Soft start control method, control device and switching power supply

Abstract

The invention discloses a soft start control method, a soft start control device and a switching power supply. The soft start control method comprises the following steps: starting at a first set frequency; comparing the output voltage Vout with the output judgment threshold Voth to judge the calculation mode of the switch preset frequency; if Vout is smaller than Voth, the switch preset frequency is calculated according to the output voltage; if Vout is greater than or equal to Voth, subtracting a fixed frequency quantity from the switching frequency of the previous judgment period as a switching preset frequency; judging whether current protection needs to be executed or not according to comparison between the resonant cavity current and a set resonant current protection threshold value; calculating the difference between the switch preset frequency and the switch frequency of the previous judgment period, comparing the difference with the maximum frequency step value and the minimum frequency step value respectively, updating the switch preset frequency, and ensuring that the switch preset frequency is adjusted within the range of the maximum frequency step value and the minimum frequency step value; and comparing the loop calculation frequency with the switch preset frequency, determining the final output switch frequency, and judging whether to end the soft start or not. The control is simple, and the monotonous rise of the output voltage within the full-load range can be realized.

Description

Technical Field

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

[0002] In the selection of high-power converter topologies, the three-phase LLC converter topology is generally accepted by R & D personnel due to its high efficiency, high power density, low noise, low switching stress and other characteristics. Especially the characteristics that the primary switching tube is easy to achieve ZVS in the full voltage and full load range, and the secondary diode is also easy to achieve ZCS, make the three-phase LLC converter widely favored in high-power application scenarios.

[0003] However, it is found in actual applications that a large current will be generated in the resonant cavity during the startup process. The reason is that the initial voltage of the output capacitor is zero at startup, which is equivalent to a short circuit on the secondary side of the transformer. The voltage on the primary side of the transformer will be clamped to zero, and the impedance of the resonant cavity is very small. When starting up, the transformer charges the output capacitor through the rectification module, and this current is very large. Then the primary resonant cavity current will also be very large. Such a large current will cause great voltage stress and current stress on power devices such as the primary switching tube, and even directly damage them. Therefore, soft start operation is required during startup. Common soft start schemes for LLC resonant converters are as follows:

[0004] (1) Frequency modulation. This scheme starts to work with a relatively high initial frequency and a control method with a duty cycle of 0.5, and then gradually reduces the frequency to the stable operating frequency to facilitate the reduction of voltage and current stress in the resonant cavity. This scheme has simple control and is easy to implement, and is also the most widely used scheme. However, the disadvantage is that it operates in an open-loop saturation state in the initial stage of startup, the output voltage is established very quickly, and the current stress is relatively large, there is a risk of damaging devices.

[0005] (2) Frequency modulation and duty cycle adjustment. This scheme adds the operation of duty cycle adjustment on the basis of the previous scheme, that is, starts startup with a very small initial duty cycle, and then gradually increases it to 0.5. This is beneficial to effectively control the current stress problem during startup, so it is widely used. However, in some application scenarios, it is required to start up with a duty cycle of 0.5, such as when driving with a transformer, this scheme cannot be applied.

[0006] (3) Pulse cluster. When starting up, it is driven at a fixed frequency and a fixed duty cycle of 0.5. After every M pulses are output, it pauses for a period of time and then continues to output M pulses. The number of M is gradually increased according to the monitoring results such as the input voltage and the output voltage. The pause time can remain unchanged or gradually decrease. In this way, the driving mode can be adjusted in real time according to the output state, and a better output effect can be achieved. However, the control of this scheme is relatively complex. In addition, under heavy load, there may be a phenomenon of a small-range drop in the output voltage during the pause time, which does not meet the requirement that the output voltage should increase monotonically during startup in general.

[0007] Therefore, a control scheme is needed that is relatively simple to control, meets the startup requirement with a duty cycle of 0.5, and can also solve the problem of excessive startup current stress. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a soft start control method, a control device and a switching power supply, which can solve at least one of the technical problems existing in the prior art to a certain extent.

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

[0010] A soft start control method is applied to a three-phase LLC resonant converter. The three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module and a rectifier module;

[0011] Among them, the soft start control method includes:

[0012] S1, the LLC resonant converter starts up at the first set frequency f1;

[0013] S2, sample the output voltage Vout, and judge the calculation method of the switch pre-adjusted frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switch pre-adjusted frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency amount △f2 from the switch frequency fpr in the previous judgment period as the switch pre-adjusted frequency fpp;

[0014] S3, sample the resonant cavity current Ir, and judge whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith;

[0015] S4. Calculate the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr of the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively. Update the switch pre-adjusted frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min;

[0016] S5. Input the output voltage Vout into the PI regulator to obtain the loop calculation frequency floop. Compare the loop calculation frequency floop with the switch pre-adjusted frequency fpp to determine the finally output switch frequency fsw, and judge whether to end the soft start.

[0017] Further, in step S3, if the resonant cavity current Ir > the resonant current protection threshold Ith, adjust the switch pre-adjusted frequency fpp to the switch frequency fpr of the previous judgment period; if the resonant cavity current Ir ≤ the resonant current protection threshold Ith, the switch pre-adjusted frequency fpp does not need to be adjusted.

[0018] Further, in step S4, compare the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr of the previous judgment period with the maximum frequency step value fst_max. If it does not exceed the maximum frequency step value fst_max, no operation is required. If it exceeds the maximum frequency step value fst_max, the value of the switch pre-adjusted frequency fpp is updated as follows:

[0019] Switch pre-adjusted frequency fpp = switch frequency fpr of the previous judgment period - maximum frequency step value fst_max

[0020] Further, in step S4, compare the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr of the previous judgment period with the minimum frequency step value fst_min. If it does not exceed the minimum frequency step value fst_min for N consecutive judgment periods and the condition that the resonant cavity current Ir ≤ the resonant current protection threshold Ith is satisfied in step 3 of the current judgment period, then subtract a fixed frequency amount △f1 from the switch frequency fpr of the previous judgment period to update the switch pre-adjusted frequency fpp, otherwise no operation is required.

[0021] Further, in step S5, if the loop calculation frequency floop > the switch pre-adjusted frequency fpp, the finally output switch frequency fsw is the switch pre-adjusted frequency fpp, and return to step S2 for the next judgment period; if the loop calculation frequency floop ≤ the switch pre-adjusted frequency fpp, the finally output switch frequency fsw is the loop calculation frequency floop, and the soft start stage ends.

[0022] Further, the first set frequency is greater than or equal to the highest frequency when the LLC resonant converter operates normally;

[0023] Further, the duty cycle of the switching tube generally does not exceed 0.5 at most.

[0024] As the second aspect of the present invention, the technical solution of the embodiment of the soft start control device is as follows:

[0025] A soft start control device is applied to a three-phase LLC resonant converter, and the three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module, and a rectifier module;

[0026] Wherein, the soft start control device is configured as a module forced to perform the following operations:

[0027] S1, the LLC resonant converter starts up at the first set frequency f1;

[0028] S2, sample the output voltage Vout, and judge the calculation method of the switching pre-adjustment frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switching pre-adjustment frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency amount Δf2 from the switching frequency fpr in the previous judgment period as the switching pre-adjustment frequency fpp;

[0029] S3, sample the resonant cavity current Ir, and judge whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith;

[0030] S4, calculate the difference between the switching pre-adjustment frequency fpp and the switching frequency fpr in the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively, and update the switching pre-adjustment frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min;

[0031] S5, input the output voltage Vout into the PI regulator to obtain the loop calculation frequency floop, compare the loop calculation frequency floop with the switching pre-adjustment frequency fpp, determine the finally output switching frequency fsw, and judge whether to end the soft start.

[0032] As the third aspect of the present invention, the technical solution of the embodiment of the switching power supply is as follows:

[0033] A switching power supply includes a three-phase LLC resonant converter, and the three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module, and a rectifier module; wherein: the switching power supply further includes the soft start control device described in any one of the above second aspects.

[0034] Further, the inverter module includes at least three groups of bridge circuits, and each group of bridge circuits includes at least two switching tubes connected in series; the resonant cavity module includes at least a resonant inductor and a resonant capacitor; the primary winding of the transformer module is connected in parallel, Y-connected or Δ-connected, and the secondary winding is connected in parallel, Y-connected or Δ-connected; the rectifier module includes at least three groups of bridge circuits, and each group of bridge circuits includes at least two diodes connected in series or switching tubes connected in series.

[0035] The present invention obtains the switching frequency by sampling the output voltage, controls in two stages, and simultaneously controls the resonant current and the frequency step amount in real time. The beneficial effects compared with the prior art are as follows:

[0036] (1) This control method only needs to reuse the output voltage sampling signal and the resonant cavity current sampling signal and combine simple judgment logic and calculation processing to achieve, and the control complexity is greatly reduced compared with the prior art;

[0037] (2) By calculating the switching frequency through real-time monitoring of the output voltage and setting the frequency step amount within a controllable range, it is possible to achieve a monotonic increase in the output voltage within the full load range;

[0038] (3) By detecting and protecting the resonant current, the problem of excessive starting current stress can be solved, and the requirement of starting at a 0.5 duty cycle is satisfied;

[0039] (4) By comparing the soft start frequency and the loop calculation frequency in real time, a smooth transition from the soft start stage to the steady state stage can be achieved. Description of the Drawings

[0040] Figure 1 It is a specific schematic diagram of the power stage circuit of the three-phase LLC resonant converter applicable to the present invention;

[0041] Figure 2 is Figure 1 The schematic diagram after adding a control device on the basis of;

[0042] Figure 3 is Figure 1 The control timing diagram of the circuit;

[0043] Figure 4 It is the step diagram of the soft start control method of the first embodiment of the present invention;

[0044] Figure 5 is Figure 1 The measured waveform diagram of the circuit adopting the soft start control method of the first embodiment of the present invention;

[0045] Figure 6 It is a schematic diagram of the frequency change of the first embodiment proposed by the present invention. Specific implementation manners

[0046] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0047] In order to enable those skilled in the art of the present technology to better understand the solution of 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 in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0048] It should be noted that the terms "first", "second", etc. in the specification, claims and the above-mentioned drawings of the present application are used to distinguish similar objects, and do not necessarily need to be used to describe a specific order or sequence. It should be understood that such used data can be interchanged under appropriate circumstances so as to describe the embodiments of the present application here. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units does not necessarily have to be limited to those clearly listed steps or units, but may include other steps or units not clearly listed or inherent to these process, method, product or device.

[0049] It should be understood that in the specification, claims and the drawings of the specification, when it is described that one step is consecutive to another step, this step can be directly consecutive to that other step, or consecutive to that other step through a third step; when it is described that an element / unit is "consecutive" to another element / unit, this element / unit can be "directly connected" to that other element / unit, or "connected" to that other element / unit through a third element / unit.

[0050] In addition, the accompanying drawings of the present disclosure are only schematic diagrams of the present disclosure and are not necessarily drawn to scale. The same reference signs in the drawings represent the same or similar parts, and thus the repeated description thereof will be omitted. Some of the block diagrams shown in the drawings are functional entities and do not necessarily have to correspond to physically or logically independent entities. These functional entities can be implemented by software, or implemented in one or more hardware modules or integrated circuits, or implemented in different networks and / or processor devices and / or microcontroller devices.

[0051] Figure 1 A specific schematic diagram of the power stage circuit of the three-phase LLC resonant converter applicable to the present invention, Figure 2 is Figure 1 the schematic diagram after adding a control device on the basis of Figure 3 is Figure 1 the control timing diagram of the circuit, as Figures 1 to 3 shown, where the LLC resonant converter includes an inverter module, a resonant cavity module, a transformer module, and a rectifier module.

[0052] The inverter module includes three groups of bridge circuits, each group of bridge circuits consists of two switching tubes, and the two switching tubes conduct complementarily with a duty cycle of 0.5; in specific implementation, to avoid shoot-through, a certain dead zone is left between the upper and lower tubes, and the dead zone time is determined according to the voltage drop time Vds between the drain and source electrodes of the two tubes within the full load range. At the same time, attention should be paid to avoiding the situation of excessive ZVS due to too large a dead zone. If necessary, the dead zone time can be adjusted in real time according to the load situation. The phase difference between the three bridge arms is 120°, as Figure 4 shown.

[0053] The resonant cavity module includes three groups of resonant inductors and a resonant capacitor.

[0054] The transformer module includes three groups of transformers. The primary windings of the three groups of transformers are connected in a Y shape, and the secondary windings are also connected in a Y shape; in specific implementation, the primary windings can also be connected in parallel or in a Δ shape, and the secondary windings can also be connected in parallel or in a Δ shape.

[0055] The rectifier module includes three groups of bridge circuits, and each group of bridge circuits consists of two diodes connected in series; in specific implementation, the diodes of the rectifier module can be replaced by switching tubes to achieve synchronous rectification to achieve higher efficiency in high-current situations.

[0056] It should be noted that Figure 1 and Figure 2 the schematic diagrams are only examples and should not limit the protection scope of the present invention. Those skilled in the art can choose how to design the three-phase LLC resonant converter circuit according to needs.

[0057] The first embodiment

[0058] This embodiment provides a soft start control method applied to a three-phase LLC resonant converter. The three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module, and a rectifier module;

[0059] Figure 4 is the step diagram of the soft start control method of the first embodiment of the present invention. Among them, the soft start control method includes:

[0060] S1, The LLC resonant converter starts up at the first set frequency f1;

[0061] S2, Sample the output voltage Vout, and determine the calculation method of the switch pre-adjusted frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switch pre-adjusted frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency amount △f2 from the switch frequency fpr in the previous judgment period as the switch pre-adjusted frequency fpp;

[0062] S3, Sample the resonant cavity current Ir, and determine whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith;

[0063] S4, Calculate the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr in the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively. Update the switch pre-adjusted frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min;

[0064] S5, Input the output voltage Vout into the PI regulator to obtain the loop calculation frequency floop, compare the loop calculation frequency floop with the switch pre-adjusted frequency fpp, determine the finally output switch frequency fsw, and judge whether to end the soft start.

[0065] The soft start control method of this embodiment is simple, can realize the monotonic increase of the output voltage in the full load range, can solve the problem of excessive starting current stress, meets the requirement of starting at a 0.5 duty cycle, and can realize a smooth transition from the soft start stage to the steady state stage.

[0066] For Figure 2 Schematic diagram. The parameters of the resonant converter are designed as follows: rated input voltage 800VDC; output voltage 230V; rated output power 16kW, the resonant frequency is set at 120kHz. Combining with the debugging effect, set the first set frequency f1 = 320kHz, duty cycle D = 0.5, set the output determination threshold Voth = 200V, set the fixed frequency amount △f2 = 0.1kHz, set the resonant current protection threshold Ith = 35A, set the maximum frequency step value fst_max = 1kHz, and set the minimum frequency step value fst_min = 0.05kHz. Figure 5 For Figure 1 The measured waveform diagram of the circuit adopting the soft start control method of the first embodiment of the present invention, where CH1 is the output voltage waveform and CH3 is the resonant current waveform.

[0067] The following combines Figure 2 and Figure 5 to describe in detail the soft - start control method of this embodiment. The specific control steps are as follows:

[0068] Step S1, the LLC resonant converter starts up at the first set frequency f1 = 320 kHz;

[0069] Preferably, the first set frequency is greater than or equal to the highest frequency when the LLC resonant converter operates normally. The first set frequency f1 should not be set too high, otherwise there may be a problem of startup failure under heavy - load conditions.

[0070] Step S2, sample the output voltage Vout. Compare the output voltage Vout with the output determination threshold Voth. In the initial state, the output voltage is 0. If it is determined that Vout < Voth, then calculate the switch pre - adjustment frequency fpp according to the output voltage Vout. The calculation formula is set through the output voltage range and the frequency range, and a function related to the output voltage Vout is obtained by fitting, such as a linear function relationship, a quadratic function relationship, or an exponential function relationship, etc. It is recommended to use a quadratic function curve, as Figure 6 shown. The reason for choosing the quadratic function is that the three - phase LLC soft start requires the frequency change law to be fast first and then slow, and the quadratic function better meets such a trend. Moreover, the three - phase LLC soft start starts from the highest frequency and continuously reduces the frequency. Therefore, the vertex of the quadratic function can be set as the lowest value of the frequency change, which is convenient for fitting. The abscissa x at this moment is the output determination threshold Voth, that is, 200V. The quadratic function obtained by fitting in this embodiment is:

[0071] y = 0.0055x 2 - 2.2014x + 339.7

[0072] Through this function, the correspondence between the output voltage and the switch frequency can be realized, so as to make the output voltage controllable.

[0073] When the output voltage Vout exceeds the output determination threshold of 200V after the soft start has been carried out for a period of time, the frequency starts to change slowly. Then, subtract a fixed frequency amount △f2 from the switch frequency fpr in the previous judgment period as the switch pre - adjustment frequency fpp. This process can facilitate the early takeover of the loop in the soft - start stage and reduce the startup overshoot.

[0074] Step S3: Sample the resonant cavity current Ir. Compare the resonant cavity current Ir with the set resonant current protection threshold Ith. If the resonant cavity current Ir > the resonant current protection threshold Ith, adjust the switch pre-adjusted frequency fpp to the switch frequency fpr of the previous judgment period. The setting of the resonant current protection threshold Ith is based on the maximum current of the resonant cavity when starting with the maximum load or capacitive load (if necessary). If it is set too small, there may be a problem that the heavy load cannot be started. The purpose of maintaining the frequency constant is to prevent the resonant current from further rising beyond the device stress. Usually, a turn-off threshold is also set above the resonant current protection threshold Ith. When the turn-off threshold is triggered, the switch is immediately turned off to protect the device. Waveform Figure 5 It can be observed that the resonant current maintains at a position of approximately 35A at most. Considering the detection accuracy, there is a slight error here, which is within an acceptable range and needs to be adjusted in combination with the actual debugging situation.

[0075] In addition, to ensure the sampling effect, the resonant current sampling in the three-phase LLC converter needs to sample the resonant current of each phase, which are Ir1, Ir2, and Ir3 respectively. The sampled results are input into the controller respectively. When making comparisons, each resonant current can be compared, or only the largest one can be selected for comparison. In some cases, the average value can also be taken for comparison. The control method of this embodiment selects to compare the largest one.

[0076] Step S4: Calculate the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr of the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively. Update the switch pre-adjusted frequency fpp according to the comparison results to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min.

[0077] If the difference between the switch pre-adjusted frequency fpp and the switch frequency fpr of the previous judgment period exceeds the maximum frequency step value fst_max, the value of the switch pre-adjusted frequency fpp is updated as follows:

[0078] Switch pre-adjusted frequency fpp = switch frequency fpr of the previous judgment period - maximum frequency step value fst_max

[0079] The purpose of setting the maximum frequency step value fst_max is to avoid the current mutation caused by the frequency mutation, which reduces the reliability of the system operation.

[0080] If it does not exceed the minimum frequency step value fst_min for N consecutive judgment cycles, and the condition that the resonant cavity current Ir ≤ the resonant current protection threshold Ith is satisfied in step S3 of the current judgment cycle, then the switching pre-adjusted frequency fpp is updated by subtracting a fixed frequency amount △f1 from the switching frequency fpr of the previous judgment cycle; otherwise, no operation is required. The purpose of setting the minimum frequency step value fst_min is to avoid the output voltage Vout staying at a certain platform due to the limited gain of the system (i.e., the three-phase LLC resonant converter) under heavy load, and the calculated switching frequency also staying at a certain point, resulting in the startup being unable to reach the expected value of the output voltage. Therefore, it is necessary to actively reduce the frequency to increase the system gain. The minimum frequency step value fst_min can be as small as 0 and can be adjusted according to the actual test phenomenon.

[0081] It should be noted that when the judgment condition that the resonant cavity current Ir > the resonant current protection threshold Ith is established in step S3, frequency modulation operation is not allowed itself. At this time, although the condition that it does not exceed the minimum frequency step value fst_min for N consecutive judgment cycles is satisfied, frequency modulation is still not allowed. That is, the priority of resonant cavity current detection is higher than that of frequency step amount detection.

[0082] Step S5: The loop calculation frequency floop is obtained by inputting the output voltage Vout into the PI regulator. Compare the loop calculation frequency floop with the switching pre-adjusted frequency fpp. If the loop calculation frequency floop > the switching pre-adjusted frequency fpp, then the finally output switching frequency fsw is the switching pre-adjusted frequency fpp, and return to step S2 for the next judgment cycle; if the loop calculation frequency floop ≤ the switching pre-adjusted frequency fpp, then the finally output switching frequency fsw is the loop calculation frequency floop, and the soft start stage ends. The purpose of this step is to ensure a smooth transition between the soft start stage and the steady state stage and avoid the adverse effects caused by frequency jumps. To reduce the startup overshoot, the loop steady state threshold can be appropriately reduced during the soft start stage to take over the soft start stage in advance.

[0083] The control method designed in this embodiment is widely applicable to LLC resonant topologies. In addition to the single-phase three-phase LLC resonant converter, it can also be used in the bidirectional three-phase LLC resonant converter or other topologies.

[0084] Second Embodiment

[0085] This embodiment provides a soft start control device applied to a three-phase LLC resonant converter. The three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module, and a rectifier module;

[0086] Among them, the soft start control device is configured as a module that is forced to perform the following operations:

[0087] S1. The LLC resonant converter starts up at the first set frequency f1;

[0088] S2. Sample the output voltage Vout, and determine the calculation method of the switch pre-adjustment frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switch pre-adjustment frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency quantity Δf2 from the switch frequency fpr in the previous judgment period as the switch pre-adjustment frequency fpp;

[0089] S3. Sample the resonant cavity current Ir, and determine whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith;

[0090] S4. Calculate the difference between the switch pre-adjustment frequency fpp and the switch frequency fpr in the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively. Update the switch pre-adjustment frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min;

[0091] S5. Input the output voltage Vout into the PI regulator to obtain the loop calculation frequency floop, compare the loop calculation frequency floop with the switch pre-adjustment frequency fpp, determine the finally output switch frequency fsw, and judge whether to end the soft start.

[0092] The technical means adopted by the control device in this embodiment correspond to those of the control method in the first embodiment, and the beneficial effects are the same, so they will not be elaborated here;

[0093] In addition, 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 units in this embodiment, and will not be described in detail one by one in this embodiment.

[0094] Third Embodiment

[0095] This embodiment provides a switching power supply, including a three-phase LLC resonant converter, and the three-phase LLC resonant converter includes: an inverter module, a resonant cavity module, a transformer module, and a rectifier module; wherein: the switching power supply further includes the soft start control device according to any one of the second aspects above.

[0096] Since the switching power supply in this embodiment further includes the soft start control device according to any one of the second aspects above, it can realize a stable, smooth and monotonically increasing output voltage within the full load range, solve the problem of excessive start-up resonant current at the same time, and meet the start-up requirement with a 0.5 duty cycle.

[0097] Preferably, the inverter module includes at least three sets of bridge circuits, and each set of bridge circuits includes at least two switching tubes connected in series; the resonant cavity module includes at least a resonant inductor and a resonant capacitor; the primary winding of the transformer module is connected in parallel, Y-connected or Δ-connected, and the secondary winding is connected in parallel, Y-connected or Δ-connected; the rectifier module includes at least three sets of bridge circuits, and each set of bridge circuits includes at least two diodes connected in series or switching tubes connected in series.

[0098] The terminology used in the above embodiments is illustrative and exemplary, rather than restrictive. Since the present invention can be embodied in many forms without departing from the spirit or essence of the invention, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be broadly construed within the spirit and scope defined by the appended claims. Therefore, all variations and modifications falling within the scope of the claims or their equivalents should be covered by the appended claims.

Claims

1. A soft start control method applied to a three-phase LLC resonant converter, the three-phase LLC resonant converter comprising: Inverter module, resonant cavity module, transformer module and rectifier module; It is characterized in that the soft start control method includes: S1, The LLC resonant converter starts up at the first set frequency f1; S2, Sample the output voltage Vout, and judge the calculation method of the switch pre-adjustment frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switch pre-adjustment frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency amount △f2 from the switch frequency fpr in the previous judgment period as the switch pre-adjustment frequency fpp; S3, Sample the resonant cavity current Ir, and judge whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith; S4, Calculate the difference between the switch pre-adjustment frequency fpp and the switch frequency fpr in the previous judgment period, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively. Update the switch pre-adjustment frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min; S5, Input the output voltage Vout into the PI regulator to obtain the loop calculation frequency floop, compare the loop calculation frequency floop with the switch pre-adjustment frequency fpp, determine the finally output switch frequency fsw, and judge whether to end the soft start.

2. The soft start control method of the three-phase LLC resonant converter according to claim 1, characterized in that In step S3, if the resonant cavity current Ir > the resonant current protection threshold Ith, adjust the switch pre-adjustment frequency fpp to the switch frequency fpr in the previous judgment period; if the resonant cavity current Ir ≤ the resonant current protection threshold Ith, the switch pre-adjustment frequency fpp does not need to be adjusted.

3. The soft start control method of the three-phase LLC resonant converter according to claim 1, characterized in that In step S4, the difference between the switch pre-adjustment frequency fpp and the switch frequency fpr in the previous judgment period is compared with the maximum frequency step value fst_max. If it does not exceed the maximum frequency step value fst_max, no operation is required. If it exceeds the maximum frequency step value fst_max, the value of the switch pre-adjustment frequency fpp is updated as follows: Switch pre-adjustment frequency fpp = switch frequency fpr in the previous judgment period - maximum frequency step value fst_max.

4. The soft start control method of the three-phase LLC resonant converter according to claim 1, characterized in that, In step S4, the difference between the switch pre-adjustment frequency fpp and the switch frequency fpr in the previous judgment period is compared with the minimum frequency step value fst_min. If it does not exceed the minimum frequency step value fst_min for N consecutive judgment periods, and in step 3 of the current judgment period, the condition that the resonant cavity current Ir ≤ the resonant current protection threshold Ith is satisfied, then subtract a fixed frequency amount △f1 from the switch frequency fpr in the previous judgment period to update the switch pre-adjustment frequency fpp, otherwise no operation is required.

5. The soft start control method of the three-phase LLC resonant converter according to claim 1, wherein In step S5, if the loop calculation frequency floop > the switch pre-adjustment frequency fpp, the finally output switch frequency fsw is the switch pre-adjustment frequency fpp, and return to step S2 for the next judgment cycle; if the loop calculation frequency floop ≤ the switch pre-adjustment frequency fpp, the finally output switch frequency fsw is the loop calculation frequency floop, and the soft start stage ends.

6. The soft start control method of the three-phase LLC resonant converter according to claim 1, wherein The first set frequency is greater than or equal to the highest frequency when the LLC resonant converter operates normally.

7. The soft start control method of the three-phase LLC resonant converter according to claim 1, characterized in that The duty cycle of the switching tubes of the three-phase LLC resonant converter does not exceed 0.5 at most.

8. A soft start control device is applied to a three-phase LLC resonant converter, and the three-phase LLC resonant converter includes: An inverter module, a resonant cavity module, a transformer module, and a rectification module; Characterized in that the soft start control device is forced to be configured as a module that performs the following operations: S1, the LLC resonant converter starts up at the first set frequency f1; S2, sample the output voltage Vout, and judge the calculation method of the switch pre-adjustment frequency according to the comparison between the output voltage Vout and the output determination threshold Voth; if Vout < Voth, calculate the switch pre-adjustment frequency fpp according to the output voltage Vout; if Vout ≥ Voth, subtract a fixed frequency amount △f2 from the switch frequency fpr in the previous judgment cycle as the switch pre-adjustment frequency fpp; S3, sample the resonant cavity current Ir, and judge whether current protection needs to be performed according to the comparison between the resonant cavity current Ir and the set resonant current protection threshold Ith; S4, calculate the difference between the switch pre-adjustment frequency fpp and the switch frequency fpr in the previous judgment cycle, and compare it with the maximum frequency step value fst_max and the minimum frequency step value fst_min respectively, and update the switch pre-adjustment frequency fpp according to the comparison result to ensure that it is adjusted within the range of the maximum frequency step value fst_max and the minimum frequency step value fst_min; S5, input the output voltage Vout into a PI regulator to obtain the loop calculation frequency floop, compare the loop calculation frequency floop with the switch pre-adjustment frequency fpp, determine the finally output switch frequency fsw, and judge whether to end the soft start.

9. A switching power supply includes a three-phase LLC resonant converter, and the three-phase LLC resonant converter includes: An inverter module, a resonant cavity module, a transformer module, and a rectification module; characterized in that: the switching power supply further includes the soft start control device according to claim 8.

10. The switching power supply according to claim 9, characterized in that: The inverter module includes at least three groups of bridge circuits, and each group of bridge circuits includes at least two series-connected switching tubes; the resonant cavity module includes at least a resonant inductor and a resonant capacitor; the primary winding of the transformer module is connected in parallel, Y-connected or △-connected, and the secondary winding is connected in parallel, Y-connected or △-connected; the rectification module includes at least three groups of bridge circuits, and each group of bridge circuits includes at least two series-connected diodes or series-connected switching tubes.

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