A Low-Profile Wide-Band Three-Port Isolation Amplifier With Coreless Printed-Circuit-Board (PCB) Transformers

Transcription

1 1180 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001 A Low-Profile Wide-Band Three-Port Isolation Amplifier With Coreless Printed-Circuit-Board (PCB) Transformers S. C. Tang, Member, IEEE, S.Y.R.Hui, Senior Member, IEEE, and Henry Shu-hung Chung, Member, IEEE Abstract Galvanic isolations are essential in many electrical patient-monitoring devices and industrial applications. In this paper, a low-profile wideband three-port isolation amplifier using coreless printed-circuit-board (PCB) transformers for isolation is studied. The PCB thickness used in the isolation amplifier is 0.4 mm. The diameters of the two coreless PCB transformers are 9.75 and mm, respectively. Operating conditions of the transformers and a design guideline of the isolation amplifier are detailed in this paper. Experimental results show that the isolation amplifier under investigation can transmit an analog signal from 20 mhz to 1.1 MHz with good linearity. Comparison of the prototype with an industrial isolation amplifier is also included. Index Terms Coreless printed-circuit-board transformers, isolation amplifier, wideband. I. INTRODUCTION ISOLATION amplifiers are used in many industrial applications. Galvanic isolation is a necessity for many electrical patient-monitoringdevices,suchaselectrocardiographs(ecgs) andelectroencephalographs(eegs),toensurethesafetyofthe patient [1]. Isolation amplifiers used in medical instrument usually transmit avery-low-frequency signal. The ECG is used to monitor the heartbeat and the EEG acquires the brain wave range from 0.5 to 100 Hz of the patient [1]. On the other hand, wide-band isolation amplifiers from near dc to a few hundred kilohertz are required in some industrial applications. In some motor control circuits and power converters with power-factor correction, Hall-effect current sensors can be used to monitor the motor current and line current. A current-sensing resistor accompanied by an isolation amplifier is an alternative to acquire the current information [2]. Commonly used isolation barriers in isolation amplifiers include transformer coupling, opto-coupler, and capacitive coupling. Transformers are commonly used because they can transmit both signal and energy across the isolation barrier. Unfortunately, from a manufacturing point of view, the manual winding cost and the fragility of thin copper windings are shortcomings of using small toroidal transformers in hybrid Manuscript received May 26, 2000; revised June 1, Abstract published on the Internet October 24, This work was supported by the Research Grant Council of Hong Kong under CERG Project S. C. Tang was with City University of Hong Kong, Kowloon, Hong Kong. He is now with the Department of Electronic Engineering, National University of Ireland, Galway, Ireland. S. Y. R. Hui and H. S. Chung are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong ( eeronhui@cityu.edu.hk). Publisher Item Identifier S (01) packages [3]. Apart from the need for magnetic cores, traditional core-based transformers have restricted bandwidth because of the limitations in the high-frequency characteristics of magnetic material. The typical bandwidth of an existing commercial transformer-coupled isolation amplifier such as an AD215 is about 120 khz [4]. High-speed opto-couplers can achieve a high bandwidth up to the megahertz range, but they are relatively expensive and can only transfer a signal to the receiver. Additional floating power supplies are necessary in both inputand output-isolated sides of the amplifier. Capacitive coupling is a simple and low-cost way to provide galvanic isolation. Two metal tracks can be etched on an insulation substrate, e.g., FR4 material, to form a coupling capacitor [5]. Similarly to the opto-coupler, the capacitive coupling method basically transmits a signal only. Floating power supplies are needed to power the input and output circuits of the isolation amplifier. Modern isolation amplifiers use various techniques to transfer the dc component of a signal over the isolation barrier. The first method is to digitize the analog signal using an analogto-digital converter (ADC), and the coded binary signal can then be transferred over the isolation barrier. This method can accurately recover the original analog signal with a digital-toanalog converter (DAC), but this technique suffers from the high cost of the ADC and DAC. The second method is to use the linear region of the opto-coupler to transfer the analog signal directly. However, the opto-coupler has nonlinear and temperature-dependent properties. Thus, feedback technique using another identical opto-coupler is normally required to reduce these shortcomings [6]. Finally, modulation/demodulation is another technique to carry a low-frequency signal to the output side of the isolation amplifier. Using frequency modulation (FM) with 50% duty cycle to carry energy and the analog signal through the isolation barrier is more beneficial than using amplitude modulation (AM) and pulsewidth modulation (PWM). Using AM technique leads to high conducting loss in the transformer drive circuit in a transformer-coupled isolation amplifier since the switches in the drive circuit have to operate in the linear region. On the other hand, using PWM method is not the optimal modulation technique to transfer maximum energy to the secondary circuit because maximum power transfer is achieved when the transformer primary voltage has a duty cycle of 50%. In this paper, we investigate a three-port isolation amplifier using coreless printed-circuit-board (PCB) transformers [7] [10], [12], [14] [17] as the isolation barriers. Coreless PCB transformers can be operated at high frequency (above a few megahertz) because they have no limitations of /01$ IEEE

2 TANG et al.: LOW-PROFILE WIDE-BAND THREE-PORT ISOLATION AMPLIFIER 1181 Fig. 1. Block diagram of the three-port isolation amplifier. the magnetic cores. They also have the advantages of high dielectric breakdown voltage, low cost, and easy fabrication in isolation amplifiers in hybrid packages, although a higher frequency operation usually implies a higher switching loss in the electronics driving the transformer windings. TABLE I DIMENSIONS OF T AND T II. OVERVIEW OF THE PROPOSED ISOLATION AMPLIFIER The three-port isolation amplifier under investigation is shown in Fig. 1. It uses two transformers ( and ), with unity turn ratios, to provide galvanic isolation. This amplifier has the same configuration as some industrial integrated circuits such as the Analog Device AD215 [4]. In this project, the core-based transformers are replaced by two coreless ones., operating at 7 MHz, is used to transfer power to the input circuit of the isolation amplifier. Operating at a center frequency of 5.2 MHz, is used to transfer both signal and power from the input circuit to the output circuit. The term three-port isolation amplifier means that the power, input, and output circuit blocks are isolated from each other. The isolation amplifier is powered by a 15-V supply. The power circuit consists of a 7-MHz pulse generator and a transformer drive circuit. The ac voltage at 7 MHz and of 15 V (peak-to-peak) is coupled to the input circuit through the coreless PCB transformer. The input circuit consists of a rectifier with a filter circuit, an input low-pass-filter, a modulator, and a transformer drive circuit that drives the primary side of another coreless PCB transformer. The ac voltage on the secondary side of is rectified and filtered as a smooth dc supply to power the components in the input circuit of the isolation amplifier. The input low-pass filter, with a cutoff frequency of 2 MHz, is used to block the high-frequency components in the incoming analog signal. The high-frequency components may cause instability in the modulation and demodulation circuits. The analog input voltage signal is converted to frequency variation by the modulator, which is formed by a voltage-controlled oscillator (VCO). The center frequency of the VCO is 5.2 MHz with about 50% duty Fig. 2. (a) Shape of T. (b) Shape of T. Fig. 3. Transformer equivalent model. cycle. The modulated signal is amplified by the transformer drive circuit and coupled to the output circuit through. The ac

3 1182 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001 TABLE II REACTIVE PARAMETERS OF T AND T voltage at 5.2 MHz is rectified and filtered to supply dc voltage to the demodulator and low-pass filter in the output circuit. The modulated signal on the secondary side of is demodulated using a digital-phase-locked-loop (DPLL). The output low-pass filter, with a cutoff frequency of 2 MHz, is used to attenuate the high-frequency noise coming from the 5.2-MHz carrier and its sidebands. The recovered analog signal can, therefore, be reconstructed at the output of the isolation amplifier. III. CHARACTERISTICS AND OPERATIONS OF CORELESS PCB TRANSFORMER In the three-port isolation amplifier, the coreless PCB transformer is used to supply isolated power to the input circuit while transfers energy and signal from the input circuit to the output circuit. Because power transfer in is less than that in, the size of is smaller than that of. For each transformer, the primary and secondary windings of the coreless PCB transformers used in the isolation amplifier are identical. The turns ratios of both transformers are unity. The primary and secondary windings are printed directly on the opposite sides of a PCB. The laminate of the coreless PCB transformer in the isolation amplifier prototype is made of FR4 material. The dielectric breakdown voltage of FR4 PCB laminate typically ranges from 15 to 40 kv [11]. The dimensions and shapes of the coreless PCB transformers are detailed in Table I and Fig. 2, respectively. The equivalent circuit model of a coreless PCB transformer is shown in Fig. 3. The intrawinding capacitances of the windings in and are negligible ( 1 pf) in the transformer model. The calculated [12] and measured values of the reactive parameters of the transformers are tabulated in Table II. The measured winding resistance of is 0.59 at 7 MHz and that of is 0.92 at 5.2 MHz. The transformer is set to operate at 7 MHz. A resonance capacitor, (Fig. 2), is connected across the secondary of to boost the transformer voltage gain energy efficiency. Using different resonance capacitors, from 200 to 1000 pf, the transformer gain and efficiency are plotted in Figs. 4 and 5, respectively. In these two plots, a 60- load resistor is connected in parallel with the resonance capacitor. This resistive load is to approximate the loading effect of the input circuit. Fig. 4 shows that, as capacitance value of increases, the transformer gain increases at 7 MHz. However, the transformer efficiency increases when the value of the resonant capacitor increases from 200 to about 800 pf. Figs. 4 and 5 give useful information to select a suitable capacitance value of. Because an appropriate voltage level is needed to supply enough power to the transformer drive circuit of, a resonance capacitor of about 600 pf is chosen as a compromise for high-voltage gain and energy efficiency in the isolation Fig. 4. Gain of transformer T versus frequency. Fig. 5. Efficiency of transformer T versus frequency. amplifier prototype. The calculated voltage gain is about 0.9 and the energy efficiency is 87%. Fig. 6 shows the measured primary and secondary waveforms of in the prototype. The positive isolated voltage source, as shown in Fig. 1, is used to supply energy to all components in the input circuit, while the negative isolated voltage source only supplies energy to the operational amplifier in the low-pass filter. Thus, the loading effect of the positive voltage source is much greater than that of the negative voltage source. With the reflection effect of the dc voltage on the power supply filter on the secondary of, the positive cycle of the secondary voltage of is clipped as shown in Fig. 6 As the power consumption of the output circuit is less than that of the input circuit, a smaller size coreless PCB transformer is adopted. Similar to, a resonance capacitor is connected across the secondary of to increase the voltage gain and efficiency of the transformer. The voltage gain and efficiency plot of with different resonance capacitors, from 1.2 to 2 nf, are shown in Figs. 7 and 8, respectively. Because the center frequency of the modulated signal passing through

4 TANG et al.: LOW-PROFILE WIDE-BAND THREE-PORT ISOLATION AMPLIFIER 1183 Fig. 6. Upper track: primary voltage of T (10 V/div); lower track: secondary voltage of T (10 V/div). Fig. 7. Gain of transformer T versus frequency. Fig. 9. Upper track: primary voltage of T (10 V/div); lower track: secondary voltage of T (10 V/div). cycle is adopted to achieve maximum power transfer. A VCO is adopted as an FM modulator to convert the input analog signal to frequency variation. On the secondary side of, a DPLL is used to demodulate the FM signal. To obtain the best linearity of the overall isolation amplifier, the VCO in the FM modulator and the VCO in the DPLL are of the same type. The circuit diagram of the DPLL is shown in Fig. 10. The output frequency of the VCO used in the isolation amplifier is a function of the input voltage as follows: The small-signal gain of the VCO is, therefore, Hz (1) Hz/V (2) or, in rad V, The transfer function of phase comparator is given by V (3) (4) Fig. 8. Efficiency of transformer T versus frequency. is 5.2 MHz, a 1.8-nF resonance is selected for high energy efficiency and the voltage gain is about unity at this frequency. Fig. 9 shows the measured primary and secondary waveforms of in the isolation amplifier prototype when the input analog signal voltage is zero. The transfer function of the loop filter is given by where and. The output voltage is obtained from Fig. 10 as follows: (5) (6) IV. DESIGN OF THE MODULATION AND DEMODULATION CIRCUITS Square voltage has the largest fundamental component at 50% duty cycle. Because the harmonics of the square voltage can be filtered out by the coreless PCB transformer with resonance capacitor, square voltage operation with 50% duty cycle has the advantage of maximum power transfer. Thus, both PWM and AM methods are ruled out in this investigation. FM with 50% duty Substituting (3) into (6), the transfer function of the phase-locked loop (PLL) becomes Substituting (5) into (7), (7) (8)

5 1184 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001 Fig. 10. Circuit diagram of the DPLL. From (8), the characteristic equation of the PLL s transfer function is (9) Thus, the natural frequency can be expressed as (10) From (5), the cutoff angular frequency of the low-pass loop filter is (11) Substituting (11) into (10), From (9), the damping factor is (12) Fig. 11. AD215. Gain versus frequency of the isolation amplifier prototype and (13) From (9) and (10), we obtain (14) (15) From (12), (14), and (15), the values of the cutoff frequency, of the low-pass filter, and the damping factor of the PLL can be selected in order to find the values of and. In the prototype, MHz and. Using pf, we get and. V. EXPERIMENTAL RESULTS The proposed isolation amplifier was designed to have 1.1-MHz bandwidth, and the carrier frequency of the FM signal is 5.2 MHz. Two Butterworth low-pass-filters, with a cutoff frequency of 2 MHz, are used to attenuate the high-frequency noise. The gain and phase plots of the isolation amplifier are shown in Figs. 11 and 12, respectively. The corresponding plots obtained from the AD215 are also included for compar- Fig. 12. AD215. Phase versus frequency of the isolation amplifier prototype and ison. The experimental results are measured by an HP4194A Gain-Phase Analyzer. The calculated results are based on (8). From Fig. 11, the 3-dB cutoff frequency of the isolation amplifier prototype is about 1.1 MHz. The isolation amplifier was tested with sinusoidal signal with 0.02 Hz, 100 khz, and 1 MHz. The measured waveforms are shown in Figs , respectively. The linearity of the amplifier can be examined

6 TANG et al.: LOW-PROFILE WIDE-BAND THREE-PORT ISOLATION AMPLIFIER 1185 Fig. 13. Upper track: input signal (2 V/div); lower track: output signal (2 V/div). Fig. 16. X axis: input signal (1 V/div); Y axis: output signal (1 V/div). Fig. 14. Upper track: input signal (1 V/div); lower track: output signal (1 V/div). Fig. 17. Upper track: input signal (1 V/div); lower track: output signal (1 V/div). Fig. 15. Upper track: input signal (1 V/div); lower track: output signal (1 V/div). in an plot [13]. The input output characteristic of the isolation amplifier at 100 Hz in mode is shown in Fig. 16. The overshoot and undershoot waveforms are shown in Figs. 17 Fig. 18. Upper track: input signal (1 V/div); lower track: output signal (1 V/div). and 18, respectively. It is shown that the propagation time delay (50% voltage level) of the isolation amplifier prototype is about 590 ns.

7 1186 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001 TABLE III POWER DISTRIBUTION IN THE PROTOTYPE BUILT WITH DISCRETE COMPONENTS It should be noted that the AD215 uses AM with a carrier frequency of about 430 khz. Without the frequency limits of the magnetic cores, the prototype amplifier uses FM with a center frequency of 5.2 MHz. The AD215 consumes about 1 W and has a bandwidth of 120 khz. In the prototype amplifier, in which the electronic circuit is built with discrete components (instead of an optimal integrated circuit), the power consumption is about 3.5 W in the tests. The power distribution is included in Table III. The major loss component occurs in the 7-MHz gate drive circuit for coreless transformer. It is envisaged that the power consumption could be lowered if customized ICs are used in the proposed isolation amplifier. VI. CONCLUSIONS An isolation amplifier using coreless PCB transformers to transfer an analog signal has been successfully demonstrated. The use of coreless PCB transformer eliminates the cost of manual winding and magnetic core. Such transformer also facilitates embedding the transformer into a hybrid package in the manufacturing process and reduces the problems of core-based transformers, such as frequency limits, magnetic saturation, manual winding cost, and the fragility of the thin windings used in small transformers. The coreless PCB transformers not only transfer the modulated signal but also energy to power the secondary circuits over the isolation barriers. Additional floating power supplies in the input and output sides of the isolation amplifier are not required. Frequency modulation and resonance techniques are used to achieve maximum power transfer. The diameters of the coreless PCB transformers used in the isolation amplifier are 9.75 and 5.86 mm, respectively. The thickness of PCB laminate is 0.4 mm. A prototype has been built, which verifies that the coreless PCB transformer-coupled isolation amplifier can transmit an analog signal from 20 mhz to 1.1 MHz with good linearity. Without the limitations of the magnetic core, such as nonlinearity and high-frequency limits, coreless PCB transformers offer a new low-cost and effective solution to implement an isolation amplifier. This investigation confirms the feasibility of using coreless PCB transformers in isolation amplifiers. Problems remaining to be tackled include the development of a customized IC to drive the transformers so as to further reduce the total power consumption. REFERENCES [1] J. G. Webster, Medical Instrumentation: Application and Design, 3rd ed. New York: Wiley, 1998, pp. 65, [2] Low cost, miniature isolation amplifier: AD202/AD204, Analog Devices Inc., Norwood, MA, [3] Design and application of transformer-coupled hybrid isolation amplifier model 3656, Burr Brown Corporation, Tucson, AZ, Application Bull., [4] 120kHz bandwidth, low distortion isolation amplifier: AD215, Analog Devices Inc., Norwood, MA, [5] W. Meinel, Plastic molded analog isolation amplifier, in Proc IEEE Int. Symp. Circuits and Systems, May 1990, pp [6] P. Waugh, Techniques used for isolation amplifiers, presented at the IEE Colloq. Measurement Techniques for Power Electronics, [7] S. Y. R. Hui, S. C. Tang, and H. Chung, Coreless printed-circuit-board (PCB) transformers for signal and energy transfer, Electron. Lett., vol. 34, no. 11, pp , May [8], Coreless PCB-based transformers for power MOSFET/IGBT gate drive circuits, IEEE Trans. Power Electron., vol. 14, pp , May [9] S. C. Tang, S. Y. R. Hui, and H. Chung, Coreless printed circuit board (PCB) transformers with multiple secondary windings for complementary gate drive circuits, IEEE Trans. Power Electron., vol. 14, pp , May [10] S. Y. R. Hui, S. C. Tang, and H. Chung, Optimal operation of coreless PCB transformer-isolated gate drive circuits with wide switching frequency range, IEEE Trans. Power Electron., vol. 14, pp , May [11] C. F. Coombs, Jr., Printed Circuits Handbook, 3rd ed. New York: Mc- Graw-Hill, 1988, p [12] S. C. Tang, S. Y. R. Hui, and H. Chung, Characterization of coreless printed circuit board (PCB) transformers, IEEE Trans. Power Electron., vol. 15, pp , Nov [13] R. A. Witte, Electronic Test Instruments: Theory and Applications. Palo Alto, CA: Hewlett-Packard, 1993, pp [14] S. C. Tang, S. Y. R. Hui, and H. Chung, A low-profile power converter using coreless PCB powertransformer shieldedwith ferrite polymercomposite, IEEE Trans. Power Electron., vol. 16, pp , July [15] S. Y. R. Hui and S. C. Tang, Coreless printed-circuit-board (PCB) transformers, U.S. Patent Application 08/ [16] S. Y. R. Hui and S. C. Tang, Coreless printed-circuit board (PCB) transformers and operating techniques, U.S. Patent Application 09/ [17] S. Y. R. Hui and S. C. Tang, Operating techniques for coreless printed-circuit-board (PCB) transformers, European Patent Application S. C. Tang (M 98) was born in Hong Kong in He received the B.Eng. degree (with first class honors) and the Ph.D degree in electronic engineering from the City University of Hong Kong, Hong Kong, in 1997 and 2000, respectively. He is currently a Post-Doctoral Research Fellow at the National University of Ireland, Galway, Ireland. His research interests include coreless PCB transformers, high-frequency magnetics, MOSFET/ IGBT gate drive circuits, isolation amplifiers, and low-profile converters. Dr. Tang won the Championship of the HKIE Young Member Paper Contest in He was awarded the Li Po Chun Scholarship and Intertek Services (ITS) Scholarship in 1996 and 1997, respectively. In 1997, he received the First Prize Award from the IEEE Hong Kong Section Student Paper Contest and was the second winner in the Hong Kong Institution of Engineers (HKIE) 50th Anniversary Electronic Engineering Project Competition. He also received Certificates of Merit in the IEEE Paper Contests (Hong Kong Section) in 1998 and 1999.

8 TANG et al.: LOW-PROFILE WIDE-BAND THREE-PORT ISOLATION AMPLIFIER 1187 S. Y. (Ron) Hui (M 87 SM 94) was born in Hong Kong in He received the B.Sc. (Hons.) degree in 1984 from the University of Birmingham, Birmingham, U.K., and the D.I.C. and the Ph.D. degree in 1987 from Imperial College of Science and Technology, University of London, London, U.K. He was a Lecturer in power electronics at the University of Nottingham, Nottingham, U.K., in In 1990, he took up a lectureship at the University of Technology, Sydney, Australia, where he became a Senior Lecturer in He joined the University of Sydney, Sydney, Australia, in 1993 and was promoted to Reader of Electrical Engineering and Director of the Power Electronics and Drives Research Group in He is currently a Chair Professor of Electronic Engineering and an Associate Dean of the Faculty of Science and Engineering, City University of Hong Kong, Hong Kong. He has authored more than 150 published technical papers, including about 80 refereed journal publications and book chapters. His research interests include all aspects of power electronics. Prof. Hui is a Fellow of the Institution of Electrical Engineers, U.K., Institution of Engineers, Australia, and Hong Kong Institution of Engineers. He has been an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS since He received the Teaching Excellence Award in 1999 and the Grand Applied Research Excellence Award in 2001 from the City University of Hong Kong. He was appointed an Honorary Professor by the University of Sydney in Henry Shu-hung Chung (S 92 M 95) received the B.Eng. degree (with first class honors) in electrical engineering and the Ph.D. degree from Hong Kong Polytechnic University, Hong Kong, in 1991 and 1994, respectively. Since 1995, he has been with the City University of Hong Kong, Hong Kong. He is currently an Associate Professor in the Department of Electronic Engineering. His research interests include time- and frequency-domain analysis of power electronic circuits, switched-capacitor-based converters, randomswitching techniques, digital audio amplifiers, and soft-switching converters. He has authored two research book chapters and more than 110 technical papers, including 50 refereed journal papers in his current research areas. He is also currently Chairman of the Council of the Sir Edward Youde Scholar s Association. Dr. Chung was the recipient of the China Light and Power Prize and was awarded the Scholarship and Fellowship of the Sir Edward Youde Memorial Fund in 1991 and 1993, respectively. He is currently an IEEE Student Branch Counselor. He was Track Chair of the Technical Committee on Power Electronics Circuits and Power Systems of the IEEE Circuits and Systems Society in 1997 and He is presently an Associate Editor of the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS PART I: FUNDAMENTAL THEORY AND APPLICATIONS.