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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 = P_out / (P_out + P_cond + P_q) × 100%

R_DS— MOSFET on-resistance (Ω)

How It Works

Class D amplifiers (switching amplifiers) use high-frequency pulse-width modulation (PWM) to drive MOSFETs alternately fully on or fully off, achieving high theoretical efficiency because MOSFETs in saturation dissipate nearly zero power. Unlike Class AB amplifiers (typically 50–65% efficient), Class D amplifiers achieve 85–98% efficiency in practice. The main loss mechanisms are: (1) Conduction losses — P_cond = I²_rms × R_DS(on) × N_MOSFETs, where I_rms is the load current and R_DS(on) is the MOSFET on-resistance. (2) Switching losses — from charging/discharging MOSFET gate capacitances at the switching frequency (typically 200 kHz–1 MHz). (3) Quiescent (idle) losses — current drawn by the control IC, gate drivers, and bootstrap circuits regardless of output power. At low output powers, quiescent current dominates and efficiency falls; at rated power, conduction losses dominate.

Worked Example

Class D module: 50 W output into 8 Ω. Supply: 36 V. MOSFETs: 4 × 50 mΩ RDS(on). Quiescent current: 30 mA. Load current (RMS, 8 Ω): I_rms = √(P/R) = √(50/8) = 2.5 A Conduction loss: P_cond = I²_rms × R_DS(on) × N = (2.5)² × 0.050 × 4 = 1.25 W Quiescent loss: P_q = 36 × 0.030 = 1.08 W Total losses ≈ 1.25 + 1.08 = 2.33 W (ignoring switching losses) Total input power: 50 + 2.33 = 52.33 W Efficiency: 50 / 52.33 = 95.5% At 5 W output (I_rms = 0.79 A): P_cond = 0.79² × 0.050 × 4 = 0.125 W; P_q = 1.08 W Efficiency = 5 / (5 + 0.125 + 1.08) = 80.7% — quiescent dominates at low power.

Practical Tips

  • ✓To maximise efficiency at low listening levels (typical home use is well below rated power), minimise quiescent current by using a Class D IC with a low-power idle mode that reduces switching frequency or enters standby when signal is absent.
  • ✓Higher supply voltage reduces the RMS current for the same power (P = V²/R), reducing conduction losses — doubling supply voltage at the same power output reduces I_rms by half and halves P_cond (I² relationship).
  • ✓Select MOSFETs with the lowest R_DS(on) × Q_gate figure of merit (a lower figure indicates better switching performance). For audio Class D at 400 kHz, R_DS(on) below 20 mΩ with Q_gate below 20 nC is achievable at modest cost.

Common Mistakes

  • ✗Assuming 100% efficiency and not budgeting for heat — even at 95% efficiency, a 200 W Class D amp dissipates 10+ W of heat, requiring thermal management. At high ambient temperatures, MOSFET R_DS(on) increases (positive tempco), worsening efficiency.
  • ✗Ignoring switching losses in the model — switching losses scale with gate charge, switching frequency, and supply voltage. At 1 MHz switching, switching losses can rival conduction losses. This calculator uses a simplified conduction + quiescent model.
  • ✗Using Class D RDS(on) from the datasheet maximum — MOSFET datasheets give RDS(on) at 25°C. At 100°C junction temperature, RDS(on) typically doubles. Use the temperature-derating curve to estimate worst-case efficiency.

Frequently Asked Questions

Even 5% dissipation in a 200 W amplifier is 10 W — enough to require a heatsink to stay within MOSFET junction temperature limits. The heatsink can be much smaller than in Class AB (which dissipates 30–50% as heat), but it is rarely zero. Some low-power Class D designs (under 20 W) use the PCB copper as the heatsink.
Class AB efficiency peaks at around 78% for a resistive load with a sine wave, and is typically 50–65% at normal listening levels. Class D efficiency is typically 85–95% at mid-to-high output levels and 75–85% at low output levels where quiescent current dominates. The improvement is most pronounced at medium-to-high power.
Modern Class D designs (e.g., Hypex, Purifi, Pascal) achieve THD+N below 0.001% and SNR above 120 dB — competitive with the best Class AB amplifiers. Earlier Class D designs suffered from EMI, poor THD, and frequency response errors. The quality difference today is negligible for listeners and is largely a matter of implementation.

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