Editor's Note: Class D amplifiers are often considered THE solution for most applications - without full engineering analysis. In this series Robert Polleros of Maxim Integrated Products Inc. takes on the thermal issues involved with audio amplifiers running in very tight enclosures like thin, flatpanel TVs.
All sets with built-in audio power amplifiers—such as stereo systems, TVs, and multichannel AV receivers—have one important specification in common: the output power. This specification gives end customers an indication of the maximum volume the set can deliver, which is an important factor for many consumers.
The manufacturer has to measure not only the output power, but also the thermal stability of the set to prove the functionality under worst case circumstances. The standards for these tests differ across companies.
There are 2 kinds of amplifiers used to generate the output power, class AB and class D amplifiers. The shift to class D was mainly the result of the introduction of flat-screen (LCD and Plasma) TVs, in which space was limited and heat dissipation became an issue. While the test standards were developed at times when only class AB was used, we want to investigate if they are still appropriate for class D.
Maximum Output Power
Maximum output power is the amount of power an amplifier can deliver within a specified frequency and total harmonic distortion (THD) range for a given time. For example, the power test specified by the Federal Trade Commission (FTC) requires one hour of preheating with a 1-kHz sinewave at one-eighth the specified output power. Afterwards, the amplifier has to deliver the specified output power for five minutes—again, within the specified THD and frequency range. The load is usually a 4 O or 8 O resistor, depending on the nominal speaker impedance.
Because most TV sets do not have external speaker connections and thus no way to measure the power amplifier output, there are no legal standards for power measurements. Usually, the nominal power is measured with a 1 kHz signal at 10% THD for at least ten minutes.
This test proves the thermal capabilities of the whole set, which is placed in a chamber with the maximum specified ambient temperature, typically 40°C. Inside the set there is some temperature rise, which brings the amplifier to an even higher ambient temperature. The set is loaded with the original speakers.
Test signals of different waveforms and amplitudes can be used as described below.
This test extends over several hours, establishing the final temperature of every part. Then, several measurements are taken with infrared thermometers or thermocouples, and these measurements are compared to those established in the safety standards, such as maximum PCB or junction temperatures. To pass the test for thermal stability, neither the amplifiers nor the speakers are permitted to suffer any damage. This functional test checks for potential damage by evaluating temperature profiles.
The thermal stability test tries to simulate a worst case real life situation. This would be audio tracks found on DVD's and TV broadcast, but engineers need a standardized signal which delivers the same result every time it is used. It should also deliver stable temperature readings once the final condition is established.
While sinewaves deliver stable readings, they do not simulate program materials like music or speech, which have amplitudes that vary with time. The amplitude of the material spans the full signal range between silence and overdrive (clipping). The amplitude distribution of the program material is best described by the crest factor, which is the ratio between the peaks and average power (of the music or speech signal) expressed in dB.
Consider, for example, the following signals:
Table 1: Analysis of various audio signals
The tables shows that the best artificial substitution for real-life signals is noise.