Analysis and Design of Boost-LLC Converter for High Power Density AC-DC Adapter

Jun-Ho Kim, Moon-Young Kim, Cheol-O Yeon, and Gun-Woo Moon

Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST),

291, Daehak-ro, Yuseong-Gu, Daejeon, 305-701, Republic of Korea, gwmoon@ee.kaist.ac.kr

Abstract- This paper proposes the size reduction methods

for two-stage Boost-LLC converter to achieve high power density

in 60W AC-DC adapter. The two-stage converter has high

component count, and the passive components such as inductor,

transformer, and capacitor occupy the most of the area. In order

to make the converter have small size, this paper proposes the

size reduction methods of passive components: the design of low

link voltage, the design of the resonant tank without additional

inductor and with high resonant frequency, and the design of C

L-C filter replacing output capacitor. A 60W (16.8V 13.65A)

prototype adapter is designed and implemented to verify the

feasibility of the proposed methods. The converter achieves

14.5W/in3 power density with the methods.

I. INTRODUCTION These days, handheld and portable devices such as smart

phone, MP3 player, and notebook PC are gaining in popularity around the world. Along with the increase on sales of the devices, the demand of adapters for the low power application is also increasing. For the adapters, it is very important to achieve the high efficiency and high power density as the main performances. The high power density is focused due to the trend towards miniaturization [1]-[3]. However, the increase of power density is limited depending on the standard for surface temperature of adapter. The surface temperature is in proportion with power loss, and in reverse proportion with surface area. Hence, the high efficiency is the foundation to fulfill high power density, and several researches have been conducted for selection of proper topology for AC-DC adapter.

The converters able to be applied to AC-DC adapter can be categorized into two types: two-stage converter and singlestage one. For AC-DC converter of lower power than 75W, single-stage structure is usually used because the high power factor (PF) is not required [4], and the structure gives the merits of small size and low cost. However, compared to twostage converter, single-stage converter has much lower efficiency due to high switching loss and rectification loss [5]. On the other hand, two-stage converter has higher efficiency than single-stage one because of the low variation of input voltage for DC-DC stage, low rectification loss, and zero voltage switching (ZVS) operation [6]. The low loss can make the adapter have low heat generation. Therefore, it can be said that two-stage structure is suitable for miniaturization of ACDC adapter, even though the increase in capacity due to the high component counts. Based on this, in [7], several two-

978-1-4799-0482-2/13/$31.00 ©2013 IEEE 6

stage converters with the same power density are examined, then it is verified that two-stage Boost-LLC converter has the highest efficiency. With the work, power density of 1O.2W/in3 is achieved.

In this paper, several methods are proposed to achieve higher power density of AC-DC adapter adopting two-stage Boost-LLC converter. The two-stage Boost-LLC converter is composed of two converters as shown in Fig. 1: boost converter for AC-DC stage and LLC converter for DC-DC stage. In order to achieve high power density, the size reduction of passive components such as link capacitor, resonant tank, transformer, and output capacitor is strongly required. The methods to decrease the size of each component are proposed and analyzed. A 60W (16.8V/3.65A) prototype adapter has been implemented to verify the feasibility of the proposed methods. With the methods, high power density of 14.5W/in3 can be achieved.

II. CONSIDERATION FOR HIGH POWER DENSITY In order to achieve the size reduction of passive

components for high power density, several methods are proposed: design of low link voltage, design of the resonant tank without additional inductor and with high resonant frequency, and design of C-L-C filter replacing output capacitor. The proposed methods are discussed and analyzed in this section.

A. Design of Low Link Voltage The size of capacitor is in proportion with rated voltage and

capacitance [8]. The size comparison can be drawn as Fig. 2 (a) with QXW series electrolytic capacitor. It is noted that the 420V electrolytic capacitor has 11.3% lower size than 450V one on average. It also means that high capacitance can be obtained with the same size by using low rated voltage capacitor. With this analysis, low rated voltage capacitor is appropriate for high power density. Hence, the method to reduce the nominal voltage of link capacitor is proposed.

In the two-stage converter, the AC-DC stage has a role doing power factor correction (PFC). However, high PF of nearly one is not required to the system with a lower level of output power than 75W according to the harmonic standard (IEC 61000). In other words, the PFC is not required in whole input range (90V AC - 264VAC). This means that the output voltage can be lower than the peak of input voltage at high line. If the desired link voltage ( Vnonunal) is lower than the maximum value

EM] filter Clink

Fig. I. Circuit diagram of two-stage Boost-LLC converter

700° 1;:======;---------- 660�-6000 ... 420V Elee. Cap.

.... 450V Elec. Cap. MS 5000 �======�-........... __::��;:;;::�;;...---� 4000t-------§ 3000 +- ��....;�;;::::��:::::::....1!!ll...--------

� > 2000 JOOO t---��-�--�--�--�-�--� 33 39 47 56

Capacitance [Il"] (a)

68 82 100

= J.l t=r:;:=�����==========�� t 1.1 --j - 420V Elec. Cap' l , n< _�: 1l 1.0 --j .... 450V Elec. Cap. �-1.07-

-a._0.9 0.8� ___ .-Q."'08t----------0.74 ...----- 0.90 .;: E . 0.66 �....-- 0.80 cr.4!f'.0.7 � -0.6 0.5\ 0.59 0.62 0.69 -= 0.5 ... * 0.4 liAR 0.54 c:: 0.3t---�-�--__,---,---.__--�-__,

33 39 47 56 68 82 \00 Capacitance [Il"]

(b) Fig. 2. Comparison between 420V and 450V electrolytic capacitor according to

capacitance (a) Volume, (b) Rated RMS ripple current

Vnominal" ........ ,'1 ... " ���:" " " " " " " " " " " " I ........... � I�:�·· .. ··· " I v: I' ,;-VAC # I I '. , max \". ,' • / I I , " I I ". : I I \ : I I ". :' I I \,/ I I \ : I I ". : I I ".

o ' " , O.5h h t

Fig. 3. Output voltage waveform of boost converter

of input voltage ( Vmax), the waveform of link voltage is directly affected by input voltage shape. As shown in Fig. 3, the boost converter operates normally during lnor, and the converter does not operate during ldis' Hence, the highest link voltage is ensured to be under the maximum voltage of input source; 373V at 264VAC. Due to the low descent speed of link voltage, the voltage waveform is similar to single-stage capacitive input filter.

As a result, the lowest link voltage level can be designed by Vnominal, and the input voltage range of DC-DC converter is also affected. In order to achieve high efficiency, it is suitable to

7

have narrow input voltage range for LLC converter. Therefore, the Vnominal cannot be designed with very low level.

In general, Vnominal is set as 400V to achieve PFC operation over the whole input range, and hence the electrolytic capacitor having a rated voltage of 450V is commonly used as link capacitor. However, the proposed operation presented above gives the chance to employ the electrolytic capacitor having rated voltage of 420V with the nominal voltage design of 366V.

To design the capacitance of link capacitor, other characteristics are needed to be considered such as RMS ripple current, voltage ripple, and hold-up time. A large size capacitor can endure high ripple current as shown in Fig. 2 (b). The RMS ripple current flowing through the link capacitor on the circuit can be calculated by (1). With the voltage ripple characteristic, the boundary of capacitance can be designed by (2). For low voltage ripple, high capacitance is required. In the adapter application, the hold-up time is only 2ms. The required capacitance can be calculated by (3), but it is less than the result of (2). Hence, capacitance is designed by the specification of voltage ripple.

llink,rms �

[ 32.J2 PlN,avg2 1 97C Vin,rmsVout,nom

_ [ y/boost x P1N,avg )2 Voul,nom

p C > out,avg link - � 2,':1 line x Voul,nom x Ll Voul,pk-pk

C > Y/lolal x I]N,avg x T Hold-up

�k- 2 2 Voul,nom - Voul,min

1 2

(1)

(2)

(3)

where Inne is the frequency of rectified input voltage, Valli. nom is nominal output voltage, L1 VOlll,pk.pk is output voltage ripple, and Po,avg is average of output power, P/N,avg is average of input power, V/N,rms is input rms voltage, Y/boosl is efficiency of boost converter, Y/IOlal is efficiency of two-stage Boost-LLC converter, and THo/d,"p is the required hold-up time.

I&I.���� � 'QR9' 'QR9' 'QR9' 'QR9' 'QR9'

� ..j �

(a)

1'QR9'����

Insulation i>-< () ffi rx-x

(b)

Pri. winding Insulation 1 I

�) i tift �A: � X . � a � � >-< ),..6 J. � M . � �� �� y J. � . � � �

(c) Fig. 4. Winding method

it-

?-< it-

it-

?-< it-

� �

t-

t-

t-

t-

Sec. winding

Insulation

Pri. winding

Sec. winding

Insulation

Pri. winding

Sec. winding

(a) General, (b) Asymmetric, (c) Sectional stacking

B. Integrated Transformer and High Resonant Frequency The conventional structure of Boost-LLC converter is

shown in Fig. 1. The LLC converter has a series resonant network comprised of an external inductor (LexJ, leakage inductance (Llkp, Llks) and magnetizing inductance (Lm) of transformer, and capacitor (C,). The resonant network occupies the high proportion of total volume. For the reason, the effort on miniaturization within the field of power electronics builds the trend towards using integrated transformer [2], [9]. As a result, the transformer is the only component having inductance in the resonant network because the external inductor is integrated into the transformer called integrated transformer. Hence, the resonant inductance (Lr) can be calculated as (4).

(4)

where Llkp is primary side leakage inductance, n is turns ratio, and Llks is secondary side leakage inductance.

As a result, the inductance for resonant tank is greatly affected by the leakage inductance of transformer. The leakage inductance is dependent on the winding method, and the exact magnitude is not predictable. Thus, practical approaches are carried out by using the transformers with the three types of winding as shown in Fig. 4.

For comparison, the turns ratio is decided as 44:4 from the voltage conversion ratio equation of LLC converter. The wire for primary side is 0.2 2strands, and 0.1 40strands is used for secondary side. The inductance can be summarized as Table I.

8

1800

1600

Mi 1400 .§. 1200 " 5 " 1000 '0 800 >

600

400

r----j 400V Film Cap / J L /

� ,----/

10 15 20 22 24 30 33 36 39 47 51 56 62 68 75 82 91 100 Capacitance (nF)

Fig. 5. Volume of capacitor according to capacitance

Table I. Comparison of inductance

Winding method Inductance

General

Asymmetric

Sectional stacking

20-24J.tH

25-30IlH

50-52J.tH

As shown in the result, the higher leakage inductance is attained when the shared area between primary and secondary winding is small, which means the magnetic coupling is poor. In summary, the leakage inductance has a tendency to be higher according to the lower coupling coefficient (K). The coupling coefficient and resonant inductor has the relation as follows.

(5) , where Lp is self-inductance. The magnitude of Lp is the same as the sum of Llkp and Lm•

From (5), there are two methods to maintain the same inductance as the case using external inductor. One method is the poor magnetic coupling between primary and secondary winding. In this case, a sectional stacking method for the winding can be applied. However, the low coupling coefficient causes the fringing effect, which increases core loss and eddy current. For this reason, it is desirable to make the coupling coefficient high even though leakage inductance is low. The other method is to make the self-inductance of transformer high. It can be realized by the increase of core size or the number of winding. Hence, large size transformer is required. By considering the efficiency and volume, the increase of transformer size is not suitable to achieve the high power density. Therefore, the low leakage inductance design of transformer is inevitable.

The resonant frequency is in reverse proportion with the inductance and capacitance as shown in (6).

f, = 1

r 27fJLrCr (6) ,

where Lr is resonant inductance, and Cr is resonant capacitance. The volume of fihn capacitor is shown in Fig. 5. The figure is based on the Pilkor box capacitor model because it is mostly used for resonant capacitor due to heat generation and low variation of capacitance. As shown in the figure, the capacitor has the smallest package under 22nF. The minimum frequency is 220kHz with this capacitance and the leakage inductance of transformer (24f..lH) having general winding method. As a result, the design with lower resonant frequency requires more space for capacitor. Hence, high resonant frequency design is appropriate for high power density.

+

Vetil

(a) (b) Fig. 6. Circuit diagram of output filter

(a) Conventional (b) C-L-C filter

+

VCo] R"

The Ap value is the criteria to detennine the resonant frequency because the value is the base for design of transfonner. For high power density, the transfonner should have small size. Therefore, the design of resonant frequency is based on it. When the secondary side has center-tapped structure, the equation of Ap value is presented as follows.

Ap = VpriD [lpn,rms +2 Ns 1sec, rms ] ku!'!.Bfsw J pri N p Jsec

(7)

where, kll is utilization factor, BMax is maximum flux density, Vpn is primary voltage of transformer, D is duty ratio, Jpri and Jsec are current density, Np and Ns are the number of turns of primary side and secondary side respectively.

With 270kHz resonant frequency, the required Ap value is 1487mm4. The primary nns current is 0.45A and secondary rms current is 2.9A based on the simulation result. The primary and secondary current density is 6A/mm2 and IOA/mm2 respectively when the wires are 0.2 2strand and O.l 40 strand. The flux density is 0.16mT, and the utilization factor is 0.15 due to the gap. The RM7 core has Ap value of 1484mm4. Therefore, the transfonner can be designed with RM7 core.

The LLC converter without the external inductor can be designed with equation as follows [10]:

(8)

(9)

(10)

(11)

Lp m= -

Lr (12)

where G is voltage gain, Ws is switching frequency, Wr is resonant frequency between Lr and C" and wp is resonant frequency between Lp and Cr.

The voltage gain is not 1 at resonant frequency. The voltage gain is higher as much as the lover coupling coefficient.

9

C. C-L-C Filter The LLC converter usually uses the capacitive filter as

output filter as shown in Fig. 6 (a). Thus, high rated current ripple characteristic is required for the output capacitor, which makes the power density of the output capacitor poor. In case of using the C-L-C filter instead of the capacitive filter, the current passing through output inductor (Lo) and rear capacitor (Co2) has low current ripple because the most of the current ripple flows through the front capacitor (COl)' Thus, the rear capacitor needs not have high rated current ripple characteristic. For the front capacitor, the multilayer chip capacitor (MLCC) can be used. The MLCC has high rated current ripple characteristic. Thus, total filter size can be smaller than capacitive filter. The current and voltage ripple condition of each component can be expressed as follows:

A _ 10 [Sin BCol Bcol ] ,-,vCol - -- ---COl 2fsw 7r.h.

!'!.ho !'!.vC02 = --"""--16 fswC02

(13)

(14)

(15)

where 10 is average output current, BCol IS defined as

BCol = arccos(2fsw J 7rfr

The parameter values of components for C-L-C filter are designed with the above equations. The specification of voltage ripple on rear capacitor can be assessed with (15). However, for selection of practical component, current ripple should be considered. The ripple is the same as a half of inductor current ripple.

III. EXPERIMENTAL RESULT In order to verify the feasibility of the proposed scheme, the

experimental prototype of 60W (16.8V/3.65A) Boost-LLC resonant converter has been built with components as Table II.

Table II. Summary of components and parameters Parameter Component Value

Boost inductor La RM7 PL-9 32511H (0. 1

Np:N, 44:4 Lm 58Ol1H L" 23.511H Lr2 23.4/lH

Resonant capacitor C, Pilkor Box cap 15nF C-L-C filter COl MLCC, 3216 2211F x2

Toroidal Lo (063060, HF) 400nH

0.6

Co2 Electrolytic cap. 1001lf Controller Boost NCPI605

LLC FAN7631

1

OA

(a)

�ink. 100V/div

ViN • 100V/div

he 1A/div

�/nk. 100V/div

ViN 100V/div

OV� he

1A/div OA

;;' J -= .. '" = �

(b) Fig. 7. Waveform of link voltage and boost inductor current

(a) at 90VAC input, (b) at 264VAC input

410

405

400

395

390

385

380

375

370

365

360

-r0il.4;o;.466 -

... '0;::;;.45"1--;;0.�44:;-7 ---------;::=====:;-i ° .5 �::::::::���::====���V�llnk����I,,�ln�='m='� �

+- ---- 0.4 '" ............. 0.343 O"'''.�cc--------i � +--___________ �_"'

0,..30

_4 __ �=-=- Oc2.:"4- 0.3 -:J" 1 -fc36'""5'"".7-'36"'6" .1 '36"6.'2 ----------.36"6.'2 -"36'T 6.3�3U. 66" .. 4 --./�'---+O.I

90 100 110 120 140 160 180 200 220 230 240 250 260 264

Input voltage[V"cJ

Fig. 8. Link voltage and rms current of link capacitor

0.2 0.4 0.6 0.8 I 1.2 1.4 1.6 1.8 Normalized Frequency

Fig. 9. Voltage gain waveform of LLC converter (fJf,)

RM7(54T) FFPF08H60S 3251'H

JPA60R190 + SJ

sJ + Clink VUllk 39)1F1420V SJ GBL406

-------OV � ----'I

Vgste 20V/div

Iprl 500mA/div

Fig. 1 1 . Waveform of primary current and output voltage on LLC converter

The operation of boost converter is shown in Fig. 7. The nominal output voltage for boost converter is set with 366V. At the low line, the boost converter operation is normal. Meanwhile, the peak of input voltage is higher than the nominal output voltage at 264V AC input. In the region, the controller skips the switching. As a result, the output voltage is the same as the peak value of input voltage. The link voltage and rms current stress of link capacitor are shown in Fig. 8. The link voltage is maintained under 373V in whole input voltage. Also, the rms current stress is measured as 0.46A. Hence, the 420V capacitor can be used with more than lO% margin of voltage and rms current ripple.

The voltage gain curve of LLC converter is represented as Fig. 9. The converter has high ratio between Lm and Lr. As a result, voltage gain curve has low-slope, but the frequency variation range can be below 50kHz because input voltage variation range is narrow. The waveform of primary current and output voltage at nominal voltage is shown in Fig. 11. Until 240V AC input, the output voltage of boost converter stays 366V range. Thus, the operation frequency is at resonant frequency for high efficiency. At high input voltage, the operation mode changes to above resonance.

The comparison of size between 450V capacitor and 420V capacitor is shown on the Fig. 12 (a). The 420V capacitor used in prototype has the diameter of I2.5mm and height of 25mm. Compared to the 450V capacitor, the diameter increase 2.5mm, and the height reduces I5mm. Therefore, the occupied area and volume of link capacitor are reduced as much as the 87.5mm2 (22%) and 73.6mm3 (2.3%) respectively.

With the LLC converter design, core can be changed from RM8 core to RM7 core as shown in Fig. 12 (b). By the change of core, the occupied area and volume reduces 74.93mm2 (25.6%) and I090mm3 (45%) respectively. Especially, the reduction of height helps to use two PCB board as shown in Fig. 13.

100)lF,25V

Fig. 1 0. Circuit diagram of two-stage Boost-LLC converter

10

� ;., " c " ';j I: �

2-n = 3---'0

(a)

RM8 (b)

RM7

-t .

(c) Fig. 1 2. Comparison of size reduction between

(a) Link capacitor, (b) Transformer, (c) C-L-C filter

(a)

(b) Fig. 13. Implementation of the proposed converter

93.50 93.00 92.50 92.00 91.50 9/.00 90.50 90.00

92.43

.� 91.32/

9089 .---' ../' .U.lV

89.50 90 115 132 180 230 Input voltage IV AC]

.!:l

264

Fig. 14. Efficiency of two-stage Boost-LLC converter

11

The output filter is designed by using C-L-C filter. The conventional capacitive filter needs to use the capacitors having high rated current ripple characteristic. Also, the high capacitance is required to meet the output voltage ripple condition (200m V). Hence, the large size capacitor is needed. With C-L-C filter, the ripple current flows through the front capacitors. Thus, the size of capacitor can be reduced by using MLCC having high rated current ripple characteristic. Also, the size of rear capacitor can be smaller. Therefore, the total volume can be reduced although the inductor is required as shown in Fig. 12 (c).

The prototype of AC-DC adapter with the proposed methods is shown as Fig. 13. As a result, the converter achieves 14.5W/in3. And also, the efficiency of two-stage Boost LLC converter can be achieved as shown in Fig. 14.

IV. CONCLUSIONS In this paper, several methods to achieve higher power

density AC-DC adapter are proposed and analyzed. In order to verify the feasibility of the methods, the AC-DC adapter prototype is implemented. As a result, the adapter system is composed in the volume of 58mm x 57mm x 22mm, and the power density of 14.5W/in3 is achieved. Therefore, the proposed methods to reduce the size of passive components are suitable for the AC-DC adapter to achieve high power density.

ACKNOWLEDGEMENT

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MEST) (No.20 12-0000981)

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