High Power Densityy High Efficiency DC/DC Converter Daocheng Huang1, David Gillham1, Weiyi Feng1, Pengju Kong2, Dianbo Fu F 3, Fred. C. Lee1 1. Center for Power Electronics Systems Virginia Tech Blacksburg, VA 24061, USA [email protected]
2. Intersil Corporation Milpitas, CA, 95035
Abstract—In this paper, the high power density high efficiency Commercial PWM trend of DC/DC converter is summarized. C dc-dc converters are discussed based on effficiency. AHB is picked to represent high efficiency PWM M converter. To increase the power density, passive In ntegrated Power Electronics Module (IPEM) technology was u utilized. However, PWM dc-dc converters meet some problems w when holdup time is required. LLC excels for its low switching loss and low circulating energy at nominal condition. At a high switching frequency, passive integration technologyy is applied to integrate Cr, Lr, Lm, and the transformer into one module (LLCT). The SR driving method is used, whicch is based on the body diode forward voltage drop of SR. One 1000 W, 800 kHz, 400 V/ 48 V LLC resonant converter prototyype is built based on optimal design procedure, LLCT, and SR drive. The power density of converter is 177W/inch3. The efficiency can reach as high as 96.4%.
Distributed power systems (DPSs) are w widely used in offline applications, such as servers, telecoom applications, laptops and flat panel TVs. In a DPS, poweer is processed in two stages: front-end converters and load coonverters. two main driving Efficiency and power density are the tw forces of front-end converters , . Fronnt-end converters normally consist of three parts: an EMI filteer, a power-factor correction (PFC) circuit, and a dc-dc conveerter. The typical structure of a front-end converter is show wn in Figure 1. Reducing bulky capacitors is one way to inncrease the power density of DPSs. For computing applicatioons, holdup time operation is required. Bulky capacitors m must be used to provide energy during the holdup time, whiich is affected by the operational input voltage range of thee dc-dc stage. A wide operation range in the dc-dc stage is necessary to reduce the holdup time capacitance.
Pulse-width modulation (PWM M) dc-dc converters are widely employed in front-end con nverters . Two-switch forward converters ,  have recceived lots of interest for their robustness and their easy transsformer reset mechanism. However, their high switching loss and large filter inductance are major concerns. To relieve r switching loss and stress, soft-switching techniques have been introduced. Phase-shift full-bridge converters - [ can achieve zerovoltage switching (ZVS) to enhan nce the efficiency of the converter. Asymmetric half bridg ge converter (AHB) is another soft switching converter ]. Since the two switches works complementarily, there is no o ringing problem caused by leakage inductance. Also, since energy e is transferred from input to output during whole switcching period, there is no circulating current as seen in phase shift s full bridge. The total size and volume of AHB are in large part determined by the size and dim mensions of the passive components. To increase the power p density, passive integrated power electronics modulee technology was utilized. The total volume, core loss, and prrofile are greatly reduced without sacrificing performance . nverters meet the problem However, most PWM dc-dc con that the duty cycle is small at nom minal conditions, when a holdup time is needed. A small duty d cycle leads to large circulating energy and a high turn-o off switching loss, which significantly sacrifices efficiency. Resonant converters are anotherr option. Series Resonant Converter (SRC) , Parallel Reesonant Converter (PRC)  and LCC resonant converter (LCC) (  are the three most popular topologies. However, they all have some limits . LLC resonant dc-dc conveerter (LLC) is another candidate [15-20] in front-end dc-dc d application. Low switching loss and low circulating g energy is achievable at nominal condition. Wide input vo oltage range is obtained. Zero-current-switching (ZCS) is achieved for output rectifiers and no reverse recovery appears. Thus, LLC has more benefits than SRC, PRC and LCC L in this application. At a high switching frequency, the passive components’ volume can be reduced. Howeverr, in the state-of-the-art resonant converters, the discrete passsive components and the heat sink still occupy more than 80% 8 of the total system volume . To reduce the vo olume of the magnetic
Figure 1. Front-end converter
This work was support by AcBel Polytech, Chicony Pow wer, Crane Aerospace, Delta Electronics, Emerson Network Power, H Huawei Technologies, International Rectifier, Intersil, Linear Technology, Lite-On Technology, Monolithic Power Systems, National Semiconductor, N NXP Semiconductors, Richtek Technology, and Texas Instruments.
978-1-4577-0541-0/11/$26.00 ©2011 IEEE
ower Systems Division 3. Po Huaweei Technologies Co., Ltd. Plano, TX, 75075, USA
components, magnetic integration has beenn proposed . However, the passive components still havee large profile. To further reduce the overall volume, passsive integration technology is applied to integrate the resonnant inductor (Lr) and capacitor (Cr), the magnetizing inducttor (Lm), and the transformer (Tx) (L-L-C-T) into one module [23-31]. For better efficiency, Secondary Recttifier (SR) drive technique is employed. Commercially, tthe drain-source voltage of secondary-side MOSFETs is ussed to drive SRs. SR would cause However, the package inductance of an S phase-shift of the sensed voltage, which introduces high body-diode conduction loss. At high switchhing frequencies, this conduction loss becomes much worse. A compensation driving scheme has been proposed to correcct the deviation in the sensed voltage and to precisely drivve the SR . Another SR driving method is based on the body diode forward voltage drop of SR. The bodyy diode forward voltage drop of SR is detected to tune the deead time [33-34]. To utilize all the techniques mentioned above, One 1000 W, 800 kHz, 400 V/ 48 V LLC resonant connverter prototype is built. The passive components are all integrated. SR driving scheme is based on . The ppower density of converter is 177W/inch3. The efficiency cann reach as high as 96.4%. C/DC converters’ In this paper, the commercial PWM DC characteristics are discussed in section II. To improve power density, the passive integration method iis also given in section II. In section III, the benefits of ressonant converters are discussed. The passive integration methhod is mentioned for LLC resonant converters. Then, two SR R driving schemes are discussed for better efficiency. One proototype including all the benefits is built to demonstrate higgh efficiency and high power density. The trend of DC/D DC converter is summarized in section IV. II.
RTER PWM DC/DC CONVER
will limit the choice of transformeer turns ratio, which will affect the performance of whole co onverter. Even with large leakage inductance, still ZVS cann not be achieved at light load. Another issue for phase shifft full bridge is the high circulating current. Symmetrical half bridge convertter is another option. It is hard switching topology. Leakage inductance i is detrimental to the performance of converter. No ormally snubber circuit is needed to absorb the ringing prob blem caused by leakage inductance during the period when both b switches are off. Asymmetrical half bridge conv verter (AHB) has some unique characteristics [9, 14]. First, AHB is a soft switching converter. Primary two switches can n achieve ZVS with help of leakage inductance. Since th he two switches works complementarily, there is no ring ging problem caused by leakage inductance. Also, since en nergy is transferred from input to output during whole switcching period, there is no circulating current as seen in phasee shift full bridge. Thus, AHB can achieve high efficiency.
Figure 2. Asymmetrical halff bridge converter
B. Passive Integration of PWM DC C/DC Converters As discussed above, AHB is takeen as example for its high efficiency. To improve power denssity of AHB, the passive integration method is utilized .
A. Comparison of PWM DC/DC Converterrs Pulse-width modulation (PWM) dc-dcc converters are widely employed in front-end converters . Two-switch forward converters ,  have received lots of interest for their robustness and their easy transformer reset mechanism. However, their high sw witching loss and large filter inductance are major concerns.
(a) Physical structure of LC cell
To relieve switching loss and stresss, soft-switching techniques have been introduced. Phase--shift full-bridge converters - can achieve zero-voltage switching (ZVS) to enhance the efficiency of the convertter. This is very helpful for high frequency operation. This ttopology also has low volt-sec on the output filter inductor. mplex. With four On the other side, this topology is com switches on primary side, control and driver circuit will be more complex. Another problem is the leaakage inductance. To achieve ZVS, large leakage inductance is needed. With large leakage inductance, the duty cycle looss due to charge and discharge leakage inductance will be significant. This
(b) Losssless symmetric model
M)/2 C (L+M (c) 1st order model for fund damental design Figure 3. LC cell structure and d equivalent circuit
In order to improve power density, passive Integrated Power Electronics Module (IP PEM) technology was developed. LC cell is the bassic element in passive integration. Its structure is shown n in Figure 3. (a). The dielectric layer is covered by coppeer layers at the both side. The lossless symmetric model of this t structure is given in Figure 3. (b). C is the capacitance of o LC cell, while L is the
self-inductance of one side copper strip and M is the mutual inductance. Its equivalent circuit measured between terminal A and D is a series of inductor and capacitor, as shown in Figure 3. (c). The integratable AHB circuit with current doubler is shown in Figure 4. As shown in Figure 5, the DC blocking capacitor is implemented in primary side winding by using the hybrid winding technology. The hybrid winding is implemented using LC cell. The inductances of the current doubler are realized by the magnetizing inductances of both transformers. The leakage inductance can be tuned by the leakage layer thickness or permeability.
Figure 7. Power density comparison of AHB converters with discrete components and passive IPEM
For asymmetrical half bridge, there are several drawbacks too. One problem is that the voltage stress on the secondary rectifier is asymmetrical and related to duty cycle. In some situations, the voltage stress on the output filter diodes could reach very high, which will limit the choice of diodes. Since asymmetrical half bridge also utilizes leakage inductance to achieve soft switching, there is similar problem as discussed for phase shift full bridge, which is lost of ZVS during light load condition.
Figure 4. Equivalent circuit of integratable AHB with current doubler
A. Benefis of LLC DC/DC Converters As discussed above, PWM dc-dc converters have the problem that the duty cycle is small at nominal conditions, when a holdup time is needed. A small duty cycle leads to large circulating energy and a high turn-off switching loss, which significantly sacrifices efficiency .
prim ary hybrid w inding Leakage layer secondary planar w inding I core prim ary & secondary planar w indings E core
Figure 5. Exploded view of L–L–C–T structure.
The size of discrete passive components is 168cm3, and the passive IPEM is 82cm3, as shown in Figure 6. Power density of AHB converters with discrete components and passive IPEM are 12W/in3 and 34W/in3, respectively, shown in Figure 7. Passive IPEM
RESONANT DC/DC CONVERTERS
Resonant converters are other choices. Series Resonant Converter (SRC) , Parallel Resonant Converter (PRC)  and LCC resonant converter (LCC)  are the three most popular topologies. However, they all have some limits that they cannot achieve high efficiency at nominal conditions with hold up time requirement . LLC resonant dc-dc converter (LLC) excels in front-end dc-dc application [15-20], as depicted in Figure 8. Low switching loss and low circulating energy is achievable at nominal condition. Wide input voltage range is obtained. Zero-current-switching (ZCS) is achieved for output rectifiers and no reverse recovery appears.
Figure 8. LLC resonant converter
Figure 6. Size comparison of passive IPEM and discrete components
Figure 11. Interconnection n of LC cells
Figure 9. Gain Curves of LLC resonant cconverter
Reference  gives a design method bbased on start-up current, efficiency and holdup time reqquirements. The combination of Ln and Q impacts the effi ficiency, and this paper uses these variables to present a treend of achieving high efficiency. However, the suitable rangee that is given for Ln and Q is based on observation. Thus, thee precise optimal efficiency point cannot be obtained, andd trial and error iterations are still necessary for this design strategy. Reference  provides a design method baased on efficiency and holdup time requirements. Based on ,  and , the design procedure takes the effects off dead time and device characteristics into account forr better system efficiency.
The electromagnetic behavior of the primary winding with the core is determined by the applied interconnections. The most-frequently applied intercconnection is connecting the cells in parallel. Fig. 12 shows the interconnections of an LLCT module with 8 cells in the primary p winding and the center-tap structure in the secondary y winding.
Based on the design procedure, 1MHz 1kW 400V/48V LLC resonant converter is built as shown inn Figure 10. The power density is 76W/in3 and the efficiencyy of full power is 92.6%. Figure 12. Interconnections and winding sttructure of an integrated LLCT module
1MHz LLC Resonant Convertter Power Density 76W/in3 Efficiency 92.6% Figure 10. Design procedure of LLC resonannt converter
B. Passive integraion of LLC DC/DC Convverters For better power density, the passsive integration technique is utilized to integrate the all passsive components (Cr, Lr, Lm, and transformer) into modules (L LLCT). As shown in Figure 8, the basic part of aan LLCT module is a transformer with a magnetizing inductaance, the leakage inductance is exploited as the integrated ressonant inductance at the primary side. By replacing the conducctor winding with internal LC cells at the primary side, the reesonant capacitor can be integrated into the module. Based on the passive integration thheory, a stacked structure is chosen to integrate the LLCT m module for better power density . The interconnection bettween LC cells is illustrated in Figure 11. The capacitor (C) iis in parallel, and inductor (L+M) is in series.
dule with the stacked structure Figure 13. Exploded view of an LLCT mod
The exploded view of an LLCT module with the stacked structure is shown in Figure 13.Two o sets of C core and I core constitute the two winding wind dows. In each winding window, four internal cells are staccked vertically. The cells are connected at the terminals thro ough the interconnections on the vertical surface at the two ends of the module as shown in Figure. 13. The cell dimensions and the properties of the dielectric substrate determine the resonant capacitance. The magnetizing inductance can be adjusted by changing the thickness of the air gap. Inserting leeakage layer between the
primary and secondary windings, the leakaage inductance is also controllable. The secondary winding is made of thin copper strips. Embedded heat extractor is used forr better thermal performance , as shown in Figure 14.
Figure 14. Integrated passive module with heat extractors
1MHz 1kW 400V/48V LLCT module iis built. The size of discrete passive components of prevvious section is 65.5cm3, and the passive IPEM is 14.6cm3, respectively, as shown in Figure 15.
D. SR driving scheme of LLC DC/D DC Converters Secondary Rectifier (SR) drive technique t is employed for better efficiency. Commercially, thee drain-source voltage of secondary-side MOSFETs is used to drive SRs. However, the package inductance of an SR would w cause phase-shift of the sensed voltage, which intro oduces high body-diode conduction loss. At high switcching frequencies, this conduction loss becomes much worse. w A compensation driving scheme has been proposed to t correct the deviation in the sensed voltage and to precisely drive d the SR . To improve previous method, an nother SR driving method is based on the body diode forward voltage drop of SR. The body diode forward voltage drop of o SR is detected to tune the dead time [33-34]. This adaptiv ve tuned driving scheme for SR in LLC resonant converters is shown in Fig.17. The turn on time of SR is the same with primary main switch, but the turn-off time is adaptively tuned d based on the sensed SR Vds. When SR is turned off, the body b diode conduction is detected by a comparator (CMP), since s its forward voltage drop is much larger when compared d with the MOSFET’s on status resistive voltage drop. If body y diode conducts, the SR pulse width is increased in next sw witch cycle; if not, the SR pulse width is decreased. Thus, SR gate driving signal is adaptively tuned to eliminate the bod dy diode conduction.
Figure 15. Size comparison of discrete components and passive IPEM
C. Improvement for symmetrical windings Due to the asymmetrical structure off secondary side winding(inner layer and outer layer), the leaakage inductances are not identical. That will cause asymmetrical waveform for each half circle. High conduction loss is intrroduced.
Figure 17. Proposed digital SR drive schem me for LLC resonant converter
Figure 18. SR turn-off time tuning process
Figure 16. Improved winding structure of an integraated LLCT module
Symmetrical secondary winding structurre is proposed for better balanced leakage inductance, as shownn in Figure 16.
The main waveforms are also o reported in Figure 18, where the sensed Vds is compared with w the threshold voltage Vth. At turn-off time of SR, the co ompared result is sent to digital logic. If body diode conductts, the comparator (CMP) output is high. As a consequence, the SR pulse width is increased in order to reduce the bod dy diode conduction time. When the comparator output is low w, there is no body diode conduction and the SR is considereed to be tuned. Since it is
not possible to detect when the SR drivingg width is higher than needed, when the comparator output iss low the SR duty is decreased by D and the tuning algoorithm alternates between the conditions reported in Figure 188 c and Figure 18 d. E. Test Results of LLC DC/DC Converters An 800 kHz 1.2kW 400V-48V LLC reesonant converter prototype is built based on previous design pprocedure, shown in Figure 19. LLCT module and precise SR R driving scheme discussed above are utilized to achieve higgh power density and high efficiency.
Figure 21. Efficiency of 800kHz 1.2kW W 400V/48V LLC resonant converter
The design parameters are shown inn Table. 1. The waveforms of full load condition are show wn in Figure 20. The efficiency of prototype is given in Figurre 21.
CONCLUSION AND A TREND
In this paper, the high power density high efficiency trend of DC/DC converter is summaarized.
Table 1. The Parameters of the LLC T prototype Design Parameter/Component Param meter Value Resonant Capacitor (Cr) 333nF Leakage Inductance outside (Lr) 0..36μH Leakage Inductance inside(Lr) 0..38μH Magnetizing Inductance (Lm) 133.1μH Core Material 3F4 Turns Ratio 44:1:1 Dielectric material N N1250 Primary Side Device 2×STW W45NM50FD Secondary Side Device 2× IRF FB4115PBF
Firstly, several widely employed d PWM dc-dc converters are discussed based on efficienccy. AHB is picked to represent high efficiency PWM co onverter. To increase the power density, the discrete electromagnetic e passive components are required to be integrated into a single, planar module. The total volume and pro ofile are greatly reduced without sacrificing performance. WM dc-dc converters have However, most commercial PW the problem that the duty cycle is small at nominal conditions, when a holdup time is needed. A small duty cycle leads to large circulating eneergy and a high turn-off switching loss, which significantly sacrifices s efficiency. LLC excels for its low switching g loss and low circulating energy at nominal condition, when a holdup time is needed. Wide input voltage range is obtained d. Zero-current-switching (ZCS) is achieved for output reectifiers and no reverse recovery appears. High switching frequency can reduce passive volume effectively. Passive integration tech hnology can push power density higher. It applied to integ grate Cr, Lr, Lm, and the transformer into one module (LL LCT). The SR driving method is utilized, which is based on the body diode forward voltage drop of SR.
Figure 19. 800kHz 1.2kW 400V/48V LLC resonant cconverter with LLCT module
One 1.2 kW, 800 kHz, 400 V/ V 48 V LLC resonant converter prototype is built based d on design procedure, LLCT, and SR drive. The power density of converter is 177W/inch3. The efficiency can reacch as high as 96.4%. The conceptual trend of hig gh power density high efficiency DC/DC converter is depiccted in Figure 22. ACKNOWLEAG GEMENT This work made use of Engin neering Research Center Shared Facilities supported by y the CPES Industry Partnership Program.
Figure 20. Waveforms of LLC resonant converrter at full load
The power density of converter is 1177W/inch3. The efficiency can reach as high as 96.4%.
Improved SR 1MHz LLC Diode Discrete 76W/in3, 92.6%
200kHz AHB Diode Integrated 34W/in 3 , 91.5%
1MHz LLC SR Discrete 96W/in3 , 95.8%
800kHz LLC Passive SR Integrated Integration 177W/in3 , 96.4%
200kHz LLC Diode Discrete 28W/in3 , 95.5%
200kHz AHB Diode Discrete 13W/in3 , 92.2%
Figure 22. The develop trend of high power density high efficency DC/DC converter
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