阅读说明：本技术 一种多电平变换器调制策略 (Modulation strategy of multi-level converter ) 是由 郭小强 王凡 王晓明 卢志刚 华长春 马瑞斯·马利诺夫斯基 乔瑟夫·格莱罗 于 2021-08-30 设计创作，主要内容包括：本发明公开了一种多电平变换器调制策略,包括如下步骤：S1、确定级联H桥个数N,电路的开关频率k,根据级联H桥个数N,确定三角载波V-(c)的移相角θ和所需要的三角载波V-(c)个数N；S2、确定电路直流侧电压V-(in),电路中阻感负载R和L；S3、确定调制度m,得到旋转SPWM调制波,在旋转SPWM调制波调制下生成相应电路拓扑的左侧桥臂调制波和右侧桥臂调制波,左侧桥臂调制波和右侧桥臂调制波均包括钳位部分和调制部分并分别与三角载波相比较,在钳位时间内,开关保持常通或者关断状态,在正常调制时间内,开关高频动作,进而驱动电路,从而完成相应调制过程。(The invention discloses a modulation strategy of a multilevel converter, which comprises the following steps: s1, determining the number N of cascaded H bridges, the switching frequency k of the circuit, and determining the triangular carrier V according to the number N of the cascaded H bridges c Phase shift angle theta of and the required triangular carrier wave V c The number N; s2, determining the DC side voltage V of the circuit in Resistance-inductance loads R and L in the circuit; s3, determining a modulation degree m to obtain a rotating SPWM modulation wave, generating a left bridge arm modulation wave and a right bridge arm modulation wave of a corresponding circuit topology under the modulation of the rotating SPWM modulation wave, wherein the left bridge arm modulation wave and the right bridge arm modulation wave both comprise a clamping part and a modulation part and are respectively compared with a triangular carrier wave, a switch keeps a normal on or off state within clamping time, and the switch acts at a high frequency within normal modulation time to drive the circuit, so that a corresponding modulation process is completed.)

1. A multilevel converter modulation strategy, characterized by: the method comprises the following steps:

s1, determining the number N of cascaded H bridges, the switching frequency k of the circuit, and determining the triangular carrier V according to the number N of the cascaded H bridges_cPhase shift angle theta of and the required triangular carrier wave V_cThe number N;

s2, determining the DC side voltage V of the circuit_inResistance-inductance loads R and L in the circuit;

s3, determining a modulation degree m to obtain a rotating SPWM (sinusoidal pulse width modulation) wave, generating a left bridge arm modulation wave and a right bridge arm modulation wave of a corresponding circuit topology under the modulation of the rotating SPWM wave, wherein the left bridge arm modulation wave and the right bridge arm modulation wave both comprise a clamping part and a modulation part and are respectively compared with a triangular carrier wave, and when the left bridge arm modulation wave or the right bridge arm modulation wave is larger than the triangular carrier wave, the generated driving signal is high; when the left bridge arm modulation wave or the right bridge arm modulation wave is smaller than the triangular carrier wave, the generated driving signal is low, the switch keeps a normally-on or off state within the clamping time, and the switch acts at a high frequency within the normal modulation time to drive the circuit, so that the corresponding modulation process is completed.

2. A multilevel converter modulation strategy according to claim 1 characterized in that: the expression of the triangular carrier phase shift angle θ in step S1 is:

3. a multilevel converter modulation strategy according to claim 2 characterized in that: in step S3, the expression of the rotating SPWM modulation wave is:

wherein, V_refIs an improved modulated wave, V, transformed from a sinusoidal conventional modulated wave_sclampThe clamp type modulation wave is represented, the positive half cycle amplitude and the negative half cycle amplitude are the same, and the amplitude in the positive half cycle and the negative half cycle is kept constant, so that the clamp type modulation wave is used for clamping a switch to enable the switch to keep a certain state;

V_refexpression:

V_sclampexpression:

4. a multilevel converter modulation strategy according to claim 3 characterized in that: in step S3, the circuit of the left arm modulated wave and the right arm modulated wave in one cycle is sequentially divided into four stages, the first stage: the switch on the upper side of the left bridge arm is normally on, the switch on the lower side of the left bridge arm is turned off, the switch on the right bridge arm acts at high frequency, and the second stage is started after the first stage is finished; the second stage is as follows: the switch on the lower side of the right bridge arm is normally on, the switch on the upper side of the right bridge arm is turned off, the switch on the left bridge arm acts at a high frequency at the same time, and the third stage is started after the second stage is finished; the third stage: the switch on the lower side of the left bridge arm is normally on, the switch on the upper side of the left bridge arm is turned off, the switch on the right bridge arm acts at high frequency, and the fourth stage is started after the third stage is finished; the fourth stage: the switch on the upper side of the right bridge arm is normally on, the switch on the lower side of the right bridge arm is turned off, and meanwhile, the switch on the left bridge arm is turned on in a high frequency mode; after the fourth phase, the periodic cycle is started.

5. The multilevel converter modulation strategy of claim 4, wherein: the specific circuit state process in the first stage is as follows: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always greater than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In a normally-on state, the lower side switch S of the left bridge arm₁₂In an off state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cTime, right side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄And off, at which point the first phase ends.

6. The multilevel converter modulation strategy of claim 4, wherein: the specific circuit state process of the second stage is as follows: the modulated wave of the right bridge arm is in the clamping part, and the modulated wave V of the right bridge arm_sRIs always greater than the triangular carrier V_cAt this time, the lower side switch S of the right bridge arm₁₄In a normally open state, the right side bridge armUpper side switch S₁₃In an off state; modulation wave V of left bridge arm_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm modulates the wave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off; when the left bridge arm modulates the wave V_sLLess than the triangular carrier V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And shut down, at which point the second phase ends.

7. The multilevel converter modulation strategy of claim 4, wherein: the third stage specifically comprises the following circuit state processes: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always less than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In an off state, the switch S is arranged at the lower side of the left bridge arm₁₂In a normally on state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cRight side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄And (4) turning off, wherein the third stage is finished.

8. The multilevel converter modulation strategy of claim 4, wherein: the fourth stage specifically comprises the following circuit state processes: the modulated wave of the right bridge arm is in the clamping part, and the modulated wave V of the right bridge arm_sRIs always less than the triangular carrier V_cRight side bridge arm lower side switch S₁₄In the off state, the upper side switch S of the right bridge arm₁₃In a conducting state; modulation wave V of left bridge arm_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm is modulatedWave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off; when the modulation wave of the left bridge arm is smaller than the triangular carrier wave V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And (4) turning off, wherein the fourth stage is finished.

Technical Field

The invention relates to the technical field of multi-level inverters, in particular to a multi-level converter modulation strategy.

Background

In recent years, multilevel inverters have been widely used in high voltage and high power systems. The multi-level inverter has the advantages of high voltage system capability, low harmonic distortion, easiness in expansion, high fault-tolerant capability and the like. Therefore, the multi-level inverter is widely applied to a high-voltage direct-current power transmission system and a renewable energy system. There are many topologies for multilevel inverters, with cascaded H-bridges being one of the most widely used topologies. There are many modulation modes of the cascaded H-bridge, including carrier phase shift, carrier stacking, discontinuous modulation, etc. And the power distribution of each module under the carrier wave laminated modulation is unbalanced. Discontinuous modulation requires balancing the relationship between the number of switches and the output waveform. The carrier phase shift modulation technology has great performance advantages, can ensure the power balance among modules and the quality of output waveforms, and is widely applied to the industrial field. However, carrier phase shift modulation has limitations such as high switching frequency, large circuit loss, and low circuit efficiency.

The carrier phase shift modulation is an excellent modulation method suitable for a high-voltage large-capacity converter. The carrier phase shift modulation technology is derived from a natural sampling SPWM theory and a multiplexing theory, and is widely applied to various fields of power electronic technology due to the good output harmonic characteristic and the high transmission bandwidth. The carrier phase shift modulation is divided into bipolar and unipolar frequency multiplication. Compared with bipolar PWM, the unipolar frequency multiplication modulation equivalent switching frequency is high, and the harmonic content in the output waveform is low, so that the performance of the PWM is better. Therefore, the following description will be made by taking unipolar frequency-doubling modulation as a representative. FIG. 1 is a schematic diagram of a conventional carrier phase-shifting single-pole frequency-doubling modulation, in which the single-pole frequency-doubling modulation is composed of two modulated waves U with the same amplitude and opposite phases_m1And U_m2And N phase shift angles theta are respectively formed by triangular carriers of 2 pi/N. The operating principle of the unipolar frequency multiplication modulation is that when the modulated wave U_m1>U_c1The switching signal S of the xth H-bridge unit_x1＝1，S_x20, otherwise S_x1＝0，S_x2When modulating wave U as 1_m2>U_c1Switching signal S of the xth bridge unit_x3＝1，S_x40, otherwise S_x3＝0，S_x41. Under the modulation, the frequency of each switch in the circuit is consistent, the power distribution is balanced, the output waveform quality is good, and the THD is small. However, in a normal state, the switch is always in a high frequency state, and the switching loss is large, which lowers the efficiency of the system.

According to the above description, it is easy to find that although the output waveform quality of the carrier phase-shifting unipolar frequency-doubling modulation is good, the switch is always in high-frequency action, as the cascade number increases, the circuit efficiency will decrease, the switch tube is heated more, and is more easily damaged, which is not beneficial to the circuit stability.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a multi-level converter modulation strategy, so that the switching times of an H-bridge unit are reduced, the circuit efficiency is improved, and the output waveform quality can be ensured; meanwhile, the H-bridge unit applies a rotating SPWM (sinusoidal pulse width modulation) method to enable switching losses among modules to be close to consistent, and a switching tube in the circuit is uniformly heated, so that the stability of the circuit is improved, and the service life of the switch is prolonged.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a multilevel converter modulation strategy comprising the steps of:

s1, determining the number N of cascaded H bridges, the switching frequency k of the circuit, and determining the triangular carrier V according to the number N of the cascaded H bridges_cPhase shift angle theta of and the required triangular carrier wave V_cThe number N;

s2, determining the DC side voltage V of the circuit_inResistance-inductance loads R and L in the circuit;

s3, determining a modulation degree m to obtain a rotating SPWM (sinusoidal pulse width modulation) wave, generating a left bridge arm modulation wave and a right bridge arm modulation wave of a corresponding circuit topology under the modulation of the rotating SPWM wave, wherein the left bridge arm modulation wave and the right bridge arm modulation wave both comprise a clamping part and a modulation part and are respectively compared with a triangular carrier wave, and when the left bridge arm modulation wave or the right bridge arm modulation wave is larger than the triangular carrier wave, the generated driving signal is high; when the left bridge arm modulation wave or the right bridge arm modulation wave is smaller than the triangular carrier wave, the generated driving signal is low, the switch keeps a normally-on or off state within the clamping time, and the switch acts at a high frequency within the normal modulation time to drive the circuit, so that the corresponding modulation process is completed.

The technical scheme of the invention is further improved as follows: the expression of the triangular carrier phase shift angle θ in step S1 is:

the technical scheme of the invention is further improved as follows: in step S3, the expression of the rotating SPWM modulation wave is:

wherein, V_refIs an improved modulated wave, V, transformed from a sinusoidal conventional modulated wave_sclampThe clamp type modulation wave is represented, the positive half cycle amplitude and the negative half cycle amplitude are the same, and the amplitude in the positive half cycle and the negative half cycle is kept constant, so that the clamp type modulation wave is used for clamping a switch to enable the switch to keep a certain state;

V_refexpression:

V_sclampexpression:

the technical scheme of the invention is further improved as follows: in step S3, the circuit of the left arm modulated wave and the right arm modulated wave in one cycle is sequentially divided into four stages, the first stage: the switch on the upper side of the left bridge arm is normally on, the switch on the lower side of the left bridge arm is turned off, the switch on the right bridge arm acts at high frequency, and the second stage is started after the first stage is finished; the second stage is as follows: the switch on the lower side of the right bridge arm is normally on, the switch on the upper side of the right bridge arm is turned off, the switch on the left bridge arm acts at a high frequency at the same time, and the third stage is started after the second stage is finished; the third stage: the switch on the lower side of the left bridge arm is normally on, the switch on the upper side of the left bridge arm is turned off, the switch on the right bridge arm acts at high frequency, and the fourth stage is started after the third stage is finished; the fourth stage: the switch on the upper side of the right bridge arm is normally on, the switch on the lower side of the right bridge arm is turned off, and meanwhile, the switch on the left bridge arm is turned on in a high frequency mode; after the fourth phase, the periodic cycle is started.

The technical scheme of the invention is further improved as follows: the specific circuit state process in the first stage is as follows: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always greater than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In a normally-on state, the lower side switch S of the left bridge arm₁₂In an off state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cTime, right side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄And off, at which point the first phase ends.

The technical scheme of the invention is further improved as follows: the specific circuit state process of the second stage is as follows: the modulated wave of the right bridge arm is in the clamping part, and the modulated wave V of the right bridge arm_sRIs always greater than the triangular carrier V_cAt this time, the lower side switch S of the right bridge arm₁₄In a normally-on state, the upper side switch S of the right bridge arm₁₃Is in a turn-off stateA state; modulation wave V of left bridge arm_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm modulates the wave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off; when the left bridge arm modulates the wave V_sLLess than the triangular carrier V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And shut down, at which point the second phase ends.

The technical scheme of the invention is further improved as follows: the third stage specifically comprises the following circuit state processes: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always less than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In an off state, the switch S is arranged at the lower side of the left bridge arm₁₂In a normally on state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cRight side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄And (4) turning off, wherein the third stage is finished.

The technical scheme of the invention is further improved as follows: the fourth stage specifically comprises the following circuit state processes: the fourth stage specifically comprises the following circuit state processes: the modulated wave of the right bridge arm is in the clamping part, and the modulated wave V of the right bridge arm_sRIs always less than the triangular carrier V_cAt this stage, the lower side switch S of the right arm₁₄In the off state, the upper side switch S of the right bridge arm₁₃In a conducting state; modulation wave V of left bridge arm_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm modulates the wave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off;when the modulation wave of the left bridge arm is smaller than the triangular carrier wave V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And (4) turning off, wherein the fourth stage is finished.

Due to the adoption of the technical scheme, the invention has the technical progress that:

the invention is different from the traditional modulation mode, the switch in the circuit is not always in high-frequency action, but is switched between high frequency and low frequency, the left side bridge arm modulation wave and the right side bridge arm modulation wave of the circuit topology are generated under the modulation of the rotating SPWM modulation wave, both the left side bridge arm modulation wave and the right side bridge arm modulation wave comprise a clamping part and a modulation part, the switching frequency can be effectively reduced, the circuit efficiency is improved, the left side bridge arm modulation wave and the right side bridge arm modulation wave are sequentially divided into four stages in one period, the switch is switched on and off in an ordered mode, and the output waveform quality can be ensured while the switching frequency is reduced; meanwhile, the H-bridge unit applies a rotating SPWM modulation method to enable switching loss between modules to be close to consistent, a switching tube in a circuit is uniformly heated, stable operation of the circuit is facilitated, and the service life of a switch can be prolonged.

Drawings

FIG. 1 is a schematic diagram of a conventional carrier phase-shifting single-pole frequency-doubling modulation control;

FIG. 2 is a single-phase cascaded H-bridge multilevel inverter topology;

FIG. 3 is a schematic diagram of a triangular carrier wave in the present invention;

FIG. 4 is a schematic diagram of the novel modulated wave generation process of the rotating SPWM of the present invention;

FIG. 5 is a schematic diagram of the rotational SPWM modulation principle of the present invention;

FIG. 6 is a schematic diagram of the rotational SPWM modulation operation of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the following examples:

as shown in fig. 2, N cascaded H-bridge cascades are provided in the graph, and compared with a conventional modulation method, when a modulation strategy of the multilevel converter provided by the present invention is applied to an H-bridge topology, the modulation method of the present invention is different from the conventional modulation method, a switch in a circuit is not always in a high-frequency action, but is switched between a high frequency and a low frequency, a left bridge arm modulation wave and a right bridge arm modulation wave of the circuit topology are generated under the modulation of a rotating SPWM modulation wave, both the left bridge arm modulation wave and the right bridge arm modulation wave include a clamping part and a modulation part, which can effectively reduce the switching frequency and improve the circuit efficiency, the circuits of the left bridge arm modulation wave and the right bridge arm modulation wave are sequentially divided into four stages in one cycle, the switches are turned on and off in an ordered manner, and the output waveform quality can be ensured while the switching frequency is reduced; meanwhile, the H-bridge unit applies a rotating SPWM modulation method to enable switching loss between modules to be close to consistent, a switching tube in a circuit is uniformly heated, stable operation of the circuit is facilitated, and the service life of a switch can be prolonged.

The method specifically comprises the following steps:

s1, as shown in figure 3, determining the number N of cascaded H bridges, the switching frequency k of the circuit, and determining the triangular carrier V according to the number N of cascaded H bridges_cPhase shift angle theta of and the required triangular carrier wave V_cThe number N; the expression of the triangular carrier phase shift angle theta is as follows:

s2, determining the DC side voltage V of the circuit_inResistance-inductance loads R and L in the circuit;

s3, determining the modulation degree m to obtain a rotating SPWM modulated wave, as shown in fig. 4(c), the expression of the rotating SPWM modulated wave is:

wherein, V_refIs an improved modulated wave transformed from a sinusoidal conventional modulated wave, as shown in fig. 4 (a); v_sclampA clamp type modulation wave is shown, as shown in fig. 4(b), the amplitudes of the positive and negative half cycles are the same, and the amplitude is kept constant in the positive and negative half cycles, so as to clamp the switch and keep the switch in a certain state;

V_refexpression:

V_sclampexpression:

generating a left bridge arm modulation wave and a right bridge arm modulation wave of a corresponding circuit topology under the modulation of a rotating SPWM modulation wave, wherein the left bridge arm modulation wave and the right bridge arm modulation wave both comprise a clamping part and a modulation part and are respectively compared with a triangular carrier wave, and when the left bridge arm modulation wave or the right bridge arm modulation wave is larger than the triangular carrier wave, the generated driving signal is high; when the left bridge arm modulation wave or the right bridge arm modulation wave is smaller than the triangular carrier wave, the generated driving signal is low, the switch keeps a normally-on or off state within the clamping time, and the switch acts at a high frequency within the normal modulation time to drive the circuit, so that the corresponding modulation process is completed.

The specific modulation process is as follows, fig. 5 is a schematic diagram of the rotational SPWM modulation principle of the single-phase cascaded H-bridge multi-level inverter proposed by the present invention, and since the upper and lower switching tubes of the same bridge arm are complementarily turned on, the left bridge arm only displays the upper switch S of the left bridge arm in fig. 5₁₁Drive signal g of₁₁The left bridge arm only displays the driving signal g of the right bridge arm side switch₁₄In the figure, V_oThe output voltage of the H bridge AC side is shown, and the input voltage of the DC side is shown as E.

The left arm modulated wave and the right arm modulated wave are sequentially divided into four stages in one cycle as shown in fig. 4 (c):

the first stage (t)₀-t₁): first stage specific circuitThe state process is as shown in fig. 6(a), the upper switch of the left bridge arm is normally on, the lower switch of the left bridge arm is off, and the switch of the right bridge arm operates at high frequency, at this stage, the modulation wave of the left bridge arm is in the clamping part, the switch of the left bridge arm is always in one state, and is on or off, at this moment, the switch of the left bridge arm is in low frequency operation, and the switching frequency of the left bridge arm is effectively reduced. And when the modulation wave of the right bridge arm is in the modulation part, the right bridge arm switch is in a high-frequency action state. The specific analysis is as follows: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always greater than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In a normally-on state, the lower side switch S of the left bridge arm₁₂In an off state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cTime, right side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄And turning off, wherein the first stage is finished, and the second stage is turned on after the first stage is finished.

The second stage (t)₁-t₂): the specific circuit state process of the second stage is shown in fig. 6 (b): the switch on the lower side of the right bridge arm is normally on, the switch on the upper side of the right bridge arm is turned off, and meanwhile, the switch on the left bridge arm acts in a high frequency mode, at this stage, the modulation wave of the right bridge arm is in a clamping part, the switch on the right bridge arm is always in one state, the switch on or the switch off is conducted, at the moment, the switch on the right bridge arm acts in a low frequency mode, and the switching frequency of the right bridge arm is effectively reduced. And when the modulation wave of the left bridge arm is in the modulation part, the switch of the left bridge arm is in a high-frequency action state. Concretely, the modulated wave of the right arm is in the clamping part, and the modulated wave V of the right arm is_sRIs always greater than the triangular carrier V_cAt this time, the lower side switch S of the right bridge arm₁₄In a normally-on state, the upper side switch S of the right bridge arm₁₃In an off state; left side ofSide bridge arm modulated wave V_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm modulates the wave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off; when the left bridge arm modulates the wave V_sLLess than the triangular carrier V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And turning off, finishing the second stage at the moment, and turning on the third stage after finishing the second stage.

The third stage (t)₂-t₃): the third stage is shown as fig. 6 (c): at this time, the modulation wave of the left bridge arm is still in the clamping part, and the difference from the first stage is that at this time, the switch on the lower side of the left bridge arm is normally on, the switch on the upper side of the left bridge arm is off, and the right bridge arm is still in high-frequency action, and the specific analysis is as follows: the modulation wave of the left bridge arm is at the clamping part, and the modulation wave V of the left bridge arm_sLIs always less than the triangular carrier V_cAt this time, the left side bridge arm upper side switch S₁₁In an off state, the switch S is arranged at the lower side of the left bridge arm₁₂In a normally on state; modulated wave V of right bridge arm_sRIn the modulation part, the right bridge arm switch is switched on and off in high frequency, and when the right bridge arm modulates the wave V_sRGreater than a triangular carrier V_cTime, the lower side switch S of the right side bridge arm₁₄On while the upper switch S of the right bridge arm₁₃Turning off; when the right bridge arm modulates the wave V_sRLess than the triangular carrier V_cRight side bridge arm upper side switch S₁₃Conducting while the lower side switch S of the right side bridge arm₁₄Turning off, finishing the third stage at the moment, and starting the fourth stage after finishing the third stage;

the fourth stage (t)₃-t₄): the fourth stage specific circuit state process is shown in fig. 6 (d): in this state, the right arm is still in the clamping portion, and the difference from the second stage is that the upper arm switch of the right arm is in a normally-on state, the lower arm switch of the right arm is in an off state, and the left arm is still in a high-frequency action. The specific analysis is as follows: modulation of right legWave is in clamping part, right bridge arm modulates wave V_sRIs always less than the triangular carrier V_cAt this stage, the lower side switch S of the right arm₁₄In the off state, the upper side switch S of the right bridge arm₁₃In a conducting state; modulation wave V of left bridge arm_sLIn the modulation part, the left bridge arm switch is switched on and off in high frequency, and when the left bridge arm modulates the wave V_sLGreater than a triangular carrier V_cTime, left side bridge arm upper side switch S₁₁Is conducted, and simultaneously the lower side switch S of the left bridge arm₁₂Turning off; when the modulation wave of the left bridge arm is smaller than the triangular carrier wave V_cLeft side bridge arm lower side switch S₁₂On while the left side bridge arm upper side switch S₁₁And (4) turning off, finishing the fourth stage at the moment, and starting periodic circulation after finishing the fourth stage.