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Signal chain basics: Save power, reduce heat with power supply volume control

Posted: 22 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Class D? audio amplifiers? power-supply volume control?

The audio level at the speaker terminals in open-loop Class D audio amplifiers can be defined by Equation 1:

Audio Level = (PWM modulation index) (power supply voltage)

A look at some of the pulse-width modulation (PWM) devices on the market may lead to a section on power-supply volume control (PSVC). Some of you may already have tinkered with this function, while others may have blown straight through the datasheets, without realizing what a cool piece of technology PSVC can be. In home audio applications, most buyers are wowed by a "1000W home theater" or "600W subwoofer."

However, in most home-TV settings, average listening levels are below 20W of power. Let's look at a standard home-audio system to see how it can be tuned for higher efficiency, lower heat dissipation, and greater signal-to-noise ratio (SNR).

Figure 1 is a block diagram of a fairly common home audio system. Multiple amplifiers all run from a fixed, high-voltage power rail. The aim is to throw any signal from the audio source with enough headroom in the power supply to handle it. Now your system is ready to pump out 1000W at a moment's notice!

Figure 1: A traditional home-audio system with a fixed power supply.

However, in the real world, most users don't run their home audio systems with the volume knob maxed out at 11. Most users have the volume knob turned down to two or three. By doing so, you've immediately lowered your maximum potential output from your amplifier. This means that your maximum PWM modulation from a 50V power rail may only be 30 percent or so.

Many amplifiers with a zero signal continue to switch at a 50-percent duty cycle, which means you have 50V being switched on and off through the FET transistors in the H-bridge. FETs have a finite (although very small) on-resistance, therefore, the power is dissipated as heat (Equation 2):

P = V2/R

By reducing the power-supply voltage by half, the power dissipated by the power stage, when muted (but still switching at 50/50), is quartered. This essentially pushes a power stage from idling at 2W per bridge-tied load (BTL) channel. Think about six of these per system for a 5.1, or eight of these in a 7.1 home theater in a box (HTIB) in the power stage to 500 mW per BTL channel, This dissipation scales up and down with the VCC of the power stage.

This directly impacts the size of the heatsink required for your amplifiers and reduces the airflow requirements, too.

Impact on Class D amplifiers
Open-loop amplifiers:In summary, power-supply modulation in open-loop amplifiers can achieve two different objectives:
1. Power idle-current consumption
2. Direct volume control.

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