Wide load range soft start control method for two-switch Buck-Boost converter

Abstract

The invention belongs to the technical field of converter control, and relates to a wide load range soft start control method for a two-switch Buck-Boost converter, and the two-switch Buck-Boost converter is a two-switch Buck-Boost cascade LLC converter. The wide load range soft start control method comprises the following steps: setting a duty ratio lower limit of a Boost bridge arm in a soft start process as k; according to the method, only Buck can be adjusted in the whole starting process by improving the lower limit of the initial duty ratio of the Boost bridge arm, PI parameters are set according to the load size, and therefore the plateau period and oscillation can be reduced, the long plateau of the gain in the Buck mode can be reduced, the total gain in the Buck mode can be improved, and the plateau period and overshoot in the starting process can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter control, and relates to a soft start control method for a two-switch Buck-Boost converter with a wide load range. Background Art

[0002] DC-DC converter modules are widely used in aircraft power supply systems. For the design of DC-DC converter modules, they need to have the characteristics of wide-range voltage regulation ability, high efficiency, and high power density. The two-switch Buck-Boost (TSBB) cascaded LLC converter is a commonly used two-stage topology for airborne power supplies. The front-stage two-switch Buck-Boost converter has an ultra-wide range of voltage regulation ability, and the rear-stage LLC has high efficiency because it can achieve ZVS for all switching tubes.

[0003] For traditional soft start control methods, Uref is directly set as a ramp signal that slowly increases from 0. At this time, the output voltage will track the given Uref signal to achieve the purpose of soft start. During the start-up process, since the output voltage increases from 0, when the output voltage is greater than the input voltage, it will experience a mode change from buck to boost. For example, in the invention CN108712074, piecewise linear, there are several inflection points in Uref, and the slopes between the inflection points are different. During the start-up process, the Buck bridge arm is started first and operates in the buck mode; then the Boost bridge arm is started and operates in the boost mode. For the duty cycle calculation method module, it usually works in the buck mode (only the Buck bridge arm acts) first and then in the boost mode (only the Boost bridge arm acts). In the actual control process, the MOSFET requires a minimum on-time and a minimum off-time to ensure full conduction and turn-off. Therefore, the on-time and off-time of the MOSFET need to be limited to ensure full conduction and turn-off. In addition, a hysteresis loop is set for the Buck bridge arm to ensure smooth switching during the switching between the Buck and Boost modes. During the start-up process, the method of gradually increasing Uref is adopted. At this time, under the control of the PI regulator, the control quantity D will continuously increase. For the case of operating in the boost mode stably, in the initial stage of the start-up process, the duty cycle of the Buck bridge arm gradually increases, and the duty cycle of the Boost bridge arm remains constant at 0.01. In the later stage of the start-up process, after the hysteresis loop process, the Buck bridge arm is always on, and the duty cycle of the Boost bridge arm is adjusted.

[0004] However, since the gain curves of the TSBB in the two states of continuous inductor current (CCM) and discontinuous inductor current (DCM) are not the same, the traditional control method can achieve an ideal soft-start process under heavy load conditions, with the output voltage rising smoothly and no obvious overshoot. However, under light load conditions, there will be a plateau and oscillation in the output voltage during the soft-start process. This starting method may lead to too long start-up time and cannot meet the requirements for ripple control, and may damage the subsequent load.

[0005] Therefore, a smoothing control device or method capable of reducing the plateau and oscillation is needed to solve the above technical problems. Summary of the Invention

[0006] The present invention proposes a smoothing control strategy for soft start. The main idea of its control method is to increase the lower limit of the initial duty cycle of the Boost leg, so that only the Buck can be adjusted throughout the start-up process, and the PI parameters are set according to the load size, thereby reducing the plateau and oscillation.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a wide-load-range soft-start control method for a two-switch Buck-Boost converter, and the two-switch Buck-Boost converter is a two-switch Buck-Boost cascaded LLC converter; the wide-load-range soft-start control method includes: setting the lower limit of the duty cycle of the Boost leg in the soft-start process to k, where the value range of k is: 0.2 to 0.3; the duty cycles of the Buck and Boost legs in the two-switch Buck-Boost converter are:

[0008]

[0009] In the formula, D S1 represents the duty cycle of the Buck leg, D S2 represents the duty cycle of the Boost leg, and D represents the control quantity.

[0010] Preferably, the value range of k is: 0.24 to 0.26.

[0011] More preferably, the value of k is: 0.25.

[0012] Preferably, the gain in the later stage of the wide-load-range soft-start control method is:

[0013]

[0014] Among them, Gain represents the later gain, d Buck represents the duty cycle of the Buck leg, d' Boost = 1 - d Boost , d Boostdenotes the initial duty cycle of the Boost leg, T s denotes the switching period, R denotes the load resistance, and L denotes the inductance value of the two-switch Buck-Boost.

[0015] The beneficial effects of the present invention are:

[0016] By increasing the lower limit of the initial duty cycle of the Boost leg, the present invention enables the entire starting process to only adjust the Buck, and sets the PI parameters according to the load size, thereby being able to reduce the plateau period and oscillation. Therefore, the present invention can reduce the long plateau of the gain in the Buck mode and increase the total gain in the Buck mode, which is beneficial to reducing the plateau period and overshoot during the starting process. Description of the Drawings

[0017] Figure 1 is the duty cycle calculation diagram of a wide load range soft start control method for a two-switch Buck-Boost converter of the present invention;

[0018] Figure 2 is the flowchart of the soft start process of the present invention;

[0019] Figure 3 is the gain curve diagram of CCM and DCM of the prior art;

[0020] Figure 4 is the drive and inductor current waveform diagram under different conditions of the soft start control strategy of the present invention;

[0021] Figure 5 is the gain curve comparison diagram between the control method of the prior art and the control method of the present invention;

[0022] Figure 6 is the TSBB simulation model diagram of the present invention;

[0023] Figure 7 is the simulation result comparison diagram between the soft start control strategy of the prior art and the soft start control strategy of the present invention;

[0024] Figure 8 is the waveform diagram of the soft start control strategy of the prior art under no-load and light-load (I = 2A) conditions;

[0025] Figure 9 is the waveform diagram of the soft start control strategy of the present invention under no-load and light-load (I = 2A) conditions. Detailed Embodiment

[0026] The following will clearly and completely describe the related technologies in the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Referring to Figures 1 to 9 as shown, the following control method is adopted in this specific embodiment. The lower limit of the duty cycle of the Boost leg in the soft start process is increased from 0.01 to 0.25. At this time, the duty cycle calculation formulas for the Buck and Boost legs are as follows:

[0028]

[0029] The schematic diagram of the specific duty cycle calculation method is as shown in Figure 1 shown.

[0030] For the specific start-up process, it can be determined whether it is operating in the light load mode according to the load size, and different PI parameter values are set accordingly for the light load condition and the heavy load condition. The flow chart of the start-up process is as shown in Figure 2 shown.

[0031] The following is the theoretical analysis:

[0032] For the traditional soft start control strategy, under the CCM condition, the gain of TSBB can be calculated according to the following formula:

[0033]

[0034] Under the DCM condition, when TSBB operates in the Buck or Boost mode (only the Buck leg or the Boost leg acts, and the other leg does not switch), the gain is:

[0035]

[0036] Among them, where L is the inductance value of TSBB, T s is the switching period, and R is the load resistance.

[0037] The gain curves in the CCM and DCM cases are plotted as shown in Figure 3 shown. Compared with the CCM state, in the DCM state, there is an obvious long platform in the second half of the corresponding region of the Buck mode (D < 1) of the gain of TSBB. When adjusting D within the platform, the gain hardly changes. After entering the Boost mode (D > 1), the DCM gain will increase rapidly, so there will be a large overshoot as D increases.

[0038] For the soft start control strategy of the present invention, first analyze the gain under light load conditions (DCM). Since the main optimization of the present invention is for the light load and no-load start waveforms in the boost case, the case where the final steady state is boost is mainly analyzed. Assume that the duty cycle of the Buck leg is d Buck , and the initial duty cycle of the Boost leg is d Boost , d′ Boost = 1 - d Boost . Figure 4 respectively gives the driving and inductor current waveforms of the two legs in two cases of d Buck < d′ Boost (V o < V in ) and d Buck > d′ Boost (V o > V in ).

[0039] At the initial stage of the start-up process, since the given Uref value is small, the output voltage is less than the input voltage, and the duty cycle of the Buck leg is low. At this time, in the working waveform, there is no overlapping part between the driving signals of S1 and S2. The operation of the Boost leg has no effect on the inductor current and the output voltage gain. At this time, the converter gain is the same as that in the Buck mode.

[0040] At the later stage of the start-up process, when the given Uref value is large, the output voltage is greater than the input voltage. At this time, there is an overlapping part between the driving signals of S1 and S2. Let T1 = t4 - t3, and the following equation can be written:

[0041] For t1 - t2, the voltage across the inductor is V in ,

[0042]

[0043] For t3 - t4, the voltage across the inductor is V o - V in ,

[0044]

[0045] The area of the triangle formed by the inductor current and the coordinate axis is equal to the integral value of the output current with respect to time for each period. Therefore, there is:

[0046]

[0047] It can be obtained that:

[0048]

[0049] Among them,

[0050]

[0051] As Figure 5 shown, a comparison chart of the gain curves of the traditional control method and the control method of the present invention can be obtained. It can be seen from the chart that the gain curve of the control method of the present invention can reduce the long platform of the gain in the Buck mode and increase the total gain in the Buck mode, which is beneficial to reducing the platform period and overshoot during the startup process.

[0052] Embodiment

[0053] In this embodiment, a simulation model is built using Simulink, and the specific simulation parameter settings are shown in Table 1:

[0054] Table 1

[0055] Simulation parameters Parameter value Input voltage 220V Output voltage 280V Rated current 29A Switching frequency 200 kHz Inductance value 117 μH Output capacitance 2 μF

[0056] The simulation results are as Figure 7 shown. There is an obvious platform in the output voltage of the prior art. During the platform period, as the duty ratio of the Buck leg increases continuously, the output voltage hardly changes. At this time, the output voltage cannot follow the given change, and the error value accumulates continuously, resulting in a rapid increase in the controlled quantity D. When the control quantity D is greater than 1, since the Boost gain curve is very steep in the DCM state at this time, a large overshoot appears in the output voltage. Under the action of the PI regulator, the generated control quantity changes continuously, causing the TSBB to switch between the Buck and Boost modes, and obvious oscillations occur in the output voltage. For the smooth control soft-start strategy of the present invention, the platform and oscillations of its output voltage are significantly reduced.

[0057] A prototype of a two-switch Buck-Boost converter cascaded with LLC-DCX is built to verify the proposed smooth soft-start control strategy. The rear-stage LLC-DCX can be equivalently regarded as a 10:1 transformer. The output voltage of the rear stage is 1 / 10 of that of the front stage, and the output current of the rear stage is 10 times that of the front stage.

[0058] In summary, the present invention increases the lower limit of the initial duty ratio of the Boost leg, enabling the startup process to only adjust the Buck throughout, and setting the PI parameters according to the load size, thereby reducing the platform period and oscillations. Therefore, the present invention can reduce the long platform of the gain in the Buck mode and increase the total gain in the Buck mode, which is beneficial to reducing the platform period and overshoot during the startup process.

[0059] It should be emphasized that the above are only the preferred embodiments of the present invention, and there is no restriction on the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A soft start control method for a two-switch Buck-Boost converter with a wide load range, characterized in that The two-switch Buck-Boost converter is a two-switch Buck-Boost cascaded LLC converter; the wide load range soft start control method includes: setting the lower limit of the duty cycle of the Boost leg in the soft start process to k, where the value range of k is: 0.2 to 0.3; the duty cycles of the Buck and Boost legs in the two-switch Buck-Boost converter are: where D S1 represents the duty cycle of the Buck leg, D S2 represents the duty cycle of the Boost leg, and D represents the control quantity.

2. The soft start control method for a wide load range of a two-switch Buck-Boost converter according to claim 1, characterized in that The value range of the said k is: 0.24 to 0.

26.

3. A wide load range soft start control method for a two-switch Buck-Boost converter according to claim 2, characterized in that, The value of the said k is: 0.

25.

4. A wide load range soft start control method for a two-switch Buck-Boost converter according to claim 1, characterized in that The gain of the wide load range soft start control method in the later stage of the start process is: Among them, Gain represents the post-stage gain, d Buck represents the duty cycle of the Buck leg, d ′ Boost = 1 - d Boost , d Boost represents the initial duty cycle of the Boost leg, T s represents the switching period, R represents the load resistance, and L represents the inductance value of the two-switch Buck-Boost.

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