A fixed frequency LLC circuit and a method for tracking its resonance frequency

Abstract

The invention discloses a fixed-frequency LLC circuit and a method for tracking its resonant frequency in the field of LLC circuits. The circuit comprises a primary full-bridge circuit, a resonant inductor, an excitation inductor, a resonant capacitor and a rectifier filter circuit; the circuit also comprises a voltage signal acquisition circuit, which is used for acquiring a bridge arm voltage and a resonant capacitor voltage and sending the acquisition to an actual phase difference operation module; the actual phase difference operation module is used for calculating the actual phase difference between the bridge arm voltage and the resonant capacitor voltage; the reference phase difference operation module is used for obtaining a reference phase difference between the bridge arm voltage and the resonant capacitor voltage when the operating frequency is equal to the resonant frequency; the phase difference comparison module is used for receiving the outputs of the actual phase difference operation module and the reference phase difference operation module, comparing the actual phase difference with the reference phase difference, and outputting the comparison result to a feedback control module; the invention can adjust the operating frequency of the LLC converter to track the resonant frequency, and solve the problem of resonant frequency point deviation caused by changes in circuit parameters.

Description

Technical Field

[0001] The invention relates to the field of LLC circuits, in particular to a fixed-frequency LLC circuit and a method for tracking its resonant frequency. Background Art

[0002] The fixed-frequency LLC converter has the advantages of fixed switching frequency, zero voltage and zero current soft switching, and good EMI characteristics. It is suitable for high efficiency, high power density, and electrical isolation. In this working state, the LLC converter's switching frequency must always be equal to the resonant frequency of the isolation transformer's resonant inductor and resonant capacitor. At this time, both the primary zero voltage turn-on and the secondary zero current turn-off can be achieved. However, in practice, due to temperature drift, device aging, and other reasons, it is difficult to control the switching frequency to always be exactly equal to the resonant frequency, causing "current backflow" and additional losses.

[0003] At present, there are several main ways to implement the resonant frequency tracking of the fixed-frequency LLC converter: 1. Direct sampling with a series resistor. This method directly connects a sampling resistor in series in the secondary circuit to accurately obtain the current zero-crossing point to achieve resonant frequency tracking, but it also introduces unacceptable losses. 2. Decoupling of the primary resonant current and the excitation current. This method is to construct an additional discrete inductor on the primary side to replace the excitation inductor, and the excitation inductor of the isolation transformer itself is used to proportionally adjust the leakage inductance, that is, the resonant inductor. The current flowing through the discrete inductor and the resonant inductor is thus decoupled, and accurate resonant frequency tracking can be achieved by capturing the current zero-crossing point of the resonant inductor. This method requires the introduction of an additional discrete inductor and a mutual inductor for collecting the resonant inductor current, which greatly increases the size of the converter. 3. Frequency tracking based on fixed threshold voltage comparison. The current flowing through the rectifier tube is reflected by detecting the change in the drain-source voltage of the switch tube, and the power tube shutdown moment is controlled by making a reasonable judgment on the voltage to achieve tracking of the resonant frequency. This method can only detect a fixed voltage of a specific SR, and has certain limitations in application. In addition, effective control can only be performed after the body diode conduction and "current backflow" are detected, which will still cause an increase in losses.

[0004] In view of the shortcomings of the existing technology, the applicant proposed an improvement plan. Summary of the invention

[0005] The object of the present invention is to provide a fixed-frequency LLC circuit and a method for tracking its resonant frequency, which can adjust the operating frequency of the LLC converter in real time to track the resonant frequency and solve the problem of resonant frequency point deviation caused by changes in circuit parameters.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A fixed-frequency LLC circuit includes a primary full-bridge circuit, a resonant inductor Lr, an excitation inductor Lm, a resonant capacitor Cres, and a secondary rectifier and filter circuit; and further includes:

[0008] The voltage signal acquisition circuit connected to the output end of the primary full-bridge circuit and the two ends of the resonant capacitor Cres is used to collect the bridge arm voltage and the resonant capacitor voltage and send them to the actual phase difference calculation module; the actual phase difference calculation module is used to calculate the actual phase difference between the bridge arm voltage and the resonant capacitor voltage

[0009] The reference phase difference calculation module is used to obtain the reference phase difference between the bridge arm voltage Vp and the resonant capacitor voltage Vcr calculated according to the parameter theory of the fixed-frequency LLC circuit when the operating frequency fc is equal to the resonant frequency fr.

[0010] The phase difference comparison module is used to receive the output of the actual phase difference calculation module and the reference phase difference calculation module, and compare the actual phase difference Phase difference with reference The size of the comparison is output to the feedback control module;

[0011] The feedback control module is used to adjust the switching frequency of the fixed-frequency LLC circuit in the next cycle according to the comparison result, and generate control pulses corresponding to the switching diodes in the primary full-bridge circuit and the rectifier and filter circuit.

[0012] As an improved solution of the present invention, the feedback control module includes a PI compensator, a frequency regulator and a pulse generator. The PI compensator is used to perform loop compensation on the fixed-frequency LLC circuit according to the output result of the phase difference comparison module. The frequency regulator is used to determine the switching frequency adjustment amount of the next cycle. The pulse generator is used to generate control pulses corresponding to the switching diodes in the primary full-bridge circuit and the rectifier and filter circuit according to the switching frequency adjustment amount.

[0013] A method for tracking the resonant frequency of a fixed-frequency LLC circuit, the method comprising the following steps:

[0014] Step 100: Establish the phase difference between the bridge arm voltage Vp, the resonant capacitor voltage Vcr and the reference phase when the operating frequency fc of the fixed-frequency LLC circuit is equal to the resonant frequency fr. The relationship between:

[0015]

[0016] Where θ represents the phase of the bridge arm voltage Vp lagging behind the resonant current ir; Ip represents the peak value of the excitation current; I represents the peak value of the resonant current; Vo represents the output voltage; n represents the isolation transformer ratio of the fixed-frequency LLC circuit; RL represents the load;

[0017] Step 200: Real-time acquisition of the bridge arm voltage Vp and the resonant capacitor voltage Vcr, and calculation and acquisition of the actual phase difference between the two after digital processing

[0018] Step 300: Set the adjustment period and regularly compare the actual phase difference Phase difference with reference When the comparison result between the two is not within the preset error range, the switching diode pulses in the primary full-bridge circuit and the rectifier filter circuit in the next working cycle are controlled by adjusting the switching frequency;

[0019] Step 400: Repeat steps 100-300 until the actual phase difference Phase difference with reference equal.

[0020] As an improved solution of the present invention, in step 300, when When the control increases the pulse frequency, When , the control reduces the pulse frequency.

[0021] Beneficial effects: Compared with the prior art, the present invention does not need to increase the volume of the LLC converter. During operation, it only needs to collect the bridge arm voltage and the resonant capacitor voltage, and adopts digital control processing, which has low power consumption and can avoid excessive additional losses and reduce efficiency. In addition, the present invention has high practicality and is suitable for popularization and application to different component selection or parameter setting. The method for tracking the resonant frequency is the same, which will not have a great impact on the technical effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the circuit principle diagram of the present invention;

[0023] Figure 2 This is a diagram showing the derivation relationship of the reference phase difference of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the 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.

[0025] like Figure 1As shown, a fixed-frequency LLC circuit is used to realize the conversion of input and output power, including a primary full-bridge circuit, a resonant inductor Lr, an excitation inductor Lm, a resonant capacitor Cres, and a secondary rectifier filter circuit. The primary full-bridge circuit includes switching diodes V1-V4, one end of the resonant inductor Lr is connected to the first output end of the primary full-bridge circuit, and the other end is connected to one end of the excitation inductor Lm, and the other end of the excitation inductor Lm is connected to the second output end of the primary full-bridge circuit. In the fixed-frequency LLC circuit, two windings are provided on the primary side of the isolation transformer, one end of the resonant capacitor Cres is connected to the non-same-name end of the first primary winding, and the other end is connected to the same-name end of the second primary winding, and the same-name end of the first primary winding and the non-same-name end of the second primary winding are respectively connected to the two ends of the excitation inductor Lm. The secondary side of the isolation transformer includes two secondary windings, and the rectifier and filter circuit includes switching diodes V5~V6. The same-name ends of the first secondary winding form a loop with the first switch, the second switch, and the non-same-name ends of the second secondary winding. The two ends of the first switch are connected in parallel with the switching diode V5 and the capacitor, and the two ends of the second switch are connected in parallel with the switching diode V6 and the capacitor.

[0026] The fixed-frequency LLC circuit needs to track the resonant frequency and also includes a voltage signal acquisition circuit, an actual phase difference calculation module, a reference phase difference calculation module, a phase difference comparison module and a feedback control module.

[0027] The voltage signal acquisition circuit is connected to the output end of the primary full-bridge circuit and the two ends of the resonant capacitor Cres, and is used to collect the bridge arm voltage Vp and the resonant capacitor voltage Vcr and send them to the actual phase difference calculation module;

[0028] The actual phase difference calculation module is used to calculate the actual phase difference between the bridge arm voltage and the resonant capacitor voltage

[0029] The reference phase difference calculation module is used to obtain the reference phase difference between the bridge arm voltage Vp and the resonant capacitor voltage Vcr calculated according to the parameter theory of the fixed-frequency LLC circuit when the operating frequency fc is equal to the resonant frequency fr.

[0030] The phase difference comparison module is used to receive the output of the actual phase difference calculation module and the reference phase difference calculation module, and compare the actual phase difference Phase difference with reference The size of the comparison is output to the feedback control module;

[0031] The feedback control module includes a PI compensator, a frequency regulator and a pulse generator. The PI compensator is used to perform loop compensation on the LLC converter according to the output result of the phase difference comparison module. The frequency regulator is used to determine the switching frequency adjustment amount of the next cycle. The pulse generator is used to generate control pulses corresponding to the switching diodes in the primary full-bridge circuit and the rectifier and filter circuit according to the switching frequency adjustment amount.

[0032] The specific calculation process of the actual phase difference calculation module, the reference phase difference calculation module, the phase difference comparison module and the feedback control module can be implemented by DSP, FPGA, single chip microcomputer, etc., and the circuit structure of the voltage signal acquisition circuit is not limited. The specific implementation methods are detailed in the prior art and will not be described in detail here.

[0033] The above-mentioned method for tracking the resonant frequency of the fixed-frequency LLC circuit comprises the following steps:

[0034] Step 100: Figure 2 As shown, for Figure 1 In the fixed-frequency LLC circuit shown in the figure, when the operating frequency fc is equal to the resonant frequency fr, the phase difference between the bridge arm voltage Vp and the resonant current ir is Determined by the excitation inductance Lm, the phase of the resonant capacitor voltage Vcr lags behind the phase of the resonant current ir Assume that the bridge arm voltage lags the resonant current phase by θ, im is the excitation current, Ip is the excitation current peak value, and the reference phase difference calculation module can theoretically establish the excitation inductance Lm, bridge arm voltage Vp, resonant capacitor voltage Vcr and reference phase difference according to the parameters of LLC circuit. The relationship between:

[0035]

[0036] Where I represents the peak value of the resonant current; Vo represents the output voltage; n represents the isolation transformer ratio of the fixed-frequency LLC circuit; RL represents the load.

[0037] Step 200: The voltage signal acquisition circuit collects the bridge arm voltage Vp and the resonant capacitor voltage Vcr in real time, and can convert them into voltage signals suitable for digital processing through isolation amplifiers and other circuits, and calculate the actual phase difference between the two through the actual phase difference calculation module using a phase-locked loop and other methods.

[0038] Step 300: Setting the adjustment cycle, the actual phase difference can be periodically compared by the phase difference comparison module at intervals of one or more working cycles. Phase difference with reference When the circuit parameters affect the resonant frequency, even if the reference phase difference When the actual phase difference changes Phase difference from reference When the comparison result between the two is not within the preset error range, the feedback control module controls the pulses of the switching diodes V1 to V6 in the next working cycle by adjusting the switching frequency. The error range is generally set according to the actual situation, for example, it can be set to As judged If the difference is not within the preset error range, the feedback control module controls to increase the pulse frequency. According to the above formula, the value of θ decreases, and the actual phase difference gradually decreases until the difference between the actual phase difference and the reference phase difference is within the preset error range. On the contrary, if it is judged that If the difference is not within the preset error range, the feedback control module will control to reduce the pulse frequency. At this time, the pulse period increases, the θ value increases, and the actual phase difference When the difference between the two is within the error range, the pulse frequency does not change. The error range is used as an allowable value to prevent the switching frequency from being changed frequently.

[0039] Step 400: Repeat steps 100-300, and feedback control continuously adjusts the circuit operating frequency until the actual phase difference Phase difference with reference equal, at this time the switching frequency is equal to the new resonant frequency, and the circuit enters a new steady state.

[0040] Although this specification is described according to implementation modes, not every implementation mode includes only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0041] Therefore, the above description is only a preferred embodiment of the present application and is not intended to limit the scope of implementation of the present application; that is, all equivalent changes made according to the scope of the claims of the present application are within the protection scope of the claims of the present application.

Claims

1. A fixed-frequency LLC circuit, comprising a primary full-bridge circuit, a resonant inductor Lr, an excitation inductor Lm, a resonant capacitor Cres and a secondary rectifier filter circuit; characterized in that: The primary side of the isolation transformer is provided with a first primary winding and a second primary winding, one end of the resonant capacitor Cres is connected to the non-same-name end of the first primary winding, and the other end is connected to the same-name end of the second primary winding, and the same-name end of the first primary winding and the non-same-name end of the second primary winding are respectively connected to the two ends of the excitation inductance Lm; Also includes: The voltage signal acquisition circuit connected to the output end of the primary full-bridge circuit and the two ends of the resonant capacitor Cres is used to collect the bridge arm voltage and the resonant capacitor voltage and send them to the actual phase difference calculation module; the actual phase difference calculation module is used to calculate the actual phase difference between the bridge arm voltage and the resonant capacitor voltage The reference phase difference calculation module is used to obtain the reference phase difference between the bridge arm voltage Vp and the resonant capacitor voltage Vcr calculated based on the hardware parameter theory of the fixed-frequency LLC circuit when the operating frequency fc is equal to the resonant frequency fr. The phase difference comparison module is used to receive the output of the actual phase difference calculation module and the reference phase difference calculation module, and compare the actual phase difference Phase difference with reference The size of the comparison is output to the feedback control module; The feedback control module is used to adjust the switching frequency of the fixed-frequency LLC circuit in the next cycle according to the comparison result, and generate control pulses corresponding to the switching diodes in the primary full-bridge circuit and the rectifier and filter circuit.

2. A fixed-frequency LLC circuit according to claim 1, characterized in that: The feedback control module includes a PI compensator, a frequency regulator and a pulse generator. The PI compensator is used to perform loop compensation on the fixed-frequency LLC circuit according to the output result of the phase difference comparison module. The frequency regulator is used to determine the switching frequency adjustment amount of the next cycle. The pulse generator is used to generate control pulses corresponding to the switching diodes in the primary full-bridge circuit and the rectifier and filter circuit according to the switching frequency adjustment amount.

3. A method for tracking the resonant frequency of a fixed-frequency LLC circuit according to any one of claims 1 to 2, characterized in that: The method comprises the following steps: Step 100: Establish the phase difference between the bridge arm voltage Vp, the resonant capacitor voltage Vcr and the reference phase when the operating frequency fc of the fixed-frequency LLC circuit is equal to the resonant frequency fr. The relationship between: Where θ represents the phase of the bridge arm voltage Vp lagging behind the resonant current ir; Ip represents the peak value of the excitation current; I represents the peak value of the resonant current; Vo represents the output voltage; n represents the ratio of the isolation transformer of the fixed-frequency LLC circuit; RL represents the load; Lm represents the magnetizing inductance; Step 200: Real-time acquisition of the bridge arm voltage Vp and the resonant capacitor voltage Vcr, and calculation and acquisition of the actual phase difference between the two after digital processing Step 300: Set the adjustment period and regularly compare the actual phase difference Phase difference with reference When the comparison result between the two is not within the preset error range, the switching diode pulses in the primary full-bridge circuit and the rectifier filter circuit in the next working cycle are controlled by adjusting the switching frequency; Step 400: Repeat steps 100-300 until the actual phase difference Equal to the reference phase difference 4. The method for tracking the resonant frequency of a fixed-frequency LLC circuit according to claim 3, characterized in that: In step 300, when When the control increases the pulse frequency, When , the control reduces the pulse frequency.

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