Taking GaN over 1200V
Power Integrations has developed a cascode gallium nitride device that operates up to 1250V.
Andy Smith at Power Integrations tells Nick Flaherty about the development of the Innoswitch3 EP-3 device with a 1250V GaN transistor.
“What we tend to do is embed GaN technology in our ICs, systems that provide value,” says Andy Smith, director of training at Power Integrations. “The EP-3 is an isolated switcher IC with primary and secondary side flyback control and a control algorithm that allows you to switch efficiently.”
This follows on from a 900V GaN version and a 1700V SiC version, with the company launching its first GaN parts back in 2019 with the integrated gate driver and isolation between the primary and secondary circuits all in one package.
“We had 900V GaN but the problem is that this is not enough margin for some industrial customers. As the voltage increases the switching losses go up dramatically so with lower losses you can use a flyback converter rather than resonant converter,” said Smith.
It is based on a normally on cascode design that combines a silicon diode with the GaN transistor to make the switch a drop in replacement in designs. The technology is aimed at higher voltage industrial and renewable energy applications.
“We like the natural performance of the GaN technology and the gate voltages are easier to organise. This makes it easier to switch and means we don’t need to put T-wells in the gate of the transistor. This needs a low voltage silicon diode but this has the same characteristics [as other Innoswitch devices] so you don’t have to redesign the EMI filter at is easy to integrate in the package.
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“1250V is the highest GaN has got and that’s because of the structure, the cascode. The thing you have to avoid is oscillation all over the circuit board. When we design the switches we decide on the size of the gate driver to make the slew rate about the same as silicon.”
The cascode used by Power Integrations for its 1250V Innoswitch3-EP
This is why the first use of the 1250V device is in the general purpose EP3 family rather than the EP4 version.
The Innoswitch 4 switches at 140kHz and this is focussed on high efficiency with active clamping and resonant switching. In contrast the Innoswitch starts with silicon then GaN then SiC versions up to lower switching frequencies up to 90kHz. “It’s the Swiss army knife family,” said Smith.
“When you make a transformer smaller you are pushing the limits for thermal issues. There is so much area loss based on non-scalable things such as creepage clearance and insulation layers you cannot get all the benefits of scaling the frequency,” said Smith. “But the higher efficiency does allow you to get rid of the heat sinks, that’s one of the areas for industrial designers. They get a more robust circuit that’s easier to build.”
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However this is just the first use of the 1250V GaN technology, and the part is shipping in volume this quarter Q4 in distribution. “The 1250V is portable so we can use it in other areas,” said Smith.
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