Download Lets start with the step

THE POWER SUPPLY
By Emiliyan Nikolov
What is it?
The power supply is a
main building block of
any electronic device.
It’s purpose is to deliver
the energy necessary for
the components and
circuits to operate and
do the job they are
meant to.
Where does it take energy from?
To deliver the energy, the
power supply has to take it
from a source. This can be:
• the power grid
• energy storage device such
as batteries, fuel cells,
supercapacitors, etc.
• alternative source such as
wind generator, solar panel,
hydroelectric generator, etc.
supercapacitors
portable wind generator
Then what?
Every device needs a
specific voltage and amount
of current to do what it has
to. So it’s up to the power
supply to convert the input
from the source to the exact
voltage needed by the device
with enough ant not too much
current. Here we can
differentiate two main types
of power supply:
•step-up power supply
•step-down power supply
the output voltage and max
current of a laptop power
brick
Step-up power supply
As the name of it may show,
it steps up the input voltage
to one higher than it, but the
output current is lower than
the input (the output power
can’t be higher than the
input). This kind of power
supply is generally the more
complicated one because
when the input is DC a
simple transformer won’t do
the job and that is why these
power supplies are always
SMPS (we will see what this
is later on).
SMPS that takes 12 volts from a battery and
outputs mains voltage to operate an ordinary
60W light bulb
Why do we need it?
Step-up power supplies are used when we need voltage
higher than we have. Examples are power supplies for
CRTs, for lasers, car inverters, etc. and even the stun gun
can be considered one.
CRT monitor
stun gun powered by batteries
that outputs several hundreds of
thousands of volts
car inverter
Step-down power supply
You guessed it! This
simple step
type of power supply
down power
takes a higher voltage
supply
and steps it down to a
lower voltage. The
output current can be
higher or lower than the
input. This type of
power supply can be as
simple as a resistor
connected in series with
the powered device and
as complicated as a
highly stabilized and
precision laboratory test
instrument.
home made
bench test
power
supply
And why do we need this?
Step-down power supplies can be found everywhere
around us. Just look around! The charger to which your
phone is connected, the power supply of your PC, the
power brick which charges your laptop. There is one for
almost every electronic household device – be it external
as an adapter or internal as part of the device itself.
DVD player internal power supply
simple adapter
PC power supply
How does it work?
Lets start with the step-down power supply.
In old days when the electronic components where still too
big almost all power supplies were based on transformers.
There would be a big transformer converting the line
voltage to a lower one and from there a rectifier and a filter
are used to turn the AC into DC if needed.
When the circuit won’t come in touch with the user, the
voltage is just too low or simplicity is needed a resistor or
something similar can be used to block the excess voltage
and current dissipating it in heat.
A voltage devider
transformerless power supply used in household
control switch – there is no electrical contact
with the user
As the electronic components got smaller and cheaper a new kind of
power supply started to take the place of the previous two – the switchmode power supply (SMPS). This type of power supply works by taking
the DC or rectifying the AC, converting it in high frequency AC and then
with the help of transformer or an inductor stepping it down to the
needed voltage.
SMPS block diagram
This type of operation has two main benefits.
As the frequency is higher the used transformer can be
several times smaller than an ordinary one and thus the
whole power supply can be very small and lightweight.
(when a transformer is used)
on the left:
100 watt mains
transformer – it is
as big as it looks
like
on the right:
100 watt
transformer for
SMPS roughly 2
by 2 centimetres.
The other benefit is that because it operates by turning the
powered device on and off at a very high frequency rather
than just burning the excess power off as heat it can be a
lot more efficient than a simple resistive method. (when an
inductor is used)
power supply used in
high-efficiency
applications
Step-up power supply operation
If the input is AC a simple transformer and SMPS can do
the job perfectly in almost the same fashion as step-down
power supply. If the output is DC only the SMPS can step
the voltage up. The principle of operation is the same as if
the voltage is stepped down (by transformer or by
inductor).
inductor
based
SMPS
operation
Stepping up with an inductor
The SMPS that steps the voltage with the help
of an inductor is also known as boost converter.
It uses an effect called back EMF which can
produce very high voltages and currents. These
voltages can be many times higher than the
input and ultimately the output voltage can be
several times higher than the input. Generally
this kind of operation is used when the output is
not much higher than the input otherwise a
transformer based SMPS is used.
here the back EMF occurs
Unregulated vs. regulated
If the powered device is more sensitive just a rectification and filtration
of the AC may not be enough. If a higher current is applied or the line
voltage changes the output of the power supply will also change. That
is where a regulator is used. This, if there is one in the power supply,
is the most complicated part of the whole unit.
unrectified
AC waveform
rectified AC
rectified, filtered and
regulated AC
(perfect DC)
rectified and
filtered AC
(almost DC)
The regulator
By itself the regulator also can be consider a power supply and as such
there are also linear (using resistance) or switch-mode regulators. It
can be made of discrete components or it can be a dedicated
integrated circuit as big as an ordinary transistor.
A very popular
linear regulator
LM7812
linear regulator
switch-mode regulator
2N5294 transistor in comparison – the
sizes of both are exactly the same
For the curious ones…
Here is a list of the resources used and sites that can explain the
matter in more details.
• http://www.howstuffworks.com/search.php?terms=power+supply
• http://www.hobbyprojects.com/block_diagrams/power_supply_units.html
• http://www.rason.org/Projects/swregdes/swregdes.htm
• http://www.circuitstoday.com/dc-power-supplies
• http://www.allaboutcircuits.com/vol_3/chpt_9/2.html
• http://sound.westhost.com/articles/power-supplies2.htm
• http://www.electronics-tutorials.com/basics/power-supply.htm
• http://www.ladyada.net/learn/powersupply/transformeracdc.html
Hope you enjoyed it!
THE END