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An Experimental Comparison of Galvanically Isolated DC-DC Converters: Isolation Technology and Integration Approach

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An Experimental Comparison of Galvanically Isolated DC-DC Converters: Isolation Technology and Integration Approach electronics Article An Experimental Comparison of Galvanically Isolated DC-DC Converters: Isolation Technology and Integration Approach Egidio Ragonese 1,* , Nunzio Spina 2, Alessandro Parisi 2 and Giuseppe Palmisano 1 1 Dipartimento di Ingegneria Elettrica Elettronica e Informatica (DIEEI), University of Catania, 95125 Catania, Italy; [email protected] 2 STMicroelectronics, 95121 Catania, Italy; [email protected] (N.S.); [email protected] (A.P.) * Correspondence: [email protected]; Tel.: +39-095-738-2331 Abstract: This paper reviews state-of-the-art approaches for galvanically isolated DC-DC converters based on radio frequency (RF) micro-transformer coupling. Isolation technology, integration level and fabrication issues are analyzed to highlight the pros and cons of fully integrated (i.e., two chips) and multichip systems-in-package (SiP) implementations. Specifically, two different basic isolation technologies are compared, which exploit thick-oxide integrated and polyimide standalone transformers, respectively. To this aim, previously available results achieved on a fully integrated isolation technology (i.e., thick-oxide integrated transformer) are compared with the experimental performance of a DC-DC converter for 20-V gate driver applications, specifically designed and implemented by exploiting a stand-alone polyimide transformer. The comparison highlights that similar performance in terms of power efficiency can be achieved at lower output power levels (i.e., about 200 mW), while the fully integrated approach is more effective at higher power levels Citation: Ragonese, E.; Spina, N.; with a better power density. On the other hand, the stand-alone polyimide transformer approach Parisi, A.; Palmisano, G. An allows higher technology flexibility for the active circuitry while being less expensive and suitable Experimental Comparison of for reinforced isolation. Galvanically Isolated DC-DC Converters: Isolation Technology and Keywords: basic isolation; electromagnetic coupling; gate drivers; integrated circuits; polyimide; Integration Approach. Electronics rectifiers; reinforced isolation; Schottky diode; silicon dioxide; system in package; transformers 2021, 10, 1186. https://doi.org/ 10.3390/electronics10101186 Academic Editor: Raed 1. Introduction A. Abd-Alhameed Safety rules for equipment operated by human beings, along with severe reliability requirements for electronics in harsh environments, call for galvanic isolation in several Received: 21 April 2021 application fields, even at very low power levels, as summarized in Figure1a [ 1]. A Accepted: 14 May 2021 Published: 15 May 2021 typical galvanically isolated system is shown in Figure1b [ 1]. It consists of two isolated domains (i.e., with different isolated ground references) that exchange data signals across Publisher’s Note: MDPI stays neutral the galvanic barrier. However, full isolation is provided only if the power supply of Domain with regard to jurisdictional claims in 2, VDD2, is provided from the power supply of Domain 1, VDD1, by means of a galvanically published maps and institutional affil- isolated DC-DC converter. Usually, power levels from a few tens of mW to about 1 W iations. can be transferred across the galvanic barrier by means of magnetic coupling. To this aim, micro-transformer devices are currently used, which exploit a dielectric layer between two stacked spiral windings to withstand an isolation voltage of several kilovolts [2,3]. Typically, by using one galvanic barrier (i.e., one transformer), the basic isolation level can be achieved (i.e., about 5–6 kV). Reinforced isolation, which is the highest level of isolation, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. requires the use of two galvanic barriers connected in series (i.e., double isolation) in order This article is an open access article to comply with the 10-kV surge test of the VDE 0884-11 standard [4]. Double isolation distributed under the terms and for a power transfer has been demonstrated by using an LC hybrid approach (i.e., an conditions of the Creative Commons isolation capacitor and an isolation transformer in a resonant mode operation), which Attribution (CC BY) license (https:// boosts efficiency with respect to a traditional two-series-connected scheme [5]. However, creativecommons.org/licenses/by/ less costly solutions for reinforced isolation rely on one galvanic barrier by means of an 4.0/). isolation transformer with increased thickness for the dielectric layer [6,7]. Electronics 2021, 10, 1186. https://doi.org/10.3390/electronics10101186 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 1186 2 of 12 However, less costly solutions for reinforced isolation rely on one galvanic barrier by Electronics 2021, 10, 1186 means of an isolation transformer with increased thickness for the dielectric layer [6,7]. 2 of 12 This paper reviews commonly used transformer-based technologies for galvanically isolated DC-DC conversion, which are based on fully integrated thick-oxide transformers [8,9] and post-processed stand-alone polyamide transformers [7,10,11], respectively. To thisHowev aim, isolationer, less technologies costly solutions are compared for by reinforced means of previously isolation available rely reonsults one and galvanic barrier by newmeans experimental of an isolation data of a DC transformer-DC converter with specifically increased designed thickness and implemented. for the Thedielectric layer [6,7]. comparison is carried out at a similar isolation rating (i.e., 5 kV) in terms of electrical per- formanceThis (i.e., paper isolated reviews output power commonly and efficiency), used transformer integration level,-based design technologies flexibility for galvanically andisolated fabrication DC costs-DC in conversion, order to highlight which the prosare basedand cons on of fullythe two integrated approaches. thick -oxide transformers [8,9The] and paper post is organized-processed as follows. stand -Sectionalone 2polyamide describes the transformerssystem architecture [7,10,11] and , respectively. To main circuit topologies for galvanically isolated DC-DC converters, while the characteris- ticsthis of theaim, two isolation isolation technologies under are comparison compared are by detailed means in of Section previously 3. Section available results and 4 newpresents experimental the design and data experimental of a DC performance-DC converter of a 5 -specificallykV 200-mW DC designed-DC converter and implemented. The Electronics 2021, 10, 1186 forcomparison gate drivers, alongis carried with a out wide at comparison a similar between isolation the ratingintegration (i.e., technologies 5 kV) in termsfor2 of 12 of electrical per- powerformance transfer (i.e., applications. isolated output power and efficiency), integration level, design flexibility and fabrication costs in order to highlight the pros and cons of the two approaches. The paper is organized as follows. Section 2 describes the system architecture and main circuit topologies for galvanically isolated DC-DC converters, while the characteris- tics of the two isolation technologies under comparison are detailed in Section 3. Section 4 presents the design and experimental performance of a 5-kV 200-mW DC-DC converter for gate drivers, along with a wide comparison between the integration technologies for power transfer applications. (a) (b) FigureFigure 1. 1. (a)( aGalvanic) Galvanic isolation isolation application application fields; fields; (b) simplified (b) simplified block diagram block diagram of a galvanically of a galvanically isolatedisolated system. system. Reproduced Reproduced from from [1]. Copyright [1]. Copyright 2019, Springer 2019, Springer Nature NatureCustomer Customer Service Centre Service Cen- GmbH. tre GmbH. 2. GalvanicallyThis paper Isolated reviews DC commonly-DC Conversion used transformer-based: System and Circuit technologies Description for galvanically isolatedA galvanically DC-DC conversion, isolated DC which-DC converter are based requires on fully a transformer integrated-based thick-oxide power link transform- to exploiters [8, 9the] and magnetic post-processed coupling between stand-alone two polyamideisolated windings transformers, according [7,10 to,11 the], respectively.general schemeTo this in aim, Figure isolation 2. When technologies isolated power are compared regulation by is means required, of previously a further isolated available data results linkand (not new represented experimental in Figure data of 2) a must DC-DC be converterincluded for specifically the feedback designed control, and which implemented. typi- callyThe comparisonuses a pulse- iswidth carried modula out attion a similar(PMW) isolationapproach rating [9]. In (i.e., common 5 kV) implementations in terms of electrical (seeperformance Figure 2), (i.e.,the DC isolated-AC conversion output power is performed and efficiency), by a radio integration frequency level, (RF) power design oscil- flexibility (a) (b) lator,and fabricationwhile a diode costs rectifier in order is used to highlight for the AC the-DC pros conversion and cons of the twoisolated approaches. voltage at the secondaryThe paper winding. is organized The isolation as follows. transformer Section can2 describes be also used the systemfor the voltage architecture step- and up,Figure as required 1. (a) byGalvanic gate driver isolation applications, application typically fields; operated (b) simplifiedat about 20 V.block diagram of a galvanically mainisolated circuit system.
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