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Class D Amplifier Efficiency

Estimate Class D amplifier efficiency from MOSFET conduction losses and quiescent current at a given output power.

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Formula

η=Pout/(Pout+Pcond+Pq)×100η = P_out / (P_out + P_cond + P_q) × 100%
R_DS— MOSFET on-resistance (Ω)

How It Works

This calculator estimates Class D amplifier efficiency based on MOSFET parameters, switching frequency, and load conditions. Power electronics engineers, audio amplifier designers, and thermal engineers use it to predict heat dissipation and select appropriate heatsinking. Class D amplifiers achieve 85-98% efficiency by operating MOSFETs as switches (fully on or off) rather than linear devices, minimizing conduction losses. Total losses comprise: conduction loss P_cond = I^2_rms R_DS(on) N_MOSFETs, switching loss P_sw proportional to f_sw V_supply Q_gate, and quiescent loss P_q from control ICs and gate drivers. Per TI and Infineon datasheets, modern Class D ICs achieve 93-95% efficiency at rated power, dropping to 70-80% at 10% power where quiescent current dominates. The IEC 60268-3 standard measures efficiency as P_out/(P_out + P_dissipated). A 200 W Class D amplifier at 93% efficiency dissipates only 15 W as heat versus 100+ W for equivalent Class AB.

Worked Example

Problem: Calculate efficiency for a 100 W Class D amplifier (TPA3255-based) at full power and at typical 10 W listening levels.

Solution at 100 W output into 8 ohms:

  1. Load current: I_rms = sqrt(100/8) = 3.54 A
  2. MOSFETs: 4 devices, R_DS(on) = 45 milliohm each (TPA3255 datasheet)
  3. Conduction loss: P_cond = (3.54)^2 0.045 4 = 2.26 W
  4. Switching frequency: 600 kHz, switching loss estimate: ~1.5 W (from datasheet graphs)
  5. Quiescent power: 36 V * 50 mA = 1.8 W
  6. Total loss: 2.26 + 1.5 + 1.8 = 5.56 W
  7. Efficiency: 100/(100 + 5.56) = 94.7%
Solution at 10 W output (typical listening level):
  1. Load current: I_rms = sqrt(10/8) = 1.12 A
  2. Conduction loss: (1.12)^2 0.045 4 = 0.23 W
  3. Switching loss: ~0.5 W (reduced with lower current)
  4. Quiescent power: 1.8 W (unchanged)
  5. Total loss: 0.23 + 0.5 + 1.8 = 2.53 W
  6. Efficiency: 10/(10 + 2.53) = 79.8%
Note: Quiescent loss dominates at low power levels, explaining the efficiency drop from 95% to 80%.

Practical Tips

  • ✓Select Class D ICs with auto-idle or low-power standby modes (TPA3255 eco-mode, MAX98357 shutdown) to improve efficiency at typical listening levels. These modes reduce quiescent current from 50-100 mA to 5-10 mA, improving low-power efficiency from 70% to 85%+ per TI application notes.
  • ✓Higher supply voltage improves efficiency: P_cond = I^2 * R, and I = P/(V*cos_phi). Doubling voltage halves current, reducing conduction losses by 4x. A 48 V Class D design achieves 96-98% efficiency where 24 V achieves 93-95% for same output power per Hypex design guidelines.
  • ✓For audio applications, prioritize low THD+N over maximum efficiency. Premium Class D (Purifi Eigentakt, Hypex nCore, Pascal) achieves THD+N < 0.0005% at 92-94% efficiency. Budget Class D (TPA3118, PAM8403) achieves 90-95% efficiency but with THD+N of 0.1-1% - audible on quality speakers.
  • ✓Thermal design rule: allow 2-3x calculated dissipation for music with high crest factor. A 100 W amplifier averaging 10 W during music dissipates ~3 W average, but peaks can reach 10+ W for 10-100 ms. Design heatsink for average dissipation but verify thermal time constant handles peaks per IEC 60268-3.

Common Mistakes

  • ✗Assuming datasheet efficiency applies across all power levels - manufacturers specify peak efficiency (typically at 50-100% rated power). At 10% power, efficiency drops 15-25 percentage points because quiescent losses become dominant. A '95% efficient' amplifier may be only 70-80% efficient during typical music playback averaging 5-10 W.
  • ✗Using R_DS(on) from datasheet without temperature derating - MOSFET R_DS(on) increases 50-100% from 25C to 100C junction temperature. A 50-milliohm MOSFET at 25C becomes 75-100 milliohms at operating temperature, increasing conduction losses by 50-100%. Use the 100C spec or apply 1.5x derating factor.
  • ✗Ignoring switching losses at high frequencies - modern Class D operates at 400 kHz - 2 MHz to push switching noise above audibility. Switching losses scale linearly with frequency: doubling f_sw doubles P_sw. A 2 MHz design may have 3-4x higher switching losses than a 500 kHz design, partially offsetting the benefits of smaller output filters.
  • ✗Forgetting inductor and capacitor losses - output LC filter inductors have DCR (0.05-0.3 ohms) and core losses (1-3 W at high power). These add 1-5 percentage points to total system losses beyond the amplifier IC itself. Budget 2-3% additional loss for passive components per typical designs.

Frequently Asked Questions

Even 5% dissipation in a 200 W amplifier equals 10 W continuous heat - enough to raise junction temperature 50-100C above ambient without heatsinking, exceeding the 150C maximum for most MOSFETs. The heatsink requirement is dramatically smaller than Class AB (which would dissipate 100+ W), but not zero. Many sub-50 W Class D designs use the PCB copper pour as a heatsink (4-6 cm2 per watt at 40C rise), while higher-power designs need aluminum heatsinks with Rth < 2-5 C/W.
Class AB theoretical maximum is 78.5% (pi/4) for sine wave into resistive load; practical Class AB achieves 50-65% due to quiescent bias current and driver stage losses. Class D theoretical maximum approaches 100%; practical Class D achieves 85-95% due to R_DS(on), switching losses, and quiescent current. At typical 10% power levels: Class AB drops to 20-30% efficiency (most input power becomes heat), while Class D maintains 70-80% efficiency. Per AES measurements, Class D provides 3-4x better efficiency across the operating range.
Modern Class D designs achieve THD+N below 0.001% and SNR above 120 dB - exceeding most Class AB amplifiers per Audio Precision measurements. Premium implementations (Purifi 1ET400A: 0.00017% THD, 133 dB SNR; Hypex nCore NC500: 0.0007% THD, 122 dB SNR) outperform virtually all Class AB designs. Early Class D (1990s-2000s) had issues with EMI, poor THD at high frequencies, and output filter ringing, but these are solved in modern designs. Blind listening tests consistently show no audible difference between well-designed Class D and Class AB.

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