A method for suppressing DC bias of phase-shifted full-bridge converter

Abstract

The present invention discloses a method for suppressing a DC bias of a phase-shifted full-bridge converter. The DC bias suppression method is implemented by 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. The virtual resistance compensation link can adaptively adjust the size of the virtual resistance according to the DC bias condition, thereby effectively improving the DC bias suppression speed. The present invention does not require an additional hardware circuit, and can calculate the equivalent DC bias according to the converter inductor current sampled in each switching cycle.

Description

Technical Field

[0001] The invention belongs to the technical field of converters, and in particular is 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, which is widely used in high-power conversion, power supply, motor drive and other fields. The phase-shifted full-bridge converter controls the output power by adjusting the phase of the input signal. The full-bridge circuit is composed of four switching elements. These switching elements are located at the four corners of the full-bridge circuit. By controlling their conduction and disconnection, the frequency conversion and phase adjustment of the output voltage can be achieved. The dual-active full-bridge DC / DC converter is a type of phase-shifted full-bridge converter. The dual-active full-bridge DC / DC converter has the characteristics of high power density and high control flexibility. It has become a key component for bidirectional power transmission and is widely used in DC microgrids, electric vehicles, battery energy storage and other fields. Changes in system parameters will increase the DC bias of the converter inductor current, thereby increasing current stress and even saturating the transformer core. Good anti-disturbance transient performance is the basis for safe and reliable operation of the converter, but the traditional phase-shift control method cannot solve the DC bias problem. Therefore, in order to improve the transient performance of the system, how to suppress the DC bias of the dual-active full-bridge DC / DC converter has become the focus of current research.

[0003] At present, in the DC bias suppression technology of dual-active full-bridge DC / DC converters, some scholars adjust the control parameters to suppress the DC bias by analyzing the changes in phase shift. However, this method is not suitable for changes in input / output voltage because the system defaults to constant parameters during the process. In addition, the control parameters can be calculated in real time according to the DC bias conditions, but accurate system parameters are required during the calculation process, and additional components or circuits are required. 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 and does not require additional components or circuits and will not increase system losses.

[0005] To achieve the above object, the present invention provides a method for suppressing DC bias of a phase-shifted full-bridge converter. The DC bias suppression method is implemented by using a single closed-loop control loop. The single closed-loop control loop 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 in each 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 shift compensation amount based on the virtual resistance compensation link to simulate the real resistance compensation, suppress the DC bias through phase shift compensation, and improve the transient characteristics of the system.

[0009] As a further solution of the present invention: 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:

[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 larger the corresponding 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 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:

[0016]

[0017] 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.

[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 prior art, 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 It is the circuit topology diagram of 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 It is the control block diagram of the converter with real and virtual resistance compensation.

[0024] Figure 4 It 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 It is a simulation waveform diagram of input voltage disturbance using the DC bias suppression method of the present invention. DETAILED DESCRIPTION

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

[0028] A method for suppressing a DC bias of a phase-shifted full-bridge converter is implemented by using a single closed-loop control loop. The single closed-loop control loop 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 following steps are included:

[0029] Step 1: The converter collects electrical quantities once in each 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 by taking a dual active full-bridge DC / DC converter as an example.

[0033] like Figure 1 As shown in FIG. 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 transformation 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 and discharging electric energy, R L The resistance of the AC circuit is zero under ideal conditions. The primary side full bridge H1 is composed of four switch tubes S1-S4 and their anti-parallel diodes D1-D4, and the secondary side full bridge H2 is composed of four switch tubes Q1-Q4 and their anti-parallel diodes M1-M4.

[0034] Taking the single phase shift modulation of the dual active full bridge DC / DC converter as an example, step 1 is as follows: The transient operation waveform of the dual active full bridge DC / DC converter is as follows: Figure 2 As shown. 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 generate 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 calculation, the virtual resistance R L The compensation link calculates the phase shift compensation amount and adds it to the PI link output to get the phase shift control amount d; the inductor current i P After calculation, the DC bias i Pdc , DC bias i Pdc After taking the absolute value Abs, it passes through the proportional control P, i.e. the proportional compensation link, and enters the virtual resistor 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 serious the bias. 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 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:

[0042]

[0043] In the formula, To compensate the pre-converter transfer function, The transfer function of the real resistor-compensated downconverter, 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 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 why it does not decay is that the inductor cannot consume the energy corresponding to the DC bias during storage and release. In actual situations, the DC bias decays slowly 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, which is T s , so the period 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 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] It can be seen that increasing the resistance of the converter AC loop can effectively suppress and eliminate the DC bias caused by changes in system parameters, but increasing the resistance also means additional losses, which leads to reduced system efficiency. To this end, the present invention suppresses transient DC bias by introducing an AC loop virtual resistance link. Since no real resistance is added, the loss of the converter will not be increased.

[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 deduced to be

[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: The converter transfer function before compensation is: And parallel link 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] In the formula, 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 the 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 the simulation experiment, input voltage disturbance is introduced to better observe transient DC bias. When the input voltage suddenly increases, the inductor current generates a DC positive bias due to the adjustment of the shift phase. Compared with the traditional control method, the DC bias of the strategy described in the present invention is significantly suppressed and has a higher drop rate.

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; The virtual resistance compensation link includes the following steps: Step 3.1: Establish the AC loop resistance R before virtual resistance compensation of the dual active full-bridge DC / DC converter L = 0 and the real resistance compensation, that is, the AC circuit resistance R L Transient mathematical model under ≠0; 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:

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.

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