Direct current / alternating current converter and control method thereof

Abstract

The invention discloses a direct-current / alternating-current converter and a control method thereof, and belongs to the technical field of electric energy conversion, and the control method comprises the steps that the turn-on time of a switch in the direct-current / alternating-current converter is realized through digital control, the turn-off time of the switch is realized through analog control, and a driving signal of the switch is generated according to the turn-on time and the turn-off time. According to the method, valley current control can be applied to the direct-current / alternating-current converter, and the current upper limit value and the current lower limit value are calculated in consideration of the condition that the current in the direct-current / alternating-current converter has positive current and negative current; under the condition that the alternating voltage changes in real time, the turn-on time is calculated in real time through digital control; under the condition that the positive and negative grid voltages are different, the inductive current is compared with the current upper limit value and the current lower limit value through analog control to obtain turn-off time, and therefore control over a switch in the direct-current / alternating-current converter is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of electric energy conversion, and in particular to a DC / AC converter and a control method thereof. Background Art

[0002] Generally speaking, valley current control is more commonly used in DC / DC converters and is easy to implement. However, there are several difficulties in applying valley current control to DC / AC converters.

[0003] One is the difference in inductor voltage difference: For DC / DC converters, such as buck converters, when the input voltage and output voltage are determined, the voltage drop across the inductor is constant, and its turn-on time T on is calculated as a constant value; however, in the DC / AC converter, since the AC voltage is changing, the on-time T on It is a real-time changing value and is difficult to calculate.

[0004] The second difference is the direction of current: in a DC / DC converter, the current direction is fixed to forward current, the given current reference value of its control is a constant DC, and the valley value of the current can remain unchanged; however, in a DC / AC converter, the current can be positive or negative.

[0005] The third difference is the comparator: in a DC / DC converter, the output voltage and output current are constant DC outputs, so only one comparator is required and no switching is required; however, in a DC / AC converter, positive and negative voltages need to be considered. Summary of the Invention

[0006] To solve the above problems, the present invention provides a DC / AC converter and a control method thereof, which can realize valley current control in the DC / AC converter.

[0007] To achieve the above object, the technical solution of the present invention is:

[0008] A control method for a DC / AC converter includes: realizing the on-time of a switch in the DC / AC converter by digital control, realizing the off-time of the switch by analog control, and generating a drive signal for the switch according to the on-time and the off-time.

[0009] In a specific embodiment, the on-time of the switch is realized by digital control, including collecting the voltage across the power inductor and calculating the voltage drop V across the power inductor. L (t), the opening time T is calculated according to the following formula on (t):

[0010] T on (t)=L*Δi(t) / V L(t),

[0011] Where L is the inductance of the power inductor, Δi(t) is the inductor current ripple;

[0012] The expression of inductor current ripple Δi(t) is as follows:

[0013] Δi(t)=i high (t)-i low (t)

[0014] Among them, i high (t) is the upper limit of current, i low (t) is the lower limit of current;

[0015] Current upper limit i high (t), current lower limit i low (t) is calculated as follows:

[0016] i high (t) = i ref (t)+I h cosωt+I B

[0017] i low (t) = i ref (t)-I h cosωt-I B

[0018] Among them, I h is the hysteresis loop width; I B is the bias current; i ref (t) is the reference current; ω is the angular frequency; t is the time;

[0019] Reference current i ref The expression of (t) is as follows:

[0020]

[0021] Among them, S ref is the apparent power; V acrms is the effective value of alternating current; is the phase difference calculated based on the power factor.

[0022] In a specific embodiment, the on-time of the switch is realized by digital control, including collecting the voltage across the power inductor and calculating the voltage drop across the power inductor. For a three-phase DC / AC converter, the on-time of each phase switch is expressed as follows:

[0023] T on_a (t)=La*Δi_a(t) / V La (t),

[0024] T on_b (t)=Lb*Δi_b(t) / V Lb (t),

[0025] T on_c (t)=Lc*Δi_c(t) / V Lc (t),

[0026] Among them, V La (t) is the voltage drop across the A-phase power inductor La, T on_a (t) is the opening time of the A-phase switch; V Lb (t) is the voltage drop across the B-phase power inductor Lb, T on_b (t) is the turn-on time of the B-phase switch; V Lc (t) is the voltage drop across the C-phase power inductor Lc, T on_c (t) is the turn-on time of the phase C switch; Δi_a(t) is the phase A inductor current ripple, Δi_b(t) is the phase B inductor current ripple, and Δi_c(t) is the phase C inductor current ripple;

[0027] The inductor current ripple of each phase is expressed as follows:

[0028] Δi_a(t)=i high_a (t)-i low_a (t)

[0029] Δi_b(t)=i high_b (t)-i low_b (t)

[0030] Δi_c(t)=i high_c (t)-i low_c (t)

[0031] Among them, i high_a (t) is the upper limit of phase A current, i low_a (t) is the lower limit of phase A current, i high_b (t) is the upper limit of phase B current, i low_b (t) is the lower limit of phase B current, i high_c (t) is the upper limit of phase C current, i low_c (t) is the lower limit of phase C current;

[0032] The upper and lower current limits of each phase are expressed as follows:

[0033] i high_a (t) = i a_ref (t)+I h cosωt+I B

[0034] i low_a (t) = i a_ref (t)-I h cosωt-I B

[0035] i high_b (t) = i b_ref (t)+I h cosωt+I B

[0036] i low_b (t) = i b_ref (t)-I h cosωt-I B

[0037] i high_c (t) = i c_ref (t)+I h cosωt+I B

[0038] i low_c (t) = i c_ref (t)-I h cosωt-I B

[0039] Among them, I h is the hysteresis loop width; I B is the bias current; i a_ref (t) is the reference current of phase A, i b_ref (t) is the reference current of phase B, i c_ref (t) is the reference current of phase C; ω is the angular frequency; t is the time.

[0040] More specifically, the on-time of the switch is realized by digital control, including reducing the on-time T when the AC voltage and the DC voltage are close. on to the set value.

[0041] In a specific embodiment, the off time of the switch is realized by analog control, including sampling the instantaneous value i of the power inductor current. L (t), in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current upper limit i high (t) Real-time comparison, when i L Not less than i high (t) indicates the off time T off (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current lower limit i low(t) Real-time comparison, when i L (t) not greater than i low (t) indicates the off time T off (t) has been reached, entering the next cycle of control.

[0042] In a specific embodiment, for a three-phase DC / AC converter, the switch off time is realized by analog control, including, for phase A, in the positive half cycle of the grid voltage, setting the instantaneous value of the power inductor current of phase A to La (t) and the upper limit value of phase A current i high_a (t) Real-time comparison, when i La (t) not less than i high_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the A-phase power inductor current i La (t) and the lower limit value of phase A current i low_a (t) Real-time comparison, when i La (t) not greater than i low_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached and enters the next cycle of control;

[0043] For phase B, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase B is i Lb (t) and the upper limit value of phase B current i high_b (t) Real-time comparison, when i Lb (t) not less than i high_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the B phase power inductor current i Lb (t) and the lower limit value of phase B current i low_b (t) Real-time comparison, when i Lb (t) not greater than i low_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached and enters the next cycle of control;

[0044] For phase C, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase C is i Lc (t) and the upper limit value of phase C current i high_c (t) Real-time comparison, when i Lc (t) not less than i high_c (t) represents the turn-off time T of the C-phase switch off_c(t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the C phase power inductor current i Lc (t) and the lower limit value of phase C current i low_c (t) Real-time comparison, when i Lc (t) not greater than i low_c (t) represents the turn-off time T of the C-phase switch off_c (t) has been reached, entering the next cycle of control.

[0045] The present invention also provides a control circuit for a DC / AC converter, which uses the above-mentioned control method for a DC / AC converter, including a voltage sampling module, a reference current calculation module, a current reference generation module and an on-time calculation module. The voltage sampling module samples the grid voltage and outputs it to the reference current calculation module. The reference current calculation module calculates a reference current based on the grid voltage and outputs it to the current reference generation module. The current reference generation module calculates a current upper limit value and a current lower limit value based on the reference current and outputs them to the on-time calculation module. The on-time calculation module calculates the on-time based on the current upper limit value and the current lower limit value.

[0046] The control circuit of the DC / AC converter further includes a current sampling module, a first comparison module, a second comparison module, a first pulse generation module, and a second pulse generation module. The current sampling module samples the instantaneous value of the power inductor current and outputs it to the first comparison module and the second comparison module. The first comparison module compares the instantaneous value of the power inductor current with the current upper limit value and outputs the turn-off time to the first pulse generation module. The second comparison module compares the instantaneous value of the power inductor current with the current lower limit value and outputs the turn-off time to the second pulse generation module. The turn-on time calculation module outputs the turn-on time to the first pulse generation module and the second pulse generation module. The first pulse generation module and the second pulse generation module generate a drive signal for a switch in the DC / AC converter based on the turn-on time and the turn-off time.

[0047] The present invention also provides a DC / AC converter, which applies the above-mentioned control method for a DC / AC converter. The DC / AC converter converts AC power into DC power or converts DC power into AC power.

[0048] Furthermore, the above-mentioned DC / AC converter includes a single-phase DC / AC converter or a three-phase DC / AC converter.

[0049] Beneficial effects: A DC / AC converter and its control method and control circuit of the present invention can realize the application of valley current control in the DC / AC converter. Taking into account the situation that the current in the DC / AC converter is both positive and negative, the current upper limit and the current lower limit are calculated; when the AC voltage changes in real time, the turn-on time is calculated in real time through digital control; when the grid voltage is positive or negative, the inductor current is compared with the current upper limit and the current lower limit respectively through analog control to obtain the turn-off time, thereby realizing the control of the switch in the DC / AC converter.

[0050] In order to make the above features and advantages of the present invention more clearly understood, embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 1 is a circuit diagram and a control block diagram of a DC / AC converter according to a specific embodiment of the present invention.

[0052] Figure 2 1 is a circuit diagram and a control block diagram of a DC / AC converter according to another specific embodiment of the present invention.

[0053] Figure 3 for Figure 2 Schematic diagram of voltage, current and switch drive waveforms of phase A in the specific embodiment. DETAILED DESCRIPTION

[0054] To make the purpose and technical solutions of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] The present invention provides a control method for a DC / AC converter, comprising: setting the on-time T of a switch in the DC / AC converter; on (t) is realized through digital control, the switch off time T off (t) is realized through analog control, according to the opening time T on (t) and the turn-off time T off (t) Generate a switch drive signal.

[0056] More specifically, the on-time T of the switch on (t) is realized through digital control, specifically including collecting the voltage across the power inductor and calculating the voltage drop V across the power inductor L (t), the opening time T is calculated according to the following formulaon (t):

[0057] T on (t)=L*Δi(t) / V L (t)

[0058] Where L is the inductance of the power inductor; Δi(t) is the inductor current ripple, which is the instantaneous value at time t; the voltage drop across the power inductor V L (t) is also the instantaneous value at time t.

[0059] From the above formula, we can see that the switch on time T on (t) is the instantaneous value at time t, which is calculated in real time based on the inductor current ripple and the voltage drop across the power inductor at time t, thereby enabling real-time control of the converter.

[0060] More specifically, the inductor current ripple Δi(t) is determined by the given output power requirement.

[0061] More specifically, the inductor current ripple Δi(t) is expressed as follows:

[0062] Δi(t)=i high (t)-i low (t)

[0063] Among them, i high (t) is the upper limit of current, i low (t) is the lower limit of current.

[0064] Furthermore, when the AC voltage and DC voltage are very close, the voltage drop V across the power inductor is L (t) is very small, and the calculated opening time T on (t) will be very large. After the inductor current linearly rises for a stage, it will oscillate. The inductor current will be distorted and the total harmonic content (THD) of the AC current will increase. At this time, shorten the on-time T on (t) to a set value to ensure that the inductor current is in the linear rising stage and then enters the shutdown stage.

[0065] Optionally, the voltage across the power inductor is acquired through analog-to-digital conversion.

[0066] More specifically, the switch off time T off (t) is realized through analog control, specifically including sampling to obtain the instantaneous value of the power inductor current i L (t). In the positive half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current upper limit i high (t) Real-time comparison, when i L (t) not less than i high(t) indicates the off time T off (t) has been reached, and the control of the next cycle has begun. In the negative half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current lower limit i low (t) Real-time comparison, when i L (t) not greater than i low (t) indicates the off time T off (t) has been reached, entering the next cycle of control.

[0067] Optionally, the instantaneous value of the inductor current i is obtained by sampling the current sensor. L (t).

[0068] Figure 1 The circuit diagram and control block diagram of a DC / AC converter of the present invention in a specific embodiment are shown in FIG. In this specific embodiment, the DC / AC converter is a single-phase topology. Figure 1 Taking as an example, a control method of a DC / AC converter of the present invention is introduced.

[0069] like Figure 1 As shown, the DC / AC converter 11 includes a full-bridge structure, specifically including a switch Q1, a switch Q2, a switch Q3, a switch Q4, a capacitor C AC With inductance L AC , switch Q1 is connected in series with switch Q2, switch Q3 is connected in series with switch Q4, and both ends of the series connection of switch Q1 and switch Q2 and both ends of the series connection of switch Q3 and switch Q4 are connected in parallel to form the first end of the DC / AC converter 11, the series midpoint of switch Q1 and switch Q2, and the inductor L AC , capacitor C AC , the midpoint of the series connection of switch Q3 and switch Q4 is connected, capacitor C AC The two ends of the DC / AC converter 11 are the second ends of the DC / AC converter 11, and the first end of the DC / AC converter 11 is connected in parallel to the DC power V dc The second end of the DC / AC converter 11 is connected in parallel with the AC power u AC .

[0070] Among them, the inductor L AC For the power inductor.

[0071] Optionally, the switch Q1 , the switch Q2 , the switch Q3 , and the switch Q4 include but are not limited to switching tubes such as MOSFET and IGBT.

[0072] In this specific embodiment, for Figure 1 In the single-phase DC / AC converter, the current upper limit i high (t), current lower limit i low (t) is calculated as follows:

[0073] i high (t) = i ref (t)+I h cosωt+I B

[0074] i low (t) = i ref (t)-I h cosωt-I B

[0075] Among them, I h is the hysteresis loop width; I B is the bias current; i ref (t) is the reference current; ω is the angular frequency.

[0076] Furthermore, the reference current i ref The expression of (t) is as follows:

[0077]

[0078] Among them, S ref is the apparent power, is a user-given value, in one embodiment, S ref Can be 10KVA;

[0079] V acrms For alternating current u AC The effective value of is the phase difference calculated based on the power factor (PF).

[0080] For example, when switch Q1 and switch Q4 are turned on, the voltage drop across the power inductor V L (t) = V dc -u AC .

[0081] Please continue to refer to Figure 1 The present invention also provides a control circuit for a DC / AC converter, which utilizes the above-mentioned control method for a DC / AC converter. More specifically, the control circuit 12 of the DC / AC converter includes a voltage sampling module 121, a reference current calculation module 122, a current reference generation module 123, and an on-time calculation module 124. The voltage sampling module 121 samples the grid voltage and outputs it to the reference current calculation module 122. The reference current calculation module 122 calculates a reference current based on the grid voltage and outputs it to the current reference generation module 123. The current reference generation module 123 calculates a current upper limit value and a current lower limit value based on the reference current and outputs them to the on-time calculation module 124. The on-time calculation module 124 calculates the on-time based on the current upper limit value and the current lower limit value.

[0082] Furthermore, the control circuit 12 of the DC / AC converter also includes a current sampling module 125, a first comparison module 126, a second comparison module 127, a first pulse generation module 128, and a second pulse generation module 129. The current sampling module 125 samples the instantaneous value of the power inductor current and outputs it to the first comparison module 126 and the second comparison module 127. The first comparison module 126 compares the instantaneous value of the power inductor current with the current upper limit and outputs the turn-off time to the first pulse generation module 128. The second comparison module 127 compares the instantaneous value of the power inductor current with the current lower limit and outputs the turn-off time to the second pulse generation module 129. The turn-on time calculation module 124 outputs the turn-on time to the first pulse generation module 128 and the second pulse generation module 129. The first pulse generation module 128 and the second pulse generation module 129 generate a drive signal for the switch in the DC / AC converter based on the turn-on time and the turn-off time.

[0083] for Figure 1 In the single-phase DC / AC converter, the voltage sampling module 121 samples the AC voltage u AC The instantaneous value is output to the reference current calculation module 122, and the reference current calculation module 122 calculates the value of the AC current u AC Calculate the reference current i ref (t) is output to the current reference generation module 123, and the current reference generation module 123 generates the reference current i ref (t) Calculate the current upper limit i high (t), current lower limit i low (t) is output to the opening time calculation module 124, and the opening time calculation module 124 calculates the opening time T on (t) is output to the first pulse generating module 128 and the second pulse generating module 129.

[0084] Furthermore, the current sampling module 125 samples the instantaneous value i of the power inductor current. LAC (t). In the positive half cycle of the grid voltage, the first comparison module 126 compares the instantaneous value of the power inductor current i LAC (t) and the current upper limit i high (t) Real-time comparison, when i LAC (t) not less than i high (t) indicates the off time T off (t) has been reached, and the output signal is sent to the first pulse generating module 128. In the negative half cycle of the grid voltage, the second comparing module 127 compares the instantaneous value of the power inductor current i LAC (t) and the current lower limit i low (t) Real-time comparison, when i LAC (t) not greater than i low (t) indicates the off time Toff (t) has been reached, and a signal is output to the second pulse generating module 129. The first pulse generating module 128 and the second pulse generating module 129 generate driving signals G11 and G12 for the switches in the DC / AC converter according to the on-time and off-time.

[0085] For example, Figure 1 In the topology, in the positive half cycle of the grid voltage, the on-time of switch Q1 and switch Q4 is T on (t), the off time is T off (t); In the negative half cycle of the grid voltage, the on-time of switch Q2 and switch Q3 is T on (t), the off time is T off (t).

[0086] Figure 2 The circuit diagram and control block diagram of a DC / AC converter of the present invention in another specific embodiment are shown in FIG. In this specific embodiment, the DC / AC converter is a three-phase topology. Figure 2 Taking as an example, a control method of a DC / AC converter of the present invention is introduced.

[0087] like Figure 2 As shown, the DC / AC converter 2 includes an A-phase bridge arm 211, a B-phase bridge arm 212, a C-phase bridge arm 213, an inductor La, an inductor Lb, and an inductor Lc. The midpoint of the A-phase bridge arm 211 is connected to the A-phase AC power supply u through the inductor La. a The midpoint of the bridge arm of phase B 212 is connected to the B phase AC voltage u through the inductor Lb. b The midpoint of the C-phase bridge arm 213 is connected to the C-phase AC voltage u through the inductor Lc. c The two ends of the A-phase bridge arm 211, the two ends of the B-phase bridge arm 212 and the two ends of the C-phase bridge arm 213 are connected in parallel, and the two ends after parallel connection are connected to the DC voltage V dc .

[0088] Among them, inductance La, inductance Lb, and inductance Lc are the power inductances of each phase.

[0089] Furthermore, the DC / AC converter 21 further includes a capacitor C1 and a capacitor C2. The capacitors C1 and C2 are connected in series and then connected in parallel to both ends of the A-phase bridge arm 211, both ends of the B-phase bridge arm 212, and both ends of the C-phase bridge arm 213.

[0090] More specifically, the A-phase bridge arm 211 includes a switch Q5, a switch Q6, a switch Q11, and a switch Q12. The two ends of the series connection of switch Q5 and switch Q6 are the two ends of the A-phase bridge arm 211. The midpoint of the series connection of switch Q5 and switch Q6 is the midpoint of the A-phase bridge arm 211. The midpoint of the A-phase bridge arm 211 is connected to one end of the reverse series connection of switch Q11 and switch Q12. The other end of the reverse series connection of switch Q11 and switch Q12 is connected to the midpoint of the series connection of capacitor C1 and capacitor C2.

[0091] More specifically, the B-phase bridge arm 212 includes a switch Q7, a switch Q8, a switch Q13, and a switch Q14. The two ends of the series connection of switch Q7 and switch Q8 are the two ends of the B-phase bridge arm 212. The midpoint of the series connection of switch Q7 and switch Q8 is the midpoint of the bridge arm of the B-phase bridge arm 212. The midpoint of the bridge arm of the B-phase bridge arm 212 is connected to one end of the reverse series connection of switch Q13 and switch Q14. The other end of the reverse series connection of switch Q13 and switch Q14 is connected to the midpoint of the series connection of capacitor C1 and capacitor C2.

[0092] More specifically, the C-phase bridge arm 213 includes a switch Q9, a switch Q10, a switch Q15, and a switch Q16. The two ends of the series connection of switch Q9 and switch Q10 form the two ends of the C-phase bridge arm 213. The midpoint of the series connection of switch Q9 and switch Q10 forms the midpoint of the C-phase bridge arm 213. The midpoint of the C-phase bridge arm 213 is connected to one end of the anti-series connection of switch Q15 and switch Q16. The other end of the anti-series connection of switch Q15 and switch Q16 is connected to the midpoint of the series connection of capacitor C1 and capacitor C2.

[0093] Optionally, the switches Q5 to Q16 include but are not limited to switching tubes such as MOSFET and IGBT.

[0094] Optionally, the DC / AC converter 21 further includes a plurality of inductors and a plurality of capacitors connected between each phase of the AC power and each phase bridge arm for filtering. More specifically, the A phase AC power u a The first end is connected to the inductor La through the inductor L1, and the B phase AC current u b The first end is connected to the inductor Lb through the inductor L2, and the C phase AC current u c The first end of the inductor is connected to the inductor Lc through the inductor L3, the midpoint of the connection between the inductor L1 and the inductor La is connected to the first end of the capacitor C3, the midpoint of the connection between the inductor L2 and the inductor Lb is connected to the first end of the capacitor C4, the midpoint of the connection between the inductor L3 and the inductor Lc is connected to the first end of the capacitor C5, and the second end of the capacitor C3, the second end of the capacitor C4, and the second end of the capacitor C5 are connected to the midpoint of the series connection of the capacitor C1 and the capacitor C2.

[0095] Furthermore, the A-phase AC current u a The second end, B phase AC u bThe second end, C phase AC u c The second end of is connected to the midpoint of the series connection of capacitor C1 and capacitor C2.

[0096] In this specific embodiment, for Figure 2 In the three-phase DC / AC converter, the inductor current ripple of each phase is calculated as follows:

[0097] First, the expression of the three-phase grid voltage is as follows:

[0098] u a (t) = U m cosωt

[0099] u b (t) = U m cos(ωt-120)

[0100] u c (t) = U m cos(ωt+120)

[0101] Among them, u a (t),u b (t),u c (t) are the instantaneous values ​​of the A-phase grid voltage, the B-phase grid voltage, and the C-phase grid voltage respectively; U m is the peak value of the three-phase grid voltage.

[0102] Secondly, the three-phase grid voltage is converted into voltage U d (t):

[0103]

[0104] Then, according to the voltage U d (t) Calculate the d-axis reference current i d_ref (t) and q-axis reference current i q_ref (t):

[0105]

[0106] Where P is the active power demand and Q is the reactive power demand, which are user-set values ​​or values ​​given by the energy management system (EMS).

[0107] According to the d-axis reference current i d_ref (t) and q-axis reference current i q_ref (t) Calculate the reference current i of phase A a_ref (t), B phase reference current i b_ref (t), C phase reference current i c_ref (t) are as follows:

[0108] i a_ref (t) = i d_ref (t)cosωt-i q_ref (t)sinωt

[0109] i b_ref (t) = i d_ref (t)cos(ωt-120)-i q_ref (t)sin(ωt-120)

[0110] i c_ref (t) = i d_ref (t)cos(ωt+120)-i q_ref (t)sin(ωt+120).

[0111] The three-phase current upper limit and the three-phase current lower limit are generated according to the three-phase reference current. The expressions are as follows:

[0112] i high_a (t) = i a_ref (t)+I h cosωt+I B

[0113] i low_a (t) = i a_ref (t)-I h cosωt-I B

[0114] i high_b (t) = i b_ref (t)+I h cosωt+I B

[0115] i low_b (t) = i b_ref (t)-I h cosωt-I B

[0116] i high_c (t) = i c_ref (t)+I h cosωt+I B

[0117] i low_c (t) = i c_ref (t)-I h cosωt-I B

[0118] Among them, i high_a (t) is the upper limit of phase A current, i low_a (t) is the lower limit of phase A current, i high_b(t) is the upper limit of phase B current, i low_b (t) is the lower limit of phase B current, i high_c (t) is the upper limit of phase C current, i low_c (t) is the lower limit of phase C current; I h is the hysteresis loop width; I B is the bias current.

[0119] The inductor current ripple of each phase is obtained based on the upper and lower current limits of each phase. The expression is as follows:

[0120] Δi_a(t)=i high_a (t)-i low_a (t)

[0121] Δi_b(t)=i high_b (t)-i low_b (t)

[0122] Δi_c(t)=i high_c (t)-i low_c (t)

[0123] Wherein, Δi_a(t) is the phase A inductor current ripple, Δi_b(t) is the phase B inductor current ripple, and Δi_c(t) is the phase C inductor current ripple.

[0124] The turn-on time of each phase switch is obtained as follows:

[0125] T on_a (t)=La*Δi_a(t) / V La (t),

[0126] T on_b (t)=Lb*Δi_b(t) / V Lb (t),

[0127] T on_c (t)=Lc*Δi_c(t) / V Lc (t),

[0128] Among them, V La (t) is the voltage drop across the A-phase power inductor La, T on_a (t) is the opening time of the A-phase switch; V Lb (t) is the voltage drop across the B-phase power inductor Lb, T on_b (t) is the turn-on time of the B-phase switch; V Lc (t) is the voltage drop across the C-phase power inductor Lc, T on_c (t) is the turn-on time of the phase C switch.

[0129] More specifically, in this embodiment, the switch off time Toff (t) is realized through analog control, specifically including sampling to obtain the instantaneous value of the power inductor current i L (t).

[0130] Furthermore, for phase A, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase A i La (t) and the upper limit value of phase A current i high_a (t) Real-time comparison, when i La (t) not less than i high_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached, and the control of the next cycle has begun. In the negative half cycle of the grid voltage, the instantaneous value of the power inductor current of phase A is i La (t) and the lower limit value of phase A current i low_a (t) Real-time comparison, when i La (t) not greater than i low_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached, entering the next cycle of control.

[0131] Furthermore, for phase B, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase B i Lb (t) and the upper limit value of phase B current i high_b (t) Real-time comparison, when i Lb (t) not less than i high_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached, and the control of the next cycle has begun. In the negative half cycle of the grid voltage, the instantaneous value of the B phase power inductor current i Lb (t) and the lower limit value of phase B current i low_b (t) Real-time comparison, when i Lb (t) not greater than i low_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached, entering the next cycle of control.

[0132] Furthermore, for phase C, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase C, i Lc (t) and the upper limit value of phase C current i high_c (t) Real-time comparison, when i Lc (t) not less than i high_c (t) represents the turn-off time T of the C-phase switch off_c (t) has been reached, and the control of the next cycle has begun. In the negative half cycle of the grid voltage, the instantaneous value of the C phase power inductor current i Lc (t) and the lower limit value of phase C current i low_c(t) Real-time comparison, when i Lc (t) not greater than i low_c (t) represents the turn-off time T of the C-phase switch off_c (t) has been reached, entering the next cycle of control.

[0133] Please continue to refer to Figure 2 The present invention also provides a control circuit for a DC / AC converter, which utilizes the above-mentioned control method for a DC / AC converter. More specifically, the control circuit 22 of the DC / AC converter includes a voltage sampling module 221, a reference current calculation module 222, a current reference generation module 223, and an on-time calculation module 224. The voltage sampling module 221 samples the grid voltage and outputs it to the reference current calculation module 222. The reference current calculation module 222 calculates a reference current based on the grid voltage and outputs it to the current reference generation module 223. The current reference generation module 223 calculates a current upper limit and a current lower limit based on the reference current and outputs them to the on-time calculation module 224. The on-time calculation module 224 calculates the on-time based on the current upper limit and the current lower limit.

[0134] Furthermore, the control circuit 22 of the DC / AC converter also includes a current sampling module 225, a first comparison module 226, a second comparison module 227, a first pulse generation module 228, and a second pulse generation module 229. The current sampling module 225 samples the instantaneous value of the power inductor current and outputs it to the first comparison module 226 and the second comparison module 227. The first comparison module 226 compares the instantaneous value of the power inductor current with the current upper limit and outputs the turn-off time to the first pulse generation module 228. The second comparison module 227 compares the instantaneous value of the power inductor current with the current lower limit and outputs the turn-off time to the second pulse generation module 229. The turn-on time calculation module 224 outputs the turn-on time to the first pulse generation module 228 and the second pulse generation module 229. The first pulse generation module 228 and the second pulse generation module 229 generate a drive signal for the switch in the DC / AC converter based on the turn-on time and the turn-off time.

[0135] for Figure 2 In the three-phase DC / AC converter, the voltage sampling module 221 samples the instantaneous value u of the A-phase grid voltage. a (t), instantaneous value of B phase grid voltage u b (t), instantaneous value of C phase grid voltage u c (t) is output to the reference current calculation module 222, which calculates the A-phase reference current i according to the three-phase grid voltage. a_ref (t), B phase reference current i b_ref (t), C phase reference current i c_ref(t) is output to the current reference generation module 223, which calculates the A-phase current upper limit value i according to the three-phase reference current. high_a (t), A phase current lower limit value i low_a (t), B phase current upper limit value i high_b (t), B phase current lower limit value i low_b (t), C phase current upper limit value i high_c (t), C phase current lower limit value i low_c (t) is output to the opening time calculation module 224, which calculates the opening time T of the A-phase switch. on_a (t), B phase switch opening time T on_b (t), C phase switch opening time T on_c (t) is output to the first pulse generating module 228 and the second pulse generating module 229.

[0136] Furthermore, the current sampling module 225 samples the instantaneous value i of the A-phase power inductor current. La (t), the instantaneous value of the B-phase power inductor current i Lb (t), the instantaneous value of the C-phase power inductor current i Lc (t). In the positive half cycle of the grid voltage, the first comparison module 226 compares the instantaneous value of the A-phase power inductor current i La (t) and the upper limit value of phase A current i high_a (t) Real-time comparison, when i La (t) not less than i high_a(t) When T is the off time of phase A off_a (t) has been reached, the output signal is sent to the first pulse generating module 228; in the positive half cycle of the grid voltage, the first comparing module 226 compares the instantaneous value of the B-phase power inductor current i Lb (t) and the upper limit value of phase B current i high_b (t) Real-time comparison, when i Lb (t) not less than i high_b (t) indicates the turn-off time T of phase B off_b (t) has been reached, the output signal is sent to the first pulse generating module 228; in the positive half cycle of the grid voltage, the first comparing module 226 compares the instantaneous value of the C-phase power inductor current i Lc (t) and the upper limit value of phase C current i high_c (t) Real-time comparison, when i Lc (t) not less than i high_c (t) represents the turn-off time T of phase C off_c (t) has been reached, and the output signal is sent to the first pulse generating module 228. In the negative half cycle of the grid voltage, the second comparing module 227 compares the instantaneous value of the A-phase power inductor current i La(t) and the lower limit value of phase A current i low_a (t) Real-time comparison, when i La (t) not greater than i low_a (t) indicates the turn-off time T of phase A off_a (t) has been reached, the output signal is sent to the second pulse generating module 229; in the negative half cycle of the grid voltage, the second comparing module 227 compares the instantaneous value of the B-phase power inductor current i Lb (t) and the lower limit value of phase B current i low_b (t) Real-time comparison, when i Lb (t) not greater than i low_b (t) indicates the turn-off time T of phase B off_b (t) has been reached, the output signal is sent to the second pulse generating module 229; in the negative half cycle of the grid voltage, the second comparing module 227 compares the instantaneous value of the C-phase power inductor current i Lc (t) and the lower limit value of phase C current i low_c (t) Real-time comparison, when i Lc (t) not greater than i low_c (t) represents the turn-off time T of phase C off_c (t) has been reached, and a signal is output to the second pulse generating module 229. The first pulse generating module 228 and the second pulse generating module 229 generate driving signals G21 and G22 of the switches in the DC / AC converter according to the on-time and off-time.

[0137] Furthermore, since the inductor current is measured by the analog comparator for the off time T off (t) is controlled, and the gain of the analog comparator is very high, so the response speed of the inductor current loop is very fast, and theoretically it can respond within one switching cycle.

[0138] In addition, in the design of DC / AC converters, a fixed frequency control scheme is generally adopted. Fixed frequency control is to constantly control the switching frequency, the switching period is fixed, and the on-time T is adjusted. on (t) realizes control, does not control the off time T off (t). The bandwidth of the PI regulator used is very low, typically one-tenth of the switching frequency. Due to the limited gain of the PI regulator, this results in a slow current loop response and poor inductor current dynamic response. The inductor current loop of the technical solution of the present invention does not include a PI regulator, thus avoiding the above problems.

[0139] Figure 3 for Figure 2Schematic diagram of voltage, current, and switch drive waveforms of phase A in a specific embodiment. Taking phase A as an example, the first pulse generation module 228 and the second pulse generation module 229 generate drive signals for the switches in the DC / AC converter based on the on-time and off-time. Specifically, in the positive half cycle of the grid voltage, switch Q11 remains on, switch Q6 remains off, and switches Q5 and Q12 are switched at high frequency. The on-time of switch Q5 is T on_a (t), the off time of switch Q5 is T off_a (t), the driving signals of switch Q12 and switch Q5 are opposite. In the negative half cycle of the grid voltage, switch Q12 remains on, switch Q5 remains off, and switch Q6 and switch Q11 are switched at high frequency. The on time of switch Q6 is T on_a (t), the off time of switch Q6 is T off_a (t), the driving signals of switch Q11 and switch Q6 are opposite.

[0140] Furthermore, the drive signal generation for the switches in phases B and C is similar to that for phase A and will not be further described here. Specifically, the drive waveform for the switches in phase B is 120° out of phase with the drive signal for the corresponding switches in phase A, and the drive signal for the switches in phase C is 240° out of phase with the drive signal for the corresponding switches in phase A.

[0141] It should be noted that the DC / AC converter referred to in the present invention can convert electric energy from DC to AC, and also can convert electric energy from AC to DC.

[0142] It should be noted that the present invention provides a DC / AC converter and a control method thereof, including but not limited to Figure 1 and Figure 2 The circuit topology shown in the embodiment is also applicable to other circuit topologies that can realize the conversion of electric energy from AC to DC or from DC to AC, and the present invention is not limited thereto.

[0143] Although the present invention has been disclosed above with reference to the embodiments, they are not intended to limit the present invention. Anyone with ordinary skill in the art may make slight changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope of the appended patent applications.

Claims

1. A control method for a DC / AC converter, characterized in that: Including, the on time of the switch in the DC / AC converter is realized by digital control, the off time of the switch is realized by analog control, and the drive signal of the switch is generated according to the on time and the off time.

2. A control method for a DC / AC converter according to claim 1, characterized in that: The on-time of the switch is realized by digital control, including collecting the voltage across the power inductor and calculating the voltage drop V across the power inductor. L (t), the opening time T is calculated according to the following formula on (t): T on (t)=L*Δi(t) / V L (t), Where L is the inductance of the power inductor, Δi(t) is the inductor current ripple; The expression of inductor current ripple Δi(t) is as follows: Δi(t)=i high (t)―i low (t) Among them, i high (t) is the upper limit of current, i low (t) is the lower limit of current; Current upper limit i high (t), current lower limit i low (t) is calculated as follows: i high (t)=i ref (t)+I h cosωt+I B i low (t)=i ref (t)―I h cosωt―I B Among them, I h is the hysteresis loop width; I B is the bias current; i ref (t) is the reference current; ω is the angular frequency; t is the time; Reference current i ref The expression of (t) is as follows: Among them, S ref is the apparent power; V acrms is the effective value of alternating current; is the phase difference calculated based on the power factor.

3. A control method for a DC / AC converter according to claim 1, characterized in that: The on-time of the switch is achieved through digital control, which includes collecting the voltage across the power inductor and calculating the voltage drop across the power inductor. For a three-phase DC / AC converter, the on-time of each phase switch is expressed as follows: T on_a (t)=La*Δi_a(t) / V La (t), T on_b (t)=Lb*Δi_b(t) / V Lb (t), T on_c (t)=Lc*Δi_c(t) / V Lc (t), Among them, V La (t) is the voltage drop across the A-phase power inductor La, T on_a (t) is the opening time of the A-phase switch; V Lb (t) is the voltage drop across the B-phase power inductor Lb, T on_b (t) is the turn-on time of the B-phase switch; V Lc (t) is the voltage drop across the C-phase power inductor Lc, T on_c (t) is the turn-on time of the phase C switch; Δi_a(t) is the phase A inductor current ripple, Δi_b(t) is the phase B inductor current ripple, and Δi_c(t) is the phase C inductor current ripple; The inductor current ripple of each phase is expressed as follows: Δi_a(t)=i high_a (t)―i low_a (t) Δi_b(t)=i high_b (t)―i low_b (t) Δi_c(t)=i high_c (t)―i low_c (t) Among them, i high_a (t) is the upper limit of phase A current, i low_a (t) is the lower limit of phase A current, i high_b (t) is the upper limit of phase B current, i low_b (t) is the lower limit of phase B current, i high_c (t) is the upper limit of phase C current, i low_c (t) is the lower limit of phase C current; The upper and lower current limits of each phase are expressed as follows: i high_a (t)=i a_ref (t)+I h cosωt+I B i low_a (t)=i a_ref (t)―I h cosωt―I B i high_b (t)=i b_ref (t)+I h cosωt+I B i low_b (t)=i b_ref (t)―I h cosωt―I B i high_c (t)=i _ref (t)+I h cosωt+I B i low_c (t)=i c_ref (t)―I h cosωt―I B Among them, I h is the hysteresis loop width; I B is the bias current; i a_ref (t) is the reference current of phase A, i b_ref (t) is the reference current of phase B, i c_ref (t) is the reference current of phase C; ω is the angular frequency; t is the time.

4. A control method for a DC / AC converter according to claim 2 or 3, characterized in that: The on-time of the switch is achieved through digital control, including reducing the on-time to a set value when the AC voltage and DC voltage are close.

5. A control method for a DC / AC converter as claimed in claim 4, characterized in that: The switch off time is realized by analog control, including sampling the instantaneous value of the power inductor current i L (t), in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current upper limit i high (t) Real-time comparison, when i L Not less than i high (t) indicates the off time T off (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the power inductor current i L (t) and the current lower limit i low (t) Real-time comparison, when i L (t) not greater than i low (t) indicates the off time T off (t) has been reached, entering the next cycle of control.

6. A control method for a DC / AC converter as claimed in claim 4, characterized in that: For the three-phase DC / AC converter, the switch off time is realized by analog control, including, for phase A, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current i La (t) and the upper limit value of phase A current i high_a (t) Real-time comparison, when i La (t) not less than i high_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the A-phase power inductor current i La (t) and the lower limit value of phase A current i low_a (t) Real-time comparison, when i La (t) not greater than i low_a (t) represents the turn-off time T of the A-phase switch off_a (t) has been reached and enters the next cycle of control; For phase B, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase B is i Lb (t) and the upper limit value of phase B current i high_b (t) Real-time comparison, when i Lb (t) not less than i high_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the B phase power inductor current i Lb (t) and the lower limit value of phase B current i low_b (t) Real-time comparison, when i Lb (t) not greater than i low_b (t) represents the turn-off time T of the B-phase switch off_b (t) has been reached and enters the next cycle of control; For phase C, in the positive half cycle of the grid voltage, the instantaneous value of the power inductor current of phase C is i Lc (t) and the upper limit value of phase C current i high_c (t) Real-time comparison, when i Lc (t) not less than i high_c (t) represents the turn-off time T of the C-phase switch off_c (t) has been reached, and the control of the next cycle begins; in the negative half cycle of the grid voltage, the instantaneous value of the C phase power inductor current i Lc (t) and the lower limit value of phase C current i low_c (t) Real-time comparison, when i Lc (t) not greater than i low_c (t) represents the turn-off time T of the C-phase switch off_c (t) has been reached, entering the next cycle of control.

7. A control circuit for a DC / AC converter, characterized in that: A control method for a DC / AC converter according to any one of claims 1 to 6 is applied, comprising a voltage sampling module, a reference current calculation module, a current reference generation module, and an on-time calculation module, wherein the voltage sampling module samples the grid voltage and outputs it to the reference current calculation module; the reference current calculation module calculates a reference current based on the grid voltage and outputs it to the current reference generation module; the current reference generation module calculates a current upper limit value and a current lower limit value based on the reference current and outputs them to the on-time calculation module; and the on-time calculation module calculates the on-time based on the current upper limit value and the current lower limit value.

8. A control circuit for a DC / AC converter as claimed in claim 7, characterized in that: It also includes a current sampling module, a first comparison module, a second comparison module, a first pulse generation module and a second pulse generation module. The current sampling module samples the instantaneous value of the power inductor current and outputs it to the first comparison module and the second comparison module. The first comparison module compares the instantaneous value of the power inductor current with the current upper limit value and outputs the turn-off time to the first pulse generation module. The second comparison module compares the instantaneous value of the power inductor current with the current lower limit value and outputs the turn-off time to the second pulse generation module. The turn-on time calculation module outputs the turn-on time to the first pulse generation module and the second pulse generation module. The first pulse generation module and the second pulse generation module generate a drive signal for the switch in the DC / AC converter according to the turn-on time and the turn-off time.

9. A DC / AC converter, characterized in that: A control method for a DC / AC converter according to any one of claims 1 to 6 is applied, wherein the DC / AC converter converts AC power into DC power or converts DC power into AC power.

10. A DC / AC converter according to claim 9, characterized in that: The DC / AC converter includes a single-phase DC / AC converter or a three-phase DC / AC converter.

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