Introduction:-
Switch mode power supply
(SMPS) is a type of an electronic power converter which switches
current at a fast rate to increase, decrease, regulate or condition
the output voltage or current.
In other words like every other
power supply SMPS transfers the power from input to the output like
every other power supply but has many advantages in the department of
efficiency, cost, size, weight and regulation.
That being said the
most important aspect of a switch mode power supply is the high
switching frequency. All SMPS operates at a very high switching
frequency typically from 25 KHz to 1MHz.
Isolated power
supplies have an electrical isolated between input and output In
order to achieve that a transformer is used.
Non-isolated power
supplies do not have an electrical isolation between input and
output. It uses an inductor for voltage conversions.
Overview:-
The electric energy is not
normally used in the form in which it was produced or distributed.
Practically all electronic systems require some form of energy
conversion. A device that transfers electric energy from a given
source to a given load using electronic circuits is referred to
as power supply.
Of course, it does not really supply power, it just converts it, so
"converter"
is a more accurate term for such a device.
A typical application of a
DC power supply unit (PSU) is to convert utility AC voltage into a
set of regulated DC voltages required for electronic equipment. The
energy flow in a modern PSU is controlled with power semiconductors,
which can operate in different modes. In original systems they
operated in linear mode. Nowadays in most PSUs semiconductors are
continuously switching on and off with high frequency. Such units are
referred to as switched mode power supplies or SMPS.
They offer greater efficiency compared with linear supplies because
they can control energy flow with low losses: when a switch is on, it
has low voltage drop and will pass any current imposed on it; when it
is off, it blocks the flow of current.
As the result,
in such a switch the power dissipation which is the product of
voltage and current, can be relatively low in both states. Switching
mode units are also smaller in size and lighter in weight due to the
reduced size of passive components and lower heat generation. The
industry trend toward miniaturization, advancements in semiconductor
technology, as well as various energy efficiency regulations have
made "switcher" the dominant type of PSU across practically
the full spectrum of applications. Most PSU manufactured today for AC
input applications also include a PFC front end.
In general, SMPS converters
can be classified into four types according to the form of input and
output voltages: AC to DC (also called off-line DC power supply), DC
to DC (voltage or current converter), AC to AC (frequency changer or
cycloconverter), and DC to AC (inverter).
Objectives:-
The objectives of the
project are:-
Achieve high efficiency (>85%).
High power factor (>0.95).
Controlling and monitoring through a
computer.
Universal input voltage.
Reduce mains noise and harmonics.
Minimize EMI (electromagnetic
interference).
Implement variable voltage regulation
Minimize RFI (radio frequency
interference).
Power
Factor:-
Power factor is the ratio of
real power to apparent power:-
Real power (watts) produces real
work; this is the energy transfer component (example
electricity-to-motor rpm).Reactive power is the power required to
produce the magnetic fields (lost power) to enable the real work to
be done, where apparent power is considered the total power that the
power company supplies.
When the power factor is not
equal to 1, the current waveform does not follow the voltage
waveform. This results not only in power losses, but May also cause
harmonics that travel down the neutral line and disrupt other devices
connected to the line. The closer the power factor is to 1, the
closer the current harmonics will be to zero since all the power is
contained in the fundamental frequency.
Since an SMPS uses DC voltage
so the mains AC voltage is rectified and filtered, circuit is shown
below:-
One problem with SMPS that they
do not use any form of power factor correction is that, the input
capacitor will only charge when input voltage is close to peak
voltage or when input voltage is greater than the capacitor voltage.
The capacitor discharges when the input voltage starts going lower
than the peak voltage and re-charges when the peak approaches. A
capacitor has a very low ESR (electrical series resistance) resulting
in very high current when recharging. As a result the current only
flows during the peak input voltages resulting in a highly distorted
and peaky current waveform having a low power factor. Illustrated in
the figure below:-
A switch mode power supply has
a power factor of 0.5 to 0.6. Which by any standards is very low.
Ideally power factor should be close to 1. With passive PFC, power
factor can be improved up to 0.75. With active PFC it can be improved
up to 0.99.
Active Power
Factor Correction (PFC):-
The power factor of an SMPS can
be improved by smoothing out the peak currents. The current draw can
be averaged out over the whole cycle improving the power factor. In
order to do that the input capacitor have to charge over the whole
cycle to accumulate energy.
The heart of active PFC is a
boost converter. It is placed between the rectifier and input
capacitor. So the boost converter receives a fully rectified AC line
voltage with no bulk filtering. So the input voltage it receives
ranges from zero to peak voltage to zero again. The frequency is
twice the line frequency. It boosts the lower voltages to allow the
input capacitor to charge during the dips in the input voltage. The
boost converter is shown below:-
PC
Interface:-
In industries all the processes
as automated i.e. controlled
by a computer for centralized control.
All the sensor data is received by the computer which then further
processes the data and acts accordingly. Power supply is the most
important component in any electrical or electronic device. It’s
monitoring and control is also necessary. It would be really
convenient if it can be monitored and controlled through a
centralized computer. It would also be easy to implement protections
and current limiting through sensor data from the equipment being
powered.
For PC interface an arduino
would be used as it has an integrated USB interface. The output
voltage and current would be monitored and the pulse width in the
SMPS controller would be varied according to the voltage and current
regulation values.
Related
Work:
PCB designing
( simulation and hardware design )
Studying Magnetic material
Power electronics
Soldering
Mains voltage safety
Protections
Thermal management.
Arduino development environment.
Hardware
and Software Requirements:
Software:-
Lab
view.
LTsplice
Cadsoft
eagle
Arduino
IDE
Hardware:-
Oscilloscope
Multimeter.
Dummy
load.
Power
factor meter.
Spectrum
analyzer (for measuring harmonics).
Advantages
And Disadvantages of SMPS:-
Advantages:
Much
more efficient as compared to linear power supplies because it uses
components as switches rather than resistive elements.
Protection
against excessive output voltage by quick acting guard circuits
Smaller
in size and lighter in weight.
Cheaper
at higher power levels (sometimes in lower power as well).
Regulation
is easy and efficient to implement.
Lower
idle power consumption.
Disadvantages:-
The
circuit is much more complex as compared to linear PSUs.
EMI/RFI
which is inherent in SMPS is difficult to suppress.
Electronic
noise at the output and input terminals.
PCB
layout is critical.
Applications
and Future:
Switch mode power
supplies have applications in various areas. A switched-mode
supply is chosen for an application when its weight, efficiency,
size, or wide input range tolerance make it preferable to linear
power supplies. Initially the cost of semiconductors made switch-mode
supplies a premium cost alternative, but current production
switch-mode supplies are nearly always lower in cost than the
equivalent linear power supply.
In industrial and high power
applications SMPS were not used because semiconductor devices were
not rugged enough. But now many manufacturers have made rugged and
powerful semiconductor devices. SMPS are now being developed and used
in high power application offering cost and space savings.
Project
timeline
TASK
|
Duration
|
Date
|
Literature
review
|
1
month
|
September
2014
|
Choosing
required Hardware
|
1
month
|
October
2014
|
Observation
of Hardware
|
1
month
|
November
2014
|
Simulation
(software) work
|
1
month
|
December
2014
|
Fabrication
of PCB
|
1
month
|
January
2015
|
Installation
of Hardware
|
1
month
|
February
2015
|
Testing
and Result
|
1
month
|
March
2015
|
Progress
Report
|
1
month
|
April
2015
|
Final
Thesis
|
1
month
|
June
2015
|
Presentation
and Demonstration
|
|
|
References:-
1)
Application Note 42047 Power Factor Correction (PFC) Basics –
Fairchild semiconductor.click
3)
Power Factor Correction (PFC) Parts Selection Guide – Infineon.power factor
4)
Control techniques for power factor correction converters - L.
Rossetto, G.Spiazzi, P. Tenti
5)
Power Supply Cookbook - Marty Brown.
6)
Practical Switching Power Supply Design - Marty Brown
7)
Switching Power Supply Design - Abraham I PressmenSPSD
