Fast load switching control method suitable for symmetric CLLC resonant converter

Abstract

The invention discloses a rapid load switching control method suitable for a symmetric CLLC resonant converter. The method comprises the following steps: step 1, building a symmetric CLLC resonant converter circuit; step 2, collecting an output current io and an output voltage vo in the symmetrical CLLC resonant converter circuit, namely, a current flowing through a load RL and voltages at two ends of the load RL; and step 3, calculating the absolute value delta io = io [n]-io [n-1] of the difference between the output current collected at the current moment and the output current collected at the previous moment, if the absolute value is less than or equal to a preset threshold current Ith, generating a driving square wave with variable frequency and a duty ratio of 50% through the collected output voltage vo by using a PI algorithm, so that the output voltage is stabilized near the reference voltage Vref, and the output voltage is stabilized near the reference voltage Vref. The circuit enters a steady state; and if the absolute value is greater than the preset threshold current Ith, load step occurs, the circuit enters a transient state from a steady state, and the transient process is quickly ended through track control, so that the next steady state is entered.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a fast load shedding control method suitable for a symmetrical CLLC resonant converter. Background Art

[0002] In power electronic converters, especially resonant converters such as CLLC (Capacitor-Inductor-Inductor-Capacitor) resonant converters, how to quickly switch to a new steady state when the load suddenly changes or changes is a technical challenge.

[0003] Traditional control methods mostly use PI controllers (Proportional Integral Controller) to adjust the output voltage or current. However, this method is difficult to quickly stabilize the system output when the load changes rapidly, resulting in long response time and poor control accuracy. Therefore, how to quickly cut the load and maintain system stability through a more efficient and responsive control strategy is an urgent problem to be solved in the current power electronics field. Summary of the invention

[0004] The present invention provides a fast load shedding control method suitable for a symmetrical CLLC resonant converter. By designing a control strategy based on a PI algorithm and combining it with a trajectory control method, the converter can quickly transition from one steady state to another when the load changes rapidly, thereby significantly improving the response speed and accuracy of the converter.

[0005] The technical solution of the present invention is as follows:

[0006] A fast load shedding control method applicable to a symmetrical CLLC resonant converter, characterized in that it comprises the following steps:

[0007] Step 1, build a symmetrical CLLC resonant converter circuit;

[0008] Step 2: collecting the output current i in the symmetrical CLLC resonant converter circuit o And the output voltage v o , that is, flows through the load R L The current and load R L The voltage across the two ends of

[0009] Step 3: Calculate the absolute value Δi of the difference between the output current collected at the current moment and the previous moment o =|i o [n]-i o [n-1]|, if the absolute value is less than or equal to the preset threshold current I th , then the collected output voltage vo , using the PI algorithm to generate a driving square wave with a frequency change and a duty cycle of 50%, so that the output voltage is stable at the reference voltage V ref If the absolute value is greater than the preset threshold current I th , indicating that a load step has occurred. At this time, the circuit enters a transient state from a steady state. The transient process is quickly ended through trajectory control, thus entering the next steady state.

[0010] In a possible implementation manner, the symmetrical CLLC resonant converter circuit in step 1 includes a primary inverter bridge, a resonant cavity, and a secondary rectifier bridge; in the primary inverter bridge, four SiC MOSFETs Q are driven 1 ~Q 4 The DC voltage V in It is converted into a two-level square wave with a duty cycle of 50%, and then passes through the resonant cavity, that is, the primary resonant inductor L 1 and resonant capacitor C 1 , high-frequency transformer with a turns ratio of n:1, secondary resonant inductor L 2 and resonant capacitor C 2 , and finally passes through switch S 1 ~S 4 The secondary rectifier bridge and output filter capacitor C o The load R is applied L The DC output voltage V o .

[0011] In a possible implementation manner, the absolute value of the current difference Δi calculated in step 3 is o As the basis for judging whether the load has a step, only when the absolute value of the current difference is greater than the preset threshold current I th When the control is turned on, it switches to trajectory control.

[0012] In a possible implementation manner, the load step in step 3 includes two situations, namely, the load resistance value increases when the load is unloaded and the load resistance value decreases when the load is loaded.

[0013] In a possible implementation, when the load is loaded, the duration of the current driving square wave is extended until the state trajectory touches the second steady-state trajectory, and then the PI control takes over the control of the driving waveform duration, and the extended time is obtained by calculation.

[0014] In a possible implementation, when the load is reduced, the duration of the next three consecutive square waves is controlled and then the PI control takes over the control, and the duration of the three consecutive square waves is obtained by calculation.

[0015] In a possible implementation manner, the carrier wave of the driving square wave generated by the PI control and trajectory control is a sawtooth wave.

[0016] The present invention has at least the following beneficial effects:

[0017] (1) Improve system response speed: By designing a trajectory control method, the system can be quickly adjusted when the load changes, thereby significantly reducing the response time.

[0018] (2) Improve system stability: By switching between PI control and trajectory control, the oscillation caused by hysteresis and over-regulation of the traditional PI controller is avoided, thereby improving the stability of the system.

[0019] (3) Reduce overshoot and oscillation: By accurately calculating the control parameters when the load changes, overshoot and oscillation are reduced and the steady-state accuracy is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 1 is a flowchart of a fast load shedding control method applicable to a symmetrical CLLC resonant converter according to an embodiment of the present invention.

[0021] Figure 2 FIG. 4 is a schematic diagram of a symmetrical CLLC resonant converter circuit in an embodiment of the present invention.

[0022] Figure 3 Schematic diagram of a control strategy for a symmetrical CLLC resonant converter in one embodiment of the present invention.

[0023] Figure 4 4 is a simulation circuit diagram of a symmetrical CLLC resonant converter in one embodiment of the present invention.

[0024] Figure 5 1 is a state mode diagram of a symmetrical CLLC resonant converter and its equivalent circuit diagram in one embodiment of the present invention.

[0025] Figure 6 The symmetrical CLLC resonant converter in one embodiment of the present invention corresponds to Figure 5 State trajectory diagram of each mode in .

[0026] Figure 7 Schematic diagram of the state trajectory of the symmetrical CLLC resonant converter trajectory control when the load is loaded (left) and MOSFETQ in one embodiment of the present invention 1 Schematic diagram of the corresponding driving waveform (right).

[0027] Figure 8 Schematic diagram of the state trajectory of the symmetrical CLLC resonant converter trajectory control during load shedding in one embodiment of the present invention (left) and MOSFETQ 1 Schematic diagram of the corresponding driving waveform (right).

[0028] Fig. 9The transient waveform of the symmetrical CLLC resonant converter when loaded in one embodiment of the present invention is divided into five parts from top to bottom. The first part is Q 1 and Q 2 Driving waveform, the second part is the resonant inductor current And the excitation inductor current waveform The third part is the waveform of the primary bridge arm midpoint voltage Vin, the transformer primary voltage Vm, and the converted secondary midpoint voltage Vo. The fourth part is the output current i o Waveform, the fifth part is the resonant capacitor voltage and Waveform.

[0029] Fig.10 for Fig. 9 Detailed view of the corresponding waveform.

[0030] Fig.11 1 is a state trajectory diagram of a symmetrical CLLC resonant converter when loaded according to an embodiment of the present invention.

[0031] Fig.12 1 is a transient waveform diagram of the output voltage of a symmetrical CLLC resonant converter when loaded according to an embodiment of the present invention.

[0032] Fig.13 This is a transient waveform of a symmetrical CLLC resonant converter during load shedding in an embodiment of the present invention, which is divided into five parts from top to bottom. The first part is Q 1 and Q 2 Driving waveform, the second part is the resonant inductor current And the excitation inductor current waveform The third part is the waveform of the primary bridge arm midpoint voltage Vin, the transformer primary voltage Vm, and the converted secondary midpoint voltage Vo. The fourth part is the output current i o Waveform, the fifth part is the resonant capacitor voltage and Waveform.

[0033] Fig.14 for Fig.13 Detailed view of the corresponding waveform.

[0034] Fig.15 This is a state trajectory diagram of a symmetrical CLLC resonant converter during load shedding in one embodiment of the present invention.

[0035] Fig.16 1 is a transient waveform diagram of the output voltage of a symmetrical CLLC resonant converter when the load is reduced in one embodiment of the present invention. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] See also Figure 1 , which is a fast load shedding control method applicable to a symmetrical CLLC resonant converter according to an embodiment of the present invention, comprises the following steps:

[0038] Step 1: Build a symmetrical CLLC resonant converter circuit;

[0039] Step 2: collecting the output current i in the symmetrical CLLC resonant converter circuit o And the output voltage v o , that is, flows through the load R L The current and R L The voltage across the two ends of the output current i o The purpose of the collection is to determine whether load shedding has occurred and whether the load is loading or unloading, and the output voltage v o The purpose of the acquisition is to achieve closed-loop PI control to stabilize the output voltage;

[0040] Step 3: Calculate the absolute value Δi of the difference between the output current collected at the current moment and the previous moment o =|i o [n]-i o [n-1]|, if the absolute value is less than or equal to the preset threshold current I th , then the collected output voltage v o , using the PI algorithm to generate a driving square wave with a frequency change and a duty cycle of 50%, so that the output voltage is stable at the reference voltage V ref If the absolute value is greater than the preset threshold current I th , indicating that a load step has occurred. At this time, the circuit enters a transient state from a steady state. The transient process is quickly ended through trajectory control, thus entering the next steady state.

[0041] In a specific application example, the schematic diagram of the symmetrical CLLC resonant converter is as follows: Figure 2 As shown in Figure 1, the topology of the symmetrical CLLC resonant converter consists of three parts: the primary inverter bridge, the resonant cavity, and the secondary rectifier bridge. In the primary inverter bridge, four SiC MOSFETs Q 1 ~Q 4 The DC V in It is converted into a two-level square wave with a duty cycle of 50%, and then passes through the resonant cavity, that is, the primary resonant inductor L1 and resonance C 1 , high-frequency transformer with a turns ratio of n:1, secondary resonant inductor L 2 and resonant capacitor C 2 , and finally passes through switch S 1 ~S 4 The secondary rectifier bridge and output filter capacitor C o Get the DC output voltage V o Here the secondary switch S 1 ~S 4 Either SiC MOSFET or SiC diode can be used for synchronous rectification.

[0042] Specifically, the input voltage is a DC voltage V in In transient state, the output voltage and output current are v o 、i o , in steady state, the output voltage and output current are V o ,I o The two resonant inductance values ​​and the resonant capacitance values ​​should satisfy L 1 =n 2 L 2 ,n 2 C r1 =C r2 , where L 1 and C 1 are the resonant inductor and capacitor in series on the primary side of the transformer, and L r2 and C r2 The symmetrical CLLC resonant converter should operate at a resonant frequency of To ensure the steady-state efficiency of the CLLC resonant converter and the accuracy of the trajectory control algorithm during load shedding. 1 and Q 3 Form a bridge arm, and Q 1 and Q 3 The driving waveform is a complementary square wave, and the two switches will not be turned on at the same time. For other switch tubes, such as Q 2 and Q 4 , S 1 and S 3 , S 2 and S 4 The same is true. And MOSFETQ 4 The driving square wave and Q 1 The same as Q 1 and Q 4 At the same time, it is turned on and off, which has an impact on Q 2 and Q 3 , S 1 and S4 , S 2 and S 3 Same reason.

[0043] Furthermore, the driving square wave generated by the PI algorithm in step 3 has a frequency that varies with the output voltage v o The absolute value of the calculated current difference Δi is adjusted according to the change of o As the basis for judging whether the load has a step, only when the absolute value of the current difference is greater than the preset threshold current I th When the control is switched to trajectory control, the overall control block diagram is as follows Figure 3 shown.

[0044] Furthermore, the load step in step 3 includes two cases, namely, the load resistance value increases when the load is unloaded and the load resistance value decreases when the load is loaded. When the load is loaded, the duration of the current driving square wave is extended until the state trajectory touches the second steady-state trajectory, and then the PI control takes over the control of the driving waveform duration, and the extended time is obtained by calculation. When the load is unloaded, the duration of the next three consecutive square waves is controlled, and then the PI control takes over the control, and the duration of the three consecutive square waves is obtained by calculation. The carrier of the driving square wave generated by PI control and trajectory control is a sawtooth wave.

[0045] Specifically, the PI algorithm in this embodiment is a positional PI algorithm, that is, the output of the PI algorithm is the primary switch Q 1 ~Q 4 The duration of high and low levels, or the frequency of change of high and low levels. 1 and Q 4 The driving waveform is consistent, MOSFETQ 2 and Q 3 The driving waveform is consistent. The collected output voltage is compared with the reference value to generate an error signal, which is then generated by the C language module OTC_PI1 to generate the primary switch Q 1 ~Q 4 The driving waveform is as follows: Figure 4 shown.

[0046] The resonant cavity state of the CLLC resonant converter can be roughly divided into six types according to the primary switching conditions and the secondary current. 1 and Q 4 Conductivity, yes There are three situations, called Mode1, Mode2 and Mode3. Figure 5 The specific states of the CLLC resonant converter in Mode 1 to Mode 3 and the corresponding equivalent circuit diagram are shown. 2 and Q 4 Conductivity, also The three situations are called Mode 4, Mode 5 and Mode 6.

[0047] When the switching frequency is at the resonant frequency When the converter is in Mode 1, the state expressions of each resonant element are as follows:

[0048]

[0049] in and

[0050]

[0051] make And through the following standardization process The parametric equation of the circular trajectory can be obtained:

[0052]

[0053] So in v sN -i sN You can draw on a flat surface Figure 6 (Left) The state trajectory, at this time the trajectory has a radius of R s , the center of the circle is located at (1-V′ oN ,0). The state trajectories of Mode 2 and Mode 3 are as follows: Figure 6 (middle), Figure 6 (right) shown.

[0054] The state trajectory diagrams of Mode 4, Mode 5 and Mode 6 are respectively compared with the state trajectory of Mode 1, Mode 2 and Mode 3 about axis v sN =0 symmetric.

[0055] Loading T 1+ and T 2+ The calculation is as follows: Assume that the load is loaded from becomes The steady-state output current is given by I o1 Change to I o2 , the radius of the steady-state trajectory is given by R s1 Change to R s2 , corresponding state trajectory diagram and switch tube Q 1 The driving waveform is shown as follows Figure 7 As shown in Figure 1, the bold part is the designed transient trajectory. The loading transient process uses the semicircular trajectory of Mode 1 and the partial elliptical trajectory of Mode 3.

[0056] Then we have:

[0057]

[0058] Combining the elliptical trajectory and the second steady-state trajectory equations, we get the horizontal coordinate x of the intersection point. 1 :

[0059]

[0060] in

[0061] Thus we get:

[0062]

[0063] in

[0064] Load reduction T 1- , T 2- and T 3- The calculation is as follows: Assume that the load is reduced from becomes The steady-state output current is given by I o2 Change to I o1 , the radius of the steady-state trajectory is given by R s2 Change to R s1 , corresponding state trajectory diagram and switch tube Q 1 The driving waveform is shown as follows Figure 7 As shown in Figure 1, the bold part is the designed transient trajectory. The three stages of the load shedding transient process use Mode 1, Mode 5, and a partial semicircle trajectory of Mode 1.

[0065]

[0066] in Indicates that the load is The corresponding parameter.

[0067]

[0068] Currently known T 1 ,T 2 , then let:

[0069]

[0070] Solve for T 3- , where R 0 (t 2 ), φ 0 (t 2 ), R 1 (t 2 ), φ1 (t 2 ) for use T 1- , T 2- Iteration results.

[0071] According to the symmetrical CLLC resonant converter example given in this embodiment, see Fig. 9 is the input voltage V in =400V, output voltage is 380V, and load steps from 380Ω to 95Ω. The transient waveform is divided into five parts from top to bottom. The first part is Q 1 and Q 2 Driving waveform, the second part is the resonant inductor current And the excitation inductor current waveform The third part is the waveform of the primary bridge arm midpoint voltage Vin, the transformer primary voltage Vm, and the converted secondary midpoint voltage Vo. The fourth part is the output current i o Waveform, the fifth part is the resonant capacitor voltage and Waveform; Fig.10 Under the same loading conditions Fig. 9 Detailed diagram of transient waveform at the moment of load step; Fig.11 It is the state trajectory diagram under the same loading conditions; Fig.12 is the output voltage V under the same loading conditions o Transient waveform diagram.

[0072] Fig.13 is the input voltage V in =400V, output voltage is 380V, and load steps from 95Ω to 380Ω. The transient waveform is divided into five parts from top to bottom. The first part is Q 1 and Q 2 Driving waveform, the second part is the resonant inductor current And the excitation inductor current waveform The third part is the waveform of the primary bridge arm midpoint voltage Vin, the transformer primary voltage Vm, and the converted secondary midpoint voltage Vo. The fourth part is the output current i o Waveform, the fifth part is the resonant capacitor voltage and Waveform; Fig.14 Under the same load reduction conditions Fig.13 Detailed diagram of transient waveform at the moment of load step; Fig.15 It is the state trajectory diagram under the same load reduction condition; Fig.16 is the output voltage V under the same load reduction condition o Transient waveform diagram.

[0073] The above-mentioned fast load shedding control method for a symmetrical CLLC resonant converter obtains the precise expression of each state variable at the resonant frequency by decoupling and analyzing the state space equation of the CLLC resonant converter. For the load loading and unloading process, the corresponding transient trajectory is designed so that the trajectory can touch the steady-state trajectory as much as possible within one switching cycle. The simulation results show that the state trajectory touches the steady-state trajectory in a very short time and tends to be stable after a small oscillation. Compared with the traditional control method, the present invention significantly reduces the overshoot of each state variable and the output voltage through transient trajectory control, shortens the adjustment time of the system, realizes the optimal trajectory design of the symmetrical CLLC resonant converter from one steady state to another, and improves the dynamic performance and control accuracy of the system.

[0074] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principles and spirit of the present invention, these embodiments, including components, are subjected to various changes, modifications, substitutions and modifications, which still fall within the scope of protection of the present invention. It should be understood that the exemplary embodiments described herein are illustrative and not restrictive. Although one or more embodiments of the present invention are described in conjunction with the accompanying drawings, it should be understood by those of ordinary skill in the art that various changes in form and detail may be made without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A fast load shedding control method for a symmetrical CLLC resonant converter, characterized in that: The following steps are involved: Step 1, build a symmetrical CLLC resonant converter circuit; Step 2: collecting the output current i in the symmetrical CLLC resonant converter circuit o And the output voltage v o , that is, flows through the load R # The current and load R # The voltage across the two ends of Step 3: Calculate the absolute value Δi of the difference between the output current collected at the current moment and the previous moment o =|i o [n]-i o [n-1]|, if the absolute value is less than or equal to the preset threshold current I $％ , then the collected output voltage v o , using the PI algorithm to generate a driving square wave with a frequency change and a duty cycle of 50%, so that the output voltage is stable at the reference voltage V re( If the absolute value is greater than the preset threshold current I $％ , indicating that a load step has occurred. At this time, the circuit enters a transient state from a steady state. The transient process is quickly ended through trajectory control, thus entering the next steady state.

2. The fast load shedding control method for a symmetrical CLLC resonant converter according to claim 1, characterized in that: The symmetrical CLLC resonant converter circuit in step 1 includes a primary inverter bridge, a resonant cavity and a secondary rectifier bridge; in the primary inverter bridge, the DC voltage V is converted to +, It is converted into a two-level square wave with a duty cycle of 50%, and then passes through the resonant cavity, namely the primary resonant inductor L1 and resonant capacitor C1, the high-frequency transformer with a turns ratio of n:1, and the secondary resonant inductor L - and resonant capacitor C - Finally, the secondary rectifier bridge formed by switches S1~S4 and the output filter capacitor C o The load R is applied # The DC output voltage V o .

3. The fast load shedding control method applicable to a symmetrical CLLC resonant converter according to claim 1, characterized in that: The absolute value of the current difference Δi calculated in step 3 o As the basis for judging whether the load has a step, only when the absolute value of the current difference is greater than the preset threshold current I $％ When the control is turned on, it switches to trajectory control.

4. The fast load shedding control method for a symmetrical CLLC resonant converter according to claim 1, characterized in that: The load step in step 3 includes two situations, namely, the load resistance value increases when the load is unloaded and the load resistance value decreases when the load is loaded.

5. The fast load shedding control method for a symmetrical CLLC resonant converter according to claim 4, characterized in that: When the load is loaded, the duration of the current driving square wave is extended until the state trajectory touches the second steady-state trajectory, and then the PI control takes over the control of the driving waveform duration. The extended time is calculated.

6. The fast load shedding control method for a symmetrical CLLC resonant converter according to claim 4, characterized in that: When the load is reduced, the control drives the duration of the next three consecutive square waves, and then the PI control takes over the control. The duration of the three consecutive square waves is calculated.

7. The fast load shedding control method for a symmetrical CLLC resonant converter according to claim 1, characterized in that: The carrier wave of the driving square wave generated by the PI control and trajectory control is a sawtooth wave.

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