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How to get higher efficiency at lower loads: A new LLC platform makes it possible

Technology News |
By eeNews Europe

For designers of power supplies, it can be challenging to sort through the various requirements for reducing power consumption. Meeting these requirements isn’t really optional – since most products can’t leave the factory without being certified compliant with one or more of them – 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 minimizing 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: maximize 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. Providing better performance at low loads while minimizing component count, 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 guarantees increased efficiency over the entire load range. The new LLC platform also delivers excellent performance at low standby power, without an auxiliary power supply, so it complies with new regulations while also reducing the overall cost. NXP is well known for its best in class Synchronous Rectifiers (SR). 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.

Two features make the LLC platform especially good at increasing efficiency at low loads: multiple operating modes and cycle-by-cycle Capacitive Voltage control.
 
Variable modes

Three operating modes – one each for burst, low power, and high power – make it possible to automatically select the best mode for each combination of power and control voltage, resulting in greater efficiency. 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. To make things simpler, the new NXP LLC platform uses a novel cycle-by-cycle architecture, as shown in Figure 1.

 

Figure 1: The cycle-by-cycle architecture
 
With cycle-by-cycle control, the output power is regulated by the primary capacitor, not by the frequency. The capacitive voltage (V(Cr)) control is linear related to the output power (Po). Here’s a closer look at how it works. Figure 2 shows what happens in the first cycle. Power is delivered from Vin. One half the power goes to Vout, while the other half goes to Cr.

 

Figure 2: First cycle of capacitive control

Figure 3 shows what happens in the second cycle. Power is delivered from Cr to the output, so V(Cr) is directly related to the power. This linear relationship makes it much easier to design the control loop.

 

Figure 3: Second cycle of capacitive control

The cycle-by-cycle technology enables well-defined, accurate regulation of the burst mode, and fast responses to dynamic loads. The low-power and burst modes are initiated by the primary feedback voltage, which is related to the primary V(Cr) voltage, and, by extension, to the output power. This linear relationship makes it much easier to design the control loop.
 
Real-world results

The LLC platform has been implemented in three new GreenChip power-supply controllers, the TEA1916 separated combo and the TEA1995. Initial tests show that these devices deliver excellent efficiency at low loads.
 
TEA1916 GreenChip resonant LLC solution

The TEA1916 is a separated combo consisting of a discontinuous conduction mode (DCM) power-factor correction (PFC) and a resonant LLC solution that serves to replace the popular TEA1716 GreenChip controller.

When compared to the older TEA1716 (or competitor LLC parts), the TEA1916 yields a significant increase in light-load efficiency. As shown in Figure 4, at 375 V, below a Po of 50 W, efficiency of the existing TEA1716 LLC part varies from moderate (around 60%) to reasonably good (around 90%), while the new TEA1916 LLC part delivers consistently excellent ratings (around 93%) throughout the range.

 

Figure 4: Efficiency of TEA1916 versus TEA1716 (LLC part only)
 

TEA1995 dual GreenChip SR controller

Designed to replace two standalone SR controllers, the TEA1995 is a dual GreenChip SR controller. NXP tested efficiency in a gaming power supply and a PC power supply by replacing two existing single SR controllers from the competition with one TEA1995 and then comparing the results. In both cases, the only change was to replace the two single SR controllers with one TEA1995. The TEA1995 consistently delivered efficiency increases of roughly six percentage points when operating at loads between 5% and 15%.
 
Both systems used an AC input of 100-240 V and 50-60 Hz. The gaming power supply produced a DC output of +12 V / 17 A (18.6 A with only 12 V loaded), and +4.7 V / 3 A. Figure 5 shows the results of benchmark testing, using the original components and the TEA1995 for low-power efficiency. The TEA1995 has a consistently higher efficiency between loads of 5% and 15%.

 

Figure 5: TEA1995 efficiency in a gaming power supply
 
The PC power supply produced a DC output of 19.5 V and 16.9 A. The results for low-power efficiency for the original system and the TEA1995 system are given in Figure 6. The efficiency readings for the TEA1995 are higher between loads of 5% and 15%.

 

Figure 6: TEA1995 efficiency in a PC power supply

NXP also tested a PC power adapter. In this test, two SR controllers, one older and one newer, were replaced with NXP’s previous-generation TEA1795 and the new TEA1995. The power adapter used a standard AC input (100-240 V, 50-60 Hz), and produced a DC output of 19.5 V and 150 W. Figure 7 shows that the two systems – the original and the one with two NXP devices – have similar efficiency between loads of 10% and 100%.

Figure 7 TEA1795 and TEA1995 efficiency in a PC power adapter

But when the output current drops below 5% (0.38A), something interesting happens. The competitor part stops switching. This causes a 3% to 4% drop in efficiency, as shown in Figure 8. Both the TEA1795 & TEA1995 controllers continue to switch below 5% and deliver consistent efficiency ratings of well above 85%.

 

Figure 8: TEA1795 and TEA1995 efficiency at low loads in PC power adapter

The two configurations provide similar efficiency over most of the operating range, but when operating at very low loads, the NXP components deliver higher efficiency.
 
Conclusion

Power supplies are complex mechanisms that require careful engineering to perform at peak efficiency. Finding new ways to minimize power losses, especially during light loads, can be particularly challenging, especially when design costs need to be kept in check.

The new LLC power platform, from NXP Semiconductors, is developed to maximize efficiency at low loads and, at the same time, creates a more streamlined design that requires fewer components and lower system cost.

Test results from the two first implementations of the LLC platform, the TEA1916 and the TEA1995, show that designers have a new way to make their high-frequency systems, such as PCs, gaming consoles, and high-definition displays, meet the requirements for “green” certifications.


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