apped-101054a confidential application information power supply unit (psu) ©2010. renesas...

APPED-101054AConfidential

Application information

Power Supply Unit (PSU)

©2010. Renesas Electronics Corporation, All rights reserved.

Part 1…Efficiency and power factor

Sep. 2011 Rev.1.0

General Purpose Systems Marketing Dept.General Purpose Systems DivisionMarketing Unit

Renesas Electronics Corporation


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

2 ©2010. Renesas Electronics Corporation, All rights reserved.


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 PSUsLearn about the role of each block in the PSUs

Contents38 pages

Learning Time30 minutes



AC voltage---voltage applied to the set (i.e. voltage from a wall outlet)

AC current---electrical current flow into the setAC voltage and frequency differ by country or area

To simplify explanation, they are assumed to bethe following in these materials unless otherwise specified.

•AC voltage: 100 V•AC frequency: 50 Hz

Definition of terms (valid in this course’s texts only)


APPED-101054A

Efficiency



Efficiency

Some power is lost at PSU!---That’s bad but unavoidable

Efficiency (η) =Input power

Output power

Input power

Input power - loss at PSU

η: Eta (Greek letter), used for expressing efficiency of PSU

=


APPED-101054A

Power Factor



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”



Well, let’s look at some examples

Current is proportional to voltage in these devices->Power factor = 1

AC voltage(AC 100 V)

+100 V

-100 V

AC current(AC 0.5 A)

+0.5 A

-0.5 A

proportional



Dimmer for electric light bulb

AC current controlled by dimmer



For the dimmer…

AC current

AC voltage

Power is consumed only during the period colored yellow(i.e. power is fed to bulb), because

Power = Voltage × CurrentAC current

AC voltage



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.



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 = distorted wave = contains much harmonics (noisy)



Peak height of the AC current wave does not change in a dimmer

Though the brightness is different, ….

How about other equipments?14 ©2010. Renesas Electronics Corporation, All rights reserved.


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



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


APPED-101054A

Structure of the PSUs



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.



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!



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



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!!



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???!!!


Switching mode power supply: SMPS

Small and light PSUs (2)


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



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


Various PSU structures


APPED-101054A

Supplement 1 Definition of power factor



Definition of power factor & various powers Power factor (PF) Ratio of real power and apparent power. (no unit) A decimal number between 0 and 1 power factor = real power / apparent power

Real power Actual consumed power at load. (unit: W) Power consumed at electric bulbs and heaters

(Nichrome) is almost all real power.Subject of electricity charges.

Apparent power The product of applied voltage (RMS) to the (unit: VA) load and current (RMS)

apparent power = voltage (RMS) × current (RMS)

Reactive power Power not consumed at load, and coming (unit: var) and going between AC outlet and the

equipment (see “AC current 2” on next page)

Relationship between these powers

RMS: Root Mean Squared – See Supplement 2

(real power)2 + (reactive power)2 apparent power =27 ©2010. Renesas Electronics Corporation, All rights reserved.


Example of real power and apparent power


APPED-101054A

Supplement 2 What is RMS?



RMS value is calculated by taking the square of the voltage, getting its average (mean), and then its square root

AC 100 V isAmplitude: 141 V (√2 times RMS)RMS ： 100 VAverage ： 90 V (about 90% of RMS)

[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

0

500

10,000

15,000

20,000

1/4 cycle 1/2 cycle 3/4 cycle 1 cycle

141

-141

AC 100 V

Square of AC 100 V

RMS: Root Mean Square


APPED-101054A

Supplement 3 Noise generated from PSUs---Harmonic current noise



Output voltage fromthe diode bridge (Vd)

AC voltage

Current to the capacitor

Voltage at thee capacitor (Vc)

AC current

(a) (b) (a) (b) (a) (b) (a)

During period (a) in left fig.,

Vd < Vc, so current does not flow from the diode bridge to the capacitor

During period (b), Vd Vc, so current ≧

flows from the diode bridge to the capacitor

The AC current flows intermittently (1)



Peak is high --- electrical energy for next half cycle has to be charged in a short period.Large and intermittent current generates much noise.

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

During this period, although there is AC voltage, there is no current (i.e. no electrical power is taken into the PSU)

Output voltage fromthe diode bridge (Vd)

AC voltage

Voltage at the capacitor (Vc)

Current to the capacitor

The AC current flows intermittently (2)



This current wave is distorted by odd order (3rd, 5th, 7th…) harmonic current

PF is far smaller than 1 in this case

AC voltage(sinusoidal)

AC current

0

1

2

3

4

5

6

3rd 5th 7th 9th

Order of harmonic current

Harm

onic

curr

ent

[A]

Fundamental = 50 Hz

(150 Hz) (250 Hz) (350 Hz) (450 Hz)

Photograph of the waveform shown on previous page

Waveform distorted by harmonic currents


APPED-101054A

Supplement 4 Inductors and magnetic saturation



Inductors

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

Photo: Wikipedia



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

Magnetic saturation and size of trans.

Drive the transformer with high frequency (see right fig.) -> Small, light and low cost transformers can be applied


apped-101054a confidential application information power supply unit (psu) ©2010. renesas...

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