ESDAB converter suitable for medium and low voltage direct current system

Abstract

The ESDAB converter comprises a three-winding transformer T, coupling inductors L1 and L2, switching tubes S1, S2, S3, S4, S5 and S6 and capacitors C1 and C2, a primary side is formed by a full bridge composed of the capacitor C1 and the switching tubes S1, S2, S3 and S4, and a secondary side is formed by connecting the capacitor C2 and the switching tube S5 in series with the coupling inductor L1 and connecting the switching tube S6 in series with the coupling inductor L2. Under the condition of the same power level, the number of low-voltage side switch tubes is reduced, the system efficiency in a low-voltage large-current occasion is improved, the excellent performance of bidirectional power flow, electrical isolation and the like of a traditional DAB converter is kept, the voltage spike problem possibly occurring on the low-voltage side is absorbed by adopting an active clamping method, and the voltage spike problem is effectively solved. Therefore, the efficiency of the device in a low-voltage large-current occasion is improved.

Description

Technical Field

[0001] The invention belongs to the field of DC / DC converters, and in particular relates to an ESDAB converter suitable for medium and low voltage DC systems. Background Art

[0002] As a bidirectional DC-DC converter, the Double Active Bridge (DAB) converter has the characteristics of easy soft switching, high power density, bidirectional energy flow, and electrical isolation. It has been widely used in energy storage systems and DC microgrids. It is often used as a solid-state transformer in a modular manner after being connected in series and parallel. It has now become an important part of medium and low voltage distribution systems.

[0003] However, the high current characteristics of the low-voltage side increase the loss of the low-voltage side switch tube, and the loss of the solid-state transformer also increases accordingly. The current solution to this problem mainly includes two aspects: one is to expand the ZVS area, and the other is to improve its circuit topology. How to improve the efficiency of the DAB converter in low-voltage and high-current situations is still an urgent problem to be solved. Therefore, some topological improvements and modulation strategy optimization based on the traditional DAB converter can provide a competitive alternative for the module topology in medium and low voltage DC systems, which has significant value and significance in practical applications. Summary of the invention

[0004] The technical problem to be solved by the present invention is: to propose an enhanced simplified double active bridge (ESDAB) converter suitable for medium and low voltage DC systems, which solves the problem of low system efficiency of a single DAB converter in medium and low voltage DC systems under low voltage and high current conditions.

[0005] The present invention adopts the following technical solutions to solve the above technical problems:

[0006] An ESDAB converter suitable for medium and low voltage DC systems, comprising a three-winding transformer T, coupled inductors L1 and L2, switch tubes S1, S2, S3, S4, S5, S6 and capacitors C1 and C2;

[0007] The primary side is composed of a full bridge consisting of capacitor C1 and switches S1, S2, S3, and S4, and the secondary side is composed of capacitor C2, switch S5 in series with inductor L1, and switch S6 in series with inductor L2.

[0008] The primary side switches S1 and S2 are turned on alternately, the switches S3 and S4 are turned on alternately, the secondary side switches S5 and S6 are turned on alternately, there is an outward shift d1 between the primary side switch S1 and the secondary side switch S5, there is an inward shift d2 between the primary side switches S1 and S4, the sum of the current waveforms flowing through the same-named ends of the coupled inductors L1 and L2 is the current waveform of the transmission inductor in the traditional DAB converter, the coupled inductors L1 and L2 are each turned on for half a cycle, and their energy is coupled with each other when the switches S5 and S6 are turned off respectively.

[0009] It also includes a buffer circuit composed of switch tubes S7, S8 and capacitors C3, C4; when a voltage spike appears on the secondary side, an active clamping method is used to absorb the voltage spike; the dead time of the switch tubes S5 and S6 is set to d B , the switch tubes S7 and S8 are 2f s The switching frequency of the switch tubes S5 and S6 works within the dead time, and its duty cycle is the dead time d B , and the driving signals are consistent.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] 1. The ESDAB converter uses a three-winding transformer and a coupled inductor, which reduces the number of low-voltage side switches and improves the system efficiency in low-voltage and high-current situations at the same power level, while maintaining the excellent performance of the traditional DAB converter, such as bidirectional power flow and electrical isolation.

[0012] 2. The ESDAB converter adopts an active clamping method to absorb the voltage spike caused by the secondary leakage inductance and feed the energy back to the circuit, thereby avoiding the problem of reduced efficiency caused by the leakage inductance. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The present invention is a circuit topology diagram of an ESDAB converter suitable for medium and low voltage DC systems.

[0014] Figure 2 It is a schematic diagram of the switch tube drive signal and voltage and current waveforms of the ESDAB converter of the present invention under EPS modulation.

[0015] Figure 3 It is a schematic diagram of the working mode of the ESDAB converter of the present invention under EPS modulation.

[0016] Figure 4 It is a circuit topology diagram of an ESDAB converter using an active clamping method according to the present invention.

[0017] Figure 5The present invention is a schematic diagram of the switch tube drive signal and voltage and current waveforms of the ESDAB converter using the active clamping method under EPS modulation.

[0018] Figure 6 The present invention is a schematic diagram of the working mode of the ESDAB converter using the active clamping method within the dead time under EPS modulation.

[0019] Figure 7 It is a schematic diagram of the current stress optimization trajectory of the ESDAB converter of the present invention under different k values ​​under EPS modulation. DETAILED DESCRIPTION

[0020] The present invention is further described below in conjunction with the accompanying drawings and embodiments. The following detailed descriptions are all exemplary and are all intended to provide further description of the present invention. Unless otherwise specified, all technical and scientific terms used in this embodiment have the same meanings as those commonly understood by those of ordinary skill in the art to which the present invention belongs.

[0021] An ESDAB converter suitable for medium and low voltage DC systems, comprising a three-winding transformer T, coupled inductors L1 and L2, switch tubes S1, S2, S3, S4, S5, S6 and capacitors C1 and C2;

[0022] The primary side is composed of a full bridge consisting of capacitor C1 and switches S1, S2, S3, and S4, and the secondary side is composed of capacitor C2, switch S5 in series with inductor L1, and switch S6 in series with inductor L2.

[0023] The primary side switches S1 and S2 are turned on alternately, the switches S3 and S4 are turned on alternately, the secondary side switches S5 and S6 are turned on alternately, there is an outward shift d1 between the primary side switch S1 and the secondary side switch S5, there is an inward shift d2 between the primary side switch S1 and S4, the sum of the current waveforms flowing through the same-named ends of the coupled inductors L1 and L2 is the current waveform of the transmission inductor in the traditional DAB converter, the coupled inductors L1 and L2 are each turned on for half a cycle, and their energy is coupled with each other when the switches S5 and S6 are turned off respectively.

[0024] To solve the voltage spike problem that may occur on the secondary side, active clamping is used to absorb the voltage spike. The switch tubes S7 and S8 and capacitors C3 and C4 form a buffer circuit, making the dead time of the switch tubes S5 and S6 d B , the switch tubes S7 and S8 are 2f s The switching frequency of the switch tubes S5 and S6 works within the dead time, and its duty cycle is the dead time d B , and the driving signals are consistent, which also reduces the complexity of control to a certain extent.

[0025] For the case of k<1, the current stress optimization with soft switching can be performed under the EPS modulation strategy. The phase shift combination after current stress optimization can achieve soft switching when the current stress is relatively small. It only needs to satisfy the following relationship:

[0026]

[0027] Specific embodiments, such as Figures 1 to 7 shown.

[0028] This embodiment proposes a circuit topology of an ESDAB converter suitable for medium and low voltage DC systems. Figure 1 As shown, the circuit topology proposed in this embodiment is composed of a three-winding transformer, a coupled inductor, a switch tube and a capacitor. The primary switch tubes S1 and S2 are turned on alternately, the switch tubes S3 and S4 are turned on alternately, and the secondary switch tubes S5 and S6 are turned on alternately. There is an outward shift d1 between the primary switch tube S1 and the secondary switch tube S5, and there is an inward shift d2 between the primary switch tubes S1 and S4. The sum of the current waveforms flowing through the same-named ends of the coupled inductors L1 and L2 is the current waveform of the transmission inductor in the traditional DAB converter. The coupled inductors L1 and L2 are each turned on for half a cycle, and their energy is coupled to each other when the switch tubes S5 and S6 are turned off respectively. The primary midpoint voltage V AB It can generate three levels: 0, V1, -V1, and the secondary midpoint voltage V CD 、V DE Two levels, v2 and -v2, can be generated. The voltage across the coupled inductor can generate six levels, V2, -V2, v1+v2, V1-V2, -V1-V2, and -V1+V2. V1 is the primary input voltage and V2 is the secondary output voltage.

[0029] The switch tube drive signal and voltage and current waveforms of the ESDAB converter proposed in this embodiment under EPS modulation. Figure 2 As shown in the figure, the EPS modulation strategy achieves bidirectional energy transfer by changing the outward shift ratio d1 and the inward shift ratio d2. The value range is [0,1]. According to the size of d1 and d2, it can be divided into Mode 1 (d1>d2) and Mode 2 (d1>d2). <d2)。

[0030] The working mode diagram of the ESDAB converter proposed in this embodiment under EPS modulation. Figure 3 As shown, taking Mode 1 (d1>d2) as an example, the EPS modulation strategy can be divided into eight modes. Ideally, all switch tubes have current flowing through their body diodes before being turned on, and ZVS conduction can be achieved.

[0031] The circuit topology of the ESDAB converter using the active clamping method proposed in this embodiment. Figure 4 As shown, the active clamping circuit is composed of switch tubes S7 and S8 and capacitors C3 and C4.

[0032] The switch tube drive signal and voltage and current waveforms of the ESDAB converter using the active clamping method under EPS modulation proposed in this embodiment. Figure 5 As shown, taking Mode 1 (d1>d2) as an example, let the dead time of switch tubes S5 and S6 be d B The active clamping method uses switches S7 and S8 with 2f s The switching frequency of the switch tubes S5 and S6 works within the dead time, and its duty cycle is the dead time d B , and the driving signals are consistent, which also reduces the complexity of control to a certain extent.

[0033] The working mode diagram of the ESDAB converter using the active clamping method proposed in this embodiment during the dead time under EPS modulation. Figure 6 As shown, taking Mode 1 (d1>d2) as an example, the ESDAB converter using the active clamping method can be divided into four working modes within the dead time under EPS modulation. The active clamping circuit absorbs and feeds back the energy stored in the leakage inductance within the dead time, which can suppress the voltage spike problem caused by the leakage inductance.

[0034] Schematic diagram of current stress optimization trajectory of the ESDAB converter proposed in this embodiment under EPS modulation with different k values. Figure 7 As shown, the current stress optimization taking into account soft switching enables the ESDAB converter to achieve soft switching when the current stress is relatively small.

[0035] This embodiment further studies the working mode of the ESDAB converter under the EPS modulation strategy, and according to the primary midpoint voltage V AB and the secondary midpoint voltage V CD 、V DE And the inductor current i L relationship, and Figure 2 The working waveform and the half-cycle symmetry of the inductor current are used to derive the transmission power expressions of Mode 1 and Mode 2. Where L1 and L2 are coupled inductors, L1=L2=L, f s is the switching frequency, V1 is the primary input voltage, V2 is the secondary output voltage, and n is the transformation ratio of the three-winding transformer.

[0036] The average power delivered by the converter during one switching cycle will be expressed as:

[0037]

[0038] To simplify the calculation, the transmission power expression is normalized and the reference value is taken as:

[0039]

[0040] Then the normalized transmission power can be expressed as:

[0041]

[0042] The present invention is an ESDAB converter suitable for medium and low voltage DC systems. The topology of the traditional DAB converter is improved. Under the same power level, the number of switches on the low voltage side is reduced, while the excellent performance of the traditional DAB converter, such as bidirectional power flow and electrical isolation, is maintained. For the voltage spike problem that may occur on the low voltage side, an active clamping method can be used to absorb it, thereby improving its efficiency in low voltage and high current situations, and it is particularly suitable for medium and low voltage DC systems.

[0043] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An ESDAB converter suitable for medium and low voltage DC systems, characterized in that: It includes a three-winding transformer T, coupled inductors L1 and L2, switches S1, S2, S3, S4, S5, S6 and capacitors C1 and C2; The primary side is composed of a full bridge consisting of capacitor C1 and switches S1, S2, S3, and S4, and the secondary side is composed of capacitor C2, switch S5 in series with inductor L1, and switch S6 in series with inductor L2. The primary side switches S1 and S2 are turned on alternately, the switches S3 and S4 are turned on alternately, the secondary side switches S5 and S6 are turned on alternately, there is an outward shift d1 between the primary side switch S1 and the secondary side switch S5, there is an inward shift d2 between the primary side switches S1 and S4, the sum of the current waveforms flowing through the same-named ends of the coupled inductors L1 and L2 is the current waveform of the transmission inductor in the traditional DAB converter, the coupled inductors L1 and L2 are each turned on for half a cycle, and their energy is coupled with each other when the switches S5 and S6 are turned off respectively.

2. The ESDAB converter suitable for medium and low voltage DC systems according to claim 1, characterized in that: It also includes a buffer circuit consisting of switch tubes S7, S8 and capacitors C3, C4; When a voltage spike appears on the secondary side, the active clamping method is used to absorb the voltage spike; the dead time of the switch tubes S5 and S6 is set to d B , the switch tubes S7 and S8 are 2f s The switching frequency of the switch tubes S5 and S6 works within the dead time, and its duty cycle is the dead time d B , and the driving signals are consistent.

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