DC-DC converter and power supply system

Abstract

To provide a compact DC-DC converter that implements an insulating function without using a transformer.SOLUTION: A DC-DC converter 100 includes first and third switch elements 111 and 113 that are connected in series between an input and an output of a high potential side, second and fourth switch elements 112 and 114 that are connected in series between an input and an output of a low potential side, an intermediate capacitor 122 that is connected between a line between the first and third switch elements and a line between the second and fourth switch elements, an output capacitor 123, and a control circuit 140. The control circuit 140 alternately repeats an intermediate capacitor charging operation of keeping the on state of the first switch element and the second switch element while keeping the off state of the third switch element and the fourth switch element, and an intermediate capacitor discharging operation of keeping the off state of the first switch element and the second switch element while keeping the on state of the third switch element and the fourth switch element.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to a DC-DC converter and a power supply system.

Background Art

[0002] For example, a connection box for connecting a plurality of solar cell units to a power conditioner for power grid connection is known (see, for example, Patent Documents 1 and 2). Such a connection box collects DC power generated by a plurality of solar cell units and supplies it to the power conditioner. For example, the connection box includes switches and reverse current prevention diodes (electrical components) provided for each solar cell unit. [[ID=1B]]

[0003] As such a power conditioner, from the viewpoints of high conversion efficiency and low cost, there are many transformerless (non-insulated) power conditioners. In a transformerless power conditioner, since the power grid side and the solar cell side are not insulated from each other, when the power grid side is grounded at the neutral point, a potential difference occurs between the positive and negative electrodes of the solar cell side and the ground (Fig. 7).

[0004] On the other hand, since the frame of the solar cell panel in the solar cell unit is usually grounded, the glass substrates on the light-receiving surface and the back surface of the solar cell panel also have the ground potential. Therefore, when the power conditioner is operating, a certain potential is applied between the solar cell and the glass substrate in the solar cell panel. Therefore, depending on the magnitude or polarity of the potential, there is a risk of adverse effects such as corrosion of the solar cell or deterioration of the power generation performance.

[0005] Also, depending on the magnitude or polarity of the potential, over time, the insulation resistance of the solar cell panel may decrease or the stray capacitance of the solar cell panel may increase. Then, in the case of a transformerless power conditioner, a ground fault current may occur, and there is a risk that the leakage breaker in the distribution board will operate.

[0006] In this regard, the inventors of the present invention have devised a method to insulate the power grid side from the solar cell side by providing an insulated DC-DC converter in the junction box (Figure 6) (see, for example, Patent Document 3).

[0007] This prevents adverse effects such as corrosion of the solar cells or a decrease in power generation performance from occurring, even in the case of a transformerless power conditioner, because no potential is applied between the solar cells and the glass substrate in the solar panel.

[0008] Furthermore, it is possible to prevent a decrease in the insulation resistance of the solar cell array and an increase in the stray capacitance of the solar cell array due to aging. Moreover, even if this phenomenon occurs, the DC-DC converter provides insulation between the power grid side and the solar cell side, so even in the case of a transformerless power conditioner, ground fault current will not occur, and unnecessary tripping of the leakage circuit breaker in the distribution board can be prevented. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2014-225614 [Patent Document 2] Japanese Patent Publication No. 2014-107370 [Patent Document 3] Japanese Patent Publication No. 2021-71750 [Overview of the project] [Problems that the invention aims to solve]

[0010] The DC-DC converter described in Patent Document 3 uses a transformer to achieve isolation, but since transformers are larger in volume and weight compared to other electronic components, there is a problem in that the volume and weight of the DC-DC converter become large.

[0011] Therefore, the present invention aims to provide a compact DC-DC converter and power supply system that achieves isolation functionality without using a transformer, in order to solve the above problems. [Means for solving the problem]

[0012] The DC-DC converter according to the present invention is a DC-DC converter that generates DC output power at a pair of output terminals by converting DC input power input to a pair of input terminals, and comprises: a first switch element having one terminal connected to the higher potential input terminal of the pair of input terminals and the other terminal; a second switch element having one terminal connected to the lower potential input terminal of the pair of input terminals and the other terminal; a third switch element having one terminal connected to the other terminal of the first switch element and the other terminal connected to the higher potential output terminal of the pair of output terminals; and the other terminal of the second switch element The device comprises a fourth switch element having one terminal connected to one terminal and the other terminal connected to the lower potential output terminal of the pair of output terminals; an output capacitor connected between the pair of output terminals; an intermediate line on the high potential side connecting the other terminal of the first switch element and the one terminal of the third switch element; an intermediate capacitor connected between the intermediate line on the low potential side connecting the other terminal of the second switch element and the one terminal of the fourth switch element; and a control circuit for controlling the first switch element, the second switch element, the third switch element, and the fourth switch element. The control circuit alternately repeats an intermediate capacitor charging operation that turns on the first and second switch elements and turns off the third and fourth switch elements, and an intermediate capacitor discharge operation that turns off the first and second switch elements and turns on the third and fourth switch elements. This maintains constant isolation between the pair of input terminals and the pair of output terminals without using a transformer.

[0013] The power supply system according to the present invention comprises a power source composed of a solar cell unit or a battery storage unit, a DC-DC converter according to any one of claims 1 to 4 having a pair of input terminals connected to the power source, and a resistive impedance element provided between the power source and the pair of input terminals of the DC-DC converter, and at least one of a high-potential input line and a low-potential input line.

[0014] Another power supply system according to the present invention comprises a power source composed of a solar cell unit or a battery storage unit, a DC-DC converter according to any one of claims 1 to 4 having a pair of input terminals connected to the power source, and a voltage source provided between the power source and the pair of input terminals of the DC-DC converter, and at least one of a high-potential input line and a low-potential input line. [Effects of the Invention]

[0015] According to the present invention, since isolation functionality can be achieved in a DC-DC converter without using a transformer, miniaturization is possible. [Brief explanation of the drawing]

[0016] [Figure 1A] This figure shows a DC-DC converter according to this embodiment. [Figure 1B] Figure 1A shows an example of a DC-DC converter. [Figure 1C] Figure 1A shows an example of a DC-DC converter. [Figure 2] This figure shows an example of a solar power generation system (power supply system) according to this embodiment. [Figure 3A] This figure shows an example of a photovoltaic power generation system (power supply system) according to a modified version of this embodiment. [Figure 3B] This figure shows an example of a photovoltaic power generation system (power supply system) according to a modified version of this embodiment. [Figure 3C] This is a diagram showing an example of a photovoltaic power generation system (power supply system) according to a modified example of this embodiment. [Figure 3D] This is a diagram showing an example of a photovoltaic power generation system (power supply system) according to a modified example of this embodiment. [Figure 4] This is a diagram showing an example of a photovoltaic power generation system (power supply system) according to a modified example of this embodiment. [Figure 5] This is a diagram showing an example of a battery system (power supply system) according to a modified example of this embodiment. [Figure 6] This is a diagram showing an example of a photovoltaic power generation system including a connection box having a DC-DC converter according to a comparative example. [Figure 7] This is a diagram showing an example of a conventional photovoltaic power generation system including a connection box.

Mode for Carrying Out the Invention

[0017] Hereinafter, an example of an embodiment of the present invention will be described with reference to the accompanying drawings. In each drawing, the same or corresponding parts will be denoted by the same reference numerals. Also, for the sake of convenience, hatching, member reference numerals, etc. may be omitted, but in such cases, other drawings shall be referred to.

[0018] (DC-DC Converter) FIG. 1A is a diagram showing a DC-DC converter according to this embodiment, and FIGS. 1B and 1C are diagrams showing an example of the DC-DC converter shown in FIG. 1A. The DC-DC converter 100 shown in FIG. 1A generates DC output power obtained by converting DC input power input to a pair of input terminals T1 and T2 at a pair of output terminals T3 and T4. The DC-DC converter 100 includes a first switch element 111, a second switch element 112, a third switch element 113, a fourth switch element 114, an input capacitor 121, an intermediate capacitor 122, an output capacitor 123, and a control circuit 140.

[0019] The first switch element 111 and the third switch element 113 are connected in series between the input terminal T1, which is on the higher potential side of a pair of input terminals (hereinafter also referred to as the positive terminal), and the output terminal T3, which is on the higher potential side of a pair of output terminals. The second switch element 112 and the fourth switch element 114 are connected in series between the input terminal T2, which is on the lower potential side of a pair of input terminals (hereinafter also referred to as the negative terminal), and the output terminal T4, which is on the lower potential side of a pair of output terminals.

[0020] Specifically, one terminal of the first switch element 111 is connected to the high-potential input terminal T1, and the other terminal of the first switch element 111 is connected to one terminal of the third switch element 113. The other terminal of the third switch element 113 is connected to the high-potential output terminal T3. One terminal of the second switch element 112 is connected to the low-potential input terminal T2, and the other terminal of the second switch element 112 is connected to one terminal of the fourth switch element 114. The other terminal of the fourth switch element 114 is connected to the low-potential output terminal T4.

[0021] The first switch element 111, the second switch element 112, the third switch element 113, and the fourth switch element 114 are not particularly limited, but include power semiconductor elements such as IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).

[0022] For example, as shown in Figure 1B, the first switch element 111, the second switch element 112, the third switch element 113, and the fourth switch element 114 may be composed of a single field-effect transistor (unidirectional switch) such as a MOSFET. These switch elements may be n-type transistors or p-type transistors. This unidirectional switch configuration is suitable when the relative positions of the input-side ground potential and the output-side ground potential remain constant.

[0023] Alternatively, as shown in Figure 1C, for example, the first switching element 111, the second switching element 112, the third switching element 113, and the fourth switching element 114 may be composed of two field-effect transistors (bidirectional switches) connected in series such that their source terminals or drain terminals face each other. This bidirectional switch configuration is suitable when the relative positions of the input-side ground potential and the output-side ground potential are undefined.

[0024] As will be described later, when the DC-DC converter 100 is applied to, for example, a solar power generation system, and the power conditioner downstream of the DC-DC converter 100 is a transformerless type, the ground voltage of the negative terminal on the output side of the DC-DC converter 100 is often negative. In this case, if the negative terminal on the input side of the DC-DC converter 100 is grounded, the relationship (input side ground potential ≥ output side ground potential) is always maintained. In this case, the DC-DC converter 100 can be realized with fewer components by using a unidirectional switch configuration as shown in Figure 1B. However, if the relationship between the input side ground potential and the output side ground potential of the DC-DC converter 100 is not limited, such as when the positive terminal on the input side is grounded, i.e., if the relationship (input side ground potential < output side ground potential) is possible, then in the configuration of Figure 1B, a reverse current will flow from the output side to the input side of the DC-DC converter 100, impairing the insulation function. In such cases, it is preferable to use a bidirectional switch configuration as shown in Figure 1C.

[0025] The input capacitor 121 is connected between a pair of input terminals T1 and T2. That is, the input capacitor 121 is connected between one terminal of the first switch element 111 and one terminal of the second switch element 112. However, the input capacitor 121 is not essential, and can be omitted if the power source connected to the input terminals is capable of supplying current intermittently, such as a storage battery.

[0026] The intermediate capacitor 122 is connected between the high-potential intermediate line connecting the other terminal of the first switch element 111 and one terminal of the third switch element 113, and the low-potential intermediate line connecting the other terminal of the second switch element 112 and one terminal of the fourth switch element 114.

[0027] The output capacitor 123 is connected between the pair of output terminals T3 and T4. That is, the output capacitor 123 is connected between the other terminal of the third switch element 113 and the other terminal of the fourth switch element 114.

[0028] The control circuit 140 controls the DC-DC converter 100 by controlling the on / off states of the first switch element 111, the second switch element 112, the third switch element 113, and the fourth switch element 114. Specifically, the control circuit 140 controls the DC-DC converter 100. - An intermediate capacitor charging operation in which the first switch element 111 and the second switch element 112 are turned ON, and the third switch element 113 and the fourth switch element 114 are turned OFF, - An intermediate capacitor discharge operation in which the first switch element 111 and the second switch element 112 are turned off, and the third switch element 113 and the fourth switch element 114 are turned on, This process is repeated alternately. As a result, the DC-DC converter 100 can maintain constant isolation between the pair of input terminals T1, T2 and the pair of output terminals T3, T4 without using a transformer (it functions as an isolated DC-DC converter). Furthermore, the DC-DC converter 100 does not use coils, nor does it perform boost or buck operations, and if voltage drop due to circuit current is ignored, the voltage between the input terminals is directly transmitted to the output terminals.

[0029] As mentioned above, the input voltage of the DC-DC converter 100 is directly transmitted to the output voltage of the DC-DC converter 100. Therefore, as will be described later, when the DC-DC converter 100 is applied to a solar power generation system, for example, the voltage-current characteristics on the output side of the DC-DC converter 100 will be the same as the voltage-current characteristics on the input side of the DC-DC converter 100, i.e., the voltage-current characteristics of the solar panel described later. As a result, the power conditioner can operate in accordance with the maximum power point of the solar panel using the MPPT function.

[0030] As described above, the DC-DC converter 100 of this embodiment achieves isolation functionality without using a transformer, thus enabling miniaturization and weight reduction.

[0031] (Solar power generation system: Power supply system) The following describes an example of applying the DC-DC converter 100 of the above-described embodiment to a solar power generation system. Figure 2 is a diagram showing an example of a solar power generation system according to this embodiment. The solar power generation system 1 shown in Figure 2 comprises a plurality of solar cell units (power sources) 20, a junction box 10, and a power conditioner 30.

[0032] Each of the solar cell units 20 includes, for example, a plurality of solar cell panels arranged in an array. Each of the solar cell panels consists of, for example, a plurality of solar cells arranged in a two-dimensional manner, a glass substrate protecting the light-receiving surface and the back surface of the plurality of solar cells, a sealing material, and a frame. Each of the solar cell units 20 may also include a bypass diode.

[0033] The junction box 10 connects multiple solar cell units 20 to a power conditioner 30. Specifically, the junction box 10 collects the DC power generated by the multiple solar cell units 20 and supplies it to the power conditioner 30. The junction box 10 may also be equipped with a switch 11 and a reverse current prevention diode 12 (electrical component) provided for each solar cell unit 20.

[0034] The junction box 10 further includes the transformerless isolated DC-DC converter 100 described above. The negative terminal (low-potential input terminal) on the input side of the DC-DC converter 100 is grounded via a series circuit of resistors 210 and 220 (resistive impedance element). In other words, the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 is connected to the earth potential via the series circuit of resistors 210 and 220.

[0035] The junction box 10 may be equipped with a ground fault detection circuit 200. The ground fault detection circuit 200 detects the ground fault current generated on the solar cell unit 20 side by detecting the voltage to ground between resistors 210 and 220. As will be described later, when the junction box 10 and power conditioner 30 are of the transformerless type, protection against ground fault current was possible by the leakage circuit breaker in the distribution board 40. However, because the junction box 10 is equipped with an isolated DC-DC converter 100, the leakage circuit breaker in the distribution board 40 cannot protect against ground fault current on the solar cell side of the junction box 10. In this regard, by providing a series circuit of resistors 210 and 220 on the ground line on the solar cell side of the isolated DC-DC converter 100 in the junction box 10 and detecting the voltage to ground between them with the ground fault detection circuit 200, the ground fault current generated on the solar cell unit 20 side can be detected.

[0036] The power conditioner 30 connects the DC power generated by the multiple solar cell units 20 to the commercial power grid. The power conditioner 30 includes a boost circuit 31, a DC-AC converter 32, and various electrical circuits 33 for power grid connection. The boost circuit 31 boosts the DC power from the multiple solar cell units 20. The DC-AC converter 32 and electrical circuits 33 convert the boosted DC power into AC power similar to commercial power. The converted AC power is connected to the commercial power grid via the distribution board 40.

[0037] Power conditioner 30 is a transformerless (non-isolated) power conditioner. Power conditioner 30 is neutral-point grounded on the power grid side.

[0038] The power conditioner 30 has a Maximum Power Point Tracking (MPPT) function. The MPPT function is a function that operates to track the optimal operating point where the output power can be maximized, i.e., the maximum power point of the solar cell unit.

[0039] Here, Figure 7 shows an example of a conventional solar power generation system equipped with a junction box. The conventional solar power generation system 1Y shown in Figure 7 differs from the solar power generation system 1 of this embodiment shown in Figure 2 in that it is equipped with a junction box 10Y instead of a junction box 10. The conventional junction box 10Y differs from the junction box 10 of this embodiment in that it does not have a DC-DC converter 100 and resistors 210, 220, and as a result the potential difference between the input side of the junction box 10Y and the ground is affected by the potential difference between the output side and the ground. This conventional solar power generation system 1Y has the following problems.

[0040] Many power conditioners 30 are transformerless (non-isolated) types, chosen for their high conversion efficiency and low cost. In transformerless power conditioners 30, there is no insulation between the power grid and the solar cell. Therefore, if the power grid is grounded at the neutral point, a potential difference occurs between the positive and negative electrodes of the solar cell and the earth.

[0041] For example, as shown in Figure 7, if the power system is a single-phase three-wire system of AC200V, then, as an example, the potential difference between the positive and negative terminals on the DC side of the DC-AC converter 32 of the power conditioner 30 and the ground is +165V and -165V, respectively. If the potential difference between the positive and negative terminals on the solar cell side of the power conditioner 30 is, for example, 234V, then the potential difference between the positive and negative terminals on the solar cell side of the power conditioner 30 and the ground is +69V and -165V, respectively.

[0042] On the other hand, since the frame of the solar panel in the solar cell unit 20 is normally grounded, the light-receiving surface and the glass substrate on the back of the solar panel are also at ground potential. Therefore, when the power conditioner 30 is operating, a certain potential (-165V in the example in Figure 7) is applied between the solar cell and the glass substrate in the solar panel. Depending on the magnitude or polarity of the potential (for example, a negative potential), adverse effects such as corrosion of the solar cell or a decrease in power generation performance may occur.

[0043] Furthermore, depending on the magnitude or polarity of the potential (for example, negative potential), the insulation resistance of the solar panel may decrease or the stray capacitance of the solar panel may increase due to aging. This can cause a ground fault current to flow back to the earth via the power conditioner 30, junction box 10X, and solar panel 20, potentially tripping the leakage circuit breaker in the distribution board 40.

[0044] In this regard, the present inventors devised a comparative example junction box in the process of arriving at the present invention. Figure 6 shows an example of a photovoltaic power generation system equipped with a junction box according to the comparative example. As shown in the comparative example in Figure 6, the present inventors devised to insulate the power grid side from the solar cell side by providing an isolated DC-DC converter 100X (DC-AC converter 110X, transformer 120X, AC-DC converter 130X, and control circuit 140X) in the junction box 10X, and to ground the positive or negative electrode on the solar cell side of the DC-DC converter 100X (stabilization of the solar cell's potential to ground). For example, as shown in Figure 6, by grounding the negative electrode on the solar cell side of the DC-DC converter 100X, the potential difference between the positive and negative electrodes on the solar cell side of the junction box 10X and the earth can be set to +234V and 0V, respectively. That is, the potential difference between the solar cell unit 20 and the earth can be set to a positive potential.

[0045] This prevents adverse effects such as corrosion of the solar cells or a decrease in power generation performance from occurring, even if the power conditioner 30 is a transformerless type, because, for example, a negative potential is not applied between the solar cells and the glass substrate in the solar panel.

[0046] Furthermore, it is possible to prevent a decrease in the insulation resistance of the solar panels and an increase in the stray capacitance of the solar panels due to aging. Moreover, even if this phenomenon occurs, the DC-DC converter 100X insulates the power grid side from the solar panel side. Therefore, even if the power conditioner 30 is a transformerless type, a ground fault current will not be generated that flows back to the earth via the power conditioner 30, junction box 10X, and solar panel 20, thus preventing unnecessary operation of the leakage circuit breaker in the distribution board 40.

[0047] However, while the DC-DC converter 100X in the comparative example can accommodate situations where basic or reinforced insulation is required for electric shock protection as defined by safety standards, the volume and weight of the transformer are larger compared to other electronic components, resulting in a larger volume and weight for the junction box 10X.

[0048] In this regard, the inventors of the present invention focus on the fact that electric shock protection as defined by safety standards can be ensured by measures such as insulation and grounding on the solar cell unit 20 side, and therefore, the insulation of the junction box 10X can be adequately provided by functional insulation that does not consider electric shock protection as defined by safety standards (for example, functional insulation that reduces leakage current (ground fault current)). Therefore, as shown in Figure 2, the inventors of the present invention devise to apply a DC-DC converter 100 to the junction box 10 that achieves functional insulation without using a transformer.

[0049] As a result, the photovoltaic power generation system 1 of this embodiment shown in Figure 2 can obtain the same advantages as the photovoltaic power generation system 1X of the comparative example shown in Figure 6 described above. Furthermore, according to the solar power generation system 1 of this embodiment, the DC-DC converter 100 in the junction box 10 achieves isolation functionality without using a transformer, making it possible to miniaturize and lighten the DC-DC converter 100. As a result, the junction box 10 with isolation functionality can be made smaller and lighter.

[0050] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications and variations are possible. In the embodiments described above, a photovoltaic power generation system 1 was illustrated, which includes a junction box 10 to which the negative electrode (low-potential input terminal) on the solar cell side of the DC-DC converter 100 is grounded. However, the present invention is not limited to this, and in the junction box 10 of the photovoltaic power generation system 1, the positive electrode (high-potential input terminal) on the solar cell side of the DC-DC converter 100 may be grounded, or the positive and negative electrodes (high-potential input terminal and low-potential input terminal) on the solar cell side of the DC-DC converter 100 may be common-mode grounded. In other words, as shown in Figure 3A, at least one of the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 and the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be grounded to the earth potential. This makes it possible to stabilize the ground potential of the solar cell.

[0051] Furthermore, in the embodiments described above, a photovoltaic power generation system 1 was illustrated that includes a junction box 10 in which the negative electrode (low-potential input terminal) on the solar cell side of the DC-DC converter 100 is grounded via a resistor. However, the present invention is not limited thereto, and as shown in Figures 3A and 4, in the junction box 10 of the photovoltaic power generation system 1, at least one of the positive electrode and negative electrode (high-potential input terminal and low-potential input terminal) on the solar cell side of the DC-DC converter 100 may be grounded via a resistive impedance element 250 or a voltage source 260. In other words, at least one of the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100, and the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100, may be grounded to the earth potential via a resistive impedance element 250 or a voltage source 260. This makes it possible to stabilize the ground potential of the solar cell. Examples of resistive impedance elements 250 include resistors, short-circuit wires, current fuses, etc.

[0052] For example, in the case of a resistor, As shown in Figure 3B, the negative terminal on the solar cell side of the DC-DC converter 100 may be grounded via the resistor 250, or in other words, the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be grounded via the resistor 250. As shown in Figure 3C, the positive terminal on the solar cell side of the DC-DC converter 100 may be grounded via the resistor 250, or in other words, the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 may be grounded via the resistor 250. As shown in Figure 3D, the positive and negative electrodes on the solar cell side of the DC-DC converter 100 may be grounded in common mode via resistors 250, specifically, the midpoint between two resistors 250 connected in series between the positive and negative electrodes may be grounded. In other words, the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 and the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be grounded in common mode via resistors 250, specifically, the midpoint between two resistors 250 connected in series between the high-potential input line and the low-potential input line may be grounded.

[0053] Similarly, for example, in the case of a short-circuit wire, The negative terminal on the solar cell side of the DC-DC converter 100 may be directly grounded by the shorting wire 250, or in other words, the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be directly grounded by the shorting wire 250. The positive terminal on the solar cell side of the DC-DC converter 100 may be directly grounded by the shorting wire 250. In other words, the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 may be directly grounded by the shorting wire 250.

[0054] Similarly, for example, in the case of a current fuse, The negative terminal on the solar cell side of the DC-DC converter 100 may be grounded via the current fuse 250, or in other words, the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be grounded via the current fuse 250. The positive terminal on the solar cell side of the DC-DC converter 100 may be grounded via the current fuse 250, or in other words, the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 may be grounded via the current fuse 250.

[0055] Similarly, for example, in the case of a voltage source, The negative terminal on the solar cell side of the DC-DC converter 100 may be grounded via the voltage source 260, or in other words, the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100 may be grounded via the voltage source 260. The positive terminal on the solar cell side of the DC-DC converter 100 may be grounded via the voltage source 260, or in other words, the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100 may be grounded via the voltage source 260.

[0056] Furthermore, the above-described embodiment illustrates a photovoltaic power generation system 1 equipped with a solar cell unit 20. However, the present invention is not limited thereto and can be applied to various power supply systems that do not require insulation for electric shock protection but supply power while providing functional insulation. For example, as shown in Figure 5, there is a battery storage system 1A equipped with a battery storage unit (power source) 20A. For example, functional insulation may be provided by installing a DC-DC converter 100 between the electrodes of the battery storage unit 20A and other equipment 30A.

[0057] Furthermore, the voltage to ground of the battery unit 20A may be adjusted so as not to be affected by fluctuations in the voltage to ground of other equipment 30A. For example, at least one of the positive and negative terminals (high-potential input terminal and low-potential input terminal) on the battery side of the DC-DC converter 100 may be grounded via a resistive impedance element 250 or a voltage source 260. In other words, at least one of the high-potential input line connected to the high-potential input terminal of the DC-DC converter 100, and the low-potential input line connected to the low-potential input terminal of the DC-DC converter 100, may be grounded to the earth potential via a resistive impedance element 250 or a voltage source 260. This makes it possible to stabilize the voltage to ground of the battery. [Explanation of Symbols]

[0058] 1. Solar power generation system (power supply system) 1A Battery Storage System (Power Supply System) 10 junction boxes 11 Switch 12 Reverse current protection diode 100 DC-DC converter 111 1st switch elements 112 Second switch element 113 Third switch elements 114. Fourth switching element 121 Input Capacitors 122 Intermediate Capacitor 123 Output Capacitor 140 Control circuits T1, T2 Input Terminals T3, T4 output terminals 200 Ground fault detection circuit 210,220 Resistors (resistive impedance elements) 250 Resistors, shorting wires, current fuses (resistive impedance elements) 260 Voltage source (resistive impedance element) 20 Solar cell units (power source) 20A Battery Unit (Power Source) 30 Power Conditioner 30A equipment 31 Boost Circuit 32 DC-AC converters 33 Electrical Circuits 40-minute distribution board

Claims

1. A power source consisting of a solar cell unit or a battery unit, A DC-DC converter having a pair of input terminals connected to the power source, which converts the DC input power input to the pair of input terminals to generate DC output power at the pair of output terminals, A transformerless power conditioner is connected to the pair of output terminals of the DC-DC converter and converts the DC power from the power source into AC power for connection to the commercial power grid. A voltage source is provided between the power source and the pair of input terminals of the DC-DC converter, at least one of the high-potential input line and the low-potential input line, and the ground potential. Equipped with, The DC-DC converter is A first switch element having one terminal connected to the higher potential input terminal of the pair of input terminals, and the other terminal, A second switch element having one terminal connected to the lower-potential input terminal of the pair of input terminals, and the other terminal, A third switch element having one terminal connected to the other terminal of the first switch element and the other terminal connected to the higher potential output terminal of the pair of output terminals, A fourth switch element having one terminal connected to the other terminal of the second switch element and the other terminal connected to the lower potential output terminal of the pair of output terminals, An output capacitor connected between the pair of output terminals, An intermediate capacitor is connected between a high-potential intermediate line connecting the other terminal of the first switch element and the one terminal of the third switch element, and a low-potential intermediate line connecting the other terminal of the second switch element and the one terminal of the fourth switch element. A control circuit for controlling the first switch element, the second switch element, the third switch element, and the fourth switch element, Equipped with, The aforementioned control circuit is An intermediate capacitor charging operation in which the first and second switch elements are turned ON, and the third and fourth switch elements are turned OFF, An intermediate capacitor discharge operation that turns the first and second switch elements to the OFF state and the third and fourth switch elements to the ON state, By repeating this alternately, Without using a transformer, the isolation between the pair of input terminals and the pair of output terminals is maintained at all times. Power supply system.

2. The power supply system according to claim 1, wherein the DC-DC converter further comprises an input capacitor connected between the pair of input terminals.

3. The power supply system according to claim 1 or 2, wherein the first switching element, the second switching element, the third switching element, and the fourth switching element are each composed of a single field-effect transistor.

4. The power supply system according to claim 1 or 2, wherein the first switch element, the second switch element, the third switch element, and the fourth switch element are composed of two field-effect transistors connected in series such that their source terminals or drain terminals face each other.

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