Lc filter parameter design method for critical conduction mode inverter

Abstract

The application discloses an LC filter parameter design method for a critical conduction mode inverter, and the LC filter parameter design mainly comprises filter inductance design and filter capacitance design. The filter inductance design is based on the relationship among the filter inductance, the effective volume of an inductance magnetic core and the switching frequency, and the optimal inductance is found by balancing the relationship between the effective volume of the magnetic core and the switching frequency. The filter capacitance design finds the extreme value of the output voltage ripple, and the selection interval of the filter capacitance is determined by constraining the output voltage ripple. The optimal filter capacitance value under the current power level is obtained by using the switching period average value of the filter capacitance current to reflect the degree of hysteresis. The application designs the LC filter parameters for the critical conduction mode inverter, and solves the problem that the selection scheme of the existing LC filter is only applicable to the continuous conduction mode and cannot be directly applied to the critical conduction mode.

Description

Technical Field

[0001] This invention belongs to the field of soft-switching technology for DC-AC voltage inverters, and relates to a method for designing LC filter parameters for inverters in critical conduction mode. Background Technology

[0002] Power electronic converters are widely used in electrical systems such as electric vehicles, photovoltaic power generation systems, and server power supplies. To date, the goal of power electronic systems has been to achieve higher efficiency, but in recent years, achieving higher power density has become a key design direction. In a power electronic converter device, the size of the device is often determined by the dimensions of the switching transistors, DC capacitors, and filters. To achieve higher power density, the required power density can be met by reducing the number of devices, using smaller components, and employing more compact packages. For example, compared to Si-based devices, GaN devices with the same current and voltage ratings have smaller packages and volumes; therefore, wide-bandgap switching devices can be used to improve power density. DC support capacitors can be replaced with smaller ceramic or film capacitors, and compact placement reduces the device size. The size of the filter is related to the converter's switching frequency. By increasing the switching frequency, the size of magnetic components is reduced, and the power density is increased.

[0003] However, traditional inverters operate in continuous conduction mode (CCM), where the switching transistors are in a hard-switching state. Higher switching frequencies not only lead to higher switching losses but also introduce electromagnetic interference (EMI) problems. To increase the switching frequency while maintaining good conversion efficiency and low EMI, soft switching is one of the best options. By operating the inverter in critical conduction mode (CRM), soft switching can be achieved. Furthermore, utilizing the operating characteristics of critical conduction mode for high-frequency operation can significantly reduce the size of the device. Therefore, soft-switching inverters based on critical conduction mode have wide applications in situations requiring high efficiency and power density. Unlike traditional inverters operating in continuous conduction mode, inverters operating in critical conduction mode are frequency conversion operations. However, existing LC filter selection schemes are based on continuous conduction mode and cannot be directly applied to inverters operating in critical conduction mode.

[0004] Therefore, how to provide a complete and reliable parameter design method for inverters in critical conduction mode, so as to find suitable filter inductor and filter capacitor parameters at different power levels. Summary of the Invention

[0005] The purpose of this invention is to provide a method for designing LC filter parameters for inverters in critical conduction mode, which solves the problem that existing LC filter selection schemes are only applicable to continuous conduction mode and cannot be directly applied to critical conduction mode.

[0006] The technical solution adopted in this invention is an LC filter parameter design method for inverters in critical conduction mode. The LC filter parameter design mainly includes the design of filter inductors and filter capacitors. The filter inductor design is based on the relationship between the filter inductor, the effective volume of the inductor core, and the switching frequency. The optimal inductance is found by balancing the relationship between the effective volume of the core and the switching frequency. The filter capacitor design determines the selection range of the filter capacitor by finding the extreme value of the output voltage ripple and constraining the output voltage ripple. The optimal filter capacitor value under the current power level is obtained by using the average value of the switching cycle of the filter capacitor current to reflect the degree of lag.

[0007] The invention is further characterized by: The inverter used consists of a power supply, an LC filter, switching transistors S1, S2, S3, and S4, and a load resistor. The LC filter includes a filter inductor and a filter capacitor. The specific structure of the inverter is as follows: the positive terminal of the power supply is connected to the drain (D) terminals of switching transistors S1 and S3, the source (S) terminals of switching transistors S1 and S3 are connected to the drain (D) terminals of switching transistors S2 and S4, and the source (S) terminals of switching transistors S2 and S4 are connected to the negative terminal of the power supply. One end of the LC filter is located between the source (S) terminal of switching transistor S1 and the drain (D) terminal of switching transistor S2, and the other end of the LC filter is located between the source (S) terminal of switching transistor S3 and the drain (D) terminal of switching transistor S4. The load resistor is connected in parallel with the capacitor.

[0008] The specific design steps for the filter inductor are as follows: Step 3.1: Determine the bus voltage, output voltage, and power frequency based on the inverter's electrical performance parameters; Step 3.2: Based on the bus voltage, output voltage, and power frequency, calculate the switching frequency and effective core volume under different inductance values ​​to obtain the range values ​​of the switching frequency and the effective core volume. Step 3.3: Calculate the number of uncontrollable switching cycles at the maximum switching frequency within the switching frequency range for different inductance values; Step 3.4: Draw the relationship between inductance and effective core volume and the relationship between inductance and number of uncontrollable switching cycles. Determine the inductance L when the effective core volume and number of uncontrollable switching cycles are optimal, thus completing the design of the filter inductor.

[0009] The formula for calculating the switching period is as follows: (1) T s For the switching cycle, i p+ This is the positive envelope of the inductor current. i p- This is the negative envelope of the inductor current. LFor inductance, V dc Bus voltage V o This is the output voltage.

[0010] The specific design steps for the filter capacitor are as follows: Step 5.1: Set the output voltage ripple rate to 5% of the rated output voltage. Based on the extreme value of the output voltage ripple under this condition, calculate the value of the filter capacitor under the inductance L. Step 5.2 Calculate the average capacitor current during the switching cycle based on the filter capacitor value obtained in Step 5.1; Step 5.3: Determine whether the voltage ripple rate meets the standard when the average capacitor current is at its minimum. If it does not meet the standard, repeat steps 5.1-5.2 until the current ripple rate meets the standard. Then the optimal capacitor value is obtained, and the design of the filter capacitor is completed.

[0011] The formula for calculating the output voltage ripple in step 5.1 is: (6) (7) Combining formulas (6) and (7), we obtain the following voltage ripple calculation formula (2): (2) in, △v c For output voltage ripple, △i L For current ripple, T s For switching frequency, C This is the value of the filter capacitor. q It is electric charge; Current ripple △i L The calculation formula is: (4) in, △i L For current ripple, i avg This is the average value of the output current. P o For output power, V o_rms This represents the average value of the output voltage.

[0012] The average capacitor current during the switching cycle in step 5.2 is: (3) in, This represents the average value of the filter capacitor current during the switching cycle.C This is the value of the filter capacitor. V o_max This is the maximum output voltage. u c The voltage of the filter capacitor.

[0013] The formula for calculating the voltage across the filter capacitor is: (5) in, u c This is the voltage across the filter capacitor. V o_max This represents the maximum output voltage.

[0014] The standard for judging whether the voltage ripple rate meets the standard in step 5.3 is whether the ratio of the voltage ripple rate to the rated output voltage is less than or equal to 5%. If it is less than or equal to 5%, it meets the standard.

[0015] The beneficial effects of this invention are as follows: The LC filter parameter design method for inverters in critical conduction mode is applicable to soft-switching inverters employing critical conduction mode, providing a direction and basis for selecting filter parameters for soft-switching inverters operating in frequency conversion mode. The filter inductor value after parameter design can achieve optimal power density while ensuring control dynamic response speed. The designed filter capacitor value can further reduce output voltage ripple and lower the THD of the output voltage while minimizing the influence of the filter capacitor current. Attached Figure Description

[0016] Figure 1 This is a circuit diagram of the LC filter module of the full-bridge inverter circuit used in the LC filter parameter design method of the inverter for critical conduction mode in this invention; Figure 2 This is a schematic diagram showing the relationship between the effective core volume and the number of uncontrollable switching cycles under different inductance values ​​in the LC filter parameter design method for inverters in critical conduction mode according to the present invention. Figure 3 This is a schematic diagram of the inductor current envelope of the CRM modulation scheme used in the LC filter parameter design method for inverters in critical conduction mode according to the present invention. Figure 4 This is a schematic diagram of the inductor current, capacitor current, and capacitor voltage during a single switching cycle in the critical conduction mode of the LC filter parameter design method for inverters in the critical conduction mode of this invention.

[0017] Figure 5 This is a schematic diagram of the switching frequency variation curve when the inductance is 41uH in the LC filter parameter design method for inverters in critical conduction mode according to the present invention.

[0018] Figure 6 This is a schematic diagram showing the relationship between the output voltage ripple and the filter capacitor value when the inductance is 41uH in the LC filter parameter design method for inverters in critical conduction mode according to the present invention. Figure 7 This is a schematic diagram showing the relationship between the size of the filter capacitor and the ripple rate and the average value of the filter capacitor current during the switching cycle in the LC filter parameter design method for inverters in critical conduction mode according to the present invention. Figure 8 This is a flowchart of the LC filter parameter design method for inverters in critical conduction mode according to the present invention. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0021] Example 1 The LC filter of the full-bridge inverter circuit used in this invention, such as... Figure 1 As shown, the inverter consists of a power supply, an LC filter, switching transistors S1, S2, S3, and S4, and a load resistor. The LC filter includes a filter inductor and a filter capacitor. The specific structure of the inverter is as follows: the positive terminal of the power supply is connected to the drain (D) terminals of switching transistors S1 and S3, respectively; the source (S) terminals of switching transistors S1 and S3 are connected to the drain (D) terminals of switching transistors S2 and S4, respectively; the source (S) terminals of switching transistors S2 and S4 are connected to the negative terminal of the power supply. One end of the LC filter is positioned between the source (S) terminal of switching transistor S1 and the drain (D) terminal of switching transistor S2, and the other end of the LC filter is positioned between the source (S) terminal of switching transistor S3 and the drain (D) terminal of switching transistor S4. The load resistor is connected in parallel with the capacitor. This filter can remove high-frequency components from the output AC signal.

[0022] Example 2 This embodiment describes a method for designing LC filter parameters for an inverter in critical conduction mode, including the design of the filter inductor and the filter capacitor. The specific process is as follows: Figure 8 As shown: The specific design steps for the filter inductor are as follows: Step 3.1: Determine the bus voltage, output voltage, and power frequency based on the inverter's electrical performance parameters; Step 3.2: Based on the bus voltage, output voltage, and power frequency, calculate the switching frequency and effective core volume under different inductance values ​​to obtain the range values ​​of the switching frequency and the effective core volume. The formula for calculating the switching period is as follows: (1) Ts For the switching cycle, i p+ This is the positive envelope of the inductor current. i p- This is the negative envelope of the inductor current. L For inductance, V dc Bus voltage V o This is the output voltage.

[0023] Step 3.3: Calculate the number of uncontrollable switching cycles at the maximum switching frequency within the switching frequency range for different inductance values; Step 3.4: Draw the relationship between inductance and effective core volume and the relationship between inductance and number of uncontrollable switching cycles. Determine the inductance L when the effective core volume and number of uncontrollable switching cycles are optimal, thus completing the design of the filter inductor.

[0024] The specific design steps for the filter capacitor are as follows: Step 5.1: Set the output voltage ripple rate to 5% of the rated output voltage. Based on the extreme value of the output voltage ripple under this condition, calculate the value of the filter capacitor under the inductance L. The formula for calculating output voltage ripple is: (6) (7) Combining formulas (6) and (7), we obtain the following voltage ripple calculation formula (2): (2) in, △v c For output voltage ripple, △i L For current ripple, T s For switching frequency, C This is the value of the filter capacitor. q It is electric charge; Current ripple △i L The calculation formula is: (4) in, △i L For current ripple, i avg This is the average value of the output current. P o For output power, V o_rms This represents the average value of the output voltage.

[0025] Step 5.2 Calculate the average capacitor current during the switching cycle based on the filter capacitor value obtained in Step 5.1; The formula for calculating the average capacitor current during the switching cycle is as follows: (3) in, This represents the average value of the filter capacitor current during the switching cycle. C This is the value of the filter capacitor. V o_max This is the maximum output voltage. u c The voltage of the filter capacitor.

[0026] The formula for calculating the voltage across the filter capacitor is: (5) in, u c This is the voltage across the filter capacitor. V o_max This represents the maximum output voltage.

[0027] Step 5.3: When the average capacitor current is at its minimum, determine whether the ratio of voltage ripple rate to the rated output voltage is less than or equal to 5%. If it is less than or equal to 5%, the standard is met, and the filter capacitor value under this condition is the optimal filter capacitor. If it does not meet the standard, repeat steps 5.1-5.2 until the current ripple rate meets the standard and the optimal capacitor value is obtained, thus completing the design of the filter capacitor.

[0028] In continuous conduction mode, the selection of inductance is often limited by the ripple of the inductor current. By limiting the maximum value of the inductor current ripple, the required inductance value in continuous conduction mode can be obtained. Unlike continuous conduction mode, when the inverter operates in critical conduction mode, the inductance value determines the range of switching frequency variation, which directly determines the effective core volume variation. Furthermore, because the switching frequency is relatively high under light load in critical conduction mode, the available time resources for the interrupt service routine are limited. Therefore, fixed-frequency control is usually used to ensure the stable operation of the control algorithm. Due to fixed-frequency control, the higher the switching frequency, the more uncontrollable switching cycles there are. For example, with a fixed-frequency control of 50kHz, if the maximum switching frequency is 300kHz, the controller cannot adjust within 6 switching cycles. Therefore, the range of switching frequency variation also affects the dynamic response speed of the converter in closed-loop mode. Thus, the selection of inductance should be determined by a trade-off between power density and the dynamic response speed of the controller. A curve showing the relationship between inductance value and core volume is plotted in the same coordinate system according to the target core specifications. In this example, the 3F36 core from Feici Company is used for analysis. Secondly, taking 50kHz fixed-frequency control as an example, the number of uncontrollable switching cycles is determined based on the maximum switching frequency under different inductance values. The trends of the two indicators with the change of inductance are as follows: Figure 2 As shown. Finally, based on the comparison of inductor volume and the number of uncontrollable switching cycles, a suitable inductance value of 41uH can be obtained for the 1000W level.

[0029] Example 3 Taking a 1000W output power, 220VAC output, 400V bus voltage, and 41uH inductance as an example, draw the inductor current envelope as follows: Figure 3 As shown, the inductor current crosses zero in each switching cycle and resets according to the designed negative current. The magnitude of the designed negative current is determined by the conditions required for the switching transistor to achieve ZVS. The positive envelope of the inductor current... i p+ and negative envelope i p- The positive and negative peak values ​​of the inductor current determine the magnitude of the inductor current ripple. In critical conduction mode, the magnitude of the inductor current ripple is equal to the magnitude of the output current, and its magnitude is: (4) Typically, the output voltage accuracy of an inverter is considered to be within ±5% of the rated voltage during steady-state operation. Therefore, the magnitude of the output voltage ripple needs to be further estimated based on the inductor current ripple. Figure 4 The diagram shows the inductor current, capacitor current, and capacitor voltage during a single switching cycle in critical conduction mode. The voltage ripple of the filter capacitor and the capacitor current are also shown. ic (t) The total charge contained in the positive portion of the waveform is related. From the expression for capacitance, Q=CV, the output voltage ripple is: (6) If the filter parameters are well designed, the filter capacitor significantly filters the switching ripple. The capacitor C is chosen to be large enough that its impedance is much smaller than the load impedance. Therefore, almost all the inductor current ripple passes through the capacitor, and thus the charge exchange on the filter capacitor is equal to the charge exchange on the filter inductor. Therefore: (7) Combining equations (6) and (7), we can obtain the expression for the output voltage ripple as follows: (2) In the formula, the inductor current ripple The size of the variable is given by equation (4). However, the size of the constraint filter capacitor also needs to be known by the switching frequency. When the inverter is operating in critical conduction mode, the expression of the switching cycle is shown in equation (1). It can be seen that the size of the switching frequency is determined by the positive and negative peak values ​​of the inductor current, the input voltage and the output voltage within the switching cycle.

[0030] (1) The calculation results of the switching frequency are plotted as follows: Figure 5 As shown, since the switching frequency is not fixed within half a power frequency cycle, the trend of output voltage ripple is not obvious. Therefore, the expression for the switching period shown in equation (1) and the expression for the current ripple shown in equation (4) are substituted into the expression for the output voltage ripple (2). The trend of the magnitude of the output voltage ripple under different filter capacitors is plotted, and the plotting results are as follows. Figure 6 As shown. It can be seen that the maximum value of the output voltage ripple is at... This occurs at the point where the output voltage change is twice the output voltage ripple. Considering that the output voltage change is within ±5% of the rated voltage during steady-state operation, the filter capacitor should be greater than 1.4uF for a 41uH inductor. Furthermore, since the size of the filter capacitor affects the average value of the filter capacitor current during the switching cycle, according to KCL, the inductor current is equal to the sum of the filter capacitor current and the output current. Therefore, it can be concluded that when the average value of the filter capacitor current increases, the output current lags behind the inductor current.

[0031] The voltage across the filter capacitor is equal to the output voltage. (5) Furthermore, based on the voltage across the filter capacitor, the average current across the filter capacitor during the switching cycle can be determined as follows: (3) From equation (3), the maximum average value of the filter capacitor current during the switching cycle is: The calculation results of the average value of the filter capacitor current switching cycle and the ripple rate under different filter capacitor conditions are as follows: Figure 7 As shown. By Figure 7 It can be seen that as the value of the filter capacitor increases, the average value of its capacitor current increases linearly. However, the slope of the change in ripple rate gradually slows down as the value of the filter capacitor increases. Considering the influence of the minimum filter capacitor current, the best filtering effect can be obtained. Therefore, it is more appropriate to select the filter capacitor value between 1.5uF and 3uF.

[0032] The design concept of this application is as follows: In critical conduction mode, the switching frequency of a full-bridge inverter circuit is related to three conditions: inductance, bus voltage, and output voltage. By selecting the application scenario and determining the converter's design specifications, the magnitudes of the bus voltage and output voltage can be determined. Once these specifications are determined, the range of switching frequency variation is determined by the inductance. Secondly, with a fixed power condition, the inductance determines the effective volume of the inductor core, thus affecting the converter's power density. By balancing the relationship between power density and control response speed, the optimal inductance can be found. Finally, based on the switching frequency variation trend within half a power frequency cycle and the inductor current ripple variation trend within the same half-power frequency cycle, the extreme value of the output voltage ripple is found. By constraining the output voltage ripple, the selection range of the filter capacitor is determined. After selecting the filter capacitor, since it causes a phase lag in the output current, the average value of the filter capacitor current over the switching cycle is used to reflect the degree of lag, yielding a suitable filter capacitor value for the current power level.

Claims

1. A method for designing LC filter parameters for an inverter in critical conduction mode, characterized in that, The design of LC filter parameters includes the design of filter inductors and filter capacitors. The design of filter inductors is based on the relationship between filter inductors, the effective volume of inductor cores, and the switching frequency. The optimal inductance is found by balancing the relationship between the effective volume of the cores and the switching frequency. The filter capacitor design determines the selection range of the filter capacitor by finding the extreme value of the output voltage ripple magnitude and constraining the output voltage ripple. The optimal filter capacitor value under the current power level is obtained by using the average value of the switching cycle of the filter capacitor current to reflect the degree of lag. The inverter used consists of a power supply, an LC filter, switching transistors S1, S2, S3, and S4, and a load resistor. The LC filter includes a filter inductor and a filter capacitor. The specific structure of the inverter is as follows: the positive terminal of the power supply is connected to the drain (D) terminals of switching transistors S1 and S3, the source (S) terminals of switching transistors S1 and S3 are connected to the drain (D) terminals of switching transistors S2 and S4, and the source (S) terminals of switching transistors S2 and S4 are connected to the negative terminal of the power supply. One end of the LC filter is located between the source (S) terminal of switching transistor S1 and the drain (D) terminal of switching transistor S2, and the other end of the LC filter is located between the source (S) terminal of switching transistor S3 and the drain (D) terminal of switching transistor S4. The load resistor is connected in parallel with the capacitor. The specific design steps for the filter inductor are as follows: Step 3.1: Determine the bus voltage, output voltage, and power frequency based on the inverter's electrical performance parameters; Step 3.2: Based on the bus voltage, output voltage, and power frequency, calculate the switching frequency and effective core volume under different inductance values ​​to obtain the range values ​​of the switching frequency and the effective core volume. Step 3.3: Calculate the number of uncontrollable switching cycles at the maximum switching frequency within the switching frequency range for different inductance values; Step 3.4: Draw the relationship between inductance and effective core volume and the relationship between inductance and number of uncontrollable switching cycles. Determine the inductance L when the effective core volume and number of uncontrollable switching cycles are optimal, thus completing the design of the filter inductor. The specific design steps for the filter capacitor are as follows: Step 5.1: Set the output voltage ripple rate to 5% of the rated output voltage. Based on the extreme value of the output voltage ripple under this condition, calculate the value of the filter capacitor under the inductance L. Step 5.2 Calculate the average capacitor current during the switching cycle based on the filter capacitor value obtained in Step 5.1; Step 5.3: Determine whether the voltage ripple rate meets the standard when the average capacitor current is at its minimum. If it does not meet the standard, repeat steps 5.1-5.2 until the current ripple rate meets the standard. Then the optimal capacitor value is obtained, and the design of the filter capacitor is completed.

2. The LC filter parameter design method for an inverter in critical conduction mode according to claim 1, characterized in that, The formula for calculating the switching period is as follows: （1） T s For the switching cycle, i p+ This is the positive envelope of the inductor current. i p- This is the negative envelope of the inductor current. L For inductance, V dc Bus voltage V o This is the output voltage.

3. The LC filter parameter design method for an inverter in critical conduction mode according to claim 2, characterized in that, The formula for calculating the output voltage ripple mentioned in step 5.1 is: （6） （7） Combining formulas (6) and (7), we obtain the following voltage ripple calculation formula (2): （2） in, △v c For output voltage ripple, △i L For current ripple, T s For switching frequency, C This is the value of the filter capacitor. q It represents electric charge.

4. The LC filter parameter design method for an inverter in critical conduction mode according to claim 3, characterized in that, The current ripple △i L The calculation formula is: （4） in, △i L For current ripple, i avg This is the average value of the output current. P o For output power, V o_rms This represents the average value of the output voltage.

5. The LC filter parameter design method for an inverter in critical conduction mode according to claim 4, characterized in that, The formula for calculating the average capacitor current during the switching cycle mentioned in step 5.2 is as follows: （3） in, This represents the average value of the filter capacitor current during the switching cycle. C This is the value of the filter capacitor. V o_max This is the maximum output voltage. u c The voltage of the filter capacitor.

6. The LC filter parameter design method for an inverter in critical conduction mode according to claim 5, characterized in that, The formula for calculating the voltage of the filter capacitor is: （5） in, u c This is the voltage across the filter capacitor. V o_max This represents the maximum output voltage.

7. The LC filter parameter design method for an inverter in critical conduction mode according to claim 6, characterized in that, The standard for judging whether the voltage ripple rate meets the standard in step 5.3 is whether the ratio of the voltage ripple rate to the rated output voltage is less than or equal to 5%. If it is less than or equal to 5%, it meets the standard.

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