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Helsinki University of Technology Applied Electronics Laboratory Series E: Electronic Publications E 3 Teknillisen korkeakoulun sovelletun elektroniikan laboratorio, Sarja E: Elektronisia julkaisuja E3 Espoo 2002

DC/DC Converter technology for distributed telecom and microprocessor power systems – a literature review Mika Sippola and Raimo Sepponen

TEKNILLINEN KORKEAKOULU TEKNISKA HÖGSKOLAN HELSINKI UNIVERSITY OF TECHNOLOGY TECHNISCHE UNIVERSITÄT HELSINKI UNIVERSITE DE TECHNOLOGIE D´HELSINKI

Helsinki University of Technology Applied Electronics Laboratory Series E: Electronic Publications E3 Teknillisen korkeakoulun sovelletun elektroniikan laboratorio, Sarja E: Elektronisia julkaisuja E3 Espoo 2002

DC/DC Converter technology for distributed telecom and microprocessor power systems – a literature review Mika Sippola and Raimo Sepponen

Helsinki University of Technology Department of Electrical and Communications Engineering Applied Electronics Laboratory Teknillinen korkeakoulu Sähkö- ja tietoliikennetekniikan osasto Sovelletun elektroniikan laboratorio

Distribution: Helsinki University of Technology Applied Electronics Laboratory P.O.Box 3000 FIN-02015 Hut, Finland

ISBN 951-22-6703-9 ISSN 1459-1111 Espoo, 2002

DC/DC Converter technology for distributed telecom and microprocessor power systems – a literature review M. Sippola and R. Sepponen Applied Electronics laboratory Helsinki University of Technology PL 3000, 02015 TKK, Finland Email: [email protected] Abstract – A literature review on DC/DC converter technology for microprocessor power delivery and telecom on-board DC/DC converters was carried out with the focus on general application requirements, topologies, packaging and magnetic components. 251 articles from IEEE database were taken into account from 1990 to 2000. Some commercial information was considered as well. In this study different packaging technologies resulted DC/DC converters with power density ranging from 10W/in3 to 130W/in3. I INTRODUCTION In this paper high power density DC/DC converters [1] – [15] and technologies for such distributed telecom and microprocessor power systems are reviewed with the focus on topologies, components, packaging and power density. The review is concluded with a comparison of DC/DC converter power densities reported in literature and found in some commercial products. The electronics development is led by the integrated circuit technology shifting from 250nm – 180nm (1999) feature sizes toward 100nm (2003). The power delivery of the resulting new ICs will be a major issue [16]. For example the microprocessor supply current is changing from 0.3 – 13A with 1A/ns slew rate to 1 – 50A with 5A/ns slew rate as the supply voltage shifts from 2.1 – 3.5V to 1 – 3V. At the same time the supply voltage tolerance tightens from 5% to 2% [17]. In order to meet such specifications distributed power systems with local voltage step down and regulation are needed. On the other hand in telecom systems distributed DC/DC converters are frequently employed for accurate local voltage regulation, hot swap ability, to make system modifications easy and for redundancy [18]. Among the high efficiency the other important requirements are high reliability, shock and vibration resistance, safety, EMC including ESD [19] and profile typically lower than 0.5inch or 12.6mm. II TOPOLOGIES Buck For lower input voltages (12V) non-isolated buck converters can be used while for higher input voltages (48V) transformer isolated converters are more viable [20]. The evolution of voltage regulator module (VRM) technology has gone from Buck, Buck with synchronous rectifier, Quasi Square wave buck to Interleaved Quasi Square wave buck. At 15 – 20A output currents a single Buck converter is adequate. For higher currents multiple Bucks are paralleled in order to reduce output filter capacitance requirement. For example 3 Bucks are used for 60A with efficiency of 80% [21]. Further in [22] and [23] the inductors of interleaved Bucks are integrated into same core in order to reduce flux ripple at center leg. Often the efficiency must be considered at light loads as well and different methods such as Discontinuous Conduction Mode (DCM) operation, synchronous rectifier shut down etc. have been developed for Interleaved Quasi Square wave bucks [24]. Isolated converters Transformer isolated converters are used with higher input voltages (48V) and large voltage step down ratios in order to maintain acceptable efficiency and also to provide galvanic isolation for telecom systems. Converter topology selection of: < 10W flyback, 10W – 50W Flyback-forward, 50W – 100W forward, 100W – 200W Forward, Push-pull or Halfbridge, >200W Half or Full-bridge is proposed in [25].

Sippola; Sepponen: DC/DC Converter technology for distributed telecom and microprocessor power systems

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Flyback is very simple but has high conduction losses and requires a large output filter. Forward converters: forward with reset winding, forward with resonant reset (FRR) and forward with active clamp (FAC) are most suitable for telecom applications [19]. Forward with resonant reset (FRR) or ZVS-MRC may be preferred for fewest components and simple transformer [25]. For these converters efficiencies of 86% and 83% are reported [18]. However these converters have waveforms not suitable for self-driven synchronous rectification and Forward with Active clamp (FAC) may be preferred instead. With 5V / 10A output the active clamp ZVS forward with litz winding achieves 92% efficiency [26]. Active clamp forward with synchronous rectifiers has high efficiency but only with slow output current slew rates. For higher output current slew rates half-bridge can be used [27] with 93% efficiency with 3.3V and 20A output [28]. Push-pull may have efficiency reduced by transformer leakage inductance caused voltage spikes. Asymmetrical half-bridge with current doubler secondary and Flyback Forward are fourth order systems and may have slow transient response. Push-pull Forward converter has low ripple input current and fast response and high efficiency. For example 86% efficiency is achieved with 48V input and 1.2V/60A output [29]. Synchronous rectifiers (SR) are used for rectifier conduction loss reduction at high current / low voltage applications. SR gate drive can be taken directly from transformer secondary or auxiliary winding (Self driven) or it may be provided by a specific IC (external drive). Self drive is suitable for some topologies only (for example Forward with active clamp FAC or Half bridge) while external drive adds cost and complexity. 3.3V schottky rectified forward converter has 85% efficiency while synchronous rectifiers improve efficiency to 89%. III COMPONENTS Switches Switching component development aims for loss minimization by optimizing on state resistance (Rds) and device charges (for example Qgate and Qdrain ) [30]. For example, fully depleted LDD MOSFET for multimegahertz VLSIBiCMOS compatible VRM [31] and UMOS and VMOS structures instead of lateral MOSFET with loss factor of 11.66 mO?nF instead of 192 mO?nF [32] have been developed. GaAs devices have an order of magnitude lower Rds · Qg product (loss factor) allowing 1 – 10MHz switching frequencies to be used. In [33] an 10MHz PWM hard switched 5W, 3.3V to 7V boost converter with power density of 500W/in3 is presented. Silicon-on-insulator (SOI) and silicon carbide (SiC) are most likely to find their first applications in higher voltage power electronics [34],[35]. Capacitors Main considerations with capacitors in low voltage / high current DC/DC converters are the volumetric efficiency (F/in3) and the minimization of effective series resistance ESR. Tantalum capacitors are provided with multiple terminals or conductive polymer cathodes [36] to reduce ESR. Alternative low ESR candidates are polymeric electrolyte and multilayer polymer capacitors [37]. Transformers Transformers and inductors are usually regarded as the bulkiest components in the switched mode power supplies. Resistors, capacitors and power semiconductors are available in SMD packages while the inductive components are often encapsulated wire-wound devices. Wire wound transformers are cost competitive but high winding losses, ferrite temperature limits and leakage inductance may become a problem. For example a toroidal transformer has low cost core with high Al-value and requires fewer turns resulting lower DC-resistance but it was replaced by higher cost planar transformer in order to increase power density [38]. Disc-type and tube-type transformer geometries are suitable for low profile applications. It has been shown that tubetype has higher power density at very low profiles. When power density is not considered the core and copper losses are usually designed to be equal. However the power density optimal disc-type design has 60%-80% copper losses with 40%-20% core losses while optimal tube-type design has 70%-50% core losses and 30%-50% copper losses [39]. Because the material development is obviously reducing the ferrite losses faster than the resistivity of copper the tubetype transformer will have even bigger advantage in the future. Single transformer may also be divided into multiple smaller devices to obtain lower profile and more effective cooling [40]. For high current / low voltage applications planar magnetics have also gained popularity. When foil conductors with optimum thickness [41] are used and primary and secondary layers are interleaved the leakage inductance, skin effects, proximity effects and intrawinding capacitance are reduced but the interwinding capacitance is increased [42].


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Planar transformers are manufactured by using: solid or litz wire on flat core, z-folded or stacked FLEX circuits, stamped and stacked metal plates, multilayer PCB [38], thin film or thick film [43] or a combination of previous methods [44]. The particular benefits of PCB integrated magnetics are: low profile, repeatability of parasitics, low proximity and skin effects, low leakage inductance and low manufacturing costs [45]. Stamped copper foil with insulator has low material costs but has adjustment problems, varying parasitics and large thermal resistance. Z-folded winding needs not primary vias but has varying parasitics. Multilayer PCB has consistent parasitics and is easy to assembly but has high cost [46]. In [47] the efficiency of different versions were 85% with PCB integrated, 87% with FLEX and 80% with thick film windings. When integrating the planar windings on PCB the number of required layers must be minimized to reduce the cost of PCB. 4 layers are used in [48] while [45] considers 8 layers with E14-3F3 core and [27] has 12 layers. Converter topology selection may be dictated by requirement for low number of turns helping to aim low layer count [27]. In high power density DC/DC converters FR4 PCBs are replaced by ceramic and IMS substrates and planar integration is not available [49]. In such applications optimized discrete planar transformers can be used. For example an 1.0kW DC/DC converter with efficiency of 95% and power density of 69.4W/in3 using z-folded transformer with 99.5% efficiency [50] and 70% copper fill factor [51] has been reported. Another z-folded transformer and inductor manufactured using CNC-cut copper foil and kapton tape [52] results efficiency of 89% for 50W forward with active clamp. In [53] the optimum efficiency vs. foot print for this transformer is suggested to be around 98.5%. In high current, single secondary turn transformers the interconnection structure causes significant proportion of leakage inductance and ac-resistance thus the terminations must be considered as an integral part of transformer design [54]. The losses at termination structure may be 25% - 33% of total losses at DC while at higher frequencies these may be 65% - 80%. Using more interconnection pads or pins doesn’t reduce AC resistance if these are not correctly interleaved [55]. Inductors For higher current levels litz wire, PCB or foil wound planar and barrel wound inductors are used. Main concern in high frequency inductor design is the high magnetic field at windings and fringing fluxes from air gaps. For high efficiency the conductors should be kept away from the air gap at distance of 3 times the gap [56], distributed air gap can be used [57] or gap may be filled with magnetic material [58]. Different manufacturing methods aimed for miniaturization and integration of inductors such as thin and thick film and copper spraying, micro machining [59],[60] and sputtering [61] have been studied. However these new methods are suitable only for lower power levels (< 10W) [62] at the moment. Ferrites For higher power densities the minimization of core losses by design optimization and material development is important. Ferrite loss mechanisms are divided into hysteresis, eddy current and dielectric losses. Eddy current losses can be reduced by increasing the grain resistance and by reducing the grain boundary capacitance [63]. NiZn ferrites have higher resistivity than MnZn yielding low eddy current losses. However their permeability is lower. Hysteresis core losses are usually modeled using Steinmetz equation, which doesn’t take into account the DC-component or nonsymmetrical magnetization [64] or pre magnetization [65]. It is recommended that ferrites are also characterized by loss measurements [66]. Examples of research on ferrite material improvement are: combination of single and polycrystalline MnZn ferrites [67], doping with Sn, Ti, Ta for 60% lower losses [68] and using fine grain (ferrite particle size 0.13um) for 2.2 times loss reduction [69]. Examples of new ferrite manufacturing methods are: magnetic powder (95 mass%) epoxies [70], ferrite coatings in liquids with assistance of ultrasound [71] and screen printed polymer/ferrite composites [72]. IV PACKAGING Electronics packaging deals with substrates and interconnections. In power electronics packaging high power loss densities must be taken into account. For example the electro-thermal performance and reliability of Direct Bonded Copper DBC, Insulated Metal Substrates IMS [73], Al/SiC and ceramic [74], CVD diamond [75], FR4 with thermal vias [76] and thick film [77] substrates have been considered. Examples of advanced methods for power module interconnections are High Density Interconnection HDI [78], Copper base plate with multiple Kapton layers [79], flipchip on FLEX and Direct Bonded Copper substrate with metal post interconnections [80]. Power semiconductors are also currently emerging in BGA [81] and flip chip packages [82].


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It has been said that the power electronics packaging has not kept in the pace with IC packaging, but that the packaging is the central driver for future power electronics [83]. In order to design and manufacture high power density / high efficiency converters for distributed power systems an integrated electro-physical approach for power electronics design is needed [84]. Traditionally 2D technologies are used in electronics design while energy storage aspect of power electronics has 3 rd dimension as well [85]. Power Sources Manufacturers Association PSMA is developing a road map for power electronics packaging. This will identify user requirements and address the bottlenecks to be solved [86]. Military and avionics have followed the commercial trends toward lower supply voltages but have had tighter packaging requirements already as a must and high power density converters have been developed for these applications. For example the High Density Interconnected (HDI) – technology based 48V to 3.3V converter with 80% silicon density and 150mils profile magnetic components achieves power density of 130W/in3 [15]. For comparison the standard FR4 PCB technology based telecom supplies have power densities of 3.7W/in3 [48], 9W/in3 [45] or 36W/in3 [76]. V SUMMARY AND DISCUSSION The power density versus output power of DC/DC converters in literature (1988 to 2001) are plotted in Figure 1. For comparison the commercial isolated 48V to 3.3V / 1.5V DC/DC converters (Appendix 1) are shown in Figure 2. Even thought the commercial converters usually have some additional features such as EMI filters etc. it can be concluded that the power density of commercial converters increases only with increasing output power while the researchers have been able to produce much wider variety of converter power densities. This suggests that commercial converter sizes tend to be fixed maybe into some industry standard (2inch3). On the other hand the effect of PCB, auxiliary circuitry, connectors, mechanics and other “overhead” components on power density is reduced as the output power increases. For higher power densities over the whole power range no waste space can be allowed in the first place and more advanced packaging schemes must be used. The best power density versus power – ratios have been obtained in [9], [8] and [15]. For example by distributing 20 [8]s around and under the microprocessor an 100W redundant VRM could be implemented into a cubic inch or less. Research DC/DC Converters

Commercial DC/DC Converters

140

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100 80

Vout = 1.5V

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Vout = 1.5V Vout = 3.3V

60 40 20 0

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Figure 1: DC/DC converter [1] – [15]

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Figure 2: Commercial DC/DC 48V to 3.3V/1.5V

It can also be concluded that the performance improvement in efficiency and power density can be achieved only by the improvements in semiconductors, magnetic components, capacitors and packaging. The topologies can not be easily judged by themselves rather the whole design, implementation and finally cost versus performance should be considered. When converter specifications are fixed the theoretical power density limit might be derived from semiconductor, ferrite and copper loss properties with cooling constraints for different topologies. The power densities of commercially available DC/DC converters depend on the industrial mass production infrastructure as well.


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APPENDIX 1: COMMERCIAL DC/DC CONVERTERS manufacturer AAK Corp Abbott Electronics Absopulse Acon inc. Artesyn Technologies Celestica Ericsson Components Lambda Electronics Lambda Novatronics Power-one Power-one Vicor SynQor SynQor Lucent Technologies Lucent Technologies

series G series NB series MIP110 E series BXI110 HHE45-033 PKN 4510PI RM100-48-3,3 MIL-28-100 HLS40ZA HLS30ZE V48B3V3C150A PQ48033HTA50 PQ48015HTA60 JAHC/JAHW-s JAHC/JAHW-s

APPENDIX 1

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Helsinki University of Technology Applied Electronics Laboratory Series E: Electronic Publications Electronical version of the report is available at http://www.hut.fi/Yksikot/Elektroniikka/reports

ISBN 951-22-6703-9 ISSN 1459-1111