JP7567282B2 - Test methods for isolated converters - Google Patents

Description

The present invention relates to a method for testing an isolated converter used for power conversion.

LLC isolated converters are a method that is widely used as on-board charging circuits for electric vehicles. Figure 2 shows a typical test device for LLC isolated converters. The DC/AC conversion means 2 consists of a smoothing capacitor Ci and four MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) M1, M2, M3, and M4. M1 to M4 are driven on and off at a specified frequency and duty ratio by a control unit and drive unit (not shown).

In Figure 2, the input voltage Vi from the DC input power supply 1 is converted to an AC voltage by the DC/AC conversion means 2 and applied to the resonant inductor 3, the primary winding of the high-frequency transformer 5, and the resonant capacitor 4. The AC voltage has a rectangular voltage waveform, but in the control of the LLC isolated converter, it is driven at a frequency close to the resonant frequency determined by the resonant inductor 3 and the resonant capacitor 4, so that a composite wave of a sinusoidal load current component and a triangular excitation current component flows in the primary winding.

In the high-frequency transformer 5, voltage conversion is performed according to the turns ratio Np:Ns of the primary winding of the input side winding and the secondary winding of the output side winding, and a DC output voltage Vo is generated by the AC/DC conversion means 6. The AC/DC conversion means 6 is composed of a diode bridge consisting of diodes D1, D2, D3, and D4, and a smoothing capacitor Co.

In this test device, the current evaluation of an insulated converter is performed by measuring the temperature rise of the high-frequency transformer 5 and resonant inductor 3 when a specified output voltage Vo and a specified output current Io are supplied from the LLC insulated converter to the load resistor 7. The temperature rise is mainly due to the iron loss and copper loss of the high-frequency transformer 5 and resonant inductor 3, and if the respective temperature rises are below a specified numerical value, the performance of the insulated converter is judged to be good. On the other hand, if the temperature rise exceeds the specified numerical value, it is judged to be poor.

Non-Patent Document 1 presents a conventional insulated converter test device. The test circuit of the proposed bidirectional insulated converter is shown in FIG. 3. This converter is called a DAB (Dual Active Bridge) type, and the DC/AC conversion means 21 is composed of a bridge section consisting of a smoothing capacitor and four IGBTs (Insulated Gate Bipolar Transistors) Q1 to Q4. The AC/DC conversion means 61 is composed of a bridge section consisting of four IGBTs Q5 to Q8 and a smoothing capacitor. Inductors 30 and 31 and a high-frequency transformer 51 are arranged between the DC/AC conversion means 21 and the bridge section of the AC/DC conversion means 61. The inductor can be replaced by the leakage inductance of the high-frequency transformer.
In order to vary the output of this converter, each IGBT is driven while controlling the phase difference between the bridge section of the DC/AC conversion means 21 and the bridge section of the AC/DC conversion means 61, which is why it is called the DAB system. Here, the turns ratio Np:Ns of the primary winding and secondary winding of the high frequency transformer 51 is 1:1. The input voltage Vi and output voltage Vo of the converter are the same 350V, and the output power is directly regenerated to the input side.

According to Non-Patent Document 1, the output power of the converter is 10 kW, but the power supplied from the input side power supply is 335 W. This 335 W is the total loss generated by each component in the test circuit, and since the input side power supply supplies only the loss in this test circuit, the test can be performed with a power supply capacity that is extremely small compared to the converter output power. In addition, since the converter output power is regenerated to the input side and circulates within the converter, there is no load. Therefore, the large DC input power supply 1, load resistor 7, and cooling equipment (not shown) required for the test device in Figure 2 are not required.

In addition, Non-Patent Document 2 discloses another example of a conventional test device for an insulated resonant converter. The evaluation circuit of the proposed insulated resonant converter is shown in FIG. 4. As shown in the figure, it has an upper converter and a lower converter, each of which includes DC/AC conversion means 120, 260, transformers 52, 53, DC/AC conversion means 160, 220, resonant inductors 32, 33, and resonant capacitors 42, 43. The input upper converter output is directly connected to the lower converter input, and the lower converter output is regenerated to the upper converter input side. Here, two other DC voltage values, MV and LV, are provided. The MV voltage is 2 kV, but since the MV side circuit is in a half-bridge configuration, the winding voltage of the MV side transformers 52, 53 is half, 1 kV. The LV voltage is 400 V, and since the LV side circuit is in a full-bridge configuration, the winding voltage of the LV side transformers 52, 53 is the same 400 V. Therefore, the turns ratio Np:Ns of the transformers 52, 53 is 2:5. The input voltage Vi of the upper converter 100 is 400V on the LV side, and the output voltage Vo of the upper converter 100 is 2kV on the MV side. Furthermore, the input side of the lower converter 200 is 2kV on the MV side, and the output side of the lower converter 200 is 400V on the LV side. The rated power presented in Non-Patent Document 2 is 166kW, but as mentioned above, this large power circulates between the upper converter 100 and the lower converter 200, and Non-Patent Document 2 states that the power supply capacity on the input side of the test device can be covered by 8kW.

Shigenori Inoue and Yasufumi Akagi, "Bidirectional isolated DC/DC converter as a core circuit for next-generation 3.3kV/6.6kV power conversion systems," IEEJ Transactions on Industrial Applications, Vol. 126 (2006) No. 3 G. Ortiz, M. Leibl, J. E. Huber, J. W. Kolar. Author: “Design and Experimental Testing of a Resonant DC-DC Converter for Solid-State Transformers” IEEE. Transactions on Power Electronics, Vol. 32, No. 10 (October 2017)

In conventional testing methods for isolated converters, when the turns ratio of the high-frequency transformer is 1:1 as in Non-Patent Document 1, the output power can be directly regenerated to the input side, but in reality, as in Non-Patent Document 2, the turns ratio is often different. In such cases, another isolated converter is required to convert the output voltage to the same voltage value as the input voltage. Furthermore, another control unit and drive unit are also required, which creates the problem that the overall circuit configuration is complex and the control method is also cumbersome. Therefore, the object of the present invention is to provide a testing method for isolated converters with a simple circuit configuration and control method.

The present invention is a method for testing an isolated converter including a DC/AC conversion means for converting DC power into AC power, a first high-frequency transformer having a turn ratio between an input winding and an output winding of n:m (where n and m are natural numbers and n ≠ m), and an AC/DC conversion means for converting AC power into DC power, in which the input winding of the first high-frequency transformer is connected to the DC/AC conversion means and the output winding of the first high-frequency transformer is connected to the AC/DC conversion means, and the first high-frequency transformer and the A method for testing an isolated converter in which a second high-frequency transformer having a turns ratio between the input winding and the output winding of m:n is provided between the AC/DC conversion means, the output winding of the first high-frequency transformer is connected to the input winding of the second high-frequency transformer, the output winding of the second high-frequency transformer is connected to the AC/DC conversion means, the output side of the AC/DC conversion means is connected to the input side of the DC/AC conversion means, and the output power of the AC/DC conversion means is regenerated to the input side of the DC/AC conversion means.

In the present invention, it is preferable that the isolated converter is an LLC resonant drive type, and has a first resonant circuit including a first resonant inductor and a first resonant capacitor connected to the input side or output side of the first high-frequency transformer, and a second resonant circuit including a second resonant inductor and a second resonant capacitor connected to the output side or input side of the second high-frequency transformer.

In the present invention, the leakage inductance of the first high-frequency transformer can be used as a first resonant inductor, and the leakage inductance of the second high-frequency transformer can be used as a second resonant inductor.

In the present invention, it is preferable that the first high-frequency transformer and the second high-frequency transformer have the same specifications.

According to the present invention, even if the turns ratio of the high-frequency transformer of the isolated converter is n:m (n ≠ m, n and m are natural numbers), an additional isolated converter is not required, so it is possible to provide a test supplementary method for an isolated converter with a simple circuit configuration and control method.

FIG. 2 is a diagram showing the configuration of a measurement circuit in a test method for an isolated converter according to an embodiment of the present invention. FIG. 1 is a diagram showing the configuration of a measurement circuit according to a typical test method for an LLC isolated converter. FIG. 1 is a diagram showing a measurement circuit configuration according to a conventional test method for a regenerative DAB insulated converter. FIG. 1 is a circuit diagram showing a conventional test method for a regenerative resonant converter. FIG. 11 is a diagram showing the configuration of a measurement circuit in a test method for an isolated converter according to another embodiment of the present invention. FIG. 11 is a perspective view of a high-frequency transformer with large leakage inductance used in another embodiment of the present invention. FIG. 11 is a cross-sectional view of a high-frequency transformer with large leakage inductance used in another embodiment of the present invention. FIG. 13 is a diagram showing the configuration of a measurement circuit in a test method for an isolated converter according to still another embodiment of the present invention.

The following describes an embodiment of the present invention with reference to the drawings.

Figure 1 is a diagram showing the configuration of a measurement circuit in a test method for an isolated converter according to one embodiment of the present invention. The difference from the measurement circuit configuration in the test method for an LLC isolated converter shown in Figure 2 described above is that a high-frequency transformer 55, a resonant inductor 35, and a resonant capacitor 45 are provided between the high-frequency transformer 5 and the AC/DC conversion means 6, the output side of the AC/DC conversion means 6 is connected to the input side of the DC/AC conversion means 2, and the output power is regenerated to the input side.

The resonant inductor 3 and the resonant inductor 35, and the resonant capacitor 4 and the resonant capacitor 45 are of the same specifications. The high-frequency transformer 5 and the high-frequency transformer 55 also have the same specifications. Here, the same specifications mean that the electrical characteristics are considered to be substantially the same. For example, in the case of a high-frequency transformer, the constituent material, shape, and magnetic path cross-sectional area of the magnetic core that constitutes it are the same, and the shape, winding diameter, winding direction, number of turns, and arrangement position of the winding are the same, and they can be recognized as having the same specifications industrially. Note that this also includes cases where the target is an equivalent circuit element rather than a specific structure. For example, the resonant inductor may be the leakage inductance of a high-frequency transformer described later, and if the electrical characteristics are equivalent, they can be considered to have the same specifications.

Even if they have the same specifications, the electrical characteristics of the inductance value of the resonant inductor and the capacitance value of the resonant capacitor may have a deviation of, for example, ±5%, and can be considered to be the same even if there is variation. Due to the variation in electrical characteristics, there is a slight difference between the resonance characteristics of the resonant circuit including resonant inductor 3 and resonant capacitor 4 and the resonance characteristics of the resonant circuit including resonant inductor 35 and resonant capacitor 45, but this can be sufficiently corrected by controlling the frequency of the converter. High-frequency transformers with the same specifications are constructed using the same materials and turns ratios, and the important turns ratio is strictly controlled when they are manufactured.

In addition, for the high frequency transformers 5 and 55 of the same specifications, the input winding and the output winding are distinguished according to whether they will be the input winding or the output winding when used. Therefore, in a state in which the output winding of the high frequency transformer 5 and the input winding of the high frequency transformer 55 are directly connected as shown in Fig. 1, the turns ratio of the input winding and the output winding of the high frequency transformer 5 is n:m, and the turns ratio of the input winding and the output winding of the high frequency transformer 55 is m:n, so that the composite turns ratio of the two high frequency transformers when viewed from the input winding of the high frequency transformer 5 to the output winding of the high frequency transformer 55 is n:n, that is, 1:1.
Therefore, in the entire circuit configuration, an output voltage Vo having the same voltage value as the input voltage Vi is generated, and the output power can be regenerated to the input side.

Generally, high-frequency transformers are designed and manufactured to maximize the degree of coupling between the input winding and the output winding. The high-frequency transformers 5 and 55 in FIG. 1 have a high degree of coupling between the input winding and the output winding. Although there is a small leakage inductance in the high-frequency transformers 5 and 55, it is small compared to the inductance value of the resonant inductors 3 and 35, so it is not shown in the figure. A method for increasing the degree of coupling between the input winding and the output winding is to increase the opposing area of the input winding and the output winding and shorten the opposing distance. For example, the input winding and the output winding can be divided into multiple parts and the input winding and the output winding can be arranged to intersect, increasing the opposing area and increasing the degree of coupling between the input winding and the output winding.

When the high frequency transformer 5 is tested at the same input voltage, drive frequency, and output power in the conventional example of FIG. 2 and the embodiment of FIG. 1, the embodiment shown in FIG. 1 differs from the conventional example in that the excitation current component on the high frequency transformer 55 side is superimposed. In this regard, if the influence of the excitation current component of the high frequency transformer 55 is small, the high frequency transformers 5 and 55 can be tested at the same time in the circuit configuration of FIG. 1. Even if there is an influence of the excitation current component, the high frequency transformer 5 in the conventional example of FIG. 2 and the embodiment of FIG. 1 are driven under the same conditions, so the measurement result on the high frequency transformer 55 side can be used to evaluate the temperature rise. As mentioned above, the high frequency transformer 5 and the high frequency transformer 55 have the same specifications, so if the measurement result of the high frequency transformer 5 is required, the high frequency transformer 5 and the high frequency transformer 55 can be swapped in the circuit configuration of FIG. 1 and tested.

So far, we have explained the testing of high-frequency transformers 5 and 55, but the same can also be applied to testing resonant inductors 3 and 35 and resonant capacitors 4 and 45.

Figure 5 shows a test method for an LLC insulated converter according to another embodiment of the present invention, and is a diagram of the measurement circuit. The difference from the circuit configuration of the embodiment of Figure 1 described above is that the resonant inductors 3 and 35 are eliminated, and high-frequency transformers 5 and 55 are replaced by high-frequency transformers 56 and 57 having leakage inductances Lr1 and Lr2.

High-frequency transformers 56 and 57 have leakage inductances Lr1 and Lr2, respectively. Strictly speaking, the high-frequency transformers 5 and 55 of the embodiment shown in Figure 1 also have leakage inductance components, but the inductance value required for an LLC insulated converter is small, and resonant inductors 3 and 35 fulfill this role. High-frequency transformers 56 and 57 of the second embodiment of the present invention in Figure 5 have a lower degree of coupling between the input winding and the output winding to increase leakage inductances Lr1 and Lr2, thereby obtaining the inductance value required for an LLC insulated converter.

Here, an example of the configuration of high-frequency transformers 56 and 57 with large leakage inductance is shown. FIG. 6 is a perspective view of the high-frequency transformer, and FIG. 7 is a cross-sectional view thereof. The high-frequency transformer includes, for example, a pair of magnetic cores 301 and 302, windings 351 and 352, and an insulator 360 arranged between the windings. The magnetic cores 301 and 302 have a center leg, a pair of side legs arranged at intervals on both sides of the center leg, and a connecting portion connecting the center leg and the side legs, and as shown in FIG. 6, the cross-sectional shape is an alphabet E shape. Since the windings 351 and 352 are arranged separately by the insulator 360, the degree of coupling between the windings is low and the leakage inductance is large. As mentioned above, the high-frequency transformers 56 and 57 have the same specifications, so here, the windings 351 and 352 are not distinguished as input side windings and output side windings.

In the circuit diagram of Figure 5, leakage inductance Lr1 is shown placed on the input winding side of high-frequency transformer 56, but strictly speaking, leakage inductance of high-frequency transformer 56 exists on both the input winding side and the output winding side. In Figure 5, leakage inductance Lr1 is expressed as including equivalently the leakage inductance component on the output winding side. High-frequency transformer 57 is also configured in the same way, and its leakage inductance Lr2 is also similar.

Here too, high frequency transformers 56 and 57 have the same specifications, and are manufactured with strict control over the important turns ratio. Therefore, when the output winding of high frequency transformer 56 and the input winding of high frequency transformer 57 are directly connected, the composite turns ratio of the two high frequency transformers when viewed from the input winding of high frequency transformer 56 to the output winding of high frequency transformer 57 is 1:1. Therefore, the entire circuit configuration generates an output voltage Vo with the same voltage value as the input voltage Vi, and the output power can be regenerated to the input side.

As in the previous embodiment, the excitation current component of high frequency transformer 57 is superimposed on the input winding side of high frequency transformer 56, so in this case too, the measurement results of high frequency transformer 57, which is the original driving condition, can be used. If the measurement results of high frequency transformer 56 are required, the high frequency transformer 56 and high frequency transformer 57 can be swapped in the circuit configuration of Figure 5 and testing can be performed. Furthermore, if the effect of the excitation current component of high frequency transformer 56 is slight, high frequency transformers 56 and 57 can be tested at the same time.

Figure 8 shows another embodiment of a test method for a DAB isolated converter, and is a diagram of the measurement circuit. The difference from the circuit configuration of the conventional example in Figure 3 described above is that high-frequency transformers 58 and 59 are provided instead of high-frequency transformer 51.

Here too, high frequency transformers 58 and 59 have the same specifications, and are manufactured with strict control over the important turns ratio. Therefore, when the output winding of high frequency transformer 58 and the input winding of high frequency transformer 59 are directly connected, the composite turns ratio of the two high frequency transformers when viewed from the input winding of high frequency transformer 58 to the output winding of high frequency transformer 59 is 1:1. Therefore, the entire circuit configuration generates an output voltage Vo with the same voltage value as the input voltage Vi, and the output power can be regenerated to the input side.

As in the previous embodiment, the excitation current component of high frequency transformer 59 is superimposed on the input winding of high frequency transformer 58, so the measurement results of high frequency transformer 59, which is the original driving condition, can be used. If the measurement results of high frequency transformer 58 are required, the high frequency transformer 58 and high frequency transformer 59 can be swapped in the circuit configuration of Figure 8 and testing can be performed. Furthermore, if the effect of the excitation current component of high frequency transformer 59 is slight, high frequency transformers 58 and 59 can be tested at the same time.

Evaluation tests were performed on the high-frequency transformers 5 and 55 and the resonant inductors 3 and 35 using the LLC insulated converter test equipment shown in Figure 1. Hitachi Metals' ML29D was used for the magnetic core of the high-frequency transformer, and the input winding of the high-frequency transformer 5 had 8 turns and the output winding had 6 turns, giving a turn ratio n:m of 4:3. The high-frequency transformer 55 has the same specifications as the high-frequency transformer 5, and the input winding of the high-frequency transformer 55 corresponds to the output winding (6 turns) of the high-frequency transformer 5, and the output winding corresponds to the input winding (8 turns) of the high-frequency transformer 5. The turn ratio m:n between the primary winding and the secondary winding of the high-frequency transformer 55 is 3:4. Hitachi Metals' ML29D was used for the magnetic core of the resonant inductor. Table 1 shows the inductance value of the resonant inductor and the capacitance value of the resonant capacitor.

The product names of the SiC (Silicon carbide) MOSFETs used in the DC/AC conversion means 2 and the SiC diodes used in the AC/DC conversion means 6 are shown in Table 2. All are manufactured by Cree, Inc., USA.

The electrical conditions for this evaluation test are shown in Table 3. Because this was a regenerative type evaluation test, the output power was 7,474 W, but the input power was only 406.6 W, so the test was conducted in an energy-saving manner.

The temperature rise δT (℃) at each part of the evaluation test results for the resonant inductors 3 and 35, and the high-frequency transformers 5 and 55 is shown in Table 4. Temperature measurements were performed using the thermocouple method, and the test was performed with a T-type thermocouple attached to each part.

In this evaluation test, the resonant inductors 3 and 35 and the high-frequency transformers 5 and 55 were attached to a water-cooled jacket, and the cooling water temperature was set to 20°C. The insulated converter was operated for two hours so that the temperature rise of each part was fully saturated, and δT was calculated by subtracting the cooling water temperature of 20°C from the saturation temperature of each part, with a value of 60°C or less being considered good. As δT at all measurement points was 60°C or less, the performance of the insulated converter was judged to be good. Moreover, it was possible to judge two sets of resonant inductors and high-frequency transformers in a single evaluation test.

1 DC input power supply 2, 21 DC/AC conversion means 3, 35 Resonant inductor 4, 45 Resonant capacitor 5, 55, 56, 57, 58, 59 High frequency transformer 6, 61 AC/DC conversion means 7 Load resistor 301, 302 Magnetic core 351, 352 Winding 360 Insulator

Claims (4)

A method for testing an isolated converter including: DC/AC conversion means for converting DC power into AC power; a first high frequency transformer having a turns ratio between an input winding and an output winding of n:m (where n and m are natural numbers and n ≠ m); and AC/DC conversion means for converting AC power into DC power, wherein an input winding of the first high frequency transformer is connected to the DC/AC conversion means and an output winding of the first high frequency transformer is connected to the AC/DC conversion means,
a second high frequency transformer having a turns ratio of m:n between the first high frequency transformer and the AC/DC conversion means of the isolated converter;
an output winding of the first high frequency transformer is connected to an input winding of the second high frequency transformer, and an output winding of the second high frequency transformer is connected to the AC/DC conversion means;
A method for testing an isolated converter in which the output side of the AC/DC conversion means and the input side of the DC/AC conversion means are connected to each other, and the output power of the AC/DC conversion means is regenerated to the input side of the DC/AC conversion means. 2. A method for testing an isolated converter according to claim 1, comprising the steps of:
A method for testing an isolated converter, wherein the isolated converter is of an LLC resonant drive type and has a first resonant circuit including a first resonant inductor and a first resonant capacitor connected to an input side of the first high-frequency transformer, and a second resonant circuit including a second resonant inductor and a second resonant capacitor connected to an output side of the second high-frequency transformer. 2. A method for testing an isolated converter according to claim 1, comprising the steps of:
the isolated converter is of an LLC resonant drive type and has a first resonant circuit including a first resonant capacitor connected to an input side of the first high frequency transformer, and a second resonant circuit including a second resonant capacitor connected to an output side of the second high frequency transformer,
a leakage inductance component on an input winding side of the first high frequency transformer is used as a first resonant inductor in the first resonant circuit ;
A method for testing an isolated converter, comprising using a leakage inductance component on an output winding side of the second high frequency transformer as a second resonant inductor in the second resonant circuit . A method for testing an isolated converter according to claim 1 or 2 , comprising the steps of:
A method for testing an isolated converter, wherein the first high frequency transformer and the second high frequency transformer have the same specifications.

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