Digital amplifier technology has made tremendous advances since it was introduced a few years ago. Where digital amplifiers once were an emerging technology, they now are common place in many everyday products including DVD receivers, flat panel TVs and MP3 docking stations. Advances in this technology are most apparent when comparing power stage power levels per channel. The amount of power available today has increased by at least ten-fold from the firstgeneration of power stages.
Available power in a digital amplifier power stage now ranges from 10 to 315W per channel. This increased power makes possible the advantages of digital amplifiers (high efficiency, digital signal integrity, etc.) for a much wider array of consumer and other electronic products. However, this exponential increase in power levels does have its design challenges.
The latest 300W power stages can handle an amazingly large amount of current. For example, the maximum shutdown current of the TAS5261 from Texas Instruments is specified at 17 amps! Most of the design challenges for high-power digital amps are related to handling this high current level, which must be taken into account for any component in the audio signal path. Additionally, the voltage required on the H-bridge is 50V. Therefore, components chosen for this design must be specified to safely operate at this voltage level.
Some of the design challenges for a high-power digital amplifier include: 1) SMPS issues including topology and high-current design issues; 2) critical components in the SMPS and high-current signal path must be specified correctly to handle the higher power and current; and 3) printed circuit board (PCB) design issues including trace widths and electromagnetic interference (EMI).
SMPS ISSUES
Typically, a stereo or multichannel product that can drive 300W per channel needs to be able to drive 600W continuously to meet the current regulatory require-ments from the Federal Trade Commission (FTC). The FTC requires that right and left channels be driven at full power for fiveminutes before a manufacturer can claim this as the rated power. Sinceswitched-mode power supplies(SMPS) are the prevalenttechnology for digital amplifiers,this requires an SMPS that cansupply the 600W to the powerstage for at least five minutes. Sincefive minutes is a relatively long timefrom a thermal perspective, forpractical purposes, the SMPS mustbe able to drive this powerindefinitely. At this high level ofpower, a push-pull, half-bridge orfull-bridge SMPS is generallyrecommended.
For lower power SMPS designs (less than 200W), "fly-back" topology is the most prevalent. A detailed explanation of why a pushpull or half-bridge SMPS are used for higher power levels in beyond the scope of the article. However, here's a quick, 10,000 foot explanation. In a fly-back SMPS, only part of the transformer's magnetic B-H curve is used (Figure 1). Also, a fly-back SMPS is simpler and cheaper. Because the high currents in a high-power SMPS create a very high magnetic flux in the SMPS transformer, using the entire curve of the B-H hysteresis loop reduces the losses in the magnetic core. A push-pull or half-bridge topology increases the power of the SMPS. However, it also increases the complexity and cost of the design.
Higher power and currents also require changes to the components used in the SMPS. The SMPS transformer must be made larger to handle the higher power and currents. The peak current in a 600W SMPS can reach 15amps for a 220VAC input. For a 110VAC design (90 to 136VAC), it is recommended you use a voltage doubler or power factor correction (PFC) after the filter. This is because the input currents will be very large for a 600W SMPS with 90 to 136VAC input. Some of the components that should be closely monitored are the primary input ac-to-dc rectifier capacitor, and the secondary dc ripple voltage elimination capacitor. The input EMI line filter also must be able to support the increasedpower load.
Because of the complexity and expertise needed to design these power supplies, it is generally recommended that you use an off-the-shelf supply for the SMPS.
COMPONENTS IN THE AUDIO SIGNAL PATH
Designing for higher ripple currents is another area for concern. For example, with an Hbridge voltage (PVDD) of 50V, 10μH inductor, and 384kHz switching frequency, the ripple current in a system using the TAS5261 can reach 1.6 amps. This means that the inductors and capacitors in the output LC filter and PVDD capacitors must be able to handle the load current as wellas this ripple current (Figure 2).
The high currents seen in the filter inductor also means that this inductor must have very low dc resistance (less than 25 milliohm is recommended). Even with low resistance, the filter inductors still will have I2R losses. The inductor, and particularly the core material, must be able to function with the resulting temperature rise. The TAS5261 reference designs include a bill of materials and part numbers of specific inductors.
PCB ISSUES
The PCB traces in a highcurrent amplifier and SMPS must have the lowest possible resistance to minimize I2R losses. Generally, this means two-ounce copper should be used as well as, makingthe traces as wide as feasible.
(Figure 3) shows the traces from the TAS5261 reference design PCB. To minimize issues with EMI and audio performance, you should follow the layout as closely as possible and copy the layout exactly on the high-voltage/power side of the power stage. The high-power traces are to the right of the integrated circuit (IC) on the top layer (see arrows).
The new, high-wattage power stages in the digital amplifier enable a wider array of products and applications to be developed. The ideas mentioned in this article should help to overcome the primary challenges that come with designing with high power.
