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 5.856 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, iso- lation amplifier, wideband. I. INTRODUCTION I SOLATION amplifiers are used in many industrial applica- tions. Galvanic isolation is a necessity for many electrical patient-monitoring devices, such as electrocardiographs (EC Gs) and electroencephalographs (E E Gs), to ensure the safety of the patient [1]. Isolation amplifiers used in medical instrument usu- ally transmit a very-low-frequency signal. The EC G is used to monitor the heartbeat and the E E G 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 ac- quire 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, 2001. Abstract published on the Internet October 24, 2001. This work was supported by the Research Grant Council of Hong Kong under CERG Project 9040446. S. C. Tang was with City Universityof 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 (e-mail: eeronhui@cityu.edu.hk). Publisher Item Identifier S 0278-0046(01)10266-2. 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 rela- tively expensive and can only transfer a signal to the receiver. Additional floating power supplies are necessary in both input- and 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 analog- to-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-to- analog 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 temper- ature-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 0278–0046/01$10.00 © 2001 IEEE