Three-level DCDC converter with flying capacitor, system and control method

Abstract

The present application discloses a three-level DCDC converter with a flying capacitor, a system and a control method, comprising: an inductor, a first switching tube, a second switching tube, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, a second voltage-dividing capacitor and a controller; the controller is used to trigger the second switching tube and the first switching tube to act simultaneously when the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, and charge the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor. As long as the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, the flying capacitor is charged and the voltage of the flying capacitor is increased. As the voltage of the flying capacitor rises, the voltage of the first voltage-dividing capacitor is adjusted, and the voltage of the second voltage-dividing capacitor is indirectly adjusted, so that the voltages of the first voltage-dividing capacitor and the second voltage-dividing capacitor are balanced, thereby protecting the safety of the two voltage-dividing capacitors.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a three-level DC-DC converter with a flying capacitor, a system, and a control method. Background Art

[0002] Currently, three-level DC-DC converters with flying capacitors (hereinafter referred to as three-level DC-DC converters) are used in many applications. For example, in photovoltaic power generation, the input of the three-level DC-DC converter is connected to the photovoltaic strings, and the output of the three-level DC-DC converter is connected to the inverter. The three-level DC-CDC converter can boost the voltage of the photovoltaic strings and output it to the inverter, which converts DC power into AC power for grid connection.

[0003] In practical applications, when a three-level DCDC converter is operating, the downstream load may be directly connected to other converters. When the downstream output voltage rises rapidly for some reason, for example, the voltage of the connected inverter suddenly increases, the output voltage of the three-level DCDC converter will rise instantaneously. Since the output end of the three-level DCDC converter is connected to series voltage-dividing capacitors, the two voltage-dividing capacitors generally have equal capacitance and evenly divide the output voltage, the voltage of the two voltage-dividing capacitors will also rise instantaneously.

[0004] However, the voltage Vcf of the flying capacitor in the three-level DCDC converter is controlled by the switching tube and cannot rise rapidly with the output voltage. In this case, the diode is cut off due to the reverse voltage. If the three-level DCDC converter continues to operate, it will cause voltage imbalance between the two voltage-dividing capacitors, and may even cause one of the voltage-dividing capacitors to overvoltage and damage. Summary of the Invention

[0005] To solve the above problems, the present application provides a three-level DCDC converter with a flying capacitor, a system and a control method, which can balance the voltages of two voltage-dividing capacitors when the output voltage increases.

[0006] The present application provides a three-level DC-DC converter with a flying capacitor, comprising: an inductor, a first switching tube, a second switching tube, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, a second voltage-dividing capacitor, and a controller;

[0007] The first end and the second end of the inductor are connected to the positive input end of the converter and the anode of the first diode respectively, and the anode and the cathode of the second diode are connected to the cathode of the first diode and the positive output end of the converter respectively;

[0008] The first end and the second end of the second switch tube are connected to the anode of the first diode and the first end of the first switch tube respectively, and the second end of the first switch tube is connected to the negative input end of the converter;

[0009] The two ends of the flying capacitor are respectively connected to the cathode of the first diode and the second end of the second switch tube, the cathode and anode of the third diode are respectively connected to the cathode of the first diode and the cathode of the fourth diode, and the anode of the fourth diode is connected to the second end of the second switch tube;

[0010] Two ends of the first voltage-dividing capacitor are connected to the positive output terminal of the converter and the anode of the third diode respectively, and two ends of the second voltage-dividing capacitor are connected to the anode of the third diode and the negative input terminal of the converter respectively;

[0011] The controller is used to trigger the second switch tube and the first switch tube to operate simultaneously when the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, so as to charge the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

[0012] Preferably, the controller is specifically configured to control the first switch tube and the second switch tube to operate in the following three modes in sequence:

[0013] First mode: controlling the first switch tube and the second switch tube to be closed;

[0014] Second mode: control the second switch tube to be disconnected and the first switch tube to be closed;

[0015] The third mode: the first switch tube and the second switch tube are both controlled to be disconnected.

[0016] Preferably, the controller is specifically used to obtain the duty cycle of the first switching tube and the duty cycle of the second switching tube based on the voltage of the flying capacitor, the voltage of the first voltage divider capacitor and the input voltage of the converter, trigger the first switching tube according to the duty cycle of the first switching tube, and trigger the second switching tube according to the duty cycle of the second switching tube.

[0017] Preferably, the controller is further configured to obtain a duty cycle adjustment value according to the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor;

[0018] The duty cycle of the first switch tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switch tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount.

[0019] Preferably, the duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor.

[0020] Preferably, the duty cycle is proportional to the first voltage and inversely proportional to the second voltage, the second voltage is the sum of the voltage of the first voltage divider capacitor and the voltage of the flying capacitor, and the first voltage is the difference between the second voltage and the input voltage of the converter.

[0021] Preferably, the controller is further configured to alternately control the operations of the second switch tube and the first switch tube when the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

[0022] The present application provides a power supply system, comprising: an inverter and at least one three-level DCDC converter with a flying capacitor as described above;

[0023] The output end of the three-level DCDC converter with a flying capacitor is connected to the input end of the inverter.

[0024] The present application also provides a control method for a three-level DCDC converter with a flying capacitor, wherein the converter includes: an inductor, a first switching tube, a second switching tube, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, and a second voltage-dividing capacitor;

[0025] The method includes:

[0026] Obtaining the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor;

[0027] When the voltage of the flying capacitor is lower than the voltage of the second voltage-dividing capacitor, the second switch tube and the first switch tube are triggered to operate simultaneously to charge the flying capacitor until the voltage of the flying capacitor is higher than or equal to the voltage of the second voltage-dividing capacitor.

[0028] Preferably, triggering the second switch tube and the first switch tube to operate simultaneously specifically includes:

[0029] The first switch tube and the second switch tube are controlled to operate in the following three modes in sequence:

[0030] First mode: controlling the first switch tube and the second switch tube to be closed;

[0031] Second mode: control the second switch tube to be disconnected and the first switch tube to be closed;

[0032] The third mode: the first switch tube and the second switch tube are both controlled to be disconnected.

[0033] Preferably, triggering the second switch tube and the first switch tube to operate simultaneously specifically includes:

[0034] The duty cycle of the first switch tube and the duty cycle of the second switch tube are obtained according to the voltage of the flying capacitor, the voltage of the first voltage-dividing capacitor and the input voltage of the converter. The first switch tube is triggered according to the duty cycle of the first switch tube, and the second switch tube is triggered according to the duty cycle of the second switch tube.

[0035] Preferably, it also includes:

[0036] Obtaining a duty cycle adjustment amount according to the voltage of the flying capacitor and the voltage of the second voltage divider capacitor;

[0037] The duty cycle of the first switch tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switch tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount;

[0038] The duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage divider capacitor.

[0039] Preferably, the duty cycle is proportional to the first voltage and inversely proportional to the second voltage, the second voltage is the sum of the voltage of the first voltage divider capacitor and the voltage of the flying capacitor, and the first voltage is the difference between the second voltage and the input voltage of the converter.

[0040] Preferably, it also includes:

[0041] When the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor, the second switch tube and the first switch tube are controlled to operate in an alternating manner.

[0042] It can be seen that this application has the following beneficial effects:

[0043] The converter provided by the present application can control the operating mode of the first switching tube and the second switching tube of the converter based on the magnitude relationship between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor. As long as the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, the flying capacitor is continuously charged, thereby increasing the voltage of the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor. Since the voltage of the flying capacitor rises, the voltage of the first voltage-dividing capacitor can be adjusted, thereby indirectly adjusting the voltage of the second voltage-dividing capacitor, so that the voltages of the first voltage-dividing capacitor and the second voltage-dividing capacitor reach a balance, thereby protecting the safety of the two voltage-dividing capacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 The circuit diagram of a three-level DCDC converter with a flying capacitor is shown in FIG.

[0045] Figure 2 The current path diagram of a three-level DCDC converter with a flying capacitor;

[0046] Figure 3 A schematic diagram of a three-level DCDC converter with a flying capacitor provided in an embodiment of the present application;

[0047] Figure 4 A path diagram corresponding to the first mode provided in an embodiment of the present application;

[0048] Figure 5 A path diagram corresponding to the second mode provided in an embodiment of the present application;

[0049] Figure 6 A path diagram corresponding to the third mode provided in an embodiment of the present application;

[0050] Figure 7 The timing and parameter change curves provided in the embodiments of the present application;

[0051] Figure 8 A schematic diagram of a power supply system provided in an embodiment of the present application;

[0052] Figure 9 This is a flow chart of a control method for a three-level DCDC converter with a flying capacitor provided in an embodiment of the present application. DETAILED DESCRIPTION

[0053] In order to enable those skilled in the art to better understand and implement the technical solutions provided in the embodiments of the present application, the circuit topology of a three-level DCDC converter with a flying capacitor is first introduced below.

[0054] See also Figure 1 ,This figure is a circuit diagram of a three-level DCDC converter with a flying capacitor.

[0055] from Figure 1 It can be seen that the three-level DC-DC converter with flying capacitor includes: an inductor L, a first switching tube S1, a second switching tube S2, a first diode D1, a second diode D2, a flying capacitor Cf, a third diode Dh, a fourth diode Df, a first voltage-dividing capacitor Co1 and a second voltage-dividing capacitor Co2.

[0056] For the convenience of description, the following embodiments refer to the three-level DCDC converter with flying capacitors as a converter.

[0057] The first and second ends of the inductor L are connected to the positive input end of the converter and the anode of the first diode D1 respectively. The anode and cathode of the second diode D2 are connected to the cathode of the first diode D1 and the positive output end of the converter respectively.

[0058] The first end and the second end of the second switch tube D2 are connected to the anode of the first diode D1 and the first end of the first switch tube respectively. The second end of the first switch tube D1 is connected to the negative input end of the converter.

[0059] The two ends of the flying capacitor Cf are respectively connected to the cathode of the first diode D1 and the second end of the second switch tube S2, the cathode and anode of the third diode Dh are respectively connected to the cathode of the first diode D1 and the cathode of the fourth diode Df, and the anode of the fourth diode Df is connected to the second end of the second switch tube S2.

[0060] Two ends of the first voltage-dividing capacitor Co1 are respectively connected to the positive output terminal of the converter and the anode of the third diode Dh. Two ends of the second voltage-dividing capacitor Co2 are respectively connected to the anode of the third diode Dh and the negative input terminal of the converter.

[0061] In addition, the converter further includes an input capacitor Cin, and both ends of the input capacitor Cin are respectively connected to the positive input terminal and the negative input terminal of the converter.

[0062] The input voltage of the converter is Vin, and the output voltage is Vout, where VL is the positive input terminal of the converter, VH is the positive output terminal of the converter, and Vcom is the common terminal of the negative input terminal and the negative output terminal of the converter.

[0063] During the operation of the converter, to avoid voltage imbalance between Co1 and Co2, it is necessary to control the minimum value Vcf(min) of the voltage Vcf of Cf to be greater than the maximum value Vco2(max) of the voltage Vco2 of Co2, that is, Vcf(min)>Vco2(max).

[0064] When the output voltage of the subsequent stage rises rapidly due to some reason, that is, Vout, Vco1, and Vco2 rise instantaneously, but the voltage Vcf of Cf is controlled by the switching tube and cannot rise rapidly following the output voltage, and cannot meet the condition of Vcf(min)>Vco2(max). In this case, D2 is reverse-biased and cut off. If the converter continues to operate, the commutation path of the current is no longer Cin-L-S2-Cf-D2-Co2-Co1. Instead, a new commutation path (Cin-L-S2-Df-Co1) is generated. For details, see Figure 2 As shown, under this commutation path, an analysis is made on whether the boost ratio N of the converter is greater than 2. The boost ratio is the ratio of the output voltage to the input voltage of the converter.

[0065] First, if the boost ratio N is greater than 2, then Vin<Vco2, the inductor L bears a forward voltage drop, and the current of the inductor L continues to rise. At the same time, the current of the inductor L directly forms a loop through Co2 until it rises to Vin and then oscillates to a steady state. In this case, it will cause voltage imbalance between Co1 and Co and is very likely to cause Co1 to be damaged due to overvoltage.

[0066] Second, if the boost ratio is less than 2, then Vin>Vco2, the inductor L bears a reverse voltage drop, and the current of the inductor L decreases. At the same time, the current of the inductor L directly forms a loop through Co2, and all the energy on the inductor L is transferred to Co2, which will also cause the voltage Vco2 across Co2 to rise rapidly. In this case, it will cause voltage imbalance between Co1 and Co2 and may even cause Co1 to be damaged due to overvoltage.

[0067] In addition, since Df is only turned on when charging Cf before operation and is in the cut-off state during normal operation, a model with relatively small current-carrying capacity is generally selected during selection. When S2 is turned on alone, the current path is as Figure 2As shown by the middle dotted line: Cin-L-S2-Df-Co1, the input current flows directly through Df. When the step-up ratio is less than 2, the inductor L is subjected to a forward voltage drop, and the input current iL flowing through Df continues to increase. In severe cases, it may exceed its current-carrying capacity and cause damage to Df.

[0068] In these situations, if the converter stops operating, it could cause damage to the entire system or even more serious harm. For example, in a photovoltaic system, the converter's downstream stage is connected to an inverter. When the inverter enters a high voltage ride-through or other conditions, the output voltage will rise instantaneously. However, there is a delay in Cf's voltage control. If the converter continues to operate in this situation, it will cause a voltage imbalance between Co1 and Co2, further affecting the safe and stable operation of the entire inverter.

[0069] Therefore, in order to solve the problem of voltage imbalance between Co1 and Co2 caused by an increase in the output voltage, the present application adopts a new control method.

[0070] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the embodiments of the present application are further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0071] See also Figure 3 , which is a schematic diagram of a three-level DCDC converter with a flying capacitor provided in an embodiment of the present application.

[0072] The structure of the converter provided in this embodiment can be found in Figure 1 The description is not repeated here.

[0073] In order to balance the voltages of the two voltage-dividing capacitors, the controller 100 in the converter provided in the embodiment of the present application is used to trigger the second switch tube S1 and the first switch tube S2 to operate simultaneously when the voltage of the flying capacitor Cf is less than the voltage of the second voltage-dividing capacitor Co2, so as to charge the flying capacitor Cf until the voltage of the flying capacitor Cf is greater than or equal to the voltage of the second voltage-dividing capacitor Co2.

[0074] The simultaneous triggering of the first switch tube S1 and the second switch tube S2 means that the actions of the two switch tubes are not staggered, but simultaneous. For example, when S1 is turned on, S2 is also turned on, that is, the simultaneous control is turned on, which is a kind of simultaneous triggering; in addition, when S2 is triggered to turn off, S1 is triggered to turn on, which is also a kind of simultaneous triggering; in addition, when S1 is triggered to turn off, S2 is triggered to turn off at the same time, which is also a kind of simultaneous triggering.

[0075] Because the voltage of the flying capacitor Cf is lower than the voltage of the second voltage-divider capacitor Co2, the second diode D2 is always in reverse cutoff. The flying capacitor Cf and the first voltage-divider capacitor Co1 can be considered as loads, ensuring that the voltage of the first voltage-divider capacitor Co1 is controllable, thereby suppressing the voltage imbalance between the two voltage-divider capacitors. At the same time, the voltage Vcf of the flying capacitor Cf is controlled and rapidly increased until Vcf(min)>Vco2(max) is satisfied, at which point this control mode is exited.

[0076] The converter provided in the embodiment of the present application can control the operating mode of the converter based on the magnitude relationship between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor. As long as the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, the flying capacitor is continuously charged, thereby increasing the voltage of the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor. Since the voltage of the flying capacitor rises, the voltage of the first voltage-dividing capacitor can be adjusted, thereby indirectly adjusting the voltage of the second voltage-dividing capacitor, so that the voltages of the first voltage-dividing capacitor and the second voltage-dividing capacitor reach a balance, thereby protecting the safety of the two voltage-dividing capacitors.

[0077] The controller is specifically used to obtain the duty cycle of the first switching tube and the duty cycle of the second switching tube according to the voltage of the flying capacitor, the voltage of the first voltage-dividing capacitor and the input voltage of the converter, trigger the first switching tube according to the duty cycle of the first switching tube, and trigger the second switching tube to operate according to the duty cycle of the second switching tube.

[0078] The duty cycle is proportional to the first voltage and inversely proportional to the second voltage. The second voltage is the sum of the voltage of the first voltage divider capacitor and the voltage of the flying capacitor. The first voltage is the difference between the second voltage and the input voltage of the converter.

[0079] The initial duty cycle D of the first switch tube and the second switch tube can be obtained by the following formula:

[0080] D=(Vco1+Vcf-–Vin) / (Vco1+Vcf)

[0081] Wherein, Vco1 is the voltage of the first voltage-dividing capacitor, Vcf is the voltage of the flying capacitor, and Vin is the input voltage of the converter.

[0082] As the adjustment proceeds, the voltage Vco2 of the second voltage-dividing capacitor increases. In order to control the voltage Vcf of the flying capacitor to reach a new voltage threshold, the duty cycle adjustment amount Dcf needs to be calculated in real time.

[0083] That is, the controller is further used to obtain a duty cycle adjustment amount according to the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor;

[0084] The duty cycle of the first switch tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switch tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount.

[0085] The duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage divider capacitor. That is, the greater the difference between the voltage of the flying capacitor and the voltage of the second voltage divider capacitor, the greater the duty cycle adjustment amount.

[0086] The duty cycle adjustment amount is superimposed on the initial duty cycle, as follows:

[0087] Finally, the duty cycles of the first switch tube S1 and the second switch tube S2 are:

[0088] Ds1 = D - Dcf;

[0089] Ds2=D+Dcf.

[0090] Wherein, Dcf is the duty cycle adjustment value, Ds1 is the duty cycle of the first switch tube after adjustment, and Ds2 is the duty cycle of the second switch tube after adjustment.

[0091] The working mode of the converter provided in the embodiment of the present application is described in detail below with reference to the accompanying drawings.

[0092] The converter and controller provided in this embodiment are specifically configured to control the first switching transistor and the second switching transistor to operate in the following three modes in sequence:

[0093] First mode: controlling the first switch tube and the second switch tube to be closed;

[0094] Second mode: control the second switch tube to be disconnected and the first switch tube to be closed;

[0095] The third mode: the first switch tube and the second switch tube are both controlled to be disconnected.

[0096] See also Figure 4 , this figure is a path diagram corresponding to the first mode provided in an embodiment of the present application.

[0097] The first switch S1 and the second switch S2 are both in the on state. The flying capacitor Cf has no charge and discharge circuit, and the voltage Vcf across it remains essentially unchanged. The input power charges the inductor L. The voltage drop VL across the inductor L is equal to the input voltage Vin, and the inductor current iL increases, that is:

[0098] VL=Vin=L*diL / dt.

[0099] See also Figure 5 , this figure is a path diagram corresponding to the second mode provided in an embodiment of the present application.

[0100] First switch S1 is on, and second switch S2 is off. First, consider the case where the boost ratio N is less than 2. The input voltage Vin is greater than the flying capacitor voltage Vcf. The inductor voltage VL continues to experience positive voltage, and the inductor current iL continues to rise, causing the inductor L to continue storing energy. Simultaneously, current flows through the flying capacitor Cf, charging it and causing the flying capacitor voltage Vcf to rise.

[0101] If the boost ratio N is greater than 2, the input voltage Vin is lower than the flying capacitor voltage Vcf. The inductor L's voltage VL continues to experience reverse voltage, and the inductor current iL begins to decrease, but still flows through the flying capacitor Cf. Energy from the inductor L is transferred to the flying capacitor Cf, causing the flying capacitor voltage Vcf to rise.

[0102] VL=Vin–Vcf=L*diL / dt;

[0103] iL / Cf=dVcf / dt.

[0104] See also Figure 6 , this figure is a path diagram corresponding to the third mode provided in an embodiment of the present application.

[0105] The first switch tube S1 and the second switch tube S2 are both disconnected, the inductor L is subjected to a reverse voltage drop, the inductor current iL decreases, and at the same time, the current passes through the flying capacitor Cf and the first voltage-dividing capacitor Co1 to charge the capacitor. The energy on the inductor L is transferred to the capacitor, and the current flowing through the inductor L and the diode Df decreases. The voltage Vcf of the flying capacitor Cf and the voltage Vco1 of the first voltage-dividing capacitor Co1 will increase.

[0106] VL=Vin–Vcf–Vco1=L*diL / dt;

[0107] iL=Cf*dVcf / dt=Co1*dVco1 / dt.

[0108] In the first mode, the inductor L is charged. The flying capacitor Cf has no charge / discharge circuit, and the voltage Vcf across the flying capacitor Cf remains essentially unchanged. However, the voltage Vcf across the first voltage-divider capacitor Co1 may drop when a load is connected. In the second mode, the flying capacitor Cf is charged independently, causing the voltage Vcf across it to rise rapidly. In the third mode, the inductor L simultaneously charges the flying capacitor Cf and the first voltage-divider capacitor Co1, using the same charging current.

[0109] According to the formula \(i_{L}=C_{f}\frac{dV_{cf}}{dt}=C_{o1}\frac{dV_{co1}}{dt}\), it can be seen that the rising amplitudes of \(V_{cf}\) and \(V_{co1}\) are mainly related to the capacitance values of \(C_{f}\) and \(C_{o1}\). In general application scenarios, the capacitance value of \(C_{f}\) is much smaller than that of \(C_{o1}\). Therefore, the rising amplitude of \(V_{cf}\) is much larger than that of \(V_{co1}\). On the one hand, \(V_{cf}\) is rapidly increased to suppress the rise of \(V_{co1}\), and on the other hand, the current flowing through diode \(D_{f}\) is reduced.

[0110] See Figure 7 , which is a curve graph of timing and parameter changes provided by the embodiment of the present application.

[0111] From Figure 7 it can be seen that the voltage \(V_{cf}\) on the flying capacitor has been rising.

[0112] The current \(i_{L}\) of the inductor decreases in the third mode and the second mode and increases in the first mode.

[0113] Moreover, for the two cases where the boost ratio \(N\) of the converter is greater than 2 and less than 2, the curves of \(i_{L}\) are different.

[0114] While suppressing the voltage imbalance of the two voltage-dividing capacitors, the voltage of the flying capacitor can be rapidly increased.

[0115] In the converter provided by the embodiment of the present application, there is no mode in which the second switch tube \(S_{2}\) conducts alone, that is, there is no current path (\(C_{in}-L-S_{2}-D_{f}-C_{o1}\)), that is, when \(V_{cf}(min)<V_{co2}(max)\), on the one hand, the voltage imbalance between the first voltage-dividing capacitor \(C_{o1}\) and the second voltage-dividing capacitor \(C_{o2}\) is suppressed, and at the same time, the flying capacitor can be rapidly charged. On the other hand, it is avoided that the current flowing through the diode \(D_{f}\) continuously rises, causing the diode \(D_{f}\) to be damaged by overcurrent. After the flying capacitor is charged or the output voltage of the subsequent stage drops and satisfies \(V_{cf}(min)>V_{co2}(max)\), the control mode provided by the embodiment of the present application is exited.

[0116] In the converter provided by the embodiment of the present application, the controller is further configured to alternately control the actions of the second switch tube and the first switch tube when the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

[0117] Based on the three-level DCDC converter with a flying capacitor provided in the above embodiments, the embodiment of the present application further provides a power supply system, which will be introduced in detail below with reference to the accompanying drawings.

[0118] See Figure 8 , which is a schematic diagram of a power supply system provided by the embodiment of the present application.

[0119] The power supply system provided in this embodiment includes: an inverter 801 and at least one three-level DCDC converter 802 with flying capacitors as described in the above embodiments;

[0120] The output end of the three-level DCDC converter 802 with flying capacitors is connected to the input end of the inverter 801 .

[0121] The embodiment of the present application does not specifically limit the DC power source connected to the input end of the three-level DCDC converter with flying capacitor 802 , and may be, for example, a photovoltaic string or a battery.

[0122] The power supply system provided in the embodiment of the present application can balance the voltages of the two voltage-dividing capacitors, especially when the voltage increase on the inverter side causes the output voltage of the three-level DCDC converter 802 with a flying capacitor to increase, thereby protecting the safety of the voltage-dividing capacitors and the diodes, and thus preventing the power supply system from being shut down.

[0123] Based on the three-level DCDC converter with a flying capacitor and a power supply system provided in the above embodiment, the embodiment of the present application further provides a control method for the three-level DCDC converter with a flying capacitor, which is described in detail below with reference to the accompanying drawings.

[0124] See also Figure 9 , which is a flow chart of a control method for a three-level DCDC converter with a flying capacitor provided in an embodiment of the present application.

[0125] This embodiment provides a control method for a three-level DC-DC converter with a flying capacitor. The converter includes: an inductor, a first switching tube, a second switching tube, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, and a second voltage-dividing capacitor.

[0126] The method includes:

[0127] S901: Obtain the voltage of the flying capacitor and the voltage of the second voltage divider capacitor;

[0128] S902: When the voltage of the flying capacitor is lower than the voltage of the second voltage-dividing capacitor, trigger the second switch tube and the first switch tube to operate simultaneously to charge the flying capacitor until the voltage of the flying capacitor is higher than or equal to the voltage of the second voltage-dividing capacitor.

[0129] The control method provided in the present application can control the operating mode of the first switching transistor and the second switching transistor of the converter based on the magnitude relationship between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor. As long as the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, the flying capacitor is continuously charged, thereby increasing the voltage of the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor. Since the voltage of the flying capacitor rises, the voltage of the first voltage-dividing capacitor can be adjusted, thereby indirectly adjusting the voltage of the second voltage-dividing capacitor, so that the voltages of the first voltage-dividing capacitor and the second voltage-dividing capacitor reach a balance, thereby protecting the safety of the two voltage-dividing capacitors.

[0130] Triggering the second switch tube and the first switch tube to operate simultaneously includes:

[0131] The first switch tube and the second switch tube are controlled to operate in the following three modes in sequence:

[0132] First mode: controlling the first switch tube and the second switch tube to be closed;

[0133] Second mode: control the second switch tube to be disconnected and the first switch tube to be closed;

[0134] The third mode: the first switch tube and the second switch tube are both controlled to be disconnected.

[0135] Triggering the second switch tube and the first switch tube to operate simultaneously includes:

[0136] The duty cycle of the first switch tube and the duty cycle of the second switch tube are obtained according to the voltage of the flying capacitor, the voltage of the first voltage-dividing capacitor and the input voltage of the converter. The first switch tube is triggered according to the duty cycle of the first switch tube, and the second switch tube is triggered according to the duty cycle of the second switch tube.

[0137] The control method provided in this embodiment further includes:

[0138] Obtaining a duty cycle adjustment amount according to the voltage of the flying capacitor and the voltage of the second voltage divider capacitor;

[0139] The duty cycle of the first switch tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switch tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount;

[0140] The duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage divider capacitor.

[0141] The duty cycle is proportional to the first voltage and inversely proportional to the second voltage. The second voltage is the sum of the voltage of the first voltage divider capacitor and the voltage of the flying capacitor. The first voltage is the difference between the second voltage and the input voltage of the converter.

[0142] The control method provided in this embodiment further includes:

[0143] When the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor, the second switch tube and the first switch tube are controlled to operate in an alternating manner.

[0144] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the systems or devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0145] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A three-level DC-DC converter with a flying capacitor, characterized in that: include: An inductor, a first switching tube, a second switching tube, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, a second voltage-dividing capacitor, and a controller; The first end and the second end of the inductor are connected to the positive input end of the converter and the anode of the first diode respectively, and the anode and the cathode of the second diode are connected to the cathode of the first diode and the positive output end of the converter respectively; The first end and the second end of the second switch tube are connected to the anode of the first diode and the first end of the first switch tube respectively, and the second end of the first switch tube is connected to the negative input end of the converter; The two ends of the flying capacitor are respectively connected to the cathode of the first diode and the second end of the second switch tube, the cathode and anode of the third diode are respectively connected to the cathode of the first diode and the cathode of the fourth diode, and the anode of the fourth diode is connected to the second end of the second switch tube; Two ends of the first voltage-dividing capacitor are connected to the positive output terminal of the converter and the anode of the third diode respectively, and two ends of the second voltage-dividing capacitor are connected to the anode of the third diode and the negative input terminal of the converter respectively; The controller is used to trigger the second switch tube and the first switch tube to operate simultaneously when the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, so as to charge the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

2. The converter according to claim 1, characterized in that The controller is specifically configured to control the first switch tube and the second switch tube to operate in the following three modes in sequence: First mode: controlling the first switch tube and the second switch tube to be closed; Second mode: controlling the second switch tube to be disconnected and the first switch tube to be closed; The third mode: controlling the first switch tube and the second switch tube to be turned off.

3. The converter according to claim 1 or 2, characterized in that The controller is specifically used to obtain the duty cycle of the first switching tube and the duty cycle of the second switching tube according to the voltage of the flying capacitor, the voltage of the first voltage divider capacitor and the input voltage of the converter, trigger the first switching tube according to the duty cycle of the first switching tube, and trigger the second switching tube to operate according to the duty cycle of the second switching tube.

4. The converter according to claim 3, characterized in that The controller is further configured to obtain a duty cycle adjustment value according to the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor; The duty cycle of the first switching tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switching tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount.

5. The converter according to claim 3 or 4, characterized in that: The duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor.

6. The converter according to claim 4 or 5, characterized in that: The duty cycle is proportional to a first voltage and inversely proportional to a second voltage, the second voltage being the sum of a voltage of the first voltage divider capacitor and a voltage of the flying capacitor, and the first voltage being the difference between the second voltage and an input voltage of the converter.

7. The converter according to any one of claims 1 to 6, characterized in that: The controller is further configured to alternately control the operations of the second switch tube and the first switch tube when the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

8. A power supply system, characterized in that: include: An inverter and at least one three-level DCDC converter with a flying capacitor according to any one of claims 1 to 7; The output end of the three-level DCDC converter with a flying capacitor is connected to the input end of the inverter.

9. A control method for a three-level DC-DC converter with a flying capacitor, characterized in that: The converter includes: an inductor, a first switching transistor, a second switching transistor, a first diode, a second diode, a flying capacitor, a third diode, a fourth diode, a first voltage-dividing capacitor, and a second voltage-dividing capacitor; the first and second ends of the inductor are respectively connected to the positive input terminal of the converter and the anode of the first diode, and the anode and cathode of the second diode are respectively connected to the cathode of the first diode and the positive output terminal of the converter; the first and second ends of the second switching transistor are respectively connected to the anode of the first diode and the first end of the first switching transistor, and the second end of the first switching transistor is connected to the negative input terminal of the converter; the two ends of the flying capacitor are respectively connected to the cathode of the first diode and the second end of the second switching transistor, the cathode and anode of the third diode are respectively connected to the cathode of the first diode and the cathode of the fourth diode, and the anode of the fourth diode is connected to the second end of the second switching transistor; the two ends of the first voltage-dividing capacitor are respectively connected to the positive output terminal of the converter and the anode of the third diode, and the two ends of the second voltage-dividing capacitor are respectively connected to the anode of the third diode and the negative input terminal of the converter; The method includes: Obtaining a voltage of the flying capacitor and a voltage of the second voltage-dividing capacitor; When the voltage of the flying capacitor is less than the voltage of the second voltage-dividing capacitor, the second switch tube and the first switch tube are triggered to operate simultaneously to charge the flying capacitor until the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor.

10. The control method according to claim 9, characterized in that: The triggering the second switch tube and the first switch tube to operate simultaneously specifically includes: The first switch tube and the second switch tube are controlled to operate in the following three modes in sequence: First mode: controlling the first switch tube and the second switch tube to be closed; Second mode: controlling the second switch tube to be disconnected and the first switch tube to be closed; The third mode: controlling the first switch tube and the second switch tube to be turned off.

11. The control method according to claim 9, characterized in that: The triggering the second switch tube and the first switch tube to operate simultaneously specifically includes: The duty cycle of the first switching tube and the duty cycle of the second switching tube are obtained according to the voltage of the flying capacitor, the voltage of the first voltage divider capacitor and the input voltage of the converter. The first switching tube is triggered according to the duty cycle of the first switching tube, and the second switching tube is triggered according to the duty cycle of the second switching tube.

12. The control method according to claim 9, characterized in that: Also includes: Obtaining a duty cycle adjustment amount according to the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor; The duty cycle of the first switch tube after adjustment is the difference between the duty cycle before adjustment and the duty cycle adjustment amount, and the duty cycle of the second switch tube after adjustment is the sum of the duty cycle before adjustment and the duty cycle adjustment amount; The duty cycle adjustment amount is proportional to the absolute value of the difference between the voltage of the flying capacitor and the voltage of the second voltage-dividing capacitor.

13. The control method according to claim 11 or 12, characterized in that: The duty cycle is proportional to a first voltage and inversely proportional to a second voltage, the second voltage being the sum of a voltage of the first voltage divider capacitor and a voltage of the flying capacitor, and the first voltage being the difference between the second voltage and an input voltage of the converter.

14. The control method according to any one of claims 9 to 12, characterized in that: Also includes: When the voltage of the flying capacitor is greater than or equal to the voltage of the second voltage-dividing capacitor, the second switching transistor and the first switching transistor are controlled to operate in an alternating manner.

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