How to use Multiple Power Sources in a Embedded System??
This article gives capability of using
multiple power sources of different voltage-current characteristics to a common
system load.
In Embedded Systems, Power System is the
main Important block of the System. The different power sources of the system
are,
1) USB
Power
2) Primary
and Secondary Batteries
3) Natural
sources like Solar, Vibration, RF and Thermal etc
4) POE
(Power Over Ethernet)
USB
Power
The USB version 1.x and 2.0 gives 5V
supply on a single wire to power connected devices. The USB version 3.0 gives voltage
level between from 4.45 to 5.25V.
The USB version 2.0 gives 100 mA to a
single load and USB version 3.0 gives 150 mA to a single load. The USB 2.0
gives 500 mA to 5 unit loads in a device at maximum. Similarly, USB 3.0 gives
900 mA to 6 unit loads in a device at maximum.
USB 1.X/2.0 Pinout :
Pin
|
Name
|
Color of the Cable
|
Description
|
1
|
VBUS
|
Red
|
+5V
|
2
|
D-
|
White
|
Data-
|
3
|
D+
|
Green
|
Data+
|
4
|
ID
|
Device Identification
|
|
5
|
GND
|
Black
|
Ground
|
Batteries
:
An
electric battery is a device consisting of one or more electrochemical cells
that convert stored chemical energy
into electrical energy. Each cell contains a positive terminal and a negative
terminal.
Primary
Batteries are non-Rechargeable Batteries whereas Secondary Batteries are
Rechargeable Batteries.
The
example of primary batteries are 2xAAA batteries and example of secondary
batteries are Li-ion and Li-polymer batteries.
Natural
Sources:
Natural
Sources are like solar, vibration, RF and thermal are ubiquitous. Using these
sources, the first step is to take convert the respective energy into DC power.
Power
Over Ethernet :
POE
is a technology that passes electrical power along with data on Ethernet Cabling.
This allows a single cable to provide both data connection and electrical power
to devices.
There
are two devices are involved in POE Design.
1)
Power Sourcing
Equipment(PSE): It is device like switch that
provides the power. The maximum allowed continuous output power per cable in
IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers
25.50 W.
When the device is a switch, it is
commonly called an endspan. Otherwise, if it's an intermediary device between a
non PoE capable switch and a PoE device, it's called a midspan.
2)
Powered Device(PD): A powered device (PD) is a device powered by a PSE and that
consumes energy.
Design
1:
Voltage Converter is required if any
natural source is using. Natural sources are ubiquitous . The conversion from
respective energy into electrical energy is required to give the power to
electrical system.
DC-DC Buck-Boost Converter :
It is device to convert one DC power
level into another DC power level which is required by the system load.
Diode :
In
electronics, a diode is a two-terminal
electronic component
with asymmetric conductance,
it has low resistance
to current flow in one direction, and high resistance in another direction.
An ideal diode should have the following characteristics:
- When
forward-biased, the diode acts as short circuited, then current
flows through it.
- When
reverse-biased, the diode acts as an open circuited, then current
does not flow through it.
Diodes used in power electronics applications are
generally required to have special characteristics, these are:
• High breakdown voltage and high current carrying
capability;
• Small switching time delays and small current
rise and fall times;
• Negligible reverse recovery (ie. charge removal
at turn OFF is negligible);
• Low voltage drop when conducting
*To
achieve very fast switching, Schottky diodes
can be used although their current and voltage ratings are restricted.
In this
case, Diodes are the key component to avoid reverse flowing current.
Using MOSFET Switch:
MOSFET devices are most preferable
devices in power related designs. Power MOSFETs are well known for fast
switching speed and they require very little gate drive power because of the
insulated voltage. In these respects, power MOSFETS approach the
characteristics of ‘Ideal Diode’.
In this case,
Switching the available power source from non-available power source can be
made by using MOSFET switch with the control of Micro controller or Logic
Enabled device.Let us assume,
Power source1 is any natural source while power source 2 is rechargeable
battery. If natural source is available, the current flows to System load by
making the MOSFET switch is OFF using control signal.
If natural
source is not available , the current flows from rechargeable battery to system
load by making the MOSFET switch is ON using the control signal.
If we connect
any GPIO pin of Micro controller to gate
pin of MOSFET switch, logic HIGH and LOW signals make enabling and disabling
the MOSFET switch.
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