Start
• Design
Requirements
•
System
Architecture
•
External
Charging Unit
•
Internal
Charging Unit
•
Charging
Coils
•
Charging
Data
• Power
Up
•
Sources
and PDF
SYSTEM
ARCHITECTURE
Our
power supply system consists of two main modules
(see Figure 1). An external charging unit (ECU)
determines when a charging current is desired
(through user interaction) and it generates and
manages that current. This charging current is
sent through an external transinductance coil
where it generates an electromagnetic field that
can be used to couple energy to the ICU.
|
(Click
here to enlarge)
|
Figure
1a—The external charging unit features an
ATmega8 microcontroller. b—The internal charging
unit also features an ATmega8 microcontroller.
The supply output is capable of providing
3.3 V at 100 mA of continuous power. |
The
transinductance coil for the ICU couples energy
from the electromagnetic field into a current
that is rectified and routed to a DC-to-DC step-up
switching voltage regulator. The voltage from
the switching regulator is used to charge one
of two battery packs in the system. The second
battery pack is routed to another switching regulator
and used to provide power to the embedded application.
The second switching regulator enables you to
select the power supply voltage. With this configuration,
the power supply voltage won’t slowly decay as
the battery is discharged. Instead, the power
supply will remain constant as the battery is
discharged. At an appropriate point, the battery
management hardware will determine that the battery
packs will be switched. The switching process
will provide power from a freshly charged pack
while the discharged pack is charged again.
The
external charging unit (ECU) features an Atmel
ATmega8 microcontroller that interacts with you
and generates the alternating current (AC) waveform
required to drive the transinductance coil. The
AC waveform is created using a power FET array.
The terminals of the transinductance coil are
alternately switched from VSUPPLY/Ground to Ground/
VSUPPLY by modulating the gate voltage of each
device in the four-FET array. The modulation frequency
optimizes energy transfer between the ECU and
ICU by trial and error. For our application and
coil geometry, a frequency of 4.7 kHz resulted
in the shortest battery charging times.
The
ICU also features an ATmega8 microcontroller for
battery-management services. The microcontroller
is interfaced via a SPI bus to a LTC1325-controlled
battery management system IC. Power is coupled
into the ICU through a transinductance coil. The
current waveform from the coil is rectified, filtered,
and passed to a National Semiconductor LM2621
DC-to-DC switching voltage regulator. The regulator
circuit is designed for an output voltage (VBOOST)
of 5 V. VBOOST is the power source the LTC1325
uses for charging the discharged battery pack.
It’s monitored by one of the ATmega8’s internal
ADCs as well.
The
output voltage of the charged battery pack is
input to a second LM2621 switching regulator.
The circuit for the second regulator can be optimized
to provide any desired power supply voltage for
powering the embedded application.
For
this project, the voltage is set to 3.3 V. This
voltage source is also used to provide power to
the internal ATmega8 microcontroller as well as
to the LTC1325 and a MAX3233. We included the
MAX3233 so there is an RS-232 interface to the
ATmega8 microcontroller for diagnostic and development
purposes.
The
microcontrollers for both the ECU and the ICU
are programmed via separate six-pin SPI-based
programming buses with an Atmel STK500 programmer.
We used the CodeVisionAVR and AVR Design Studio
to develop the source code in C language.
