JP2020092556A - Power conversion device - Google Patents

Abstract

To suppress a current flowing through an electrolytic capacitor to suppress an increase in the number of parallel electrolytic capacitors when a low-capacitance capacitor is used together with the electrolytic capacitor.SOLUTION: A power conversion device 100 includes a rectifier circuit 2 that converts an AC voltage to a DC voltage, and a converter 4 that converts the output voltage of the rectifier circuit to another output voltage. The converter includes a first capacitor 15 as an electrolytic capacitor, a second capacitor 17 connected in parallel with the first capacitor, having a capacitance smaller than that of the first capacitor, and having an equivalent series resistance smaller than that of the first capacitor, and a resistance element 16 connected in series with the first capacitor.SELECTED DRAWING: Figure 1

Description

The present invention relates to a power conversion device.

The AC-DC converter is, for example, a rectifier circuit that converts AC power into DC power, a smoothing circuit that smoothes a pulsating current output from the rectifier circuit, and a smoothed DC voltage that is further converted into another voltage. And a converter that operates. The converter is provided with a capacitor for absorbing a switching surge. Ripple (hereinafter referred to as “AC power supply ripple”) based on the input AC power, for example, the frequency (50 Hz or 60 Hz) of the commercial AC power supply, and a switching signal (several hundred kHz) for switching the rectifier circuit are included in the capacitor. )) and a ripple (hereinafter, referred to as “switching ripple”) mixed therein flows.

As a capacitor included in the converter, a large-capacity electrolytic capacitor is often used in order to absorb switching ripple and switching surge. Further, when the allowable ripple current of the electrolytic capacitor is not sufficient, a low-capacity capacitor such as a ceramic capacitor or a film capacitor may be used together with the electrolytic capacitor (for example, refer to Patent Document 1).

Generally, an electrolytic capacitor has a large capacity per unit volume, and has a low impedance in a frequency region of about several hundred kHz used as a switching frequency, but has a relatively large ESR (equivalent series resistance). Therefore, many electrolytic capacitors have an allowable ripple current of about several amperes. On the other hand, a ceramic capacitor or a film capacitor has a capacity per unit volume inferior to that of an electrolytic capacitor, but since the ESR is small, the allowable ripple current is relatively large.

In the power conversion circuit, the capacity of the electrolytic capacitor is designed based on the ripple of the commercial frequency specified in the design specifications, but if the allowable ripple current is not sufficient, increase the number of electrolytic capacitors or use a larger capacity electrolytic capacitor. I need to do it. However, increasing the number of electrolytic capacitors or using a large-capacity electrolytic capacitor causes a decrease in power conversion efficiency, and does not meet the demand for downsizing and weight reduction of electric devices.

JP 2012-055084A

The present invention provides a power conversion device capable of suppressing the current flowing through the electrolytic capacitor when the low-capacity capacitor is also used together with the electrolytic capacitor, thereby suppressing an increase in the number of parallel electrolytic capacitors. The purpose is to

を満たすことを特徴とする。 A power converter according to the present invention includes a rectifier circuit that converts an alternating voltage into a direct current, and a converter that converts an output voltage of the rectifier circuit into another output voltage, and the converter is a first electrolytic capacitor. A capacitor, a second capacitor connected in parallel with the first capacitor, having a capacitance smaller than the first capacitor, and having an equivalent series resistance smaller than the first capacitor, and connected in series with the first capacitor And a resistance element that is formed.
When the resistance value of the resistance element is R _s , the capacitance value of the first capacitor is C _c , the capacitance value of the second capacitor is C _f , and the switching frequency in the converter is f _sw , the resistance value R _s is ,

It is characterized by satisfying.

を満たすのが好適である。 In addition to the above [Equation 1], when the ripple frequency of the AC voltage is f _ac ,
The resistance value R _s is

It is preferable to satisfy.

According to the present invention, when a capacitor having a lower capacity than that of an electrolytic capacitor is used together with the electrolytic capacitor, it is possible to suppress a current flowing through the electrolytic capacitor and thereby suppress an increase in the number of parallel electrolytic capacitors. A power converter can be provided.

It is a circuit diagram explaining power converter 100 concerning an embodiment of the invention. It is a circuit diagram explaining the power converter device which concerns on a comparative example. 7 is a graph for explaining the effect of the embodiment.

The present embodiment will be described below with reference to the accompanying drawings. In the accompanying drawings, functionally the same elements may be represented by the same numbers. It should be noted that although the accompanying drawings show embodiments and implementation examples according to the principles of the present disclosure, these are for understanding of the present disclosure, and are used for limiting interpretation of the present disclosure. is not. The description herein is merely exemplary in nature and is not intended to limit the scope or claims of the disclosure in any way.

Although the present embodiment has been described in detail enough for those skilled in the art to carry out the present disclosure, other implementations/forms are also possible without departing from the scope and spirit of the technical idea of the present disclosure. It is necessary to understand that the structure and structure can be changed and various elements can be replaced. Therefore, the following description should not be limited to this.

With reference to FIG. 1, the overall configuration of the power conversion device 100 according to the present embodiment will be described. The power conversion device 100 includes a rectifier circuit 2 that converts AC power supplied from a three-phase AC power supply 1, for example, commercial AC power into DC power, and a smoothing circuit 3 that smoothes a pulsating current component of DC power. , A converter 4 for converting the DC voltage output from the smoothing circuit 3 into another DC voltage. Although illustration is omitted, the AC power supply 1 may be a power supply device of a single-phase AC power supply instead of the three-phase AC power supply, and the rectifying circuit 2 also rectifies the single-phase AC power into DC power. May be

When three-phase AC is input, the rectifier circuit 2 is configured by bridge-connecting six diodes D1 to D6, and outputs a DC voltage having an AC ripple component from its output terminal. The smoothing circuit 3 includes, for example, a capacitor 11 connected between both output terminals of the rectifier circuit 2 (between the first line L1 and the second line L2).

Further, the converter 4 is configured as a step-up chopper including an inductor 12, a transistor 13 as a switching element, a diode 14, a first capacitor 15, a resistance element 16, and a second capacitor 17. That is, the converter 4 is a device that converts the DC voltage Vsm output by the smoothing circuit 3 into a different DC voltage Vout.

The inductor 12 and the diode 14 are connected in series to the (+) side output terminal of the rectifier circuit 2. The diode 14 is connected so that the inductor 12 side serves as an anode and the opposite side serves as a cathode. On the other hand, the transistor 13 is connected between the connection node of the inductor 12 and the diode 14 and the (−) side output terminal, and is turned on/off at a predetermined switching frequency (duty ratio). By adjusting this duty ratio, the boost ratio of the converter (boost chopper) is adjusted.

The first capacitor 15 and the second capacitor 17 are connected in parallel to the output terminal of the converter 4 (between the first power line L1 and the second power line L2) and reduce (smooth) the ripple component included in the output of the converter 4. It also has the role of absorbing the switching surge. The input voltage (Vsm) of the converter 4 includes AC power supply ripple (about 300 to 360 Hz in the case of three-phase AC) based on the AC voltage from the three-phase AC power supply 1. In addition, the switching operation of the transistor 13 causes switching ripple (several tens to several hundreds of kHz) based on the switching frequency. The first capacitor 15 and the second capacitor 17 have a role of further reducing such ripple components. The first capacitor 15 has a larger capacity than the second capacitor 17, and has a main role in smoothing the output voltage. The second capacitor 17 is a capacitor provided as an auxiliary to the first capacitor 15 in order to cope with larger ripples.

Therefore, the electrolytic capacitor is preferably used as the first capacitor 15. One end thereof is on the cathode side of the diode 14 and is connected to the first power line L1 extending from the (+) side output terminal. The electrolytic capacitor used as the first capacitor 15 has, for example, a capacitance of about several hundred μF. Electrolytic capacitors have a relatively large equivalent series resistance (ESR), which limits the allowable ripple current to a few amps. The capacitance tolerance is about ±20%.

The resistance element 16 is connected in series with the first capacitor 15 between the other end of the first capacitor 15 and the second power line L2 connected to the (−) side output terminal. As will be described later, the resistance element 16 has a role of suppressing a current based on the switching ripple from flowing to the first capacitor 15 which is an electrolytic capacitor. By suppressing the current based on the switching ripple from flowing through the first capacitor 15, it is possible to use a large electrolytic capacitor or reduce the number of electrolytic capacitors connected in parallel. This contributes to improvement of power conversion efficiency and reduction in size and weight of electric equipment.

The second capacitor 17 is a capacitor having a smaller capacity than the first capacitor 15, as described above. As the second capacitor 17, for example, a film capacitor or a laminated ceramic capacitor is preferably used. The second capacitor 17 is connected in parallel with the first capacitor 15 between the first power line L1 and the second power line L2. As an example, the capacitance of the second capacitor 17 is set to about 1/100 to 1/10 of the capacitance of the first capacitor 15 (electrolytic capacitor). For example, when the capacitance of the first capacitor 15 is several hundred μF, the capacitance of the second capacitor 17 can be set to, for example, several μF to several tens μF. When the second capacitor 17 is a film capacitor or a laminated ceramic capacitor, its equivalent series resistance (ESR) is smaller than the equivalent series resistance (ESR) of the first capacitor 15. Therefore, the rated allowable current or allowable ripple current of the second capacitor 17 is also larger than that of the first capacitor 15.

As described above, when an electrolytic capacitor is used as the first capacitor 15 and a film capacitor or a laminated ceramic capacitor is used as the second capacitor 17, the second capacitor 17 having a smaller capacity has a larger impedance than the first capacitor 15. Become. Therefore, when the first capacitor 15 and the second capacitor 17 are simply connected in parallel, most of the current based on the switching ripple of 10 kHz to several hundred kHz flows in the first capacitor 15 and almost flows in the second capacitor 17. Absent. In this case, the current of the first capacitor 15 exceeds the allowable ripple current, and heating of the first capacitor 15 or the like becomes a problem. However, in the present embodiment, this problem is solved by connecting the resistance element 16 in series with the first capacitor 15 and setting the resistance value appropriately.

Here, as a comparative example, FIG. 2 shows a device in which the resistance element 16 is excluded from the power conversion device 100 of FIG. Further, FIG. 3 shows a graph for explaining the effect of the present embodiment as compared with this comparative example. The graph of FIG. 3 is a graph showing the relationship between the frequency f and the impedance. In FIG. 3, the rectangular dot curve shows the change of the impedance of the first capacitor 15 (470 μF electrolytic capacitor) with respect to the frequency f, and the diamond dot curve shows the change of the second capacitor 17 (20 μF film capacitor). The change of the impedance with respect to the frequency f is shown, and the curve of the triangular dot shows the change of the impedance of the series circuit of the first capacitor 15 (470 μF electrolytic capacitor) and the resistance element 16 (0.1Ω) with respect to the frequency f. ..

As shown in FIG. 3, the electrolytic capacitor and the film capacitor generally show the characteristic that the impedance decreases as the frequency f increases in the frequency region of several hundreds kHz or less. However, when the capacitance of the electrolytic capacitor is significantly larger than that of the film capacitor, the impedance of the film capacitor is significantly larger than that of the electrolytic capacitor even in the vicinity of the frequency of switching ripple (300 to 360 kHz). Therefore, in the configuration in which the first capacitor 15 and the second capacitor 17 are simply connected in parallel as in the comparative example (FIG. 2), most of the current based on the switching ripple frequency (about 300 kHz to 360 kHz) is generated. The current flows through the first capacitor 15, and the second capacitor 17 hardly flows. In this case, the current flowing through the first capacitor 15 exceeds the allowable ripple current, which may cause the first capacitor 15 to be heated or even destroyed.

When the resistance element 16 (example: 0.1Ω) is connected in series to the first capacitor 15 (example: 470 μF electrolytic capacitor) as in the present embodiment, the frequency f-impedance characteristic is as shown in FIG. Therefore, the decrease (slope) of the impedance with the increase of the frequency f becomes gentler than that of the comparative example (FIG. 2). This is because the impedance based on the capacitance component decreases in the high frequency region, while the impedance based on the resistance component of the resistance element 16 is constant, and the impedance based on the resistance component becomes dominant in the high frequency region. By appropriately setting the resistance value of the resistance element 16, the impedance of the series circuit of the first capacitor 15 and the resistance element 16 near the switching ripple frequency (300 to 360 kHz) is made larger than the impedance of the second capacitor 17. can do. In this case, most of the current based on the switching ripple can be passed through the second capacitor 17. Therefore, it is not necessary to increase the capacity of the first capacitor 15 or increase the number of parallel connections.

As described above, in the power conversion device according to the present embodiment, the impedance of the series circuit of the first capacitor 15 and the resistance element 16 near the switching ripple frequency (300 to 360 kHz) is larger than the impedance of the second capacitor 17. The resistance value Rs of the resistance element 16 is set so as to do so (hereinafter, referred to as “first condition”). For that purpose, the following formula must be satisfied. Here, the resistance value of the resistance element 16 is R _s , the capacitance of the first capacitor 15 is C _c , the capacitance of the second capacitor 17 is C _f , and the switch frequency of the second capacitor 17 in the capacitance rectifying circuit 2 is f _sw .

From this [Formula 3], the resistance value R _s of the resistance element 16 is set so as to satisfy the following Formula [Formula 4].

As an example, when f _ac =360 Hz, f _sw =150 kHz, C _c =470 μF, and C _f =20 μF, the resistance element 16 needs to be selected so that Rs≧0.0508 [Ω].

In addition to this [Equation 4], the resistance value Rs is preferably set to 20% or less of the impedance of the first capacitor 15 at the ripple frequency (AC power supply ripple) f _ac based on the frequency of the AC power supply (hereinafter, This is called "second condition"). The second condition is based on the capacitance tolerance of the electrolytic capacitor being about ±20%. By satisfying the second condition, the dependence of the impedance of the series circuit of the first capacitor 15 and the resistance element 16 on the frequency f is reduced, and as a result, the frequency-impedance characteristic with a small inclination as shown in FIG. 2 is obtained. be able to. For that purpose, the following formula [Equation 5] needs to be satisfied.

As an example, when f _ac =360 Hz, f _sw =150 kHz, C _c =470 μF, and C _f =20 μF, the resistance element 16 needs to be selected so that Rs≦0.188 [Ω]. When it is required to satisfy both the first condition and the second condition, it is required that [Equation 4] and [Equation 5] be satisfied at the same time.

As described above, by setting the resistance value R _s of the resistance element 16 so that the first condition is satisfied, most of the current based on the switching ripple is supplied to the second capacitor 17 instead of the first capacitor 15. In addition, the current flowing through the first capacitor 15 can be suppressed. This eliminates the need to increase the capacity of the first capacitor 15 or increase the number of parallel connections. In addition to this, if the resistance value R _s is set so as to satisfy the second condition as well, the frequency dependence of the impedance of the series circuit of the first capacitor 15 and the resistance element 16 becomes small, and the first capacitor 15 near the switching frequency. It is possible to reliably suppress the current.
The first capacitor 15, the resistance element 16, and the second capacitor 17 are not necessarily one, but a plurality of them may be provided. The total capacitance value and resistance value of such a plurality of elements may be set so as to satisfy the above conditions.

The present invention is not limited to the above embodiment, and various modifications are included. For example, the above embodiments have been described in detail for the purpose of explaining the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add/delete/replace other configurations with respect to a part of the configurations of the respective embodiments.

DESCRIPTION OF SYMBOLS 1... Three-phase alternating current power supply, 2... Rectifier circuit, 3... Smoothing circuit, D1-D6... Diode, L1... 1st line, L2... 2nd line, 11... Capacitor, 12... Inductor, 13... Transistor, 14... Diode, 15... 1st capacitor, 16... Resistance element, 17... 2nd capacitor.

Claims (2)

A rectifier circuit that converts AC voltage to DC,
A converter that converts the output voltage of the rectifier circuit into another output voltage,
Equipped with
The converter is
A first capacitor as an electrolytic capacitor,
A second capacitor connected in parallel with the first capacitor, having a capacitance smaller than that of the first capacitor, and having an equivalent series resistance smaller than that of the first capacitor;
A resistance element connected in series with the first capacitor;
When the resistance value of the resistance element is R _s , the capacitance value of the first capacitor is C _c , the capacitance value of the second capacitor is C _f , and the switching frequency in the converter is f _sw , the resistance value R _s is ,

A power conversion device, which satisfies: When the ripple frequency of the AC voltage is f _ac ,
The resistance value R _s is

The power conversion device according to claim 1, which satisfies:

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