LLC converter control device and method based on extended harmonic impedance model

Abstract

The application provides an LLC converter control device and method based on an extended harmonic impedance model. The proposed synchronous rectification method needs to calculate the sampling signals of output direct current voltage and current in a digital signal processor (DSP), and establishes an impedance model in a frequency domain by considering harmonics, so that the synchronous rectification conduction time is accurately calculated. The turn-on time of the synchronous rectification tube is consistent with the primary side switch tube device, and the turn-off time of the synchronous rectification tube is equal to the turn-on time plus the calculated synchronous rectification conduction time. The method not only minimizes the conduction time of the synchronous rectification body diode, has high calculation accuracy, and has high anti-interference on high switching frequency noise, and the proposed method greatly improves the efficiency of the LLC converter.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter technology, specifically relating to an LLC converter control device and method based on an extended harmonic impedance model. Background Technology

[0002] LLC resonant converters can achieve soft switching across the entire load range, resulting in extremely high operating efficiency. Due to their soft-switching capability and magnetic integration advantages, high-frequency GaN and SiC power devices are suitable for use in resonant converters to achieve high efficiency and high power density. Synchronous rectification technology, which uses MOSFETs instead of rectifier diodes, significantly reduces the rectification conduction losses of the rectifier circuit and is one of the most effective methods for achieving high efficiency in LLC converters. Therefore, this technology has become a research hotspot in DC-DC conversion for various switching power supplies.

[0003] Generally, LLC synchronous rectification control methods can be divided into three types: current sensing method, voltage sensing method, and model-based calculation method. Current sensing and voltage sensing methods are effective in low-voltage, high-current applications, but they are highly susceptible to the high dv / dt caused by the use of high-frequency GaN devices. Due to parasitic inductance and high dv / dt, the sensing circuit may malfunction. Considering these effects, a model-based calculation method for high-immunity synchronous rectification was studied. However, traditional model-based calculation methods only consider the fundamental harmonics and ignore higher harmonics, resulting in low accuracy and high conduction time of the body diode in the synchronous rectifier. Summary of the Invention

[0004] This invention addresses the shortcomings and deficiencies of existing technologies by proposing a control device and method for LLC converters based on an extended harmonic impedance model. This method is a synchronous rectification control approach based on an extended harmonic impedance model. By considering harmonics, an impedance model is established in the frequency domain to accurately calculate the synchronous rectification turn-on time, minimizing the turn-on time of the synchronous rectifier diode. This reduces conduction losses, improves converter efficiency, enhances anti-interference capabilities, effectively avoids the impact of high dv / dt on the synchronous rectification turn-on time, and achieves high immunity to switching noise caused by GaN devices during operation. This invention can accurately calculate the synchronous rectification turn-on time to minimize the turn-on time of the synchronous rectifier diode and achieve low conduction losses.

[0005] To solve its technical problem, the present invention adopts the following specific technical solution:

[0006] An LLC converter control device based on an extended harmonic impedance model includes an LLC converter, a sampling circuit, a digital signal processor, and an isolation drive circuit.

[0007] The LLC converter includes a primary-side half-bridge converter circuit, a resonant circuit, and a secondary-side full-wave rectifier circuit; the primary-side half-bridge converter circuit includes a first switch Q1 and a second switch Q2; the resonant circuit includes a resonant inductor L. r Resonant capacitor C r And a transformer, wherein the inherent magnetizing inductance L in the transformer m The excitation inductor L m Located on the primary side of the transformer; the midpoint of the first switch Q1 and the second switch Q2 is connected to the resonant inductor L. r Connected in series, and then with the excitation inductor L m One end is connected to the excitation inductor L m The other end and the resonant capacitor C r One end is connected to the resonant capacitor C. r The other end is connected to the other end of Q2; the secondary full-wave rectifier circuit includes a third switch S1 and a fourth switch S2, one end of the third switch S1 and the fourth switch S2 are respectively connected to the two ends of the secondary winding of the transformer; the midpoint of the secondary winding of the transformer is connected to the load R. o At one end, the third switch S1 and the fourth switch S2 are connected in parallel to the load R. o The other end.

[0008] Furthermore, the first to fourth switching transistors Q1 to Q2 and S1 to S2 are all power switching transistors.

[0009] The present invention also provides a synchronous rectification control method for an LLC converter control device based on an extended harmonic impedance model, comprising the following steps:

[0010] Step (1) Acquire the output current i o and output voltage v o The signal is input to the digital signal processor (DSP) via a sampling circuit. This signal is compared with a reference output voltage within the DSP to obtain an error signal. This error signal is then calculated by a proportional-integral controller (PIC) to obtain a pulse frequency modulation (PWM) signal. The PWM signal is input to the isolation drive circuit to obtain the drive signals for the first and second switches Q1-Q2 on the primary side of the primary-side half-bridge converter, thereby controlling the output current i. o and output DC voltage v o Control;

[0011] Step (2) Utilize the sampled output DC voltage v o and output current i o The equivalent load of the output is calculated, and the switching frequency is calculated based on the proportional-integral controller. By considering harmonics, an impedance model is established in the frequency domain to accurately calculate the synchronous rectification conduction time.

[0012] Step (3) The turn-on time of the third and fourth synchronous switches S1 to S2 is the same as that of the first and second switches Q1 to Q2 on the primary side. The turn-off time of the third and fourth synchronous switches S1 to S2 is determined by the calculated synchronous rectification conduction time.

[0013] Furthermore, in step (3), the turn-off time of the third and fourth switching transistors S1 to S2 is equal to the turn-on time plus the calculated synchronous rectification conduction time.

[0014] Furthermore, the method for establishing the impedance model is as follows:

[0015] Impedance Z 1k yes:

[0016] Z 1k =a 1k +jb 1k (1)

[0017] a 1k and b 1k The impedances Z are respectively 1k The real and imaginary parts are expressed as follows:

[0018] a 1k =R ek / (R ek 2 C j 2 k 2 ω 2 +1) (2)

[0019]

[0020] Where ω is the switching frequency, and the unit is angle; C j It is the equivalent output capacitance of the synchronous rectifier MOSFET, where k represents the k-th harmonic; the equivalent output resistance R ek yes:

[0021] R ek =8n 2 R o / (k 2 π 2 (4)

[0022] Where, r e It is the output load, and n is the transformer turns ratio; therefore, Z 1k The impedance angle is:

[0023] θ 1k =arctan(a 1k / b 1k (5)

[0024] Considering the magnetizing inductance L m Z 2k for:

[0025] Z 2k =Z 1k / / (jkωL m )=(a 2k +jb 2k ) / (c 2k +jd 2k (6)

[0026] Wherein, the intermediate parameter a 2k ,b 2k ,c 2k and d 2k Calculated separately as follows:

[0027]

[0028] b 2k =kωL m R ek (8)

[0029] c 2k =R ek (9)

[0030]

[0031] Impedance angle θ 2k The calculation is as follows:

[0032]

[0033] Output impedance Z 3k The calculation formula is as follows:

[0034]

[0035] Wherein, the intermediate parameter a 3k ,b 3k ,c 3k and d 3k yes:

[0036]

[0037]

[0038]

[0039] d 3k =kωC r R ek (16)

[0040] Among them, f nequals ω / ω r ω r It is the resonant frequency, λ equals L m / L r ;

[0041] Z 3k The impedance angle is:

[0042]

[0043] The secondary current is:

[0044]

[0045] Among them, I sk It is the effective value of the secondary current, v in The input voltage for the LLC converter;

[0046] Therefore, the secondary current can be expressed in the time domain as:

[0047]

[0048] When the secondary current i' s When (t) crosses zero, the phase angle corresponding to the secondary current is θ. f That is, ωt=θ f The secondary current is at this time:

[0049]

[0050] The above equation has no analytical solution. θ is obtained by using the bisection method in mathematical theory. f ;

[0051] The synchronous rectification conduction time is obtained in the following way:

[0052] Δt on =0.5f s -1 -θ f / ω (21).

[0053] The present invention has the following beneficial effects:

[0054] 1. Based on the proposed extended harmonic model, this invention can accurately calculate the synchronous rectification conduction time, thereby minimizing the conduction time of the synchronous rectification diode, significantly reducing conduction losses, and improving conversion efficiency;

[0055] 2. This invention uses digital control and mathematical models to easily implement the proposed synchronous rectification without the need for additional detection of high-frequency signals.

[0056] 3. This invention has high immunity to switching noise caused by high-frequency switching devices during operation. Attached Figure Description

[0057] Figure 1 This is a topology and control principle diagram of the present invention.

[0058] Figure 2 This is a voltage and current waveform diagram of the present invention at different switching frequencies.

[0059] Figure 3 This is the LLC equivalent circuit of the present invention.

[0060] Figure 4 This is the working waveform of the present invention.

[0061] Figure 5 This is a waveform diagram showing the generation of the synchronous rectification drive signal of the present invention.

[0062] Figure 6 This is a flowchart of the synchronous rectification control of the present invention.

[0063] Explanation of component symbols in the diagram:

[0064] v in LLC input DC voltage L r Resonant inductor

[0065] v o Output DC voltage C r Resonant capacitor

[0066] i o Output current L m Magnetizing inductor

[0067] ADC1, ADC2 First AD sampling, Second AD sampling Lr Resonant current

[0068] Q1~Q2 First and second switching transistors i S1 i S2 rectifier side current

[0069] S1~S2 Third and fourth switching transistors n Transformer turns ratio

[0070] R o Output load V ref Reference voltage

[0071] C o Filter capacitor i Lm Magnetizing current Detailed Implementation

[0072] The present invention will now be described in further detail with reference to the accompanying drawings. The specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of the invention.

[0073] The LLC converter control device based on the extended harmonic impedance model involved in this invention includes an LLC converter, a sampling circuit, a DSP (digital signal processor), and an isolation drive circuit, such as... Figure 1 , Figure 3 As shown, the LLC converter includes a primary-side half-bridge converter circuit, a resonant circuit, and a secondary-side full-wave rectifier circuit; the primary-side half-bridge converter circuit includes a first switch Q1 and a second switch Q2; the resonant circuit includes a resonant inductor L. r Resonant capacitor C r And a transformer, wherein the inherent magnetizing inductance L in the transformer m The excitation inductor L m Located on the primary side of the transformer; the midpoint of the first switch Q1 and the second switch Q2 is connected to the resonant inductor L. r Connected in series, and then with the excitation inductor L m One end is connected to the excitation inductor L m The other end and the resonant capacitor C r One end is connected to the resonant capacitor C. r The other end is connected to the other end of the second switch Q2; the secondary full-wave rectifier circuit includes a third switch S1 and a fourth switch S2, one end of which is connected to each end of the secondary winding of the transformer. The midpoint of the secondary winding of the transformer is connected to the load R. o At one end, the third switch S1 and the fourth switch S2 are connected in parallel to the load R. o The other end. Figure 1 In this context, SRs represents synchronous rectifier diodes, PI represents proportional-integral controllers, and f s This indicates the switching frequency. VCO stands for Voltage Controlled Oscillator, which converts the signal output from the proportional-integral controller into the switching frequency.

[0074] The LLC converter control method based on the extended harmonic impedance model of this invention requires calculation of the sampled signals of output DC voltage and current in the DSP. By considering harmonics, an impedance model is established in the frequency domain to accurately calculate the synchronous rectification turn-on time. The turn-on time of the synchronous rectification is consistent with the turn-on time of the primary-side switch, while the turn-off time of the synchronous rectification is equal to the turn-on time plus the calculated synchronous rectification turn-on time.

[0075] The first to fourth switching transistors Q1 to Q2 and S1 to S2 are all MOSFETs.

[0076] The method for establishing the impedance model is as follows:

[0077] Impedance Z 1k yes:

[0078] Z1k =a 1k +jb 1k (twenty two)

[0079] intermediate parameter a 1k and b 1k yes:

[0080] a 1k =R ek / (R ek 2 C j 2 k 2 ω 2 +1) (23)

[0081]

[0082] Where ω is the switching frequency, and the unit is angle. C j This is the equivalent output capacitance of the synchronous rectifier MOSFET, where k represents the k-th harmonic. The equivalent output resistance R... ek yes:

[0083] R ek =8n 2 R o / (k 2 π 2 (25)

[0084] Where, r e This is the output load, and n is the transformer turns ratio. Therefore, Z 1k The impedance angle is:

[0085] θ 1k =arctan(a 1k / b 1k (26)

[0086] Considering the magnetizing inductance L m Z 2k for:

[0087] Z 2k =Z 1k / / (jkωL m )=(a 2k +jb 2k ) / (c 2k +jd 2k (27)

[0088] Wherein, the intermediate parameter a 2k ,b 2k ,c 2k and d 2k yes:

[0089]

[0090] b 2k =kωL m R ek (29)

[0091] c 2k =R ek (30)

[0092]

[0093] Impedance angle θ 2k The calculation is as follows:

[0094]

[0095] Output impedance Z 3k The calculation formula is as follows:

[0096]

[0097] Wherein, the intermediate parameter a 3k ,b 3k ,c 3k and d 3k yes:

[0098]

[0099]

[0100]

[0101] d 3k =kωC r R ek (37)

[0102] Among them, f n equals ω / ω r ω r It is the resonant frequency, λ equals L m / L r .

[0103] Z 3k The impedance angle is:

[0104]

[0105] The secondary current is:

[0106]

[0107] Among them, I sk It is the effective value of the secondary current, v in This is the input voltage for the LLC converter.

[0108] Therefore, the secondary current can be expressed in the time domain as:

[0109]

[0110] When the secondary current i' s When (t) crosses zero, the phase angle corresponding to the secondary current is θ. f That is, ωt=θ f .

[0111]

[0112] The synchronous rectification conduction time can be obtained in the following way:

[0113] Δt on =0.5f s -1 -θ f / ω (42)

[0114] Since (20) has no analytical solution, it is difficult to obtain θ directly using mathematical expressions. f However, due to the advantages of digital control, θ can be calculated using the bisection method in mathematical theory. f .

[0115] like Figure 1 As shown, the main control steps of the LLC converter during operation are as follows:

[0116] Step (1) Acquire the output current i o and output DC voltage v o The signal is input to the DSP via a sampling circuit; this signal is compared with the reference output voltage within the DSP to obtain an error signal. This error signal is then calculated by a proportional-integral (PI) controller to obtain a pulse frequency modulation signal. The pulse frequency modulation signal is input to the isolation drive circuit to obtain the drive signal for the primary-side switches Q1 to Q2 in the primary-side half-bridge converter circuit, thereby controlling the output current i. o and output DC voltage v o Control;

[0117] Step (2) Utilize the sampled output DC voltage v o and output current i o The equivalent output load is calculated, and the switching frequency is determined based on the proportional-integral controller. By considering harmonics, an impedance model is established in the frequency domain to accurately calculate the synchronous rectification conduction time. This signal utilizes existing sampling circuitry, eliminating the need for additional circuitry. Using the closed-loop obtained switching frequency and the established impedance model, θ is obtained through the mathematical bisection method. f Substituting into equation (42), the synchronous rectification conduction time is calculated.

[0118] Step (3) The turn-on time of the third and fourth synchronous switches S1 to S2 is the same as that of the first and second switches Q1 to Q2 on the primary side. The turn-off time of the third and fourth synchronous switches S1 to S2 is determined by the calculated synchronous rectification conduction time.

[0119] Based on the calculated synchronous rectification turn-on time from the established model, the synchronous rectifier turn-off time is equal to the primary-side turn-on time plus the calculated turn-on time. In the DSP, the synchronous rectification turn-on time is converted to obtain the comparison value of the comparator register, resulting in a pulse frequency modulation signal. Using an isolation chip and driver, the synchronous rectifier drive signal is output.

[0120] When the LLC converter is operating, the required output DC voltage v is controlled by closed-loop control. o and output current i o Calculate the equivalent load output.

[0121] Figure 2 The image shows the waveform of an LLC synchronous rectifier. (The waveform is derived from...) Figure 2 The changes of various circuit parameters over time under different switching frequencies can be obtained.

[0122] Figure 3 and Figure 4 The equivalent model and operating waveforms of the LLC converter are shown below. ab The voltage at the midpoint of the primary side bridge arm is v. in i Lr It is a resonant current, C j It is the equivalent output capacitance of the synchronous rectifier. The power devices, resonant LC pair and transformer are ideal cases. In the analysis, the resonant inductor generates leakage inductance and the dead time is ignored. n is the transformer turns ratio and k is the kth harmonic. From the above parameters in the equivalent model diagram, equations (22)(27)(33) can be obtained, and then equation (42) can be derived. Although equation (41) does not have an analytical solution, due to the advantages of digital control, θ can be obtained by using the bisection method in mathematical theory. f And thus the synchronous rectification conduction time is obtained.

[0123] Figure 5 A digital implementation scheme for the proposed synchronous rectification control method is presented. The scheme utilizes a TI DSP (TMS320F280049C) with a clock frequency up to 100MHz. The DSP enhances the clock frequency f of the pulse width modulation. clk Also 100MHz. According to... Figure 5 Using the rising-falling counting pattern, the PRD is calculated as f. clk / (2f s A cmp and B cmpThe formula is calculated based on the synchronous rectification conduction time:

[0124] A cmp =f clk ·Δt on (43)

[0125] B cmp =PRD-f clk ·Δt on (44)

[0126] Among them, f clk Indicates the DSP clock frequency, A cmp and B cmp Δt represents the count value in the comparison register. on It represents the conduction time of synchronous rectification in the time domain, and PRD represents the switching cycle of the rising-falling calculation mode.

[0127] When the value in the compare register equals A cmp Or B cmp At that time, the enhanced pulse width modulation digital signal will output the synchronous rectification drive signal.

[0128] Figure 6 A flowchart of the synchronous rectification is provided. Figure 6 As can be seen from this, the switching frequency f is calculated from the output of the proportional-integral controller using a pulse frequency modulation signal. s Calculate the synchronous rectification conduction time according to equation (21), and then obtain A. cmp Or B cmp To ensure safe operation, upper and lower limits are used to provide protection. Therefore, the proposed synchronous rectification control can be easily implemented in a DSP without the need for additional detection circuitry.

[0129] In summary, this invention is applicable to LLC converters. By considering harmonics and establishing an impedance model in the frequency domain, the synchronous rectification conduction time is accurately calculated, thereby minimizing the conduction time of the synchronous rectifier diode, significantly reducing conduction losses, and improving synchronous rectification accuracy and LLC conversion efficiency. Due to the advantages of digital control and mathematical models, the proposed synchronous rectification can be easily implemented without additional high-frequency signal detection, and it exhibits high immunity to switching noise caused by high-frequency switching devices during operation.

[0130] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A synchronous rectification control method for an LLC converter control device based on an extended harmonic impedance model, characterized in that, The LLC converter control device based on the extended harmonic impedance model includes an LLC converter, a sampling circuit, a digital signal processor, and an isolation drive circuit; the LLC converter includes a primary-side half-bridge converter circuit, a resonant circuit, and a secondary-side full-wave rectifier circuit; the primary-side half-bridge converter circuit includes a first switch Q1 and a second switch Q2; the resonant circuit includes a resonant inductor L. r Resonant capacitor C r And a transformer, wherein the inherent magnetizing inductance L in the transformer m The excitation inductor L m Located on the primary side of the transformer; the midpoint of the first switch Q1 and the second switch Q2 is connected to the resonant inductor L. r Connected in series, and then with the excitation inductor L m One end is connected to the excitation inductor L m The other end and the resonant capacitor C r One end is connected to the resonant capacitor C. r The other end is connected to the other end of Q2; the secondary full-wave rectifier circuit includes a third switch S1 and a fourth switch S2, one end of the third switch S1 and the fourth switch S2 are respectively connected to the two ends of the secondary winding of the transformer; the midpoint of the secondary winding of the transformer is connected to the load R. o At one end, the third switch S1 and the fourth switch S2 are connected in parallel to the load R. o At the other end; the first to fourth switching transistors Q1 to Q2 and S1 to S2 are all power switching transistors; The synchronous rectification control method includes the following steps: Step (1) Acquire the output current i o and output voltage v o The signal is input into the digital signal processor via the sampling circuit; The error signal is obtained by comparing the signal with the reference output voltage in the digital signal processor. This error signal is then calculated by the proportional-integral controller to obtain a pulse frequency modulation signal. This pulse frequency modulation signal is input to the isolation drive circuit to obtain the drive signals for the first and second switches Q1~Q2 on the primary side of the primary-side half-bridge converter circuit, thereby controlling the output current i. o and output DC voltage v o Control; Step (2) Utilize the sampled output DC voltage v o and output current i o The equivalent load of the output is calculated, and the switching frequency is calculated based on the proportional-integral controller. By considering harmonics, an impedance model is established in the frequency domain to accurately calculate the synchronous rectification conduction time. The impedance model is established as follows: Impedance Z 1k yes: (1) a 1k and b 1k The impedances Z are respectively 1k The real and imaginary parts are expressed as follows: (2) (3) Where ω is the switching frequency, and the unit is angle; C j It is the equivalent output capacitance of the synchronous rectifier MOSFET, where k represents the k-th harmonic; the equivalent output resistance R ek yes: (4) Where, r e It is the output load, and n is the transformer turns ratio; therefore, Z 1k The impedance angle is: (5) Considering the excitation inductance L m Z 2k for: (6) Wherein, the intermediate parameter a 2k , b 2k , c 2k and d 2k Calculated separately as follows: (7) (8) (9) (10) Impedance angle θ 2k The calculation is as follows: (11) Output impedance Z 3k The calculation formula is as follows: (12) Wherein, the intermediate parameter a 3k , b 3k , c 3k and d 3k yes: (13) (14) (15) (16) Among them, f n equals ω / ω r ω r It is the resonant frequency, λ equals L m / L r ; Z 3k The impedance angle is: (17) The secondary current is: (18) Among them, I sk It is the effective value of the secondary current, v in The input voltage for the LLC converter; Therefore, the secondary current can be expressed in the time domain as: (19) When the secondary current i ’ s (t) When it crosses zero, the phase angle corresponding to the secondary current is θ. f That is, ωt=θ f The secondary current is at this time: (20) The above equation has no analytical solution. θ is obtained by using the bisection method in mathematical theory. f ; The synchronous rectification conduction time is obtained in the following way: （21）； Step (3) The turn-on time of the third and fourth synchronous switches S1~S2 is the same as that of the first and second switches Q1~Q2 on the primary side. The turn-off time of the third and fourth synchronous switches S1~S2 is determined by the calculated synchronous rectification conduction time. The turn-off time of the third and fourth switches S1~S2 is equal to the turn-on time plus the calculated synchronous rectification conduction time.

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