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Perform thermal tests on Class D amps

Posted: 17 Jan 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Class D amps? output power? thermal test?

By Robert Polleros
Maxim Integrated Products Inc.

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 thermal tests differ across companies.

There are two 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. Since the thermal test standards were developed when only class output power 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 U.S. Federal Trade Commission (FTC) requires one hour of preheating with a 1kHz 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 or 8 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 1kHz signal at 10 percent THD for at least ten minutes.

Thermal stability
This test proves the thermal capabilities of the whole set, which is placed in a chamber with the maximum specified ambient temperature, typically 40C. 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; this is discussed in the next paragraph.

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.

Test signals
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 signals listed in the table. The best artificial substitution for real-life signals is noise.

Table

So far we were dealing with source signals, but for thermal evaluation we are concerned about the output signal of the amplifier. The signal chain also has volume and sound control, allowing for significant gain and fixed supply that limits the peak output voltage. Therefore the crest factor changes if one turns up the volume: the peaks are limited while the average energy still rises, so the crest factor goes down (it differs from the input signal of the amplifier).

The lowest possible crest factor depends on the amount of distortion considered acceptable by the customer and the maximum gain settings of the set. The ideal worst-case test signal would have the lowest crest factor found in any consumer application.

Also, speaker manufacturers have been investigating suitable test signals. Their speakers have to process the amplifier signal without damage or serious distortion. Most manufacturers have settled for a standard called IEC268-5, which describes a test signal: Pink noise!a mix of frequencies that are filtered (40Hz highpass, 5kHz lowpass, 2nd Order) to resemble the long-term frequency distribution of music as shown in Figure 1.

Fig1

The IEC268-5 test signal has a 6dB crest factor, which is considered a worst-case specification. The average power that the speaker can handle using this signal is called the "continuous power" specification, but most manufacturers publish the "program power," which is 3dB higher and measured with an intermittent signal (one minute on, one minute off, etc.). So speakers can handle clipped signals with a 9dB crest factor.

The peak power, which is referred to by the crest factor, is the peak output power delivered by the amplifier. The rated output power of the amplifier is measured with sinewaves, which are 3dB; therefore, the long-term power handling of the speakers is 6dB less than the rated amplifier power. The worst-case long-term test signal for the whole set is an IEC268 noise whose RMS power is 9dB below the peak output or 6dB below the largest sinewave, which is the maximum output power from the sinewave test.

When designing the thermal capability of the amplifier, there is no reason to request more than the speakers can handle. Integrated amplifiers usually have thermal protection, so the worst thing that happens is muting, which is automatically reset after the amplifier cools down again. Since speaker overload results in permanent damage, setting the thermal limit of the amplifier to a lower level can actually be considered a means of speaker protection.

Amplifier classes
TVs use either of two types of audio amplifiers: Class AB and Class D. We want to analyze how these types perform in the previously mentioned tests. Class AB amplifiers have been the low-cost workhorse solution, but they have significant power dissipation issues and thus require large heat sinks. Class D amplifiers are known for having better efficiency, but the downside is a higher price for the silicon of the amplifier. This can be compensated for since less cooling effort!smaller or no heat sinks!will be required and smaller IC packages can be used. However, the system still has to pass the thermal test. So the test strategy defines the cost of the amplifier.

To simplify the comparison of the two classes, let us assume that both use FET, not bipolar, output transistors. Then, the maximum output voltage for a given supply voltage (VCC), load (Rl), and RDSON (the resistance of the fully conducting output transistors) is the same for both classes, as is the maximum output power.

Let us also assume a bridge-tied-load (BTL) output!the output current flows through two transistors and RDSON counts twice Figure 2.

Fig2

Dissipation is quite different for each amplifier class. Let's start with the DC analysis for an output voltage Ua (giving the output power P= Ua2/Rl):

Class AB:

Dab=[(Ua/Rl*(VCC- Ua)] + IQ* VCC

Dissipation equals the output current multiplied by the voltage drop on the output transistors.

Class D:

Dd=(Ua/Rl)2*2*RDSON +IQ* VCC

Dissipation consists mainly of resistive losses, (Output Current)2 * R.

Both amplifier classes have a constant factor: IQ*VCC, IQ being the quiescent current. Class AB amplifiers use this current to reduce crossover distortions, whereas for Class D amplifiers this current represents switching losses. This current is of similar magnitude for both classes.

Further analysis can be done by simulation. Let us choose a common TV application that uses a 12V supply and 8 speakers, and we will use the following numbers for parameters:

VCC = 12V

Rl = 8

RDSON = 0.3

IQ = 0.02A

First, we must determine efficiency, which is calculated with the following equation:

Equation1

Figure 3 illustrates the efficiency of the sinewave input and also shows the distortion of the output signal. That distortion is caused by clipping, and the clipping, in turn, is due to the limited supply voltage.

Fig3

The following equation is used to calculate the maximum output-voltage swing:

Equation2

At 10 percent, THD the output power is 10W, which is the specified maximum output power of the system.

As the graph in Figure 3 indicates, Class D amplifiers offer much better efficiency versus output power than Class AB amplifiers. There are only two points in the graph at which Class D amplifiers do not outperform Class AB amplifiers:

  • Zero input: both amplifiers consume only quiescent power, which is assumed to be the same;

  • "Infinite overload: produces a squarewave output, which is always saturated, even with Class AB. At this point, both amplifiers have the same efficiency, dissipation, output power (15.56W), and distortion (43.5 percent).

While efficiency figures are important for battery operated systems, designers of mains powered equipment are more interested in the dissipation characteristics of amplifiers. Figure 4 shows the dissipation that each amplifier class creates with a sinewave input and varying gain:

Fig4

At the rated power of 10W, the dissipation is 2.53W for Class AB and only 0.994W for Class D amplifiers. At lower inputs Class D dissipation decreases, while Class AB dissipation increases.

How does this relate to real life, when the amplifier is used for music or speech?

This is best simulated using noise signals, which have an amplitude distribution similar to music and lead to consistent results.

To compare the results to real listening situations and to the speakers? power handling capabilities, we must change the x-axis from power to crest factor. The crest factor gives the relationship between average output power and the peak power of this system, which is 15.56W.

Ideal noise sources have infinite crest factors: their amplitude distribution follows a "normal distribution" with well defined variance but no limit to the peak voltage. This distribution changes when we route the signal through our simulated amplifier!the output signal is limited by the supply rails. The average (RMS) voltage changes as the gain of the system changes. Increasing the RMS voltage decreases the crest factor, as the peak reference is constant.

While clipping rarely occurs at high crest factors, it occurs more frequently as the gain is increased. Figure 5 shows noise with a 3dB crest factor, where the output signal is heavily clipped.

Fig5

For the purposes of this simulation, we are not concerned with the "color" of the noise; real tests, however, should use the IEC268-5 signal because some amplifiers have reduced efficiency at high frequencies. As we change the gain, we can calculate the dissipation for all possible crest values (see figure 5).

Figure 6 shows high output power to the right, now expressed as low crest factors.

Fig6

  • At 15dB to 12dB even very dense music gets heavily clipped, which leads most listeners to turn the volume down.

  • 9dB is considered the worst allowable crest factor by speaker manufacturers.

  • 0dB produces a full-scale squarewave output.

At 9dB, which would be the most reasonable point for thermal evaluation, the dissipation is 3.05W for Class AB, and 0.388W for Class D.

The ratio in this case is 3.05/0.388 = 7.86, while for the output power test it was only 2.53/0.994 = 2.55.

This simulation has important implications:

For a Class AB amplifier, the thermal challenge is to pass the noise test. Once the amplifier is designed to absorb those 3.05W per channel there are no more thermal problems at the rated output power with 2.53W dissipation per channel. The rated output power can be delivered forever.

Since the power dissipation is similar in both tests, it became common practice to use the sinewaves for both the output power and for the thermal test. Of course, a test using sinewaves is slightly easier to set up, but it leads to less dissipation than the recommended noise test.

In other words, using sinewaves for thermal evaluation leads to class AB amplifiers with less power handling than speakers of the same wattage.

For Class D amplifiers, the situation is reversed. The noise test creates a 0.388W dissipation, while at rated output power 1W is dissipated, which is 2.56 times more. So it makes quite some difference which signal is used for thermal evaluation.

Using sinewaves for thermal evaluation of class D amplifiers leads to largely oversized systems, which drives up the cost:

  • IC suppliers need bigger die sizes to reduce RDSON, one of the main contributors to efficiency.

  • Class D amplifiers need bigger packages to have a low thermal resistance between the junction and PCB or heat sink.

  • Manufacturers need to provide either small heat sinks or multilayered PCBs to realize low Rthja values, which is the junction-to-ambient thermal resistance.

  • If the PCB is used for heat sinking, careful layout is needed to have large uninterrupted areas of copper. The copper is used for thermal transfer and there should also be good thermal connections between the layers by using multiple vias.

Burn-in test

Sometimes an even more severe test is used for thermal evaluation, called the "burn-in test". This test applies the maximum voltage available from the sound processor at the input of the power amp, leading to a squarewave-like output signal. This test dissipates up to 1.41W per channel in the example amplifier used in this article and there is no more difference to a Class AB amp. To pass this test class D amplifiers require 3.6 times better cooling as compared to the noise test.

The transition from CRT to flat screen TV has led to the need for smaller amplifiers with less heat dissipation, so class D amplifiers were introduced. Even when using the traditional sinewave tests the new designs were able to reduce the heat by a factor of 2.5X.

Engineers had to cope with new challenges!solving EMI, designing output filters and dealing with small amplifier packages using exposed pads for cooling. Now it is time to rethink the test methods to unveil the full potential of cost savings involved with class D.

This is the recommended method:

Check output power in burst mode: Apply full-power sinewaves only as long as needed to obtain THD values.

Check thermal capability with noise signals or the worst-case practical application (speech or music). This should be accompanied by gain settings that limit amplifier clipping so that even full-volume settings produce acceptable sound.

About the author
Robert Polleros
is a senior member of the technical staff for applications at Maxim Integrated Products Inc.




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