A frequency compensation-based control method for a full-bridge LLC resonant converter

Abstract

The application discloses a kind of full-bridge LLC resonant converter control methods based on frequency compensation, comprising the following steps: step one, sampling is carried out;Step two, set t1 time;Step three, the frequency loss f ff1 That needs to be compensated in a resonant period is calculated;Step four, the frequency compensation f ff2 In ΔT time period with t1 time as starting is calculated;Step five, the full-bridge LLC resonant converter carries out frequency loss in a resonant period according to f ff1 , frequency compensation is carried out in ΔT time period with t1 time as starting according to f ff2 The application only needs to sample input bus voltage Vin, output voltage V o And load current I o , control is simple;Output voltage precision is high, ripple is small, response speed is fast.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electricity, in particular to a full-bridge LLC resonant converter control method based on frequency compensation. BACKGROUND

[0002] LLC resonant converter has been widely used in modern power electronics field due to its excellent performance, such as high efficiency, high power density and low electromagnetic interference (EMI) and other advantages, especially in high demand scenarios such as data centers, distributed energy systems, energy storage devices and electric vehicles. Compared with traditional hard-switching converters, LLC resonant converters can achieve higher efficiency and lower electromagnetic interference through resonant operation, which makes them more competitive in power conversion.

[0003] However, due to the core working mechanism of LLC resonant converter depends on the resonance of current and voltage, it shows significant nonlinear characteristics in dynamic process. This nonlinear characteristic makes it difficult for traditional linear control methods, such as PID control, state feedback control, etc., to effectively cope with the control requirements under complex working conditions such as load change, input voltage fluctuation and external disturbance.

[0004] Currently, in the application of full-bridge LLC resonant converter, the problem of excessive dynamic response overshoot and slow adjustment speed is still an important factor affecting the system performance. In order to solve these problems, the traditional method is to increase the sampling of output current sensor and adopt closed-loop control strategy to accurately adjust the current. Through real-time feedback of the change of output current, the controller can quickly adjust the operating state of the converter to optimize the dynamic response. However, this method usually introduces additional sensor cost and hardware complexity, and depends on high-precision current measurement, which may be limited by sensor accuracy, noise interference and sensor bandwidth, etc. This makes it difficult to achieve ideal control accuracy and response speed under some high-speed changing load conditions.

[0005] Meanwhile, the low-frequency ripple existing in the input power bus, especially the 100Hz secondary voltage harmonic introduced by the power factor correction (PFC) circuit, is also an important factor affecting the stability and performance of the LLC resonant converter. Since the PFC circuit is usually connected with the full-bridge LLC resonant converter, the voltage fluctuation at the input end will cause a large low-frequency ripple, which will directly affect the output performance of the converter, resulting in fluctuations of the output voltage and current, and even unnecessary noise and electromagnetic interference. In order to suppress these low-frequency ripples, the traditional solutions include parallel connection of a large number of electrolytic capacitors at the input end, addition of an LC filter or use of an active decoupling circuit. The electrolytic capacitor can provide a higher charge storage capacity, which helps to smooth the input voltage and reduce the low-frequency fluctuation; and the LC filter effectively reduces the influence of low-frequency ripples on the performance of the converter by filtering signals of a specific frequency band. However, these methods usually increase the volume, cost and complexity of the system, especially in high-power, high-frequency applications, the volume and weight of the electrolytic capacitor limit its effectiveness. In practical applications, the traditional control method often faces problems such as excessive overshoot in dynamic response, slow adjustment speed, and excessive output voltage ripple, especially in high-performance systems, such a control effect is difficult to meet the requirements of accurate power regulation, fast response and low output ripple. SUMMARY

[0006] To solve the above technical problems, the present application provides a full-bridge LLC resonant converter based on frequency compensation.

[0007] The object of the present application is achieved by the following technical solutions:

[0008] A full-bridge LLC resonant converter control method based on frequency compensation, comprising the following steps:

[0009] Step one, sampling the input voltage V in , output voltage V0 and output current I0 of the full-bridge LLC resonant converter;

[0010] Step two, setting the second MOS tube Q2 and the third MOS tube Q3 of the primary side circuit in the full-bridge LLC resonant converter to be in the off state, the first MOS tube Q1 and the fourth MOS tube Q4 to be in the on state, and the time when the resonant current is equal to the excitation current as t1 time;

[0011] Step three, calculating the frequency loss f ff1 to be compensated in a resonant period;

[0012] Step four, calculating the frequency compensation f ff2 in the time period of ΔT starting from t1 time;

[0013] Step five, the full-bridge LLC resonant converter carries out frequency loss in a resonant period ff1 frequency compensation in the ΔT time period starting from t1 moment ff2

[0014] Further improvement, the full-bridge LLC resonant converter includes a transformer, the primary side of the transformer is electrically connected with the primary side circuit, the secondary side is electrically connected with the secondary side circuit, and the secondary side circuit is electrically connected with the load ZL; the primary side circuit includes first MOS tube Q1, second MOS tube Q2, third MOS tube Q3 and fourth MOS tube Q4; one end of the primary side of the transformer is electrically connected with the source of the first MOS tube Q1 and the drain of the second MOS tube Q2 through the filter circuit, and the other end is electrically connected with the source of the third MOS tube Q3 and the drain of the fourth MOS tube Q4; the drain of the first MOS tube Q1 is electrically connected with the drain of the third MOS tube Q3, and the source of the second MOS tube Q2 is electrically connected with the source of the fourth MOS tube Q4.

[0015] Further improvement, when the full-bridge LLC resonant converter operates, the moment when the second MOS tube Q2 and the third MOS tube Q3 are in the off state and the first MOS tube Q1 and the fourth MOS tube Q4 are in the on state in a resonant period is t0 moment;

[0016] The moment when the second MOS tube Q2 and the third MOS tube Q3 are in the off state, the first MOS tube Q1 and the fourth MOS tube Q4 are in the on state, and the resonant current is equal to the excitation current is t1 moment;

[0017] The moment when the second MOS tube Q2, the third MOS tube Q3, the first MOS tube Q1 and the fourth MOS tube Q4 are all off is t2 moment;

[0018] The moment when the first MOS tube Q1 and the fourth MOS tube Q4 maintain off, and the second MOS tube Q2 and the third MOS tube Q3 are on is t3 moment;

[0019] The moment when the first MOS tube Q1 and the fourth MOS tube Q4 maintain off, the second MOS tube Q2 and the third MOS tube Q3 are in the on state, and the resonant current is equal to the excitation current is t4 moment; The moment when the first MOS tube Q1 and the fourth MOS tube Q4, the second MOS tube Q2 and the third MOS tube Q3 are off again is t5 moment;

[0020] The moment when the second MOS tube Q2 and the third MOS tube Q3 are in the off state, and the first MOS tube Q1 and the fourth MOS tube Q4 are in the on state in the next resonant period of the full-bridge LLC resonant converter is t6 moment.

[0021] Further improvement, the specific method of step three is as follows: ​

[0022] The frequency loss f needed to compensate the time period t1-t6 is calculated ff1 :

[0023]

[0024] Where, L r represents the resonant inductance, L m represents the excitation inductance, C r represents the resonant capacitance, f r represents the resonant frequency, n represents the transformer turns ratio, V ref represents the output target voltage, f s represents the power device switching frequency, Z0 represents the characteristic impedance value.

[0025] Further improvement, the specific method of step four is as follows:

[0026] The time period ΔT needed to speed up the dynamic response and the frequency compensation f are calculated ff2 :

[0027]

[0028] Where, t n represents the completion time of the nth iteration, V CrN represents the normalized value of the resonant capacitor voltage, I n represents the current value at the nth iteration; I H represents the maximum load current, i.e. the maximum value of I0; I L represents the minimum load current, i.e. the low value of I0

[0029] The beneficial effects of the present application are:

[0030] (1) Only the input bus voltage Vin, output voltage V o and load current I o need to be sampled;

[0031] (2) The control is simple;

[0032] (3) The output voltage has high precision, small ripple and fast response speed. BRIEF DESCRIPTION OF DRAWINGS

[0033] The present application is further illustrated by the accompanying drawings, but the contents of the drawings do not constitute any limitation on the present application.

[0034] Figure 1 It is a full-bridge LLC resonant circuit topology.

[0035] Figure 2A It is an equivalent circuit I during the operation process of the full-bridge LLC resonant circuit topology.

[0036] Figure 2B Equivalent circuit two for full-bridge LLC resonant circuit topology during operation.

[0037] Figure 2C Equivalent circuit three for full-bridge LLC resonant circuit topology during operation.

[0038] Figure 3 Current and voltage analysis diagram for LLC operation.

[0039] Figure 4A Resonant voltage and current trajectory curve Figure 1 .

[0040] Figure 4B Resonant voltage and current trajectory curve diagram two.

[0041] Figure 5 Frequency prediction control block diagram.

[0042] Figure 6A Resonant current and voltage when load changes Figure 1 .

[0043] Figure 6B Resonant current and voltage diagram two when load changes.

[0044] Figure 7 Frequency compensation algorithm flow chart.

[0045] Figure 8 Output ripple effect diagram.

[0046] Figure 9 Dynamic response effect diagram. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical scheme and advantages of the application more clear and obvious, the application will be further described in detail below in combination with the drawings and examples.

[0048] System principle description

[0049] The topology structure of the full-bridge LLC resonant converter is shown in Figure 1 . Among them, V i is the input voltage, V out is the output voltage, Z L is the output equivalent impedance. Q1~Q4 are the upper and lower power tubes on the primary side, Q5~Q6 are the secondary side rectifier tubes. L r is the resonant inductance, C r is the resonant capacitance, L m is the excitation inductance, C o is the output filter capacitance, and n is the transformer turns ratio. i Lr , V Cr , iLm These are the resonant inductor current, resonant capacitor voltage, and magnetizing inductor current, respectively. The equivalent circuit during operation can be summarized into the following three states: Figure 2A , Figure 2B , Figure 2C As shown. Depending on the actual working process, the change process is essentially a combination of the following three states.

[0050] From equivalent Figure 2A According to Kirchhoff's voltage law, we have:

[0051]

[0052] Find the general solution of the quadratic differential equation from (1) and (2):

[0053]

[0054] k1 and k2 are determined by the initial conditions, and can be deduced as follows:

[0055]

[0056] In the formula, V Cr0 I Cr0 To obtain the initial values ​​of the resonant voltage and current at time t = t0, The characteristic impedance value, The resonant frequency is determined by normalizing (3) and (4) and multiplying all voltage values ​​by a coefficient 1 / V. in Current quantity multiplied by coefficient Z0 / V in , can be obtained

[0057]

[0058] In (5) and (6), N represents the normalized variable. Combining (5) and (6), we can obtain...

[0059]

[0060] Similarly, from the equivalent diagrams (2) and (3), we can obtain...

[0061]

[0062] in,.

[0063]

[0064] From (7), (8), and (9), we can see that Figure 2A , Figure 2B The stable operating state is about V CrN I CrN The trajectory of the circle, Figure 2Cis an elliptical trajectory. Wherein, the center of the trajectory circle and the focus of the trajectory ellipse are determined by the specific working state. In the process of full-bridge LLC resonant operation, the driving signal, voltage and current waveforms are as shown in Figure 3 . Figure 2A , t1-t3 corresponds to the equivalent circuit Figure 2C , t3-t4 corresponds to the equivalent circuit Figure 2B .

[0065] Implementation of frequency prediction control strategy

[0066] According to the above rule, this paper proposes a control strategy based on frequency compensation, and the implementation block diagram of the strategy is as shown in Figure 5 . The control target is the output voltage, and the frequency compensation method is used to speed up the dynamic response and eliminate the output voltage ripple. As can be seen from Figure 4B , to eliminate the voltage ripple, the frequency loss f ff1 in the t1-t3 and t4-t0(t6) time periods needs to be compensated; and to speed up the dynamic response, the frequency compensation f ff2 in the ΔT time period is needed.

[0067] (1) In the t1-t3 and t4-t0(t6) time periods, the resonant current can be regarded as a constant value i Lr_1 , and the trajectory is symmetrical; the resonant voltage change value, i.e. the voltage u 13 between O1 and O2.

[0068]

[0069] From (10), (11), (12), (13), and replacing V ref with V o , we can get

[0070]

[0071] (2) When the load changes, the resonant current and voltage change conditions are as shown in Figure 6A and Figure 6B . From Figure 6A , the inductance current cannot be changed, and the inductance current remains unchanged in the t1-t3 time period:

[0072]

[0073] From Figure 6B According to geometric operation, we can get:

[0074]

[0075] Through iterative calculation, we can deduce:

[0076]

[0077] It should be pointed out finally that the above embodiments are only used for illustrating the technical solutions of the present application but not for limiting the protection scope of the present application, and although the present application has been described in detail with reference to the preferred embodiments, it should be appreciated by those skilled in the art that the technical solutions of the present application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A control method for a full-bridge LLC resonant converter based on frequency compensation, characterized in that, Includes the following steps: Step 1: Measure the input voltage V of the full-bridge LLC resonant converter. in Sampling of output voltage V0 and output current I0; Step 2: Set the second MOSFET (Q2) and the third MOSFET (Q3) of the primary side circuit to be off and the first MOSFET (Q1) and the fourth MOSFET (Q4) to be on during one resonant cycle of the full-bridge LLC resonant converter, and set the time when the resonant current and the excitation current are equal to time t1. Step 3: Calculate the frequency loss f that needs to be compensated within one resonant period. ff1 ; Step 4: Calculate the frequency compensation f during the time interval ΔT starting from time t1. ff2 ; Step 5: Within one resonant cycle, the full-bridge LLC resonant converter operates according to f... ff1 Frequency loss is performed, within a time interval ΔT starting from time t1, according to f ff2 Perform frequency compensation; The full-bridge LLC resonant converter includes a transformer. The primary side of the transformer is electrically connected to a primary circuit, and the secondary side is electrically connected to a secondary circuit. The secondary circuit is electrically connected to a load (ZL). The primary circuit includes a first MOSFET (Q1), a second MOSFET (Q2), a third MOSFET (Q3), and a fourth MOSFET (Q4). One end of the primary side of the transformer is electrically connected to the source of the first MOSFET (Q1) and the drain of the second MOSFET (Q2) through a filter circuit, and the other end is electrically connected to the source of the third MOSFET (Q3) and the drain of the fourth MOSFET (Q4). The drain of the first MOSFET (Q1) is electrically connected to the drain of the third MOSFET (Q3), and the source of the second MOSFET (Q2) is electrically connected to the source of the fourth MOSFET (Q4). When the full-bridge LLC resonant converter is running, during one resonant cycle, the second MOSFET (Q2), the third MOSFET (Q1), the fourth MOSFET (Q4), and the fourth MOSFET (Q3) are in the off state, and the first MOSFET... The time when it is in the activated state is time t0; The second MOSFET (Q2) and the third MOSFET (Q3) are in the off state, the first MOSFET (Q1) and the fourth MOSFET (Q4) are in the on state, and the moment when the resonant current and the excitation current are equal is time t1. The time when the second MOSFET (Q2), the third MOSFET (Q3), the first MOSFET (Q1), and the fourth MOSFET (Q4) are all turned off is time t2; The first MOSFET (Q1) and the fourth MOSFET (Q4) remain off, and the second MOSFET (Q2) and the third MOSFET (Q3) are turned on at time t3. The first MOSFET (Q1) and the fourth MOSFET (Q4) remain off, the second MOSFET (Q2) and the third MOSFET (Q3) are on, and the moment when the resonant current equals the excitation current is t4. The time when the first MOSFET (Q1) and the fourth MOSFET (Q4), the second MOSFET (Q2) and the third MOSFET (Q3) are all turned off again is time t5; In the next resonant cycle of the full-bridge LLC resonant converter, the second MOSFET (Q2) and the third MOSFET (Q3) are in the off state, and the first MOSFET (Q1) and the fourth MOSFET (Q4) are in the on state at time t6. The specific method for step three is as follows: The calculated frequency loss f during the time period t1-t6 needs to be compensated. ff1 : Among them, L r L represents the resonant inductance. m C represents the magnetizing inductance. r Represents the resonant capacitance, f r Represents the resonant frequency, n represents the transformer turns ratio, and V ref Indicates the target output voltage, f s Z0 represents the characteristic impedance value of the power device, where Z represents the switching frequency of the power device. The specific method for step four is as follows: The calculations yielded the required ΔT time period and frequency compensation f to accelerate the dynamic response. ff2 : Among them, t n V represents the completion time of the nth iteration. CrN I represents the normalized value of the resonant capacitor voltage. n I represents the current value at the nth iteration. H This represents the maximum load current, i.e., the maximum value of I0; I L This represents the minimum load current, i.e., the minimum value of I0.

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