As Data Centers Expand, How are Engineers Upping AC/DC Conversion Efficiency?
The demand for data center efficiency is only expanding in 2021. Here are some methods that EEs use to increase A/C/ power conversion efficiency—including a new option of GaN-powered AC/DC power supplies.
In almost all wall-powered electronic circuits, the first to-do item is AC/DC conversion. EEs are familiar with this ubiquitous and seemingly simple task, where, in many cases, the classic full-bridge rectifier circuit will suffice. Yet, it turns out that in many high power environments—such as data centers—the classic full-wave rectifier circuit simply doesn’t cut it in terms of power efficiency.
Classic full-wave rectifier circuit. Image used courtesy of ROHM Semiconductor
In fact, about 50% of all power in a data center is either wasted in conversion, distribution, or thermal management.
This reality has become increasingly consequential as data centers experience booming processing loads with digitization and at-home work at an all-time high. Data Center Frontier recently reported that Microsoft has purchased 900 acres of property in Mecklenburg County, Virginia, which is located nearby its 1.1 million square feet of data center land in Boydton. The tech giant has also has big plans for data center land acquisition in Iowa, Northern Virginia, and Phoenix.
Microsoft data center in Boydton, Virginia. Image used courtesy of Microsoft and Data Center Frontier
This unprecedented boom in data centers means that current efficiency numbers will no longer suffice. In this article, we’ll talk about techniques engineers have used to increase AC/DC power conversion efficiency and highlight recent efforts from Bel Power Solutions and Transphorm to increase power efficiency in data centers using GaN FETs.
Power Factor Correction
One of the main sources of inefficiency in traditional AC/DC conversion comes down to the power factor.
Power factor is the ratio between deliverable power (measured in kW) and total power (measured in kVA). In essence, the figure describes how much of a circuit’s power is transformed into useful work. In an ideal circuit, the power factor equals 1.
The figure on the left shows a circuit with a power factor of 1. The figure on the right shows a circuit with a power factor less than 1. Image used courtesy of Toshiba (PDF)
However, in the non-ideal world of electrical engineering, the power factor is often much less. This is normally a result of a significant phase difference between the voltage and current at the load terminals of a circuit.
The solution to this problem is called power factor correction (PFC) (PDF) and can take on many forms.
In a conventional bridge rectifier, the smoothing capacitor causes a phase shift between current and voltage, where the current leads the voltage. To counteract this, engineers will insert an inductor, which has the opposite phase effect of a capacitor in the circuit, working to restore the power factor to 1.
Example of an active boost PFC rectifier circuit. Image used courtesy of Lazar Rozenblat
Still, this is not a perfect solution, since other sources of power loss limit efficiency. For example, according to Texas Instruments, the input bridge can consume approximately 2% of the input power at the low line of a wide mains application.
Totem-Pole Bridgeless PFC
To further improve power efficiency in AC/DC conversion, engineers often turn to a “totem-pole bridgeless” circuit architecture. This architecture replaces the bridge with a series of high-frequency MOSFET switches, controlled in such a way that they behave as a rectifier. In some cases, the circuit can even be set up as a boost converter.
In this way, conduction losses are significantly reduced.
A bridgeless rectifier with PFC implemented with SiC transistors. Image used courtesy of Texas Instruments
The issue with this circuit, however, is that when the AC input changes from the positive half to the negative half, the duty cycle of the corresponding FETs must also change from 0-100% (or vice versa). Silicon MOSFETs have a slow reverse recovery because of the body diode of the FET and cannot be used for this reason.
GaN-Powered AC/DC Power Supplies
This week, Bel Power Solutions announced that it was able to solve this problem by using GaN FETs from Transphorm.
Combining the bridgeless totem-pole rectifier PFC circuit with GaN FETs proved to be an effective solution. The result is the industry’s first AC/DC supply that has, according to the press release, achieved Titanium power efficiency ratings—the highest efficiency rating possible. The new converter is said to reach up to 96% efficiency at high line with a main output of 12 VDC.
Basic specs of the GaN-powered TET series. Image used courtesy of Transphorm
This news is particularly relevant as data centers continue to expand. With a new AC/DC converter achieving such high efficiencies, it seems promising that future data centers will be able to employ similar technology to save power in their servers.
The reduced power factor in full wave rectification is not from the phase of the current being shifted from that of the voltage, but from the current flowing only at the peaks of the voltage waveform. Once the reservoir caps on the output of the rectifiers have charged, the diodes do not conduct until the instantaneous input voltage is higher than the voltage stored in the caps, enough to forward bias the diodes into conduction (~0.7 V for silicon). The result is that the current is a series of alternating positive and negative spikes coincident with the voltage peaks. In a lightly loaded supply, the conduction angle is very small and these spikes are very narrow; the power factor then is very low. With a heavily loaded supply, the reservoir caps lose more voltage and so the diodes conduct for a longer time; the larger conduction angle thus increases the power factor.