A control method for soft start applied to CLLC bidirectional DC / DC converter

Abstract

The present invention discloses a control method for soft start applied to a CLLC bidirectional DC / DC converter. It includes the following steps: A. Determine the maximum switching frequency f max ; B. Determine the condition for the conversion between the half-bridge and full-bridge structures of the CLLC resonant bidirectional DC / DC converter circuit: First, start with the half-bridge structure and PWM method at the maximum switching frequency f max , and then start with the full-bridge structure and PWM method; C. Determine the duty cycle change function expressions for PWM control of the half-bridge structure and the full-bridge structure, and finally obtain the change law of the switching frequency from f max decreasing to the resonant frequency f r . The advantages are: The starting current of the CLLC bidirectional DC / DC converter is greatly reduced, the resonant current impact during the starting stage of the CLLC bidirectional converter is effectively reduced, and a better soft start effect can be achieved through the change of the duty cycle.

Description

Technical Field

[0001] The present invention relates to a power electronic power conversion technology, and more specifically to a control method for soft start applied to a CLLC bidirectional DC / DC converter. Background Art

[0002] Due to the advantages that the primary side switching devices of the transformer in the CLLC bidirectional DC / DC converter can achieve zero-voltage turn-on and the secondary side switching devices can achieve zero-current turn-off, the efficiency can reach a very high level, and it is widely used in occasions such as microgrids, new energy vehicle charging and discharging, etc., so it has attracted much attention.

[0003] Since the output side of the CLLC bidirectional DC / DC converter is only filtered by a single filter capacitor, when starting, this large-capacity filter capacitor will be charged, resulting in a large inrush current. Therefore, a soft start control strategy needs to be adopted for the CLLC bidirectional DC / DC converter. In the prior art, common soft start methods for the CLLC bidirectional DC / DC converter include high-frequency start, phase-shift start, optimal trajectory control, etc.; among them, the high-frequency start starts the converter with a switching frequency several times higher than the resonant frequency. Due to the frequency limitation of the switching devices and the fact that the gain of the CLLC at high frequencies cannot be reduced very low, the high-frequency soft start scheme cannot well limit the start current. To solve this problem, there are soft start schemes that separately use the half-bridge / full-bridge switching method, and there are also soft start schemes that separately introduce the PWM control method, but there are still large inrush currents. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a control method for soft start applied to a CLLC bidirectional DC / DC converter that can not only greatly reduce the start current but also achieve a better soft start effect through the change of the duty cycle.

[0005] To solve the above technical problem, the control method for soft start applied to a CLLC bidirectional DC / DC converter of the present invention includes the following steps:

[0006] A. Determine the maximum switching frequency f max ;

[0007] B. Determine the conditions for the half-bridge and full-bridge structure conversion of the CLLC resonant bidirectional DC / DC converter circuit: first start with the half-bridge structure and the PWM method at the maximum switching frequency f max and then start with the full-bridge structure and the PWM method;

[0008] C. Determine the duty cycle change function expressions for the PWM control of the half-bridge structure and the duty cycle change function expressions for the PWM control of the full-bridge structure, and finally obtain the switching frequency from f maxReduced to the resonant frequency f r Variation law of

[0009] The CLLC resonant bidirectional DC / DC converter circuit includes a primary - side full - bridge circuit composed of four MOSFETs S1 - S4 and a secondary - side full - bridge circuit composed of four MOSFETs S5 - S8. The mid - points of the bridge arms of the four MOSFETs in the primary - side full - bridge circuit and the mid - points of the bridge arms of the four MOSFETs in the secondary - side full - bridge circuit are respectively led out and connected to both ends of the resonant cavity. The two resonant cavities are connected through the high - frequency transformer T.

[0010] The primary - side full - bridge circuit is connected to the energy - storage battery V battery and the secondary - side full - bridge circuit is connected to the DC bus V bus .

[0011] In step B, during the start - up stage of the half - bridge structure, the switching tubes S1 and S2 are asymmetrically complementary and the duty cycle of S1 linearly increases. During the start - up stage of the full - bridge structure, S3 and S4 are asymmetrically complementary and the duty cycle of S3 linearly increases.

[0012] When the duty cycles of both S1 and S2 become 0.5, the start - up of the half - bridge structure is switched to the start - up of the full - bridge structure.

[0013] When the CLLC resonant bidirectional DC / DC converter starts and operates, at the maximum switching frequency f max , first, the half - bridge structure and the symmetric PWM control method are adopted. At this time, the driving signals of S1 and S2 are complementary. The duty cycle of S1 gradually increases from 0 to 0.5, while the driving signal of S3 remains 0 and the driving signal of S4 remains 1. When the duty cycles of both S1 and S2 become 0.5, the full - bridge structure and the asymmetric PWM control method are adopted. At this time, the switching frequency still remains at f max . S1 and S2 are complementary and the duty cycle always remains 0.5. The driving signals of S3 and S4 are complementary and the duty cycle of the driving signal of S3 gradually increases from 0 to 0.5, while the duty cycle of the driving signal of S4 gradually decreases from 1 to 0.5.

[0014] The duty - cycle function expressions of S1 and S3 are as follows:

[0015]

[0016]

[0017] D s1 is the duty cycle of S1, D s3 is the duty cycle of S3. t is time, t c is the conversion time of the half - bridge / full - bridge, t o is the end time of the full - bridge start - up and the start time of the frequency - reduction start - up, t endis the soft start end time. k1 and k2 are the slopes of the linear changes in the duty cycles of S1 and S3 respectively, both taking constant values;

[0018]

[0019] Equation (3) is the variation law of the switching frequency f in the frequency reduction stage. k3 is the slope of the linear frequency change, taking a negative value; b is the intercept, and t end is the soft start end time.

[0020] The advantages of the present invention are as follows:

[0021] In the start-up stage of the CLLC resonant bidirectional DC / DC converter, at the maximum switching frequency, a method of first starting with a half-bridge structure and pulse width modulation (PWM) method and then starting with a full-bridge structure and PWM method is adopted, which greatly reduces the start-up current of the CLLC bidirectional DC / DC converter and effectively reduces the resonance current impact in the start-up stage of the CLLC bidirectional converter. In particular, the PWM control introduced on the basis of the half-bridge / full-bridge soft start scheme and the duty cycle change function expressions of the PWM control of the half-bridge structure and the full-bridge structure are designed, and a better soft start effect can be achieved through the change of the duty cycle. Brief Description of the Drawings

[0022] Figure 1 is the circuit topology diagram of the CLLC bidirectional DC / DC converter in the present invention;

[0023] Figure 2 is the driving signal diagram of the CLLC soft start switch tube in the present invention;

[0024] Figure 3 is the PWM duty cycle waveform diagram of the half-bridge structure of the CLLC bidirectional DC / DC converter in the present invention;

[0025] Figure 4 is the PWM duty cycle waveform diagram of the full-bridge structure of the CLLC bidirectional DC / DC converter in the present invention;

[0026] Figure 5 is the duty cycle waveform diagram of the frequency reduction stage of the CLLC bidirectional DC / DC converter in the present invention;

[0027] Figure 6 is the experimental waveform diagram of the half-bridge / full-bridge soft start (current and voltage) scheme of the traditional CLLC bidirectional DC / DC converter;

[0028] Figure 7 is the experimental waveform diagram of the soft start (current and voltage) scheme of the CLLC bidirectional DC / DC converter in the present invention. Detailed Embodiments

[0029] The control method for soft start of the CLLC bidirectional DC / DC converter applied in the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0030] As shown in the figure, the CLLC resonant bidirectional DC / DC converter is an asymmetric structure. The CLLC resonant bidirectional DC / DC converter circuit involved in the present invention includes a primary-side full-bridge circuit composed of four MOSFETs S1 - S4 and a secondary-side full-bridge circuit composed of four MOSFETs S5 - S8. The primary-side full-bridge circuit is connected to an energy storage battery V battery , and the secondary-side full-bridge circuit is connected to a DC bus V bus . The midpoints of the bridge arms of the four MOSFETs in the primary-side full-bridge circuit and the midpoints of the bridge arms of the four MOSFETs in the secondary-side full-bridge circuit are respectively led out and connected to both ends of the resonant cavity. The battery-side resonant cavity is composed of a resonant inductor L r and a resonant capacitor C r1 . The DC bus-side resonant cavity is composed of a resonant capacitor C r2 . The high-frequency transformer T connecting the two resonant cavities has a turns ratio set to n:1.

[0031] The control method for soft start of the CLLC bidirectional DC / DC converter applied in the present invention includes the following steps:

[0032] A. Determine the maximum switching frequency f max ;

[0033] B. Determine the conditions for the half-bridge and full-bridge structure conversion of the CLLC resonant bidirectional DC / DC converter circuit: First, start with the half-bridge structure and PWM method at the maximum switching frequency f max , and then start with the full-bridge structure and PWM method. Specifically, during the half-bridge structure startup stage, the switching tubes S1 and S2 are asymmetrically complementary and the duty cycle of S1 linearly increases. During the full-bridge structure startup stage, S3 and S4 are asymmetrically complementary and the duty cycle of S3 linearly increases. To achieve a smooth switch between the half-bridge and full-bridge structure startups, it is selected to switch from the half-bridge structure startup to the full-bridge structure startup when the duty cycles of both S1 and S2 become 0.5.

[0034] C. Determine the duty cycle change function expressions for the half-bridge structure PWM control and the full-bridge structure PWM control, and finally obtain the change law of the switching frequency from f max to the resonant frequency f r .

[0035] Furthermore, as shown in the figure, in this embodiment, when the CLLC resonant bidirectional DC / DC converter starts and operates, at the maximum switching frequency f maxAt this point, the half-bridge structure and symmetrical PWM control method are first used. At this time, the driving signals of S1 and S2 are complementary, and the duty cycle of S1 gradually increases from 0 to 0.5, while the driving signal of S3 remains at 0 and the driving signal of S4 remains at 1. When the duty cycles of S1 and S2 both become 0.5, the full-bridge structure and asymmetrical PWM control method are used. At this time, the switching frequency remains at f max At this point, S1 and S2 are complementary and the duty cycle is always kept at 0.5. The driving signals of S3 and S4 are complementary and the duty cycle of the driving signal of S3 gradually increases from 0 to 0.5, while the duty cycle of the driving signal of S4 gradually decreases from 1 to 0.5. In order to achieve smooth switching from the half-bridge structure to the full-bridge structure, the conversion condition of this scheme is selected when S1 increases to 0.5. Since the upper and lower tubes of the same bridge arm are complementary, the duty cycle function expressions of S1 and S3 are given below:

[0036]

[0037]

[0038] D s1 is the duty cycle of S1, D s3 is the duty cycle of S3. t is the time, t c is the half-bridge / full-bridge conversion time, t o is the time when full-bridge startup ends and frequency reduction startup begins, t end is the end time of soft start. k1 and k2 are the slopes of the linear change of duty cycle of S1 and S3, respectively, and both are constants;

[0039]

[0040] Formula (3) is the change law of the switching frequency f in the frequency reduction stage, k3 is the slope of the linear change of frequency, which takes a negative value; b is the intercept, t end The soft start end time.

[0041] In order to verify the feasibility and advancement of the proposed control method, a CLLC test circuit was built. The maximum switching frequency in the soft start phase was f max =200kHz, under the same conditions, the traditional half-bridge / full-bridge soft-start control method and the soft-start control method proposed in the present invention are compared to verify the superiority and practicality of the soft-start strategy of the present invention.

[0042] The main circuit parameters and experimental conditions are as follows: the transformer primary inductance is 45.5μH, the transformer secondary inductance is 174.8μH, and the excitation inductance L m is 8.6μH, C r1 The value is 326.5nF, C r2 The value is 132nF. The primary voltage V batteryis 10V, the secondary side voltage V bus is 16.8V, the starting frequency is the maximum frequency f max = 200kHz, the resonant frequency f r = 123kHz.

[0043] It can be seen from the experimental diagrams of the traditional scheme and this scheme that the soft start time of this scheme is shorter, the starting current is smaller, and the sudden change of the starting current is avoided.

[0044] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A control method for soft start of a CLLC bidirectional DC / DC converter, comprising the following steps: A. Determine the maximum switching frequency f max ; B. Determine the conditions for the half-bridge and full-bridge structure conversion of the CLLC resonant bidirectional DC / DC converter circuit: First, start with the half-bridge structure and PWM mode at the maximum switching frequency f max and then start with the full-bridge structure and PWM mode. The CLLC resonant bidirectional DC / DC converter circuit includes a primary-side full-bridge circuit composed of four MOSFETs S1 - S4; During the start-up phase of the half-bridge structure, the switching tubes S1 and S2 are asymmetrically complementary and the duty cycle of S1 increases linearly. During the start-up phase of the full-bridge structure, S3 and S4 are asymmetrically complementary and the duty cycle of S3 increases linearly; when the duty cycles of S1 and S2 both become 0.5, the start-up of the half-bridge structure is switched to the start-up of the full-bridge structure; C. Determine the expression of the duty cycle change function for PWM control of the half-bridge structure, specifically as follows: （1） D s1 is the duty cycle of S1, where t is time, t c is the switching moment of the half-bridge, and k1 is the slope of the linear change of the duty cycle of S1, taking a constant value; Determine the expression of the duty cycle change function for PWM control of the full-bridge structure, specifically as follows: （2） D s3 is the duty cycle of S3, where t is time, t c is the conversion moment of the full bridge, t o is the moment when the full bridge startup ends and the frequency reduction startup begins, and k2 is the slope of the linear change of the S3 duty cycle, taking a constant value; Finally, the variation law of the switching frequency decreasing from f max to the resonance frequency f r is obtained.

2. The control method for soft start applied to a CLLC bidirectional DC / DC converter according to claim 1, characterized in that: The CLLC resonant bidirectional DC / DC converter circuit further includes a secondary-side full-bridge circuit composed of four MOSFETs S5 - S8. The midpoints of the bridge arms of the four MOSFETs in the primary-side full-bridge circuit and the midpoints of the bridge arms of the four MOSFETs in the secondary-side full-bridge circuit are respectively led out and connected to both ends of the resonant cavity, and the two resonant cavities are connected through the high-frequency transformer T.

3. The control method for soft start applied to a CLLC bidirectional DC / DC converter according to claim 2, characterized in that: The primary side full-bridge circuit is connected to an energy storage battery V battery , and the secondary side full-bridge circuit is connected to a DC bus V bus .

4. The control method for soft start applied to a CLLC bidirectional DC / DC converter according to claim 3, characterized in that: When the CLLC resonant bidirectional DC / DC converter starts and operates, at the maximum switching frequency f max firstly, the half-bridge structure and the symmetric PWM control method are adopted. At this time, the driving signals of S1 and S2 are complementary. The duty cycle of S1 gradually increases from 0 to 0.5, while the driving signal of S3 remains 0 and the driving signal of S4 remains 1. When the duty cycles of S1 and S2 both become 0.5, the full-bridge structure and the asymmetric PWM control method are adopted. At this time, the switching frequency still remains at f max S1 and S2 are complementary and the duty cycle always remains 0.

5. The driving signals of S3 and S4 are complementary and the duty cycle of the driving signal of S3 gradually increases from 0 to 0.5, while the duty cycle of the driving signal of S4 gradually decreases from 1 to 0.

5.

5. The control method for soft start applied to a CLLC bidirectional DC / DC converter according to claim 4, characterized in that: The switching frequency decreases from f max to the resonance frequency f r and the variation law is as follows: （3） Equation (3) is the variation law of the switching frequency during the frequency reduction stage f , k where 3 is the slope of the linear frequency variation, taking a negative value; b is the intercept, t end and end is the end time of soft start.

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