Three-degree-of-freedom control method for resonant dc / dc converter based on cllc topology

Abstract

The application discloses a three-freedom-degree control method of a resonant DC / DC converter based on a CLLC topology, which comprises three-freedom-degree adjustment of bus voltage, frequency and phase-shifting angle, double-loop control of a voltage outer loop and a current inner loop, so as to realize control and switching of three working modes of constant voltage mode CV, constant current mode CC and constant power mode CP of battery charging. The application introduces bus voltage adjustment on the basis of frequency adjustment control and phase-shifting control, can reduce the frequency adjustment range and the phase-shifting angle, thereby overcoming the problem of large resonant cavity current caused by phase-shifting control, and realizing wide voltage gain.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics, in particular to a control method of a resonant DC / DC converter based on CLLC topology. BACKGROUND

[0002] With the aggravation of energy shortage and environmental problems, the number of electric vehicles continues to rise, and the on-board charger (OBC) as the core component connecting the vehicle battery and the power grid is particularly important in electric vehicle charging. Among them, the isolated DC / DC converter is an important part of the OBC, and the resonant DC / DC converter with high power density, high efficiency and wide voltage gain characteristics is most widely used.

[0003] CLLC is the mainstream topology of isolated resonant DC / DC converter, and its characteristics are: zero voltage turn-on at high frequency; can realize step-up and step-down, wide gain range; under suitable resonant cavity parameter design conditions, the gain characteristics are basically the same in forward and reverse directions. CLLC topology is the most advantageous topology for the DC / DC circuit in the later stage of the on-board charger. The traditional CLLC control follows the frequency modulation control of LLC, and the monotonic frequency has the disadvantages of large switching frequency range and small gain range at light load. In view of this problem, many academic papers and invention patents have analyzed and proposed solutions, for example: "A modulation method for CLLC bidirectional isolated DC-DC converter: 201910148780.7", Xi'an Jiaotong University, 2019.02.28, proposes a hybrid control strategy by introducing phase shift modulation based on frequency modulation, which increases the gain range and has the advantage of high efficiency at light load. However, under phase shift modulation, the problem of large resonant cavity current caused by large gain and phase shift angle may occur. In addition, for vehicle battery charging, different working modes in the battery charging curve are not considered, which cannot meet the demand of vehicle battery charging and the problem of large voltage and current overshoot during working mode switching in single-loop control. SUMMARY

[0004] The purpose of the present application is to solve the problems in the prior art and propose a three-degree-of-freedom control method of a resonant DC / DC converter based on CLLC topology, in order to introduce input bus voltage regulation as the third degree of freedom under the hybrid control strategy of frequency modulation control and phase shift control, to reduce the frequency modulation range and phase shift angle, overcome the problem of large resonant cavity current caused by phase shift control, and realize wide voltage gain. At the same time, voltage outer loop and current inner loop double-loop control are adopted to realize the control and switching of constant voltage mode CV, constant current mode CC and constant power mode CP three working modes of battery charging.

[0005] In order to achieve the above-mentioned application purposes, the technical scheme adopted by the present application is as follows:

[0006] The application discloses a three-degree-of-freedom control method of a resonant DC / DC converter based on a CLLC topology, the resonant DC / DC converter is a post-circuit of an on-board charger (OBC) and is used for charging an on-board battery, and the CLLC topology of the resonant DC / DC converter comprises an inverter network, a resonant cavity and a rectifier network.

[0007] Step 1: selecting an input bus voltage of the resonant DC / DC converter in a current period according to an on-board battery voltage;

[0008] Step 2: selecting a working mode of the resonant DC / DC converter in the current period according to the on-board battery voltage, and comparing the working mode with a working mode in a previous period, if the working modes are the same, executing step 3, or executing working mode switching.

[0009] Step 3: acquiring an actual output voltage or / and current of the resonant DC / DC converter in the current period, and according to a given voltage or / and current, respectively calculating voltage difference or / and current difference between the actual output quantity in the current period and the given quantity, then inputting the voltage difference or / and current difference into respective PI controllers to realize frequency control and phase shift control of a voltage outer loop and a current inner loop, so as to obtain a frequency and a phase shift angle for controlling the output voltage or the output current of the resonant DC / DC converter in a next period, and generating a driving signal of a switching tube to control on-off of the switching tube, so as to realize closed-loop control of stable output voltage or current.

[0010] The three-degree-of-freedom control method of the resonant DC / DC converter based on the CLLC topology has the following characteristics: the input bus voltage in step 1 is selected according to the following conditions:

[0011] When U o = U omin / n~U imin / n, U i = U imin ;

[0012] When U o = U imin / n~U imax / n, U i = U0x n;

[0013] When U0= U imax / n~U omax , U i = U imax ;

[0014] Wherein, U i represents the input bus voltage, and U iminUmin represents the minimum value of the input bus voltage imax Umax represents the maximum value of the input bus voltage o Uout represents the output voltage omin Umin represents the minimum value of the output voltage omax Umax represents the maximum value of the output voltage, n represents the transformer ratio in the resonant cavity.

[0015] The working mode in step 2 is selected by the following process:

[0016] According to the charging curve of the vehicle-mounted battery, the charging process of the vehicle-mounted battery is divided into three working modes, namely constant voltage mode CV, constant current mode CC and constant power mode CP; wherein the constant voltage mode CV is a double-loop control of voltage outer loop and current inner loop; the constant current mode CC and the constant power mode CP are current inner loop controls;

[0017] When charging the vehicle-mounted battery, the voltage of the vehicle-mounted battery is first detected, and the position of the voltage in the charging curve is found, so that the working mode of the position to which the voltage corresponds is selected to start charging according to the position of the voltage, and the working mode switching is performed during the charging process.

[0018] The frequency modulation control and phase shift control in step 3 include the following process:

[0019] Step a, selecting the over-resonance region of the resonant DC / DC converter as the frequency modulation range f min ~ f max ;

[0020] Step b, if the switching frequency of the resonant DC / DC converter is within the frequency modulation range f min ~ f max , the output voltage or current requirement can be met, and only the frequency modulation control of the resonant DC / DC converter is performed;

[0021] If the switching frequency of the resonant DC / DC converter reaches the minimum frequency f min or the maximum frequency f max , the output voltage or current requirement cannot be met, then the phase shift control of the resonant DC / DC converter is performed at the minimum frequency f min or the maximum frequency f max .

[0022] The working mode switching is performed by the following process:

[0023] The switching sequence of the working mode of the vehicle-mounted battery charging process is CV-CC-CP-CV, and the current inner loop is always used as the frequency and phase shift angle output of the frequency modulation control and phase shift control;

[0024] If the working mode in the current cycle is CV, and the voltage of the vehicle-mounted battery reaches the first voltage threshold, the working mode in the next cycle is switched from CV to CC;

[0025] If the working mode in the current cycle is CC, and the voltage of the vehicle-mounted battery reaches the second voltage threshold, the working mode in the next cycle is switched from CC to CP;

[0026] If the working mode in the current cycle is CP, and the voltage of the vehicle-mounted battery reaches the third voltage threshold, the working mode in the next cycle is switched from CP to CV;

[0027] When CV is switched to CC, the voltage outer ring is not controlled, and only the control of the current inner ring is reserved;

[0028] When CC is switched to CP, the voltage outer ring is not controlled, and the output power of the resonant DC / DC converter is calculated on the basis of the control of the current inner ring to obtain the reference output current as the input of the current inner ring;

[0029] When CP is switched to CV, the voltage outer ring and the current inner ring are controlled.

[0030] The frequency modulation control includes:

[0031] (1) In the constant voltage mode CV, the voltage outer ring and the current inner ring participate in control at the same time, including: obtaining a given voltage value and inputting the voltage difference obtained after the difference between the actual output voltage value and the given voltage value into the PI controller of the voltage outer ring, so as to obtain a current reference value, simultaneously obtaining an actual output current value, and inputting the current difference obtained after the difference between the actual output current value and the current reference value into the frequency modulation PI controller of the current inner ring to obtain a closed-loop frequency, so as to control the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network;

[0032] (2) In the constant current mode CC, the current inner ring is controlled, including: obtaining a given current value and inputting the current difference obtained after the difference between the actual output current value and the given current value into the frequency modulation PI controller of the current inner ring to obtain a closed-loop frequency, so as to control the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network;

[0033] (3) In the constant power mode CP, the current inner ring is controlled, including: obtaining a given power value and an actual output voltage value, dividing the given power value by the actual output voltage value to obtain a current reference value, simultaneously obtaining an actual output current value, and inputting the current difference obtained after the difference between the actual output current value and the current reference value into the frequency modulation PI controller of the current inner ring to obtain a closed-loop frequency, so as to control the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network.

[0034] The phase shift control includes:

[0035] (1) Constant voltage mode CV, voltage outer ring, current inner ring control at the same time, including: the given voltage value is obtained and the actual output voltage value is subtracted to obtain the voltage difference value input PI controller of voltage outer ring, thereby obtaining the current reference value, at the same time, the actual output current value is obtained, and the current reference value is subtracted to obtain the current difference value input phase shift PI controller of current inner ring, the phase shift angle is obtained, which is used to adjust the phase difference between the H bridge first bridge arm switch tube of rectifier network and the corresponding switch tube of H bridge first bridge arm of inverter network, so as to adjust the output gain to realize closed loop control;

[0036] (2) Constant current mode CC, controlled by current inner ring, including: the given current value is obtained and the actual output current value is subtracted to obtain the current difference value input phase shift PI controller of current inner ring, the phase shift angle is obtained, which is used to adjust the phase difference between the H bridge first bridge arm switch tube of rectifier network and the corresponding switch tube of H bridge first bridge arm of inverter network, so as to adjust the output gain to realize closed loop control;

[0037] (3) Constant power mode CP, controlled by current inner ring, including: the given power value and the actual output voltage value are obtained, the given power value is divided by the actual output voltage value to obtain the current reference value, at the same time, the actual output current value is obtained, and the current reference value is subtracted to obtain the current difference value input phase shift PI controller of current inner ring, the phase shift angle is obtained, which is used to adjust the phase difference between the H bridge first bridge arm switch tube of rectifier network and the corresponding switch tube of H bridge first bridge arm of inverter network, so as to adjust the output gain to realize closed loop control.

[0038] Compared with the prior art, the beneficial effects of the present application are as follows:

[0039] 1. The present application is based on the resonant DC / DC converter based on CLLC topology, on the basis of frequency modulation control and phase shift control, the bus voltage regulation control is introduced, and the bus voltage regulation is taken as the first priority, which overcomes the problem of large resonant cavity current caused by phase shift control when the gain requirement is large, achieves the purpose of wide voltage output range, not only reduces the frequency modulation range, but also reduces the phase shift angle, thereby reducing the resonant cavity current and the loss, and improving the efficiency.

[0040] 2. The present application is based on the resonant DC / DC converter based on CLLC topology, considering that the load is the working mode corresponding to the battery charging curve of vehicle-mounted battery, the output constant voltage in constant voltage mode CV, the output constant current in constant current mode CC, and the output constant power in constant power mode CP are realized, so as to meet the battery charging requirements.

[0041] 3, The application is aimed at a resonant DC / DC converter based on CLLC topology, considers the working mode switching in the vehicle battery charging process, adopts double-loop control of voltage outer loop and current inner loop, only switches or removes the outer loop at mode switching, and the current inner loop remains unchanged, so as to overcome the problem of large voltage and current overshoot caused by single-loop control at switching, and make the mode switching more smooth. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is the topology diagram of the CLLC resonant DC / DC converter circuit in the application;

[0043] Figure 2 is the control block diagram of the CLLC resonant DC / DC converter in the application;

[0044] Figure 3 is the three-degree-of-freedom control flow chart of the CLLC resonant DC / DC converter in the application;

[0045] Figure 4 is the bus voltage regulation curve of the CLLC resonant DC / DC converter in the application;

[0046] Figure 5 is the CV-CC-CP loop switching diagram of the CLLC resonant DC / DC converter in the application;

[0047] Figure 6 is the normalized gain curve of the CLLC resonant DC / DC converter in the application under frequency modulation control;

[0048] Figure 7 is the relationship curve between the gain and the phase shift angle of the CLLC resonant DC / DC converter in the application under phase shift control;

[0049] Figure 8 is the driving control timing diagram of the frequency modulation and phase shift control of the CLLC resonant DC / DC converter in the application;

[0050] Figure 9 is the experimental result diagram of the CLLC resonant DC / DC converter in the application. DETAILED DESCRIPTION

[0051] In order to more clearly illustrate the technical solutions implemented by the application, the application will be described in more detail below with reference to the drawings and examples.

[0052] In this embodiment, a three-degree-of-freedom control method of a resonant DC / DC converter based on CLLC topology, wherein the resonant DC / DC converter is a post-circuit of an on-board charger OBC and is used for charging a vehicle battery, the CLLC topology of the resonant DC / DC converter comprises an inverter network, a resonant cavity and a rectifier network, likeFigure 1 As shown in S P1 -S P4 The four switch tubes constitute an H-bridge of the inverter network, S S1 -S S4 The four switch tubes constitute an H-bridge of the rectifier network, and the inverter network is connected with the output of the OBC pre-stage PFC as the bus voltage input U Bus , and the rectifier network is connected with the battery as the output U Batt . The resonant cavity is a two-port network composed of a primary side resonant inductance L r , a primary side resonant capacitance C r , a secondary side resonant inductance L s , a secondary side resonant capacitance C s , and a high-frequency transformer with a transformation ratio of n:1. The midpoint of the two bridge arms of the H-bridge of the inverter network is connected with the primary side port of the resonant cavity, and the midpoint of the two bridge arms of the H-bridge of the rectifier network is connected with the secondary side port of the resonant cavity.

[0053] As shown in Figure 2 , it is a control block diagram of the CLLC resonant DC / DC converter. For wide range input voltage application occasions, on the basis of frequency modulation control and phase shift control, bus voltage regulation is introduced to realize three degrees of freedom control. The three degrees of freedom control method includes the following steps:

[0054] Step 1: according to the vehicle battery voltage, select the input bus voltage of the resonant DC / DC converter in the current period;

[0055] Step 2: according to the vehicle battery voltage, select the working mode of the resonant DC / DC converter in the current period, and compare it with the working mode in the previous period. If they are the same, execute step 3, otherwise execute working mode switching;

[0056] Step 3: obtain the actual output voltage or / and current of the resonant DC / DC converter in the current period, and according to the given voltage or / and current, calculate the voltage difference or / and current difference between the actual output quantity and the given quantity in the current period, and then input into the respective PI controller to realize the frequency modulation control and phase shift control of the voltage outer loop and the current inner loop, so as to obtain the frequency and phase shift angle for controlling the output voltage or output current of the resonant DC / DC converter in the next period, and generate the driving signal of the switch tube to control the on-off of the switch tube, so as to realize the closed-loop control of stable output voltage or current, as shown in Figure 8 , which shows the driving control timing of the switch tube of the CLLC resonant DC / DC converter in the CP mode.

[0057] Among them, the driving signal of the switch tube is a PWM square wave signal.

[0058] The PWM waves of the upper and lower switches in the same arm of the H-bridge of the inverter network are complementary, with a duty cycle of 0.5 and a dead time set to prevent the switches in the same arm from being directly turned on. The PWM waves of the diagonal switches of the H-bridge of the inverter network are the same.

[0059] The PWM waves of the upper and lower switching tubes in the first arm of the H-bridge of the rectifier network are complementary, with a duty cycle of 0.5, and a dead time is set to prevent the switching tube in the same arm from being directly turned on; the PWM wave of the switching tube in the second arm of the H-bridge of the rectifier network is generated by synchronous rectification control.

[0060] When there is a phase shift requirement, there is a phase difference between the PWM wave of the first arm switch tube of the H bridge of the rectifier network and the PWM wave of the first arm switch tube of the H bridge of the inverter network, that is, external phase shift control;

[0061] When there is no phase shift requirement, the PWM wave of the first-arm switch tube of the H-bridge of the rectifier network is generated by synchronous rectification control.

[0062] like Figure 4 As shown, the input bus voltage in step 1 is selected according to the following conditions:

[0063] When U o =U omin ~U imin / n, U i =U imin ;

[0064] When U o =U imin / n~U imax / n, U i =U o ×n;

[0065] When U o =U imax / n~U omax When U i =U imax ;

[0066] Among them, U i Indicates the input bus voltage, U imin Indicates the minimum value of the input bus voltage, U imax Indicates the maximum value of the input bus voltage, Uo indicates the output voltage, U omin Indicates the minimum value of the output voltage, U omax It represents the maximum value of the output voltage, and n represents the transformation ratio of the transformer in the resonant cavity.

[0067] like Figure 3 As shown, the regulation of bus voltage is the highest priority. While achieving the same wide range of output voltage, the frequency modulation range and phase shift angle can be reduced, thereby reducing the resonant cavity current, reducing losses and improving efficiency.

[0068] The working mode in step 2 is selected according to the following process:

[0069] According to the charging curve of the vehicle-mounted battery, the charging process of the vehicle-mounted battery is divided into three working modes, namely constant voltage mode CV, constant current mode CC and constant power mode CP; wherein the constant voltage mode CV is a double-loop control of voltage outer loop and current inner loop; the constant current mode CC and the constant power mode CP are current inner loop control;

[0070] When charging the vehicle-mounted battery, the voltage of the vehicle-mounted battery is first detected, and the position of the voltage in the charging curve is found, so that the working mode of the corresponding position to which the voltage corresponds is selected to start charging, and the working mode switching is performed during the charging process.

[0071] As shown in Figure 5 The working mode switching is performed according to the following process:

[0072] The switching sequence of the working mode of the vehicle-mounted battery charging process is CV-CC-CP-CV, and the current inner loop is always used as the frequency and phase shift angle output of the frequency control and phase shift control;

[0073] If the working mode in the current period is CV, and the voltage of the vehicle-mounted battery reaches the first voltage threshold, then the working mode in the next period is switched from CV to CC;

[0074] If the working mode in the current period is CC, and the voltage of the vehicle-mounted battery reaches the second voltage threshold, then the working mode in the next period is switched from CC to CP;

[0075] If the working mode in the current period is CP, and the voltage of the vehicle-mounted battery reaches the third voltage threshold, then the working mode in the next period is switched from CP to CV;

[0076] When CV is switched to CC, the voltage outer loop is not controlled, and only the current inner loop control is retained, that is, the voltage outer loop is discarded;

[0077] When CC is switched to CP, the voltage outer loop is not controlled, and on the basis of the current inner loop control, the output power of the resonant DC / DC converter is calculated to obtain the reference output current as the input of the current inner loop;

[0078] When CP is switched to CV, the voltage outer loop and the current inner loop are controlled, that is, the power calculation process is switched to the voltage outer loop.

[0079] The frequency control and phase shift control in step 3 include the following process:

[0080] Step a, selecting the over-resonance region of the resonant DC / DC converter as the frequency control range f min ~ fmax ;

[0081] Step b, if the switching frequency of the resonant DC / DC converter is within the frequency modulation range f min ~ f max , the output voltage or current requirement of the resonant DC / DC converter can be met, only frequency modulation control is performed on the resonant DC / DC converter;

[0082] If the switching frequency of the resonant DC / DC converter reaches the minimum frequency f min or the maximum frequency f max , the output voltage or current requirement of the resonant DC / DC converter cannot be met, then phase shift control is performed on the resonant DC / DC converter at the minimum frequency f min or the maximum frequency f max .

[0083] The frequency modulation range is set to the over-resonance region of the CLLC converter to ensure the monotonicity of the gain-frequency curve. Under the analysis of the fundamental equivalent model, the normalized gain-frequency curve of the frequency modulation control of the CLLC converter is shown in Figure 6 , and the voltage gain formula is:

[0084]

[0085] wherein, is the normalized frequency (f s is the switching frequency, and f r is the resonant frequency), is the quality factor, is the excitation inductance ratio, is the inductance ratio (L' s = n 2 L s ), is the capacitance ratio (C' s = C s / n 2 ).

[0086] The purpose of introducing the phase shift control is mainly to overcome the narrow gain of the frequency modulation control under certain conditions. As shown in Figure 7 , the introduction of the external phase shift on the basis of the frequency modulation can increase the gain and widen the gain range of the CLLC converter. The range of the phase shift angle is -180°~180°, when the phase shift angle is negative, the voltage reduction can be realized; when the phase shift angle is positive, the voltage increase can be realized.

[0087] In this embodiment, the frequency modulation control includes:

[0088] (1) Constant voltage mode CV, voltage outer loop and current inner loop participate in control at the same time, including: the given voltage value is obtained and the actual output voltage value is subtracted to obtain the voltage difference value input into the PI controller of the voltage outer loop, so as to obtain the current reference value, at the same time, the actual output current value is obtained and the current reference value is subtracted to obtain the current difference value input into the frequency modulation PI controller of the current inner loop, so as to obtain the closed loop frequency, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network.

[0089] (2) Constant current mode CC, controlled by the current inner loop, including: the given current value is obtained and the actual output current value is subtracted to obtain the current difference value input into the frequency modulation PI controller of the current inner loop, so as to obtain the closed loop frequency, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network.

[0090] (3) Constant power mode CP, controlled by the current inner loop, including: the given power value and the actual output voltage value are obtained, the given power value is divided by the actual output voltage value to obtain the current reference value, at the same time, the actual output current value is obtained and the current reference value is subtracted to obtain the current difference value input into the frequency modulation PI controller of the current inner loop, so as to obtain the closed loop frequency, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network.

[0091] In the embodiment, the phase shift control includes:

[0092] (1) Constant voltage mode CV, voltage outer loop and current inner loop participate in control at the same time, including: the given voltage value is obtained and the actual output voltage value is subtracted to obtain the voltage difference value input into the PI controller of the voltage outer loop, so as to obtain the current reference value, at the same time, the actual output current value is obtained and the current reference value is subtracted to obtain the current difference value input into the phase shift PI controller of the current inner loop, so as to obtain the phase shift angle, used for adjusting the phase difference between the switch tube of the first bridge arm of the H-bridge of the rectifier network and the corresponding switch tube of the first bridge arm of the H-bridge of the inverter network, thereby adjusting the output gain to realize the closed loop control requirement;

[0093] (2) Constant current mode CC, controlled by the current inner loop, including: the given current value is obtained and the actual output current value is subtracted to obtain the current difference value input into the phase shift PI controller of the current inner loop, so as to obtain the phase shift, used for adjusting the phase difference between the switch tube of the first bridge arm of the H-bridge of the rectifier network and the corresponding switch tube of the first bridge arm of the H-bridge of the inverter network, thereby adjusting the output gain to realize the closed loop control requirement.

[0094] (3) In the constant power mode CP, the current inner loop is used for control, including: obtaining a given power value and an actual output voltage value, dividing the given power value by the actual output voltage value to obtain a current reference value, and obtaining the actual output current value at the same time, and subtracting it from the current reference value. The obtained current difference is input into the phase-shifted PI controller of the current inner loop to obtain a phase shift angle, which is used to adjust the phase difference between the first arm switch tube of the H-bridge of the rectifier network and the corresponding switch tube of the first arm of the H-bridge of the inverter network, thereby adjusting the output gain to achieve the closed-loop control requirements.

[0095] In order to verify the feasibility of the proposed technical solution, an experimental circuit based on CLLC topology was built.

[0096] The main circuit parameters and experimental conditions are as follows: the input bus voltage range is set to 650-800V, the output voltage range is set to 200-500V, the switching frequency range is set to 100kHZ-250kHz, the primary side resonant inductor value is 16μH, the secondary side resonant inductor value is 4μH, the primary side resonant capacitor value is 0.21μF, the secondary side resonant capacitor value is 0.56μF, the excitation inductor value is 2.4mH, and the transformer ratio is 2:1. Figure 9 Figure 2 shows the experimental results of the output voltage of the CLLC resonant DC / DC converter in CC, CP, and CV modes. The experimental results show that this technical solution has high feasibility.

Claims

1. A three-degree-of-freedom control method for a resonant DC / DC converter based on a CLLC topology. The resonant DC / DC converter is a post-stage circuit of an onboard charger (OBC) and is used to charge an onboard battery. The CLLC topology of the resonant DC / DC converter includes: Inverter network, resonant cavity and rectifier network; characterized in that the three-degree-of-freedom control method comprises the following steps: Step 1: selecting the input bus voltage of the resonant DC / DC converter in the current period according to the voltage of the vehicle-mounted battery; The input bus voltage in step 1 is selected as follows: When time, ; When Time, ; When Time, ; wherein, represents the input bus voltage, represents the minimum value of the input bus voltage, represents the maximum value of the input bus voltage, represents the output voltage, represents the minimum value of the output voltage, represents the maximum value of the output voltage, represents the transformer ratio of the transformer in the resonant cavity; Step 2: selecting the working mode of the resonant DC / DC converter in the current period according to the voltage of the vehicle-mounted battery, and comparing it with the working mode of the previous period, if they are the same, then executing step 3, otherwise executing working mode switching; The working mode in step 2 is selected as follows: According to the charging curve of the vehicle-mounted battery, the charging process of the vehicle-mounted battery is divided into three working modes, namely constant voltage mode CV, constant current mode CC and constant power mode CP; wherein the constant voltage mode CV is double-loop control of voltage outer loop and current inner loop; the constant current mode CC and the constant power mode CP are current inner loop control; When charging the vehicle-mounted battery, first detect the voltage of the vehicle-mounted battery, and find the position of the voltage in the charging curve, so as to select the working mode to which the position belongs according to the position of the voltage to start charging, and perform working mode switching during the charging process; The working mode switching is performed as follows: Let the switching sequence of the working mode of the vehicle-mounted battery charging process be CV-CC-CP-CV, and always take the current inner loop as the frequency and phase shift angle output of the frequency and phase shift control of the voltage outer loop and the current inner loop; Step 3: obtaining the actual output voltage or / and current of the resonant DC / DC converter in the current period, and according to the given voltage or / and current, calculating the voltage difference or / and current difference between the actual output quantity and the given quantity in the current period, and then inputting it into the respective PI controller to realize the frequency control and phase shift control of the voltage outer loop and the current inner loop, so as to obtain the frequency and phase shift angle for controlling the output voltage or output current of the resonant DC / DC converter in the next period, and generate the driving signal of the switching tube to control the on-off of the switching tube, thereby realizing the closed-loop control of stable output voltage or current.

2. The three-degree-of-freedom control method of a resonant DC / DC converter based on a CLLC topology according to claim 1, characterized in that, The frequency control and phase shift control in step 3 include the following process: Step a, selecting the over resonant region of the resonant DC / DC converter as the frequency modulation range ; Step b. If the switching frequency of the resonant DC / DC converter is within a frequency modulation range that can satisfy its output voltage or current requirement, only frequency modulation control is performed on the resonant DC / DC converter. If the switching frequency of the resonant DC / DC converter cannot meet its output voltage or current requirement when it reaches a minimum frequency or a maximum frequency , the resonant DC / DC converter is phase-shifted controlled at the minimum frequency or the maximum frequency .

3. The three-degree-of-freedom control method of the resonant DC / DC converter based on CLLC topology according to claim 1, characterized in that: If the working mode in the current period is CV, and the voltage of the vehicle-mounted battery reaches the first voltage threshold, then the working mode in the next period is switched from CV to CC; If the working mode in the current period is CC, and the voltage of the vehicle-mounted battery reaches the second voltage threshold, then the working mode in the next period is switched from CC to CP; If the working mode in the current period is CP, and the voltage of the vehicle-mounted battery reaches the third voltage threshold, then the working mode in the next period is switched from CP to CV; When CV is switched to CC, the voltage outer loop is not controlled, only the control of the current inner loop is retained; When CC is switched to CP, the voltage outer loop is not controlled, and on the basis of the control of the current inner loop, the output power of the resonant DC / DC converter is calculated to obtain the reference output current as the input of the current inner loop; When the CP is switched to the CV, the voltage outer loop and the current inner loop are controlled.

4. The three-degree-of-freedom control method of a CLLC topology-based resonant DC / DC converter according to claim 3, characterized in that: The frequency modulation control comprises: (1) in the constant voltage mode CV, the voltage outer loop and the current inner loop participate in the control simultaneously, comprising: the given voltage value is obtained and the voltage difference value is obtained after the difference between the actual output voltage value and the given voltage value, the voltage difference value is input into the PI controller of the voltage outer loop, thereby obtaining the current reference value, the actual output current value is obtained, and the current difference value is obtained after the difference between the actual output current value and the current reference value, the current difference value is input into the frequency modulation PI controller of the current inner loop, the closed loop frequency is obtained, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network; (2) in the constant current mode CC, the current inner loop controls, comprising: the given current value is obtained, the current difference value is obtained after the difference between the actual output current value and the given current value, the current difference value is input into the frequency modulation PI controller of the current inner loop, the closed loop frequency is obtained, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network; (3) in the constant power mode CP, the current inner loop controls, comprising: the given power value and the actual output voltage value are obtained, the current reference value is obtained by dividing the given power value by the actual output voltage value, the actual output current value is obtained, and the current difference value is obtained after the difference between the actual output current value and the current reference value, the current difference value is input into the frequency modulation PI controller of the current inner loop, the closed loop frequency is obtained, thereby controlling the on-off of the four switch tubes of the H-bridge of the inverter network and the two switch tubes of the first bridge arm of the H-bridge of the rectifier network.

5. The three-degree-of-freedom control method of a CLLC topology-based resonant DC / DC converter according to claim 4, characterized in that: The phase shift control comprises: (1) in the constant voltage mode CV, the voltage outer loop and the current inner loop participate in the control simultaneously, comprising: the given voltage value is obtained and the voltage difference value is obtained after the difference between the actual output voltage value and the given voltage value, the voltage difference value is input into the PI controller of the voltage outer loop, thereby obtaining the current reference value, the actual output current value is obtained, and the current difference value is obtained after the difference between the actual output current value and the current reference value, the current difference value is input into the phase shift PI controller of the current inner loop, the phase shift angle is obtained, which is used for adjusting the phase difference between the switch tube of the first bridge arm of the H-bridge of the rectifier network and the corresponding switch tube of the first bridge arm of the H-bridge of the inverter network, thereby adjusting the output gain to realize the closed loop control; (2) in the constant current mode CC, the current inner loop controls, comprising: the given current value is obtained, the current difference value is obtained after the difference between the actual output current value and the given current value, the current difference value is input into the phase shift PI controller of the current inner loop, the phase shift angle is obtained, which is used for adjusting the phase difference between the switch tube of the first bridge arm of the H-bridge of the rectifier network and the corresponding switch tube of the first bridge arm of the H-bridge of the inverter network, thereby adjusting the output gain to realize the closed loop control; (3) in the constant power mode CP, the current inner loop controls, comprising: the given power value and the actual output voltage value are obtained, the current reference value is obtained by dividing the given power value by the actual output voltage value, the actual output current value is obtained, and the current difference value is obtained after the difference between the actual output current value and the current reference value, the current difference value is input into the phase shift PI controller of the current inner loop, the phase shift angle is obtained, which is used for adjusting the phase difference between the switch tube of the first bridge arm of the H-bridge of the rectifier network and the corresponding switch tube of the first bridge arm of the H-bridge of the inverter network, thereby adjusting the output gain to realize the closed loop control.

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