Thermal Dissipation
Designers of multimedia products are faced with demands to provide versatile, high-quality audio functions, including high-output speaker modes. To make this possible, high performance is required of the audio amplifiers. However, the linear amplifier efficiency in most practical situations is around 50%, and a small increase in output power comes at the cost of a relatively large increase in current consumption. This often leads to significant heat dissipation that requires system-level cooling measures to spread and remove heat. Typically, bulky heatsinks are attached to audio amplifiers for this purpose. For automotive audio systems, where space and cost are at a premium, these measures can be quite expensive and even disruptive to the design flow of the entire automotive platform.

By using a Class-D amplifier, less heat is generated. This permits the use of a smaller heatsink, and head units can be offered with extra output channels without the need for an expensive external amplifier. Therefore, a Class-D amplifier provides tremendous advantages to automotive OEMs. Specifically, it can deliver high sound quality, while minimizing packaging costs. It can also reduce the capacity ratings needed for the thermal management components as well as the power supply.

On average, a realistic audio signal (such as that for music) applied to a Class-D audio amplifier spends a very short time at peak output power. This results in a much lower rms output power for a continuous audio signal. This property allows for a much smaller heatsink than that required for linear amplifiers rated to support a continuous sine wave. As a rule of thumb, the heatsink can be sized to accommodate a steady-state power value that is half the peak output power value for which the Class-D amplifier is rated.

However, it is ultimately up to the system design engineer to determine the thermal management approach for a specific application based on size and cost, including the use of techniques that compliment an external heatsink. For example, the amplifier’s PCB can also assist in removing heat from the amplifier IC. Specifically, using large IC copper pads and maximizing the widths of all traces that connect to the IC are effective measures that can be implemented at relatively low cost.

Efficiency
Efficiency is the ratio of output power across the load to the input power drawn from the supply, defined in the following equation, which expresses efficiency as a percentage: η (%) = (P_LOAD / P_SUPPLY) x 100. As stated previously, linear audio amplifiers are highly inefficient, since they often produce more power as heat dissipated into the ambient environment than as electricity delivered to the speaker. This is because a linear amplifier uses the resistance of the driving transistors to produce the desired output voltage. The power dissipated by each of the transistors is then partly a function of the voltage difference between the output and the connected power rail (either positive or negative) developed across the transistor.

One crude method for improving the efficiency of a linear amplifier is to minimize this voltage difference by expanding the output voltage signal to be mainly at the supply-rail voltages. While this produces a type of switching action that is similar to that of the Class-D audio amplifier, this method also causes severe distortion, because the voltage of the audio signal is being clipped. Distortion severely degrades the sound quality and can permanently damage the speaker, so this method is usually not a viable option. Therefore, a high-efficiency amplifier such as a Class-D amplifier nearly always requires far less power than a linear amplifier for a given audio application.

While the ideal Class-D amplifier is 100% efficient, this is not quite achieved in reality due to the non-zero on-state resistance, RDS(ON), in each of the output transistors. Moreover, other circuit elements in the output path have non-zero resistances that also contribute to the overall power loss of an audio amplifier. This is illustrated in Figure 3, which shows a dc equivalent circuit with all major resistive losses for a typical Class-D amplifier having a double-ended output stage configured as an H-bridge. The circuit represents a steady state where two complimentary transistors on opposite sides of the bridge are turned on to power the load.

In the Figure 3 circuit, R_DS(ON) is the output on resistance for a transistor in the on state, R_PARASITIC is the parasitic resistance in the output circuit path (metal interconnects, bond wires, lead frame, and PCB traces), R_FILTER is the combined series resistance of the low-pass filter used for the output path, and R_LOAD is the load resistance. From this circuit, the efficiency of the Class-D amplifier system can be estimated by the formula:

Which simplifies to the following when R^FILTER and R_PARASITIC can be neglected:

Another contributor to overall system power loss is the combined switching losses from the output transistors (P_SWITCH). These are caused by rise and fall times that are greater than zero (Figure 4), leading to brief intervals of voltage-current products in the transistors that result in power pulses that must be dissipated by the devices. The switching losses can be ignored at high power levels, but they must be taken into account at low power levels.

The system efficiency can then be estimated with greater accuracy by this formula:

Which simplifies to:

Where PDISS is the total power dissipation contributed by parasitic losses, and by the conductive and switching losses in the output transistors of the Class-D amplifier stage of the audio system.