Dc-dc circuit, control method thereof, and photovoltaic power generation system

Abstract

The application discloses a DC-DC circuit and a control method thereof and a photovoltaic power generation system. The DC-DC circuit comprises an inductor, a switch tube and a diode. The method comprises the following steps: acquiring an input voltage of the DC-DC circuit; determining an inductor current target value according to the acquired input voltage and a fitting curve of the input voltage and the inductor current; and controlling a switching action of the switch tube according to the inductor current target value. The fitting curve of the input voltage and the inductor current can be used to determine the inductor current target value corresponding to the input voltage, so that the switching action of the switch tube can be controlled, the problem of overheating damage of the switch tube is avoided, and the reliability of the system is improved.

Description

Technical Field

[0001] This application relates to the field of photovoltaic power generation technology, and in particular to a DC-DC circuit and its control method, and a photovoltaic power generation system. Background Technology

[0002] For converters with wide-range voltage input requirements, the power switching transistors experience inconsistent losses under different input voltages. For example... Figure 1 As shown, the photovoltaic power generation system uses a DC-DC unit for tracking MPPT (Maximum Power Point Tracking), with an input voltage u. in Generally, the voltage range can be from 200V to 1000V. However, due to factors such as sunlight, temperature, and photovoltaic cell string configuration, the input power of the DC-DC unit is inconsistent under different input voltage conditions. At this time, the loss of the power switch Q inside the DC-DC is also in an uncertain state. In the face of sudden weather changes or other situations with large power fluctuations, the power switch may overheat and be damaged.

[0003] Due to cost and other factors, NTC temperature measurement is generally used to monitor the overheating of power switching transistors. This solution can only monitor the steady-state thermal equilibrium of the power switching transistor; it cannot monitor the instantaneous junction temperature in real time. In other words, the solution cannot provide feedback on the transient junction temperature of the power switching transistor, and therefore cannot effectively protect against transient junction temperature failures. Summary of the Invention

[0004] This application provides a DC-DC circuit and its control method, as well as a photovoltaic power generation system, to avoid the problem of overheating and damage to the switching transistor in the DC-DC circuit.

[0005] One aspect of this application provides a control method for a DC-DC circuit, the DC-DC circuit including an inductor, a switching transistor, and a diode; the method includes:

[0006] Obtain the input voltage of the DC-DC circuit;

[0007] Based on the obtained input voltage and the fitting curve of input voltage and inductor current, the target value of inductor current is determined;

[0008] The switching action of the switching transistor is controlled according to the target value of the inductor current.

[0009] Another aspect of this application provides a DC-DC circuit, including an inductor, a switching transistor, and a diode; it also includes a controller configured to execute steps of a control method for implementing the DC-DC circuit.

[0010] Another aspect of this application provides a photovoltaic power generation system, the photovoltaic power generation system including photovoltaic modules and the DC-DC circuit.

[0011] The DC-DC circuit and its control method, as well as the photovoltaic power generation system provided in this application, can determine the target value of the inductor current corresponding to the input voltage by fitting the curve of the input voltage and the inductor current, thereby controlling the switching action of the switching transistor, avoiding the problem of overheating and damage to the switching transistor, and improving the reliability of the system. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a DC-DC circuit provided in an embodiment of this application;

[0013] Figure 2 A schematic diagram of the fitting curve of inductor current versus duty cycle provided for an embodiment of this application;

[0014] Figure 3 A schematic diagram of another fitting curve of inductor current versus duty cycle provided for an embodiment of this application;

[0015] Figure 4 A schematic diagram of the fitting curve of input voltage and inductor current provided for an embodiment of this application;

[0016] Figure 5 A schematic diagram of another input voltage versus inductor current fitting curve provided for an embodiment of this application;

[0017] Figure 6 This is a schematic diagram of a control method for a DC-DC circuit provided in an embodiment of this application.

[0018] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0020] In the description of this application, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," and "right," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] For ease of description, let's continue with... Figure 1 The following explanation uses a DC-DC circuit as an example. The DC-DC circuit includes an input capacitor C. in Inductor L, switching transistor Q, diode D, and output capacitor C o One end of the inductor L is the positive terminal of the input voltage (i.e., u). in The positive terminal of the inductor L is connected to the first terminal of the switching transistor Q and the anode terminal of the diode D; the second terminal of the switching transistor Q is either the negative terminal of the input voltage or the negative terminal of the output voltage (i.e., u). in The negative extreme or u o The negative terminal of diode D is the positive terminal of the output voltage (i.e., u). o The positive terminal of the transistor Q is used to receive the control signal from the controller, which can be used to control the switching transistor Q to turn on or off; the input capacitor C... in The output capacitor C is connected between the positive and negative terminals of the input voltage. o It is connected between the positive and negative terminals of the output voltage.

[0022] The current in inductor L is denoted as I. L The ripple current is ΔI L The input voltage is u in The output voltage is u o The duty cycle d of the switching transistor Q is defined by the following formula:

[0023]

[0024] The total loss P of the switching transistor Q loss It consists of two parts: switching loss P switch and on-state loss P on The specific calculation formula is as follows:

[0025] P loss =P switch +P on =f s (UI+I 2 R on (2)

[0026] Where f s U represents the switching frequency, I represents the voltage when the switch is on or off, and R represents the current when the switch is on or off. on This indicates the resistance when the switching transistor is turned on.

[0027] For the current I of the MOSFET Mos Inductor current I Land ripple current ΔI L The following calculation relationship exists between them.

[0028]

[0029] Substituting formula (3) into formula (2), we can obtain the functional relationship between the switching transistor loss, duty cycle, and inductor current, as shown below:

[0030] P loss =F(d,I L (4)

[0031] To prevent the switching transistor Q from overheating and being damaged, the maximum loss P of the switching transistor Q can be calculated using formula (4). Loss_max The switching transistor loss should not exceed the maximum loss P. Loss_max Based on this rule, the maximum allowable value of the duty cycle d under different currents can be calculated. Accordingly, a relationship curve between the duty cycle d and the inductor current can be established: d = F(I L If the entire system operates according to this set curve, the system can effectively protect the switching transistors.

[0032] A feasible relationship curve is as follows: Figure 2 As shown, the inductor current I L Throughout the entire range, there is a one-to-one correspondence with the duty cycle d. The overall machine design must meet the following requirements. Figure 2 The relationship curve shown ensures that the switching transistor Q will not overheat and be damaged. This relationship curve only sets a maximum limit for the inductor current I. L The curve of the duty cycle d can be in any constrained form, as long as the Q loss of the switching transistor does not exceed the maximum allowable loss.

[0033] Another feasible relationship curve is as follows: Figure 3 As shown. The requirements for the relationship curve are the same as those described above, and will not be elaborated here.

[0034] In another example, based on the relationship between the losses of the switch Q and the current flowing through the switch and the duty cycle d, the relationship between the switch losses and the switch current I can be obtained. Mos (Considering practical applications, the switching current I here) Mos Using inductor current I L (to express), input voltage u in and output voltage u o The relationship is as follows. Generally, the converter's output voltage is system-controlled and can remain stable. Changes in the input voltage determine the magnitude of the current allowed to flow through the switching transistor Q. Based on this, and following the calculation methods for duty cycle and current, the input voltage u can be designed during the design phase. in With the maximum allowable current I of the switching transistor QMos (Considering practical applications, the switching current I here) Mos Using inductor current I L To express the relationship curve between ( ), the curve is as follows: Figure 4 As shown.

[0035] Another feasible relationship curve is as follows: Figure 5 As shown, the main considerations are the upper limit of output power and the maximum power transistor current limit, which will result in the following... Figure 5 The current limit shown is for a given current limit. Of course, there are other similar current limit curves, such as piecewise or linear curves, which will not be listed here.

[0036] Based on the above principles, such as Figure 6 As shown, one embodiment of this application provides a control method for a DC-DC circuit, the method comprising the following steps:

[0037] S11. Obtain the input voltage of the DC-DC circuit;

[0038] S12. Determine the target value of the inductor current based on the obtained input voltage and the fitting curve of the input voltage and inductor current.

[0039] S13. Control the switching action of the switching transistor according to the target value of the inductor current.

[0040] In one example, based on the principle that the loss of the switching transistor is not greater than the maximum loss value of the switching transistor, the fitting curve of the input voltage and the inductor current is obtained.

[0041] In one example, the method includes:

[0042] Determine the maximum loss value of the switching transistor;

[0043] Multiple sets of input voltage and inductor current data are acquired, and a fitting curve of the input voltage and inductor current is obtained by fitting the multiple sets of input voltage and inductor current data.

[0044] In one example, determining the target value of the inductor current based on the acquired input voltage and the fitted curve of the input voltage versus the inductor current includes:

[0045] If the obtained input voltage is less than or equal to the minimum input voltage, the target value of the inductor current is the inductor current corresponding to the minimum input voltage.

[0046] If the obtained input voltage is greater than the minimum input voltage and less than the maximum input voltage, or if the obtained input voltage is greater than the minimum input voltage and less than the preset input voltage, then the target value of the inductor current is the inductor current corresponding to the input voltage obtained in the fitted curve.

[0047] If the obtained input voltage is greater than or equal to the maximum input voltage, or if the obtained input voltage is greater than or equal to the preset input voltage, then the target value of the inductor current is the inductor current corresponding to the maximum input voltage.

[0048] The preset input voltage is between the maximum input voltage and the minimum input voltage.

[0049] Please combine Figure 4 To understand this, when the input voltage u in Less than or equal to the minimum input voltage u inmin When the inductor current target value is the minimum input voltage u, then the inductor current target value is the minimum input voltage u. inmin The corresponding inductor current I Lmin When the input voltage u in Greater than the minimum input voltage u inmin And less than the maximum input voltage u inmax When the inductor current target value is Figure 4 The input voltage u in the fitted curve in The corresponding inductor current; when the input voltage u in Greater than or equal to the maximum input voltage u inmax When the inductor current target value is the maximum input voltage u, then the inductor current target value is the maximum input voltage u. inmax The corresponding inductor current I Lmax .

[0050] Please combine Figure 5 To understand this, when the input voltage u in Less than or equal to the minimum input voltage u inmin When the inductor current target value is the minimum input voltage u, then the inductor current target value is the minimum input voltage u. inmin The corresponding inductor current I Lmin When the input voltage u in Greater than the minimum input voltage u inmin And less than the preset input voltage u inm When the inductor current target value is Figure 5 The input voltage u in the fitted curve in The corresponding inductor current; when the input voltage u in Greater than or equal to the preset input voltage u inm When the inductor current target value is the maximum input voltage u, then the inductor current target value is the maximum input voltage u. inmax The corresponding inductor current I Lmax .

[0051] Another embodiment of this application provides a DC-DC circuit, the components of which and their connections are described above. The DC-DC circuit further includes a controller configured to execute the steps of the control method for the DC-DC circuit as described in the foregoing embodiments.

[0052] Another embodiment of this application provides a photovoltaic power generation system, which includes a photovoltaic module and the DC-DC circuit described in the foregoing embodiments. The voltage output terminal of the photovoltaic module is connected to the positive and negative terminals of the input voltage in the DC-DC circuit.

[0053] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. A control method for a DC-DC circuit, wherein the DC-DC circuit includes an inductor, a switching transistor, and a diode; characterized in that, The method includes: Obtain the input voltage of the DC-DC circuit; Based on the obtained input voltage and the fitting curve of input voltage and inductor current, the target value of inductor current is determined; The switching action of the switching transistor is controlled according to the target value of the inductor current; Based on the principle that the loss of the switching transistor is not greater than the maximum loss value of the switching transistor, the fitting curve of the input voltage and the inductor current is obtained; The method includes: Determine the maximum loss value of the switching transistor; Multiple sets of input voltage and inductor current data are acquired, and a fitting curve of the input voltage and inductor current is obtained by fitting the multiple sets of input voltage and inductor current data. The step of determining the target value of the inductor current based on the acquired input voltage and the fitting curve of the input voltage versus the inductor current includes: If the obtained input voltage is less than or equal to the minimum input voltage, the target value of the inductor current is the inductor current corresponding to the minimum input voltage. If the obtained input voltage is greater than the minimum input voltage and less than the maximum input voltage, or if the obtained input voltage is greater than the minimum input voltage and less than the preset input voltage, then the target value of the inductor current is the inductor current corresponding to the input voltage obtained in the fitted curve. If the obtained input voltage is greater than or equal to the maximum input voltage, or if the obtained input voltage is greater than or equal to the preset input voltage, then the target value of the inductor current is the inductor current corresponding to the maximum input voltage. The preset input voltage is between the maximum input voltage and the minimum input voltage.

2. A DC-DC circuit, comprising an inductor, a switching transistor, and a diode; characterized in that, It also includes a controller configured to perform the steps of the control method for the DC-DC circuit as described in claim 1.

3. The DC-DC circuit according to claim 2, characterized in that, One end of the inductor is the positive terminal of the input voltage, and the other end of the inductor is connected to the first terminal of the switching transistor and the anode terminal of the diode; the second terminal of the switching transistor is either the negative terminal of the input voltage or the negative terminal of the output voltage; the cathode terminal of the diode is the positive terminal of the output voltage; the control terminal of the switching transistor is used to receive the control signal from the controller.

4. The DC-DC circuit according to claim 3, characterized in that, The DC-DC circuit also includes an input capacitor and / or an output capacitor; The input capacitor is connected between the positive and negative terminals of the input voltage, and the output capacitor is connected between the positive and negative terminals of the output voltage.

5. A photovoltaic power generation system, characterized in that, The photovoltaic power generation system includes photovoltaic modules and a DC-DC circuit according to any one of claims 2-4.

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