CN103326587A - Light load control method and device of LLC (Liquid Level Control) resonant converter - Google Patents

Abstract

The invention provides a light load control method and device of an LLC resonant converter, belonging to the field of power conversion. It solves the problem of loss of soft switching environment for zero voltage switching and zero current switching. This method: A. Judging whether it is working under a light load state; B. When it is not working under a light load state, carry out frequency adjustment control mode; C. When it is light load, carry out constant frequency pulse width control mode; D. Output corresponding Switch tube control signal. The device includes a controller, a frequency modulation module and a width adjustment module, a current detection unit and a voltage detection unit, the controller is connected to a feedback compensation error amplifier, the voltage detection unit is connected to a feedback compensation error amplifier, the current detection unit is connected to the controller, and the controller is connected to a The working module switching unit, the frequency modulation module and the width adjusting module are respectively connected to the controller and the working module switching unit, and the working module switching unit is connected to the switching tube. The device and method realize the optimization of soft switching environment and resonance parameters in the full load range.

Description

LLC resonant converter light load control method and device

technical field

The invention belongs to the field of power conversion, and relates to a light load control method and device of an LLC resonant converter.

Background technique

With the development of switching power supply technology, high efficiency and high power density have become the development trend. Therefore, the application of LLC resonant converter is born and becomes more and more extensive. The LLC resonant converter realizes the zero-voltage turn-on of the primary-side switch tube and the zero-current turn-off of the secondary-side rectifier tube in a wide frequency range. But as the load is reduced, in order to obtain the same DC gain, the LLC resonant converter needs to increase the switching frequency. The increase of switching frequency, on the one hand, increases the switching loss; on the other hand, it will also cause the current of the diode on the rectification side to be turned off when it does not return to zero, and the soft switching environment of the zero-current switch is lost, further increasing the switching loss. At the same time, the light load will reduce the Q value of the circuit quality factor, and the load change will seriously affect the voltage gain. Therefore, as the switching frequency increases under light load, the efficiency of the LLC resonant converter will be affected. Compared with its high efficiency under heavy load, it is difficult to take into account the efficiency improvement under light load at the same time.

With the higher requirements for the efficiency of power converters, LLC converters need to obtain higher light-load efficiency. In the prior art, a control method with a hysteresis at the output terminal and a comparator circuit is often used. When the output voltage rises to a certain upper limit When the control signal of the switching tube is turned off, the converter enters the idle state; when the output voltage drops to a certain lower limit value, the frequency modulation control signal of the switching tube is generated, and the converter enters the working state of output power. This method is only suitable for very light load or standby state. Other light-load efficiency improvement strategies include adding switching tubes or diode auxiliary branches in the primary and secondary, and PWM control schemes with a fixed duty cycle. These schemes can reduce the loss at no-load or light load, but because of adding hardware to the main circuit, it will have a certain impact on the reliability of the circuit, and it is difficult to take into account the efficiency improvement at heavy load at the same time.

Chinese patent literature discloses a control method and device for an LLC resonant converter with the application number 200910221471.4. The device judges the load state by inputting the conduction frequency of the switch tube, and then performs frequency conversion and width adjustment control according to the load state. To a certain extent The circuit loss is reduced, but the frequency conversion width adjustment control has no constraints, which will cause the loss of the soft switching environment of the zero-voltage switching of the switching tube and the zero-current switching of the rectifier tube of the LLC resonant converter, so that the voltage gain of the resonant cavity is uncertain.

Contents of the invention

Aiming at the above-mentioned problems in the existing technology, the present invention proposes a light-load control method and device for an LLC resonant converter. The method and device adopt light-load constant-frequency pulse width control, thereby realizing soft switching environment and resonance in the full load range Parameter optimization to improve the efficiency of the converter, especially the light load efficiency.

The present invention is realized through the following technical solutions: a light-load control method of an LLC resonant converter, which is characterized in that it includes a hardware circuit and a load control circuit of the LLC resonant converter, and the specific steps of the light-load control method of the LLC resonant converter as follows:

A. According to the output voltage signal and output current signal adjusted by the hardware circuit of the LLC resonant converter, it is judged whether the output end of the resonant converter is working under light load state;

B. When the output end of the resonant converter is not working in a light load state, the load control circuit performs frequency adjustment control mode;

C. When the output terminal of the resonant converter is working in a light load state, the load control circuit performs a constant frequency pulse width control mode;

D. Outputting a corresponding switching tube control signal to the switching tube of the LLC resonant converter.

Use the output voltage signal and output current signal adjusted by the hardware circuit of the LLC resonant converter to judge whether the output terminal of the resonant converter is working under a light load state. The working states of the output terminal of the resonant converter include heavy load, rated load and light load. Wait for three states. When the output terminal of the resonant converter is working at heavy load or rated load, the load control circuit performs frequency regulation control mode. When the output end of the resonant converter is working in a light load state, the load control circuit performs a constant frequency pulse width control mode. This process realizes the zero-voltage switching of the switch tube and the zero-current switching software environment of the rectifier tube in the full load range. Thereby greatly reducing the switching loss.

In the above light-load control method of the LLC resonant converter, in the step A, the output voltage signal is calculated through feedback compensation to obtain an error voltage signal and an output current signal to determine whether the output terminal of the resonant converter is working under a light-load state. Here, the corresponding error voltage signal and output current signal obtained through feedback compensation calculation can more accurately judge the working state of the output end of the resonant converter.

In the above light load control method of the LLC resonant converter, in the step C, in the periodic processing of the fixed frequency pulse width control mode, the fixed switching frequency is the resonant frequency, and the PWM is calculated according to different load output voltage regulation requirements. Duty cycle D. PWM is pulse width regulation, and the fixed frequency pulse width control mode in this method is a pulse width regulation control method with a fixed frequency. Here, determining the relationship between the fixed switching frequency and the load output voltage regulation requirement and the PWM duty ratio D has the effect of optimizing the parameters of the resonant network.

In the above light load control method of the LLC resonant converter, the PWM duty cycle is used to adjust the voltage gain of the resonant network in the resonant converter. At this time, the formula for calculating the voltage gain of the resonant network in the light load is: $G_{\mathrm{ac}}=\frac{1-\cos \left(2\mathrm{\πD}\right)}{2}.$

其中fr为谐振频率，即固定开关频率，f_r2为第二谐振频率。在额定负载时采用频率调节控制模式，此时开关频率在f_r2<f≤f_r内变动。这里确定轻负载谐振网络电压增益的占空比D变化范围跟重负载和额定负载相连接使固定频率取值更加稳定。In the above light load control method of the LLC resonant converter, the range of the duty ratio D for adjusting the voltage gain of the light load resonant network is

Among them, fr is the resonant frequency, that is, the fixed switching frequency, and f _r2 is the second resonant frequency. When the rated load is used, the frequency adjustment control mode is adopted, and the switching frequency changes within f _r2 <f≤f _r at this time. Here, it is determined that the duty ratio D variation range of the light load resonant network voltage gain is connected with the heavy load and the rated load to make the value of the fixed frequency more stable.

In the above light load control method of the LLC resonant converter, the resonant network in the resonant converter works at the switching frequency f=f _r under the rated load, and the switching frequency f=f _r is the frequency adjustment control mode and the fixed frequency pulse width The critical operating point of the control mode. Here, the switching frequency under the rated load working condition is the fixed switching frequency under the light load working condition, and it is used as the critical working point of the frequency regulation control mode and the constant frequency pulse width control mode. It is convenient to optimize the resonance parameters of the resonant network in the resonant converter.

In the light-load control method of the LLC resonant converter described above, the switching between the frequency adjustment control mode and the constant frequency pulse width control mode is performed within a dead time after one switching period ends. Here, the switching between the frequency adjustment control mode and the fixed frequency pulse width control mode is realized within the dead time without being affected by the previous switching cycle, so that the control is more accurate.

A light load control device for an LLC resonant converter, characterized in that the device includes a controller, a frequency modulation module for making the resonant converter work in a frequency regulation control mode and a frequency modulation module for making the resonant converter work in a fixed frequency pulse width The width adjustment module under the control mode, the voltage detection unit and the current detection unit for respectively detecting the output voltage signal and the output current signal of the hardware circuit conditioning of the LLC resonant converter, the controller is also connected with a feedback compensation error amplifier, so The voltage detection unit and the reference voltage power supply are respectively connected to the input end of the feedback compensation error amplifier, the current detection unit is connected to the input end of the controller, and the output end of the controller is connected to a working module switching unit. The wide module is respectively connected to the controller and the switching unit of the working module, the switching unit of the working module is connected to the control terminal of the switching tube of the resonant converter, and the error voltage signal output by the controller after the output voltage detected by the voltage detection unit is fed back and compensated by the error amplifier Judging whether the LLC resonant converter is working under light load with the output current signal detected by the current detection unit; if it is not working under light load, select the frequency modulation module to work through the working module switching unit and perform frequency adjustment control mode; if it is working under light load , the width adjustment module is selected to work through the working module switching unit, and the constant frequency pulse width control mode is performed to realize the duty ratio to adjust the voltage gain, and the corresponding switching tube control signal is output to the switching tube control terminal through the working module switching unit.

LLC resonant converter hardware circuit bus supply voltage is DC voltage. The load control circuit includes a controller, a voltage detection unit, a current detection unit, a frequency modulation module and a width adjustment module. The voltage detection unit collects the DC output load voltage signal of the LLC resonant converter hardware circuit, sends it to the feedback compensation error amplifier, and compares it with the reference voltage, and the compared error voltage signal is sent to the controller through the feedback compensation error amplifier. The controller receives the output current signal collected by the current detection unit at the same time, the controller judges whether the load is rated load, heavy load or light load according to the received error voltage signal and output current signal, and selects the corresponding control mode through the working module switching unit; if it is working In heavy load or rated load, the frequency modulation module is selected to work through the working module switching unit, and the frequency adjustment control mode is performed; if the work is under light load, the width adjustment module is selected to work through the working module switching unit, and the constant frequency pulse width control mode is performed. Realize the duty ratio to adjust the voltage gain, and output the corresponding switching tube control signal to the switching tube control terminal through the working module switching unit.

In the above-mentioned LLC resonant converter light load control device, the width adjustment module is used to generate a fixed-frequency pulse width signal with a fixed frequency and to adjust the duty ratio according to the output voltage signal of the resonant converter. Arm switch tube symmetrical output form. Here it is realized that under light load conditions, the control signals received by the resonant converter are two PWM signals with the same frequency, the same duty cycle and a timing difference of half a switching period.

In the light-load control device of the LLC resonant converter described above, the inner resonant network of the LLC resonant conversion is low excitation inductance. On the one hand, low magnetizing inductance will cause higher circulating current when the primary side of the transformer is light-loaded, and on the other hand, it will cause more conduction and switching losses when the resonant current is large under heavy load. Low excitation inductance can obtain narrower switching frequency under the same gain, which has a certain effect on optimizing the size of the transformer and the control circuit.

Compared with the prior art, the LLC resonant converter light load control method and device have the following advantages:

1. The device and method adopt light-load constant-frequency pulse width control to realize the soft-switching environment and optimized design of resonance parameters in the full load range, and improve the efficiency of the resonant converter, especially the light-load efficiency.

2. The device and method directly select the working mode according to the voltage and current conditions of the load. At the same time, the present invention also clearly points out that the critical switching point of the two working modes is at the rated load resonance frequency. Ensure the soft switching environment of the zero-voltage switch of the switching tube and the zero-current switch of the rectifier tube in the whole control process, so as to effectively determine the voltage gain of the resonant cavity.

Description of drawings

Fig. 1 is a schematic diagram of the circuit of the present invention;

Fig. 2 is a control flowchart of the present invention;

Fig. 3 is a waveform diagram of the switch tube control signal generated by the width adjustment module and the key current of the resonant cavity during the light load regulation of the present invention. Where T _r =1/f _r , a=D·T _r ;

Fig. 4 is a graph of the normalized gain characteristic curve of the resonant cavity in the constant frequency pulse width control mode of the present invention;

Fig. 5 is an experimental waveform diagram of the key current of the resonant cavity in the constant frequency pulse width control mode of the present invention.

Detailed ways

The following are specific embodiments of the present invention, and further describe the technical solution of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments.

As shown in Figure 1-5, the light load control method of the LLC resonant converter includes the hardware circuit and the load control circuit of the LLC resonant converter. The specific steps of the light load control method of the LLC resonant converter are as follows:

A. According to the output voltage signal and output current signal adjusted by the hardware circuit of the LLC resonant converter, it is judged whether the output terminal of the resonant converter is working under a light load state; the output voltage signal is calculated by feedback compensation to obtain the error voltage signal and output current signal It is judged whether the output terminal of the resonant converter is working in a light load state.

B. When the output end of the resonant converter is not working in a light load state, the load control circuit performs frequency adjustment control mode;

C. When the output end of the resonant converter is working in a light load state, the load control circuit performs a constant frequency pulse width control mode. In the periodic processing of the fixed-frequency pulse width control mode, the fixed switching frequency is the resonant frequency, and the PWM duty ratio D is calculated according to different load output voltage regulation requirements.

D. Outputting a corresponding switching tube control signal to the switching tube of the LLC resonant converter.

f_r为谐振频率，即固定开关频率，f_r2为在频率调节控制模式下的开关最低频率，即第二谐振频率。谐振变换器内谐振网络工作于额定负载下的开关频率f=f_r，开关频率f=f_r为频率调节控制模式和定频脉宽控制模式的临界工作点。这里额定负载工作状态下的开关频率即为轻负载工作状态下的固定开关频率，且作为频率调节控制模式和定频脉宽控制模式的临界工作点。便于优化谐振变换器内谐振网络的谐振参数。频率调节控制模式和定频脉宽控制模式的切换在一个开关周期结束后的死区时间内进行。The PWM duty cycle is used to adjust the voltage gain of the resonant network in the resonant converter. At this time, the formula for calculating the voltage gain of the resonant network with light load is The duty cycle D to adjust the voltage gain of the light-load resonant network varies from

f _r is the resonant frequency, that is, the fixed switching frequency, and f _r2 is the lowest switching frequency in the frequency regulation control mode, that is, the second resonant frequency. The resonant network in the resonant converter works at the switching frequency f= _fr under the rated load, and the switching frequency f= _fr is the critical operating point of the frequency regulation control mode and the constant frequency pulse width control mode. Here, the switching frequency under the rated load working condition is the fixed switching frequency under the light load working condition, and it is used as the critical working point of the frequency regulation control mode and the constant frequency pulse width control mode. It is convenient to optimize the resonance parameters of the resonant network in the resonant converter. The switching between the frequency regulation control mode and the fixed frequency pulse width control mode is carried out within the dead time after a switching cycle ends.

The LLC resonant converter light load control device includes a controller, a frequency modulation module for making the resonant converter work in the frequency regulation control mode and a width adjustment module for making the resonant converter work in the fixed frequency pulse width control mode, A voltage detection unit and a current detection unit for respectively detecting the output voltage signal and the output current signal of the hardware circuit conditioning of the LLC resonant converter, the controller is also connected with a feedback compensation error amplifier, the voltage detection unit and the reference voltage power supply The input ends of the feedback compensation error amplifier are respectively connected, the current detection unit is connected to the input end of the controller, the output end of the controller is connected to a working module switching unit, and the frequency modulation module and the width adjustment module are connected to the controller and the working module respectively. A module switching unit, the working module switching unit is connected to the control terminal of the switching tube of the resonant converter, and the controller outputs an error voltage signal after the output voltage detected by the voltage detection unit is fed back to compensate the error amplifier and the output current detected by the current detection unit The signal judges whether the LLC resonant converter is working in the light load state; if it is not working in the light load, select the frequency modulation module to work through the working module switching unit, and perform frequency adjustment control mode; if it is working in the light load, select through the working module switching unit The width adjustment module works in a fixed-frequency pulse width control mode to realize duty ratio adjustment voltage gain, and outputs a corresponding switching tube control signal to the switching tube control terminal through the working module switching unit.

The width adjustment module is used to generate a fixed-frequency pulse-width signal with a fixed frequency and a variable duty ratio adjusted according to the output voltage signal of the resonant converter. The upper and lower arm switches of the resonant converter have a symmetrical output form.

The light load control method and device of the LLC resonant converter of the present invention can be applied to resonant power converters of full bridge, half bridge or push-pull structure. A resonant converter generally includes an electronic switch, an output terminal of a half-bridge or full-bridge structure, which is composed of a resonant inductor, a resonant capacitor, and the primary winding of a transformer or an electromagnetic induction coil in series, wherein the resonant inductor can be magnetically integrated with the transformer or exist alone. The LLC resonant converter selected in this embodiment is a half-bridge LLC resonant converter as shown in Figure 1, including: input DC voltage V _in , a square wave generator driven by switching tube Q1 and switching tube Q2, and a series resonant capacitor Resonance network composed of C _r , series resonant inductance L _r and parallel excitation inductance L _m , ideal transformer T with ratio n:1:1, rectifier circuit composed of diode VD ₁ and diode VD ₂ , filter capacitor C _O and load Resistance R _O . Among them, the series resonant inductor L _r can be an independent component, and can also be magnetically integrated with the transformer T. The figure shows the timing waveforms of the control signals control of the switch tube Q ₁ and the switch tube Q ₂ and the working module selection signal mode at critical points. The above-mentioned voltage detection unit and current detection unit can be composed of resistors, or can be composed of Hall elements or voltage and current transformers; wherein the controller, frequency modulation module, width adjustment module and working module selection circuit can be realized by analog circuits, or It can be realized by digital circuits, including programmable controllers with software computing capabilities.

As shown in Figures 1 and 2, FM in the figure represents the frequency regulation control mode, PWM represents the fixed frequency pulse width regulation control mode, and the LLC resonant converter hardware circuit bus supply voltage is DC voltage. The load control circuit includes a controller, a voltage detection unit, a current detection unit, a frequency modulation module and a width regulation module. The voltage detection unit collects the DC output load voltage signal of the LLC resonant converter hardware circuit, sends it to the feedback compensation error amplifier, and compares it with the reference voltage, and the compared error voltage signal is sent to the controller through the feedback compensation error amplifier. The controller receives the output current signal collected by the current detection unit at the same time. The controller judges whether the load is rated load, heavy load or light load according to the received error voltage signal and output current signal, and selects the corresponding control mode through the working module switching unit; if it is working In heavy load or rated load, the frequency modulation module is selected to work through the working module switching unit, and the frequency adjustment control mode is performed; if the work is under light load, the width adjustment module is selected to work through the working module switching unit, and the constant frequency pulse width control mode is performed. Realize the duty ratio to adjust the voltage gain, and output the corresponding switching tube control signal to the switching tube control terminal through the working module switching unit. If the fixed frequency pulse width control mode is selected, the controller will fix the switching frequency as the resonant frequency in the period processing of the width adjustment module, and calculate the PWM duty cycle according to the voltage gain formula of the fixed frequency pulse width control mode according to different load output voltages ; In the cycle processing of the frequency modulation module, the controller has a fixed duty ratio of 1/2 (ignoring the dead time), and the switching frequency is calculated by the voltage gain formula of FM mode according to different load output voltages; The wide module or frequency modulation module works, and outputs the control signal of the switching tube. The controller adjusts the duty ratio of the width adjustment module according to different error voltage values, thereby adjusting the voltage gain of the circuit and keeping the output load voltage of the LLC resonant converter stable. The specific adjustment process includes:

The frequency adjustment control mode of frequency adjustment gain is adopted at the rated load, and the switching frequency changes within f _r2 <f≤f _r , and the signal flow diagram in the figure shows that Q ₁ and Q ₂ are switch tube Q1 and switch tube Q2 in Figure 1 corresponding drive signal. The duty cycle of the driving signals Q ₁ and Q ₂ of the switching tubes is fixed at 1/2. The resonant network voltage gain is:

${\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\sqrt{{<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

in $K=\frac{L_m}{L_r},f_r=\frac{1}{2\mathrm{\&pi;}\sqrt{L_r{C}_r}},{\mathrm{<figure-callout\; id="2"\; label="f\; r"\; filenames="HDA00003526076500031.png"\; state="\{\{state\}\}">f</figure-callout>}}_r=\frac{1}{2\mathrm{\&pi;}\sqrt{(L_r+L_m)C_r}},$ Quality factor $Q=\frac{2{\mathrm{\&pi;}}^3{f}_r{L}_r}{8{\mathrm{no}}^2{R}_o}.$ In the formula, C _r is the capacitance value of the series resonant capacitor C _r , L _r is the inductance value of the series resonant inductor Lr, L _m is the inductance value of the excitation inductance L _m , and K is the ratio of the excitation inductance to the resonance inductance.

When the load becomes lighter, the fixed-frequency pulse width control mode with the switching frequency fixed at f= _fr is adopted, and the voltage gain is adjusted by the PWM duty cycle D. The formula for calculating the gain of the resonant network circuit is $G_{\mathrm{ac}}^{\&prime;}=\frac{1-\cos \left(2\mathrm{\&pi;D}\right)}{2},$ where the duty cycle D varies in the range of $1-\frac{f_r}{2{\mathrm{<figure-callout\; id="2"\; label="f\; r"\; filenames="HDA00003526076500031.png"\; state="\{\{state\}\}">f</figure-callout>}}_r}<\mathrm{D.}\&le;\frac{1}{2}.$

每半个开关周期为T，负载电流Io由谐振电流i_Cr和励磁电流i_Lm之差经变压器耦合得到，满足：

I_o与i_Cr和i_Lm围成的面积成正比。输出功率时，励磁电流i_Lm因为整流侧的嵌位保持了恒定的线性变化。因此，当负载减轻时，谐振电流i_Cr必须减小才能适应这种变化。因为开关频率固定在f=f_r，没有改变谐振网络的阻抗特性，可以通过PWM占空比D控制输入电压来适应谐振电流i_Cr的减小。When the PWM signal with a duty ratio of D is used to control the LLC resonant converter half-bridge switching tube, through Fourier decomposition, the amplitude of the fundamental component of the DC voltage V _in input to the resonant network by the bus power supply is: However, the amplitude of the fundamental component in the traditional frequency adjustment control mode is

Every half switching period is T, and the load current Io is obtained by coupling the difference between the resonant current i _Cr and the excitation current i _Lm through the transformer, which satisfies:

I _o is proportional to the area surrounded by i _Cr and i _Lm . When outputting power, the excitation current i _Lm maintains a constant linear change because of the clamping of the rectification side. Therefore, when the load is reduced, the resonant current i _Cr must decrease to adapt to this change. Because the switching frequency is fixed at f= _fr , the impedance characteristics of the resonant network are not changed, and the input voltage can be controlled by PWM duty ratio D to adapt to the reduction of resonant current i _Cr .

According to the above-mentioned analysis, the results obtained by the present invention can be fully embodied in Fig. 3, 4, 5, as shown in Fig. 3, the operating mode analysis of the fixed-frequency pulse width regulation control mode under the light load state is as follows:

It can be seen in Figure 3 that at time t ₀ , the drive signal Q ₂ is turned off, the excitation current i _Lm reverses and starts to rise linearly, the resonant current i _Cr rises according to the resonant frequency f _r , and the resonant current i _Cr > the excitation current i _Lm , the diode VD ₁ on the rectification side is turned on, and the output terminal obtains energy. Because of the existence of the duty cycle, the Q ₁ drive signal has not arrived at this time;

At the time t ₁ before the resonant current i _Cr crosses zero, apply the Q ₁ driving signal; due to the light load, the resonant current i _Cr is lower than the rated load, and the resonant current i _Cr = excitation current i _Lm occurs before reaching half a cycle At time _t2 , the diode VD ₁ on the rectification side is cut off, and the energy is cut off from the input stage to the output stage. The resonant current i _Cr and the exciting current i _Lm are connected in series, and the resonant network resonates at f _r2 frequency. See that diode VD ₁ is turned off for zero current switching;

At time _t3 , Q ₁ drive signal is turned off, the excitation current i _Lm is reversed, and begins to decline linearly, the resonant current i _Cr drops according to the resonant frequency f _r , and the resonant current i _Cr < excitation current i _Lm , the diode VD ₂ on the rectification side conducts through rectification;

At time t ₄ before i _Cr crosses zero, apply Q ₂ drive signal; resonant current i _Cr = excitation current i _Lm occurs at time t ₅ before reaching the half cycle, at this time the diode VD ₂ on the rectifier side is cut off, and the energy is transferred from the input stage to the output stage is isolated. The resonant current i _Cr and the exciting current i _Lm are connected in series, and the resonant network resonates at f _r2 frequency. Diode VD ₂ is turned off for zero current switching;

At t ₆ moment, Q ₂ driving signal is turned off, i _Lm reverses, and enters the next working cycle.

To sum up the analysis of Figure 3, it can be seen that the zero-current switching off of the rectifier side diode is realized by using the fixed-frequency pulse width modulation control mode under light load conditions. If the driving signal can be ensured to be applied before the resonant current crosses zero, the switching The zero-voltage switching off of the tube can also be realized, and the zero-voltage switching off of the fixed-frequency pulse width adjustment control module is guaranteed by the value range of the duty cycle D.

The resonant network is designed to work at the switching frequency f=f _r under the rated load, which is also the critical operating point of the frequency adjustment mode and the fixed pulse width adjustment mode. The switching between the frequency regulation mode and the fixed-frequency pulse width regulation control mode is carried out within the dead time after a switching cycle ends. The fixed pulse width regulation mode has the same upper and lower arm symmetrical drive form as the frequency regulation mode.

The selection of the working module and the calculation of the error voltage signal can adopt analog circuit or digital control PI algorithm.

Fig. 4 is a characteristic curve diagram of the normalized gain of the resonant cavity when the fixed-frequency pulse width modulation control mode is used in the embodiment of the utility model. The parameters selected for the curve are: L _r =46μH, Lm=1.38mH, C _r =55nF, Q=0.126, K=30. The X-axis is the duty cycle D, and the Y-axis is the normalized voltage gain. In the figure, the switching frequency is f=f _r , and the minimum voltage gain G _min is determined by the minimum value of the duty cycle D _min . D _min can be determined when designing the resonant network parameters. It can be seen from the figure that the gain calculation formula can effectively determine the voltage gain of the resonant cavity. The conventional frequency adjustment mode will change the circuit quality factor when the load changes, causing the circuit gain to vary with the load. Using the light-load gain characteristic curve shown in Figure 4 can effectively improve the above-mentioned effects, making the circuit gain and The load switch is only controlled by the duty cycle D.

FIG. 5 is a waveform diagram of a key current experiment of a resonant cavity using a fixed-frequency pulse width modulation control mode in this embodiment. As shown in the figure, the driving signal _Q1 is 5V/div, _Q2 is 5V/div, under the action of i _Cr is the resonant current 500mA/div, the voltage of the switching tube Q1, the switching tube Q2 and the current of the rectifier diode VD ₁ and diode VD ₂ Waveform diagram, Fig. 5 further verifies the analysis of Fig. 3 above. Wherein the X axis is i _Cr =0. The experiment shown in the figure proves that the light load adopts the PWM duty ratio D to control the voltage gain, so that the LLC resonant converter realizes the zero-voltage switching off of the switching tube and the zero-current switching off of the rectifier tube. ZVS is used to represent zero voltage in the figure. switch off and ZCS meter is zero current switch off. Figure 5 shows the current waveforms of the resonant current i _Cr and the rectifier tubes VD ₁ and VD ₂ under the action of the driving signals Q ₁ and Q ₂ in the fixed-frequency pulse width modulation control mode. The fixed-frequency pulse width adjustment control mode enables the converter to realize ZCS at light load, as shown in Figure 5, at a certain moment before the Q ₁ driving signal is turned off, i _VD2 reaches 0 to realize zero-current shutdown, and then the Q ₁ driving signal is turned off Off, the resonant current i _Cr phase delays the voltage and then begins to decline in reverse, i _VD1 rises, and before i _Cr crosses zero, the Q ₂ drive signal arrives. Therefore, this mode not only realizes the ZVS of the switch tube, but also realizes the ZCS of the rectifier tube.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, but they will not deviate from the spirit of the present invention or go beyond the definition of the appended claims range.

Claims (10)

1. a LLC controlled resonant converter underload control method is characterized in that, comprises hardware circuit and the load control circuit of LLC controlled resonant converter, and the concrete steps of described LLC controlled resonant converter underload control method are as follows:

Whether A, the output of judging controlled resonant converter according to output voltage signal and the output current signal of the conditioning of the hardware circuit of LLC controlled resonant converter work under the light-load state;

B, when the output of controlled resonant converter be not that load control circuit carries out the frequency adjustment control model when working in light-load state;

C, when the output of controlled resonant converter works in light-load state, load control circuit carries out deciding frequency pulse width control pattern;

D, output respective switch management and control signal processed are to the switching tube of LLC controlled resonant converter.

2. LLC controlled resonant converter underload control method according to claim 1, it is characterized in that, in the described steps A, output voltage signal calculates error voltage signal through feedback compensation and output current signal judges whether the output of controlled resonant converter works under the light-load state.

3. LLC controlled resonant converter underload control method according to claim 1 and 2 is characterized in that, among the described step C, in deciding the processing of frequency pulse width control pattern cycle, fixed switching frequency is resonance frequency, and according to different load output voltage voltage stabilizing requirements, calculates PWM duty ratio D.

4. LLC controlled resonant converter underload control method according to claim 3 is characterized in that, described PWM duty ratio D is used for regulating the gain of controlled resonant converter interior resonance network voltage, and underload resonant network voltage gain computing formula is

5. LLC controlled resonant converter underload control method according to claim 4 is characterized in that, the duty ratio D excursion of described adjusting underload resonant network voltage gain is

F wherein _rBe resonance frequency, i.e. fixed switching frequency, f _R2It is the second resonance frequency.

6. LLC controlled resonant converter underload control method according to claim 5 is characterized in that, described controlled resonant converter interior resonance network works in the switching frequency f=f under the nominal load _r, switching frequency f=f _rBe frequency adjustment control model and the fixed frequently critical working point of pulse width control pattern.

7. LLC controlled resonant converter underload control method according to claim 6 is characterized in that, switching in the Dead Time after the switch periods end of described frequency adjustment control model and fixed frequently pulse width control pattern carried out.

8. LLC controlled resonant converter underload control device, it is characterized in that, this device comprises controller, be used for making controlled resonant converter be operated in the frequency modulation module under the frequency adjustment control model and be used for that controlled resonant converter is operated in deciding the wide module of accent under the frequency pulse width control pattern, be used for detecting respectively voltage detection unit and the current detecting unit of LLC controlled resonant converter output voltage signal and output current signal, described controller also is connected with the feedback compensation error amplifier, described voltage detection unit and reference voltage power supply connect respectively the input of feedback compensation error amplifier, described current detecting unit connects the input of controller, the output of described controller is connected with the operational module switch unit, described frequency modulation module be connected wide module to connect respectively controller and operational module switch unit, described operational module switch unit connects controlled resonant converter switch controlled end.

9. LLC controlled resonant converter underload control device according to claim 8, it is characterized in that, the wide module of described accent is fixed for generation of frequency and is decided frequency pulse width signal, the symmetrical output form of the up and down arm switch pipe of described controlled resonant converter according to what the controlled resonant converter output voltage signal was regulated change in duty cycle.

10. LLC controlled resonant converter underload control device according to claim 9 is characterized in that, described resonant network parameter is large magnetizing inductance value and little resonant inductance value.

Images