A modulation system and method for a cycloconversion type high frequency link inverter

Abstract

The present invention discloses a modulation system and method for a cycloconverter-type high-frequency link inverter, which includes a main circuit topology, a dual closed-loop controller, and a switch modulation strategy module. The method is as follows: starting from the voltage stress of the switch tube, the relationship between the voltage stress and the switch timing is analyzed, and the switch combination under different modulation strategies is obtained through the state of the transformer leakage inductance current under different modulation strategies, so as to realize the energy transmission between the primary and secondary sides of the converter; then, a dynamic mathematical model of the converter is established to obtain the transfer function of the system; a discrete controller is designed to obtain a reference modulation wave, and finally, by designing the logical relationship between the corresponding switch tubes, the safe soft commutation and converter function of the cycloconverter are realized. The present invention provides an effective modulation strategy for the high-frequency link inverter, and the transformer leakage inductance current naturally drops to zero, which can reduce the voltage stress generated by the discontinuous inductance current when the switch tube is commutated, and can realize the soft switching of the converter, which has high practicality.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a modulation system and method of a cycloconversion type high frequency link inverter. Background Art

[0002] Common inverters can be divided into power frequency isolation and high-frequency isolation based on transformer isolation. Compared to traditional power frequency transformers, the use of high-frequency transformers reduces system size and increases converter power density. Compared to non-isolated inverters, high-frequency link inverters achieve electrical isolation between the input and output sides, while also ensuring voltage matching. This is crucial for applications with relatively low input voltages. High-frequency link inverters can also be used in rectification stages, enabling bidirectional energy flow and employing special modulation strategies to achieve soft switching in all semiconductor devices. High-frequency link inverters offer a wide range of topologies, offering flexible adaptability to applications of varying power and voltage levels. These high-frequency link inverters can be used in uninterruptible power supplies (UPS), photovoltaic power generation systems, electric vehicle charging stations, audio amplifiers, and other applications. Therefore, research into high-frequency link (HF-Link) inverter technology is of great significance.

[0003] High-frequency link inverters are widely used in grid-connected applications such as photovoltaics. Cycloconverter-type high-frequency link inverters offer advantages such as single-stage power conversion, bidirectional energy flow, and electrical isolation. However, they present safety commutation issues, impacting the reliability of cycloconverters. For example, during operation, the transformer leakage inductance and output filter inductance are both treated as current sources in single-stage high-frequency link inverters. Analysis of the converter's operating modes under two common modulation strategies reveals that both strategies overlook the issue of transformer leakage inductance commutation. Although both modulation strategies incorporate dead-band overlap during secondary-side commutation to ensure soft commutation by fully turning on all secondary switches, leakage inductance current still exists during commutation. When the switches are turned off, the current in the commutation switches has no path to flow and is forced to drop to zero, generating large voltage spikes across the secondary switches and posing a hard-off problem for the cycloconverter's inductor current. Summary of the Invention

[0004] The object of the present invention is to provide a modulation system and method for a cycloconverter-type high-frequency link inverter, which can realize soft switching within the full load range on the primary side, and the secondary side cycloconverter can realize soft switching, while providing synchronous rectification function in the energy transfer stage to improve the efficiency of the converter.

[0005] The technical solution for achieving the purpose of the present invention is: a modulation system for a cycloconversion type high frequency link inverter, comprising a main circuit topology, a dual closed loop controller, and a switch modulation strategy module;

[0006] The main circuit topology includes a front-stage full-bridge converter, a rear-stage cycloconverter, and an LC filter. The front-stage full-bridge converter converts the DC input voltage into a positive and negative square wave voltage with a 50% duty cycle, which is applied to the primary side of the transformer. The converted voltage consists of high-frequency voltage components, and the driving signal for the primary-side switch tube is achieved by dividing the rising edge of the carrier by two. The rear-stage cycloconverter demodulates the voltage transmitted from the primary side of the transformer into a unipolar SPWM voltage signal. Using a set modulation method, it implements soft commutation of the cycloconverter, converting high-frequency AC into low-frequency AC with a fundamental component, achieving single-stage power conversion. The LC filter is used to remove high-frequency harmonic components.

[0007] The dual closed-loop controller consists of a voltage outer loop and a current inner loop, and the output voltage reference value V ref Subtract the output voltage sampling value V of the main circuit topology o After that, the voltage outer loop controller is used as the reference value of the current inner loop i ref ,i ref The filter inductor current sampling value i of the main circuit topology Lf After comparison, it passes through the current inner loop controller as a reference sinusoidal modulation wave;

[0008] In the switch modulation strategy module, the driving signal of the switch tube in the front-stage full-bridge converter is obtained by dividing the sawtooth carrier by two on the rising edge and adding the dead time. The initial driving signal is obtained by the PWM comparator with reference to the sinusoidal modulation wave and the sawtooth wave. The initial driving signal is converted into the driving signal of the switch tube in the subsequent-stage cycloconverter through logical transformation.

[0009] A modulation method for a cycloconversion type high frequency link inverter is provided. The method is based on the modulation system of the cycloconversion type high frequency link inverter and comprises the following steps:

[0010] Step 1: Based on the modulation system of the cycloconverter high-frequency link inverter, the duty cycle command-output voltage transfer function of the high-frequency link inverter is obtained by the state space averaging method, which is used to design a discrete controller and its compensation parameters;

[0011] Step 2: Sampling to obtain the output voltage sampling value V of the main circuit topology o , filter inductor current sampling value i Lf , obtain the reference sinusoidal modulation wave through the discrete controller designed in step 1, and determine the level switching time of the positive and negative half cycles; according to the working mode under different switch combinations, select the working mode under the '0101' switching sequence, and give the soft switching characteristics under different switching sequences;

[0012] Step 3. Design the corresponding modulation strategy by demodulating the subsequent cycloconverter: first determine the operating range of the high-frequency link inverter, and the previous full-bridge converter adopts 50% duty cycle control through frequency division; refer to the sinusoidal modulation wave and the sawtooth wave through the PWM comparator to obtain the initial drive signal, and the initial drive signal is converted into an intermediate logic signal through logic transformation, and then the logic signal of the subsequent cycloconverter is obtained, and finally the delay time is used to shut down to achieve safe soft commutation of the cycloconverter.

[0013] Compared with the prior art, the present invention has the following significant advantages: (1) Taking into account the transformer leakage inductance factor, the secondary side cycloconverter achieves zero current shutdown and natural soft commutation, and does not produce voltage overshoot due to the discontinuous leakage inductance current; (2) The primary side full-bridge MOS tube achieves soft switching in the full load range, optimizing the soft switching range; (3) In the energy transfer stage, the cycloconverter belongs to synchronous rectification, and the current flows through the channel of the MOS tube rather than the body diode, which reduces the circuit conduction loss and improves the efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the structural block diagram of the modulation system of the cycloconversion high-frequency link inverter.

[0015] Figure 2 The figure is a flow chart of the modulation method of the cycloconversion type high frequency link inverter.

[0016] Figure 3 Simplify logic timing for high-frequency link inverter.

[0017] Figure 4 This is the timing principle diagram of the new modulation strategy.

[0018] Figure 5 This is a timing diagram of the circuit operation status within one cycle.

[0019] Figure 6 This is the positive half-cycle logic block diagram under the new modulation strategy.

[0020] Figure 7 This is the soft commutation waveform of the secondary side current of the converter. DETAILED DESCRIPTION

[0021] Adopting an appropriate modulation strategy to address commutation issues is crucial for stable inverter operation and improved power quality. To address this issue, this paper proposes a novel switching modulation strategy. This strategy, based on the principle of allowing the transformer leakage current to naturally drop to zero, enables soft switching of the power switches and soft commutation of the output filter inductor and transformer leakage inductance.

[0022] The present invention provides a modulation system for a cycloconversion high-frequency link inverter, comprising a main circuit topology, a dual closed-loop controller, and a switch modulation strategy module;

[0023] The main circuit topology includes a front-stage full-bridge converter, a rear-stage cycloconverter, and an LC filter. The front-stage full-bridge converter converts the DC input voltage into a positive and negative square wave voltage with a 50% duty cycle, which is applied to the primary side of the transformer. The converted voltage consists of high-frequency voltage components, and the driving signal for the primary-side switch tube is achieved by dividing the rising edge of the carrier by two. The rear-stage cycloconverter demodulates the voltage transmitted from the primary side of the transformer into a unipolar SPWM voltage signal. Using a set modulation method, it implements soft commutation of the cycloconverter, converting high-frequency AC into low-frequency AC with a fundamental component, achieving single-stage power conversion. The LC filter is used to remove high-frequency harmonic components.

[0024] The dual closed-loop controller consists of a voltage outer loop and a current inner loop, and the output voltage reference value V ref Subtract the output voltage sampling value V of the main circuit topology o After that, the voltage outer loop controller is used as the reference value of the current inner loop i ref ,i ref The filter inductor current sampling value i of the main circuit topology Lf After comparison, it passes through the current inner loop controller as a reference sinusoidal modulation wave;

[0025] In the switch modulation strategy module, the driving signal of the switch tube in the front-stage full-bridge converter is obtained by dividing the sawtooth carrier by two on the rising edge and adding the dead time. The initial driving signal is obtained by the PWM comparator with reference to the sinusoidal modulation wave and the sawtooth wave. The initial driving signal is converted into the driving signal of the switch tube in the subsequent-stage cycloconverter through logical transformation.

[0026] Furthermore, the front-stage full-bridge converter includes an input voltage source V in , the first to fourth power switch tubes M1, M2, M3, M4 of the front stage; the drains of M1 and M2 are connected to V in At one end, the sources of M1 and M2 are connected to the drains of M3 and M4 respectively, and the sources of M3 and M4 are connected to V in At the other end, the midpoints A and B of the two bridge arms are connected to the two ends of the primary side of the transformer T; the leakage inductance L is connected in series between the midpoint A of the bridge arm and the transformer T. r , leakage inductance L r Provide soft switching conditions; excitation inductance L m Connected in parallel at both ends of the transformer T, it plays the role of transferring energy;

[0027] The front-stage full-bridge converter provides a positive and negative alternating square wave voltage in one switching cycle and operates under a driving signal with a 50% duty cycle.

[0028] Furthermore, the subsequent cycloconverter is composed of four sets of bidirectional switches (S xa 、S xb), x=1,2,3,4, each set of bidirectional switches consists of two MOS tubes connected in reverse series; S 1a 、S 2a The drains of the transformers are connected to one end of the secondary side of the transformer T. 1b The drain connection S 3a The drain and common terminal are E, S 2b The drain connection S 4a The drain and common terminal are F, S 3b 、S 4b The drains are connected to the other end of the secondary side of the transformer T; the common end E is connected to the other end of the secondary side of the transformer T through the filter inductor L. f Connect filter capacitor C f One end of the common terminal F is connected to the filter capacitor C f The other end of the filter inductor L f , filter capacitor C f A low-pass filter is formed, and the output voltage after filtering is applied to the load R o ;

[0029] The subsequent cycloconverter supplies power to the load R in one switching cycle. o Energy is transferred twice to convert the square wave high-frequency signal into a low-frequency output signal.

[0030] The present invention provides a modulation method for a cycloconversion type high frequency link inverter. The method is based on the modulation system of the cycloconversion type high frequency link inverter and includes the following steps:

[0031] Step 1: Based on the modulation system of the cycloconverter high-frequency link inverter, the duty cycle command-output voltage transfer function of the high-frequency link inverter is obtained by the state space averaging method, which is used to design a discrete controller and its compensation parameters;

[0032] Step 2: Sampling to obtain the output voltage sampling value V of the main circuit topology o , filter inductor current sampling value i Lf , obtain the reference sinusoidal modulation wave through the discrete controller designed in step 1, and determine the level switching time of the positive and negative half cycles; according to the working mode under different switch combinations, select the working mode under the '0101' switching sequence, and give the soft switching characteristics under different switching sequences;

[0033] Step 3. Design the corresponding modulation strategy by demodulating the subsequent cycloconverter: first determine the operating range of the high-frequency link inverter, and the previous full-bridge converter adopts 50% duty cycle control through frequency division; refer to the sinusoidal modulation wave and the sawtooth wave through the PWM comparator to obtain the initial drive signal, and the initial drive signal is converted into an intermediate logic signal through logic transformation, and then the logic signal of the subsequent cycloconverter is obtained, and finally the delay time is used to shut down to achieve safe soft commutation of the cycloconverter.

[0034] Furthermore, the duty cycle command-output voltage transfer function described in step 1 is as follows:

[0035]

[0036] Among them: G Vd (s) is the duty cycle command-output voltage transfer function of the cyclic conversion high-frequency link inverter, s refers to the time in the frequency domain, V in is the converter input voltage, n is the transformer T turns ratio, R o is the output load, r is the internal resistance of the output filter inductor, L f is the output filter inductor, C f is the output filter capacitor.

[0037] Furthermore, the level switching time of the positive and negative half cycles is determined as described in step 2. Based on the working modes under different switch combinations, the working mode under the '0101' switching sequence is selected, and the soft switching characteristics under different switching sequences are given as follows:

[0038] (1) Judgment conditions for positive and negative half cycles:

[0039] When the output voltage sampling value of the main circuit topology V o , filter inductor current sampling value i Lf After passing through the dual closed-loop controller, a reference sinusoidal modulation wave is obtained. Since the output voltage changes positively and negatively within a power frequency cycle, the basis for determining the modulation strategy within the positive and negative half cycles is:

[0040]

[0041] The U s For the reference sinusoidal modulation wave after passing through the voltage outer loop and the current inner loop, it is necessary to determine the switching time between the positive and negative half cycles, thereby obtaining the logical principle of the modulation strategy under the two polarities;

[0042] (2) Switch combination selection basis:

[0043] In the power frequency cycle, the average value of the output voltage in one switching cycle is expressed as:

[0044] <V EF >=±nV in m(t)

[0045] in <V EF > is the average output voltage in one switching cycle, m(t) is the duty cycle of the sinusoidal change, n is the turns ratio of the transformer T, V in Input voltage to the converter;

[0046] where m(t)T s / 2 is the energy transfer time of the circuit in half a switching cycle, and m(t) is expressed as:

[0047]

[0048] Where m(t) is the duty cycle of the sinusoidal change, V m is the carrier amplitude, V in is the converter input voltage, n is the transformer turns ratio, ω is the sine wave angular frequency, and t is time;

[0049] Within a switching cycle, the output inductor current freewheeling phase is defined as the '0' state, and the energy transfer phase from the primary side to the secondary side is defined as the '1' state. Different switching sequences produce different operating modes. Based on the operating modes under different switching combinations, the operating mode under the '0101' switching sequence is selected. After the energy transfer is completed, the primary diagonal switch transistors are switched. Due to the presence of load current during commutation, sufficient energy is ensured in the primary leakage inductance to charge and discharge the parasitic capacitance within the dead time, thereby achieving zero-voltage turn-on of the primary MOS transistor.

[0050] Furthermore, in step 3, the corresponding modulation strategy is designed by demodulating the subsequent cycloconverter, as follows:

[0051] (1) Logic judgment signal of the working range of high-frequency link inverter:

[0052]

[0053] The U s is the reference sinusoidal modulation wave after passing through the voltage outer loop and the current inner loop, u1 is the positive half-cycle logic signal of the reference sinusoidal modulation wave, and u2 is the negative half-cycle logic signal of the reference sinusoidal modulation wave, which are used to determine the operating range of the high-frequency link inverter;

[0054] (2) Primary side MOS tube drive logic signal:

[0055] The primary MOS tube switching logic obtained after the carrier signal Uc passes through the rising edge frequency division trigger is:

[0056]

[0057] described It is the driving logic signal of the primary MOS tube M1; It is the driving logic signal of the primary MOS tube M2; It is the driving logic signal of the primary MOS tube M3; It is the driving logic signal of the primary MOS tube M4;

[0058] (3) Intermediate logic signal judgment:

[0059]

[0060] K is a logic signal obtained by comparing the reference sinusoidal modulation wave with the carrier wave; It is the driving logic signal of the primary MOS tube M2; S9~S 11 All are intermediate logic signals;

[0061] (4) Secondary side cycloconverter MOS tube drive logic signal:

[0062]

[0063]

[0064] The u1 is the positive half-cycle logic signal of the reference sinusoidal modulation wave, It is the driving logic signal of the primary MOS tube M1; It is the driving logic signal of the primary MOS tube M2, S9~S 11 Are intermediate logic signals, MOS tube S for the secondary side cycloconverter 1a ~S 4b Drive logic signals;

[0065] (5) Delayed shutdown time setting:

[0066] Through S 1a The switch-off signal of the switch tube is delayed to ensure that the transformer leakage current naturally drops to zero during the transition period, thus achieving S 1a The switch tube is turned off at zero current; the soft switching of the secondary side cycloconverter is achieved by delaying the turn-off of the switch tube, allowing the current to naturally drop to 0, thereby achieving zero current turn-off; the delayed turn-off time is:

[0067]

[0068] Where Δt1 is the parasitic capacitance discharge from the input voltage V in The time it takes for the current to drop to 0; Δt2 is the time it takes for the secondary cycloconverter’s freewheeling diode to turn on. s Time to drop to 0; t delay is the delayed turn-off time of the switch tube; C oss is the parasitic capacitance of the MOS tube, V in is the converter input voltage, L δ is the transformer leakage inductance, i p is the primary current, i Lf is the filter inductor current;

[0069] In normal circuit operation, since the inductor current is a sinusoidal envelope within the power frequency cycle, a fixed time is added to the corresponding switch tube to delay shutdown. When setting the delay time, t delay The minimum value is not less than Δt1+Δt 2max .

[0070] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0071] Example

[0072] Figure 1 This is the overall control block diagram of the cycloconversion high-frequency link inverter. Figure 2 This is a flow chart of the modulation method of the cycloconversion high-frequency link inverter. After the output voltage and current are sampled, they enter the closed-loop controller. The controller outputs a sinusoidal reference voltage, which is compared with the carrier and converted by the logic module to generate the switch tube drive signal. The simplified logic sequence of the converter is as follows: Figure 3 As shown, the primary side switch tube and the diagonal switch tube are turned on and off at the same time, and the upper and lower switches of the same bridge arm are complementary and turned on 180 degrees. The secondary side cycloconverter switch tube is shifted by the phase angle m(t)T s / 2 to transfer energy, and the phase shift angle changes sinusoidally.

[0073] According to the equivalent model of the converter, its duty cycle command-output voltage transfer function is obtained;

[0074]

[0075] Where: V in is the converter input voltage, n is the transformer turns ratio, R o is the output load, r is the internal resistance of the output filter inductor, L f is the output filter inductor, C f is the output filter capacitor.

[0076] When the output voltage V o and the filter inductor current i Lf After passing through the dual-loop controller, it is treated as a sinusoidal modulation wave. Since the output voltage changes positively and negatively within a power frequency cycle, the basis for determining the modulation strategy within the positive and negative half cycles is:

[0077]

[0078] The U s For a sinusoidal modulation wave after passing through the voltage outer loop and the current inner loop, it is necessary to determine the switching time between the positive and negative half cycles, thereby obtaining the logical principle of the modulation strategy under the two output voltage polarities.

[0079] Depend on Figure 4 It can be obtained that the high-frequency sawtooth carrier signal Uc The rising edge frequency divider module can obtain the primary diagonal switch tube drive signal, and then after the 'inversion' operation, a small amount of dead time is added as the diagonal complementary switch tube drive signal. 1b The driving signal of the switch S is the same as the driving signal of the primary side switches M1 and M4. 1a The driving signal of the switch S is a two-frequency signal with a small amount of delay time added to delay the shutdown and realize the soft commutation of the transformer current. 3a The driving signal of switch S is the same as that of switches M2 and M3. 3b The turn-off driving signal also adds a small delay time, and the switch S 1a Similar. Switch S 4b The driving signal is the modulation wave u after feedback s The intermediate signal K obtained by comparing with the carrier is then subjected to a logical 'exclusive OR' operation with the drive signals of switches M2 and M3. After its logic is 'inverted' and the dead time is added, the switch S 2a The driving signal of switch S 2b The driving signal of is obtained by performing a logic 'AND' operation on the intermediate signal K obtained by comparison and the driving signals of switches M2 and M3. The corresponding S 4a The switch drive signal is obtained by performing a logic 'AND' operation on the intermediate signal K obtained by comparison and the drive signals of switches M1 and M4. This is the generation principle of the drive signal of each switch tube in the positive half cycle. The specific gate-level drive logic block diagram is as follows Figure 5 As shown, the driving signal of the switch tube in the negative half cycle is symmetrical, and the generation principle is similar to that of the positive half cycle.

[0080] Figure 6 This is a timing diagram of the converter's circuit operation within a single cycle. Energy is transferred from the DC side to the AC side, and the output voltage is greater than 0. Within a high-frequency switching cycle, the circuit's operation can be divided into 16 operating modes, with energy transferred twice from the primary to the secondary side of the transformer. Figure 7 Shows the transformer secondary current i s With S 1a The switch drive waveform shows that the MOS transistor turns off only after the inductor current naturally drops to zero, achieving zero-current shutdown. Simultaneously, the secondary inductor current commutates naturally, eliminating forced commutation. This solves the commutation issues of the cycloconverter and reduces voltage stress across the switch.

[0081] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A modulation method for a cycloconversion type high frequency link inverter, characterized in that: The method is based on a modulation system of a cycloconversion high-frequency link inverter, the system comprising a main circuit topology, a dual closed-loop controller, and a switch modulation strategy module; The main circuit topology includes a front-stage full-bridge converter, a rear-stage cycloconverter, and an LC filter. The front-stage full-bridge converter converts the DC input voltage into a positive and negative square wave voltage with a 50% duty cycle, which is applied to the primary side of the transformer. The converted voltage consists of high-frequency voltage components, and the driving signal for the primary-side switch tube is achieved by dividing the rising edge of the carrier by two. The rear-stage cycloconverter demodulates the voltage transmitted from the primary side of the transformer into a unipolar SPWM voltage signal. Using a set modulation method, it implements soft commutation of the cycloconverter, converting high-frequency AC into low-frequency AC with a fundamental component, thus achieving single-stage power conversion. The LC filter is used to remove high-frequency harmonic components. The dual closed-loop controller consists of a voltage outer loop and a current inner loop, and the output voltage reference value V ref Subtract the output voltage sampling value V of the main circuit topology o After that, the voltage outer loop controller is used as the reference value of the current inner loop i ref ,i ref The filter inductor current sampling value i of the main circuit topology Lf After comparison, it passes through the current inner loop controller as a reference sinusoidal modulation wave; In the switch modulation strategy module, the driving signal for the switch in the front-stage full-bridge converter is obtained by dividing the sawtooth carrier by two on the rising edge and adding dead time. The initial driving signal is obtained by using a PWM comparator with reference to the sinusoidal modulation wave and the sawtooth wave. The initial driving signal is then converted into the driving signal for the switch in the rear-stage cycloconverter through logical transformation. The modulation method comprises the following steps: Step 1: Based on the modulation system of the cycloconverter high-frequency link inverter, the duty cycle command-output voltage transfer function of the high-frequency link inverter is obtained by the state space averaging method, which is used to design a discrete controller and its compensation parameters; Step 2: Sampling to obtain the output voltage sampling value V of the main circuit topology o , filter inductor current sampling value i Lf , obtain the reference sinusoidal modulation wave through the discrete controller designed in step 1, and determine the level switching time of the positive and negative half cycles; according to the working mode under different switch combinations, select the working mode under the '0101' switching sequence, and give the soft switching characteristics under different switching sequences; Step 3. Design the corresponding modulation strategy by demodulating the subsequent cycloconverter: first determine the operating range of the high-frequency link inverter, and the previous full-bridge converter adopts 50% duty cycle control through frequency division; refer to the sinusoidal modulation wave and the sawtooth wave through the PWM comparator to obtain the initial drive signal, and the initial drive signal is converted into an intermediate logic signal through logic transformation, and then the logic signal of the subsequent cycloconverter is obtained, and finally the delay time is used to shut down to achieve safe soft commutation of the cycloconverter.

2. The modulation method of the cycloconversion high frequency link inverter according to claim 1, characterized in that: The duty cycle command-output voltage transfer function described in step 1 is as follows: ； Among them: G Vd (s) is the duty cycle command-output voltage transfer function of the cyclic conversion high-frequency link inverter, s refers to the time in the frequency domain, V in is the converter input voltage, n is the transformer T turns ratio, R o is the output load, r is the internal resistance of the output filter inductor, L f is the output filter inductor, C f is the output filter capacitor.

3. The modulation method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: Determine the level switching time of the positive and negative half cycles as described in step 2. Based on the operating modes under different switch combinations, select the operating mode under the '0101' switching sequence and give the soft switching characteristics under different switching sequences, as follows: (1) Judgment conditions for positive and negative half cycles: When the output voltage sampling value of the main circuit topology V o , filter inductor current sampling value i Lf After passing through the dual closed-loop controller, a reference sinusoidal modulation wave is obtained. Since the output voltage changes positively and negatively within a power frequency cycle, the basis for determining the modulation strategy within the positive and negative half cycles is: ； The U s For the reference sinusoidal modulation wave after passing through the voltage outer loop and the current inner loop, it is necessary to determine the switching time between the positive and negative half cycles, thereby obtaining the logical principle of the modulation strategy under the two polarities; (2) Switch combination selection basis: In the power frequency cycle, the average value of the output voltage in one switching cycle is expressed as: ； in <V EF > is the average output voltage in one switching cycle, m(t) is the duty cycle of the sinusoidal change, n is the turns ratio of the transformer T, V in Input voltage to the converter; where m(t)T s / 2 is the energy transfer time of the circuit in half a switching cycle, and m(t) is expressed as: ； Where m(t) is the duty cycle of the sinusoidal change, V m is the carrier amplitude, V in is the converter input voltage, n is the transformer turns ratio, is the angular frequency of the sine wave, and t is the time; Within a switching cycle, the output inductor current freewheeling phase is defined as the '0' state, and the energy transfer phase from the primary side to the secondary side is defined as the '1' state. Different switching sequences produce different operating modes. Based on the operating modes under different switching combinations, the operating mode under the '0101' switching sequence is selected. After the energy transfer is completed, the primary diagonal switch transistors are switched. Due to the presence of load current during commutation, sufficient energy is ensured in the primary leakage inductance to charge and discharge the parasitic capacitance within the dead time, thereby achieving zero-voltage turn-on of the primary MOS transistor.

4. The modulation method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: Step 3 describes the design of a corresponding modulation strategy by demodulating the subsequent cycloconverter, as follows: (1) Logic judgment signal of the working range of high-frequency link inverter: ； The U s is the reference sinusoidal modulation wave after passing through the voltage outer loop and the current inner loop, u1 is the positive half-cycle logic signal of the reference sinusoidal modulation wave, and u2 is the negative half-cycle logic signal of the reference sinusoidal modulation wave, which are used to determine the operating range of the high-frequency link inverter; (2) Primary side MOS tube drive logic signal: The primary MOS tube switching logic obtained after the carrier signal Uc passes through the rising edge frequency division trigger is: ； described It is the driving logic signal of the primary MOS tube M1; It is the driving logic signal of the primary MOS tube M2; It is the driving logic signal of the primary MOS tube M3; It is the driving logic signal of the primary MOS tube M4; (3) Intermediate logic signal judgment: ； K is a logic signal obtained by comparing the reference sinusoidal modulation wave with the carrier wave; It is the driving logic signal of the primary MOS tube M2; S9~S 11 All are intermediate logic signals; (4) Secondary side cycloconverter MOS tube drive logic signal: ； ； The u1 is the positive half-cycle logic signal of the reference sinusoidal modulation wave, It is the driving logic signal of the primary MOS tube M1; It is the driving logic signal of the primary MOS tube M2, S9~S 11 Are intermediate logic signals, MOS tube S for the secondary side cycloconverter 1a ~S 4b Drive logic signals; (5) Delayed shutdown time setting: Through S 1a The switch-off signal of the switch tube is delayed to ensure that the transformer leakage current naturally drops to zero during the transition period, thus achieving S 1a The switch tube is turned off at zero current; the soft switching of the secondary side cycloconverter is achieved by delaying the turn-off of the switch tube, allowing the current to naturally drop to 0, thereby achieving zero current turn-off; the delayed turn-off time is: ； in To discharge the parasitic capacitance from the input voltage V in Time to drop to 0; When the secondary side cycloconverter freewheeling diode is turned on, the current flows from i s Time to drop to 0; t delay is the delayed turn-off time of the switch tube; C oss is the parasitic capacitance of the MOS tube, V in is the converter input voltage, is the transformer leakage inductance, i p is the primary current, i Lf is the filter inductor current; In normal circuit operation, since the inductor current is a sinusoidal envelope within the power frequency cycle, a fixed time is added to the corresponding switch tube to delay shutdown. When setting the delay time, t delay The minimum value is not less than .

5. The modulation method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: The front-stage full-bridge converter includes an input voltage source V in , the first to fourth power switch tubes M1, M2, M3, M4 of the front stage; the drains of M1 and M2 are connected to V in At one end, the sources of M1 and M2 are connected to the drains of M3 and M4 respectively, and the sources of M3 and M4 are connected to V in At the other end, the midpoints A and B of the two bridge arms are connected to the two ends of the primary side of the transformer T; the leakage inductance L is connected in series between the midpoint A of the bridge arm and the transformer T. r , leakage inductance L r Provide soft switching conditions; excitation inductance L m Connected in parallel at both ends of the transformer T, it plays the role of transferring energy; The front-stage full-bridge converter provides a positive and negative alternating square wave voltage in one switching cycle and operates under a driving signal with a 50% duty cycle.

6. The modulation method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: The subsequent cycloconverter is composed of four sets of bidirectional switches (S xa 、S xb ), x=1,2,3,4, each set of bidirectional switches consists of two MOS tubes connected in reverse series; S 1a 、S 2a The drains of the transformers are connected to one end of the secondary side of the transformer T. 1b The drain connection S 3a The drain and common terminal are E, S 2b The drain connection S 4a The drain and common terminal are F, S 3b 、S 4b The drains are connected to the other end of the secondary side of the transformer T; the common end E is connected to the other end of the secondary side of the transformer T through the filter inductor L. f Connect filter capacitor C f One end of the common terminal F is connected to the filter capacitor C f The other end of the filter inductor L f , filter capacitor C f A low-pass filter is formed, and the output voltage after filtering is applied to the load R o ; The subsequent cycloconverter supplies power to the load R in one switching cycle. o Energy is transferred twice to convert the square wave high-frequency signal into a low-frequency output signal.

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