSFET (31) with extremely low conduction loss. Thereby, the conduction loss in the inverter circuit (30) can be reduced. Further, since there is no need to separately provide a freewheeling diode, it is possible to reduce the size and cost of the power conversion device (10). As described above, the synchronous rectification according to the present embodiment turns on the SiC-MOSFET (31) at a predetermined timing when a reverse current flows through the parasitic diode (32) inside the SiC-MOSFET (31). is there. More specifically, the synchronous rectification of the present embodiment turns off the SiC-MOSFET (31) at the timing when the reverse current flows through the parasitic diode (32) inside the SiC-MOSFET (31), Turns off the SiC-MOSFET (31) at a predetermined timing after turning it off (after the elapse of a predetermined dead time period), and applies a reverse current to the inside of the SiC-MOSFET (31) excluding the parasitic diode (32). It is a flow.

    Further, the same synchronous rectification is possible even if the switching element (31) of the present embodiment is configured by JFET or HFET. In this case, the parasitic diode (32) is omitted from the switching element (31) as described above. Here, the synchronous rectification in the case of JFET (31) will be described as a representative. In this case, first, the forward current flows while the JFET (31) of the upper arm is on. Next, the JFET (130) of the upper arm is turned off, and the JFET (31) of the lower arm is turned off at the timing when the reverse current flows, and the reverse current is applied to the JFET (31) of the lower arm. Flows inside. That is, during the dead time period, the reverse current flows through the JFET (31) in the off state. When the dead time period elapses, the lower-arm JFET (31) is turned on, and the reverse current flows through the on-state JFET (31) as it is. Here, the conduction state loss is higher in the off state of the JFET (31) than in the on state. Therefore, during the dead time period in which the reverse current flows through the off-state JFET (31), a voltage drop occurs as in the case of the SiC-MOSFET (31). However, since the reverse current flows through the on-state JFET (31) after the dead time period has elapsed, the voltage drop can be avoided. That is, in this case, by performing synchronous rectification, the reverse current hardly flows to the off-state JFET (31), and flows to the on-state JFET (31) with extremely low conduction loss. Thereby, the conduction loss in the inverter circuit (30) can be reduced. In the case of JFET (31), SiC is used as the wide band gap semiconductor, and in the case of HFET (31), GaN (gallium nitride), GaAs (gallium arsenide), or the like is used as the wide band gap semiconductor.

    Here, as a conventional technique for performing synchronous rectification using a parasitic diode as a free-wheeling diode, there is a technique using a parasitic diode of a Si-MOSFET as a free-wheeling diode. However, since the rise voltage (ON voltage) of the parasitic diode of the Si-MOSFET is low (about 0.7V), the parasitic diode immediately conducts even after synchronous rectification. Therefore, the effect of synchronous rectification is small. On the other hand, when the parasitic diode (32) of the SiC-MOSFET (31) is used as a freewheeling diode as in this embodiment, the rising voltage (ON of the parasitic diode (32) of the SiC-MOSFET (31) is turned on. Since the voltage is high (approximately 3V), the parasitic diode (32) will not conduct unless the current increases when performing synchronous rectification. Therefore, if the parasitic diode (32) of the SiC-MOSFET (31) is used as a freewheeling diode as in this embodiment, the effect of synchronous rectification is greater than when the parasitic diode of the Si-MOSFET is used as a freewheeling diode. Become.

    In FIG. 1, a configuration in which synchronous rectification is applied to all six switching elements (31) in the inverter circuit (30) using the parasitic diode (32) as a freewheeling diode is applied. It is also possible to apply only to the element (31).

    Next, the efficiency of the air conditioner and the selection conditions for the switching element (31) will be described. In the inverter circuit (30) of this embodiment, the SiC-MOSFET (31) is energized by synchronous rectification, and an SiC-SBD (Schottky barrier diode) is connected in parallel to the conventional configuration (eg, SiC-MOSFET (31)). In the configuration used as a freewheeling diode), SiC-SBD is energized. As shown in FIG. 5, the SiC-MOSFET (31) exhibits a constant resistance characteristic. The rising voltage (ON voltage) of the SiC-SBD is about 1V, and the rising voltage (ON voltage) of the parasitic diode (32) of the SiC-MOSFET (31) is about 3V. The rising voltage is determined by the physical property value and cannot be set arbitrarily.

    When the characteristics of the configuration of the present embodiment are compared with those of the conventional configuration, the present embodiment is more efficient when the terminal voltage is equal to or lower than the rising voltage of the SiC-SBD. However, if more current flows, the conventional configuration becomes more efficient. Therefore, the conventional configuration using SiC-SBD is more efficient in operating conditions where a large current flows, such as rated conditions and heavy loads. On the other hand, at a light load, the effect of SiC-SBD is low, and the configuration of this embodiment with only the SiC-MOSFET (31) is more efficient.

Here, when i = (√2) I _rms sin θ is passed as the current, the losses of this embodiment and the conventional configuration are expressed by the following (Equation 1) to (Equation 3), respectively.
_-This embodiment (Formula 1) R _on × I _rms ²
・ Conventional configuration (Formula 2) Vf x (2√2 / π) Irms… However, Vf = const.
(Equation 3) αI _rms ² + β (2 (√2) / π) I _rms ... where Vf (i) = αi + β
I _rms is the effective current value, R _on is the on-resistance value of the SiC-MOSFET (31), and Vf is the terminal voltage of the SiC-SBD. (Equation 2) approximates Vf with a constant value, and (Equation 3) approximates Vf with a first order.

    As can be seen from FIG. 5 and the above equation, the loss of the present embodiment increases at the rated load and the heavy load, but the loss of the present embodiment decreases at the light load. Unlike general loads where efficiency at the rated load is important, operating time at light loads is long in air conditioning applications. For this reason, in order to save energy, the driving efficiency at light load is required. The operating condition called heating intermediate load, which is the condition for producing a half of the heating rated capacity, has the most influence on actual energy saving in Japan.

_{Select the} SiC-MOSFET (31) so that the conditions shown in (Equation 4) and (Equation 5) below are satisfied when the current effective value at the heating intermediate load is I _rms1 .
(Equation 4) I _rms1 <(2 (√2) / π) Vf / R _on ... However, Vf = const.
(Formula 5) I _rms1 <(2 (√2) / π) β / (R _on −α) where Vf (i) = αi + β
By selecting a switching element and performing synchronous rectification in this way, it is possible to achieve the same or higher efficiency in the heating intermediate load without using SiC-SBD, and to achieve both cost reduction and high efficiency. Become.

Further, considering that the rising voltage of the SiC-SBD is about 1V, if Vf is 1V, the above (formula 4) can be simplified as the following (formula 6).
(Formula 6) I _rms1 <0.9 / R _on
This further facilitates switching element selection.

    In the above embodiment, the SiC-MOSFET is shown as an example of a unipolar element using a wide band gap semiconductor, but the same can be applied to a unipolar element using another wide band gap semiconductor such as GaN or diamond.

    As described above, the present invention is useful for a power conversion device including a converter circuit that performs voltage doubler rectification and an inverter circuit having a switching element.

1 AC power supply
10 Power converter
20 Converter circuit
30 Inverter circuit
31 Switching element
40 Control unit

Claims (4)

A converter circuit (20) connected to a 200V AC power supply (1) and configured to double-voltage rectify the current from the AC power supply (1);
An inverter circuit (30) having a plurality of switching elements (31) each made of a unipolar element, and converting the DC voltage rectified by the converter circuit (20) into alternating current by an on / off operation of the switching element (31); equipped with a,
At least one of the switching elements (31) is a unipolar element using a wide band gap semiconductor,
The inverter circuit (30) is composed of an upper arm and a lower arm each having the switching element (31), and one of the switching elements (31) of the upper arm and the lower arm is a normally-on type. <br/> A power converter characterized by the above. Oite to claim 1,
A control unit (40) for turning on the switching element (31) and performing synchronous rectification of the inverter circuit (30) at a predetermined timing when a reverse current flows in the switching element (31); A power conversion device. Oite to claim 1,
A power conversion device used for an air conditioner. In claim 3,
Relationship between effective current value (I _rms ) and on-resistance (R _on ) of the switching element (31) under a heating intermediate load condition, which is a condition that produces a half of the heating rated capacity of the air conditioner Is configured so that I _rms <0.9 / R _on .

Images