Understanding Class-D amplifier power supply requirements

Most regulatory agencies currently specify a warm-up or pre-conditioning period at 1/8 of the continuous power rating followed by a short period of time at the continuous rated power output as a sufficient test of an amplifier's capability. It is generally felt that 1/8 power is a fair indicator of the average music content of a typical CD. The duration of the warm-up period and the time at continuous rated power varies with the specifying agency.

Notes:
1. Each channel is tested individually, while all other channels run at 1/8 power.
2. All channels in the same frequency range tested at full power. Subwoofer tests are performed separately.

However, this methodology is not representative of all CDs. Measurements show that the average power output of most rock and heavy metal CDs ranged around 20% of the continuous power rating. Classical CDs vary widely from very little power up to higher levels than the rock CDs, depending on the composer and the recording level. The worst-case RMS power that we have found so far was found on a "mega-bass" CD. The average power measured over the length of this entire CD was about 40% of the continuous power rating for the amplifier.

Determining the Average Power Requirements
Consumers almost never need 40% power on a continuous basis. Most people operate their amplifiers well below the maximum rated power, with the outputs only approaching full power for short periods of time during peaks in the music. This difference between maximum available power and typical usage is the reason behind the power rating requirements of the various regulatory agencies.

Power supplies can be designed in different ways depending on the needs of their target market. Amplifiers for applications such as professional recording studios or laboratory applications may need to be able to support the full output power on a continuous basis. In this case the power supply would also have to be able to deliver full output power continuously. This approach is expensive and is normally only used when the application requires it.

FTC and EIA specifications only require amplifiers to operate continuously at 1/8 of their rated output power. This allows designers to save money in the design of both the amplifier and the power supply. Most consumer amplifiers use this approach. In either case, the power supply components must rated for the maximum output current, which is a function of the amplifier's peak output power and not its continuous output power capability.

The ability of an amplifier and its power supply to dissipate power limits the length of time that an amplifier can operate at full power. An amplifier and power supply with high operating efficiency and/or good power dissipation capability may be able to operate at the full rated power continuously. At a minimum, the transformer, heatsinks, and other components in the power supply (and amplifier) must be sized so that they can sustain the output power requirement long enough to meet the applicable regulatory requirement without overheating. The thermal mass of these components, together with their thermal resistance to ambient, keeps them from overheating long enough to meet the regulatory requirements.