Power Supply Design Seminar

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Topic 1 Presentation:

High Power Factor and High Efficiency — You Can Have Both

Reproduced from2008 Texas Instruments Power Supply Design Seminar

SEM1800, Topic 1TI Literature Number: SLUP282

© 2008, 2011 Texas Instruments Incorporated

SLUP254

Topic 1

High Power Factor and High Efficiency: You Can Have Both

I C h d Bi LIsaac Cohen and Bing Lu

SLUP254

Agenda

Definition of power factor and discussion of the applicable standards

Effect of the power factor on power-distribution losses

Benefits of active PFC

Effect of the input-voltage range on PFC efficiency

Configurable PFC topologiesConfigurable PFC topologies

The buck PFC: a solution for universal input-voltage applicationsapplications

Conclusions

1-2

SLUP254

Definition of Power Factor:A Quick ReviewQu c e e

PF = Real Power / Apparent Powerpp

Real Power = T

0

1vidt

T ∫

Apparent Power = VRMS × IRMS

0T ∫

Definition valid for arbitrary current and voltage waveformswaveforms

1-3

SLUP254

Linear and Nonlinear Loads

Sinusoidal source, linear load

Both the voltage and the current are sinusoidal but gnot in phase; power factor less than unity

PF known as : cos(Φ) or “displacement power factor”( ) p p

Sinusoidal source, nonlinear load

PF i d t i d b h l d h iPF is determined by phase angle and harmonics

Harmonics increase apparent power, the power factor l l th italso less than unity

Reducing harmonics increases the power factor

1-4

SLUP254

Standards for Power-Factor Correction

EN61000-3-2

Focuses on line-current harmonics

Four categories according to different end equipments

Most power supplies are Class D

Energy Star®

Power supplies with greater than or equal to 100 W inputPower supplies with greater than or equal to 100-W input power must have a true power factor of 0.9 or greater at 100% of rated load when tested at 115 V, 60 Hz

Universal-input power supplies need to meet requirements of both standards

1-5

SLUP254

Power Factor and EN61000-3-2 Class D

Meeting EN61000-3-2 harmonic standard is not enough to meet the Energy Star power-factor requirementrequirement

Harmonic Order n

75 W < P < 600 WMaximum Permissible Harmonic

Current (mA/W)3 3.45 1.9

7 1.0

9 0.5

11 0.3513 0.269

15 ≤ n ≤ 39 3.85/n

1IPF 0 726= =DisplacementF 1=

1-6

392

nn 1,3

PF 0.726

I=

= =

∑Displacement DistortionPF F F= ×

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Harmonics for Energy Star

A power supply drawing square-wave line current is well within Energy Star PF limits but will fail the IEC harmonic-content standardharmonic content standard

PF 0.9=

IACIAC

AC

11I 0.56 mA / W 0.35 mA / W= >

1-7

SLUP254

Meeting Both Standards

The PFC circuit must increase the PF to the

Energy Star limit AND attenuate the

harmonics below the limits of EN61000-3-2harmonics below the limits of EN61000 3 2

1-8

SLUP254

The Power Factor and the Power-Distribution Losseso e st but o osses

If the PF is low, an increase in RMS current is required to deliver a given amount of powerrequired to deliver a given amount of power

10

(A)

8

6MS

Cur

rent

4

zed

Line

RM

2

Nor

mal

i

1-9

0 0 0.2 0.4Power Factor

0.80.6

SLUP254

Normalized Power-Distribution Losses

Wiring 0.05 R =w Ω

G t LoadR = 1load Ω

GeneratorVgen

+–

Where:Vgen = Effective AC source voltageRw = Distribution wiring resistanceR L d i t

1-10

Rload = Load resistance

SLUP254

Effect of the Power Factor on Power-Distribution Losseso e st but o osses

PFC reduces

0.92

0.94

ency

, η

distribution losses

Assuming 5% 0.9

0.88

0 86utio

n Ef

ficie

distribution loss, the system will break even if the efficiency

0.84

0.86

0 82

Dis

trib

ueven if the efficiency of the PFC circuit is 0.953 0.82

0.4 0.5 0.6 0.7 0.8 0.9 1Power Factor (PF)

1-11

SLUP254

Direct Benefits of PFC/Harmonics AttenuationC/ a o cs tte uat o

Meet the requirements of EN61000-2-3 and Energy Star

Higher power factor

Allows higher power draw from 115-V linesg p

0.5 PF = 719 W, 0.9 PF = 1294 W (intermittent rating)

Reduced stresses on neutral conductorsReduced stresses on neutral conductors

Improved electrical system distribution efficiency (Only if the PFC efficiency is high enough!)(Only if the PFC efficiency is high enough!)

Reduced VA rating of standby power systems

1-12

SLUP254

Indirect Benefits of Active PFC/Harmonics Attenuationct e C/ a o cs tte uat o

Facilitates power supply holdupp pp y p

Universal input voltage capability ( 85 to 265 VAC)( 85 to 265 VAC)

Improved efficiency of downstream DC/DC converters

1-13

SLUP254

Active PFC/Harmonic Attenuation Circuits

Most popular active PFC circuit: The boost converterconverter

This additional conversion stage adds power dissipation

Efficiency of the PFC stage is stronglyEfficiency of the PFC stage is strongly dependent on the difference between the input and the output voltage

1-14

SLUP254

Analysis of the Boost PFC Efficiency

To examine the effect of the differenceTo examine the effect of the difference between the input and output voltages on efficiency, we define the “Boost Factor” BF:y,

outVBF out

in(pk)

VBF = V

1-15

SLUP254

Analysis of the Boost PFC Efficiency

A nonisolated converter is similar to an autotransformer

Separate the power flow by separating the power transferred directly to the output from p y ppower processed by the PFC circuit

Emulate the PFC boost converter by addingEmulate the PFC boost converter by adding the output of a flyback converter (“ΔV converter”) in series with the rectified( ΔV converter ) in series with the rectified line-input voltage

The model is eq i alent to a boost con erter

1-16

The model is equivalent to a boost converter

SLUP254

The ΔV Converter

A little algebra shows that the DC transfer function of the proposed circuit is identical to that of a boost converter

ΔV Converter

converter

PFCOut

ΔV Converter

In Out

+Fl

ybac

k

lyba

ck O–

F Fl

out in inDV =V V

1 DV

+ ×−

1-17

inV1 D

=−

SLUP254

Peak-Power Rating of the ΔV Converter as a Function of Input Voltageas a u ct o o put o tage

1 22

1.36BF = 3 118

0.95

1.09

1.22

r, P Δ

V

BF = 3.118(V = 85 V)in

0.68

0.82

ized

Pow

er

BF = 1.418(V = 187 V)in

0 27

0.41

0.54

Nor

mal

i

BF = 1(V = 265 V)in

0

0.14

0.27

0 63 1 26 1 88 2 51 3 14 3 77 4 4 5 03 5 650

1-18

ωt0.63 1.26 1.88 2.51 3.14 3.77 4.4 5.03 5.650

SLUP254

Average-Power Rating of ΔV Converter as a Function of Input Voltageas a u ct o o put o tage

The average power delivered by the ΔV

0.73

ave)

delivered by the ΔV converter is a strong function of BF

0.62

0.5Pow

er, P

V(a

Δ

An increasing BF also corresponds to

0.5

0.38

ed A

vera

ge P

also corresponds to an increase in boost-PFC size and cost

0.27

0.151 1 44 1 88 2 32 2 76 3 2

Nor

mal

ize

1 1.44 1.88 2.32 2.76 3.2Boost Factor (BF)

1-19

SLUP254

Overall PFC Efficiency versus BF

ΔV converter efficiency = 90%

98.32

96 38

97.0297.67

%)

95.0895.7396.38

cien

cy,

(%η

93 13

93.7894.43

Effic

91.84

92.4893.13

1.44 1.88 2.32 2.76 3.21

1-20

1.44 1.88Boost Factor (BF)

2.32 2.76 3.21

SLUP254

Discussion of the Results

As the boost factor increases, the converter processes more power and the efficiency decreases

Component ratings and converter efficiency are driven by the lowest operating Boost Factor

For “local” voltagesDesigned for Boost Factor of 1.417

ΔV-converter average- and peak-power ratings are 40.1% and 58.9% of the PFC-stage output power

For universal input voltageFor universal input voltageDesigned for Boost Factor of 3.118

ΔV converter average- and peak-power ratings are 73% and

1-21

ΔV converter average and peak power ratings are 73% and 136% of the PFC-stage output power

SLUP254

Universal-Input Voltage Issues

Must or preference?

Logistics commonalityLogistics, commonality

“Regional” products

HVDC bus voltage

BOM f th PFC t d th d tBOMs for the PFC converter and the downstream DC/DC converter(s)

Trade-offs

1-22

SLUP254

Improving PFC Efficiency: Reduce Boost Factor!educe oost acto

Ideas:

Boost follower

Configurable PFC stage

Three level PFCThree-level PFC

Buck instead of boost PFC

1-23

SLUP254

Boost-Follower PFCO t t V lt (C ti l)

3.5

400

300V)

Output Voltage (Conventional)

e (Boost Follower)3

2 5r (B

F)

Volta

ge ( V

200Output Voltage (B

Input Voltage

2.5

Boo

st F

acto

r

2

ConventionalBoost PFC

100

0100 20 30 40 50 60 70 80 90 100

B

1.5

1100 150 200 250

Boost-Follower PFC

Conventional PFC maintains same output voltage for different

100 20 30 40 50 60 70 80 90 100Time (ms)

100 150 200 250Input Voltage, V (V)in

line voltages, the boost factor is 3.3 at 85-V inputBoost follower PFC changes its output voltage according to the AC line voltage (following AC line voltage)

1-24

Boost factor is greatly reduced and efficiency can be improved but Vout is not constant

SLUP254

Boost-Follower PFC Implementation

By adjusting the voltage-sensing divider network, output voltage can be changed with most off-the-shelf PFC controllersPFC controllers

UCC3817

RectifiedLine Voltage

V(Boost Output

Voltage)

out

UCC3817PFC Controller

VoltageAmplifier

R1998 kΩ

R3998 kΩ

Amplifier

Internal

R214.3 kΩ

R4Q1C1

+

–

ReferenceR439.2 kΩ

R539.2 kΩ

2N22226.6 µF

1-25“Simple circuitry gets that old PFC controller working in a boost-follower PFC Application,” By Michael O’Loughlin

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Design Considerations

“Boost Follower” reduces BF but:T f t ti i th d tTransformer turns ratio in the downstream converters must be lower, so:

Primary currents are higher, VA rating increases

Higher voltages on rectifiers, higher Vf

Larger filter inductor

Larger capacitors on the HV busLarger capacitors on the HV bus

1-26

SLUP254

Configurable Topologies

Use identical components for both 115-V and 230-V inputs

Same HVDC bus voltage

C f fConfiguration on assembly line for 115 V or 230 V

Nearly equal efficiencies for 115-V or 230-V operation

1-27

SLUP254

“Ideal” Configurable PFCL li i t fi tiHi h li i t fi ti Low line-input configurationHigh line-input configuration

At high line input, the MOSFETs, diodes, and inductors are in

At low line input, the MOSFETs, diodes, and

series

Output voltage is equal to the peak of the high line voltage

inductors are in parallel

Output voltage is only half the peak of the high line p g g

(same as conventional PFC)

Equal stresses on the converter components at either “High” or “Low” line-input configuration!

p gvoltage

1-28

Low line-input configuration!

SLUP254

Boost Factor of Ideal Configurable PFC3 5

)

3

3.5

ConventionalBoost PFC Converter

ost F

acto

r (B

F)2.5

2

Boo

2

1.5

Ideal Configurable

Input Voltage, V (V)in

1100 150 200 250

gPFC Converter

At “high” line-input condition, the BF and the output voltage are the same as for a conventional PFCAt “low” line-input condition the BF and the output

1-29

At low line-input condition, the BF and the output voltage are half the values of a conventional PFC

SLUP254

“Real-Life” Solutions

The “ideal” circuit is not practical

Voltage balancing for series devices

D i i i it f i i itDriving circuit for series circuits

Practical circuits will be less efficient than the ideal

1-30

SLUP254

Configurable Three-Level PFC

High line-input condition: two boost PFCs are in series

Low line-input condition: only one boost PFC is operating at each half lineseries operating at each half line cycle

1-31Note: Output-bus voltage does not change for different configurations

SLUP254

Boost Factor and Efficiency Estimation3 5

F)

3

3.5

ConventionalBoost PFC Converter

98

%)

Boo

st F

acto

r (B

2.5

2

96

94

Effic

ienc

y, η

(

B

1.5

1100 150 200 250

Ideal ConfigurablePFC Converter

92

9050 100 150 200 250 300

Configurable three-level PFC maintains same output voltage for different line conditions

Input Voltage, V (V)in

100 150 200 250 50 100 150 200 250 300Input Voltage, V (V)in

At low line input, each boost PFC operates half-line cycle and the equivalent output voltage is half of the total PFC output voltage

1-32

gThis reduced output voltage reduces the boost factor at low line input and improves efficiency

SLUP254

Benefits and Challenges

Benefits

Reduced boost factor improves overall system efficiency

Three-level structure, a lower-voltage switches and diodes can be used to improve efficiency

At low line conditions, the bridge is reconfigured as a doubler (bridge diodes can be paralleled)

ChallengesChallenges

At low line input, each boost PFC only operates half of the line cycle (component utilization)cycle (component utilization)

Devices referenced to different grounds: drive, current sensing

Voltage balancing on the output capacitors

1-33

Voltage balancing on the output capacitors

SLUP254

The Buck PFC

A buck converter can also shape the input currentHandles least power at lowest input (“Buck Factor”)

Iin

Vboost+

+

Outboost

––

Control(Modified(Modified

Peak CMC)

1-34

SLUP254

Idealized AC-Line Current with a Buck PFCt a uc C

1.51.000.5

0–0.5

1 0I

(A)

loadLow Line Input

(Normalized input t) –1.0

–1.50 5 10 15 20

Time (ms)

current)

1.51.00 5A

) 0.50

–0.5–1.0

I (A

loadHigh Line Input

(Normalized input current)

1-35

–1.50 5 10 15 20

Time (ms)

SLUP254

Buck PFC and Prevailing Standards

The buck PFC draws current only if the input voltage is higher than the output voltagevoltage is higher than the output voltage

The optimal output voltage for universal input voltage is 80 V

Waveform is “ugly” – but passes both EPAWaveform is ugly – but passes both EPA and IEC standards

1-36

SLUP254

Benefits of the Buck PFC

Solves the problem of low efficiency at low input voltage

Allows optimization of the downstream DC/DC converter

Inherent inrush-current control

“Gentler” on the HV diode than the boost converter it replaces

1-37

SLUP254

Efficiency of a 90-W Buck PFC versus the AC-Input Voltagee sus t e C put o tage

9896969492(%

)

9088

ficie

ncy

(

868482

Eff

8280

90 130110 170 190150 210 230 250 270AC Input (V)

1-38

p ( )

SLUP254

Conclusions

PFC provides many benefits but adds losses that are strongly affected by the Boost Factor

Reconfigurable-PFC topology is available that can reduce the effective BF and use the same components for both 115-V and 230-V lines—components for both 115 V and 230 V linesothers to be invented?

It remains to be seen if the pressure for ever higher e a s o be see e p essu e o e e g eefficiency will drive adoption of more complex topologies

The buck PFC is an attractive solution for universal input voltages at power levels below 600 W

1-39

SLUP282

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