design considerations for 500w class d automotive subwoofer amplifiers
class d designs promise higher output power within a given automotive electronics system volume. but converting to a class d from a class ab amp is challenging because the modes of operation are significantly different and circuit protection schemes must also be adapted for the different topologies.

by johan strydom and jun honda, international rectifier

in audio applications there is a trend towards class d audio amplifiers as replacements for traditional class ab electronics. the main driving forces are improved efficiency and space savings. converting to class d designs promises higher output power within the same system volume or miniaturization at existing power levels-both desirable for automotive audio and infotainment applications.
converting to class d for traditional class ab amplifier designers can be quite daunting as the modes of operation are significantly different. moreover, the circuit protection schemes must also be adapted for the different topologies. in an attempt to simplify these issues, the basic design procedure for a 500w, 2ω automotive class d subwoofer amplifier is presented here.

the class d amplifier

the major differences between a class ab amplifier and a class d amplifier are tabulated in table 1 below. it can be seen that the main advantages of class d amplifiers are efficiency, stability, and inherently low output impedance (voltage source), which are all beneficial for driving speaker loads. (for a more complete discussion on the differences between the class ab and class d amplifiers, please refer to application note an-1071 [reference 1].)

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design of a class d amplifier

for the class d amplifier design shown in figure 1 above, consider the following requirements, given in table 2 below. for this design a half bridge topology was chosen, as it tends to be the most cost effective solution if the proportionally higher bus voltage requirements can be met. as can be seen in figure 1, the supply voltages, active devices, output filter, gate drive, and protection circuits have to be designed.

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selection of power devices

the first step for selecting the switching devices is to estimate the minimum bus voltage, which is dependent on output voltage swing. in this respect, the design task starts out identical to that for a class ab amplifier. considering the power requirement is for 1% thd (total harmonic distortion), the required peak-to-peak voltage swing (without clipping) at full load is given by equation 1:

where m is the maximum modulation index. for this design, a modulation index of 100% is possible. thus neglecting supply regulation and device on-resistance, the minimum bus voltages (+b or "b) for a half bridge topology will each be half of the peak-to-peak voltage swing in equation 1 above.

as with other hard switching converters, the voltage rating of the switching devices should be on the order of 50% higher than the overall bus voltage to allow for powe