2011 International Conference on Information and Electronic Engineering IPCSIT vol.6 (2011) (2011) IACSIT Pre, Singapore Deign of a digitally-controlled LLC reonant converter Jia-Wei huang 1, Shun-Chung Wang 2 and Yi-Hua Liu 1 1 National Taiwan Univerity of Science and Technology, Taipei, Taiwan, R.O.C. 2 Lunghwa Univerity of Science and Technology, Taoyuan County, Taiwan, R.O.C. Abtract. Nowaday, liquid crytal diplay (LCD) panel are widely ued in application uch a monitor, notebook and televiion. In large cale LCD TV (>30 ), the power upply typically require an output power range from 200 W to 600 W. Generally, a half-bridge LLC reonant converter with zero voltage witching (ZVS) capability i utilized a the DC/DC power tage. In thi paper, the deign of a digitallycontrolled LLC reonant converter i preented. The LLC reonant topology allow for ZVS of the main witche, thereby dramatically lowering witching loe and booting efficiency. A digital ignal controller (dpic30f2020) from Microchip corp. i utilized a the digital controller of the LLC reonant converter. In order to deign the compenation circuit correctly, the large-ignal and mall-ignal model of the utilized LLC converter i alo derived in thi paper uing the phaor tranformation method. Finally, experimental reult are preented to validate the correctne of the propoed ytem. Keyword: LLC reonant converter, digital control, phaor tranformation 1. Introduction Nowaday, liquid crytal diplay (LCD) panel are widely ued in application uch a monitor, notebook and televiion [1]-[2]. In large cale LCD TV (>30 ), the power upply typically require an output power range from 200 W to 600 W. A the creen area increae, the power required for the 24-Vdc rail continue to rie until it i no longer practical to implement the SMPS uing a flyback topology. A a reult, a variety of higher-power topologie, including the half bridge LLC topology, ha been conidered to achieve high efficiency in a compact pace with low-emi generation [3]-[4]. In thi paper, the deign of a digitally-controlled LLC reonant converter i preented. The LLC reonant topology allow for ZVS of the main witche, thereby dramatically lowering witching loe and booting efficiency. A digital ignal controller (dpic30f2020) from Microchip corp. i utilized a the digital controller of the LLC reonant converter. Digital power alo provide intelligent adaptability and flexibility to atify any complex power requirement with the traightforward ability to monitor, proce and adapt to ytem condition [5]. In order to deign the compenation circuit correctly, the large-ignal and mall-ignal model of the utilized LLC converter i alo derived in thi paper uing the phaor tranformation method [6]-[7]. Finally, experimental reult are preented to validate the correctne of the propoed ytem. The meaured efficiency of the whole ytem achieved 87 %. 2. Hardware Decription Fig. 1 how the circuit configuration of the propoed digitalized LLC reonant converter. From Fig. 1, the whole hardware ytem can be divided into two major part: LLC reonant converter DC/DC power tage and digital controller. Detailed decription about each part will be given in the following ubection. 2.1. LLC reonant converter The half-bridge LLC reonant topology i utilized in thi paper to convert the output voltage of power factor corrector front tage (typically 400 V) into voltage needed to upply the functional block uch a backlighting, microproceor and interfacing circuit, etc. LLC reonant converter i widely ued in conumer application uch a LCD TV or plama diplay panel with output power level range from 200 W up to 600 W. The advantage of LLC converter include: ZVS capability over the entire load range 55
Lower electromagnetic interference (EMI) Zero current turnoff of the econdary diode No increaed component count comparing to conventional half bridge topology. From Fig. 1, the primary ide of the LLC converter i a half-bridge configuration. The econdary ide i a center-tapped rectifier followed by a capacitive filter. Switche S 1 and S 2 are both driven by 50 % duty cycle gating ignal, with a mall amount of dead time introduced between the conecutive tranition. The circuit ha three paive component, L r, C r and L m, where L m i the magnetizing inductance that act a a hunt inductor, L r i the erie reonant inductor, and C r i the reonant capacitor. Fig. 2 how the typical waveform of the preented LLC reonant converter. From Fig. 2, the operation of half of witching cycle can be divided into four mode. (1) Mode 1 (t 0 < t < t 1 ):at t=t 0, S 1 turned on. During thi mode, output rectifier diode D 1 conduct. The tranformer voltage i clamped at V o. L m i linearly charged with output voltage, o it doen t participate reonant during thi period and V p = n V out. The current i Lr and i Lm increae. The energy flow through the reonant tank and tranformer and to the load. Thi mode end when i Lr current i the ame a i Lm current. Output current reach zero. (2) Mode 2 (t 1 < t < t 2 ):at t=t 1,, the two inductor current i Lr and i Lm are equal. Output current reache zero. Both output rectifier diode D 1 and D 2 i revere biaed. Tranformer econdary voltage i lower than output voltage. Output i eparated from tranformer. During thi period, ince output i eparated from primary, L m i freed to participate reonant. Thi mode end when S 1 i turned off. (3) Mode 3 (t 2 < t < t 3 ):at t=t 2, S 1 i turned off. During thi mode, S 1 and S 2 are both off. The reonant current i Lr charge (dicharge) the paraitic capacitance C o1 ( C o2 ) of the power witche. When the voltage acro C o1 equal V in, the body diode of S 2 i turned on. (4) Mode 4 (t 3 < t < t 4 ):The body diode of S 2 i turned on in previou mode, which create a ZVS condition for S 2. Gate ignal of S 2 hould be applied during thi mode. When S 2 i turned on, i Lr decreae and thi will force econdary diode D 2 conduct and i out begin to increae. Alo, from thi moment, tranformer ee output voltage on the econdary ide. L m i clamped with contant voltage Vp = n Vout, o it doen t participate reonant during thi period. For next half cycle, the operation i ame a analyzed above and i omitted here. 2.2. Digital controller In thi paper, the dpic30f2020 digital ignal controller (DSC) from Microchip corp. i ued to control the preented LLC reonant converter. The propoed digital power upply offer a new freedom for uer to control the power output of the LCD power upply according to the power requirement, thi lead to an energy economy. When compared to an analog one, digital control boat everal advantage uch a the poibility for implementing ophiticated control law allow the deigner to modify the control trategy without ignificant hardware modification higher tolerance to ignal noie and the complete abence of ageing effect or thermal drift the poibility to provide man to machine interface (MMI) The digital controller utilized in thi paper i a conventional PID controller; the digital control algorithm can be deigned a: un ( ) = un ( 1) + K0 en ( ) + K1 en ( 1) + K2 en ( 2) (1) where E(n) i the error ignal and U(n) i the input ignal. It hould be noted that the LLC converter work with variable frequency control. Thi implie that power flow can be controlled by changing the operating frequency of the converter in uch a way that a reduced power demand from the load produce a frequency rie, while an increaed power demand caue a frequency reduction. Unlike the conventional PWM control trategie in which the duty cycle i the control variable. For LLC converter, the control variable i the witching frequency. Therefore, for the preented digital controller, the output of PID controller hould be fed into the Period regiter of the utilized PWM 56
module while the PDC (duty cycle) regiter hould be et a half the value of that in Period regiter to obtain 50 % duty cycle. Fig. 1 Circuit configuration of the digitalized LLC reonant converter Fig. 2 Typical waveform of LLC Reonant Converter 3. Small-ignal Model of the LLC Reonant Converter In order to deign the parameter (K p, K i and K d ) of the compenation circuit, a mall-ignal model hould firt be obtained. In thi paper, the phaor tranformation method a preented in [6] i utilized. In [6], a modified phaor tranform a hown in Eq. (2) can be ued to apply to variable frequency operation. j () tdt xt () = Re[ xte () ] (2) where x() t i the time-varying phaor correponding to x(t), which preent the amplitude of x(t), and () t i time-varying intantaneou frequency. Uing Eq. (2), the phaor equation for reitor, inductor and capacitor can be derived repectively. Uing the ame technique, the phaor preentation of a general witching network and a rectifier with a tranformer can alo be obtained. Since the LLC reonant converter only conit the above-mentioned part, the large ignal and mall ignal model of the LLC reonant converter can be obtained and are preented in Fig. 3. Uing thi mall-ignal model, the tranfer function can be derived and the compenation circuit can be deigned accordingly. Fig. 4 how the open loop and cloe loop bode plot of the preented ytem. From the cloe loop bode plot, the cut-off frequency i 20 khz and the phae-margin i 55.8 degree. The obtained compenation circuit parameter can be decribed a G () = G c c0 z (1 + ) z2 (1 + )(1 + ) p 3 3.31 10 4 K I 5.34 + + 11.89 10 = KP + + KD (3) 57
(a) Large ignal model (b) Small-ignal model Fig. 3 Large and mall ignal model of LLC Reonant Converter (a) Open loop (b) Cloe loop Fig. 4 Open loop and cloe loop bode plot of the utilized LLC Reonant Converter 4. Experimental Reult In order to verify the correctne of the propoed ytem, ome experiment are carried out. Due to limited pace, only elected waveform are diplayed in thi ection. The pecification of the preented prototype ytem i 1) Input voltage: 400 Vdc 2) Output voltage: 24 Vdc 3) Output current: 14.6 A (350 W) Fig. 5 how the meaured key waveform of the propoed LLC converter. From Fig. 15, the propoed ytem operate in LLC reonant mode correctly. Fig. 6 how the meaured efficiency of the propoed LLC converter. From Fig. 6, the efficiency of the propoed LLC i higher than 87 %. Fig. 7 how the photo of the propoed ytem. (a) light load (3 A) (b) full load (14.6 A) (VGS 10 V/div, VDS 500 V/div, Vp 500 V/div, ir 2 A/div, Time 10 μ/div) Fig. 5 Meaure key waveform of the propoed LLC converter 58
Fig. 6 Meaured efficiency of the propoed LLC converter 5. Concluion Fig. 7 Photo of the propoed ytem In thi paper, the deign of a digitalized LLC reonant converter i preented. Detailed decription of the LLC reonant converter tage and the digital controller i preented. Since the LLC converter work with variable frequency control, the deign conideration of implementing frequency control uing commercially available microcontroller i alo preented. In order to deign the compenation circuit correctly, the largeignal and mall-ignal model of the utilized LLC converter i alo derived in thi paper uing the phaor tranformation method. According to the experimental reult, the efficiency of the propoed LLC converter i higher than 87 % for the whole load range. 6. Reference [1] Y. H. Liu, Deign and Implementation of an FPGA-Baed CCFL Driving Sytem With Digital Dimming Capability, IEEE Tran. on Indutrial Electronic, vol. 54, no. 6, Dec. 2007, pp. 3307-3316. [2] H. J. Chiu, S. J. Cheng, LED Backlight Driving Sytem for Large-Scale LCD Panel, IEEE Tran. on Indutrial Electronic, vol. 54, no. 5, Oct. 2007, pp. 2751-2760. [3] F. Krimer and J. W. Kolar, Accurate mall-ignal model for the digital control of an automotive bidirectional dual active bridge, IEEE Tran. on Power Electronic, VOL. 24, NO. 12, pp. 2756-2768, Dec. 2009. [4] M. M. Peretz and S. Ben-Yaakov, Digital control of reonant converter: reolution effect on limit cycle, IEEE Tran on Power Electronic, VOL. 25, NO. 6, pp. 1652-1661, Jun. 2010. [5] Yan-Fei Liu; Meyer, E.; Xiaodong Liu, Recent Development in Digital Control Strategie for DC/DC Switching Power Converter, IEEE Tranaction on Power Electronic, VOL. 24, NO. 11, pp. 2567-2577, Nov. 2009. [6] J. Tian, J. Petzoldt, T. Reimann, M. Scherf, G. Deboy, M.Maerz, G.Berger, Envelope Model for Reonant Converter and Application in LLC Converter, 2007 European Conference on Power Electronic and Application, pp.1-7, 2007. [7] J. Tian, J. Petzoldt, T. Reimann, M. Scherf, G. Berger, Modelling of aymmetrical pule width modulation with frequency tracking control uing phaor tranformation for half-bridge erie reonant induction cooker, 11th European Conference on Power Electronic and Application (EPE), September, 2005, Dreden, Germany. 59