Non-monotonic control method for gain of CLLC resonant converter

Abstract

The application provides a CLLC resonant converter gain non-monotonic control method, which comprises the following steps: step one, taking the primary side resonant capacitor, the secondary side resonant capacitor, the primary side resonant inductor current and the secondary side resonant inductor current as state variables, establishing a time domain model and solving the minimum working frequency value under different loads; step two, detecting the output voltage and the load current of the CLLC resonant converter and judging the load state when the CLLC resonant converter is working; step three, if it is light load, the system is in normal working mode; step four, if it is heavy load, the system enters the gain non-monotonic working mode; the system calculates the minimum frequency corresponding to the current situation by calculating the size of the load current, and limits the PI output result to above the minimum frequency; step five, the system gives the driving signal of the switch tube of the CLLC resonant converter. The application realizes the gain non-monotonic control method by reasonably improving the PI control strategy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic converters, and in particular to a gain non-monotonicity control method for CLLC resonant converters. BACKGROUND

[0002] Currently, society has entered an era of high energy consumption, and the use of fossil energy is gradually increasing, which has a more serious impact on the ecological environment of the earth. Therefore, energy issues and the realization of carbon neutralization have attracted widespread attention. New energy fields, such as wind energy and solar energy, have developed rapidly, but their proportion of total energy consumption is still small, and they may cause harmonic pollution to the power grid. In terms of energy application, the number of electric new energy vehicles has increased significantly, even exceeding traditional gasoline vehicles. The battery energy storage characteristics of electric vehicles provide new ideas for optimizing the power system.

[0003] V2G (Vehicle to Grid) technology takes full advantage of the energy storage characteristics of electric vehicle batteries as a distributed energy storage device to balance the fluctuations of new energy generation and optimize power grid operation. The development of this technology helps to more efficiently manage energy, improve energy utilization efficiency, and promote sustainable development. In the V2G system, a bidirectional DC / DC converter is a key power conversion device. The DC / DC converter in the on-board charger is mainly used to achieve voltage matching, control the charging and discharging power, and electrical isolation, while also ensuring the bidirectional flow of power. Therefore, these converters need to meet the requirements of high efficiency and high power density, and the design is difficult. Bidirectional CLLC resonant converter is a common bidirectional isolated DC / DC topology structure, which realizes bidirectional symmetric power flow on the basis of LLC resonant converter, has the advantages of wide soft switching range, high operating efficiency, low control complexity, and no magnetic balance problem.

[0004] Because the CLLC resonant converter has one more resonant element than the LLC resonant converter, the CLLC resonant converter has two gain peaks in the under-resonance region under heavy load conditions, which will cause the gain to be non-monotonic under heavy load. The use of traditional PI modulation will cause loss of control. However, the research on this feature of the CLLC resonant converter is not deep enough, and there is a lack of effective methods to prevent gain non-monotonicity. SUMMARY

[0005] The present application provides a CLLC resonant converter gain non-monotonicity control method, which can be used to solve the technical problem of lack of effective methods to prevent gain non-monotonicity.

[0006] The present application provides a CLLC resonant converter gain non-monotonicity control method, which comprises the following steps:

[0007] Step one, the primary side resonant capacitor u Cr1 , the secondary side resonant capacitor uCr2 primary side resonant inductor current i Lr1 secondary side resonant inductor current i Lr2 As a state variable, a time-domain model is established, and the minimum frequency value under different loads is solved;

[0008] Step two, when the CLLC resonant converter is working, the output voltage and load current of the CLLC resonant converter are detected, and the load state is judged;

[0009] Step three, according to step two, if it is light load, the system is in normal working mode;

[0010] Step four, according to step two, if it is heavy load, the system enters the gain non-monotonic working mode; and the working frequency is limited to above the minimum frequency value under the current load;

[0011] Step five, according to steps three and four, the system gives the driving signal of the switch tube of the CLLC resonant converter.

[0012] Further, step one establishes a time-domain model to solve the frequency boundary of the mode, that is, the minimum working frequency, and the steps are as follows:

[0013] Step 11, confirm the parameters of the CLLC resonant converter;

[0014] The CLLC resonant converter topology structure includes switch tubes S1-S8, anti-parallel diodes D S1 -D S8 and parasitic capacitor C oss1 -C oss8 ; primary side resonant inductor L r1 , primary side resonant capacitor C r1 , transformer excitation inductor L m , transformer with turn ratio n:1, secondary side resonant inductor L r2 , secondary side resonant capacitor C r2 , output capacitor C s , DC input voltage U in and output resistor R; resonant inductor L r1 , L r2 , resonant capacitor C r1 , C r2 and transformer with turn ratio n:1 constitute the resonant cavity of the CLLC resonant converter;

[0015] The positive electrode of the DC input voltage U in is connected to the source electrode of the primary side switch tubes S1 and S3 at the same time; the negative electrode of the DC input voltage U in is connected to the drain electrode of the primary side switch tubes S2 and S4 at the same time; the output capacitor C s is connected in parallel with the output resistor R;

[0016] Step 12, establish CLLC resonant converter time domain model, using time domain analysis to derive CLLC resonant converter i Lr1N , Lr2N , Cr1N , Cr2N under-resonance time domain expression;

[0017] Step 13, solve the CLLC resonant converter boundary conditions;

[0018] Step 14, solve the frequency boundary.

[0019] Further, step 11, confirm the CLLC resonant converter parameters, including:

[0020] CLLC resonant converter works in the original side two-element series resonance frequency f r , its two-element series resonance frequency as follows:

[0021]

[0022] In the formula, L r1 is the original side resonant inductance, C r1 is the original side resonant capacitance; CLLC resonant converter voltage variable according to the input voltage U in , all current variables according to U in / Z r , where Z r is the original side resonant inductance L r1 and the original side resonant capacitance C r1 characteristic impedance:

[0023]

[0024] Further, step 12, establish CLLC resonant converter time domain model, using time domain analysis to derive CLLC resonant converter i Lr1N , Lr2N , Cr1N , Cr2N under-resonance time domain expression; including:

[0025] When the CLLC resonant converter worker PO mode, assuming that the O-mode excitation current does not change; solve the CLLC resonant converter under-resonance PO mode i Lr1N , Lr2N , Cr1N , Cr2N unit expression:

[0026] under-resonance overload P-mode i Lr1N , Lr2N , Cr1Nu Cr2N The dimensionless expression of u

[0027]

[0028] where ω r is the angular frequency of the resonant converter, ω k1 =k1ω r is the second angular frequency of the resonant converter, k=L m / L r1 , L m is the magnetizing inductance in the transformer, L r1 is the primary side resonant inductance; the dimensionless value of the output voltage U o M=U in / nU o , f n is the dimensionless value of the operating frequency f n =f s / f r , and R is the output resistance;

[0029] The dimensionless expression of i Lr1N ,i Lr2N ,u Cr1N ,u Cr2N under the under-resonant heavy load O-mode is as follows:

[0030]

[0031] where u Cr1N (0.5T r ) is the dimensionless value of the primary side resonant capacitor voltage at 0.5T r , i Lr1N (0.5T r ) is the dimensionless value of the primary side resonant inductor current at 0.5T r , and u Cr2N (0.5T r ) is the dimensionless value of the secondary side resonant capacitor voltage at 0.5T r ; the time-domain expression of the CLLC resonant converter under the under-resonant region is derived.

[0032] Further, in step 13, the boundary conditions of the CLLC resonant converter are solved, including:

[0033] The CLLC resonant converter under the under-resonant region has OPO / PO / PON / PN modes; the gain of the OPO / PO mode is monotonic, the heavy load PON mode enters the non-monotonic gain interval, and the heavy load PN mode will definitely enter the non-monotonic gain interval; first, the frequency and load range conditions for entering the PON and PN modes are calculated;

[0034] The boundary between the PO mode and the PON mode is described as follows: the resonant capacitor will not overcharge before the end of the 0 mode, causing the secondary diode to turn on prematurely; similarly, for the PON and PN modes, the resonant capacitor will not overcharge before the end of the N mode, thus causing the secondary diode to turn on prematurely; based on this, the following relationship equation is derived:

[0035]

[0036] Where T s T is the actual period of the resonant converter. r M is the resonant period of the resonant converter, and M is the output voltage U. o The per-unit value M = U in / nU o u Cr1N (0.5T s ) is 0.5T s The voltage value of the first resonant capacitor on the original side at time u Cr2N (0.5T s ) is 0.5T s The per-unit value of the second resonant capacitor voltage on the original side at time u Cr1N (0.5T s ) is 0.5T s The per-unit value of the first resonant capacitor voltage on the original side at time u Cr2N (0.5T s ) is 0.5T s The per-unit value of the voltage of the second resonant capacitor on the original side at that moment.

[0037] Further, step 14, solves for the frequency boundaries, including:

[0038] Time-domain analysis shows that part of the PON mode exhibits monotonic gain, while another part exhibits non-monotonic gain. The boundaries between the PO and PON modes are calculated to avoid the non-monotonic gain region and maintain a certain margin. The frequency boundary between the PO and PON modes is obtained as follows:

[0039]

[0040] Where f s It is the actual operating frequency, I o For the output current, f r It is the resonant frequency, Z. r is the characteristic impedance of the resonant converter, and n is the transformer turns ratio;

[0041] That is, the minimum operating frequency of the CLLC resonant converter is:

[0042]

[0043] When the frequency f s is greater than the minimum value, the resonant converter works in the monotonic gain mode; when the frequency f s is less than the minimum value, the resonant converter works in the non-monotonic operation mode.

[0044] Further, step four, according to step two, if it is heavy load, the system enters the non-monotonic gain operation mode; and the working frequency is limited above the minimum frequency value under the load, including:

[0045] According to step two, if it is heavy load, the system enters the non-monotonic gain operation mode;

[0046] The system calculates the corresponding minimum frequency under the current condition by calculating the size of the load current, and limits the working frequency above the minimum frequency.

[0047] The non-monotonic gain control method proposed in the application effectively prevents the non-monotonic gain caused by the two gain peaks of the CLLC resonant converter itself, and the analysis and control are simple. Through the improved PI control, the CLLC resonant converter is prevented from entering the non-monotonic gain interval, thereby avoiding the possibility of control disorder, and the reliability of the CLLC resonant converter is improved. At the same time, the control method can also effectively reduce the load shedding time and overshoot, so the control method not only has strong universality, but also has very good control effect. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is the circuit diagram of the controlled object CLLC resonant converter of the application.

[0049] Figure 2 is the CLLC resonant converter under-resonance operation waveform schematic diagram of the application.

[0050] Figure 3 is the non-monotonic gain interval diagram of the CLLC resonant converter of the application.

[0051] Figure 4 is the PO-PON-PN mode gain interval analysis schematic diagram of the application.

[0052] Figure 5 is the traditional PI load shedding control phase schematic diagram of the CLLC resonant converter of the application.

[0053] Figure 6 is the traditional PI voltage boosting control phase schematic diagram of the CLLC resonant converter of the application.

[0054] Figure 7 is the improved PI control flow schematic diagram of the application.

[0055] Figure 8 is the gain comparison diagram of the application and the traditional PI control.

[0056] Figure 9 is the simulation waveform diagram of the traditional PI in the loading process.

[0057] Figure 10 is the resonance waveform diagram of the traditional PI after the gain adjustment in the loading process.

[0058] Figure 11 is the simulation waveform diagram of the control method in the loading process.

[0059] Figure 12 is the resonance waveform diagram of the control method in the loading process. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical scheme and advantages of the present application more clear, the embodiments of the present application will be further described in detail below with reference to the drawings.

[0061] Firstly, the embodiments of the present application will be introduced below with reference to the drawings.

[0062] Step one, taking the primary side resonance capacitor u Cr1 , the secondary side resonance capacitor u Cr2 , the primary side resonance inductor current i Lr1 , the secondary side resonance inductor current i Lr2 as the state variables, a time domain model is established, and the frequency boundary of the mode is solved;

[0063] Step two, when the CLLC resonant converter works, the output voltage and the load current of the CLLC resonant converter are detected, and the load state is judged;

[0064] Step three, according to step two, if it is light load, the system is in normal working mode;

[0065] Step four, according to step two, if it is heavy load, the system enters the gain non-monotonic working mode; the system calculates the minimum frequency corresponding to the current situation by calculating the size of the load current, and limits the PI output result to be above the minimum frequency;

[0066] Step five, according to step three and step four, the system gives the driving signal of the switch tube of the CLLC resonant converter.

[0067] In a further embodiment, the step of establishing the time domain model to solve the frequency boundary of the mode, i.e. the minimum working frequency, in step one is as follows:

[0068] Step 11, confirming the parameters of the CLLC resonant converter: such as Figure 1 is the CLLC resonant converter topology, including switch tubes S1-S8, anti-parallel diodes D S1 -D S8and parasitic capacitance C oss1 -C oss8 primary side resonant inductance L r1 , primary side resonant capacitance C r1 , transformer excitation inductance L m , transformer with a turn ratio of n:1, secondary side resonant inductance L r2 , secondary side resonant capacitance C r2 , output capacitance C s , DC input voltage U in and output resistance R; resonant inductance L r1 , L r2 , resonant capacitance C r1 , C r2 and transformer with a turn ratio of n:1 constitute a resonant cavity of a CLLC resonant converter;

[0069] DC input voltage U in positive pole is connected to the source of primary side switch S1, S3 at the same time; DC input voltage U in negative pole is connected to the drain of primary side switch S2, S4 at the same time; output capacitance C s is connected in parallel with output resistance R.

[0070] The CLLC resonant converter operates at the two-element series resonant frequency f r of the primary side, where the two-element series resonant frequency is as follows:

[0071]

[0072] In the formula, L r1 is the primary side resonant inductance, C r1 is the primary side resonant capacitance; the voltage variable of the CLLC resonant converter is normalized according to the input voltage U in , and all current variables are normalized according to U in / Z r , where Z r is the characteristic impedance of the primary side resonant inductance L r1 and the primary side resonant capacitance C r1 :

[0073]

[0074] Step 12, establish the time domain model of the CLLC resonant converter, and use time domain analysis to derive the time domain expression of i Lr1N , i Lr2N , u Cr1N , u Cr2N under-resonance of the CLLC resonant converter;

[0075] When the CLLC resonant converter worker PO mode, assuming that the O-mode excitation current does not change; solve out CLLC resonant converter under-resonance PO mode i Lr1N , Lr2N , Cr1N , Cr2N The per-unit expression of:

[0076] Under-resonance heavy load P-mode i Lr1N , Lr2N , Cr1N , Cr2N The per-unit expression is as follows:

[0077]

[0078] Where ω r is the angular frequency of the resonant converter, ω k1 =k1ω r is the second angular frequency of the resonant converter, k=L m / L r1 , L m is the excitation inductance in the transformer, L r1 is the primary side resonant inductance; The per-unit value of the output voltage U o M=U in / nU o , f n The per-unit value of the operating frequency f n =f s / f r , R is the output resistance;

[0079] Under-resonance heavy load O-mode i Lr1N , Lr2N , Cr1N , Cr2N The per-unit expression is as follows:

[0080]

[0081] Where u Cr1N (0.5T r ) is the 0.5T r time primary side resonant capacitor voltage per-unit value, i Lr1N (0.5T r ) is the 0.5T r time primary side resonant inductance current per-unit value, u Cr2N (0.5T r ) is the 0.5T r time secondary side resonant capacitor voltage per-unit value; Based on the derivation of the CLLC resonant converter under-resonance region time domain expression. Its working waveform is Figure 2 .

[0082] Step 13, solve the boundary conditions of CLLC resonant converter: according to Figure 3 , CLLC resonant converter is easy to enter the gain non-monotonic interval at heavy load, causing control out of control, and easy to cause damage to the equipment. Because a method needs to be used to control the gain non-monotonic interval. CLLC resonant converter exists OPO / PO / PON / PN mode in under-resonance region; among them, the gain of OPO / PO mode is monotonic, the heavy load PON mode exists to improve into the non-monotonic gain interval, and the heavy load PN mode will definitely enter the non-monotonic gain interval; therefore, first calculate the frequency and load range conditions of entering PON and PN mode;

[0083] The boundary between PO mode and PON mode is described as follows: the resonant capacitor will not be overcharged before the end of O mode, causing the secondary side diode to turn on too early; similarly, for PON mode and PN mode, the resonant capacitor will not be overcharged before the end of N mode, thereby causing the secondary side diode to turn on in advance; based on this, the following relationship equation is derived:

[0084]

[0085] Where T s is the actual period of the resonant converter, T r is the resonant period of the resonant converter, M is the unit value of the output voltage U o of the resonant converter, M=U in / nU o , u Cr1N (0.5T s ) is the voltage value of the first resonant capacitor on the primary side at 0.5T s , u Cr2N (0.5T s ) is the unit value of the voltage of the second resonant capacitor on the primary side at 0.5T s , u Cr1N (0.5T s ) is the unit value of the voltage of the first resonant capacitor on the primary side at 0.5T s , and u Cr2N (0.5T s ) is the unit value of the voltage of the second resonant capacitor on the primary side at 0.5T s .

[0086] Step 14, solve the frequency boundary: time domain analysis shows that part of the PON mode presents monotonic gain, and the other part presents non-monotonic gain; calculate the boundary of PO mode and PON mode to avoid the non-monotonic gain region and keep a certain margin; it is obtained that the frequency boundary between PO mode and PON mode is:

[0087]

[0088] where f s is the actual operating frequency, I o is the output current, f r is the resonant frequency, Z r is the characteristic impedance of the resonant converter, and n is the transformer ratio;

[0089] That is, the minimum value of the operating frequency of the CLLC resonant converter is:

[0090]

[0091] When the frequency f s is greater than the minimum value, the resonant converter operates in the monotonic gain mode; when the frequency f s is less than the minimum value, the resonant converter operates in the non-monotonic gain mode.

[0092] Step two, when the CLLC resonant converter is operating, the output voltage and load current of the CLLC resonant converter are detected, and the load state is determined;

[0093] Step three, according to step two, if the load state is light load, the system is in normal operation mode;

[0094] Step four, according to step two, if the load state is heavy load, the system enters the non-monotonic gain control mode, and the system calculates the minimum frequency corresponding to the situation by calculating the size of the load current, and limits the PI output result to be above the minimum frequency.

[0095] Step five, according to steps three and four, the system gives the driving signal of the switch tube of the CLLC resonant converter.

[0096] Step four is as follows: the traditional PI control has a lag phenomenon, which can cause numerical overshoot and can also cause the system to enter the non-monotonic gain region, such as Figure 5 and Figure 6 . Figure 5 represents the lag phenomenon when the load is switched, Figure 6 represents the lag phenomenon when the voltage is switched. The lag phenomenon mainly manifests that, due to the influence of the output capacitor, the change of the output voltage is slower than the change of the frequency when the load is switched, which can cause the frequency to be easily overshoot, and thus the system enters the non-monotonic gain region. Therefore, the frequency calculated by the PI control needs to be limited, and the limit is calculated by the above analysis. In order to ensure that the resonant converter operates in the PO mode, the size of the operating frequency f s is limited, that is, the calculated frequency needs to be limited, and the limit is calculated by the above analysis of the present application.

[0097] Let the resonant converter operate at the frequency fs above the minimum value. By using the output current I o as the feedback variable. However, when the output value of the PI controller reaches the set cut-off level, the PI controller is still calculating and generating output. When the output voltage reaches the set value, since the actual output of the PI controller has exceeded the set value, adjustment is needed to return it to the set value. This requires a long feedback calculation time. In order to make the CLLC resonant converter switch the load or voltage faster, an additional controller S c is designed. When the frequency reaches the minimum value, S c is set to 1, and the PI controller stops working until the output voltage reaches the set value (U o,set ). Its control mode is shown in Figure 7 . Figure 8 The difference between the traditional PI and the non-monotonic control method proposed by the present application is shown. The traditional PI control enters the gain non-monotonicity.

[0098] Figure 9 The simulation graph of the traditional PI control method during the loading process is given. When the light load is switched to the heavy load, the output voltage of the CLLC resonant converter gradually decreases and cannot return to the set 480V voltage, enters the gain non-monotonicity interval, and the resonant converter output state is unstable. Figure 10 The specific resonant current waveform graph after the load is cut is shown. It can be seen that the resonant current completely enters the PN state, the excitation current is no longer clamped, the waveform is no longer regular, and the converter efficiency sharply decreases.

[0099] Figure 11 The simulation graph of the non-monotonic gain control method proposed by the present application during the loading process is given. When the light load is switched to the heavy load, the output voltage of the CLLC resonant converter initially decreases and then returns to the set 480V voltage, and the resonant converter output state is stable. Figure 12 The specific resonant current waveform graph after the load is cut is shown. It can be seen that the resonant current enters the PO state, the excitation current is clamped, the waveform is regular, and the converter efficiency is guaranteed.

[0100] The present application provides a control method which is simple in calculation, small in error and does not require additional devices, realizes the control of the wide range of converter voltage, reduces the influence of the non-monotonic gain, and improves the working performance of the converter.

[0101] The embodiments of the present application described above do not constitute a limitation on the protection scope of the present application.

Claims

1. A method of controlling the gain of a CLLC resonant converter, characterized in that The method comprises: Step one, the original side resonant capacitor u Cr1 , the secondary side resonant capacitor u Cr2 , the original side resonant inductance current i Lr1 , the secondary side resonant inductance current i Lr2 As a state variable, the time domain model is established, and the minimum frequency value under different loads is solved; Step two, when the CLLC resonant converter is working, detecting the output voltage and load current of the CLLC resonant converter, and judging the load state; Step three, according to step two, if it is light load, the system is in normal working mode; Step four, according to step two, if it is heavy load, the system enters the gain non-monotonic working mode; and the working frequency is limited above the minimum frequency value under the current load; Step five, according to step three and step four, the system gives the driving signal of the switch tube of the CLLC resonant converter; Step one establishes a time domain model to solve the frequency boundary of the mode, i.e. the minimum working frequency, and the steps are as follows: Step 11, confirming the parameters of the CLLC resonant converter; The CLLC resonant converter topology comprises switching transistors S1-S8, anti-parallel diodes D S1 – D S8 and a parasitic capacitance C oss1 – C oss8 ; a primary side resonant inductance L r1 , a primary side resonant capacitance C r1 , a transformer excitation inductance L m , a transformer with a turn ratio of n:1, a secondary side resonant inductance L r2 , a secondary side resonant capacitance C r2 , an output capacitance C s , a DC input voltage U in and an output resistance R; a resonant inductance L r1 , L r2 , a resonant capacitance C r1 , C r2 and a transformer with a turn ratio of n:1 form a resonant tank of the CLLC resonant converter; DC input voltage U in The positive terminal is simultaneously connected to the source terminals of the primary-side switching transistors S1 and S3; DC input voltage U in The negative terminal is simultaneously connected to the drain of both primary-side switching transistors S2 and S4; the output capacitor C s Connected in parallel with the output resistor R; Step 12, the CLLC resonant converter time domain model is established, and the i Lr1N , Lr2N , Cr1N , Cr2N under resonance time domain expression; including: When the CLLC resonant converter worker PO mode, assuming that the O mode excitation current does not change; solve out the CLLC resonant converter under-resonance PO mode i Lr1N , Lr2N , Cr1N , Cr2N The unit expression: P-mode i under off-resonance heavy loading Lr1N , Lr2N , Cr1N , Cr2N The normalized expression of u is as follows: where ω r is the angular frequency of the resonant converter, ω k1 = k1ω r is a second angular frequency of the resonant converter, k = L m / L r1 , L m is the magnetizing inductance in the transformer, L r1 is the primary-side resonant inductance; M output voltage U o in normalized units M = U in / nU o , f n is the operating frequency in normalized units f n = f s / f r , R is the output resistance; Z r is the characteristic impedance of the primary-side resonant inductance L r1 and the primary-side resonant capacitance C r1 ; O-mode i Lr1N , Lr2N , Cr1N , Cr2N The normalized expression of U is as follows: Where u Cr1N (0.5T r ) is 0.5T r The per-unit value of the primary side resonant capacitor voltage at time i Lr1N (0.5T r ) is 0.5T r The per-unit value of the resonant inductor current on the primary side at time u Cr2N (0.5T r ) is 0.5T r The per-unit value of the resonant capacitor voltage on the secondary side at any given time; the time-domain expression of the underresonant region of the CLLC resonant converter is derived based on this; Step 13, solving the boundary conditions of the CLLC resonant converter; including: The OPO / PO / PON / PN mode exists in the under-resonance region of the CLLC resonant converter; wherein the gain of the OPO / PO mode is monotonic, the heavy load PON mode exists to improve into the non-monotonic gain interval, and the heavy load PN mode will definitely enter the non-monotonic gain interval; first, the frequency and load range conditions of entering the PON and PN mode are calculated; The boundary between the PO mode and the PON mode is described as follows: the resonant capacitor will not be overcharged before the end of the O mode, resulting in the early conduction of the secondary side diode; similarly, for the PON mode and the PN mode, the resonant capacitor will not be overcharged before the end of the N mode, thereby causing the early conduction of the secondary side diode; based on this, the following relationship equation is derived: wherein T s is the actual period of the resonant converter, T r is the resonant period of the resonant converter, M is the normalized value of the output voltage U o of the resonant converter, M = U in / nU o , u Cr1N (0.5T s ) is the primary side first resonant capacitor voltage value at 0.5T s , u Cr2N (0.5T s ) is the normalized value of the primary side second resonant capacitor voltage at 0.5T s , u Cr1N (0.5T r ) is the primary side first resonant capacitor voltage normalized value at 0.5T r , u Cr2N (0.5T r ) is the normalized value of the primary side second resonant capacitor voltage at 0.5T r ; Step 14, solving the frequency boundary.

2. The method of claim 1, wherein, Step 11, confirming the parameters of the CLLC resonant converter, including: The CLLC resonant converter operates at a primary side two-element series resonant frequency f r with a two-element series resonant frequency as follows: wherein L r1 is the primary side resonant inductance, r1 is the primary side resonant capacitance; the voltage variable of the CLLC resonant converter is normalized according to the input voltage U in , and all current variables are normalized according to U in / Z r , wherein Z r is the characteristic impedance of the primary side resonant inductance L r1 and the primary side resonant capacitance C r1 . 。 3. The method of claim 1, wherein, Step 14, solving the frequency boundary, including: The time domain analysis shows that part of the PON mode presents monotonic gain, and another part presents non-monotonic gain; the boundary of the PO mode and the PON mode is calculated to avoid the non-monotonic gain region and keep a certain margin; it is obtained that the frequency boundary between the PO mode and the PON mode is: where f s is the actual operating frequency, I o is the output current, f r is the resonant frequency, Z r is the characteristic impedance of the resonant converter, and n is the transformer turns ratio; That is, the minimum value of the working frequency of the CLLC resonant converter is: When the frequency f s is greater than the minimum value, the resonant converter operates in a monotonic gain mode; when the frequency f s is less than the minimum value, the resonant converter operates in a non-monotonic operating mode.

4. The method of claim 1, wherein, Step four, according to step two, if it is heavy load, the system enters the gain non-monotonic working mode; and the working frequency is limited above the minimum frequency value under the load, including: According to step two, if it is heavy load, the system enters the gain non-monotonic working mode; The system calculates the minimum frequency corresponding to the current situation by calculating the size of the load current, and limits the working frequency above the minimum frequency.

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