The Major Advantage of Series-Mounted Diode - Research Paper Example

Wireless Energy Harvesting: 3 different topologies of Rectennas By Department January 9th, Three Different Topologies of Rectennas The last couples of decades have been characterized by massive development in wide range of portable electronic devices like smart phones and wireless sensor networks. In order to increase their portability and integration in environment, a continuous trend in miniaturization of these devices has been observed. Owing to their mobile nature, the majority of these devices use batteries for their energy source; however for small compact devices these batteries occupy most of the available volume of a device. Since batteries need to be periodically replaced and recharged therefore the miniaturization and autonomy of these devices are limited to the trade off on batteries regarding the size and power density (Agbinya 2012). To improve the portability, the availability and user-friendliness of these mobile electronic devices, one approach is to frequently recharge the device battery by using wireless power transfer (WPT) technique. The WPT is carried out in three stages namely; DC-RF conversion, RF propagation and RF-DC conversion as shown in figure 1. The following discussion is focused on analysis and simulation of various low-power topologies of RF-DC conversion stage. The simulation is carried out by the application of OrCad PSPICE as shown in (simulation screen shots) in simulation section. . The RF-DC conversion is the third stage of WPT. The basic unit that carries out RF-DC conversion is known as “Rectifying Antenna” or in short “Rectenna” as shown in schematic diagram in figure 2. Single series-mounted diode rectenna The most common and simplest configuration of rectanna is single series mounted diode (D1) topology as shown in figure 3. The configuration structure is based on a single diode connected in series with RF signal source. The series mounted diode acts as half-wave rectifier. During negative half as D1 blocks the input sine wave, at this occasion the input filter is charged. Through the positive half-wave, D1 is in conduction mode and allows energy to move from the source to the input filter towards the output filter, which will in turn block the high frequency harmonics from reaching the load. The major advantage of series mounted diode is that it minimizes diode loss, which is proportional to the diode junction resistance. The single series mounted diode rectenna topology is normally dedicated to low power levels (below 0 dBm), where the capability of power handling can be traded with high sensitivity (Agbinya 2012). It is important to note that rectenna circuits’ behavior is non-linear in single series diode mounted topology due to the diode rectification behavior. Practically it is not possible to design sub-parts independently namely due to their mutual interaction and dependency among each other. The load of the input filter highly depends on the diode and the output filter at the diode extremity. Global circuit optimization is used for dimensioning the passive component of filter elements. The efficiency for single series diode mounted rectenna topology is usually measured with respect to the rectenna load at -5 dBm of power input. Maximum efficiency is obtained for 2.4 kΩ when the output voltage is 620mV. The process can be indicated in figures 4 and 5. The graphical representation given in figure 4 and 5 show single series-mounted diode rectenna traces the evolution of RF-DC energy conversion efficiency as a function of the incident RF power level. The rectenna load has been tuned to give the maximum power point efficiency of a factor of 5 between -5 dBm and 0 dBm (1 mW) of incident power. When the power is lower, efficiency is also lower since the threshold voltage of the diode which is comparable to the amplitude of the incident signal. On the other hand the power levels are high, the internal losses of the diode becomes significant due to in series resistance of the diode. Single shunt-diode mounted Rectenna Topology The figure (6) above indicates a single shunt mounted rectenna topology; the diode in this case is parallel between the two input and output filters, where the anode is connected to the electrical ground. This makes the diode to be directly polarized by the DC voltage. During the negative half wave, diode (D1) is conductive mode and the input filter is charged. During the positive half wave, (D1) locks the energy flow from the source and the input filter towards the output filter – a similar analogy as to the series-mounted diode structure. The advantage of shunt-mounted diode rectenna topology is that it minimizes diode loss, which is proportional to the diode junction resistance (Shams and Ali 2007). For the Shunt mounted diode rectenna structure the input is generally applied in 0-20 dBm range as shown in figure 7. The corresponding power levels make the threshold voltage to have a lesser impact on the performance of the circuit. The objective of this configuration is to minimize the internal loss of the rectifier diode used in rectenna structure. The input and output filters should be dimensioned using the same optimization technique as for single series mounted diode rectenna topology. In such situation the structure can attain maximum conversion efficiencies of about 70% for an input power of +15 dBm. There is a similar dependency with respect to the impendence of the load, which has an optimal load of 750 Ω for +15 dBm input power at a frequency of 2.45 GHz (Agbinya 2012). For shunt-mounted rectenna configuration the maximum efficiency level is attained against +15dB input power and conversion efficiency exhibits dependency with respect to load impedance as shown in Pin vs. efficiency curve. It implies that shunt-mounted rectenna configuration with a particular resistance, acts as a voltage source. Compared to single series mounted diode rectenna the shunt mounted diode exhibits a low efficiency for low power levels. Additionally by increasing the power levels above 15 dBm the efficiency will decrease rapidly due to the internal diode ohmic loss. Diode Bridge Additional to the low frequency rectification, an important application of the diode bridge (as shown in figure 8) is its use as rectenna in RF-DC conversion. The principle of full length rectification entails the reinstitution of the complete incident wave at the load level complementary to half-wave rectifiers with characteristics whereby the load is disconnected for half the time from the source. The key benefit of diode bridge topology is its grater power handling capability due to application of diodes as bridge. However, the application of diode bridge topology is limited as it cannot be adapted to low incident power levels. At low power level the input signal has to overcome the three thresholds thereby increasing the loss. For high RF-DC conversion by diode bridge rectenna, the RF signal received through antenna is converted into effective voltage supply. The part of the incoming signal power is converted to DC by the rectifier for the power supply. The behavior of the diodes creates a supporting environment for the rectifier to rectify the signal based on the level of the input signal. Hence the diode either allows or prevents the passage of current. A basic diode bridge rectifier is made up of two distinct levels having capacitor connected either in series or parallel. When the rectifier is connected in series and then connected by a parallel diode, the resulting rectifier configurations are observed as shown in figures 8 (a, b, c). The output voltage from the energy harvesting circuit is highly influenced by the different stages of the rectifier. The relationship between the two is directly proportional such that the output voltage of the energy is proportionate to the stage level of the harvesting process. It implies that the efficiency of a rectifier can be increased by the number of stages the process is comprised of the configuration. Since as a rectifier with optimum stages, has to produce sufficient output voltage, many stages lead to dampening the effect of the high Q-resonator while few stages have proved to produce a low level of output voltage. Hence the number of stages should be regulated at an optimum level in order to ensure sufficient energy harvesting through the various stages as shown in the figures 9 (a, b). The multiple stage topologies impede energy harvesting capability of the circuit hence affect the efficiency of the RF-DC conversion as well as load impedance. Additionally the application of diode bridge topology has limitations as it cannot be adapted to low incident power levels when the incident power is low. Hence while designing diode bridge rectenna for energy harvesting all such factors are overcome by using a fast switching diode such as the Schottky diode which is preferred due to its use of a metal-semiconductor junction that ensures that the junction operates a very fast pace thereby leading to an increase in output voltage (Agbinya 2012; Schmitz and Eles 2003). Simulation The simulation of above discussed rectenna topologies is obtained by using OrCAD PSPICE as shown in following screen shots (figures 12 – 19) along with their circuit diagrams. The type of rectifier is convectional full-wave and simple rectifier with peak detection. The full-wave rectifier transforms the negative cycles of the alternating current to the positive. The peak detector reduces the ripples while creating a direct current output. For all three topologies the efficiency measurements are carried out by a comparison between input and output parameters (voltages) ratios of the circuit. The graph produced below is the transient of the demonstration of the alternation current and voltage source and the DC rectifier current and voltage. Simulation Results Analysis For single series mounted diode rectenna the peak efficiency (Vout/Vin) of rectenna is concentrated between 40 nsec – 80 nsec (4GHz – 8 GHz) intervals while for shunt mounted diode rectenna the peak efficiency (Vout/Vin) of rectenna is concentrated between 0 - 10 nsec (0Hz – 1GHz) intervals as shown in fig. 11 and 14 respectively. After this time interval a rapid decline in efficiency has been observed after that interval that can be attributed to internal diode ohmic loss. The decline in Vout/Vin value is more obvious in shunt diode rectenna than the series mounted diode that exhibits a good approximation of efficiency curves (figures 5 and 7) as given in discussion section for single series- and shunt mounted diode rectenna topologies. The corresponding power output at load (R1) obtained through simulation are also shown in figs. 12, and 15. The maximum values of output power at load (R1) obtained through single series- and shunt diode rectenna are 100.5µW (- 9.59 dBm) and 16.5 NW (- 0.22 dBm). However, the value of power at load for drops drastically particularly for shunt - mounted diode rectenna topology. For diode bridge rectenna the Vout/Vin vales is obtained between 5nsec – 6 nsec time intervals as shown in figs. 18 and 19. The maximum power value through bridge diode rectanna is concentrated between – 20 dBm to – 18.50dBm intervals as shown in figure 17. Fig. 10 Single series-mounted diode retenna circuit diagram Fig. 11 Vout/Vin (vs. time and frequency) spikes for simulated single series-mounted diode retenna circuit given in fig. 10. Fig. 12 Simulated Power W (R1) at load (R1) for single series-mounted diode mounted retenna. Fig. 13 Shunt diode retenna circuit diagram Fig. 14 Vout/Vin (vs. time and frequency) spikes for simulated shunt diode retenna circuit given in fig. 13. Fig. 15 Simulated Power W (R1) at load (R1) for shunt diode retenna circuit given in fig. 13. Fig. 16 Diode Bridge Retenna circuit diagram Fig. 17 Simulated Power W (R1) at load (R1) for bridge diode retenna circuit given in fig. 16. Fig. 18 Vout/Vin (vs. time) spikes for bridge diode retenna circuit given in fig. 16. Fig. 19 Vout/Vin (vs. frequency) spikes for bridge diode retenna circuit given in fig. 18. References Agbiny, J. I., (2012). “Wireless Power Transfer,” Aaborg. River Publisher. Shams, K. M. Z. and M. Ali, (2007) “Wireless power transmission to a buried sensor in concrete,” IEEE Sensors Journal, vol. 7, no. 12, pp. 1573–1577. Schmitz, M. B. Al-Hashimi, and P. Eles, (2003). System Level Design Techniques of Energy-Efficient Embedded Systems. Kluwer Acad. Publishers. Read More