Method of Diagnosing Open-Circuit Fault of Dual Active Bridge Converter

Abstract

A method for diagnosing an open-circuit fault in a dual active bridge converter is disclosed. Among a pair of switches forming a positive current flowing from the primary side to the transformer and a pair of switches forming a negative current flowing out from the transformer to the primary side, a pair of switches causing an open-circuit fault is determined. After determining the pair of switches, a bridge causing an open-circuit fault among a first bridge and a second bridge is determined. Additionally, by applying a zero vector and an effective vector, an open-circuit fault is determined for the switch of the fault bridge.

Description

Technology Field

[0001] The present invention relates to a Dual Active Bridge (DAB) converter, and more specifically, to a method for diagnosing a fault in a DAB converter that can diagnose a fault within a short period of time. Background Technology

[0002] The DAB converter is a circuit widely used in power conversion systems due to its simple structure and ease of control. It offers the advantage of simultaneous power supply and recovery as it allows current to flow in both directions. Widely used in DC-DC converters, DAB converters are employed in various fields, such as electric vehicle charging systems and renewable energy systems, thanks to their high conversion efficiency and excellent power density.

[0003] Figure 1 is a circuit diagram illustrating a conventional DAB converter.

[0004] Referring to FIG. 1, the DAB converter has a first bridge (110), a transformer (120), and a second bridge (130).

[0005] One bridge consists of four switches. Each bridge forms a current path according to the supplied switching signal, and the transformer (120) performs voltage conversion and current conversion operations according to the ratio of the number of turns.

[0006] Input voltage V in This is supplied, and the output voltage V is supplied through the transformer (120). out This is formed. Input voltage V in and output voltage V out The transformer (120) placed between them has a primary pole voltage V according to the winding ratio n. pri and secondary pole voltage V sec It has. Inductance L represents the leakage inductance of the transformer, and the inductor current i flowing through it. L The current actually becomes the current flowing through the primary coil of the transformer (120).

[0007] If a specific switch constituting the two bridges (110, 130) has an open circuit fault, the current flowing through the transformer (120) is skewed to one polarity and causes inductance saturation. It is necessary to stop the power conversion system through fault diagnosis before the inductance saturates.

[0008] To diagnose open-circuit faults in switches, current detection via a current sensor is required. In addition to the current sensor, additional circuitry and sensors for detecting voltage are also required. This places a significant circuit burden on systems employing DAB converters.

[0009] Therefore, a fault diagnosis method is required that can rapidly determine open circuit failures in the DAB converter through an algorithm and accurately identify the fault switch. The problem to be solved

[0010] The technical problem that the present invention aims to solve is to provide a fault diagnosis method for a DAB converter switch that can determine an open-circuit fault of a switch solely by detecting or sampling the current flowing on the primary side of a transformer and quickly identify the location of the faulty switch. means of solving the problem

[0011] The present invention, for achieving the aforementioned technical problem, provides a method for diagnosing an open-circuit fault in a dual active bridge converter having a transformer connected between a first bridge and a second bridge, wherein the transformer is converted according to the primary side voltage and the winding ratio of a coil connected to the first bridge that converts the input voltage, and generates a secondary side voltage connected to the second bridge, comprising the steps of: determining a switch pair having an open-circuit fault among a first switch pair forming a positive inductor current having a direction flowing from the first bridge to the transformer and a second switch pair forming a negative inductor current having a direction flowing out from the transformer to the first bridge; determining an open-circuit fault bridge among the first bridge and the second bridge among the open-circuit fault switch pairs; and determining a switch having an open-circuit fault within the open-circuit fault bridge. Effects of the invention

[0012] According to the present invention described above, double sampling is performed to detect current at the minimum and maximum values ​​of the carrier wave. A pair of switches in which an open circuit fault has occurred is determined through the sampled primary side current. Additionally, among the two bridges within the determined pair of switches, a bridge in which an open circuit fault has occurred is determined.

[0013] Finally, an upper zero vector is applied to the bridge where the open fault occurred, and an effective vector is applied to other normal bridges. Through this, the presence or absence of current flowing through the bridge where the open fault occurred is determined, and the specific open fault of the switch is determined based on the determination of the presence or absence of current.

[0014] Therefore, rapid diagnosis of open-circuit faults can be achieved solely through sampling of the current flowing through the transformer, and the location of the fault switch can be detected without any additional devices. Brief explanation of the drawing

[0015] Figure 1 is a circuit diagram illustrating a conventional DAB converter. FIG. 2 is a graph showing the operating waveform of the DAB converter of FIG. 1 during normal operation according to a preferred embodiment of the present invention. Figure 3 is a circuit diagram illustrating the operating state of the switches of Figure 1 to explain the waveform diagram of Figure 2. FIG. 4 is a circuit diagram illustrating the operation when an open circuit fault occurs at switch S1 in FIG. 1 according to a preferred embodiment of the present invention. FIG. 5 is a graph showing the current-voltage characteristics according to the operation of the circuit of FIG. 4 in accordance with a preferred embodiment of the present invention. FIG. 6 is a circuit diagram illustrating the operation when an open circuit fault occurs at switch S6 in FIG. 1 according to a preferred embodiment of the present invention. FIG. 7 is a graph showing the current and voltage according to the operation of the circuit of FIG. 6 in accordance with a preferred embodiment of the present invention. Specific details for implementing the invention

[0016] The present invention is susceptible to various modifications and may take various forms; therefore, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the invention to the specific disclosed forms, and it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention. Similar reference numerals have been used for similar components in the description of each drawing.

[0017] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0018] Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the attached drawings.

[0020] Examples

[0021] FIG. 2 is a graph showing the operating waveform of the DAB converter of FIG. 1 during normal operation according to a preferred embodiment of the present invention.

[0022] Referring to FIG. 2, under the assumption that all switches of the bridge are operating normally, the voltage and current on the primary side according to the switch state are initiated.

[0023] Primary pole voltage V pri is the voltage difference V across the leakage inductance L. L and secondary pole voltage V sec The voltage nV appearing in the transformer due to sec It is the sum of. Here, n is the winding ratio of the primary side to the secondary side. That is, nV sec can be described as the voltage appearing in the primary coil due to the influence of the transformer's mutual inductance. In the present invention, nV sec This is referred to as induced voltage. Also, the induced voltage nV sec is phase displacement D Φ It has a phase delayed by that amount. Also, the inductor voltage V L Integrating this and dividing by the inductance L gives the inductor current i L This appears. The above inductor current i LIt sets the direction of flow from the first bridge to the transformer to a positive value, and the direction of flow from the transformer to the first bridge to a negative value.

[0024] Figure 3 is a circuit diagram illustrating the operating state of the switches of Figure 1 to explain the waveform diagram of Figure 2.

[0025] Referring to FIGS. 2 and FIGS. 3, the primary pole voltage V pri and secondary pole voltage V sec Phase shift D between Φ The voltage difference V across the terminals of inductor L is due to L This occurs, and the inductor current i L Power is transmitted to the secondary side through this.

[0026] For example, switches S1 and S4 are turned on in sections [a] and [b], and the inductor current i L It forms a path with a positive value. However, depending on the state of the secondary switch, i L The amount of current changes.

[0027] Section [a] is the case where switches S6 and S7 on the secondary side are short-circuited, and the inductor current i on the primary side L It tends to increase with a steep slope. Therefore, the inductor voltage V L ... has a positive value. Section [b] represents the current path when all secondary switches are open while the primary switches remain the same as in section [a]. In the above section, the primary inductor current i L It has a lower slope compared to interval [a] and increases gradually.

[0028] That is, inductor current i L When this has a positive value, the inductor current i has a positive value due to the activation operation of switches S1, S4, S6, and S7. L This is formed. This is referred to as the first switch pair.

[0029] Inductor current i in sections [c] and [d] L ...has a negative value. To implement this, switches S2 and S3 on the primary side are short-circuited. Additionally, depending on the state of switches S5 and S6 on the secondary side, the inductor current i L The size of is determined.

[0030] In section [c], switches S5 and S8 are short-circuited, and the inductor current i L It exhibits a decreasing trend with a relatively large negative slope. Additionally, in section [d], switches S5 and S8 are open, and a current path is formed through the diodes of the open switches S6 and S7. Therefore, the inductor current i L It shows a tendency to gradually decrease in the negative direction.

[0031] That is, inductor current i L When this has a negative value, the activation operation of switches S2, S3, S5, and S8 results in a negative inductor current i L It forms. The switches are referred to as the second switch pair.

[0032] In the present invention, activation of the switch refers to entering a short-circuit state.

[0033] In addition, the inductor current iL at a specified point in the waveform of Fig. 2 can be modeled as shown in Equation 1 below.

[0034] [Formula 1]

[0035]

[0036] In Equation 1 above, Ts is the period of the switching operation or the carrier period, and t0 is positive i L The initial time t1 before this is formed is the positive inductor current i L This is generated, and at the point when the secondary switches are off, t2 is the positive inductor current i L In this flowing state, negative inductor current i LAt the point in time when the switching operation is switched for this to be formed, t3 is negative i L It refers to the point in time when this is formed and the secondary switches are turned off. L_avg is the average inductor current, with the minimum value i L (t0) and maximum value i L It represents the average value between (t3). That is, when the switches constituting the bridge are operating normally, the inductor current i L It can be seen that the average inductor current, which is the average of the sum of the minimum and maximum values, is 0.

[0037] FIG. 4 is a circuit diagram illustrating the operation when an open circuit fault occurs at switch S1 in FIG. 1 according to a preferred embodiment of the present invention.

[0038] Referring to FIG. 4, if an open circuit fault occurs at switch S1 on the primary side, a short-circuit state is not realized even when a switching signal is applied. That is, in the error section [E1], a path is formed through the diode of switch S2 and the short-circuited switch S4, and an inductor current i with a positive value L It forms. Also, in the section [E2] where the secondary switches S6 and S7 are open, the inductor current i L decreases, and as time passes, the inductor current i L It has a value of 0.

[0039] FIG. 5 is a graph showing the current-voltage characteristics according to the operation of the circuit of FIG. 4 in accordance with a preferred embodiment of the present invention.

[0040] Referring to FIG. 5, the inductor current i in the error interval [E1] of FIG. 4 L increases with a positive value, and the inductor current i due to the off operation of the secondary switch in the error interval [E2]. L decreases, and due to the formation of a single closed loop through the diodes on the primary side, the inductor current i LIt has a value of 0. That is, the error interval [E3] appears compared to the normal interval [b].

[0041] Inductor current i when an open circuit fault occurs at switch S1 on the primary side of the first switch through the above-described operation L It is modeled by the following Equation 2.

[0042] [Equation 2]

[0043]

[0044] In the above Equation 2, t1 * is the point in time when the secondary switches of the first switch pair open after an open-circuit fault occurs, and t2 * This is the point in time when switching operation through the second switch pair is initiated after an open circuit failure of the first switch pair occurs.

[0045] Referring to Equation 2, the inductor current i within the interval where the switching operation through the first switch pair is performed L It is confirmed that a negative offset current is formed where the sum of the minimum and maximum values ​​is not zero.

[0046] That is, if an open-circuit fault occurs in the primary switch of the first switch pair, the average inductor current i at the time the first switch pair is activated L_avg It cannot have a value of 0 and has a negative offset value.

[0047] FIG. 6 is a circuit diagram illustrating the operation when an open circuit fault occurs at switch S6 in FIG. 1 according to a preferred embodiment of the present invention.

[0048] Referring to FIG. 6, the section in which switch S6 is activated corresponds to section [a] of FIG. 3. Assuming that S7 of the first switch pair is normal, if an open circuit fault occurs at switch S6 on the secondary side, a current path is formed through the diode of switch S5 and switch S7, and the output voltage V outA closed loop is formed that cannot be passed through. This is defined as the error interval [E4].

[0049] FIG. 7 is a graph showing the current and voltage according to the operation of the circuit of FIG. 6 in accordance with a preferred embodiment of the present invention.

[0050] Referring to FIG. 7, the primary switches S1 and S4 of the first switch pair operate normally. However, the normal section [a] is replaced by an error section [E4] due to an open circuit failure of the secondary switch S6. In the error section [E4], the inductor current i L This increasing slope is reduced. However, even if the secondary switches S6 and S7 are turned off in section [b] of Figure 3, the current path is formed normally due to the diode operation of S5 and S8.

[0051] When an open circuit fault occurs in the secondary switch S6 of the first switch pair, the current modeling follows Equation 3 below.

[0052] [Equation 3]

[0053]

[0054] In Equation 3 above, if an open-circuit fault occurs in the secondary switch, the inductor current i within the section where the switches of the first switching pair are activated L i, which is the average of the maximum and minimum values ​​of L_avg It is confirmed that it has a negative offset value.

[0055] In the present invention, a determination regarding a faulty switch pair is performed first. That is, it is determined whether an open-circuit fault has occurred in either the first switch pair or the second switch pair. When an open-circuit fault occurs in a switch, regardless of whether there is a fault on the primary or secondary side, a phenomenon occurs in which the average values ​​of the maximum and minimum currents in a specific section are all skewed toward one polarity.

[0056] Therefore, the criterion for determining a faulty switch pair is based on the offset value of the average inductor current generated upon a failure of the secondary switch. This is because the magnitude of the offset current value resulting from an open-circuit failure of the secondary switch is smaller than that resulting from an open-circuit failure of the primary switch. Additionally, considering sensing errors, the average value i of the offset current in Equation 3 above L_avg Set half of the switch pair threshold.

[0057] In addition, the inductor current i at the minimum point ZERO and the maximum point PRD of the triangular carrier wave L Sample and compare with the switch pair threshold. Inductor current i sampled at the carrier wave minimum point ZERO. L ul i L_ZERO Let be the inductor current i sampled at the PRD point of the carrier wave's maximum value. L ul i L_PRD It is defined as the average value i of the double-sampled inductor current. L_AVG eun (i L_ZERO +i L_PRD With a value of ) / 2, the switch group in which an open circuit failure has occurred is determined according to the following Equation 4 through comparison with the switch pair threshold.

[0058] [Equation 4]

[0059]

[0060] If the first equation in the above Equation 4 is satisfied, it is determined that there is an open circuit fault of the first switch pair affecting the (+) current. Also, if the second equation is satisfied, it is determined that there is an open circuit fault of the second switch pair affecting the (-) current.

[0061] Next, when the faulty switch pair is determined, the faulty bridge is determined through the magnitude of the double-sampled current. That is, a determination is made as to whether an open-circuit fault has occurred in the primary bridge or in the secondary bridge.

[0062] If a fault occurs in a switch constituting the primary bridge, the current path is blocked in section [E3], which replaces section [b], and due to diode operation, the inductor current appears close to zero at the ZERO point, which represents the minimum value of the carrier wave. On the other hand, if an open-circuit fault occurs in a switch constituting the secondary bridge, a current with a constant value may be formed due to diode operation, and this is i at the ZERO sampling point. L (t1 * It forms a current greater than )

[0063] In this invention, the current at the zero point where the carrier minimum is formed is modeled by assuming a secondary bridge failure. To model the current at the zero point, the current sampled at the PRD point where the carrier maximum appears is used. The modeled inductor current i L It follows Formula 5 below.

[0064] [Formula 5]

[0065]

[0066] Referring to Equation 5 above and Fig. 7, the modeling current i at the ZERO point through Equation 5 L_ZERO_m is time t1 * and time t2 * It is derived as the average value of the current modeled in.

[0067]

[0068] This is the current predicted at the ZERO sampling point when an open circuit fault occurs in the switch on the secondary side. On the other hand, if an open circuit fault occurs in the switch on the primary side, the current predicted at the ZERO point, as disclosed in Figure 5 above, shows a value of 0 and is interpreted as no current flowing.

[0069] Considering sensing errors and parameter errors, i L_ZERO_m Half of the bridge threshold i, which is the criterion value for determining a failed bridge.L_side_th It is used as follows. That is, in determining a faulty bridge, the following Equation 6 is applied.

[0070] [Equation 6]

[0071] Primary side failure

[0072] Secondary side failure

[0073] In the above Equation 6, i L_ZERO is the inductor current i sampled at the ZERO point. L is, i L_side_th is the bridge threshold. In addition, Equation 6 above indicates that the faulty switch pair has a positive inductor current i L It is assumed that this is the case of a first switch pair forming a...

[0074] If a fault occurs in the second switch pair, it is necessary to determine whether the fault lies in the primary or secondary bridge within the second switch pair. Since the second switch pair is assumed to generate a negative current, the fault bridge determination threshold is negative, so -i L_side_th This becomes the case. Therefore, the current i sampled at the zero point of the carrier wave L_ZERO The fault bridge is determined according to the following Equation 7.

[0075] [Equation 7]

[0076] Primary side failure

[0077] Secondary side failure

[0078] When a faulty bridge is determined, the faulty switch within the determined bridge is identified.

[0079] Once the fault bridge is identified, the process of determining the open-circuit fault switch within the fault bridge is carried out. The identification of the fault switch within the fault bridge is achieved by setting the transformer voltage of the fault bridge to a zero vector and applying an effective vector to the transformer voltage of the bridge opposing the fault bridge. If the transformer voltage within the fault bridge is a zero vector and the transformer voltage of the opposing bridge has an effective vector component, an induced electromotive force is formed in the transformer voltage connected to the fault bridge by the effective vector component. At this time, depending on whether the switch within the fault bridge that realizes the zero vector is an open-circuit fault, the inductor current i through the fault bridge L This may or may not be formed.

[0080] The above operations are summarized in Table 1 below.

[0081] Faulty switch pair Broken bridge Fault switch Authorization conditions First switch pair (positive current formation) 1st bridge S1 or S4 1st side: Upper zero vector Secondary side: (-)effective vector 2nd bridge S6 or S7 Primary side: (+) effective vector Secondary side: Upper zero vector Second switch pair (negative current formation) 1st bridge S2 or S3 1st side: Upper zero vector Secondary side: (+) effective vector 2nd bridge S5 or S8 1st side: (-) effective vector Secondary side: Upper zero vector

[0082] In Table 1 above, applying the upper zero vector means applying a switching signal so that the transformer voltage becomes the zero vector. That is, applying the upper zero vector to the primary side means the transformer's primary side voltage V pri Indicates that it is a zero vector. However, the upper zero vector is implemented by turning on the upper switch on the bridge. For example, on the first bridge, Vpri can be set to the zero vector by activating S1 and S3. Additionally, on the second bridge, V can be set by activating S5 and S7. sec It can be set to the zero vector.

[0083] In addition, the condition for applying a (+) effective vector is to set the voltage of the transformer connected to the bridge determined to be normal as the converter voltage. For example, applying a (+) effective vector to the primary side is V through the activation of switches S1 and S4. pri Ga V in It is to set it to be identical to, and applying a (+) effective vector to the secondary side is V through the activation of switches S5 and S8. sec Ga Vout It is to set it to be the same as.

[0084] In addition, the application of a (-) effective vector sets the voltage of the transformer connected to the bridge determined to be normal to the negative value of the converter voltage. For example, the application of a (-) effective vector to the secondary side activates switches S6 and S7 to V sec -V out It is to set it to have a value of.

[0085] When a zero vector or an effective vector is applied according to the application conditions of Table 1 above, the fault switch is determined.

[0086] For example, if an open fault is determined in the first switch pair and it is to determine the faulty switch within the primary bridge, an upper zero vector is applied to the primary side. To this end, switches S1 and S3 are activated. Therefore, V pri becomes the zero vector. Also, a (-) effective vector is applied to the second-order bridge. That is, V sec -V out It has the value of. Current is generated on the primary side by the voltage on the secondary side. However, if switch S1 is an open-circuit fault, i L ...does not occur. Therefore, under the above conditions, i L If this occurs and is measured, switch S1 is normal, and it is determined that there is an open circuit failure of switch S4. However, i L If this does not occur, it is determined to be an open circuit failure of switch S1.

[0087] Additionally, if it is determined that the second switch pair has an open fault and the secondary bridge has a fault, either switch S5 or S8 has an open fault. Since the primary bridge is normal, an effective vector is applied, but a negative effective vector is applied. To this end, S2 and S3 of the primary bridge are activated, and V pri -V inIt has the value of. In addition, the upper zero vector is applied to the secondary bridge. That is, through the activation of switches S5 and S7, V sec It has a zero vector. If switch S5 is open-circuited, no current is detected flowing through the secondary side. Also, if switch S5 is normal and switch S8 is open-circuited, current flows through switch S5. That is, the switch having an open-circuited fault can be distinguished by detecting the current flowing through switch S5.

[0088] However, the application of the upper zero vector and effective vector in Table 1 above needs to be performed for a limited period. In particular, since the zero vector state is interpreted as a substantial short-circuit state between the two terminals, excessive current may flow on the side where the zero vector is formed upon the application of the effective vector, thereby paralyzing the functions of various components constituting the converter.

[0089] Therefore, the present invention prevents damage to components caused by the generation of excessive current by specifying the period for applying the effective vector. The primary side current i that can flow in a normal converter through the application of a switching signal. L The maximum value of i L_ds It follows Formula 8 below.

[0090] [Equation 8]

[0091]

[0092] In Equation 8 above, ds represents the duty cycle to which the effective vector is applied, and Ts represents the period of the switching signal. The above equation represents the voltage nV induced on the primary side. sec It was inferred under the assumption that is constant.

[0093] If normal operation is assumed, then during normal operation, i of Fig. 2 above L (t1) or i L (t2) has a large current value. Therefore, the maximum value i of the primary side current when the effective vector and the zero vector are applied L_ds is iL (t1) or i L It needs to be limited to (t2).

[0094] For example, under the condition of applying an effective vector and a zero vector, the maximum value of the current flowing through the primary switch is i L If the duty cycle ds is adjusted to be equal to (t1), then the following Equation 9 holds.

[0095] [Formula 9]

[0096]

[0097] That is, when an effective vector is applied, the protection operation of the converter circuit can be performed by applying it to have the duty ds within one period Ts. By applying the effective vector to have a constant duty within one period, the protection operation of the circuit constituting the converter is performed, and the location of the switch where an open circuit fault has occurred is quickly identified.

[0099] In the present invention described above, double sampling is performed to detect current at the minimum and maximum values ​​of the carrier wave. A pair of switches in which an open circuit fault has occurred is determined through the sampled primary-side current. Additionally, among the two bridges within the determined pair of switches, a bridge in which an open circuit fault has occurred is determined.

[0100] Finally, an upper zero vector is applied to the bridge where the open fault occurred, and an effective vector is applied to other normal bridges. Through this, the presence or absence of current flowing through the bridge where the open fault occurred is determined, and the specific open fault of the switch is determined based on the determination of the presence or absence of current.

[0101] Therefore, rapid diagnosis of open-circuit faults can be achieved solely through sampling of the current flowing through the transformer, and the location of the fault switch can be detected without any additional devices. Explanation of the symbols

[0102] 110: 1st Bridge 120: Transformer 130 : 2nd Bridge

Claims

Claim 1 A method for diagnosing an open-circuit fault in a dual active bridge converter having a transformer connected between a first bridge and a second bridge, wherein the transformer is converted according to the primary side voltage and the winding ratio of the coil connected to the first bridge that converts the input voltage, and generates a secondary side voltage connected to the second bridge, comprising: a step of determining a switch pair having an open-circuit fault among a first switch pair forming a positive inductor current having a direction flowing from the first bridge to the transformer and a second switch pair forming a negative inductor current having a direction flowing out from the transformer to the first bridge; a step of determining an open-circuit fault bridge among the first bridge and the second bridge among the open-circuit fault switch pairs; and a step of determining a switch having an open-circuit fault within the open-circuit fault bridge, wherein the step of determining the open-circuit fault switch pair compares the average value of the inductor current sampled at the minimum value of the carrier wave and the inductor current sampled at the maximum value of the carrier wave with a switch pair threshold value, and the switch pair threshold value in the open-circuit fault switch pair An open-circuit fault diagnosis method characterized by using an offset current value modeled according to an open-circuit fault in a second bridge. Claim 2 delete Claim 3 A method for diagnosing open-circuit faults according to claim 1, wherein the determination of the open-circuit fault pair is characterized by determining the first switch pair as faulty if the following Equation 1 is satisfied, and determining the second switch pair as faulty if the following Equation 2 is satisfied.[Equation 1] [Equation 2] In the above Equations 1 and 2, i L,AVG is the average value of the sampled current above, and i L (t1) is the inductor current at the point in time when the above positive inductor current is generated during normal operation and the switches of the second bridge are in the off state, and V pri ε is the primary side voltage of the above transformer, n is the primary side winding ratio for the secondary side of the above transformer, and V sec represents the secondary voltage of the above transformer. Claim 4 A method for diagnosing an open-circuit fault according to claim 1, wherein the step of determining the open-circuit fault bridge involves determining the inductor current sampled from the minimum value of the carrier wave by comparing it with a bridge threshold value, and the bridge threshold value utilizes the predicted current when an open-circuit fault occurs in the second bridge. Claim 5 A method for diagnosing an open fault according to claim 4, characterized in that if the first switch pair has an open fault, the first bridge is determined to have an open fault according to the following Equation 3, and the second bridge is determined to have an open fault according to the following Equation 4.[Equation 3] [Equation 4] In the above Equations 3 and 4, i L_ZERO is the inductor current sampled at the minimum value of the carrier wave, and i L (t1 * ) is the inductor current predicted at the time when the switches of the second bridge of the first switch pair are opened after an open-circuit fault occurs, and i L (t2 * ) is the inductor current predicted at the time when switching operation through the second switch pair is initiated after an open circuit fault occurs. Claim 6 A method for diagnosing an open fault according to claim 5, characterized in that if the second switch pair has an open fault, the first bridge is determined to have an open fault according to the following Equation 5, and the second bridge is determined to have an open fault according to the following Equation 6.[Equation 5] [Equation 6] Claim 7 An open fault diagnosis method according to claim 1, wherein the determination of the open fault switch is characterized by applying a zero vector to a terminal of a transformer connected to the open fault bridge and applying an effective vector to a transformer terminal opposite to the terminal to which the zero vector is applied.

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