A Multi-Degree-of-Freedom Optimal Control Method for Four-Switch Buck-Boost Converter

Abstract

The present invention discloses a multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter, and belongs to the field of power conversion technology in power electronics technology. In the prior art, the traditional mode switching control method has the problems of discontinuous switching points, nonlinear gain, and inability to soft switch. Although some new control methods proposed in recent years can realize soft switching, they need to record a large number of multidimensional data tables and rely on hardware current sensors dedicated to zero voltage switching (ZVS) current detection. The present invention proposes a multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter, which is based on the constraints of realizing zero voltage switching of all switch tubes and the minimum average value of inductor current, and minimizing the root mean square value of inductor current as the objective function, and solving the optimal combination of Buck half-bridge duty cycle, Boost half-bridge duty cycle, two half-bridge phase shift duty cycle, and switching frequency under any input voltage and load current. The method can realize zero voltage switching of all switch tubes without serially connecting a current sensor and a comparison circuit dedicated to detecting the zero voltage switching current value, thereby saving the number of sensors. And there is no need to record a multidimensional data table, which effectively saves the storage space of the microcontroller. This method also achieves the minimum RMS value of the inductor current and significantly reduces the conduction loss of the switching device.

Description

Technical Field

[0001] The invention belongs to the technical field of power conversion in power electronics technology, and specifically relates to a multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter. Background Art

[0002] Buck-boost DC-DC converters are gaining more and more attention and applications. Traditional DC-DC converters can only make the output voltage higher or lower than the input voltage, while buck-boost converters can achieve continuous regulation of voltage reduction, equalization and boost, so the voltage regulation range is wider.

[0003] Classic non-isolated converters with buck-boost functions include Buck-Boost, Cuk, Sepic, Zeta, and Boost cascade Buck. Boost and Buck cascade converters are essentially two basic conversion units cascaded together, adjusting the duty cycle of the front and rear stages to achieve the buck-boost function, but the converter needs to use two inductors. Buck-Boost converter uses only one inductor to achieve the buck-boost function, and the number of its components is the same as Buck (or Boost), without adding additional components. However, the output voltage of this converter is reverse polarity, and the voltage stress of the switching device is the sum of the input side voltage and the output side voltage, and the device voltage stress is high. The Cuk converter uses two inductors, and the output voltage is reverse polarity. It does not have an advantage in terms of the number of components and voltage stress, but its significant advantage is that the input power supply current and the output load current are continuous, and the pulsation is very small, which greatly reduces the pressure on the input and output filter capacitors. Sepic and Zeta converters use 2 inductors, 2 capacitors, 1 switch tube, and 1 diode respectively. The difference between the two is the position of the inductor and the switch tube. The output voltage of both converters is positive.

[0004] Nowadays, many scholars have focused their attention on the four-switch Buck-Boost converter. This converter consists of four switches, one inductor, and two capacitors. Compared with the various buck-boost converters mentioned above, the FSBB converter has significant advantages: (1) there is only one inductor; (2) the input voltage and output voltage have the same polarity; (3) the duty cycle of the Buck bridge arm and the Boost bridge arm can be adjusted independently.

[0005] In recent years, domestic and foreign scholars have conducted extensive research on the control strategy of four-switch converters. Reference [1] proposed a fixed-frequency control strategy that can achieve ZVS and minimize I L-RMS , but the controller needs to record a large number of multidimensional data tables, and the serial access is dedicated to I zvsThe sensors used for detection will increase hardware cost and loss, and also have extremely high requirements for response time. Reference [2] proposed a simplified real-time calculation method that reduces the offline data dimension from three dimensions to two dimensions, but still requires the use of I zvs The sensor used for detection. Reference [3] uses logical reasoning from the perspective of physical meaning to explain why the four-stage inductor current shape is the optimal shape. Reference [4] proposes a simplified real-time calculation method to avoid the use of multi-dimensional data tables, but it can only achieve a smaller inductor current instead of the minimum inductor current, and it also depends on I zvs Detection.

[0006] In summary, in the prior art, the traditional mode switching control method has problems such as discontinuous switching points, nonlinear gain, and inability to soft switch. Although some new control methods proposed in recent years can achieve soft switching, they need to record a large number of multidimensional data tables, occupy a large amount of microcontroller storage space, and rely on hardware current sensors dedicated to ZVS current detection, which increases hardware consumption and circuit loss and reduces operating efficiency.

[0007] [1]Z.Zhou, H.Li, and

[0008] [2]F.Liu,J.Xu,Z.Chen,R.Huang,and

[0009] [3] J.Fang,

[0010] [4] L.Tian, Summary of the invention

[0011] In view of the defects existing in the prior art, the present invention discloses a multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter. The method takes the realization of zero voltage turn-on of all switch tubes and the minimum average value of inductor current as constraints, takes minimizing the root mean square value of inductor current as the objective function, and solves the optimal combination of Buck half-bridge duty cycle, Boost half-bridge duty cycle, two half-bridge phase shift duty cycle, and switching frequency under any input voltage and load current. The method can realize the zero voltage turn-on of all switch tubes without serial connection of current sensors and comparison circuits dedicated to detecting zero voltage turn-on current values, saving the number of sensors. And there is no need to record multidimensional data tables, which effectively saves the storage space of the microcontroller. The method also realizes the minimum root mean square value of inductor current, greatly reducing the conduction loss of the switching device.

[0012] To achieve the above purpose, the technical solution adopted by the present invention is a multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter, wherein the four-switch Buck-Boost converter is composed of four switch tubes, one inductor, and two capacitors, wherein the switch tubes S1 and S2 constitute the left Buck half-bridge, the switch tubes S3 and S4 constitute the right Boost half-bridge, the inductor L is connected to the midpoint of the two half-bridges, and the capacitors C1 and C2 are connected in parallel at the input side and the output side, respectively. The control method comprises the following steps:

[0013] (1) Select Buck half-bridge duty cycle D 1 , Boost half-bridge duty cycle D 2 , the phase shift duty ratio of the two half bridges Switching frequency f s As a four-switch Buck-Boost converter degree of freedom;

[0014] (2) Establishing ZVS constraints and minimum inductor current average constraints, and selecting the optimal inductor current shape based on these two constraints;

[0015] (3) Taking the minimum inductor current root mean square (RMS) value as the objective function, and the ZVS condition, the mathematical condition of the optimal inductor current shape, the gain relationship, and the frequency range as constraints, the optimization problem is established and solved;

[0016] (4) Record the optimal degree of freedom combination under different input and output conditions, draw the relationship curve between each input and output quantity and each degree of freedom offline, and record the turning point of each curve and the piecewise fitting polynomial expression;

[0017] (5) The turning point coordinate value data and the mathematical expression of the fitting polynomial are written into the controller. The controller obtains the optimal controllable degree of freedom combination based on the real-time collected input and output of the converter, and then converts it into a drive signal through a modulation link to control the four-switch converter, thereby achieving the purpose of soft switching, high efficiency, and simplified calculation.

[0018] Furthermore, in step (2), the conditions that need to be met to achieve ZVS of S1 to S4 are: <1> S1 and S4 are connected when the inductor current is less than -I zvs Open during the period; <2> S2 and S3 are connected when the inductor current is greater than I zvs During the period. zvs It is the minimum current value to achieve ZVS of the switching tube.

[0019] Furthermore, in step (2), the selected switch sequence combination corresponding to the inductor current shape is S1S4→S1S3→S2S3→S2S4 or S1S4→S1S3→S2S3.

[0020] Furthermore, in step (3), the RMS value of the inductor current is I L-RMS , the converter input voltage is U in , the converter output voltage is U out , the upper limit of the switching frequency is f s-max , the lower limit of the switching frequency is f s-min , the inductor current valley value is I 0 , the turning point current of S1S4→S1S3 is I B , the turning point current of S1S3→S2S3 is IC , the mathematical description of the optimization problem is:

[0021]

[0022]

[0023] Furthermore, in step (5), what is written into the controller is the turning point coordinate value data of the relationship curve between each input and output quantity and each control degree of freedom and the mathematical expression of the fitting polynomial, rather than a multidimensional data table.

[0024] The effects of the present invention are:

[0025] 1. All switch tubes of the four-switch Buck-Boost converter can achieve ZVS, which reduces the turn-on loss of the converter and greatly improves the operating efficiency of the converter.

[0026] 2. The inductor current is minimized, the conduction loss of the switch tube is reduced, and the operating efficiency of the converter is improved.

[0027] 3. It is not necessary to connect the dedicated I zvs The current sensor only needs to collect the output current I of the FSBB converter. o This method reduces the number of current sensors, eliminates the loss caused by the sensors connected in series, and improves the efficiency and versatility of the converter.

[0028] 4. There is no need to record huge multi-dimensional data tables. Instead, only the turning points and polynomial fitting expressions of each degree of freedom need to be recorded to achieve wide-range closed-loop optimization operation of input voltage and load current, which greatly saves the storage space of the controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the topology diagram of the four-switch Buck-Boost converter.

[0030] Figure 2 These are the four basic operating modes of a four-switch Buck-Boost converter.

[0031] Figure 3 It is the driving signal and inductor current waveform that meets ZVS of all switch tubes in boost, equal pressure and buck states.

[0032] Figure 4 It is a four-segment inductor current waveform.

[0033] Figure 5 It is a block diagram of the overall closed-loop operation strategy of the multi-degree-of-freedom optimal control system.

[0034] Figure 6 It is the optimal D under the conditions of fixed 200V output voltage, different input voltage and load current. 1 Change curve.

[0035] Figure 7 The optimal D under different input voltage and load current conditions with a fixed 200V output voltage is 2 Change curve.

[0036] Figure 8 It is the optimal output voltage of 200V under different input voltage and load current conditions. Change curve.

[0037] Fig. 9 The optimal f under fixed 200V output voltage, different input voltage and load current conditions s Change curve. DETAILED DESCRIPTION

[0038] The technical solution and implementation effects of the present invention will be further described in detail below with reference to the accompanying drawings.

[0039] The topology of the four-switch Buck-Boost (FSBB) converter is shown in Figure 1 As shown in Figure 1, the converter consists of 4 switching tubes, 1 inductor, and 2 capacitors. 1 , S 2 Its body diode D 1 , D 2 The Buck bridge is formed, and the switch tube S 3 , S 4 Its body diode D 3 , D 4 The inductor L is connected to the midpoint of the two half bridges, and the two half bridges and the inductor together form an H bridge. 1 and C 2 They are the voltage stabilizing capacitors on the input and output sides of the converter respectively.

[0040] The FSBB converter has four degrees of freedom, namely, the upper tube S of the Buck bridge arm 1 Duty cycle D 1 , Boost bridge arm upper tube S 3 Duty cycle D 2 , the phase shift angle between the two bridge arms Switching frequency f s .

[0041] There are 4 basic operating modes of FSBB converter, such as Figure 2 As shown in Figure 1, the positive direction of the inductor current is assumed to be from left to right. Figure 2 (a), S 1 S4 Turn on S 2 S 3 When the inductor is turned off, the voltage stress of the inductor is the input voltage U in , inductor current i L Increase; Mode 2 is as follows Figure 2 (b) S 1 S 3 Turn on S 2 S 4 When the inductor is turned off, the voltage stress of the inductor is (U in -U out ), if U in >U out Then i L Positive increase, if U in out Then i L Decrease, if the two are equal, then i L Remain unchanged; Mode 3 is as follows Figure 2 (c), S 2 S 3 Turn on S 1 S 4 When the inductor is turned off, the voltage stress of the inductor is -U out ,i L Mode 4 is as follows Figure 2 (d), S 2 S 4 On, S 1 S 3 When turned off, the voltage stress of the inductor is 0 and the rate of change of the inductor current is 0.

[0042] According to the volt-second balance law, the voltage gain relationship of the FSBB converter can be obtained, which is only related to the duty cycle D of the two half-bridges. 1 , D 2 is related to the phase shift angle and frequency f s It is irrelevant. The expression of voltage gain K is:

[0043]

[0044] First, the inductor current shape should be optimized. The change of phase shift angle does not affect the voltage gain relationship and the average value of the inductor current of the FSBB converter, but it can affect the shape, ripple size and RMS value of the inductor current. The inductor current shape plays a decisive role in whether the switching device ZVS and minimum conduction loss can be achieved.

[0045] Definition D 1 The rising edge of is the starting position of each cycle. Yes D 2 The rising edge lags behind D 1 ​The phase shift duty cycle of the rising edge. According to the size of the duty cycle, it can be divided into three categories: D 1 >D 2 , D 1 =D 2 , D 1 <D 2 . According to 1-D 1 With D 2 Size, divided into several subcategories based on each major category. And the current change rate under four modes, any D 1 , D 2 , All inductor current shapes under combination.

[0046] The first optimal constraint is the inductor current average value constraint. The DC component of the inductor current is a key factor affecting the volume of passive components, especially the volume of magnetic components, and is also the main factor affecting the RMS value. Reducing the average value can often significantly reduce the RMS value.

[0047] According to the power conservation relationship:

[0048] U in I in =U mid I L =U out I out

[0049] The average value of the inductor current I L-avg for:

[0050]

[0051] Where P is the system power.

[0052] At the same voltage gain, D 1 and D 2 The larger the value of the virtual intermediate voltage U mid The larger the inductor current, the higher the average value I L-avg The smaller. When U out >U in When I L-avg The duty ratios corresponding to the minimum values ​​are D 1 =1,D 2 =1 / K; when U out in When I L-avg The duty ratios corresponding to the minimum values ​​are D 1 =K,D 2 = 1. After comprehensive analysis and selection of the average value of the inductor current and all the inductor current shapes, it is found that when D 1 or D​2 When approaching 1, 1-D 1 >D 2 is clearly not valid. Therefore, the only inductor current shape that meets the requirements can be 1-D 1 <D 2 .

[0053] The second optimization condition is the soft-switching (ZVS) constraint of the switching transistor. The condition for a power switching device to achieve ZVS is that the terminal voltage drops to 0V or the charge of the output capacitor C oss drops to 0 during the dead time before the switching transistor turns on. For the FSBB converter, the current flows through two switching transistors in each mode. Therefore, the expression for achieving ZVS is:

[0054] |I ZVS |t dead ≥2C oss U S

[0055] where I zvs is the minimum current value to achieve ZVS of the switching device, t dead is the dead time, and U s is the voltage stress of the switching device before the dead time arrives.

[0056] If ZVS of S 1 ~S 4 is to be achieved, the following two conditions need to be met:

[0057] <1>S 1 and S 4 turn on during the period when the inductor current is less than -I zvs .

[0058] <2>S 2 and S 3 turn on during the period when the inductor current is greater than I zvs .

[0059] Based on the above conditions, the remaining inductor current shapes are further screened. After screening, it is found that the inductor current shapes that meet the ZVS conditions have the following characteristics: (1) The turn-off time of S3 is always within the off-state time range of S1; (2) The switching sequence combination is always S1S4 → S1S3 → S2S3 → S2S4 or S1S4 → S1S3 → S2S3; (3) D 1 , D 2 , always satisfy the inequality

[0060] The drive signals and inductor current waveforms that meet ZVS of all switching transistors in boost, equal-pressure, and buck states are as Figure 3 shown.

[0061] Secondly, we need to find the objective function and constraints and construct the optimization problem.

[0062] The current shape of the four-stage inductor is as follows Figure 6 As shown. Figure 6 The average value of the inductor current I can be derived L-avg , valley value I 0 and the RMS value I L-RMS The mathematical expression of .

[0063] The piecewise expression of the inductor current in each cycle is:

[0064]

[0065] The average value of the inductor current I L-avg The expression is:

[0066]

[0067] Assuming R is the equivalent load resistance, the average value of the inductor current satisfies:

[0068]

[0069] Combining the above two equations, we can get:

[0070]

[0071] The current expression at point B is:

[0072]

[0073] The current expression at point C is:

[0074]

[0075] The inductor current root mean square value expression is:

[0076]

[0077] With minimum I L-RMS The optimization problem of the objective function can be described as:

[0078]

[0079]

[0080] Next, the optimization problem needs to be solved and the solution written into the microcontroller for calling.

[0081] For any given set of input conditions (U in , U out ,I0 ) into the above optimization problem, we can get a set of corresponding optimal controllable freedom output D 1 , D 2 , and f s .

[0082] I L-RMS The expression is complex and the microcontroller has limited computing power, so it is impossible to solve the optimization problem online in real time. If the traditional table lookup method is used, it is necessary to store a huge three-dimensional data table (including input quantity data table and output quantity data table), which consumes a lot of storage space. Therefore, it is necessary to find the relationship between input quantity and output quantity to reduce the data dimension.

[0083] Optimal D under different input voltage and load current conditions with fixed 200V output voltage 1 The change curve is as Figure 6 As shown, the optimal D 2 The change curve is as Figure 7 As shown, the best The change curve is as Figure 8 As shown, the optimal f s The change curve is as Fig. 9 As shown. Figure 6-Figure 9 It can be seen that, first, the optimal point of each degree of freedom changes continuously without discontinuity. Secondly, when the input voltage and output voltage are fixed, as the load changes, the curve of the optimal controllable degree of freedom has an obvious turning point. In addition, when the input voltage is different, the turning point and curvature of each curve are also different. Based on the above three characteristics, only the information of the turning point + fitting polynomial can be stored to obtain the optimal D corresponding to different input voltages or loads. 1 , D 2 , and f s combination.

[0084] The overall closed-loop operation strategy of the proposed FSBB converter multi-degree-of-freedom optimal control system is as follows: Figure 5 As shown. in , U out ,I out is sampled and used as the input variable of the controller. ref and U out After the difference is made, it enters the PI controller and generates D 1 In each interrupt cycle, the microcontroller collects real-time U in and U out To extract the corresponding turning points and polynomial expressions stored offline, and then convert the real-time I out Substituting into the polynomial expression, we get the optimal D 1 , D 2 , and f s Finally, the driving signals of S1 to S4 are generated.

[0085] The present invention can be implemented in other specific forms without departing from its spirit or essential characteristics. The described embodiments are considered to be illustrative and non-restrictive in all aspects, for example:

[0086] 1) The devices used in the four-switch Buck-Boost converter are not limited to the types in the embodiment;

[0087] 2) The operating conditions (input voltage, output voltage, load current, etc.) of the four-switch Buck-Boost converter are not limited to the parameters in the embodiment.

[0088] The scope of the invention is therefore indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalent technical solutions of the claims are embraced within their scope.

Claims

1. A multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter, wherein the four-switch Buck-Boost converter is composed of four switch tubes, one inductor, and two capacitors, wherein the switch tubes S1 and S2 constitute a left Buck half-bridge, the switch tubes S3 and S4 constitute a right Boost half-bridge, the inductor L is connected to the midpoint of the two half-bridges, and the capacitors C1 and C2 are connected in parallel at the input side and the output side respectively; It is characterized in that The control method comprises the following steps: (1) Select Buck half-bridge duty cycle D 1 , Boost half-bridge duty cycle D 2 , the phase shift duty ratio of the two half bridges Switching frequency f s As a control freedom of the four-switch Buck-Boost converter; (2) Establish the zero voltage switch (ZVS) constraint of the switch tube and the minimum inductor current average value constraint conditions, and select the optimal inductor current shape based on these two constraints; (3) Taking the minimum inductor current root mean square (RMS) value as the objective function, and the ZVS condition, the mathematical condition of the optimal inductor current shape, the gain relationship, and the frequency range as constraints, the optimization problem is established and solved; (4) Record the optimal controllable degree of freedom combination under different input and output conditions, draw the relationship curve between each input and output quantity and each degree of freedom offline, and record the turning point coordinate value of each curve and the mathematical expression of the piecewise fitting polynomial; (5) The turning point coordinate value data and the mathematical expression of the fitting polynomial are written into the controller. The controller obtains the optimal control degree of freedom combination based on the real-time collected input and output of the converter, and then converts it into a drive signal through a modulation link to control the four-switch converter, thereby achieving the purpose of soft switching, high efficiency, and simplified calculation.

2. A four-switch Buck-Boost converter multi-degree-of-freedom optimization control method as claimed in claim 1, It is characterized in that In step (2), the conditions that need to be met to achieve ZVS of S1 to S4 are: <1> S1 and S4 are connected when the inductor current is less than -I zvs Open during the period; <2> S2 and S3 are connected when the inductor current is greater than I zvs During the period, I zvs It is the minimum current value to achieve ZVS of the switching tube.

3. A four-switch Buck-Boost converter multi-degree-of-freedom optimization control method as claimed in claim 1, It is characterized in that In step (2), the selected switching sequence combination corresponding to the inductor current shape is S1S4→S1S3→S2S3→S2S4 or S1S4→S1S3→S2S3.

4. A multi-degree-of-freedom optimization control method for a four-switch Buck-Boost converter as claimed in claim 1, It is characterized in that In step (3), the RMS value of the inductor current is I L-RMS , the input voltage of the converter is U in , the converter output voltage is U out , the upper limit of the switching frequency is f s-max , the lower limit of the switching frequency is f s-min , the inductor current valley value is I 0 , the turning point current of S1S4→S1S3 is I B , the turning point current of S1S3→S2S3 is I C , the mathematical description of the optimization problem is: Among them, min means minimization and st means the constraint condition.

5. A four-switch Buck-Boost converter multi-degree-of-freedom optimization control method as claimed in claim 1, It is characterized in that In step (5), what is written into the controller is the turning point coordinate value data of the relationship curve between each input and output quantity and each control degree of freedom and the mathematical expression of the fitting polynomial, rather than a multidimensional data table.

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