Higher efficiency at low loads – how it’s possible

BY ERWIN SEINEN,  
International Product Marketer,
NXP,
www.nxp.com

As global demand for electricity rises, so do the efforts toward making appliances more energy efficient. One area receiving attention from regulatory bodies is the low / no load power performance, and stringent limits are being set. This is particularly challenging for manufacturers in the cost-sensitive 90 W to 500 W device segment, which includes popular consumer items such as TVs, PCs and gaming consoles. NXP’s new resonant LLC platform provides the ideal solution. Combining all the benefits associated with the resonant LLC architecture with excellent light load performance and low BoM, it enables more easy-to-design power supplies.

Over the past few decades, electronic systems have become ever more practical, feature-rich, versatile and convenient. This has led to us using an increasing number of electronic devices in virtually every walk of life, with new applications continuing to spring up every day. Consumer devices such as TVs, gaming consoles and computers are incredibly popular, with most homes possessing one or more of each type. The problem is that all these electronic devices demand power – and their combined thirst for electricity presents a global challenge.

Technological advances in energy efficiency need to be made continuously just to keep pace with the growing demand for power. And while today’s electronic systems are more efficient than ever before, there is still room for improvement. One area that shows particular promise is in optimizing power performance at low loads.

Meeting new light-load regulations

Improving low load power consumption is also crucial for appliance manufacturers as it receives particular attention from regulatory bodies. New regulatory requirements impose specific maximums for low loads and low, no-load standby power. This makes finding ways to boost the efficiency of light-load operation a critical issue for many developers.

These new regulations on power consumption have particular relevance for designers developing 90 W to 500 W power supplies. Supplies in this rating category cover a broad range of products including LCD and plasma TVs, notebook adapters, desktop and all-in-one PCs, and gaming systems. These systems are, by and large, relatively cost-sensitive. Therefore, the challenge for designers is how to meet new regulatory requirements while still keeping the bill of materials (BoM) low.

A resonant LLC that’s easy to design

To help designers meet this challenge, NXP has developed a new, high-performance power platform using its GreenChip technology. The platform is based on the LLC resonant topology – one that will be very familiar to many designers of 75+ W power supplies. However, power supplies built on resonant LLC topologies are often considered both difficult to design and expensive. However, by employing a novel cycle-by-cycle architecture, NXP’s new platform enables highly efficient power supplies which, in design effort, come closer to flyback, but with all the additional benefits of resonant LLC (excellent efficiency, low harmonic content, removal of snubber losses, long hold-up time, etc.).

Fig. 1: The TEA1916 integrates a resonant LLC control circuit with a discontinuous conduction mode PFC controller 

The new LLC platform uses a secondary synchronous rectifier control with adaptive gate drive, without minimum on-time and reverse current, which guarantees increased efficiency. As a result, low load performance in particular is excellent, even without an auxiliary power supply. This means that it complies with the latest regulations while also reducing overall cost.

Best efficiency in every mode

When aiming to increase efficiency at low loads, there are two special features of the platform that give designers a distinct advantage. To begin with, three operating modes, one each for burst, low power and high power, allow the best mode to be chosen for each level of output power. This ensures the most efficient use of power during any operation.

Cycle-by-cycle control

Secondly, cycle-by-cycle capacitive voltage control overcomes a complexity introduced by the traditional method of frequency control. In the new resonant LLC architecture, the output voltage (V_out ) is regulated using the capacitance voltage (V_cr ). The usual method of frequency control can be difficult to manage, since it involves high gain in the control loop. Even small frequency deviations can result in much higher output power.

Cycle-by-cycle capacitive voltage (Vcap) control addresses this issue by controlling the energy transfer from the primary side to the output at every switching cycle. This means there is a linear relationship between the control parameter and the required output power (P_o ). The cycle-by-cycle format means the low power and burst modes are driven by both the secondary feedback voltage and the primary Vcap voltage, and consequently, by the output power. This relationship makes it much easier to design the control loop. Furthermore, such a control loop enables a more accurate burst mode and reduces audible noise.

A solution for the consumer market

Combining the TEA1916 and TEA1995 ICs will help reduce power consumption in consumer appliances including desktop PC/AiO, gaming consoles, notebook adapters and large panel TVs. The two-chip solution keeps component count to a minimum for low BoM, and combines energy savings with market-leading ease-of-design.

The platform delivers high efficiency over the entire load range, especially at light loads of 10 to 30%. It also meets energy efficiency regulations including Energy Star v6, CoC tier 2, 80+ platinum and EuP lot 6. In addition, the TEA1916 features a low no-load power,  below 75 mW, exceeding the requirements of Energy Star, DoE, CoC tier 2, etc. The LLC resonant topology also makes it easier to meet common mode noise requirements and 200% peak power requirements when compared to a flyback topology.

First impressions

A next-generation version of the highly successful TEA1716, the TEA1916 integrates resonant LLC control circuit with a discontinuous conduction mode (DCM) power-factor correction (PFC) controller. First silicon shows very impressive efficiency at light loads. At 375 V, below a P_o of 50 W, the LLC part achieves consistently high efficiency ratings of above 90%, throughout the range. By comparison, efficiency of the older TEA1716 LLC part varies from 65% to just below 90%, showing that the TEA1916 delivers a significant performance increase.

The TEA1995 is a dual GreenChip SR controller which can replace two separate SR controllers. The IC has been tested in several existing systems, including a gaming power supply and a PC power adapter, to see how it performs at low loads. The only change made to the systems was to replace the two existing SR controllers with a single TEA1995. In each case, the TEA1995 delivered efficiency increases of roughly six percentage points when operating at loads between 5% and 15%.

Worldwide energy savings

The global drive for energy efficiency underscores the importance of driving low-load efficiency performance. With an easy-to-use design offering all the advantages of resonant LLC, the new platform makes cost-effective, high-efficiency power supplies more accessible to developers all over the world. Together with supporting overall sustainability, it allows manufacturers to exceed requirements of the latest power regulations while helping lower consumers’ electricity bills.