Control method of three-level flying capacitor type Buck converter in valley current mode

Abstract

The invention discloses a control method for a three-level flying capacitive Buck converter in a valley current mode, and the method comprises the steps: firstly building a converter control system which comprises a flying capacitive Buck converter and a valley current mode controller; secondly, sampling parameters of the flying capacitor type Buck converter to obtain sampling values of output voltage, output current and inductive current; finally, the difference value of the output voltage reference value and the output voltage of the flying capacitance type Buck converter is input into a voltage compensator to obtain an inductive current reference value, the inductive current reference value and a slope compensation signal are subtracted and then input into the inverted input end of a comparator, an inductive current sampling value is input into the in-phase input end of the comparator, and the output voltage of the flying capacitance type Buck converter is obtained. And the output of the comparator is connected with the PWM module to obtain driving signals of each switching tube of the flying capacitor type Buck converter. According to the method, the conditions for avoiding current subharmonic oscillation and ensuring that the current in the whole working range is stable and the minimum slope compensation slope meets are provided, high frequency of the converter is achieved, and the power density of the converter is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of three-level flying capacitor type buck converter control, in particular to a control method of a three-level flying capacitor type Buck converter in a valley current mode. Background Art

[0002] In order to solve the problem of energy supply fluctuation of renewable energy, energy storage devices are usually added to the DC microgrid system. The energy storage medium in the energy storage device is usually connected to the DC bus through a bidirectional DC / DC converter, so as to realize the bidirectional flow of energy between the DC bus side and the energy storage medium side. The traditional two-level DC / DC converter is difficult to adapt to the increase of voltage level due to the voltage resistance limit of silicon devices. In this regard, there are currently two solutions. One is to use high-voltage and high-power devices, such as series-parallel silicon devices or new devices, but there will be problems such as uneven voltage and current between series-parallel devices and high cost of new devices. Another method is to use a multi-level topology, which is widely used because it can effectively reduce the voltage stress of the switch tube. Among them, the flying capacitor (FC) type three-level bidirectional DC converter has the advantages of low voltage stress of the switch tube and small current ripple on the low-voltage side, so as to effectively improve the power density. In order to achieve the lowest device voltage stress, the FC voltage needs to be stably controlled at half of the high-voltage side voltage. At present, the control strategies for FC voltage are mainly divided into two categories: passive balancing strategy and closed-loop control strategy. The passive balancing strategy mainly utilizes the self-balancing characteristics of FC to maintain the voltage. The passive balancing strategy is an open-loop control strategy. Due to the existence of non-ideal factors, a closed-loop control loop is usually required to balance the FC voltage.

[0003] The voltage control mode is based on the output voltage of the converter. The output voltage sampling signal of the converter is compared with the reference voltage to generate an error signal. The voltage loop controller generates the control reference of the converter based on the error signal. The control reference generated by the voltage loop controller is compared with the sawtooth wave signal to generate a dynamically changing duty cycle to adjust the converter output. In this control mode with only a single voltage loop, the dynamic response speed of the system is slow and is not suitable for applications with high requirements for the dynamic response performance of the converter.

[0004] Unlike the voltage control mode, the current control mode controls the converter by measuring the current, and two nested control loops are usually used in the controller: the voltage outer loop and the current inner loop. Compared with the voltage control mode, the current control mode has a faster response speed and is more widely used. The valley current control mode controls the converter by detecting the valley value of the inductor current cycle by cycle, so the response speed of the valley current control mode is faster than that of the voltage control mode. However, in the valley current control mode, there are problems such as static instability of the inductor current and loss of control of the flying capacitor voltage. Summary of the invention

[0005] In view of the prior art, the technical problem to be solved by the present invention is to provide a control method for a three-level flying capacitor type Buck converter in a valley current mode.

[0006] The present invention solves the technical problem by adopting the following technical solution:

[0007] A control method for a three-level flying capacitor type Buck converter in a valley current mode, characterized in that it comprises the following steps:

[0008] The first step is to build a converter control system, including a flying capacitor Buck converter and a valley current mode controller.

[0009] The second step is to sample the parameters of the flying capacitor Buck converter to obtain the output voltage V o , output current I o and the inductor current sampling value I L ;

[0010] Step 3: Set the output voltage reference value V of the flying capacitor Buck converter ref and output voltage V o The difference is input into the voltage compensator to obtain the inductor current reference value I ref , I ref The inductor current sampling value I L The input is input to the non-inverting input terminal of the comparator, and the output of the comparator is connected to the PWM module to obtain the driving signal of each switch tube of the flying capacitor type Buck converter.

[0011] Furthermore, the minimum slope compensation slope for stabilizing the inductor current in the entire operating range satisfies:

[0012]

[0013] In the formula, S e is the slope compensation slope, V g is the input voltage, L oIt's an inductor.

[0014] Furthermore, when slope compensation exists, the static stability condition of the system is:

[0015]

[0016] Where M is the voltage conversion ratio.

[0017] Furthermore, the topology diagram of the flying capacitor type Buck converter includes a DC power supply, an input filter capacitor, a flying capacitor, a load, an output filter capacitor, an inductor, an upper bridge arm and a lower bridge arm; the upper bridge arm includes a first switch tube and a second switch tube, and the lower bridge arm includes a third switch tube and a fourth switch tube; the source of the first switch tube is connected to the drain of the second switch tube as a first common end, the source of the second switch tube is connected to the drain of the third switch tube as a second common end, and the source of the third switch tube is connected to the drain of the fourth switch tube as a third common end; the two ends of the flying capacitor are respectively connected to the first common point and the third common point; the DC power supply is connected in parallel with the input filter capacitor, the load is respectively connected in parallel with the output filter capacitor, and one end of the inductor is connected to the common end; in steady-state operation, the conduction phase of the first switch tube and the second switch tube differs by 180°, the first switch tube and the fourth switch tube are complementary to each other, and the second switch tube and the third switch tube are complementary to each other.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] Compared with the voltage control mode and the average current control mode, the dynamic response speed of the valley current control mode is fast, and when the valley current control mode is used for the flying capacitor type Buck converter, not only slope compensation is required to suppress subharmonic oscillation, but also flying capacitor voltage balance needs to be considered, so the present invention analyzes the stability and control characteristics of the three-level flying capacitor type Buck converter under the valley current control mode, and proposes the conditions for avoiding current subharmonic oscillation and ensuring the minimum slope compensation slope of current stability in the entire working range. The method of the present invention not only has a fast response speed, but also can balance the flying capacitor voltage, thereby helping to realize the high frequency of the converter and improve its power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is the circuit topology diagram of the flying capacitor type three-level Buck converter;

[0021] Figure 2 is a control flow chart of the present invention;

[0022] Figure 3 Figure 2 shows the current steady-state and perturbation waveforms of V-CMC, including the compensation ramp when M < 0.5.

[0023] Figure 4Is the steady-state waveform of the valley current mode, including the compensation ramp, and considering a small positive perturbation of the FC voltage (case M < 0.5);

[0024] Figure 5 Waveform diagrams of the flying capacitor voltage, output voltage, and inductor current without ramp compensation when 0.5 < M < 0.75;

[0025] Figure 6 Waveform diagrams of the flying capacitor voltage, output voltage, and inductor current with ramp compensation when 0.5 < M < 0.75;

[0026] Figure 7 Waveform diagrams of the flying capacitor voltage, output voltage, and inductor current without ramp compensation when 0.75 < M < 1;

[0027] Figure 8 Waveform diagrams of the flying capacitor voltage, output voltage, and inductor current when the load suddenly changes from 6.7 A to 8 A. Specific implementation mode

[0028] The technical solution of the present invention will be introduced in detail below in conjunction with the accompanying drawings and specific implementation modes, but the protection scope of this application is not limited thereby.

[0029] The present invention provides a control method for a three-level flying capacitor type Buck converter in valley current mode (referred to as the method, see Figures 1 to 8 ) and includes the following steps:

[0030] First step, build a converter control system, including a flying capacitor type Buck converter and a valley current mode controller. Figure 1 Is the topology diagram of the flying capacitor type Buck converter, including a DC power supply, an input filter capacitor C1, a flying capacitor, a load, an output filter capacitor C2, an inductor L, an upper bridge arm, and a lower bridge arm; the upper bridge arm includes a first switching tube Q1 and a second switching tube Q2, and the lower bridge arm includes a third switching tube Q3 and a fourth switching tube Q4. The source of the first switching tube Q1 is connected to the drain of the second switching tube Q2 as the first common terminal, and the source of the second switching tube Q2 is connected to the drain of the third switching tube Q3 as the second common terminal; the source of the third switching tube Q3 is connected to the drain of the fourth switching tube Q4 as the third common terminal; both ends of the flying capacitor are respectively connected to the first common point and the third common point; the DC power supply is connected in parallel with the input filter capacitor C1, and the load is respectively connected in parallel with the output filter capacitor C2; one end of the inductor L is connected to the common terminal; in the case of steady-state operation, the conduction phases of the first switching tube Q1 and the second switching tube Q2 differ by 180°, the first switching tube Q1 and the fourth switching tube Q4 are complementary in conduction, and the second switching tube Q2 and the third switching tube Q3 are complementary in conduction.

[0031] Second step, sample the parameters of the flying capacitor type Buck converter to obtain the output voltage Vo , output current I o and the inductor current sampling value I L ;

[0032] Step 3: First, set the output voltage reference value V of the flying capacitor Buck converter ref and output voltage V o The difference is input into the voltage compensator to obtain the inductor current reference value I ref , I ref After subtracting the slope compensation signal, it is input to the inverting input of the comparator. L The output of the comparator is connected to the PWM module to obtain the driving signals of the first switch tube Q1, the second switch tube Q2, the third switch tube Q3 and the fourth switch tube Q4. D1, D2, D3 and D4 respectively represent the duty cycles of the first to fourth switch tube driving signals, and D1 = 1-D4, D2 = 1-D3.

[0033] In the third step, the specific analysis of the impact of slope compensation on the flying capacitor and converter stability under the valley current mode control strategy is as follows:

[0034] The subharmonic oscillation boundary of the conventional CMC buck converter is usually calculated by studying the effect of the disturbance on the inductor current (Δi k+1 =KΔi k ), the stable region is defined as the region where |K|<1, which means that the effect of the disturbance disappears over time. In V-CMC, the inductor current discharge phase is started by a clock with a fixed period of Ts / 2, while the charging phase is determined by the intersection of the inductor current and the current loop set value Iref. The steady-state and perturbation current waveforms of valley current modulation are shown in Figure 3 The absolute slope m of the inductor current charging and discharging is shown in ON and m OFF satisfy:

[0035]

[0036] Where M is the voltage conversion ratio, V g is the input voltage, L o is the inductor;

[0037] The current perturbation at the beginning of the (k+1)th switching cycle is given as a function of the current perturbation of the previous switching cycle. The static stability of the CMC 3LFC converter can be extended to the entire operating range of the studied operating mode using an external ramp, which gives:

[0038]

[0039] In the formula, S e is the slope compensation slope, Δik+1 , Δi k are the inductor currents after the k+1th and kth disturbances respectively;

[0040] When M < 0.5, the minimum value of the slope compensation slope satisfies the following formula:

[0041]

[0042] The steady-state waveform of the CMC 3LFC converter when M<0.5 is as follows: Figure 4 As shown, it is assumed that the FC voltage is Vg / 2, Due to this mismatch, the current programming control generates two different duty cycles for the switch group, D1 and D2, given by

[0043]

[0044] Here, the stability of FC voltage is evaluated by the sign of FC average current Ifly. fly and has the same sign, the FC voltage will further deviate from V g / 2, leading to instability; if I fly and The sign of the FC voltage is opposite to that of the FC voltage at V g / 2. The average FC current is given by:

[0045]

[0046] Among them, A p and A n are the positive and negative areas under the FC current curve, respectively;

[0047] The exact expression of FC average current

[0048]

[0049] When M<0.5, substitute D1 and D2 in equation (6) and equation (7) into equation (9), then I fly Approximately:

[0050]

[0051] Select the slope compensation slope as S e ≥V g / 4L o , so that the sign of the FC average current depends on I fly . Write the above result as load current I o And the inductor current static ripple ΔIL, a more profound expression can be obtained:

[0052]

[0053] In the case of V-CMC (21), the slope compensation slope is selected as S e ≥V g / 4L o The FC average current is forced to have the opposite sign to the FC voltage perturbation. Therefore, V-CMC inherently stabilizes the FC voltage once the subharmonic current oscillations are suppressed.

[0054] Extended to the working mode of M>0.5, the absolute slope of the inductor current charging and discharging is m ON and m OFF satisfy:

[0055]

[0056] When slope compensation exists, the static stability condition of the system is:

[0057]

[0058] The minimum slope compensation slope of the stable inductor current in the entire operating range still satisfies equation (5);

[0059] The average FC current meets the following requirements:

[0060]

[0061] Qualitatively, the case with M > 0.5 shows no major differences relative to the case with M < 0.5. The V-CMC can be adequately stabilized with an appropriate choice of the compensation ramp slope.

[0062] Simulation verification:

[0063] The effectiveness of the control method of the present invention is verified by simulation below. The main simulation parameters are: input voltage V g =100V, output voltage V o =80V, output filter capacitor C2 = 100μF, inductor L = 10μH, switching frequency f S =100kHz, the load changes from 6.7A to 8A. The simulation results show Figures 5 to 8 The obtained simulation waveform diagram; Figure 5 , 6 As can be seen from Figure 7, the influence of ramp compensation on the stability of inductor current and flying capacitor voltage at different duty cycles is as follows: Figure 8 It can be seen that the converter responds quickly when the load changes suddenly.

[0064] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A control method for a three-level flying capacitor Buck converter in valley current mode, characterized in that: The following steps are involved: The first step is to build a converter control system, including a flying capacitor Buck converter and a valley current mode controller. The second step is to sample the parameters of the flying capacitor Buck converter to obtain the output voltage, output current and inductor current sampling values; The third step is to input the difference between the output voltage reference value and the output voltage of the flying capacitor type Buck converter into the voltage compensator to obtain the inductor current reference value, and then input the difference between the inductor current reference value and the slope compensation signal into the inverting input terminal of the comparator. The inductor current sampling value is input into the non-inverting input terminal of the comparator, and the output of the comparator is connected to the PWM module to obtain the driving signal of each switch tube of the flying capacitor type Buck converter.

2. The control method of a three-level flying capacitor type Buck converter in valley current mode according to claim 1, characterized in that: The minimum slope compensation slope for stabilizing the inductor current in the entire operating range satisfies: In the formula, S e is the slope compensation slope, V g is the input voltage, L o It's an inductor.

3. The control method of a three-level flying capacitor type Buck converter in valley current mode according to claim 1 or 2, characterized in that: When slope compensation exists, the static stability condition of the system is: Where M is the voltage conversion ratio.

4. The control method of a three-level flying capacitor Buck converter in valley current mode according to claim 1, characterized in that: The topology diagram of the flying capacitor type Buck converter includes a DC power supply, an input filter capacitor, a flying capacitor, a load, an output filter capacitor, an inductor, an upper bridge arm and a lower bridge arm; the upper bridge arm includes a first switch tube and a second switch tube, and the lower bridge arm includes a third switch tube and a fourth switch tube; the source of the first switch tube is connected to the drain of the second switch tube as a first common end, the source of the second switch tube is connected to the drain of the third switch tube as a second common end, and the source of the third switch tube is connected to the drain of the fourth switch tube as a third common end; the two ends of the flying capacitor are respectively connected to the first common point and the third common point; the DC power supply is connected in parallel with the input filter capacitor, the load is respectively connected in parallel with the output filter capacitor, and one end of the inductor is connected to the common end; in steady-state operation, the conduction phases of the first switch tube and the second switch tube differ by 180°, the first switch tube and the fourth switch tube are complementary to each other, and the second switch tube and the third switch tube are complementary to each other.

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