Class D Amp Notes

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