LLC resonant converter feedforward control method based on state trajectory analysis

Abstract

The invention discloses an LLC resonant converter feedforward control method based on state trajectory analysis. The LLC resonant converter feedforward control method comprises the following steps: S1, establishing a differential equation of an LLC resonant converter in a phase shift modulation typical working mode; s2, drawing a state trajectory equation of the LLC resonant converter in a positive half period under phase shift modulation, and drawing a phase plane curve graph; s3, combining and reasonably simplifying the state trajectory and geometric calculation, and estimating a phase shift angle; s4, the phase shift angle is fed forward to PI control output, and a pulse is generated and sent to the LLC resonant converter, the complex calculation process of solving a transcendental equation is avoided, state track jumping from heavy load to light load is achieved, and the transient control performance of the LLC resonant converter is optimized.

Description

Technical Field

[0001] The invention belongs to the technical field of LLC resonant converter control, and in particular relates to a feedforward control method of an LLC resonant converter based on state trajectory analysis. Background Art

[0002] In new energy ships, traditional fossil fuels are partially or completely replaced by new energy and power batteries. Integrating new energy into the ship power system effectively reduces fuel consumption, significantly improves environmental friendliness, and greatly improves the ship's endurance. Therefore, electric ships that integrate new energy and batteries are recognized as an important part of the future shipping industry. Energy storage converters are used in new energy ship power systems to transmit electricity from new energy to power batteries. Among all isolated DC / DC converters, LLC resonant converters show advantages in soft switching operation range and power density, making them more suitable for application scenarios of new energy ship power distribution.

[0003] New energy ship power systems face complex surface or underwater operating environments, and their propulsion loads change rapidly and are difficult to predict. The existing variable frequency modulation is characterized by simple implementation and high technical maturity. However, this method requires a large range of switching frequency adjustment, which brings challenges such as low utilization of magnetic components, difficult design of resonant circuit parameters, and large reactive power. In this regard, phase-shift modulation can be used to circumvent the above problems. In order to improve the dynamic response capability of the LLC resonant converter, researchers have proposed a series of LLC resonant converter state trajectory optimization control strategies based on state trajectory, but they focus on variable frequency modulation strategies, and there is no state trajectory control based on phase-shift modulation. Summary of the invention

[0004] The object of the present invention is to provide a feedforward control method for an LLC resonant converter based on state trajectory analysis to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A feedforward control method for an LLC resonant converter based on state trajectory analysis comprises the following steps:

[0007] S1. Establish the differential equation of LLC resonant converter in the typical working mode of phase shift modulation;

[0008] S2. Draw the state trajectory equation of the LLC resonant converter in the positive half cycle under phase shift modulation, and draw the phase plane curve diagram;

[0009] S3, after combining the state trajectory with the geometric calculation and making reasonable simplifications, the phase shift angle is estimated;

[0010] S4, feed forward the phase shift angle to the PI control output and generate pulses to send to the LLC resonant converter.

[0011] Preferably, in S1:

[0012] Ignoring the influence of dead time, the LLC resonant converter under time-shift phase modulation is divided into six different modes: P mode, Px mode, N mode, Nx mode, O mode and Ox mode. Each mode corresponds to different conditions of the input voltage, output voltage and resonant state of the resonant tank. The differential equation is as follows:

[0013] In P mode, the differential equation is:

[0014]

[0015] In the Px mode, the differential equation is:

[0016]

[0017] In the Ox mode, the differential equation is:

[0018]

[0019] Among them, Lr, Cr and Lm are the resonant inductor, resonant capacitor and excitation inductor respectively, Tr is the high-frequency transformer, S1~S4 and D1~D4 are the switching devices and diodes on the primary side and secondary side respectively, VI and VO are the input voltage and output voltage, iLr, vCr and iLm are the resonant inductor current, resonant capacitor voltage and excitation inductor current respectively, ip and is are the currents on both sides of the transformer respectively.

[0020] Preferably, in S2, the state trajectory equation of the LLC resonant converter in P mode is:

[0021]

[0022] The state trajectory equation of LLC resonant converter in Px mode is:

[0023]

[0024] The state trajectory equation of LLC resonant converter in Ox mode is:

[0025]

[0026] Where vCrn and iLrn represent the per-unit values ​​of vCr and iLr respectively, VCr0n_S and ILr0n_S represent vCrn and iLrn at the initial moment of S mode, S∈{P,Px,Ox}, k represents the ratio of excitation inductance to resonant inductance, that is, k=Lm / Lr, M represents the converter voltage gain, that is, M=nVO / VI, and n is the transformer turns ratio.

[0027] Preferably, in S3, since Lm>>Lr, the resonant current in the Ox mode is much smaller than the resonant current in the P mode and the Px mode, the state trajectory of Ox is simplified to a straight line on the horizontal axis, and the phase shift angle ∠AOPB is calculated:

[0028]

[0029] Where OPOPx is the length between points OP and OPx in the simplified trajectory diagram, ILmn is the per-unit value of the excitation inductor current, ρP and ρPx are the state trajectory circle radii corresponding to the P mode and Px mode respectively.

[0030] Technical effects and advantages of the present invention:

[0031] The present invention analyzes the state trajectory of the LLC resonant converter under phase-shift modulation, and avoids the complex calculation process of solving transcendental equations through differential equations, trajectory equations and shift angles, realizes the state trajectory jump from heavy load to light load, and optimizes the transient control performance of the LLC resonant converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flowchart of the control method of the present invention;

[0033] Figure 2 It is the specific structure diagram of LLC resonant converter;

[0034] Figure 3 This is the resonance tank waveform diagram of PPxOx working mode;

[0035] Figure 4 is the P-mode state trajectory;

[0036] Figure 5 is the Px modal state trajectory;

[0037] Figure 6 is the Ox modal state trajectory;

[0038] Figure 7 The state trajectory of the LLC resonant converter in the PPxOx working mode under different loads;

[0039] Figure 8 This is a schematic diagram of equivalent calculation of phase shift angle;

[0040] Fig. 9 To simplify the state trajectory diagram;

[0041] Fig.10 is the deviation between the feedforward estimate and the actual displacement;

[0042] Fig.11 This is the control block diagram for adding feedforward control;

[0043] Fig.12 Comparison of transient performance between PI control and the proposed control;

[0044] Fig.13 This is the state trajectory diagram of the dynamic process of unloading 50%. DETAILED DESCRIPTION

[0045] The following will be combined with the attached embodiment of the present invention Figure 1-Figure 13 , the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0046] Example 1

[0047] like Figure 1 As shown, a feedforward control method for LLC resonant converter based on state trajectory analysis includes the following steps:

[0048] S1. Establish the differential equation of LLC resonant converter in the typical working mode of phase shift modulation;

[0049] Ignoring the influence of dead time, the LLC resonant converter under time-shift phase modulation is divided into six different modes: P mode, Px mode, N mode, Nx mode, O mode and Ox mode.

[0050] P mode (Primary Switching Mode):

[0051] In P mode, the main switch (usually the upper switch of a half-bridge or full-bridge) is turned on, and energy is transferred from the input source to the resonant cavity through the switch and the excitation inductor.

[0052] Px mode (Primary Switching with Body Diode Conduction Mode):

[0053] The Px mode involves the body diode of the switch being turned on, which usually occurs when the switch is turned off. Due to the need for current continuity, the body diode will be turned on briefly.

[0054] N mode (Negative Cycle Mode):

[0055] N mode refers to the stage in half a switching cycle when the current direction in the resonant cavity is opposite to that in the P mode. At this time, the secondary rectifier diode is turned on and energy is transferred from the resonant cavity to the output.

[0056] Nx mode (Negative Cycle with Body Diode Conduction Mode):

[0057] The Nx mode is similar to the Px mode, but occurs when the current direction is the same as the N mode, involving the body diode conduction of the secondary side switch.

[0058] O mode (Off-state Mode):

[0059] In the O mode, all main switches are in the off state, and the energy in the resonant cavity is transferred through parasitic parameters (such as the parasitic capacitance of the switch tube and the leakage inductance of the transformer).

[0060] Ox mode (Off-state with Body Diode Conduction Mode):

[0061] The Ox mode refers to the situation in which, based on the O mode, the energy is transferred in an unexpected manner due to the conduction of the switch body diode.

[0062] Each mode corresponds to different conditions of the input voltage, output voltage and resonant state of the resonant tank. Since the PPxOx working mode allows the LLC resonant converter to maintain zero voltage switching on the primary side and zero current switching on the secondary side under non-light load conditions, the PPxOx operating mode is adopted. The differential equation is as follows:

[0063] In P mode, the differential equation is:

[0064]

[0065] In the Px mode, the differential equation is:

[0066]

[0067] In the Ox mode, the differential equation is:

[0068]

[0069] Among them, Lr, Cr and Lm are the resonant inductor, resonant capacitor and excitation inductor respectively, Tr is the high-frequency transformer, S1~S4 and D1~D4 are the switching devices and diodes on the primary side and secondary side respectively, VI and VO are the input voltage and output voltage, iLr, vCr and iLm are the resonant inductor current, resonant capacitor voltage and excitation inductor current respectively, ip and is are the currents on both sides of the transformer respectively.

[0070] S2. Draw the state trajectory equation of the LLC resonant converter in the positive half cycle under phase shift modulation, and draw the phase plane curve diagram. The state trajectory equation of the LLC resonant converter in P mode is:

[0071]

[0072] The state trajectory equation of LLC resonant converter in Px mode is:

[0073]

[0074] The state trajectory equation of LLC resonant converter in Ox mode is:

[0075]

[0076] Where vCrn and iLrn represent the per-unit values ​​of vCr and iLr respectively, VCr0n_S and ILr0n_S represent vCrn and iLrn at the initial moment of S mode, S∈{P,Px,Ox}, k represents the ratio of excitation inductance to resonant inductance, that is, k=Lm / Lr, M represents the converter voltage gain, that is, M=nVO / VI, and n is the transformer turns ratio.

[0077] S3. After combining the state trajectory with geometric calculation and reasonably simplifying it, the phase shift angle is estimated. Since Lm>>Lr, the resonant current in the Ox mode is much smaller than the resonant current in the P mode and the Px mode. The state trajectory of Ox is simplified to a straight line on the horizontal axis, and the phase shift angle ∠AOPB is calculated:

[0078]

[0079] Where OPOPx is the length between points OP and OPx in the simplified trajectory diagram, ILmn is the per-unit value of the excitation inductor current, ρP and ρPx are the state trajectory circle radii corresponding to the P mode and Px mode respectively.

[0080] S4, feed forward the phase shift angle to the PI control output and generate pulses to send to the LLC resonant converter.

[0081] Figure 2The specific structure diagram of LLC resonant converter is given, where Cin and Cout are the input and output filter capacitors respectively. Lr, Cr and Lm are the resonant inductor, resonant capacitor and excitation inductor respectively. Tr is the high-frequency transformer. S1~S4 and D1~D4 are the switching devices and diodes on the primary and secondary sides respectively. VI and VO are the input voltage and output voltage. iLr, vCr and iLm are the resonant inductor current, resonant capacitor voltage and excitation inductor current respectively. ip and is are the transformer currents on both sides respectively.

[0082] Figure 3 The resonant tank waveform of the LLC resonant converter in a typical PPxOx working mode is given, where t1, t2 and t3 are the start times of the P, Px and Ox modes respectively.

[0083] In the PPxOx working mode, the per-unit values ​​of the initial values ​​of the state variables in each mode are VCr0n_P, VCr0n_Px, VCr0n_Ox and ILr0n_P, ILr0n_Px, ILr0n_Ox respectively.

[0084] In P mode, the time domain state equation is:

[0085] (1)

[0086] The state equation of LLC resonant converter in P mode is:

[0087] (2)

[0088] Figure 4 It is the phase plane of LLC resonant converter in P mode.

[0089] In the Px mode, the time domain state equation is:

[0090] (3)

[0091] The state equation of LLC resonant converter in Px mode is:

[0092] (4)

[0093] Figure 5 It is the phase plane of LLC resonant converter in Px mode.

[0094] In the Ox mode, the time domain state equation is:

[0095] (5)

[0096] The state equation of LLC resonant converter in Ox mode is:

[0097] (6)

[0098] Figure 6 It is the phase plane of LLC resonant converter in Ox mode.

[0099] Figure 7 In order to combine the state trajectories of the above three modes, the state trajectory of the LLC resonant converter in the positive half-cycle is plotted into a plane curve diagram. For clarity, only the complete state trajectories of each mode in the positive half-cycle are drawn in the figure, while only the actual operation state trajectory of the negative half-cycle is drawn. It can be seen that the state trajectories are different under different loads, and the state trajectory of heavy load wraps up the state trajectory of light load.

[0100] Figure 8 This is a typical state trajectory curve under the PPxOx working mode, where OP and OPx represent the center of the state trajectory corresponding to the P mode and Px mode, respectively, A and B represent the initial state point and final state point of the P mode, respectively, and C and D are the projections of A and B on the horizontal axis. Under the PPxOx working mode, the angle corresponding to the P mode is the phase shift angle, so to realize feedforward control based on state trajectory analysis, it is only necessary to calculate the size of ∠AOPB in the figure.

[0101] make:

[0102] (7)

[0103] Obviously there are:

[0104] (8)

[0105] The present invention proposes a calculation method combining state trajectory with geometric calculation to simplify the calculation and improve the calculation accuracy. Considering Lm>>Lr, the resonant current in the Ox mode is much smaller than the resonant current in the P mode and the Px mode.

[0106] Fig. 9 is the simplified Ox state trajectory, which is a straight line on the horizontal axis. In the figure, θ1 corresponds to the phase shift angle. Points A, B and C in the figure correspond to the initial points of modes P, Px and Ox respectively. Combined with the state trajectory circle expression, we can get;

[0107] (9)

[0108] Considering the symmetry of the half-cycle waveform, we have

[0109] (10)

[0110] in:

[0111] (11)

[0112] Since Lm>>Lr, it can be considered that iLm is always a constant value ILm under mode Ox, so the full-cycle waveform of iLm is simplified to a trapezoidal wave. At the same time, in order to simplify the analysis, it is assumed that the duration of mode Px is consistent with that of mode Ox, then the expression of ILm is:

[0113] (12)

[0114] and then

[0115] (13)

[0116] Therefore, combining (10), (12) and (13) we have

[0117] (14)

[0118] Considering the conservation of energy,

[0119] (15)

[0120] Substituting formula (7) into (12) and (13), we can obtain:

[0121] (16)

[0122] Next, combine Fig.10 The geometric relationship shown,

[0123] (17)

[0124] Among them, according to the state trajectory center expression, OPOPx=1.

[0125] Iteratively calculate the feedforward expression of the phase shift angle:

[0126] (18)

[0127] Fig.10 Figure 2 is the relationship between the steady-state value of the feedforward quantity and the steady-state value of the displacement under different working conditions. It can be seen that the maximum error within the working condition range shown in the figure is 5.89%, which is within the allowable range.

[0128] Fig.11 This is the block diagram of the feedforward control strategy based on state trajectory analysis proposed in the present invention. First, the state trajectory corresponding to the P mode and Px mode is combined with the geometric calculation and reasonably simplified to estimate the phase shift angle. Then, the phase shift angle is fed forward to the PI control output and a pulse is generated and sent to the LLC resonant converter for control.

[0129] Fig.12In order to compare the effects of the proposed traditional PI control strategy and the proposed feedforward control strategy under transient conditions of load addition and reduction, the two strategies use the same PI parameters, kP=0.005, kI=5, and the load power switches between 1kW and 2kW. After adding feedforward control, the converter voltage pulsation is reduced due to the application of additional feedforward control, and it can converge more quickly. The adjustment time here is the time for the voltage to converge to the steady-state value of ±1V.

[0130] Fig.13 The state trajectory changes of the load addition and reduction process are shown respectively for pure PI control and after adding feedforward control. It can be seen that after adding feedforward control, the state trajectory jump from heavy load to light load is realized, the gradual approach process is avoided, and the dynamic performance of the LLC resonant converter is optimized.

[0131] In summary, the present invention analyzes the state trajectory of the LLC resonant converter under phase-shift modulation, and avoids the complex calculation process of solving transcendental equations through differential equations, trajectory equations and shift angles, thereby achieving a state trajectory jump from heavy load to light load, and optimizing the transient control performance of the LLC resonant converter.

[0132] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A feedforward control method for LLC resonant converter based on state trajectory analysis, characterized in that: The following steps are involved: S1. Establish the differential equation of LLC resonant converter in the typical working mode of phase shift modulation; S2. Draw the state trajectory equation of the LLC resonant converter in the positive half cycle under phase shift modulation, and draw the phase plane curve diagram; S3, after combining the state trajectory with the geometric calculation and making reasonable simplifications, the phase shift angle is estimated; S4, feed forward the phase shift angle to the PI control output and generate pulses to send to the LLC resonant converter.

2. The LLC resonant converter feedforward control method based on state trajectory analysis according to claim 1, characterized in that: In S1: Ignoring the influence of dead time, the LLC resonant converter under time-shift phase modulation is divided into six different modes: P mode, Px mode, N mode, Nx mode, O mode and Ox mode. Each mode corresponds to different conditions of the input voltage, output voltage and resonant state of the resonant tank. The differential equation is as follows: In P mode, the differential equation is: In the Px mode, the differential equation is: In the Ox mode, the differential equation is: Among them, Lr, Cr and Lm are the resonant inductor, resonant capacitor and excitation inductor respectively, Tr is the high-frequency transformer, S1~S4 and D1~D4 are the switching devices and diodes on the primary side and secondary side respectively, VI and VO are the input voltage and output voltage, iLr, vCr and iLm are the resonant inductor current, resonant capacitor voltage and excitation inductor current respectively, ip and is are the currents on both sides of the transformer respectively.

3. The LLC resonant converter feedforward control method based on state trajectory analysis according to claim 1, characterized in that: In S2, the state trajectory equation of the LLC resonant converter in P mode is: The state trajectory equation of LLC resonant converter in Px mode is: The state trajectory equation of LLC resonant converter in Ox mode is: Where vCrn and iLrn represent the per-unit values ​​of vCr and iLr respectively, VCr0n_S and ILr0n_S represent vCrn and iLrn at the initial moment of S mode, S∈{P,Px,Ox}, k represents the ratio of excitation inductance to resonant inductance, that is, k=Lm / Lr, M represents the converter voltage gain, that is, M=nVO / VI, and n is the transformer turns ratio.

4. The LLC resonant converter feedforward control method based on state trajectory analysis according to claim 1, characterized in that: In S3, since Lm>>Lr, the resonant current in the Ox mode is much smaller than the resonant current in the P mode and the Px mode. The state trajectory of Ox is simplified to a straight line on the horizontal axis, and the phase shift angle ∠AOPB is calculated: Where OPOPx is the length between points OP and OPx in the simplified trajectory diagram, ILmn is the per-unit value of the excitation inductor current, ρP and ρPx are the state trajectory circle radii corresponding to the P mode and Px mode respectively.

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