Primary side current peak value calculation method and application thereof

Abstract

The invention provides a primary side current peak value calculation method and application thereof, the primary side current peak value calculation method can be applied to an electric appliance using a flyback circuit, and the electric appliance comprises a flyback switching power supply, a flyback circuit LED driver and a flyback micro inverter. Correspondingly, the influence of a half resonance period on a primary side current peak value is considered on the basis of a traditional BCM mode; and when n is greater than or equal to 3, the value of n can be automatically set according to the requirement of the flyback circuit on the switching frequency corresponding to the improved DCM mode, so that the switching frequency of the DCM mode is a variable-frequency DCM mode.

Description

Technical Field

[0001] The present invention relates to the technical field of primary current peak value calculation, and in particular to a primary current peak value calculation method and application thereof. Background Art

[0002] As the core grid-connected device of photovoltaic power generation system, inverter has a broad prospect for development. In order to meet different product requirements, academia and industry are also developing various types of photovoltaic inverters. Micro inverter has the advantages of high efficiency, high safety, strong adaptability and long life. Among them, flyback topology is widely used due to its simple structure, easy control and high energy conversion efficiency.

[0003] Energy conversion efficiency is the most important technical indicator of flyback micro inverters. Without increasing the control complexity and hardware cost, the conventional practice is to use the DCM / BCM hybrid mode to control the operation of the flyback circuit. The traditional DCM / BCM hybrid mode uses independent formulas to calculate the respective primary current peaks; in the BCM mode, the influence of the resonant period of the flyback circuit on the primary current peak is ignored; in the DCM mode, a fixed switching frequency is used, which cannot guarantee that the switch tube is always turned on at the bottom of the drain-source resonant voltage, that is, the switching loss of the switch tube cannot be minimized.

[0004] In order to improve the energy conversion efficiency of flyback micro-inverters, under the premise of saving hardware costs and not increasing the complexity of the control algorithm, most studies start from the characteristics of flyback micro-inverters themselves, and compare the control complexity and energy loss of the inverters in the three modes of DCM, CCM, and BCM, and usually use the control method of DCM / BCM hybrid mode. However, the traditional DCM and BCM modes use two different formulas to calculate the primary current peak, which leads to the control algorithm to switch between DCM and BCM modes at appropriate times when using the DCM / BCM hybrid mode to control the inverter; and the traditional DCM mode uses a constant switching frequency, and the drain-source voltage value is random when the switch tube is turned on, which cannot guarantee that the switch tube is turned on at the resonance valley of the drain-source voltage in each switching cycle, and the switching loss of the switch tube cannot be minimized; the traditional BCM mode does not consider the impact of the resonance cycle on the primary current peak, which will lead to poor harmonic distortion rate THD of the grid-connected current.

[0005] Therefore, the technical problem that needs to be solved urgently is: how to ensure that the switch tube is always turned on at the bottom of the drain-source resonant voltage while taking into account the influence of the resonant period of the flyback circuit on the primary current peak. Summary of the invention

[0006] The present invention is made to solve the above-mentioned problem, and aims to provide a primary current peak value calculation method and application thereof.

[0007] The present invention provides a primary current peak value calculation method, which has the following characteristics and specifically includes the following steps: first, assuming that the system power device is an ideal device, then the average value of the transformer secondary current in one cycle is equal to the instantaneous value of the grid-connected current, and the average value of the secondary current can be expressed as:

[0008]

[0009] In the formula, I g is the effective value of the grid-connected current, θ g is the grid fundamental voltage phase angle;

[0010] According to the transformer ratio, the relationship between the peak value of the transformer primary current and the average value of the transformer secondary current can be expressed as:

[0011]

[0012] In the formula, i p,peak (θ g ) is the peak value of the primary current of the transformer, N is the transformer turns ratio, t off (θ g ) is the time taken for the secondary current of the flyback transformer to drop to zero, T sw (θ g ) is the switching period;

[0013] Substituting the secondary current average value expression into the above formula, we can get:

[0014]

[0015] In the traditional DCM mode, the switching frequency of the switch tube is DCM (θ g ) is a fixed value, and accordingly T DCM (θ g ) is a fixed constant; the switching frequency f of the traditional BCM mode BCM (θ g ) is:

[0016]

[0017] Where P o is the transformer secondary output power, V pv is the primary input voltage of the flyback circuit, V g is the effective value of the grid voltage, T BCM (θ g ) is the switching cycle in BCM mode,

[0018] Consider the resonant period of the flyback circuit and denote the half-resonant period as T R , whose expression is:

[0019]

[0020] Where, L m is the primary inductance of the transformer, C oss Q is the switch tube M The parasitic capacitance between the drain and source,

[0021] Without distinguishing between DCM and BCM modes, the switching cycle of the switch tube is uniformly defined as:

[0022] T sw =t on +t off +n·T R (n is an odd number)

[0023] The switching period T sw Substitute i p,peak (θ g ) is calculated as follows:

[0024]

[0025] The time required for the primary current of the flyback transformer to rise to the peak value and the time required for the secondary current to drop to zero are expressed as:

[0026]

[0027] Substitute the above formula into i p,peak (θ g ) can be calculated by p,peak (θ g ) of the quadratic equation:

[0028]

[0029] According to the formula for solving quadratic equations, we can get:

[0030]

[0031] In the formula,

[0032]

[0033] The method for calculating the primary current peak value of a flyback micro-inverter provided by the present invention may also have the following characteristics: wherein, in the expression of the method for calculating the primary current peak value, when n is 1, it corresponds to taking into account the influence of half a resonant cycle on the primary current peak value on the basis of the traditional BCM mode; when n is greater than or equal to 3, it corresponds to the improved DCM mode, and the value of n can be set according to the demand of the flyback circuit for the switching frequency, thereby, the switching frequency of the DCM mode is a variable frequency DCM mode.

[0034] The present invention also provides an application of the primary current peak value calculation method in an electrical appliance, wherein the electrical appliance is an electrical appliance using a flyback circuit.

[0035] In the application of the primary current peak value calculation method provided by the present invention in an electrical appliance, the electrical appliance using the flyback circuit is a flyback micro inverter.

[0036] In the application of the primary current peak value calculation method provided by the present invention in electrical appliances, the electrical appliances using the flyback circuit also include a flyback switching power supply and a flyback circuit LED driver.

[0037] Functions and Effects of the Invention

[0038] The primary current peak value calculation method and application thereof involved in the present invention are mainly applied to flyback micro-inverters. The calculation method of the present invention does not distinguish the working mode of the flyback circuit, and uses a set of formulas to calculate the primary current peak values ​​in DCM and BCM modes, which can reduce the complexity of the flyback circuit control algorithm; the influence of the resonant period of the flyback circuit on the switching period in DCM and BCM modes is taken into account, which can reduce the harmonic distortion rate THD of the grid-connected current of the flyback micro-inverter and improve the quality of the grid-connected current; the switching period of the DCM and BCM modes is limited by an odd number of half-resonant periods, which ensures that the switch tube can be turned on at the resonant valley bottom of the drain-source voltage in each switching period, which can minimize the switching loss of the flyback circuit and improve the energy conversion efficiency of the flyback circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a structural schematic diagram of a single-channel flyback micro-inverter in the prior art;

[0040] Figure 2 The embodiment of the present invention is different from the conventional method in DCM mode. DS A schematic diagram comparing the waveform with the primary current peak waveform; and

[0041] Figure 3 The V of the embodiment of the present invention and the conventional method under BCM DS Schematic diagram of the comparison between the waveform and the primary current peak waveform. DETAILED DESCRIPTION

[0042] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or mutual communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0043] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the following embodiments and the accompanying drawings specifically illustrate the primary current peak value calculation method and its application of the present invention.

[0044] Figure 1 It is a structural schematic diagram of a single-channel flyback micro-inverter in the prior art.

[0045] like Figure 1 As shown in the figure, the single-channel flyback micro inverter consists of three parts: the primary circuit, the transformer T, and the secondary circuit. The primary circuit contains an electrolytic capacitor C in , photovoltaic modules and main switch MOS tube Q M , its connection method is: electrolytic capacitor C in Connected in parallel at both ends of the photovoltaic module, the electrolytic capacitor C in The positive pole is connected to the positive pole of the photovoltaic module and to one end of the primary winding of the transformer T. The other end of the primary winding is connected in series with the main switch tube Q. M The drain stage, the main switch tube Q M The source stage is connected to the electrolytic capacitor C in The secondary circuit includes a rectifier diode D M , secondary side decoupling capacitor C o , an H-bridge inverter circuit consisting of four MOS tubes Q1, Q2, Q3, and Q4, and a CL filter circuit consisting of a filter capacitor Cf and a filter inductor Lf. The connection method is: the opposite end of the transformer T is connected to the rectifier diode D M The positive pole is connected to the decoupling capacitor C o Connected in parallel at both ends of the secondary side of transformer T, one end is connected to the rectifier diode D MThe negative pole, the other end is connected to the same-name terminal of the secondary side of the transformer T, and then connected in parallel with the H-bridge inverter circuit. The drains of Q1 and Q4 of the H-bridge are connected, the source of Q1 is connected to the drain of Q2, the sources of Q2 and Q3 are connected, the source of Q4 is connected to the drain of Q3, the filter capacitor Cf is connected in parallel between the source of Q1 and the drain of Q3, one end of the filter inductor Lf is connected to one end of the filter capacitor Cf, and the other end is connected to one end of the power grid Vg, and the other end of the power grid Vg is connected to the other end of the filter capacitor Cf and the drain of Q3. Among them, Q1 and Q3 of the H-bridge are diagonal, Q2 and Q4 are diagonal, and in the H-bridge control strategy, the diagonals are synchronously turned on.

[0046] The primary current peak value calculation method of the present invention can be used in application scenarios where a flyback circuit can be used, including flyback switching power supplies, flyback circuit LED drivers, especially flyback micro inverters, motor drivers and other scenarios. Once the BCM and DCM modes in the flyback circuit are used, the primary current peak value calculation method considering the resonance time described in the present invention can be used to achieve the purpose of optimizing the system control algorithm; reducing the current harmonic distortion rate THD; reducing the system switching loss and improving the energy conversion efficiency.

[0047] Taking the flyback micro inverter as an example, the calculation method of the primary current peak value of the flyback micro inverter is as follows:

[0048] First, assuming that the system power devices are ideal devices, the average value of the transformer secondary current in one cycle is equal to the instantaneous value of the grid-connected current. The average value of the secondary current can be expressed as:

[0049]

[0050] In the formula, I g is the effective value of the grid-connected current, θ g is the grid fundamental voltage phase angle;

[0051] According to the transformer ratio, the relationship between the peak value of the transformer primary current and the average value of the transformer secondary current can be expressed as:

[0052]

[0053] In the formula, i p,peak (θ g ) is the peak value of the primary current of the transformer, N is the transformer turns ratio, t off (θ g ) is the time taken for the secondary current of the flyback transformer to drop to zero, T sw (θ g ) is the switching period;

[0054] Substituting the secondary current average value expression into the above formula, we can get:

[0055]

[0056] In the traditional DCM mode, the switching frequency of the switch tube is DCM (θ g ) is a fixed value, and accordingly T DCM (θ g ) is a fixed constant; the switching frequency f of the traditional BCM mode BCM (θ g ) is:

[0057]

[0058] Where P o is the transformer secondary output power, V pv is the primary input voltage of the flyback circuit, V g is the effective value of the grid voltage, T BCM (θ g ) is the switching cycle in BCM mode.

[0059] Consider the resonant period of the flyback circuit and denote the half-resonant period as T R , whose expression is:

[0060]

[0061] Where, L m is the primary inductance of the transformer, C oss Q is the switch tube M Parasitic capacitance between drain and source.

[0062] In the present invention, no distinction is made between DCM and BCM modes, and the switching cycle of the switch tube is uniformly defined as:

[0063] T sw =t on +t off +n·T R (n is an odd number).

[0064] The switching period T sw Substitute i p,peak (θ g ) is calculated as follows:

[0065]

[0066] The time required for the primary current of the flyback transformer to rise to the peak value and the time required for the secondary current to drop to zero can be expressed as:

[0067]

[0068] Substitute the above formula into i p,peak (θ g) can be calculated by p,peak (θ g ) of the quadratic equation:

[0069]

[0070] According to the formula for solving quadratic equations, we can get:

[0071]

[0072] In the formula,

[0073]

[0074] The primary current peak expression obtained above, when n is 1, corresponds to the traditional BCM mode taking into account the influence of half a resonant cycle on the primary current peak; when n is greater than or equal to 3, it corresponds to the improved DCM mode. Since the value of n can be set according to the switching frequency requirements of the flyback circuit, this means that the switching frequency of the DCM mode is not constant at this time, and it is a variable frequency DCM mode.

[0075] Figure 2 The embodiment of the present invention is different from the conventional method in DCM mode. DS Schematic diagram of the comparison between the waveform and the primary current peak waveform. Figure 3 The V of the embodiment of the present invention and the conventional method under BCM DS Schematic diagram of the comparison between the waveform and the primary current peak waveform.

[0076] like Figure 2-3 As shown, the waveforms in the DCM mode and the BCM mode in the embodiments of the present invention are different from the prior art in the envelopes of the primary currents; the voltage of the DCM mode waveform when turned on is the resonant valley voltage, which is different from the random voltage of the prior art, and the voltage of the BCM mode waveform when turned on is the resonant valley voltage, which is different from the resonant peak voltage of the prior art.

[0077] Functions and Effects of the Embodiments

[0078] The primary current peak value calculation method and application thereof involved in the present invention are mainly applied to flyback micro-inverters. The calculation method of the present invention does not distinguish the working mode of the flyback circuit, and uses a set of formulas to calculate the primary current peak values ​​in DCM and BCM modes, which can reduce the complexity of the flyback circuit control algorithm; the influence of the resonant period of the flyback circuit on the switching period in DCM and BCM modes is taken into account, which can reduce the harmonic distortion rate THD of the grid-connected current of the flyback micro-inverter and improve the quality of the grid-connected current; the switching period of the DCM and BCM modes is limited by an odd number of half-resonant periods, which ensures that the switch tube can be turned on at the resonant valley bottom of the drain-source voltage in each switching period, which can minimize the switching loss of the flyback circuit and improve the energy conversion efficiency of the flyback circuit.

[0079] The primary current peak value calculation method of the present invention is not limited to application in flyback micro-inverters, but can be used in other application scenarios where flyback circuits can be used, such as flyback switching power supplies, flyback circuit LED drivers, motor drivers, etc. Once the BCM and DCM modes in the flyback circuit are used, the primary current peak value calculation method considering the resonance time described in the present invention can be used to achieve the purpose of optimizing the system control algorithm; reducing the current harmonic distortion rate THD; reducing the system switching loss and improving the energy conversion efficiency.

[0080] Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for calculating the peak value of primary current, characterized in that: The specific steps include: First, assuming that the system power devices are ideal devices, the average value of the transformer secondary current in one cycle is equal to the instantaneous value of the grid-connected current. The average value of the secondary current can be expressed as: In the formula, I g is the effective value of the grid-connected current, θ g is the grid fundamental voltage phase angle; According to the transformer ratio, the relationship between the peak value of the transformer primary current and the average value of the transformer secondary current can be expressed as: In the formula, i p,peak (θ g ) is the peak value of the primary current of the transformer, N is the transformer turns ratio, t off (θ g ) is the time taken for the secondary current of the flyback transformer to drop to zero, T sw (θ g ) is the switching period; Substituting the secondary current average value expression into the above formula, we can get: In the traditional DCM mode, the switching frequency of the switch tube is DCM (θ g ) is a fixed value, and accordingly T DCM (θ g ) is a fixed constant; the switching frequency f of the traditional BCM mode BCM (θ g ) is: Where P o is the transformer secondary output power, V pv is the primary input voltage of the flyback circuit, V g is the effective value of the grid voltage, T BCM (θ g ) is the switching cycle in BCM mode, Consider the resonant period of the flyback circuit and denote the half-resonant period as T R , whose expression is: Where, L m is the primary inductance of the transformer, C oss Q is the switch tube M The parasitic capacitance between the drain and source, Without distinguishing between DCM and BCM modes, the switching cycle of the switch tube is uniformly defined as: T sw =t on +t off +n·T R (n is an odd number) The switching period T sw Substitute i p,peak (θ g ) is: The time required for the primary current of the flyback transformer to rise to the peak value and the time required for the secondary current to drop to zero are expressed as: Substitute the above formula into i p,peak (θ g ) can be calculated by p,peak (θ g ) of the quadratic equation: According to the formula for solving quadratic equations, we can get: In the formula, a＝1, 2. The primary current peak value calculation method according to claim 1, characterized in that: in, In the expression of the primary current peak calculation method, when n is 1, it corresponds to the traditional BCM mode in which the influence of half a resonant cycle on the primary current peak is taken into account; when n is greater than or equal to 3, it corresponds to the improved DCM mode, and the value of n can be set according to the switching frequency requirement of the flyback circuit. Therefore, the switching frequency of the DCM mode is a variable frequency DCM mode.

3. An application of the primary current peak value calculation method as claimed in claim 1 in an electrical appliance, wherein the electrical appliance is an electrical appliance using a flyback circuit.

4. The application of the primary current peak value calculation method according to claim 3 in electrical appliances is characterized in that: The electrical appliance using the flyback circuit is a flyback micro inverter.

5. The application of the primary current peak value calculation method according to claim 3 in electrical appliances is characterized in that: The electrical appliances using the flyback circuit also include a flyback switching power supply and a flyback circuit LED driver.

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