Prof R T KennedyPOWER ELECTRONICS 21

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

Prof R T KennedyPOWER ELECTRONICS 23 CR L CR L CR L

Prof R T KennedyPOWER ELECTRONICS gate driver load L C SYNCHRONOUS BUCK SMPS + - E /A gate driver load L C + - gate driver load L C CLASS ‘D’ AMPLIFIER

Prof R T KennedyPOWER ELECTRONICS gate driver load L C SYNCHRONOUS BUCK SMPS  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

Prof R T KennedyPOWER ELECTRONICS gate driver load L C SYNCHRONOUS BUCK SMPS the MOSFETs are optimised differently higher duty cycle mosfet optimised for lower on-state resistance lower duty cycle mosfet optimised for lower gate charge

Prof R T KennedyPOWER ELECTRONICS E /A gate driver load L C + - gate driver load L C CLASS ‘D’ AMPLIFIER  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

Prof R T KennedyPOWER ELECTRONICS E /A gate driver load L C + - gate driver load L C CLASS ‘D’ AMPLIFIER both mosfets are optimised for low on-state resistance

Prof R T KennedyPOWER ELECTRONICS 29 PWM 25 kHz 400 kHz

Prof R T KennedyPOWER ELECTRONICS gate driver load L C SYNCHRONOUS BUCK SMPS 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

Prof R T KennedyPOWER ELECTRONICS E /A gate driver load L C + - gate driver load L C CLASS ‘D’ AMPLIFIER 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

Prof R T KennedyPOWER ELECTRONICS E /A gate driver load L C + - gate driver load L C CLASS ‘D’ AMPLIFIER 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

Prof R T KennedyPOWER ELECTRONICS 213 gate drive L C gate drive L C

Prof R T KennedyPOWER ELECTRONICS 214

Prof R T KennedyPOWER ELECTRONICS 215 L C gate drive L gate drive L C gate drive L gate drive

Prof R T KennedyPOWER ELECTRONICS 216 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

Prof R T KennedyPOWER ELECTRONICS 217 a few nano-seconds gate switching timing error produces non –linearity  >1% of THD onoff on highside deadtime lowside deadtime

Prof R T KennedyPOWER ELECTRONICS 218 Higher efficiency, increased power density and better audio performance are driving the increased use of Class D amplifiers; an approach first proposed in 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%).

Prof R T KennedyPOWER ELECTRONICS 219 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

Prof R T KennedyPOWER ELECTRONICS 220 POWER SUPPLY OUTPUT VOLTAGE AND CURRENT FACTORS amplifier output power rated amplifier 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'

Prof R T KennedyPOWER ELECTRONICS 221 V power supply I power supply P pk POWER MAXIMUM ‘UNDISTORTED‘ POWER

Prof R T KennedyPOWER ELECTRONICS 222 V load I load P pk POWER R parasitic R load V amp

Prof R T KennedyPOWER ELECTRONICS 223

Prof R T KennedyPOWER ELECTRONICS 224 R T : sum of all of the DC resistances in series with the load: R T = R load + R dson + R ind + R pcb + R ps,out R load : loudspeaker resistance: R dson : mosfet on -state resistance: HB R dson FB 2 R dson R ind : filter inductor DC resistance R pcb : board traces, connectors, and wires R ps,out : power supply output impedance (use the component's resistance at maximum operating temperature) D sw,max : amplifier maximum output duty cycle D sw is also referred to as the modulation index (M) M MAX is the maximum modulation factor

Prof R T KennedyPOWER ELECTRONICS 225 amplifier peak output power occurs at loudspeaker's peak voltage or peak current

Prof R T KennedyPOWER ELECTRONICS 226 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

Prof R T KennedyPOWER ELECTRONICS 227

Prof R T KennedyPOWER ELECTRONICS 228

Prof R T KennedyPOWER ELECTRONICS 229 P FB,max amplifier maximum output power20 W η max amplifier maximum efficiency90% D swmax amplifier maximum duty cycle0.8 R load amplifier loudspeaker impedance8 Ω RTRT total output resistance8.2 Ω power supply output voltage tolerance ±10 %

Prof R T KennedyPOWER ELECTRONICS 230

Prof R T KennedyPOWER ELECTRONICS 231

Prof R T KennedyPOWER ELECTRONICS 232

Prof R T KennedyPOWER ELECTRONICS 233

Prof R T KennedyPOWER ELECTRONICS 234 Cell Phone Application for TI's TPA2012D2 Class D Amplifier