Figure 3 is an example of a 2 channel 370W+370W Class D amplifier. This design features the IRF6775M DirectFET® from International Rectifier rated at 150V, 47m ohms, 25nC in a surface mountable package that enables small footprint while optimizing PCB layout.
Figure 3: Gate driver features (IRS2052M)
Although power dissipation of a Class D amplifier is significantly smaller compared to its conventional Class AB counterpart, thermal design is still not a negligible part of the effort. Unlike Class AB, efficiency is a function of the MOSFET temperature. The higher die temperature increases RDS(ON) , thus conduction loss in the MOSFET. On the other hand, a good portion of the switching loss comes from reverse recovery charge Qrr in the body diode which has a positive temperature coefficient. Higher junction temperature increases overshoot/undershoot due to the higher Qrr.
The IRS2052M integrates all four essential functional blocks along with clock oscillator and thermal protection. A clean THD+Noise vs. output power waveform in Figure 4 indicates successful noise management in the controller IC.
Figure 4: THD+N vs. Power
Thanks to the low Qg in the MOSFET making gate drive easier, the control IC can also deal with a noise sensitive analog section on the same die. In addition, the low Qg in the MOSFET allows low gate drive power so that all four gate driver stages fit in a small MLP package.
Figure 5: Picture of IRAUDAMP10
The resulting design example is a 4" x 2.8" board space that offers a complete design in the size of an iPod at the efficiency of 90% while providing comparable audio performance to an equivalent output power Class AB amplifier. The latest Class D design brings higher audio quality with superior power efficiency at lower system cost which makes this topology achieve ever close to the ideal amplifier.
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