A soft-start control method, control device and switching power supply

Abstract

This invention discloses a soft-start control method, control device, and switching power supply. The soft-start control method is applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switching transistors connected in series. The soft-start control method involves controlling the duty cycle of the first to fourth switching transistors so that the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetrical state and a transition state, ultimately entering the symmetrical state required for steady-state operation. During the soft-start control process, the following is maintained: the switching frequencies of the four switching transistors are the same; the driving of the second switching transistor is complementary to that of the first switching transistor; and the driving of the fourth switching transistor is complementary to that of the third switching transistor. This invention achieves smooth pre-charging of the resonant capacitor voltage without adding additional circuitry, ensuring that all MOSFETs in the three-level switching network achieve ZVS throughout the soft-start process.

Description

Technical Field

[0001] This invention relates to the field of switching converter technology, and in particular to a soft-start control method, control device, and switching power supply. Background Technology

[0002] With the vigorous development of new energy, the demand for high-performance, high-reliability, and low-cost switching converters is increasing. LLC resonant converters are favored by industry professionals due to their high efficiency, soft switching, and ability to achieve higher power density. In applications such as new energy vehicles and residential photovoltaic energy storage charging, where the input voltage is high and the input voltage variation range is wide, a multi-level topology can be used to reduce the voltage stress on the switching transistors, facilitating device selection and reducing hardware costs. Series half-bridge three-level LLC resonant converters (such as...) Figure 1 Its advantages, such as simple topology, low device cost, and high degree of control freedom, make it a perfect match for application requirements.

[0003] Similar to a conventional half-bridge LLC, during normal steady-state operation, the resonant capacitor voltage has a DC voltage bias, often half the input voltage. However, in the initial state before startup, the resonant capacitor voltage is 0. If a soft start is performed directly using the normal steady-state drive timing, the resonant capacitor voltage will become unbalanced for a period of time, causing the switching transistor to lose its zero-voltage turn-on characteristic and easily leading to overvoltage stress damage. At the same time, in the initial state before startup, the voltage of the large filter capacitor on the secondary side of the resonant converter is also zero (equivalent to a short circuit). This means that during startup, the primary side magnetizing inductance of the transformer is essentially short-circuited, resulting in a low resonant cavity impedance and a large resonant current surge during startup.

[0004] To address the aforementioned issues, patent CN117498671A, entitled "A Segmented Soft-Start Method for a Half-Bridge Series LLC Circuit," proposes a segmented soft-start control method. This method charges the resonant capacitor in segments by maintaining a minimum duty cycle. After charging, the duty cycle is gradually increased to approximately 0.5 for gain adjustment. However, this method has the following drawbacks: First, when maintaining a minimum duty cycle to pre-charge the resonant capacitor, the two switches in the same switching arm are in a non-complementary state, preventing the switches from achieving zero-voltage turn-on, increasing switching losses. Furthermore, hard switching during high-voltage startup can easily cause excessive voltage stress on the switches, leading to damage. Second, this method involves segmented drive timing switching control during the soft-start process, increasing control complexity. Moreover, the switching process can easily cause overshoot and undershoot fluctuations in the output voltage, making it impractical for engineering applications.

[0005] The following is based on Figure 2The following is a detailed description of the main waveform timing diagrams of the soft-start process when using the segmented soft-start method proposed in patent CN117498671A. Here, VGS1 / 3 represents the driving waveforms of the first switch S1 and the third switch S3, and VGS2 / 4 represents the driving waveforms of the second switch S2 and the fourth switch S4; Vds1~Vds4 represent the drain-source voltage waveforms of the first switch S1~the fourth switch S4, respectively; VAB represents the voltage waveform injected into the LLC resonant cavity network III; and Vcr represents the resonant capacitor. C r The voltage waveform across the two ends; Ir represents the current flowing through the resonant inductor. L r The resonant current waveform.

[0006] During soft-start, the system maintains a minimum duty cycle. When the first switch S1 and the third switch S3 are turned on, the voltage VAB injected into the LLC resonant cavity network III is 1 / 2Vin, and the resonant inductor current Ir increases, supplying power to the resonant capacitor. C r As the capacitor charges, the resonant capacitor voltage Vcr gradually increases. When the first switch S1 and the third switch S3 are turned off, the resonant inductor current Ir freewheels through the body diodes of the second switch S2 and the third switch S3. Since the resonant capacitor voltage Vcr is low at this time, the slope of the resonant current Ir's decrease is less than the slope of its increase. As a result, the resonant current Ir is still very small, either positive or negative, before the second switch S2 and the fourth switch S4 are turned on. This makes it impossible for the second switch S2 and the fourth switch S4 to achieve ZVS, and the switching losses increase. Furthermore, due to the hard switching of the second switch S2 and the fourth switch S4, the output parasitic capacitance Coss of the second switch S2 and the fourth switch S4 discharges instantaneously at the moment of turn-on, generating a large current. Since PCB trace inductance is unavoidable in actual products, this inductance resonates with the output parasitic capacitance Coss of the first switch S1 and the third switch S3. This causes significant voltage spikes in the drain-source voltages Vds1 and Vds3 of the first switch S1 and the third switch S3 at the moment of turn-on of the second switch S2 and the fourth switch S4. Similarly, the first switch S1 and the third switch S3 cannot achieve ZVS, resulting in significant voltage spikes in the drain-source voltages Vds2 and Vds4 of the second switch S2 and the fourth switch S4 at the moment of turn-on of the first switch S1 and the third switch S3.

[0007] Therefore, it is necessary to propose an improved control strategy to overcome the shortcomings of existing technologies. Summary of the Invention

[0008] In view of this, the technical problem to be solved by the present invention is to propose a soft-start control method, control device and switching power supply, which can achieve smooth pre-charging of resonant capacitor voltage without adding additional circuitry, improve the ZVS (zero voltage turn-on) performance of primary-side switching transistor during soft-start process, avoid the problem of MOSFET overvoltage stress damage caused by hard switching, and solve the problem of excessive resonant current surge during soft-start process.

[0009] As a first aspect of the present invention, the technical solution of the provided soft-start control method is as follows:

[0010] A soft-start control method is applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switches connected in series, forming two sets of series-connected switch bridge arms. The four switches are, in sequence, a first switch, a second switch, a third switch connected to the positive input terminal of the resonant converter, and a fourth switch connected to the negative input terminal of the resonant converter. When the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switches are the same; the driving of the second switch is complementary to the driving of the first switch; the driving of the fourth switch is complementary to the driving of the third switch; and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch. The soft-start control method includes:

[0011] By controlling the duty cycle of the first to the fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation.

[0012] During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor and the driving of the first switching transistor are complementary, and the driving of the fourth switching transistor and the driving of the third switching transistor are complementary.

[0013] Wherein: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switching transistors in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback adjustment until the output voltage is established, and the soft start ends.

[0014] Furthermore, when the resonant converter operates in non-frequency doubling mode, controlling the drive duty cycle of the first to fourth switches includes: starting the power-on at a first set switching frequency, gradually increasing the drive duty cycle of the first switch from a set minimum duty cycle, and gradually decreasing the drive duty cycle of the third switch from being complementary to the drive duty cycle of the first switch inwards until it is equal to the drive duty cycle of the first switch and lags behind the drive duty cycle of the first switch by half a cycle and is then maintained.

[0015] Furthermore, controlling the end of the soft start includes: if the output voltage has been established before the drive duty cycle of the first switch reaches the first set duty cycle, then the soft start is directly ended when the output voltage is established; otherwise, the drive duty cycle of the first switch is maintained unchanged after reaching the first set duty cycle, and the switching frequency of the four switches is gradually reduced until the output voltage is established before the soft start is ended.

[0016] Preferably, the first set duty cycle is 0.5.

[0017] Preferably, the first set switching frequency is the highest frequency when the resonant converter is working normally; or the first set switching frequency is 1.1 to 3 times the series resonant frequency of the LLC resonant cavity network.

[0018] Furthermore, when the resonant converter operates in frequency doubling mode, controlling the driving duty cycle of the first switch to the fourth switch includes: starting the power-on at a second set switching frequency, gradually increasing the driving duty cycle of the first switch from a set minimum duty cycle, and ensuring that the driving duty cycle of the fourth switch is equal to that of the first switch and lags behind the driving duty cycle of the first switch by half a cycle.

[0019] Furthermore, controlling the soft start to end includes: after the drive duty cycle of the first switching transistor reaches a second set duty cycle, it remains unchanged, and the soft start ends by gradually reducing the switching frequency of the four switching transistors until the output voltage is established.

[0020] Preferably, the second set duty cycle is 0.25.

[0021] Preferably, the second set switching frequency is less than or equal to the first set switching frequency; or the second set switching frequency is 0.4 to 1 times the first set switching frequency; the first set switching frequency is the switching frequency set when the resonant converter is started in non-frequency doubling mode.

[0022] Furthermore, during the soft-start control process, the resonant converter is prohibited from switching between non-frequency doubling mode and frequency doubling mode. The resonant converter can only operate in one of the non-frequency doubling mode and frequency doubling mode to achieve a monotonically increasing output voltage of the resonant converter during the soft-start process.

[0023] As a second aspect of the present invention, the technical solution of the provided soft-start control device is as follows:

[0024] A soft-start control device is applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switches connected in series, forming two sets of series-connected switch bridge arms. The four switches are, in sequence, a first switch, a second switch, a third switch connected to the positive input terminal of the resonant converter, and a fourth switch connected to the negative input terminal of the resonant converter. When the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switches are the same; the driving of the second switch is complementary to the driving of the first switch; the driving of the fourth switch is complementary to the driving of the third switch; and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch. The soft-start control device is configured as follows:

[0025] By controlling the duty cycle of the first to the fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation.

[0026] During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor and the driving of the first switching transistor are complementary, and the driving of the fourth switching transistor and the driving of the third switching transistor are complementary.

[0027] Wherein: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switching transistors in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback adjustment until the output voltage is established, and the soft start ends.

[0028] As a third aspect of the present invention, the technical solution of the provided switching power supply embodiment is as follows:

[0029] A switching power supply includes a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switching transistors connected in series, forming two sets of series-connected switching transistor bridge arms. The four switching transistors are, in sequence, a first switching transistor, a second switching transistor, a third switching transistor connected to the positive input terminal of the resonant converter, and a fourth switching transistor connected to the negative input terminal of the resonant converter. When the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switching transistors are the same; the driving of the second switching transistor is complementary to the driving of the first switching transistor; the driving of the fourth switching transistor is complementary to the driving of the third switching transistor; and the driving of the third switching transistor is phase-shifted by 180° relative to the driving of the first switching transistor. The switching power supply also includes the soft-start control device described in any of the second aspects above.

[0030] Based on the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) The soft-start control embodiment proposed in this invention adopts asymmetric complementary driving timing. Without adding additional circuits, by optimizing the control timing, the resonant converter can achieve smooth pre-charging of the resonant capacitor voltage during the startup process, regardless of whether it is operating in non-frequency doubling mode or frequency doubling mode. This enables all switches in the three-level switching network to achieve full ZVS during the soft-start process, avoiding the problem of overvoltage stress damage to the switches due to hard switching. At the same time, it can reduce the resonant current spike during the soft-start process, which is beneficial to reduce startup loss and improve the reliability of the resonant converter.

[0032] (2) The soft-start control embodiment proposed in this invention further requires that the resonant converter be prohibited from switching between non-frequency doubling mode and frequency doubling mode during the soft-start control process. That is, soft start is performed according to a specific mode first, and mode switching is allowed only after the soft start is completed. This makes it applicable to wide-gain resonant converters with multi-mode combination control. The gain closed-loop control is controllable during the soft start process and does not involve mode switching, which simplifies the switching control logic. At the same time, it can ensure that the output voltage increases monotonically during the soft start process, thereby improving the performance and reliability of the resonant converter. Attached Figure Description

[0033] Figure 1 This is a typical topology diagram of the resonant converter used in this invention;

[0034] Figure 2 The following is a timing diagram of the main waveforms of the soft start process when using the segmented soft start method proposed in patent CN117498671A;

[0035] Figure 3This is a schematic diagram of the soft-start control timing in non-frequency doubling mode according to a preferred embodiment of the present invention;

[0036] Figure 4 This is a schematic diagram of the soft-start control timing in frequency multiplication mode according to a preferred embodiment of the present invention;

[0037] Figure 5 The following is a waveform timing diagram of the main waveforms of the prototype using the soft-start control method of the present invention in non-frequency doubling mode.

[0038] Figure 6 The following is a waveform timing diagram of the main waveforms of the prototype using the soft-start control method of the present invention in frequency doubling mode. Detailed Implementation

[0039] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0040] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0041] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate for the purposes of describing embodiments of this application herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0042] It should be understood that in the specification, claims, and drawings, when a step is described as continuing into another step, the step may directly continue into that other step or be continued into that other step through a third step; when an element / unit is described as "continuing" into another element / unit, the element / unit may be "directly connected" to that other element / unit or "connected" to that other element / unit through a third element / unit.

[0043] Furthermore, the accompanying drawings are merely illustrative of this disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions thereof will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0044] like Figure 1 The diagram shown is a typical topology of the resonant converter used in this invention. It is a series three-level LLC resonant converter, including an input voltage divider capacitor I, a three-level switch network II, an LLC resonant cavity network III, a transformer IV, and a secondary-side rectifier and filter circuit V.

[0045] In practical implementation, its structure is as follows:

[0046] The input voltage divider capacitor I includes a first capacitor C1 and a second capacitor C2 connected in series. The other ends of the first capacitor C1 and the second capacitor C2 are connected to the positive and negative terminals of the power supply, respectively. The three-level switching network II includes four switching transistors connected in series, forming two sets of series-connected switching transistor bridge arms. The four switching transistors specifically include a first switching transistor S1, a second switching transistor S2, a third switching transistor S3, and a fourth switching transistor S4, each with a body diode or an external parallel diode. The drain and source of the first switching transistor S1, the second switching transistor S2, the third switching transistor S3, and the fourth switching transistor S4 are connected in series. The drain of the first switching transistor S1 and the source of the fourth switching transistor S4, located at both ends of the switching bridge arm, are connected to the positive and negative terminals of the power supply, respectively. The intermediate connection point of the second switching transistor S2 and the third switching transistor S3 is connected to the intermediate connection point of the first capacitor C1 and the second capacitor C2. The LLC resonant cavity network III includes resonant capacitors connected in series. C r Resonant inductor L r Equivalent magnetizing inductance of transformer L m resonant capacitor C r The other end is connected to the intermediate connection point of the first switch S1 and the second switch S2 in the three-level switch network II, denoted as point A. The transformer magnetizing inductance... L mThe other end is connected to the intermediate connection point between the third switch S3 and the fourth switch S4 in the three-level switch network II, which is denoted as point B; the secondary side of transformer IV is connected to the input terminal of rectifier and filter circuit V; the secondary rectifier and filter circuit V includes a full-wave rectifier circuit composed of the first synchronous rectifier SR1 and the second synchronous rectifier SR2 on the secondary side, and an output filter capacitor. C o .

[0047] For those skilled in the art, Figure 1 The circuit shown also includes, but is not limited to, the following variations:

[0048] (1) Replace the secondary rectifier filter circuit V with a bridge rectifier structure consisting of four switching transistors or diodes;

[0049] (2) Exchange resonant capacitor C r and resonant inductor L r The location.

[0050] First Embodiment

[0051] This embodiment provides a soft-start control method applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switches connected in series to form two sets of series-connected switch bridge arms. The four switches are, in order, the first switch S1, the second switch S2, the third switch S3 connected to the positive input terminal of the resonant converter, and the fourth switch S4 connected to the negative input terminal of the resonant converter.

[0052] When the resonant converter is in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switches are the same, the driving of the second switch is complementary to the driving of the first switch, the driving of the fourth switch is complementary to the driving of the third switch, and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch.

[0053] It should be noted that the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch. This can be either leading by 180° or lagging by 180°.

[0054] The soft-start control method provided in this embodiment applies to a series three-level LLC resonant converter that can operate in either non-frequency doubling mode or frequency doubling mode during steady-state operation. Regardless of the mode, the voltage injected into the LLC resonant cavity network remains symmetrical due to the aforementioned control strategy, ensuring that the voltage applied to the resonant capacitor is balanced. C rThe charging and discharging currents are also symmetrical, and the resonant capacitor voltage Vcr fluctuates under a DC voltage bias.

[0055] Furthermore, during steady-state operation, when the resonant converter operates in non-frequency doubling mode, it is further divided into PWM mode and PFM mode. The gain adjustment capability of the two modes is continuous, that is, the PWM mode and PFM mode in non-frequency doubling mode can be seamlessly and smoothly switched, as detailed below:

[0056] When the input voltage is low, the series three-level LLC resonant converter operates in non-frequency doubling mode. The output voltage Vo is adjusted by changing the duty cycle of the first switch S1 and the third switch S3. This is the PWM mode. If the output voltage Vo gain adjustment is still not satisfied when the duty cycle of the first switch S1 and the third switch S3 is increased to about 0.5, the duty cycle of the first switch S1 and the third switch S3 is maintained at about 0.5. The adjustment of the switching frequency of the first switch S1 and the third switch S3 is then changed to meet the adjustment requirement of the output voltage Vo. This is the PFM mode.

[0057] When the input voltage is high, the series three-level LLC resonant converter is operated in frequency doubling mode. At this time, the duty cycle of the first switch S1 and the fourth switch S4 is maintained at about 0.25. The output voltage Vo is adjusted by adjusting the switching frequency of the first switch S1 and the fourth switch S4.

[0058] When the input voltage changes from low voltage to high voltage or from high voltage to low voltage during steady-state operation, the series three-level LLC resonant converter will switch between non-frequency doubling mode and frequency doubling mode to achieve output voltage regulation under wide gain variation.

[0059] The above describes the control details of the series three-level LLC resonant converter under steady-state operation in which the soft-start control method provided in this embodiment is applied. The following section provides a detailed description of the series three-level LLC resonant converter used in the soft-start control method of this embodiment. The soft-start control method of this embodiment includes:

[0060] By controlling the duty cycle of the first to fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation.

[0061] During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor is complementary to the driving of the first switching transistor, and the driving of the fourth switching transistor is complementary to the driving of the third switching transistor.

[0062] Among them: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switches in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback regulation until the output voltage is established and the soft start ends.

[0063] It should be noted that the zero-voltage turn-on (ZVS) of the present invention is not limited to the turn-on when the voltage across the switching transistor is exactly zero. As long as the voltage across the switching transistor is near zero, it is considered to be zero-voltage turn-on. The specific deviation of the voltage from zero depends on the design requirements, as long as it is within an acceptable range.

[0064] In particular, in order to achieve a monotonically increasing output voltage of the resonant converter during the soft-start process, the soft-start process of the series three-level LLC resonant converter of the present invention smoothly transitions from the asymmetric control stage to the symmetric control stage regardless of the mode used during steady-state operation. However, the specific drive control timing of the first switch S1 to the fourth switch S4 differs in different modes. Therefore, before soft-start, the operating mode is determined based on the magnitude of the input voltage, and then soft-start is performed according to the specific drive timing of different operating modes. Furthermore, the mode switching between the non-frequency doubling mode and the frequency doubling mode is not performed during the soft-start process.

[0065] like Figure 3 The diagram shown is a schematic of the soft-start control timing in non-frequency doubling mode according to a preferred embodiment of the present invention. The driving of the first switch S1 is complementary to the driving of the second switch S2, and the driving of the third switch S3 is complementary to the driving of the fourth switch S4. The switching frequencies of switches S1 to S4 are the same. During the soft-start process, the device starts at the set maximum switching frequency (e.g., twice the series resonant frequency). The duty cycle of the first switch S1 gradually increases from the set minimum duty cycle, while the duty cycle of the third switch S3 gradually decreases from being complementary to the duty cycle of the first switch S1. At this time, the time during which the first switch S1 and the fourth switch S4 are simultaneously conducting is less than the time during which the second switch S2 and the third switch S3 are simultaneously conducting. This is called an asymmetrical state. That is, when the first switch S1 and the fourth switch S4 are simultaneously conducting, the input voltage Vin is injected into the LLC resonant cavity network to supply the resonant capacitor. C rCharging occurs, but the charging time is short, and the resonant capacitor voltage rises slowly. When the second switch S2 and the third switch S3 are both on, the voltage injected into the LLC resonant cavity network is zero. The resonant capacitor is slowly charged only by the forward freewheeling current Ir. Since the second switch S2 and the third switch S3 are on for a relatively long time, the resonant current Ir will continue to flow in reverse after the rectified freewheeling current drops to zero. At this time, the resonant capacitor voltage Vcr will slowly discharge. Because the charging amount is still greater than the discharging amount, the resonant capacitor voltage Vcr can achieve a slow and smooth rise. Furthermore, because the resonant current Ir can reverse in an asymmetrical state, ZVS (Zero-Voltage-Side Variable) can be achieved for switches S1~S4. Additionally, due to the high start-up frequency, the equivalent impedance of the LLC resonant cavity network is large, which reduces the resonant current surge during the soft-start process.

[0066] As the driving duty cycle of the third switch S3 gradually decreases to be equal to that of the first switch S1 and lags behind the first switch S1 by half a cycle, the time during which the first switch S1 and the fourth switch S4 are turned on together is the same as the time during which the second switch S2 and the third switch S3 are turned on together. This means that they enter a symmetrical state. At this time, the charging and discharging currents of the resonant capacitor are symmetrical, the voltage Vcr of the resonant capacitor tends to be stable, and the output voltage increases as the driving duty cycle of the first switch S1 gradually increases. If the output voltage is established before the drive duty cycle of the first switch S1 reaches the first set duty cycle (e.g., 0.5), the soft start ends, and the series three-level LLC resonant converter operates stably in PWM mode without frequency multiplication. If the output voltage is still not established after the drive duty cycle of the first switch S1 reaches the first set duty cycle, the drive duty cycle of the first switch S1 is maintained at the first set duty cycle, and its switching frequency is further reduced to achieve output voltage regulation until the output voltage is established. Then the soft start ends, and the series three-level LLC resonant converter operates stably in PFM mode without frequency multiplication.

[0067] It should be noted that the driving duty cycle of the first switch S1 reaching the first set duty cycle is not limited to the first switch S1 driving duty cycle being exactly equal to the first set duty cycle. As long as the driving duty cycle of the first switch S1 is close to the first set duty cycle, it is considered to have reached the first set duty cycle. The specific deviation of this duty cycle from the first set duty cycle is related to the design requirements, as long as it is within the acceptable range.

[0068] It should also be noted that during the soft start process, as the drive duty cycle of the first switching transistor S1 gradually increases, the switching frequency can be adjusted to gradually decrease at the same time. This ensures that the switching transistors S1~S4 can achieve ZVS while shortening the rise time of the output voltage, which can meet the application requirements for the output voltage settling time.

[0069] like Figure 4The diagram shown is a schematic of the soft-start control timing in frequency multiplication mode according to a preferred embodiment of the present invention. The driving of the first switch S1 is complementary to the driving of the second switch S2, and the driving of the third switch S3 is complementary to the driving of the fourth switch S4. The switching frequencies of switches S1 to S4 are the same. During the soft-start process, the device starts at the set maximum switching frequency (e.g., the series resonant frequency). The duty cycle of the first switch S1 gradually increases from the set minimum duty cycle. The duty cycle of the fourth switch S4 is the same as that of the first switch S1. The duty cycle is equal to and lags behind the driving half-cycle of the first switch S1. At this time, the time when the first switch S1 and the third switch S3 are on together, and the time when the second switch S2 and the fourth switch S4 are on together, are both less than the time when the second switch S2 and the third switch S3 are on together. This is called an asymmetrical state. That is, when the first switch S1 and the third switch S3 are on together, or the second switch S2 and the fourth switch S4 are on together, half of the input voltage Vin / 2 is injected into the LLC resonant cavity network to supply the resonant capacitor. C r Charging occurs, but the charging time is short, and the resonant capacitor voltage rises slowly. When the second switch S2 and the third switch S3 are both on, the voltage injected into the LLC resonant cavity network is zero. Only the resonant current Ir provides slow charging to the resonant capacitor in the forward freewheeling direction. Furthermore, because the second switch S2 and the third switch S3 are on for a relatively long time, the resonant current Ir will continue to flow in the reverse direction after the rectified freewheeling current drops to zero. At this time, the resonant capacitor voltage Vcr will slowly discharge. Since the charging amount is still greater than the discharging amount, the resonant capacitor voltage Vcr can achieve a slow and smooth rise. Moreover, because the resonant current Ir can reverse in the asymmetrical state, ZVS (Zero-Voltage Switching) can be achieved for switches S1~S4. In frequency multiplication mode, the voltage injected into the LLC resonant cavity network operates at twice the switching frequency of switches S1~S4, increasing the equivalent impedance of the LLC resonant cavity network and reducing the resonant current surge during soft-start.

[0070] As the driving duty cycle of the first switch S1 gradually increases to the second set duty cycle (e.g., 0.25), the driving duty cycle of the first switch S1 is maintained at the second set duty cycle, i.e., it enters a symmetrical state. At this time, the switching frequency of the switches S1 to S4 is further reduced to achieve output voltage regulation until the output voltage is established. The soft start ends, and the series three-level LLC resonant converter operates stably in frequency doubling mode.

[0071] It should be noted that the driving duty cycle of the second switch S1 reaching the second set duty cycle is not limited to the driving duty cycle of the first switch S1 being exactly equal to the second set duty cycle. As long as the driving duty cycle of the first switch S1 is near the second set duty cycle, it is considered to have reached the second set duty cycle. The specific deviation of this duty cycle from the second set duty cycle is related to the design requirements, as long as it is within the acceptable range.

[0072] It should also be noted that during the soft start process, as the drive duty cycle of the first switching transistor S1 gradually increases, the switching frequency can be adjusted to gradually decrease at the same time. This ensures that the switching transistors S1~S4 can achieve ZVS while shortening the rise time of the output voltage, which can meet the application requirements for the output voltage settling time.

[0073] Second Embodiment

[0074] This embodiment provides a soft-start control device applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switches connected in series, forming two sets of series-connected switch bridge arms. The four switches are, in sequence, a first switch, a second switch, a third switch connected to the positive input terminal of the resonant converter, and a fourth switch connected to the negative input terminal of the resonant converter. When the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switches are the same; the driving of the second switch is complementary to the driving of the first switch; the driving of the fourth switch is complementary to the driving of the third switch; and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch. The soft-start control device is configured as follows:

[0075] By controlling the duty cycle of the first to fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation.

[0076] During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor is complementary to the driving of the first switching transistor, and the driving of the fourth switching transistor is complementary to the driving of the third switching transistor.

[0077] Among them: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switches in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback regulation until the output voltage is established and the soft start ends.

[0078] The control device in this embodiment uses the same technical means as the control method in the first embodiment, and has the same beneficial effects, so it will not be described in detail.

[0079] Furthermore, the preferred technical means or further improved means in each step of the control method in the first embodiment can be extended to the corresponding unit in this embodiment, and will not be described in detail in this embodiment.

[0080] Third Embodiment

[0081] This embodiment provides a switching power supply, including a resonant converter. The primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network. The three-level switching network includes four switching transistors connected in series, forming two sets of series-connected switching transistor bridge arms. The four switching transistors are, in sequence, a first switching transistor, a second switching transistor, a third switching transistor connected to the positive input terminal of the resonant converter, and a fourth switching transistor connected to the negative input terminal of the resonant converter. When the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical. The control strategy is as follows: the switching frequencies of the four switching transistors are the same; the driving of the second switching transistor is complementary to the driving of the first switching transistor; the driving of the fourth switching transistor is complementary to the driving of the third switching transistor; and the driving of the third switching transistor is phase-shifted by 180° relative to the driving of the first switching transistor. The switching power supply also includes any of the soft-start control devices described in the second embodiment.

[0082] according to Figure 1 A prototype of a series three-level LLC resonant converter was designed. The prototype has a rated output power of 960W, an input voltage Vin range of 250~1000V, a nominal output voltage of 24V, and a series resonant frequency of 100kHz. Measurements were taken under input voltages of 300V and 1000V in non-frequency doubling mode (e.g., ...). Figure 5 (as shown) and frequency multiplication mode (such as) Figure 6 The soft-start process experimental waveform is shown below, where V o For the output voltage waveform, V GS1~ V GS4 These are the driving waveforms for the first switch S1 to the fourth switch S4, respectively; Vds1 to Vds4 are the drain-source voltage waveforms for the first switch S1 to the fourth switch S4, respectively; VAB represents the voltage waveform injected into the LLC resonant cavity network; Vcr represents the resonant capacitance. C r The voltage waveform across the two ends; Ir represents the current flowing through the resonant inductor. L r The resonant current waveform. (From...) Figure 5 , Figure 6The experimental waveforms show that during the soft-start process, the resonant capacitor voltage Vcr rises smoothly, and all switches S1 to S4 can achieve ZVS. There are no obvious voltage spikes in the drain-source voltages of switches S1 to S4, and the resonant current Ir also has no obvious current surge. The output voltage also rises smoothly and monotonically, thus verifying the effectiveness of the soft-start control method of the present invention.

[0083] In the above description of the embodiments, the soft-start control method of the resonant converter of the present invention, by employing asymmetric complementary drive timing during the soft-start process and combining a duty cycle and frequency hybrid modulation strategy, can achieve smooth pre-charging of the resonant capacitor voltage without adding additional circuitry. This ensures that all switches in the three-level switching network achieve ZVS throughout the soft-start process, avoiding overvoltage stress damage to the switches due to hard switching. At the same time, it can reduce the resonant current spike during the soft-start process, providing a stable and reliable preferred soft-start control scheme for the series three-level LLC resonant converter with current requirements for high efficiency, high power density, and wide voltage range.

[0084] It should be noted that the embodiments described above are merely illustrative examples of the technical solutions and content of the present invention. It should be pointed out that the above embodiments should not be considered as limitations on the present invention. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, but these will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims, and should also be considered within the protection scope of the present invention.

Claims

1. A soft-start control method applied to a resonant converter, wherein the primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network; the three-level switching network includes four switches connected in series to form two sets of series-connected switch bridge arms, the four switches being a first switch, a second switch, a third switch connected to the positive input terminal of the resonant converter, and a fourth switch connected to the negative input terminal of the resonant converter; when the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical, and the control strategy is as follows: the switching frequencies of the four switches are the same, the driving of the second switch and the driving of the first switch are complementary, the driving of the fourth switch and the driving of the third switch are complementary, and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch; characterized in that, The soft-start control method includes: By controlling the duty cycle of the first to the fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation. During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor and the driving of the first switching transistor are complementary, and the driving of the fourth switching transistor and the driving of the third switching transistor are complementary. Wherein: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switches in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback adjustment until the output voltage is established, and the soft start ends.

2. The soft-start control method according to claim 1, characterized in that, When the resonant converter operates in non-frequency doubling mode, controlling the drive duty cycle of the first to fourth switches includes: starting the power-on at a first set switching frequency, gradually increasing the drive duty cycle of the first switch from a set minimum duty cycle, and gradually decreasing the drive duty cycle of the third switch from being complementary to the drive duty cycle of the first switch inward until it is equal to the drive duty cycle of the first switch and lags behind the drive duty cycle of the first switch by half a cycle and is then maintained.

3. The soft-start control method according to claim 2, characterized in that, Controlling the termination of the soft start includes: if the output voltage has been established before the drive duty cycle of the first switch reaches the first set duty cycle, then the soft start is terminated directly when the output voltage is established; otherwise, the drive duty cycle of the first switch is maintained unchanged after reaching the first set duty cycle, and the switching frequency of the four switches is gradually reduced until the output voltage is established before the soft start is terminated.

4. The soft-start control method according to claim 3, characterized in that: The first set duty cycle is 0.

5.

5. The soft-start control method according to claim 2, characterized in that: The first set switching frequency is the highest frequency when the resonant converter is working normally; or the first set switching frequency is 1.1 to 3 times the series resonant frequency of the LLC resonant cavity network.

6. The soft-start control method according to claim 1, characterized in that, When the resonant converter operates in frequency multiplication mode, controlling the driving duty cycle of the first switch to the fourth switch includes: starting the power-on at a second set switching frequency, gradually increasing the driving duty cycle of the first switch from a set minimum duty cycle, and having the driving duty cycle of the fourth switch equal to that of the first switch and lagging behind the driving duty cycle of the first switch by half a cycle.

7. The soft-start control method according to claim 6, characterized in that, Controlling the soft start to end includes: after the drive duty cycle of the first switching transistor reaches a second set duty cycle, it remains unchanged, and the soft start ends by gradually reducing the switching frequency of the four switching transistors until the output voltage is established.

8. The soft-start control method according to claim 7, characterized in that: The second duty cycle is set to 0.

25.

9. The soft-start control method according to claim 6, characterized in that: The second set switching frequency is less than or equal to the first set switching frequency; or the second set switching frequency is 0.4 to 1 times the first set switching frequency; the first set switching frequency is the switching frequency set when the resonant converter is started in non-frequency doubling mode.

10. The soft-start control method according to claim 1, characterized in that: During the soft-start control process, the resonant converter is prohibited from switching between non-frequency doubling mode and frequency doubling mode. The resonant converter can only operate in one of the non-frequency doubling mode and frequency doubling mode to achieve a monotonically increasing output voltage of the resonant converter during the soft-start process.

11. A soft-start control device applied to a resonant converter, wherein the primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network; the three-level switching network includes four switches connected in series to form two sets of series-connected switch bridge arms, the four switches being a first switch, a second switch, a third switch connected to the positive input terminal of the resonant converter, and a fourth switch connected to the negative input terminal of the resonant converter; when the resonant converter is operating in steady state, the voltage injected into the LLC resonant cavity network is symmetrical, and the control strategy is as follows: the switching frequencies of the four switches are the same, the driving of the second switch and the driving of the first switch are complementary, the driving of the fourth switch and the driving of the third switch are complementary, and the driving of the third switch is phase-shifted by 180° relative to the driving of the first switch; characterized in that, The soft-start control device is configured to: By controlling the duty cycle of the first to the fourth switching transistors, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters the symmetric state during steady-state operation. During the soft-start control process, the following are always maintained: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor and the driving of the first switching transistor are complementary, and the driving of the fourth switching transistor and the driving of the third switching transistor are complementary. Wherein: the asymmetric state is used to achieve smooth pre-charging of the resonant capacitor voltage, thereby achieving zero-voltage turn-on of all switches in the three-level switching network during the soft start process; during the transition state, the output voltage of the resonant converter is monotonically increased through closed-loop feedback adjustment until the output voltage is established, and the soft start ends.

12. A switching power supply, comprising a resonant converter, wherein the primary circuit of the resonant converter includes an input voltage divider capacitor, a three-level switching network, and an LLC resonant cavity network; the three-level switching network includes four switching transistors connected in series to form two sets of series-connected switching transistor bridge arms, the four switching transistors being, in sequence, a first switching transistor, a second switching transistor, a third switching transistor connected to the positive input terminal of the resonant converter, and a fourth switching transistor connected to the negative input terminal of the resonant converter; the voltage injected into the LLC resonant cavity network during steady-state operation of the resonant converter is symmetrical, and the control strategy is as follows: the switching frequencies of the four switching transistors are the same, the driving of the second switching transistor and the driving of the first switching transistor are complementary, the driving of the fourth switching transistor and the driving of the third switching transistor are complementary, and the driving of the third switching transistor is phase-shifted by 180° relative to the driving of the first switching transistor; characterized in that: The switching power supply also includes the soft-start control device as described in claim 11.

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