Being new to power electronics, I've been going back through the literature to find out how engineers use Advanced Design System for their switched-mode power supplies (SMPSs) and wireless power transfer (WPT) systems. I'm going highlight a selection of these in this series of postings I call Case Studies. The first is a paper from 2009 on WPT. I initially posted this on my LinkedIn blog but with the launch of this new blog, I'm migrating over to this new site. A more polished version is available as a PDF "Ericsson and Freiburg U. Design High Q Power Harvesting Circuit Using ADS."
This Case Study is about a paper titled RF Energy Harvesting Design Using High Q Resonators by Xavier Le Polozec of Ericsson and Tolgay Ungan, William Walker, and Leonhard Reindl of the University of Freiburg. Their goal was wireless power of a couple of hundred nanowatts at 1V to power a sensor node. The transmitted power was at 24 MHz. They used ADS harmonic balance to explore the design space particularly around the innovative impedance transformation using a high Q quartz crystal resonator to transform the low voltage from the low impedance antenna to a voltage high enough to overcome the Schottky diode junction voltage (about 0.3V).
The tricky part here is that government regulations generally limit the transmitted power such that the harvesting antenna only receives at most -30 dBm (1 μW). Prior solutions had only ~10% total efficiency. To power their sensor they needed a conversion efficiency > 20%. They added a high Q quartz crystal resonator to introduce an impedance transformation between the antenna and the rectifier. The antenna impedance was 50 ohms so voltage with 1 μW is only about 7 mV, not enough to forward bias the Schottky microwave detector diodes (Avago HSMS-2862). The transformer boosts the voltage so that diodes, used as rectifiers in a half bridge configuration, are forward biased and conducting on their respective positive half cycles. The challenge was to satisfy the impedance matching requirement when the antenna, resonator, and matching components were loaded with the nonlinear diodes.
The authors used the ADS Harmonic Balance simulator to help design the impedance matching circuit. Unlike SPICE simulators, which typically only handle linear circuits in the frequency domain ("AC analysis"), ADS Harmonic Balance handles nonlinear effects and gives you the periodic steady state solution without having to wait for the startup transient to die down. In contrast ordinary time domain ("transient analysis") requires this time consuming step. They could quickly explore the design space with a set of trustworthy simulations before committing to fabrication. Once the design was optimized, they invested in building the prototype.
“ADS harmonic balance gives us trustworthy simulations before committing to fabrication. We can estimate the influence of each parameter over the global circuit response, tuning and optimizing the circuit before committing to fabrication. We get good correlation with subsequent measurements on hardware.”
-- Xavier Le Polozec, Senior Technical Subject Matter Expert, Ericsson
They achieved excellent correlation with measurement and the circuit delivered the required efficiency. The simulation correctly predicted that the peaks of the family of curves of efficiency versus frequency would shift. The nonlinearity of the diodes cause the peak to shift about 200 Hz as the input power is increased from -50 dBm to -20 dBm. This subtle effect was seen in both simulation and measurement.
An input of only 7 mV rms at 24 MHz (1 μW into the 50 ohm antenna) yielded an output of 1 V DC and a measured overall efficiency of 22.6%.
Are you working on wireless power transfer? Do you face the same challenge? Or something else? Please log in and leave a comment and/or "like" this posting!
PS Here's the link to request an ADS evaluation license if you want to try it.