IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 3, MAY 2001 311 A Low-Profile Low-Power Converter with Coreless PCB Isolation Transformer S. C. Tang, Member, IEEE, S. Y. (Ron) Hui, Senior Member, IEEE, and Henry Shu-hung Chung, Member, IEEE Abstract—Very small manually wound transformers for sub- double-sided printed circuit board (PCB), the manual winding watt dc–dc converters are notorious for their relatively high cost cost is eliminated and automation manufacturing process is and low reliability. In this paper, an isolated low-profile low-power facilitated. Because the transformer windings are etched on 8 MHz soft-switching power converter using a coreless printed cir- cuit board (PCB) transformer is described. Coreless PCB trans- the PCB surfaces, packaging processes and materials, such as formers eliminate several problems of their core-based counter- resin epoxy, can be excluded. Apart from the device package, parts in low-power applications. The diameter of the coreless PCB magnetic core is eliminated so the height of device can be transformer is merely 0.46 cm. The converter’s power output is reduced substantially –. about 0.5W with a typical transformer efficiency of 63%. The high- One major challenge of designing low-profile power con- frequency capability, high reliability and the low-profile structure make coreless PCB transformers a viable and attractive option for verters with coreless transformers is the high-frequency oper- reliable mega-hertz switching converters and micro-circuits. ation of the coreless PCB transformers. The power converter Index Terms—Coreless PCB transformers, low-power con- on the primary side of the transformer has to be switched at verters, low-profile converters, megahertz switching, packaging, several Mega-Hertz. In order to ensure that the overall power passive components. consumption of the converter is competitive when compared with existing core-based transformer isolated circuits, it is nec- essary to minimize not just the transformer loss, but also the I. INTRODUCTION switching loss of the converter electronics . In this paper, a low-profile and low-power power converter that incorporates L OW-PROFILE low-power power converters for portable electronic applications are in high demand especially for laptop, notebook, and palmtop computers. Many commercial the soft-switching technique and the coreless PCB transformer is presented. It will be demonstrated that the coreless PCB trans- power supply products such as small outline package (SOP) formers offer a viable solution to megahertz switching power power semiconductor chips  and low-profile transformer converters. have been manufactured for various PC Cards specifications. For low-power (sub-watt) power converters with isolated II. OPTIMAL OPERATION OF CORELESS PCB TRANSFORMERS voltage output, two major practical problems arise. First, the cost of the core-based manually wound transformer may The dimensions of the coreless PCB transformer (named Tr8), contribute to about 80% of the total manufacturing cost of the which provides isolation in the dc–dc converter, are shown in converter. This is mainly due to the labor and material costs Fig. 1(a). The diameter of the coreless PCB transformer is 4.6 involved in the manufacturing process of transformer. Second, mm. Both of the primary and secondary have 10-turns. The con- the wire is usually very thin and can be damaged easily in ductor width is 0.1 mm and separation between conducting track the winding process. The very thin transformer winding, that is 0.2 mm. The thickness of the printed circuit board laminate is carries the weight of the magnetic core, can easily be broken 0.4 mm. The copper track thickness is 35 m. Two spiral wind- when the transformer is subject to shock conditions (that often ings are printed “directly” on both side of a double-sided PCB. occur in the transportation process). Thus, reliability of very The PCB laminate is made of FR4 material that has high break- small manually wound transformer is not high. down voltage (15 kV to 40 kV) . The area of the coreless PCB Research into the development of micro-transformers has transformer, Tr8, is 0.6648 cm . Fig. 1(b) shows the photograph been documented –. Ferrite cores or materials are usually of a typical core-based pulse transformer, an 8-pin SOIC package employed in these transformers to provide a closed magnetic integrated circuit and the actual coreless PCB transformer, Tr8, path. However, the current rating and operating frequency are used in the power converter. The size of the transformer is smaller limited by the ferrite material because of magnetic saturation than the surface mount electronics used in the primary circuit. and Eddy current loss. The use of planar transformers without The transformer circuit model derived from  is shown in Fig. 2. any ferrite cores have been studied in –. By printing The winding resistances depend on operating frequency due to the windings of a planar transformer on both sides of a skin effect. The primary and secondary winding resistance are measured which are given by Manuscript received November 29, 1999; revised January 31, 2001. This work was supported by the Hong Kong Research Grant Council (CERG 9040446). Recommended by Associate Editor K. Ngo. (1) The authors are with the Department of Electronic Engineering, City Uni- versity of Hong Kong, Kowloon, Hong Kong, China (e-mail: eeronhui@cityu. edu.hk). Publisher Item Identifier S 0885-8993(01)04041-8. (2) 0885–8993/01$10.00 © 2001 IEEE 312 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 3, MAY 2001 included in the transformer model as part of the secondary leakage inductance. The stray inductance of the secondary winding is about 30 nH. The differences between the mea- sured (at 10 MHz) and calculated values are mainly due to the high frequency effects such as proximity effects. Because the intra-winding capacitance, and , are much smaller than the external capacitor, their effects are neglected to simplify (a) the following analysis. The inter-winding capacitance, , is measured and calculated. The calculated and measured capac- itances, at 10 MHz, are 2.106 pF and 2.08 pF, respectively. is negligible and now represents the external capacitance in the calculation. Based on this equivalent circuit and the theory described in –, several characteristics of this transformer can be ob- tained. It has been pointed out that by connecting an external ca- pacitor across the secondary winding, the no-load resonant fre- quency can be determined. This feature is an important factor in designing the power converter for a specific switching fre- quency range. The equation of the no-load resonant frequency is given as (3) (b) Fig. 1. (a) Dimensions of the coreless PCB transformer (Tr8) used converter. where and . (Here (b) A photograph of a typical core-based pulse transformer, a 8-pin SOIC package surface mount IC and the coreless PCB transformer (Tr8). is the load capacitance.) Because the transformer only consists of a few turns, it is al- most a short circuit at low operating frequency. It is necessary to operate the transformer at high frequency in order to mini- mize the magnetizing current of the transformer. Based on the transformer model in Fig. 2, the input impedance and phase plot of the transformer can be determined. A 1.2 nF resonant capac- itor is chosen so that the maximum input impedance is set at about 8.2 MHz. Fig. 3(a) shows the calculated and measured magnitude of the transformer input impedance, when the transformer is loaded with a 50 resistor. The phase plot of the input impedance is shown in Fig. 3(b). As frequency increases Fig. 2. Equivalent circuit of a coreless PCB transformer with an external resonant capacitor. from dc, the phase of input impedance increases and approaches 90 degree within the frequency from about 1 MHz to 8.5 MHz. In this frequency range, the input terminal of the transformer TABLE I is highly inductive. This phenomenon is desirable in a low-loss MEASURED AND CALCULATED INDUCTANCES OF THE TRANSFORMER EQUIVALET CIRCUIT zero-voltage-switching (ZVS) power converter . The voltage gain of the transformer is defined as the ratio be- tween the secondary voltage and the primary voltage, . The calculated and measured voltage gain of the transformer with 1.2 nF resonant capacitor and 50 load are plotted in Fig. 4. As previously mentioned, the transformer’s voltage gain can be boosted by an external capacitor . The high voltage gain region (greater than unity) is between 7 MHz and 11 MHz. III. OPERATION OF THE SOFT-SWITCHED CONVERTER The leakage and mutual inductances are measured and sim- As a power transformer, the operating frequency should ulated. The results are given in Table I. The inductances are be chosen so that the maximum efficiency can be achieved measured by an HP4194A impedance/gain-phase analyzer. The after considering the switching loss factors for the power lead wire stray inductance of the primary winding can be com- electronics. Fig. 5 shows the calculated and measured energy pensated with the aid of the impedance analyzer. However, efficiency of the transformer versus operating frequency when the lead wire stray inductance between the secondary winding the primary winding is fed with a sinusoidal voltage and the and the external capacitor cannot be compensated. It has to be secondary winding is loaded with a resistor of 50 . The TANG et al.: LOW-PROFILE LOW-POWER CONVERTER 313 optimal operating frequency of the transformer, Tr8, with 1.2 nF resonant capacitor, is about 8 MHz. An transformer efficiency of about 76% is recorded when the transformer is fed with a sinusoidal voltage. At such frequency, soft switching can also be achieved for the power switches because the transformer load is still sufficiently inductive. Therefore, the operating frequency of the coreless PCB transformer is chosen to be 8 MHz. Soft-switching technique is used to reduce the switching loss in the converter circuit. The proposed transformer isolated converter circuit, shown in Fig. 6, is a low-loss half-bridge converter that uses zero- voltage-switching (ZVS) technique. The high side switch, , is (a) a P-channel MOSFET instead of an N-channel MOSFET. This configuration eliminates a voltage level shifter that may com- plicate the converter circuit and dissipate more energy . The timing sequences of the switches, and , are shown in Fig. 7. Before the time, , is in the on-state and is in the off- state. The drain current of increases linearly from zero. At , is turned off. Capacitor is charged gradually and it diverts the transformer primary current from the drain of . There- fore, the product of is small and the turn-off loss of the is minimized. From to , is charged and is discharged through the inductive primary winding of the trans- former. The input impedance of the transformer with resonant capacitor, , has to be inductive at the operating frequency and the primary current must be high enough to remove the charges (b) of before is turned on. At , the voltage across is Fig. 3. (a) Magnitude of input impedance of the transformer (Tr8) with discharged to zero. The transformer primary current forces the 1.2 nF//50 load. (b) Phase of input impedance of the transformer (Tr8) diode, , to turn on. Drain-to-source voltage of is, there- with 1.2 nF//50 load. fore, zero (neglecting the forward voltage drop of ) between to . is turned on at to achieve zero-voltage-switching of . At , is turned off. Similar to , is used to minimize the turn-off loss of . As is charged to 12 V, the drain-to-source voltage of is zero and is forced to turn on at by the primary current of the transformer. Again, the is turned on under zero-voltage condition at to com- plete one switching cycle. IV. EXPERIMENTAL VERIFICATION The converter has been tested with a load of 50 . The input voltage is 12 V. The secondary output voltage is regulated to 5 V. The switching frequency of the converter is set at 8 MHz. Fig. 4. Voltage gain jV =V j against frequency of the transformer (Tr8) with The output power is 0.5 W. A linear low dropout voltage reg- 1.2 nF//50 load. ulator LM2937IMP-5.0 is used to regulate the output voltage. Fig. 8 shows the measured drain current ( : 0.2 A/div.) and the drain-source voltage of one of the power switches. It can be seen that soft switching has been achieved. The voltages and currents of the primary and secondary windings of the coreless trans- former were measured and are displayed in Fig. 9. It can be seen that the primary current, secondary current and voltage do not consist of fast transients. At 8 MHz operation, the transformer’s stray inductance provides the filtering effect and smoothes out all sharp edges in the waveforms. When fed with a nonsinu- soidal primary voltage under these test conditions, a transformer efficiency of 63% has been achieved. This efficiency value is less that recorded when the transformer is fed with sinusoidal Fig. 5. Energy efficiency of Tr8 when load with 1.2 nF//50 load. voltage (76%). The reason is due to the harmonic losses. The 314 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 3, MAY 2001 Fig. 6. Schematic of the isolated low-profile converter with the coreless PCB transformer. Fig. 9. Measured primary voltage (V ), primary current (I ), secondary voltage (V ) and secondary current (I ) of the coreless PCB transformer from the circuit in Fig. 6. Fig. 7. Timing diagram of the soft-switched converter. Fig. 10. Measured output voltage (V : 2.5 V/div) and output current (I : Fig. 8. Measured drain current I and drain-source voltage V . 50 mA/div). output voltage and current of the dc–dc converter are regulated. The measured efficiency of the overall converter at shown in Fig. 10. It can be seen that the output voltage is well 0.5 W output is about 34%. TANG et al.: LOW-PROFILE LOW-POWER CONVERTER 315 V. CONCLUSIONS  W. A. Tabixz and F. C. Lee, “Zero-voltage switching multi-resonant technique—A novel approach to improve performance of high-frequency A low-profile, low-power power converter with soft quasiresonant converters,” in Proc. IEEE PESC’88, 1988, pp. 9–17. switching and coreless PCB isolation transformer has been  D. Carter and R. A. McMahon, “Electronic level shifter for use in half- bridges operating at 13.56 MHz,” Electron. Lett., vol. 31, no. 16, pp. demonstrated successfully. The half-bridge converter uses zero- 1301–1302, 1995. voltage-switching (ZVS) technique to minimize the switching loss. The transformer has diameter of 4.6 mm and thickness of 0.4 mm, which is smaller than an 8-pin surface mount SOIC S. C. Tang (M’98) was born in Hong Kong in 1972. package integrated circuit. The choice of optimal frequency He received the B.Eng. (with first class honors) and range of the coreless PCB transformer for ZVS converter has the Ph.D. degrees in electronic engineering from the City University of Hong Kong, Kowloon, in 1997 and been addressed. This experimental study confirms that it is 2000, respectively. feasible to use coreless PCB transformers for developing low- He is presently a Research Fellow in the City profile power converters with mega-hertz switching operation. University of Hong Kong. His research interests In this study, transformer efficiency of 76% and 63% are include coreless PCB transformers, high-frequency magnetics, MOSFET/IGBT gate drive circuits, iso- recorded when the transformer is fed with sinusoidal voltage lation amplifiers and low-profile power converters. and nonsinusoidal voltage, respectively. Coreless PCB trans- Dr. Tang is the Champion of the Institution of formers eliminate the disadvantages of core-based manually Electrical Engineers (IEE) Hong Kong Younger Member Section Paper Contest 2000. He received the Li Po Chun Scholarships and Intertek Testing Services wound transformers and provide a relatively low-cost and (ITS) Scholarships, in 1996 and 1997, respectively, the First Prize Award from highly reliable solution to the manufacturing of sub-watt con- the IEEE HK Section Student Paper Contest’97, was the second winner in verters with isolated voltage output. The favorable features at the Hong Kong Institution of Engineers (HKIE) 50th Anniversary Electronics Engineering Project Competition, and received the Certificates of Merit in the high frequency range, high reliability and the drastic reduction IEEE Paper Contests (Hong Kong Section), in 1998 and 1999, respectively. in the vertical dimension make coreless PCB transformers an attractive option for low-profile and mega-hertz applications. The successful implementation has demonstrated their potential S. Y. (Ron) Hui (SM’94) was born in Hong Kong for reliable mega-hertz switching converters and micro-circuits. in 1961. He received the B.Sc. degree (with honors) from the University of Birmingham, U.K. in 1984, and the D.I.C. and Ph.D degrees from the Imperial REFERENCES College of Science and Technology, University of  Max845 Evaluation Kit, Maxim Integrated Products, Inc., Oct. 1997. London, London, U.K., in 1987.  N. Dai, A. W. Lofti, G. Skutt, W. Tabisz, and F. C. Lee, “A comparative He was a Lecturer in power electronics at the study of high-frequency, low-profile planar transformer technologies,” University of Nottingham, U.K., from 1987 to 1990. in Proc. IEEE APEC’94, 1994, pp. 226–232. In 1990, he took up a lectureship at the University  K. Onda, A. Kanouda, T. Takahashi, S. Hagiwara, and H. Horie, “Thin of Technology, Sydney, Australia, where he became type dc/dc converter using a coreless wire transformer,” in Proc. IEEE a Senior Lecturer in 1991. He joined the University PESC’94, 1994, pp. 1330–1334. of Sydney in 1993 and was promoted to Reader of Electrical Engineering and  Y. Yamaguchi, S. Ohnuma, T. Imagawa, J. Toriu, H. Matsuki, and K. Director of Power Electronics and Drives Research Group in 1996. Presently, Murakami, “Characteristics of a thin film microtransformer with circular he is a Chair Professor of Electronic Engineering and an Associate Dean of spiral coils,” IEEE Trans. Magn., vol. 29, pp. 2232–2237, Sept. 1993. the Faculty of Science and Engineering, City University of Hong Kong. He  X. Huang, K. Ngo, and G. Bloom, “Design techniques for planar wind- has published over 150 technical papers, including about 80 refereed journal ings with low resistance,” in Proc. APEC’95, 1995, pp. 533–539. publications and book chapters. He has been an Honorary Professor at the  J. M. Bourgeois, “PCB based transformer for power MOSFET drive,” University of Sydney, since 2000. His research interests include all aspects of Proc. IEEE APEC’94, pp. 238–244, 1994. power electronics.  H. Chung, S. Y. R. Hui, and S. C. Tang, “Development of a multi- Dr. Hui received the Teaching Excellence Award in 1999 and the Grand Ap- stage current-controlled switched-capacitor step-down dc/dc converter plied Research Excellence Award in 2001 from the City University of Hong with continuous input current,” IEEE Trans. Circuits Syst. I, vol. 47, pp. 1017–1025, July 2000. Kong. He is a Fellow of the IEE, the IEAust, and the HKIE. He has been an As-  S. Y. R. Hui, S. C. Tang, and H. Chung, “Coreless printed-circuit board sociate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS since 1997. transformers for signal and energy transfer,” Electron Lett., vol. 34, no. 11, pp. 1052–1054, 1998.  S. Y. R. Hui, H. Chung, and S. C. Tang, “Coreless printed circuit board Henry Shu-hung Chung (S’92–M’95) received the (PCB) transformers for power MOSFETS/IGBT gate drive circuits,” B.Eng. degree (with first class honors) in electrical IEEE Trans. Power Electron., vol. 14, pp. 422–430, May 1999. engineering and the Ph.D. degree from The Hong  S. Y. R. Hui, S. C. Tang, and H. Chung, “Optimal operation of core- less PCB transformer-isolated gate drive circuits with wide switching Kong Polytechnic University in 1991 and 1994, frequency range,” IEEE Trans. Power Electron., vol. 14, pp. 506–514, respectively. May 1999. Since 1995, he has been with the City University  S. C. Tang, S. Y. R. Hui, and H. Chung, “Coreless printed circuit board of Hong Kong. He is currently an Associate Pro- (PCB) transformer with multiple secondary windings for complemen- fessor in the Department of Electronic Engineering. tary gate drive circuits,” IEEE Trans. Power Electron., vol. 14, pp. His research interests include time-domain and 431–437, May 1999. frequency-domain analysis of power electronic  F. I. Haqua, Inside PC Card: Card Bus and PCMCIA Design, U.K.: circuits, switched-capacitor-based converters, Newnes, 1996. randomswitching techniques, digital audio amplifiers, and soft-switching  J. C. Maxwell, A Treatise on Electricity and Magnetism. New York: converters. He has authored two research book chapters and over 110 technical Dover, 1954. papers, including 50 refereed journal papers in his current research area.  W. G. Hurley and M. C. Duffy, “Calculation of self and mutual imped- Dr. Chung received the China Light and Power Prize and was awarded the ances in planar magnetic structures,” IEEE Trans. Magn., vol. 31, pp. Scholarship and Fellowship of the Sir Edward Youde Memorial Fund in 1991 2416–2422, July 1995. and 1993, respectively. He is currently Chairman of the Council of the Sir Ed-  S. C. Tang, S. Y. R. Hui, and H. Chung, “A naturally soft-switched high- ward Youde Scholar’s Association and IEEE student branch counselor. He was frequency gate drive circuit for power MOSFETs/IGBTs,” in Proc. IEEE Track Chair of the Technical Committee on Power Electronics Circuits and PEDS, Hong Kong, July 1999, pp. 246–252. Power Systems, IEEE Circuits and Systems Society, from 1997 to 1998. He  C. F. Coombs Jr., Printed Circuits Handbooks, 3rd ed. New York: Mc- is presently an Associate Editor of the IEEE TRANSACTIONS ON CIRCUITS AND Graw-Hill, 1988, pp. 6–32. SYSTEMS—PART I: FUNDAMENTAL THEORY AND APPLICATIONS.