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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?

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 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 2: First cycle of capacitive control.

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 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.

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 stand-alone 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%.

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