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RF Rectifiers with Voltage Doubler for Wireless
Implantable Devices
Doha H. Hussein1, a) and Ahmed M A Sabaawi1, b)
1 Electronics Engineering Dept., College of Electronics Engineering, Ninevah University, Mosul, Iraq
a) doha.hasan2020@stu.uoninevah.edu.iq
b) Corresponding author: ahmed.sabaawi@uoninevah.edu.iq
Abstract. In this paper, single- and double-stage RF rectifiers operating at 433 MHz and 915 MHz for wireless implantable
devices were designed. The designed circuits were simulated by using ADS simulation tools. The circuit is simulated on a
FR-4 substrate layer with dielectric constant of 4.1 and a thickness of 1.6 mm. In addition, microstrip lines were added to
the circuit to connect the circuit elements and mimic the real case and achieve more accurate results. The results showed
that the achieved output voltage of the singe stage rectifier was around 3.5 V with a total conversion efficiency up to 60 %.
On the other hand, the 915 MHz rectifier was able to provide an output voltage of more than 1 V and a conversion efficiency
of 7%. The contribution of this work is the attempt to compare the performance of the same RF rectifier but at different
frequency bands.
Keywords. RF rectifiers, implantable device, wireless powering, far-field power transfer, voltage Doubler.
INTRODUCTION
Due to the rapid progress of technological approaches and instruments, our world is evolving at a breakneck pace.
Human requirements in various living styles, such as health and leisure, serve as the foundation for such progress.
There is no doubt that health takes priority over most other necessities because it is directly related to human life. It
has a thriving market and attracts a huge amount of research investment. As a result, industrial and academic
researchers are working hard to design and manufacture health instruments and medical gadgets that are more
dependable, safer, and convenient. Medical equipment can be planted in the human body for a variety of purposes,
including monitoring, medicine delivery, and activation. In 1960, a pacemaker became the first implantable device to
work successfully inside the human body [1]. Inductive coils and radio frequency (RF) antennas are the two most
common methods for establishing communication linkages for biomedical implants. Through inductively connected
coils, the working frequency bands range from a few kHz to many MHz. Within the near field zone, the communication
range between the inductively coupled implanted device and the programmer is limited to very few centimeters [2, 3].
In response to a petition from Medtronic, the US Federal Communications Commission (FCC) awarded the Medical
Implant Communication Service (MICS) band of 402–405 MHz in 1999. This will allow implantable devices to
communicate via a mobile wireless device. The RF spectrum can be used to enhance the range of a communication
link, [4].The regulation limits the equivalent isotropically radiated power (EIRP) of MICS devices to -16 dBm in order
to prevent interference between these devices [5].The European Telecommunications Standards Institute (ETSI)
eventually adopted this frequency in 2002, making it the target spectrum for implantable antenna designers [6]. In
2009, the Medical Device Radio Communication (MedRadio) band (401-406 MHz) was added to the MICS band.
The MICS band, on the other hand, remains the heart of the MedRadio system, and it is allocated for implantable
device transmission only, with a channel bandwidth of 300 kHz. The industrial, scientific, and medical frequency
bands (ISM, 433–434 MHz, 902–908 MHz, 2.4–2.48 GHz, 5.715–5.875 GHz), and the wireless medical telemetry
service frequency band (WMTS, 1.395–1.4 GHz). In addition, certain foreign locations have approved ultra-wideband
frequency bands (UWB, 3.1–10.6 GHz) for high-quality communication [11]. Primary batteries are used to power
most implants. Non-rechargeable batteries with a predetermined lifetime, for example, power deep brain
neurostimulators and pacemakers. These batteries have a lifespan of 5 to 7 years, depending on the device's function
when the implantable device's battery life expires, it must be surgically replaced, which comes at a significant expense
and poses a danger of infection to the patient. Infection rates associated with pacemaker replacements range from 1
percent to 19 percent [6]. Furthermore, several implantable devices that work in direct contact with infected, such as
pH and glucose sensors, cannot use batteries due to the risk of poisoning in the event of a leak [7]. Batteries take up
more than 50% of the volume of some of the other gadgets. The usage of such batteries as an alternate solution results
in a significant reduction in implant size. It also ensures the implant's long-term usage and is cost-effective.
Information on the power consumption of medical devices is extremely valuable in determining the best power
sources. The usual power consumption of an implanted device ranges from tens to hundreds of milliwatts. The
pacemaker, for example, requires between 10 µW to 70 µW under typical conditions [7, 8].The nerve stimulation
equipment utilizes 100 µW [9], while the glucose monitoring system uses 48 µW [10]. Several designs were presented
in literature employing numerous different approaches with the aim to increase the RF-DC conversion efficiency [11-
15] In this work, two rectifier circuits will be designed and simulated at frequencies 433MHz and 915MHz, which are
considered within the range of frequencies allowed by the World Health Organization with the aim to increase the
system conversion efficiency. The simulations will be carried out by using ADS (Advanced Design System) program.
SINGLE STAGE RECTIFIER AT 433 MHZ
A single stage rectifier is designed and simulated by using ADS as shown in Fig.1. The circuit consists of a power
source, which represents antenna part in such systems. The power source feeds the circuit with 10 dBm AC power at
433 MHz and has a 50 Ω internal impedance. The rectifier circuit also has a matching network circuit, which consists
of capacitance and inductance that is selected to match the rectifier circuit with the antenna. There is also HSMS2028
diodes were used, which works to convert the signal from AC to DC. The series capacitance (68 pF) at the input acts
as voltage doubler [16]. In addition, the output contains a capacitance for smoothing the DC output to be stored in a
battery or to feed the load as DC power. It is worth mentioning that the circuit below is supposed to be printed on a
FR-4 substrate with a dielectric constant of 4.1 and a thickness of 1.6 mm. The substrate was chosen because it is low-
cost and obtainable.
FIGURE 1. Single stage rectifier operating at 433 MHz.
The waveforms of the designed single stage rectifier at 433MHz is shown in Fig.2. The figure illustrates the
input voltage, output voltage and the load current. It can be clearly seen from Fig.2 that the achieved output voltage
is more than 4 V with an obvious ripple that can be easily eliminated by using a better smother. In addition, the
recorded output current was around 1.9 mA. The output current can be adjusted by changing the load value.
FIGURE 2. The waveforms of single stage rectifier at 433MHz, a) input voltage, b) output voltage and c) load current.
In addition to the value of Vout and its quality, the performance of the rectifier is measured by the conversion
efficiency. The impact of the input power (Pin) on the output voltage and the efficiency has been studied. Fig.3 shows
the relationship between Pin (dBm) and output voltage (Vout) of single stage at 433MHz.
FIGURE 3. Pin (dBm) and Vout (V) of single stage rectifier at 433MHz.
It can be seen from Fig.3 that the output voltage increases exponentially when the input power increases. Thus it
gives an indication that the system performance can be improved dramatically when the input power increase mildly.
In other words, changing the input power from 0 dBm to 10 dBm can increase the output voltage from 1 V to 3.5 V.
Furthermore, Fig. 4 demonstrates the effect of the input power on the rectifier conversion efficiency. The results
showed that increasing the input power leads to increase the conversion efficiency. It is observed that changing the
input power from -10 dBm to 10 dBm has increased from 14% to 60%.
FIGURE 4. Efficiency versus input power of single stage rectifier at 433MHz.
(a)
(b)
(c)
(dBm)
(dBm)
%
DOUBLE STAGE RECTIFIER AT 433 MHZ
A double stage rectifier is also designed and simulated by replicating the previous single stage rectifier and
connecting both circuits to one source and one load but with reversed diodes as shown in Fig.5. The waveforms of the
double stage rectifier are shown in Fig.6. A zoom in focus is clearly shown in the curve of output voltage (Fig.6 b) to
show the voltage ripple, which is observed to be around 1 mV.
FIGURE 5. Double stage rectifier at 433 MHz.
FIGURE 6. The waveforms of double stage rectifier at 433MHz, (a) input voltage, (b) output voltage and (c) load current.
The relationship between the input power and the output voltage for the double stage rectifier at 433MHz is illustrated
in Fig.7. The result in this figure showed that increasing the input power leads to an increased output voltage in a way
similar to the single stage rectifier. Same observation applies to the relation between the input power and as can be
seen from Fig.1, rectangular microstrip lines were added to the circuit in to connect the elements on the printed circuit
board (PCB). The substrate details were added to the simulations in order to add the required reality to the simulations
and to achieve more accurate results. Thus, a correct width of the strip lines must be selected that matches the
impedance of the source (50 Ω) on this substrate and at this specific frequency (433 MHz). A study to determine the
effect of the strip line width on the overall performance is conducted and the result is shown in Fig. 9. It can be seen
from Fig.6 that the best width that achieves highest efficiency is 3 mm, which exactly matches the number found by
theoretical calculations.
(a)
(b)
(c)
FIGURE 7. Pin (dBm) and Vout (V) of double stage rectifier at 433MHz.
FIGURE 8. Efficiency versus input power of double stage rectifier at 433MHz.
FIGURE 9. Efficiency versus strip line width (mm) at 433 MHz.
SINGLE STAGE RECTIFIER AT 915 MHZ
A single stage rectifier operating at 915 MHz is designed and simulated as shown in Fig. 10. An AC power source
operating at 915 MHz is feeding the circuit and is mimicking a microstrip antenna designed to operate at 915 MHz.
The waveforms of the single stage rectifier at 915 MHz are shown in Fig.11. The results showed that a less output
voltage and hence a lower efficiency are achieved due to the higher frequency and the losses associated with. In
addition, changing the frequency to a higher value could alter the impedance matching condition of the circuit and
leads to higher mismatching losses.
%
(dBm)
%
(mm)
(dBm)
FIGURE 10. Single stage rectifier at 915 MHz.
FIGURE 11. Waveforms of the single stage rectifier at 915MHz, (a) input voltage, (b) output voltage and (c) load current.
By looking at the relation between the input power and the output voltage as well as the relation of the efficiency
with the input power for the single stage rectifier at 915 MHz, it can be seen that the obtained output voltage was
around 1V only leading to a conversion efficiency not more than 2.5% as illustrated in Figs. 12 and 13. It is thus
concluded that the single stage rectifier failed to provide a good output voltage and efficiency. This gives a motivation
to try out the double stage rectifier in order to increase the output voltage and improve the system performance.
FIGURE 12. Pin (dBm) and Vout (V) of single stage rectifier at 915MHz.
(a)
(b)
Pin
Vout
(c)
(dBm)
FIGURE 13. Efficiency versus input power of single stage rectifier at 915 MHz.
For the sake of further improving the system performance in terms of output voltage and conversion efficiency, a
double stage rectifier at 915 MHz is designed and simulated as shown in Fig. 14. Similarly, the single stage rectifier
circuit is replicated but with reversed diodes and the two parts are connected to one source and one load. The double
stage rectifier is expected to achieve higher efficiency.
FIGURE 14. Double stage rectifier at 915 MHz.
The waveforms of the double stage rectifier at 915MHz are shown in Fig. 15. It is noted that the converted DC
voltage is more than 1 V and the recorded load current is around 0.5 mA.
FIGURE 15. Waveforms of the double stage rectifier at 915MHz, (a) input voltage, (b) output voltage and (c) load current.
(b)
(a)
(c)
(dBm)
%
By looking at the relation between the input power and the output voltage as well as the relation of the efficiency
with the input power for the single stage rectifier at 915 MHz as depicted in Figs. 16 and 17, it can be seen that the
obtained output voltage was around 1.2 V when the input power was 10 dBm leading to a conversion efficiency around
7%.
FIGURE 16. Pin (dBm) and Vout (V) of double stage rectifier at 915MHz.
FIGURE 17. Efficiency versus input power of double stage rectifier at 915 MHz
CONCLUSION
In this paper, RF rectifiers for wirelessly powered implanted devices were presented and discussed. The designed
rectifiers are operating at 433 MHz and 915 MHz to cover all available frequency bands dedicated for this application.
Two versions of each circuit (double and single) stages were designed and simulated. The designed rectifiers were
simulated on FR-4 substrate by using ADS simulation tool. The results showed that for both the 433 MHz and 915
MHz rectifiers the input power has a huge influence on the rectifier performance. It was shown that the output voltage
and the conversion efficiency can be increased by increasing the input voltage. The studied range of the input voltage
was from -15 dBm to 10 dBm. The highest recorded output voltage for the 433 MHz rectifier was around 3.5 V while
the highest recorded efficiency was 65%. In contrast, the highest recorded output voltage and efficiency for the 915
MHz rectifier were 1.1 V and 7%, respectively. It worth mentioning that these devices are operating at ultra-low power
and the chosen frequency bands are not harmful to human body. The main contribution of this work was the attempt
to compare the performance of the same RF rectifier but at different frequency bands.
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