8. Basic Technologies for Batteryless Power Supply
8.2 Inductive power transmission
A system for inductive power transmission is basically composed of two components:
• Reader, intended to generate a low frequency magnetic field and to read/elaborate information.
• Transponder (tag), activated by the magnetic field and able to send back data to the reader.
Due to the low operating frequency, wavelength is very high (2.4 km @ 125 kHz and 22 m @ 13.56 MHz). If the distances are much lower than the wavelength of the signal, a near field
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magnetic inductive coupling is achieved. Both, a reader and a transponder are equipped with antenna coils and electronic circuits. In particular, the reader comprises an AC voltage generator (usually square wave, either half or full bridge) which supplies a series resonant circuit consisting of the antenna coil and a capacitor. The resonant circuit is tuned at the operating frequency.
Electronic means are provided in order to demodulate sent back data as resonant current variations.
The transponder comprises of a parallel resonant circuit which is also tuned at the operating frequency consisting. This resonant circuit consists of its antenna coil and a capacitor. Rectifying means are used in order to provide DC power supply for the tag which is represented by the load in Figure 8.2-1. For data transmission to the reader a digitally controlled switch is used to load the secondary resonant circuit. The digitally controlled switch is a part of the tag (see Chapter 7).
A block diagram of an inductive transmission system consisting of a reader (base station) and a transponder (batteryless system) is presented in Figure 8.2-1. [IPT01 – IPT07]
Base-station (e.g. vehicle)Base-station (e.g. vehicle) Battery-less system (e.g. tyre) Coil
driver
Primary antenna and series resonant
capacitor
Secondary antenna and parallel resonant
capacitor
Rectifier
bridge Energy storage
Load
Inductive coupling
Figure 8.2-1: Block diagram of inductive transmission system.
Some characteristics of an inductive system can be summarised as follows:
• Distance is limited to the ranges of the lines of force emitting from a magnetic field generator.
• To spread the lines of flux the antenna sizes need to be large.
• 125 kHz operating tags have numerous turns of fine wire on ferrite rod. The practical distance is in the order of few centimeters.
• This kind of transponder is used in immobilizer anti-theft systems.
• 13,56 MHz operating tags have few turns etched on flexible printed circuit substrate to which a single chip is bonded. The practical distance is in the order of tens of centimeters.
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The equivalent circuit diagram of an inductive power transmission system is shown in Figure 8.2-2.
Figure 8.2-2: Principle scheme of inductive power transmission.
For a flat circular coil in free air the following equation for calculation of H-field (H magnetic field strength) can be used:
From this relationship two considerations can be immediately obtained:
• for d >> rR field decreases as d3, so this kind of operation is not efficient at long distances and
• at a given distance (d), a maximum field exists for rR = √ 2 d .
Reader Transponder
NR turns rR
d H
IR
NR IR rR2 H =
2 (rR2 + d2)3/2
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For an efficient power transmission reader coil radius must be in the order of magnitude of the distance. This condition is practical for ranges of few centimeters (as for the immobilizer application in which the transponder is in the keyfob and the reader coil on ignition block).
H-field values for 1A turns (rR = 5 cm, 10 cm, 15 cm) are shown in Figure 8.2- 3:
Figure 8.2-3: H-field depending on distance (air gap).
Figure 8.2-4 shows H-field peak values for a distance of 10 cm as a function of reader coil radius:
Figure 8.2-4: H-field peak values for a distance of 10 cm.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
radius (m)
H (A/m)
0 0.05 0.1 0.15 0.2 0.25 0.3
0 1 2 3 4 5 6 7 8 9 10
distance (m)
H (A/m)
r= 5 cm r=10 cm r=15 cm
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For a low coupling factor (Kcou) an approximation of the peak transponder voltage (VT) can be calculated by the following equation:
The parameters in this equation are the reader and transponder coils inductance values (LR) and (LT), coupling factor (Kcou), reader coil voltage (VR) and transponder quality factor (QT).
The equation for the peak transponder voltage (VT) is valid for the following assumptions: free air conditions for transmission, no load and perfect tuning of primary and secondary resonant circuits with operation frequency.
Besides the aspects of a big antenna size described above, a further limitation to the use of the low frequency inductive transponder principle are effects caused by of metal parts close to the system.
Low resistive metals allow the flow of induced eddy currents the effect of which is to generate a magnetic field in opposition to the main one. This effect causes an attenuation of useful field strength and results in a reduction of the possible operating range in terms of distance. Another effect of the same cause is the reduction of self inductance of the reader coil and an increase of its equivalent series resistance.
LT
VT ≈ VR Kcou QT
LR
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The summary of inductive power transmission possibilities in automotive applications is presented in the following table (Table 8.2-1). [IPT01 – IPT07]
Table 8.2-1: Summary of inductive power transmission possibilities in vehicle applications.
Low frequency (rotational transformer) Typical freq. 100-300KHz Two ferrite half cores, the first one located on the vehicle close to the wheel, the second one on the wheel itself. An air gap (as small as possible) exists between the two half cores allowing mutual rotation.
Advantages:
high power transfer capability
fair efficiency (good coupling factor)
possibility of data transfer little influence of surrounding metal parts
no tuning issues Disadvantages:
mechanical issues
cost effective for power level much greater than what estimated for the application
Low frequency (transponder concept) Typical freq. 125KHz Resonant circuits mutually coupled. Receiving coil may be wound on a small ferrite rod. Typically used for few centimeters distance range.
Advantages:
low cost driving electronics possibility of data transfer (backscattering)
well established concept
Disadvantages:
achievable distance limited by generator coil size very low coupling factor high influence of surrounding metal parts
critical tuning
high turn number required
Medium frequency (transponder concept) Typical freq. 13.56MHz Resonant circuits mutually coupled. Coils usually realized by means of copper tracks on flexible substrate.
Distances in the order of 1 meter achievable.
Advantages:
flat design
possibility of data transfer (backscattering)
low turn number required well established concept
Disadvantages:
achievable distance limited by generator coil size very low coupling factor high influence of surrounding metal parts
critical tuning
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