ENERGY EFFICIENCY EVALUATION OF VOLTAGE

CONTROL AND FREQUENCY CONTROL OVER AN

INDUCTIVE POWER LINK FOR BIOMEDICAL IMPLANTS

Pablo Aqueveque, Marcial Saez and Rodrigo Rosales

Electrical Engineering Department, University of Concepcion, Concepcion, Chile

Keywords: Efficiency, Implanted Devices, Inductive power Link, Wireless Power Transmission.

Abstract: This article presents the analysis of the efficiency of two control systems used to regulate the DC voltage in

an implanted device fed by an inductive power link. Both control systems work outside the body,

eliminating the voltage regulator in the implanted circuit (inside the body). These ways of voltage control

reduce the power and heat dissipated inside the body. The first control system involves regulation of power

supply voltage to the high frequency amplifier. The second control system adjusts the frequency of the

inductive link. A laboratory prototype was built and experimental results were obtained. It is shown that for

a range of distance between 0 mm and 11.8 mm the efficiency of the system is greater when using

amplitude voltage control. Above that distance, the efficiency of frequency control is better. A difference of

20% was obtained in the optimal points.

1 INTRODUCTION

Over the past few decades, the inductive powering

link has been studied and used for transmitting

energy wirelessly to Implantable Microelectronic

Devices (IMDs), such as cochlear prosthesis, visual

prosthesis, cortical neuromotor prosthesis and

implantable sensors, to name just a few. This kind of

transmission energy avoids wires crossing the skin,

which have a high risk of infection or other

problems for the patient. It is also used to avoid

placing rechargeable batteries inside the body,

because they have a limited lifetime and will require

new surgery to replace them (Ma et. al., 2010).

A basic remote powering link scheme has four

main parts: A high frequency power amplifier, an

inductive link, a high frequency rectifier, and a

voltage regulator. MOSFET based Class D and

Class E amplifiers are the most used high frequency

power amplifiers because of their higher efficiency

at high frequencies (Atluri and Ghovanloo, 2006). In

the inductive link, the common resonance topology

is a series L-C tuned primary circuit and parallel L-C

tuned secondary circuit due to its high voltage gain

and efficiency characteristics (over 80%) at low

coupling ranges (Ali, Ahmad and Khan, 2009). The

distance between coils varies the mutual inductance

between primary and secondary coils, and hence, the

coupling coefficient (k); therefore, the choice of

shape, number of turns, diameter and size of the

primary and secondary coils allows a good position

tolerance. The high frequency rectifier is normally

half or full bridge with very fast switching Schottky

diodes (Dissanayake et. al., 2009).

External batteries are used to feed the wireless

power transmission for implanted devices. These

applications need to have a small size and light

weight external control box for patient mobility and

comfort. Battery lifetime must be maximized in

order to give the patient more autonomy without the

need of frequent changing the external battery.

Most of the implantable electronic devices need

a constant supply voltage to work properly, which

means that the voltage on the secondary coil has to

be regulated for some kind of control system that

produces a constant voltage. There are two main

approaches: Magnitude voltage control and

frequency control. The first one varies the

magnitude of the power supply voltage that feeds the

high frequency amplifier, in order to vary the power

delivered to the inductive link (and consequently to

the load) (Si et. al., 2007). And the second one

varies the frequency of the voltage that feeds the

inductive link, moving the frequency value around

the resonant situation (Si et. al., 2008).

359

Aqueveque P., Saez M. and Rosales R..

ENERGY EFFICIENCY EVALUATION OF VOLTAGE CONTROL AND FREQUENCY CONTROL OVER AN INDUCTIVE POWER LINK FOR

BIOMEDICAL IMPLANTS.

DOI: 10.5220/0003792403590363

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 359-363

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 1: Four main parts of the circuit of the system.

Both control systems have the goal of maintaining a

constant voltage at the load, improving the overall

reliability of the system.

This paper analyzes the inductive link efficiency

of both control systems when the distance between

the primary and secondary coils is changed.

2 INDUCTIVE POWERING LINK

When analyzing the inductive powering link it is

necessary to consider several parts.

2.1 System

The system used in this study consists in four main

parts (showed in figure 1): High Frequency power

amplifier, inductive link, high frequency rectifier,

and load. A detailed description of each part is

presented below.

2.1.1 Power Amplifier

There are several classes of power amplifiers (A,

AB, B, C, D and E) that can be used. Class D and

class E power amplifiers are the most used for

inductive links, because of their high efficiency,

over 80%. In this paper, a Class D Power Amplifier

based on push-pull Mosfet’s configuration is used.

The amplifier works at 1MHz and is fed by a

variable 30V power supply. The schematic circuit is

shown in figure 2.

2.1.2 Inductive Link

The inductive link is composed of the external

primary coil and the implanted secondary coil. There

are different options of circuit configurations, where

the resonance topology is the most effective option,

because it makes the final circuit band limited,

improving the voltage gain and link efficiency.

Studies proposed that the series-tuned primary and

parallel-tuned secondary circuit is the ideal

configuration because of its good displacement

tolerance, high voltage gain and efficiency

characteristics at low coupling ranges (Ali, Ahmad

and Khan, 2009).

Figure 2: Power amplifier, driver and series tuned primary

coil.

We decided to work with circular coils. The

parameters of the resultant inductive link are listed

in table 2.

Table 1: Parameters of the series tuned primary and

parallel tuned secondary coils.

Parameters Primary Secondary

Number of turns 30 20

Coil Diameter 50 mm 50 mm

Wire Diameter 0.28 mm 0.28mm

Inductance 71 μH 42 μH

Resistance 1.1 Ω 0.7 Ω

2.1.3 Rectifier

A simple half-wave rectifier is used to rectify the

AC signal generated on the implanted tuned coil.

This topology is appropriate to this application

because it has less consumption than a full-wave

rectifier (because it has only one diode). The

rectifier is composed of a 1N5819 Schottky diode.

The high frequency of the inductive link (1MHz)

reduces the size of the parallel capacitor used as a

filter, in order to have a continuous voltage with an

acceptable ripple (less than 10%).

2.2 Model of Inductive Link

We need to consider model of the inductive link, in

order to gain more knowledge about the coupling

coefficient, critical coupling, voltage transfer ratio,

and efficiency of the link.

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

360

2.2.1 Coupling coefficient and Critical

Voltage Transfer Ratio

The coupling coefficient of the system is defined by

(1).

=









∙



(1)

Where k is the coupling coefficient of the

inductive link, M is the mutual inductance between

the two coils, L

is the self-inductance of the

external coil (primary), and L

is the self-inductance

of the implanted coil (secondary).

The inductive link frequency was selected

considering the limitation on power density within

the body (<80mW/cm

) (Zumsteg, 2004) in order to

avoid tissue damage. In this case the frequency used

was 1MHz. In resonance condition, the resonant

frequency (ω

) of the system is defined by (2)











∙









∙



(2)

2.2.2 Definition of Overall Efficiency

The model presented in (Donaldson and Perkins,

1983) shows that the transmitter has a resistance R

and the receiver has a resistance R

, which means

that in every stage there are power losses, which

change the overall efficiency (η

) of the system. This

efficiency can be expressed by (3).













+



+



+



+



+



+



(3)

Composed of the power consumption off the

oscillator (P

), dissipation in ouput stage not

accounted by the oscillator (P

), and the power

consumption dissipated in circuits; on the transmitter

side: by the power in the resistance of the power

supply voltage (P

), power in the resistance of the

primary coil (P

), and in the receiver side, by the

power in the resistance of the secondary coil (P

the power in the rectifier (P

), and the useful power

3 CONTROL METHODS FOR A

REGULATED VOLTAGE

The control of the load voltage is important to obtain

a steady output voltage during coupling variations

between the primary and the secondary coil, which

could be used as the power supply to some

implanted circuit. This can be done by two main

methods: Amplitude voltage control and frequency

control. The amplitude voltage control and

frequency control can be produced in several ways.

One of the proposed options is controlling from the

external system (Silay, Dehollain and Declercq,

2010), and the other one is controlling from the

internal circuit (Donaldson, 1985). For simplicity,

we used the first one in this paper.

Figure 3: Inductive link prototype for measurements.

4 OVERALL EFFICIENCY OF

THE SYSTEM AND

EXPERIMENTAL RESULTS

The overall efficiency of the system was empirically

established by using two control methods.

Amplitude voltage control and frequency control, for

a fixed voltage at the load of 5V. The position of the

primary coil was fixed, and the distance of the

secondary coil was incremented by 2 mm for every

measurement, in order to see the difference of

efficiency produced by the change of the distance

between the coils. The circuit and instruments used

are shown in figure 3. All the measures were done

with a 100MHz digital oscilloscope and computed in

Matlab.

4.1 Measurements with Amplitude

Voltage Control

The voltage controller varied the supply voltage of

the primary circuit to maintain a constant load

voltage at the secondary circuit for each distance,

with a constant frequency of the system of 1 MHz.

The measurements started 0 mm between the coils,

and were increased until 25 mm, with measurements

every 2 mm. The input voltage shows an exponential

behavior from 5V to 32V with respect to the

distance, which means that it is not possible to reach

a greater distance between the coils, because it

would supply an input voltage greater than the

maximum voltage permitted by electronic

components.

ENERGY EFFICIENCY EVALUATION OF VOLTAGE CONTROL AND FREQUENCY CONTROL OVER AN

INDUCTIVE POWER LINK FOR BIOMEDICAL IMPLANTS

361

4.2 Measurements with Frequency

Control

The frequency controller varied the frequency of the

system from 1.28 MHz to 1.05 MHz in order to

maintain a constant load voltage at the secondary

circuit for each distance, with a constant supply

voltage of 7 V. The measurements started at 0 mm

between the coils, and were increased until 35 mm,

with measurements every 2 mm.

Figure 4: Efficiency of the amplitude voltage control and

frequency control at each distance.

It was not possible to maintain a constant output

voltage with a value of 5V for a distance between

the coils was greater than 35 mm.

4.3 Efficiency of Both Control Systems

The overall efficiency of both control systems was

measured in order to compare the two ways of

maintaining a constant output voltage at a certain

value. Figure 4 shows that the efficiency of the

amplitude voltage control is greater at distances

between 0 mm and 11.8 mm, and over that distance,

the efficiency of the frequency control is greater.

The amplitude voltage control has a greater

maximum voltage, but is more sensitive to distance

changes. On the other hand, the frequency control

has fewer variations of efficiency at different

distances, but its maximum efficiency value is less

than the maximum efficiency of the amplitude

voltage value.

5 CONCLUSIONS

An analysis of the efficiency of two control systems

used to regulate the DC voltage in an implanted

device fed by an inductive power link was presented.

Both control systems work outside the body,

eliminating the voltage regulator in the implanted

circuit (inside the body). These ways of voltage

control reduce the power and heat dissipated inside

the body. The first control system regulates the

power supply voltage and the second adjusts the

frequency of the inductive link. Experimental results

show that the efficiency of the system is greater

when using amplitude voltage control for a range of

distance between 0 mm and 11.8 mm. Above that,

frequency control is better. A difference of 20% was

obtained at the optimal points.

ACKNOWLEDGEMENTS

This paper was developed with support of

postdoctoral project 3100136 of the Chilean Fund

for Science and Technology Development

(FONDECYT).

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INDUCTIVE POWER LINK FOR BIOMEDICAL IMPLANTS

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