Direct-current bias suppression method for phase-shifted full-bridge converter

Abstract

The invention discloses a direct current bias suppression method for a phase-shifted full-bridge converter, the direct current bias suppression method is realized by adopting a single closed-loop control loop, the single closed-loop control loop comprises a virtual resistance compensation-based direct current bias suppression strategy, and the virtual resistance compensation-based direct current bias suppression strategy comprises a virtual resistance compensation link; in the virtual resistance compensation link, the size of the virtual resistance can be adaptively adjusted according to the direct-current bias condition, and the direct-current bias suppression speed is effectively increased; according to the method, an additional hardware circuit is not needed, and the equivalent direct current bias can be calculated according to the converter inductive current sampled in each switching period.

Description

Technical Field

[0001] The invention belongs to the technical field of converters, and in particular relates to a method for suppressing direct current bias of a phase-shifted full-bridge converter. Background Art

[0002] The phase-shifted full-bridge converter is a typical power electronic conversion circuit widely used in high-power conversion, power supply, motor drive, and other fields. A phase-shifted full-bridge converter controls output power by adjusting the phase of the input signal. It consists of four switching elements located at the four corners of the full-bridge circuit. By controlling the on / off switching of these elements, the output voltage can be frequency- and phase-adjusted. A dual-active full-bridge DC / DC converter is a type of phase-shifted full-bridge converter. Featuring high power density and control flexibility, dual-active full-bridge DC / DC converters have become key components for bidirectional power transmission and are widely used in DC microgrids, electric vehicles, and battery energy storage. System parameter changes can increase the DC offset of the converter's inductor current, leading to increased current stress and even transformer core saturation. Good transient immunity is essential for safe and reliable converter operation, but traditional phase-shifted control methods cannot address this DC offset issue. Therefore, to improve system transient performance, suppressing the DC offset of dual-active full-bridge DC / DC converters has become a research priority.

[0003] Currently, in the DC bias suppression technology of dual-active full-bridge DC / DC converters, some scholars adjust the control parameters to suppress DC bias by analyzing the changes in phase shift. However, this method is not applicable to changes in input / output voltage because the system default parameters are constant during the derivation process. In addition, the control parameters can be calculated in real time according to the DC bias situation, but the calculation process requires precise system parameters and additional components or circuits. Increasing the resistance of the AC loop can suppress transient DC bias, but it will increase system losses. Summary of the Invention

[0004] The object of the present invention is to provide a method for suppressing DC bias of a phase-shifted full-bridge converter, which can effectively suppress DC bias without requiring additional components or circuits and without increasing system losses.

[0005] To achieve the above objectives, the present invention provides a method for suppressing DC bias in a phase-shifted full-bridge converter. The DC bias suppression method is implemented using a single closed-loop control circuit. The single closed-loop control circuit includes a DC bias suppression strategy based on virtual resistance compensation. The DC bias suppression strategy based on virtual resistance compensation includes a virtual resistance compensation link. Specifically, the method includes the following steps:

[0006] Step 1: The converter collects electrical quantities once per switching cycle and calculates the equivalent DC bias;

[0007] Step 2: Adjust the virtual resistance value according to the severity of the DC bias in step 1. The more severe the bias, the larger the virtual resistance.

[0008] Step 3: Calculate the phase compensation amount based on the virtual resistance compensation link to simulate the real resistance compensation. Use phase compensation to suppress DC bias and improve the transient characteristics of the system.

[0009] As a further solution of the present invention: in step 1, the converter is a dual-active DC / DC converter, when the system parameter switching frequency f of the dual-active DC / DC converter is s When the phase shift ratio d changes, the inductor current i P Will generate a DC bias i Pdc ; Assume that the inductor current i P At the beginning of the switching cycle, there is an equivalent DC bias i Pdc , then the DC bias i Pdc The inductor current value i can be sampled at the initial moment P (0) and system parameter calculation, namely:

[0010]

[0011] Where U in is the input voltage, k=NU in / U o , U o is the output voltage, N is the high-frequency transformer ratio, and L is the inductance.

[0012] As a further solution of the present invention: the equivalent DC bias in step one is to calculate its absolute value, and the virtual resistance value in step two is obtained through the proportional compensation link. The larger the absolute value, the more severe the bias, and the corresponding larger the virtual resistance, the faster the DC bias suppression speed.

[0013] As a further solution of the present invention: the virtual resistance compensation link includes the following steps:

[0014] Step 3.1: Establish the dual active full-bridge DC / DC converter virtual resistance compensation (AC loop resistance R L =0) and real resistance compensation (AC circuit resistance R L ≠0) under transient mathematical model;

[0015] Step 3.2: Derive the small signal linear model of the dual-active full-bridge DC / DC converter before compensation and under true resistance compensation. Calculate the corresponding converter link transfer function based on the control target. The converter transfer functions under output voltage control are:

[0016]

[0017] Where, To compensate the pre-converter transfer function, The transfer function of the downconverter is compensated for real resistance, D and I Pdc are the phase shift steady-state point and DC bias steady-state point respectively, C is the output side capacitance, s is the s-domain parameter of the transfer function, and R is the load resistance.

[0018] Step 3.3: Calculate the transfer function of the virtual resistance compensation link of the system so that the overall transfer function of the control system is equivalent to the system transfer function under real resistance compensation. The transfer function of the virtual resistance compensation link is:

[0019]

[0020] Compared with the existing technology, the present invention uses a control loop to enable the phase-shifted full-bridge converter to calculate the phase shift ratio compensation in real time under disturbance, thereby simulating the converter characteristics of the real resistance compensation, and realizing the suppression of DC bias current without increasing additional losses. The implementation method is simple; the virtual resistance compensation link can adaptively adjust the virtual resistance size according to the DC bias situation, effectively improving the DC bias suppression speed; the present invention does not require additional hardware circuits, and the equivalent DC bias can be calculated based on the converter inductor current sampled in each switching cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the circuit topology diagram of the dual active full-bridge DC / DC converter.

[0022] Figure 2 This is the transient operation waveform of the dual active full-bridge DC / DC converter.

[0023] Figure 3 This is the converter control block diagram for real and virtual resistance compensation.

[0024] Figure 4 This is a system block diagram of the DC bias suppression strategy based on virtual resistance compensation.

[0025] Figure 5 This is the simulation waveform of input voltage disturbance under traditional single closed-loop control.

[0026] Figure 6 This is a waveform diagram of the input voltage disturbance simulation using the DC bias suppression method of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to the accompanying drawings.

[0028] A method for suppressing DC bias of a phase-shifted full-bridge converter is implemented using a single closed-loop control circuit. The single closed-loop control circuit includes a DC bias suppression strategy based on virtual resistance compensation. The DC bias suppression strategy based on virtual resistance compensation includes a virtual resistance compensation link. Specifically, the method includes the following steps:

[0029] Step 1: The converter collects electrical quantities once per switching cycle and calculates the equivalent DC bias;

[0030] Step 2: Adjust the virtual resistance value according to the severity of the DC bias in step 1. The more severe the bias, the larger the virtual resistance.

[0031] Step 3: Calculate the phase shift compensation amount based on the virtual resistance compensation link to simulate the real resistance compensation, and suppress the DC bias through phase shift compensation.

[0032] The method of the present invention is specifically described below using a dual active full-bridge DC / DC converter as an example.

[0033] like Figure 1 As shown in Figure 1, the dual active full-bridge DC / DC converter consists of a primary side full bridge H1, an inductor L, a high-frequency transformer with a ratio of N:1, a secondary side full bridge H2, and an output side capacitor C. Among them, the inductor L is a switching cycle T s The key component for charging, discharging and transmitting electric energy, R L The resistance of the AC circuit is zero under ideal conditions. The primary-side full-bridge H1 consists of four switches S1–S4 and their antiparallel diodes D1–D4, while the secondary-side full-bridge H2 consists of four switches Q1–Q4 and their antiparallel diodes M1–M4.

[0034] Taking the single phase shift modulation of the dual active full bridge DC / DC converter as an example, the specific step 1 is: the transient operation waveform of the dual active full bridge DC / DC converter is as follows: Figure 2 As shown. Among them, each switch tube has a switching period of T s , the duty cycle is 50% signal drive, the phase shift time of the primary and secondary side full bridge is dT s , where d is the phase shift ratio; when system parameters such as switching frequency f s When the phase shift ratio d changes, the inductor current i P Will produce a DC bias; assuming the inductor current i P At the beginning of the switching cycle, there is an equivalent DC bias i Pdc , then i Pdc The inductor current value i can be sampled at the initial moment P (0) and system parameter calculation, namely:

[0035]

[0036] In the formula, k is defined as NUin / U o , U in is the input voltage, U o is the output voltage, N is the high-frequency transformer ratio, and L is the inductance.

[0037] like Figure 4 As shown in the dashed box below, taking the single phase shift (SPS) modulation of the dual active full-bridge DC / DC converter as an example, the DC bias suppression strategy based on virtual resistance compensation adopts a single closed-loop control loop, which includes: the output voltage reference value U of the dual active full-bridge DC / DC converter ref The actual output voltage u of the dual active full-bridge DC / DC converter o After a PI link calculation, the virtual resistance R L The phase shift compensation amount calculated by the compensation link is added to the output of the PI link to obtain the phase shift control amount d; the inductor current i P The DC bias i is obtained by calculation Pdc , DC bias i Pdc After taking the absolute value Abs, it passes through the proportional control P, that is, the proportional compensation link, and enters the virtual resistance R L 'compensation link; finally, the shift phase control quantity d enters the SPS modulation to control the dual active full-bridge DC / DC converter.

[0038] Step 2 calculates the absolute value of the equivalent DC bias in step 1, and obtains the virtual resistance value through the proportional compensation link. The larger the absolute value, the more severe the bias, and the larger the corresponding virtual resistance, the faster the DC bias suppression speed.

[0039] The derivation process of the virtual resistance compensation link in step 3 is as follows:

[0040] Step 3.1: Establish the dual active full-bridge DC / DC converter before compensation (AC loop resistance R L =0) and real resistance compensation (AC circuit resistance R L ≠0) under transient mathematical model;

[0041] Step 3.2: Derive the small signal linear model of the dual-active full-bridge DC / DC converter before compensation and under true resistance compensation. Calculate the corresponding converter link transfer function based on the control target. The converter transfer functions under output voltage control are:

[0042]

[0043] Where, To compensate the pre-converter transfer function, The transfer function of the downconverter is compensated for real resistance, D and I Pdcare the phase shift steady-state point and DC bias steady-state point respectively, C is the output side capacitance, s is the s-domain parameter of the transfer function, and R is the load resistance.

[0044] Step 3.3: Calculate the transfer function of the virtual resistance compensation link of the system so that the overall transfer function of the control system is equivalent to the system transfer function under real resistance compensation. The transfer function of the virtual resistance compensation link is:

[0045]

[0046] Firstly, the circuit characteristics of the dual active full-bridge DC / DC converter before the compensation resistor are analyzed.

[0047] The AC circuit resistance of the converter before compensation can be approximated to an ideal state, that is, the AC circuit resistance R L = 0, the converter's inductor current i p It can be expressed as:

[0048]

[0049] It can be seen that after a switching cycle T s After that, the DC bias of the inductor current i Pdc remains unchanged, resulting in a DC bias i Pdc The reason for no attenuation is that the inductor cannot consume the energy corresponding to the DC bias during storage and release. In practice, the DC bias will slowly decay due to the presence of parasitic resistance. When the inductor current has a DC bias, the pulsation period of the converter output voltage or current is different from that in normal operation without a DC bias, and is T s , so the periodic expression of the output current i2 is:

[0050]

[0051] Next, the circuit characteristics of the compensation resistor of the dual active full-bridge DC / DC converter are analyzed.

[0052] according to Figure 2 , compared with the compensation resistor before, the compensation resistor (AC circuit resistance R L ≠0) when the DC bias of the converter i Pdc Can be effectively suppressed because the compensation resistor in the AC circuit can consume the energy corresponding to the DC bias and cause it to decay rapidly. L = 0, the transient DC component will decay exponentially with time. Therefore, the three-element method is used to derive the AC circuit resistance R L The inductor current expression when ≠0 is:

[0053]

[0054] Where, τ = L / R L is the decay time constant.

[0055] As can be seen, increasing the resistance of the converter's AC circuit can effectively suppress and eliminate DC offsets caused by system parameter changes. However, this increased resistance also results in additional losses, which reduces system efficiency. Therefore, the present invention introduces a virtual resistance link in the AC circuit to suppress transient DC offsets. Since no actual resistance is added, this also does not increase converter losses.

[0056] Finally, the control loop of the DC bias suppression method based on virtual resistance compensation is designed.

[0057] When the AC circuit resistance R L = 0, the average output current ignoring the ripple can be deduced to be

[0058]

[0059] According to the above formula, the DC bias of the converter does not affect the average output current when there is no virtual resistance compensation. Its average model consists of a nonlinear differential equation system, which can be linearized by constructing a small signal model. Assume that the state variable x can be regarded as a steady-state value X and a small signal state The superposition of Then, after ignoring the high-order terms, the linear small signal model can be extracted as

[0060]

[0061] Therefore, the AC circuit resistance R L = 0, the transfer function of the converter control output can be derived as

[0062]

[0063] When the AC circuit resistance R L ≠0, the average output current can be derived as

[0064]

[0065] Similarly, under real resistance compensation (AC circuit resistance R L ≠0) The transfer function of the converter control output can be derived as

[0066]

[0067] like Figure 3 As shown, the transfer function of the dual active full-bridge DC / DC converter with true resistance compensation is It can be regarded as the parallel connection of two links, which is: the converter transfer function before compensation and parallel links Considering that real resistance compensation can suppress DC bias but will lead to high loss, the present invention compensates a virtual resistance link in the controller loop. Make it equivalent to a real resistor. Assume Then the transfer function of the virtual resistance link can be approximated as

[0068]

[0069] Where, the virtual resistance R L 'It can be adjusted adaptively according to the severity of DC bias. The system block diagram is as follows Figure 4 shown.

[0070] The input voltage disturbance simulation waveforms under traditional single closed-loop control and the DC bias suppression method of the present invention are shown as follows: Figure 5 Figure 6 As shown in the figure, in simulation experiments, input voltage disturbances were introduced to better observe transient DC bias. When the input voltage suddenly increases, the inductor current generates a positive DC bias due to the shift phase adjustment. Compared with traditional control methods, the DC bias of the strategy described in this invention is significantly suppressed and has a higher rate of decline.

Claims

1. A method for suppressing DC bias of a phase-shifted full-bridge converter, characterized in that: The DC bias suppression method is implemented by using a single closed-loop control loop, wherein the single closed-loop control loop includes a DC bias suppression strategy based on virtual resistance compensation, and the DC bias suppression strategy based on virtual resistance compensation includes a virtual resistance compensation link; and the following steps are included: Step 1: The converter collects electrical quantities once in each switching cycle and calculates the equivalent DC bias; Step 2: Adjust the virtual resistance value according to the severity of the DC bias in step 1. The more severe the bias, the larger the virtual resistance. Step 3: Calculate the phase shift compensation amount based on the virtual resistance compensation link to simulate the real resistance compensation, and suppress the DC bias through phase shift compensation.

2. A method for suppressing DC bias of a phase-shifted full-bridge converter according to claim 1, characterized in that: In the step 1, the converter is a dual-active DC / DC converter. When the system parameter switching frequency f of the dual-active DC / DC converter is s When the phase shift ratio d changes, the inductor current i P Will generate a DC bias i Pdc ; Assume that the inductor current i P At the beginning of the switching cycle, there is an equivalent DC bias i Pdc , then the DC bias i Pdc The inductor current value i can be sampled at the initial moment P (0) and system parameter calculation, namely: Where U in is the input voltage, k = NU in / U o , U o is the output voltage, N is the high frequency transformer ratio, and L is the inductance.

3. A method for suppressing DC bias of a phase-shifted full-bridge converter according to claim 1, characterized in that: The equivalent DC bias in step one is to calculate its absolute value, and the virtual resistance value in step two is obtained through the proportional compensation link. The larger the absolute value, the more serious the bias, and the larger the corresponding virtual resistance, the faster the DC bias suppression speed.

4. A method for suppressing DC bias of a phase-shifted full-bridge converter according to claim 1, characterized in that: The virtual resistance compensation link includes the following steps: Step 3.1: Establish the dual active full-bridge DC / DC converter virtual resistance compensation (AC loop resistance R L =0) and real resistance compensation (AC circuit resistance R L ≠0) under transient mathematical model; Step 3.2: Derive the small signal linear model of the dual active full-bridge DC / DC converter before compensation and under real resistance compensation, and find the corresponding converter link transfer function according to the control target. The converter transfer functions under output voltage control are: In the formula, To compensate the pre-converter transfer function, The transfer function of the real resistor-compensated downconverter, D and I Pdc are the phase shift steady-state point and DC bias steady-state point respectively, C is the output side capacitance, s is the s-domain parameter of the transfer function, and R is the load resistance; Step 3.3: Calculate the transfer function of the virtual resistance compensation link of the system so that the overall transfer function of the control system is equivalent to the system transfer function under real resistance compensation. The transfer function of the virtual resistance compensation link is:

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