JP7108526B2 - power converter - Google Patents

Description

The present invention relates to power converters.

An AC-DC converter includes, for example, a rectifier circuit that converts AC power into DC power, a smoothing circuit that smoothes the pulsating current output by the rectifier circuit, and a smoothed DC voltage that is further converted into another voltage. and a converter for The converter is provided with a capacitor to absorb the switching surge. This capacitor has a ripple (hereinafter referred to as "AC power 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. ) (hereinafter referred to as “switching ripple”) flows.

As capacitors included in converters, large-capacity electrolytic capacitors are often used to absorb switching ripples and switching surges. Moreover, 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 (see, for example, Patent Document 1).

In general, electrolytic capacitors have a large capacitance per unit volume and low impedance in a frequency range of about several hundred kHz used as a switching frequency, but have a relatively large ESR (equivalent series resistance). Therefore, many electrolytic capacitors have an allowable ripple current of several amperes. On the other hand, ceramic capacitors and film capacitors are inferior to electrolytic capacitors in terms of capacitance per unit volume, but their ESR is small, so their allowable ripple current is relatively large.

In the power conversion circuit, the capacity of the electrolytic capacitor is designed based on the commercial frequency ripple of the design specification, but if the allowable ripple current is not sufficient, the number of electrolytic capacitors is increased or a larger capacity electrolytic capacitor is used. It is necessary to However, increasing the number of electrolytic capacitors or using large-capacity electrolytic capacitors causes a decrease in power conversion efficiency, and does not meet the demand for miniaturization and weight reduction of electrical equipment.

JP 2012-055084 A

SUMMARY OF THE INVENTION The present invention provides a power converter capable of suppressing an increase in the number of parallel electrolytic capacitors by suppressing current flowing through the electrolytic capacitor when a low-capacity capacitor is used together with the electrolytic capacitor. for the purpose.

を満たすことを特徴とする。 A power conversion device according to the present invention includes a rectifier circuit that converts an AC voltage into a DC voltage, and a converter that converts the output voltage of the rectifier circuit into another output voltage. a capacitor, a second capacitor connected in parallel with the first capacitor, having a smaller capacitance than the first capacitor and having a smaller equivalent series resistance than the first capacitor, and a second capacitor connected in series with the first capacitor. and a resistive element.
When the resistance value of the resistive 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 ,

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 _fac ,
The resistance value R _s is

is preferably satisfied.

According to the present invention, when a capacitor with a lower capacity than the electrolytic capacitor is used together with the electrolytic capacitor, it is possible to suppress the current flowing through the electrolytic capacitor, thereby suppressing 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. 9 is a graph for explaining effects of the embodiment;

Hereinafter, this embodiment will be described with reference to the accompanying drawings. In the accompanying drawings, functionally identical elements may be labeled with the same numbers. It should be noted that although the attached drawings show embodiments and implementations in accordance with the principles of the present disclosure, they are for the purpose of understanding the present disclosure and are in no way used to interpret the present disclosure in a restrictive manner. is not. The description herein is merely exemplary and is not intended to limit the scope or application of this disclosure in any way.

Although the present embodiments are described in sufficient detail to enable those skilled in the art to practice the present disclosure, other implementations and configurations are possible and do not depart from the scope and spirit of the present disclosure. It is necessary to understand that it is possible to change the composition/structure and replace various elements. Therefore, the following description should not be construed as being limited to this.

The overall configuration of a power converter 100 according to the present embodiment will be described with reference to FIG. This power converter 100 includes a rectifier circuit 2 that converts AC power supplied from a three-phase AC power supply 1, such as commercial AC power, into DC power, and a smoothing circuit 3 that smoothes the pulsating current component of the DC power. , and a converter 4 for converting the DC voltage output from the smoothing circuit 3 into another DC voltage. Although not shown, the AC power supply 1 may be a single-phase AC power supply instead of a three-phase AC power supply, and the rectifier circuit 2 is also a circuit for rectifying single-phase AC power into DC power. may be

The rectifier circuit 2 is configured by bridge-connecting six diodes D1 to D6 when a three-phase AC is input, and outputs a DC voltage having an AC ripple component from its output terminal. The smoothing circuit 3 is composed of, 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).

Also, the converter 4 is configured as a boost 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 is the anode and the other side is the 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 and off at a predetermined switching frequency (duty ratio). The step-up ratio of the converter (step-up chopper) is adjusted by adjusting the duty ratio.

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) to reduce (smooth) the ripple component included in the output of the converter 4. In addition, it has a role of absorbing switching surge. The input voltage (Vsm) of converter 4 includes an AC power supply ripple (about 300 to 360 Hz in the case of three-phase AC) based on the AC voltage from three-phase AC power supply 1 . Also, the switching operation of the transistor 13 causes a switching ripple (about several tens to several hundred kHz) based on its switching frequency. The first capacitor 15 and the second capacitor 17 serve to further reduce such ripple components. The first capacitor 15 has a larger capacity than the second capacitor 17 and plays a major 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 be able to cope with a larger ripple.

Therefore, an 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. An electrolytic capacitor used as the first capacitor 15 has a capacitance of about several hundred μF, for example. Electrolytic capacitors have a relatively high equivalent series resistance (ESR), which generally limits the allowable ripple current to about several amperes. Also, 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 resistive element 16 has a role of suppressing current based on switching ripple from flowing to the first capacitor 15, which is an electrolytic capacitor. By suppressing the flow of the current based on the switching ripple in the first capacitor 15, it is possible to use a large electrolytic capacitor and reduce the number of parallel connections of electrolytic capacitors. This contributes to improvement in power conversion efficiency and reduction in size and weight of electrical equipment.

The second capacitor 17 is a capacitor with a smaller capacity than the first capacitor 15 as described above. A film capacitor or a laminated ceramic capacitor is preferably used as the second capacitor 17, for example. 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 approximately 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 of μF. If the second capacitor 17 is a film capacitor or a laminated ceramic capacitor, its equivalent series resistance (ESR) is smaller than that 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 through the first capacitor 15 and most of it flows through the second capacitor 17. do not have. In this case, the current of the first capacitor 15 exceeds the allowable ripple current, causing problems such as heating of the first capacitor 15 . However, in this embodiment, this problem is solved by connecting the resistance element 16 in series with the first capacitor 15 and appropriately setting the resistance value thereof.

Here, as a comparative example, FIG. 2 shows a device in which the resistance element 16 is removed from the power conversion device 100 of FIG. Also, FIG. 3 shows a graph for explaining the effect of this embodiment compared to this comparative example. The graph in FIG. 3 is a graph showing the relationship between frequency f and impedance. In FIG. 3, the curve of rectangular dots shows the change in the impedance of the first capacitor 15 (470 μF electrolytic capacitor) with respect to the frequency f, and the curve of diamond dots shows the change of the impedance of the second capacitor 17 (20 μF film capacitor). The curve of triangular dots 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, electrolytic capacitors and film capacitors generally exhibit the characteristic that the impedance decreases as the frequency f increases in the frequency range of several hundred 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 near the switching ripple frequency (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) 15 and hardly flows to the second capacitor 17 . In this case, the current flowing through the first capacitor 15 may exceed the allowable ripple current, causing the first capacitor 15 to be heated and even destroyed.

When the resistance element 16 (eg, 0.1Ω) is connected in series to the first capacitor 15 (eg, an electrolytic capacitor of 470 μF) as in the present embodiment, the frequency f-impedance characteristic is as shown in FIG. Therefore, the decrease (slope) of the impedance with an increase in the frequency f becomes gentler than in the comparative example (FIG. 2). This is because the impedance based on the capacitance component decreases in the high frequency range, while the impedance based on the resistance component of the resistance element 16 is constant, so the impedance based on the resistance component becomes dominant in the high frequency range. 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 switching ripple can flow 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 of 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 set larger than the impedance of the second capacitor 17. The resistance value Rs of the resistive element 16 is set so as to (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 switching frequency of the second capacitor 17 in the capacitance rectifier circuit 2 is Let _fsw .

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

As an example, if f _ac =360 Hz, f _sw =150 kHz, C _c =470 μF, and C _f =20 μF, the resistance element 16 should be selected such 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 ripple) _fac based on the frequency of the AC power supply (hereinafter referred to as This is called “second condition”). This second condition is based on the fact that the capacitance tolerance of electrolytic capacitors is 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 becomes small, thereby obtaining a frequency-impedance characteristic with a small slope as shown in FIG. be able to. For that purpose, the following formula [Equation 5] must be satisfied.

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

As described above, by setting the resistance value _Rs of the resistance element 16 so as to satisfy the first condition, most of the current based on the switching ripple flows through the second capacitor 17 instead of the first capacitor 15. Also, 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 _Rs is set so as to satisfy the second condition, 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 current can be reliably suppressed.
The number of the first capacitor 15, the resistance element 16 and the second capacitor 17 is not necessarily one, and a plurality of them may be provided. It is sufficient that the total capacitance value and resistance value of such a plurality of elements are set so as to satisfy the above conditions.

The present invention is not limited to the above embodiments, and includes various modifications. For example, the above embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the described configurations. Also, 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. Moreover, it is possible to add, delete, or replace a part of the configuration of each embodiment with another configuration.

DESCRIPTION OF SYMBOLS 1... Three-phase alternating current power supply 2... Rectifier circuit 3... Smoothing circuit D1-D6... Diode L1... First line L2... Second line 11... Capacitor 12... Inductor 13... Transistor 14... Diode 15 First capacitor 16 Resistance element 17 Second 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;
with
The converter is
a first capacitor as an electrolytic capacitor;
a second capacitor connected in parallel with the first capacitor, having a smaller capacitance than the first capacitor and a smaller equivalent series resistance than the first capacitor;
a resistive element connected in series with the first capacitor,
When the resistance value of the resistive 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 characterized by satisfying When the ripple frequency of the AC voltage is _fac ,
The resistance value R _s is

The power converter according to claim 1, satisfying:

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