PC Power Supplies Explained

PC Power Supplies
Instructor
Bruce Youmans
Deep Dive Into

Power Supply
- Most failure prone component in PC !
- Most often replaced component !
- Malfunctioning power supply can:
- Cause other components to malfunction
- Damage other components

Primary Function
- Convert electrical power to energy.
- .
- Are “constant voltage switching” type.
- Constant voltage means power supply
outputs same voltage regardless of AC
current input or wattage capacity.

- Switching refers to design and power
regulation technique.
- Switching design provides an efficient
and inexpensive power source and
generates minimum heat.
- Low price and small size.
Primary Function

Voltage Rails
- PSU normally supplies +3.3V, +5V, and +12V
- Digital electronics use +3.3V or +5V.
- HDD motors and fans use +12V power.
- Each rail is a separate power circuit
- Rail: a “power supply within the power supply”

- The +3.3V, +5V, and +12V rails are
technically independent in supply.
- Cheaper designs share circuitry, making
them less independent !
- But, processors and video cards vary
power consumption by their activity.
Voltage Rails

-Transitioning from an idle desktop to a 3D
video game can double the +12V draw.
- On cheaper models, this causes voltages
on other rails to fall out of spec (5%+),
making the system crash !
- Better designed models feature truly
independent rails tolerance (1-3%).
Voltage Rails

Voltage Rails
PSU Tolerance Table
Voltage
Rail
Tolerance Minimum
Voltage
Maximum
Voltage
+3.3 VDC ± 5% +3.135 VDC +3.465 VDC
+5 VDC ± 5% +4.750 VDC +5.250 VDC
+5 VSB ± 5% +4.750 VDC +5.250 VDC
-5 VDC ± 10% -4.500 VDC -5.500 VDC
+12 VDC ± 5% +11.400 VDC +12.600 VDC
-12 VDC ± 10% -10.800 VDC - 13.200 VDC

- PS must deliver clean, steady DC to PC.
- Devices that need voltages other than
3.3v, 5v or 12v must be indirectly powered
through on-board regulators.
- Processors also need voltages (~1.3V)
supplied by Voltage Regulator Module
(VRM) built into MB.
Voltage Regulators

PS_ON turns power supply on or off via
software. Soft-power feature
PS_ON uses Advanced Configuration and Power
Interface (ACPI).
On PC shut down, Windows turns the computer
off after the shutdown sequence.
Soft Power Feature (PS_ON)

Power Good Signal
Power supply completes internal checks and
tests before allowing system to start.
If successful, sends Power_Good signal to MB.
If AC voltages dip and PS can't maintain outputs
within tolerance, Power_Good signal is
withdrawn and system resets. System will not
restart until Power_Good returns.

Power_Good (PG) signal is (2.4V < PG < 6V)
asserted by PS 100 - 500 ms after power on.
Received by processor timer chip that controls
reset line to the processor.
If PG absent: timer chip holds CPU reset line
preventing system from running.
If PG present: timer chip releases CPU reset line
and CPU starts @ FFFF0h (MB ROM)
Power Good Signal

If PS can't maintain proper output levels
(brownout) PG signal is withdrawn.
By withdrawing PG, before voltage falls too
low, system never sees bad power!
Bad power (unstable power levels) causes
memory parity errors and other problems.
Power Good Signal

PG is on pin 8 (gray wire) PS connector.
Properly designed PS delays arrival of PG until
all voltages stabilize upon turning system on.
Poorly designed PS in low-cost systems, do not
delay PG enabling processor to start too soon.
Improper timing also causes CMOS memory
corruption in some systems!
Power Good Signal

Cheaper supplies may not have a PG circuit,
may just tie any +5V line to that signal !
Some MBs are sensitive to improperly designed
PG signals, causing intermittent startup issues.
Example: a new MB may seem defective, but is
the power supply that is poorly designed !
Power Good Signal

Positive Pressure Design: Fan draws air in
from rear of chassis to blow inside across
MB and pressurizing case. Problem –
requires powerful fan!
Negative Pressure Design: Fan blows air
out rear of chassis pulling air across
motherboard and CPU. Solution - greater
cooling capacity for fan speed/flow!
Power Supply Cooling

Power Supply Form
Factors
Shape and general physical layout of a
component is called the Form Factor (FF).
Choosing a popular standard PS insures
inexpensive replacement parts are available.
Using/Following standards allows you to upgrade
and repair systems by easily.

All modern ATX form motherboards with PCI express
slots have two main power connectors:
- 24-pin ATX main connector
- 4-pin +12V connector.
Power Supply Form
Factors

Any power supply not conforming to one of
these standards is considered proprietary.
Avoid systems that use proprietary power
supplies because replacements and
upgrades are generally not available.
Power Supply Form
Factors

ATX / ATX12V Form
Factor
ATX12V PSUs popular with ATX & BTX MBs
ATX12V 2.0 specification (Feb 2003):
- dropped the 6-pin aux connector
- changed main power connector to 24 pins
- required serial ATA power connectors
ATX12V 2.2 (Mar 2005), contains minor changes
Molex High Current System (HCS) terminals.

Includes intelligently designed power plugs.
Power_On (PS-On) and 5V_Standby (5VSB)
outputs, known collectively as “Soft Power”.
Soft Power allows a signal from modem or
network adapter wake up and power on a PC.
A wakeup time can also be set to turn on PC so
it can perform specific time-sensitive tasks.
Check BIOS for control of these features.
ATX / ATX12V
Form Factor

SFX / SFX12V
Form Factor
SFX defines five different physical shapes,
some are not directly interchangeable.
When replacing SFX / SFX12V-type PSUs,
buy correct size for the chassis.
A 60mm cooling fan on rear of PS exhausts
air out the rear of chassis (negative
pressure design).

EPS/EPS12V
Form Factor
Server System Infrastructure (SSI) is an industry
initiative for industry-standard form factors.
SSI created the Entry level Power Supply (EPS)
specification to define an industry-standard
power supply form factor for entry-level
pedestal (standalone tower chassis) servers.
Initial EPS standard was based on ATX but with
several enhancements, such as the use of a
24-pin main power connector.

EPS originally used form factor identical to ATX,
but later extended to support higher power out
Higher power requires a deeper PSU body.
EPS extends depths to 180mm and 230mm.
Most 500+ watt PSUs are made in EPS12V FF
If room in your ATX chassis and need the
power, an EPS12V PSU is good choice.
EPS / EPS12V
Form Factor

TFX12V Form Factor
TFX12V is for smaller chassis 9-15 liters in
volume, using microATX, FlexATX, or
Mini-ITX motherboards.
Designed to deliver 180-300 watts output.
Only one mechanical form for TFX12V.

CFX12V Form Factor
CFX12V is for mid-size chassis 10-15 liters
in volume, using microBTX motherboards.
Shape of PSU includes a ledge so it can
extend over motherboard, reducing size.

LFX12V Form Factor
LFX12V is for ultra-small chassis 6-9 liters
in volume, w/ PicoBTX or nanoBTX MBs.
Shape of PSU includes a ledge so it can
extend over motherboard, reducing size.

PSU manufacturer named Fortron Souce Power
introduced the FlexATX standard.
Now used by HP/Compaq, IBM and others.
Designed to deliver nominal outputs 180-270w.
a.k.a 1U power supplies as they fit in 1U chassis.
Usually include one or two internal 40mm fans.
FlexATX Form Factor

CPU Power Connectors
Power for CPUs comes from the voltage
regulator module (VRM) in motherboards.
VRM senses CPU voltage requirements via
sense pins on processor, and calibrates
itself to provide proper voltage to run CPU.
VRM runs on +5V or +12V input power.

Early VRM designs used +5V regulators.
Most systems now use +12V regulators.
Higher voltage reduces current draw.
Pulling 12V at 8A over 18-gauge wire can
melt a 6A-rated ATX connector!
CPU Power Connectors

Four-pin +12V CPU
Power Connectors
To augment +12V power to MB, Intel
created new +12V power supply spec.
The 4-pin +12V power connector is
specified for ATX 12V standards.
This connector consists of Molex Mini-Fit Jr.
connector housing with female terminals.

Have two +12V power pins, each rated 8A
w/ standard or 11A w/ HCS terminals.
Provides up to 16A more current to MB, for
total of 22A of +12V on 20-pin main.
Combining 20-pin main plus 4-pin +12V
provides 443 watts max to motherboard.
Adding +12V connector supports up to
500w without overload/meltdown !
Four-pin +12V CPU
Power connectors

Four-pin +12V CPU
Power Connector

Peripheral To 4-Pin +12V
CPU Power Adapters
If installing a MB in system w/o +12V connection
for CPU voltage regulator, be careful!
Some power adapters can convert one extra
peripheral power connector to +12V.
There are two +12V terminals in a +12V 4-pin
connector but only one in peripheral connector.
Not recommended to use these adapters on any
processor that requires more than 75w !

Peripheral To 4-Pin +12V
CPU Power Adapters
Not Recommended for >75w CPUs!
https://en.wikipedia.org/wiki/List_of_CPU_power_dissipation_figures

Backward and
Forward Compatibility
Can you connect your new power supply with a
24-pin connector to your MB w/ 20-pin socket?
Can you connect a 24-pin motherboard to your
power supply with a 20-pin connector?
Adapters to convert 24-pin to 20-pin connectors
are available but may not be necessary.
Compatibility has been engineered into
connectors, power supplies, and motherboards.

All 24-pin power supply connectors have extra 4 pins
placed separately on the end of the connector.
This allows you to plug a 24-pin connector into a 20-pin
motherboard without an adapter.
Just position the connector so that the extra 4 pins are
empty. But beware component blockage.
Backward and Forward
Compatibility

Compatibility

For 20-pin connectors plugged into a 24-pin
motherboard, four terminals at the end of the
motherboards sockets are not connected.
This will work because the first twenty sockets
are the same as the 20 pins on the connector.
Caution! Does the motherboard draws more
current than the 20-pin connector can handle?
Compatibility

Compatibility
Yes You Can Do This!

Floppy Power Connectors
Red = +5v / 2A Yellow = +12v / 2A

Peripheral Power Connectors
Yellow = +12v / 5A Red = +5v / 5A

Serial ATA (SATA) Power
Connectors
Yellow = +12v / 4.5A Orange = 3.3v / 4.5A
Red = +5v / 2A

PCI Express Auxiliary
Graphics Power Connectors
Designed to power video card drawing up to
75 watts max through PCIe x16 slot.
For graphics cards needing more power,
two standards for supplying additional
power via addl. graphics connectors
exist.....

PCI Express x16 Graphics
150W-ATX Specification
A 6-pin (2x3) auxiliary power connector that
delivers an extra 75 watts to a graphics
card directly from power supply for a total
of 150 watts to the graphics card.
A 6-pin Molex Mini-Fit Jr Connector is used.

PCI Express 225W/300W
Specification
An 8-pin (2x4) auxiliary power connector
that delivers an extra 150 watts to a
graphics card directly from power supply
for a total of 300 watts to graphics card.
A 8-pin Molex Mini-Fit Jr Connector is used.

PCI Express Auxiliary Graphics Power
Connectors

PCI Express 150W or 300W
Connector Specifications
If your graphics card does not
detect that ENOUGH power is
being supplied – the graphics
card may shut down!

If graphics card has 8-pin socket, but power supply has
6-pins, you can plug 6-pin into 8-pin socket.
If a graphics card needs full 300w and has both 8 and
6-pin connector on board, if you attach two 6-pin
connectors the card may shut down!

6-pin use two +12V wires for 75 watts.
8-pin uses three +12V wires carrying 150 watts.
Often called PCI Express Graphics (PEG)
Scalable Link Interface (SLI) or
CrossFire power connectors.

SLI and CrossFire are NVIDIA and AMD
methods of using two video cards in
unison.
Most power supplies are rated as SLI and
CrossFireX-ready.

Many Power Supplies are linear design.
Linear Design
Output voltage follows input voltage so
AC input fluctuations seen on DC output.
A transformer is required.
AC input (60 Hz) hard to filter out requiring
large filter capacitors and rectifiers.
Higher weight and cost.
Linear Power Supplies

Switching Power Supplies
PC Power Supplies are switching design.
Switching Design
Uses high-speed oscillator circuit to convert
higher AC voltage to lower DC voltage.
No transformer is required.
More efficient in size weight and energy.
MUST have a load on at all times.

Some have built-in load resistors
and can run no-load.
Most do not and need a
small load to operate properly.
To bench test a power supply place a load
on all positive voltage outputs.
Power Supply Loading

Mean Time Between Failures (MTBF)
Calculated average hours the power supply
is expected to operate before failing.
Over Voltage Protection
Defines trip points for each output at which
the power supply shuts down.
Power Supply Specs

Power Factor Correction
Power Factor measures how effectively
electrical power is being used on a scale
of 0 to 1 (zero to one).
High Power Factor means power is being
used efficiently, a low figure is inefficient.

Two types of loads are placed on AC lines:
Resistive
Power to heat, light, motion, or work.
Inductive
Electromagnetic field: transformer or motor.

Resistive load is working power and is
measured in kilowatts (kW).
Inductive loads are reactive power and is
measured in kilovolt-amperes-reactive
(kVAR)

Working power and reactive power together
make up apparent power, measured in
kilovolt-amperes (kVA).

Power factor is ratio of working power to
apparent power, or kW / kVA.
Ideal power factor is 1, where working
power and apparent power are the same.

Resistive load is easy to understand.
A lightbulb that consumes 100W of power
generates 100W worth of heat and light
energy. This is a pure resistive load.

With inductive loads some power is required to
saturate the windings and generate the
magnetic field, even if no work is being done.
A power transformer not connected to anything is
an example of a pure inductive load.
Apparent power draw exists to generate the
fields, but no working power exists because the
device isn't doing anything.

When a transformer is connected to a load, it uses both
working power and reactive power.
Power is consumed to do work, and apparent power is
used to maintain electromagnetic fields intransformer.
In AC circuits, these two loads can become out of sync
or phase - they don't peak at the same time.
Generates harmonic distortions back into power line.

PFC adds capacitance to the circuit to
maintain the inductive load without
drawing additional power from line.
This makes the working power and
apparent power the same which results in
power factor 1.

Passive Power Factor Correction
Adding capacitors to a circuit.
Active Power Factor Correction
Requires a more intelligent circuit designed
to match resistive and inductive loads so
electrical outlet sees them as same.

Active Power Factor Corrected PS
Draws low distortion current from AC source
and has power factor of .9 or greater.
Non-Power Factor Corrected PS
Draws distorted current (nonlinear load)
with a power factor of .6 - .8
Only 60% of apparent power consumed is
actually doing work.

Active PFC may not lower your power usage.
But active PFC reduces the load on wiring.
With PFC all power input is converted to work.
With PFC wiring is less overworked.

Always use PC power supplies with Active PFC.
80 PLUS certification requires Active PFC.
Main benefit of PFC is power supplies do not
overheat building wiring or distort AC source
waveform, which causes less interference.

Scalable Link Interface (SLI) Ready
and CrossFireX Certifications
NVIDIA and AMD certify power supplies for multiple
graphics cards in SLI or CrossFire configuration.
These configurations put extreme demands on the
PSU, as it has to power up to three video cards,
which may be drawing 300w or more each.
Certification process involves PSU manufacturers
sending PSUs in for testing.

Power-Use Calculations
When expanding or upgrading your PC, ensure
the power supply is capable of providing
sufficient current to power all internal devices.
Power consumption can vary greatly for different
devices such as CPUs and video cards.
Consult data sheets or technical manuals for
specific components.

After summing all component power
consumption, multiply total power consumed
by all components by 1.5 to determine size of
power supply required.
Power supply wattage calculators:
https://us.msi.com/power-supply-calculator/
Power-Use Calculations

80 PLUS Certification
2004-Northwest Energy Efficiency Alliance
(NEEA) funded 80 PLUS program to
encourage computer manufacturers to
install highly efficient power supplies at
80% efficiency or higher.
Systems with more efficient power supplies
consume 15 - 30% less power than
conventional models.

80 PLUS Certification
Power Supply Efficiency
% of Rated
Load
10% 20% 50% 100%
80 PLUS – 80% 80% 80%
80 PLUS Bronze – 82% 85% 82%
80 PLUS Silver – 85% 88% 85%
80 PLUS Gold – 87% 90% 87%
80 PLUS Platinum – 90% 92% 89%
80 PLUS Titanium 90% 92% 94% 90%

Power Cycling
Results in thermal or temperature shock.
As a system warms up it's components expand.
As it cools off the components contract.
Various materials in system have different
thermal expansion coefficients, and they
expand and contract at different rates.
Over time, thermal shock deteriorates system.

Insulate system from thermal shock !
On power-up MB components go from
ambient temp to 185 F in 30 minutes.
On shut-down, components cool back to
ambient temperature.
Thermal Shock

Largest cause of component failure.
Chip cases split, allowing moisture to enter.
Internal wiring and contacts can break.
Circuit boards develop stress cracks.
Thermal Shock

SMT components expand and contract causing
enormous stress at solder joints.
Components with heat sinks overheat and fail
as adhesives deteriorate and break thermal
bonds.
Socketed devices / connectors loosen or creep.
Thermal Shock

Thermal expansion / contraction also affect hard drives.
Most drives have thermal compensation routines that
adjust every 5 - 30 min and every 30 min thereafter.
Best solution is keep the system at the same
temperature by either leaving it on or leaving it off.
Biggest problem with system on 24/7 is wasted energy.
Thermal Shock

A typical desktop PC consumes 75w to 300w
when idling and 150w to 600w when in use.
Turning a system off when not in use
saves energy.
When in sleep mode, a system saves the full
system context (system state, RAM, etc.).
Energy Use

With improved power management
capabilities of modern hardware,
combined with stability and control
features built into modern OSs,
systems can sleep and resume almost
instantly.
Energy Use

Power Supply
Troubleshooting
A parity error message may indicate a problem
with power supply.
Frequent parity errors at the same memory
location the problem is defective memory.
Random parity errors at different memory
locations the problem is power supply.

PC problems often related to power supply:
Any power-on or startup failures or lockups
Spontaneous rebooting or intermittent lockups
Intermittent parity check or other memory errors
Power Supply
Troubleshooting

PC problems often related to power supply:
Hard disk / fan both failing (no +12V).
Overheating due to fan failure.
Small brownouts that cause system to reset.
Electric shocks felt on system case / connectors.
Power Supply
Troubleshooting

Static discharges that disrupt system operation.
Erratic recognition (enumeration)
of bus powered USB peripherals.
Some power supply problems are very obvious:
Computer dead / smoking / blown breaker.
Power Supply
Troubleshooting

AC Power Input: Reseat or replace power cord.
DC Power Connections: Reseat connectors.
DC Power Output: Measure / verify proper voltages.
Check installed peripherals:
- Remove all boards and drives and retest system.
- Add items one at a time until system fails again.
Power Supply
Troubleshooting

Overloaded Power
Supplies
Cheaper Power Supplies
Noisy / unstable power.
Cause problems with the system.
Under-Engineered Power Supplies
Run hot.
Cause system overheating.

Inadequate Cooling
Building or upgrading your own system?
You are responsible for properly cooling it!
Place hottest-running boards near fan or
ventilation.

Inadequate Cooling
Measurement Tools

Running system with cover off:
- Can cause system to overheat !
- Verify fans blow in the right direction.
- System must be cooled by convection.
- Fill empty slot positions with brackets.
Inadequate Cooling

Measuring Voltage
“Back Probing”
“Front Probing”
Short Pins 15 & 16
on 24-pin Connector
Pin 15 = Black = Ground
Pin 16 = Green = PS_ON

Power Supply
Recommendations
Use industry-standard PS such as ATX12V
Adequate power connectors for MB used
Adequate power output (watts) for system
80 PLUS certification
Active Power Factor correction (80-Plus)
SLI or CrossFIre certification
Single +12V rail design

Modular Cables
Modular cables adds resistance due to addl contacts.
Connectors are Molex Mini-Fit Jr type (10 milli-ohms).
Most power supply cables use 18AWG, which has a
resistance of 6.4 milli-ohms per foot
Adding an extra connector on modular cable is equal to
about 1.5 feet of wire in additional resistance.

Modular cables add cost to power supply construction
as reflected in a higher final price.
They can also create clearance issues with other
components in the system.
Modular cables are easily lost or misplaced.
To prevent this, additional cables can be stored in the
case when building a system.
Modular Cables

Power Protection Systems
Easiest: power-off / unplug your PC in a thunderstorm.
Automatic shutdown of PC during power glitches is a
built-in function of most high quality supplies.
Power can be reset by cycling power switch.
Auto-restart shuts down system 3-6 secs delay before
resetting itself and powering back on.

Surge Suppressors/Protectors
Simplest form of power protection.
Inserted between the system and power line.
Absorbs high voltage transients (lightning) using
Metal Oxide Varistor (MOV) to clamp and
shunt all voltages above a certain level.
Designed to accept voltages as high as 6,000V
and divert any power beyond 200V to ground.

Can handle normal surges, but powerful surges like
direct lightning strikes blow through them.
Surge protectors need status light to indicate when a
surge large enough to blow the MOV has occurred.
Without this light the protector is useless-you never
know when it has stopped protecting.

Underwriters Laboratories UL1449 standard.
Any UL1449 standard offers protection beyond
what your PC Power supply already offers.
If UL1449 is not on the package, DON'T BUY IT!
Another good feature in surge suppressors are
circuit breakers that can be manually reset.

Power Line Conditioners
Wires can act as antennas and have voltage induced in
them by nearby EM fields (other wires, motors, etc.).
Digital circuitry responds with to noise of even 1-2V,
making those inductions troublesome.
Line conditioners handle these problems: filter power,
bridge brownouts, suppress high-voltage and current
conditions, and buffer between power line & system.

Standby Power Supplies
Off-line devices functioning only when power disrupted.
When SPS system senses loss of AC line current it
switches to battery / power inverter.
Power inverter converts battery power to 120V AC,
which is then supplied to system.
Provides 15 minutes or less of operation enabling
survival of brief power interruptions.

Uninterruptable Power Supply
On-line devices always supplying power to PC.
System always operates from battery.
Battery charger keeps battery charged at rate equal to
or greater than power consumption rate.
When AC current interrupted it continues functioning
undisturbed as only battery charging is lost.

Real-Time Clock / Nonvolatile
RAM (CMOS RAM) Batteries
CMOS RAM chip: real time clock (RTC) with
at 64 bytes of nonvolatile RAM (NVRAM).
Clock enables software to read/preserve
date/time even system powered off.
NVRAM also stores basic system config,
including memory /drives installed, PnP
configuration, power-on passwords, etc.

Modern CMOS Batteries
Motherboard NVRAM (CMOS RAM) batteries
typically lithium design as they last 2-5 years.
Most use a manganese dioxide (MnO2) cathode,
designated by a CR prefix in the part number,
others use a carbon monoflouride (CF)
Cathode, designated by a BR prefix.
Because CR series is cheaper and easier to find,
they are most common in PCs.

CMOS Battery
Troubleshooting
Symptoms indicating battery is about to fail:
-need to reset clock on PC each reboot
-problems in POST e.g. such as drive-detection
If these occur:
Note of your system's CMOS settings and
replace battery as soon as possible.
When you replace a battery, existing data stored
in NVRAM is lost.

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