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Maximising efficiency at lower loads

Posted: 08 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power supplies? power consumption? LLC? resonant topology? PC power adapter?

Designers of power supplies face the challenging task of sorting through the various requirements for reducing power consumption. Meeting these requirements is not really optional 每 since most products cannot leave the factory without being certified compliant with one or more of them 每 but sifting through the details can be both confusing and frustrating. Issued by different governing authorities, in different regions, these various initiatives and directives cover different types of end equipment, and may have different requirements for different power levels.

There are, to name a few, publications from the California Energy Commission (CEC) and EnergyStar program (including 80+), the U.S. Department of Energy's (DOE's) External Power Supply (EPS) guidelines, the One-Watt Initiative, issued by the International Energy Agency (IEA), Australia's House Energy Rating program, the European Code of Conduct (CoC) for power supplies (Tier 2), and the Eco-Design Directive for Energy-Using Products (EuP) requirements (Lot 6).

The guidelines don't always agree, but, having boiled it all down, there is an emphasis on reducing "phantom" or standby power consumption, which involves minimising waste at light and no-load conditions. Every milliwatt counts, and it can be difficult, especially in high-power systems, like PCs, gaming consoles, and high-definition displays, to reach the most recent target of consuming no more than 0.5 W during standby. Add to this the fact that most electronic systems compete in highly cost-sensitive markets, and you have a two-fold challenge: maximise light-load efficiency, without increasing the bill of materials (BoM).

Meeting the low-load challenge
A new power platform, developed by NXP Semiconductors, aims to help designers meet this two-fold challenge. The new platform builds on what is, for many designers of high-power power supplies, a familiar format: the LLC resonant topology.

The new NXP LLC platform uses synchronous rectifier (SR) control with patented gate drive, without minimum on-time and without reverse current, so it is touted to guarantee increased efficiency over the entire load range. The new LLC platform also performs at low standby power, without an auxiliary power supply, so it complies with new regulations while also reducing the overall cost. Having a dominant share in SR controllers for flyback power supplies, NXP now has a new SR control solution for LLC resonant power supplies with the new TEA1995.

This LLC platform has two notable features: multiple operating modes and cycle-by-cycle Capacitive Voltage control.

Variable modes
Three operating modes 每 one each for burst, low power, and high power 每 make it possible to automatically select the best mode for each combination of power and control voltage. The burst and low-power modes switch at lower loads and use switching frequencies that are outside the audible spectrum, therefore reducing acoustic noise.

Cycle-by-cycle control
The output voltage (Vout) is regulated using the capacitance voltage V(Cr) of the LLC resonant tank. The traditional kind of frequency control can be difficult to manage, since it involves high gain in the control loop, meaning even small deviations in the frequency can produce much higher output power. The new NXP LLC platform uses a cycle-by-cycle architecture, as shown in figure 1.

Figure 1: The cycle-by-cycle architecture.

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