RF-DC Converter Efficiency OptimizationUsing Source-Pull Techniques

Luis Filipe Ribeiro Dias, Alírio Boaventua and Nuno Borges de CarvalhoInstituto de Telecomunicações, Dep. Electrónica, Telecomunicações e Informática, Universidade de Aveiro

Abstract—In this paper a source-pull approach is followed tooptimize RF-DC converter efficiency in Wireless Power Transmis-sion/Wireless harvesters systems.This approach is applied to simpleone stage RF/DC rectifier but also to a voltage multiplier ina Dickson charge pump configuration. The optimization processwas made in two scenarios, while in the first case it was usedto maximize the efficiency of the simple rectifier, in the secondcase, the circuit was optimized to generate higher values of outputvoltage. Both circuits were dimensioned to harvest energy from anemitting an Radio Station, and thus optimized for operation at the100MHz band. The simple experimental rectifier was optimizedfor efficiency and the Dickson multiplier was optimized for outputvoltages maximization.

I. INTRODUCTION

TECHNOLOGY is advancing at a frenetic pace, as ev-eryday breakthroughs are made in different areas, and

innovative products are released to the market, due to the ex-perienced bigger proximity between the investigation and thepublic needs. Energy-aware RF circuits have been followingthis trend, and although they have been used for some time inWSN applications,[1], both for military and civil purposes, aswell as in RFID [2], for tracking, automation, access control orpublic transportation ticketing, an increased interest has roseover the last years in this field.

In fact, large electronics companies are already investing inthis, which can be seen as a good indicator for the feasibilityof the technology in a near future. Nokia for instance [3]is currently developing a battery-free low-power consumingcellphone that harvests from many RF sources from 500MHzto 10GHz, while Intel created a wall-mounted weather stationwith an LCD screen using a TV antenna pointed at a local TVstation 4km away [4].

As of the solutions found in the literature, very interestingresults are presented in the group of papers [5], [6] and [7]were good results for relatively simple circuits are obtained,going over 60%.

The reason for this is in the careful analysis of the involvedpower magnitudes in the circuit, where the reflections aremade minimal by the use of accurately selected matchingcomponents and transmission lines for connecting devices andfiltering the harmonics on the output.

These works, as well as the circuits presented here, haveemployed a Schottky diode, due to it’s low forward voltage,together with valid responses up to higher frequencies.

As of the issue of obtaining higher output voltage valuesit is generally solved employing voltage multipliers, eitheron the Dickson or the Villard configuration, presenting both

similar results for the traditional scheme. A straightforwardapplication of the former is found on [8], were a 5-stagesmultiplier is employed, resulting in V o = 1V for an inputof −16.5dbm, while a more complex approach is presentedin [9], where two resonant structures are added to the systemresulting in more interesting results..

In this article, the first section introduces the motivation andproblematic of the work together with some of the state of theart solutions, the second presents the both circuits studied withsome considerations on it together with the mechanism appliedfor obtaining the matching and the last clarifies the simulationand experimental results obtained for the proposed circuits.

II. SYSTEM DESCRIPTION

In this section the circuit topologies and the source pullmechanism approach will be described.

A. TopologyWithin this paper, two rectifying circuits were considered,

one with only one diode in series representing a simple RF-DC converter, and another one with a more complex voltagemultiplier topology, being depicted in Fig. 1 and Fig. 2,respectively.

Figure 1. Series Diode Rectifier

Figure 2. Dickson Voltage Multipler

In addition to the diode(s) each of the circuits contains afew more components, that will be useful for the optimizationof the input power that gets to the input of the diode(s), that isthe input matching circuit, and some others used for filteringaspects, and also for the charge pump effect in the Dicksonapproach, case of Cc. The values of each component for theinput matching network, were obtained through the use of thesource-pull mechanism which is detailed in the next section.

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B. Source Pull Mechanism

In order to optimize the RF-DC conversion efficiency thecircuit should be optimized for reducing the waste of energy asmuch as possible. One of the first component optimization is toselect the best matching network to maximize the conversionof RF energy from the antenna to the input diode.

In order to achieve this input matching, the sourceimpedance should equal the input impedance complex con-jugate, as in Γs = Γ∗

in, where:

Γin = S11 +S12S21ΓL

1 − S22ΓL(1)

However for real rectifier circuits, generally the inputimpedance is dependent of the input power, which is whathappens for the studied circuits, as can be seen from table I.It should be noted that in this table only the values of S11, orequivalently Γin.

PIN (dBm) S11 (figure 1) S11 (figure 2)-20 0.97∠− 139.7◦ 0.92∠− 8.1◦

-10 0.95∠− 139.7◦ 0.89∠− 10.3◦

0 0.93∠− 139.8◦ 0.87∠− 15.9◦

10 0.91∠− 139.7◦ 0.79∠− 23.1◦

20 0.90∠− 139.6◦ 0.77∠− 30.3◦

Table IS11 FOR BOTH CIRCUITS

In order to obtain the optimum input source load to max-imize the efficiency or the output voltage the source pullmethod can be employed, which works by systematicallyvarying the impedance presented to a Device Under Test(DUT), in this case the circuits 1 and 2, and to assess it’sperformance under this variation.

The source pull approach makes use of a series of equationsand smith charts to plot the information from the source-pullsimulation in constant efficiency and output voltage circles. Itallows both the possibility of obtaining the impedances thatresult in the highest efficiency or output voltage, but also tofind specific constant valued circles of these quantities, whichis not easily done with the traditional matching mechanisms,all this recurring to a very simple and intuitive tool.

Both circuits used this technique to obtain the best resultsfor the optimization of the matching networks in differentperspectives.

III. EXPERIMENTAL AND SIMULATION RESULTS

In this section the results from both the implemented cir-cuits and respective simulations are presented, the source-pullmechanism used for optimizing the simple rectifier efficiencyis presented in more detail, while for the voltage multiplieronly the final results are shown.

A. Simple Rectifier Simulations

Before implementing this circuit, an analysis of the inputimpedance with the objective to optimize the efficiency, asexplained before, and for this a couple of graphics containingconstant valued circles and respective values of the measuredmagnitudes were obtained.

Figure 3. Diode in Series with PIN = −10dBm

Figure 4. Diode in Series with PIN = 0dBm

Figure 5. Diode in Series with PIN = 10dBm

Figures 3, 4 and 5 contain the values of interest, fromwhich it can be seen that: The circles of constant outputvoltage are relatively coincident with the efficiency circles,as would be expected, since the output power is dependent onthe voltage and on the resistor that is maintained constant foreach essay; The constant circles and maximum point values aredependent on the input power, since they rotate their locationfor different PIN ; As the input power increases, the circles ofsame efficiency are less restrictive; The best efficiencies varyfor the different Pin since they also depend on the load andoutput filter tuning.

An alternate study was made by fixing the input impedancetaken from the source-pull mechanism and studying the depen-dence of the efficiency with the load resistor, being picturedon figure 6 for two different matching networks. It should benoted that the location of components shown in the graphics(C1 and C2) is properly identified in figure 1.

The obtained results from these simulations corroboratesome of the made deductions and introduce a few others:

As can be seen the matching network depends on the inputpower, and also the load resistance is highly affected by thematching network, which means that a carefully selectionof both input matching circuit and DC load components isfundamental.

Figure 6. Diode in Series with Different Matching Networks

Having obtained very reasonable efficiencies for the sim-ulations allowed the implementation of a real circuit, whoseresults are presented in the next section.

B. Simple Rectifier Measurements

Fig. 7 presents the measured results, superimposed withthe ideal and worst simulated results cases considering 5%tolerance in the components. The obtained results were notas good as the ones simulated in our opinion due to severalreasons as: Incomplete diode model (without the packageparasitic elements); Considering infinite quality factor in-ductors; Use of roughly approximated components, insteadof exact valued one’s; Poorly characterized subtract (FR-4);Components tolerance excessive (5%);

Figure 7. Efficiency Optimization

From these results a couple of inferences can be taken:The tolerance of the components has a large influence in thefinal results; The experimental results are inside the margin ofuncertainty; Increasing PIN leads to better results, probablyto a better nonlinear behavior approach; Very high efficiencies(≈ 80%) may be obtained in a controlled board development.

C. Voltage Multiplier Measurements

In the case of the voltage multiplier study the source-pullmechanism was used in a different perspective, which con-sisted in analysing the output voltage instead of the efficiency.This was made due to the fact that in certain occasions themaximization of the output voltage is a must.

The final experimental results are presented in Fig. 8.

Figure 8. Voltage Maximization

For this circuit, the following can be concluded; the outputvoltage values are optimized for PIN = 10; the outputvoltage is around 8V , the measured results are below the worstcases due to the fact that the modelling of the concentratedcomponents is very challenging for this type of results.

IV. CONCLUSION

In this paper the application of a source pull mechanism wasapplied to a simple RF-DC rectifier and to a Dickson voltagemultiplier, the measured results were not significant due to thefact that better modelling approaches should be used for theused components.

REFERENCES

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