CN103326587B - LLC resonant converter underload control method and device - Google Patents
LLC resonant converter underload control method and device Download PDFInfo
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Abstract
The invention provides a kind of LLC resonant converter underload control method and device, belong to power conversion field.The problem that the Sofe Switch environment that it solves zero voltage switch and Zero Current Switch is lost.This method: A, judge whether to work in light-load state under; B, when not being when working in light-load state, carry out frequency adjustment control model; C, when underload, carry out determining frequency pulse width control pattern; D, output respective switch pipe control signal.This device comprises the wide module of controller, FM module and tune, current detecting unit and voltage detection unit, controller is connected with feedback compensation error amplifier, voltage detection unit connects feedback compensation error amplifier, current detecting unit connection control device, controller is connected with operational module switch unit, FM module and the wide module of tune connection control device and operational module switch unit respectively, operational module switch unit connecting valve pipe.The Sofe Switch harmonious environment that this device and method realizes full-load range shakes parameter optimization.
Description
Technical field
The invention belongs to power conversion field, relate to a kind of LLC resonant converter underload control method and device.
Background technology
Along with the development of switch power technology, high efficiency and high power density become development trend.Therefore LLC resonant converter is applied and gives birth to also more and more extensive.The no-voltage that LLC resonant converter realizes primary side switching tube in wider frequency range is opened and secondary side rectifying tube zero-current switching.But along with alleviating of load, in order to obtain identical DC current gain, LLC resonant converter needs to improve switching frequency.The rising of switching frequency, adds switching loss on the one hand; On the other hand, also will the electric current of rectification side diode be caused just to turn off when not having back to zero, the Sofe Switch environment of Zero Current Switch be lost, and increases switching loss further.Meanwhile, underloading makes quality factor of circuit Q value reduce, and load variations can have a strong impact on voltage gain.Therefore, along with the increase of switching frequency under LLC resonant converter underloading, the efficiency of transducer will be affected, and compare the high efficiency of its heavy duty, and the efficiency be difficult to when simultaneously taking into account underloading improves.
Along with the requirements at the higher level to supply convertor efficiency, LLC converter needs to obtain higher light-load efficiency, prior art often adopts the control method of output sluggishness band comparison circuit, the on-off switching tube control signal when output voltage is elevated to a certain higher limit, and converter enters idle condition; When output voltage reduces to producing switching tube frequency modulation(FM) control signal during a certain lower limit, converter enters the operating state of power output.The method is suitable only for pole underload or holding state.Other light load efficiency improvement alternative also has the scheme increasing switching tube or diode auxiliary branch in primary and secondary level, also has the PWM control program that duty ratio is fixing.These schemes can make loss when zero load or underload reduce, but owing to increasing hardware to realize in main circuit, bring certain impact to the reliability of circuit, and the efficiency be also difficult to when simultaneously taking into account heavy duty improves.
Chinese patent literature discloses a kind of LLC resonant converter control method and the device that application number is 200910221471.4, this device judges load condition by input switch pipe turn-on frequency, carry out frequency conversion respectively according to load condition, adjust wide control again, reduce circuit loss to a certain extent, but this frequency conversion adjusts wide control without any constraints, the switch tube zero voltage switch of LLC resonant converter and the Sofe Switch environment of rectifying tube Zero Current Switch can be made to lose, thus make cavity voltage gain uncertain.
Summary of the invention
The present invention is directed to existing technology and there are the problems referred to above, propose a kind of LLC resonant converter underload control method and device, the method and device adopt underload frequency pulse width control surely, the parameter optimization thus the Sofe Switch harmonious environment realizing full-load range shakes, improves the efficiency especially light-load efficiency of converter.
The present invention is realized by following technical proposal: a kind of LLC resonant converter underload control method, it is characterized in that, comprise hardware circuit and the load control circuit of LLC resonant converter, the concrete steps of described LLC resonant converter underload control method are as follows:
Under A, the output voltage signal nursed one's health according to the hardware circuit of LLC resonant converter and output current signal judge whether the output of controlled resonant converter works in light-load state;
B, when the output of controlled resonant converter be not that when working in light-load state, load control circuit carries out frequency adjustment control model;
C, when the output of controlled resonant converter works in light-load state, load control circuit carries out determining frequency pulse width control pattern;
D, output respective switch pipe control signal are to the switching tube of LLC resonant converter.
Under the output voltage signal nursed one's health by the hardware circuit of LLC resonant converter and output current signal judge whether the output of controlled resonant converter works in light-load state, wherein the operating state of the output of controlled resonant converter has three kinds of states such as heavy duty, nominal load and underload.When the output of controlled resonant converter works in heavy duty, nominal load, load control circuit carries out frequency adjustment control model.When the output of controlled resonant converter works in light-load state, load control circuit carries out determining frequency pulse width control pattern.The zero voltage switch of switching tube and the Zero Current Switch software environment of rectifying tube in this process implementation full-load range.Thus greatly reduce switching loss.
In above-mentioned LLC resonant converter underload control method, in described steps A, output voltage signal is under feedback compensation calculates error voltage signal and output current signal judges whether the output of controlled resonant converter works in light-load state.Here corresponding error voltage signal is calculated by feedback compensation and output current signal can judge the operating state of the output of controlled resonant converter more accurately.
In above-mentioned LLC resonant converter underload control method, in described step C, determining in the process 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.PWM is pulse-width regulated, and the frequency pulse width control pattern of determining in this method is the pulse-width regulated control mode being fixed frequency.Here determine to there is the relation that fixed switching frequency and load output voltage stabilizing requirement and PWM duty ratio D exist the effect optimizing resonant network parameter.
In above-mentioned LLC resonant converter underload control method, described PWM duty ratio is for regulating the gain of controlled resonant converter interior resonance network voltage, and now, underload resonant network voltage gain computing formula is
In above-mentioned LLC resonant converter underload control method, the duty ratio D excursion of described adjustment underload resonant network voltage gain is
wherein fr is resonance frequency, i.e. fixed switching frequency, f
r2it is the second resonance frequency.Adopt frequency adjustment control model when nominal load, now switching frequency is at f
r2<f≤f
rinterior variation.Here determining that the duty ratio D excursion of underload resonant network voltage gain is connected with nominal load with heavy duty makes fixed frequency value more stable.
In above-mentioned LLC resonant converter underload control method, described controlled resonant converter interior resonance network works in the switching frequency f=f under nominal load
r, switching frequency f=f
rfor frequency adjustment control model and the critical working point of determining pulse width control pattern frequently.Here the switching frequency under nominal load operating state is the fixed switching frequency under light load operation state, and as frequency adjustment control model and the critical working point of determining pulse width control pattern frequently.Be convenient to the resonant parameter optimizing controlled resonant converter interior resonance network.
In above-mentioned LLC resonant converter underload control method, described frequency adjustment control model and fixed pulse width control pattern frequently switch in a switch periods terminate after Dead Time in carry out.Here the impact of a switch periods that is not subject in Dead Time achieving frequency adjustment control model and the switching of fixed pulse width control pattern frequently makes control more accurate.
A kind of LLC resonant converter underload control device, it is characterized in that, this device comprises controller, FM module under frequency adjustment control model is operated in and for making controlled resonant converter be operated in determine the wide module of tune under frequency pulse width control pattern for making controlled resonant converter, for detecting the output voltage signal of the hardware circuit conditioning of LLC resonant converter and the voltage detection unit of output current signal and current detecting unit respectively, described controller is also connected with feedback compensation error amplifier, described voltage detection unit is connected the input of feedback compensation error amplifier respectively with reference voltage power supply, the input of described current detecting unit connection control device, the output of described controller is connected with operational module switch unit, described FM module and the wide module of tune connection control device and operational module switch unit respectively, described operational module switch unit connects controlled resonant converter switch controlled end, the error voltage signal that the output voltage that described controller detects according to voltage detection unit exports after feedback compensation error amplifier and the output current signal that current detecting unit detects judge whether LLC resonant converter works in light-load state, if be not operated in underload, then select FM module work by operational module switch unit, carry out frequency adjustment control model, if be operated in underload, then select to adjust wide module work by operational module switch unit, carry out determining frequency pulse width control pattern, realize duty cycle adjustment voltage gain, and export corresponding switch controlled signal to switch controlled end through operational module switch unit.
LLC resonant converter hardware circuit bussed supply voltage is direct voltage.Load control circuit comprises controller, the wide module of voltage detection unit, current detecting unit, FM module and tune.Voltage detection unit gathers the direct current output loading voltage signal of LLC resonant converter hardware circuit, send to feedback compensation error amplifier, and compare with reference voltage, the error voltage signal relatively sends to controller through feedback compensation error amplifier.The output current signal of controller received current detecting unit collection simultaneously, according to the error voltage signal received and output current signal, controller judges that load is nominal load, heavy duty or underload, select corresponding control model through operational module switch unit; If be operated in heavy duty or nominal load, then select FM module work by operational module switch unit, carry out frequency adjustment control model; If be operated in underload, then select to adjust wide module work by operational module switch unit, carry out determining frequency pulse width control pattern, realize duty cycle adjustment voltage gain, and export corresponding switch controlled signal to switch controlled end through operational module switch unit.
In above-mentioned LLC resonant converter underload control device, the wide module of described tune fix for generation of frequency and regulate change in duty cycle according to controlled resonant converter output voltage signal determine frequency pulse width signal, the symmetrical output form of the pipe of arm switch up and down of described controlled resonant converter.Here achieve under light load conditions, the two-way pwm signal that the control signal that controlled resonant converter receives is same frequency, same duty cycle and sequential differ half switch periods.
In above-mentioned LLC resonant converter underload control device, described LLC resonant transformation is interior resonance network is low magnetizing inductance.Circulating current higher when low magnetizing inductance can cause transformer primary side underloading on the one hand, makes again, when heavy duty resonance current is larger, to bring more conducting and switching loss on the other hand.The switching frequency that low magnetizing inductance is narrower under can obtaining identical gain, has certain effect to optimization transformer size and control circuit tool.
Compared with prior art, this LLC resonant converter underload control method and device have the following advantages:
1, this device and method adopts underload frequently pulse width control surely, and the Sofe Switch harmonious environment realizing full-load range shakes Parameters Optimal Design, improves the efficiency especially light-load efficiency of controlled resonant converter.
2, this device and method is directly carried out the selection of mode of operation by the voltage of load, current condition.Meanwhile, the present invention has also explicitly pointed out the critical switching point of two kinds of mode of operations at nominal load resonance frequency place.Ensure the Sofe Switch environment of switch tube zero voltage switch in whole control procedure and rectifying tube Zero Current Switch, thus effectively determine cavity voltage gain.
Accompanying drawing explanation
Fig. 1 is circuit diagram of the present invention;
Fig. 2 is control flow chart of the present invention;
The switch controlled signal resonant cavity critical current oscillogram that Fig. 3 adjusts wide module to produce when being underload of the present invention regulation and control.Wherein T
r=1/f
r, a=DT
r;
Fig. 4 is the present invention's resonant cavity normalized gain performance diagram when determining frequency pulse width control pattern;
Fig. 5 is the present invention resonant cavity critical current experimental waveform figure when determining frequency pulse width control pattern.
Embodiment
Be below specific embodiments of the invention, and by reference to the accompanying drawings technical scheme of the present invention be further described, but the present invention is not limited to these embodiments.
As Figure 1-5, this LLC resonant converter underload control method, comprises hardware circuit and the load control circuit of LLC resonant converter, and the concrete steps of LLC resonant converter underload control method are as follows:
Under A, the output voltage signal nursed one's health according to the hardware circuit of LLC resonant converter and output current signal judge whether the output of controlled resonant converter works in light-load state; Output voltage signal is under feedback compensation calculates error voltage signal and output current signal judges whether the output of controlled resonant converter works in light-load state.
B, when the output of controlled resonant converter be not that when working in light-load state, load control circuit carries out frequency adjustment control model;
C, when the output of controlled resonant converter works in light-load state, load control circuit carries out determining frequency pulse width control pattern.Determining in the process 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.
D, output respective switch pipe control signal are to the switching tube of LLC resonant converter.
PWM duty ratio is for regulating the gain of controlled resonant converter interior resonance network voltage, and now, underload resonant network voltage gain computing formula is
the duty ratio D excursion of adjustment underload resonant network voltage gain is
f
rfor resonance frequency, i.e. fixed switching frequency, f
r2for the switch low-limit frequency under frequency adjustment control model, i.e. the second resonance frequency.Controlled resonant converter interior resonance network works in the switching frequency f=f under nominal load
r, switching frequency f=f
rfor frequency adjustment control model and the critical working point of determining pulse width control pattern frequently.Here the switching frequency under nominal load operating state is the fixed switching frequency under light load operation state, and as frequency adjustment control model and the critical working point of determining pulse width control pattern frequently.Be convenient to the resonant parameter optimizing controlled resonant converter interior resonance network.Frequency adjustment control model and fixed pulse width control pattern frequently switch in a switch periods terminate after Dead Time in carry out.
This LLC resonant converter underload control device comprises controller, FM module under frequency adjustment control model is operated in and for making controlled resonant converter be operated in determine the wide module of tune under frequency pulse width control pattern for making controlled resonant converter, for detecting the output voltage signal of the hardware circuit conditioning of LLC resonant converter and the voltage detection unit of output current signal and current detecting unit respectively, described controller is also connected with feedback compensation error amplifier, described voltage detection unit is connected the input of feedback compensation error amplifier respectively with reference voltage power supply, the input of described current detecting unit connection control device, the output of described controller is connected with operational module switch unit, described FM module and the wide module of tune connection control device and operational module switch unit respectively, described operational module switch unit connects controlled resonant converter switch controlled end, the error voltage signal that the output voltage that described controller detects according to voltage detection unit exports after feedback compensation error amplifier and the output current signal that current detecting unit detects judge whether LLC resonant converter works in light-load state, if be not operated in underload, then select FM module work by operational module switch unit, carry out frequency adjustment control model, if be operated in underload, then select to adjust wide module work by operational module switch unit, carry out determining frequency pulse width control pattern, realize duty cycle adjustment voltage gain, and export corresponding switch controlled signal to switch controlled end through operational module switch unit.
Adjust wide module to fix for generation of frequency and regulate change in duty cycle according to controlled resonant converter output voltage signal determine frequency pulse width signal, the symmetrical output form of the pipe of arm switch up and down of described controlled resonant converter.
LLC resonant converter underload control method of the present invention and device are applicable to the resonance type power converter of full-bridge, half-bridge or push-pull configuration.Controlled resonant converter generally comprises electronic switch, the output of half-bridge or full bridge structure, by the armature winding of resonant inductance, resonant capacitance and transformer or electromagnetic induction coil in series, wherein resonant inductance can be integrated with transformer magnetic, or individualism.The LLC resonant converter selected in the present embodiment be LLC half bridge resonant as shown in Figure 1, comprising: input direct voltage V
in, the squarer driven by switching tube Q1, switching tube Q2, by series resonant capacitance C
r, series resonance inductor L
rwith shunt excitation inductance L
mthe resonant network of composition, no-load voltage ratio is the ideal transformer T of n:1:1, diode VD
1, diode VD
2the rectification circuit of composition, filter capacitor C
oand load resistance R
o.Wherein series resonance inductor L
rcan be an independent component, also can be integrated with transformer T magnetic.Switching tube Q is depicted in figure
1, switching tube Q
2control signal control and Working mould block selection signal mode at the timing waveform of critical point.Above-mentioned voltage detection unit, current detecting unit can be made up of resistance, also can be made up of Hall element or voltage, current transformer; The wide module of its middle controller, FM module, tune and operational module selection circuit can adopt analog circuit to realize, and also can adopt digital circuit to realize, comprise the programmable controller with software operational capability.
As shown in Figure 1, 2, in figure, FM represents frequency adjustment control model, and PWM represents solid frequency pulse-width regulated control model, and LLC resonant converter hardware circuit bussed supply voltage is direct voltage.Load control circuit comprises controller, the wide module of voltage detection unit, current detecting unit, FM module and tune.Voltage detection unit gathers the direct current output loading voltage signal of LLC resonant converter hardware circuit, send to feedback compensation error amplifier, and compare with reference voltage, the error voltage signal relatively sends to controller through feedback compensation error amplifier.The output current signal of controller received current detecting unit collection simultaneously, according to the error voltage signal received and output current signal, controller judges that load is nominal load, loyal load or underload, select corresponding control model through operational module switch unit; If be operated in heavy duty or nominal load, then select FM module work by operational module switch unit, carry out frequency adjustment control model; If be operated in underload, then select to adjust wide module work by operational module switch unit, carry out determining frequency pulse width control pattern, realize duty cycle adjustment voltage gain, and export corresponding switch controlled signal to switch controlled end through operational module switch unit.If select solid pulse width control pattern frequently, controller is in the period treatment adjusting wide module, and fixed switching frequency is resonance frequency, and according to the voltage gain formulae discovery PWM duty ratio of different load output voltage by solid frequency pulse width control pattern; Controller is in the period treatment of FM module, and fixed duty cycle is that 1/2(ignores Dead Time), switching frequency is according to the voltage gain formulae discovery of different load output voltage by FM pattern; Controller is selected to adjust wide module or FM module work, output switch pipe control signal according to loading condition.Controller, according to different error voltage value, is exchanged wide module and is carried out duty ratio adjustment, thus the voltage gain of regulating circuit, make the output loading voltage of LLC resonant converter keep stable.Concrete adjustment process comprises:
Adopt the frequency adjustment control model of frequency adjustment gain when nominal load, switching frequency is at f
r2<f≤f
rinterior variation, in the drawings signal flow graph represents Q
1, Q
2for the drive singal that Fig. 1 switching tube Q1 and switching tube Q2 is corresponding.The drive singal Q of switching tube
1, Q
2duty ratio is fixed as 1/2.Resonant network voltage gain is:
Wherein
Quality factor
C in formula
rfor series resonant capacitance C
rcapacitance, L
rfor inductance value, the L of series resonance inductor Lr
mfor magnetizing inductance L
minductance value, K is the ratio of magnetizing inductance and resonant inductance.
When load lightens, switching frequency is adopted to be fixed as f=f
rsolid frequency pulse width control pattern, carry out regulation voltage gain by PWM duty ratio D.Resonant network resistance gain computing formula is
Wherein duty ratio D excursion is
When the pwm signal that duty ratio is D goes to control LLC resonant converter half-bridge switch pipe, by fourier decomposition, bussed supply direct voltage V
inthe fundametal compoment amplitude being input to resonant network is:
and the fundametal compoment amplitude under traditional frequency adjustment control model is
every half switch periods is T, and load current Io is by resonance current i
crwith exciting current i
lmdifference obtain through transformer coupled, meet:
i
owith i
crand i
lmthe area surrounded is directly proportional.During power output, exciting current i
lmbecause the clamped of rectification side maintains constant linear change.Therefore, when load reduction, resonance current i
crmust reduce to adapt to this change.Because switching frequency is fixed on f=f
r, do not change the impedance operator of resonant network, resonance current i can be adapted to by PWM duty ratio D control inputs voltage
crreduction.
Can fully demonstrate according to above-mentioned analysis the result that the present invention obtains in Fig. 3,4,5, the operational modal analysis of carrying out solid frequency pulse-width regulated control model as Fig. 3 under light-load state is as follows:
Can see in figure 3, at t
0moment, drive singal Q
2turn off, exciting current i
lmoppositely, start linearly to rise, resonance current i
crby resonance frequency f
rrise, resonance current i
cr> exciting current i
lm, rectification side diode VD
1conducting, output obtains energy.Because the existence of duty ratio, now Q
1drive singal does not arrive;
At resonance current i
crt before zero passage
1in the moment, apply Q
1drive singal; Because underload makes resonance current i
crcompared to low during nominal load, resonance current i
cr=exciting current i
lmoccur in the t before arriving the half period
2moment, now rectification side diode VD
1cut-off, energy is cut off from input stage to output stage.Resonance current i
crwith exciting current i
lmserial connection, resonant network presses f
r2frequency resonance.See diode VD
1for Zero Current Switch turns off;
At t
3moment, Q
1drive singal turns off, exciting current i
lmoppositely, start linearly to decline, resonance current i
crby resonance frequency f
rdecline, resonance current i
cr< exciting current i
lm, rectification side diode VD
2conducting rectification;
At i
crt before zero passage
4in the moment, apply Q
2drive singal; Resonance current i
cr=exciting current i
lmoccur in the t before arriving the half period
5moment, now rectification side diode VD
2cut-off, energy is cut off from input stage to output stage.Resonance current i
crwith exciting current i
lmserial connection, resonant network presses f
r2frequency resonance.Diode VD
2for Zero Current Switch turns off;
At t
6moment, Q
2drive singal turns off, i
lmoppositely, the subsequent work cycle is entered.
The Zero Current Switch shutoff achieving rectification side diode under light-load state by solid pulse-width regulated control model frequently to sum up can be found out to the analysis of Fig. 3, if drive singal can be guaranteed to apply before resonance current zero passage, then switch tube zero voltage switch turns off and also can realize, and is ensured by duty ratio D span Gu the mould of pulse-width regulated control frequently realizes zero voltage switch shutoff.
Resonant network works in switching frequency f=f under being designed to nominal load
r, this is also the critical working point of frequency modulation mode and fixed pulse width shaping modes.Frequency modulation mode and the admittedly switching of pulse-width regulated control model frequently carry out in the Dead Time after a switch periods terminates.Fixed pulse width shaping modes has the symmetrical drive form of the upper underarm same with frequency modulation mode.
The selection of operational module and the calculating of error voltage signal can adopt analog circuit or digital control PI algorithm.
Fig. 4 is resonant cavity normalized gain performance diagram when using solid pulse-width regulated control model frequently in the utility model embodiment.Parameter selected by curve is: L
r=46 μ H, Lm=1.38mH, C
r=55nF, Q=0.126, K=30.X-axis is duty ratio D, and Y-axis is normalized voltage gain, figure breaker in middle frequency f=f
r, by duty ratio minimum value D
mindetermine minimum voltage gain G
min.D
mincan determine when designing resonant network parameter.As can be seen from the figure cavity voltage gain can effectively be determined by this gain computing formula.Conventional frequency modulation mode can make quality factor of circuit change when load variations, cause circuit gain to change along with load, use underload gain characteristic curve as shown in Figure 4, can effectively improve above-mentioned impact, make circuit gain and load switch, only control by duty ratio D.
Fig. 5 uses the solid resonant cavity of pulse-width regulated control model frequently critical current experimental waveform figure in the present embodiment.As figure shows drive singal Q
1for 5V/div, Q
2for the lower i of 5V/div effect
crfor voltage and the rectifier diode VD of resonance current 500mA/div, switching tube Q1, switching tube Q2
1, diode VD
2current waveform figure, Fig. 5 demonstrates the analysis of above-mentioned Fig. 3 further.Wherein X-axis is i
cr=0.Prove that underload adopts the gain of PWM duty ratio D control voltage as schemed experiment, the zero voltage switch making LLC resonant converter achieve switching tube turns off and the Zero Current Switch of rectifying tube turns off, and represents that zero voltage switch turns off and ZCS table is that Zero Current Switch turns off respectively in the drawings with ZVS.As shown in Figure 5 at solid frequency pulse-width regulated control model drive singal Q
1, Q
2the lower resonance current i of effect
cr, rectifying tube VD
1, VD
2current waveform.Gu pulse-width regulated control model makes converter achieve ZCS when underloading frequently, as shown in Figure 5, Q
1certain moment before drive singal turns off, i
vD2arrive 0 and realize zero-current switching, after this Q
1drive singal turns off, resonance current i
crstart after phase delay voltage oppositely to decline, i
vD1rise, at i
crbefore zero passage, Q
2drive singal arrives.Therefore, this pattern had both achieved switching tube ZVS, achieved again rectifying tube ZCS.
Specific embodiment described herein is only to the explanation for example of the present invention's spirit.Those skilled in the art can make various amendment or supplement or adopt similar mode to substitute to described specific embodiment, but can't depart from spirit of the present invention or surmount the scope that appended claims defines.
Claims (7)
1. a LLC resonant converter underload control method, is characterized in that, comprises hardware circuit and the load control circuit of LLC resonant converter, and the concrete steps of described LLC resonant converter underload control method are as follows:
Under A, the output voltage signal nursed one's health according to the hardware circuit of LLC resonant converter and output current signal judge whether the output of controlled resonant converter works in light-load state;
B, when the output of controlled resonant converter be not that when working in light-load state, load control circuit carries out frequency adjustment control model;
C, when the output of controlled resonant converter works in light-load state, load control circuit carries out determining frequency pulse width control pattern; Determining in the process 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; Described PWM duty ratio D is for regulating the gain of controlled resonant converter interior resonance network voltage, and underload resonant network voltage gain computing formula is
the duty ratio D excursion of described adjustment underload resonant network voltage gain is
wherein f
rfor resonance frequency, i.e. fixed switching frequency, f
r2it is the second resonance frequency;
D, output respective switch pipe control signal are to the switching tube of LLC resonant converter.
2. LLC resonant converter underload control method according to claim 1, it is characterized in that, in described steps A, output voltage signal is under feedback compensation calculates error voltage signal and output current signal judges whether the output of controlled resonant converter works in light-load state.
3. LLC resonant converter underload control method according to claim 1 and 2, is characterized in that, switching frequency f=f
rfor frequency adjustment control model and the critical working point of determining pulse width control pattern frequently.
4. LLC resonant converter underload control method according to claim 3, is characterized in that, described frequency adjustment control model and fixed pulse width control pattern frequently switch in a switch periods terminate after Dead Time in carry out.
5. a LLC resonant converter underload control device, it is characterized in that, this device comprises controller, FM module under frequency adjustment control model is operated in and for making controlled resonant converter be operated in determine the wide module of tune under frequency pulse width control pattern for making controlled resonant converter, for detecting voltage detection unit and the current detecting unit of LLC resonant converter output voltage signal and output current signal respectively, described controller is also connected with feedback compensation error amplifier, described voltage detection unit is connected the input of feedback compensation error amplifier respectively with reference voltage power supply, the input of described current detecting unit connection control device, the output of described controller is connected with operational module switch unit, described FM module and the wide module of tune connection control device and operational module switch unit respectively, described operational module switch unit connects controlled resonant converter switch controlled end, the output current signal of controller received current detecting unit collection simultaneously, according to the error voltage signal received and output current signal, controller judges that load is nominal load, heavy duty or underload, corresponding control model is selected through operational module switch unit, if be operated in underload, then select to adjust wide module work by operational module switch unit, switching frequency is adopted to be fixed as f=f
rsolid frequency pulse width control pattern, carry out regulation voltage gain by PWM duty ratio D, resonant network resistance gain computing formula is
wherein duty ratio D excursion is
wherein f
rfor resonance frequency, i.e. fixed switching frequency, f
r2it is the second resonance frequency.
6. LLC resonant converter underload control device according to claim 5, it is characterized in that, the wide module of described tune fix for generation of frequency and regulate change in duty cycle according to controlled resonant converter output voltage signal determine frequency pulse width signal, the pipe of arm switch up and down of described controlled resonant converter is the symmetrical output form of control voltage.
7. LLC resonant converter underload control device according to claim 6, is characterized in that, described resonant network parameter is large magnetizing inductance value and little resonant inductance value.
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