Control method of cascade system and cascade system

Abstract

The invention provides a cascade system control method and a cascade system, the cascade system comprises a first inductor and N power modules (N is an integer greater than or equal to 2), each power module is provided with a first side and a second side, a branch formed by connecting the first sides of the N power modules in series bears a first voltage, and the second side of the N power modules bears a second voltage. The second side of each power module is subjected to a second voltage. Each power module includes a switch operating with a switching cycle. And determining the number of the power modules working in the high-frequency modulation mode according to the ratio of the first voltage to the second voltage, so that the moment when the current flowing through the switch is 0 exists in one switching period. The invention provides a soft switching solution, which can reduce the turn-on loss and is favorable for improving the power density of the cascade system. The condition that ZVS cannot be realized when the duty ratio is about 0 or 1 in a carrier cascading modulation means and the condition that ZVS cannot be realized due to ripple cancellation in a carrier phase-shifting modulation means can be avoided.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a control method for a cascaded system and a cascaded system. Background Technology

[0002] In step-down applications, module cascading is commonly used, such as with solid-state transformers. Compared to traditional power frequency transformers, solid-state transformers leverage the high-frequency advantages of power electronic devices to achieve high power density, small size, and high efficiency. The achievable switching frequency of a module typically depends on switching losses. Operating the switching transistors in soft-switching mode, such as using ZVS (Zero Voltage Switch), is one way to reduce switching losses. While existing modular cascaded systems have achieved high frequencies in the DC-DC converter stage using soft-switching, the AC-DC converter stage still employs hard-switching. This results in high switching losses, low frequency, and large filter inductance, which have become obstacles to further increasing the power density of solid-state transformers.

[0003] Currently, single-module AC-DC circuits typically employ TCM (Triangle Current Mode) to achieve soft switching. For example, by adjusting the switching frequency, the inductor current is reversed to discharge the charge on the switch's parasitic capacitance after crossing zero, thus achieving the ZVS turn-on condition for the switching transistor. Cascaded AC-DC circuits generally use two modulation methods: one is inter-module carrier phase shifting, and the other is carrier stacking, where several modules maintain a 0 or 1 level, while only one module is activated at a time for high-frequency operation.

[0004] However, if inter-module carrier phase shifting is used in a cascaded system and TCM technology is applied directly, the carrier phase shifting causes ripple cancellation, so ZVS cannot be achieved at duty cycles close to integer multiples of 1 / N. If carrier phase shifting is abandoned and the cascaded modules are synchronized, the voltage change across the inductor is large, requiring a high-value filter inductor, which severely weakens the effectiveness of TCM technology. If carrier stacking is used in a cascaded system and TCM technology is applied directly to modules performing high-frequency operations, the duty cycle changes around 0 or 1 whenever a module is added or removed, resulting in ripple that is too small to achieve ZVS. Summary of the Invention

[0005] This application provides a control method and a cascaded system for a cascaded system, which provides a soft-switching solution for the AC-DC circuit of the cascaded system, overcoming the shortcomings of the prior art that directly applies the single-module TCM technology to the cascaded system to achieve soft switching.

[0006] In a first aspect, this application provides a control method for a cascaded system, the cascaded system comprising a first inductor and N power modules, where N is an integer greater than or equal to 2, each power module having a first side and a second side, the first sides of the N power modules and the first inductor being connected in series to form a branch, the branch bearing a first voltage, the second side of each power module bearing a second voltage, each power module including a switch, the switch operating in a switching cycle; the control method comprising:

[0007] The number of power modules operating in high-frequency modulation mode is determined based on the ratio of the first voltage to the second voltage, such that there is a moment in one switching cycle when the current flowing through the switch is 0.

[0008] In one possible design, determining the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage to the second voltage includes:

[0009] If the ratio is close to an integer, the duty cycle of each power module is set according to the ratio, so that at least two power modules operate in the high-frequency modulation mode, and the duty cycle of the remaining power modules is 0 or 1;

[0010] The ratio is the sum of the duty cycles of the N power modules.

[0011] In one possible design, the difference between the ratio and the integer is greater than -0.2 and less than 0.2.

[0012] In one possible design, each of the power modules includes an upper switch and a lower switch connected in series, controlling the lower switch of the power module operating in the high-frequency modulation mode to be turned on simultaneously but not turned off simultaneously.

[0013] In one possible design, each power module includes two sub-power modules and a second inductor. Each sub-power module includes a power frequency bridge arm and a high frequency bridge arm. The power frequency bridge arm and the high frequency bridge arm of the same sub-power module are connected in parallel. The midpoint of the power frequency bridge arm of the two sub-power modules is connected to the first side. The midpoint of the high frequency bridge arm of the two sub-power modules is connected through the second inductor.

[0014] In one possible design, the power frequency bridge arm comprises two active switches connected in series.

[0015] In one possible design, the power frequency bridge arm comprises two diodes connected in series.

[0016] In one possible design, each power module includes a first sub-power module, a second sub-power module, and multiple third inductors. Both the first and second sub-power modules include a power frequency bridge arm and multiple high-frequency bridge arms. The midpoint of the power frequency bridge arm of the first and second sub-power modules is connected to the first side. The midpoints of the multiple high-frequency bridge arms of the first sub-power module are connected one-to-one with the midpoints of the multiple high-frequency bridge arms of the second sub-power module through the corresponding third inductors. The multiple high-frequency bridge arms of the first sub-power module operate in phase-shifted mode, and the multiple high-frequency bridge arms of the second sub-power module also operate in phase-shifted mode.

[0017] In one possible design, the cascaded system further includes N capacitors, which are connected in parallel to the first side of the N power modules.

[0018] In one possible design, determining the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage to the second voltage includes:

[0019] Based on the ratio, the modulation ratio of the current working cycle is obtained;

[0020] The number of power modules operating in the high-frequency modulation mode during the current working cycle is determined based on the modulation ratio of the current working cycle.

[0021] In one possible design, obtaining the modulation ratio of the current duty cycle based on the ratio includes:

[0022] The current flowing through the first inductor in the previous cycle is sampled, and the average value of the first inductor current is compared with a current reference value. The comparison result is then passed through a regulator and superimposed with the ratio to obtain the modulation ratio of the current working cycle.

[0023] In one possible design, each of the power modules includes an up switch and a down switch connected in series;

[0024] When (M) integer -bw1) <M total <(M integer When +bw2), the two power modules are controlled to operate in the high-frequency modulation mode, and the duty cycle of the upper switch of one of the two power modules operating in the high-frequency modulation mode is controlled to be (1-bw1)+a*(M). total -M integer +bw1), wherein the duty cycle of the other upper switch is (1-a)*(M) total -M integer +bw1), controlling (M) except for the two power modules operating in the high-frequency modulation mode.integer -1) The duty cycle of the upper switch of the power module is 1, and the duty cycle of the upper switch of the remaining power modules is 0;

[0025] When M total <(M integer -bw1) or M total >(M integer When +bw2), control one power module to operate in the high-frequency modulation mode, and control the duty cycle of the upper switch of the power module operating in the high-frequency modulation mode to be (M). total -M integer ), controlling M except for one of the power modules operating in the high-frequency modulation mode. integer The duty cycle of the upper switch of one of the power modules is 1, and the duty cycle of the upper switch of the other power modules is 0.

[0026] Among them, M total The modulation ratio of the current working cycle is given by rem(M). total When M > (1-bw1), integer =int(M total +1, otherwise, M integer =int(M total ), the rem(M total ) is the M total The decimal part of the int(M) total ) is the M total The integer part, a = bw1 / (bw2+bw1), bw1 = bw2, 0 <bw1<0.2。

[0027] In one possible design, the N power modules include a first power module and a second power module. Both the first and second power modules include a series-connected upper and lower switch. The first and second power modules operate in the high-frequency modulation mode, while the remaining power modules operate in a non-high-frequency modulation mode. The control method further includes estimating the duration of the next switching cycle according to the following formula.

[0028] t 1_est (n)=T s (n-1)*(1-D s21 )

[0029] t 2_est (n)=T s (n-1)*(D S21 -D s11 )

[0030]

[0031] T s_est (n)=t 1_est (n)+t 2_est (n)+t 3_est (n)

[0032] Among them, t 1_est (n) is an estimated value of the duration of the lower switch of the second power module being turned on within the current switching cycle, t 2_est (n) is an estimated value of the duration during which the lower switch of the first power module is turned on and the lower switch of the second power module is turned off within the current switching cycle, t 3_est (n) is an estimated value of the duration of the lower switch of the first power module being turned off during the current switching cycle, T s (n-1) represents the duration of the previous switching cycle, D s21 D is the duty cycle of the upper switch of the second power module. s11 Let i0(n) be the duty cycle of the upper switch of the first power module, and i0(n) be the sampled value of the minimum current flowing through the first inductor in the current switching cycle. set The absolute value of the minimum current flowing through the first inductor that enables the upper and lower switches to achieve zero-voltage conduction is k. 1_est For the t 1_est (n) The slope of the current flowing through the first inductor, k 2_est For the t 2_est (n) The slope of the current flowing through the first inductor, k 3_est For the t 3_est (n) The slope of the current flowing through the first inductor, V in (n) represents the magnitude of the first voltage during the current switching cycle, V dc L represents the magnitude of the second voltage. g T is the inductance of the first inductor. s_est (n) is an estimated value of the duration of the current switching cycle.

[0033] In one possible design, the method further includes: limiting the estimated duration of the current switching cycle based on preset maximum and minimum frequencies to obtain the duration of the current switching cycle.

[0034] Secondly, this application provides a cascaded system, comprising: a first inductor and N power modules, where N is an integer greater than or equal to 2, each power module having a first side and a second side, the first sides of the N power modules being connected in series to form a branch, the branch bearing a first voltage, the second side of each power module bearing a second voltage, each power module including a switch, the switch operating in a switching cycle; and

[0035] The control unit determines the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage and the second voltage, such that there is a moment in one switching cycle when the current flowing through the switch is 0.

[0036] In one possible design, the control unit is specifically used for:

[0037] If the ratio is close to an integer, the duty cycle of each power module is set according to the ratio, so that at least two power modules operate in the high-frequency modulation mode, and the duty cycle of the remaining power modules is 0 or 1;

[0038] The ratio is the sum of the duty cycles of the N power modules.

[0039] In one possible design, the difference between the ratio and the integer is greater than -0.2 and less than 0.2.

[0040] In one possible design, each of the power modules includes an up switch and a down switch connected in series; the control unit is further configured to:

[0041] The lower switches of the power module operating in the high-frequency modulation mode are simultaneously turned on but not simultaneously turned off.

[0042] In one possible design, each power module includes two sub-power modules and a second inductor. Each sub-power module includes a power frequency bridge arm and a high frequency bridge arm. The power frequency bridge arm and the high frequency bridge arm of the same sub-power module are connected in parallel. The midpoint of the power frequency bridge arm of the two sub-power modules is connected to the first side. The midpoint of the high frequency bridge arm of the two sub-power modules is connected through the second inductor.

[0043] In one possible design, the power frequency bridge arm comprises two active switches connected in series.

[0044] In one possible design, the power frequency bridge arm comprises two diodes connected in series.

[0045] In one possible design, each power module includes a first sub-power module, a second sub-power module, and multiple third inductors. Both the first and second sub-power modules include a power frequency bridge arm and multiple high-frequency bridge arms. The midpoint of the power frequency bridge arm of the first and second sub-power modules is connected to the first side. The midpoints of the multiple high-frequency bridge arms of the first sub-power module are connected one-to-one with the midpoints of the multiple high-frequency bridge arms of the second sub-power module through the corresponding third inductors. The multiple high-frequency bridge arms of the first sub-power module operate in phase-shifted mode, and the multiple high-frequency bridge arms of the second sub-power module also operate in phase-shifted mode.

[0046] In one possible design, the cascaded system further includes N capacitors, which are connected in parallel to the first side of the N power modules.

[0047] In one possible design, the control unit is further configured to:

[0048] Based on the ratio, the modulation ratio of the current working cycle is obtained;

[0049] The number of power modules operating in the high-frequency modulation mode during the current working cycle is determined based on the modulation ratio of the current working cycle.

[0050] In one possible design, the control unit is further configured to:

[0051] The current flowing through the first inductor in the previous cycle is sampled, and the average value of the first inductor current is compared with a current reference value. The comparison result is then passed through a regulator and superimposed with the ratio to obtain the modulation ratio of the current working cycle.

[0052] In one possible design, each of the power modules includes an up switch and a down switch connected in series;

[0053] When (M) integer -bw1) <M total <(M integer When +bw2), the control unit is used to:

[0054] The two power modules are controlled to operate in the high-frequency modulation mode, and the duty cycle of the upper switch of one of the two power modules operating in the high-frequency modulation mode is controlled to be (1-bw1)+a*(M). total -M integer +bw1), wherein the duty cycle of the other upper switch is (1-a)*(M) total -M integer +bw1), controlling (M) except for the two power modules operating in the high-frequency modulation mode. integer-1) The duty cycle of the upper switch of the power module is 1, and the duty cycle of the upper switch of the remaining power modules is 0;

[0055] When M total <(M integer -bw1) or M total >(M integer When +bw2), the control unit is used to:

[0056] Controlling a power module to operate in the high-frequency modulation mode, and controlling the duty cycle of the upper switch of the power module operating in the high-frequency modulation mode to be (M total -M integer ), controlling M except for one of the power modules operating in the high-frequency modulation mode. integer The duty cycle of the upper switch of one of the power modules is 1, and the duty cycle of the upper switch of the other power modules is 0.

[0057] Among them, M total The modulation ratio of the current working cycle is given by rem(M). total When M > (1-bw1), integer =int(M total +1, otherwise, M integer =int(M total ), the rem(M total ) is the M total The decimal part of the int(M) total ) is the M total The integer part, a = bw1 / (bw2+bw1), bw1 = bw2, 0 <bw1<0.2。

[0058] In one possible design, the N power modules include a first power module and a second power module, both of which include a series-connected upper and lower switch. The first and second power modules operate in the high-frequency modulation mode, while the remaining power modules operate in a non-high-frequency modulation mode. The control unit is further configured to estimate the duration of the next switching cycle according to the following formula.

[0059] t 1_est (n)=T s (n-1)*(1-D s21 )

[0060] t 2_est (n)=T s (n-1)*(D S21 -D s11 )

[0061]

[0062] T s_est (n)=t 1_est (n)+t 2_est (n)+t 3_est (n)

[0063] Among them, t 1_est (n) is an estimated value of the duration of the lower switch of the second power module being turned on within the current switching cycle, t 2_est (n) is an estimated value of the duration during which the lower switch of the first power module is turned on and the lower switch of the second power module is turned off within the current switching cycle, t 3_est (n) is an estimated value of the duration of the lower switch of the first power module being turned off during the current switching cycle, T s (n-1) represents the duration of the previous switching cycle, D s21 D is the duty cycle of the upper switch of the second power module. s11 Let i0(n) be the duty cycle of the upper switch of the first power module, and i0(n) be the sampled value of the minimum current flowing through the first inductor in the current switching cycle. set The absolute value of the minimum current flowing through the first inductor that enables the upper and lower switches to achieve zero-voltage conduction is k. 1_est For the t 1_est (n) The slope of the current flowing through the first inductor, k 2_est For the t 2_est (n) The slope of the current flowing through the first inductor, k 3_est For the t 3_est (n) The slope of the current flowing through the first inductor, V in (n) represents the magnitude of the first voltage during the current switching cycle, V dc L represents the magnitude of the second voltage. g T is the inductance of the first inductor. s_est (n) is an estimated value of the duration of the current switching cycle.

[0064] In one possible design, the control unit is further configured to:

[0065] Based on the preset highest and lowest frequencies, the estimated duration of the current switching cycle is limited to obtain the duration of the current switching cycle.

[0066] This application provides a control method and a cascaded system for a cascaded system. The cascaded system includes a first inductor and N power modules (N is an integer greater than or equal to 2). Each power module has a first side and a second side. The first sides of the N power modules are connected in series to form a branch, which bears a first voltage. The second side of each power module bears a second voltage. Each power module includes a switch that operates in a switching cycle. The number of power modules operating in high-frequency modulation mode can be determined based on the ratio of the first voltage and the second voltage, such that there is a moment in one switching cycle when the current flowing through the switch is 0. Compared with the prior art of directly using TCM technology, this method avoids the situation in the carrier stacking modulation method of the cascaded system where the duty cycle is close to 0 or 1, which prevents ZVS from being achieved. It also avoids the situation in the carrier phase shift modulation method of the cascaded system where ZVS cannot be achieved due to ripple cancellation. Thus, it provides a soft-switching solution for the AC-DC circuit of the cascaded system, which can reduce turn-on losses and improve the power density of the cascaded system. Attached Figure Description

[0067] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0068] Figure 1 A schematic diagram of a cascaded system provided in an embodiment of this application;

[0069] Figure 2 This is a schematic diagram of another cascaded system provided in an embodiment of this application;

[0070] Figure 3 A schematic diagram of the inductor current and switch drive signal waveforms of a cascaded system provided in an embodiment of this application;

[0071] Figure 4a and 4b A simulation effect diagram provided for an embodiment of this application;

[0072] Figure 5 This is a schematic diagram of another cascaded system provided in an embodiment of this application;

[0073] Figure 6 This is a schematic diagram of the structure of another cascaded system provided in the embodiments of this application;

[0074] Figure 7Another schematic diagram of the inductor current and switch drive signal waveforms of the cascaded system provided in the embodiments of this application;

[0075] Figure 8 This is another simulation effect diagram provided for an embodiment of this application;

[0076] Figure 9 This is a schematic diagram of the structure of another cascaded system provided in the embodiments of this application;

[0077] Figure 10 A schematic flowchart of a control method for a cascaded system provided in an embodiment of this application;

[0078] Figure 11 A schematic diagram illustrating the effect of another implementation of the control method for the cascaded system provided in the embodiments of this application;

[0079] Figure 12 This is a schematic diagram illustrating another implementation effect of the control method for the cascaded system provided in the embodiments of this application. Detailed Implementation

[0080] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of methods and apparatus consistent with some aspects of this application as detailed in the appended claims.

[0081] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0082] Currently, single-module AC-DC circuits typically employ TCM (Transmission Controlled Motion) for soft switching. Cascaded AC-DC circuits generally utilize two modulation methods: inter-module carrier phase shifting and carrier stacking, where several modules maintain a 0 or 1 level while only one module is engaged for high-frequency operation at a time. However, for the former modulation method, directly applying TCM results in ripple cancellation due to carrier phase shifting, preventing zero-voltage switching (ZVS) at duty cycles close to integer multiples of 1 / N. If carrier phase shifting is abandoned and cascaded modules are synchronized, the voltage change across the inductor becomes significant, necessitating a large value for the filter inductor, severely weakening the effectiveness of TCM. For the latter modulation method, directly applying TCM to the high-frequency operating module results in ripple too small to achieve ZVS due to duty cycle variations near 0 or 1 whenever a module is engaged or disengaged.

[0083] To address the aforementioned problems in the prior art, this application provides a control method and a cascaded system for a cascaded system. The inventive concept of the control method for the cascaded system provided in this application is as follows: the cascaded system includes a first inductor and N power modules, where N is an integer greater than or equal to 2. Each power module has a first side and a second side. The first sides of the N power modules and the first inductor are connected in series to form a branch, which bears a first voltage. The second side of each power module bears a second voltage. Each power module includes a switch, which operates in one switching cycle. Based on the ratio of the first voltage and the second voltage, the number of power modules operating in high-frequency modulation mode is determined, such that there is a moment within one switching cycle when the current flowing through the switch is 0. This provides a soft-switching solution for the AC-DC circuit of the cascaded system and avoids the situation where the duty cycle is near 0 or 1, preventing ZVS, due to only one power module operating in high-frequency modulation mode being activated at a time in carrier stacking. It also avoids the situation where ZVS cannot be achieved due to carrier phase shift ripple cancellation.

[0084] Figure 1 This is a schematic diagram of a cascaded system provided in an embodiment of this application. Figure 1 As shown, the cascaded system includes a first inductor L g There are N power modules, where N is an integer greater than or equal to 2. Each power module has a first side and a second side. The first sides of the N power modules and a first inductor Lg are connected in series to form a branch, which is subjected to a first voltage V. in Each power module's second side is subjected to a second voltage V. dc Each power module includes a switch that operates according to a switching cycle. Based on the first voltage V... in Second voltage V dc The ratio between them is M = V in / Vdc From the aforementioned N power modules, determine the number of power modules operating in high-frequency modulation mode, such that a current i can flow through the switch during one switching cycle. g The moment when it is 0.

[0085] In some embodiments, if the ratio M is close to an integer, the duty cycle of each power module can be set according to the ratio M, such that at least two power modules operate in high-frequency modulation mode, while the duty cycles of the remaining power module switches are 0 or 1. The sum of the duty cycles of the N power modules is the ratio M = V. in / V dc =∑m i m i Let be the duty cycle of the switch of the i-th power module.

[0086] For example, when the value of M is close to an integer (e.g., M belongs to [n-0.1, n+0.1], where n is an integer), at least two power modules are engaged in high-frequency modulation mode during each switching cycle, while the duty cycles of other power modules can remain unchanged at 0 or 1. For instance, assuming N=6, if M=4.1, then m1=m2=m3=1, m4+m5=1.1, m6=0; if M=3.9, then m1=m2=m3=1, m4+m5=0.9, m6=0. Here, m4 and m5 are the duty cycles of the two power modules operating in high-frequency modulation mode. The carrier synchronization of the power modules operating in high-frequency modulation mode can have the same or different duty cycles.

[0087] Optionally, the difference between the ratio M and an integer can be greater than -0.2 and less than 0.2.

[0088] As can be seen, the control method of the cascaded system provided in this application differs from the modulation method of carrier stacking in that it can avoid the situation where only one power module is put into operation in high-frequency modulation mode each time in carrier stacking, resulting in a duty cycle of 0 or near 1 and thus failing to achieve ZVS; and it differs from the modulation method of carrier phase shifting in that it can avoid the situation where carrier phase shifting ripple cancels out and thus fails to achieve ZVS.

[0089] In some embodiments, each power module includes an upper switch and a lower switch connected in series, the upper switch and the lower switch in each power module being as follows: Figure 1 S in power module 1 11 and S 12 S in power module 2 21 and S 22 ...and S in power module N N1 and S N2The duty cycle of the module switches mentioned above refers to the duty cycle of the lower switch in the module. By controlling the lower switch of each power module operating in high-frequency modulation mode to be turned on simultaneously, ZVS turn-on is achieved simultaneously. However, in order to minimize the current peak, the power modules operating in high-frequency modulation mode are not turned off simultaneously.

[0090] Taking cascaded Boost as an example, such as Figure 2 As shown, Figure 2 This is a schematic diagram of another cascaded system provided in an embodiment of this application. The cascaded system includes three power modules, i.e., N = 3. Assume V... dc =1000V, V in =1900V, then M=1.9, and the duty cycles of the lower switches of power modules 1 to 3 can be m1=0.2, m2=0.7, and m3=1 respectively. That is, the duty cycles of the lower switches S of power modules 1 and 2 are... 12 and S 22 Switches can be turned on simultaneously but not off simultaneously. Switch control can be achieved using methods such as... Figure 3 The quadrilateral soft-switching method shown implements control, and within one switching cycle, the current i g The waveform is quadrilateral. This allows for ZVS switching of the AC-DC converter when the input voltage changes, reducing turn-on losses. Furthermore, compared to TCM technology, the input current ripple is lower, which can reduce turn-off and turn-on losses.

[0091] Figure 4a and 4b This application provides a simulation effect diagram, specifically, Figure 4a and 4b for Figure 3 The diagram shows the simulation effect of the cascaded system, where the simulation parameter V... in A sinusoidal, dome-shaped wave with a peak value of 1900V at 50Hz. b V is the total bridge arm voltage of the cascaded system. dc =1000V. Figure 4a Modules 1, 2, and 3 shown are power module 1, power module 2, and power module 3, respectively. From... Figure 4a and Figure 4b Simulation results show that the control method for the cascaded system provided in this application embodiment can achieve ZVS of the cascaded system switches, and the current i g It has a quadrilateral shape and a relatively small peak current.

[0092] exist Figure 1 On this basis, Figure 5 This is a schematic diagram of another cascaded system provided in an embodiment of this application. For example... Figure 5 As shown, each power module includes two sub-power modules and a second inductor, for example... Figure 5 The power module 1 shown includes sub-power modules 11 and 12 and a second inductor L1; the power module 2 includes sub-power modules 21 and 22 and a second inductor L2; and the power module N includes sub-power modules N1 and N2 and a second inductor L1. N .

[0093] Each sub-power module includes a power frequency bridge arm and a high frequency bridge arm. The power frequency bridge arm and the high frequency bridge arm of the same sub-power module are connected in parallel. The midpoint of the power frequency bridge arm of the two sub-power modules is connected to the first side of the module. The midpoint of the high frequency bridge arm of the two sub-power modules is connected through a second inductor.

[0094] The high-frequency bridge arm of the two sub-power modules included in each power module can be equivalent to, for example: Figure 1 The two cascaded Boost modules shown can also be implemented using the control method of the cascaded system provided in the embodiments of this application to achieve ZVS, thus achieving the same result as... Figure 1 The same technical effect.

[0095] In some embodiments, each power frequency bridge arm may include two active switches connected in series, wherein the active switch is only a functional switch, and in actual working conditions, multiple switches may be connected in series or in parallel to achieve the function of one switch.

[0096] In some embodiments, each power frequency bridge arm may include two diodes connected in series, for example Figure 5 The power frequency bridge arm of the sub-power module 11 of the power module 1 shown includes diodes D connected in series. 111 and D 112 The power frequency bridge arm of sub-power module 12 of power module 1 includes diodes D connected in series. 121 and D 122 .

[0097] Each power frequency bridge arm contains two active switches connected in series, enabling ZVS turn-on. If each power frequency bridge arm contains two diodes connected in series, it can operate in critical conduction mode, partially achieving ZVS.

[0098] It should be noted that the two diodes in each power frequency bridge arm described above are only functionally two diodes. In actual operating conditions, multiple diodes can be connected in series or parallel to achieve the function of a single diode. Figure 5 The two diodes shown in each power frequency bridge arm are for illustrative purposes only and are not intended to limit the application of the technology.

[0099] exist Figure 1 On this basis, Figure 6 This is a schematic diagram of another cascaded system provided in an embodiment of this application. For example... Figure 6As shown, each power module (e.g., power module 1 to power module N) includes a first sub-power module, a second sub-power module, and multiple third inductors, for example... Figure 6 The power module 1 shown includes a first sub-power module 11, a second sub-power module 12, and multiple third inductors (such as L). 1a L 1b ... L 1n’ The power module 2 includes a first sub-power module 21, a second sub-power module 22, and multiple third inductors (such as L). 2a L 2b ... L 2n’ ), and power module N includes a first sub-power module N1 and a second sub-power module N2 and multiple third inductors (such as L). Na L Nb ... L Nn’ Each power module includes a first sub-power module and a second sub-power module, each comprising a power frequency bridge arm and multiple high-frequency bridge arms. The midpoints of the power frequency bridge arms of the first and second sub-power modules in each power module are connected to the first side of the power module. The midpoints of the multiple high-frequency bridge arms of the first sub-power module in each power module are connected one-to-one with the midpoints of the multiple high-frequency bridge arms of its second sub-power module via corresponding third inductors. For example, in power module 1, the midpoints of the multiple high-frequency bridge arms of the first sub-power module 11 in power module 1 are connected via corresponding third inductors (L1, L2, L3, L4, L5, L6, L7, L8, L9, L1, L9, L1, L1, L1, L1, L2 ... 1a L 1b ... L 1n’ It is connected one-to-one with the midpoints of multiple high-frequency bridge arms of its second sub-power module 12.

[0100] In each power module, multiple high-frequency bridge arms of the first sub-power module operate in phase-shifted mode, and in the second sub-power module of each power module, multiple high-frequency bridge arms operate in phase-shifted mode. Each sub-power module has more than two high-frequency bridge arms, and the carrier phase shifts between the high-frequency bridge arms result in different phases of the inductor currents, thus reducing the ripple of the total current. Specifically, when there are two high-frequency bridge arms, the carrier phase shifts between these two bridge arms are 180 degrees.

[0101] Figure 7 This is another schematic diagram of the inductor current and switch drive signal waveforms of the cascaded system provided in an embodiment of this application. For example... Figure 6 The cascaded system shown has a circuit with two high-frequency bridge arms. Implementing the control method for the cascaded system provided in this application has the advantage that all switches can achieve ZVS, and the ripple of the high-frequency bridge arms is canceled, reducing the total current ripple. Optionally, in actual device selection, the diodes included in the power frequency bridge arm carry power frequency current. During unidirectional rectification operation, power frequency diodes can be selected to save costs. It should be noted that... Figure 7 i inL This represents the total current across the multiple third inductors in power module 1.

[0102] Figure 8 This is another simulation effect diagram provided for an embodiment of this application. Specifically, Figure 8 for Figure 7 Simulation of the control method shown. The simulation parameter is V. dc =1000V, V in It is a sine wave with an amplitude of 1500V and L = 40uH. From Figure 8 The simulation results show that the control method for the cascaded system provided in this application can achieve ZVS for the switches of the cascaded system. The carrier phase shift between the two sets of high-frequency bridge arms results in different ripple phases, reducing the high-frequency ripple in the synthesized total grid current. The simulation results verify this. Figure 7 The implementation effect.

[0103] Furthermore, such as Figure 6 The cascaded system shown can also include N capacitors, each connected in parallel to the first side of one of the N power modules. These N capacitors are as follows: Figure 6 The capacitors C1, C2 to C3 shown in the figure N Each power module has a capacitor connected to its input terminal. This capacitor filters out high-frequency current ripple, eliminating high-frequency components in the current. Furthermore, due to the voltage regulation effect of the capacitor, the terminal voltages of the power modules are independent, thus decoupling the control between the modules.

[0104] exist Figure 6 On this basis, Figure 9 This is a schematic diagram of another cascaded system provided in an embodiment of this application. The control method for the cascaded system provided in this embodiment is also applicable to, for example... Figure 9 The circuit of the bidirectional cascaded system shown is as follows: Figure 9 As shown, this circuit can operate in four quadrants. Each power module's power frequency bridge arm can contain series-connected power frequency operated switches. For example, Q111, Q112, Q121, and Q122 in power module 1 are power frequency operated switches. In power module 1, Q111 and Q121 conduct when the current is greater than zero, and Q112 and Q122 conduct when the current is less than zero.

[0105] In some embodiments, low-cost Si MOSFETs or IGBTs can be used for power frequency operating switches, while SiC MOSFETs or GaN MOSFETs can be used for high frequency operating switches to maximize the capabilities of the devices.

[0106] The cascaded systems provided in the above embodiments Figure 10 This is a schematic flowchart illustrating a control method for a cascaded system provided in an embodiment of this application. Figure 10As shown, the embodiments of this application include:

[0107] S101: Based on the ratio of the first voltage and the second voltage, the modulation ratio of the current working cycle is obtained.

[0108] Referring to the above-described cascaded system, the cascaded system includes a first inductor and N power modules, where N is an integer greater than or equal to 2. Each power module has a first side and a second side. The first sides of the N power modules and the first inductor are connected in series to form a branch. The branch is subjected to a first voltage. The second side of each power module is subjected to a second voltage. Each power module includes a switch, which operates in a switching cycle.

[0109] The current flows through the first inductor L in the previous working cycle before sampling g current i g The first inductor current i g The average value i gmean and a current reference value i gRef The comparison is performed, and the result, after passing through a regulator, is superimposed with the ratio M to obtain the modulation ratio M of the current working cycle. total .

[0110] In some embodiments, i gmean By i g It can be obtained by sampling, or by sampling at the moment of switching action, through i g It can be obtained directly by sampling after hardware filtering, or by sampling i at the moment of switching action. g It is obtained through subsequent calculations. For example, using... Figure 1 Taking the cascaded system shown as an example, the i can be obtained by using the equal area method, as shown in formula (1) below. gmean .

[0111]

[0112] Among them, such as Figure 11 As shown, t1(n-1) is the previous period S 12 S 22 The time during which both are on is t2(n-1), which is the time of the previous cycle S. 12 On and S 22 The shutdown time, t3(n-1), is the time of the previous cycle S. 12 S 22 The time of joint shutdown.

[0113] S102: Based on the modulation ratio of the current working cycle, determine the number of power modules operating in high-frequency modulation mode in the current working cycle, so that there is a moment in a switching cycle when the current flowing through the switch is 0.

[0114] As described in the foregoing embodiments, each power module includes an up switch and a down switch connected in series.

[0115] As shown in the previous steps, M is obtained. total The modulation ratio for the current working cycle, when rem(M total When M > (1-bw1), integer =int(M total )+1, that is, M integer For M total Round up to the nearest integer; otherwise, M integer =int(M total ), that is, M integer For M total The value rounded down. Where rem(M) total ) is M total The decimal part, int(M total ) is M total The integer part, a = bw1 / (bw2+bw1), bw1 = bw2, 0 <bw1<0.2。

[0116] For example, when (M integer -bw1) <M total <(M integer When +bw2), the two power modules are controlled to operate in high-frequency modulation mode, and the duty cycle of the upper switch of one of the two power modules operating in high-frequency modulation mode is (1-bw1)+a*(M). total -M integer +bw1), where the duty cycle of the other upper switch is (1-a)*(M total -M integer +bw1), controls (M) all except the two power modules operating in high-frequency modulation mode. integer -1) The duty cycle of the upper switch of the power module is 1, and the duty cycle of the upper switch of the remaining power modules is 0.

[0117] When M total <(M integer -bw1) or M total >(M integer When +bw2), control one power module to operate in high-frequency modulation mode, and control the duty cycle of the upper switch of the power module operating in high-frequency modulation mode to be (M total -M integer ), controlling M except for one power module operating in high-frequency modulation mode. integer The duty cycle of the upper switch of one power module is 1, and the duty cycle of the upper switch of the other power modules is 0.

[0118] The above method can determine the number of power modules operating in high-frequency modulation mode in the current working cycle based on the modulation ratio of the current working cycle. This ensures that there is a moment when the current flowing through the switch is 0 within a switching cycle, providing a soft-switching solution for cascaded AC-DC circuits. It can also avoid the situation where ZVS cannot be achieved due to the duty cycle being close to 0 or 1 when only one high-frequency modulation mode power module is put into operation at a time in carrier stacking, and can also avoid the situation where ZVS cannot be achieved due to carrier phase shift ripple cancellation.

[0119] Furthermore, for the cascaded system provided in the above embodiments, for example, the N power modules of the cascaded system include a first power module and a second power module, that is, both the first power module and the second power module include an upper switch and a lower switch connected in series. If the first power module and the second power module operate in a high-frequency modulation mode, and the remaining power modules operate in a non-high-frequency modulation mode, the control method of the cascaded system provided in this application embodiment further includes estimating the duration of the next switching cycle according to the following formulas (2)-(7) as shown in formula (9):

[0120] t 1_est (n)=T s (n-1)*(1-D s21 (2)

[0121] t 2_est (n)=T s (n-1)*(D S21 -D s11 (3)

[0122]

[0123] T s_est (n)=t 1_est (n)+t 2_est (n)+t 3_est (n) (8)

[0124] Among them, such as Figure 12 As shown, t 1_est (n) is an estimated value of the duration of the second power module's lower switch conduction within the current switching cycle, t 2_est (n) is an estimated value of the duration during which the lower switch of the first power module is turned on and the lower switch of the second power module is turned off within the current switching cycle, t. 3_est (n) is an estimated value of the duration of the next switch-off of the first power module in the current switching cycle, T. s (n-1) represents the duration of the previous switching cycle, D s21 D represents the duty cycle of the upper switch of the second power module. s11Let i0(n) be the duty cycle of the upper switch of the first power module, and i0(n) be the current current flowing through the first inductor L during the current switching cycle. g The minimum current sample value, I set To enable zero-voltage conduction of the upper and lower switches, current flows through the first inductor L. g The absolute value of the minimum current, k 1_est For t 1_est (n) flows through the first inductor L g The slope of the current, k 2_est For t 2_est (n) flows through the first inductor L g The slope of the current, k 3_est For t 3_est (n) flows through the first inductor L g The slope of the current, V in (n) represents the magnitude of the first voltage during the current switching cycle, V dc L represents the magnitude of the second voltage. g T is the inductance of the first inductor. s_est (n) is an estimated value of the duration of the current switching cycle.

[0125] In some embodiments, the duration T of the current switching cycle can also be adjusted according to the preset highest and lowest frequencies of the cascaded system. s_est The estimated value of (n) is limited to obtain the duration T of the current switching cycle. s (n).

[0126] The cascading system of this application embodiment also includes:

[0127] The control unit is configured to implement the control method of the cascaded system provided in the above embodiments, for example, determining the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage and the second voltage, such that there is a moment in a switching cycle when the current flowing through the switch is 0.

[0128] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this application are indicated by the claims.

[0129] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this application is limited only by the appended claims.

Claims

1. A control method for a cascaded system, characterized in that, The cascaded system includes a first inductor and N power modules, where N is an integer greater than or equal to 2. Each power module has a first side and a second side. The first side of the N power modules and the first inductor are connected in series to form a branch. The branch is subjected to a first voltage. The second side of each power module is subjected to a second voltage. Each power module includes a switch, which operates in a switching cycle. The control method includes: The number of power modules operating in high-frequency modulation mode is determined based on the ratio of the first voltage to the second voltage, such that there is a moment in one switching cycle when the current flowing through the switch is 0.

2. The control method according to claim 1, characterized in that, Determining the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage to the second voltage includes: If the ratio is close to an integer, the duty cycle of each power module is set according to the ratio, so that at least two power modules operate in the high-frequency modulation mode, and the duty cycle of the remaining power modules is 0 or 1; The ratio is the sum of the duty cycles of the N power modules.

3. The control method according to claim 2, characterized in that, The difference between the ratio and the integer is greater than -0.2 and less than 0.

2.

4. The control method according to claim 2, characterized in that, Each of the power modules includes an upper switch and a lower switch connected in series, which controls the lower switch of the power module operating in the high-frequency modulation mode to be turned on simultaneously but not turned off simultaneously.

5. The control method according to any one of claims 1-4, characterized in that, Each power module includes two sub-power modules and a second inductor. Each sub-power module includes a power frequency bridge arm and a high frequency bridge arm. The power frequency bridge arm and the high frequency bridge arm of the same sub-power module are connected in parallel. The midpoint of the power frequency bridge arm of the two sub-power modules is connected to the first side. The midpoint of the high frequency bridge arm of the two sub-power modules is connected through the second inductor.

6. The control method according to claim 5, characterized in that, The power frequency bridge arm includes two active switches connected in series.

7. The control method according to claim 5, characterized in that, The power frequency bridge arm contains two diodes connected in series.

8. The control method according to any one of claims 1-4, characterized in that, Each power module includes a first sub-power module, a second sub-power module, and multiple third inductors. The first sub-power module and the second sub-power module each include a power frequency bridge arm and multiple high frequency bridge arms. The midpoint of the power frequency bridge arm of the first sub-power module and the second sub-power module is connected to the first side. The midpoints of the multiple high frequency bridge arms of the first sub-power module are connected one-to-one with the midpoints of the multiple high frequency bridge arms of the second sub-power module through the corresponding third inductors. The multiple high frequency bridge arms of the first sub-power module operate in phase-shifted mode, and the multiple high frequency bridge arms of the second sub-power module operate in phase-shifted mode.

9. The control method according to claim 8, characterized in that, The cascaded system also includes N capacitors, which are connected in parallel to the first side of the N power modules.

10. The control method according to claim 1, characterized in that, Determining the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage to the second voltage includes: Based on the ratio, the modulation ratio of the current working cycle is obtained; The number of power modules operating in the high-frequency modulation mode during the current working cycle is determined based on the modulation ratio of the current working cycle.

11. The control method according to claim 10, characterized in that, The step of obtaining the modulation ratio of the current working cycle based on the ratio includes: The current flowing through the first inductor in the previous cycle is sampled, and the average value of the first inductor current is compared with a current reference value. The comparison result is then passed through a regulator and superimposed with the ratio to obtain the modulation ratio of the current working cycle.

12. The control method according to claim 10, characterized in that, Each of the power modules includes an upper switch and a lower switch connected in series; When (M) integer -bw1) <M total <(M integer When +bw2), the two power modules are controlled to operate in the high-frequency modulation mode, and the duty cycle of the upper switch of one of the two power modules operating in the high-frequency modulation mode is controlled to be (1-bw1)+a*(M). total -M integer +bw1), wherein the duty cycle of the other upper switch is (1-a)*(M) total -M integer +bw1), controlling (M) except for the two power modules operating in the high-frequency modulation mode. integer -1) The duty cycle of the upper switch of the power module is 1, and the duty cycle of the upper switch of the remaining power modules is 0; When M total <(M integer -bw1) or M total >(M integer When +bw2), control one power module to operate in the high-frequency modulation mode, and control the duty cycle of the upper switch of the power module operating in the high-frequency modulation mode to be (M). total -M integer ), controlling M except for one of the power modules operating in the high-frequency modulation mode. integer The duty cycle of the upper switch of one of the power modules is 1, and the duty cycle of the upper switch of the other power modules is 0. Among them, M total The modulation ratio of the current working cycle is given by rem(M). total When M > (1-bw1), integer =int(M total +1, otherwise, M integer =int(M total ), the rem(M total ) is the M total The decimal part of the int(M) total ) is the M total The integer part, a = bw1 / (bw2+bw1), bw1 = bw2, 0 <bw1<0.2。 13. The control method according to claim 10, characterized in that, The N power modules include a first power module and a second power module. Both the first and second power modules include a series-connected upper and lower switch. The first and second power modules operate in the high-frequency modulation mode, while the remaining power modules operate in a non-high-frequency modulation mode. The control method further includes estimating the duration of the next switching cycle according to the following formula. t 1_est (n)=T s (n-1)*(1-D s21 ) t 2_est (n)=T s (n-1)*(D S21 -D s11 ) T s_est (n)=t 1_est (n)+t 2_est (n)+t 3_est (n) Among them, t 1_est (n) is an estimated value of the duration of the lower switch of the second power module being turned on within the current switching cycle, t 2_est (n) is an estimated value of the duration during which the lower switch of the first power module is turned on and the lower switch of the second power module is turned off within the current switching cycle, t 3_est (n) is an estimated value of the duration of the lower switch of the first power module being turned off during the current switching cycle, T s (n-1) represents the duration of the previous switching cycle, D s21 D is the duty cycle of the upper switch of the second power module. s11 Let i0(n) be the duty cycle of the upper switch of the first power module, and i0(n) be the sampled value of the minimum current flowing through the first inductor in the current switching cycle. set The absolute value of the minimum current flowing through the first inductor that enables the upper and lower switches to achieve zero-voltage conduction is k. 1_est For the t 1_est (n) The slope of the current flowing through the first inductor, k 2_est For the t 2_est (n) The slope of the current flowing through the first inductor, k 3_est For the t 3_est (n) The slope of the current flowing through the first inductor, V in (n) represents the magnitude of the first voltage during the current switching cycle, V dc L represents the magnitude of the second voltage. g T is the inductance of the first inductor. s_est (n) is an estimated value of the duration of the current switching cycle.

14. The control method according to claim 13, characterized in that, Also includes: Based on the preset highest and lowest frequencies, the estimated duration of the current switching cycle is limited to obtain the duration of the current switching cycle.

15. A cascaded system, characterized in that, include: The system comprises a first inductor and N power modules, where N is an integer greater than or equal to 2. Each power module has a first side and a second side. The first side of the N power modules and the first inductor are connected in series to form a branch. The branch is subjected to a first voltage. The second side of each power module is subjected to a second voltage. Each power module includes a switch that operates in a switching cycle. as well as The control unit determines the number of power modules operating in high-frequency modulation mode based on the ratio of the first voltage and the second voltage, such that there is a moment in one switching cycle when the current flowing through the switch is 0.

16. The cascaded system according to claim 15, characterized in that, The control unit is specifically used for: If the ratio is close to an integer, the duty cycle of each power module is set according to the ratio, so that at least two power modules operate in the high-frequency modulation mode, and the duty cycle of the remaining power modules is 0 or 1; The ratio is the sum of the duty cycles of the N power modules.

17. The cascaded system according to claim 16, characterized in that, The difference between the ratio and the integer is greater than -0.2 and less than 0.

2.

18. The cascaded system according to claim 16, characterized in that, Each of the power modules includes an upper switch and a lower switch connected in series; the control unit is further configured to: The lower switches of the power module operating in the high-frequency modulation mode are simultaneously turned on but not simultaneously turned off.

19. The cascaded system according to any one of claims 15-18, characterized in that, Each power module includes two sub-power modules and a second inductor. Each sub-power module includes a power frequency bridge arm and a high frequency bridge arm. The power frequency bridge arm and the high frequency bridge arm of the same sub-power module are connected in parallel. The midpoint of the power frequency bridge arm of the two sub-power modules is connected to the first side. The midpoint of the high frequency bridge arm of the two sub-power modules is connected through the second inductor.

20. The cascaded system according to claim 19, characterized in that, The power frequency bridge arm includes two active switches connected in series.

21. The cascade system according to claim 19, characterized in that, The power frequency bridge arm contains two diodes connected in series.

22. The cascaded system according to any one of claims 15-18, characterized in that, Each power module includes a first sub-power module, a second sub-power module, and multiple third inductors. The first sub-power module and the second sub-power module each include a power frequency bridge arm and multiple high frequency bridge arms. The midpoint of the power frequency bridge arm of the first sub-power module and the second sub-power module is connected to the first side. The midpoints of the multiple high frequency bridge arms of the first sub-power module are connected one-to-one with the midpoints of the multiple high frequency bridge arms of the second sub-power module through the corresponding third inductors. The multiple high frequency bridge arms of the first sub-power module operate in phase-shifted mode, and the multiple high frequency bridge arms of the second sub-power module operate in phase-shifted mode.

23. The cascade system according to claim 22, characterized in that, The cascaded system also includes N capacitors, which are connected in parallel to the first side of the N power modules.

24. The cascaded system according to claim 15, characterized in that, The control unit is also used for: Based on the ratio, the modulation ratio of the current working cycle is obtained; The number of power modules operating in the high-frequency modulation mode during the current working cycle is determined based on the modulation ratio of the current working cycle.

25. The cascaded system according to claim 24, characterized in that, The control unit is also used for: The current flowing through the first inductor in the previous cycle is sampled, and the average value of the first inductor current is compared with a current reference value. The comparison result is then passed through a regulator and superimposed with the ratio to obtain the modulation ratio of the current working cycle.

26. The cascaded system according to claim 24, characterized in that, Each of the power modules includes an upper switch and a lower switch connected in series; When (M) integer -bw1) <M total <(M integer When +bw2), the control unit is used to: The two power modules are controlled to operate in the high-frequency modulation mode, and the duty cycle of the upper switch of one of the two power modules operating in the high-frequency modulation mode is controlled to be (1-bw1)+a*(M). total -M integer +bw1), wherein the duty cycle of the other upper switch is (1-a)*(M) total -M integer +bw1), controlling (M) except for the two power modules operating in the high-frequency modulation mode. integer -1) The duty cycle of the upper switch of the power module is 1, and the duty cycle of the upper switch of the remaining power modules is 0; When M total <(M integer -bw1) or M total >(M integer When +bw2), the control unit is used to: Controlling a power module to operate in the high-frequency modulation mode, and controlling the duty cycle of the upper switch of the power module operating in the high-frequency modulation mode to be (M total -M integer ), controlling M except for one of the power modules operating in the high-frequency modulation mode. integer The duty cycle of the upper switch of one of the power modules is 1, and the duty cycle of the upper switch of the other power modules is 0. Among them, M total The modulation ratio of the current working cycle is given by rem(M). total When M > (1-bw1), integer =int(M total +1, otherwise, M integer =int(M total ), the rem(M total ) is the M total The decimal part of the int(M) total ) is the M total The integer part, a = bw1 / (bw2+bw1), bw1 = bw2, 0 <bw1<0.2。 27. The cascaded system according to claim 24, characterized in that, The N power modules include a first power module and a second power module. Both the first and second power modules include a series-connected upper and lower switch. The first and second power modules operate in the high-frequency modulation mode, while the remaining power modules operate in a non-high-frequency modulation mode. The control unit is further configured to estimate the duration of the next switching cycle according to the following formula. t 1_est (n)=T s (n-1)*(1-D s21 ) t 2_est (n)=T s (n-1)*(D S21 -D s11 ) T s_est (n)=t 1_est (n)+t 2_est (n)+t 3_est (n) Among them, t 1_est (n) is an estimated value of the duration of the lower switch of the second power module being turned on within the current switching cycle, t 2_est (n) is an estimated value of the duration during which the lower switch of the first power module is turned on and the lower switch of the second power module is turned off within the current switching cycle, t 3_est (n) is an estimated value of the duration of the lower switch of the first power module being turned off during the current switching cycle, T s (n-1) represents the duration of the previous switching cycle, D s21 D is the duty cycle of the upper switch of the second power module. s11 Let i0(n) be the duty cycle of the upper switch of the first power module, and i0(n) be the sampled value of the minimum current flowing through the first inductor in the current switching cycle. set The absolute value of the minimum current flowing through the first inductor that enables the upper and lower switches to achieve zero-voltage conduction is k. 1_est For the t 1_est (n) The slope of the current flowing through the first inductor, k 2_est For the t 2_est (n) The slope of the current flowing through the first inductor, k 3_est For the t 3_est (n) The slope of the current flowing through the first inductor, V in (n) represents the magnitude of the first voltage during the current switching cycle, V dc L represents the magnitude of the second voltage. g T is the inductance of the first inductor. s_est (n) is an estimated value of the duration of the current switching cycle.

28. The cascade system according to claim 27, characterized in that, The control unit is also used for: Based on the preset highest and lowest frequencies, the estimated duration of the current switching cycle is limited to obtain the duration of the current switching cycle.

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