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PCIM: Watch for SiC and GaN at the power show

Momentum has been building behind silicon carbide and gallium nitride power semiconductors, with SiC transistors available and GaN devices not far behind.


PCIM 2013 is likely to be the show where we see which of the vying SiC technologies are being adopted, and the show where real GaN devices emerge from the hype.

Both materials offer fast, low-loss, high-voltage switching.

And both offer real design challenges as devices made from either have unique characteristics that mean new drivers and changes associated power components.

With silicon carbide, interesting arguments are to be had over what type of transistor to develop.

Depletion-mode (normally-on) JFETs are the default answer, however now-defunct SemiSouth was developing enhancement-mode devices, Cree is backing SiC mosfets, and a number of firms including Fairchild prefer SiC bipolar power transistors - which are far faster and more efficient than their aged bipolar silicon cousins.

For example, this year Rohm will be showing its second generation of n-channel SiC mosfets (SCT2xx series, no Schottky diode, TO-247, 175°C Tjmax) and it will be interesting to see how their switching characteristics compare with any new SiC JFETs announced at the show.

If you want to watch GaN then don’t go looking for bipolars or mosfets because GaN transistors have to be HEMTs (high electron-mobility transistors) to take advantage of the miracle-working 2d electron gas. Do look for novel ways to build HEMTs, and maybe this will be the year where someone achieves a vertical 2d electron gas with all the advantages that could spring from that.

Since there were a lot of nearly-but-not-quite capability announcements last year over GaN-on-Si wafers for GaN HEMT production - flatness, defects, bowing and six-inch were words on the lips of the epitaxy guys - expect some reasonable 6in wafer results this year.

The GaN device guys, or perhaps more correctly their marketing colleagues, will be talking-up operation away from the 1,200V sweet-spot - up to 1,700V to compete with SiC, and particularly down to 600V to compete with silicon.

Both SiC JFETs and GaN HEMTs are normally-on and need unusual drive waveforms to make them work and fail-safe, unless they are operated in cascode with a low-voltage silicon mosfet.

This simple strategy all depends on complementary characteristics between the two transistors to get top-notch performance, and it will be interesting to see of anyone is co-packaging such Si-GaN or Si-SiC pairs this year.

One of the best places to get a GaN technology update will be on Thursday when Dr Michael Briere will reveal the status of 600V GaN devices at International Rectifier.

IR is in a great place to know what is going on as it has had Si mosfets since they were invented and has been quietly using GaN transistors, made on its own GaN-on-silicon process, in its own products for a while now.

The above makes it sound like shiny III-V and IV-IV technologies have taken over from silicon, but this is far from the truth as silicon is still extraordinarily good at switching high-voltages.

Silicon IGBTs are the switching stalwarts if operating frequency is not too high: motor control and induction cookers, for example, and it seems like IGBT makers develop a new-generation every year.

2013 sees IR, for example, moving to Gen8 IGBTs for motor control.

Where a bit more speed is required at mains voltages, super-junction mosfets (Infineon CoolMOS is the classic example) with their on-resistance-cheating cunning are still the technology to beat – which is why GaN advocates will have such a fight on their hands at 600V.

Rohm is going to unveil something called 'hybrid MOS', aimed at the power factor correction (PFC) stage of power supplies, which it claims will push super-junctions into IGBT territory.

“Hybrid MOS combines the high-speed switching characteristics and low-current performance of the mosfet, and high breakdown voltage of the IGBT", said the firm, "Rohm adopted a new structure taking advantage of super-junction technology, making it possible to maintain the characteristics of super-junction mosfets at large currents and higher temperatures."

Energy savings are claimed over the full range "from small to large currents", and sampling is scheduled for the summer.

High-power is not all about the active devices.

For example, current sensor firm LEM will be showing its HO series of for motor drives and inverters, and the HLSR series which is intended to knock the cost out of sensing up to 50A.

PCIM is not all about high-voltage devices and mains power.

DC-DC converters, regulator chips, and microcontrollers with power control peripherals abound.

As well as presenting papers, Vicor will be there promoting its recently announced customer-configurable 'VI Chip PRM' modules. Inside is high-frequency zero-voltage-switched buck-boost technology that is said to deliver up to 500W, at above 97% efficiency, at up to 1,700W/in3 (100W/cm3).

Right down at the tiny end, Vishay will be showing off its 1x1.5mm slew-rate-controlled p-channel high-side load switches.

Designed for 1.5-5.5V operation, SiP32458 and SiP32459 offer on-resistance down to 20mΩ (at >3.3V), and a turn-on slew rate of 3ms. Both switches support 3A continuously and quiescent current is 4.2µA. 32458 is reverse-blocking when off, while the 32459 has an output discharge switch.

Diodes is aiming at similar territory with 2x2mm current-limited power switches with an on-resistance of 70mΩ and handling currents out to 2.5A. AP2501 and AP2511 are, respectively, active-low and active-high enabled. Both have an open-drain, over-current fault flag with a 7ms deglitch to prevent false reporting.

See also: Picture Gallery - PCIM 2012, Nuremberg


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