Three-port DC-DC converter applied to photovoltaic power generation system

Abstract

The utility model relates to a three-port DC-DC converter applied to a photovoltaic power generation system. The three-port DC-DC converter comprises a main loop Buck circuit, a battery charging and discharging Buck / Boost circuit and a PID control module. According to the utility model, the main loop Buck circuit and the battery charging and discharging Buck / Boost circuit are connected in parallel, the main loop Buck circuit is connected with output ports of renewable energy sources such as a photovoltaic battery through a chopper circuit, and the battery charging and discharging Buck / Boost circuit is connected with an energy storage system such as an energy storage battery through a bidirectional DC-DC circuit. The PID control module carries out proportional-integral adjustment by collecting related current signals or voltage signals to obtain corresponding control signals, and automatic switching of voltage-stabilizing output and charging and discharging modes is achieved. The three-port converter can solve the problems that an existing three-port converter is complex in system, cumbersome in control, large in number of switch elements and not beneficial to improving power quality and density.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a three-port DC-DC converter applied to a photovoltaic power generation system. Background Art

[0002] In the prior art, power electronic converters are usually used to convert electricity from renewable energy into electricity that meets load demand and grid requirements. With the increasing depletion of fossil energy and the increasing awareness of environmental protection, the application of renewable energy such as solar energy and wind energy is becoming more and more widespread. In order to convert electricity from renewable energy into stable electricity that meets load demand, power electronic converters are needed.

[0003] The traditional power electronic converter structure usually includes multiple DC converters connected to the DC bus. However, this structure has disadvantages such as complex circuits and a large number of components. In addition, the traditional converter structure may not be able to efficiently combine various ports such as renewable energy power generation facilities, nor can it maximize the use of photovoltaic power. In order to overcome these shortcomings, a three-port converter is proposed. By using a three-port converter to connect multiple ports, the number of switching tubes can be greatly reduced, thereby increasing power density and reducing system costs. In addition, the three-port converter can also realize power management and control of the input source, energy storage facilities and load ports, simplifying the system structure and reducing costs.

[0004] The three-port converters in the prior art mainly include three types: non-isolated, partially isolated and fully isolated. Although the non-isolated converter has higher power density and efficiency, it lacks electrical isolation, has potential safety hazards, and has low voltage gain effect, which limits its scope of application. Although the partially isolated converter has solved the problem of electrical isolation to a certain extent, the system is complex, the control is cumbersome, and the number of switching elements is large, which is not conducive to improving power density and efficiency. Although the fully isolated converter has high safety, it has low integration, complex system, and a large number of switching elements, which is also not conducive to improving power density and efficiency. Summary of the invention

[0005] The purpose of this application is to provide a non-isolated three-port DC-DC converter for photovoltaic power generation systems to solve the problems of the existing three-port converter system being complex, cumbersome to control, having a large number of switching elements, and being unfavorable for improving power quality and density.

[0006] The technical solution adopted by the utility model is: a three-port DC-DC converter applied to a photovoltaic power generation system, used for connecting a first external power supply, a second external power supply and a load, comprising a main loop Buck circuit, a battery charging and discharging Buck / Boost circuit and a PID control module; the main loop Buck circuit is a chopper circuit, used for step-down conversion, so that the first external power supply can match the voltage requirement of the load, the input end of the main loop Buck circuit serves as the first port of the three-port DC-DC converter, the first port is the input port, used for connecting the first external power supply, the output end of the main loop Buck circuit is connected to the battery charging and discharging Buck / Boost circuit, and at the same time serves as the second port of the three-port DC-DC converter, the second port is the output port, used for connecting the load The battery charge and discharge Buck / Boost circuit is used to perform step-up and step-down conversion, manage the charge and discharge of the second external power supply, and enable the second external power supply to match the voltage requirement of the load. The first connection end of the battery charge and discharge Buck / Boost circuit is connected to the output end of the main loop Buck circuit, and the second connection end serves as the third port of the three-port DC-DC converter. The third port is a bidirectional port for connecting the second external power supply. The PID control module is connected to the main loop Buck circuit and the battery charge and discharge Buck / Boost circuit, and is used to collect voltage and current signals of the first external power supply and the battery charge and discharge Buck / Boost circuit, and generate a control signal to control the operation of the main loop Buck circuit and the battery charge and discharge Buck / Boost circuit.

[0007] Furthermore, the main loop Buck circuit includes a first resistor, a second resistor, a third resistor, a first capacitor, a second capacitor, a first inductor, a first transistor, a first diode and a second diode; after one end of the first capacitor is connected in series with one end of the first resistor, the other end of the first capacitor and the other end of the first resistor serve as the input end of the main loop Buck circuit; the collector of the first transistor is connected to the first capacitor; the cathode of the first diode is connected to the emitter of the first transistor and the second resistor, and the anode is connected to the first resistor and the third resistor and grounded; the second resistor, the first inductor, the second diode, the second capacitor and the third resistor are connected in series, the anode of the second diode is connected to the first inductor, the cathode is connected to the second capacitor, and the cathode of the second diode and one end of the third resistor connected to the anode of the first diode serve as the output end of the main loop Buck circuit.

[0008] Further, the battery charge and discharge Buck / Boost circuit includes a fifth resistor, a sixth resistor, a seventh resistor, a third capacitor, a second inductor, a second transistor and a third transistor; after one end of the third capacitor is connected in series with one end of the fifth resistor, the other end of the third capacitor is connected to the fifth resistor and the seventh resistor, the other end of the sixth resistor is grounded, and the other end of the fifth resistor and the other end of the sixth resistor serve as the second connection end of the battery charge and discharge Buck / Boost circuit; one end of the second inductor is connected to the seventh resistor, and the other end is connected to the collector of the second transistor and the emitter of the third transistor, and the emitter of the second transistor is connected to one end of the sixth resistor that is grounded; the collector of the third transistor and the emitter of the second transistor serve as the first connection end of the battery charge and discharge Buck / Boost circuit, wherein the collector of the third transistor is connected to the cathode of the second diode, and the emitter of the second transistor is connected to the third resistor.

[0009] Further, the PID control module includes a triangle wave generator 1, a triangle wave generator 2, a pulse generator, a difference amplifier 1, a difference amplifier 2, a difference amplifier 3, a comparator 1, a comparator 2, a comparator 3, a constant comparator, an SR latch, a switch 1 and a switch 2; the output voltage Vout of the second port and the reference value 1 signal are input to the difference amplifier 1, the output signal of the difference amplifier 1 and the triangle wave signal 1 emitted by the triangle wave generator 1 enter the comparator 1, the comparator 1 outputs the control signal PWMA of the first transistor and is connected to the base of the first transistor; the input voltage Vin of the first port is input to the constant comparator, and the constant comparator outputs a flag value signal; The current signal IB of the third port and the reference value 2 signal are input to the difference amplifier 2, and the triangular wave signal 2 emitted by the triangular wave generator 2 and the output signal of the difference amplifier 2 are input to the comparator 2; the output voltage Vout of the second port and the reference value 3 signal are also input to the difference amplifier 3, the current signal IL of the second transistor and the output signal of the difference amplifier 3 are input to the comparator 3, and the pulse signal emitted by the pulse generator and the output signal of the comparator 3 are input to the SR latch; the output signal of the comparator 2, the mark value signal output by the constant comparator and the reference value 4 signal are input to the switching switch 1, and the switching switch 1 outputs the control signal PWMBS of the third transistor and is connected to the base of the third transistor; the output signal of the SR latch, the mark value signal output by the constant comparator and the reference value 4 signal are input to the switching switch 2, and the switching switch 2 outputs the control signal PWMB of the second transistor and is connected to the base of the second transistor.

[0010] The beneficial effects of the utility model are:

[0011] (1) The utility model simplifies the overall structure of the power electronic converter, reduces the system complexity, and reduces the number of required components, thereby improving the reliability and maintainability of the system; at the same time, the utility model effectively improves the power conversion efficiency, reduces energy loss, and significantly improves the overall efficiency of the photovoltaic power generation system;

[0012] (2) The three-port design of the utility model allows efficient energy exchange between photovoltaic cells, battery energy storage and loads; the PID control module collects voltage and current signals in real time and generates control signals to accurately adjust the working state of the main loop Buck circuit and the battery charging and discharging Buck / Boost circuit, thereby realizing closed-loop voltage regulation control of the output voltage and maximizing the use of photovoltaic power;

[0013] (3) The utility model can simultaneously meet the needs of photovoltaic power generation, battery charging and discharging, and load power supply, thereby achieving flexibility in energy management; when the photovoltaic power is sufficient, the battery can be charged; when the photovoltaic power is insufficient, the system can automatically switch to battery power supply mode to ensure stable operation of the load. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present utility model, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0015] Figure 1 A circuit diagram of an embodiment of the utility model;

[0016] Figure 2 4 is a circuit diagram of a PID control module in an embodiment of the present utility model. DETAILED DESCRIPTION

[0017] In order to more clearly understand the above-mentioned purpose, features and advantages of the utility model, the utility model is further described in detail below in conjunction with the accompanying drawings and specific embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the utility model, but the utility model can also be implemented in other ways different from those described herein, and therefore, the utility model is not limited to the limitations of the specific embodiments disclosed below.

[0018] Unless otherwise defined, the technical or scientific terms used herein shall have the usual meanings understood by persons of ordinary skill in the field described in this application. The words "first", "second" and similar terms used in this patent application specification and claims do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "one" or "an" do not indicate a quantity limitation, but rather indicate the existence of at least one. Words such as "connected" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship also changes accordingly.

[0019] like Figure 1 As shown, a three-port DC-DC converter applied to a photovoltaic power generation system, the three-port DC-DC converter is a non-isolated DC-DC converter, used to connect a first external power source, a second external power source and a load. The three-port DC-DC converter includes a main loop Buck circuit, a battery charge and discharge Buck / Boost circuit and a PID control module; the main loop Buck circuit is a chopper circuit, used to perform a step-down conversion so that the first external power source can match the voltage requirement of the load, the input end of the main loop Buck circuit serves as the first port of the three-port DC-DC converter, the first port is the input port, used to connect the first external power source, the output end of the main loop Buck circuit is connected to the battery charge and discharge Buck / Boost circuit, and at the same time serves as the second port of the three-port DC-DC converter, the second port is the output port, used to connect the load; the battery charge and discharge Buck / Boost circuit is used to perform The step-up / down conversion is performed to manage the charge and discharge of the second external power supply, and the second external power supply can match the voltage requirement of the load. The first connection end of the battery charge and discharge Buck / Boost circuit is connected to the output end of the main loop Buck circuit, and the second connection end serves as the third port of the three-port DC-DC converter. The third port is a bidirectional port for connecting the second external power supply. The PID control module is connected to the main loop Buck circuit and the battery charge and discharge Buck / Boost circuit, and is used to collect voltage and current signals of the first external power supply and the battery charge and discharge Buck / Boost circuit, and generate a control signal to control the operation of the main loop Buck circuit and the battery charge and discharge Buck / Boost circuit.

[0020] The main loop Buck circuit includes a first resistor R1, a second resistor R2, a third resistor R3, a first capacitor C1, a second capacitor C2, a first inductor L1, a first transistor FETD1, a first diode D1 and a second diode D2; after one end of the first capacitor C1 is connected in series with one end of the first resistor R1, the other end of the first capacitor C1 and the other end of the first resistor R1 serve as the input end of the main loop Buck circuit; the collector of the first transistor FETD1 is connected to the first capacitor C1; the cathode of the first diode D1 is connected to the emitter of the first transistor FETD1 and the second resistor R2, and the anode is connected to the first resistor R1 and the third resistor R3 and is grounded; the second resistor R2, the first inductor L1, the second diode D2, the second capacitor C2 and the third resistor R3 are connected in series, the anode of the second diode D2 is connected to the first inductor L1, and the cathode is connected to the second capacitor C2. The first capacitor C1 and the first resistor R1 are used for filtering and voltage division. The high-frequency noise in the input voltage Vin of the first external power supply is filtered out by the first capacitor C1. The first resistor R1 and the first capacitor C1 together form a voltage division network to perform appropriate voltage division processing on the input voltage Vin of the first external power supply. The first transistor FETD1, the first inductor L1, and the first diode D1 jointly realize the core function of the Buck chopper circuit. The first transistor FETD1 acts as a switch tube to control the charging and discharging process of the first inductor L1, thereby adjusting the output voltage. The first diode D1 acts as a freewheeling diode to provide a current path when the first transistor FETD1 is turned off, ensuring that the current of the first inductor L1 can flow continuously. The second capacitor C2 plays the role of energy storage and filtering, further smoothing the ripple of the output voltage.

[0021] The battery charge and discharge Buck / Boost circuit includes a fifth resistor R5, a sixth resistor R6, a seventh resistor R7, a third capacitor C3, a second inductor L2, a second transistor FETD2 and a third transistor FETD3; after one end of the third capacitor C3 is connected in series with one end of the fifth resistor R5, the other end of the third capacitor C3 is connected to the fifth resistor R5 and the seventh resistor R7, the other end of the sixth resistor R6 is grounded, and the other end of the fifth resistor R5 and the other end of the sixth resistor R6 serve as the second connection end of the battery charge and discharge Buck / Boost circuit; one end of the second inductor L2 is connected to the seventh resistor R7, and the other end is connected to the collector of the second transistor FETD2 and the emitter of the third transistor FETD3; the emitter of the second transistor FETD2 is connected to one end of the sixth resistor R6 that is grounded; the collector of the third transistor FETD3 and the emitter of the second transistor FETD2 serve as the first connection end of the battery charge and discharge Buck / Boost circuit, wherein the collector of the third transistor FETD3 is connected to the cathode of the second diode D2, and the emitter of the second transistor FETD2 is connected to the third resistor R3. The second inductor L2, the second transistor FETD2 and the third transistor FETD3 jointly realize the function of the bidirectional DC-DC converter and the switching of the charging and discharging mode of the second external power supply. The second transistor FETD2 controls the charging process of the second external power supply, the third transistor FETD3 controls the discharging process of the second external power supply, and the second inductor L2 is used to store and release energy to realize the voltage rise and fall conversion. The third capacitor C3 plays the role of energy storage and filtering in the charging and discharging circuit of the second external power supply to ensure the stability of the voltage output from the second external power supply port.

[0022] The PID control module includes a triangular wave generator 1, a triangular wave generator 2, a pulse generator, a difference amplifier 1, a difference amplifier 2, a difference amplifier 3, a comparator 1, a comparator 2, a comparator 3, a constant comparator, an SR latch, a switching switch 1 and a switching switch 2; the output voltage Vout of the second port and the reference value 1 signal are input to the difference amplifier 1, the output signal of the difference amplifier 1 and the triangular wave signal 1 emitted by the triangular wave generator 1 enter the comparator 1, the comparator 1 outputs the control signal PWMA of the first transistor and is connected to the base of the first transistor FETD1; the conduction time of the first transistor FETD1 is controlled by adjusting the duty cycle of the control signal PWMA of the first transistor, thereby adjusting the output voltage Vout of the second port to adapt to the load demand. The input voltage Vin of the first port is input to the constant comparator, and the constant comparator outputs the mark value signal; the current signal IB of the third port and the reference value 2 signal are input to the difference amplifier 2, and the triangular wave signal 2 emitted by the triangular wave generator 2 and the output signal of the difference amplifier 2 are input to the comparator 2; the output voltage Vout of the second port and the reference value 3 signal are also input to the difference amplifier 3, the current signal IL of the second transistor and the output signal of the difference amplifier 3 are input to the comparator 3, and the pulse signal emitted by the pulse generator and the output signal of the comparator 3 are input to the SR latch; the output signal of the comparator 2, the mark value signal output by the constant comparator and the reference value 4 signal are input to the switching switch 1, and the switching switch 1 outputs the control signal PWMBS of the third transistor and is connected to the base of the third transistor FETD3; the conduction time of the third transistor FETD3 is controlled by adjusting the duty cycle of the control signal PWMBS of the third transistor, thereby controlling the discharge current of the energy storage battery to ensure that the energy storage battery is discharged within a safe range. The output signal of the SR latch, the flag value signal output by the constant comparator and the reference value 4 signal are input to the switch 2, and the switch 2 outputs the control signal PWMB of the second transistor and is connected to the base of the second transistor FETD2. PWMB realizes the switching of the working mode of the battery charging and discharging Buck / Boost circuit and the charging function of the energy storage battery by controlling the state of the switch tube FETD2.

[0023] In an embodiment of the utility model, the first external power source is a photovoltaic cell, the second external power source is an energy storage battery, and the load connected to the second port is an adjustable load.

[0024] The embodiment of the utility model has two working states in the photovoltaic power generation system to achieve different functions, specifically including:

[0025] Working status 1:

[0026] When the output voltage of the photovoltaic cell is greater than or equal to the first threshold value (the first threshold value is 50V in the embodiment of the utility model) and the output current is 1.2A, the photovoltaic cell supplies power to the load and charges the energy storage battery at the same time. The on-off state of the transistor at this time is: the first transistor FETD1 of the main loop Buck circuit is turned on and off according to the control signal PWMA of the first transistor to adjust the output voltage of the photovoltaic cell to adapt to the load. The second transistor FETD2 and the third transistor FETD3 of the battery charging and discharging Buck / Boost circuit work in the charging mode, and the energy storage battery is charged by the control signal PWMB of the second transistor and the control signal PWMBS of the third transistor. The inductance state at this time is: the first inductance L1 in the main loop Buck circuit stores energy when the first transistor FETD1 is turned on, and continues to flow when it is turned off to stabilize the output voltage. The second inductance L2 in the battery charging and discharging Buck / Boost circuit also performs energy storage and continuous flow operations in the charging mode. The capacitance state at this time is: the second capacitor C2 of the main loop Buck circuit is charged when the first transistor FETD1 is turned off, and discharged when it is turned on, which is used to filter and smooth the output voltage. The capacitor in the battery charge and discharge Buck / Boost circuit plays a corresponding role in energy storage or discharge during the charging or discharging process according to the specific circuit configuration. The current flow direction in working state 1 is: the current output by the photovoltaic cell flows to the load after being regulated by the main loop Buck circuit, and at the same time, part of the current flows to the energy storage battery through the battery charge and discharge Buck / Boost circuit for charging.

[0027] Working status 2:

[0028] When the output voltage of the photovoltaic cell is less than the second threshold value (the second threshold value is 35V in the embodiment of the utility model) and the output current is 1.2A, the output of the photovoltaic cell is insufficient to meet all the requirements of the load. At this time, the photovoltaic cell and the energy storage battery simultaneously power the load. The transistor on-off state at this time: the first transistor FETD1 of the main loop Buck circuit is still turned on and off according to the control signal PWMA of the first transistor to adjust the output voltage of the photovoltaic cell to adapt to the load. The second transistor FETD2 and the third transistor FETD3 of the battery charge and discharge Buck / Boost circuit work in the discharge mode, respectively controlled by the control signal PWMB of the second transistor and the control signal PWMBS of the third transistor to release the energy of the energy storage battery. The inductance state at this time is: the first inductance L1 in the main loop Buck circuit is the same as the working state 1, and performs energy storage and freewheeling operations. The inductance in the battery charge and discharge Buck / Boost circuit performs a discharge operation in the discharge mode. The capacitance state at this time is: similar to the working state 1, the second capacitor C2 of the main loop Buck circuit is used to filter and smooth the output voltage. The capacitor in the battery charge and discharge Buck / Boost circuit may play a corresponding discharge role in the discharge process according to the specific circuit configuration. The current flow direction in working state 2 is: the current output by the photovoltaic cell flows to the load after being regulated by the main loop Buck circuit. Due to the insufficient output of the photovoltaic cell, the energy storage battery releases energy through the battery charging and discharging Buck / Boost circuit, and together with the photovoltaic cell, supplies power to the load.

[0029] The embodiment of the utility model can be used in a photovoltaic power generation system to realize the function of photovoltaic cells supplying power to loads and charging energy storage batteries at the same time, or photovoltaic cells and energy storage batteries supplying power to loads at the same time, and ensure the stability of the output voltage through closed-loop control. The embodiment of the utility model adopts two DC-DC converters in parallel, that is, the main loop Buck circuit and the battery charging and discharging Buck / Boost circuit are connected in parallel. In the main loop Buck circuit, a Buck chopper circuit is used to connect the output port of renewable energy such as photovoltaic cells, and the battery charging and discharging Buck / Boost circuit uses a Buck / Boost bidirectional DC-DC circuit to connect energy storage systems such as energy storage batteries. The PID control module collects relevant current signals or voltage signals for proportional integral regulation to obtain corresponding control signals to control the conduction and shutdown of transistors, thereby realizing automatic switching between voltage stabilization output and charging and discharging modes.

[0030] The above description is only the preferred embodiment of the utility model, and is not intended to limit the utility model. For those skilled in the art, the utility model can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the utility model shall be included in the protection scope of the utility model.

Claims

1. A three-port DC-DC converter applied to a photovoltaic power generation system, used to connect a first external power source, a second external power source and a load, characterized in that: It includes a main loop Buck circuit, a battery charging and discharging Buck / Boost circuit and a PID control module; the main loop Buck circuit is a chopper circuit, which is used to perform step-down conversion so that the first external power supply can match the voltage requirement of the load. The input end of the main loop Buck circuit serves as the first port of the three-port DC-DC converter, the first port is the input port, and is used to connect the first external power supply. The output end of the main loop Buck circuit is connected to the battery charging and discharging Buck / Boost circuit and serves as the second port of the three-port DC-DC converter. The second port is the output port, and is used to connect the load. The battery charging and discharging Buck / Boost circuit is used to perform step-up and step-down conversion. The second external power supply is managed for charging and discharging, and the second external power supply is enabled to match the voltage requirement of the load. The first connection end of the battery charging and discharging Buck / Boost circuit is connected to the output end of the main loop Buck circuit, and the second connection end serves as the third port of the three-port DC-DC converter. The third port is a bidirectional port for connecting the second external power supply. The PID control module is connected to the main loop Buck circuit and the battery charging and discharging Buck / Boost circuit, and is used to collect voltage and current signals of the first external power supply and the battery charging and discharging Buck / Boost circuit, and generate a control signal to control the operation of the main loop Buck circuit and the battery charging and discharging Buck / Boost circuit.

2. A three-port DC-DC converter for photovoltaic power generation system according to claim 1, characterized in that: The main loop Buck circuit includes a first resistor, a second resistor, a third resistor, a first capacitor, a second capacitor, a first inductor, a first transistor, a first diode and a second diode; after one end of the first capacitor is connected in series with one end of the first resistor, the other end of the first capacitor and the other end of the first resistor serve as the input end of the main loop Buck circuit; the collector of the first transistor is connected to the first capacitor; the cathode of the first diode is connected to the emitter of the first transistor and the second resistor, and the anode is connected to the first resistor and the third resistor and grounded; the second resistor, the first inductor, the second diode, the second capacitor and the third resistor are connected in series, the anode of the second diode is connected to the first inductor, the cathode is connected to the second capacitor, and the cathode of the second diode and one end of the third resistor connected to the anode of the first diode serve as the output end of the main loop Buck circuit.

3. A three-port DC-DC converter for photovoltaic power generation system according to claim 2, characterized in that: The battery charge and discharge Buck / Boost circuit includes a fifth resistor, a sixth resistor, a seventh resistor, a third capacitor, a second inductor, a second transistor and a third transistor; after one end of the third capacitor is connected in series with one end of the fifth resistor, the other end of the third capacitor is connected to the fifth resistor and the seventh resistor, the other end of the sixth resistor is grounded, and the other end of the fifth resistor and the other end of the sixth resistor serve as the second connection end of the battery charge and discharge Buck / Boost circuit; one end of the second inductor is connected to the seventh resistor, and the other end is connected to the collector of the second transistor and the emitter of the third transistor, and the emitter of the second transistor is connected to one end of the sixth resistor connected to the ground; the collector of the third transistor and the emitter of the second transistor serve as the first connection end of the battery charge and discharge Buck / Boost circuit, wherein the collector of the third transistor is connected to the cathode of the second diode, and the emitter of the second transistor is connected to the third resistor.

4. A three-port DC-DC converter for photovoltaic power generation system according to claim 3, characterized in that: The PID control module includes a triangle wave generator 1, a triangle wave generator 2, a pulse generator, a difference amplifier 1, a difference amplifier 2, a difference amplifier 3, a comparator 1, a comparator 2, a comparator 3, a constant comparator, an SR latch, a switch 1 and a switch 2; the output voltage Vout of the second port and the reference value 1 signal are input to the difference amplifier 1, the output signal of the difference amplifier 1 and the triangle wave signal 1 emitted by the triangle wave generator 1 enter the comparator 1, the comparator 1 outputs the control signal PWMA of the first transistor and is connected to the base of the first transistor; the input voltage Vin of the first port is input to the constant comparator, and the constant comparator outputs a flag value signal; the The current signal IB of the three ports and the reference value 2 signal are input to the difference amplifier 2, and the triangular wave signal 2 emitted by the triangular wave generator 2 and the output signal of the difference amplifier 2 are input to the comparator 2; the output voltage Vout of the second port is also input to the difference amplifier 3 with the reference value 3 signal, the current signal IL of the second transistor and the output signal of the difference amplifier 3 are input to the comparator 3, and the pulse signal emitted by the pulse generator and the output signal of the comparator 3 are input to the SR latch; the output signal of the comparator 2, the mark value signal output by the constant comparator and the reference value 4 signal are input to the switching switch 1, and the switching switch 1 outputs the control signal PWMBS of the third transistor and is connected to the base of the third transistor; the output signal of the SR latch, the mark value signal output by the constant comparator and the reference value 4 signal are input to the switching switch 2, and the switching switch 2 outputs the control signal PWMB of the second transistor and is connected to the base of the second transistor.

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