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.

There are two ways to design the power multiplexing using multiple power sources:

 Design 1:

                        















  Design 2:

                                                                                                          





Voltage Converter :

     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.






Article by:-

      Venkatesh Enjam
      Project Engineer - 1
      CDAC R & D
      Bangalore