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Not much fanfare about GaN at APEC 2016

Posted: 31 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:GaN? Gallium Nitride? transistors?

After years of touting their Gallium Nitride (GaN) product developments or raising issues about GaN technology on the Applied Power Electronics Conference (APEC) floor, GaN semiconductor developers were strangely subdued about the topic at the March 20-24 conference in Long Beach, California.

GaN product developments were once characterised as "high school science fair projects," a reaction to the proliferation of technical papers on developers' web sites, and a then noticeable absence of actual data sheets.

Now, with the introduction of GaN transistors with various voltage and current ratings, package designs, and increasing distributor support, it seems we've gone over a hump in user acceptance. Manufacturers should have been popping champagne corks on the show floor (though a few were popped offsite). Instead, the mood on the floor was strangely subdued. There was a certain excitement from EPC's Alex Lidow, who continues to find new applications for low-voltage GaN transistors, and who views the GaN revolution as a reincarnation of the MOSFET transistor revolution he triggered in the late '70s. A new startup, Navitas Semiconductor, in an APEC plenary speech, announced they had discovered the key to extracting the promised switching potential of GaN: Use aluminum on GaN (AlGaN) driver ICs for the GAN FETs!preferably on the same chip with GaN power transistors!and clock the resulting DC-DC converters at 40MHz. What a difference GaN would make for miniature power adapters, declared Tony Sagneri of FinSIX Corp. (another plenary speaker). GaN transistors could enable 65 watt tablet adapters with 680W-per-cubic-inch densities, and 96% efficiency, he said.

Silicon-based Vcore power train modules

Infineon, which had acquired International Rectifier's GaN business more than a year ago, answered written questions on their progress with GaN transistors, but declined an on-site interview on the subject. Infineon's written position stressed relationships with existing customers for server and data centre applications!how important it was to provide products that enhanced THEIR success!and reiterated Infineon's messaging on how important it was to bring only tested and highly reliable products to market. Infineon's APEC promotions emphasised silicon-based Vcore power train modules for computing power applications. (The multi-chip module combines Infineon's MOSFETs and driver ICs in a very small 5mm x 6mm x 0.9mm package. The module will output up to 5.5V with a 70-A load, and a claimed efficiency better than 95%.)

Transphorm, an early GaN pioneer, announced on-resistance reductions to its line of 650-volt transistors, but its marketing and development partner, ON Semiconductor, remained quiet regarding their GaN product developments, focusing instead on server power train improvements and the introduction of 1200-Volt IGBTs for industrial applications.

Panasonic was touting its own X-GAN transistors at the conference, and manufacturing (barely acknowledging an earlier partnership with Infineon). Panasonic's GAN thrusts contrast with other Japanese transistor makers, like Rohm who is banking on Silicon Carbide (SiC) to provide high-voltage switchers for industrial applications. The Panasonic X-GaN transistors exhibited at APEC are rated for 400 watts (600V) and packaged in a TO-220 transistor package.

Texas Instruments announced they had developed a 600V GaN process and manufacturing capability. The 600V process would allow the build out of a GaN infrastructure, coupled with TI's controllers and power management devices, the company said. TI introduced its first half-bridge GaN module last year, a 100V power stage that integrated drivers and GaN FETs in the same package. The company's goal is to continue on an integration path using the new 600V process and to enable designers to reach new levels of power density and performance. The company showcased a 1KW totem-pole AC-to-DC converter using its new GaN technology and digital power controller. The power factor correction (PFC) demonstrated a 99.2% energy transfer efficiency.

King of low-voltage applications

Navitas and EPC (Efficient Power Conversion) were in full proselytisation mode. Both companies touted the higher switching speeds enabled by GaN and how this would reduce the size and costs of ancillary components like inductors and capacitors. But Navitas put a new wrinkle on the driver circuitry required. GaN transistors require a regulated 4.5- to 5.0-V gate drive, but few silicon driver circuits could clock the circuit fast enough, explained Navitas CEO Gene Sheridan. New GaN devices need to take advantage of the material's horizontal conduction path. An AlGaN layer would enable GaN FETs and their drivers to share the same substrate.

In Navitas experiments with Stanford University researchers, a 27MHz DC-to-AC converter in a 5mm x 6mm x 0.85mm QFN package [remember those dimensions] delivered 150 watts with 96% efficiency. Putting the driver circuitry on the same chip with the power transistor resolves impedance matching issues, Sheridan said. The GaN converter circuit could be clocked at 40MHz, he admitted, though in the lab there was a drop in the efficiency at that speed (to 93%) and resulting output power (115watts). The Stanford experiments used 650V GaN transistors and drivers. Navtas' prospectus to investors identified laptops, mobile phones, and tablet chargers as target markets!applications using considerably less than 600V.

Perhaps the king of low-voltage is EPC, whose transistor line includes 40-, 100- and 200-V devices!eschewing the 650 industrial applications most competitors (including Silicon Carbide vendors) were chasing. EPC's APEC floor demonstrations!like a poster session!included devices for automotive Lidar, wireless power transfers (for cell phones on automotive consoles), RF envelop tracking for base stations, asteroid mining, and medical diagnostics. Practically all of the applications discussed operate at considerably lower voltages than 600 or 650, but all depend very high switching frequencies for their resolution and dynamic range.

Less heat dissipation

For example, the charging standard devised by the Air Fuel Alliance (formerly the alliance for wireless power, A4WP) uses resonant magnetics, operating at 6.78 MHz. The Air Fuel standard describes a number of power levels up to 50 watts. While the power pulse train can be initiated with legacy silicon (RF transistors, for example), the GaN devices operate with greater efficiency and far less heat dissipation. Charging system implementers include Gill Electronics, who builds wireless charging transmitters for automotive consoles and WiTricity, who specialises in charging pads, EPC explained.

Perhaps most relevant to server and communications power conservation, EPC demonstrated a "One Step" power converter that would drop voltage from 48 volts to 1 volt without going through intermediate stages. The DC-DC converter uses Texas Instruments' LMG5200 GaN FET driver. It outputs 40 A under load, and keeps its temperature to 26-degrees C.

As expected, digital power management was a dominant theme in APEC presentations and in the exhibition. Eliminating conversion steps remains a primary technique for promoting data centre efficiency. The ability to go from 48V DC to 5V (or 3.3V or lower) in one conversion step requires a high switching frequency!1.5 MHZ or more!to effect a 95 or 96% efficiency. TI says its TPS53632G analog controller is optimised for GaN enables 48-to-1V conversion. In fact, all the elements of a 40A DC-DC point-of-load supply (POL)-- including the shrunken passives can fit in a 5mm x 6mm x 0.85mm package.

STMicroelectronics introduced a DC-DC converter also said to execute a One Step conversion from 48V DC. ST's part, the STRG06, uses a digital controller with both primary side and secondary side pulse control, clocked up to 1.5MHz.

Second thoughts on PMBus

Despite the market shifts toward high-voltage industrial products, a large number of power management IC vendors continue to show solutions to server and data centre power management problems. Vishay, for example is building a power stage for driving ASICs, FPGAs and Intel CPUs. The power stage includes the power MOSFETs and DrMOS driver circuitry in one package, and reports its current consumption to a central power controller through a PMBus implementation. What Vishay calls its "senseFETs" includes a PMBus interface and enables current reporting accuracy to within 1.5%!an improvement on the 7% accuracy of other PMBus implementations.

Rohm contribution to the computing power arsenal is a synchronous bus regulator with SMBus and PMBus digital interfaces. The Rohm module, the PV9111, includes control logic, PWM, drivers and MOSFETs all in a 7mm x 7mm MLP package. It is optimised for intermediate bus inputs (12V), outputs 5V at 40A with up to 96% efficiency. The control logic is provided by an adaptive DSP, one that came from Rohm's acquisition of Powervation.

Intersil has become the champion of digital control for power supplies and modules at a time when the construction of data centres is increasingly cost sensitive. In its most simple form, digital power management puts a digital control element!a microcontroller, a simple state machine, or even a DSP!into the feedback loop of POL power controller. The insertion of a control element does not necessarily increase the efficiency of the voltage control loop. But it does allow software monitoring for the loop, and!for phase and frequencies!a certain amount of throttle control. Advocates see this as essential for modern power management applications (especially servers and communications switching stations), where it is important to visualise how much power is being handles by back-end POLs.

Appealing to traditional analog designers

Intersil introduced a 50A module with a digital interface at last year's (2015) APEC conference. The part represented something of a breakthrough for Intersil in its ability to pack all DC-DC converter components (passives as well as semiconductors) into one 18mm x 23mm x 7.5mm. But its PMBus interface needed programmer's attention.

Intersil's current introductions (its ISL68200 and ISL68201) is intended to appeal to more traditional analog designers. The full PMBus interface needs lot of Non-Volatile Memory, explains product marketing engineer Brandon Howell. The newest devices can be pin-strapped to set outputs, and includes a condensed PMBus command set. In server systems (hooked up to MOSFETs and drivers), the single-phase module will output up to 40A. The parts support data centre servers, wired and wireless infrastructure, and FPGA power requirements.

Rather than a direct conversion, CUI's rack-mounted power supplies utilise an intermediate bus. Its VSP-200 series will take a 120VAC or 220VAC line input and bring it down to -48V (-12,-5 or -3.3 in some models), with output currents as high as 40A. Software monitoring tools what managers call Data Center Infrastructure Management (DCIM) visually display what currents are consumed by elements on the computer system rack (including the battery backup). The visualisation enables data centre resource allocation.

Some 10% of the world's electricity is consumed by data centres, estimates CUI vice president Mark Adams. "Thanks to the 'magic of peak shaving'," he says, "it is no longer necessary to over-specify power supply requirements." A 10kWatt rack, he points out, no longer requires a 16-kwatt power supply (acknowledging at time when up to 60% of data centre power consumption was devoted to cooling and air conditioning).

Using CANbus interface

CUI promotes a "true power echo system" in which the power consumption of all cards in a rack can be carefully monitored and controlled. This allows data centres to consume more power from the existing architecture, Adams says. The visual interface allows monitoring of loads, utility power draws, battery charging, power allocation among cards in a rack, as well as the battery discharge profile.

The CUI machinery uses a CANbus interface, rather than the PMBus, to communicate power consumption of elements in a rack. Mark Adams claims the existing PMBus command structure is flawed. The interpretation of PMIBus instructions will be different from one PMBus component supplier to the next. The parts are not interoperable. A command to (say) "output 1 Volt" will be interpreted differently by a Texas Instruments' digital POL than by an Intersil or a Maxim POL. The difference in resulting outputs could be as high as 8.0 volts.

-Stephen Ohr,

EE Times

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