A heric inverter soft switching topology based on coupled inductance and a modulation method thereof

Abstract

The application discloses a Heric inverter soft switching topology based on coupled inductance and a modulation method thereof. The soft switching topology mainly comprises a Heric inverter and a soft switching auxiliary circuit. The soft switching auxiliary circuit comprises a coupled inductance, an auxiliary inductance and an auxiliary bridge arm. The coupled inductance is composed of a small inductance and a main filter inductance. The auxiliary inductance is a small inductance connected in series with the coupled inductance. The coupled inductance and the auxiliary inductance are both symmetrical structures. The soft switching auxiliary bridge arm is a bidirectional switch, which is generally composed of two unidirectional switches connected in reverse series. The modulation strategy is a modulation strategy for realizing the soft switching condition by means of the soft switching auxiliary bridge arm and the coupled inductance. The application combines the Heric inverter and the soft switching auxiliary circuit, solves the reliability problem caused by the leakage current of the grid-connected inverter, realizes the soft switching operation of the grid-connected inverter, and improves the operation efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of soft switching technology of inverters, and particularly relates to a Heric inverter soft switching topology based on coupled inductance and a modulation method thereof. BACKGROUND

[0002] The topology of photovoltaic inverters is generally divided into isolated and non-isolated types. The non-isolated grid-connected inverter has a small weight, a high power density, a low cost, and a higher efficiency, and is more widely applied than the isolated grid-connected inverter. However, there is a parasitic capacitance between the photovoltaic array and the ground, and a common-mode current will be generated when the common-mode voltage between the two ends of the parasitic capacitance changes. In the non-isolated topology, the photovoltaic array and the grid are directly connected, and the leakage current will significantly affect the system performance, efficiency, and threaten the safety of personnel. With the continuous progress of switching devices, the pulse width modulation technology is adopted in the inverter, and the influence of the turn-on and turn-off losses of the power switch on the efficiency cannot be ignored.

[0003] In order to reduce the leakage current and introduce the soft switching technology to reduce the switching loss in the operation of the converter, researchers have proposed various grid-connected inverter topologies and control strategies for realizing leakage current suppression or soft switching. One is to use a passive buffer circuit to realize the soft switching condition of the inverter, and only a small number of passive devices are used to realize it. However, this method aggravates the common-mode voltage phenomenon and deteriorates the output performance. Some scholars introduce an auxiliary resonant rectifier pole into the grid-connected inverter, and introduce a resonant DC branch and a resonant AC branch to realize the soft switching condition. As for the leakage current suppression, some scholars have proposed a Heric inverter, which realizes the suppression of leakage current through its symmetric structure, and the suppression effect has been verified to be very effective. SUMMARY

[0004] The present application aims to overcome the above-mentioned defects of the prior art, and proposes a Heric inverter soft switching topology based on coupled inductance and a modulation method thereof, to realize the soft switching operation of the inverter and improve the system operation efficiency.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0006] A Heric inverter soft switching topology based on coupled inductance, comprising a Heric inverter, further comprising a soft switching auxiliary circuit connected to the Heric inverter, which is composed of a coupled inductance L c , an auxiliary inductance L a and a soft switching auxiliary switch composed of a bidirectional switch, and one coupled inductance L cThe main filter inductance and the coupling inductance are reversely connected in series in the main circuit, and the added bidirectional switch is connected across the two main filter inductances L and the coupling inductance L of the Heric inverter AC circuit c The added bidirectional switch is composed of two switch tubes S7 and S8 reversely connected in series, and the added two auxiliary inductances La are connected in series with the coupling inductance Lc, the inductance values of the auxiliary inductances and the coupling inductance are all smaller than the main inductance of the inverter, so that the current flowing through the auxiliary inductance can be quickly reversed to provide ZVS opening conditions for the H-bridge switches S1-S4 in the commutation process, and the equivalent filter inductance meets the design requirements of the AC current ripple.

[0007] The switch topology is further improved, and the inverter bridge and the Heric auxiliary bridge arm are composed of switches S1-S6, wherein D S1 -D S6 , C S1 -C S6 are the parasitic capacitance and the parasitic diode of the switch.

[0008] The modulation method of the Heric inverter soft switching topology based on the coupling inductance is as follows: in the positive half cycle of the power grid, the switches S1 and S4 are high-frequency operated, S2 and S3 are in the off state, S6 is in the constant-on state, S7 is in the constant-off state, and S5 and S8 need to be turned on in advance before the switches S1 and S4 are turned on to force the current of the auxiliary inductance L a to be reversed, so as to provide the zero-voltage opening condition of the switches S1 and S4, and the positive half cycle of the power grid contains seven working states, and the working principle of the inverter in the negative half cycle of the power grid is similar and has symmetry.

[0009] Mode 1 time period [0-t1], in this stage, the switches S1 and S4 are turned on, the main filter inductance and the auxiliary inductance of the inverter are connected in series, the energy is transmitted from the DC side to the AC side, the inductance current continuously rises under the DC voltage, and the current change process is as shown in the following formula:

[0010]

[0011] Mode 2 time period [t1-t2], this stage is a time period before the dead-time commutation of the inverter bridge arm, in this stage, the switches S1 and S4 are turned off, the inductance current flows through the parasitic capacitance of the switch, the parasitic capacitance C S1 , C S4 is charged, the energy is transmitted from the DC side to the parasitic capacitance and the AC side, the parasitic capacitance C S1 , C S4 is charged, and the voltage across the two ends continuously rises, and the current change process is as shown in the following formula

[0012]

[0013] Mode 3 time period [t2-t3], this stage is the auxiliary inductor current i La A very short period of time begins before the reverse direction, during which the switch S6 is turned on. La Continue to decrease. In this stage, the energy in the inductor is transferred to the AC side. Under the action of the grid side voltage, the inductor current continues to decrease. The current change process in this stage is shown as follows

[0014]

[0015] Mode 4 time period [t3-t4] is a period before the auxiliary inductor current reverses. In this period, switch S8 is turned on. With the turning on of S8, the primary side of the coupled inductor and the AC grid are no longer connected in series with the secondary side of the coupled inductor and the auxiliary inductor. The auxiliary inductor current continues to decrease until it reaches zero under the action of the induced voltage of the coupled inductor. In this period, energy is transferred from the grid to the auxiliary inductor. For the auxiliary inductor and the secondary side of the coupled inductor, the grid voltage does not directly affect them. Only the voltage induced by the secondary side of the coupled inductor acts on the auxiliary inductor and the secondary side of the coupled inductor, causing the current i La It drops rapidly to less than zero, while the current of the primary inductance drops slowly under the action of the grid voltage. The current change process in this stage is as follows;

[0016]

[0017] Mode 5 time period [t4-t5], this stage is a very short period of time after the auxiliary inductor current reverses. In this stage, S5 is turned on to provide a freewheeling channel for the auxiliary inductor current after the reverse. The auxiliary inductor current continues to increase in the reverse direction under the action of the coupled inductor induced voltage. At the beginning of this stage, the auxiliary inductor current i La It is zero and has a tendency to continue to decrease. The opening of switch S5 provides it with a reverse flow path. Therefore, in this stage i La It continues to increase in the negative direction. The current change process in this stage is as follows

[0018]

[0019] Mode 6 time period [t5-t6] is a very short period before switches S1 and S4 are turned on. During this period, S5 is turned off. Since the inductor current cannot change suddenly, the current iLa flowing through the auxiliary inductor can only flow through the parasitic capacitor C S1 , C S4 To continue the flow, so as to S1 , C S4 Discharge is performed to provide ZVS conditions for subsequent opening. At this time, i La reaches its reverse maximum value, due to the shutdown of S5, i La Can only flow through the parasitic capacitance CS1 and C S4 The parasitic capacitance is discharged by freewheeling. When the parasitic capacitance is discharged, S1 and S4 are turned on, and the switch ZVS is turned on. The current change process in this stage is as follows

[0020]

[0021] Mode 7, time period [t6-t8], this stage is the time period after switches S1 and S4 are turned on. La The rate of increase is much greater than i L The rate of increase, so i a Continuously decreasing, when i a When the voltage decreases to zero, switch S8 is turned off, thus achieving ZCS turn-off. At the beginning of this stage, switches S1 and S4 are turned on. Since their parasitic capacitance has been discharged in the previous stage, ZVS is achieved. In this stage, due to the effect of the DC side voltage, i La It increases rapidly in the positive direction, and since the primary inductance of the coupled inductor is much larger than the auxiliary inductance and the secondary inductance of the coupled inductor, i La The rising speed is greater than i L , so the current i flowing through the soft switching auxiliary bridge arm a It continues to decrease, and when it decreases to zero, the switch S8 is turned off, and this stage ends.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) By adding a soft-switching auxiliary circuit to a traditional inverter, the present invention achieves zero-voltage turn-on of all high-frequency switches of the inverter, reduces switching losses, and facilitates the converter to achieve high-frequency and high-power density operation;

[0024] 2) The auxiliary bidirectional switch added in the present invention has good zero-current characteristics during both the turn-on and turn-off processes, effectively reducing additional switching losses;

[0025] 3) The auxiliary inductor added in the present invention is only used for reverse current construction during high-frequency switching. In other modes, it is connected in series with the main filter inductor to form the total filter inductor of the inverter, which improves the utilization rate of the inductor device and is conducive to improving the output performance of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a topology diagram of the leakage current suppression type soft switching grid-connected inverter in the present invention;

[0027] Figure 2 Schematic diagram of the soft switching modulation process and steady-state current waveform in the present invention;

[0028] Figure 3 Schematic diagram of Mode 1 in the present invention;

[0029] Figure 4 Schematic diagram of Mode 2 in the present invention;

[0030] Figure 5 Schematic diagram of Mode 3 in the present invention;

[0031] Figure 6 Schematic diagram of Mode 4 in the present invention;

[0032] Figure 7 Schematic diagram of Mode 5 in the present invention;

[0033] Figure 8 Schematic diagram of Mode 6 in the present invention;

[0034] Figure 9 Schematic diagram of mode 7 in the present invention. DETAILED DESCRIPTION

[0035] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0036] The following is a best example of a Heric inverter soft switching topology based on coupled inductors and a modulation method thereof described in the present invention, which does not limit the scope of protection of the present invention.

[0037] like Figure 1 As shown in the figure, a new topology of leakage current suppression type soft switching grid-connected inverter based on coupled inductor is shown. Based on the traditional Heric inverter, a coupled inductor L is added. c , auxiliary inductor L a And a soft-switching auxiliary circuit composed of a soft-switching auxiliary switch composed of a bidirectional switch. First, a coupling inductor with an inductance much smaller than the main filter inductor is integrated on each of the two symmetrical main filter inductors L of Heric. The main filter inductor and the coupling inductor are connected in series in reverse order in the main circuit. The added bidirectional switch is connected between the two main filter inductors and their coupling inductors in the AC circuit of the Heric inverter. The bidirectional switch is generally composed of two switch tubes S7 and S8 connected in series in reverse order. The two added auxiliary inductors La are respectively connected in series with the coupling inductor Lc. The coupling inductance of the auxiliary inductor and the main inductor is generally much smaller than the main inductor of the inverter to ensure that the current flowing through the auxiliary inductor during the commutation process can quickly reverse to provide ZVS opening conditions for the H-bridge switches S1-S4, and the equivalent filter inductor meets the AC current ripple design requirements.

[0038] The inverter bridge and Heric auxiliary bridge arm are composed of switches S1-S6, where D S1 -D S6 , C S1 -CS6 parasitic diode of the switch;

[0039] The soft switching modulation strategy is described, taking the positive half cycle of the power grid as an example. The switches S1 and S4 are high-frequency operated, S2 and S3 are in the off state, S6 is in the constant on state, S7 is in the constant off state, and S5 and S8 need to be turned on in advance before the opening of switches S1 and S4 to force the current of the auxiliary inductor L a to reverse, so as to provide the condition for zero voltage turn-on of switches S1 and S4. The working principle of the inverter in the negative half cycle of the power grid is similar and has symmetry.

[0040] Taking the positive half cycle of the power grid as an example, the converter contains seven working modes in one switching cycle under the modulation strategy. The analysis process of various modes is introduced as follows.

[0041] Mode 1 is shown in Figure 3 , in which stage S1 and S4 are turned on, and the current flows according to the path in the figure. The inductor current continuously rises under the DC side voltage, and the current change process is as follows.

[0042]

[0043] Mode 2 is shown in Figure 4 , in which stage switches S1 and S4 are turned off at the beginning. The inductor current flows through the parasitic capacitor C S1 , C S4 is charged, and the voltage across the two ends continuously rises. The current change process is as follows.

[0044]

[0045] Mode 3 is shown in Figure 5 , in which stage switch S6 is turned on at the beginning. Obviously, after switch S6 is turned on, the inductor current continues to flow through the path Figure 5 , and the inductor current continuously decreases under the action of the grid side voltage. The current change process in this stage is as follows.

[0046]

[0047] Mode 4 is shown in Figure 6 , in which stage switch S8 is turned on at the beginning. With the opening of S8, the grid voltage does not directly act on the auxiliary inductor and the secondary side of the coupled inductor. Only the voltage induced by the secondary side of the coupled inductor acts on the auxiliary inductor and the secondary side of the coupled inductor, so that the current i La flowing through them rapidly decreases to less than zero, while the current of the primary inductor slowly decreases under the action of the grid voltage. The current change process in this stage is as follows.

[0048]

[0049] Mode 5 is as follows Figure 7 As shown, the switch S5 is turned on at the beginning of this phase, and the auxiliary inductor current i La It is zero and has a tendency to continue to decrease. The opening of switch S5 provides it with a reverse flow path. Therefore, in this stage i La It continues to increase in the negative direction. The current change process in this stage is as follows;

[0050]

[0051] Mode 6 Figure 8 As shown, switch S5 is turned off at the beginning of this phase, and i La reaches its reverse maximum value, due to the shutdown of S5, i La Can only flow through the parasitic capacitance C S1 and C S4 The parasitic capacitance is discharged by freewheeling. When the parasitic capacitance is discharged, S1 and S4 are turned on, and the switch is turned on by ZVS. The current change process in this stage is as follows;

[0052]

[0053] Mode 7 as Figure 9 As shown, switches S1 and S4 are turned on at the beginning of this phase. Since their parasitic capacitance has been discharged in the previous phase, ZVS is achieved. In this phase, due to the effect of the DC side voltage, i La It increases rapidly in the positive direction, and since the primary inductance of the coupled inductor is much larger than the auxiliary inductance and the secondary inductance of the coupled inductor, i La The rising speed is greater than i L , so the current i flowing through the soft switching auxiliary bridge arm a It continues to decrease, and when it decreases to zero, the switch S8 is turned off, and this stage ends.

[0054] Specifically, in order to further analyze the characteristics of this topology, its steady-state operating point is modeled. In order to simplify the analysis, the influence of the resonant commutation stage is ignored and the definition is Figure 2 The occupancy ratios of each stage are as follows;

[0055]

[0056] Specifically, based on the duty cycle defined above, the voltage transformation ratio of this topology can be deduced as follows through the inductor volt-second balance. It can be seen that the voltage transformation ratio is no different from that of the conventional topology.

[0057] V g =DV DC

[0058] Specifically, since the auxiliary inductor and the coupled inductor secondary side are much smaller than the coupled inductor primary side, the impact of the auxiliary inductor on the main filter inductor is ignored, and the topology operating characteristics do not change significantly. Based on this simplification, the size of the coupled inductor primary side required to meet the 20% inductor current ripple requirement can be approximately calculated as follows

[0059]

[0060] Specifically, according to the topology analysis above, to achieve ZCS characteristics of the auxiliary switch turn-on and turn-off, the coupled inductor secondary side inductance value and the auxiliary inductor value also need to meet certain conditions, and the relationship is as follows

[0061]

[0062] So far, the technical route of the present application has been described in detail through examples and in conjunction with the drawings, but it is easy for those skilled in the art to understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to related technical features without deviating from the principles of the present application, and the technical solutions after these changes or replacements will all fall within the protection scope of the present application.

Claims

1. A modulation method for a Heric inverter soft switching topology based on coupled inductors, wherein the Heric inverter soft switching topology based on coupled inductors comprises an inverter bridge, a Heric auxiliary bridge arm, two auxiliary inductors, L a , two coupled inductors L c , soft switching auxiliary bridge arm, two main filter inductors L ; The inverter bridge consists of switches S 1 and switch S 2 in series and the first bridge arm consisting of switches S 3 and switch S The second bridge arm is composed of 4 series connections, the series connection point of the first bridge arm is point A, and the series connection point of the second bridge arm is point B; the Heric auxiliary bridge arm includes switches connected in series S 5 and switch S 6. The first and second ends of the Heric auxiliary bridge arm are connected to points A and B respectively. The first end of the Heric auxiliary bridge arm and the first auxiliary inductor L a , the first coupled inductor L c , the first main filter inductor L, AC power supply V g , the second main filter inductor L, the second coupling inductor L c , the second auxiliary inductor L a , the second end of the Heric auxiliary bridge arm is connected in series; the soft switch auxiliary bridge arm is composed of the switch S 7 and switch S 8 are connected in reverse series, and the two ends of the soft switch auxiliary bridge arm are connected across the first coupled inductor L c The series connection point of the first main filter inductor L and the second coupling inductor L c Between the series connection point of the second main filter inductor L; diode D S1 -D S6 For switch S 1- S 6 parasitic diode, capacitor C S1 -C S4 For switch S 1- S 4 parasitic capacitance; It is characterized in that The working process of the positive half cycle of the power grid is as follows: switches S1 and S4 operate at high frequency, S2 and S3 are in the off state, S6 is in the constant on state, S7 is in the constant off state, and S5 and S8 need to be turned on in advance before switches S1 and S4 are turned on to force the auxiliary inductor L a The current is reversed to provide the zero voltage opening condition for switches S1 and S4. There are seven working states in the positive half cycle of the grid. The working principle of the inverter in the negative half cycle of the grid is similar and symmetrical. Mode 1 time period [0-t1], during this stage, switches S1 and S4 are turned on, the inverter main filter inductor L and the auxiliary inductor L a When connected in series, energy is transferred from the DC side to the AC side. The inductor current continues to rise under the DC side voltage. The current change process is solved as follows: ； Mode 2 time period [t1-t2], this stage is a period of time before the inverter bridge arm dead zone commutation, during this stage switches S1 and S4 are turned off, because the inductor current cannot change suddenly, the inductor current flows through the switch parasitic capacitance, the parasitic capacitance C S1 , C S4 Charging, energy is transferred from the DC side to the parasitic capacitor and the AC side, the parasitic capacitor C S1 , C S4 During charging, the voltage at both ends continues to rise, and the current change process is as follows: ； Mode 3 time period [t2-t3], this stage is the auxiliary inductor current i La A very short period of time begins before the reverse direction. During this period, the switch S6 is turned on and the auxiliary inductor current i La Continue to decrease. In this stage, the energy in the inductor is transferred to the AC side. Under the action of the grid side voltage, the inductor current continues to decrease. The current change process in this stage is as follows: ； Mode 4 time period [t3-t4], this stage is the auxiliary inductor current i La In the period before the reverse direction, the switch S8 is turned on. With the turning on of S8, the coupled inductor L c The primary side and the AC grid are no longer connected to the coupling inductor L c Secondary side and auxiliary inductor L a In series, the auxiliary inductor current i La Only in the coupled inductor L c The induced voltage continues to decrease until it reaches zero. At this stage, the energy is transferred from the grid to the auxiliary inductor L. a , for the auxiliary inductor L a and the coupled inductor L c On the secondary side, the grid voltage does not directly affect the circuit, only the coupling inductor L c The voltage induced on the secondary side acts on the auxiliary inductor L a and the coupled inductor L c The secondary side makes the current i flowing through it La It drops rapidly to less than zero, while the current of the primary inductance drops slowly under the action of the grid voltage. The current change process in this stage is as follows: ； Mode 5 time period [t4-t5], this stage is the auxiliary inductor current i La A very short period of time after the reverse, during which S5 is turned on, is the auxiliary inductor current i after the reverse La Provides a freewheeling channel to assist the inductor current i La The coupled inductor L c The inductive voltage continues to increase in the reverse direction. At the beginning of this stage, the auxiliary inductor current i La It is zero and has a tendency to continue to decrease. The opening of switch S5 provides it with a reverse flow path. Therefore, in this stage i La It continues to increase in the negative direction. The current change process in this stage is as follows: ； Mode 6 time period [t5-t6], this stage is a very short period of time before switches S1 and S4 are turned on. During this stage, S5 is turned off. Since the inductor current cannot change suddenly, the auxiliary inductor current i La Can only flow through the parasitic capacitance C S1 , C S4 To continue the flow, so as to S1 , C S4 Discharge is performed to provide ZVS conditions for subsequent opening. At this time, i La reaches its reverse maximum value, due to the shutdown of S5, i La Can only flow through the parasitic capacitance C S1 and C S4 The parasitic capacitance is discharged by freewheeling. When the parasitic capacitance is discharged, S1 and S4 are turned on, and the switch is turned on by ZVS. The current change process in this stage is as follows: ； Mode 7, time period [t6-t8], this stage is the time period after switches S1 and S4 are turned on. La The rate of increase is much greater than i L The rate of increase, so i a Continuously decreasing, when i a When the voltage decreases to zero, switch S8 is turned off, thus achieving ZCS turn-off. At the beginning of this stage, switches S1 and S4 are turned on. Since their parasitic capacitance has been discharged in the previous stage, ZVS is achieved. In this stage, due to the effect of the DC side voltage, i La It increases rapidly in the positive direction, and due to the coupling inductance L c The primary inductance is much larger than the auxiliary inductance L a and the coupled inductor L c Secondary inductance, i La The rising speed is greater than i L , so the current i flowing through the soft switching auxiliary bridge arm a It continues to decrease, and when it decreases to zero, the switch S8 is turned off, and this stage ends.

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