Energy efficiency optimization model of low-output variable-power power supply resonant converter

Abstract

According to the energy efficiency optimization model of the low-output variable-power power supply resonant converter, the element parameter selection method of the resonant converter is determined through the steady-state characteristics and the state expression of the converter, and the mathematical model is established for the power loss device of the resonant converter in combination with the operation parameters and the element parameters; converting an optimization objective function according to the established loss model, taking switching frequency as an optimization variable, taking inverter efficiency as an objective, combining element parameters and constraint conditions formed in a steady-state analysis expression, determining an optimization objective, and optimizing the overall energy efficiency of the resonant converter with variable output power; when the resonant converter operates at a high control frequency, the switching energy consumption is greatly reduced, zero-voltage turn-on and turn-off are easily completed, the converter can operate at the high control frequency, the energy efficiency of the power supply resonant converter can be optimized to the greatest extent under the condition of low output variable power, and the safety and stability of a power supply system can be ensured.

Description

Technical Field

[0001] The present application relates to an energy efficiency optimization model for a power resonant converter, and in particular to an energy efficiency optimization model for a low-output variable-power power resonant converter, belonging to the technical field of power resonant converter optimization. Background Art

[0002] As the application of DC / DC power supply in industries such as computers, communications and the Internet becomes more and more widespread, people have higher and higher requirements on the volume and power density of power converters, and their performance and efficiency have become the focus of attention. Since resonant converters have high frequency characteristics and can meet the above requirements, their research, development and application have attracted attention.

[0003] Nowadays, distributed power supply systems are very popular, and people have higher requirements for the power consumption and heat dissipation capabilities of DC converters. How to efficiently use energy, especially the efficient use of electric energy, has become a research and development hotspot. On the one hand, it is because more attention is paid to energy consumption and environmental issues. On the other hand, compared with traditional power supplies, people are now paying attention to the power efficiency of electrical appliances with constantly changing actual loads, rather than the full-load efficiency of traditional power supplies. Therefore, in order to save electric energy efficiently, the average efficiency of electrical appliances under load changes rather than the full-load efficiency of electrical appliances has begun to be pursued. At the same time, since the power supply of many information devices is often in standby or light load during use, such as laptops, mobile phone chargers and other devices, although their standby power consumption is small, in the case of a huge base, the number of devices and their power loss are still very huge and amazing. The efficiency of light-load electrical power supplies and the energy loss of standby are regarded as important factors in power supply manufacturing.

[0004] The shape and volume of DC / DC converter power supplies are becoming smaller and smaller, resulting in higher requirements for their power density. High-frequency control methods can directly reduce the space volume of DC / DC converter power supplies and improve their power density and operating efficiency. Therefore, if you want to improve the power density of the DC converter and its efficiency during operation, you need to optimize the operating state and performance of the converter at a higher control frequency, so that it has better efficiency during operation and reduces power loss. The structure of the half-bridge PWM converter is similar to that of the resonant converter, but the control method is completely different. The voltage regulation of the PWM converter is achieved by changing the duty cycle of the switch tube, while the voltage change of the resonant converter is related to the change of the switching frequency. The control method of the resonant converter belongs to soft switching control, while that of the PWM converter belongs to hard switching, so its loss will be greater than the former. And when the floating range of the input voltage drop is large, if the PWM converter wants to change the output voltage drop, it needs a duty cycle with a large variable range, which will cause serious deviation of the duty cycle during feedback adjustment, but there is no such situation in the resonant converter.

[0005] Resonant converters are one of the mainstream methods to solve power application problems, such as laptop adapters. In recent years, the dynamic performance of adapters has been required to be better and better. Current shocks often occur on the load side, such as a sudden change from zero load to full load, so the dynamic performance of resonant converters is the focus of research and development.

[0006] The problems that need to be solved in optimizing the energy efficiency of the power resonant converter in the prior art and the key technical difficulties of this application include:

[0007] (1) The energy efficiency of the prior art resonant converter has a large room for optimization, especially at low output power, the working efficiency is low, there is a lack of efficiency optimization methods based on loss models, and there is a lack of its application in variable power resonant converters. The input voltage limit range and control frequency range involved in the steady-state characteristics are not considered, the operating state of the converter at different frequencies is not analyzed, and the steady-state research cannot be carried out according to the operating process. The relationship characteristics between its parameters are not understood. There is no detailed analysis and calculation of the operating parameters of the resonant converter, the loss expression is not established according to the calculated effective values ​​of the operating parameters and the characteristics of different components, and there is a lack of a comprehensive efficiency expression as an optimization target. There is a lack of loss models for resonant converters, and it is impossible to design appropriate parameters according to their characteristic curves and customer needs. The optimization variable is not selected as the control frequency, and the efficiency cannot be optimized with the control frequency as a variable. There is a lack of MATLAB calculation and simulation of the optimization process and results, and the optimal solution of the model cannot be obtained. The energy efficiency model of the power resonant converter in the prior art is quite different from the actual situation, and the accuracy and timeliness are relatively poor, which is not conducive to optimizing the energy efficiency of the power resonant converter.

[0008] (2) The prior art lacks research on the loss of resonant converters, and cannot determine the method for selecting the component parameters of resonant converters through the steady-state characteristics and state expressions of the converters. It does not establish a mathematical model for the power loss devices of resonant converters by combining the operating parameters and component parameters, and cannot transform the optimization objective function according to the established loss model. It lacks the optimization objective determined by combining the switching frequency as the optimization variable and the inverter efficiency as the target, and the constraint conditions formed by the component parameters and the steady-state analysis expression. It cannot optimize the overall energy efficiency of the resonant converter with variable output power. Moreover, the prior art optimization algorithm is difficult to quantitatively prove, has randomness, and its convergence is probabilistic. The actual specific performance is uncontrollable, and the convergence speed is relatively slow. It has high computational complexity, too high hardware requirements, and average convergence speed. In addition, the objective function only has one variable to be optimized, the switching frequency. The prior art algorithm is too complex and not very practical. However, because the optimization problem contains complex constraints, it is difficult to directly optimize and solve it. Therefore, it is necessary to simplify the optimization model appropriately. The switching frequency and efficiency of the prior art resonant converter are not optimized enough, and the converter cannot achieve high energy efficiency.

[0009] (3) The energy efficiency model of the low-output power resonant converter in the existing technology has many errors. It is impossible to obtain the optimal frequency and the most efficient curve with the output power. The optimized frequency is not optimal. The loss test results are quite different from the theoretical values. The feasibility of loss modeling is low. The correctness of the resonant converter energy efficiency model cannot be guaranteed. Therefore, it cannot be well applied to the actual operation of the DC converter. The power resonant converter has low energy efficiency and poor stability. The optimization algorithm is not effective. It is impossible to timely and accurately diagnose the resonant converter fault and cannot handle the converter fault in time, which is not conducive to ensuring the safety and stability of the power supply system. Summary of the invention

[0010] The present application determines the component parameter selection method of the resonant converter through the steady-state characteristics and state expressions of the converter, and establishes a mathematical model for the power loss device of the resonant converter in combination with the operating parameters and component parameters. According to the established loss model, the optimization objective function is transformed, and the switching frequency is used as the optimization variable and the inverter efficiency is used as the target. The optimization target is determined in combination with the component parameters and the constraints formed in the steady-state analysis expression, and the overall energy efficiency of the resonant converter with variable output power is optimized; the energy consumption of the converter is effectively reduced, and the control frequency of the power tube and the overall efficiency of the converter are significantly improved. In the soft switching topology, the resonant converter greatly reduces the switching energy consumption when operating at a higher control frequency, and its zero voltage turn-on and turn-off are easily completed. The converter can operate at a higher control frequency. Under the condition of low output variable power, the energy efficiency of the power resonant converter can also be optimized to the greatest extent, which is beneficial to ensure the safety and stability of the power supply system.

[0011] In order to achieve the above technical effects, the technical solutions adopted in this application are as follows:

[0012] Preferably,

[0013] Compared with the prior art, the innovations and advantages of this application are:

[0014] (1) The present application determines the component parameter selection method of the resonant converter through the steady-state characteristics and state expression of the converter, and establishes a mathematical model for the power loss device of the resonant converter in combination with the operating parameters and component parameters. According to the established loss model, the optimization objective function is transformed, and the switching frequency is used as the optimization variable and the inverter efficiency is used as the target. The optimization target is determined in combination with the component parameters and the constraints formed in the steady-state analysis expression, and the overall energy efficiency of the resonant converter with variable output power is optimized; the energy consumption of the converter is effectively reduced, and the control frequency of the power tube and the overall efficiency of the converter are significantly improved. In the soft switching topology, the resonant converter greatly reduces the switching energy consumption when operating at a higher control frequency, and its zero voltage start and shutdown are easily completed. The converter can operate at a higher control frequency. Under the condition of low output variable power, the energy efficiency of the power resonant converter can also be optimized to the greatest extent, which is beneficial to ensure the safety and stability of the power supply system.

[0015] (2) The present application establishes a low-output converter variable power loss model, analyzes the instantaneous and effective current at the input end and the instantaneous and effective current at the output end during the circuit operation process, determines the operating parameters of the resonant converter, establishes a model for the loss of the power MOS tube, inductor and rectifier tube, and jointly calculates to obtain the overall energy consumption expression to establish the optimization target; the calculation and selection of converter parameters are more scientific and reasonable, and the components are designed according to the determined parameters, the withstand voltage value of the switch tube and the maximum current allowed to pass are set, and the switch delay time is determined to meet the dead zone requirements and the start-up voltage meets the control voltage involved in its own drive; the filter inductor is selected to set the core model and the diameter and number of turns of the coil, and according to the known inductor parameters, the effective internal resistance of the inductor and the magnetic loss density of the core are reduced, the energy consumption of the inductor during operation is reduced, and the converter efficiency is improved; the power supply energy consumption is effectively reduced, so that the control frequency of the power tube and the overall efficiency of the converter are significantly improved, and the resonant converter can greatly reduce the switching energy consumption when operating at a higher control frequency, and its zero voltage start and shutdown are easily completed. The converter can operate safely and stably at a higher control frequency while ensuring high energy efficiency.

[0016] (3) The present application optimizes the variable power efficiency of the converter loss model. The specific mathematical model and actual function of the components of the resonant converter are analyzed through formula transformation. The calculated component parameters are brought into the loss model, and the optimization is performed according to the total loss. After the output power changes, the optimization variable is set to the control frequency, and the efficiency is optimized with the control frequency as the variable. The optimization process and results are simulated by MATLAB to obtain the optimal solution of the model, and the optimal control frequency is found to control the converter, so that the converter efficiency is maximized, and the volume and power density of the power converter meet the increasingly high requirements, and its performance and efficiency are greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a comparison chart of switch tube losses under different k values ​​based on experimental data.

[0018] Figure 2 This is a diagram showing the calculation and selection results of the converter parameters.

[0019] Figure 3 is the variable power efficiency optimization flow chart of the converter loss model.

[0020] Figure 4 This is a graph of converter loss versus output power.

[0021] Figure 5 This is the curve of optimal efficiency changing with output power.

[0022] Figure 6 It is a comparison curve of optimization and constant frequency control efficiency.

[0023] Figure 7 This is a schematic diagram comparing simulation and saber verification parameters.

[0024] Figure 8 This is a schematic diagram comparing simulation and saber verification parameters.

[0025] Fig. 9 It is a schematic diagram comparing simulation and saber verification parameters.

[0026] Fig.10 This is a comparison chart between model calculation and Saber experimental data. DETAILED DESCRIPTION

[0027] The following, in conjunction with the accompanying drawings, further describes the technical solution of the energy efficiency optimization model of the low output variable power supply resonant converter provided in the present application, so that technicians in this field can better understand the present application and implement it.

[0028] As the application of DC / DC power supplies in industries such as computers, communications, and the Internet becomes more and more widespread, people have higher and higher requirements for the volume and power density of power converters, and their performance and efficiency have also become the focus of attention. Because resonant converters have high-frequency characteristics and can meet the above requirements, people have paid great attention to their research, development, and application in recent years. This application proposes an efficiency optimization method based on a loss model to address the energy efficiency issues of resonant converters, especially at lower output powers, and applies it to variable-power resonant converters.

[0029] This application first analyzes the operation mode and loss source of the resonant converter. Based on the operating waveform of the resonant converter, the working process of the converter at different stages and the overall steady-state characteristics are analyzed, so as to facilitate the calculation and analysis of the operating parameters of the resonant converter. Secondly, the component parameter selection method of the resonant converter is determined by the steady-state characteristics and state expression of the converter, and a mathematical model of the power loss device of the resonant converter is established in combination with the operating parameters and component parameters. According to the established loss model, the optimization objective function is transformed, with the switching frequency as the optimization variable and the inverter efficiency as the target. Combined with the constraints formed in the component parameters and the steady-state analysis expression, the working goal of this application is determined: to optimize the overall energy efficiency of the LLC resonant converter with variable output power.

[0030] Finally, combined with the existing optimization algorithm, the optimization method of the problem is established. Through simulation calculation, the effects of constant frequency and optimized frequency on the efficiency of the resonant converter are compared, and specific conclusions are drawn. The model optimization results are verified using the "hardware" simulation software Saber, which shows that the established mathematical optimization model is effective and can be applied in real resonant converters.

[0031] 1. Establishing a low output converter variable power loss model

[0032] First, the power parameter values ​​of the converter are determined, and then the power value is used to build a mathematical model of the power device, and the final loss and total efficiency model of the converter is comprehensively established.

[0033] (I) Analysis and determination of resonant converter operating parameters

[0034] In half a switching cycle, the resonant converter is divided into two working phases, and the analysis frequency is less than f r In this case, the parameters at other frequencies are included in the expression for this case.

[0035] The first operation phase includes the time from t0 to t1. During this period, D1 of the downstream stage is turned on and only the series device C is in the resonant network. r With L rUnder the action of input and output voltage, the transformer voltage drop at one end of the front stage is clamped at +nV0 by the voltage drop at the rear stage. m At this moment, the current is charged and increases linearly, which does not resonate with the previous two. The voltage differential equation of the entire circuit loop is:

[0036]

[0037] Where L r is the series resonant inductor, C r is the resonant capacitor, V in is the input DC voltage, i p (t) is the current of the resonant circuit at the primary end of the converter, V Cr is the resonant capacitor C r The voltage across the two ends of , n is the turns ratio of the transformer, and V0 is the output voltage;

[0038] Pre-stage L m The current I Lm The magnitude is equal and the direction is opposite in the whole control cycle, and t2-t0=T s / 2,T s is the control period, we get:

[0039] i Lm (t0) = -i Lm (t2) Formula 3

[0040] The duration of phase 2 is short, and the approximate current I Lm In the second operation phase, we get:

[0041] i Lm (t1) = i Lm (t2) Equation 4

[0042] i Lm (t0) = -i Lm (t1) Equation 5

[0043] During the second stage, the output current is equal to 0, and the input main circuit current is equal to L m The current is equal to:

[0044] i p (t0) = i Lm (t0) = -i Lm (t1)=-i p (t1) Equation 6

[0045] In the first stage, the primary resonant current is a sinusoidal resonant current, and we get:

[0046]

[0047] Where T r is the series resonant cycle; in stage 1, the inductor L m The voltage is nV0, combined with the formula of inductor voltage and current changes Get the inductance L m The amount of change in phase 1:

[0048]

[0049] Substitute the equation of equation 6 into equation 8 and solve for I Lm (t0) and I Lm (t1):

[0050]

[0051] Where f r is the series resonant frequency, the series resonant capacitor C r When participating in resonance, the DC is blocked, C r The terminal voltage will contain a DC component, C r The DC component on the in / 2, and it also contains an AC component, but the mean value of the AC component during period T is 0, and it is symmetrical in positive and negative directions, so we get:

[0052]

[0053] The effective mean current of the converter's rear stage is I0, while the current flowing through the rectifier tube during the second operation is always 0, L m The current I Lm In half of the operating cycle T r The average value during the period of / 2 is equal to 0, and the input current I p The average current during the first stage is equal to the average current of the primary side of the transformer, and the average current value of the front stage transformer can be converted by the average current value of the rear stage.

[0054] 2. Modeling of Resonant Converter Component Losses

[0055] 1. Loss model of power switch tube

[0056] The energy consumption of the converter's front-stage power tube is divided into three parts: driving loss, on-state loss, and off-state loss. The driving loss of the field effect transistor is P dr_s for:

[0057] P dr_s =Q g_s V gs_s f s Formula 12

[0058] Where Q g_s is the gate charge of the previous field effect transistor, Vgs_s is the voltage drop value of the power tube when it is turned on, f s Control frequency for power tube;

[0059] The heat dissipation of the power tube conduction loss of the converter at the time of the switch tube turning on is affected by the on resistance of the field effect transistor, and its expression Pcon_s is:

[0060] P con_s =I p_rms 2 R s_on Formula 13

[0061] Where I p_rms I is the converter front-stage main circuit current p The effective value, R s_on It is the resistance value when the power tube is turned on;

[0062] Converter switch turn-off loss P off_s The calculation is as follows: The front-end bridge arm power tube Q of the resonant converter s1 The shutdown process is equivalent to the current value I during the shutdown period off Constant, assuming that during the off period the power tube Q s1 The current is at t f The linear decrease to 0 within a certain time period, the expression of the switch tube current during the shutdown process is:

[0063]

[0064] Assumption C s1 =C s2 , then flows through C s1 , C s2 The current is:

[0065]

[0066] During shutdown, C s1 Voltage:

[0067]

[0068] Switching tube Q s1 The turn-off loss is:

[0069]

[0070] Where I off Represents the off current, that is, I Lm The maximum value of I Lm_pk , t f Represents the power tube current drop time, C s1 Represents the equivalent output junction capacitance of the power tube, f s To control the frequency.

[0071] 2. Resonant inductor loss model

[0072] Ignoring the skin effect and proximity effect of the wire, assume that the internal resistance of the resonant inductor is R L , the effective value of the primary current of the converter through the coil is I rms , then the copper loss of the resonant inductor in the circuit is:

[0073] P cu_L =I rms 2 R L Formula 18

[0074] The inductor in the converter uses iron oxide with a gap as the magnetic core, and its maximum magnetic induction intensity is expressed as:

[0075]

[0076] n L is the number of inductors L, I p_pk is the resonant current peak value, l L is the air gap length of the core.

[0077] 3. Transformer loss model

[0078] Energy consumption is divided into two parts, copper loss and magnetic loss. Transformer copper loss is divided into two parts, one is generated by the primary inductance resistance, and the other is generated by the secondary inductance. The specific expression is:

[0079] P cu_T =I 2 p_rms R pri_ac +I 2 s_rms R sec_ac Formula 20

[0080] Where I p_rms is the primary current of the transformer I p Effective value, current I s_rms is the effective value of the secondary side of the transformer, R pri_ac and R sec_ac are the transformer primary and secondary AC equivalent resistances,

[0081] The magnetic loss of the transformer is determined by the magnetic induction intensity. The converter is divided into two operating intervals during this operating cycle. The maximum magnetic induction intensity of the transformer in different intervals is different. It is divided into two intervals. In the first interval, the voltage drop across the front transformer is v p is +nV0. In the second stage, the excitation inductance L mThe current flowing through the transformer does not change, and the magnetic flux in the transformer core does not change.

[0082] 4. Rectifier diode loss

[0083] When the converter is running in the rectification period, when the current flows through the converter's post-stage rectifier tube, part of the power consumption is dissipated in the form of heat. The forward voltage drop of the rectifier tube is 0.7V. In one cycle, the conduction time of each rectifier tube is π / 2, and the current passing through it is the output current.

[0084] 2. Calculation and selection of converter parameters

[0085] The normalized voltage gain M(f) (hereinafter referred to as voltage gain) is defined as:

[0086]

[0087] VIN F is the effective value of the fundamental component, Vo F is the fundamental effective value, H(s) is the transfer function of the resonant network, and after fixing the inductance ratio λ and the transformer coil ratio n, the gain M is obtained when the circuit quality factor Q value is different. dc With the frequency ratio f n The changing relationship M(f) is obtained as:

[0088]

[0089] The known condition is the input voltage range V max and V min , rated input voltage V in , rated output power V O , resonant frequency f r , the control frequency works at f r When the load change does not cause the gain change, the values ​​of the following variables are obtained:

[0090] 1) When the converter is operating under all rated variables, the control frequency operates at frequency f r , at this time f n =1,M=1,V O / V in =M dc =1 / 2n, find the value of transformer winding ratio n;

[0091] 2) By V max 、V min and V in Find M max (f), M min (f): M max =2n*VO / V min ,M min =2n*V O / V max ;

[0092] 3) Let k = 1 / λ. Under the same load variation range, the smaller the k value, the narrower the frequency adjustment range in which the gain M remains unchanged. Figure 1 This is the data diagram obtained from engineering and experiments. When the frequency and capacitance are determined, the inductance L r If it is determined, the k value becomes larger, L m As it gets bigger, the energy consumption of the power tube decreases. The value of k is better between 2 and 6. Select k = 3.5, that is, A = 0.2857;

[0093] 4) When the load is zero, Q = 0, M = M min When , we can find the maximum limiting frequency:

[0094]

[0095] The minimum frequency needs to be found with an appropriate Q value;

[0096] 5) At the same gain, the value of Q increases, and the frequency ratio f n Reduce, find the maximum Q value to determine the minimum frequency, the impedance Z of the resonant circuit network in for:

[0097]

[0098] The imaginary part in the above formula is 0, which is the capacitive-inductive dividing line. The converter works in the inductive region to achieve zero voltage switching and obtain the maximum Q value Q max :

[0099]

[0100] Q max Value and M max Substituting into equation 22, the minimum switching frequency f is calculated. min :

[0101]

[0102] 6) The value of Q is 95% of Q max , leaving some margin for rated power output;

[0103] By Q, R ac , f r , λ calculates L r , C r , L m , according to the above parameter design process, the required parameters are obtained, such as Figure 2 .

[0104] After calculating and determining the parameter values ​​of specific components, design the components according to the determined parameters. Consider whether all parameters of the switch tube are suitable for the circuit characteristics and the impact on the circuit performance. First, set the withstand voltage value of the switch tube and the maximum current allowed to pass. Then determine whether its switch delay time meets the dead zone requirements, whether the maximum switching frequency meets the requirements, and whether the start-up voltage meets the control voltage involved in its own drive.

[0105] Select the filter inductor to set the core model, coil diameter, and number of turns. Based on the known inductor parameters, try to reduce the effective internal resistance of the inductor and the magnetic loss density of the core, thereby reducing the energy consumption of the inductor during operation and improving the efficiency of the converter.

[0106] 3. Variable Power Efficiency Optimization of Converter Loss Model

[0107] According to the constructed resonant converter component loss model, the loss of the converter as a whole is obtained as:

[0108] P loss =2*(P dr_s +P con_s +P off_s )+P cu_L +P fe_L +P cu_T +P fe_T +P diode Formula 27

[0109] Ignoring capacitor losses, the overall efficiency of the converter is:

[0110]

[0111] In the formula, the working efficiency η is a function of the switching frequency f. The switching frequency under different output powers is different, and the efficiency is also different. s Optimize to achieve the best efficiency, set:

[0112]

[0113] To optimize the goal.

[0114] (I) Constraints

[0115] The switching frequency of the power switch tube is set with clear upper and lower limits f min and f max , and should be greater than L r , C r , L m The control frequency f when the three resonate together m , the frequency constraint is Max(fm , f min )<f s <f max , the power change of the subsequent load is also constrained, that is, it cannot exceed the rated load power P O , 0<P out <P O At the same power, the input voltage value is different, the control frequency of the switch is also different, the input voltage value and limit range also need to be considered, that is, V min <V in <V max ;

[0116] The optimal switching frequency problem of the resonant converter at different powers is converted into the problem of finding the maximum target value under constraints:

[0117]

[0118] The converter loss-to-power efficiency model is obtained.

[0119] (II) Optimization Method

[0120] Taking into account the characteristics and shortcomings of traditional optimization and intelligent optimization methods, the actual situation needs to be considered when choosing the two methods. The differences between the two methods can be roughly divided into the following points:

[0121] (1) The optimization problems faced by traditional optimization algorithms generally have specific structures, and the constraints of the problems are relatively clear. For example, optimization problems such as linear programming, quadratic programming, integrated programming, integer programming, and programming with or without constraints are problems with relatively clear structural information. However, the problems faced by intelligent optimization algorithms are more universal, and their structural parameters and other information are often unclear.

[0122] (2) The optimization problems solved by traditional optimization algorithms all belong to the category of concave function or convex function optimization problems, and have relatively clear global optimal values; while the optimization problems solved by intelligent optimization algorithms all contain many extreme values. The advantage is that when the algorithm is running, it can find the global optimal solution while not letting the algorithm fall into the range of local optimal values. When facing optimization problems with a single extreme value, traditional optimization algorithms can effectively solve them. In comparison, intelligent optimization algorithms have no advantages in solving such problems. However, when facing optimization problems with multiple extreme values, intelligent optimization algorithms can not only jump out of the local optimal solution, but also ensure good convergence, and then find the global optimal solution, which is incomparable to traditional optimization algorithms. However, after improving the algorithms with special structures, traditional optimization algorithms still have a wide range of applications.

[0123] (3) Traditional optimization algorithms have clear and fixed structures and parameters for their optimization objectives. They are all deterministic algorithms, and their algorithm convergence and computational complexity can be analyzed theoretically. Intelligent optimization algorithms are all heuristic algorithms that can be analyzed qualitatively, but quantitative proof is difficult. In addition, most optimization algorithms are random, and their convergence is probabilistic. The actual performance is uncontrollable, and the convergence speed is relatively slow, with high computational complexity.

[0124] According to the characteristics and application analysis of the above two optimization algorithms, the problem to be solved in this application belongs to nonlinear optimization problem. However, due to the relatively complex calculation of intelligent optimization algorithm, the high hardware requirements, the average convergence speed, and the special optimization problem of this application, there is only one variable to be optimized in the objective function, which is the switching frequency. The use of intelligent optimization algorithm is too complicated and not very practical. However, because the optimization problem contains more complex constraints, it is difficult to directly optimize and solve it, so it is necessary to simplify the optimization model appropriately.

[0125] The load power at the output of the resonant converter is P out When the output voltage is the expected value and fixed, I O The size of the load power is P out To ensure the maximum conversion efficiency of the resonant converter is to ensure that the operating loss of the resonant converter under this power is P loss The optimization problem is finally transformed into the minimum sum of the following four component losses:

[0126] min P loss =P MosFet +P L +P T +P diode

[0127] stf min ≤f s ≤f max

[0128] V min ≤V in ≤V max Formula 33

[0129]

[0130] When the voltage drop V0 at the downstream load is determined, the total loss of the converter is only related to the output power (output current) and the switching frequency f s Related, sorted out P loss for:

[0131]

[0132] in:

[0133] K=R Lr_ac +R pri +R ds_on_s Formula 37

[0134]

[0135] The optimization objective becomes:

[0136]

[0137] For the above P loss The derivative is:

[0138]

[0139] Then for P' loss The derivative is:

[0140]

[0141] The α parameter of the core is greater than 1, each of the second-order derivatives is positive, and the whole is greater than 0, P loss It is a concave function with a minimum value. Let P' loss =0, solve the optimal control frequency f optimal , the frequency change is limited by the interval, and the final optimized frequency f final for:

[0142]

[0143] In summary, the overall steps for optimizing the converter efficiency and control frequency are as follows:

[0144] Step 1: When the converter output power is P out Under this condition, the corresponding output current I is obtained. O ;

[0145] Step 2: Calculate the values ​​of coefficients K, K0, K1, K2, K3, K4 and K5 based on the resonant converter operating parameters and device parameters;

[0146] Step 3: Get the upper and lower limits of the switching frequency f min 、f max ; Now calculate M(f min ) and M(f max ), if both are in M max and M min That is, the input voltage always changes between V min and V max If M(f max) is lower than M min , let f1 = f min , f2=f max ;

[0147] Step 4: Let f = (f1 + f2) / 2, and calculate the value of M(f) and input voltage at this switching frequency;

[0148] Step 5: If M(f)>M min , then let f1=(f1+f2) / 2, f2 remains unchanged, if M<M min , then let f2=(f1+f2) / 2, f1 remains unchanged;

[0149] Step 6: Repeat steps 4 and 5 until M(f) equals the minimum range M min Or f2-f1≤100Hz, stop the calculation, and let f max =f, and similarly we can get f min , and then according to formula 46, the optimal frequency value f is obtained optimal .

[0150] The specific process of the optimization algorithm adopted is as follows Figure 3 .

[0151] 4. Simulation Results and Verification

[0152] Based on the above calculations and analysis, simulation software is used to perform efficiency optimization and verification tests on the established component model, thereby determining the efficiency optimization method of the resonant converter.

[0153] (I) Simulation optimization results

[0154] After the loss modeling, optimization function and optimization method are established, MATLAB simulation is used to analyze the loss and optimal frequency.

[0155] Figure 4 is the loss change curve at the optimal frequency. As the load output power (load current I0) increases, the loss corresponding to the optimal efficiency becomes larger and larger. Figure 5 ,Through the analysis and calculation of the optimization steps, the optimal efficiency of the ,resonant converter under different output powers and the corresponding switching frequency under ,the highest efficiency are obtained. The optimal efficiency will first increase and then decrease when the power increases, and the corresponding ,control switching frequency will gradually decrease.

[0156] In order to verify the optimization results, the efficiency of the converter under optimal frequency control and constant frequency control is compared, such as Figure 6When the control frequency of the switch tube is constant at 20kHz, the efficiency of the converter is lower than the efficiency at the optimal control frequency, indicating that the efficiency at the optimal control frequency calculated by analysis is the best. This application is relatively small in power and is studied at a relatively low power, so the efficiency is lower than that of other high-power converters.

[0157] (II) Comparison results of efficiency experiments

[0158] The efficiency simulation optimization is performed through MATLAB software, and the tool for verifying the efficiency and model correctness is Saber;

[0159] Before starting the run, you need to set the parameters of each component. The simulation step is set to 100 nanoseconds and the duration is 50 milliseconds.

[0160] Run the simulation, view the waveform through the CosmosScope of the Saber software, and use the waveform calculator tools such as measurement and calculate in the software to calculate the input power and output power. Display the waveforms of input voltage and input current, output voltage and output current, and use the calculator to multiply them respectively. Since it is a DC voltage, the power is the average product of voltage and current, so as to obtain the input and output power, and finally divide them to obtain the efficiency value. Using the method described in the figure below, the efficiency of the model is verified through Saber, and the efficiency of the converter controlled by 15 groups of optimal frequencies is calculated, and the verification is performed three times on average under different powers.

[0161] Figure 7 , Figure 8 , Fig. 9 They are the loss calculation and operation efficiency generated by the theoretical calculation value and the Saber simulation circuit operation. It can be seen that at low power moments, the theoretical loss is less than the circuit experimental value, and the efficiency is higher. As the loss increases, the theoretical loss is greater than the experimental data of the circuit operation. This is because the slope of the curve of the theoretical transformer copper loss, conduction loss, and diode loss increases as the output current increases, and the growth is faster. When the current is low, the loss is smaller than that measured in Saber, and when the current is high, the loss is larger than that measured in Saber, resulting in the theoretical efficiency growth not being as fast as the efficiency measured in Saber, resulting in the above phenomenon.

[0162] Since the data is not as obvious as the graph, Figure 7 , Figure 8 , Fig. 9 The efficiency data of Fig.10The figure is a comparison of efficiencies at different output currents. The dotted lines represent the efficiency of the converter experimental value, and the squares represent the efficiency calculated by the theoretical model. By comparison, it can be seen that the converter efficiency obtained by the optimal efficiency obtained by the established model and the converter efficiency obtained by the ratio of output to input power at the optimal control switching frequency in the Saber experimental circuit has a floating difference, but it is not very large and the error is within an acceptable range. Therefore, the correctness of the model establishment can be verified and proved.

[0163] The experiment uses MATLAB and appropriate optimization algorithms to optimize the established optimization objectives, and obtains the curves of the optimal frequency and the most efficient with the output power. In addition, the operating efficiency of the converter between the constant frequency and the optimal frequency is compared to ensure that the optimized frequency is optimal. Saber is used to build a resonant converter, and the simulation and results of the model theoretical calculation are verified by modifying the calculated parameters and controlling the frequency. The loss test results are close to the theoretical values, with a small error, which proves the feasibility of the loss modeling of this application. This ensures the correctness of this application, and the proposed method can be well applied to the actual operation of the DC converter.

Claims

1. Energy efficiency optimization model of low output variable power resonant converter, characterized by: The component parameter selection method of the resonant converter is determined by the steady-state characteristics and state expressions of the converter, and a mathematical model of the power loss device of the resonant converter is established in combination with the operating parameters and component parameters. The optimization objective function is transformed according to the established loss model, with the switching frequency as the optimization variable and the inverter efficiency as the target. The optimization target is determined in combination with the constraints formed by the component parameters and the steady-state analysis expression, and the overall energy efficiency of the resonant converter with variable output power is optimized. 1) Establish a variable power loss model for low-output converters, analyze the instantaneous and effective currents at the input and output during the circuit operation process, determine the operating parameters of the resonant converter, establish a model for the losses of power MOS tubes, inductors and rectifiers, and jointly calculate the overall energy consumption expression to establish the optimization goal; 2) Calculation and selection of converter parameters: Design components according to the determined parameters, set the withstand voltage value of the switch tube and the maximum current allowed to pass, and ensure that its switch delay time meets the dead zone requirements and the start-up voltage meets the control voltage involved in its own drive; Select the filter inductor to set the core model, coil diameter, and number of turns. Based on the known inductor parameters, reduce the effective internal resistance of the inductor and the magnetic loss density of the core, reduce the energy consumption of the inductor during operation, and improve the efficiency of the converter. 3) Variable power efficiency optimization of converter loss model: The specific mathematical model and actual function of the components of the resonant converter are analyzed through formula transformation, and the calculated component parameters are brought into the loss model. The optimization is performed according to the total loss. After the output power changes, the optimization variable is set to the control frequency, and the efficiency is optimized with the control frequency as the variable. The optimization process and results are simulated by MATLAB to obtain the optimal solution of the model, and the optimal control frequency is found to control the converter to maximize the converter efficiency.

2. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1 is characterized in that: Resonant converter operating parameters are analyzed and determined: In half a switching cycle, the resonant converter is divided into two working stages, and the analysis frequency is less than f r In the case of , the parameters at other frequencies are included in the expressions for this case; The first operation phase includes the time from t0 to t1. During this period, D1 of the downstream stage is turned on and only the series device C is in the resonant network. r With L r Under the action of input and output voltage, the transformer voltage drop at one end of the front stage is clamped at +nV0 by the voltage drop at the rear stage. m At this moment, the current is charged and increases linearly, which does not resonate with the previous two. The voltage differential equation of the entire circuit loop is: Where L r is the series resonant inductor, C r is the resonant capacitor, V in is the input DC voltage, i p (t) is the current of the resonant circuit at the primary end of the converter, V Cr is the resonant capacitor C r The voltage across the two ends of , n is the turns ratio of the transformer, and V0 is the output voltage; Pre-stage L m The current I Lm The magnitude is equal and the direction is opposite in the whole control cycle, and t2-t0=T s / 2,T s is the control period, we get: i Lm (t0)=-i Lm (t2) Equation 3 The duration of phase 2 is short, and the approximate current I Lm In the second operation phase, we get: i Lm (t1)=i Lm (t2) Equation 4 i Lm (t0) = i Lm (t1) Formula 5 During the second stage, the output current is equal to 0, and the input main circuit current is equal to L m The current is equal to: i p (t0) = i Lm (t0) = -i Lm (t1) = -i p (t1) Equation 6 In the first stage, the primary resonant current is a sinusoidal resonant current, and we get: Where T r is the series resonant cycle; in stage 1, the inductor L m The voltage is nV0, combined with the formula of inductor voltage and current changes Get the inductance L m The amount of change in phase 1: Substitute the equation of equation 6 into equation 8 and solve for I Lm (t0) and I Lm (t1): Where f r is the series resonant frequency, the series resonant capacitor C r When participating in resonance, the DC is blocked, C r The terminal voltage will contain a DC component, C r The DC component on the in / 2, and it also contains an AC component, but the mean value of the AC component during period T is 0, and it is symmetrical in positive and negative directions, so we get: The effective mean current of the converter's rear stage is I0, while the current flowing through the rectifier tube during the second operation is always 0, L m The current I Lm In half of the operating cycle T r The average value during the period of / 2 is equal to 0, and the input current I p The average current during the first stage is equal to the average current of the primary side of the transformer, and the average current value of the front stage transformer can be converted by the average current of the rear stage.

3. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1 is characterized in that: Power switch tube loss model: The energy consumption of the converter front-stage power tube is divided into three parts: driving loss, on-state loss and off-state loss. The driving loss of the field effect transistor is P dr_s for: P dr_s =Q g_s V gs_s f s Formula 12 Where Q g_s is the gate charge of the previous field effect transistor, V gs_s is the voltage drop value of the power tube when it is turned on, f s Control frequency for power tube; The heat dissipation of the power tube conduction loss of the converter at the time of the switch tube turning on is affected by the on resistance of the field effect transistor, and its expression Pcon_s is: P con_s =I p_rms 2 R s_on Formula 13 Where I p_rms The converter front-stage main circuit current I p The effective value, R s_on It is the resistance value when the power tube is turned on; Converter switch turn-off loss P off_s The calculation is as follows: The front-end bridge arm power tube Q of the resonant converter s1 The shutdown process is equivalent to the current value I during the shutdown period off Constant, assuming that during the off period the power tube Q s1 The current is at t f The linear decrease to 0 within the time, then the expression of the switch tube current during the shutdown process is: Assumption C s1 =C s2 , then flows through C s1 , C s2 The current is: During shutdown, C s1 Voltage: Switching tube Q s1 The turn-off loss is: Where I off Represents the off current, that is, I Lm The maximum value of I Lm_pk , t f Represents the power tube current drop time, C s1 Represents the equivalent output junction capacitance of the power tube, f s To control the frequency.

4. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1 is characterized in that: Resonant inductor loss model: Assume that the internal resistance of the resonant inductor wire is R L , the effective value of the primary current of the converter through the coil is I rms , then the copper loss of the resonant inductor in the circuit is: P cu_L =I rms 2 R L Formula 18 The inductor in the converter uses iron oxide with a gap as the magnetic core, and its maximum magnetic induction intensity is expressed as: n L is the number of inductors L, I p_pk is the resonant current peak value, l L is the air gap length of the core.

5. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1, characterized in that: Rectifier diode loss: When the converter is running in the rectification period, when the current flows through the converter's post-stage rectifier tube, part of the power consumption is dissipated in the form of heat. The forward voltage drop of the rectifier tube is 0.7V. In one cycle, the conduction time of each rectifier tube is π / 2, and the current passing through it is the output current.

6. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1, characterized in that: Calculation and selection of converter parameters: Define the normalized voltage gain M(f) as: VIN F is the effective value of the fundamental component, Vo F is the fundamental effective value, H(s) is the transfer function of the resonant network, and after fixing the inductance ratio λ and the transformer coil ratio n, the gain M is obtained when the circuit quality factor Q value is different. dc With the frequency ratio f n The changing relationship M(f) is obtained as: The known condition is the input voltage range V max and V min , rated input voltage V in , rated output power V O , resonant frequency f r , the control frequency works at f r When the load change does not cause the gain to change, the values ​​of the following variables are obtained: 1) When the converter is operating under all rated variables, the control frequency operates at frequency f r , at this time f n =1,M=1,V O / V in =M dc =1 / 2n, find the value of transformer winding ratio n; 2) By V max 、V min and V in Find M max (f), M min (f): M max =2n*V O / V min ,M min =2n*V O / V max ; 3) Let k = 1 / λ. When the load variation range is the same, the smaller the k value, the narrower the frequency adjustment range in which the gain M remains unchanged. Select k = 3.5, that is, λ = 0.2857; 4) When the load is zero, Q = 0, M = M min When , we can find the maximum limiting frequency: The minimum frequency needs to be found with an appropriate Q value; 5) At the same gain, the value of Q increases, and the frequency ratio f n Reduce, find the maximum Q value to determine the minimum frequency, the impedance Z of the resonant circuit network in for: The imaginary part in the above formula is 0, which is the capacitive-inductive dividing line. The converter works in the inductive region to achieve zero voltage switching and obtain the maximum Q value Q max : Q max Value and M max Substituting into equation 22, the minimum switching frequency f is calculated. min : 6) The value of Q is 95% of Q max , leaving some margin for rated power output; By Q, R ac , f r , λ calculates L r , C r , L m , according to the above parameter design process, the required parameters are obtained.

7. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1, characterized in that: Variable power efficiency optimization of converter loss model: Based on the constructed resonant converter component loss model, the converter loss is obtained as follows: P loss =2*(P dr_s +P con_s +P off_s )+P cu_L +P fe_L +P cu_T +P fe_T +P diode Formula 27 Ignoring capacitor losses, the overall efficiency of the converter is: In the formula, the working efficiency η is a function of the switching frequency f. The switching frequency under different output powers is different, and the efficiency is also different. s Optimize to achieve the best efficiency, set: To optimize the goal.

8. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1, characterized in that: Variable power efficiency constraints of the converter loss model: The switching frequency of the power switch tube is set with clear upper and lower limits f min and f max , and should be greater than L r , C r , L m The control frequency f when the three resonate together m , the frequency constraint is Max(f m , f min )<f s <f max , the power change of the subsequent load is also constrained, that is, it cannot exceed the rated load power P O , 0<P out <P O At the same power, the input voltage value is different, the control frequency of the switch is also different, the input voltage value and the limit range also need to be considered, that is, V min <V in <V max ; The optimal switching frequency problem of the resonant converter at different powers is converted into the problem of finding the maximum target value under constraints: s.t.f min ≤f s ≤f max V min ≤V in ≤V max Formula 30 The converter loss-to-power efficiency model is obtained.

9. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 1, characterized in that: Variable power efficiency optimization method of converter loss model: When the load power at the output of the resonant converter is P out When the output voltage is the expected value and fixed, the value of I0 is obtained. When the load power is P out To ensure the maximum conversion efficiency of the resonant converter is to ensure that the operating loss of the resonant converter under this power is P loss The optimization problem is finally transformed into the minimum sum of the following four component losses: min P loss =P MosFet +P L +P T +P diode s.t.f min ≤f s ≤f max V min ≤V in ≤V max Formula 33 When the voltage drop V0 at the downstream load is determined, the total loss of the converter is only related to the output power (output current) and the switching frequency f s Related, sorted out P loss for: in: K=R Lr_ac +R pri +R ds_on_s Formula 37 K5=2Kn 2 V o 2 T r 2 +0.7I o +1.19 Formula 40 The optimization objective becomes: s.t.f min ≤f s ≤f max V min ≤V in ≤V max Formula 41 For the above P loss The derivative is: Then for P' loss The derivative is: The α parameter of the core is greater than 1, each of the second-order derivatives is positive, and the whole is greater than 0, P loss It is a concave function with a minimum value. Let P' loss =0, solve the optimal control frequency f optimal , the frequency variation is limited by the interval: Get the final optimized frequency f final .

10. The energy efficiency optimization model of the low output variable power supply resonant converter according to claim 9, characterized in that: The overall steps for optimizing the converter efficiency and control frequency are as follows: Step 1: When the converter output power is P out Under, obtain the corresponding output current I0 at this time; Step 2: Calculate the values ​​of coefficients K, K0, K1, K2, K3, K4 and K5 based on the resonant converter operating parameters and device parameters; Step 3: Get the upper and lower limits of the switching frequency f min 、f max ; Now calculate M(f min ) and M(f max ), if both are in M max and M min That is, the input voltage always changes between V min and V max If M(f max ) is lower than M min , let f1 = f min , f2=f max ; Step 4: Let f = (f1 + f2) / 2, and calculate the value of M(f) and input voltage at this switching frequency; Step 5: If M(f)>M min , then let f1=(f1+f2) / 2, f2 remains unchanged, if M<M min , then let f2=(f1+f2) / 2, f1 remains unchanged; Step 6: Repeat steps 4 and 5 until M(f) equals the minimum range M min Or f2-f1≤100Hz, stop the calculation, and let f max =f, and similarly we can get f min , and then according to formula 46, the optimal frequency value f is obtained optimal .

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