阅读说明：本技术 一种llc谐振变换器线性补偿的软启动方法 (Soft start method for linear compensation of LLC resonant converter ) 是由 李光耀 邵李焕 俞文昌 于 2021-09-03 设计创作，主要内容包括：本发明公开了一种LLC谐振变换器线性补偿的软启动方法,包括如下步骤：步骤S1：设置软启动控制模型,其中,设置占空比的系统增益模型并根据该模型设计占空比D的变化函数；以及设置频率的系统增益模型,并控制输出的频率增益使系统增益为线性,以此得到频率f的变化函数；步骤S2：系统启动后,利用步骤S1设置的软启动控制模型控制PWM信号以控制开关管的状态实现软启动。本发明采用新的软启动控制模型,通过同时控制PWM的占空比和开关频率,实现快速的占空比从0到50％的变化,且电流增益变化在可控范围内,有效减小了开关损耗。同时引入频率的规律递减进行增益补偿,使得总体增益能够线性的上升,使得整个系统电流平稳增加。(The invention discloses a soft start method for LLC resonant converter linearity compensation, comprising the following steps: step S1: setting a soft start control model, wherein a system gain model of the duty ratio is set, and a change function of the duty ratio D is designed according to the model; setting a system gain model of the frequency, and controlling the output frequency gain to make the system gain linear so as to obtain a change function of the frequency f; step S2: after the system is started, the soft start control model set in step S1 is used to control the PWM signal to control the state of the switching tube to realize soft start. The invention adopts a new soft start control model, realizes the rapid change of the duty ratio from 0 to 50 percent by simultaneously controlling the duty ratio and the switching frequency of the PWM, and effectively reduces the switching loss because the current gain change is in a controllable range. Meanwhile, the regular decrement of the frequency is introduced for gain compensation, so that the overall gain can be increased linearly, and the current of the whole system is increased stably.)

1. A soft start method for LLC resonant converter linearity compensation is characterized by comprising the following steps:

step S1: setting a soft start control model, wherein a system gain model of the duty ratio is set, and a change function of the duty ratio D is designed according to the model; setting a system gain model of the frequency, and controlling the output frequency gain to make the system gain linear so as to obtain a change function of the frequency f;

step S2: after the system is started, the soft start control model set in step S1 is used to control the PWM signal to control the state of the switching tube to realize soft start, wherein step S2 at least includes the following steps:

step S21: setting initial parameters of a soft start control model, wherein the switching frequency of PWM is set as the highest starting frequency fm; its duty cycle is set to the lowest duty cycle 0%;

step S22: starting according to the set parameters, and simultaneously adjusting the switching frequency and the duty ratio of PWM by the soft start control model;

step S23: when in useAfter the duty ratio is saturated, only adjusting the switching frequency until the switching frequency is the resonant frequency; ending the soft start;

in step S1, the system gain model of the duty ratio D is:

wherein M is system gain, a, k are reference adjusting parameters, λ is 1/H is Lr/Lm, Q is Zo/Rac,

in order to enable the gain to change linearly and smoothly, the duty ratio D change function is set as follows:

whereinDmax is the maximum duty cycle, a, b, c are the adjustment parameters,the time taken to reach maximum duty cycle, t, is a time variable;

in the frequency system gain model, the system normalized gain M is:

or

Wherein f is_nFor normalized frequency, i.e. f_n＝f/fr；

f_rFor the resonant frequency, the calculation formula is:

in the above formula, Lr is the inductive reactance of the resonant inductor, and Cr is the capacitance value of the resonant capacitor;

the variation function of the frequency f is set to make the system gain linear, and the formula is as follows:

in the above equation, tp is the time taken to reach the resonant frequency, and fm is the maximum switching frequency that can be set.

2. The method of claim 1, wherein f is_mSet to 2fr to 3 fr.

3. The method of claim 1, wherein the model of the system gain of the duty cycle D is obtained from experimental data.

Technical Field

The invention relates to the technical field of LLC resonant converters, in particular to a soft start method for LLC resonant converter linearity compensation.

Background

The LLC resonant converter has the advantages of simple structure, convenient magnetic integration of the structure and high working frequency, so that higher power density can be realized. Because the resonant device can form resonance to realize soft switching when the resonant device works, the conversion efficiency of the LLC resonant converter is high, and meanwhile, when the resonant device is electrified and started, the capacitor is charged, so that a large impact current can be generated at the moment, and a circuit system and electric equipment are damaged. Soft start techniques are the most common way to effectively reduce the inrush current at the start of a resonant converter.

In the prior art, there are several soft start schemes altogether, the first is to increase the resonant impedance at start, keep the duty cycle at 50%, start by a switching frequency far higher than the resonant frequency to reduce the start current, and then gradually reduce to the resonant frequency. However, the higher starting frequency cannot ensure the starting current to be small, and the higher frequency has higher requirements on the driving circuit, which will correspondingly increase a lot of cost, and the soft starting time of the high-frequency starting is also longer. The second is to keep the resonant frequency constant and increase the duty cycle from 0 to 50% by adjusting it. The method has long regulation time, so that when the resonant converter works at the duty ratio of less than 50%, soft switching cannot be realized, the switching power consumption is increased, and the efficiency is reduced. In a third method for adjusting the operating frequency and the duty cycle simultaneously, the duty cycle is increased from 0 to 50%, the frequency is decreased from the highest operating frequency to the resonant frequency, and the conversion process can be linear conversion or exponential conversion. And the third scheme combines the first two schemes, but the gain in the adjusting process cannot be linearly converted, the current has burrs, the switching device is still greatly impacted, the duty ratio adjusting time is too long, and the loss is not reduced.

In summary, there are many technical problems with the existing soft start scheme, such as: the starting current can not be controlled within the range smaller than the resonant current, the duty ratio adjusting time is too long, the loss is too large, the gain is not linear change, the current has a burr phenomenon and the like.

Therefore, it is necessary to provide a technical solution to solve the technical problems of the prior art.

Disclosure of Invention

In view of the above, it is necessary to provide a soft start method for linear compensation of an LLC resonant converter, which employs a new soft start control model, and by controlling the duty cycle and the switching frequency of PWM simultaneously, realizes a fast change of the duty cycle from 0 to 50%, and the current gain change is within a controllable range, effectively reducing the switching loss. Meanwhile, the regular decrement of the frequency is introduced for gain compensation, so that the overall gain can be increased linearly, and the current of the whole system is increased stably.

In order to solve the technical problems in the prior art, the technical scheme of the invention is as follows:

a soft start method for LLC resonant converter linearity compensation comprises the following steps:

step S1: setting a soft start control model, wherein a system gain model of the duty ratio is set, and a change function of the duty ratio D is designed according to the model; setting a system gain model of the frequency, and controlling the output frequency gain to make the system gain linear so as to obtain a change function of the frequency f;

step S2: after the system is started, the soft start control model set in step S1 is used to control the PWM signal to control the state of the switching tube to realize soft start, wherein step S2 at least includes the following steps:

step S21: setting initial parameters of a soft start control model, wherein the switching frequency of PWM is set as the highest starting frequency fm; its duty cycle is set to the lowest duty cycle 0%;

step S22: starting according to the set parameters, and simultaneously adjusting the switching frequency and the duty ratio of PWM by the soft start control model;

step S23: when in useAfter the duty ratio is saturated, only adjusting the switching frequency until the switching frequency is the resonant frequency; ending the soft start;

in step S1, the system gain model of the duty ratio D is:

wherein M is system gain, a, k are reference adjusting parameters, λ is 1/H is Lr/Lm, Q is Zo/Rac,

in order to enable the gain to change linearly and smoothly, the duty ratio D change function is set as follows:

where Dmax is the maximum duty cycle, a, b, c are the tuning parameters,the time taken to reach maximum duty cycle, t, is a time variable;

in the frequency system gain model, the system normalized gain M is:

or

Wherein f is_nFor normalized frequency, i.e. f_n＝f/fr；

f_rFor the resonant frequency, the calculation formula is:

in the above formula, Lr is the inductive reactance of the resonant inductor, and Cr is the capacitance value of the resonant capacitor;

the variation function of the frequency f is set to make the system gain linear, and the formula is as follows:

in the above equation, tp is the time taken to reach the resonant frequency, and fm is the maximum switching frequency that can be set.

As a further improvement, f_mSet to 2fr-3 fr.

As a further improvement, a system gain model of the duty ratio D is obtained according to experimental data.

Compared with the prior art, the invention adopts a new soft start control model, realizes the rapid change of the duty ratio from 0 to 50 percent by simultaneously controlling the duty ratio and the switching frequency of the PWM, and effectively reduces the switching loss because the current gain change is in a controllable range. And meanwhile, the regular decrement of the frequency is introduced for gain compensation, so that the overall gain can be linearly increased, namely the gain curve is M (ht), and the current of the whole system is stably increased. By adopting the method, the quick gain of starting the LLC resonant converter can be realized, the starting current is small, the circuit loss is small, and the starting time is short.

Drawings

Fig. 1 is a schematic diagram of a circuit model of an LLC resonant converter in the present invention.

Fig. 2 is a flow chart of a soft start method for LLC resonant converter linearity compensation of the present invention.

Fig. 3 is a gain curve in the constant frequency control.

Fig. 4 is a schematic diagram of a circuit configuration of an LLC resonant converter in accordance with another preferred embodiment of the present invention.

FIG. 5 is a diagram of the start-up current of the method of the present invention.

Fig. 6 is a diagram of the startup current for the no soft start scheme.

Fig. 7 is a diagram of the start-up current of the first prior art scheme.

Fig. 8 is a diagram of a second prior art scheme start-up current.

Fig. 9 is a diagram of the start-up current of a third prior art scheme.

The following specific embodiments will further illustrate the invention in conjunction with the above-described figures.

Detailed Description

The technical solution provided by the present invention will be further explained with reference to the accompanying drawings.

Referring to fig. 1, a model diagram of an LLC resonant full-bridge converter of the present invention is shown, including a main circuit, a control circuit and its associated filter circuit; the main circuit is divided into a primary side and a secondary side, the primary side is composed of an MOSFET bridge type conversion circuit and an LLC resonance circuit, the middle is a power conversion circuit, and the secondary side is composed of an output rectification circuit. The primary side is controlled by a digital chip control driving circuit to drive the MOSFET to adjust.

Referring to fig. 2, there is shown a flow chart of a soft start method for LLC resonant converter linearity compensation according to the present invention, wherein a soft start control procedure is executed in a digital chip, and the method at least includes the following steps:

step S1: setting a soft start control model, wherein a system gain model of the duty ratio is set, and a change function of the duty ratio D is designed according to the model; setting a system gain model of the frequency, and controlling the output frequency gain to make the system gain linear so as to obtain a change function of the frequency f;

step S2: after the system is started, the soft start control model set in step S1 is used to control the PWM signal to control the state of the switching tube to realize soft start, wherein step S2 at least includes the following steps:

step S21: setting initial parameters of a soft start control model, wherein the switching frequency of PWM is set as the highest starting frequency fm; its duty cycle is set to the lowest duty cycle 0%;

step S22: starting according to the set parameters, and simultaneously adjusting the switching frequency and the duty ratio of PWM by the soft start control model;

step S23: when in useAfter the duty ratio is saturated, only adjusting the switching frequency until the switching frequency is the resonant frequency; end of soft start；

In step S1, the system gain model of the duty ratio D is:

wherein M is system gain, a, k are reference adjusting parameters, λ is 1/H is Lr/Lm, Q is Zo/Rac,

in order to enable the gain to change linearly and smoothly, the duty ratio D change function is set as follows:

where Dmax is the maximum duty cycle, a, b, c are the tuning parameters,the time taken to reach maximum duty cycle, t, is a time variable;

in the frequency system gain model, the system normalized gain M is:

or

Wherein f is_nFor normalized frequency, i.e. f_n＝f/fr；

f_rFor the resonant frequency, the calculation formula is:

in the above formula, Lr is the inductive reactance of the resonant inductor, and Cr is the capacitance value of the resonant capacitor;

the proposed mathematical model of duty cycle gain is not capable of linearly increasing the duty cycle gain, and the present invention contemplates compensation by frequency gain in order to make the system gain linear.

Applicants have discovered by studying the relationship of gain to frequency and duty cycle that the total gain is the product of the duty cycle gain and the frequency gain. Based on the discovery, creatively provides that regular decrement of the frequency f is introduced at the same time to compensate the duty ratio gain, so that the overall gain can be increased linearly, namely the gain curve is M-ht, and the starting current of the whole system is increased smoothly.

The variation function of the frequency f is set to make the system gain linear, and the formula is as follows:

in the above equation, tp is the time taken to reach the resonant frequency, and fm is the maximum switching frequency that can be set.

In the technical scheme, the duty ratio and the switching frequency of the PWM are controlled simultaneously, so that the rapid change of the duty ratio from 0 to 50 percent is realized, the current gain change is in a controllable range, and the switching loss is effectively reduced. And meanwhile, the regular decrement of the frequency is introduced for gain compensation, so that the overall gain can be linearly increased, namely the gain curve is M (ht), and the current of the whole system is stably increased. By adopting the method, the quick gain of starting the LLC resonant converter can be realized, the starting current is small, the circuit loss is small, and the starting time is short.

In a preferred embodiment, the system gain model for duty cycle D is derived from experimental data. Since an explicit expression of the duty ratio cannot be mathematically derived, by assigning D, a curve of the gain M with respect to D at different Q values when operating at the resonance frequency when λ is 0.1 is thus made, see fig. 3, which shows the gain curve in the case of fixed frequency control.

Typically we set Q between 0.1 and 1, thus we can derive a system gain M for duty cycle D that is approximately:

wherein a, k are reference adjusting parameters, λ 1/H Lr/Lm, Q Zo/Rac,

in a preferred embodiment, f_mSet to 2fr-3 fr.

Referring to fig. 4, a schematic diagram of a circuit structure of an LLC resonant converter in accordance with another preferred embodiment of the present invention is shown, including a main circuit, a control circuit and its related circuits; the main circuit is divided into a primary side and a secondary side, the primary side is composed of an MOSFET bridge type conversion circuit and an LLC resonance circuit, the middle is a power conversion circuit, and the secondary side is composed of an output rectification circuit. One end of the MOSFET bridge type conversion circuit is connected with a power input, the other end of the MOSFET bridge type conversion circuit is connected with the LLC resonance circuit, the LLC resonance circuit is connected with the power conversion circuit and the primary side current detection circuit, the power conversion circuit is connected with the secondary side output rectifying circuit, and the output rectifying circuit is connected with the output end. The control circuit consists of a voltage and current sampling module, a digital chip controller module and an isolation driving module, wherein the isolation driving module and the digital chip controller module are driven by an auxiliary power supply module. One end of a voltage and current acquisition module is connected with the output end of a secondary side circuit, the other end of the voltage and current acquisition module is connected with an ADC channel of a digital chip, a digital chip control module is respectively connected with an isolation driving module and a primary side current detection circuit, and the other end of the isolation driving module is connected with an MOSFET bridge type conversion circuit.

Further, the specific parameters of the circuit are set as follows: vin is 24V; lr is 6.582 uH; cr is 1.539 uf; the transformer transformation ratio is 5:5: 5; the filter capacitor C is 1 mf; the leakage inductance Lm is 32.911uH, and the equivalent resistance is 5.76 omega; the rated output voltage Vo is 24V.

The control process of the LLC resonant converter is as follows:

(1) after the system is powered on, relevant software and hardware settings are initialized, and a soft start stage can be started after no fault exists;

(2) the soft start will set the PWM on frequency and duty cycle at the beginning: the switching frequency is set to be the highest opening frequency fm which is 150 kHz; the duty cycle is set to the lowest duty cycle of 0%.

(3) Soft start mathematical model setting of duty cycle.

According to the above model, the parameter Q is set to 0.5, a to 1, k to 4, and Dmax to 0.49.

The system gain M of the duty cycle is:

in order to solve the technical problems of slow starting process, unstable gain in the later starting period and the like in the prior art, the invention optimizes the starting process on the basis of the technical problems, thereby leading the gain to show linear and stable change. The duty cycle D variation function with time t is set according to equation (2) as:

in order to enable the duty ratio to reach a target value as soon as possible, realize soft switching, reduce loss and optimize a duty ratio change function, the duty ratio change function D is also provided along with the time t. Further, the maximum duty ratio Dmax is set to 0.49, a is set to 1, b is set to 1, and c is set to 0, and the approximate time t taken to reach the maximum duty ratio is set to 0.618.

The duty ratio D variation function is now set to:

(4) soft start mathematical model setting of frequency.

According to the principle of the LLC resonant converter, the obtained resonant frequency fr is:

the parameters λ and Q are substituted to 0.2 and 0.5, where the normalized starting frequency fn is used on the abscissa.

The system normalized gain M can be obtained as:

the duty cycle gain cannot be increased linearly due to the previous duty cycle gain function, and the frequency gain is set to make the system gain linear. At present, the invention introduces regular decrement of frequency to compensate duty ratio gain, so that the overall gain can be increased linearly, namely, the gain curve is M-ht, and the current of the whole system is increased and decreased stably.

The invention sets parameter tp as 1S, the working frequency is decreased from the maximum switching frequency, and the variation function of approximate f is obtained as follows:

(5) the soft start is started according to the set parameters, after the time t is 0.618S, the duty ratio is saturated first, the gain is approximately 1 at the moment, the soft switching mode is entered in advance, only the frequency is adjusted until the time 1S is reached, the frequency is the resonance frequency at the moment, and the soft start is finished.

(6) And ending the soft start state and entering a normal operation state.

By adopting the technical scheme, the method has the following technical effects:

1. by quickly saturating the duty cycle, the loss is effectively reduced and the start-up time is reduced.

2. By studying the relationship between gain and frequency and duty cycle, it is found that the total gain of the system is the product of the duty cycle gain and the frequency gain. The invention introduces a law of decreasing frequency and compensates duty ratio gain, so that the overall gain can be increased linearly, namely the gain curve is M-ht, the current of the whole system is increased stably, overcurrent is avoided, the current burr phenomenon is obviously reduced, and circuit components are protected better.

In order to verify the technical effect of the method, the size of the primary current is compared with the size of the primary current when the scheme in the prior art is started through simulation. Wherein, fig. 5 is a starting current diagram of the method of the present invention. Fig. 6 is a diagram of the startup current for the no soft start scheme. Fig. 7 is a diagram of the start-up current of the first prior art scheme. Fig. 8 is a diagram of a second prior art scheme start-up current. Fig. 9 is a diagram of the start-up current of a third prior art scheme.

Compared with the starting current of the first scheme and the second scheme in the prior art, the starting circuit can realize the gradual gain starting of the current, the starting current is smaller, no burr phenomenon exists in the starting current increasing process, circuit components and parts cannot be damaged by overcurrent, and the soft switching state can be quickly started to reduce the switching loss compared with the second scheme. Compared with the third scheme in the prior art, the method can enter a soft switching state faster (about half the time of the third scheme), reduces switching loss, obviously reduces starting current burrs, and well protects circuit components.

The above description of the embodiments is only intended to facilitate the understanding of the method of the invention and its core idea. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.