DAB circuit with dc blocking or resonant capacitor and design method thereof

Abstract

The application discloses a DAB circuit with a direct-current isolation capacitor or a resonant capacitor and a design method thereof, and can provide certain basis for the design of the size of the capacitor for the direct-current isolation or resonance in an alternating current loop. The application firstly defines an evaluation index of a direct-current isolation resonance degree, which is used for guiding the ratio design of a resonance period and a switching period. When guiding the design of the direct-current isolation capacitor, a larger direct-current isolation resonance degree is selected to ensure that the direct-current isolation capacitor has the least influence on the original characteristics of the circuit. When guiding the design of the resonant capacitor, a smaller direct-current isolation resonance degree is selected to reduce the switching loss. Secondly, according to the relationship among the maximum phase shift angle and 10% thereof, the ratio of the resonance period and the switching period, and the direct-current isolation resonance degree, the ratio size or the ratio reference range of the resonance period and the switching period is calculated, and finally the size of the required direct-current isolation capacitor or resonant capacitor is calculated.

Description

Technical Field

[0001] This invention relates to a DAB circuit with DC blocking or resonant capacitor and its design method, belonging to the field of power electronics technology. Background Technology

[0002] Traditional dual-active bridge (DAB) steady-state analysis does not consider DC blocking capacitors because, in the mathematical equivalent model, they are negligible and have almost no impact on the circuit's operating principle and steady-state characteristics. However, during transient processes, such as sudden load increases, large-scale power transmission switching, or instantaneous changes in the output voltage reference, DABs without DC blocking capacitors suffer from transient magnetic bias problems in high-frequency transformers. In current operational engineering projects, many high-frequency transformers use nanocrystalline iron cores, especially toroidal cores, which have large magnetizing inductances, poor anti-saturation capabilities, and are more prone to saturation problems. Although DC blocking capacitors are only one method to solve transient magnetic bias, they are the simplest and most reliable method compared to others, requiring no changes to the control strategy. For more complex systems, the DAB is only one sub-component, and in practical applications, overly complex control is undesirable to minimize the increase in control complexity and mutual interference, thereby affecting the stability of the entire system. Therefore, in practical engineering, after comprehensively weighing various factors, some systems still choose to use DC blocking capacitors. DC blocking capacitors increase the size and cost of the system, so it is desirable to keep them as small as possible without affecting the operation of the original circuit. However, current methods for designing the minimum value of DC blocking capacitors are mostly based on experience, and there is no publicly available quantitative design method.

[0003] Traditional methods for capacitor design for resonant functions primarily focus on fully resonant operation, where the circuit operates at the resonant point, thus the resonant capacitor size is essentially fixed. However, for phase-shift controlled resonant DAB circuits, even a slight change in the resonant capacitor can significantly affect the control performance. Currently, the design of resonant capacitors for resonant DAB circuits is mostly based on simulation results, and there is no definitive design methodology.

[0004] Quantitative design methods for the minimum value of DC blocking capacitors and the design range of resonant capacitors are rarely disclosed. Patent [CN112968612A] discloses a modular high-ratio isolated DC-DC converter, which uses DC blocking capacitors in its series-parallel sub-module topology, but does not discuss the design method of the DC blocking capacitors. Patent [CN112953230A] provides a triple phase-shift control method and control device for a dual active bridge circuit, which also uses DC blocking capacitors in its topology, but does not discuss the design method of the DC blocking capacitors. Patent [CN112713780B] provides an asymmetric voltage matching phase-shift control method for a dual active bridge converter, in which DC blocking capacitors are also used to solve the asymmetry problem when the control pulse is mismatched, but does not discuss the design method of the DC blocking capacitors. Patent [CN111817574A] discloses a method, system, and storage medium for designing LCC resonant converter parameters, but it is aimed at non-DAB circuits. Patent [CN114665723A] discloses a method for designing the resonant inductor and capacitor parameters of a series resonant three-phase DAB converter. It starts from three aspects: return power, converter maximum current, and converter current RMS value. The main design is of the parameters of the resonant inductor. The resonant capacitor is directly calculated based on the specified resonant frequency, and the deviation between the actual value and the design value of the inductor and capacitor is not considered. It is not intuitive and practical in guiding the design of soft switching. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a DAB circuit with DC blocking or resonant capacitor and its design method, thus solving the problems disclosed in the background art.

[0006] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0007] In a first aspect, the present invention provides a DAB circuit with DC blocking or resonant capacitors, comprising a DC / AC module, an LC unit, a high-frequency transformer, and an AC / DC module connected sequentially from left to right. The DC / AC module converts the input DC current into a high-frequency square wave, which is then transmitted to the AC / DC module via the LC unit and the high-frequency transformer. The AC / DC module converts the high-frequency square wave back into DC current and outputs it. The LC unit includes a transmission inductor L. r DC blocking or resonant capacitor C r .

[0008] Furthermore, the AC / DC module and DC / AC module adopt two-level, three-level, or other multi-level circuits.

[0009] Secondly, the present invention provides a design method for a DAB circuit with DC blocking or resonant capacitor according to any one of the foregoing claims, the method comprising:

[0010] The current value at the inflection point within half a cycle is obtained from the real-time waveform of the steady-state resonant current, denoted as I2. The amplitude of the equivalent square wave current is obtained from the power balance, denoted as I1. The evaluation index DC blocking resonance degree δ=I2 / I1 is defined. When δ is close to 1, the AC circuit capacitor is used as a DC blocking capacitor. When δ is close to 0, the AC circuit capacitor is used as a resonant capacitor.

[0011] Calculate the range of values ​​for the AC circuit capacitor when it is used as a DC blocking capacitor or a resonant capacitor.

[0012] Based on the calculated range of values ​​for the DC blocking capacitor or resonant capacitor, select an appropriate value for the DC blocking capacitor or resonant capacitor C in the circuit. r This forms the corresponding DC blocking capacitor circuit or resonant capacitor circuit.

[0013] Furthermore, methods for calculating the value range of the AC circuit capacitor when it is used as a DC blocking capacitor include:

[0014] Set the outer phase shift angle under rated power, and calculate the relationship between the ratio of the resonant period to the switching period k and δ at the outer phase shift angle under rated power;

[0015] Based on the above relationship, the maximum deviation Δ of the inflection point current at the outward phase angle under rated power is set, and the required value of the minimum DC blocking capacitor is calculated.

[0016] Furthermore, the formula for calculating the range of values ​​for the AC circuit capacitor when it is used as a DC blocking capacitor is as follows:

[0017] δ=1-Δ, and according to Further calculations yielded the ratio k of the resonant frequency to the switching frequency, where... The set outward phase angle;

[0018] According to k=T r / T, where T is the switching frequency, T r The resonant frequency, Due to the switching frequency T and the transmission inductance L r Given the fixed parameters, calculate the DC blocking capacitance C. r The value that can be taken.

[0019] Furthermore, methods for calculating the range of values ​​for the AC circuit capacitance when it is used as a resonant capacitor include:

[0020] The maximum values ​​of the external phase shift angle and DC blocking resonance δ at the rated power are set, and the ratio of the resonant period to the switching period at this time is calculated as the maximum value of k. Then, considering the deviation of the device parameters, there is a minimum value of k to ensure that the resonant period is greater than the switching period. Then, considering whether I2 is greater than the minimum soft switching current at 10% rated load, a suitable value of k is finally selected, and the required value of the resonant capacitor is calculated.

[0021] Furthermore, the calculation formula for the AC circuit capacitance when it is used as the resonant capacitance is as follows:

[0022] Set the upper limit value of δ and the phase angle value of rated power downward shift. pass Obtain the corresponding first k value as the upper limit of k value;

[0023] Assuming resonant inductance L r and resonant capacitor C r There is a deviation; to ensure the resonant period is greater than the switching period, i.e.

[0024] but

[0025] In the formula, ε represents the resonant period deviation caused by electrical parameter errors;

[0026] Calculate the second k value at this point as the lower limit of the value of k;

[0027] During the second k-value and the first k-value phase, by judging 10%*δ*I 1e The value of k is adjusted to determine whether the minimum soft-switching current of the power device is met, until the minimum soft-switching current is satisfied, where I... 1e I1 at rated load;

[0028] Based on the final selected value of k, through k=T r / T, where T is the switching frequency, T r The resonant frequency, Due to the switching frequency T and the transmission inductance L r Given the fixed parameters, calculate the DC blocking capacitance C. r The value that can be taken.

[0029] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0030] This invention provides a DAB circuit with DC blocking or resonant capacitors and its design method. A DC blocking resonance degree is defined as an evaluation index to guide the design of the ratio of resonant period to switching period. When guiding the design of the DC blocking capacitor, a larger DC blocking resonance degree is selected to minimize the impact of the DC blocking capacitor on the original characteristics of the circuit. When guiding the design of the resonant capacitor, a smaller DC blocking resonance degree is selected to ensure a suitable soft-switching effect. Secondly, based on the relationship between the ratio of resonant period to switching period at the maximum phase shift angle and its 10% ohm, and the DC blocking resonance degree, the required ratio of resonant period to switching period or a reference range for the ratio is calculated. Finally, the minimum required size of the DC blocking capacitor or the range of values ​​for the resonant capacitor is calculated. The DAB circuit with DC blocking or resonant capacitors designed accordingly is more accurate and practical. Attached Figure Description

[0031] Figure 1 This refers to a DAB circuit with a capacitor in an AC loop and its equivalent circuit.

[0032] Figure 2 This is a waveform diagram of a capacitor in an AC circuit when it is used as a DC blocking capacitor or a resonant capacitor.

[0033] Figure 3 This is a schematic diagram illustrating the definition of DC blocking resonance.

[0034] Figure 4 The diagram shows the relationship between the ratio of the resonant period to the switching period and the DC blocking resonance (a), as well as the detailed design diagrams of the DC blocking capacitor (b) and the resonant capacitor (c). Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0036] Example 1

[0037] like Figure 1 As shown in (a), this embodiment introduces a DAB circuit with DC blocking or resonant capacitors. From left to right, the components are a DC / AC module, an LC unit, a high-frequency transformer, and an AC / DC module. Taking forward power transmission as an example, the DC / AC module converts the input DC power into a high-frequency square wave, which is then transmitted to the AC / DC module via the LC unit and the high-frequency transformer. The AC / DC module then converts the high-frequency square wave back into DC power and outputs it.

[0038] The AC / DC module and the DC / AC module can use two-level, three-level, or other multi-level circuits. Here, a two-level full-bridge circuit is used as an example. The LC unit consists of a transmission inductor L... r DC blocking or resonant capacitor C r Series connection, Tr For high-frequency transformers, the turns ratio is set to 1:1 to simplify the analysis. Figure 1 The equivalent circuit of (a) is as follows Figure 1 As shown in (b).

[0039] The advantage of adding DC blocking or resonant capacitors to circuits is that they can most simply and reliably solve the transient magnetic bias problem of high-frequency transformers during transient processes, such as sudden load increases, large-scale power transmission switching, or instantaneous changes in output voltage reference. For circuits with relatively complex overall structures, the sub-module circuits are simple to control, which can simplify the complexity of the overall control software and reduce logical loopholes.

[0040] The working principles after adding DC blocking or resonant capacitors are as follows: Figure 2 (a) and Figure 2 As shown in (b), the gray portion of the current section represents the operating current of a traditional DAB circuit without a DC blocking resonant capacitor under the same transmission power. It can also be considered as the square wave current equivalent to the resonant current, such as... Figure 3 The equivalent of the shaded area in (a). Define the DC blocking resonance δ=I2 / I1, as follows. Figure 3 As shown in (b). Where I2 is the inflection point current at time t2. I1 is the amplitude of the equivalent square wave current at times t1 to t2, which is also the amplitude of the current corresponding to the traditional non-DC blocking resonant capacitor topology under the same transmission power.

[0041] Example 2

[0042] This embodiment provides a design method for a DAB circuit with DC blocking or resonant capacitor according to any one of Embodiment 1, the method comprising:

[0043] The current value at the inflection point within half a cycle is obtained from the real-time waveform of the steady-state resonant current, denoted as I2. The amplitude of the equivalent square wave current is obtained from the power balance, denoted as I1. The evaluation index DC blocking resonance degree δ=I2 / I1 is defined. When δ is close to 1, the AC circuit capacitor is used as a DC blocking capacitor. When δ is close to 0, the AC circuit capacitor is used as a resonant capacitor.

[0044] Calculate the range of values ​​for the AC circuit capacitor when it is used as a DC blocking capacitor or a resonant capacitor.

[0045] Based on the calculated range of values ​​for the DC blocking capacitor or resonant capacitor, select an appropriate value for the DC blocking capacitor or resonant capacitor C in the circuit. r This forms the corresponding DC blocking capacitor circuit or resonant capacitor circuit.

[0046] Specifically, the methods for calculating the value range of the AC circuit capacitor when it is used as a DC blocking capacitor include:

[0047] Set the external phase shift angle under rated power, and calculate the relationship between the resonant period and the ratio of the switching period k and δ at the rated power phase shift angle;

[0048] Based on the above relationship, the maximum deviation Δ of the inflection point current at the rated power phase shift angle is set, and the required minimum DC blocking capacitor value is calculated.

[0049] Specifically, the formula for calculating the value range of the AC circuit capacitor when it is used as a DC blocking capacitor is as follows:

[0050] δ=1-Δ, and according to Further calculations yielded the ratio k of the resonant frequency to the switching frequency, where... The set outward phase angle;

[0051] According to k=T r / T, where T is the switching frequency, T r The resonant frequency, Due to the switching frequency T and the transmission inductance L r Given the fixed parameters, calculate the DC blocking capacitance C. r The value that can be taken.

[0052] Specifically, methods for calculating the value range of the AC circuit capacitor when it is used as a resonant capacitor include:

[0053] The maximum values ​​of the external phase shift angle and DC blocking resonance degree δ at rated power are set, and the ratio of the resonant period to the switching period at this point is calculated as the maximum value of k. Considering the deviation of device parameters, a minimum value of k is required to ensure that the resonant period is greater than the switching period. Then, considering whether I2 is greater than the minimum soft-switching current at 10% rated load, a suitable value of k is finally selected, and the required value of the resonant capacitor is calculated. Specifically, the calculation formula for the AC circuit capacitor as the resonant capacitor is as follows:

[0054] Set the upper limit value of δ and the phase angle value of rated power downward shift. pass Obtain the corresponding first k value as the upper limit of k value;

[0055] Assuming resonant inductance L r and resonant capacitor C r There is a deviation; to ensure the resonant period is greater than the switching period, i.e.

[0056] but

[0057] In the formula, ε represents the resonant period deviation caused by electrical parameter errors.

[0058] Calculate the second k value at this point as the lower limit of the value of k;

[0059] During the second k-value and the first k-value phase, by judging 10%*δ*I 1e The value of k is adjusted to determine whether the minimum soft-switching current of the power device is met, until the minimum soft-switching current is satisfied, where I... 1e I1 at rated load;

[0060] Based on the final selected value of k, through k=T r / T, where T is the switching frequency and Tr is the resonant frequency. Due to the switching frequency T and the transmission inductance L r Given the fixed parameters, calculate the DC blocking capacitance C. r The value that can be taken.

[0061] The design method for a DAB circuit with DC blocking or resonant capacitor provided in this embodiment involves the following steps in its application:

[0062] (1) When guiding the design of DC blocking capacitors

[0063] Step 1: Calculate the relationship between the ratio of resonant frequency to switching frequency and the DC blocking resonant degree. The closer δ is to 1, the smaller the effect. Under the same power transmission, the voltage and current integrals are equal. When the voltage conditions are also the same, the areas of S0 and S1 are equal. The t0~t1 part is the return power of the circuit; the positive and negative power cancel each other out, so this part can be ignored. Therefore, only the t1~t2 part needs to be considered. Figure 3 In (a), the current with and without DC blocking capacitors are superimposed into a single waveform, such as... Figure 3 As shown in (b). t m Let t be the midpoint between t1 and t2. Due to symmetry, let t be... m -t1=t2-t m =xT r T r For L r and C r The resonant period. Figure 3 (b) T 12 =2xT r At the same time, there are in For the outward phase angle, T r This represents the switching cycle of the DAB circuit.

[0064] For ease of analysis, let t be set. m =0. Therefore, since the areas of S0 and S1 are equal, we have...

[0065]

[0066] Further derivation

[0067]

[0068] And according to Figure 3 (b) Derivable

[0069]

[0070] Substituting equation (2) into equation (1) yields

[0071]

[0072] Moreover, according to

[0073]

[0074] achievable

[0075]

[0076] Preferably, the rated power is shifted downwards by a phase angle. remember So

[0077]

[0078] Substituting equation (5) into equation (3) yields

[0079]

[0080] Equation (6) is the formula for the relationship between k and δ near the rated power.

[0081] Simultaneously, the relationship between k and δ can be calculated near 10% of the rated power. Considering the approximate linearity of power change near the phase shift angle of 0, the phase shift angle can be assumed to be 10% of the rated power, i.e. So

[0082]

[0083] Equations (6) and (7) can be plotted in the same figure, such as... Figure 4 As shown in (a).

[0084] Step two, preferably, setting the acceptable deviation to 5%, then the corresponding δ = 1 - 5% = 0.95. According to formula (6) or Figure 4 (b) allows us to calculate the corresponding k value, k = 3.4. From Figure 4(a) It can be seen that as k increases, the rate of increase in DC blocking resonance becomes extremely slow, and becomes increasingly flat when δ = 0.9, meaning that k has little effect on δ. Verifying k = 3.4, the curve corresponding to formula (7) shows δ = 0.928, with little change. Therefore, here we only need to consider the relationship between k and δ at rated power. From... Figure 4 (b) It can be seen that the value of k is 3.4.

[0085] Step 3, according to k=T r / T=3.4, given T=333.33μs, L r =180μH, C can be calculated r =181μF. When the k value is already selected, as the circuit power gradually decreases from the rated value, along... Figure 4 (b) shows the change in arrow direction.

[0086] (2) When guiding the design of resonant capacitors

[0087] Step 1: Calculate the ratio of resonant frequency to switching frequency and the relationship formula between it and DC blocking resonance degree, which is the same as in specific implementation method (1). However, focus on areas where the k value is relatively small. I2 is the inflection point current, and the magnitude of this current directly determines the magnitude of the DAB switching loss; therefore, it is desirable for I2 to be as small as possible. Preferably, the upper limit of δ is set to 0.6, i.e., δ≤0.6. Preferably, the phase angle of rated power downward shift is set. Then according to formula (6) or Figure 4 (c) yields a k value of 1.25, which is the upper limit of the k value. Figure 4 Point E in (c).

[0088] Step two: As the value of k decreases, the switching loss decreases, but the smaller I2 is, the closer it is to the resonant point, resulting in poorer controllability. In extreme cases, at the resonant point, the power becomes uncontrollable. Therefore, it should not be allowed to approach the resonant point. Preferably, the resonant inductance L is also considered. r and resonant capacitor C r There will be some deviation during mass production; assuming this deviation is 10%, then the maximum deviation in the resonant period will also be 10%. To ensure that the resonant period is greater than the switching period, the value of k must be at least 1.1. Figure 4 Point D in (c).

[0089] Step 3: When the power decreases, according to steps 1 and 2, point D will move towards D′, and point E will move towards E′. D′ and E′ represent the δ values ​​corresponding to the minimum power. From... Figure 4 (c) It can be seen that within the k∈[1.1,1.25] range, the change of k has a great influence on the change of δ, which is much more significant than when k is larger. Therefore, what we obtain here is a range of k values.

[0090] Step four: In practical engineering, to ensure zero-voltage soft switching, I2 has a minimum value limit; below this value, zero-voltage soft switching cannot be achieved. Preferably, setting the minimum limit to 10% of the rated load current enables zero-voltage soft switching. Ignoring the dead-time effect, this can be approximated as 10% of the maximum phase shift angle. Therefore, δ is calculated; taking D′ as an example, δ = 0.22. This is determined by 10% * 0.22 * I. 1e (where I) 1e To adjust the value of k, we need to determine whether I1 at rated load meets the minimum soft-switching current of the power device. Figure 4 (c) Move in the direction of the double arrow until the minimum soft-switching current is met. If the maximum k value is exceeded, the maximum phase shift angle needs to be readjusted, the maximum phase shift angle setting should be appropriately reduced, and steps one through four should be repeated.

[0091] Step 5, assuming k = T is ultimately selected. r / T=1.15, given T=333.33μs, L r =180μH, C can be calculated r =20.7μH.

[0092] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A design method for a DAB circuit with DC blocking or resonant capacitor, characterized in that, The DAB circuit includes, from left to right, a DC / AC module, an LC unit, a high-frequency transformer, and an AC / DC module. The DC / AC module converts the input DC power into a high-frequency square wave, which is then transmitted to the AC / DC module via the LC unit and the high-frequency transformer. The AC / DC module converts the high-frequency square wave back into DC power and outputs it. The LC unit includes transmission inductors connected in series. DC blocking or resonant capacitor The AC / DC module and DC / AC module adopt two-level, three-level, or other multi-level circuits; The design method includes: The inflection point current value within half a cycle is obtained from the real-time waveform of the steady-state resonant current, and is denoted as follows: The amplitude of the equivalent square wave current is obtained based on power balance and denoted as... Define the evaluation index DC blocking resonance degree ,when When the value is close to 1, the AC circuit capacitor is used as a DC blocking capacitor. When the value is close to 0, the AC circuit capacitor is used as a resonant capacitor. Calculate the range of values ​​for the AC circuit capacitor when it is used as a DC blocking capacitor or a resonant capacitor. Based on the calculated range of values ​​for the DC blocking capacitor or resonant capacitor, select an appropriate size for the DC blocking capacitor or resonant capacitor in the circuit. These can be used to form corresponding DC blocking capacitor circuits or resonant capacitor circuits. Methods for calculating the range of values ​​for AC circuit capacitors when used as DC blocking capacitors include: Set the outward phase shift angle at rated power, and calculate the ratio of the resonant period to the switching period at the outward phase shift angle at rated power. and The relationship between them; Based on the above relationship, the maximum deviation Δ of the inflection point current at the phase shift angle outside the rated power is set, and the required minimum value of the DC blocking capacitor is calculated. Methods for calculating the range of values ​​for the AC circuit capacitor when it acts as a resonant capacitor include: External phase shift angle and DC blocking resonance when setting rated power The maximum value of is used to calculate the ratio of the resonant period to the switching period at this point, which is then used as... Considering the maximum value of k and the deviation of device parameters, to ensure that the resonant period is greater than the switching period, k has a minimum value. Furthermore, considering a 10% rated load... Whether it exceeds the minimum soft-switching current, the appropriate choice will ultimately be made. The required value of the resonant capacitor is calculated.

2. The design method of the DAB circuit with DC blocking or resonant capacitor according to claim 1, characterized in that, The formula for calculating the range of values ​​for the AC circuit capacitor when it is used as a DC blocking capacitor is as follows: And according to Further calculations yielded the ratio k of the resonant frequency to the switching frequency, where... The set outward phase angle; And according to In the formula, T is the switching frequency. The resonant frequency, Due to the switching frequency T and the transmission inductance Calculate the DC blocking capacitance using known fixed parameters. The value that can be taken.

3. The design method of the DAB circuit with DC blocking or resonant capacitor according to claim 1, characterized in that, The formula for calculating the AC circuit capacitance when it is used as a resonant capacitor is as follows: set up The upper limit value and the phase angle value of the rated power shift. ,pass Obtain the corresponding first k value, which serves as the upper limit of the k value; Assuming resonant inductance and resonant capacitor There is a deviation; to ensure the resonant period is greater than the switching period, Right now ; but ; In the formula The resonant period deviation is caused by electrical parameter errors; Choose the second k value at this time as the lower limit of the value of k; During the second k-value and the first k-value stages, by judging Adjustment based on whether the minimum soft-switching current of the power device is met. The value is maintained until the minimum soft-switching current is met, where, When at rated load ; Based on the final selected k value, through In the formula, T is the switching frequency. The resonant frequency, Due to the switching frequency T and the transmission inductance Calculate the resonant capacitance using known fixed parameters. The value that can be taken.

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