Light load control method for LLC resonant converter

Abstract

The invention discloses a light load control method for an LLC resonant converter, and the method comprises the steps: dividing the work of the converter into repeated large periods T, and each large period T comprises an energy transfer period T1 and a non-energy transfer period T2; in the T1, the inverter unit transmits energy to the load according to a continuous pulse train in a preset switching period Ts, and the continuous pulse train comprises n1 continuous switching periods; in the T2, the resonant cavity enters a ternary resonant state formed by a resonant inductor Lr, a resonant capacitor Cr and an excitation inductor Lm by controlling a switch combination of the inversion unit, the T2 comprises n2 ternary resonant periods Tr2, and the ternary resonant periods Tr2 refer to a natural resonant period jointly determined by Lr, Cr and Lm; and dynamically adjusting the time ratio of T1 to T2 or adjusting the values of n1 and n2 based on the load condition and output voltage feedback so as to actively change the equivalent quality factor QEFF of the converter, thereby changing the voltage gain characteristic of the converter under the light load condition, realizing effective voltage reduction and maintaining soft switching and high efficiency.

Description

Technical Field

[0001] This invention relates to the field of resonant converter technology, and in particular to a light-load control method for LLC resonant converters. Background Technology

[0002] LLC resonant converters, due to their soft-switching characteristics and advantages such as high efficiency and high power density, occupy an important position in medium- and high-power power supply applications and have been widely used in high-end power systems such as data centers, new energy, and electric vehicles. However, these resonant converters face control challenges under light-load conditions caused by their inherent gain characteristics, especially in the high-frequency operating range. Traditional PFM (pulse frequency modulation) methods cannot effectively reduce voltage, leading to the failure of traditional frequency converter control logic under light-load conditions.

[0003] Specifically, as the load gradually decreases, the voltage gain curve of the LLC resonant converter exhibits a significant "upward tilt" in the high-frequency region, meaning that the voltage gain increases with further increases in the switching frequency. This characteristic completely contradicts the conventional buck regulation logic of "increasing the frequency to reduce the output voltage." This means that under light load conditions, the frequency increase command issued by the control system to reduce the output voltage may instead trigger an uncontrolled increase in the output voltage, leading to system instability, startup failure, and even the risk of damaging the load.

[0004] To address the aforementioned light-load operation issues, existing technologies typically employ light-load control strategies such as burst mode, reducing the average switching frequency through intermittent operation to avoid gain spikes in the high-frequency region. However, this type of burst mode control inevitably introduces significant low-frequency current ripple and audible audio noise, and its alternating "operation-sleep" mechanism also leads to slower dynamic response. Furthermore, due to the inherent uncertainty in the turn-on and turn-off timings in burst mode, it is difficult to maintain soft-switching conditions continuously during certain operating periods, resulting in increased switching losses and decreased overall efficiency. This makes it difficult to meet the demands of high-performance power supply applications that require a balance of high efficiency, low noise, and fast dynamic response.

[0005] Therefore, there is an urgent need to propose a new light-load control method that can effectively solve the gain sag problem of resonant converters under light-load conditions, while taking into account soft-switching characteristics and system efficiency, so as to improve the stability and application value of LLC resonant converters across the entire load range. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies, solve the core technical problem of the inherent upward tilt of the voltage gain curve of the converter under light load conditions, which prevents stable voltage reduction, and prevent the modulated equivalent quality factor Q from being reduced. EFFExcessive gain spikes can lead to gain imbalance, lack of soft switching, and oscillation problems. This paper presents a light-load control method for LLC resonant converters. Through frequency modulation and pulse sequence management, it ensures the converter always operates in a safe and efficient state, fundamentally overcoming the runaway problem caused by gain spikes under light loads. This provides key technical support for achieving efficient and stable operation of resonant converters under light loads. The paper also analyzes the boundary equivalent quality factor Q. EFF This improves security and reliability.

[0007] The objective of this invention is achieved through the following technical solution: A light-load control method for LLC resonant converters The converter operation is divided into repeated large cycles T, each large cycle T including an energy transfer cycle T1 and a non-energy transfer cycle T2. In the energy transfer cycle T1, the inverter unit transfers energy to the load in a predetermined switching cycle Ts and in continuous pulse groups, the continuous pulse groups containing n1 continuous switching cycles. In the non-energy transfer cycle T2, the switching combination of the inverter unit is controlled to make the resonant cavity enter a three-element resonance state composed of resonant inductor Lr, resonant capacitor Cr and excitation inductor Lm. The non-energy transfer cycle T2 contains n2 three-element resonance cycles Tr2, the three-element resonance cycle Tr2 refers to an inherent resonance period determined by the resonant inductor Lr, resonant capacitor Cr and transformer excitation inductor Lm. It also includes: dynamically adjusting the time ratio of the energy transfer cycle T1 and the non-energy transfer cycle T2 based on load conditions and output voltage feedback, or adjusting the values ​​of n1 and n2 to actively change the equivalent quality factor Q of the converter. EFF This allows the voltage gain characteristics of the converter to be changed under light load conditions, achieving effective voltage reduction while maintaining soft switching and high efficiency.

[0008] Furthermore, during the energy transfer cycle T1, the inverter unit is controlled to operate in a differential drive state: by making the switching transistors located in different bridge arms conduct in pairs to generate differential drive voltages of opposite polarities alternately, energy is transferred to the secondary side. During the non-energy transfer cycle T2, the control inverter unit simultaneously turns on a pair of switching transistors located on different bridge arms but on the same polarity side according to the time interval, so that the primary side of the transformer is placed in a zero differential voltage state, preventing energy transfer to the load, and causing the resonant cavity to enter a three-element resonance state in the primary side circuit of the transformer.

[0009] Furthermore, during the positive half-cycle of the energy transfer cycle T1, a set of switching transistors located on opposite polarity sides of each bridge arm are turned on in a complementary manner; during the negative half-cycle, another set of switching transistors located on the other polarity side of each bridge arm are turned on in a complementary manner, forming an alternating polarity driving voltage on the primary side of the transformer to transfer energy to the load.

[0010] Furthermore, by measuring the output voltage and load current, and selecting a set of preset (n1,n2) control modes based on the measured load percentage or equivalent load current, the preset group includes: n1=3,n2=1; n1=2,n2=1; n1=1,n2=2; n1=1,n2=1; during operation, the (n1,n2) control modes are smoothly switched to reduce output voltage fluctuations and current stress.

[0011] Furthermore, an energy flow model is established based on energy conservation, relating the remaining energy of the resonant cavity to the energy required for energy transfer. This energy flow model is then used to derive and determine the maximum permissible equivalent quality factor Q that ensures no secondary-side energy transfer occurs during the non-transfer period T2. EFF The value is used to avoid energy transfer, soft switching failure, or oscillation during the non-energy transfer cycle T2.

[0012] Furthermore, the equivalent quality factor Q EFF The initial energy stored in the resonant cavity, E initial Work done to overcome the secondary side reflected voltage (W) reflect It is calculated based on the law of conservation of energy, and is calculated and assigned values ​​by table lookup in real time or periodically.

[0013] Preferably, the present invention also provides a light-load controller for an LLC resonant converter, comprising: The measurement unit is used to sample the output voltage and load current; The calculation unit is used to calculate the load condition based on the sampled signal obtained by the measurement unit and determine the time ratio of (n1, n2) and T1 / T2, and calculate the equivalent quality factor Q according to the energy conservation model. EFF ; The drive unit generates drive signals for the inverter unit according to the output of the computing unit, so that the converter operates according to the timing sequence of T1 and T2.

[0014] The controller contains a preset control mode table or lookup table, which is used to map different load ranges to different (n1,n2) combinations and control modes; and the computing unit can be fine-tuned online during operation.

[0015] Preferably, the present invention also provides a power conversion system, including an LLC resonant converter and a controller. The converter's topology, resonant elements, and magnetizing inductor structure are conventionally arranged. The controller is used to control the inverter unit within the converter to operate according to a light-load control method. The power conversion system also includes a monitoring unit for recording and analyzing the resonant cavity current, resonant cavity voltage, and secondary-side conduction state during the non-energy transfer cycle T2, for offline or online verification of the equivalent quality factor Q. EFF And optimize control strategies.

[0016] Preferably, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the light-load control method.

[0017] Compared with the prior art, the beneficial effects of the technical solution of the present invention are: 1. The control strategy of this invention divides a complete large cycle T into an energy transfer cycle T1 and a non-energy transfer cycle T2, and operates alternately with pulse groups (n1×Ts) and three-element resonant cycles (n2×Tr2). By concentrating energy transfer in the short energy transfer phase T1, the actual load during T1 is greater than the average load, thus "equivalent" to a higher load state under the fundamental frequency approximation. This allows the gain corresponding to the selected operating frequency during the energy transfer period to be reduced (i.e., the operating point in the light load region is mapped to the gain drop segment), thereby overcoming the runaway caused by gain rise. Unlike the traditional Burst method, this method uses a controlled three-element resonant cycle in T2 instead of a simple shutdown, maintaining the periodic equivalence of the cavity energy cycle, thus avoiding large low-frequency ripple and audible noise, and preserving better soft-switching conditions, thereby improving light load efficiency and dynamic response. This addresses the issues of voltage gain "increasing" in the high-frequency region under light load in traditional LLC circuits, which prevents voltage reduction by increasing the switching frequency; and the problems of audio noise, increased ripple, and poor dynamic response caused by PFM / Burst mode.

[0018] 2. This invention uses a pulse group of n1 consecutive switching cycles (according to a predetermined Ts) within the energy transfer cycle T1, and n2 three-element resonant cycles Tr2 (determined by Lr, Cr, and Lm) within T2. ​​The continuous pulse group (n1) ensures sufficient average power output in each energy transfer segment; T2 is based on a complete three-element resonant cycle (not an arbitrary off-time), making the energy of the resonant cavity approximately equal at the beginning and end of T2. i Lr(ta) = i Lr(tb) This approach maintains macroscopic energy conservation, ensuring the continued applicability of fundamental wave analysis and guaranteeing system stability, without introducing additional bias magnetism or output drift caused by asymmetric energy storage. It addresses how to achieve concentrated energy transfer mechanically / temporally while maintaining the periodic equivalence of the cavity's energy state during non-energy transfer phases, avoiding energy disturbances caused by the non-energy transfer period T2.

[0019] 3. Actively change the equivalent quality factor Q by adjusting the T1 / T2 time ratio (or n1 / n2). EFF To change the shape of the gain curve (making the gain decrease in the high-frequency range). Q EFF This is a key parameter affecting the gain-frequency characteristics of an LLC circuit. By changing the energy transfer ratio in the time domain, the circuit exhibits a higher instantaneous Q within the energy transfer window. EFF(or equivalent changes) are made to adjust the gain curve, turning the curve that originally sloping upwards at high frequencies into a falling segment or reducing the degree of upward sloping, thereby achieving effective voltage reduction control. Under light loads, traditional frequency converter voltage regulation (increasing the frequency to reduce voltage) is ineffective or even reverses (gain sloping upwards).

[0020] 4. This invention establishes a cavity energy flow model based on energy conservation and derives Q. EFF Safety boundaries; limiting Q in control EFF Located within the safety boundary. Through energy conservation / threshold energy calculations (considering Einitial and Wreflect), the maximum Q that prevents energy transfer at T2 is obtained. EFF This establishes boundaries, providing theoretical and engineering safety constraints to prevent control strategies from exceeding limits and causing hazards. This measure improves system reliability and safety margins while ensuring effective voltage reduction.

[0021] 5. Real-time or preset lookup table implementation within the controller enables (n1,n2) selection and frequency / timing adjustment based on output voltage and load measurements. Measurement and closed-loop control allow the control strategy to adapt to load and input voltage changes, maintain output voltage stability, reduce overshoot and undershoot, and adjust Q based on observations. EFF To avoid overstepping boundaries and improve robustness and engineering feasibility.

[0022] 6. During T2, a specific switching combination (e.g., Q1 and Q3 conducting simultaneously, or Q2 and Q4 conducting simultaneously) is used to achieve a primary-side short circuit and induce a three-element resonance. The primary-side short circuit isolates the DC side from the secondary side, allowing the resonant cavity to release / exchange energy to the magnetizing inductor and resonant elements through its own resonance, without transferring energy to the load. This achieves a smooth sleep effect while retaining the advantages of soft switching. Precise switching combinations can evenly distribute heat dissipation and avoid asymmetrical current stress.

[0023] 7. Provide several (n1, n2) examples in the design / simulation / experiment (e.g., n1=2, n2=1; n1=1, n2=2; n1=3, n2=1) and verify them through simulation / experiment. The examples provide the operating point under actual parameters, and a comparison of simulated and experimental waveforms, which can serve as a reference for selecting (n1, n2) in actual engineering projects. They also demonstrate the effectiveness of the method in the 10%~25% load range, enhancing the credibility of the application. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a full-bridge LLC topology.

[0025] Figure 2 This is a diagram illustrating the control strategy.

[0026] Figure 3 The waveform diagram is shown under the control mode of n1=2 and n2=1.

[0027] Figure 4 This is the working mode diagram for the positive half-cycle of time period T1.

[0028] Figure 5 This is the working mode diagram for the positive half-cycle of the T2 time period.

[0029] Figure 6a and Figure 6b The equivalent circuit diagrams for time period T1 and time period T2 are shown respectively.

[0030] Figure 7 The equivalent circuit diagram shows the energy transfer phenomenon that occurs during the first half-switching cycle (Ts / 2) of time period T2.

[0031] Figure 8 Equivalent quality factor Q EFF Simulation waveforms within boundary conditions.

[0032] Figure 9 Equivalent quality factor Q EFF Simulation waveform diagram when the value is too large.

[0033] Figure 10 The simulation waveform is shown for 15% load—n1=2, n2=1.

[0034] Figure 11 The simulation waveform is shown for 10% load—n1=1, n2=2.

[0035] Figure 12 The experimental waveform diagram is for 15% load—n1=2, n2=1.

[0036] Figure 13 The experimental waveform diagram is for 10% load—n1=1, n2=2.

[0037] Figure 14 Gain curves were obtained experimentally under different control modes at 250Ω (10% load). Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0039] Example 1 This embodiment provides a light-load control method for LLC resonant converters, such as... Figure 1The diagram shows the topology of a full-bridge LLC resonant converter. The inverter unit consists of switches Q1-Q4 (the primary side of the LLC), Lr is the resonant inductor, Cr is the resonant capacitor, and Lm is the magnetizing inductor. Rectifier diodes D1-D4 constitute the rectifier unit (the secondary side of the LLC), and the transformer turns ratio is n:1. AB , i Lr and i Lm These are the resonant cavity voltage, resonant current (resonant cavity current), and excitation current, respectively. The specific control method is as follows: By controlling the drive signals of the switching transistors on the inverter unit side, a "start-stop" control strategy is adopted. This embodiment's control scheme generates drive signals with specific timing sequences. (Refer to the appendix.) Figure 2 The key cavity voltage (V) shown AB ) and current waveform ( i Lr In this waveform, a complete operating cycle T consists of two distinct phases: "start"—the energy transfer cycle T1; and "stop"—the non-energy transfer cycle T2. That is, T = T1 + T2. The equivalent quality factor Q of the circuit can be improved by adjusting its switching frequency and pulse sequence distribution pattern. EFF Active control of the value solves the problem of voltage reduction in high-frequency bands under light load conditions.

[0040] When the converter is in the "on"—power transfer cycle T1: the upper and lower switches of the two bridges of the inverter unit are complementary and conduct, that is, during the positive half-cycle, switches Q1 and Q4 are conducting. At this time, the voltage applied to the resonant cavity is the DC side voltage V. in During the negative half-cycle, switching transistors Q2 and Q3 are turned on, and the voltage applied to the resonant cavity is -V. in Energy is transferred from the DC side to the resonant cavity and the load side. This cycle consists of a continuous group of switching pulses, with a total duration of T1 = n1 × Ts. Where Ts is the switching cycle in the current operating state, and n1 is the number of continuous switching pulses included in this energy transfer phase.

[0041] When the converter is in the "stop"—non-energy transfer cycle T2: the upper and lower switches of the two bridges of the inverter unit are simultaneously and alternately turned on, that is, switches Q1 and Q3 are turned on. At this time, the primary side of the resonant cavity is short-circuited, the system stops transferring energy to the load, and enters a silent period. Until the next non-energy transfer cycle arrives, switches Q2 and Q4 are turned on simultaneously. During this stage, the resonant cavity is in a resonant state, the duration of which is T2 = n2 × Tr2. Wherein, Tr2 is the three-element resonant period under this specific operating condition (an inherent resonant period Tr2 determined by the resonant inductance Lr, the resonant capacitance Cr, and the transformer magnetizing inductance Lm), and n2 is the number of three-element resonant periods contained in this non-energy transfer stage.

[0042] To further clarify the effectiveness of this control method, this embodiment provides a corresponding theoretical analysis model. (See attached...) Figure 2 As shown, the resonant current i Lr At the beginning time t of the non-energy transfer cycle T2 a With end time t b Its current amplitude and phase exhibit equivalence, that is i Lr(ta) = i Lr(tb) This key feature indicates that, from an energy state perspective, the electromagnetic energy states stored in the resonant cavity are approximately equal at the beginning and end of the energy transfer period. This periodic equivalence of energy states signifies that the entire system has entered a stable energy cycle state macroscopically. It proves that although this embodiment introduces a non-energy transfer cycle, altering the energy transfer method, the overall operating characteristics of the converter still satisfy the applicability conditions of the classical fundamental wave analysis method. Therefore, the validated fundamental wave analysis method can continue to be used to model and calculate its gain characteristics. Furthermore, this modified energy transfer method can reduce the switching frequency and turn-off current, thereby improving efficiency under light loads.

[0043] In this embodiment, the quality factor Q is adjusted by controlling the frequency of the inverter unit's switching transistors and the pulse sequence. EFF To change the gain characteristics, therefore, to prevent the adjusted Q... EFF The larger value ensures energy transfer during the "stop" phase, avoiding voltage regulation imbalance, lack of soft switching, excessive current stress caused by excess energy, and DC bias problems during non-energy transfer cycles. The energy conservation principle is used to analyze the complex interaction between the energy stored in the resonant cavity and the energy required to overcome the reflected voltage during non-energy transfer periods. An energy flow model is established, and by establishing the dynamic balance between the remaining energy in the cavity, the energy stored in the resonant elements, and the energy loss under the reflected voltage, the maximum allowable Q to ensure stable system operation is derived. EFF Value boundary.

[0044] By measuring the output voltage and load current, and selecting a set of preset (n1,n2) control modes based on the measured load percentage or equivalent load current, the preset groups include: n1=3,n2=1; n1=2,n2=1; n1=1,n2=2; n1=1,n2=1; and during operation, the (n1,n2) control modes are smoothly switched to reduce output voltage fluctuations and current stress. Specifically, this embodiment takes the preset (2,1) control mode, i.e., the control strategy of n1=2 and n2=1, as an example. Figure 3The key waveform diagram shown includes the drive signals G1~G4 of the primary-side switching transistors and the resonant cavity voltage V. AB Resonant cavity current i Lr and load current i o Waveform. Two Ts are generated within the energy transfer cycle T1 by controlling the complementary conduction of the upper and lower switches of the inverter bridge. During the energy transfer period T1, from t0 to t1, primary-side switches Q1 and Q4 are turned on, and secondary-side rectifier diodes S1 and S4 are turned on, resulting in a DC input voltage V. in Energy is applied across the two ends of the resonant network, transferring from the DC side to the carrier side. Due to its half-cycle symmetry, only the positive half-cycle is analyzed, and its positive half-cycle operating mode is as follows: Figure 4 As shown. Its equivalent circuit is... Figure 6a As shown. The expressions for its resonant period Tr and the three-element resonant period Tr2 are as follows: (1); (2); The inverter bridge switches are controlled to alternately conduct simultaneously, generating one Tr2 within the non-energy transfer cycle T2. During the non-energy transfer period T2, from t4 to t5, primary-side switches Q1 and Q3 are turned on, the resonant cavity voltage is zero, and the system enters a three-element resonant state. The operating mode diagram is shown below. Figure 5 As shown, to ensure uniform heat dissipation of the primary-side switches, primary-side switches Q2 and Q4 are turned on during the period t9-t10. The equivalent circuit is as follows: Figure 6b As shown. Since the non-energy transfer period is a three-element resonant period, neglecting loop losses, the energy stored in the resonant inductor Lr and the resonant capacitor Cr is equal at the start and end of the non-energy transfer period, that is: (3); This is equivalent to assigning the entire energy transfer task within the entire time interval T to the energy transfer phase T1. V Cr The voltage across the resonant capacitor allows the LLC to transition from a light-load to a heavy-load condition, hence the following relationship can be obtained: (4); Where I o_act and R o_act It is the actual load current and equivalent load during the T1 energy transfer stage, I o_ave and R o This represents the average load current and actual load within the large cycle T.

[0045] An approximate equivalent analysis of the LLC resonant converter is performed using the Fundamental Harmonic Approximation (FHA) method, and its equivalent AC resistance is determined. R ac The energy consumed is approximately equal to the energy consumed by the load, therefore (5); V o For the output voltage, therefore, R ac It can be represented as (6); According to the definition of voltage gain of DC-DC converters nV o / V in The DC voltage gain of LLC is approximately derived using the equivalent fundamental frequency analysis method. M The expression is (7); in , representing characteristic impedance; , which is the equivalent quality factor; , indicating the inductance ratio; , f n Represents the normalized frequency. f s Indicates the operating frequency. f r This indicates the resonant frequency.

[0046] This embodiment establishes an energy flow model based on energy conservation, relating the remaining energy of the resonant cavity to the energy required for energy transfer. Using this energy flow model, it derives and determines the maximum permissible equivalent quality factor Q that ensures no secondary-side energy transfer occurs during the non-energy transfer period T2. EFF The value is set to avoid energy transfer failures, soft switching omissions, or oscillations during the non-energy transfer cycle T2. Specifically: During the control process in LLC, if the final modulated equivalent quality factor Q EFF If the value is too large, it will cause oscillatory energy transfer phenomena during the T2 time period, and the energy state of the energy storage element will change between the beginning and end of the T2 time period. This situation will lead to problems such as voltage offset, soft switching losses, and increased core losses. Therefore, to address the above-mentioned problems, the boundary Q is adjusted. EFFThe analysis focuses on the boundary condition: during time period T2, the secondary side is just conducting, and energy is transferred to or not transferred to the secondary side. The condition for the secondary side to conduct is that the voltage generated by the primary side resonant current on the magnetizing inductor is sufficient to overcome the secondary side reflected voltage and the diode voltage drop. Therefore, from an energy perspective, the boundary condition should be the energy storage state E of the resonant cavity at the beginning of time period T2. initial Able to overcome secondary side reflected voltage and do work W reflect The work done by the diode voltage drop W diode and loop loss W loss Only then can the secondary side conduct. Therefore, based on the principle of energy conservation, we can obtain: (8); At the beginning of stage T2 (t=0), let the inductor current be I. ro The resonant capacitor voltage is V cro The initial energy stored in the resonant cavity It can be represented as: (9); To represent the energy of this resonant cavity at this moment, let's assume that the current amplitude I... o Switching in the case of ( ): (10); (11); Precise circuit impedance model Solving for: (12); Threshold energy calculation. The energy required to overcome for a half-wave phenomenon to occur includes: 1) Doing work to overcome the secondary side reflected voltage: (13); 2) Overcome diode voltage drop (V f )acting: (14); 3) Overcoming loop losses by doing work: (15); R loss This represents the overall loop impedance, including wire resistance, and Rds(on) of MOS and Diode. Where τ is the integration time, expressed as: (16); ω r It is the resonant angular frequency.

[0047] The current model is then constructed. To prevent the absence of energy transfer (half-wave appearance) during the half-switching cycle (Ts) at the beginning of time period T2, the analysis focuses on the resonant current oscillation decay state during this half-cycle. The energy required for the resonant cavity to overcome the reflected voltage in this state includes the electromagnetic energy of the magnetizing inductor Lm and the energy transferred from the AC load Rac. The cavity supplies energy to the magnetizing inductor Lm and the AC resistance Rac, thus it can be equivalently represented as an equivalent resistance Re. Its equivalent circuit is shown below. Figure 7 As shown.

[0048] (17); in ω s This is the actual operating angular frequency. The circuit equations for this attenuation process follow the classic differential equations for a series RLC circuit. According to KVL, the sum of the voltages on all components in the loop is zero, and the relationships are as follows: (18); in V Lr (t) represents the voltage across the resonant inductor Lr. V Re (t) is the voltage across the equivalent impedance. V Cr (t) represents the voltage across the resonant capacitor.

[0049] Substitute the voltage-current relationship for each component: Inductor voltage: (19); Resonant capacitor voltage: (20); Resistance voltage: (twenty one); Substituting equations (19), (20), and (21) into the KVL equation, we obtain: (twenty two); Simultaneously change both sides of equation (22) with respect to time t Differentiate. We get: (twenty three); Let the damping coefficient be The resonant angular frequency is The damped oscillation angular frequency is .

[0050] The general solution is: (twenty four); A1 and A2 are determined by the initial conditions, namely: (25); in: (26); (27); Then the attenuation coefficient : (28); The final expression for the damped oscillation model of the current is: (29); in I o It is the current amplitude. φ It is the initial phase.

[0051] The boundary conditions in this study are based on the law of conservation of energy, so only the work done to overcome the secondary side reflected voltage is considered. For conservatism, an extreme case is considered: the resonant current is positive throughout the half-cycle, i.e., the initial value is 0, and... The current reaches its maximum value during this period. Therefore, the current model during this period simplifies to the following equation: (30); Ignoring diode voltage drop and circuit losses, , but .

[0052] Therefore, the energy boundary is: (31); in This is the threshold coefficient.

[0053] (32); The result obtained is: (33); (34); Since the transmission efficiency is highest and the loss is lowest at the resonant point, and considering that the defects of the fundamental wave analysis method become more significant the further away from the resonant point, the Q value corresponding to the resonant point is... EFF The value boundary is the minimum boundary. Therefore, it is only necessary to find the Q value at the resonant point. EFF Just the boundary.

[0054] Will , and Substituting into it, we get: (35); Define the discriminant function: (36); So the final Q EFF Boundary values ​​satisfy: (37)

[0055] Example 2 This embodiment provides a supplementary explanation of the light-load control method for the LLC resonant converter described above, based on specific applications and data, as follows: The input voltage range is rated at 100V, the resonant capacitor is 1.20uF, and the resonant inductor... L r The magnetizing inductance is 2.20uH. L m The transformer turns ratio is 13.83uH. n The ratio is 2:5, the output voltage is 250V, and the rated power is 2500W.

[0056] Perform boundary equivalent quality factor Q EFF For the numerical solution of the value, to be conservative, this embodiment takes λ as 0.7, and the boundary value of the obtained Q value is 0.66. Therefore, during the entire frequency modulation process, Q EFF The maximum value does not exceed 0.66.

[0057] When n1=3 and n2=1, its normal operating waveform is as follows: Figure 8 As shown in the diagram, during the energy transfer period T1, energy is transferred to the secondary side over three cycles. Afterwards, during the non-energy transfer period T2, a three-element resonant state occurs, and the energy storage element has approximately the same energy state at the beginning and end of this period. The output voltage is approximately 242V, consistent with the above analysis principle. Increasing the load to improve Q... EFF When the value is set, the final modulated Q is achieved. EFF When the stability boundary is exceeded, the simulated waveform is as follows: Figure 9 As shown, it can be seen that the energy transferred in each of the three small cycles during the energy transfer period is not uniform, and an oscillation phenomenon with gradually increasing amplitude occurs, which poses a certain danger. Secondly, a half-wave phenomenon occurs during the non-energy transfer period, which disrupts the inherent three-element resonant cycle, causing changes in the energy stored in the energy storage element, resulting in output voltage imbalance (output voltage is about 230V), bias magnetization, and lack of soft switching.

[0058] To verify its solution to the voltage reduction problem under light load, simulation experiments were conducted under different control modes at loads ranging from 10% to 25%. Taking 15% load and 10% load conditions as examples, simulation experiments were performed with different control modes. Figure 10 The simulated waveform at the resonant point with n1=2 and n2=1 under 15% load is shown. Figure 11 The simulated waveforms are for the resonant points with n1=1 and n2=2 under 10% load.

[0059] An experimental prototype with a rated power of 2500W was used, and its electrical parameters were consistent with the simulation parameters mentioned above. Experiments were conducted under different control modes and frequencies between 10% and 25% load. Taking 15% load and 10% load conditions as examples, at 15% load, n1=3, n2=2; n1=1, n2=1; n1=2, n2=1 were set respectively. At 10% load, n1=3, n2=2; n1=1, n2=1; n1=1, n2=2 were set respectively. Experiments were also conducted at different frequencies within these modes. Figure 12 The experimental waveform diagram of the resonant point under the control mode of n1=2 and n2=1 at 15% load; Figure 13 The experimental waveform at the resonant point is shown under the control mode of n1=1 and n2=2 at 10% load. The gain curve obtained from the 10% load experiment is shown below. Figure 14 As shown. From Figure 14 As can be seen, the LLC under traditional PFM control exhibits a gain spike, and frequency modulation cannot achieve a voltage reduction effect. Meanwhile, the control strategy of this invention can effectively solve this problem.

[0060] Example 3 Based on the same inventive concept, embodiments of this application also provide a light-load controller for LLC resonant converters, which can be used to implement the methods described in the above embodiments, as shown in the following embodiments. Since the principle of the light-load controller for LLC resonant converters in solving the problem is similar to that of the light-load control method for LLC resonant converters, the implementation of this controller can refer to the aforementioned method implementation, and repeated details will not be elaborated further. As used below, the terms "unit" or "module" can refer to a combination of software and / or hardware that implements a predetermined function. Although the system described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0061] Embodiments of the present invention provide a specific implementation of a controller capable of implementing a light-load control method for LLC resonant converters, specifically including the following: The measurement unit is used to sample the output voltage and load current; The calculation unit is used to calculate the load condition based on the sampled signal obtained by the measurement unit and determine the time ratio of (n1, n2) and T1 / T2, and calculate the equivalent quality factor Q according to the energy conservation model. EFF ; The drive unit generates drive signals for the inverter unit according to the output of the computing unit, so that the converter operates according to the timing sequence of T1 and T2.

[0062] The controller contains a preset control mode table or lookup table, which is used to map different load ranges to different (n1,n2) combinations and control modes; and the computing unit can be fine-tuned online during operation.

[0063] Preferably, embodiments of this application also provide a power conversion system, including an LLC resonant converter and a controller. The converter's topology, resonant elements, and excitation inductor structure are conventionally arranged. The controller is used to control the inverter unit within the converter to operate according to the light-load control method described in claim 1. The power conversion system also includes a monitoring unit for recording and analyzing the resonant cavity current, resonant cavity voltage, and secondary-side conduction state during the non-energy transfer cycle T2, for offline or online verification of the equivalent quality factor QEFF and optimization of the control strategy.

[0064] Preferably, embodiments of this application also provide a specific implementation of an electronic device capable of implementing all steps in the light-load control method for LLC resonant converters described in the above embodiments. The electronic device specifically includes the following components: Processor, memory, communications interface, and bus; The processor, memory, and communication interface communicate with each other via a bus; the communication interface is used to realize information transmission between server-side devices, metering devices, and user-side devices.

[0065] The processor is used to call a computer program in memory, and when the processor executes the computer program, it implements all the steps in the light-load control method for LLC resonant converters in the above embodiments.

[0066] Embodiments of this application also provide a computer-readable storage medium capable of implementing all steps of the light-load control method for LLC resonant converters in the above embodiments. The computer-readable storage medium stores a computer program that, when executed by a processor, implements all steps of the light-load control method for LLC resonant converters in the above embodiments.

[0067] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on its differences from other embodiments. In particular, hardware + program embodiments are relatively simple in description because they are fundamentally similar to method embodiments; relevant parts can be referred to the descriptions in the method embodiments.

[0068] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired result. In some embodiments, multitasking and parallel processing are possible or may be advantageous.

[0069] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0070] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0071] This invention is not limited to the embodiments described above. The above description of specific embodiments is intended to illustrate and explain the technical solutions of this invention. The specific embodiments described above are merely illustrative and not restrictive. Without departing from the spirit and scope of the claims, those skilled in the art can make many specific modifications based on the teachings of this invention, and these modifications all fall within the scope of protection of this invention.

Claims

1. A light-load control method for an LLC resonant converter, characterized in that, The converter operation is divided into repeated large cycles T, each large cycle T including an energy transfer cycle T1 and a non-energy transfer cycle T2. In the energy transfer cycle T1, the inverter unit transfers energy to the load in a predetermined switching cycle Ts and in continuous pulse groups, the continuous pulse groups containing n1 continuous switching cycles. In the non-energy transfer cycle T2, the switching combination of the inverter unit is controlled to make the resonant cavity enter a three-element resonance state composed of resonant inductor Lr, resonant capacitor Cr and excitation inductor Lm. The non-energy transfer cycle T2 contains n2 three-element resonance cycles Tr2, the three-element resonance cycle Tr2 refers to an inherent resonance period determined by the resonant inductor Lr, resonant capacitor Cr and transformer excitation inductor Lm. It also includes: dynamically adjusting the time ratio of the energy transfer cycle T1 and the non-energy transfer cycle T2 based on load conditions and output voltage feedback, or adjusting the values ​​of n1 and n2 to actively change the equivalent quality factor Q of the converter. EFF This allows the voltage gain characteristics of the converter to be changed under light load conditions, achieving effective voltage reduction while maintaining soft switching and high efficiency.

2. The light-load control method according to claim 1, characterized in that, During the energy transfer cycle T1, the inverter unit is controlled to operate in differential drive mode: by turning on the switches located in different bridge arms in pairs to generate differential drive voltages of opposite polarity alternately, energy is transferred to the secondary side. During the non-energy transfer cycle T2, the control inverter unit simultaneously turns on a pair of switching transistors located on different bridge arms but on the same polarity side according to the time interval, so that the primary side of the transformer is placed in a zero differential voltage state, preventing energy transfer to the load, and causing the resonant cavity to enter a three-element resonance state in the primary side circuit of the transformer.

3. The light-load control method according to claim 2, characterized in that, During the positive half-cycle of the energy transfer cycle T1, a set of switching transistors on the opposite polarity side of each bridge arm are turned on in a complementary manner; during the negative half-cycle, another set of switching transistors on the other polarity side of each bridge arm are turned on in a complementary manner, forming an alternating polarity driving voltage on the primary side of the transformer to transfer energy to the load.

4. The light-load control method according to claim 1, characterized in that, By measuring the output voltage and load current, and selecting a set of preset (n1,n2) control modes based on the measured load percentage or equivalent load current, the preset groups include: n1=3,n2=1; n1=2,n2=1; n1=1,n2=2; n1=1,n2=1. During operation, the (n1,n2) control modes are smoothly switched to reduce output voltage fluctuations and current stress.

5. The light-load control method according to claim 1, characterized in that, Based on the law of energy conservation, an energy flow model is established between the remaining energy of the resonant cavity and the energy required for energy transfer. Using this energy flow model, the maximum permissible equivalent quality factor Q that guarantees no secondary-side energy transfer occurs during the non-energy transfer period T2 is derived and determined. EFF The value is used to avoid energy transfer, soft switching failure, or oscillation during the non-energy transfer cycle T2.

6. The light-load control method according to claim 1, characterized in that, The equivalent quality factor Q EFF The initial energy stored in the resonant cavity, E initial Work done to overcome the secondary side reflected voltage (W) reflect It is calculated based on the law of conservation of energy, and is calculated and assigned values ​​by table lookup in real time or periodically.

7. A light-load controller for an LLC resonant converter, based on the light-load control method according to any one of claims 1-6, characterized in that, include: The measurement unit is used to sample the output voltage and load current; The calculation unit is used to calculate the load condition based on the sampled signal obtained by the measurement unit and determine the time ratio of (n1, n2) and T1 / T2, and calculate the equivalent quality factor Q according to the energy conservation model. EFF ; The drive unit generates drive signals for the inverter unit according to the output of the computing unit, so that the converter operates according to the timing sequence of T1 and T2.

8. The light-load controller according to claim 7, characterized in that, The controller contains a preset control mode table or lookup table, which is used to map different load ranges to different (n1,n2) combinations and control modes; and the computing unit can be fine-tuned online during operation.

9. A power conversion system, comprising an LLC resonant converter and the controller as described in claim 7, characterized in that, The converter's topology, resonant elements, and magnetizing inductor structure are conventionally configured. The controller is used to control the inverter unit within the converter to operate according to the light-load control method described in claim 1. The power conversion system also includes a monitoring unit for recording and analyzing the resonant cavity current, resonant cavity voltage, and secondary-side conduction state during the non-energy transfer cycle T2, used for offline or online verification of the equivalent quality factor Q. EFF And optimize control strategies.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the steps of the light load control method according to any one of claims 1 to 6.

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