Transcript PSU_Part1_EfficiencyPowerFactor - Renesas e

Application information
Power Supply Unit (PSU)
Part 1…Efficiency and power factor
Renesas Electronics Corporation
General Purpose Systems Marketing Dept.
General Purpose Systems Division
Marketing Unit
Sep. 2011
Rev.1.0
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Course Introduction
 Purpose
This course is intended to expand your knowledge of power supply units to
help you educate customers about Renesas products. Included in this is
product information on Renesas ICs and discrete devices for power supply
units.
 Objectives
 Learn about the structure of PSUs
 Learn about types of PFC ICs and isolated DC/DC converter ICs,
their functions and line-up
 Learn about discrete devices used in PSUs
 Contents
 This course consists of five parts
–Part 1…Efficiency and power factor
–Part 2…Standards or regulations for PSUs
–Part 3…PFCs (types, modes, how they work, roadmap, etc.)
–Part 4…Isolated DC/DC (types, how they work, etc.)
–Part 5…Discrete devices (lineup, roadmap, etc)
 Each part has its own introduction
--- please refer to each introduction for further information
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Introduction to Part1
 Purpose
Part1 provides basic knowledge of power supply units
 Objectives
Learn about efficiency and power factor
Learn about the structure of the PSUs
Learn about the role of each block in the PSUs
 Contents
38 pages
 Learning Time
30 minutes
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Definition of terms
(valid in this course’s texts only)
 AC voltage---voltage applied to the set
(i.e. voltage from a wall outlet)
 AC current---electrical current flow into the set
 AC voltage and frequency differ by country or area
To simplify explanation,
they are assumed to be
the following in these
materials unless
otherwise specified.
•AC voltage: 100 V
•AC frequency: 50 Hz
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Efficiency
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Efficiency
 Some power is lost at PSU!
---That’s bad but unavoidable
Efficiency (η) =
Output power
Input power
=
Input power - loss at PSU
Input power
η: Eta (Greek letter), used for expressing efficiency of PSU
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Power Factor
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Remember this from your
university classes?
Power factor (PF) = real power / apparent power
(a decimal number between 0 and 1)
Generally, there is no symbol for power factor,
but it is often abbreviated as “PF” in tables and figures.
Real power (P): actual power consumed at load (unit: watt (W))
Electrical charge is calculated based on this value
Reactive power (Q): unconsumed power coming and going between
(unit: var) wall outlet and load
Apparent power (S): the product of the effective values of
(unit: volt-ampere (VA))
voltage and current
It's all Greek to me --- William Shakespear “Julius Caesar”
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Well, let’s look at some examples
Current is proportional to
voltage in these devices
->Power factor = 1
+100 V
AC voltage
(AC 100 V)
-100 V
proportional
+0.5 A
AC current
(AC 0.5 A)
-0.5 A
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Dimmer for electric light bulb
AC current controlled
by dimmer
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For the dimmer…
AC voltage
AC current
AC voltage
 Power is consumed only during the
period colored yellow
(i.e. power is fed to bulb), because
Power = Voltage × Current
AC current
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The meaning of PFC is …
PF is 0.6
 Power Factor (PF) is the index which shows how the
waveforms of AC voltage and AC current overlap each
other
(refer to supplement-1 for the strict definition of PF).
 A dimmer changes the brightness of a light bulb by
changing the PF.
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Harmonic current
 Harmonic current is an integer multiple of the
fundamental frequency (e.g. 50 Hz in Tokyo)
100 Hz (2nd harmonic), 150 Hz (3rd harmonic),
200 Hz (4th harmonic)…
 PF < 1
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= distorted wave
= contains much harmonics (noisy)
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Peak height of the AC current wave
does not change in a dimmer
Though the brightness is
different, ….
How about other equipments?
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In electronic equipment
--- the peak height changes
 The smoothing circuit cannot take in the
current continuously
(refer to “Supplement 3”)
 This noise goes back to the AC outlet and
is fed to other equipment, and may have a
negative effect on that equipment.
If the PF were 1, such a small current would be
enough for the equipment
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If PF is low ……
 Electric power companies have to generate more
electricity corresponding to the peak of the current.
-> need more fuel …not environmentally friendly
 “TV doesn’t work while the microwave oven is operating.
Help!” …from users
 “Hi-Fi set hums when the air conditioner is turned on.
Uncomfortable” …from users
Let’s correct the power factor -> PFC ICs are needed
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Structure of the PSUs
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The simplest: primary side of PSUs (1)
 Transformer changes AC voltage (eg. AC 100 V -> AC 12 V),
and electrically isolates the secondary side from the primary
side.
 Diode bridge rectifies AC voltage (eg. AC 12 V). The smoothing
capacitor smoothes the output voltage from the diode bridge.
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Why are transformers needed?
 If any of the parts on the red path in the figure
are out of order, AC 100 V may appear on the
secondary side
 We are obligated by law to isolate all exposed
conductive surfaces from the AC line to avoid the
risk of electrical shock (e.g. PSE law, etc.)
->So, transformers are needed!
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The simplest primary side of PSUs (2)
 The disadvantage of this type of PSU is that it is HEAVY and
LARGE!!
 Let’s apply higher frequency (>50Hz) to the transformer
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Switching mode power supply: SMPS
Small and light PSUs (1)
 DC 100 V is converted to 100-300 kHz pulses by MOSFET and
Isolated DC/DC IC, then it’s fed to a transformer
 This enables the use of a smaller transformer
-> Small, light, GOOD!!
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Switching mode power supply: SMPS
Small and light PSUs (2)
 The main issue for this type of SMPS is the
generation of harmonic current as shown before.
(refer to Supplement 3)
 The harmonic current goes back to the AC outlet
→What should we do???!!!
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Better SMPS:
--- Isolated DC/DC + PFC
 PFC IC filters out the harmonic and prevents the
harmonic current from going back to the AC outlet
→GREAT! WONDERFUL! SPLENDID!
 This is the solution to satisfy harmonic regulations
and EnergyStar
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Summary
PFC
•Corrects the power factor (reduces the harmonic current back to AC outlet)
Isolated DC/DC
•drive the transformer with a few hundred kHz pulse
There are COMBO ICs which have both PFC function and isolated DC/DC
function on one chip
Besides these, there is an auxiliary regulator on the primary side that
supplies power to the PFC IC and isolated DC/DC IC
Auxiliary regulator
•Small power regulator which supplies regulated voltage to PFC IC, isolated
DC/DC IC, receiver for remote controller, etc..
•Aux. regulators have their own regulator on-chip
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Various PSU structures
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Supplement 1
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Definition of
power factor
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Definition of power factor &
various powers
 Power factor (PF)
(no unit)
Ratio of real power and apparent power.
A decimal number between 0 and 1
power factor = real power / apparent power
 Real power
(unit: W)
Actual consumed power at load.
Power consumed at electric bulbs and heaters
(Nichrome) is almost all real power.
Subject of electricity charges.
 Apparent power
(unit: VA)
The product of applied voltage (RMS) to the
load and current (RMS)
apparent power = voltage (RMS) × current (RMS)
RMS: Root Mean Squared – See Supplement 2
 Reactive power
(unit: var)
Power not consumed at load, and coming
and going between AC outlet and the
equipment (see “AC current 2” on next page)
 Relationship between these powers
apparent power =
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(real power)2 + (reactive power)2
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Example of real power and
apparent power
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Supplement 2 What is RMS?
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RMS: Root Mean Square
20,000
 RMS value is calculated
by taking the square of
the voltage, getting its
average (mean), and
then its square root
Square of AC 100 V
15,000
10,000
500
AC 100 V
[Note:]
• Average of AC voltage (or current)
is defined as average of a half cycle
• Average of AC 100 V (RMS) is
about 90 V
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141
0
-141
1/4 cycle
1/2 cycle
3/4 cycle
AC 100 V is
 Amplitude: 141 V (√2 times RMS)
 RMS： 100 V
 Average： 90 V (about 90% of RMS)
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1 cycle
Supplement 3 Noise generated
from PSUs
---Harmonic current noise
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The AC current flows intermittently (1)
AC voltage
Output voltage from
the diode bridge (Vd)
 During period (a) in left
fig.,
Vd < Vc, so current does
not flow from the diode
bridge to the capacitor
Voltage at thee
capacitor (Vc)
Current to
the capacitor
(a) (b) (a) (b) (a) (b) (a)
AC current
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 During period (b),
Vd ≧ Vc, so current
flows from the diode
bridge to the capacitor
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The AC current flows intermittently (2)
AC voltage
Output voltage from
the diode bridge (Vd)
Voltage at the
capacitor (Vc)
Current
to the capacitor
Peak is high --- electrical
energy for next half cycle
has to be charged in a
short period.
Large and intermittent
current generates much
noise.
During this period,
although there is AC
voltage, there is no current
(i.e. no electrical power is
taken into the PSU)
Electric power companies have to prepare excessive facilities and to
generate electricity corresponding to the peak of the current
-> It is neither energy saving nor environmentally friendly
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Waveform distorted by harmonic
currents
 Photograph of the waveform
shown on previous page
AC voltage
(sinusoidal)
 This current wave is
distorted by odd order
(3rd, 5th, 7th…) harmonic
current
 PF is far smaller than 1 in
this case
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Harmonic current [A]
AC current
6
5
Fundamental = 50 Hz
4
3
2
1
0
3rd
5th
7th
9th
(150 Hz) (250 Hz) (350 Hz) (450 Hz)
Order of harmonic current
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Supplement 4
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Inductors and
magnetic saturation
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Inductors
Photo: Wikipedia
 Current flowing in inductors generates magnetic
fields
 When magnetic force (magnetic flux) through the
inductor varies, the inductor generates voltage to
prevent variation
 When DC voltage is applied to the inductor, current
increases with time
 During the period when current is increasing, the
electric power is used to form and to strengthen
the magnetic force
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Magnetic saturation and size of trans.
 The state at which the core cannot have
any larger magnetic force is called
magnetic saturation.
 In this state, the magnetic field does not
change correctly with input voltage.
In case of a transformer, electric power
applied on the primary side is not
conducted to the secondary side
correctly.
 Preventing saturation
 Enlarge the core or select material that
is difficult to saturate for the core
-> Transformer becomes large and heavy
 Drive the transformer with high frequency (see right fig.)
-> Small, light and low cost transformers can be applied
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Thank you
Thank You
Renesas Electronics Corporation.
©2011. Renesas Electronics Corporation. All rights reserved.