1.2kV SiC JFET challenges IGBTs

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SemiSouth, a little-know semiconductor firm from Mississippi, is introducing high-power silicon carbide JFETs to replace silicon IGBTs and silicon mosfets in high power inverters.

The firm already makes SiC Schottky diodes for the power factor correction and solar inverter markets.

Even ignoring the choice of material, a power JFET seems an odd choice for a switching device when enhancement mode power mosfets are frequently the component of choice.

Enhancement-mode devices like power mosfets and IGBTs are ‘off’ when the gate is at zero volts.

Silicon JFETs operate in depletion mode – they are ‘on’ with zero volts on the gate and n-JFETs, for example, need a negative bias to turn them off.

“We are the only company in the world to offer a normally-off JFET,” Dan Schwob, v-p sales and marketing at SemiSouth, told Electronics Weekly.

How?

“In a JFET, you are basically creating a junction for the gate. Because of the wide band-gap of SiC, there is a broader range of possibility compared with silicon and we are able to move over the gate threshold,” explained Schwob.

He claims that, with the same current and voltage rating, his SiC JFET die are one tenth the size of Si mosfets and IGBTs.

And that compared with IGBTs, even with the smaller die, they have almost around a seventh of the power losses – due mainly to reduced switching losses.

The JFET structure is a trench design – see diagram below.

“We are the only company in the world who can etch a trench in SiC,” claimed Schwob. “For the devices, ours is a seven mask process, which is the same number of steps as an IGBT, although the steps are different.”

SemiSouth is selling 1,200V transistors with 4.5m?/cm² specific on-resistance, and claims to be able to make 2.5m?/cm² devices.

A production example is the SJEP120R100, a 20A 1,200V 100m? JFET with a 4.5mm2 die.

Gate capacitance is 576pF, and Schwab said it sells for $12 – which he compares to $4 for a similarly rate IGBT.

With no minority carrier storage, rise and fall times are fast: 20 and 15ns respectively.

This enables SiC power inverters to be operated faster than silicon IGBT types – Schwob talks of 50kHz compared with 20kHz for a 20kW inverter.

Higher frequency means smaller magnetics, but the designer is going to have to watch out for switching transients.

“dv/dt and di/dt is so high that we have to teach customers about parasitics,” he said. “Sometimes you have to slow the device down or do some wave shaping.”

Although Schwob claims the SiC JFETs can sometimes be a direct drop-in for Si IGBTs, this is not going to happen very often because 60V/ns transients mean very short connections and careful driving are both critical at the gate.

As such, SemiSouth has issued a suggested opto-isolated gate driver based around one Ixys IXDN404 4A dual driver chip per gate – see circuit below.

itemid-54122-getasset.jpg Even though these are enhancement devices, the suggested driver takes the gate down to -15V to make sure the transistor stays off when it should do – particularly when it is the top transistor of a half-bridge.

“You don’t have to throw the whole inverter design away,” said Schwob, “you will have to do a little bit of work on the gate circuit.”

There is a 50m? 9mm² device under development, and a 25m? device has been announced, but at 15mm², “this will not be a cheap part because of defect densities”, said Schwob. “We tell customers to use the 100m? part or the 63m? part which has two 100m? die, and there is a new 75m? part.”

All of the 1,200V devices are aimed at inverters which produce single and three phase mains voltages, particularly from solar arrays – more of which later.

For high-voltage flyback converters, which are used to produce auxiliary power in ac motor drives – from 800V dc for example – the firm has a 500m? 1,700V JFET.

“Long term, the temperature characteristics are attractive for the electric vehicle market,” said Schwob. “If we specifically make some 600V transistors for EVs, it will cut RDSon by 25%.”

Solar inverters are the application that most excites SemiSouth, both for its transistors and its more established SiC diodes – where it competes head-on with Cree and Infineon – with ST and Fairchild aiming to get into SiC diodes as well.

Conversely, Cree – and other firms including Rohm – are getting into SiC mosfets where they will compete against SemiSouth’s JFETs.

SiC Schottky diodes are a drop-in replacement for the fast high-voltage silicon diodes used in the boost converter stage of solar inverters.

They are expensive.

Schwob’s sales pitch goes like this:

Switching from silicon diodes ($0.50-0.75) to $4 SiC Schottkys in a 20kW solar inverter gains 0.5% efficiency with no other modifications, he claims.

Four 10A diodes are needed in a two-phase 20kW inverter – which Schwob says are around 97% efficient.

Gaining 0.5% on a 20kW inverter is a saving of 100W which, at an installed cost of $4/W for solar panels, is worth $400.

A saving of $400 for $16 worth of diodes.

SemiSouth’s diode product range includes 1,200V 5 and 10A devices in TO-220 packages, and 10, 20, and 30A in TO-247.

“The 30A rated SDP30S120 is industry’s highest current 1,200V part to be commercially available,” said SemiSouth at its launch. “We expect to expand our silicon carbide diode Schottky power diode family to include 60A parts in the very near future. “

Schwob insists SemiSouth is an independent supplier of SiC devices.

“We get our substrates from three vendors: one German and two US, none of which is Cree,” he said.

All wafer processing including epi layer growth is done in-house on 75mm wafers, with 100mm by the end of the year and 150mm wafers when they become available, said Schwob, which he estimates will be the end of 2 012.

itemid-54121-getasset.jpg ”We have spent 7-8 years perfecting the profiled epitaxy,” he added.

In September, the firm announced the development of a 100A 1,200V all-SiC solar inverter module.

“Utilising parallel combinations of enhancement mode SiC JFETs totalling 36mm², and 23mm² of Schottky diodes, a total on-resistance of only 10m? [2.7m?/cm²] was achieved at drain currents of 100A in the commercially available standard module configured as a half-bridge circuit,” said SemiSouth.

Tags: IGBT, inverter, JFET, mosfet, SemiSouth, Solar

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