一种应用于LLC中的过流保护电路的分析和设计

Analysis and Optimization of LLC Resonant Converter With a Novel Over-CurrentProtection Circuit

Xiaogao Xie,Junming Zhang,Chen Zhao,Zhuo Zhao,and Zhaoming Qian,Senior Member,IEEE

Abstract—A novel over-current protection(OCP)method for a LLC resonant converter has been proposed in this paper.Thismethod is very attractive for its inherent current limit ability,espe-cially under short-circuit

http://www.doczj.com/doc/1e5116ed31b765ce050814d1.html bined with the frequency increasing method,good current limitfeatures can be achieved. Compared to the conventional LLC resonant converter with fre-quency increasing method,LLCwith the proposed OCP method can reduce the maximum frequency to an acceptable value.De-tailed theoretical analysisand optimization design considerations have been presented.An experimental prototype converter based on the proposedmethod has been built up to verify the theoretical analysis.Experimental results meet the theoretical analysis very well.Index Terms—LLC,over-current protection(OCP).I.I NTRODUCTION

I N RECENT years,the LLC resonant converter has drawn

more and more attention because of some of its unique char-acteristics and improvements over other topologies[1]–[5],suchas zero voltage switching(ZVS)capability from zero to full load range,low voltage stress on secondary recti?er,high ef?ciencyat high input voltage,etc.

To make practical use of this converter,there are still some issues to be solved.Over-current protection(OCP)is one criticalissue,which will be discussed in this paper.The purpose of OCP is to limit the current stress in the circuit under overload orshort-circuit condition,or limit the inrush current during start up so that the power converter will not be damaged under thoseconditions.

Three OCP methods have been available for LLC resonant converters up until now.The?rst method is increasing theswitching frequency when over-current occurs.It’s simple,but the frequency becomes too high to keep acceptable current,which results in great losses on devices and brings more critical demand on thermal management.Another concern is thatmagnetic design will be greatly affected by this high switching frequency.

Manuscript received October26,2005;revised June1,2006.This paper ap-peared in part at Intelec’05.This work was

supported by the China National Science Fund under Contract50237030and by the China Postdoctoral Science Fund underContract2005038277.Recommended for publication by Associate Editor S.Y.Hui.

The authors are with the College of Electrical Engineering,Zhejiang Univer-sity,Hangzhou310027,China(e-mail:vrm@http://www.doczj.com/doc/1e5116ed31b765ce050814d1.html ).Color versions of one or more of the?gures in this paper are available online athttp://www.doczj.com/doc/1e5116ed31b765ce050814d1.html .Digital Object Identi?er10.1109/TPEL.2006.889919

The second method is a combination of varied-frequency control and pulsewidth modulation(PWM)control.With this method,alower maximum frequency can be chosen compared to the?rst method.The PWM control is used to limit the current so thatmagnetic and semiconductor components don’t need to be over designed.However,the primary switches will lose ZVS underOCP mode.

The last method achieves the current limitation function by modifying the original LLC topology with splitting resonant ca-pacitors and clamping diodes[3],[4].The current can be auto-matically limited by clamping the voltage across the splittingresonant capacitors and ZVS of primary switches can still be achieved under OCP mode.The main problem of this method isthat the current-limit point is a function of input voltage.Un-fortunately,the primary current and voltage across resonant ca-pacitors are larger at low line voltage than they are at high line voltage.Hence,the current-limit-point at high line voltage ismuch higher so that the current limitation function almost loses. Moreover,the voltage across the splitting capacitors shouldbe designed to be smaller than1/2input voltage,which makes it dif?cult to optimize the current limitation range.

In this paper,a novel OCP circuit for the LLC resonant con-verter as shown in Fig.1is proposed.By adding a simple aux-iliarycircuit,the current limitation feature of the original con-verter can be improved greatly.The detailed principle analyses will beillustrated in Section II.Section III will present param-eters design considerations.Experimental results will be pre-sented inSection IV.Finally,some useful conclusions will be achieved in Section V.

II.T HEORETICAL A NALYSIS

As shown in Fig.1,the proposed OCP circuit for the LLC resonant converter is formed by a small

transformer and two clamping

diodes

and.In normal operation conditions, the protection circuit doesn’t work and the circuit works in the same way with the

conventional LLC resonant converter. When over-current or short-circuit condition occurs,the primary current will increaserapidly.At the same time the voltage across the resonant

capacitor will also increase.When the voltage across the resonant

capacitor is larger than the output voltage referred to the primary side of

transformer, the clamping

diodes

and begins to conduct alternatively. Hence,the voltage

across is clamped and the primary current is also clamped.Because the output voltage is usually constant, the current-limit-point range is relatively narrow compared to that of the modi?ed LLC topology with splitting capacitors and clamping diodes.0885-8993/$25.00?2007IEEE

Fig.1.LLC topology with proposed current limitationcircuit.

Fig.2.Waveforms and simpli ?ed models of LLC topology with proposed cur-rent limitation circuit under clamping mode.

The key waveforms and simpli ?ed equivalent circuit models of LLC topology with proposed OCP circuit under over-currentcondition are shown in http://www.doczj.com/doc/1e5116ed31b765ce050814d1.html ually,there are three operating modesin half a switching period.1)Mode.1:At

time ,the primary current increases from negative to positive.The primary circuit enters a resonant mode and the equivalentcircuit model is shown in Fig.2(a).Be-cause the magnetizinginductor

is far larger than the reso-nant

inductor ,for simpli ?cation it is ignored in the simpli ?ed equivalent circuit model.In thisperiod

(1)

where,

is the turns ratio of thetransformer

(2)

where1

;is the clamping voltage,which is equalto;

is the turns ratio of the auxiliary trans-former.

2)Mode.2:At time ,the resonant capacitor

voltage increasesto

and is clamped.Theperiod canbe

Fig.3.Operating frequency range versusL =L (full load).calculatedas(3)

The equivalent circuit model is shown in Fig.2(b).In thisperiod(4)(5)3)

Mode.3:At time ,the input voltage becomes

negative.In thisperiod(6)(7)

De ?ne the periods of Mode 2and Mode 3areand,

respectively.It can be derivedthat(8)(9)

In a LLC converter,the operation frequency range relatesto

,which can be derivedas(10)

where is the minimum operation frequency,which occursat the lowest input voltage.

According to (10),the relationship curve of the operating fre-quency rangeversus can be plotted as shown in Fig.3.From Fig.3we can see thatsmaller can help achieve a narrower operating frequency range.