Frequency conversion-phase shift hybrid control method based on bidirectional LLC resonant converter

Abstract

The invention discloses a control method for a bidirectional LLC resonant converter, and the method comprises the following steps: firstly, judging the working mode of the bidirectional LLC resonant converter, and if the working mode is a forward mode, if the voltage gain is Mgt, judging that the voltage gain is Mgt; if the voltage gain is 1, a frequency conversion control mode is adopted; 1, a phase shift control mode is adopted; when the working mode is a reverse mode, because the excitation inductor is always clamped by a reverse output potential, the converter is always in a voltage gain Mlt; for the state of 1, a control method of primary side frequency conversion and secondary side phase shift is provided to realize reverse boosting, if the voltage gain is Mgt in a reverse working mode; if 1, a phase shift-frequency conversion boost control mode is adopted, and if the voltage gain of the input voltage is Mlt; 1, a phase shift voltage reduction control mode is adopted, ZVS and ZCS soft switching can be realized through forward frequency modulation and phase shift, the system loss is greatly reduced, and the system efficiency is improved;

Description

Technical Field

[0001] The invention relates to the technical field of bidirectional LLC resonant converter modulation, and in particular to an improved frequency conversion-phase shifting hybrid control method of an LLC resonant converter. Background Art

[0002] The dual LLC resonant converter is widely used in distributed power generation, electric vehicles, microgrid energy storage systems, and smart grids due to its simple structure, wide voltage gain range, and good soft switching characteristics.

[0003] Resonant DC / DC converters can achieve ZVS of the primary switch tube and ZCS of the secondary switch tube in a wide voltage range and a wide load range, which can reduce losses and have high efficiency. Among them, LLC bidirectional resonant converters have been widely studied due to their simple circuit structure and easy soft switching. However, the voltage gain adjustable range of the traditional LLC bidirectional resonant converter is not wide, and it is subject to many restrictions when used in wide input voltage or wide output voltage applications. And when working in reverse, it is equivalent to an LC series resonant converter, the resonant state is inconsistent with the forward operation, the voltage gain is always less than 1, and the boost cannot be achieved.

[0004] In view of the problem that the voltage gain of LLC resonant converter is always less than 1 when it is running in reverse, and the single modulation system based on PI regulator is difficult to achieve fast dynamic response, which affects the performance and stability of the converter. The above-mentioned problems limit the transmission efficiency of LLC resonant converter, affect the system stability of LLC resonant converter, and limit the application scope of LLC resonant converter. It is necessary to improve and adjust it. Summary of the invention

[0005] The purpose of the present invention is to propose a frequency conversion-phase shifting hybrid control method to solve the problem that the voltage gain of the LLC resonant converter is always less than 1 during reverse operation and the single modulation method is difficult to respond quickly dynamically.

[0006] In order to achieve the above object, the technical solution of the present invention is: a frequency conversion-phase shift hybrid control method based on a bidirectional LLC resonant converter, comprising the following steps:

[0007] Step 1: Establish a model of a bidirectional LLC resonant converter, analyze the principle of a frequency conversion-phase shifting hybrid control method based on the bidirectional LLC resonant converter, and derive the working characteristics of the bidirectional LLC resonant converter under the frequency conversion-phase shifting hybrid control method.

[0008] Step 2: According to the working characteristics of the bidirectional LLC resonant converter, establish the voltage and current dual closed-loop control model of the bidirectional LLC resonant converter during forward operation and the voltage and current dual closed-loop control model of the bidirectional LLC resonant converter during reverse operation. 0 and the actual V o_ref It is the input signal of the voltage loop PI controller, referring to the output current I 0 The output signal of the voltage loop PI controller is the input signal of the current loop PI controller. 1 With S 4 , S 2 With S 3 The output voltage is controlled by changing the switching frequency of the switch tube S in the phase shift mode. 1 With S 4 , S 2 With S 3 The output voltage is controlled by the phase shift angle.

[0009] Step 3: Reference output voltage V during reverse operation in and the actual V in_ref It is the input signal of the voltage loop PI controller, referring to the output current I in The output signal of the voltage loop PI controller is the input signal of the current loop PI controller. In the variable frequency phase shift hybrid control mode, the secondary side switch tube S is controlled 1 With S 5 , S 8 The switch tube S is turned on by staggering a phase shift angle. 3 and S 1 Complementary conduction, the secondary switch tube S 6 , S 7 The conduction mode remains unchanged and the variable frequency control mode is used. 5 , S 8 The output voltage is controlled by complementary conduction. In the phase-shift control mode, the switch S 5 With S 8 , S 6 With S 7 The output voltage is controlled by the phase shift angle.

[0010] Step 4: Determine whether the working mode of the bidirectional LLC resonant converter is the forward mode or the reverse mode. If the working mode is the forward mode, further calculate the voltage gain of the bidirectional LLC resonant converter. If the voltage gain M>1, the frequency conversion control method is adopted. If the voltage gain M<1, the phase shift control method is adopted. If the working mode is the reverse mode, the working state of the bidirectional LLC resonant converter will always be in the state of voltage gain M<1. To solve this problem, a hybrid control method of primary-side frequency conversion and secondary-side phase shift is proposed to achieve reverse boost.

[0011] Step 5: First determine the voltage gain of the bidirectional LLC resonant converter in the reverse working mode. If it works in a lower voltage gain mode, a phase shift control method is used. If it works in a higher voltage gain mode, a frequency conversion-phase shift hybrid control method is used. At this time, the working mode switching point is selected as a voltage value. If the input voltage is between 250 and 320V, a phase shift-frequency conversion boost control method is used. If the input voltage is between 320 and 430V, the control method of the bidirectional LLC resonant converter is a phase shift buck control method.

[0012] Furthermore, the step 1 comprises the following steps:

[0013] Step 1.1: Find the expression of the transfer function of the LLC resonant converter:

[0014]

[0015] Among them, V AB_FHA(t) V is the square wave fundamental component of the input DC bus voltage after full-bridge inversion. CD_FHA(t) is the square wave fundamental component of the output voltage after full-bridge rectification, L r , C r is the value of the capacitance and inductance to the primary side of the transformer, R eq is the equivalent load converted to the original side;

[0016] Step 1.2: Find the resonant frequency when the voltage across Lm is clamped by the output voltage. The first resonant frequency is:

[0017]

[0018] Step 1.3: Find the resonant frequency when Lm is no longer clamped by the output voltage. The second resonant frequency is:

[0019]

[0020] Furthermore, the step 2 specifically includes the following steps:

[0021] Step 2.1: Calculate the voltage gain expression of the LLC resonant converter in forward operation:

[0022]

[0023] Where, k is the inductance coefficient, Q is the quality factor; f n is the normalized frequency, f s is the actual switching frequency;

[0024] Step 2.2: When the voltage gain M>1, the frequency conversion control method is adopted; when the voltage gain M<1, the phase shift control method is adopted;

[0025] Step 2.3: Derive the process by which the control mode of the LLC resonant converter changes with the voltage gain during forward operation.

[0026] Furthermore, the step 3 specifically includes the following steps:

[0027] Step 3.1: Calculate the voltage gain expression of the LLC resonant converter in reverse operation:

[0028]

[0029] Step 3.2: At the resonant frequency point of the LLC resonant converter, the voltage gain of the converter is always 1, regardless of the changes in circuit parameters and load. The rated operating point is usually set here, so the turns ratio n of the transformer can be obtained as:

[0030]

[0031] The forward input rated voltage V in The rated output voltage is 400V and 320V;

[0032] Step 3.3: Calculate the voltage switching point when the LLC resonant converter is in reverse operation:

[0033]

[0034] Where n is the ratio of the primary side to the secondary side of the transformer, and the reverse input rated voltage V in The rated output voltage is 320V and 400V;

[0035] Step 3.4: The 320V input voltage is the switching voltage of the converter working mode. When the reverse input voltage is in the range of 250-320V, the voltage gain is always greater than 1. When the reverse input voltage is in the range of 320-430V, the voltage gain is always less than 1.

[0036] Step 3.5: Derive the process by which the control mode of the LLC resonant converter changes with the voltage gain when it is in reverse operation.

[0037] Compared with the prior art, the present invention has the following advantages and positive effects:

[0038] By improving the control method of the bidirectional LLC resonant converter, the problem that the voltage gain is always less than 1 when the converter is running in reverse is solved. In addition, the forward frequency conversion and phase shift can achieve ZVS and ZCS soft switching; the reverse LC frequency modulation can achieve ZVS soft switching, and the LC phase shift leading bridge can achieve ZVS soft switching. The voltage and power can meet the requirements. The system loss is greatly reduced and the system efficiency is improved.

[0039] It broadens the practical application scope of LLC resonant converter and makes certain contributions to the future development of LLC resonant converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0041] Figure 1 It is the overall control strategy block diagram of the bidirectional LLC resonant converter;

[0042] Figure 2 This is the equivalent circuit diagram of the bidirectional LLC resonant converter when it is running in the forward direction;

[0043] Figure 3 It is the equivalent circuit diagram of the bidirectional LLC resonant converter when it is running in reverse direction;

[0044] Figure 4 The voltage gain diagram of the bidirectional LLC resonant converter under the variable frequency-phase shift control in forward operation;

[0045] Figure 5 The voltage gain diagram of the bidirectional LLC resonant converter under reverse operation variable frequency-phase shift control;

[0046] Figure 6 It is the waveform diagram of main variables under the forward phase shift control of bidirectional LLC resonant converter;

[0047] Figure 7 It is the waveform diagram of main variables under forward frequency conversion control of bidirectional LLC resonant converter;

[0048] Figure 8 It is the waveform diagram of main variables under reverse frequency conversion-phase shift control of bidirectional LLC resonant converter;

[0049] Fig. 9 This is the waveform diagram of the main variables under the reverse phase-shift control of the bidirectional LLC resonant converter.

Claims

1. A frequency conversion + phase shift hybrid control method based on a bidirectional LLC resonant converter, characterized in that: The following steps are involved: Step 1: Establish a model of a bidirectional LLC resonant converter, analyze the principle of a frequency conversion-phase shift hybrid control method based on the bidirectional LLC resonant converter, and derive the working characteristics of the bidirectional LLC resonant converter under the frequency conversion-phase shift hybrid control method; Step 2: According to the working characteristics of the bidirectional LLC resonant converter, a voltage-current dual closed-loop control model of the bidirectional LLC resonant converter during forward operation and a voltage-current dual closed-loop control model of the bidirectional LLC resonant converter during reverse operation are established. As shown in Figure 1, the reference output voltage V0 and the actual V o_ref The reference output current I0 and the output signal of the voltage loop PI controller are the input signals of the current loop PI controller. In the variable frequency mode, the output voltage is controlled by changing the switching frequency of the switch tubes S1 and S4, S2 and S3. In the phase shift mode, the output voltage is controlled by changing the phase shift angle of the switch tubes S1 and S4, S2 and S3. Step 3: Reference output voltage V during reverse operation in and the actual V in_ref It is the input signal of the voltage loop PI controller, referring to the output current I in The output signal of the voltage loop PI controller is the input signal of the current loop PI controller. In the variable frequency phase shift hybrid control mode, the secondary side switch tubes S1 and S5, S8 are turned on by staggering a phase shift angle, the switch tubes S3 and S1 are turned on complementary, and the secondary side switch tubes S6 and S7 are turned on in a variable frequency control mode, and the output voltage is controlled by the switch tubes S5 and S8. In the phase shift control mode, the output voltage is controlled by changing the phase shift angles of the switch tubes S5 and S8, and S6 and S7; Step 4: Determine whether the working mode of the bidirectional LLC resonant converter is the forward mode or the reverse mode. If the working mode is the forward mode, further calculate the voltage gain of the bidirectional LLC resonant converter. If the voltage gain M>1, use the frequency conversion control method. If the voltage gain M<1, use the phase shift control method. If the working mode is the reverse mode, the working state of the bidirectional LLC resonant converter will always be in the state of voltage gain M<1. To solve this problem, a hybrid control method of primary-side frequency conversion and secondary-side phase shift is proposed to achieve reverse boost. Step 5: First determine the voltage gain of the bidirectional LLC resonant converter in the reverse working mode. If it works in a lower voltage gain mode, a phase shift control method is used. If it works in a higher voltage gain mode, a frequency conversion-phase shift hybrid control method is used. At this time, the working mode switching point is selected as a voltage value. If the input voltage is between 250 and 320V, a phase shift-frequency conversion boost control method is used. If the input voltage is between 320 and 430V, the control method of the bidirectional LLC resonant converter is a phase shift buck control method.

2. The operating characteristics of the frequency conversion and phase shift control based on LLC resonant converter according to claim 1 are analyzed using fundamental wave analysis method, characterized in that: The specific process of step 1 is as follows: Step 1.1: Find the expression of the transfer function of the LLC resonant converter: Among them, V AB_FHA(t) V is the square wave fundamental component of the input DC bus voltage after full-bridge inversion. CD_FHA(t) is the square wave fundamental component of the output voltage after full-bridge rectification, L r , C r is the value of the capacitance and inductance to the primary side of the transformer, R eq is the equivalent load converted to the original side; Step 1.2: Find the resonant frequency when the voltage across Lm is clamped by the output voltage. The first resonant frequency is: Step 1.3: Find the resonant frequency when Lm is no longer clamped by the output voltage. The second resonant frequency is:

3. The operating characteristics of the frequency conversion and phase shift control based on LLC resonant converter according to claim 2 are analyzed using fundamental wave analysis method, characterized in that: The specific process of step 2 is as follows: Step 2.1: Calculate the voltage gain expression of the LLC resonant converter in forward operation: Where, k is the inductance coefficient, Q is the quality factor; f n is the normalized frequency, f s is the actual switching frequency; Step 2.2: When the voltage gain M>1, the frequency conversion control method is adopted; when the voltage gain M<1, the phase shift control method is adopted; Step 2.3: Derive the process by which the control mode of the LLC resonant converter changes with the voltage gain during forward operation.

4. The reverse working characteristic analysis based on LLC resonant converter according to claim 3, characterized in that: The specific process of step 3 is as follows: Step 3.1: Calculate the voltage gain expression of the LLC resonant converter in reverse operation: Step 3.2: At the resonant frequency point of the LLC resonant converter, the voltage gain of the converter is always 1, regardless of the changes in circuit parameters and load. The rated operating point is usually set here, so the turns ratio n of the transformer can be obtained as: The forward input rated voltage V in The rated output voltage is 400V and 320V; Step 3.3: Calculate the voltage switching point when the LLC resonant converter is in reverse operation: Where n is the ratio of the primary side to the secondary side of the transformer, and the reverse input rated voltage V in The rated output voltage is 320V and 400V; Step 3.4: The 320V input voltage is the switching voltage of the converter working mode. When the reverse input voltage is in the range of 250-320V, the voltage gain is always greater than 1. When the reverse input voltage is in the range of 320-430V, the voltage gain is always less than 1. Step 3.5: Derive the process by which the control mode of the LLC resonant converter changes with the voltage gain when it is in reverse operation.

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