Noty AN138fa Linear Technology

Wireless Power User Guide

Trevor Barcelo

Application Note 138

October 2013

OVERVIEW

An inductive wireless power system consists of a transmit
ter that generates a high frequency alternating magnetic
field and a receiver that collects power from that field.
The resonant coupled system described here provides for
increased power transmit distance and reduced alignment
sensitivity, with no need for a coupling core.

To build a wireless power system four items are required:
transmitter electronics, transmit coil, receive coil and

­receiver electronics. The LTC4120 wireless synchronous

buck charger combined with minimal external circuitry
comprises the receiver electronics (Figure 1). Please see
the LTC4120 data sheet for more detail.

L, LT, LT C, LT M, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.

Figure 1. LTC4120 Receiver Demo Board

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Application Note 138

Figure 2. Implementation of Basic Transmitter Reference Design

Figure 3. Proxi-Point Transmitter Figure 4. Proxi-2D Transmitter

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Application Note 138

Transmitter Solutions

Currently there are four transmitter options available for
design or off-the-shelf purchase:

1. Basic: This wireless power design (Figure 2) was
developed by collaboration between PowerbyProxi
Inc. and Linear Technology. It is provided as an open
source reference design that can be used to integrate
the LTC4120 into a wireless power system. The details
of the push-pull current-fed resonant converter are
described later in this document.

2. Proxi-Point: This is an advanced transmitter (Figure 3)
that is available from PowerbyProxi. For further infor­mation visit www.powerbyproxi.com. It is ready to use
or incorporate directly into a product. Unlike the basic
transmitter, it offers features such as foreign metal detec­tion, low standby power and a stable crystal-controlled
operating frequency. The transmit coil is built in.

3. Proxi-2D: This is an advanced transmitter (Figure 4) that
is available from PowerbyProxi. For further informa­tion visit www.powerbyproxi.com. It is ready to use or
incorporate directly into a product. Unlike Proxi-Point,
it is capable of charging multiple receivers simultane­ously in any orientation on the 2D charging surface.
The transmit coil is built in.

4. Proxi Custom: If the above options are not suitable for
your application, a custom transmitter can be designed
and manufactured to meet your requirements. Please con­tact PowerbyProxi at [e-mail] for further
information and pricing or visit www.powerbyproxi.com.

BASIC TRANSMITTER

The basic transmitter for the LTC4120, described in the
following sections, combined with a receive coil and
LTC4120-based receiver electronics can be used to
produce a wireless battery charging system. This wire­less battery charging system enables evaluation of the
LTC4120 using standard components.

Basic is a resonant DC-AC transmitter. It is a simple, easy
and inexpensive transmitter designed to work with the
LTC4120. Pre-regulation is required to provide a relatively
precise DC input voltage to meet a given set of receive
power requirements. The basic transmitter does not feature
foreign object metal detection and can therefore cause
these objects to heat up. Furthermore, the operating fre
quency of the basic transmitter can vary with component
selection and load.

The system draws power from a DC power supply to wire
lessly charge multi-chemistry batteries. A block diagram
of the system is shown in Figure 5.

While the basic transmitter can be used to build a wire
less battery charging system, a Proxi-Point or Proxi-2D
transmitter is recommended for applications requiring
enhanced features as described in the Appendix.

-

-

-

DC POWER SUPPLY

COUPLING (TX + RX COIL)

R

X

LTC4120
CIRCUIT

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TX CIRCUIT

Figure 5. Functional Block Diagram of Wireless Battery Charging System

TX COIL RX COIL

T

X

WIRELESS BATTERY CHARGING SYSTEM

BATTERY

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System Functional Block Description

LTC4120-based wireless battery charging systems use
wireless power transfer technology with Dynamic Harmo
nization Control (DHC), a patented technique that enables
optimal wireless power transfer across a variety of condi­tions while providing thermal management and overvoltage
protection. The resonant coupled system described here
eliminates both the need for precise mechanical alignment
as well as the need for a coupling core. The charging
system is composed of transmitter electronics, transmit
coil, receive coil and receiver electronics.

The transmit coil, L

, is energized by the transmitter

X

electronics to generate a high frequency magnetic field
(typically around 130kHz, though the operating frequency
varies depending on the load at the receiver and the cou­pling to the receive coil). This magnetic field induces a
voltage in the power receive coil, L

. After being tuned

R

with a capacitor, this induced voltage is managed by the
LTC4120 in order to control the power transfer. A typical
transmitter generates an AC coil current of about 2.5A RMS.

The receive coil, L

, is configured in a resonant circuit

R

followed by a rectifier and the LTC4120. Please see the

LTC4120 data sheet for more detail. The receive coil

presents a

load reflected back to the transmitter through

the mutual inductance between LR and LX. The reflected

impedance of the receiver may influence the operating
frequency of the transmitter. Likewise, the power output
by the transmitter depends on the load at the receiver.

­The charging system, consisting of both the transmit

ter and LTC4120 charger, provides an efficient method
for wireless battery charging. The power output by the
transmitter varies automatically based on the power used
to charge a battery.

Circuit Description

The basic transmitter is a current-fed push-pull transmit­ter capable of delivering 2W to the battery output of the
LTC4120. The basic transmitter schematic is shown in
Figure 6. The switches in this push-pull transmitter are
driven from the voltage on the opposing leg and no ad
ditional control circuitry is required to drive them. The
switch driving circuitry consists of a resistor, turn-off
diode, gate capacitor and a Zener diode for each switch.

The voltage rating of the Zener diodes D1 and D4 is cho­sen to fully turn on M1 and M2 while protecting them
from overvoltage.

The current limiting gate resistors R1 and R2 are selected
according to the maximum V

of M1, M2 and the current

DS

rating of the Zener diodes.

-

-

V

DC

5V

LB1

• •

68µH

C4

0.01µF

R1

100Ω

D3

M1 M2

D1
BZX84C16

Figure 6. Schematic of a Basic Transmitter for LTC4120

LB2

68µH

C5

0.01µF

R2

100Ω

D2

D4

BZX84C16

TRANSMITTER

L

C

X

X

0.3µF

5µH

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Application Note 138

1

The resultant voltage waveforms across LX are shown in
Figure 7.

The basic transmitter design is simple, easy to assemble
and test. Table 1 lists components used to build the basic
transmitter. The resonant operating frequency of the trans­mitter should match that of the receiver. The operating
frequency is calculated as follows:

fO=

2π LXC

X

Basic Design Recommendations

Due to the high frequency magnetic fields generated by
the transmitter electronics, there is a potential for the
induction of eddy currents in foreign metal objects that

TEK RUN

V

LX

are within range of the transmitter coil’s field. These eddy
currents can result in heat or small induced voltages in
these objects. In order to ensure users and devices are not
exposed to such hazards it is recommended that:

• A thermal detection system be integrated with the ba

sic transmitter. This detection system should turn the
magnetic field off if elevated temperature is detected.

• Electronic devices that are intended to be used with the

basic transmitter be thoroughly tested to ensure there
is no damage to the device or hazard to the user.

• All practical measures (e.g., labeling and user in-

struction) be taken to ensure electronic devices not
intended for usage with the basic transmitter are not
placed on the L

coil.

X

-

MATH FREQ

130.0kHz

V

DS

(M1, M2)

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Figure 7. System Waveforms (with Receiver and 1.7W Load). Drain Voltage of M1
(CH1), Drain Voltage of M2 (CH4), and Output AC Voltage Across LX.

Table 1. Components Used to Build the Basic Transmitter

CIRCUIT CODE DESCRIPTION VALUE (PARAMETERS) VENDOR VENDOR PART NUMBER

L

X

C

X

LB1, LB2 Inductors 68µH TDK VLCF5028T-680MR40-2
M1, M2 MOSFET V
D1, D4 Zener Diode V
D2, D3 Schottky Diode 40V, 1A On Semi NSR10F40NXT5G
C4, C5 Gate Capacitor 0.01µF, 50V Kemet C0402C103K5RACTU
L

R

Tx Coil 5µH TDK WT-505060-8K2-LT

CX Capacitors

Rx Coil 47µH Embedded PCB Coil Link to DC1967A Files

Rx Coil Ferrite 25mm Diameter TDK B67410-A223-X195

2 × 0.15µF

= 60V, R

DS

=16V, PD = 350mW Diodes BZX84C16

Z

= 11mΩ Vishay Si4108-TI-GE3

DS(ON)

Panasonic ECHU1H154GX9

MATH PK-PK

32.8V

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Measured Data

Tables 2 to 4 list circuit parameters that can be verified
during testing of the basic transmitter. The testing reflected
here was done using the components shown in Table 1.
Testing was conducted with Tx and Rx coils with gaps
of 4.5mm, 7.5mm and 10.5mm. Figure 8 shows battery
charger power curves with respect to transmit and receive
coil separation and coil center-to-center offset. Figure 9
shows a typical charge profile with this wireless power
configuration. Actual data will vary with component toler
ance and specific setup.

Table 2.Basic Transmitter Circuit Parameters (4.5mm Gap)

WITHOUT

SPECIFICATION
Operational Frequency 130.5kHz 128.7kHz 128.9kHz
Input Voltage 4.99V 4.99V 4.95V
Input Current 0.15A 0.173A 0.676A

RMS Value of T
Output AC Voltage

Peak Value of T
Output AC Voltage

Receiver Output DC
Voltage

Standby Loss 0.75W 0.873W N/A
Efficiency N/A N/A 47.1%

X

X

RECEIVER

(STANDBY)

10.9V 10.8V 10.4V

15.2V 15.2V 15.2V

N/A 34.9V 27V

WITH

RECEIVER

(NO LOAD)

WITH

RECEIVER

(1.58W LOAD)

Table 3. Basic Transmitter Circuit Parameters (7.5mm Gap)

SPECIFICATION
Operational Frequency 129.5kHz 128.8kHz
Input Voltage 4.99V 4.96V
Input Current 0.154A 0.602A

RMS Value of T
Output AC Voltage

Peak Value of T
Output AC Voltage

Receiver Output DC

-

Voltage
Standby Loss 0.768W N/A
Efficiency N/A 51.4%

X

X

WITH RECEIVER

(NO LOAD)

10.9V 10.5V

15.2V 15.2V

23.9V 17.5V

WITH RECEIVER

(1.535W LOAD)

Table 4. Basic Transmitter Circuit Parameters (10.5mm Gap)

SPECIFICATION
Operational Frequency 130.2kHz 128.8kHz
Input Voltage 4.99V 4.95V
Input Current 0.156A 0.658A

RMS Value of T
Output AC Voltage

Peak Value of T
Output AC Voltage

Receiver Output
DC Voltage

Standby Loss 0.77W N/A
Efficiency N/A 46.9%

X

X

WITH RECEIVER

(NO LOAD)

10.8V 10.5V

15.2V 15.2V

17.4V 13.9V

WITH RECEIVER

(1.53W LOAD)

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Figure 8. Battery Charger Power vs RX-TX Coil Location

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5

4

V

BAT

3

, V

CHRG

(V)

2

1

0

500

1.75Ah LI-ION BATTERY

400

300

(mA)

BAT

I

200

100

0

0

1

I

BAT

V

BAT

V

CHRG

2 3 4 5 6 7

TIME (HRS)

Figure 9. Typical Battery Charge Profile Using LTC4120 and the Basic Transmitter.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa­tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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Application Note 138

APPENDIX: PROXI-POINT AND PROXI-2D

The patented Proxi-Point and Proxi-2D transmitters
are available as fully assembled, tested and certified
off-the-shelf solutions from PowerbyProxi. For further
information visit, www.powerbyproxi.com.

The receive coil is configured in a resonant circuit followed
by a rectifier and the LTC4120. The transmitter frequency
is controlled by a crystal oscillator and will not vary
significantly from the designed value. The power output
by the transmitter depends on the load at the receiver.
The impedance of the resonant receiver presents a load
reflected back to the transmitter, so the transmitted power
will automatically vary depending on receiver power as the
LTC4120 charges the battery. The wireless power charging
system—consisting of either the Proxi-Point or Proxi-2D
transmitter and the LTC4120-based receiver—provides
an efficient method for wireless battery charging.

Table 5 compares features offered by the various trans­mitter options.

Further details regarding Proxi-Point, Proxi-2D and Proxi
custom solutions can be found at www.powerbyproxi.com

Table 5. Features and Functions of Transmitter Options

FEATURES AND FUNCTIONS BASIC PROXI-POINT PROXI-2D

Rated Power 2W 2W 2W per

Receivers per Transmitter Single Single Multiple
Freedom of Placement
Intelligent Foreign Metal Object

Detection*
EMC/EMI Compliant Off-The-Shelf

Fixed Operating Frequency

Supplied AC/DC Adaptor
Reverse-Polarity Protection
Built-In Transmit Coil
Low Power Standby**
Available for Purchase
*This feature is a way of preventing foreign metal objects from heating

when they are placed over the transmit coil.

**This feature allows the transmitter to autonomously enter a low power

state when there is no receiver within charging range of a transmitter
or if the receiver in range does not require power.

× ×

×

×

×

×

×

×

×

×

√ √

√ √

√ √

√ √
√ √
√ √
√ √
√ √

Receiver

√

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

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LT 0114 REV A • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2013