A Control Method for a Bidirectional Symmetric LLC Resonant Converter

Abstract

The present invention relates to the field of electronic circuits, and discloses a control method for a bidirectional symmetric LLC resonant converter, including the following steps: S1. Determine whether the operating mode of the bidirectional symmetric LLC resonant converter is a voltage gain mode or a power modulation mode. If the operating mode is the power modulation mode, the modulation method of the bidirectional symmetric LLC resonant converter is a fixed-frequency quasi-multiphase shift modulation method; otherwise, jump to S2; S2. Determine whether the operating state of the bidirectional symmetric LLC resonant converter is a high-gain operating state or a low-gain operating state. If the operating state is the high-gain operating state, the modulation method of the bidirectional symmetric LLC resonant converter is a variable-frequency control method; otherwise, the modulation method of the bidirectional symmetric LLC resonant converter is a fixed-frequency quasi-multiphase shift modulation method. The present invention can not only achieve stable operation under a wide input range and a wide load range, but also achieve a power modulation function.

Description

Technical Field

[0001] The present invention relates to the field of electronic circuits, and particularly to a control method for a bidirectional symmetric LLC resonant converter. Background Art

[0002] With the rapid development of technology, people's requirements for the living material conditions are gradually increasing, and the requirements for the reliability, comfort, energy conservation and cleanliness of household energy storage, portable energy storage and in-vehicle storage power supplies are further improved. With the continuous development of energy storage technology, bidirectional DC / DC converters that realize the energy interaction between energy storage systems and DC power grids have been widely studied. People have put forward higher requirements for indicators such as the efficiency, power density, and noise of power supplies, and have increasingly wider requirements for the input voltage range and load modulation range, and stricter control over the output power. Bidirectional symmetric resonant converters have received extensive attention in recent years due to their advantages such as high switching frequency, wide gain range, and consistent forward and reverse operating characteristics.

[0003] The existing control methods for bidirectional symmetric resonant converters are mainly frequency conversion control. When the parameters of the resonant cavity are determined, the gain characteristics of the system are fixed. When the voltage gain is greater than 1, frequency conversion control can obtain a larger voltage gain. When the voltage gain is less than 1, when both the input voltage range and the load range increase, the adjustment range of the switching frequency also needs to increase accordingly. At this time, the frequency conversion control has very limited adjustment of the gain, and the zero-current turn-off ability of the secondary side of the converter is lost at this time, and the output voltage ripple and power supply efficiency are also difficult to meet the requirements. Summary of the Invention

[0004] The present invention provides a control method for a bidirectional symmetric LLC resonant converter, which can not only achieve stable operation under a wide input range and a wide load range, but also achieve a power modulation function.

[0005] The present invention is realized through the following technical solutions:

[0006] A control method for a bidirectional symmetric LLC resonant converter includes the following steps:

[0007] S1. Determine whether the operating mode of the bidirectional symmetric LLC resonant converter is a voltage gain mode or a power modulation mode. If the operating mode is the power modulation mode, the modulation method of the bidirectional symmetric LLC resonant converter is a fixed-frequency quasi-multiphase shift modulation method; otherwise, jump to S2;

[0008] S2. Determine whether the operating state of the bidirectional symmetric LLC resonant converter is a high-gain operating state or a low-gain operating state. If the operating state is a high-gain operating state, the modulation method of the bidirectional symmetric LLC resonant converter is a variable-frequency control method; otherwise, the modulation method of the bidirectional symmetric LLC resonant converter is a fixed-frequency quasi-multiphase shift modulation method.

[0009] As an optimization, in S1, the basis for determining the operating mode of the bidirectional symmetric LLC resonant converter is set by the user according to actual needs.

[0010] As an optimization, the basis for the determination in S2 is: the sampled values of the input voltage, output voltage, and output current.

[0011] As an optimization, in S2, before determining the operating mode of the bidirectional symmetric LLC resonant converter, it is also necessary to determine whether the bidirectional symmetric LLC resonant converter is in a forward operating state or a reverse operating state.

[0012] As an optimization, in the power modulation mode, the bidirectional symmetric LLC resonant converter controls the output by adjusting the magnitude of the inter-bridge phase shift angle θ between the primary side and the secondary side of the bidirectional symmetric LLC resonant converter.

[0013] As an optimization, in the low-gain operating state of the voltage gain mode, the bidirectional symmetric LLC resonant converter controls the output by adjusting the in-bridge phase shift φ p of the bidirectional symmetric LLC resonant converter.

[0014] As an optimization, in the high-gain operating state of the voltage gain mode, the bidirectional symmetric LLC resonant converter controls the output by adjusting the switching frequency of the bidirectional symmetric LLC resonant converter.

[0015] As an optimization, the forward operating state is: the input of the bidirectional symmetric LLC resonant converter is on the primary side and the output is on the secondary side; the reverse operating state is: the input of the bidirectional symmetric LLC resonant converter is on the secondary side and the output is on the primary side.

[0016] As an optimization, the circuit of the bidirectional symmetric LLC resonant converter has a symmetric structure, with high-voltage and low-voltage DC buses at both ends. The two DC buses are respectively connected to two full bridges. Each full bridge is composed of two series-connected switching tubes in parallel, and a wire is led out from the midpoint of each group of series-connected switching tubes to connect to the resonant cavity. The resonant cavity is a two-port network, one port is connected to the midpoint of the two bridge arms on the high-voltage side, and the other port is connected to the midpoint of the two bridge arms on the low-voltage side. There is a transformer in the middle of the resonant cavity, and a high-voltage side inductor L r is connected in series on the high-voltage side of the transformer and a high-voltage side capacitor C r, and finally connected in series to the high-voltage side port, and the high-voltage side inductor L is not provided at the high-voltage side port r and the high-voltage side capacitor C r One end of r is connected to a first inductor L m and a second inductor L c One of the ends of m , the first inductor L m and the second inductor L c The other ends of are respectively connected to the mutually remote ends of the high-voltage side inductor L r and the high-voltage side capacitor C r

[0017] As an optimization, the turns ratio of the transformer is n, and the corresponding parameter relationship is: n is an integer.

[0018] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0019] The present invention proposes a hybrid control strategy that combines a variable-frequency control method and a fixed-frequency quasi-multi-phase shift control method for a bidirectional symmetric LLC resonant converter. When the converter gain is greater than 1, the variable-frequency control method is adopted. When operating in the gain range greater than 1, the variable-frequency control can achieve zero-voltage turn-on of the primary-side switching tube and zero-current turn-off of the secondary-side switching tube, thereby reducing the switching loss and improving the converter operating efficiency. Moreover, in this range, the output voltage of the variable-frequency control is stable and the output ripple is small; when the converter gain is less than 1, the fixed-frequency quasi-multi-phase shift control method is adopted, which can stably achieve the gain modulation with a gain less than 1, has a large adjustment range and at the same time has a gain characteristic independent of the load; in addition, power modulation can be achieved according to the required functions, and the above control methods each have only one control variable in each working state, which is convenient for variable control. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings. In the drawings:

[0021] Figure 1 is a topology diagram of a bidirectional symmetric LLC resonant converter according to the present invention;

[0022] Figure 2 is a flowchart of a control method for a bidirectional symmetric LLC resonant converter according to an embodiment of the present invention;

[0023] Figure 3 ​This is the switching tube drive waveform diagram of the quasi-multiphase shift control method in the embodiment of the present invention at a fixed frequency. Specific embodiments

[0024] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments and drawings. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0025] Embodiment

[0026] In the embodiment of the present invention, a control method under a bidirectional symmetric LLC loop divides the working modes and states of the power supply according to the real-time input voltage, output current and voltage sampling results, and then selects the corresponding control method according to the working state. After selecting the corresponding control method, the system's own loop control method (PI, 2P2Z or other higher-order negative feedback control methods) is used to change the frequency and three phase angles of the real-time drive signal to adjust the gain and power of the system. When operating on the frequency and phase angle of the drive signal, the frequency and phase angle of the drive signal should be slowly and smoothly changed according to a fixed step size to achieve a stable transition effect, so that stable operation can be achieved within the full voltage range and full load range. It can not only achieve stable operation under a wide input range and wide load range, but also achieve the power modulation function, which specifically includes the following steps:

[0027] First, the user inputs and selects the working mode, that is, the voltage gain mode and the power modulation mode, and turns on the sampling module. According to the real-time input voltage, output current and voltage sampling results, the working state of the converter is determined, and it is judged whether the working state of the DC converter is the forward working state or the reverse working state; the forward working state means input from the primary side and output from the secondary side; the reverse working state means input from the secondary side and output from the primary side;

[0028] Furthermore, when the bidirectional symmetric LLC resonant converter operates in the voltage gain mode, it is divided into a high-gain working state and a low-gain working state according to the sampled values of the input voltage, output voltage and output current, and the corresponding modulation method is selected according to the working state. In the high-gain working state, a variable-frequency control method is adopted, and in the low-gain working state, a quasi-multiphase shift modulation method with a fixed frequency is adopted; when operating in the power modulation mode, the modulation method is the same as that in the low-gain mode, and a quasi-multiphase shift modulation method with a fixed frequency is adopted, but the specific phase angle variable to be adjusted is different.

[0029] 1. Voltage gain mode

[0030] The input voltage, output voltage, and output current of a bidirectional symmetric LLC resonant converter are sampled in real time. According to the sampled values of the input voltage, output voltage, and output current, it is divided into a high-gain operating state (gain greater than 1) and a low-gain operating state (gain less than 1), and the corresponding loop control method is selected according to the operating state of the converter.

[0031] High-gain loop control. In the high-gain operating state, a variable-frequency control method is adopted. The LLC loop control changes the frequency of the real-time drive signal calculated by the feedback loop to control the system gain. Under variable-frequency modulation, the gain characteristic of the converter can be expressed as:

[0032]

[0033] In the formula, M represents the voltage gain, f n represents the normalized switching frequency, m represents the ratio of the magnetizing inductance to the resonant inductance, and Q represents the quality factor. At this time, the operating frequency of the converter is between the low-impedance resonant frequency and the high-impedance resonant frequency . Within this range, the primary side of the converter can achieve zero-voltage turn-on of the primary-side switch tube and zero-current turn-off of the secondary-side switch tube, which is the best switching frequency range to ensure efficiency under the variable-frequency control method of LLC resonant converters.

[0034] Low-gain loop control. In the low-gain operating state, a quasi-multiphase shift modulation method with a fixed frequency is adopted. At this time, the converter operates at a fixed low-impedance resonant frequency f r , and at the same time, the phase shift θ between the primary-side bridge and the secondary-side bridge is set to 0. At this time, the gain characteristic of the converter can be expressed as:

[0035]

[0036] In the formula, φ p represents the phase shift within the primary-side bridge arm, and φ s represents the phase shift within the secondary-side bridge arm. By adjusting φ p and φ s to control the voltage gain, the converter can operate stably. And from Equation (2), it can be seen that the voltage gain range is independent of the load at this time, and is only related to the phase shift φ p and φ s , and voltage modulation over a wide load range can be achieved.

[0037] 2. Power modulation mode

[0038] In the power modulation mode, the same control method as in the low-gain operating state is adopted. At this time, the converter operates at a fixed frequency of f s = 1.75f r , and the phase shift φ p within the bridge arm is fixed at φs = π, and the output power of the converter is adjusted by adjusting the inter-bridge phase shift θ between the primary side and the secondary side.

[0039] Therefore, the operating states of the bidirectional symmetric LLC resonant converter are divided into six states, which are respectively:

[0040] (1) Forward high-voltage input low-gain mode;

[0041] (2) Forward low-voltage input high-gain mode;

[0042] (3) Reverse high-voltage input low-gain mode;

[0043] (4) Reverse low-voltage input high-gain mode;

[0044] (5) Forward power modulation mode;

[0045] (6) Reverse power modulation mode.

[0046] When the bidirectional symmetric LLC resonant converter operates in the forward (reverse) high-voltage input low-gain mode, the converter control loop selects a fixed-frequency quasi-multiphase shift control method, and controls the output by adjusting the phase shift φ p (φ s ) When operating in the forward (reverse) low-voltage input high-gain mode, the converter control loop selects a variable-frequency control method and controls the output by adjusting the switching frequency; when operating in the forward (reverse) power modulation mode, the converter control loop selects a fixed-frequency quasi-multiphase shift control method, and the converter controls the output by adjusting the magnitude of the inter-bridge phase shift angle θ between the primary side and the secondary side. Through such a hybrid control method, the converter can not only achieve stable operation under a wide input range and a wide load range, but also achieve the power modulation function.

[0047] The topological structure of the bidirectional symmetric LLC resonant converter is as Figure 1 shown. The circuit of the bidirectional symmetric LLC resonance transformation is of a symmetric structure, with high-voltage and low-voltage DC buses at both ends. The two DC buses are respectively connected to two full bridges. Each full bridge is composed of two series-connected switching tubes in parallel, and a wire is led out from the midpoint of each group of series-connected switching tubes to connect to the resonant cavity; the resonant cavity is a two-port network, one port is connected to the midpoint of the two bridge arms on the high-voltage side, and the other port is connected to the midpoint of the two bridge arms on the low-voltage side; there is a transformer in the middle of the resonant cavity, and a high-voltage side inductor L r and a high-voltage side capacitor C r are connected in series on the high-voltage side of the transformer, and finally connected in series to the high-voltage side port. And one end of the high-voltage side inductor L r and the high-voltage side capacitor C r is connected to a first inductor L m and a second inductor Lc One end of the first inductor L m and the second inductor L c The other ends are respectively connected to the ends of the high-voltage side inductor L r and the high-voltage side capacitor C r which are far away from each other.

[0048] The control implementation flowchart is as Figure 2 shown.

[0049] 1. The user inputs to select the working mode, namely the voltage gain mode and the power modulation mode;

[0050] 2. Turn on the sampling module, and determine the working state of the converter according to the real-time input voltage, output current and voltage sampling results, which is used as the basis for determining the control method;

[0051] 3. When the converter works in the forward high-voltage input low-gain mode, the converter control loop selects the fixed-frequency quasi-multiphase shift control method. At this time, the fixed switching frequency f s = f r , the inter-bridge phase shift θ = 0, the intra-arm phase shift 0 ≤ φ p ≤ π, φ s = π. The converter samples the output voltage on the secondary side and inputs the sampled value and the reference voltage into the comparator at the same time. The output end of the comparator is connected to a PI regulator. After passing through the PI regulator, the phase shift controller determines the size of the intra-arm phase shift angle φ p on the primary side and outputs the corresponding drive signal to control the conduction state of the switching tube, so that the converter works stably in the forward high-voltage input low-gain mode;

[0052] 4. When the converter works in the forward low-voltage input high-gain mode, the converter control loop selects the variable-frequency control method. The converter samples the output voltage on the secondary side and inputs the sampled value and the reference voltage into the comparator at the same time. The output end of the comparator is connected to a PI regulator. After passing through the PI regulator, the voltage-controlled oscillator outputs the corresponding real-time variable-frequency drive signal, and uses this signal to drive the switching tube of the converter to work so that the converter works stably in the forward low-voltage input high-gain mode;

[0053] 5. When the converter works in the reverse high-voltage input low-gain mode, the converter control loop selects the fixed-frequency quasi-multiphase shift control method. At this time, the fixed switching frequency f s = f r , the inter-bridge phase shift θ = 0, the intra-arm phase shift 0 ≤ φ s ≤ π, φ p = π. The converter samples the output voltage on the primary side and inputs the sampled value and the reference voltage into the comparator together. The output end of the comparator is connected to a PI regulator. After passing through the PI regulator, the phase shift controller determines the intra-arm phase shift angle φ of the primary sides and output corresponding drive signals to control the conduction status of the switching tubes, so that the converter operates stably in the reverse high-voltage input low-gain mode.

[0054] 6. When the converter operates in the reverse low-voltage input high-gain mode, the converter control loop selects the variable-frequency control method. The converter samples the output voltage on the primary side and inputs the sampled value and the reference voltage into the comparator at the same time. The output terminal of the comparator is connected to a PI regulator. After passing through the PI regulator, it outputs corresponding real-time variable-frequency drive signals through a voltage-controlled oscillator, and uses this signal to drive the switching tubes of the converter to work so that the converter operates stably in the reverse low-voltage input high-gain mode;

[0055] 7. When the converter operates in the forward power modulation mode, the converter control loop selects the fixed-frequency quasi-multiphase shift control method. At this time, the converter operates at a fixed frequency of f s = 1.75f r . The internal phase shift φ of the fixed bridge arm p = φ s = π. By adjusting the inter-bridge phase shift θ between the primary side and the secondary side, the output power of the converter is adjusted. At this time the converter samples the output voltage on the secondary side and inputs the sampled value and the reference voltage into the comparator at the same time. The output terminal of the comparator is connected to a PI regulator. After passing through the PI regulator, it determines the magnitude of the inter-bridge phase shift angle θ between the primary side and the secondary side through a phase shift controller and outputs corresponding drive signals to control the conduction status of the switching tubes, so that the converter operates stably in the forward power modulation mode;

[0056] 8. When the converter operates in the reverse power modulation mode, the converter control loop selects the fixed-frequency quasi-multiphase shift control method. At this time, the converter operates at a fixed frequency of z. The internal phase shift φ of the fixed bridge arm p = φ s = π. By adjusting the inter-bridge phase shift θ between the primary side and the secondary side, the output power of the converter is adjusted. At this time the converter samples the output voltage on the primary side and inputs the sampled value and the reference voltage into the comparator at the same time. The output terminal of the comparator is connected to a PI regulator. After passing through the PI regulator, it determines the magnitude of the inter-bridge phase shift angle θ between the primary side and the secondary side through a phase shift controller and outputs corresponding drive signals to control the conduction status of the switching tubes, so that the converter operates stably in the reverse power modulation mode. The drive waveform diagram of the switching tubes in the fixed-frequency quasi-multiphase shift control method is as shown in Figure 3 . Table 1 is the operation mode list of the present invention.

[0057] Table 1

[0058]

[0059]

[0060] The specific embodiments described above further elaborate on the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A control method for a bidirectional symmetric LLC resonant converter, characterized in that, It includes the following steps: S1. Determine whether the operating mode of the bidirectional symmetric LLC resonant converter is the voltage gain mode or the power modulation mode. If the operating mode is the power modulation mode, the modulation method of the bidirectional symmetric LLC resonant converter is the fixed-frequency quasi-multiphase shift modulation method; otherwise, jump to S2; S2. Determine whether the operating state of the bidirectional symmetric LLC resonant converter is the high-gain operating state or the low-gain operating state. If the operating state is the high-gain operating state, the modulation method of the bidirectional symmetric LLC resonant converter is the variable-frequency control method; otherwise, the modulation method of the bidirectional symmetric LLC resonant converter is the fixed-frequency quasi-multiphase shift modulation method; The circuit of the bidirectional symmetric LLC resonant conversion has a symmetric structure, with the high-voltage and low-voltage DC buses at both ends. The two DC buses are respectively connected to two full bridges. Each full bridge is composed of two series-connected switching tubes in parallel, and a wire is led out from the midpoint of each group of series-connected switching tubes to connect to the resonant cavity. The resonant cavity is a two-port network, one port is connected to the midpoint of the two bridge arms on the high-voltage side, and the other port is connected to the midpoint of the two bridge arms on the low-voltage side. In the middle of the resonant cavity is a transformer, and a high-voltage side inductor L r and a high-voltage side capacitor C r are finally connected in series to the high-voltage side port, and the high-voltage side inductor L r and the high-voltage side capacitor C r are not provided at the high-voltage side port. One end of the high-voltage side capacitor C m is connected to one end of a first inductor L c and a second inductor L m The other ends of the first inductor L c and the second inductor L r are respectively connected to the ends of the high-voltage side inductor L r and the high-voltage side capacitor C that are far away from each other. The turns ratio of the transformer is n, and the corresponding parameter relationship is: n is an integer, where V1 and V2 are the high-voltage DC bus voltage and the low-voltage DC bus voltage respectively.

2. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, In S1, the basis for determining the operating mode of the bidirectional symmetric LLC resonant converter is set by the user according to actual needs.

3. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, The basis for the judgment in S2 is: the sampled values of the input voltage, output voltage, and output current.

4. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, In S2, before judging the operating mode of the bidirectional symmetric LLC resonant converter, it is also necessary to judge whether the bidirectional symmetric LLC resonant converter is in the forward operating state or the reverse operating state.

5. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, In the power modulation mode, the bidirectional symmetric LLC resonant converter controls the output by adjusting the magnitude of the inter-bridge phase shift angle θ between the primary side and the secondary side of the bidirectional symmetric LLC resonant converter.

6. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, When the bidirectional symmetric LLC resonant converter is in the low-gain operating state of the voltage gain mode, the output is controlled by adjusting the in-phase shift φ within the bridge arm of the bidirectional symmetric LLC resonant converter. p Control the output.

7. The control method for a bidirectional symmetric LLC resonant converter according to claim 1, characterized in that, In the high-gain operating state of the voltage gain mode, the bidirectional symmetric LLC resonant converter controls the output by adjusting the switching frequency of the bidirectional symmetric LLC resonant converter.

8. The control method for a bidirectional symmetric LLC resonant converter according to claim 4, characterized in that, The forward operating state is: the input of the bidirectional symmetric LLC resonant converter is on the primary side and the output is on the secondary side; the reverse operating state is: the input of the bidirectional symmetric LLC resonant converter is on the secondary side and the output is on the primary side.

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