1,700V GaN mosfet IC from Power Integrations

Power Integrations has announced industry’s first 1,700V GaN transistor – as far as the company can tell.

It is integrated within its latest series of InnoMux-branded multi-output fly-back ac-dc converter ICs, in his case intended to work from 1,000V rails in industrial applications.

“InnoMux 2 is the first vehicle because a lot of applications that need 1,700V are industrial with two or three outputs” company director of training Andy Smith told Electronics Weekly.

The transistor has been developed within the company, which does not disclose where the fab that makes it is located. The wafer is brought in, and Power Integrations grows, the epi, is all that Smith would reveal. He also remained tight-lipped on whether it was GaN-on-silicon technology, or not. but did reveal that the transistor itself is a cascode device.

The motivation for high-voltage GaN here is simply cost saving: silicon carbide transistors, which the company already uses in high-voltage products (and will continue to used in those products, said Smith), are expensive because the boule from which wafers are sliced takes so long to grow.

Are the 1,700V GaN transistors fast?

“There is no need for speed in fly-back. We don’t run more than 140kHz anyway” responded Smith. “We can run at 300kHz, but the need for creepage and clearance means magnetic core volume does not scale 2:1.”

GaN transistors lack the avalanche-based over-voltage protection that can be designed into silicon or silicon carbide mosfets.
Instead, GaN transistors are designed with a large over-voltage margin – margin that has yet to be standardised between manufacturers.

How is Power Integrations playing this?

“Our GaN devices break down around twice the rated voltage,” said Smith. Above the rated voltage “a high-voltage spike causes electrons to shoot our of the channel into surrounding GaN, imparting a charge and inhibiting the channel – Rds(on) increases by 4-5%. The electrons will drift back eventually, and Rds(on) gets back to normal over a number of hours. This is non-destructive until 2x the rated voltage.”
Is the result as tough as a SiC dc-dc converter?

“Silicon carbide is exceptional,” said Smith. “I think GaN is more resilient than silicon.”

How is reliability?

“We have 14 product families that use GaN, and we have developed a good set of reliability data accelerated by high-temperature and high voltage,” said Smith. Our FIT is around 0.9.”

FIT is failures-in-time, here assuming 55°C and 0.7eV activation energy at a 60% confidence level.

Are standard heat and voltage acceleration techniques valid for GaN?

“The Arrhenius equations holds true for GaN as well as silicon if you don’t go too far,” said Smith.

InnoMux is the company’s multi-output fly-back dc-dc architecture and it has sized its first 1.7kV GaN transitor product for outputs up to 100W, making it suitable for industrial auxiliary power supplies.

The topology works using one primary-side switch – the Gan HEMT in this case – and one or two secondary-side low-voltage mosfet switches – one for a dual output and two for a triple output.

The highest voltage output has no controlling series switch, allowing the one or two secondary mosfets to ‘steal’ power for their outputs when needed. This can happen for all outputs during each primary switching cycle.

The control IC contains both primary and secondary control die, an inductive isolated feedback link, and the GaN primary switch.

This primary switch is exposed to the input voltage, plus the output voltage reflected back through the turns ratio, plus any switching and rectifier recovery transients.

These add up to less than 1,360V in the company’s 1,000V input RDR-1053 design example and report, said Smith, leaving 340V of clear headroom for unexpected transients before the transistor experiences even temporary Rds(on) changes.

Efficiency tests have been done comparing the company’s earlier 750V GaN transistor and its ‘StackFet’ switch stacks a high-voltage silicon mosfet on top of the 750V GaN transistor (see graph).

Using the same PCB and inductors, efficiency remains above 90% for inputs between 150V and 1kV with the 1,700V GaN transistor, and identical to the 750V transistor with up to 400V on the input.

The stackFet version works up to 1kV, but sacrifices 2% in efficiency at 400V, increasing to 8% at 1kV.

Silicon carbide tests have yet to be done as a one-off SiC InnoMux 2 circuit has to be constructed, said Smith but “I expect SiC would be similar to the 1,700V GaN results.”

Images: Power Integrations

See also: Cambridge GaN Devices drives it like a MOSFET