COMMERCIALIZING CLASS D AMPLIFIER TECHNOLOGIES

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COMMERCIALIZING CLASS D AMPLIFIER TECHNOLOGIES

• Paul Mathews Rick JeffsRane CorporationMukilte
  o, WA 98275 USApaulm_at_rane.com, rickj_at_rane.com

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Smaller, lighter power amplifiers

• Commercial Marketplace Trends
• Multichannel sound
• More audio zones
• Redundancy for safety systems
• Reduced power consumption
• Product response switchmode power supplies and
  amplifiers in small packages

Dont expect to compete with highly-evolved
linear technologies on cost alone.
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Product Requirements

• Mains power
• Universal mains
• High power factor, low inrush
• Low EMI radiation and susceptibility
• Inputs pro audio 4 to 22 dBu
• Outputs
• 100 watts adequate for many multi-channel
  applications
• Audio signals design for crest factor 12 dB,
  i.e., avoid overly-conservative power and thermal
  design
• Design for load faults and non-constant load
  impedances
• Cost
• Cost/watt similar to older products no adder for
  new features

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Product Requirements (contd)

• Reliability and redundancy
• Commercial users demand reliability
• Safety applications require redundancy
• Enclosure
• Smaller is better, 1U
• Certifications
• UL, CE mark, FCC
• Primary challenges radiated EMI, safety

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System Design Considerations

• Power requirements vary greatly due to
• Signal dynamics
• Complex load impedance
• Amplifier and Supply efficiencies

• These variables impact
• Amplifier size and cost
• Thermal management requirements
• ac mains current draw and EMI

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Amplifier Technologies

• Switchmode amplifiers have size and weight
  advantage
• Monolithic switchmode technologies
• Advantages cost, size, EMI
• Disadvantages IC process and packaging voltage,
  current, thermal limits
• Digital-in/digital-out disadvantage little or no
  benefit of feedback
• Our technology selection
• fully-integrated, analog-in/PWM out,
  silicon-on-insulator IC
• Full-bridge outputs to maximize power output
  within voltage limits
• High fixed PWM clock (307 kHz) for simplified
  output filtering

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Inherent Limitations

• All amplifier technologies have limitations
• Voltage, Current and Thermal Dissipation
• Exceeding any limit can cause unacceptable signal
  distortion or disruption

• Challenge of designing within voltage/current
  limits
• Must know
• Signal dynamics, load impedance, temperature
• These are non-stationary parameters
• For component amplifiers load impedance is
  almost totally unknown in advance

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Over Design?

• One solution is to over-design the system

• Over designing adds cost
• Power Amplifier selection
• Size and weight
• Thermal management
• ac mains requirements

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Highly-Integrated Solution

• Integrating
• Supervisory control of all functions
• Load and temperature aware dynamics control
• Efficient power supply with PFC
• Efficient class d amplifiers
• Allows
• Small size and moderate power
• Optimum performance into a given load
• Reliable audio signal integrity
• Easy system design and setup
• New features

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Integrated Power Amplifier
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Highly-Integrated Solution
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Supervisory Host

• Operates from auxiliary supply, controls higher
  power circuits
• Control all clocks to high power circuits,
  monitoring
• Mains and internal supply rail voltages and
  currents
• Load impedance, Amplifier OK status,
  over-temperature
• Works with DSP to set Limiter Threshold, Fault
  status, and load impedance equalization
  parameters
• Amplifier temperature is feedback for
• Compressor Threshold
• Fan Speed / Fault status

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Power Supply Technologies

• Switchmode for size and weight reductions
• Power Factor Correction benefits
• Reduced EMI
• Improved mains circuit utilization
• Reduced regulation burden for dcdc conversion

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Power Supply PFC

• Power Factor Correction reduces
• peak current, rms current
• ac mains harmonics
• dcdc stage peak currents
• bulk capacitance
• Interleaving with dcdc converter reduces EMI

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Inrush Reduction with PFC

• PFC high V (385 V)
• high ripple tolerance
• Allows small bulk C
• Ripple removed by post-PFC regulation
• Small capacitor reduces charging energy
• Faster RC time constant for given peak current
  limit

Top PFC front end, 220 mF _at_ 340V Bottom
Transformer/rectifier, 10000 mF _at_ 60V
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Switchmode Magnetics

• Requirements
• Low profile, low loss at high frequency
• Thermal performance consistent with audio
  signals, low air flow
• Technologies
• Sendust distributed-gap toroidal cores for
  inductors (carrying significant dc current)
• Single-layer and progressive windings to minimize
  capacitances
• Ferrite core dcdc converter transformer, Litz
  windings, inter-winding shield
• Toroidal ferrite common mode transformers on
  mains input and dc output

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Switchmode Magnetics (contd)
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Power Supply Attributes

• Very high incremental efficiency
• Quiescent power largely set by switching losses
• Maximal use of mains circuits
• Many more 100 watt channels per breaker
• High tolerance for brownouts and dropouts
• Internal and external fault self-diagnosis and
  soft shutdown

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Power Amplifiers

• Efficient, class d design
• Low power loss/thermal handling requirements
• Enables voltage limiting without efficiency loss
• Silicon-on-insulator process
• Zero dead-time
• 300 kHz switching rate
• Self test, self-protection supply imbalance,
  over-temperature, over-current
• External clock and remote start/stop

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Power Amplifiers (contd)

• Less than ideal properties
• Very fast output overcurrent protection
• Shuts off output MOSFETs ? undesirable sound
  effects
• Voltage clipping (as with all amplifiers)
• Finite thermal capabilities
• Consumer audio input and gain levels

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Power Amplifiers (contd)

• Integration solution
• Control power amplifier drive signals for
  trouble-free operation
• This approach also allows sensible control of
  gain structure
• Basis of control Load Impedance Estimation

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Signal Processing
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Load Estimation

• Maintain load voltage and supply current
  statistics to estimate Z
• Average impedance for actual signals
• Not impedance at a particular frequency
• Used to set Limiter Threshold
• Margins for departures from estimated Z
• Bonus feature load status to user

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DSP Functions

• Limiter
• Prevents voltage and current clipping
• Set automatically for
• sensitivity setting
• Average load impedance

• Compressor
• Limits long-term average power
• Limits amplifier heating
• Tied to Limit Threshold

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DSP Limiter / Compressor

• Allowable load currents (allowing 3 dB margin)
• 8.0 W load Ipeak voltage limit, 40 V swing
• 4.0 W Ipeak limit 7.7 A
• 1.3 W Ipeak limit 7.3 A

• Compressor characteristics
• Tied to limiter threshold
• Long time constant
• 4 dB max compression

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DSP Functions

• High-pass filter
• Adjusted for speaker or distribution transformer
• Sensitivity
• Set gain maximum output at maximum input
• Expander
• Reduce source noise with no signal

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Metering

• Load sensitive peak headroom
• Limit/Compress/Expand indicators
• Fault status
• Load status
• Off/Standby/Ready indicator

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Fault Reporting and Redundancy

• Three basic features
• Internal fault reporting
• External fault reporting
• Load switching
• Enable
• Fault reporting to a control system
• Automatic redundancy switching

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Integration Benefits

• Predictable and low power consumption
• Reduced thermal stress
• Improved reliability
• Improved signal integrity
• no voltage or current clipping
• no thermal cycling
• no blown fuses
• New features

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Mechanical/Thermal/EMI Design

• Integrated design is essential. Examples
• 1U form best with low mass (heatsinks, chassis,
  etc)
• Switchmode heatsinks can radiate or, preferably,
  act as shields
• Airflow vents can compromise shielding
• Customers dislike airflow noises
• Supervisory control monitors temperature,
  controls fan speed, power dissipation

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Squeezing It In

• Small chassis makes shielding difficult
• EMI generating components close together, close
  to chassis ? must minimize parasitics
• Floor planning and careful placement and
  orientation of every power component is essential
• Minimize capacitance of high dv/dt structures
• Minimize area of high di/dt loops and arrange for
  cancellation or perpendicularity where possible
• Minimize di/dt and dv/dt by circuit design where
  possible
• Recognize that most power components have high
  dv/dt and low dv/dt sides

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Magnetic Loops Example
Similar analyses for dv/dt and parasitic
capacitances
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Thermal Management
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Conclusion

• Market demands smaller, lighter, more channels
  and features with no added cost
• Our response
• All switchmode 4-channel amplifier 4 x 100 W
• Integrated power management, audio signal
  control, reliability enhancement, and other
  ease-of-use features

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Thanks for your attention

• Anyone with biographical knowledge of Danish
  physicist Soren Larsen (1871-1957), said to be
  among the first to analyze the Larsen Effect
  also known as feedback howl, please contact me.