Design Challenges of High Power Digital Amplifiers

Design challenges for high power digital amplifiers include:

　　1) SMPS issues, including topology and high-current design;

　　2) The SMPS and key components in the high-current signal path must be properly specified to handle the higher power and current;

　　3) Printed circuit board (PCB) design issues, including signal line width and electromagnetic interference (EMI).

　　SMPS Issues

　　Generally speaking, a stereo or multi-channel product that can reach 300 W per channel needs to be able to reach 600 W continuously to comply with current regulations set by the US Federal Trade Commission (FTC). According to the FTC regulations, the left and right channels must be at full power for five minutes before the manufacturer can claim this power as rated power. Since the switch-mode power supply (SMPS) is the most common power supply technology currently used in digital amplifiers, this requires the SMPS to be able to provide a power level of 600 W for at least five minutes. From a heat dissipation perspective, five minutes is a relatively long time, and in reality, the SMPS must be able to reach this power continuously. For this high power, push-pull, half-bridge or full-bridge SMPS are generally recommended.

　　As for low power SMPS design (less than 200 W), the flyback topology is most commonly used. This article does not go into detail on why push-pull or half-bridge SMPS are suitable for high power levels, etc. The following is just a brief description. In a flyback SMPS, only a portion of the transformer magnetic BH curve is used (see Figure 1). In addition, the flyback SMPS is simpler to construct and less expensive.

Figure 1. BH hysteresis curve of SMPS transformer magnetics

　　Since the high current of high power SMPS will cause very high magnetic flux in the SMPS transformer, using the entire BH hysteresis loop curve can reduce the losses in the magnetic core. Push-pull or half-bridge topologies can increase the power of SMPS, however, the design complexity and cost also increase.

　　The components used in the SMPS also need to be changed to achieve high power and high current. The SMPS transformer must also be enlarged to handle the high power and high current. For 220 VAC input, the peak current of a 600 W SMPS can reach 15 amps. For 110 VAC designs (90 VAC to 136 VAC), it is recommended to use a voltage doubler or power factor correction (PFC) after the filter because the input current will be quite large for a 600 W SMPS with 90 VAC to 136 VAC input. Among the components that need to be closely monitored are the main input AC to DC rectifier capacitors and the auxiliary DC ripple voltage elimination capacitors. In addition, the input EMI line filter must also be able to support the increased power load.

　　Since designing these power supplies is quite complex and requires expertise, it is generally recommended to use an existing SMPS power supply.

　　Components of the Audio Signal Path

　　Designing for higher ripple currents presents additional considerations. For example, with the circuit shown in Figure 2, a system using the TAS5261 can see 1.6 amps of ripple current with an H-bridge voltage (PVDD) of 50V, a 10µH inductor, and a switching frequency of 384kHz. This means that the inductor and capacitor in the output LC filter and PVDD capacitor must be able to handle the load current and this ripple current. The presence of high currents in the filter inductor also means that the inductor must have a fairly low DC resistance (less than 25 milliohms is recommended), however, even with fairly low resistance, the filter inductor will experience I2R losses. The inductor must be able to handle the resulting temperature rise, especially the core material. The TAS5261 reference design includes a bill of materials and specific inductor part numbers.

Figure 2. PVDD capacitor and output LC filter components of the TAS5261 reference design

　　PCB Design Issues

　　The PCB signal traces for high current amplifiers and SMPSs must have minimal resistance to minimize I2R losses. Generally, this means using 2 ounce copper and making the signal traces as wide as possible. Figure 3 shows the signal traces for the TAS5261 reference design PCB. To minimize EMI and audio performance issues, the configuration should be followed as closely as possible and applied exactly to the high voltage/high power side of the power stage. The high power signal traces are located to the right of the integrated circuit (IC) on the top layer (as indicated by the arrow). Figure 3 also shows the PCB configuration for the TAS5261 reference design. 

Figure 3. Example of high current signal traces on the TAS5261 reference design PCB.

　　New high-wattage power levels for digital amplifiers are enabling the development of a wider variety of products and applications, and the concepts described in this article can help overcome the main challenges encountered when designing for high power.