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Rarely Asked Questions—Issue 162
Enablement of Batteryless Applications
with Wireless Power
By Mark Vitunic
Question:
My application doesn’t have a battery. Is it possible to power it wirelessly?
Answer:
Yes, sure—a simple integrated nanopower solution originally designed for
energy harvesting is available.
A wireless power transfer
(
WPT
)
system is composed of two parts separated
by an air gap: transmitter
(
Tx
)
circuitry with a transmit coil, and receiver
(
Rx
)
circuitry with a receive coil
(
see Figure 1
)
. Much like in a typical transformer
system, ac generated in the transmit coil induces ac in the receive coil via a
magnetic field. However, unlike in a typical transformer system, the coupling
between the primary
(
transmitter
)
and secondary
(
receiver
)
is typically very
low. This is due to the nonmagnetic material
(
air
)
gap.
Transmitter
Circuit
Air Gap
Low Coupling Between Coils
L
TX
Coil L
RX
Coil
R
L
V
IN
Receiver
Circuit
Figure 1. Wireless power transfer system.
Most wireless power transfer applications in use today are configured as
wireless battery chargers. A rechargeable battery resides on the receiver
side and is charged wirelessly whenever in the presence of a transmitter.
After charging is complete, and when the battery is subsequently taken off
of the charger, the rechargeable battery then powers the end application.
Downstream loads can connect either directly to the battery, indirectly to the
battery through a PowerPath
™
ideal diode, or to the output of a battery-
powered regulator integrated into the charger IC. In all three scenarios
(
see
Figure 2
)
, the end application can run either on or off the charger.
But what if a particular application does not have a battery at all, and
instead what is desired is just providing a regulated voltage rail when wire-
less power is available? Examples of such applications abound in remote
sensors, metering, automotive diagnostics, and medical diagnostics. For
example, if a remote sensor does not need to be continuously powered, then
it doesn’t have to have a battery, which would either need to be replaced
periodically
(
if it is a primary cell
)
or charged
(
if it is rechargeable
)
. If that
remote sensor only needs to give a reading when the user is in the vicinity
of it, then it can be wirelessly powered on demand.
(a) (b) (c)
L
RX
C
RX
C
IN
4.7 µF
R
PROG
953 Ω
I
CHARGE
ACIN
BAT
PROG
LTC4123
GND
V
CC
CHRG
NiMH
1.5 V
+
To
Loads
L
RX
C
RX
Li-Ion
4.2 V
100 kΩ
V
CC
V
CC
I
CHARGE
ACIN
ACIN
DCIN
VSEL2
V
SEL
LBSEL
PRECHG
VSEL1
ISEL1
ISEL2
BAT NTC
OUT
BAT
ACPR
STAT2
STAT1
EN
NTC
LTC4124
LTC4126
GND
GND
+
To
Loads
CHRG
CHRG
L
RX
C
RX
I
CHARGE
Li-Ion
4.2 V
+
To
Loads
Digital I/O
2.2 µF
1.2 V
Figure 2. Wireless Rx battery chargers with downstream loads connected to a) a battery, b) a PowerPath ideal diode, and c) a regulator output.
1
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Analog Dialogue 53-02, February 2019
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