How to reduce audible noise and power consumption at standby in your AC/DC design


To meet evolving energy regulations that require lower standby power and higher efficiency, engineers are designing power supplies that use light-load modes such as burst-mode operation. Although light-load modes help minimize standby power, they introduce a new challenge: the potential for the power-supply magnetics to emit high-pitch ringing. If you have ever heard one of these irritating sounds coming from your laptop power adapter, then you understand why it is so important to eliminate.

To minimize standby power, minimize audible noise and still keep designs cost-effective, you need to carefully optimize your full AC/DC system solution. It is becoming more critical that all components of the system – the boost power factor controller (PFC), the isolated DC/DC using an inductor-inductor-capacitor (LLC) and synchronous rectification (SR) – all work together.

Audible noise

When designing a power supply for consumer electronics such as TVs or AC/DC adapters, power designers face the challenge of meeting stringent audible-noise requirements in standby mode. A common method to reduce standby power at light loads is to use a burst-mode function built into PFC and LLC controllers. Burst mode is an operating mode that periodically disables switching when there is no load or light load on the output. The disabling of switching limits power consumption of the supply when the load needs only minimal power. One drawback of burst mode is that the burst frequency can be low enough where it enters the audible range. This is a problem when the power supply is targeted for consumer applications – no one wants to hear their TV hum at night!

By slowly ramping the resonant current up and down during burst, the UCC28056 transition-mode PFC controller and UCC256403 LLC resonant controller help prevent noticeable audible noise, as shown in Figure 1.

Figure 1: UCC256403 and UCC256404 burst-mode pattern 

The ability to slowly ramp the magnitude of the resonant current up or down is a key feature of the UCC256403 and UCC256404’s optimized burst-mode algorithm. Combined with hybrid hysteretic control, this optimized burst-mode algorithm helps minimize audible noise while enabling low standby power consumption and a best-in-class transient response. For reference, audible noise measurements are included in section 9.6 (page 18) of the “UCC25640EVM-020 Evaluation Module User’s Guide.”

Always-on PFC

In addition to reducing audible noise at light loads, the UCC256403 and UCC256404 can further improve the light-load performance of your AC/DC power supply when combined with the UCC28056 critical conduction mode PFC controller. The burst mode of the UCC28056 significantly improves light-load efficiency and reduces no-load standby power. This improvement enables applications to keep the PFC always on while still meeting rigorous efficiency and standby targets. For TVs in the U.S., Energy Star version 8.0 requires standby passive power consumption to be less than 0.5 W. Other regional standards, such as Ecodesign Directive Lot 6 in the EU, demand similar specifications.

Combined with the integrated high-voltage startup function of the UCC256404, you can eliminate the need for an auxiliary flyback converter as well as the small-signal and isolation circuitry needed to enable and disable the PFC. This configuration leads to reduced size, cost and complexity. Figure 2 shows a typical AC/DC power supply for TV applications, illustrating the system savings and benefits that using the UCC28056 and UCC256403 solution can help achieve.

Figure 2: TI’s PFC-LLC-SR system for TV applications, highlighting the component savings of the TI chipset 

Synchronous rectification

Synchronous rectifiers (SR) can even further enhance the efficiency of an AC/DC power supply, improving efficiency by replacing the rectification diodes with actively controlled MOSFETs on the secondary side. The conduction loss of SR MOSFETs is significantly lower than the conduction losses of Schottky diodes. The UCC24624 is a dual-channel SR controller that specifically works with an LLC topology. This controller features a proportional gate drive that prolongs SR conduction time to minimize body diode conduction, enabling increased efficiency. In addition, the UCC24624 has an automatic sleep mode to further reduce the standby power consumption of the system.

Conclusion

TI designed the UCC28056, UCC256404 and UCC24624 to work together, with a system-level solution in mind. For this reason, you can expect the best performance when designing to meet stringent standby power and audible noise standards.

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