Patent Publication Number: US-2023155499-A1

Title: Three-level boost converter and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application a continuation of International Application No. PCT/CN2021/081970, filed on Mar. 22, 2021, which claims priority to Chinese Patent Application No. 202010713160.6, filed on Jul. 22, 2020. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The embodiments relate to the field of power electronics technologies, a three-level boost converter, and a control method. 
     BACKGROUND 
     A boost converter is a boost power converter circuit that steps up an input voltage and then outputs an increased voltage, thereby implementing power conversion. Boost converters are classified into two-level boost converters and three-level boost converters. The two-level boost converter is generally applied to a scenario with a low voltage level, and the two-level boost converter corresponds to two input levels. The three-level boost converter is applied to a scenario with a higher voltage level. The three-level boost converter can realize power conversion of three or more input levels. 
     Compared with the two-level boost converter, the three-level boost converter implements three levels by improving a topology structure of the three-level boost converter, and then implements a high-voltage and high-power output. With a same input voltage, the three-level boost converter has a distinct advantage that a voltage stress of a power component can be reduced to a half of a voltage stress of the two-level boost converter, so that a high-voltage output can be achieved through the power component with a low withstand voltage. 
     Most of the current three-level boost converters include flying capacitors. A volume and a cost of a three level flying-capacitor boost converter are limited by an inductor inside the boost converter. Because the inductor has a large volume, the three-level boost converter has a large size, causing a high cost. 
     SUMMARY 
     The embodiments may provide a three-level boost converter and a control method to reduce a volume of an inductor and therefore reduce a volume of the three-level boost converter. 
     According to a first aspect, an embodiment may provide a three-level boost converter. The converter is a typical three level flying-capacitor boost, and includes a first switching transistor, a second switching transistor, an inductor, a flying capacitor, and a controller, where the first switching transistor and the second switching transistor are primary power transistors. The controller sends asymmetric driving signals to the first switching transistor and the second switching transistor, to increase a charge and discharge frequency of the inductor and further reduce a ripple current of the inductor. When a voltage of the flying capacitor is less than or equal to a half of a bus voltage, the controller may send a first driving signal to the first switching transistor and may send a second driving signal to the second switching transistor; or when a voltage of the flying capacitor is greater than a half of a bus voltage, the controller may send a first driving signal to the second switching transistor, and may send a second driving signal to the first switching transistor. The bus voltage is an output voltage of the three-level boost converter. The second driving signal has N pulses in each period of the first driving signal, the inductor has N+1 charge and discharge periods in each period of the first driving signal, and N is an integer greater than or equal to 2. In this way, the charge and discharge frequency of the inductor is increased. 
     Because the charge and discharge frequency of the inductor is increased, the ripple current of the inductor can further be reduced. After the ripple current is reduced, an inductor with a smaller inductance can be used, which has a smaller volume and a lower cost. Therefore, the embodiment can reduce a size and a cost of the converter. 
     The converter may further include a third switching transistor and a fourth switching transistor. A first terminal of the inductor is connected to a first input of the three-level boost converter, and a second terminal of the inductor is connected to a first node. A first terminal of the first switching transistor is connected to the first node, and a second terminal of the first switching transistor is connected to a second node. A first terminal of the second switching transistor is connected to the second node, and a second terminal of the second switching transistor is connected to a second input of the three-level boost converter. An anode and a cathode of the third switching transistor are respectively connected to the first node and a third node, and an anode and a cathode of the fourth switching transistor are respectively connected to the third node and a first output of the three-level boost converter. A first terminal of the flying capacitor is connected to the second node, and a second terminal of the flying capacitor is connected to the third node. The controller sending the asymmetric driving signals to the first switching transistor and the second switching transistor can still increase the charge and discharge frequency of the inductor and further reduce the ripple current of the inductor. 
     To increase the charge and discharge frequency of the inductor, the second driving signal may have N pulses in a low-level time period of each period of the first driving signal, so that the inductor has N+1 charge and discharge periods existing in one period of the first driving signal. In other words, the inductor completes N+1 times of charging and discharging in one period of the first driving signal. 
     To configure asymmetric driving signals for the first switching transistor and the second switching transistor, frequencies corresponding to at least two pulses of the N pulses of the second driving signal may be made different. 
     A pulse of the second driving signal may have the following several implementations. 
     Frequencies corresponding to all the N pulses of the second driving signal may be made different. 
     The N pulses may have a same pulse width. 
     N may be 2. 
     N may be greater than or equal to 3. 
     Frequencies of the first N−1 pulses of the N pulses may be the same and a frequency of the last pulse of the N pulses may be less than the frequency of the first N−1 pulses, so that a first driving signal and a second driving signal that are asymmetric can be formed. Then the asymmetric driving signals are sent to the first switching transistor and the second switching transistor. This increases the charge and discharge frequency of the inductor and reduces the ripple current of the inductor. 
     In each period of the first driving signal, a rising edge of the first pulse of the N pulses may follow a falling edge of the first driving signal. 
     Because the ripple current of the inductor is affected by a time lag between the first pulse of the N pulses and the first driving signal, the controller may determine the time lag through the ripple current of the inductor. To further reduce the ripple current of the inductor and keep the ripple current of the inductor at a low value, the controller controls a charge current of the inductor to be equal to a discharge current of the inductor in each charge and discharge period of the inductor, so as to determine the time lag. 
     The controller may be further configured to determine the time lag based on an input voltage and the output voltage of the three-level boost converter, a voltage of the flying capacitor, and the period of the first driving signal. 
     The controller may be configured to obtain the time lag, which is (D1+D2)Tsw, according to the following formula: 
       ( V   in   +V   c   V   o ) D   1   T   sw =( V   o   −V   in ) D   2   T   sw    
     where V in  is the input voltage of the three-level boost converter, V o  is the output voltage of the three-level boost converter, V c  is the voltage of the flying capacitor, T sw  is the period of the first driving signal, D 1  is a duty cycle of the three-level boost converter, and D 2  T sw  is a delay time between the rising edge of the first pulse of the N pulses and the falling edge of the first driving signal. 
     To further reduce a loss generated in a circuit, in each period of the first driving signal, the time lag between the first pulse of the N pulses and the first driving signal may be determined based on a loss of the first switching transistor and/or a loss of the second switching transistor. 
     to reduce both the ripple current of the inductor and the loss generated in the circuit, the time lag may be determined using a combination of the two manners of determining the time lag described above. In each period of the first driving signal, the time lag between the first pulse of the N pulses and the first driving signal may be determined based on the ripple current of the inductor and a loss of at least one switching transistor. 
     Another three level flying-capacitor boost may further include a fifth switching transistor. A first terminal of the fifth switching transistor is connected to the second node, and a second terminal of the fifth switching transistor is connected to the first terminal of the flying capacitor. The fifth switching transistor included in the converter can protect the second switching transistor. When a power supply is turned on, the fifth switching transistor is first controlled to be switched off. In this case, a voltage of the power supply is not applied on the second switching transistor. This reduces a voltage applied on the second switching transistor at a moment when the second switching transistor is switched on. 
     Another three level flying-capacitor boost may further include a third diode and a fourth diode. An anode of the third diode is connected to a midpoint of the output voltage of the three-level boost converter, and a cathode of the third diode is connected to the third node. An anode of the fourth diode is connected to the second node, and a cathode of the fourth diode is connected to the midpoint of the output voltage. In a topology structure of the converter, a voltage of two terminals of the second switching transistor can also be reduced at a moment when the power supply is turned on, and a voltage stress withstood by the second switching transistor is further reduced, thereby protecting the second switching transistor. 
     According to a second aspect, an embodiment may provide a control method for a three-level boost converter. The three-level boost converter includes a first switching transistor, a second switching transistor, an inductor, a flying capacitor, and a controller, where the first switching transistor and the second switching transistor are primary power transistors. 
     The Method Includes: 
     when a voltage of the flying capacitor is less than or equal to a half of a bus voltage, sending a first driving signal to the first switching transistor, and sending a second driving signal to the second switching transistor; or when a voltage of the flying capacitor is greater than a half of a bus voltage, sending a first driving signal to the second switching transistor, and sending a second driving signal to the first switching transistor. 
     The bus voltage is an output voltage of the three-level boost converter. 
     The second driving signal has N pulses in each period of the first driving signal, the inductor has N+1 charge and discharge periods in each period of the first driving signal, and N is an integer greater than or equal to 2. 
     The second driving signal may have N pulses in a low-level time period of each period of the first driving signal. 
     Frequencies corresponding to at least two pulses of the N pulses may be different. 
     The embodiments may have at least the following advantages: 
     The embodiments may provide a three-level boost converter, including a first switching transistor, a second switching transistor, an inductor, a flying capacitor, and a controller, where the first switching transistor and the second switching transistor are primary power transistors. A volume and a cost of the three-level boost converter are limited by a volume and a cost of the inductor. A ripple current of the inductor needs to be reduced to reduce the volume of the inductor. Because a higher frequency indicates a smaller ripple current, the controller reduces the ripple current of the inductor by increasing a charge and discharge frequency of the inductor, and further reduces an inductance of the inductor. A smaller inductance of the inductor indicates a smaller volume of the inductor. Therefore, the volume of the three-level boost converter that includes the inductor is also reduced. 
     The controller may send asymmetric driving signals to the first switching transistor and the second switching transistor, and the two driving signals have different frequencies. When a voltage of the flying capacitor is less than or equal to a half of a bus voltage, the controller sends a first driving signal to the first switching transistor, sends a second driving signal to the second switching transistor, and controls the second driving signal to have N pulses in each period of the first driving signal and control the inductor to have N+1 charge and discharge periods in each period of the first driving signal, where N is an integer greater than or equal to 2. 
     Through the foregoing driving manner, the inductor has at least three charge and discharge periods in each period of the first driving signal. This increases the charge and discharge frequency of the inductor and further reduces the ripple current of the inductor. Similarly, when the voltage of the flying capacitor is greater than the half of the bus voltage, the controller sends a first driving signal to the second switching transistor, and sends a second driving signal to the first switching transistor. This also increases the charge and discharge frequency of the inductor and reduces the ripple current of the inductor. A smaller ripple current indicates a smaller inductance required for the inductor, and the inductor with a smaller inductance has a smaller volume. Therefore, the volume of the three-level boost converter is reduced, and the size and cost of the three-level boost converter are reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a topology diagram of a three-level boost converter according to an embodiment; 
         FIG.  2    is a waveform graph of driving signals according to an embodiment; 
         FIG.  3    is a waveform graph of symmetrical driving signals; 
         FIG.  4 A  is another waveform graph of driving signals according to an embodiment; 
         FIG.  4 B  is still another waveform graph of driving signals according to an embodiment; 
         FIG.  4 C  is yet another waveform graph of driving signals according to an embodiment; 
         FIG.  5 A  is still yet another waveform graph of driving signals according to an embodiment; 
         FIG.  5 B  is a waveform graph of driving signals according to an embodiment; 
         FIG.  5 C  is another waveform graph of driving signals according to an embodiment; 
         FIG.  6 A  is still another waveform graph of driving signals according to an embodiment; 
         FIG.  6 B  is yet another waveform graph of driving signals according to an embodiment; 
         FIG.  6 C  is still yet another waveform graph of driving signals according to an embodiment; 
         FIG.  7    is a further waveform graph of driving signals according to an embodiment; 
         FIG.  8    is another topology diagram of a three-level boost converter according to an embodiment; 
         FIG.  9    is still another topology diagram of a three-level boost converter according to an embodiment; and 
         FIG.  10    is a flowchart of a control method for a three-level boost converter according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     To make a person skilled in the art better understand the embodiments, an operating principle of a three-level boost converter is first described below. 
     For ease of description, the three-level boost converter is referred to as a converter below. 
     Embodiment 1 of Converter 
     A controller in the converter provided in this embodiment may send asymmetric driving signals to a first switching transistor and a second switching transistor, to increase a charge and discharge frequency of an inductor and further reduce a ripple current of the inductor. After the ripple current is reduced, an inductor with a smaller inductance can be used, which has a smaller volume and a lower cost. Therefore, the embodiment can reduce a size and a cost of the converter. 
       FIG.  1    is a topology diagram of a three-level boost converter according to an embodiment. 
     The converter includes a first switching transistor T 1 , a second switching transistor T 2 , an inductor L, a flying capacitor C fly , a first diode D3, a second diode D4, and a controller (which is not shown in the figure). 
     A first terminal of the inductor L is connected to a first input of the converter, and a second terminal of the inductor L is connected to a first node A. 
     A first terminal of the first switching transistor T 1  is connected to the first node A, and a second terminal of the first switching transistor T 1  is connected to a second node B. 
     A first terminal of the second switching transistor T 2  is connected to the second node 
     B, and a second terminal of the second switching transistor T 2  is connected to a second input of the converter. 
     An anode of the first diode D3 is connected to the first node A, and a cathode of the first diode D3 is connected to a third node C. 
     An anode of the second diode D4 is connected to the third node C, and a cathode of the second diode D4 is connected to a first output of the converter. 
     The first switching transistor T 1  and the second switching transistor T 2  are primary power transistors. 
     A first terminal of the flying capacitor C fly  is connected to the second node B, and a second terminal of the flying capacitor C fly  is connected to the third node C. 
     V in  is an input voltage, V o  is a bus voltage on a circuit output side of the converter, that is, an output voltage, and C bus , is a bus capacitance on the circuit output side of the converter. 
     In addition, D3 and D4 in the figure may be replaced with switching transistors, that is, respectively replaced with a third switching transistor and a fourth switching transistor, provided that both the third switching transistor and the fourth switching transistor are controlled to implement operating modes of the diodes. In other words, the third switching transistor and the fourth switching transistor may be of the same type as the first switching transistor and the second switching transistor. 
     Driving signals sent by the controller to the first switching transistor T 1  and the second switching transistor T 2  are different for different magnitude relationships between a voltage V c  of the flying capacitor and a half of the bus voltage V o , that is, different magnitude relationships between V c  and 0.5 V o . However, in both cases, a charge and discharge frequency of the inductor L can be increased and a ripple current of the inductor L can be reduced. 
     For ease of understanding, both cases are described in detail below. 
     In a First Case: 
     when V c  is less than or equal to 0.5 V o , the controller sends a first driving signal to the first switching transistor T 1 , and sends a second driving signal to the second switching transistor T 2 . 
     The second driving signal has N pulses in each period of the first driving signal, the inductor L has N+1 charge and discharge periods in each period of the first driving signal, and N is an integer greater than or equal to 2. 
     Therefore, the inductor L has at least three charge and discharge periods in each period of the first driving signal. Further, the controller increases a charge and discharge frequency of the inductor L in each period of the first driving signal. 
     For example, when N is equal to 2, the inductor L has three charge and discharge periods in each period of the first driving signal. When N is equal to 3, the inductor L has four charge and discharge periods in each period of the first driving signal. 
     A value of N is not limited and N is an integer greater than or equal to 2. For example, N may be 2 or 3, or may be an integer with a larger value. For ease of description, an example that N is equal to 2 is used below for detailed description. 
       FIG.  2    is a waveform graph of driving signals according to an embodiment. 
     PWM 1  represents the first driving signal, PWM 2  represents the second driving signal, and iL represents a current of the inductor. 
     The controller sends PWM 1  to the first switching transistor. Therefore, the controller can control an on/off state of the first switching transistor by using PWM 1 . 
     Similarly, the controller can also control an on/off state of the second switching transistor by using PWM 2 . 
     When PWM 1  is at a high level, the first switching transistor is switched on, and when PWM 1  is at a low level, the first switching transistor is switched off. 
     When PWM 2  is at a high level, the second switching transistor is switched on, and when PWM 2  is at a low level, the second switching transistor is switched off. 
     Periods of PWM 1  and PWM 2  are different. In the figure, periods (a+b+c+d+e+f) are a period of PWM 1 . 
     PWM 2  may have a plurality of periods in each period of PWM 1  and PWM 2  may have two pulses in a low-level time period in each period of PWM 1 . Therefore, the inductor has three charge and discharge periods in one period of PWM 1 , that is, the inductor completes three times of charging and discharging in one period of PWM 1 . In other words, a charge and discharge frequency of the inductor is increased. 
     Using one period of PWM 1  as an example, time period a is a high-level time period of PWM 1 , and time period b, time period c, time period d, time period e, and time period f are all low-level time periods of PWM 1 . 
     PWM 2  has pulses in time periods c and e, that is, PWM 2  is at a high level in time periods c and e. PWM 2  has no pulse in time periods a, b, d, and f, that is, PWM 2  is at a low level in time periods a, b, d, and f. 
     PWM 2  has two pulses, that is, a pulse corresponding to time period c and a pulse corresponding to time period e, in the low-level time periods of one period of PWM 1 . Therefore, the inductor completes one time of charging and discharging in time period a and time period b, completes another time of charging and discharging in time period c and time period d, and completes another time of charging and discharging in time period e and time period f. In this way, the charge and discharge frequency of the inductor in one period of PWM 1  is increased. Because the charge and discharge frequency of the inductor is increased, the ripple current of the inductor is reduced. 
     For ease of understanding by a person skilled in the art, the following describes in detail, with reference to  FIG.  1   , a process in which the controller controls PWM 2  to have two pulses in a low-level time period of each period of PWM 1  to increase a quantity of charging and discharging times of the inductor, that is, increase the charge and discharge frequency of the inductor. 
     For ease of description, a charge and discharge process of the inductor is described below by using one period of PWM 1  as an example. 
     Within time period time a, PWM 1  is at a high level, and PWM 2  is at a low level, that is, the controller controls T 1  to be switched on, controls T 2  to be switched off, and controls L to charge, so that iL gradually increases; and within time period b, both PWM 1  and PWM 2  are at a low level, that is, the controller controls T 1  to be switched off, controls T 2  to be switched off, and controls L to discharge, so that iL gradually decreases. In this case, L completes one time of charging and discharging. 
     Within time period time c, PWM 1  is at a low level, and PWM 2  is at a high level, that is, the controller controls T 1  to be switched off, controls T 2  to be switched on, and controls L to charge, so that iL gradually increases; and within time period d, both PWM 1  and PWM 2  are at a low level, that is, the controller controls T 1  to be switched off, controls T 2  to be switched off, and controls L to discharge, so that iL gradually decreases. In this case, L completes another time of charging and discharging. 
     Similarly, L completes another time of charging and discharging in time period e and time period f Therefore, L completes a total of three times of charging and discharging in one period of PWM 1 . 
     The controller sends the asymmetric PWM 1  and PWM 2  to T 1  and T 2 , to increase the quantity of charging and discharging times of L in each period of PWM 1 , that is, increase the charge and discharge frequency of L, and therefore reduce the ripple current of L. Therefore, because the ripple current is reduced, an inductor with a small inductance can be used in the converter provided in this embodiment, where the inductor with a small inductance has a small volume and a low cost. Therefore, a volume and a cost of the converter can be reduced. 
     In a Second Case: 
     when V c  is greater than 0.5 V o , the controller sends a second driving signal to the first switching transistor T 1  and sends a first driving signal to the second switching transistor T 2 . 
     Similarly, the controller can also increase the charge and discharge frequency of the inductor in each period of PWM 1 . Compared to the first case in which L performs at least two times of charging and discharging when T 1  is in an off state, in the second case, L performs at least two times of charging and discharging when T 2  is in an off state. 
     Therefore, the controller may further send asymmetric driving signals to T 1  and T 2  based on a magnitude relationship between V c  and 0.5 V o , to increase the charge and discharge frequency of the inductor. In both the first case and the second case, the controller can send asymmetric driving signals to T 1  and T 2 , to increase the charge and discharge frequency of the inductor, and further reduce the ripple current of the inductor, so that an inductor with a small inductance can be selected. 
     The following analyzes advantages of the waveform graph provided in  FIG.  2    over a waveform graph provided in  FIG.  3    with reference to the waveform graph of symmetric driving signals provided in  FIG.  3   . 
       FIG.  3    is a waveform graph of symmetrical driving signals. 
     PWM 3  and PWM 4  in  FIG.  3    are symmetrical, that is, the two driving signals have a same period and a same duty cycle. 
     PWM 3  represents the first driving signal, PWM 4  represents the second driving signal, and iL represents the current of the inductor. 
     As shown in  FIG.  3   , in one period of PWM 3 , PWM 3  has one pulse, PWM 4  has one pulse, the inductor performs two times of charging and discharging, and iL has two periods, iL may rise at a high level of PWM 3  and may fall at a low level of PWM 3 , and iL may rise at a high level of PWM 4  and may fall at a low level of PWM 4 . 
     However, in  FIG.  2   , PWM 1  and PWM 2  are asymmetric driving signals. In one period of PWM 1 , PWM 1  has one pulse, PWM 2  has at least two pulses, and the inductor performs at least three times of charging and discharging. 
     By comparison between  FIG.  2    and  FIG.  3   , the driving signals provided in the embodiment are asymmetric and a frequency of one of the driving signals is increased, thereby increasing the charge and discharge frequency of the inductor and reducing the ripple current of the inductor. 
     The foregoing describes the process in which the controller sends the asymmetric driving signals to the first switching transistor and the second switching transistor to increase the charge and discharge frequency of the inductor. The following describes in detail several implementation forms of the driving signal. 
     Embodiment 2 of Converter 
     The embodiments may not limit a quantity of pulses of a second driving signal in a low-level time period of each period of a first driving signal. In other words, N may be 2 or more than 2, for example, 3 or 4. 
     Because the first driving signal is simple, an implementation of the corresponding second driving signal in one period of the first driving signal is described below. A pulse width of each of N pulses of the second driving signal is not limited. The pulse widths may be the same or may be different. 
     For ease of understanding by a person skilled in the art, the following provides detailed descriptions in three cases. 
     In a first case, frequencies corresponding to at least two pulses of the N pulses are different. 
     For ease of understanding, the following provides descriptions by using an example in which the second driving signal has three pulses in a low-level time period of each period of the first driving signal. In other words, when N=3, frequencies corresponding to two pulses of the three pulses are different. 
       FIG.  4 A  is another waveform graph of driving signals according to an embodiment. 
     P 1 , P 2 , and P 3  represent three pulses of the second driving signal existing in a low-level time period in one period of the first driving signal. 
     T 1  is a period corresponding to P 1 , T 2  is a period corresponding to P 2 , and T 3  is a period corresponding to P 3 . 
     Period T 1  corresponding to P 1  may be the same as period T 3  corresponding to P 3 , that is, T 1 =T 3 , and therefore, a frequency corresponding to P 1  may be equal to a frequency corresponding to P 3 . However, the periods of P 1  and P 3  are different from period T 2  corresponding to P 2 , and therefore, the frequencies of P 1  and P 3  are different from a frequency corresponding to P 2 . T 3  may be greater than T 2 . 
     P 0  represents a pulse of the first driving signal in one period, and pulse widths of P 1 , P 2 , and P 3  are the same. 
       FIG.  4 B  is still another waveform graph of driving signals according to an embodiment. P 1 , P 2 , and P 3  represent three pulses of the second driving signal in one period of the first driving signal, and P 0  represents the pulse of the first driving signal in one period. Period T 1  corresponding to P 1  may be the same as period T 3  corresponding to P 3 , that is, T 1 =T 3 , and therefore, a frequency corresponding to P 1  may be equal to a frequency corresponding to P 3 . However, the periods of P 1  and P 3  are different from period T 2  corresponding to P 2 , and therefore, the frequencies of P 1  and P 3  are different from a frequency corresponding to P 2 . T 3  may be greater than T 2 . 
     In addition, pulse P 0  of the first driving signal overlaps with the first pulse P 1  of the three pulses of the second driving signal. In a time period in which pulse P 0  and pulse P 1  overlap, the inductor is charged, and discharged until pulse P 1  is at a low level. 
     Two pulses, namely, P 2  and P 3 , still exist in a low-level time period in one period of the first driving signal. By configuring asymmetric driving signals shown in  FIG.  4 B , the controller can still increase a charge and discharge frequency of the inductor, and further reduce a ripple current of the inductor. 
     In another situation, pulse widths of P 1 , P 2 , and P 3  may be different or partially the same. For example, P 2 =P 3 , and P 1  is not equal to P 3 .  FIG.  4 C  is yet another waveform graph of driving signals according to an embodiment. 
     In  FIG.  4 C , features of periods and frequencies corresponding to P 1 , P 2 , and P 3  are the same as those in  FIG.  4 A , with differences from  FIG.  4 A  as follows: In the waveform graph of driving signals shown in  FIG.  4 C , a pulse width of P 1  is different from a pulse width of P 2 , and the pulse width of P 1  is different from a pulse width of P 3 , but the pulse width of P 2  is the same as the pulse width of P 3 . 
     A difference of  FIG.  4 C  from  FIG.  4 B  lies in that, there is a delay time between a rising edge of the first pulse of the second driving signal and a falling edge of the pulse of the first driving signal, which will be described in detail in subsequent embodiments. 
     The controller increases the quantity of pulses of PWM 2  in the low-level time period of PWM 1  to increase a quantity of charging and discharging times of the inductor, and further increase a charge and discharge frequency of the inductor. Therefore, even if the pulse widths of the pulses of PWM 2  are different, the ripple current of the inductor can also be reduced. 
     In a second case, frequencies corresponding to the N pulses are different. 
     For ease of understanding, the following describes in detail an implementation form of the driving signal by using an example in which the second driving signal has three pulses in a low-level time period of each period of the first driving signal. In other words, when N=3, frequencies corresponding to the three pulses are different. 
       FIG.  5 A  is still yet another waveform graph of driving signals according to an embodiment. 
     P 1 , P 2 , and P 3  correspond to three pulses of the second driving signal existing in a low-level time period in one period of the first driving signal. 
     T 1  is a period corresponding to P 1 , T 2  is a period corresponding to P 2 , and T 3  is a period corresponding to P 3 . 
     Period T 3  corresponding to P 3  may be greater than period T 2  corresponding to P 2  and period T 2  corresponding to P 2  may be greater than period T 1  corresponding to P 1 , that is, T 3 &gt;T 2 &gt;T 1 . Therefore, P 1 , P 2 , and P 3  may correspond to different frequencies. 
     Pulse widths of the three pulses are not limited in this embodiment. For example, the pulse widths of P 1 , P 2 , and P 3  are the same. In another case, the pulse widths of P 1 , P 2 , and P 3  may be partially the same, or may be different. 
       FIG.  5 B  is a waveform graph of driving signals according to an embodiment. 
     In  FIG.  5 B , features of periods and frequencies corresponding to P 1 , P 2 , and P 3  are the same as those in  FIG.  5 A , with differences from  FIG.  5 A  as follows: In the waveform graph of driving signals shown in  FIG.  5 B , a pulse width of P 3  is different from a pulse width of P 1 , and the pulse width of P 3  is different from a pulse width of P 2 , but the pulse width of P 1  is the same as the pulse width of P 2 . 
     The controller increases the quantity of pulses of PWM 2  in the low-level time period of PWM 1  to increase the quantity of charging and discharging times of the inductor, and further increase the charge and discharge frequency of the inductor. Therefore, even if the pulse widths of the pulses of PWM 2  are different, the ripple current of the inductor can also be reduced. 
     The first and second cases described above are described only by using an example in which N is equal to 3. N may alternatively be an integer greater than 3, for example, 4 or 5. 
     When N is equal to 2, the first and second cases may be combined. In other words, when N is equal to 2, PWM 2  has two pulses in each period of PWM 1 . For the first case in which frequencies corresponding to at least two pulses in the two pulses are different, frequencies corresponding to the two pulses are different. For the second case in which frequencies corresponding to the two pulses are different, frequencies corresponding to the two pulses are different. 
     When N=2, the foregoing first and second cases may be combined into one form of driving signal.  FIG.  5 C  is another waveform graph of driving signals according to an embodiment. 
     P 1  and P 2  represent two pulses of PWM 2  in each period of PWM 1 . 
     T 1  is a period corresponding to P 1 , and T 2  is a period corresponding to P 2 . 
     Period T 2  corresponding to P 2  may be greater than period T 1  corresponding to P 1 , that is, T 2 &gt;T 1 . Therefore, P 1  and P 2  correspond to different frequencies. 
     A third case is described below. The third case is applicable to a situation that N is greater than 2. 
     In a third case, frequencies of the first N−1 pulses in the N pulses are the same, and a frequency of the last pulse of the N pulses is less than the frequency of the first N−1 pulses. 
     For ease of understanding, the following describes in detail an implementation form of the driving signal by using an example in which the second driving signal has three pulses in a low-level time period of each period of the first driving signal. In other words, when N=3, frequencies corresponding to the first two pulses in the three pulses are the same, and a frequency corresponding to the last pulse is less than the frequency corresponding to the first two pulses. 
       FIG.  6 A  is still another waveform graph of driving signals according to an embodiment. 
     P 1 , P 2 , and P 3  correspond to three pulses of the second driving signal existing in a low-level time period in one period of the first driving signal. 
     T 1  is a period corresponding to P 1 , T 2  is a period corresponding to P 2 , and T 3  is a period corresponding to P 3 . 
     Period T 3  corresponding to P 3  may be greater than period T 2  corresponding to P 2 , period T 3  corresponding to P 3  may be greater than period T 1  corresponding to P 1 , and period T 1  corresponding to P 1  may be equal to period T 2  corresponding to P 2 , that is, T 3 &gt;T 2 =T 1 . Therefore, P 1  and P 2  correspond to a same frequency, and a frequency corresponding to P 3  is less than the frequency corresponding to P 1  and P 2 . 
     Pulse widths of P 1 , P 2 , and P 3  are all the same. In another case, the pulse widths of P 1 , P 2 , and P 3  may be different.  FIG.  6 B  is yet another waveform graph of driving signals according to an embodiment. 
     In  FIG.  6 B , features of periods and frequencies corresponding to P 1 , P 2 , and P 3  are the same as those in  FIG.  6 A , with differences from  FIG.  6 A  as follows: In the waveform graph of driving signals shown in  FIG.  6 B , a pulse width of P 2  is different from a pulse width of P 1 , and the pulse width of P 2  is different from a pulse width of P 3 . 
     The controller increases the quantity of pulses of PWM 2  in the low-level time period of PWM 1  to increase the quantity of charging and discharging times of the inductor, and further increase the charge and discharge frequency of the inductor. Therefore, even if the pulse widths of the pulses of PWM 2  are different, the ripple current of the inductor can also be reduced. 
     In addition, if N is equal to 4, the third case will be that, frequencies corresponding to the first three pulses in the four pulses are the same, and a frequency corresponding to the last pulse is less than the frequency corresponding to the first three pulses. If N is an integer greater than 3, the third case will be that, frequencies corresponding to the first N−1 pulses in the N pulses are the same, and a frequency corresponding to the last pulse is less than the frequency corresponding to the first N−1 pulses. 
     For ease of description, the following describes a waveform graph of driving signals after a change of the third case by using an example in which N is equal to 4. 
       FIG.  6 C  is still yet another waveform graph of driving signals according to an embodiment. 
     P 1 , P 2 , P 3 , and P 4  correspond to four pulses of PWM 2  in each period of PWM 1 . 
     T 1  is a period corresponding to P 1 , T 2  is a period corresponding to P 2 , T 3  is a period corresponding to P 3 , and T 4  is a period corresponding to P 4 . 
     Periods of the first three pulses may be the same and a period of the last pulse may be greater than the period of the first three pulses. In other words, T 1  is equal to T 2  and T 3 , and T 1 , T 2 , and T 3  are all less than T 4 , that is, T 1 =T 2 =T 3 &lt;T 4 . Therefore, frequencies corresponding to P 1 , P 2 , and P 3  are the same, and a frequency corresponding to P 4  is less than the frequency corresponding to P 1 , P 2 , and P 3 . 
     Through implementation forms of the driving signals, the controller can increase the charge and discharge frequency of the inductor in each period of the first driving signal, and further reduce the ripple current of the inductor. 
     In each period of the first driving signal, a rising edge of the first pulse of the N pulses follows a falling edge of the first driving signal. The following describes in detail a manner of obtaining a time lag between the rising edge of the first pulse of the N pulses and a rising edge of a pulse of the first driving signal. 
     Embodiment 3 of Converter 
     For ease of understanding by a person skilled in the art, the following describes in detail the time lag by using an example in which frequencies corresponding to the first N−1 pulses of the N pulses of the second driving signal are the same, and a frequency of the last pulse is less than the frequency of the first N−1 pulses. 
       FIG.  7    is a further waveform graph of driving signals according to an embodiment. 
     In one period of PWM 1 , P 1  represents the first pulse of PWM 2 , P 2  represents the second pulse of PWM 2 , PN represents the N th  pulse of PWM 2 , and P 0  represents a pulse of PWM 1 . 
     D 1  Tsw is high-level duration of PWM 1 , D 2  Tsw is a time that a rising edge of P 1  lags behind a falling edge of P 0 , and (D 1 +D 2 )Tsw is a time lag between the first pulse of the N pulses of the second driving signal and a first driving signal. 
     In an actual operation process, different influence factors can be selected to determine (D 1 +D 2 )Tsw for different requirements. The following provides descriptions in three cases. 
     In a first case, in each period of the first driving signal, the time lag (D 1 +D 2 )Tsw between the first pulse of the N pulses and the first driving signal is determined based on a ripple current of an inductor. 
     Because the ripple current of the inductor is affected by (D 1 +D 2 )Tsw, the controller may determine (D 1 +D 2 )Tsw through the ripple current of the inductor. 
     To further reduce the ripple current of the inductor and keep the ripple current of the inductor at a low value, the controller controls a charge current of the inductor to be equal to a discharge current of the inductor in each charge and discharge period of the inductor, so as to determine (D 1 +D 2 )Tsw. After determining (D 1 +D 2 )Tsw, the controller may obtain a ripple current with a small value based on (D 1 +D 2 )Tsw. 
     The following describes in detail, with reference to the topology diagram of the converter shown in  FIG.  1   , a process in which the controller obtains the time lag (D 1 +D 2 ) Tsw. 
     The controller determines the time lag based on an input voltage V in  of the converter, an output voltage V o , a voltage V c  of a flying capacitor C fly , and the period T sw  of the first driving signal. 
     With reference to  FIG.  2   , using an example in which the controller sends the first driving signal to a first switching transistor and sends a second driving signal to a second switching transistor, period a and period b correspond to one charge and discharge period of the inductor. In this case, period a corresponds to T 1  switched on, and period b corresponds to T 1  switched off. The inductor satisfies the following equation in the charge and discharge period: 
       ( V   in   +V   c   −v   o ) D   1   T   sw =( V   o   V   in ) D   2   T   sw    
     where V in  is the input voltage of the converter, V o  is the output voltage of the converter, V c  is the voltage of the flying capacitor, T sw  is the period of the first driving signal, D 1  is a duty cycle of the converter, and D 2  T sw  is the delay time between the rising edge of the first pulse of the N pulses and the falling edge of the first driving signal. 
     The duty cycle D 1  of the converter satisfies the following equation: 
     
       
         
           
             
               D 
               1 
             
             = 
             
               1 
               - 
               
                 
                   V 
                   
                     i 
                     ⁢ 
                     n 
                   
                 
                 
                   V 
                   o 
                 
               
             
           
         
       
     
     where D 1  is the duty cycle of the converter, V in  is the input voltage of the converter, and V o  is the output voltage of the converter. 
     Therefore, the controller may obtain the time lag (D 1 +D 2 )Tsw according to the foregoing two equations, and further reduce the ripple current of the inductor. 
     In a second case, in each period of the first driving signal, the time lag between the first pulse of the N pulses and the first driving signal is determined based on a loss of the first switching transistor and/or a loss of the second switching transistor. 
     To further reduce a loss generated in a circuit, (D 1 +D 2 )Tsw may be determined based only on the loss of the first switching transistor, or may be determined based only on the loss of the second switching transistor, or may be determined based on both the loss of the first switching transistor and the loss of the second switching transistor. 
     For example, when the controller determines (D 1 +D 2 )Tsw based only on the loss of the first switching transistor, (D 1 +D 2 )Tsw corresponding to a smallest loss of the first switching transistor is determined; when the controller determines (D 1 +D 2 )Tsw based only on the loss of the second switching transistor, (D 1 +D 2 )Tsw corresponding to a smallest loss of the second switching transistor is determined; and when the controller determines (D 1 +D 2 )Tsw based on both the loss of the first switching transistor and the loss of the second switching transistor, (D 1 +D 2 )Tsw corresponding to a moment when the loss of the first switching transistor and the loss of the second switching transistor are evenly distributed is determined. 
     Therefore, the controller may determine (D 1 +D 2 )Tsw based on the loss of the switching transistor, thereby reducing the loss generated in the circuit and improving conversion efficiency of the converter. 
     In a third case, in each period of the first driving signal, the time lag between the first pulse of the N pulses and the first driving signal is determined based on the ripple current of the inductor and a loss of at least one switching transistor. 
     To reduce both the ripple current of the inductor and the loss generated in the circuit, the controller may determine (D 1 +D 2 )Tsw based on a combination of the influence factors introduced in the first and second cases. 
     Therefore, the controller can obtain (D 1 +D 2 )Tsw corresponding to a low ripple current of the inductor and a low loss of at least one switching transistor. 
     The foregoing embodiments describe a method for controlling a driving signal. The method for controlling a driving signal is also applicable to a modification of the topology diagram of the converter shown in  FIG.  1   . The modified topology diagram is described in detail below. 
     Embodiment 4 of Converter 
     The topology diagram of the converter is not limited, and a person skilled in the art may perform adaptive modification on the topology diagram based on an actual requirement. 
       FIG.  8    is another topology diagram of a three-level boost converter according to an embodiment. 
     On the basis of the topology diagram shown in  FIG.  1   , the converter further includes a fifth switching transistor T 3 . 
     A first terminal of the fifth switching transistor T 3  is connected to the second node B, and a second terminal of the fifth switching transistor T 3  is connected to the first terminal of the flying capacitor C fly . 
     By sending asymmetric driving signals to the first switching transistor T 1  and the second switching transistor T 2 , the controller can also increase a charge and discharge frequency of the inductor L, and therefore reduce a ripple current of the inductor L. 
     A circuit shown in  FIG.  8    has the following advantages over the circuit shown in  FIG.  1   : 
     When a power supply is turned on, T 3  is first controlled to be switched off, so that L, D 3 , C fly , and T 2  cannot form a loop for a current to pass through. In this case, a voltage Vin of the power supply is not applied to T 2 , and therefore, T 2  is protected. For example, when Vin is 1400 V, switching transistors with a withstand voltage of 950 V are usually used for T 1  and T 2 . If T 3  is not added, a voltage on the flying capacitor C fly  is 0 at a moment when the power supply is turned on. In other words, Vin is almost completely applied to T 2 . This causes T 2  to withstand a voltage that exceeds a voltage stress of the component, which could damage T 2 . 
       FIG.  9    is still another topology diagram of a three-level boost converter according to an embodiment. 
     On the basis of the topology shown in  FIG.  1   , the converter further includes a third diode D 5 , and a fourth diode D 6 . 
     An anode of the third diode D 5  is connected to a midpoint of an output voltage of the three-level boost converter, that is, the anode of the third diode D 5  is connected to a first terminal of a first capacitor C 1 , the first terminal of the first capacitor C 1  is connected to a first terminal of a second capacitor C 2 , a second terminal of the first capacitor C 1  is connected to a first output, and a second terminal of the second capacitor C 2  is connected to a second output. 
     A cathode of the third diode D 5  is connected to the third node C. A function of D 5  is to clamp a pressure drop withstood by D 4  to prevent D 4  from withstanding an entire direct current bus voltage (Vo) when T 2  is switched on. 
     An anode of the fourth diode D 6  is connected to the second node B, and a cathode of the fourth diode D 6  is connected to the midpoint of the output voltage, that is, the cathode of the fourth diode D 6  is connected to the first terminal of the first capacitor C 1 . 
     By sending asymmetric driving signals to the first switching transistor T 1  and the second switching transistor T 2 , the controller can also increase the charge and discharge frequency of the inductor L, and therefore reduce the ripple current of the inductor L. 
     A function of  FIG.  9    is consistent with a function of  FIG.  8   , both to reduce a voltage stress withstood by T 2  when the power supply is turned on, that is, when the power supply is connected. A circuit shown in  FIG.  9    has the following advantages over the circuit shown in  FIG.  1   : 
     D 5  and D 6  are added in  FIG.  9    so that when the power supply is turned on, C fly  and Cl are connected in parallel, Vin charges C fly , and C fly  does not have a voltage of 0. Therefore, the voltage Vin of the power supply is not completely applied to T 2 , which reduces the voltage stress withstood by T 2 , thereby protecting T 2 . 
     D 3  and D 4  in  FIG.  8    and  FIG.  9    may both be replaced with switching transistors, that is, respectively replaced with a third switching transistor and a fourth switching transistor, provided that both the third switching transistor and the fourth switching transistor are controlled to implement operating modes of the diodes. 
     Method Embodiment 1 
     The converter is described in the foregoing embodiments, and a control method for a converter is described in detail below. 
       FIG.  10    is a flowchart of a control method for a three-level boost converter according to an embodiment. 
     For a topology diagram of the converter, described in the foregoing embodiments, to which the method is applied, refer to  FIG.  1   . Details are not described herein again. 
     The method includes the following steps. 
     Step  1001 : When a voltage of a flying capacitor is less than or equal to a half of a bus voltage, send a first driving signal to a first switching transistor, and send a second driving signal to a second switching transistor. 
     The bus voltage is an output voltage of the three-level boost converter. The second driving signal has N pulses in each period of the first driving signal, an inductor has N+1 charge and discharge periods in each period of the first driving signal, and N is an integer greater than or equal to 2. 
     Therefore, the inductor has at least three charge and discharge periods in each period of the first driving signal, thereby increasing a charge and discharge frequency of the inductor in each period of the first driving signal. 
     A value of N is not limited and N is an integer greater than or equal to 2. For example, N may be 2 or 3, or may be an integer with a larger value. For ease of description, an example that N is equal to 2 is used below for detailed description. 
     With reference to  FIG.  2   , PWM 2  has a plurality of periods in each period of PWM 1 , and PWM 2  has two pulses in a low-level time period in each period of PWM 1 . Therefore, the inductor has three charge and discharge periods in one period of PWM 1 , that is, the inductor completes three times of charging and discharging in one period of PWM 1 . In other words, the charge and discharge frequency of the inductor is increased. 
     Step  1002 : When the voltage of the flying capacitor is greater than the half of the bus voltage, send a first driving signal to the second switching transistor, and send a second driving signal to the first switching transistor. 
     Similarly, when the voltage of the flying capacitor is greater than the half of the bus voltage, the charge and discharge frequency of the inductor in each period of PWM 1  can also be increased. Compared with the first case that the voltage of the flying capacitor is less than or equal to the half of the bus voltage, in which the inductor performs at least two times of charging and discharging when the first switching transistor is in an off state, in the second case, the inductor performs at least two times of charging and discharging when the second switching transistor is in an off state. 
     Therefore, based on a magnitude relationship between the voltage of the flying capacitor and the half of the bus voltage, asymmetric driving signals are sent to the first switching transistor and the second switching transistor in the method, to increase the charge and discharge frequency of the inductor. In both the first case and the second case, a controller can send the asymmetric driving signals to the first switching transistor and the second switching transistor, to increase the charge and discharge frequency of the inductor, and further reduce a ripple current of the inductor, so that an inductor with a small inductance can be selected. 
     It is assumed that a rising edge of the first pulse of the second driving signal lags behind a falling edge of a pulse of the first driving signal, and the second driving signal has N pulses in a low-level time period of each period of the first driving signal, where N is greater than or equal to 2. With reference to  FIG.  2   , an example in which N is equal to 2 is used for description. PWM 1  and PWM 2  are asymmetric driving signals. In one period of PWM 1 , PWM 1  has one pulse, PWM 2  has two pulses, and the inductor performs three times of charging and discharging. The method can increase the charge and discharge frequency of the inductor and reduce the ripple current of the inductor. 
     The control method for a converter is described above, and several implementation forms of the driving signal are described in detail below. 
     Because the first driving signal is simple, an implementation of the corresponding second driving signal in one period of the first driving signal is described below. 
     Frequencies corresponding to at least two pulses of the N pulses of the second driving signal are different. 
     For ease of understanding, the following provides descriptions by using an example in which the second driving signal has three pulses in the low-level time period of each period of the first driving signal. In other words, when N=3, frequencies corresponding to two pulses of the three pulses are different. 
     As shown in  FIG.  4 A , period T 1  corresponding to P 1  may be the same as period T 3  corresponding to P 3 , that is, T 1 =T 3 , and therefore, a frequency corresponding to P 1  may be equal to a frequency corresponding to P 3 . However, the periods of P 1  and P 3  are different from period T 2  corresponding to P 2 , and therefore, the frequencies of P 1  and P 3  are different from a frequency corresponding to P 2 . T 3  may be greater than T 2 . 
     In another case, with reference to  FIG.  4 B , features of frequencies of the pulses are the same as features of frequencies of the pulses in  FIG.  4 A , with differences from  FIG.  4 A  as follows: The first pulse of the second driving signal overlaps with the pulse of the first driving signal in terms of time, for example, pulse P 0  of the first driving signal overlaps with the first pulse P 1  of the three pulses of the second driving signal. In a time period in which pulse P 0  overlaps with pulse P 1 , the inductor is charged, and discharged until pulse P 1  is at a low level. 
     Two pulses, namely, P 2  and P 3 , still exist in the low-level time period in one period of the first driving signal. Through the asymmetric driving signals shown in  FIG.  4 B , the charge and discharge frequency of the inductor can still be increased, and the ripple current of the inductor can further be reduced. 
     Comparison between  FIG.  4 A  and  FIG.  4 B  is further performed. In  FIG.  4 B , a pulse width of P 1  is different from a pulse width of P 2 , the pulse width of P 1  is different from a pulse width of P 3 , but the pulse width of P 2  is the same as the pulse width of P 3 . In  FIG.  4 A , P 1 , P 2 , and P 3  have a same pulse width. 
     Because of the increase of a quantity of pulses of PWM 2  in the low-level time period of PWM 1 , a quantity of charging and discharging times of the inductor can be increased, and the charge and discharge frequency of the inductor is further increased. Therefore, even if the pulse widths of the pulses of PWM 2  are different, the ripple current of the inductor can also be reduced. 
     It should be understood that, “at least one (piece)” means one or more, and “plurality” means two or more. The term “and/or” is used for describing an association relationship between associated objects, and represents that three relationships may exist. For example, “A and/or B” may represent the following three cases: only A exists, only B exists, and both A and B exist, where A and B may be singular or plural. The character “/” usually indicates an “or” relationship between associated objects. “At least one of the following items (pieces)” or a similar expression thereof refers to any combination of these items, including any combination of singular items (pieces) or plural items (pieces). For example, at least one of a, b, or c may indicate a, b, c, a and b, a and c, b and c, or a, b, and c, where a, b, and c may be singular or plural. 
     The foregoing descriptions are merely embodiments and are not intended as limiting. Although the examples are described above, the embodiments are not intended to limit. By using the method disclosed above, any person of ordinary skill in the art can make a plurality of possible changes and modifications without departing from the scope of the of embodiments. Therefore, any simple amendment, equivalent change, or modification made to the foregoing embodiments shall fall within the scope of the embodiments.