Prof R T KennedyPOWER ELECTRONICS 21. Prof R T KennedyPOWER ELECTRONICS 22 Class D audio amplifiers switching - PWM amplifiers -V cc

Prof R T KennedyProf R T Kennedy POWER ELECTRONICS 2POWER ELECTRONICS 2 11

Prof R T Kennedy POWER ELECTRONICS 2 2

Class D audio amplifiers

switching - PWM amplifiers

-Vcc


C

R

L

C

R

L

C

R

L


+

-

gatedriver

load

L

C

SYNCHRONOUS BUCK SMPS

+

-E /A gate

driverload

L

C

+

-

gatedriver

load

L

C

CLASS ‘D’ AMPLIFIER


+

-

gatedriver

load

L

C


the reference signal is a DC voltage from the error amplifier output

determined by sensing the output voltage switch duty cycle

load current is unidirectional from source to load


+

-

gatedriver

load

L

C


the MOSFETs are optimised differently • higher duty cycle mosfet optimised for lower on-state resistance

• lower duty cycle mosfet optimised for lower gate charge


+

-E /A gate

driverload

L

C

+

-

gatedriver

load

L

C


the reference signal is a continuously varying audio signal

resulting in a continuously varying switch duty cycle

load current in the half-bridge class 'D' amplifier is bidirectional

inductor energy from one half of the bridge is returning to the supply


+

-E /A gate

driverload

L

C

+

-

gatedriver

load

L

C


• both mosfets are optimised for low on-state resistance


PWM

25 kHz

400 kHz


+

-

gatedriver

load

L

C


the filter capacitor is selected to

• store energy and smooth the output DC voltage low esr to minimise losses

the inductor is selected to minimise ripple current

• the inductor core material is based on energy storage & losses

and not to saturate at the DC current level


+

-E /A gate

driverload

L

C

+

-

gatedriver

load

L

C


the LOW PASS filter corner frequency is selected to

attenuate the switching noise in the output waveform

pass the audio signal to the loudspeaker

capacitor selected to have minimum capacitance change versus voltage to reduce

distortion


+

-E /A gate

driverload

L

C

+

-

gatedriver

load

L

C


INDUCTOR selected

not to saturate at the DC current level

to have minimum inductance change versus load current (< 10%) core material selected based on harmonic distortion and low hysteresis loss


gatedriv

e

L

C

gatedriv

e

L

C



L

C

gatedriv

e

L gatedriv

e

L

C

gatedriv

e

L gatedriv

e


• Half-Bridge simpler & lower cost

• Full-Bridge has better audio performance differential output structure can inherently cancel even

order harmonic distortion DC offsets

• feedback from the output in H-B compensates for variation in the ± bus voltages (voltage pumping)

• open loop class ‘D’ operation is common in FB


a few nano-seconds

gate switching timing error

produces non –linearity >1% of THD

on off

offon

highside deadtime

lowside deadtime


• Higher efficiency, increased power density and better audio performance are driving the increased use of Class D amplifiers; an approach first proposed in 1958.

• A Class D audio amplifier is basically a switching amplifier or PWM amplifier with the switches being either fully on or fully off resulting in reduced transistor power losses and higher efficiency (90-95%).


POWER requirements vary considerably dependent on the application

application specific power supplies needed to optimise the system performance

UNDER-DESIGNED power supplies amplifier does not meet

performance specifications

OVER-DESIGNED power supplies increase product cost


POWER SUPPLY OUTPUT VOLTAGE AND CURRENT FACTORS

• amplifier output power• rated amplifier power @ min supply output

voltage• load (speaker) impedance • amplifier output configuration single-ended or BTL (bridge-tied load) output• amplifier maximum achievable duty cycle • parasitic output path resistance • under-voltage lockout to avoid poor low input voltage performance• output voltage 'pumping'


Vpower supply

Ipower supply

Ppk

POWER

2

1

2

,

2

,

2

,

pkloadav

load

pkloadav

rmsloadav

PP

R

VP

R

VP

MAXIMUM ‘UNDISTORTED‘

POWER


Vload

Iload

Ppk

POWER

22

max,,

2

2

,

2

2

,

2

,

2,

2

)(

2

2

total

loadccswloadav

total

loadpkloadav

paraload

loadpkloadav

loadparaload

rmsloadav

loadrmsloadav

R

RVDP

R

RVP

RR

RVP

RRR

VP

RIP

Rparasitic

RloadVamp


avHBT

loadCCswavFB P

R

RVDP ,2

2max,

, 42

)(

48

)( ,2

2max,

,avFB

T

loadCCswavHB

P

R

RVDP


RT : sum of all of the DC resistances in series with the load:

RT = Rload + Rdson + Rind + Rpcb + Rps,out

Rload : loudspeaker resistance:

Rdson : mosfet on -state resistance: HB Rdson FB 2 Rdson

Rind : filter inductor DC resistance

Rpcb : board traces, connectors, and wires

Rps,out : power supply output impedance

(use the component's resistance at maximum operating temperature)

• Dsw,max: amplifier maximum output duty cycle

Dsw is also referred to as the modulation index (M)MMAX is the maximum modulation factor


amplifier peak output power occurs at loudspeaker's peak voltage or peak current

avHBT

loadout

T

loadoutloadoutpkHB P

R

RV

R

RVRIP ,2

2

2

22

, 24)2(

avFBT

loadoutloadoutFBpk P

R

RVRIP ,2

22 2


power supply voltage is determined at the lower limit

of the power supply's output voltage tolerance

based on the amplifier's rated output power

with unclipped output voltage


tolerance

VV

R

P

D

R

RD

RPV

CCFBnomCCFB

load

FB

sw

T

loadsw

TFBCCFB

1

2

)(

)(2

min,,

max2

max

2

min,

tolerance

VV

R

P

D

R

RD

RPV

CCHBnomCCHB

load

HB

sw

T

loadsw

THBCCHB

1

22

)(

)(8

min,,

max2

max

2

min,


)(max

)(max

amp

ampoutPSout

PP

T

CCswit R

VDI min,max,

min,lim


PFB,max amplifier maximum output power

20 W

ηmax amplifier maximum efficiency 90%

Dswmax amplifier maximum duty cycle

0.8

Rload amplifier loudspeaker impedance

8 Ω

RT total output resistance 8.2 Ω

power supply output voltage tolerance

±10 %


Vtolerance

VV

VV

R

P

D

R

RD

RPV

CCCCnom

CC

load

FB

sw

T

loadsw

TFBCC

466.251.01

919.22

1

919.228

202

8.0

2.8

2

)()(

)(2

min

min

max2

max

2

min


AR

VDI

T

CCswit 236.2

2.8

919.228.0minmaxminlim

WP

Pamp

ampoutPSout 22.22

9.0

20

)(max

)(max




Cell Phone Application for TI's TPA2012D2 Class D Amplifier

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Prof R T KennedyPOWER ELECTRONICS 21. Prof R T KennedyPOWER ELECTRONICS 22 Class D audio amplifiers switching - PWM amplifiers -V cc