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# Notes on LLC Design for Audio Amplifiers

Special consideration must be given when designing power supplies for audio amplifiers. The nonlinear nature of audio signals presents different design challenges than standard isolated power supplies. This power supply tip covers the essentials for designing a Half-Bridge LLC Series Resonant Converter (HB LLC-SRC) for audio applications.

Special consideration must be given when designing power supplies for audio amplifiers. The nonlinear nature of audio signals presents different design challenges than standard isolated power supplies. This power supply tip covers the essentials for designing a Half-Bridge LLC Series Resonant Converter (HB LLC-SRC) for audio applications.

audio power

One thing you find in the broad field of electrical engineering is that different industries and even companies may use different languages ​​to describe the same topic. For a successful design, power and audio engineers must understand each other.

The first two terms that need to be defined are peak power and continuous power. Peak power is the ZD instantaneous audio power. It will determine how much power is designed for the physical output of the power supply. Continuous power is the audio power averaged over a period of time. In the context of power supply design, continuous power is the specified output power that a system can deliver without exceeding component temperature or average current ratings. Figure 1 provides examples of peak and continuous audio levels. They are related to the crest factor, which is a measure of the ratio of the peak value of a waveform to the root mean square (RMS) value.

Figure 1 This graph shows continuous and peak power audio levels.

It can also be expressed in decibels using the following equation:

Formula for calculating audio level

RMS is a misnomer when it comes to audio power, as this value is not technically the calculated RMS value of the power waveform. Another article could be written on how to specify the intricacies of audio amplifiers. Knowing industry standards for rated amplifier power levels does not necessarily specify what power supply requirements are in terms of peak and continuous power.

For example, consider an LLC series resonant converter (LLC-SRC) design for a 400W audio amplifier. Without prior knowledge of audio systems, you can design an excellent 400 W power supply. But when it’s time to power up the amplifier, the power supply fails, or the audio quality is poor. LLC converter gain curves are typically designed for ZD loads and operate around the series resonant frequency under ZX line conditions. This approach usually produces a perfect 400-W LLC-SRC, but in real audio systems the peak power will actually be greater than the amplifier’s 400-W rating. Before starting a power supply design, at least continuous power and peak power should be specified.

For the 400 W amplifier example, an appropriate power level for a consumer product to play compressed music might be 200 W continuous and 800 W peak for 15 milliseconds. This represents a crest factor of 12 dB, which is typical for processing music. Unprocessed audio is around 18-20 dB, movie audio can be greater than 20 dB. Ultimately, the ratio of peak power to continuous power depends on the specific application, so it is important to clearly define these early in the design process. Duration requirements for different load levels also help to optimize the design. Keep in mind that the efficiency of the audio amplifier needs to be taken into account as there will be losses in the amplifier which will result in a higher load on the power supply.

LLC-SRC Design

Once the specifications are finalized, you can move on to the power supply design. Depending on the region and application power quality standards, you may need a power factor corrected (PFC) power supply for this power level design. The PFC front end will provide a stable 400VDC bus for the LLC-SRC input.

As with most resonant converters, the DY step in designing an LLC-SRC is to select the resonant tank components. This will set the resonant frequency and shape the gain curve. In this step, it is ensured that the output voltage can reach the peak power level. If the resonant tank cannot achieve the desired gain, the output voltage will drop at audio peaks, reducing audio quality or turning off the amplifier. For output capacitors, the peak power duration requirement is often too long to maintain the output voltage, so the power supply needs to be able to practically supply the entire peak load.

Add some extra headroom to peak gain. Physical limitations of transformer construction do not always reach the exact number of turns or inductance. For audio designs that need to achieve high peak power, it is advantageous to use discrete resonant inductors to ensure a more JQ resonant and magnetizing inductance.

At peak power, it is important to choose a component rated to handle the peak current. When designing magnetics, make sure they don’t saturate. At continuous power, it is important to select components and packages based on continuous thermal performance. Designers can downsize some packages and use PCBs for thermal management instead of heat sinks.

As with any LLC-SRC, shaping the gain curve is an iterative process. Trying to reach a specific operating frequency, resonant current and voltage and balance the design between peak and continuous power levels is a challenge. When doing the calculations, you need to adjust the magnetizing inductance, resonant inductance, turns ratio, and resonant capacitance. 100 kHz is a common resonant frequency target for silicon-based designs. For audio applications, a target frequency of 100 kHz for the continuous power operating point makes sense. Figure 2 shows the gain curve for the above example. The operating frequency range is 83C139 kHz.

Figure 2 This gain curve is shaped for the LLC-SRC design.

burst mode

An important aspect of modern LLC-SRC designs is burst-mode operation that achieves light-load efficiency. Burst Mode is also used to meet industry standby power regulations. Audible noise is a problem when the burst packet frequency is in the audible noise range, but some LLC resonant controllers (such as the UCC256404) use a burst mode control law to prevent audible noise from burst frequencies. Here are three methods, and possible reasons for choosing them:

1. Enable Burst Mode: Use Burst Mode to reduce standby power consumption without shutting down the main output. Power will be supplied to the amplifier immediately, with no delay due to power on.

2. Disable Burst Mode: In standby, the converter needs to use standard switching operations to regulate the output. This reduces light-load efficiency, but reduces complexity and further eliminates any audible noise issues, such as secondary-side rectifier parasitics affecting the gain curve. Figure 3 shows how the gain curve actually starts to rise at higher frequencies. If the ZX gain cannot be achieved, the power supply will lose regulation.

Figure 3 With Burst Mode disabled, the gain curve will start to rise at higher frequencies.

External Controller Disable: Use an external disable circuit to shut down the controller when the audio amplifier is not running. This further reduces standby power consumption compared to Burst Mode, but adds cost as the system now requires auxiliary power. There is also a start-up delay when the amplifier is ready to output audio.

LLC-SRC is a high performance topology suitable for continuous power ranges between 100 and 500 W. It is an excellent topology for AC-DC systems that require high efficiency and ZX electromagnetic interference (EMI). Resonant converter design is challenging enough, even before being applied to complex audio systems. The DY step is the mutual understanding between the power engineer and the audio engineer about the peak and continuous power levels required by the amplifier. Consider the above strategies as the starting point for a successful LLC-SRC audio application design.