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IEEE TRANSACTIONS ON POWER , VOL. 16, NO. 4, JULY 2001 493 A Low-Profile Power Converter Using Printed-Circuit Board (PCB) Power with Ferrite Polymer Composite S. C. Tang, Member, IEEE, S. Y. Ron Hui, Senior Member, IEEE, and Henry Shu-Hung Chung, Member, IEEE

Abstract—A new design of low-cost and low-profile power transformer is presented in this paper. The cost of a power transformer can be reduced using the proposed printed-cir- cuit board (PCB) transformer. The transformer windings are etched on the opposite sides of a double-sided PCB. Self-adhesive ferrite polymer composite (FPC) sheets are stuck on the two PCB surfaces to shield the magnetic flux induced from the transformer windings. The PCB transformer does not require manual winding and bobbin. A power converter employing the PCB transformer has been implemented. The technique of choosing the optimum switching frequency of the power converter using PCB transformer is addressed in this paper. The maximum power delivered from the prototype is 94 W. The maximum efficiency of the power converter is 83.5%. Fig. 1. Dimensions of the transformer. Index Terms—Coreless printed-circuit-board , low- profile converter. II. STRUCTURE OF THE PRINTED-CIRCUIT BOARD (PCB) TRANSFORMER WITH FERRITE POLYMER COMPOSITE (FPC) I. INTRODUCTION SHEETS HE continuous improvement of power electronics has T made it possible to increase the switching speeds of power The printed-circuit board (PCB) transformer in the low-pro- devices. Switching frequency of several hundreds of kilo-Hertz file power converter is used to provide isolation and transfer is commonly adopted in switching power supplies. High-fre- power. The primary and secondary windings, shown in Fig. 1, quency operation can to a reduction of magnetic size and are respectively printed on the opposite sides of a printed-cir- an increase of power density. As the frequency increases, the cuit board. When the PCB transformer is operated in low- core loss becomes a limiting factor that cannot be ignored. power-level applications, such as isolated gate drive circuit Recently, coreless printed-circuit-board (PCB) transformers for power and IGBTs, can be elim- have been investigated for signal and power transfer [1]–[9]. inated [1]–[9]. Previous research shows that the magnetic flux It has been found that coreless PCB transformers can be generated from coreless PCB transformer employed in gate used for high-frequency operation from about 200 kHz and drive circuit for power MOSFET is negligible compared with upwards. The elimination of magnetic cores and the use of the magnetic flux generated from the high current-carrying printed windings open a new way to manufacture low-profile tracks connected to the drain terminal of the power transformers with high power density. MOSFET [7]. When the PCB Width transformer is utilized for In this paper, the study of a low-profile power converter using power transfer applications, the magnetic flux generated from coreless PCB transformer is reported. The special feature of the the transformer has to be confined. Two self-adhesive ferrite proposed converter is the use of a coreless PCB transformer as polymer compose (FPC) sheets [10] are stuck on both sides of the isolation transformer for power transfer [13]. The recently the PCB transformer to shield the magnetic flux. Fig. 2 shows developed ferrite polymer composite is used to enclose the mag- the cross-section of the PCB transformer. The geometric param- netic field of the transformer. eters are listed in Table I. The primary and secondary windings lay on the oppose sides of FR-4 resin glass fiber lam- inate. In the manufacturing process of PCB, a layer of high electrical breakdown mask is sprayed on the surfaces of Manuscript received March 27, 2000; revised April 6, 2001. This work was supported by the Research Grant Council of Hong Kong under CERG Project the PCB. Solder mask is used to prevent the PCB tracks from 9040446. Recommended by Associate Editor K. Ngo. mis-. In the case of the PCB transformer, the solder The authors are with the Department of Electronic , City Univer- sity of Hong Kong, Kowloon, Hong Kong (e-mail: [email protected]). mask can also provide isolation between the copper windings Publisher Item Identifier S 0885-8993(01)05959-2. and the FPC sheets.

0885–8993/01$10.00 ©2001 IEEE 494 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 4, JULY 2001

Fig. 3. Equivalent circuit of PCB transformer with FPC sheet (includes g , and resistive load, ‚ ).

and , are equal to 0.206 H. The mutual Fig. 2. Cross section of the PCB power transformer with FPC sheets. is 2.173 H. From [9], the voltage gain, , and input impedance, , of the transformer can be boosted by an TABLE I GEOMETRIC PARAMETERS OF PCB POWER TRANSFORMER external resonant . The resonant frequency can be chosen flexibly with appropriate value of . The voltage gain and the input impedance of the transformer can be expressed by as follows:

Voltage Gain (2)

(3)

where

III. CHARACTERISTICS OF THE PCB TRANSFORMER In this section, the characteristics of the PCB transformer in high frequency range, 100 kHz to 10 MHz, are described. The equivalent circuit model (includes the resonate capacitor, , and resistive load, ) of the PCB transformer with FPC sheet is shown in Fig. 3. The circuit model is similar to the accurate high frequency circuit model for coreless PCB transformer [5]. The loss in the FPC sheets can, in principle, be represented by a , , connected across the mutual inductance, . and However, it has been pointed out that [11] the loss in the ferrite substrates is negligible compared with the proximity effects in the conductor windings in planar sandwich . Thus, the resistor, , can be ignored in the circuit model in order to Input power to the transformer simplify the analysis. The distributive parameters of the PCB transformer are measured by an impedance analyzer HP4194A. (4) Because the geometry of the primary and secondary windings is identical, the winding resistances are the same and follow the Output power delivered from the transformer empirical equations: (5)

(1) Efficiency of the transformer From (1), the winding resistances increase with operating fre- quency. This phenomenon is mainly due to skin effects and proximity effects. The capacitive and inductive parameters are measured at 10 MHz. The interwinding , , is 86 pF. The (6) of the primary and secondary windings, TANG et al.: LOW-PROFILE POWER CONVERTER 495

Fig. 4. Voltage gain of the PCB transformer using FPC sheet with g a QWH pF and various resistive load, ‚ . Fig. 6. Circuit schematic of the half-bridge converter.

IV. POWER CONVERTER PROTOTYPE AND EXPERIMENTAL RESULTS The PCB transformer is driven by a half-bridge converter as shown in Fig. 6. Both the upper and lower side are IRF630 power MOSFET ( V and ). The power MOSFET is in TO-220 package. In the con- verter prototype, the back metal plate of the TO-220 package (connected to the drain of the MOSFET) is soldered directly on the copper pad of the printed-circuit board. This technique facil- itates heat dissipation from the MOSFETs’ dies and reduces the lead inductances of the power MOSFETs. Moreover, the con- Fig. 5. Efficiency of the PCB transformer using FPC sheet with g a QWH pF verter can be very thin. The thickness of the TO-220 package and various resistive load, ‚ . power MOSFET is 4.69 mm. The thickness of the PCB trans- former is 1.0 mm. When surface mount components are em- In the paper, the PCB transformer is used to drive a resistive ployed in the converter prototype, the thickness of the power load, , through a full-wave bridge as shown in Fig. 6. converter is determined by the thickness of the power MOSFET From [12], the equivalent resistance, , in the (2)–(6) can package. be approximately related to as The supply voltage of the half-bridge is 120 V dc. The res- onant capacitor, , is 390 pF so that the maximum efficiency (7) of the power converter is between 2 MHz and 4 MHz as de- scribed in Section III. In the half-bridge converter, a 40 ns dead From (2), the voltage gain of the PCB transformer using FPC time is introduced between the two MOSFETs’ gating signals. sheet with 390 pF resonant capacitor is plotted in Fig. 4. The This dead time prevents shoot-through current from flowing into efficiency plot of the PCB transformer, shown in Fig. 5, can be the two MOSFETs at the transition time. The power converter obtained from (6). Since the PCB transformer acts as a power is tested with various switching frequencies (from 1 MHz to transformer, high power efficiency is desirable. The maximum- 4 MHz) and load resistance (from 30 to 200 ). Under an efficiency-frequency (MEF) is about 4 MHz. At this frequency, open-loop condition, the output voltage, , of the power con- the transformer efficiency is about 94% when . verter is plotted in Fig. 7. Operating the transformer at this MEF is the most favorable. Fig. 8 shows the measurement setup of the PCB transformer However, when the transformer is driven by power MOSFET in the half-bridge power converter. Because neither of the trans- switches, the switching losses of the power MOSFETs must be former terminals is common to the converter’s , differen- considered. Because the switching losses of the power MOS- tial voltage probes P5205 from Tektronix are used to measure FETs are directly proportional to the switching frequency, the the primary and secondary voltages of the PCB transformer. switching frequency of the power converter should be as low as The primary and secondary currents of the PCB transformer possible. The transformer losses, therefore, should be consid- are measured by Tektronix current probe AM503B with current ered together with the switching losses of power MOSFETs. In probe A6302. Time delay of the differential voltage the frequency range from 2 MHz to 7.5 MHz, the transformer is probes and the current probe have to be considered. The time in the high efficiency region ( ). Therefore, the optimum delays of the differential probes and current probe are about 10 switching frequency of the power converter using the PCB trans- ns and 17 ns, respectively, in the testing environment. The input former is between 2 MHz and 4 MHz. This point will be verified power, , to the transformer primary is the average value of the in the next section. Fig. 4 shows that the voltage gain of the PCB product of the instantaneous primary voltage, , and current, transformer, in the optimum switching frequency region, is be- in one cycle. Similarly, the output power, , delivered tween 0.84 and 1.0. from the secondary of the transformer is the average value of 496 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 4, JULY 2001

Fig. 9. Measured efficiency of the PCB transformer operated in the half-bridge power converter. Fig. 7. Output voltage of the power converter with various switching frequency and load resistance.

Fig. 8. Efficiency measurement setup of the PCB transformer in the half-bridge power converter.

Fig. 10. Measured efficiency of the overall power converter using PCB the product of the instantaneous secondary voltage, , and transformer with FPC sheet. current, , in one cycle. In mathematical form predicted result in Fig. 5. The predicted transformer efficiency, in (6) and Fig. 5, produces useful information to predict the (8) optimum switching frequency of the PCB transformer. As mentioned in Section III, because the switching losses and of power MOSFETs are directly proportional to the switching frequency, the optimum switching frequency of the overall (9) power converter should be lower than the maximum switching frequency of the PCB transformer. Accordingly, the optimum where one switching period. switching frequency of the power converter is between 1.5 MHz The power efficiency, , of the PCB transformer is defined and 3.5 MHz. Fig. 10 shows the measured efficiency of the as the ratio of the output power to the input power. Thus power converter using PCB transformer with FPC sheet. The gate drive power consumption is not included. The converter (10) efficiency is increasing in the frequency range between 1 MHz and 2.5 MHz. The increasing efficiency of the power converter The measured transformer efficiency versus switching fre- is attributed to the increasing efficiency of the transformer in quency with different load resistances is plotted in Fig. 9. the frequency range between 1 MHz and 3.5 MHz as shown in Because the transformer is driven by a half-bridge totem Fig. 9. When the switching frequency is greater than 2.5 MHz, pole pair, the primary voltage of the PCB transformer is the switching loss of the power switches become significant. As a square wave as shown in Fig. 14. The rectangular wave a result, the efficiency of the power converter, which consists voltage contains not only the fundamental frequency but also of the MOSFET half-bridge, the PCB transformer, the the harmonics. However, the prediction in Fig. 5 considers rectifier and the output capacitor, decreases. From Fig. 10, the the fundamental frequency only (sinusoidal excitation). The maximum efficiency frequency is found to be about 2.5 MHz efficiency plot in Fig. 9 indicates that the switching frequency, which is considered as the optimum switching frequency of the at which maximum efficiency of the PCB transformer occurs, power converter. is about 3.5 MHz. The predicted results in Fig. 5 show that At the maximum efficiency frequency of the power converter, the maximum efficiency is about 4 MHz. This error in the MHz, the converter efficiency was measured with prediction is due to the assumption that the voltage excitation in different load resistance. The results are plotted in Fig. 11. The the transformer is sinusoidal. In practice, the voltage excitation efficiency of the power converter is 83.49% with 80 load re- is in form of a rectangular waveform. Nonetheless, the trend of sistance. With a light load, , the power converter the measured transformer efficiency, in Fig. 9, is similar to the efficiency is 76.2%. TANG et al.: LOW-PROFILE POWER CONVERTER 497

Fig. 11. Measured output power of the power converter using PCB transformer with FPC sheets.

Fig. 14. † @IHH VaDivA, † @IHH VaDivA, s @S AaDivA, and s @S AaDivA (from top to bottom).

Fig. 12. Measured output power of the power converter using PCB transformer with FPC sheets.

Fig. 15. Photograph of the coreless PCB transformers with and without ferrite polymer composite.

current, , of the PCB transformer are shown in Fig. 14. A pho- tograph of the coreless PCB transformers with and without the ferrite polymer composite is included in Fig. 15.

V. C ONCLUSIONS A new design of low-cost and low-profile power transformer is examined. The transformer windings are etched on the opposite sides of a double-sided printed-circuit board (PCB). Self-adhesive ferrite polymer composite (FPC) sheets are stuck on the two PCB surfaces to shield the magnetic flux from the transformer windings. The PCB transformer does not require manual winding and bobbin. The total transformer thickness is 1.0 mm. The radius of the outermost winding is about 1.8 Fig. 13. † @IH VaDivA, † @IH VaDivA, † @IHH VaDivA, and cm. The operation of the PCB transformer is demonstrated in s @S AaDivA. (from top to bottom). a half-bridge power converter. The technique for choosing the optimum switching frequency of the power converter using The measured output power of the power converter is plotted PCB transformer with FPC sheets is addressed in this paper. in Fig. 12. The maximum power delivered from the power con- The maximum power delivered from the prototype is 94 W verter is about 94 W when the load resistance is 30 . The wave- and the maximum efficiency of the overall power is 83.5%. forms were also captured when the converter was operated at 2.5 This investigation indicates that it is feasible to use coreless MHz switching frequency and with a 40 resistive load. Fig. 13 PCB transformers in power converters up to about 100 W at a shows the gate-to-source voltages of and ( and reasonably high-energy efficiency. The shielding effect of the ), the drain-to-source voltage of and the pri- self-adhesive ferrite polymer composite is presently limited mary current of the PCB transformer. The primary voltage, by its low permeability. Future research should focus on the , secondary voltage, , primary current, , and secondary improvement of this shielding material. 498 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 4, JULY 2001

REFERENCES S. Y. Ron Hui (SM’94) was born in Hong Kong in 1961. He received the B.Sc. degree (with honors) [1] S. Y. R. Hui, S. C. Tang, and H. Chung, “Coreless printed-circuit board from the University of Birmingham, U.K. in 1984, transformers for signal and energy transfer,” Electron. Lett., vol. 34, no. and the D.I.C. and Ph.D. degrees from the Imperial (11), pp. 1052–1054, 1998. College of Science and , University of [2] S. Y. R. Hui, H. Chung, and S. C. Tang, “Coreless (PCB) transformers for power MOSFET/IGBT gate drive circuits,” London, London, U.K., in 1987. IEEE Trans. Power Electron., vol. 14, pp. 422–430, May 1999. He was a Lecturer in power electronics at the Uni- [3] S. C. Tang, S. Y. R. Hui, and H. Chung, “Coreless printed circuit versity of Nottingham, U.K., from 1987 to 1990. In board (PCB) transformers with multiple secondary windings for 1990, he had a lectureship at the University of Tech- complementary gate drive circuits,” IEEE Trans. Power Electron., vol. nology, Sydney, Australia, where he became a Senior 14, pp. 431–437, May 1999. Lecturer in 1991. He joined the University of Sydney [4] S. Y. R. Hui, S. C. Tang, and H. Chung, “Optimal operation of core- in 1993 and was promoted to Reader of electrical engineering and Director of less PCB transformer-isolated gate drive circuits with wide switching Power Electronics and Drives Research Group in 1996. Presently, he is a Chair frequency range,” IEEE Trans. Power Electron., vol. 14, pp. 506–514, Professor of and an Associate Dean of the Faculty of Sci- May 1999. ence and Engineering, City University of Hong Kong. He has published over [5] , “An accurate circuit model for coreless PCB-based transformers,” 150 technical papers, including about 80 refereed journal publications and book in Proc. Eur. Power Electron. Conf., Trondheim, Norway, Sept. 1997. chapters. His research interests include all aspects of power electronics. [6] S. C. Tang, S. Y. R. Hui, and H. Chung, “Characterization of coreless Dr. Hui received the Teaching Excellence Award in 1999 and the Grand printed circuit board transformer,” IEEE Trans. Power Electron., vol. Applied Research Excellence Award in 2001 from the City University of 15, pp. 1275–1282, Nov. 2000. Hong Kong. He has been appointed an Honorary Professor by the University [7] , “Some electromagnetic aspects of coreless PCB transformers,” of Sydney, Australia since 2000. He is a Fellow of the WE, the IEAust, and IEEE Trans. Power Electron., vol. 15, pp. 805–810, July 2000. the HKTE. He has been an Associate Editor of the IEEE TRANSACTIONS ON [8] H. Chung, S. Y. R. Hui, and S. C. Tang, “Development of a multistage POWER ELECTRONICS since 1997. current-controlled switched-capacitor step-down DC/DC converter with continuous input current,” IEEE Trans. Circuits Syst., vol. 47, pp. 1017–1025, July 2000. [9] S. Y. R. Hui, S. C. Tang, and H. Chung, “’Coreless planar printed-cir- cuit-board (PCB) transformers -A fundamental concept for signal and Henry Shu-Hung Chung (S’92–M’95) received the energy transfer’,” IEEE Trans. Power Electron., vol. 15, pp. 931–941, B.Eng. degree (with first class honors) in electrical Sept. 2000. engineering and the Ph.D. degree from The Hong [10] Siemens Data Sheet—Ferrite Polyner Composite Sheet C351. Kong Polytechnic University in 1991 and 1994, [11] W. G. Hurley and M. C. Duffy, “Calculation of self and mutual imped- respectively. ances in planar magnetic structures,” IEEE Trans. Magn., vol. 31, pp. Since 1995, he has been with the City University 2416–2422, July 1995. of Hong Kong. He is currently an Associate Pro- [12] R. W. Erickson, Fundamentals of power electronics. New York: fessor in the Department of Electronic Engineering. Chapman & Hall, 1997, ch. 19. His research interests include time-domain and [13] S. Y. R. Hui and S. C. Tang, “Coreless printed-circuit-board (CPCB) frequency-domain analysis of power electronic transformers and operating techniques,” 09 316 735, 2001. circuits, switched-capacitor-based converters, random-switching techniques, , and soft-switching converters. He has authored two research book chapters, and over 110 technical papers including 50 refereed journal papers in the current research area. Dr. Chung received the China Light and Power Prize and the Scholarship S. C. Tang (M’98) was born in Hong Kong in 1972. and Fellowship of the Sir Edward Youde Memorial Fund, in 1991 and 1993, He received the B.Eng. (with first class honors) and respectively. He is currently Chairman of the Council of the Sir Edward Youde Ph.D. degrees in electronic engineering from the Scholar’s Association and an IEEE student branch counselor. He was track City University of Hong Kong, Kowloon, in 1997 chair of the technical committee on power electronics circuits and power and 2000, respectively. systems of IEEE Circuits and Systems Society, from 1997 to 1998. He is an He is a Research Fellow at the City University of Associate Editor of the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS: Hong Kong. His research interests include coreless PART I—FUNDAMENTAL THEORY AND APPLICATIONS. PCB transformers, high-frequency magnetics, MOSFET/IGBT gate drive circuits, isolation amplifiers, and low-profile power converters. Dr. Tang is the Champion of the Institution of Elec- trical Engineers (IEE) Hong Kong Younger Member Section Paper Contest, 2000. He received the Li Po Chun Scholarships and Intertek Testing Services (ITS) Scholarships, in 1996 and 1997, respectively, the First Prize Award from the IEEE HK Section Student Paper Contest’97, was the second winner in the Hong Kong Institution of Engineers (HKIE) 50th Anniversary Electronics Engi- neering Project Competition, and received the Certificates of Merit in the IEEE Paper Contests (Hong Kong Section) in 1998 and 1999, respectively.