A bidirectional buck-boost four-quadrant partial power converter and a control method thereof

Abstract

The application discloses a bidirectional voltage-lifting four-quadrant partial power converter and a control method thereof. The bidirectional voltage-lifting four-quadrant partial power converter comprises a bipolar bidirectional DC / DC. The first positive terminal and the second positive terminal of the bidirectional DC / DC are respectively connected to a direct current bus for grounding. The first negative terminal of the bidirectional DC / DC is grounded, and the second negative terminal is connected to a storage battery for grounding. The voltage of the direct current bus is equal to the voltage of the storage battery plus the voltage of the second positive terminal of the bidirectional DC / DC. When the voltage of the direct current bus is greater than the voltage of the storage battery, the voltage of the second positive terminal of the bidirectional DC / DC is a positive voltage. When the voltage of the direct current bus is less than the voltage of the storage battery, the voltage of the second positive terminal of the bidirectional DC / DC is a negative voltage. The bidirectional DC / DC only processes a small part of the total power of the system, so that the efficiency and the power density of the partial power converter system are greatly improved.

Description

Technical Field

[0001] This invention belongs to the technical field of DC battery energy storage converters, specifically relating to a bidirectional buck-boost four-quadrant partial power converter and its control method. Background Technology

[0002] Renewable energy generation is intermittent and unstable, making traditional AC systems unsuitable. Therefore, there is a strong push to develop DC power distribution systems, along with the widespread integration of battery energy storage devices. DC-DC battery energy storage converters (bidirectional DC / DC converters), serving as the power electronic interface connecting the battery to the DC bus, play a crucial role in adjusting voltage, controlling battery charging and discharging, and enabling bidirectional power transmission.

[0003] Most existing DC battery energy storage converters adopt a full-power structure, meaning that all electrical energy transmitted between the battery and the DC bus flows through the converter; very few DC battery energy storage converters adopt a partial-power structure, meaning that the bidirectional DC / DC converter inside the converter only processes a portion of the total system power to achieve the transmission of the entire system power.

[0004] For traditional full-power structures, as the power level of battery energy storage devices increases, the power level of the full-power DC battery energy storage converter, which serves as the power conversion interface, also increases, leading to larger size, higher cost, lower efficiency, and more difficult heat dissipation design. For partial-power structures, they can solve the drawbacks of full-power solutions, but existing partial-power DC battery energy storage converters can only achieve two quadrants of operation: boost battery charging and battery discharging and voltage reduction, or boost battery discharging and battery charging and voltage reduction. This prevents the superior performance of partial power from being fully utilized, meaning that the partial power processed by the DC / DC converter inside the converter cannot reach its minimum value. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a bidirectional buck-boost four-quadrant partial power converter and its control method, which solves the technical problem that partial power DC-DC battery energy storage converters can only operate in two quadrants, i.e., battery charging boost and battery discharging buck or battery discharging boost and battery charging buck, resulting in a large partial power of bidirectional DC / DC processing inside the partial power converter.

[0006] The present invention adopts the following technical solution:

[0007] A bidirectional buck-boost four-quadrant power converter includes a bipolar bidirectional DC / DC converter. The first and second positive terminals of the bidirectional DC / DC converter are respectively connected to ground via a DC bus. The first negative terminal of the bidirectional DC / DC converter is grounded, and the second negative terminal is connected to ground via an energy storage battery. The voltage of the DC bus is equal to the energy storage battery voltage plus the voltage of the second positive terminal of the bidirectional DC / DC converter. When the voltage of the DC bus is greater than the energy storage battery voltage, the voltage of the second positive terminal of the bidirectional DC / DC converter is positive; when the voltage of the DC bus is less than the energy storage battery voltage, the voltage of the second positive terminal of the bidirectional DC / DC converter is negative.

[0008] Specifically, the bidirectional DC / DC converter includes a front-end bidirectional unipolar LLC resonant circuit, which is cascaded with a rear-end bidirectional bipolar H-bridge circuit. The bidirectional unipolar LLC resonant circuit is used for pre-voltage regulation and soft switching. The bidirectional bipolar H-bridge circuit is used to output positive or negative voltage, and secondary voltage regulation is used to compensate for the voltage difference between the energy storage battery and the DC bus, while controlling the charging and discharging of the energy storage battery.

[0009] Furthermore, the bidirectional unipolar LLC resonant circuit uses fixed-frequency control, while the bidirectional bipolar H-bridge circuit uses dual closed-loop PI control with an outer voltage loop and an inner current loop.

[0010] Furthermore, the bidirectional unipolar LLC resonant circuit includes an input filter capacitor C1. One end of the input filter capacitor C1 is connected to the DC bus, and the other end is connected to one end of the output filter capacitor C2 in sequence through the primary inverter circuit, the resonant circuit, the isolation transformer T, and the secondary rectifier circuit. The other end of the output filter capacitor C2 is connected to the subsequent bidirectional unipolar H-bridge circuit.

[0011] Furthermore, the primary-side inverter circuit includes switches S1, S2, S3, and S4. The drains of switches S1 and S3 are connected to the positive terminal of the DC bus, while the sources of switches S2 and S4 are connected to the negative terminal of the DC bus. The source of switch S1 and the drain of switch S2 are connected in sequence to a resonant capacitor C. r and resonant inductance L r It then splits into two paths, one of which connects to the transformer's magnetizing inductor L. m One end is connected to the primary side of the isolation transformer T, and the source of switch S3 and the drain of switch S4 are connected to the transformer's magnetizing inductor L. m The other end is connected to the primary side of the isolation transformer T; the secondary rectifier circuit includes switching transistors S5, S6, S7 and S8. The drains of switching transistors S5 and S7 are connected to one end of the output filter capacitor C2, and the sources of switching transistors S6 and S8 are connected to the other end of the output filter capacitor C2. The sources of switching transistors S5 and S7 and the drains of switching transistors S6 and S8 are connected to the secondary side of the isolation transformer T.

[0012] Furthermore, the peak current I of the magnetizing inductor pk With dead time T dead The relationship is as follows:

[0013]

[0014] Resonant inductor L r and resonant capacitor C r The relationship is as follows:

[0015]

[0016] Among them, C oss V is the junction capacitance of the switching transistor. LLCin For LLC input voltage, f s This refers to the system switching frequency.

[0017] Furthermore, the bidirectional bipolar H-bridge circuit includes an inverter circuit, which is divided into two paths. One path is connected to the positive terminal of the DC bus and one end of the filter capacitor C3 via the filter inductor L. The second path is connected to the positive terminal of the energy storage battery via the other end of the filter capacitor C3.

[0018] Furthermore, the inverter circuit includes switching transistor S9 and switching transistor S... 10 Switch S 11 and switching transistor S 12 Switch S9 and switch S 11 The drain of the transistor is connected to one end of the preceding bidirectional unipolar LLC resonant circuit, and the switching transistor S... 10 and switching transistor S 12 The drain of the transistor is connected to the other end of the preceding bidirectional unipolar LLC resonant circuit, and the source of the switching transistor S9 is connected to the source of the switching transistor S... 10 The drain of the transistor is connected to one end of the filter inductor L, and the switching transistor S... 11 The source and switch S 12 The drain of the battery is connected to the positive terminal of the energy storage battery, and the negative terminal of the energy storage battery is connected to the ground along with the negative terminal of the DC bus.

[0019] Furthermore, the inductor current ripple ΔI L The relationship is as follows:

[0020]

[0021] The relationship of the filter capacitor C3 is as follows:

[0022]

[0023] Among them, U Hin f is the input voltage of the H-bridge. s This refers to the system switching frequency.

[0024] Another technical solution of the present invention is a control method for a bidirectional buck-boost four-quadrant partial power converter. The bidirectional unipolar LLC resonant circuit uses a fixed-frequency PWM control strategy and always works at the resonant point. The output voltage is adjusted by changing the duty cycle of the switching transistor or by changing the turns ratio of the isolation transformer T in the circuit parameter design.

[0025] The bidirectional bipolar H-bridge circuit uses voltage and current dual closed-loop PI control. The outer loop is the voltage loop, which controls the voltage of the H-bridge filter capacitor C3 to track the voltage difference between the DC bus and the battery. The inner loop is the current loop, which controls the current of the H-bridge filter inductor L to track the charging and discharging current of the battery. The output value is used as a modulation signal to perform PWM control on the switching transistor.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects:

[0027] This invention discloses a bidirectional buck-boost four-quadrant partial power converter. The bidirectional DC / DC converter can output a bipolar voltage, thus achieving four-quadrant operation: battery charging when the battery voltage is higher than the DC bus voltage, battery discharging when the battery voltage is higher than the DC bus voltage, battery charging when the battery voltage is lower than the DC bus voltage, and battery discharging when the battery voltage is lower than the DC bus voltage. This four-quadrant operation significantly reduces the proportion of power processed by the internal bidirectional DC / DC converter relative to the total system power, further improving efficiency and power density, and effectively controlling battery charging and discharging. The converter's specific connection method allows the internal bidirectional DC / DC converter to process only a small portion of the total system power, achieving partial power output. The internal bidirectional bipolar DC / DC converter can change the sign of the output voltage, thereby compensating for the voltage difference between the battery and the DC bus in real time.

[0028] Furthermore, within the bidirectional DC / DC converter, the front-stage bidirectional unipolar LLC resonant circuit, in addition to meeting the bidirectional energy flow requirements for battery charging and discharging, also solves the problem of low efficiency caused by an excessively large voltage regulation ratio between the DC bus voltage and the DC bus-battery voltage difference, playing a pre-regulation role and assisting the subsequent H-bridge in precise voltage regulation. Simultaneously, soft-switching technology is used to further improve efficiency. The subsequent bidirectional bipolar H-bridge circuit, in addition to meeting the bidirectional energy flow requirements for battery charging and discharging, also functions to compensate for the voltage difference between the battery and the DC bus in real time, outputting either positive or negative voltage—that is, voltage bipolarity. Furthermore, since the H-bridge output current is the same as the battery charging and discharging current, it directly controls battery charging and discharging.

[0029] Furthermore, to optimize efficiency, the bidirectional unipolar LLC resonant circuit uses fixed-frequency PWM control to ensure that the circuit always operates at the optimal operating point, achieving soft switching while achieving optimal efficiency. To compensate for the voltage difference between the battery and the DC bus in real time and to control battery charging and discharging, the bidirectional bipolar H-bridge circuit uses dual closed-loop PI control with an outer voltage loop and an inner current loop.

[0030] Furthermore, filter capacitor C1 is used to smooth out DC bus voltage fluctuations, reduce ripple, and provide a relatively stable voltage to the LLC, facilitating LLC voltage regulation; filter capacitor C2 is used to smooth out LLC output voltage fluctuations, reduce ripple, and provide a relatively stable voltage to the H-bridge, facilitating H-bridge voltage regulation.

[0031] Furthermore, the DC / DC converter's internal front-end is a bidirectional unipolar LLC resonant circuit, with its isolation transformer T's primary side being an inverter circuit that inverts the DC voltage of the DC bus into an AC voltage, which is then transmitted to the subsequent stage through the isolation transformer T's winding. The isolation transformer T's secondary side is a rectifier circuit that rectifies the AC voltage of the isolation transformer winding into a DC voltage, which is then transmitted to the subsequent stage H-bridge through the filter capacitor C2.

[0032] Furthermore, to achieve zero-voltage turn-on of the LLC switch, the peak current I of the magnetizing inductor... pk Must be within dead time T dead The junction capacitance C of the internal switching transistor oss Upon complete discharge, the switched-off transistor simultaneously discharges its junction capacitance C. oss Charging. LLC operates at its optimal operating point, where the resonant frequency equals the switching frequency, allowing the calculation of the resonant inductor and capacitor parameters.

[0033] Furthermore, to achieve partial power, the H-bridge inverter circuit is divided into two paths. One path is connected to the positive terminal of the DC bus, with the aim of enabling most of the system's power to be directly fed between the battery and the DC bus. The other path is connected to the positive terminal of the energy storage battery, with the aim of enabling the remaining small portion of the system's power to be transmitted bidirectionally via the internal DC / DC converter.

[0034] Furthermore, the purpose of the H-bridge inverter circuit is to change the voltage polarity, thereby satisfying both positive and negative voltage differences between the battery and the DC bus, while simultaneously controlling the charging and discharging of the battery.

[0035] Furthermore, the filter inductor L and filter capacitor C3 are used to smooth out H-bridge output voltage fluctuations, reduce ripple, and better compensate for the voltage difference between the DC bus and the battery. Based on the voltage-current relationship of the inductor, the expression for the inductor current ripple can be obtained. When the H-bridge output voltage is half the input voltage, the inductor current ripple is at its maximum, thus allowing the determination of the minimum value of the filter inductor L. Under PWM modulation, the output harmonics are higher harmonics at or above the switching frequency. Therefore, the minimum cutoff frequency can be taken as 1 / 10 of the lowest output harmonic frequency (switching frequency), thus obtaining the minimum value of the filter capacitor C3.

[0036] A control method for a bidirectional buck-boost four-quadrant partial power converter is disclosed. To optimize system efficiency, a fixed-frequency PWM control strategy is used for the bidirectional unipolar LLC resonant circuit, which always operates at its optimal operating point, i.e., the resonant point, to achieve the best LLC efficiency. The output voltage is adjusted by changing the duty cycle of the switching transistors or the turns ratio of the isolation transformer T. To better track the voltage difference between the battery and the DC bus and control battery charging and discharging, a bidirectional bipolar H-bridge circuit uses voltage and current dual closed-loop PI control. The outer loop is a voltage loop, controlling the voltage of the H-bridge filter capacitor C3 to track the voltage difference between the DC bus and the battery; the inner loop is a current loop, controlling the current of the H-bridge filter inductor L to track the battery charging and discharging current. Its output value serves as a modulation signal for PWM control of the switching transistors.

[0037] In summary, this invention improves upon existing DC-DC energy storage converters by proposing a bidirectional buck-boost four-quadrant partial power converter and its control method. On one hand, the internal DC / DC converter handles only a small portion of the total system power, effectively improving the converter's power density, efficiency, and cost. On the other hand, the internal bidirectional DC / DC converter features a bidirectional bipolar topology, enabling four-quadrant operation. Compared to existing two-quadrant partial power converters, the four-quadrant partial power converter handles less power through its internal DC / DC converter, further enhancing converter performance. In terms of control, it can achieve four operating modes corresponding to the four quadrants, while simultaneously optimizing efficiency and effectively controlling battery charging and discharging.

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

[0039] Figure 1 This is a partial power structure diagram used in this invention;

[0040] Figure 2 This is a block diagram of a four-quadrant power converter system with bidirectional buck-boost capability;

[0041] Figure 3 The main circuit topology of a four-quadrant power converter with bidirectional buck-boost capability is shown.

[0042] Figure 4 The diagram shows a comparison of the percentage of power processed by the two-quadrant and four-quadrant partial power converters, where (a) represents the boost mode, (b) represents the buck mode, and (c) represents the buck-boost mode. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "one side," "one end," and "one side," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0045] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0046] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0047] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0048] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0049] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0050] Please see Figure 1 The present invention provides a bidirectional buck-boost four-quadrant power converter, including a bidirectional DC / DC converter, a DC bus and an energy storage battery. The two positive terminals of the bidirectional DC / DC converter are respectively connected to ground via the DC bus, one negative terminal of the bidirectional DC / DC converter is grounded, and the other negative terminal is connected to ground via the energy storage battery.

[0051] In this invention, the following are selected: Figure 1 The partial power structure shown has the advantage of being able to transmit the full power of the system by only handling a portion of the total system power through the bidirectional DC / DC converter inside the converter. Its advantage also lies in clamping the DC bus voltage, which fluctuates relatively little compared to the battery voltage, to one end of the bidirectional DC / DC converter inside the partial power converter, making the design of the control strategy simpler.

[0052] The four-quadrant operation requirement of the bidirectional buck-boost four-quadrant power converter is that the internal bidirectional DC / DC converter must be bipolar, that is, after inputting a positive voltage, the output voltage is either positive or negative. Figure 1 DC bus voltage V dc Equal to battery voltage V bat Add the internal bidirectional DC / DC terminal voltage V2; when V dc Greater than V bat When V2 is positive, V2 needs to be a positive voltage; when V dc Less than V bat At that time, V2 needs to be a negative voltage.

[0053] Please see Figure 2The bidirectional buck-boost four-quadrant power converter internally includes a bidirectional unipolar LLC resonant circuit in the front stage and a bidirectional bipolar H-bridge circuit in the rear stage. The bidirectional unipolar LLC resonant circuit in the front stage is cascaded with the bidirectional bipolar H-bridge circuit in the rear stage.

[0054] Among them, the bidirectional unipolar LLC resonant circuit plays the role of pre-voltage regulation and realizes soft switching to reduce losses. In terms of control strategy, fixed frequency control is used to make the LLC work at the optimal operating point to optimize efficiency.

[0055] The bidirectional bipolar H-bridge circuit can output positive or negative voltage, and secondary voltage regulation is used to compensate for the voltage difference between the battery and the DC bus. At the same time, it controls the charging and discharging of the battery. The control strategy uses a dual closed-loop PI control with an outer voltage loop and an inner current loop.

[0056] Please see Figure 3 The circuit topology of the bidirectional buck-boost four-quadrant power converter of the present invention specifically includes a bidirectional unipolar LLC resonant circuit and a bidirectional bipolar H-bridge circuit.

[0057] The front-end bidirectional unipolar LLC resonant circuit includes an input filter capacitor C1, a primary-side inverter circuit, a resonant circuit, an isolation transformer T, a secondary-side rectifier circuit, and an output filter capacitor C2.

[0058] One end of the input filter capacitor C1 is connected to the DC bus, and the other end is connected in sequence to the primary inverter circuit, the resonant circuit, the isolation transformer T, the secondary rectifier circuit, and one end of the output filter capacitor C2. The other end of the output filter capacitor C2 is connected to the subsequent bidirectional bipolar H-bridge circuit.

[0059] The primary-side inverter circuit includes four switching transistors S1, S2, S3, and S4. The drains of transistors S1 and S3 are connected to the positive terminal of the DC bus, and the sources of transistors S2 and S4 are connected to the negative terminal of the DC bus. The source of transistor S1 and the drain of transistor S2 are connected in sequence to a resonant capacitor C. r Resonant inductor L r and transformer magnetizing inductance L m At one end, the source of switch S3 and the drain of switch S4 are connected to the transformer magnetizing inductor L. m The other end.

[0060] The resonant circuit includes a resonant inductor L r Resonant capacitor C r and transformer magnetizing inductance L m Resonant capacitor C r One end is connected to the source of switch S1 and the drain of switch S2, and the other end is connected to the resonant inductor L. r Connect the transformer magnetizing inductor L respectively mAnd the primary side of the isolation transformer T, the transformer magnetizing inductance L m The other end is connected to the source of switch S3 and the drain of switch S4.

[0061] The secondary rectifier circuit includes four switching transistors S5, S6, S7, and S8. The drains of switching transistors S5 and S7 are connected to one end of the subsequent bidirectional bipolar H-bridge circuit, and the sources of switching transistors S6 and S8 are connected to the other end of the subsequent bidirectional bipolar H-bridge circuit. The sources of switching transistors S5 and S7 and the drains of switching transistors S6 and S8 are connected to the secondary side of the isolation transformer T.

[0062] For the front-end bidirectional unipolar LLC resonant circuit, the main consideration is the transformer magnetizing inductance L. m Resonant inductor L r Resonant capacitor C r The design.

[0063] Transformer magnetizing inductance L m :

[0064] To achieve zero-voltage turn-on of the switching transistor, the peak current I of the magnetizing inductor... pk The junction capacitance of the switching transistor must be completely discharged within the dead time; for transistors that have been turned off, their junction capacitance C must be discharged simultaneously. oss Charge to the input voltage. The LLC input voltage is V. LLCin The LLC output voltage is V LLCout The system switching frequency is f s The transformer turns ratio is n.

[0065] The peak current I of the magnetizing inductor should be satisfied. pk With dead time T dead The relationship is as follows:

[0066]

[0067] Peak current I of magnetizing inductor pk The calculation is as follows:

[0068]

[0069] Obtain the dead time T dead The constraints are:

[0070]

[0071] The transformer magnetizing inductance L can then be obtained. m The maximum value.

[0072] Resonant inductor L r Resonant capacitor C r :

[0073] The resonant frequency f of LLC r for:

[0074]

[0075] LLC operates at its optimal operating point. Setting the switching frequency equal to the resonant frequency, we obtain:

[0076]

[0077] This leads to the resonant inductance L. r With resonant capacitor C r The specific parameters of the two can be obtained by multiplying them by the actual component selection.

[0078] The subsequent bidirectional bipolar H-bridge circuit includes an inverter circuit, a filter inductor L, and a filter capacitor C3.

[0079] The inverter circuit includes four switching transistors S9, S1, S2, S3, S4, S5, S6, S7, S8, S9 ... 10 S 11 S 12 Switch S9 and S 11 The drain of the transistor is connected to one end of the preceding bidirectional unipolar LLC resonant circuit, and the switching transistor S... 10 and S 12 The drain of the transistor is connected to the other end of the preceding bidirectional unipolar LLC resonant circuit, and the source of the switching transistor S9 is connected to the source of the switching transistor S... 10 The drain of the transistor is connected to one end of the filter inductor L. The other end of the filter inductor L splits into two paths: one path is connected to the positive terminal of the DC bus, and the other path is connected to the positive terminal of the energy storage battery via the filter capacitor C3. The switching transistor S... 11 The source and switch S 12 The drain of the battery is connected to the positive terminal of the energy storage battery, and one end of the positive terminal of the energy storage battery is grounded.

[0080] For the subsequent bidirectional bipolar H-bridge circuit, the design of the filter inductor L and filter capacitor C3 is the main consideration. The calculation of the filter circuit parameters mainly considers two aspects: current ripple and cutoff frequency.

[0081] Filter inductor L:

[0082] For current ripple, the value of the filter inductor under PWM modulation is generally determined by the maximum ripple of the inductor current, and the maximum ripple ΔI of the inductor current is taken as the value. Lmax Battery charging and discharging current I bat 10%, that is:

[0083] ΔI Lmax =10%×I bat

[0084] H-bridge output voltage U Hout With input voltage U HinThe ratio is the equivalent duty cycle D, as detailed below:

[0085]

[0086] Based on the voltage-current relationship of the inductor, the inductor current ripple ΔI is obtained. L The expression:

[0087]

[0088] When U Hout =0.5U Hin At that time, the inductor current ripple ΔI L The maximum, expressed as:

[0089]

[0090] This yields the minimum value of the filter inductance.

[0091] Filter capacitor C3:

[0092] Cutoff frequency f c The calculation formula is:

[0093]

[0094] Under PWM modulation, the output harmonic is the switching frequency f. s For higher harmonics and above, the cutoff frequency f can be chosen. c The minimum value is the lowest output harmonic frequency (switching frequency). Right now:

[0095]

[0096] The minimum value of the filter capacitor can then be obtained.

[0097] This invention discloses a control method for a bidirectional buck-boost four-quadrant power converter, comprising the following steps:

[0098] The bidirectional unipolar LLC resonant circuit uses a fixed-frequency PWM control strategy, which always operates at the optimal operating point, i.e., the resonant point, to achieve the best efficiency of the LLC. The output voltage is adjusted by changing the duty cycle of the switching transistor or changing the turns ratio of the isolation transformer T.

[0099] The bidirectional bipolar H-bridge circuit uses voltage and current dual closed-loop PI control. The outer loop is the voltage loop, which controls the voltage of the H-bridge filter capacitor C3 to track the voltage difference between the DC bus and the battery. The inner loop is the current loop, which controls the current of the H-bridge filter inductor L to track the charging and discharging current of the battery. Its output value is used as a modulation signal to perform PWM control on the switching transistor.

[0100] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0101] Taking the operating conditions of the partial power converter system in Table 1 as an example, we implemented four-quadrant operation of the partial power converter. We recorded the DC bus voltage V at a certain steady-state moment under each of the four operating modes. dc Battery voltage V bat The percentage of power processed by the bidirectional DC / DC converter within the power converter relative to the total system power (K) pr See Table 2.

[0102] Table 1 Operating conditions of some power converters

[0103]

[0104] Table 2 shows the four-quadrant operation results of some power converters.

[0105]

[0106] As can be seen from Table 2, in the battery charging mode where the battery voltage is greater than the DC bus voltage, the total system power is the highest and the operating conditions are the worst when the battery voltage is close to the peak value. However, the power processed by the bidirectional DC / DC converter inside the power converter accounts for only 16.03% of the total system power.

[0107] The efficiency of the partial power converter was calculated under extreme operating conditions of highest battery voltage, battery charging, and maximum system power. At the same time, a full-power converter was designed under the same operating conditions for efficiency comparison. The results are shown in Table 3.

[0108] Table 3 Efficiency Comparison

[0109]

[0110] As shown in Table 3, the efficiency of the bidirectional DC / DC converter inside the partial power converter is only 86.94%. However, since it only handles 16.03% of the total system power, the efficiency of the partial power converter system is as high as 97.91%, while the efficiency of the full power converter under the same operating conditions is only 94.11%. The efficiency advantage of the partial power converter is significant.

[0111] Please see Figure 4 This paper compares the differences between two-quadrant and four-quadrant operation to highlight the advantages of four-quadrant operation. Assuming a voltage regulation range of 30%, some power converters in the four-quadrant operate in buck-boost mode, and their internal bidirectional DC / DC converters can handle a maximum of only 15% of the total system power; while some power converters in the two-quadrant operate in boost or buck mode, and their internal bidirectional DC / DC converters can handle a maximum of 30% of the total system power.

[0112] In summary, this invention provides a bidirectional buck-boost four-quadrant partial power converter and its control method. The converter's internal DC / DC converter handles only a small portion of the total system power, effectively improving the converter's power density, efficiency, and cost. The internal bidirectional DC / DC converter features a bidirectional bipolar topology, enabling four-quadrant operation. Compared to existing two-quadrant partial power converters, the four-quadrant partial power converter handles less power through its internal DC / DC converter, further enhancing converter performance. In terms of control, it can achieve four operating modes corresponding to the four quadrants, while simultaneously optimizing efficiency and effectively controlling battery charging and discharging.

[0113] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A bidirectional buck-boost four-quadrant partial power converter, characterized in that, This includes a bipolar, two-way DC / DC converter. The first and second positive terminals of the two-way DC / DC converter are respectively connected to ground via a DC bus. The first negative terminal of the two-way DC / DC converter is grounded, and the second negative terminal is connected to ground via an energy storage battery. The voltage of the DC bus is equal to the energy storage battery voltage plus the voltage of the second positive terminal of the two-way DC / DC converter. When the voltage of the DC bus is greater than the energy storage battery voltage, the voltage of the second positive terminal of the two-way DC / DC converter is positive; when the voltage of the DC bus is less than the energy storage battery voltage, the voltage of the second positive terminal of the two-way DC / DC converter is negative.

2. The bidirectional buck-boost four-quadrant power converter according to claim 1, characterized in that, The bidirectional DC / DC converter includes a front-end bidirectional unipolar LLC resonant circuit, which is cascaded with a rear-end bidirectional bipolar H-bridge circuit. The bidirectional unipolar LLC resonant circuit is used for pre-voltage regulation and soft switching. The bidirectional bipolar H-bridge circuit is used to output positive or negative voltage, and secondary voltage regulation is used to compensate for the voltage difference between the energy storage battery and the DC bus, while controlling the charging and discharging of the energy storage battery.

3. The bidirectional buck-boost four-quadrant power converter according to claim 2, characterized in that, The bidirectional unipolar LLC resonant circuit uses fixed-frequency control, while the bidirectional bipolar H-bridge circuit uses dual closed-loop PI control with an outer voltage loop and an inner current loop.

4. The bidirectional buck-boost four-quadrant power converter according to claim 2, characterized in that, The bidirectional unipolar LLC resonant circuit includes an input filter capacitor C1, a primary-side inverter circuit, a resonant circuit, an isolation transformer T, a secondary-side rectifier circuit, and an output filter capacitor C2. The two ends of the input filter capacitor C1 are connected to the positive and negative terminals of the DC bus, respectively. The DC side of the primary-side inverter circuit is connected in parallel to the two ends of the input filter capacitor C1. The output terminal of the primary-side inverter circuit is connected to the primary side of the isolation transformer T via the resonant circuit. The secondary side of the isolation transformer T is connected to the secondary-side rectifier circuit. The output filter capacitor C2 is connected in parallel to the output terminal of the secondary-side rectifier circuit. The subsequent bidirectional bipolar H-bridge circuit is connected in parallel to the two ends of the output filter capacitor C2.

5. The bidirectional buck-boost four-quadrant power converter according to claim 4, characterized in that, The primary-side inverter circuit includes switching transistors S1, S2, S3, and S4. The drains of transistors S1 and S3 are connected to the positive terminal of the DC bus, and the sources of transistors S2 and S4 are connected to the negative terminal of the DC bus. The source of transistor S1 and the drain of transistor S2 are connected in sequence to a resonant capacitor. and resonant inductor It then splits into two paths, one of which connects to the transformer's magnetizing inductor. One end is connected to the primary side of the isolation transformer T, and the source of switch S3 and the drain of switch S4 are connected to the transformer's magnetizing inductor. The other end is connected to the primary side of the isolation transformer T; the secondary rectifier circuit includes switching transistors S5, S6, S7 and S8. The drains of switching transistors S5 and S7 are connected to one end of the output filter capacitor C2, and the sources of switching transistors S6 and S8 are connected to the other end of the output filter capacitor C2. The sources of switching transistors S5 and S7 and the drains of switching transistors S6 and S8 are connected to the secondary side of the isolation transformer T.

6. The bidirectional buck-boost four-quadrant partial power converter according to claim 5, characterized in that, peak current of magnetizing inductor With dead time The relationship is as follows: Resonant inductor and resonant capacitor The relationship is as follows: in, The junction capacitance of the switching transistor. For LLC input voltage, This refers to the system switching frequency.

7. The bidirectional buck-boost four-quadrant power converter according to claim 2, characterized in that, The bidirectional bipolar H-bridge circuit includes an inverter circuit. The output of the inverter circuit is divided into two paths. One path is connected to the positive terminal of the DC bus and one end of the filter capacitor C3 via the filter inductor L. The second path is connected to the positive terminal of the energy storage battery via the other end of the filter capacitor C3.

8. The bidirectional buck-boost four-quadrant power converter according to claim 7, characterized in that, The inverter circuit includes switching transistor S9 and switching transistor S2. 10 Switch S 11 and switching transistor S 12 Switch S9 and switch S 11 The drain of the transistor is connected to one end of the preceding bidirectional unipolar LLC resonant circuit, and the switching transistor S... 10 and switching transistor S 12 The drain of the transistor is connected to the other end of the preceding bidirectional unipolar LLC resonant circuit, and the source of the switching transistor S9 is connected to the source of the switching transistor S... 10 The drain of the transistor is connected to one end of the filter inductor L, and the switching transistor S... 11 The source and switch S 12 The drain of the battery is connected to the positive terminal of the energy storage battery, and the negative terminal of the energy storage battery is connected to the ground along with the negative terminal of the DC bus.

9. The bidirectional buck-boost four-quadrant power converter according to claim 8, characterized in that, Inductor current ripple The relationship is as follows: Filter capacitor The relationship is as follows: in, This is the input voltage of the H-bridge. This refers to the system switching frequency.

10. A control method for a bidirectional buck-boost four-quadrant power converter as described in claim 1, characterized in that, The bidirectional unipolar LLC resonant circuit uses a fixed-frequency PWM control strategy and always operates at the resonant point. The output voltage is adjusted by changing the duty cycle of the switching transistor or by changing the turns ratio of the isolation transformer T during parameter design. The bidirectional bipolar H-bridge circuit uses voltage and current dual closed-loop PI control. The outer loop is the voltage loop, which controls the voltage of the H-bridge filter capacitor C3 to track the voltage difference between the DC bus and the battery. The inner loop is the current loop, which controls the current of the H-bridge filter inductor L to track the charging and discharging current of the battery. The output value is used as a modulation signal to perform PWM control on the switching transistor.

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