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Class D Amp Notes

Class D Amp Notes. Putzey’s Open Loop PWM Model. AES Convention Paper 6690, 2006 “All amplifiers are analogue, but some amplifiers are more analogue than others”. Open Loop PWM Challenges. Output is highly dependent on power supply quality

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Class D Amp Notes

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  1. Class D Amp Notes

  2. Putzey’s Open Loop PWM Model • AES Convention Paper 6690, 2006 • “All amplifiers are analogue, but some amplifiers are more analogue than others”

  3. Open Loop PWM Challenges • Output is highly dependent on power supply quality • High currents modulate the signal and result in distortion products • Power supply should be a switcher with sense feedback or a linear regulator for best results • Output filter is load-dependent • No feedback from filter results in some interaction with the loudspeaker (peaking or roll-off) • Fixed clock frequency results in higher EMI • Power concentrated in fixed frequencies • Modulator linearity

  4. Output Stage Feedback • Feedback taken after output devices improves Power Supply rejection (PSRR) • Most analog input Class D chips use output feedback (including Tripath) • TI and Apogee do not use output feedback, but new TI output stage uses local feedback for improved PSRR • Output feedback can improve modulator linearity • Carrier-based modulators benefit most from output feedback • Output feedback can improve output stage fidelity • This was an issue for early Class D designs with “dead-time” issues, but not a big driver for new designs

  5. Modulators • Two major classes in use today • Sigma-Delta • Carrier-based (triangle wave/comparator) • Lowest distortion achieved with Sigma-Delta + feedback • Analog Devices achieves .005% distortion levels • “Open Loop” Sigma-Delta achieves good results • Basis for Apogee and TI designs—distortion approx .02% • Carrier-based with feedback is also good • Feedback reduces distortion to .01% levels • Feedback difficult to optimize (power-level dependent) • See “Carrier Distortion in Hysteretic Self-Oscillating Class-D Audio Power Amplifiers”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 3, MARCH 2009

  6. Other Design Issues • Self-oscillating vs clocked • Self-oscillating reduces EMI and results in simplified design for analog-input Class D • Digital input is always clocked, but some digital modulators (e.g. Apogee) use spread spectrum to reduce EMI • Filter feedback • Results in lower output impedance and less interaction with the loudspeaker • Low noise • Early Class D amps suffer from Fab process that have high noise levels in analog stages (e.g. Tripath) • Digital input • Simplifies the design for multi-channel (6-8 DAC’s with I-V converters and filters consumes a lot of board space)

  7. Output Power • Three distinct tiers at higher outputs: • 125W: 50V H-bridge chips (STA517B or TAS5631) • 300W: TDA8954 or Sanyo STK-428 (plus and minus 40V) • 500W+: Discrete outputs (plus and minus 60 to 80) • Note: power ratings are for 8ohms—double for 4ohm • Tripath version at 500W: • TA0105A/TA2500 (or RA2500), based on TC2000 with drivers plus discrete outputs • More complex than newer IRF or ICE designs and with higher distortion • Poor reputation for reliability

  8. Other high power options • UCD self-oscillating amps (carrier-based) • Marketed by Hypex; chief evangelist is Bruno Putzeys • Elegantly simple design • Fairly low distortion • Uses output feedback (after the filters) to work with many loads • IR IRF2092 • Simple, low-distortion design • Uses Sigma-Delta modulator with output stage feedback (but before the filters) • Zetex (Diodes, Inc) • Used in NAD M2 • High power and very low distortion • Direct Digital Feedback Amplifier uses DSP to control modulator with feedback after the filters.

  9. Chip Comparison

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