Analysis and Optimization of LLC Resonant Converter With a Novel Over-Current
Protection 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.This method is very attractive for its inherent current limit ability,espe-cially under short-circuit https://www.doczj.com/doc/1617252143.html,bined with the frequency increasing method,good current limit features can be achieved. Compared to the conventional LLC resonant converter with fre-quency increasing method,LLC with the proposed OCP method can reduce the maximum frequency to an acceptable value.De-tailed theoretical analysis and optimization design considerations have been presented.An experimental prototype converter based on the proposed method 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],such as zero voltage switching(ZVS)capability from zero to full load range,low voltage stress on secondary recti?er,high ef?ciency at 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 critical issue,which will be discussed in this paper.The purpose of OCP is to limit the current stress in the circuit under overload or short-circuit condition,or limit the inrush current during start up so that the power converter will not be damaged under those conditions.
Three OCP methods have been available for LLC resonant converters up until now.The?rst method is increasing the switching 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 that magnetic 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 under Contract2005038277.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@https://www.doczj.com/doc/1617252143.html,).
Color versions of one or more of the?gures in this paper are available online at https://www.doczj.com/doc/1617252143.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,a lower maximum frequency can be chosen compared to the?rst method.The PWM control is used to limit the current so that magnetic and semiconductor components don’t need to be over designed.However,the primary switches will lose ZVS under OCP 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 splitting resonant capacitors and ZVS of primary switches can still be achieved under OCP mode.The main problem of this method is that 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 is much higher so that the current limitation function almost loses. Moreover,the voltage across the splitting capacitors should be 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-iliary circuit,the current limitation feature of the original con-verter can be improved greatly.The detailed principle analyses will be illustrated in Section II.Section III will present param-eters design considerations.Experimental results will be pre-sented in Section 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 increase rapidly.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.
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Fig.1.LLC topology with proposed current limitation
circuit.
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-current condition are shown in https://www.doczj.com/doc/1617252143.html,ually,there are three operating modes in 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 equivalent circuit model is shown in Fig.2(a).Be-cause the magnetizing
inductor
is far larger than the reso-nant
inductor ,for simpli ?cation it is ignored in the simpli ?ed equivalent circuit model.In this
period
(1)
where
,
is the turns ratio of the
transformer
(2)
where
1
;is the clamping voltage,which is equal
to
;
is the turns ratio of the auxiliary trans-former.
2)Mode.2:At time ,the resonant capacitor
voltage increases
to
and is clamped.The
period can
be
Fig.3.Operating frequency range versus
L =L (full load).
calculated
as
(3)
The equivalent circuit model is shown in Fig.2(b).In this
period
(4)(5)
3)
Mode.3:At time ,the input voltage becomes
negative.In this
period
(6)(7)
De ?ne the periods of Mode 2and Mode 3
are
and
,
respectively.It can be derived
that
(8)(9)
In a LLC converter,the operation frequency range relates
to
,which can be derived
as
(10)
where is the minimum operation frequency,which occurs
at the lowest input voltage.
According to (10),the relationship curve of the operating fre-quency range
versus can be plotted as shown in Fig.3.From Fig.3we can see that
smaller can help achieve a narrower operating frequency range.