Transcript 150LECTURE20 POWER SUPPLIES Lecture Notes Page

POWER SUPPILES
LECTURE 20
LINEAR POWER SUPPLY
Circuit Skills: Power Supply .
http://www.youtube.com/watch?v=qPt0X5s-1JU
SMPS (SWITCH MODE POWER SUPPLY)
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 AC voltage into
regulated DC voltages required for electronic equipment. Original PSUs were linear.
Nowadays most of them are of SMPS type. What is it? The units in which the energy
flow is controlled with semiconductors that are continuously switching on and off with
high frequency are referred to as switch mode power supplies or SMPS.
SMPS offer greater efficiency compared with linear supplies because a switch 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.
In general, SMPS can be classified into four types according to the form of input and
output voltages: AC to DC ,DC to DC (voltage or current converter), AC to AC and DC to
AC (inverter).
SWITCH MODE POWER SUPPLY BLOCK DIAGRAM
Circuit Skills: PWM (Pulse Width Modulation)
http://www.youtube.com/watch?v=Lf7JJAAZxEU
SMPS power handling electronic components are continuously switching on and off with
high frequency in order to provide the transfer of electric energy via energy storage
components (inductors and capacitors). By varying duty cycle, frequency or a relative
phase of these transitions the average value of output voltage or current is controlled.
The frequency range of a commercial SMPS units varies typically from 50 kHz to several
MHz.
The lowpass EMI filter is designed to reduce high frequency currents getting from
PSU into the AC line to an acceptable level. This is necessary to prevent the PSU
from causing interference on the other devices connected to the input wiring.
The AC is rectified by a full-wave bridge rectifier. The rectified
voltage is next applied to the power factor correction (PFC) preregulator followed by output DC-DC converter(s).
TRUE POWER
IS THE ACTUAL POWER USED BY THE CIRCUIT.
IT IS MEASURED WITH A WATTMETER.
APPARENT POWER
POWER IN A CIRCUIT WHEN VOLTAGE AND POWER ARE MEASURED
SEPARATELY IT IS CALCULATED IN UNITS OF VOLTAMPERE. (VA)
PF: It measures how effectively AC power is
being used by a device. The difference
between true power and
POWER FACTOR
IS THE RATIO OF TRUE POWER/APPARENT POWER.
WHEN CURRENT AND VOLTAGE ARE IN PHASE THE POWER FACTOR = 1.
IF 90° OUT OF PHASE THE POWER FACTOR = 0.
POWER FACTOR OF A CIRCUIT CAN VARY BETWEEN O AND 1.
The rectified input voltage is next fed into (PFC) stage, which increases power factor
(PF). In the process it usually boosts the voltage to a regulated 370-400 VDC. PF by
definition is the ratio between watts and volt-amps. A PFC pre-regulator controls input
AC current so that it is in phase with mains AC voltage and its waveform repeats the
input voltage waveform. Without this, the input current would be delivered to the SMPS
in short high peak pulses, which have a high harmonic content. The current harmonics
do not supply any real power to the load, but cause additional heating in the wiring and
distribution equipment. They also reduce the maximum amount of electricity that can be
taken from a standard wall outlet, since home circuit breakers are rated by current rather
than by watts..
Locating the primary and the secondary.
SMPS Repair
http://www.youtube.com/watch?v=v7nTAQOdT-Y
Transient filter second stage.
Electrolytic capacitors from the voltage doubler.
Rectifying bridge.
Active POWER FACTOR CORRECTION components.
Components attached to the primary heatsink.
Power supply transformers.
PWM control circuit.
Power supply secondary stage.
Rectifying diodes for the –12 V and –5V lines.
Components found on the secondary heatsink of a high-end power supply.
This power supply uses optocouplers instead of
using a transformer to isolate the PWM circuit.