A parameter design method for optimizing the circuit of a fixed-frequency LCC topology

Abstract

For the fixed-frequency LCC topology resonant converter, considering that it is extremely prone to losing the ZVS operating state, resulting in problems such as large switching losses, low system efficiency, and large EMI interference, the present invention adopts an auxiliary network composed of a series connection of an auxiliary inductor and a DC-blocking capacitor and applies it to the resonant converter with the fixed-frequency LCC topology. And a parameter design method for optimizing the resonant converter with the fixed-frequency LCC topology is given, so as to achieve ZVS of the switching tube within a wide range of input voltage and load variations, and have a small resonant current, limiting the circulating current loss of the converter, providing a design reference for converters that require a wide ZVS range for the switching tube, and being beneficial to device selection.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and particularly to a parameter design method for optimizing a fixed-frequency LCC topology circuit. Background Art

[0002] With the development of power electronics technology, the industry's requirements for the power density and volume of converters are increasing day by day. It is imperative to increase the switching frequency to reduce the volume of the converter. Among them, the resonant converter with the LCC topology combines the advantages of the commonly used series resonant converter and the parallel resonant converter, and has good load regulation characteristics and small light-load circulating current loss. Generally, the resonant converter with the variable-frequency LCC topology is easy to achieve wide-range ZVS, but its switching frequency changes with the input voltage and load, and its magnetic components can only be designed according to the minimum switching frequency, thus limiting the reduction of the converter volume. Therefore, a resonant converter using a fixed-frequency LCC topology has emerged, which has the advantages of simple control circuit and easy design of magnetic components. However, when the input voltage increases or the load is reduced, the converter with the fixed-frequency LCC topology is extremely likely to lose the ZVS operating state, resulting in problems such as large switching losses, low system efficiency, and large EMI interference.

[0003] In response to the above problems of the resonant converter with the fixed-frequency LCC topology, the industry has also proposed some solutions. For example, discrete self-sustaining phase-shift control is used to achieve ZVS in a wide load range of the converter, but its control loop is complex, and it is difficult to determine the parameters in the control circuit, so it is not easy to implement in practical applications. There are also auxiliary current source networks or auxiliary inductors used to broaden the ZVS range, but they respectively have problems such as limiting the converter volume, power density, and high-order harmonics, and there is no complete design process, which is not conducive to engineers' design. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a parameter design method for optimizing a resonant converter with a fixed-frequency LCC topology.

[0005] The technical solution adopted by the invention is as follows: The present invention includes the following steps:

[0006] Step 1: Define the input and output characteristics of the resonant converter with the fixed-frequency LCC topology, and confirm the requirements of various technical indicators, including input voltage, output voltage, output power, and working frequency;

[0007] Step 2: Based on the resonant conversion circuit with the fixed-frequency LCC topology, add an auxiliary network composed of an auxiliary inductor and a DC-blocking capacitor between the two bridge arms, and list the corresponding ZVS conditions of the resonant converter according to the improved circuit;

[0008] Step 3: Adopt the fundamental wave analysis method to conduct the steady-state analysis of the fixed-frequency LCC topology resonant converter and solve the resonant current, and list the corresponding derivation relationships;

[0009] Step 4: According to the input-output characteristics of the resonant converter, select the appropriate series-parallel resonant capacitance ratio and the quality factor required for the resonant converter under rated load, and maximize the conduction angle of the resonant converter, and draw the corresponding gain-frequency characteristic curve;

[0010] Step 5: According to the quality factor of the rated load, mark the maximum gain point of the voltage on the curve, and at the same time set the gain margin. From the gain-frequency characteristic curve, mark the normalized switching angular frequency corresponding to the gain on the curve;

[0011] Step 6: Solve the transformer turns ratio and calculate the resonant parameters of each item of the resonant converter;

[0012] Step 7: According to the relationship derivation in Steps 2 and 3, list the resonant current expressions of the two bridge arms, and calculate and derive the conduction angle expression of the fixed-frequency LCC structure topology resonant converter;

[0013] Step 8: According to the resonant current expressions of the two bridge arms in Step 7, draw the relationship curve of the resonant current - auxiliary inductor, solve the auxiliary inductor value that satisfies the ZVS condition of the fixed-frequency LCC structure topology resonant converter, and leave a certain margin.

[0014] Further, the ZVS condition of the converter in Step 2 is as follows:

[0015]

[0016] Among them, in formula (1): ω s is the switching angular frequency, t1 and t2 are the time parameters of the switching tubes S1 and S2 respectively, i s2 (ω s t2) and i s4 (ω s t1) are the currents flowing through the switching tubes S1 and S2 respectively, i La (ω s t1) and i r (ω s t1) are the auxiliary network current and resonant current flowing through the switching tube S1 respectively, i La (ω s t2) and i r (ω s t2) are the auxiliary network current and resonant current flowing through the switching tube S2 respectively.

[0017] Further, in Step 3, the fundamental wave analysis method is used to perform a steady-state analysis on the improved resonant converter with a fixed-frequency LCC topology, and the quality factor Q and voltage gain M of the resonant converter can be obtained as follows:

[0018]

[0019] Among them, in Equations (2) and (3), Q is the quality factor of the resonant converter, M is the voltage gain of the resonant converter, R L is the load resistance, A is the ratio of series and parallel resonant capacitors, ω s is the switching angular frequency, ω r is the series resonant angular frequency, n is the transformer turns ratio, and δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch;

[0020] From the voltage gain expression (3), the expressions of the resonant currents flowing through the switch tubes S1 and S2 are as follows:

[0021]

[0022] Among them, in Equation (4): i r (ω s t1) and i r (ω s t2) are the resonant currents flowing through the switch tubes S1 and S2 respectively, t1 and t2 are the time parameters of the switch tubes S1 and S2 respectively, I r is the peak value of the resonant current, δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch, and θ is the impedance angle.

[0023] Further, in Step 4, according to the input and output characteristics of the resonant converter listed in Step 1, select the appropriate ratio A of series and parallel resonant capacitors and the quality factor Q of the resonant converter required at the rated load, and make the conduction angle δ of the resonant converter equal to π. Then, according to the voltage gain expression (3) in Step 3, draw the normalized voltage gain-frequency characteristic curve.

[0024] Further, in Step 5, according to the gain-frequency characteristic curve in Step 4, mark the maximum gain point Mmax, and at the same time set the gain margin k, let M = kMmax, and from the gain-frequency characteristic curve, mark the normalized switching angular frequency ω n = ωs / ω r .

[0025] Further, in Step 6, from the maximum gain point Mmax and the normalized switching angular frequency parameter ω n obtained in Step 5, solve the turns ratio of the resonant transformer according to the following formula:

[0026] n = kM max Vin_min / V0 (5)

[0027] Among them, in formula (5): n is the transformer turns ratio, k is the voltage gain margin, Mmax is the maximum gain point, V in_min is the minimum value of the input voltage, and V0 is the output voltage;

[0028] At the same time, calculate the parameters L r 、C p 、C s of the resonant converter as follows:

[0029]

[0030] Among them, in formulas (6), (7) and (8): L r is the resonant inductor, C p is the parallel resonant capacitor, C s is the series resonant capacitor, k is the voltage gain margin, Mmax is the maximum gain point, V in_min is the minimum value of the input voltage, ω n is the normalized switching angular frequency corresponding to the gain, Q is the quality factor of the converter, f s is the operating frequency, and P 0_max is the maximum output power.

[0031] Furthermore, the resonant current expressions of the two bridge arms in step seven are as follows:

[0032]

[0033] Among them, in formula (9): δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch, f s is the operating frequency, L a is the value of the auxiliary inductor, V in is the input voltage, I r is the peak value of the resonant current, θ is the impedance angle, and i r2 (ω s t2) and i s4 (ω s t1) are the currents flowing through the switch tubes S1 and S2 respectively;

[0034] Combined with the voltage gain formula (3), the expression of the phase-shifted control conduction angle δ of the leading-arm switch and the lagging-arm switch can be obtained as follows:

[0035]

[0036] Among them, in formula (10): δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch, ω nis the normalized switching angular frequency corresponding to the gain, M is the voltage gain of the converter, A is the ratio of the series and parallel resonant capacitors, Q is the quality factor of the converter, and n is the transformer turns ratio.

[0037] Further, according to the resonant current expressions of the two bridge arms in step seven, the relationship curves of the resonant current - auxiliary inductor are plotted respectively when the input voltage is maximum and minimum. From the two relationship curves of the resonant current - auxiliary inductor, when the auxiliary inductor L a is less than XuH, the currents i s2 (ω s t2) and i s4 (ω s t1) are always less than 0, satisfying the ZVS condition of the switching tube. X is the value taken from the relationship curve of the two resonant current - auxiliary inductors according to the technical index requirements of the actual situation; in addition, considering the error of the fundamental wave analysis method, a certain margin is reserved, and the auxiliary inductor L a is selected to be around (X - 3)uH.

[0038] The beneficial effects of the present invention are as follows: Therefore, in view of the problem of narrow ZVS range of the resonant converter with a fixed - frequency LCC topology, considering that it is very easy to lose the ZVS operating state, resulting in large switching losses, low system efficiency, and large EMI interference, etc., the present invention adopts an auxiliary network composed of a series connection of an auxiliary inductor and a DC - blocking capacitor and applies it to the resonant converter with a fixed - frequency LCC topology, and accordingly gives a parameter design method for optimizing the resonant converter with a fixed - frequency LCC topology, enabling the ZVS of the switching tube to be achieved within a wide range of input voltage and load variations, and having a small resonant current, limiting the circulating current loss of the converter, providing a design reference for converters that require a wide ZVS range for the switching tube, and being beneficial to device selection. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a parameter design flow chart for optimizing the resonant converter with a fixed - frequency LCC topology;

[0040] Figure 2 is the circuit topology diagram of the improved resonant converter with a fixed - frequency LCC topology;

[0041] Figure 3 is the normalized voltage gain - frequency characteristic curve graph;

[0042] Figure 4 is the relationship curve of the resonant currents of the two bridge arms and the auxiliary inductor when the input voltage is minimum;

[0043] Figure 5 is the relationship curve of the resonant currents of the two bridge arms and the auxiliary inductor when the input voltage is maximum. DETAILED DESCRIPTION OF THE INVENTION

[0044] As Figures 1 to 5 shown, in this embodiment, the present invention includes Step 1: This example mainly focuses on a power electronic conversion system based on an LCC topology resonant converter, with a required output power of 500 W, an operating frequency fs set to 100 kHz, an input voltage Vin of 100 - 200 V, and a maximum output voltage V0 of 48 V;

[0045] Step 2: Based on the resonant conversion circuit with a fixed-frequency LCC topology, an auxiliary network composed of an auxiliary inductor and a DC-blocking capacitor is added between the two bridge arms to obtain the improved resonant converter circuit topology diagram with a fixed-frequency LCC topology as shown in Figure 2 Figure; From this circuit topology diagram, the ZVS conditions of the resonant converter can be preliminarily obtained as follows:

[0046]

[0047] Where in Equation (1): ω s is the switching angular frequency, t1 and t2 are the time parameters of switch tubes S1 and S2 respectively, i s2 (ω s t2) and i s4 (ω s t1) are the currents flowing through switch tubes S1 and S2 respectively, i La (ω s t1) and i r (ω s t1) are the auxiliary network current and resonant current flowing through switch tube S1 respectively, i La (ω s t2) and i r (ω s t2) are the auxiliary network current and resonant current flowing through switch tube S2 respectively.

[0048] Step 3: Analyze the improved resonant converter with a fixed-frequency LCC topology using the fundamental wave analysis method, and the quality factor Q and voltage gain M of the resonant converter can be obtained as follows:

[0049]

[0050] Where in Equation (2) and Equation (3), Q is the quality factor of the resonant converter, M is the voltage gain of the resonant converter, R L is the load resistance, A is the series-parallel resonance capacitance ratio, ω s is the switching angular frequency, ω r is the series resonance angular frequency, n is the transformer turns ratio, j is the mathematical complex number symbol, and δ is the phase-shifted control conduction angle between the leading-arm switch tube and the lagging-arm switch tube;

[0051] From the voltage gain expression, the resonant current expressions flowing through the switching transistors S1 and S2 are as follows:

[0052]

[0053] Among them, in Equation (4): i r (ω s t1) and i r (ω s t2) are the resonant currents flowing through the switching transistors S1 and S2 respectively, t1 and t2 are the time parameters of the switching transistors S1 and S2 respectively, I r is the peak value of the resonant current, δ is the phase-shifted control conduction angle of the leading-arm switching transistor and the lagging-arm switching transistor, and θ is the impedance angle.

[0054] Step 4: According to the input and output characteristics of the resonant converter listed in Step 1, select a suitable series-parallel resonant capacitor ratio A of 1 and a required quality factor Q of 2 for the resonant converter under rated load here. The reason for taking Q as 2 here is that the quality factor is often set to 2 during design, and there is also the reason of facilitating calculation here. And make the conduction angle δ of the resonant converter equal to π. Draw the normalized voltage gain-frequency characteristic curve according to the voltage gain expression (3) in Step 3 as Figure 3 ;

[0055] Step 5: From the Figure 3 gain-frequency characteristic curve, mark the maximum gain point Mmax, and at the same time set the gain margin k = 0.85 - 0.95, let M = kMmax, and from the Figure 3 gain-frequency characteristic curve, mark the normalized switching angular frequency ω n = ω s / ω r ;

[0056] Step 6: According to the gain parameter and the normalized angular frequency parameter obtained in Step 5, solve the transformer turns ratio according to the following formula:

[0057] n = kM max V in_min / V0 (5)

[0058] Among them, in Equation (5): n is the transformer turns ratio, k is the voltage gain margin, Mmax is the maximum gain point, V in_min is the minimum value of the input voltage, and V0 is the output voltage;

[0059] At the same time, calculate the resonant converter parameters L r , C p , C s as follows:

[0060]

[0061] In formulas (6), (7) and (8): L r is the resonant inductor, C p is the parallel resonant capacitor, C s is the series resonant capacitor, k is the voltage gain margin, Mmax is the maximum gain point, V in_min is the minimum value of the input voltage, ω n is the normalized switching angular frequency corresponding to the gain, Q is the quality factor of the resonant converter, f s is the operating frequency, P 0_max is the maximum output power;

[0062] Substituting the index parameters in Step 1, the parameter of the resonant inductor L r = 85 uH, the capacitor C s = C p = 44 nF, and the transformer turns ratio n = 2.2.

[0063] Step 7: According to the relationship derivation in Steps 2 and 3, list the resonant current expressions of the two bridge arms as follows:

[0064]

[0065] In formula (9): δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch, f s is the operating frequency, L a is the value of the auxiliary inductor, V in is the input voltage, I r is the peak value of the resonant current, θ is the impedance angle, i s2 (ω s t2) and i s4 (ω s t1) are the currents flowing through the switches S1 and S2 respectively.

[0066] Combined with the voltage gain formula (3), the expression of the phase-shifted control conduction angle δ of the leading-arm switch and the lagging-arm switch is as follows:

[0067]

[0068] In formula (10): δ is the phase-shifted control conduction angle of the leading-arm switch and the lagging-arm switch, ω n is the normalized switching angular frequency corresponding to the gain, M is the voltage gain of the resonant converter, A is the ratio of the series and parallel resonant capacitors, Q is the quality factor of the resonant converter, and n is the transformer turns ratio.

[0069] Step 8: According to the resonant current expressions of the two bridge arms in Step 7, draw the relationship curve of the resonant current - auxiliary inductor, where Figure 4It is the relationship curve between the resonant currents of the two bridge arms and the auxiliary inductor when the input voltage is the minimum. Figure 5 It is the relationship curve between the resonant currents of the two bridge arms and the auxiliary inductor when the input voltage is the maximum.

[0070] According to Figure 4 and Figure 5 , it can be read that when the auxiliary inductor L a is less than 24 uH, the currents i s2 (ω s t2) and i s4 (ω s t1) are always less than 0, meeting the ZVS condition of the switching tubes; in addition, the error of the fundamental wave analysis method needs to be considered, and a certain margin is reserved. The auxiliary inductor L a can be selected to be about 21 uH. Thus, according to the requirements of the input-output characteristics, the parameter design of a resonant converter for optimizing the fixed-frequency LCC topology is completed.

[0071] Although the embodiments of the present invention are described with actual schemes, they do not constitute a limitation to the meaning of the present invention. For those skilled in the art, the modifications of its implementation schemes according to this specification and the combinations with other schemes are obvious.

Claims

1. A parameter design method for optimizing a fixed-frequency LCC topology circuit, characterized in that, It includes the following steps: Step 1: Define the input and output characteristics of the resonant converter with a fixed-frequency LCC topology, and confirm the requirements for various technical indicators, including input voltage, output voltage, output power, and switching frequency; Step 2: Based on the resonant conversion circuit with a fixed-frequency LCC topology, add an auxiliary network composed of an auxiliary inductor and a DC-blocking capacitor between the two bridge arms, and list the ZVS conditions of the corresponding resonant converter according to the improved circuit; Step 3: Adopt the fundamental wave analysis method to conduct the steady-state analysis of the fixed-frequency LCC topology resonant converter and solve the resonant current, and list the corresponding derivation relationships; Step 4: According to the input and output characteristics of the resonant converter, select the appropriate series-parallel resonance capacitance ratio and the quality factor of the resonant converter required under the rated load, and make the conduction angle of the resonant converter the largest, and draw the corresponding gain-frequency characteristic curve; Step 5: According to the quality factor of the rated load, mark the maximum gain point of the voltage on the curve, and at the same time set the gain margin. From the gain-frequency characteristic curve, mark the normalized switching angular frequency corresponding to the gain on the curve; Step 6: Solve the transformer turns ratio and calculate the resonant parameters of each resonant converter; Step 7: According to the relationship derivation in Steps 2 and 3, list the resonant current expressions of the two bridge arms, and calculate and derive the conduction angle expression of the fixed-frequency LCC structure topology resonant converter; Step 8: According to the resonant current expressions of the two bridge arms in Step 7, draw the relationship curve between the resonant current and the auxiliary inductor, solve the value of the auxiliary inductor that satisfies the ZVS condition of the fixed-frequency LCC structure topology resonant converter, and leave a certain margin.

2. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 1, characterized in that: The ZVS conditions of the converter in Step 2 are as follows: Among them, in formula (1): ω s is the switching angular frequency, t1 and t2 are the time parameters of switching transistors S1 and S2 respectively, and i s2 (ω s t2) and i s4 (ω s t1) are the currents flowing through switching transistors S1 and S2 respectively, and i La (ω s t1) and i r (ω s t1) are the auxiliary network current and the resonant current flowing through switching transistor S1 respectively, and i La (ω s t2) and i r (ω s t2) are the auxiliary network current and the resonant current flowing through switching transistor S2 respectively.

3. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 2, characterized in that: By using the fundamental wave analysis method to conduct the steady-state analysis of the improved fixed-frequency LCC topology resonant converter in Step 3, the quality factor Q and voltage gain M of the resonant converter can be obtained as follows: Among them, in Formula (2) and Formula (3), Q is the quality factor of the resonant converter, M is the voltage gain of the resonant converter, R L is the load resistance, L r is the resonant inductor, C s is the series resonant capacitor, A is the ratio of the series-parallel resonant capacitors, ω s is the switching angular frequency, ω r is the series resonant angular frequency, n is the transformer turns ratio, and δ is the phase-shifted control conduction angle of the leading-arm switch tube and the lagging-arm switch tube; From the voltage gain expression (3), the resonant current expressions flowing through the switching tubes S1 and S2 respectively are as follows: Wherein in formula (4): i r (ω s t1) and i r (ω s t2) are the resonant currents flowing through the switching tubes S1 and S2 respectively, t1 and t2 are the time parameters of the switching tubes S1 and S2 respectively, I r is the peak value of the resonant current, δ is the phase-shifted control conduction angle of the leading-arm switching tube and the lagging-arm switching tube, and θ is the impedance angle.

4. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 3, characterized in that: In Step 4, according to the input and output characteristics of the resonant converter listed in Step 1, select the appropriate series-parallel resonance capacitance ratio A and the quality factor Q of the resonant converter required under the rated load, and make the conduction angle δ of the resonant converter equal to π. Then, according to the voltage gain expression (3) in Step 3, draw the normalized voltage gain-frequency characteristic curve graph.

5. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 4, characterized in that: In step five, mark the maximum gain point M according to the gain-frequency characteristic curve in step four max , and set the gain margin k, making M = kM max， And from the gain-frequency characteristic curve, mark the normalized switching angular frequency ω corresponding to the gain n = ω s / ω r .

6. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 5, characterized in that: The maximum gain point M obtained from Step 5 in Step 6 max and the normalized switching angular frequency parameter ω n , solve the turns ratio of the resonant transformer according to the following formula: n = kM max V in_min / V0 (5) Among them, in formula (5): n is the transformer turns ratio, k is the voltage gain margin, M max is the maximum gain point, V in_min is the minimum value of the input voltage, V0 is the output voltage; Calculate the resonant converter parameters L r , C p , C s as follows: In formulas (6), (7) and (8): L r is the resonant inductor, C p is the parallel resonant capacitor, C s is the series resonant capacitor, k is the voltage gain margin, M max is the maximum gain point, V in_min is the minimum value of the input voltage, ω n is the normalized switching angular frequency corresponding to the gain, Q is the quality factor of the converter, f s is the operating frequency, P 0_max is the maximum output power.

7. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 2, characterized in that: The resonant current expressions of the two bridge arms in Step 7 are as follows: Among them, in formula (9): δ is the phase-shifted control conduction angle of the leading-arm switch tube and the lagging-arm switch tube, f s is the operating frequency, L a is the value of the auxiliary inductor, V in is the input voltage, I r is the peak value of the resonant current, θ is the impedance angle, i s2 (ω s t2) and i s4 (ω s t1) are the currents flowing through the switch tubes S1 and S2 respectively; Combined with the voltage gain formula (3), the expression of the phase-shifted control conduction angle δ of the leading-arm switching tube and the lagging-arm switching tube can be obtained as follows: In formula (10): δ is the phase-shifted control conduction angle of the leading-arm switch tube and the lagging-arm switch tube, ω n is the normalized switching angular frequency corresponding to the gain, M is the voltage gain of the converter, A is the series-parallel resonant capacitor ratio, Q is the quality factor of the converter, and n is the transformer turns ratio.

8. A parameter design method for optimizing a fixed-frequency LCC topology circuit according to claim 7, characterized in that: According to the resonance current expressions of the two bridge arms in step 7, draw the relationship curves of the resonance current - auxiliary inductor when the input voltage is maximum and minimum respectively. Read from the two resonance current - auxiliary inductor relationship curves that when the auxiliary inductor L a is less than XuH, the currents i s2 (ω s t2) and i s4 (ω s t1) are always less than 0, meeting the ZVS condition of the switching tube. X is the value taken from the relationship curve of the two resonance current - auxiliary inductors according to the technical index requirements of the actual situation; in addition, the error of the fundamental wave analysis method needs to be considered and a certain margin is reserved. The auxiliary inductor L a is selected to be around (X - 3)uH.

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