CN109256955B - Backflow power suppression method of double-active bridge based on modal analysis - Google Patents

Abstract

The invention discloses a method for restraining and analyzing the backflow power of a double-active bridge based on modal analysis, which analyzes the working process of DAB controlled by SPS in three working modes of inductive current being larger than zero, equal to zero and smaller than zero at the moment of secondary side switching action; then defining a DAB mode according to the circuit characteristics of each working mode, and reconstructing the working process of various working modes of the DAB based on the mode; analyzing the working process of the DAB with the modal reconstruction in multiple working modes, and analyzing the reasons for generating the reflux power; secondly, based on the reason of the generation of the reflux power, an extended phase-shifting control method is adopted to change the size of a phase-shifting duty ratio, change the timing sequence of a DAB primary side switch driving signal, and add a follow current mode similar to a basic DC/DC converter and follow current through a diode in the DAB mode evolution process to reduce the reflux power; and finally analyzing the reflux power after DAB inhibition, and determining the value of the control parameter for reducing the reflux power.

Description

Backflow power suppression method of double-active bridge based on modal analysis

Technical Field

The invention belongs to the technical field of double-active drive bridge application, and particularly relates to a backflow power suppression method of a double-active drive bridge based on modal analysis.

Background

The Dual Active Bridge (DAB) has the advantages of bidirectional energy flow, input and output electrical isolation, high power density, easy realization of soft switching and the like, and is a dc converter topology with great research and application prospects. The single-phase-shift control-based improved multi-phase-shift control strategy brings complex working modes and working analysis processes to DAB, and a circuit mechanism for generating and inhibiting reflux power is lacked, so that a DAB analysis method which is simple in analysis process and can be flexibly applied is needed.

The existing DAB control strategy is improved based on the traditional phase shift control (SPS), and the improved phase shift control mainly comprises extended phase shift control (EPS), double phase shift control (DPS) and triple phase shift control (TPS). When the existing analysis method is used for analyzing the working process of the DAB, the conduction conditions of a current circulation path and a switching device are analyzed in a time-sharing mode according to the relation between the primary side H-bridge voltage and the secondary side H-bridge voltage of the transformer, so that the working process of the DAB in the whole switching period is obtained, and the analysis process is complex. When the DAB works in different working modes, the whole working process needs to be analyzed again; for the defect of large backflow power in SPS control, an improved item shifting control strategy can inhibit the backflow power through the control of an inner item shifting angle, however, the existing analysis method lacks the reason for reducing the backflow power from the perspective of a circuit mechanism, and the backflow power inhibition scheme has poor expansibility.

Disclosure of Invention

The invention aims to provide a method for inhibiting reflux power of a double-active bridge based on modal analysis, which aims to solve the problems that the analysis process of the existing DAB analysis method is complex, DAB works in different working modes and needs to be analyzed again, and the existing DAB reflux power inhibition method for SPS control has poor expansibility.

The technical scheme adopted by the invention is that the backflow power suppression method of the double-active bridge based on modal analysis comprises the following specific steps:

step S1, analyzing the working process of the traditional phase shift controlled double active bridge, namely DAB controlled by SPS, in three working modes of inductive current larger than zero, equal to zero and smaller than zero at the moment of secondary side H bridge switch action;

step S2, defining DAB modes according to the circuit characteristics of each working mode of DAB, and reconstructing the working process of various working modes of DAB based on the DAB modes;

step S3, analyzing the working process of the DAB multiple working modes reconstructed by the modes, and analyzing the reasons for generating the reflux power;

step S4 is to suppress the return power from the circuit mechanism based on the cause of the return power generation.

Further, the circuit characteristic in step S2 means that the current flow paths in DAB are different, and the two ends of the inductor have different voltage levels and current directions.

Further, in the step S2, the defined DAB modality is used to replace each operation modality of the original operation procedures of each operation mode of DAB, so as to reconstruct the operation procedures of multiple operation modes of DAB.

Further, the DAB modality of step S2 includes a power return modality and a power transmission modality.

Further, the step S3 analyzes the operation process of the modal reconstruction DAB multiple operation modes, and the determined reflux power generation cause is: when the inductor unloads energy, the DAB primary side switching device and the secondary side switching device act simultaneously, and a similar follow current channel formed by a diode connected with the inductor in parallel and the inductor in a basic DC/DC converter does not exist in the DAB working process, so that the follow current flows to an input power supply through a follow current diode connected with the inductor in series, and the reflux power is generated.

Further, in step S4, based on the reason for generating the backflow power, a specific method for suppressing the backflow power from the circuit mechanism is: the method adopts an extended phase-shift control method, changes the size of a phase-shift duty ratio, changes the time sequence of a primary side switch driving signal of the DAB, adds a follow current mode in the evolution process of the DAB mode, provides a corresponding follow current channel for the energy discharge of an inductor, ensures that follow current does not flow to an input power supply, further controls the working time of the follow current mode and a power transmission mode, and reduces or completely eliminates the reflux power.

Further, the controlling of the working time of the follow current mode and the power transmission mode is to determine a control parameter value for reducing the reflux power on the basis of analyzing the reflux power after the DAB suppression, and the specific steps are as follows:

step S41, defining the working time of the power transmission mode in one period of DAB as M1T, the working time of the freewheel mode as M2T, the power transmission coefficient C being M1+ M2, M1 being the working time coefficient of the freewheel mode, and M2 being the working time coefficient of the power transmission mode;

step S42, determining an inductive current i0 ' at the initial moment of DAB in a period, an inductive current i1 ' at the action moment of a primary side switch and an inductive current i2 ' at the action moment of a secondary side switch according to a volt-second balance law;

step S43, according to i0 ', i1 ' and i2 ', calculating the integral of the voltage-current product of the input side of the DAB half period, and determining the transmission power of the DAB including M1 and C;

step S44, selecting a transmission power reference value, performing per-unit on DAB transmission power and analyzing the per-unit transmission power;

step S45, classifying the DAB working modes again according to different states of the DAB inductive currents i0 'and i 1' in one period; determining the limitation conditions of each working mode classification of the follow current mode working time coefficient M1 and the power transmission coefficient C according to the ratio n of the actual input voltage to the output voltage to the primary voltage of the transformer;

step S46, analyzing the reflux power of the DAB operation modes after reclassification according to i0 ', i1 ' and i2 ';

and step S47, determining values of M1 and C which can completely eliminate the DAB reflux power or reduce the reflux power and keep the transmission power unchanged in a normal working state based on the reflux power and per unit transmission power analysis.

Further, the operation modes of DAB reclassified in step S45 and the restriction conditions for each mode classification are as follows:

mode I: i.e. i₀`≥0，i₁The gradient is more than or equal to 0; the classification limitation condition is

Mode II: i.e. i₀`＜0，i₁The gradient is more than 0; the classification limitation condition is

C∈[0,1]；

Mode III: i.e. i₀`＜0，i₁The strain is less than 0; the classification limitation condition is

Further, in step S47, the value ranges of M1 and C, which enable the reflux power of DAB to be reduced and the transmission power to be unchanged in the normal operating state, are as follows: c belongs to [ M0 ', 1], M1 belongs to [ M0, M0' ]; wherein, M0 and M0' are two solutions of M1 under the condition that the transmission power is not changed during SPS control, M0 ∈ [0,0.5], and M0 ═ 1-M0 ∈ [0.5,1 ]; the value ranges of M1 and C which can completely eliminate the DAB reflux power and keep the transmission power unchanged are the values of M1 and C when the DAB works in the mode I.

Further, the determination of the values of M1 and C in step S47 is to select any C and then obtain M1 according to the transmission power expression or select any M1 and then obtain C according to the transmission power expression within the value ranges of C and M1 that reduce or eliminate the DAB reflux power during SPS control.

The method has the advantages that the whole working process of DAB can be reconstructed through the modal evolution process for a specific control strategy based on the modal analysis based on the backflow power suppression method of the double-active bridge, and the analysis method is simple and convenient; different working modes are different only in individual derivative modes, and the complex working mode of the DAB under the control of multiple degrees of freedom is simplified and expressed; when the improved phase shift control is adopted, the working process analysis can be carried out on the basis of the evolution of the SPS control mode, so that the analysis process is greatly simplified; the method is characterized in that the reason for generating the reflux power is explained from a circuit mechanism, an extended phase-shift control method is adopted based on the reason, the phase-shift duty ratio is changed, the time sequence of a primary side switch driving signal of DAB is changed, a follow current mode similar to a basic DC/DC converter and follow current through a diode is added in the evolution process of the DAB mode, the DAB reflux power controlled by SPS after the follow current mode is added is analyzed, a proper control parameter range is determined, and the reflux power is reduced or inhibited.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a DAB topology circuit diagram;

FIG. 2 is a DAB main waveform diagram for SPS control;

FIG. 3 is a DAB node current i with SPS control₁>DAB working process analysis chart at 0;

FIG. 4 is a DAB operation process analysis diagram for three modes of operation using SPS control;

FIG. 5 is a Buck mode diagram;

FIG. 6 is a timing diagram of evolution of DAB mode;

FIG. 7 is a graph of analysis of the reflux power generation mechanism;

FIG. 8 is a diagram of analysis of the backflow power suppression mechanism;

FIG. 9 is a DAB modal evolution timing diagram;

FIG. 10 is a main waveform diagram after DAB is added to the freewheel mode;

FIG. 11 is a schematic diagram of per-unit transmission power;

FIG. 12 is a schematic view of the operating mode region;

FIG. 13 shows that C is 0.8, M₁The primary and secondary side voltages of the transformer and the inductance current waveform diagram at 0.2;

FIG. 14 shows that C is 0.8, M₁The primary and secondary side voltage and inductance current waveform of the transformer at 0.75 time;

FIG. 15 shows that C is 0.9, M ₁13/30 time transformer primary and secondary side voltage and inductance current waveform diagram;

FIG. 16 shows that C is 0.9, M₁23/30 time transformer primary and secondary side voltage and inductance current waveform diagram;

FIG. 17 is the SPS control M₀The primary side voltage and the secondary side voltage of the transformer and the inductance current waveform at 0.7;

FIG. 18 shows that C is 0.8, M₁The primary and secondary side voltages of the transformer and the inductance current waveform diagram at 0.2;

FIG. 19 shows that C is 0.8, M₁The primary and secondary side voltage and inductance current waveform of the transformer at 0.75 time;

FIG. 20 shows that C is 0.9, M ₁13/30 time transformer primary and secondary side voltage and inductance current waveform diagram;

FIG. 21 shows that C is 0.9，M₁23/30 time transformer primary and secondary side voltage and inductance current waveform diagram.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The DAB topological circuit is shown in FIG. 1 and comprises a high-frequency transformer, an inductor L (external inductor plus transformer leakage inductor), an H-bridge circuit on an original side and a secondary side, and capacitors C1 and C2; wherein, the transformation ratio of the high-frequency transformer is k: 1.

FIG. 2 shows DAB main waveforms including switch driving signal and H-bridge DC side voltage u_ABAnd u_CDInductor current i_lAnd an input current i_V1And (4) waveform. In each switching period of DAB, two IGBTs at opposite corners of an H bridge are simultaneously switched on and off, two groups of opposite-corner bridge arms are respectively switched on alternately for T time to operate, and power is defined to flow from a primary side to a secondary side to be a positive direction. As can be seen from fig. 2, the switching frequency f of the device is 1/2T, d is the two H-bridge phase-shift duty cycle, and when d is greater than d>At 0, power transmission is positive, d<At 0, power delivery is negative. The magnitude and direction of power transfer can be varied by controlling d. Considering the power bidirectional flow characteristic of DAB and the symmetry of the structure thereof, d is more than or equal to 0 and less than or equal to 1 and V is used for simplifying analysis₁>kV₂For example, the working process of DAB was analyzed.

In FIG. 2, at t₀At the beginning of a switching period, the inductor L current is at t₀～t₂The time period is increased positively at t₂～t₄The time period is reversely reduced and at t₂The time takes the positive maximum. According to the symmetry and volt-second balance law of the DAB working process, the inductive current is at t₁Value of time i₁Presence of i₁>0、i ₁0 and i₁< 0 three modes of operation.

i₁>At 0, there are 6 operation modes of DAB in one switching period, as shown in fig. 3:

the first mode is as follows: t is t₀～t₀Period' A: t is t₀Before time, Q₂、Q₃And Q₆、Q₇On, inductor L current is reversed at t₀Time Q₁And Q₄Can not be conducted at once and is composed of an antiparallel diode VD₁And VD₄Follow current, secondary side by anti-parallel diode VD₆And VD₇Follow current, current edge V₁-VD₄-VD₇-V₂-VD₆-VD₁-V₁The loop flows. t is t₀At' time, inductor L current increases to zero, Q₁And Q₄And naturally conducting.

Mode two: t is t₀′～t₁Time period: at t₀At time, inductor current Lcurrent begins to flow in the forward direction, Q₆And Q₇On, current edge V₁-Q₁-L-Q₆-V₂-Q₇-Q₄-V₁The loop flows.

Mode three: t is t₁～t₂Time period: t is t₁Time, Q₆And Q₇Off, Q₅And Q₈On, current edge V₁-Q₁-L-VD₅-V₂-VD₈-Q₄-V₁The loop flows, the inductor L current continues to increase in the positive direction, Q₅And Q₈And cannot be conducted.

And a fourth mode: t is t₂～t₂Period' A: t is t₂Time, Q₁And Q₄When the current of the inductor L is turned off, the current of the inductor L cannot be immediately reversed, and continues to flow in the forward direction, and Q₂And Q₃Can not be conducted at once and is composed of an antiparallel diode VD₂And VD₃Freewheeling diode VD₅And VD₈Continued conduction with current edge V₁-VD₂-L-VD₅-V₂-VD₈-VD₃-V₁The loop flows. To t₂At time, inductor current decreases to zero and inductor L current begins to flow in the opposite directionAnd (6) moving.

A fifth mode: t is t₂′～t₃Time period: t is t₂' time, Q₂And Q₃Naturally on, Q₅And Q₈Also simultaneously conduct current along V₁-Q₃-Q₈-V₂-Q₅-L-Q₂-V₁The loop flows.

A sixth mode: t is t₃～t₄Time period: t is t₃Time, Q₅And Q₈Off, Q₆And Q₇Opening, inductor current passing VD₆And VD₇Follow current, current edge V₁-Q₃-VD₇-V₂-VD₆-L-Q₂-V₁The loop flows. At t₄Time, Q₂And Q₃Off, Q₁And Q₄Turning on, entering the next period of operation, circuit working state and t₀The time is consistent.

At t₀～t₀' and t₂～t₂' time period, inductor current i₁And the primary side voltage u_ABThe phases are opposite. During this time, the transmission power is negative, and the portion of the power flowing back to the power supply is called the return power, as shown by the shaded portion in fig. 2. It is easy to find that, at a certain output power, the larger the return power is, the larger the power that needs to be transmitted in the forward direction is, so as to compensate the return power, which will result in the increase of the power circulation and the current stress of the converter, and further increase the loss of the power device and the magnetic component, and reduce the efficiency of the converter.

i₁When 0, there are 4 operating modes of DAB in one switching period:

modality (one): t is t₀～t₁Time period: t is t₀Before time, Q₂、Q₃And Q₆、Q₇On, inductor L current is reversed at t₀Time Q₁And Q₄Can not be conducted at once and is composed of an antiparallel diode VD₁And VD₄Follow current, secondary side by anti-parallel diode VD₆And VD₇Follow current, current edge V₁-VD₄-VD₇-V₂-VD₆-VD₁-V₁The loop flows. t is t₁At that moment, the inductor L current increases to zero.

Modality (ii): t is t₁～t₂Time period: t is t₁Time, Q₁And Q₄Naturally on while Q₆And Q₇Off, Q₅And Q₈On, current edge V₁-Q₁-L-VD₅-V₂-VD₈-Q₄-V₁The loop flows, the inductor L current continues to increase in the positive direction, Q₅And Q₈And cannot be conducted.

Modality (iii): t is t₂～t₃Time period: t is t₂Time, Q₁And Q₄When the current of the inductor L is turned off, the current of the inductor L cannot be immediately reversed, and continues to flow in the forward direction, and Q₂And Q₃Can not be conducted at once and is composed of an antiparallel diode VD₂And VD₃Freewheeling diode VD₅And VD₈Continued conduction with current edge V₁-VD₂-L-VD₅-V₂-VD₈-VD₃-V₁The loop flows. To t₃At that moment, the inductor current decreases to zero.

Modality (iv): t is t₃～t₄Time period: t is t₃Time, Q₅And Q₈Off, Q₆And Q₇Opening, inductor current passing VD₆And VD₇Follow current, current edge V₁-Q₃-VD₇-V₂-VD₆-L-Q₂-V₁The loop flows. At t₄Time, Q₂And Q₃Off, Q₁And Q₄Turning on, entering the next period of operation, circuit working state and t₀The time is consistent.

i₁<At 0, there are 6 working modes of DAB in one switching period:

mode 1: t is t₀～t₁Time period: t is t₀Before time, Q₂、Q₃And Q₆、Q₇On, inductor L current is reversed at t₀Time Q₁And Q₄Can not be conducted at once,by antiparallel diodes VD₁And VD₄Follow current, secondary side by anti-parallel diode VD₆And VD₇Follow current, current edge V₁-VD₄-VD₇-V₂-VD₆-VD₁-V₁The loop flows.

Mode 2: t is t₁～t₁Period' A: at t₁Time, Q₆And Q₇Off, Q₅And Q₈Conduction, no sudden change of inductive current, and current edge V₁-VD₄-Q₈-V₂-Q₅-L-VD₁-V₁The loop flows. To t₁At' time, the inductor current increases to zero.

Modality 3: t is t₁′～t₂Time period: t is t₁' time, Q₁And Q₄Naturally on, current edge V₁-Q₁-L-VD₅-V₂-VD₈-Q₄-V₁The loop flows, the inductor L current continues to increase in the positive direction, Q₅And Q₈And cannot be conducted.

Modality 4: t is t₂～t₃Time period: t is t₂Time, Q₁And Q₄When the current of the inductor L is turned off, the current of the inductor L cannot be immediately reversed, and continues to flow in the forward direction, and Q₂And Q₃Can not be conducted at once and is composed of an antiparallel diode VD₂And VD₃Freewheeling diode VD₅And VD₈Continued conduction with current edge V₁-VD₂-L-VD₅-V₂-VD₈-VD₃-V₁The loop flows.

Mode 5: t is t₃～t₃Period' A: t is t₃Time, Q₅And Q₈Off, Q₆And Q₇On, inductor current can not change suddenly, current edge V₁-VD₂-L-Q₆-V₂-Q₇-VD₃-V₁Loop flow to t₃At' time, the inductor current is reduced to zero.

Modality 6: t is t₃′～t₄Time period: t is t₃' time, Q₂And Q₃Nature of natureOn, current edge V₁-Q₃-VD₇-V₂-VD₆-L-Q₂-V₁The loop flows. At t₄Time, Q₂And Q₃Off, Q₁And Q₄Turning on, entering the next period of operation, circuit working state and t₀The time is consistent.

From the operation modes of the above operation modes, in i₁>0、i ₁0 and i₁<In 0 three operation states, the working process of DAB in one switching period is symmetrical, and in order to simplify the analysis process, the working process of the previous half switching period is taken as an example, DAB in i₁>0、i ₁0 and i₁<The operation processes of 0 three operation modes are respectively shown in fig. 4. Can find that i₁>The mode one and the mode three in the 0 mode working process are common to the three working modes, and i ₁0 mode having only i₁>Two modes of mode one and mode three in the 0 mode working process.

With i₁>0 as an example, the voltage across the inductor in the 'three modes' common to the three operating modes of the DAB is V₁-kV₂The inductor current rises linearly and power is transmitted in the forward direction. Due to V₁>kV₂The DAB operates in Buck mode, so from the circuit characteristic point of view, this mode is similar to the basic Buck converter switch when it is on, as shown in fig. 5, and therefore this mode is defined herein as "Buck mode", which is a power transmission mode due to forward power transmission in the Buck mode. And for the common mode one, after the Buck mode, namely mode six, of the second switching period is finished, the primary side H-bridge switch acts, and the mode is inductance energy discharge. However, due to the low degree of freedom of SPS control, the diagonal switches of the primary H-bridge operate simultaneously, so that the energy discharge channel allows an inductive current to flow back to the input-side power supply through the freewheeling diode, and a backflow power is generated, which is defined herein as a "power backflow mode".

From the viewpoint of satisfying volt-second balance, when DAB works, only Buck mode and power reflux mode existWhen the DAB is in use, the DAB can normally work, and at the moment, the DAB works in step i₁In the 0 state, the current decreases from the maximum value to 0 in the power return mode, and increases from 0 to the maximum value in the Buck mode. However, when the circuit parameters are fixed, the transmission power is also fixed, and effective adjustment of the transmission power cannot be realized. In order to realize adjustable transmission power, the phase-shift duty ratio needs to be changed, the action time of the secondary side H-bridge switch relative to the primary side H-bridge switch is changed, and the inductive current can have three states of more than zero, equal to zero and less than zero when the secondary side switch acts, i₁And 0 is an intermediate state, and acts before and after the zero-crossing time of the inductive current respectively to generate a Buck derivative mode and a power backflow derivative mode. In FIG. 4, i₁<The 0 mode 'two mode' and 'three mode' circuit structure is the same and the inductive current direction is opposite, i₁>The 0 mode "two mode" and "one mode" circuit structure are the same and the direction of the inductive current is opposite.

Based on the foregoing analysis, the working processes of the multiple working modes of DAB are reconstructed through modal evolution, that is, each modality in the original analysis method in step 1 is replaced by a newly defined modality, so that the working processes of the multiple working modes of DAB can be clearly presented, as shown in fig. 6. The lower part of the P axis shows that the mode has backflow power, and the upper part of the P axis shows the forward transmission power of the mode; as can be seen from fig. 6, the power return mode is present in all operating modes of DAB, i.e. there must be a return power in DAB with SPS control.

For a basic DC/DC converter, such as a Buck converter, since there are few switching devices and the circuit structure is simple, current only flows through the inductor in one direction, the inductor respectively stores and discharges energy when the switch is turned on and off, and current flows through the freewheeling diode when the inductor discharges energy, there is no backflow power, as shown in fig. 7.

In the Buck mode, DAB is the same as the conduction of a Buck converter switch, while transmitting power, the inductor stores energy, and after the mode is finished, the switching device on the primary side H bridge of the transformer acts, as shown in fig. 7. Due to the fact that the SPS control degree of freedom is low, Q1 and Q4 are turned off and Q2 and Q3 are turned on simultaneously under the condition that no dead zone is considered, DAB enters a power backflow mode, the energy discharging process of the inductor is carried out by enabling current to flow back to the input side, and backflow power is generated. It is conceivable that even if the switching devices on the secondary H-bridge of the transformer are active, return power will be generated before the current crosses zero. The existence of the reflux power greatly reduces the efficiency of DAB power transmission, and SPS control with low control freedom degree only has a single control parameter, namely H-bridge phase-shift duty ratio variable, transmission power and reflux power are mutually restrained, and the control flexibility is poor.

From the foregoing analysis, it can be known that, in the method for suppressing the reflux power in the circuit mechanism, a freewheeling mode needs to be added in the DAB mode evolution process, and a freewheeling channel similar to the switching-off of the Buck converter switch is provided for the energy discharging of the inductor, so that the freewheeling current does not flow to the input side. By further controlling the duration of the follow current mode and the Buck mode, partial or complete replacement of the power backflow mode is achieved, namely backflow power is reduced or even eliminated. Meanwhile, the control dimensionality is increased, the containment relation between the transmission power and the backflow power can be more flexibly relieved, and the efficiency is increased.

Since the generation of the mode is determined by the operation of the switching device and the characteristics of the inductor current, and the operation of the transformer secondary side H-bridge switching device cannot suppress the return power from the circuit mechanism. Therefore, by changing the timing sequence of the driving signal of the primary H-bridge switch of the transformer, the DAB has a freewheeling mode similar to the switching-off of the Buck converter switch after the Buck mode is ended, as shown in fig. 8, where a newly added freewheeling mode is in a dashed frame. With i₁>The mode 0 is taken as an example, a freewheeling mode is added when the switching frequency is not changed, and the mode evolution of the working process of the DAB is shown in fig. 9. It can be seen that the addition of the freewheeling mode affects the working time of the original modes, i.e. the magnitudes of the return power and the transmission power.

The DAB working process generates a follow current mode by adopting a phase-shift control method on a primary side H bridge. When the switching frequency is not changed, the Buck mode, namely the power transmission mode working time is set to be M1T, and the duration of the introduced free-wheeling mode is set to be M2T (M2 epsilon [0,1 ]). The main waveforms for the new DAB operation are shown in fig. 10. Wherein the dotted line is the original inductive current waveform, and the solid line is the waveform of the new working process. It can be seen that the working time of each mode changes due to the addition of the follow current mode, and the magnitude of the backflow power (shown by the shaded part) also changes. The modal operating time is analyzed below in relation to the return power and the transmission power.

In SPS control, for the convenience of subsequent analysis and comparison, the time used by the Buck mode in time T is assumed to be MT, i.e., M is 1-d. At this point, the balance is obtained from volt-seconds:

l represents an inductance; i.e. i₀For the initial time of the power return mode, i.e. t₀Current value at time i₁For secondary side H-bridge switching device action t₁The current at the moment. The following equations (1) to (2) can be solved:

wherein n is V₁/kV₂N is the ratio of the actual input voltage to the output voltage converted to the primary voltage of the transformer; the switching frequency f of the device is 1/2T; with reference to equations (3) to (4), the transmission power in half a switching cycle can be obtained as:

and the power of the reflux is according to i₁Will vary in scope.

i₁When not less than 0,0<M≤(n+1)/2n：

i₁<At 0, (n +1)/2n<M<1：

Derivation of equation (7) and making it 0 can be obtained:

as is clear from the expressions (6) to (9), n is>1，0<M<1，i₁>The return power at 0 is a decreasing function in the value range of M, i₁<The return power at 0 decreases and then increases as M increases.

And DAB still satisfies volt-second equilibrium law after the afterflow mode is added, so that:

in the formula, i is distinguished from SPS₀' is the current value at the initial moment of the power reflux mode i₁"is the current i of the secondary side H-bridge switching device at the moment of action₂And' is the current of the primary side H-bridge switching device at the action moment.

Definition ofC＝(M₁+M₂)∈[0,1]When M is₁And M₂When the DAB is not zero, the modal evolution of the DAB necessarily comprises a Buck mode and a follow current mode, and the complete working process is completed by adding a new mode on the basis of the work of the Buck converter, wherein C is called a Buck coefficient. See the relation between the value of C and the DAB operation mode and transmission power in detail below.

Solving the formulas (10) to (12) and substituting the Buck coefficient C to obtain the current of each node about C and M₁The expression of (a) is as follows:

according to i₀`、i₁' and i₂"integrate the voltage-current product at the input side for half a cycle, determine the transmission power expression of DAB with respect to M1 and C as:

maximum transmission power value P under SPS control_N＝kV₁V₂Where (8fL) is a transmission power reference value, the transmission power expressions (5) and (16) are per unit:

p represents the per-unit power transmission during the SPS control, and P represents the per-unit power transmission after the follow current mode is introduced during the SPS control; as shown in FIG. 11, per unit power transmission in SPS control is introduced as a single curve in the figureAfter freewheel mode, for each fixed value C ∈ [0,1]]P is M₁About an axis of symmetry of₁1/2, and each function has a value in the range of M₁∈[0,C]The transmission power range is shaded in fig. 11.

Let the transmission power be P₀When there is a definite Buck mode time M₁＝M₀∈[0,0.5]At this point, there are always two solutions for the Buck coefficient C: c₁＝C₀∈[0,0.5]And C₂＝1-C₀，C₀Is a constant; and when C₁<C<C₂Then, as can be seen from FIG. 11, M₁When not changed, the transmission power increases and then decreases with the increase of C and is always greater than P₀When C is 0.5, there is a maximum per unit transmission power function:

P^*′ _max＝2M₁(1-M₁)+0.5,M₁∈[0,0.5]；(18)

when 0 is present<C<C₁Or C₂<C<The transmission power at 1 time is respectively monotonically increased and decreased along with C and is always less than P₀. And when C ═ 0 and C ═ 1, P ═ has a minimum value of 0 and with respect to M, respectively₁Curve of the minimum function:

when the determined solution M is₁＝M₀`∈[0.5,1]When the method is used, only one Buck coefficient is used for solving the C-C ═ C<sub>0</sub>`∈[0.5,1]And when M is₁Invariable, C>C₀When the transmission power is less than P₀(ii) a When C is present<C₀When the transmission power is more than P₀。

The node current state at the moment when the DAB switch is operated within the half period T is known to determine the DAB operation mode. After a follow current mode is introduced, the timing of a control signal is changed, so that the intermediate node current is except i₀All too much, also increase i₁`。

According to i₀' and i₁Different states of the' can approximately model the working of DABIs divided into₀`≥0、i<sub>1</sub>`≥0，i₀`<0、i<sub>1</sub>`>0，i₀`<0 and i<sub>1</sub>`<And (4) class 0. According to the Buck coefficient C and the Buck mode working time coefficient M₁The classification constraints are as indicated in table 1. The power transmission ranges of the different operation modes are distinguished in the figure, as shown by the left slash table area, the grid area and the right slash area in fig. 12, respectively. It can be seen that when the value of n is a fixed value in practice, the Buck coefficient C and the Buck mode M₁Different values correspond to different working modes and transmission powers, and different working modes can be provided when the same power is transmitted, so that the flexibility of transmission power control is greatly improved compared with SPS control.

TABLE 1 Buck coefficient C and Buck modal time coefficient M of different working modes of DAB₁Value of

The Buck coefficient C and the Buck modal time coefficient M are utilized herein₁On the basis of dividing the DAB working mode, firstly, the characteristics of the reflux power of the three working modes are analyzed, and the influence of the addition of the follow current mode on the change of the reflux power is emphatically analyzed when the DAB works in a working mode II with the widest transmission power range.

The working mode I: the reflux power is 0;

and a working mode II: the reflux power expression is:

and a working mode III: the reflux power expression is:

as shown in fig. 11, the per unit power transmission during SPS control is a single curve in the graph, and after the freewheeling mode is introduced, for each fixed value C ∈ [0,1], P ×' is a series of unitary quadratic functions of M1 with respect to the symmetry axis M1 ∈ [ 1/2 ], and the value range of each function is M1 ∈ [0, C ], and the transmission power range is the shaded part in fig. 11.

As can be seen from fig. 11, the operation mode ii can realize full-range power transmission, and the operation mode i can realize DAB zero-reflow power operation, so that the analysis of the two modes has important guiding significance for optimizing DAB reflow power and selecting control parameters. For comparison with the SPS control, the present invention analyzes only the operating mode ii return power characteristics.

From the equation (20), the change in the reflux power and i₀"related to. Therefore, the change of the reflux power after adding the follow current mode can be realized by delta i₀`＝|i<sub>0</sub>`|-|i₀Comparison is performed under the expression of' |:

Δi₀`＝|i<sub>0</sub>|-|i<sub>0</sub>`|＝m[(n+2)C-nM₁-2M]； (22)

wherein m is a constant; i.e. i₀"is the initial current of power reflux mode under SPS control, and there is M₁＝C＝M。

For comparison with SPS control, the return power is analyzed to ensure that the transmission power is constant. There may be two solutions M-M for SPS control in operating mode II₀E [0,0.5) and M ═ M₀`＝(1-M₀)∈[0.5,1]. When the freewheeling mode is introduced, as can be seen from FIG. 12, M is now set₁And C have countless solutions. Therefore, the analysis introduces a free-wheeling mode on the basis of SPS control, and classification and discussion are needed.

(1)M₀∈[0,0.5]：

If the time coefficient is M in Buck mode₀On the basis of the method, a follow current mode is introduced, and meanwhile, the transmission power is ensured to be unchanged. As can be seen from the previous section, when there is a Buck coefficient C ∈ [1-M ]₀,1]∈[0.5,1]While, M can be increased simultaneously₁And C, due to C ″)>M₀Therefore, equation (22) is constantly greater than zero and the reflux power is always reduced. When the Buck coefficient C is equal to (0.5, 1-M)₀) While, M can be reduced simultaneously₁And C; c is belonged to (0, M)₀)∈[0,05) while decreasing M₁And C is increased, and the condition that the formula (22) is larger than zero is satisfied in order to reduce the backflow power.

(2)M₀`＝(1-M₀)∈[0.5,1]

As can be seen from FIG. 12, the time coefficient in Buck mode is M₀Introducing follow current mode on the basis of 'M', two groups of solutions C ═ M must be provided₀`，M<sub>1</sub>＝M<sub>0</sub>And C ═ M<sub>0</sub>，M<sub>1</sub>＝M<sub>0</sub>. Changing C to M<sub>0</sub>`，M₁＝M₀Can be substituted by formula (22)

Δi₀`＝mn(M<sub>0</sub>`-M₀)＞0； (23)

That is, the return power is reduced based on the SPS control, and C is equal to M₀，M₁＝M₀The substitution of formula (22) can be:

Δi₀`＝2m(M<sub>0</sub>-M<sub>0</sub>`)＜0； (24)

that is, the return power is increased on the basis of the SPS control; as can be seen from the results of equations (22) to (24) in FIG. 12, when there is a solution C ∈ (M)₀`,1)∈[0.5,1]，M<sub>1</sub>∈[M<sub>0</sub>，M<sub>0</sub>`]The reflux power is necessarily reduced; when there is a solution C<M₀The equation (25) is satisfied to ensure that the addition of the follow current mode can reduce the reflux power.

Δi₀`＝m[(n+2)C`-nM₁-2M₀`]； (25)

From the above analysis, for DAB in the operating mode II, if the reflux power is to be reduced on the basis of SPS control and the transmission power is ensured to be unchanged, the control parameter is selected to be C e (M)₀`,1)∈[0.5,1]，M<sub>1</sub>∈[M<sub>0</sub>，M<sub>0</sub>`]The reflux power must be reduced.

The mode III after the freewheeling mode is added in the SPS control process usually appears when the converter is in light load, and at this time, the DAB cannot realize ZVS operation of the secondary side H bridge switch, and the operating state needs to be avoided as much as possible in practice, so that the values of M1 and C with reduced backflow power and unchanged transmission power do not need to be determined by further analyzing the backflow power of the mode III, and the DAB operates in the mode III only by subsequently determining the values of M1 and C, so that the converter can be effectively prevented from operating in the mode III. In particular, in the case of improved phase shift control, the transmission power flexibility is high, for example, in the case of extended phase shift control, the mode ii can already meet the requirements for the overall transmission power, so that DAB operates predominantly in this mode in the normal operating state.

Simulation and experiment:

(1) DAB operating mode simulation

The simulation parameters set input voltage 120V, output voltage 80V, transformer transformation ratio 1, inductance L0.6 mH, and device switching frequency f 2 kHz. According to the analysis, when the actual input-output voltage ratio is constant, the Buck coefficient C takes different values, and DAB is in the whole M₁The value ranges have different operating modes. From the constraints in Table 1, it can be calculated that there is C>5/7 and M₁When E is 0, C), DAB can work in mode I, when C is present>6/7 and M₁E (1+1/n-C, C) DAB can operate in mode III. Next, the coefficient according to Buck C and M is verified through Simulink simulation₁Correctness of classifying the DAB working mode.

As can be seen from table 1, when C is 0.8, the DAB operates in only two modes, i and ii, no matter M₁No change in the operating mode iii occurs. M₁0.2 is the critical point for both modes of operation, when i is present₀`＝0，i<sub>1</sub>`>0. The primary and secondary side voltage and inductor current waveforms of the transformer are shown in fig. 13. FIG. 14 shows that C is 0.8, M₁When the primary and secondary side voltage and the inductance current of the transformer are 0.75, i can be seen₀`<0，i<sub>1</sub>`>0, i.e. DAB is in mode ii.

When C is 0.9, according to M₁The value ranges of the DAB are different, the working modes of the DAB can be three working modes I, II and III, wherein when M is taken₁13/30 and M₁23/30, which is the critical point for operating modes i and ii, ii and iii, respectively, i.e. two times each have i₀`＝0，i<sub>1</sub>`>0 and i₀`<0，i ₁0. The waveforms of the primary and secondary side voltages and the inductor current of the transformer are shown in fig. 15 and 16, respectively.

(2) Return power comparison

Assuming that the transmission power is 750W, for SPS control, the transmission power is [0.5,1] at this time according to equation (5)]The solution in the range is M ═ 0.75. The reflux power at this time was determined to be 200W according to the formula (6). After the free-wheeling mode is added, according to the theoretical analysis, when C is 0.8, M at the moment is calculated₁When the reflux power is equal to 0.687, the reflux power is 106.6W by equation (20). When C is 0.31, M is₁At 0.2, the reflux power was 588W. The current waveforms for the three states are shown in fig. 17, and the return power regions are distinguished by different shaded regions, respectively.

(3) Experimental waveform

In order to verify the correctness of theoretical analysis, different working modes of DAB are verified firstly, and Buck coefficients C are 0.8 and M are taken respectively₁＝0.2、M₁0.75 and C0.9, M₁＝13/30、M₁The experimental waveforms obtained for the four cases of 23/30 are shown in fig. 18-21.

As can be seen from fig. 18 and 19, when C is 0.8, M is₁The larger the value is taken in the value range of (i)₁The closer to 0 and always greater than zero, i.e. the only two operating modes of DAB are i and ii. And when M is₁When 0.2 is taken, DAB works at the critical point of modes I and II, i₀`＝0。

As can be seen from fig. 20 and 21, when C is 0.9, M is the same as M₁The value ranges of the DAB are different, and the working modes of the DAB can be three working modes I, II and III. When M is₁When 13/30 and 23/30 are taken, DAB respectively works at the critical points of mode I and mode II and mode III, namely node current i ₀0 and i ₁0. Therefore, the modal parameters are selected in different ranges, the DAB can have different operation mode types, and the conclusion is the same as that of theoretical analysis and simulation.

The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims (6)

1. The backflow power suppression method of the double-active bridge based on the modal analysis is characterized by comprising the following specific steps:

step S1, analyzing the working process of the double active driving bridge controlled by single phase shift in three working modes of inductive current larger than zero, equal to zero and smaller than zero at the moment of the secondary side H bridge switch action in one switching cycle;

step S2, defining the mode of the double-active bridge according to the circuit characteristics of each working mode of the double-active bridge, and reconstructing the working process of multiple working modes of the double-active bridge based on the mode of the double-active bridge;

step S3, analyzing the working process of the mode reconstruction double-active bridge in various working modes, and analyzing the reason for generating the reflux power;

step S4, based on the reason of the reflux power generation, the reflux power is restrained from the circuit mechanism;

in step S4, based on the reason for generating the backflow power, the specific method for suppressing the backflow power from the circuit mechanism is: an extended phase-shift control method is adopted, the phase-shift duty ratio is changed, the time sequence of a primary side switch driving signal of the double-active driving bridge is changed, a follow current mode is added in the mode evolution process of the double-active driving bridge, a corresponding follow current channel is provided for the energy discharge of the inductor, follow current does not flow to an input power supply, and the working time of the follow current mode and the power transmission mode is further controlled to reduce or completely eliminate the reflux power;

the method for controlling the working time of the follow current mode and the power transmission mode is to determine the value of a control parameter for reducing the reflux power on the basis of analyzing the reflux power after the follow current mode is added into the single-phase-shift control double-active bridge mode, and comprises the following specific steps of:

step S41, defining the working time of a power transmission mode in one period of the double active driving bridges as M1T, the working time of a follow current mode as M2T, the power transmission coefficient C = M1+ M2, M1 is the working time coefficient of the follow current mode, and M2 is the working time coefficient of the power transmission mode;

step S42, determining the inductive current of the double active drive bridge at the initial moment in a period according to the volt-second balance lawi ₀Inductive current at primary side switch action momenti ₁Inductive current at the moment of' and secondary-side switching actioni ₂`；

Step S43, according toi ₀`、i ₁' an andi ₂integrating the product of voltage and current at the half-period input side of the double-active bridge, and determining a transmission power expression of the double-active bridge including M1 and C;

step S44, selecting a transmission power reference value, performing per-unit on the transmission power of the double active bridges, and analyzing the per-unit transmission power;

step S45, according to the inductive current of the double active bridges in one periodi ₀' an andi ₁classifying the working modes of the double active driving bridges again in different states of the driving bridge; and converting the ratio of the actual input voltage to the output voltage to the primary voltage of the transformernDetermining the limiting conditions of each operation mode classification of the free-wheeling mode operation time coefficient M1 and the power transmission coefficient C;

step S46, according toi ₀`、i ₁' an andi ₂analyzing the reflux power of each working mode of the double-active drive bridge after reclassification;

and step S47, determining values of M1 and C which can completely eliminate the reflux power of the double-active bridge or reduce the reflux power and keep the transmission power unchanged in a normal working state based on the reflux power and per unit transmission power analysis.

2. The method of claim 1, wherein the circuit characteristic in step S2 is that the current flowing through the dual active bridge is different, and the voltage level and the current direction across the inductor are different.

3. The method for suppressing backflow power of a dual-active bridge based on modal analysis according to claim 1, wherein the step S2 is to use the defined dual-active bridge power backflow mode and power transmission mode to replace each operation mode of the original operation process of each operation mode of the dual-active bridge, so as to reconstruct the operation process of multiple operation modes of the dual-active bridge.

4. The method for suppressing backflow power of a dual-active bridge based on modal analysis according to claim 1, wherein the operating modes of the dual-active bridge re-classified in step S45 and the limiting conditions of each mode classification are as follows:

mode I:i ₀`≥0，i ₁the gradient is more than or equal to 0; the classification limitation condition is

，

；

Mode II:i ₀`＜0，i ₁the gradient is more than 0; the classification limitation condition is

And 0<M₁<C，C∈[0，1]；

Mode III:i ₀`＜0，i ₁the strain is less than 0; the classification limitation condition is

，

。

5. The method for suppressing backflow power of a dual-active bridge based on modal analysis as claimed in claim 4, wherein M in step S47 enables backflow power of the dual-active bridge to be reduced and transmission power to be unchanged under normal operating conditionsThe value ranges of 1 and C are as follows:C∈[M ₀`,1]，M <sub>1</sub>∈[M <sub>0</sub>,M <sub>0</sub>`](ii) a Wherein the content of the first and second substances,M ₀andM ₀under the condition of unchanged transmission power when the phase is controlled by single phase shiftM ₁There are two solutions to the effect that,M ₀∈[0,0.5]，M ₀`=(1-M ₀)∈[0.5,1](ii) a The value ranges of M1 and C which can completely eliminate the return power of the double-active bridge and keep the transmission power unchanged are the values of M1 and C when the double-active bridge works under the condition of zero return power.

6. The method as claimed in claim 5, wherein the determination of the values of M1 and C in step S47 is performed by selecting any C and then obtaining M1 according to the transmission power expression or selecting any M1 and then obtaining C according to the transmission power expression, within the range of values of C and M1 that reduces or eliminates the backflow power of the dual-active bridge during the single-phase shift control.

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