Soft start control method of LLC resonant converter and LLC resonant converter

Abstract

The application provides a soft start control method of an LLC resonant converter and the LLC resonant converter. In the soft start process, a dynamic current limit threshold gradually increasing over time is generated; the switching frequency of the resonant converter is gradually reduced from a starting high frequency to a resonant frequency; the resonant current of the LLC resonant cavity is detected in real time, and the resonant current is compared with the current dynamic current limit threshold; when the resonant current reaches or exceeds the current dynamic current limit threshold, the reduction action of the switching frequency is frozen, so that the LLC resonant converter is maintained at the current frequency. The application effectively overcomes the current mutation problem caused by the gain nonlinearity of the LLC resonant converter, makes the resonant current envelope smooth and controllable, avoids the resonant cavity current peak value exceeding the expectation, reduces the device current stress, and improves the stability and reliability of the soft start process.

Description

Technical Field

[0001] This application belongs to the field of LLC resonant converter technology, and particularly relates to a soft-start control method for an LLC resonant converter and an LLC resonant converter. Background Technology

[0002] LLC resonant converters are widely used in medium- and high-power power supplies due to their ability to achieve zero-voltage switching of the primary-side switch and zero-current switching of the secondary-side rectifier diodes. Most existing LLC resonant converters employ a linear frequency sweep soft-start scheme with direct frequency control, which linearly reduces the frequency from a starting frequency much higher than the resonant frequency until closed-loop control is entered. However, the gain characteristics of LLC resonant converters exhibit a highly nonlinear relationship with the switching frequency. During soft-start, even small changes in the switching frequency can easily cause abrupt changes in the resonant current amplitude, resulting in a highly irregular amplitude envelope of the resonant current and causing the peak current within the resonant cavity to exceed expectations. Summary of the Invention

[0003] This application provides a soft-start control method and an LLC resonant converter, which can solve the problem that the amplitude envelope of the resonant current in existing LLC resonant converters is extremely irregular, causing the peak current in the resonant cavity to exceed expectations.

[0004] In a first aspect, embodiments of this application provide a soft-start control method for an LLC resonant converter, applied to an LLC resonant converter, the soft-start control method for the LLC resonant converter comprising: During the soft start process, a dynamic current limit threshold is generated that gradually increases over time; The switching frequency of the resonant converter is gradually reduced from the initial high frequency toward the resonant frequency. The resonant current of the LLC resonant cavity is detected in real time, and the resonant current is compared with the current dynamic current limit threshold. When the resonant current reaches or exceeds the current dynamic current limit threshold, the reduction action of the switching frequency is frozen, so that the LLC resonant converter maintains the current frequency operation.

[0005] In one possible implementation of the first aspect, before generating the dynamic current limit threshold that gradually increases over time during the soft-start process, the method further includes: Turn on the lower bridge arm switch in the LLC resonant converter; During the first switching cycle of the soft-start, the upper bridge arm switch of the LLC resonant converter is forcibly turned on.

[0006] In one possible implementation of the first aspect, generating a dynamic current limit threshold that gradually increases over time during the soft-start process includes: At the start of soft start, the reference voltage of the overcurrent protection comparator is set to the first voltage value; Within a preset time interval, the reference voltage is linearly increased over time to a second voltage value, where the second voltage value is the system maximum current protection setpoint. The reference voltage is used as the dynamic current limit threshold. During each switching cycle, the resonant current is compared with the current dynamic current limit threshold in real time to define a dynamic safety envelope for the amplitude increase of the resonant current.

[0007] In one possible implementation of the first aspect, the switching frequency of the control resonant converter gradually decreases from an initial high frequency toward the resonant frequency, including: Set the initial maximum on-time limit and determine the highest starting frequency corresponding to soft start; By continuously charging the charging capacitor with a controlled current source, the maximum on-time limit increases linearly with time, and the switching frequency decreases linearly. The maximum on-time limit is used to control the on-time of the switching transistors in the LLC resonant converter.

[0008] In one possible implementation of the first aspect, freezing the reduction of the switching frequency when the resonant current reaches or exceeds the current dynamic current limit threshold includes: When the resonant current is detected to reach or exceed the current dynamic current limit threshold, an overcurrent protection flag signal is generated; The overcurrent protection flag signal activates the bypass logic and outputs a bypass pulse; the bypass pulse is used to bypass the controlled current source and forcibly stop the controlled current source from charging the charging capacitor.

[0009] In one possible implementation of the first aspect, the method further includes: During the soft start process, the resonant current is detected in real time and a zero-crossing signal of the resonant current is output. Each time the resonant current zero-crossing signal is triggered, the on-time timer is started. Based on the timing duration of the conduction timer, the conduction duration after the resonant current crosses zero is controlled, thereby adjusting the amount of charge accumulation flowing into the resonant cavity.

[0010] In one possible implementation of the first aspect, the method further includes: During the soft start process, the voltage slope at the midpoint of the switching node is captured in real time; The dead time is dynamically adjusted based on the midpoint voltage slope.

[0011] In one possible implementation of the first aspect, the method further includes: The output power of the LLC resonant converter is detected in real time, and the output power is compared with the system closed-loop set value; When the output power reaches the system closed-loop set value, the freeze control of the switching frequency reduction action is released. Switch to the closed-loop control algorithm and configure the maximum on-time limit to the maximum limit allowed by the system.

[0012] Secondly, embodiments of this application provide an LLC resonant converter, comprising: The resonant unit includes a resonant inductor, a resonant capacitor, and a transformer magnetizing inductor. The bridge arm switch unit includes an upper bridge arm switch tube and a lower bridge arm switch tube; A current detection unit is used to detect the resonant current of the LLC resonant cavity in real time. The control unit is electrically connected to the bridge arm switch unit and the current detection unit, and the control unit is used to execute the soft-start control method of the LLC resonant converter as described in any one of the first aspects.

[0013] In one possible implementation of the second aspect, the control unit includes a first timer generating circuit, a second timer generating circuit, and a ramp frequency increasing circuit. The first timer generating circuit and the second timer generating circuit are both electrically connected to the ramp frequency increasing circuit. The first timer generating circuit and the second timer generating circuit are both electrically connected to the bridge arm switching unit. The first timer generating circuit, the second timer generating circuit, and the ramp frequency increasing circuit are all electrically connected to the current detection unit. The first timer generation circuit is used to output an upper bridge arm conduction timer based on the resonant current negative zero-crossing signal, the soft-start end signal, the upper bridge arm drive signal, and the maximum conduction time limit. The second timer generation circuit is used to output a lower bridge arm conduction timer based on the resonant current positive zero-crossing signal, the soft-start end signal, the lower bridge arm drive signal, and the maximum conduction time limit. The ramp frequency growth circuit is used to generate the maximum conduction time limit based on the soft-start end signal, the dynamic current limit threshold, and the overcurrent protection flag signal, so as to control the switching frequency to gradually decrease and freeze.

[0014] The beneficial effects of the embodiments in this application compared with the prior art are: The soft-start control method for LLC resonant converters provided in this application first generates a dynamic current limit threshold that gradually increases over time during the soft-start process. Then, it controls the switching frequency of the resonant converter to gradually decrease from the initial high frequency towards the resonant frequency. Based on this, the resonant current of the LLC resonant cavity is detected in real time and compared with the current dynamic current limit threshold. When the resonant current reaches or exceeds the current dynamic current limit threshold, the frequency reduction action is frozen, allowing the LLC resonant converter to maintain its current frequency operation. Therefore, the soft-start control method for LLC resonant converters in this application, by generating a dynamic current limit threshold that gradually increases over time, combined with the gradual decrease in switching frequency and real-time comparison of the resonant current, freezes the frequency reduction action when the resonant current reaches the threshold, thereby forming a dynamic envelope constraint on the resonant current. This method effectively overcomes the current surge problem caused by the gain nonlinearity of the LLC resonant converter, making the resonant current envelope stable and controllable, avoiding the resonant cavity current peak from exceeding expectations, reducing device current stress, and improving the stability and reliability of the soft-start process.

[0015] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in an embodiment of this application; Figure 2 This is a schematic diagram of the asymmetric startup waveform during the initial soft-start stage of an LLC resonant converter provided in an embodiment of this application; Figure 3 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 4 This is a schematic diagram of the dynamic current limiting and frequency control waveforms during the soft-start phase of an LLC resonant converter according to an embodiment of this application; Figure 5 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 6 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 7 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 8 This is a circuit connection diagram of the first timer generating circuit provided in an embodiment of this application; Figure 9 This is a circuit connection diagram of the second timer generating circuit provided in an embodiment of this application; Figure 10 This is a circuit connection diagram of a ramp frequency growth circuit provided in an embodiment of this application; Figure 11 This is a schematic diagram of the dynamic current limiting and frequency control waveforms during the soft-start phase of an LLC resonant converter according to an embodiment of this application; Figure 12 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 13 This is a flowchart illustrating a soft-start control method for an LLC resonant converter provided in another embodiment of this application; Figure 14 This is a waveform diagram illustrating the entire soft-start process of an LLC resonant converter from initial to final stages according to an embodiment of this application. Figure 15 This is a schematic diagram of the multi-parameter joint operation waveform of the soft-start process of an LLC resonant converter provided in an embodiment of this application. Detailed Implementation

[0018] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0019] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0020] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0021] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if [the described condition or event] is detected" may be interpreted, depending on the context, as "once determined," "in response to determination," "once [the described condition or event] is detected," or "in response to detection of [the described condition or event]."

[0022] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0023] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0024] LLC resonant converters are widely used in medium- and high-power power supplies due to their ability to achieve zero-voltage switching of the primary-side switch and zero-current switching of the secondary-side rectifier diodes. Most existing LLC resonant converters employ a linear frequency sweep soft-start scheme with direct frequency control, which linearly reduces the frequency from a starting frequency much higher than the resonant frequency until closed-loop control is entered. However, the gain characteristics of LLC resonant converters exhibit a highly nonlinear relationship with the switching frequency. During soft-start, even small changes in the switching frequency can easily cause abrupt changes in the resonant current amplitude, resulting in a highly irregular amplitude envelope of the resonant current and causing the peak current within the resonant cavity to exceed expectations.

[0025] To address the aforementioned issues, the soft-start control method for LLC resonant converters provided in this application first generates a dynamic current limit threshold that gradually increases over time during the soft-start process. Then, it controls the switching frequency of the resonant converter to gradually decrease from the initial high frequency towards the resonant frequency. Based on this, the resonant current of the LLC resonant cavity is detected in real time and compared with the current dynamic current limit threshold. When the resonant current reaches or exceeds the current dynamic current limit threshold, the frequency reduction action is frozen, allowing the LLC resonant converter to maintain its current frequency operation. Therefore, the soft-start control method for LLC resonant converters in this application, by generating a dynamic current limit threshold that gradually increases over time, combined with the gradual decrease in switching frequency and real-time comparison of the resonant current, freezes the frequency reduction action when the resonant current reaches the threshold, thereby forming a dynamic envelope constraint on the resonant current. This method effectively overcomes the current surge problem caused by the gain nonlinearity of the LLC resonant converter, making the resonant current envelope stable and controllable, avoiding unexpected peak current in the resonant cavity, reducing device current stress, and improving the stability and reliability of the soft-start process.

[0026] To illustrate the technical solution described in this application, specific embodiments are provided below.

[0027] The LLC resonant converter mainly consists of a resonant unit, a bridge arm switching unit, a current sensing unit, and a control unit. The resonant unit includes a resonant inductor, a resonant capacitor, and a transformer magnetizing inductor, used to form a resonant circuit and achieve resonant energy transfer and soft-switching conditions. The bridge arm switching unit consists of upper and lower bridge arm switches, which convert the input voltage into a high-frequency pulse voltage through controlled on / off switching, providing excitation for the resonant unit. The current sensing unit collects the resonant current within the resonant cavity in real time, providing current feedback signals to the control unit. The control unit is electrically connected to the bridge arm switching unit and the current sensing unit, used to execute the soft-start control method and output drive signals, precisely controlling the operating timing and frequency of the switches to achieve smooth startup and reliable operation of the converter.

[0028] The following is combined Figure 1 The soft-start control method of the LLC resonant converter in this application is described in detail.

[0029] Figure 1 A schematic flowchart illustrating a soft-start control method for an LLC resonant converter according to an embodiment of this application is shown. See also... Figure 1 As shown, the soft-start control method for the LLC resonant converter includes steps S101 to S104.

[0030] Step S101: During the soft start process, a dynamic current limit threshold that gradually increases over time is generated.

[0031] Specifically, the dynamic current limit threshold is used to dynamically constrain the growth rate of the resonant current during soft start, so that the restriction conditions are gradually relaxed as the soft start process progresses, providing a time-varying reference value for subsequent real-time comparison of the resonant current.

[0032] Step S102: Control the switching frequency of the resonant converter to gradually decrease from the initial high frequency toward the resonant frequency.

[0033] Specifically, in the initial stage of soft start, a starting operating frequency much higher than the resonant frequency is set to avoid excessive inrush current at the moment of startup due to the operating point being close to the resonant region. Then, according to the preset control rhythm, the switching frequency is gradually and orderly reduced over time, allowing the converter's operating state to gradually approach the resonant operating point, ensuring that the entire frequency reduction process is smooth and controllable, and avoiding drastic current fluctuations caused by frequency abrupt changes.

[0034] Step S103: Real-time detection of the resonant current of the LLC resonant cavity, and comparison of the resonant current with the current dynamic current limit threshold.

[0035] Specifically, the resonant current in the resonant cavity is continuously sampled and monitored in real time to obtain the instantaneous current value at each moment. At the same time, the resonant current detected in real time is compared with the dynamic current limit threshold that is currently in effect to determine whether the resonant current is within the safe allowable range, providing a direct and reliable basis for determining whether the frequency control strategy needs to be adjusted in the future.

[0036] Step S104: When the resonant current reaches or exceeds the current dynamic current limit threshold, freeze the reduction action of the switching frequency to keep the LLC resonant converter running at the current frequency.

[0037] Specifically, once the real-time comparison results show that the resonant current reaches or exceeds the current dynamic current limit threshold, indicating that the resonant current is approaching the safe upper limit, a frequency freeze operation is immediately executed to suspend further reduction of the switching frequency, ensuring that the converter operates stably at the current operating frequency. By locking the frequency in a timely manner, further abnormal increases in the resonant current due to continued frequency reduction are avoided, effectively suppressing the risk of current over-limit and ensuring stable operation of the converter under safe conditions.

[0038] It should be noted that once the resonant current falls below the current dynamic current limit threshold, the freezing of the switching frequency reduction action is lifted, and the switching frequency continues to decrease until the soft start is complete. Specifically, during the soft start process, once the resonant current touches the dynamic current limit threshold, the system freezes the switching frequency reduction action to suppress further current increase. When load changes or energy regulation cause the resonant current to naturally fall below the current threshold, it indicates that the current has escaped the risk of exceeding the limit and returned to the safe range. At this time, the system automatically lifts the freeze restriction on the frequency reduction action and resumes the gradual decrease of the switching frequency. The freeze-thaw adaptive adjustment process of this application will continue to cycle until the converter completes the soft start and reaches the preset steady-state operating state, ensuring that the entire soft start process proceeds smoothly under current safety constraints without abnormal fluctuations.

[0039] It should be noted that before step S101, there are also steps S1001 and S1002.

[0040] Step S1001: Turn on the lower bridge arm switch in the LLC resonant converter.

[0041] Specifically, in the initial preparation phase before the soft start officially begins, the lower bridge arm switch is turned on first, so that the resonant capacitor forms a controllable discharge circuit through the lower bridge arm, releasing the initial charge on the resonant capacitor to a safe potential (controlled low potential), avoiding abnormal current surges in the first cycle due to the uncertainty of the initial voltage of the resonant capacitor; at the same time, this operation also provides a pre-charging path for the bootstrap capacitor of the upper bridge arm drive circuit, ensuring that the upper bridge arm switch can be reliably turned on subsequently, establishing a stable and controllable initial working state for the entire soft start process.

[0042] Step S1002: In the first switching cycle of soft start, the upper bridge arm switch of the LLC resonant converter is forcibly turned on.

[0043] Specifically, after initial preparation, during the first working cycle of soft start, the upper bridge arm switch is actively and forcibly turned on to establish a definite initial current direction in the resonant circuit, eliminating the current randomness caused by the uncertainty of the initial state of the resonant cavity. Through asymmetric start logic, the resonant current is smoothly established and rises from the preset potential, effectively suppressing the current spike in the first cycle and ensuring that the current change in the first cycle of soft start is smooth and controllable.

[0044] It should be noted that, Figure 2The diagram illustrates the asymmetric startup waveform during the initial soft-start phase of an LLC resonant converter. Here, VBOOT is the capacitor voltage waveform providing timing for soft-start, HG is the upper bridge arm switch drive waveform, LG is the lower bridge arm switch drive waveform, Vreset is the reset signal waveform indicating the end of the soft-start drive timing, Vset is the set signal waveform indicating the start of the soft-start drive, VHB is the midpoint voltage waveform of the upper and lower bridge arms of the LLC resonant converter, Vcr is the resonant capacitor voltage, BBC1 and BBC2 are the upper and lower thresholds controlled by BBC, OCP is the current dynamic current limit threshold, and iLr is the resonant current. Furthermore, Figure 2 The initial stage frequency marked as being set by the soft-start Ton means that at the very beginning of soft-start, the switching frequency is not determined by the closed loop, but by the soft-start conduction time (Ton). HG ZVS on means that the upper arm switch is zero-voltage turned on (ZVS), indicating that at this moment, the voltage across HG has dropped to near zero, with no hard-turn-on losses or spikes, representing a safe and efficient turn-on state. When the resonant current (iLr) reaches the current dynamic current limit threshold (OCP), it will not immediately turn off the upper switch for protection (traditional solutions would directly turn it off), but will freeze Ton, preventing it from increasing further, thus stopping the switching frequency at the current frequency and entering a current-limited constant-frequency state.

[0045] like Figure 3 As shown, step S101 includes steps S1011 to S1014.

[0046] Step S1011: At the start of soft start, the reference voltage of the overcurrent protection comparator is set to the first voltage value.

[0047] Specifically, at the initial moment of soft start, the system is in a low energy and low current state. In order to avoid starting failure due to excessively strict initial restrictions and to prevent current surges caused by excessively loose restrictions, the reference voltage of the overcurrent protection comparator is set to a low and safe initial voltage value (such as 0.75V). This voltage value corresponds to a small initial current limit, providing a relaxed and controllable current constraint starting point for the initial stage of soft start.

[0048] In step S1012, within a preset time interval, the reference voltage is linearly increased to a second voltage value over time, where the second voltage value is the system maximum current protection setpoint.

[0049] Specifically, during the soft-start process, the converter gradually builds up energy, the load current rises steadily, and the reference voltage no longer maintains its initial low value. Instead, it gradually and continuously increases at a linear slope within a preset time interval (e.g., 20ms) according to a preset fixed time step, until it reaches a pre-calibrated second voltage value (e.g., 1.5V). This second voltage value corresponds to the maximum current protection threshold allowed by the system, ensuring that the current limit at the end of the soft start can meet the maximum current requirements for normal operation.

[0050] Step S1013: Use the reference voltage as the dynamic current limit threshold.

[0051] Specifically, the reference voltage, after initial setting and linear boosting, is directly mapped to a dynamic current limit threshold that takes effect in real time during the soft-start phase, establishing a one-to-one quantitative relationship between the voltage signal and the current limit. The threshold provided in this application dynamically changes with the soft-start process, replacing the traditional fixed current limit and enabling the current constraint condition to adaptively adjust with the startup state.

[0052] In step S1014, within each switching cycle, the resonant current is compared with the current dynamic current limit threshold in real time to define a dynamic safety envelope for the amplitude increase of the resonant current.

[0053] Specifically, during each complete switching cycle of the soft-start process, the instantaneous value of the resonant current within the resonant cavity is continuously sampled and precisely compared with the corresponding dynamic current limit threshold at the current moment, cycle by cycle. Through real-time comparison, a dynamic safety boundary that gradually widens over time is constructed for the amplitude growth of the resonant current, ensuring that the current always grows smoothly within the boundary. In other words, the amplitude of the resonant current is forced to grow along a preset linear trajectory, avoiding over-limit spikes and achieving linear, smooth, and controllable current amplitude envelope.

[0054] It should be noted that, Figure 4 A schematic diagram of dynamic current limiting and frequency control waveforms during the soft-start phase of an LLC resonant converter is shown, where Voptocoupler is the optocoupler voltage, and tset is the soft-start timing setting. Furthermore, Figure 4 iLr (annotated in the middle) When the OCP peak reaches the OCP point, MAX is reached. TON stops decreasing the frequency and maintains the current switching frequency. Constant power maintaining output current means that when the resonant current increases to the dynamic current limit threshold (OCP point), the system determines that the current has reached the safe upper limit and stops increasing the maximum conduction time limit. In other words, it stops decreasing the switching frequency and fixes the frequency at the current value. At a fixed frequency, the converter enters constant power current limiting mode, the output current is clamped and will not continue to increase, preventing current spikes and device overstress. The soft-start frequency in the SS stage is changed from MAX... The ramp setting for TON refers to the following: during the soft-start (SS) phase, the switching frequency is not determined by the voltage closed loop, but entirely by MAX. TON ramp signal control. The maximum on-time limit increases slowly from small to large, causing the switching frequency to decrease slowly from high to low, forming a controllable soft-start frequency trajectory to ensure a smooth, controllable, and shock-free start-up.

[0055] like Figure 5 As shown, step S102 includes steps S1021 to S1023.

[0056] Step S1021: Set the initial maximum on-time limit and determine the highest starting frequency corresponding to soft start.

[0057] Specifically, before the soft start begins, the system pre-configures a small initial maximum on-time limit, which corresponds to the highest starting switching frequency of the LLC resonant converter during the soft start phase. By setting a higher starting frequency, the converter's initial operating point can be kept far from the resonant frequency, thus avoiding the inrush current caused by excessively high resonant gain at startup and establishing reliable initial operating conditions for subsequent smooth frequency reduction and safe startup.

[0058] In step S1022, the charging capacitor is continuously charged by a controlled current source, so that the maximum conduction time limit increases linearly with time and the switching frequency decreases linearly.

[0059] Specifically, during the soft-start process, the system uses a controlled current source to charge a preset charging capacitor at a constant current, causing the voltage across the capacitor to rise linearly over time. Since this voltage change directly maps to a linear increase in the maximum on-time limit, and the on-time is inversely proportional to the switching frequency, this achieves a linear and smooth decrease in the switching frequency from high to low, equivalent to the switching frequency decreasing linearly along a preset trajectory. This controllable linear frequency sweep method allows the converter to gradually approach the resonant point, effectively suppressing current fluctuations caused by sudden frequency changes.

[0060] Step S1023: The conduction time of the switching transistor in the LLC resonant converter is controlled by the maximum conduction time limit.

[0061] Specifically, during the soft-start process, the system uses the real-time updated maximum conduction time limit as a constraint benchmark for the conduction time of the switching transistors, precisely controlling the actual conduction time of the upper and lower bridge arm switching transistors to not exceed this limit. By directly limiting the conduction time, precise control of the switching frequency is achieved, ensuring that the frequency reduction process strictly follows the preset trajectory, and guaranteeing the stability and controllability of frequency regulation during the soft-start phase.

[0062] like Figure 6 As shown, step S104 includes steps S1041 to S1042.

[0063] Step S1041: When the resonant current is detected to reach or exceed the current dynamic current limit threshold, an overcurrent protection flag signal is generated.

[0064] Specifically, during the soft-start process, the system continuously samples the resonant current of the resonant cavity in real time, cycle by cycle, and accurately compares it with the currently effective dynamic current limit threshold. Once the instantaneous value of the resonant current is detected to reach or exceed the dynamic current limit threshold, indicating that the current is approaching the safe upper limit and there is a risk of exceeding the limit, the system immediately generates and outputs a high-level effective overcurrent protection flag signal to trigger subsequent frequency freeze control actions, ensuring safe current limiting during the soft-start phase.

[0065] In step S1042, the overcurrent protection flag signal activates the bypass logic and outputs a bypass pulse; the pulse bypass is used to bypass the controlled current source and forcibly stop the controlled current source from charging the charging capacitor.

[0066] Specifically, upon receiving an overcurrent protection flag signal, the system immediately activates the preset bypass control logic, generating and outputting a bypass pulse signal of fixed duration (e.g., at least 200µs). This pulse signal directly acts on the controlled current source in the control loop, bypassing it and forcibly cutting off its charging circuit to the charging capacitor. This stops the charging capacitor voltage from rising, thereby locking the maximum on-time limit to prevent further increase and ultimately freezing the switching frequency. This effectively prevents the resonant current from further increasing due to continued frequency reduction, ensuring the converter operates within a safe current range. During this stage, the system transitions from a frequency sweep state to a constant power output state, effectively limiting further increases in current amplitude. Once the resonant current no longer reaches the current dynamic current limit threshold, the bypass is released, and the switching frequency continues to decrease.

[0067] like Figure 7 As shown, the method also includes steps S105 to S107.

[0068] Step S105: During the soft start process, the resonant current is detected in real time and a resonant current zero-crossing signal is output.

[0069] Specifically, during the entire soft-start operation, the system continuously performs high-precision real-time sampling and acquisition of the resonant current inside the LLC resonant circuit, continuously tracks the instantaneous value and current flow direction of the resonant current, accurately captures the zero-crossing critical point when the resonant current changes from positive to negative or from negative to positive, and synchronously generates the corresponding resonant current zero-crossing feedback signal (resonant current negative zero-crossing signal and resonant current positive zero-crossing signal), providing a precise synchronous trigger reference signal for subsequent precise control of the conduction timing.

[0070] Step S106: Whenever the resonant current crosses zero signal, the on-time timer is triggered to start counting.

[0071] Specifically, once the system receives a stable and valid resonant current zero-crossing signal, it immediately starts the preset conduction timing module to enter the working state. It starts to count the duration from the moment the current completes the zero-crossing switch, strictly relying on the moment of current zero-crossing as the timing start node to ensure that the timing start point is completely synchronized with the current state change of the resonant circuit, and to ensure precise control of the conduction duration of the switching tube.

[0072] Step S107: Based on the timing duration of the conduction timer, control the conduction duration after the resonant current crosses zero, and adjust the amount of charge accumulation flowing into the resonant cavity.

[0073] Specifically, based on the actual timing duration obtained from the conduction timer, the system precisely limits the actual conduction time of the power switch after the resonant current completes the zero-crossing switch. By controlling the effective inflow time after the current crosses zero, this solution essentially achieves precise management of the charge integral flowing into the resonant cavity, orderly regulates the amount of electrical energy delivered to the resonant cavity within a unit cycle, and reasonably adjusts the charge accumulation rate and total amount inside the resonant cavity. This makes the rise and fall of the resonant current more gradual and orderly, fundamentally solving the current runaway phenomenon caused by traditional frequency sweeping from an energy perspective, and further optimizing the energy establishment process of the resonant circuit during the soft-start phase.

[0074] It should be noted that, in order to achieve the above steps, this application also provides the circuit composition of the control unit. For example... Figure 8 , Figure 9 and Figure 10 As shown, the control unit includes a first timer generation circuit ( Figure 8 ), second timer generating circuit ( Figure 9 ) and ramp frequency growth circuit ( Figure 10 The first timer generating circuit and the second timer generating circuit are both electrically connected to the ramp frequency growth circuit. The first timer generating circuit and the second timer generating circuit are both electrically connected to the bridge arm switch unit. The first timer generating circuit, the second timer generating circuit and the ramp frequency growth circuit are all electrically connected to the current detection unit.

[0075] The first timer generation circuit outputs the upper bridge arm conduction timer MOT_HG based on the resonant current negative zero-crossing signal ILR_ZCD_NEG, the soft-start end signal SS_END_FLAG, the upper bridge arm drive signal D_H, and the maximum conduction time limit TON_MAX_SET, which is used to control the on / off state of the upper bridge arm switch. The second timer generation circuit outputs the lower bridge arm conduction timer MOT_LG based on the resonant current positive zero-crossing signal ILR_ZCD_POS, the soft-start end signal SS_END_FLAG, the lower bridge arm drive signal D_L, and the maximum conduction time limit TON_MAX_SET, which is used to control the on / off state of the lower bridge arm switch. The ramp frequency growth circuit generates the maximum conduction time limit TON_MAX_SET based on the soft-start end signal SS_END_FLAG, the dynamic current limit threshold, and the overcurrent protection flag signal, in order to control the switching frequency to gradually decrease and achieve frequency freezing.

[0076] It should be noted that, Figure 8 and Figure 9 The circuits shown all contain logic gates, capacitors, diodes, and comparators. For details on the connections between these components, please refer to [reference needed]. Figure 8 and Figure 9 As shown, logic gates are used for logical operations to realize the logical combination and timing judgment of various control signals; capacitors are used to realize signal delay, level holding and soft-start ramp generation to ensure smooth transition of control timing; diodes are used for level clamping to protect the stability of circuit node voltage; comparators are used to compare the voltage of two input terminals and output high and low level logic signals to realize current threshold judgment and overcurrent protection triggering functions. Figure 10 V27 shown can be used as the initial maximum on-time limit set in step S1021. I7 is used as a controlled current source and C15 is used as a charging capacitor. I7 can continuously charge C15, causing the voltage across C15 to rise linearly, thereby generating a linearly increasing maximum on-time limit TON_MAX_SET. When an overcurrent protection flag signal is received, I7 is forced to stop charging C15, and the voltage of C15 no longer increases, thus achieving the freezing control of the switching frequency.

[0077] It should be noted that, Figure 11 The diagram shows the dynamic current limiting and frequency control waveforms during the soft-start phase of an LLC resonant converter. In the initial stage of soft start, the resonant current remains below the dynamic current limiting threshold.

[0078] like Figure 12 As shown, the method also includes steps S108 to S109.

[0079] Step S108: During the soft start process, the voltage slope at the midpoint of the switching node is captured in real time.

[0080] Specifically, the soft-start process continuously collects the voltage waveform data at the midpoint of the switching node connected to the upper and lower bridge arm switches in the LLC resonant converter, calculates the rate of rise and fall of the voltage signal over time in real time, accurately extracts the characteristic value of the voltage change slope under different start-up conditions and different frequency states, and comprehensively grasps the dynamic change law of the switching node voltage, providing an intuitive and reliable basis for the adaptive adjustment of dead time.

[0081] Step S109: Dynamically adjust the dead time based on the midpoint voltage slope.

[0082] Specifically, the system uses the real-time voltage slope of the switching node as the basis for judgment, and performs logical judgment in combination with the current operating conditions of the converter. Based on the magnitude and trend of the slope, the dead time interval between the upper and lower bridge arm switches is adaptively increased or decreased. This can reasonably extend the dead time when the voltage change rate is fast to avoid bridge arm shoot-through and circuit damage, and appropriately reduce the dead time when the voltage change is slow to effectively reduce the switching losses of the switching transistors and ensure that the switching transistors always achieve reliable zero-voltage turn-on during the soft start phase.

[0083] like Figure 13 As shown, the method also includes steps S110 to S112.

[0084] Step S110: Detect the output power of the LLC resonant converter in real time and compare the output power with the system closed-loop setpoint.

[0085] Specifically, during the soft start process, the voltage and current parameters at the converter output terminal are continuously collected, and the actual output power of the whole machine is calculated in real time through calculation. Then, the actual output power detected in real time is compared with the steady-state closed-loop power setting value stored in the system in advance to determine whether the current output power meets the power standard for normal and stable operation of the system.

[0086] Step S111: When the output power reaches the system closed-loop set value, the freeze control of the switching frequency reduction action is released.

[0087] Specifically, once the actual detected output power reaches the system closed-loop set power, it means that the converter has completed the previous current-limiting and smooth start-up stage and has the basic conditions to enter steady-state operation. At this time, the system automatically cancels the frequency freeze constraint command previously set to limit the current increase and restores the operating mechanism of autonomous and gradual reduction of the switching frequency, that is, smoothly switches to the closed-loop control algorithm (such as the BBC algorithm based on resonant capacitor voltage control).

[0088] Step S112: Switch to closed-loop control algorithm and configure the maximum on-time limit to the maximum limit allowed by the system.

[0089] Specifically, after the frequency freeze control is lifted, the system officially exits the soft-start open-loop control mode and seamlessly switches to a dual closed-loop steady-state control strategy for voltage and current, relying on closed-loop feedback to achieve precise voltage and current regulation of output parameters. At the same time, the maximum conduction time limit, which was progressively increased in the previous soft-start phase, is directly configured and set to the maximum conduction time allowed for normal operation of the entire system, to ensure that the soft-start logic does not interfere with normal steady-state regulation, completing a seamless transition from frequency sweep open-loop to steady-state closed-loop, and the LLC resonant converter fully enters the rated steady-state operating state.

[0090] It should be noted that, as Figure 14 As shown, during the initial soft-start phase, the system operates in an open-loop frequency sweep and current limiting mode, with power gradually increasing. Once the output power reaches the set threshold, the frequency freeze logic is canceled, the soft-start open-loop mode is exited, and the system officially switches to voltage and current closed-loop control. At the same time, the on-time limit configuration is completed, and the entire machine enters the rated stable operating state. Figure 14 The main focus is on the soft-start timing voltage setting (yellow), which is set to its maximum value upon completion of the soft start to avoid affecting TON and actual operation. Specifically, the ramp voltage of the maximum conduction time limit gradually increases linearly from an initial 0.1V to 1V, precisely corresponding to the maximum allowable conduction time of the switching transistor in the hardware circuit, ranging from 1µs to 10µs. This linear relationship between the ramp voltage and conduction time, achieved through a gradual increase in the ramp voltage, ensures an orderly increase in conduction time, thereby achieving a smooth and linear decrease in the switching frequency from high to low. This completes the progressive frequency sweep control during the soft start phase of the LLC resonant converter, ensuring a regular and controllable start-up frequency adjustment process. Figure 15 The diagram shows the waveforms of multiple parameters operating together during the soft-start process of the LLC resonant converter. In the diagram, blue VHB is the voltage waveform at the midpoint of the upper and lower bridge arms, red iLr is the current waveform of the resonant inductor, orange Vcr is the voltage waveform across the resonant capacitor, green LG is the driving waveform of the lower bridge arm switch, and yellow VOUT is the output voltage waveform of the LLC resonant converter. Figure 15 It can intuitively present the changing patterns of various electrical quantities in the entire process of soft-start pre-initialization, dynamic current limiting and frequency sweeping, frequency freezing and current limiting, and soft-start exit switching closed-loop control. It clearly reflects the timing coordination and dynamic coupling relationship between various signals, and intuitively verifies the actual effect of the soft-start strategy of this solution on resonant current spike suppression, device voltage stress control, and stable establishment of output voltage.

[0091] In summary, the soft-start control method for LLC resonant converters provided in this application has the following advantages compared to the prior art: 1. Achieve precise and linear controllable resonant current envelope, avoiding current surges caused by nonlinear gain. Traditional linear frequency sweep startup methods fail to adequately consider the inherently highly nonlinear voltage gain of LLC resonant converters. Relying solely on time-based frequency downsampling easily leads to abrupt, abrupt changes in the resonant current amplitude during startup, resulting in unpredictable current variations. This application sets a dynamic current limit threshold that rises linearly over time, defining a progressively wider safe growth boundary for the resonant current. This constrains the instantaneous value of the resonant current to rise strictly according to a preset, stable trajectory, ultimately achieving a linear and smooth change in the overall amplitude envelope of the resonant inductor current and the transformer primary current. This completely overcomes the current runaway problem caused by nonlinear gain at the control level.

[0092] 2. Effectively reduces current stress on power devices and magnetic components, improving the overall hardware reliability. Existing conventional soft-start solutions are prone to generating extremely large inrush currents at full load power-on, which not only easily causes the power MOSFET to operate close to or even beyond its safe operating range, but also significantly increases the probability of core saturation in resonant components, shortening device lifespan. This application combines dynamic current threshold with maximum conduction time frequency freezing control logic. Once the resonant current reaches the dynamic limit, the system immediately locks the current operating frequency and automatically switches to constant power current-limiting operation, suppressing abnormal surges in startup peak current from the source. This effectively reduces the current surges and electrothermal stresses borne by core components such as switching transistors, resonant inductors, and high-frequency transformers, comprehensively enhancing the long-term operational stability of power supply equipment.

[0093] 3. Suppress electromagnetic vibration during startup and completely eliminate power supply operating audio noise. Traditional startup schemes exhibit drastic fluctuations in current amplitude, with alternating electromagnetic forces acting on the high-frequency transformer core. This magnetostriction effect induces high-frequency vibrations in the core and structural components, generating audible noise that is clearly perceptible to the human ear, making them unsuitable for high-end silent power supply applications. This application ensures a smooth and gradual change in the resonant current envelope throughout the soft-start process, without abrupt increases or decreases in amplitude. This significantly reduces the instantaneous electromagnetic impact force acting on the core, fundamentally suppressing mechanical vibration and effectively eliminating audible noise during startup, thus meeting the high-level NVH (Noise, Vibration, and Harshness) design requirements of power supply equipment.

[0094] 4. Enhanced startup capability under complex operating conditions, significantly improving startup robustness across the entire load range. Traditional fixed-threshold overcurrent protection schemes have poor adaptability, and are prone to triggering false protection in heavy-load startup and large-capacitive load startup scenarios, leading to adverse phenomena such as system hiccups and restarts, and startup failures. This application abandons the hard protection logic of directly shutting down the drive when the current exceeds the limit, and adopts a flexible control strategy of freezing the frequency when the current reaches the threshold and restoring the frequency sweep when the current falls back. It can adaptively adjust the frequency sweep progress rhythm according to the actual load conditions. Whether under full-load conditions or large-capacitive load conditions, it can complete the power-on startup smoothly in one go, eliminating false protection and repeated restart problems, and significantly broadening the product's operating condition adaptability range.

[0095] 5. Optimize the dynamic characteristics of mode switching to achieve seamless connection between soft start and steady-state closed-loop control. This application employs a deterministic asymmetric pre-initialization control method to pre-commit operations such as bridge arm switch pre-conduction and resonant cavity charge release, establishing a unified and stable initial electrical state upon power-on. Simultaneously, a time-shifted precise energy management strategy accurately controls resonant cavity charge accumulation and energy transfer efficiency. When switching to steady-state closed-loop control algorithms such as BBC at the end of the soft-start process, the overall output voltage exhibits no significant overshoot or voltage drop fluctuations. The transition between the two control modes is smooth and natural, effectively optimizing the dynamic response performance during mode switching and ensuring a continuous and stable power supply output from the backend.

[0096] 6. Adapts to multiple operating modes, balancing smooth startup and overall machine efficiency. This application can be used in conjunction with auxiliary control logic such as adaptive dead-time adjustment and zero-crossing current timing control to ensure reliable zero-voltage turn-on of the switching transistor throughout the entire process, reducing switching losses, while achieving smooth soft-start current limiting control. Simultaneously, it can automatically adapt the variable frequency or fixed-frequency operation logic based on the input voltage range and the actual load, addressing many pain points of LLC resonant converters such as difficult start-up, high current, and high noise, while also considering the overall power conversion efficiency under both light and heavy load conditions, resulting in a more prominent comprehensive application advantage.

[0097] Since the processing and functions implemented by the LLC resonant converter in this embodiment are basically the same as the embodiments, principles and examples of the aforementioned soft-start control method for the LLC resonant converter, any details not covered in this embodiment can be found in the relevant descriptions in the aforementioned embodiments, and will not be repeated here.

[0098] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A soft-start control method for an LLC resonant converter, characterized in that, The soft-start control method for LLC resonant converters includes: During the soft start process, a dynamic current limit threshold is generated that gradually increases over time; The switching frequency of the resonant converter is gradually reduced from the initial high frequency toward the resonant frequency. The resonant current of the LLC resonant cavity is detected in real time, and the resonant current is compared with the current dynamic current limit threshold. When the resonant current reaches or exceeds the current dynamic current limit threshold, the reduction action of the switching frequency is frozen, so that the LLC resonant converter maintains the current frequency operation. Before generating the dynamic current limit threshold that gradually increases over time during the soft-start process, the method further includes: Turn on the lower bridge arm switch in the LLC resonant converter; During the first switching cycle of soft start, the upper bridge arm switch of the LLC resonant converter is forcibly turned on. The process of generating a dynamic current limit threshold that gradually increases over time during soft start includes: At the start of soft start, the reference voltage of the overcurrent protection comparator is set to the first voltage value; Within a preset time interval, the reference voltage is linearly increased over time to a second voltage value, where the second voltage value is the system maximum current protection setpoint. The reference voltage is used as the dynamic current limit threshold. During each switching cycle, the resonant current is compared with the current dynamic current limit threshold in real time to define a dynamic safety envelope for the amplitude increase of the resonant current.

2. The soft-start control method for an LLC resonant converter according to claim 1, characterized in that, The switching frequency of the controlled resonant converter gradually decreases from the initial high frequency towards the resonant frequency, including: Set the initial maximum on-time limit and determine the highest starting frequency corresponding to soft start; By continuously charging the charging capacitor with a controlled current source, the maximum on-time limit increases linearly with time, and the switching frequency decreases linearly. The maximum on-time limit is used to control the on-time of the switching transistors in the LLC resonant converter.

3. The soft-start control method for an LLC resonant converter according to claim 2, characterized in that, The action of freezing the reduction of the switching frequency when the resonant current reaches or exceeds the current dynamic current limit threshold includes: When the resonant current is detected to reach or exceed the current dynamic current limit threshold, an overcurrent protection flag signal is generated; The overcurrent protection flag signal activates the bypass logic and outputs a bypass pulse; the bypass pulse is used to bypass the controlled current source and forcibly stop the controlled current source from charging the charging capacitor.

4. The soft-start control method for an LLC resonant converter according to any one of claims 1-3, characterized in that, The method further includes: During the soft start process, the resonant current is detected in real time and a zero-crossing signal of the resonant current is output. Each time the resonant current zero-crossing signal is triggered, the on-time timer is started. Based on the timing duration of the conduction timer, the conduction duration after the resonant current crosses zero is controlled, thereby adjusting the amount of charge accumulation flowing into the resonant cavity.

5. The soft-start control method for an LLC resonant converter according to any one of claims 1-3, characterized in that, The method further includes: During the soft start process, the voltage slope at the midpoint of the switching node is captured in real time; The dead time is dynamically adjusted based on the midpoint voltage slope.

6. The soft-start control method for an LLC resonant converter according to claim 5, characterized in that, The method further includes: The output power of the LLC resonant converter is detected in real time, and the output power is compared with the system closed-loop set value; When the output power reaches the system closed-loop set value, the freeze control of the switching frequency reduction action is released. Switch to the closed-loop control algorithm and configure the maximum on-time limit to the maximum limit allowed by the system.

7. An LLC resonant converter, characterized in that, include: The resonant unit includes a resonant inductor, a resonant capacitor, and a transformer magnetizing inductor. The bridge arm switch unit includes an upper bridge arm switch tube and a lower bridge arm switch tube; A current detection unit is used to detect the resonant current of the LLC resonant cavity in real time. The control unit is electrically connected to the bridge arm switch unit and the current detection unit, and the control unit is used to execute the soft-start control method of the LLC resonant converter according to any one of claims 1 to 6.

8. The LLC resonant converter according to claim 7, characterized in that, The control unit includes a first timer generating circuit, a second timer generating circuit, and a ramp frequency increasing circuit. The first timer generating circuit and the second timer generating circuit are both electrically connected to the ramp frequency increasing circuit. The first timer generating circuit and the second timer generating circuit are both electrically connected to the bridge arm switch unit. The first timer generating circuit, the second timer generating circuit, and the ramp frequency increasing circuit are all electrically connected to the current detection unit. The first timer generation circuit is used to output an upper bridge arm conduction timer based on the resonant current negative zero-crossing signal, the soft-start end signal, the upper bridge arm drive signal, and the maximum conduction time limit. The second timer generation circuit is used to output a lower bridge arm conduction timer based on the resonant current positive zero-crossing signal, the soft-start end signal, the lower bridge arm drive signal, and the maximum conduction time limit. The ramp frequency growth circuit is used to generate the maximum conduction time limit based on the soft-start end signal, the dynamic current limit threshold, and the overcurrent protection flag signal, so as to control the switching frequency to gradually decrease and freeze.

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