Soft start control method, control device and switching power supply

Abstract

The invention discloses a soft start control method, a control device and a switching power supply, the soft start control method is applied to a resonant converter, and a primary side circuit of the resonant converter comprises an input voltage dividing capacitor, a three-level switching network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes which are sequentially connected in series, and the soft start control method comprises the steps that voltage injected into the LLC resonant cavity network sequentially goes through an asymmetric state and a transition state by controlling the driving duty ratio of the first to fourth switch tubes, and finally enters a symmetric state during steady-state work; in the soft start control process, the switching frequencies of the four switching tubes are always kept to be the same, the driving of the second switching tube and the driving of the first switching tube are complementary, and the driving of the fourth switching tube and the driving of the third switching tube are complementary. Under the condition that no extra circuit is added, smooth pre-charging of the resonant capacitor voltage can be achieved, and it is ensured that all MOS transistors in the three-level switching network achieve ZVS in the whole soft start process.

Description

Technical Field

[0001] The present invention relates to the technical field of switching converters, and in particular to a soft start control method, a control device and a switching power supply. Background Art

[0002] With the vigorous development of new energy, the demand for high-performance, high-reliability, and low-cost switching converters is increasing. The LLC resonant converter is favored by industry insiders for its advantages such as high efficiency, soft switching, and higher power density. In application scenarios such as new energy vehicles and household photovoltaic storage and charging, the input voltage is high and the input voltage range is wide. A multi-level topology can be used to reduce the voltage stress of the switch tube to facilitate device selection and reduce hardware costs. The series half-bridge three-level LLC resonant converter (such as Figure 1 ) It can perfectly match application requirements due to its simple topology, low device cost, and high control freedom.

[0003] Like the conventional half-bridge LLC, during normal steady-state operation, there is a DC voltage bias in the resonant capacitor voltage, which is often half of the input voltage. In the initial state before starting, the resonant capacitor voltage is 0. If soft-start is performed directly with the normal steady-state drive timing, the resonant capacitor voltage will be unbalanced for a period of time, causing the switch tube to lose its zero-voltage turn-on characteristic, which can easily cause overvoltage stress damage to the switch tube. At the same time, in the initial state before starting, the voltage of the large filter capacitor on the secondary side of the resonant converter is also zero (equivalent to a short-circuit state), so that during the startup process, the primary excitation inductance of the transformer is equivalent to being short-circuited, and the resonant cavity impedance is low, which can easily cause a large resonant current shock problem during startup.

[0004] To solve the above problems, a segmented soft-start control method is proposed in the patent "A segmented soft-start method for a half-bridge series LLC circuit" with publication number CN117498671A. The resonant capacitor is charged by segmented control by maintaining the minimum duty ratio. After charging is completed, the duty ratio is gradually increased to about 0.5 for gain adjustment. This method has the following defects: First, when maintaining the minimum duty ratio to pre-charge the resonant capacitor, the two switch tubes in the same switch tube bridge arm are in a non-complementary state, resulting in the switch tube being unable to achieve zero voltage switching, increasing the switching loss, and hard switching during high-voltage startup is likely to cause the voltage stress of the switch tube to be too high and damaged; second, this method involves switching control of the segmented drive timing in the soft-start process, which increases the complexity of the control, and is likely to cause overshoot and undershoot fluctuations in the output voltage during the switching process, which is not practical in engineering.

[0005] According to the following Figure 2, that is, the main waveform timing diagram of the soft start process when the segmented soft start method proposed in patent CN117498671A is used is described in detail, wherein VGS1 / 3 represents the driving waveform of the first switch tube S1 and the third switch tube S3, VGS2 / 4 represents the driving waveform of the second switch tube S2 and the fourth switch tube S4; Vds1~Vds4 represent the drain-source voltage waveforms of the first switch tube S1~the fourth switch tube S4 respectively; VAB represents the voltage waveform injected into the LLC resonant cavity network III; Vcr represents the resonant capacitor C r Voltage waveform at both ends; Ir represents the current flowing through the resonant inductor L r The resonant current waveform.

[0006] During the soft start process, the minimum duty cycle is maintained. When the first switch tube S1 and the third switch tube 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, giving the resonant capacitor C r The resonant capacitor voltage Vcr also gradually increases during charging. When the first switch tube S1 and the third switch tube S3 are turned off, the resonant inductor current Ir is freewheeling through the body diodes of the second switch tube S2 and the third switch tube S3. Since the resonant capacitor voltage Vcr is relatively low at this time, the slope of the resonant current Ir decreases less than the slope of the increase, resulting in the resonant current Ir still being positive or having a very small negative current before the second switch tube S2 and the fourth switch tube S4 are turned on, making it impossible for the second switch tube S2 and the fourth switch tube S4 to achieve ZVS, and the switching loss increases. And due to the hard switching of the second switch tube S2 and the fourth switch tube S4, at the moment of turning on, the output parasitic capacitance Coss of the second switch tube S2 and the fourth switch tube S4 is discharged instantly, generating a large current. Due to the inevitable existence of PCB routing inductance in actual products, the routing inductance resonates with the output parasitic capacitance Coss of the first switch tube S1 and the third switch tube S3, resulting in a large voltage spike stress on the drain-source voltage Vds1 and Vds3 of the first switch tube S1 and the third switch tube S3 at the moment of turning on the second switch tube S2 and the fourth switch tube S4. Similarly, the first switch tube S1 and the third switch tube S3 cannot achieve ZVS, resulting in a large voltage spike stress on the drain-source voltage Vds2 and Vds4 of the second switch tube S2 and the fourth switch tube S4 at the moment of turning on the first switch tube S1 and the third switch tube S3.

[0007] Therefore, it is necessary to propose an improved control strategy to overcome the shortcomings of the existing technology. 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, a control device and a switching power supply, which can realize smooth pre-charging of the resonant capacitor voltage without adding additional circuits, improve the ZVS (zero voltage switch-on) performance of the primary side switch tube during the soft start process, avoid the problem of overvoltage stress damage to the MOS tube due to hard switching, and at the same time solve the problem of excessive resonant current impact during the soft start process.

[0009] As a first aspect of the present invention, the technical solution of the embodiment of the soft start control method provided is as follows: A soft start control method is applied to a resonant converter, wherein the primary circuit of the resonant converter comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are a first switch tube connected to a positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to a negative input end of the resonant converter in sequence; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is as follows: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is phase-shifted by 180° relative to the drive of the first switch tube; wherein the soft start control method comprises: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, it is always maintained that: the switching frequencies of the four switch tubes are the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switch tubes in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase until the output voltage is established and the soft start ends.

[0010] Furthermore, when the resonant converter operates in a non-frequency doubling mode, the size of the driving duty cycle of the first switch tube to the fourth switch tube is controlled, including: starting with a first set switching frequency, the driving duty cycle of the first switch tube gradually increases from a set minimum duty cycle, and the driving duty cycle of the third switch tube gradually decreases inward from being complementary to the driving duty cycle of the first switch tube until it is equal to the driving duty cycle of the first switch tube and is maintained after lagging behind the driving duty cycle of the first switch tube by half a cycle.

[0011] Furthermore, controlling the end of the soft start includes: if the output voltage has been established before the driving duty cycle of the first switch tube reaches a first set duty cycle, then directly ending the soft start when the output voltage is established; otherwise, maintaining the driving duty cycle of the first switch tube unchanged after reaching the first set duty cycle, and gradually reducing the switching frequency of the four switch tubes until the output voltage is established to end the soft start.

[0012] Preferably, the first set duty cycle is 0.5.

[0013] Preferably, the first set switching frequency is the highest frequency when the resonant converter works normally; or the first set switching frequency is 1.1 to 3 times the series resonant frequency of the LLC resonant cavity network.

[0014] Furthermore, when the resonant converter operates in the frequency doubling mode, the size of the driving duty cycle of the first switch tube to the fourth switch tube is controlled, including: starting with a second set switching frequency, the driving duty cycle of the first switch tube gradually increases from a set minimum duty cycle, the driving duty cycle of the fourth switch tube is equal to the driving duty cycle of the first switch tube and lags behind the driving duty cycle of the first switch tube by half a cycle.

[0015] Furthermore, controlling the end of the soft start includes: maintaining the driving duty cycle of the first switch tube unchanged after reaching a second set duty cycle, and gradually reducing the switching frequency of the four switch tubes until the output voltage is established to end the soft start.

[0016] Preferably, the second set duty cycle is 0.25.

[0017] 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 operates in a non-frequency doubling mode.

[0018] Furthermore, during the soft start control process, the resonant converter is prohibited from switching between the non-frequency doubling mode and the frequency doubling mode, and the resonant converter can only operate in one of the non-frequency doubling mode and the frequency doubling mode, so as to achieve a monotonic increase in the output voltage of the resonant converter during the soft start process.

[0019] As a second aspect of the present invention, the technical solution of the embodiment of the soft start control device provided is as follows: A soft start control device is applied to a resonant converter, wherein the primary circuit of the resonant converter comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are a first switch tube connected to a positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to a negative input end of the resonant converter in sequence; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is as follows: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is phase-shifted by 180° relative to the drive of the first switch tube; wherein the soft start control device is configured as follows: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, it is always maintained that: the switching frequencies of the four switch tubes are the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switch tubes in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase until the output voltage is established and the soft start ends.

[0020] As a third aspect of the present invention, the technical solution of the embodiment of the switching power supply provided is as follows: A switching power supply comprises a resonant converter, wherein the primary circuit of the resonant converter comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are, in sequence, a first switch tube connected to a positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to a negative input end of the resonant converter; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is phase-shifted by 180° relative to the drive of the first switch tube; wherein: the switching power supply further comprises the soft start control device described in any one of the second aspects above.

[0021] Based on the above technical solution, compared with the prior art, the present invention has the following beneficial effects: (1) The soft start control embodiment proposed in the present invention adopts an asymmetric complementary drive timing. Without adding any additional circuits, only by optimizing the control timing, the resonant converter can realize smooth pre-charging of the resonant capacitor voltage during the startup process regardless of whether it is working in the non-frequency doubling mode or the frequency doubling mode, so that all the switch tubes in the three-level switch network can achieve full ZVS during the soft start process, avoiding the problem of overvoltage stress damage to the switch tubes due to hard switching. At the same time, the resonant current spike during the soft start process can be reduced, which is beneficial to reducing the startup loss and improving the reliability of the resonant converter. (2) The soft start control embodiment proposed in the present invention further requires that during the soft start process, the resonant converter is prohibited from switching between the non-frequency doubling mode and the frequency doubling mode, that is, the soft start is first performed according to a specific mode, and the mode switching is allowed only after the soft start is completed. Therefore, it can be applied to a wide gain resonant converter with multi-mode combination control. The gain closed-loop of the soft start process is controllable, and no mode switching is involved, which can simplify the switching control logic. At the same time, it can ensure that the output voltage rises monotonically during the soft start process, thereby improving the performance and reliability of the resonant converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A typical topological structure diagram of the resonant converter used in the present invention; Figure 2 This is the main waveform timing diagram of the soft start process when the segmented soft start method proposed in patent CN117498671A is adopted; Figure 3 Schematic diagram of soft start control timing in non-frequency doubling mode according to a preferred embodiment of the present invention; Figure 4Schematic diagram of soft start control timing in frequency doubling mode according to a preferred embodiment of the present invention; Figure 5 It is a main waveform timing diagram of the soft start process of a prototype machine adopting the soft start control method of the present invention in a non-frequency doubling mode; Figure 6 The main waveform timing diagram of the soft start process of the prototype machine adopting the soft start control method of the present invention in the frequency doubling mode. DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0024] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present application.

[0025] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application are described here. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, the process, method, system, product or equipment comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or equipment.

[0026] It should be understood that, in the specification, claims and drawings, when a step is described as being connected to another step, the step may be directly connected to the other step, or be connected to the other step through a third step; when an element / unit is described as being "connected" to another element / unit, the element / unit may be "directly connected" to the other element / unit, or be "connected" to the other element / unit through a third element / unit.

[0027] In addition, the drawings of the present disclosure are only schematic diagrams of the present disclosure and are not necessarily drawn to scale. The same symbols in the drawings represent the same or similar parts, and thus repeated descriptions thereof will be omitted. Some of the 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 using software, or implemented in one or more hardware modules or integrated circuits, or implemented in different networks and / or processor devices and / or microcontrollers.

[0028] like Figure 1 As shown, a typical topological structure diagram of the resonant converter applied in the present invention is a series three-level LLC resonant converter, including an input voltage-dividing capacitor I, a three-level switch network II, an LLC resonant cavity network III, a transformer IV and a secondary side rectifier filter circuit V; In specific implementation, its structure is as follows: The input voltage-dividing capacitor I includes a first capacitor C1 and a second capacitor C2 connected in series, and the other ends of the first capacitor C1 and the second capacitor C2 are respectively connected to the positive and negative electrodes of the power supply; the three-level switch network II includes four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, and the four switch tubes specifically include a first switch tube S1, a second switch tube S2, a third switch tube S3 and a fourth switch tube S4 with a body diode or an external parallel diode, wherein the drain and source electrodes of the first switch tube S1, the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are connected in series in sequence, and the drain electrode of the first switch tube S1 and the source electrode of the fourth switch tube S4 located at both ends of the switch bridge arm are respectively connected to the positive and negative electrodes of the power supply, and the middle connection point of the second switch tube S2 and the third switch tube S3 is connected to the middle connection point of the first capacitor C1 and the second capacitor C2; the LLC resonant cavity network III includes resonant capacitors connected in series in sequence C r , resonant inductor L r and transformer equivalent magnetizing inductance L m , resonant capacitor C r The other end is connected to the middle connection point of the first switch tube S1 and the second switch tube S2 in the three-level switch network II, and the middle connection point is marked as point A. L m The other end of is connected to the middle connection point of the third switch tube S3 and the fourth switch tube S4 in the three-level switch network II, and the middle connection point is recorded as point B; the secondary side of the transformer IV is connected to the input end of the rectifier filter circuit V; the secondary side rectifier filter circuit V includes a full-wave rectifier circuit composed of a secondary side first synchronous rectifier tube SR1 and a secondary side second synchronous rectifier tube SR2, and an output filter capacitor Co .

[0029] For those skilled in the art, Figure 1 The circuit shown also includes but is not limited to the following variations: (1) Replace the secondary side rectifier filter circuit V with a bridge rectifier structure consisting of four switching tubes or diodes; (2) Exchange resonant capacitor resonant capacitor C r and resonant inductor L r location.

[0030] First embodiment The present embodiment provides a soft start control method, which is applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network includes four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, and the four switch tubes are, in sequence, a first switch tube S1 connected to the positive input end of the resonant converter, a second switch tube S2, a third switch tube S3 and a fourth switch tube S4 connected to the negative input end of the resonant converter.

[0031] When the resonant converter is working in steady state, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is 180° phase-shifted relative to the drive of the first switch tube.

[0032] It should be noted that the driving of the third switch tube is phase-shifted by 180° relative to the driving of the first switch tube, and can be 180° ahead or 180° behind.

[0033] The series three-level LLC resonant converter applied by the soft start control method provided in this embodiment can work in a non-frequency doubling mode or a frequency doubling mode in steady-state operation. In steady-state operation, no matter which mode it works in, due to the use of the above control strategy, the voltage injected into the LLC resonant cavity network is in a symmetrical state, so that the resonant capacitor C r The charging and discharging currents are also symmetrical, and the resonant capacitor voltage Vcr fluctuates under a DC voltage bias.

[0034] In addition, in steady-state operation, when the resonant converter works in non-frequency doubling mode, it is 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 follows: When the input voltage is in the low voltage segment, the series three-level LLC resonant converter operates in the non-frequency doubling mode, and the output voltage Vo is regulated by adjusting the duty cycle of the first switch tube S1 and the third switch tube S3. At this time, it is the PWM mode; if the output voltage Vo gain regulation is still not satisfied when the duty cycle of the first switch tube S1 and the third switch tube S3 is increased to about 0.5, the duty cycle of the first switch tube S1 and the third switch tube S3 is maintained at about 0.5, and the output voltage Vo regulation requirement is further satisfied by adjusting the switching frequency of the first switch tube S1 and the third switch tube S3. At this time, it is the PFM mode; When the input voltage is in the high voltage segment, the series three-level LLC resonant converter operates in the frequency doubling mode. At this time, the duty cycle of the first switch tube S1 and the fourth switch tube S4 is maintained at about 0.25, and the output voltage Vo is adjusted by adjusting the switching frequency of the first switch tube S1 and the fourth switch tube S4.

[0035] In steady-state operation, when the input voltage changes from low voltage to high voltage or from high voltage to low voltage, the series three-level LLC resonant converter will involve switching between non-frequency doubling mode and frequency doubling mode to achieve output voltage regulation under wide gain changes.

[0036] The above describes the control details of the series three-level LLC resonant converter in steady-state operation to which the soft start control method provided by the present embodiment is applied. Next, the series three-level LLC resonant converter to which the soft start control method of the present embodiment is applied is described in detail. The soft start control method of the present embodiment includes: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, the switching frequencies of the four switch tubes are always maintained to be the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switches in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase until the output voltage is established and the soft start ends.

[0037] It should be noted that the zero voltage switching (ZVS) of the present invention is not limited to the switching when the voltage at both ends of the switch tube is exactly equal to zero. As long as the voltage at both ends of the switch tube is close to zero, it is regarded as zero voltage switching. The specific deviation of the voltage from zero voltage is related to the design requirements, as long as it is within an acceptable range.

[0038] In particular, in order to achieve a monotonic increase in the output voltage of the resonant converter during the soft start process, the series three-level LLC resonant converter of the present invention smoothly transitions from the asymmetric control stage to the symmetric control stage in the soft start process regardless of the mode adopted during steady-state operation. However, the specific drive control timings of the first switch tube S1 to the fourth switch tube S4 are different in different modes. Therefore, before the soft start, the working mode is first determined according to the size of the input voltage, and then the soft start is performed with specific drive timings according to different working modes, and the mode switching between the non-frequency doubling mode and the frequency doubling mode is not performed during the soft start process.

[0039] like Figure 3 The figure shows a schematic diagram of the soft start control timing of a preferred embodiment of the present invention in a non-frequency doubling mode, which controls the drive of the first switch tube S1 to be complementary to the drive of the second switch tube S2, the drive of the third switch tube S3 to be complementary to the drive of the fourth switch tube S4, and the switching frequencies of the switches S1~S4 are the same; in the soft start process, the machine is started with a set maximum switching frequency (such as set to 2 times the series resonant frequency), the drive duty cycle of the first switch tube S1 gradually increases from the set minimum duty cycle, and the drive duty cycle of the third switch tube S3 gradually decreases inward from the complementary drive duty cycle of the first switch tube S1. At this time, the time when the first switch tube S1 and the fourth switch tube S4 are jointly turned on is less than the time when the second switch tube S2 and the third switch tube S3 are jointly turned on. This is called an asymmetric state, that is, when the first switch tube S1 and the fourth switch tube S4 are jointly turned on, the input voltage Vin is injected into the LLC resonant cavity network to give the resonant capacitor C r However, the charging time is short, and the resonant capacitor voltage rises slowly. When the second switch tube S2 and the third switch tube S3 are turned on together, the voltage injected into the LLC resonant cavity network is zero, and the resonant capacitor is slowly charged by the forward continuous flow of the resonant current Ir. Since the second switch tube S2 and the third switch tube S3 are turned on together for a long time, when the resonant current Ir rectifies and continues to flow after it drops to zero, it will continue to flow in the reverse direction. At this time, the resonant capacitor voltage Vcr will discharge slowly. Since the charging amount is still greater than the discharge amount, the resonant capacitor voltage Vcr can be slowly and smoothly increased. Since the resonant current Ir can achieve current reversal in an asymmetric state, the ZVS of the switch tubes S1~S4 can be achieved. In addition, since the starting frequency is high, the equivalent impedance of the LLC resonant cavity network is large, which can reduce the impact of the resonant current during the soft start process.

[0040] As the driving duty cycle of the third switch tube S3 gradually decreases to be equal to the driving duty cycle of the first switch tube S1 and lags behind the driving half cycle of the first switch tube S1, the time when the first switch tube S1 and the fourth switch tube S4 are turned on together is the same as the time when the second switch tube S2 and the third switch tube S3 are turned on together, that is, it enters a symmetrical state. At this time, the positive and negative charging and discharging currents of the resonant capacitor are symmetrical, the resonant capacitor voltage Vcr tends to be stable, and the output voltage increases as the driving duty cycle of the first switch tube S1 gradually increases. If the output voltage is established before the driving duty cycle of the first switch tube S1 reaches the first set duty cycle (for example, 0.5), the soft start ends, and the series three-level LLC resonant converter operates stably in the PWM mode of the non-frequency doubling mode; if the output voltage is still not established after the driving duty cycle of the first switch tube S1 reaches the first set duty cycle, the driving duty cycle of the first switch tube 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, the soft start ends, and the series three-level LLC resonant converter operates stably in the PFM mode of the non-frequency doubling mode.

[0041] It should be noted that the driving duty cycle of the first switch tube S1 reaching the first set duty cycle is not limited to the driving duty cycle of the first switch tube S1 being exactly equal to the first set duty cycle. As long as the driving duty cycle of the first switch tube S1 is near the first set duty cycle, it is regarded as reaching the first set duty cycle. The specific deviation of the duty cycle from the first set duty cycle is related to the design requirements, as long as it is within an acceptable range.

[0042] It should also be noted that during the soft start process, while the driving duty cycle of the first switch tube S1 gradually increases, the switching frequency can be adjusted to gradually decrease. While ensuring that the switch tubes S1~S4 can achieve ZVS, the rise time of the output voltage can be shortened, which can meet the application scenarios with strict requirements on the output voltage establishment time.

[0043] like Figure 4FIG. 1 is a schematic diagram of the soft start control timing of a preferred embodiment of the present invention in the frequency doubling mode, wherein the driving of the first switch tube S1 and the driving of the second switch tube S2 are controlled to be complementary, the driving of the third switch tube S3 and the driving of the fourth switch tube S4 are controlled to be complementary, and the switching frequencies of the switches S1 to S4 are the same; during the soft start process, the machine is started at the set maximum switching frequency (such as the series resonant frequency), the driving duty cycle of the first switch tube S1 gradually increases from the set minimum duty cycle, and the driving duty cycle of the fourth switch tube S4 is the same as the driving duty cycle of the first switch tube S1. The duty cycle is equal and lags behind the driving half cycle of the first switch tube S1. At this time, the time when the first switch tube S1 and the third switch tube S3 are turned on together and the time when the second switch tube S2 and the fourth switch tube S4 are turned on together are both shorter than the time when the second switch tube S2 and the third switch tube S3 are turned on together. This is called an asymmetric state, that is, when the first switch tube S1 and the third switch tube S3 are turned on together or the second switch tube S2 and the fourth switch tube S4 are turned on together, half of the input voltage Vin / 2 is injected into the LLC resonant cavity network to give the resonant capacitor C r Charging, but the charging time is short, the resonant capacitor voltage rises slowly, and when the second switch tube S2 and the third switch tube S3 are turned on together, the voltage injected into the LLC resonant cavity network is zero, and the resonant capacitor is slowly charged by the forward continuous flow of the resonant current Ir, and because the second switch tube S2 and the third switch tube S3 are turned on together for a long time, when the resonant current Ir rectifier continuous flow drops to zero, it will continue to reverse continuous flow, and the resonant capacitor voltage Vcr will slowly discharge at this time. Since the charging amount is still greater than the discharge amount, the resonant capacitor voltage Vcr can be slowly and smoothly increased, and since the resonant current Ir can achieve current reversal in an asymmetric state, the ZVS of the switch tubes S1~S4 can be achieved. In addition, in the frequency doubling mode, the voltage operating frequency injected into the LLC resonant cavity network is twice the switching frequency of the switch tubes S1~S4, which increases the equivalent impedance of the LLC resonant cavity network and reduces the resonant current impact during the soft start process.

[0044] As the driving duty cycle of the first switch tube S1 gradually increases to the second set duty cycle (for example, 0.25), the driving duty cycle of the first switch tube S1 is maintained at the second set duty cycle, that is, the symmetrical state is entered. At this time, the switching frequency of the switch tubes S1~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 the frequency doubling mode.

[0045] It should be noted that the driving duty cycle of the second switch tube S1 reaching the second set duty cycle is not limited to the driving duty cycle of the first switch tube S1 being exactly equal to the second set duty cycle. As long as the driving duty cycle of the first switch tube S1 is near the second set duty cycle, it is regarded as reaching the second set duty cycle. The specific deviation of the duty cycle from the second set duty cycle is related to the design requirements, as long as it is within an acceptable range.

[0046] It should also be noted that during the soft start process, while the driving duty cycle of the first switch tube S1 gradually increases, the switching frequency can be adjusted to gradually decrease. While ensuring that the switch tubes S1~S4 can achieve ZVS, the rise time of the output voltage can be shortened, which can meet the application scenarios with strict requirements on the output voltage establishment time.

[0047] Second embodiment The present embodiment provides a soft start control device, which is applied to a resonant converter. The primary circuit of the resonant converter includes an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network. The three-level switch network includes four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series. The four switch tubes are a first switch tube connected to the positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to the negative input end of the resonant converter in sequence. When the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state. The control strategy is as follows: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is 180° phase-shifted relative to the drive of the first switch tube. The soft start control device is configured as follows: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, the switching frequencies of the four switch tubes are always maintained to be the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switches in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase until the output voltage is established and the soft start ends.

[0048] The technical means adopted by the control device of this embodiment correspond to and are consistent with the control method of the first embodiment, and have the same beneficial effects, so they will not be described in detail. In addition, the preferred technical means or further improved means of each step in the control method of the first embodiment can be extended to the corresponding units of this embodiment, and this embodiment will not describe them one by one.

[0049] Third embodiment The present embodiment provides a switching power supply, including a resonant converter, wherein the primary circuit of the resonant converter includes an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network includes four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, and the four switch tubes are, in sequence, a first switch tube connected to the positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to the negative input end of the resonant converter; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is phase-shifted by 180° relative to the drive of the first switch tube; wherein: the switching power supply also includes any one of the soft start control devices in the second embodiment.

[0050] according to Figure 1 The prototype of the series three-level LLC resonant converter is designed. The rated output power of the prototype is 960W, the input voltage Vin varies from 250 to 1000V, the nominal output voltage is 24V, and the series resonant frequency is 100kHz. Under the conditions of input voltage of 300V and 1000V, the measured results in the non-frequency doubling mode (such as Figure 5 as shown) and frequency doubling mode (as shown Figure 6 The soft start process experimental waveform is shown below, where V o is the output voltage waveform, V GS1~ V GS4 are the driving waveforms of the first switch tube S1 to the fourth switch tube S4, Vds1 to Vds4 are the drain-source voltage waveforms of the first switch tube S1 to the fourth switch tube S4, VAB represents the voltage waveform injected into the LLC resonant cavity network; Vcr represents the resonant capacitor C r Voltage waveform at both ends; Ir represents the current flowing through the resonant inductor L r The resonant current waveform. Figure 5 , Figure 6 It can be seen from the experimental waveform that during the soft start process, the resonant capacitor voltage Vcr rises smoothly, the switch tubes S1~S4 can achieve ZVS, the drain-source voltage of the switch tubes S1~S4 has no obvious voltage spikes, and the resonant current Ir has no obvious current impact, and the output voltage also rises smoothly and monotonically, which verifies the effectiveness of the soft start control method of the present invention.

[0051] In the description of the above embodiments, a soft start control method for a resonant converter of the present invention adopts an asymmetric complementary drive timing in the soft start process, and combines the duty cycle and frequency hybrid modulation strategy. Without adding additional circuits, the resonant capacitor voltage can be smoothly pre-charged, ensuring that all switch tubes in the three-level switch network achieve ZVS throughout the soft start process, avoiding the problem of overvoltage stress damage to the switch tube due to hard switching, and reducing the resonant current spike in the soft start process, providing a stable and reliable preferred soft start control scheme for the current series three-level LLC resonant converter with high efficiency, high power density and wide voltage range requirements.

[0052] It should be noted that the embodiments described above are merely examples of the technical solutions and contents of the present invention, and it should be noted that the above embodiments should not be regarded as limiting the present invention. For those skilled in the art, several improvements and modifications may be made without departing from the spirit and scope of the present invention, but they will not deviate from the spirit of the present invention or exceed the scope defined by the attached claims, and should also be regarded as 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 comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are a first switch tube connected to the positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to the negative input end of the resonant converter in sequence; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is 180° phase-shifted relative to the drive of the first switch tube; characterized in that The soft start control method comprises: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, it is always maintained that: the switching frequencies of the four switch tubes are the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switch tubes in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase 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 a non-frequency doubling mode, the size of the driving duty cycle of the first switch tube to the fourth switch tube is controlled, including: starting with a first set switching frequency, the driving duty cycle of the first switch tube gradually increases from a set minimum duty cycle, and the driving duty cycle of the third switch tube gradually decreases inward from being complementary to the driving duty cycle of the first switch tube until it is equal to the driving duty cycle of the first switch tube and is maintained after lagging behind the driving duty cycle of the first switch tube by half a cycle.

3. The soft start control method according to claim 2, characterized in that: Controlling the end of the soft start includes: if the output voltage has been established before the driving duty cycle of the first switch tube reaches a first set duty cycle, then directly ending the soft start when the output voltage is established; otherwise, maintaining the driving duty cycle of the first switch tube unchanged after reaching the first set duty cycle, and gradually reducing the switching frequency of the four switch tubes until the output voltage is established to end the soft start.

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 the frequency doubling mode, the size of the driving duty cycle of the first switch tube to the fourth switch tube is controlled, including: starting with a second set switching frequency, the driving duty cycle of the first switch tube gradually increases from the set minimum duty cycle, the driving duty cycle of the fourth switch tube is equal to the driving duty cycle of the first switch tube and lags behind the driving half cycle of the first switch tube.

7. The soft start control method according to claim 6, characterized in that: Controlling the end of the soft start includes: maintaining the driving duty cycle of the first switch tube unchanged after reaching the second set duty cycle, and gradually reducing the switching frequency of the four switch tubes until the output voltage is established to end the soft start.

8. The soft start control method according to claim 7, characterized in that: The second set duty cycle is 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 operates in a 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 the non-frequency doubling mode and the frequency doubling mode. The resonant converter can only operate in one of the non-frequency doubling mode and the frequency doubling mode to achieve a monotonic increase in the 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 comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are a first switch tube, a second switch tube, a third switch tube connected to the positive input end of the resonant converter in sequence, and a fourth switch tube connected to the negative input end of the resonant converter; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is 180° phase-shifted relative to the drive of the first switch tube; characterized in that The soft start control device is configured as follows: By controlling the driving duty cycle of the first switch tube to the fourth switch tube, the voltage injected into the LLC resonant cavity network sequentially experiences an asymmetric state and a transition state, and finally enters a symmetric state during the steady-state operation; During the soft start control process, it is always maintained that: the switching frequencies of the four switch tubes are the same, the driving of the second switch tube is complementary to the driving of the first switch tube, and the driving of the fourth switch tube is complementary to the driving of the third switch tube; Among them: the asymmetric state is used to realize smooth pre-charging of the resonant capacitor voltage, thereby realizing zero-voltage turn-on of all switch tubes in the three-level switch network during the soft start process; during the transition state, the output voltage of the resonant converter is adjusted by closed-loop feedback to achieve monotonic increase 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 comprises an input voltage-dividing capacitor, a three-level switch network and an LLC resonant cavity network; the three-level switch network comprises four switch tubes connected in series in sequence, forming two groups of switch tube bridge arms connected in series, wherein the four switch tubes are a first switch tube connected to the positive input end of the resonant converter, a second switch tube, a third switch tube and a fourth switch tube connected to the negative input end of the resonant converter in sequence; when the resonant converter is in steady-state operation, the voltage injected into the LLC resonant cavity network is in a symmetrical state, and the control strategy is: the switching frequencies of the four switch tubes are the same, the drive of the second switch tube is complementary to the drive of the first switch tube, the drive of the fourth switch tube is complementary to the drive of the third switch tube, and the drive of the third switch tube is phase-shifted by 180° relative to the drive of the first switch tube; characterized in that: The switching power supply also includes the soft start control device according to claim 11.

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