Switched Mode Power Supply
DC to DC
converters and DC to AC inverters belong to the category of switched mode power
supplies (SMPS). Besides, there are SMPS, operating from mains, called off-line
switching supplies. An off-line switching supply can be distinguished from a
conventional AC--DC supply, as in case of former AC mains rectified and filtered
without using an Input transformer, and DC voltage so obtained is then used as
an Input to a switching type DC to DC converter.
In switching
power supply, the active device that provides regulation is always operated in
switched Mode, i.e. it is operated either in cut-off or in saturation. The
Input DC is chopped at a high freq. (10 kHz to 100 kHz) using an active device and
converter transformer. The transformed, chopped waveform is rectified and
filtered. A sample of Output voltage is used as feedback signal for the drive
circuit for switching transistor to achieve regulation.
Power
electronics is entirely devoted to switch mode power conversion and deals with
modem problems in analysis, design and synthesis of electronic circuits as
applied to efficient conversion, control and regulation and electrical energy.
Design and optimization of DC to DC converters which offer the highest power
efficiency, small size and weight and high performance, are also included in
power electronics.
Basic Block Diagram of SMPS
Figure shows
the block diagram of the PSU. The input from mains is first filtered to
suppress any spikes/surges entering the power supply circuit. This is an
important part of the circuit and helps in preventing data loss or erroneous working
during power line disturbances.
Filtered mains
supply is then rectified by a full-wave bridge rectifier to produce +150V DC
for the power converter section. The input supply is switched with the help of
this power converter and the energy transferred to the output through a high
frequency ferrite transformer.
The power converter consists of two
externally driven transistors operating in push-pull configuration. These are
driven by converter driver. The power converter transformer produces low
voltage switched wave-forms on the secondary side which are rectified and
filtered to produce well-regulated ±5 V and ±12 V DC output voltages.
The ±5 V DC
output voltage is sensed by control section of the PSU which essentially consists
of a pulse width modulator (PWM) controller IC. This PWM produces suitable
drive pulses at about 22 kHz for the Converter driver. The width of these
pulses is controlled by the modulator depending upon the output sense voltages,
thus ensuring correct operating voltages at the output and providing necessary
protection to the PSU.
The input
mains supply is applied to the circuit via an on-off switch of fuse F1. Power
converters operating directly ‘off-line’ like this one draw heavy current when
switched on. This inrush of current causes great stress on input components, switches,
rectifiers and capacitors.
For low power
applications, simple series resistor is used to limit the initial high voltage
and high current. Special high current surge rated resistors are best suited
for this application. However, adequately rated wire-wound resistor also serves
the purpose and is frequently used.
Connectors from SMPS
Molex Connectors
The first and
most common type of connection is called the Molex. The Molex connector is
primarily used for devices that need both 12 V and 5 V of power. The Molex
connector has chambers (notches), which make for easy installation. These
chambers can be defeated if one pushes hard enough, so Always inspect the Molex
connection to ensure proper orientation before one installs it.
Mini Connectors
Most systems
also provide a mini connector. The mini is used primarily on 3.5-inch floppy
drives, because floppy drive makers have adopted the mini connector for that
use.
Motherboard Connectors
AT Connectors
Industry
standard PC, XT, AT, Baby-AT, and LPX motherboards all use the same type of
main power supply connectors. These supplies feature two main power connectors
(P8 and P9), each with 6 pins that attach the power supply to the motherboard. P8
and P9 are identically sized and shaped; the only difference is the colors of
wires and the order in which they appear. You have to get them plugged in right
or you will irreparably smoke your motherboard. Just keep this in mind: black together.
Both connectors have a black wire; when the connectors are plugged in properly,
the black wire on P8 will be next to the black wire on P9.
ATX Connectors
The industry
standard ATX power-supply-to motherboard main connector is the Molex 39-29-9202
(or equivalent) 20-pin ATX style connector. It is used in the ATX, Mini-ATX, Micro-ATX
& SFX form factors. This is a 20-pin keyed connector with pins configured
as shown in Table 16.4. The colors for the wires listed are those recommended
by the ATX standard; however, they are not required for compliance to the
specification, so they could vary from manufacturer to manufacturer.
ATX 12 V Connector
To augment the supply of +12 V power to the motherboard, Intel created a new ATX 12 V power supply specification. This adds a third power connector, called the ATX 12 V connector, specifically to supply additional +12 V power to the board.
Power Switch
Every PC needs
a power switch. Power switch utilization creates one of the major differences
between AT and ATX power supplies. AT power switches simply turn the system on
or off, whereas ATX power supplies use a feature called soft power.
AT Power Switch
AT power
switches come in only two common
Types: rocker and
plunger. Each of these switches has four tab connectors that attach to four
color-coded wires leading from the power supply.
The ends of
the cable are fitted with spade connector lugs, which plug onto the spade
connectors on the power switch. The cable from the power supply to the switch
in the case contains four color-coded wires. In addition, a fifth wire
supplying a ground connection to the case might be included. The switch was
usually included with the power supply and heavily shrink-wrapped or insulated
where the connector lugs attached to prevent electric shock. The four or five
wires are color-coded as follows:
Brown and blue: These wires are the live and neutral feed wires from the 110 V power cord to the power supply. These are always hot when the power supply is plugged in.
• Black and
white: These wires carry the AC feed from the switch back to the power supply.
These leads should be hot only when the power supply is plugged in and the switch
is turned on.
• Green or green
with a yellow stripe: This is the ground lead. It should be connected to the PC
case and should help ground the power supply to the case.
Troubleshooting tips for SMPS
• Check the
wall outlet. The outlet should be providing between 220-250 V AC current. Just
set the voltage-ohmmeter (VOM) to read AC voltage and put one lead in each hole
of the outlet.
• Check the
power cord. It should be firmly plugged into the power supply. If you have a
spare cord, swap cords. Yes, power cords do fail.
• Is power
getting to the power supply? The fan gets it first, so if it isn't turning on,
the power supply isn't getting power. When some power supplies are first turned
on, the speaker emits a low click.
• Check to
make sure the power supply is connected to the motherboard using the right
connectors, whether P8 and P9 (AT) or the single-piece, 20-pin ATX connector.
• To conclude
whether the problem is with the motherboard or SMPS, short the PS ON pin (Green
pin) with any black pin i.e. the ground, and observe whether the SMPS fan has
started. If it has, then it can be concluded that the source of the problem is
with the motherboard, else it will be with the SMPS.
• If all those
things check out correctly, try swapping in a different power supply.
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