MOSFET versus IGBT: round 1

The MOSFET or IGBT question is a recurring one. We all have faced this question once. Knowing which type of FET to go for. And the question is still going-on. You can look for the web, and you find an IGBT manufacturer’s e-book about how IGBT’s are such a good solutions for so many applications. But you may also find an e-book teaching you about MOSFETs, their superiority, and how you could use them in almost all of your designs. If this book happens to be written by a famous MOSFET manufacturer, it’s a total coincidence of course…

It reminds me of Transphorm’s former CEO presentation during a Keynote at APEC. He showed us a slide (actually one that I made back when I was at Yole Développement), that he twisted to mock how SiC, GaN and Super Junction MOSFET actors view the same situation.

A great way to teach how marketing and vision alter a message supposed to be objective at first (and why consulting companies like Point The Gap have clear view :-) ).

Figure 1: Transphorm’s CEO presented these picture during APEC keynote sessions: The truth about marketing your products.

So, I’m not here to reveal you a hidden truth for generations (of IGBTs). There is no hidden truth.

MOSFET or IGBT: « It’s trade-off issue », they said!

It’s what « they » said… All the people you met and asked the question. But each time you read a new article on the subject, it becomes blurry. Let’s make things a little bit clearer for everyone.

There are a few cases where the MOSFET or IGBT question is non-applicable:

• All applications requiring 1700V or higher breakdown voltage for the switches
  • Rail traction, Wind turbines, Grid T&D, Central inverters for utility scale PV, high power motor drives…
  • This breakdown voltage requirement comes mostly from the DC-bus voltage. This bus in motor drive or generator as an example, is the one that conveys rectified DC power to the DC/AC conversion part. This bus, loaded with capacitors to stabilize and filter current, may have a design at 1000V+. This make you have to use 1700V devices or more.
• All applications at very low voltage:
  • Apart from flash light triggering in cameras (300V IGBTs). You will always go for a MOSFET. From 5V to 400V. Period. It’s smaller, faster, better and sometimes stronger.
  • This includes a load of consumer applications, embedded power conversion to power-up all sorts of functions inside all the electronics you have at home or carry everywhere.

So where does the question applies? For all the stuff between 400V and 1700V.

You can have:

• Consumer applications: like AC/DC adapters for laptops, tablets and electronics
• Renewable energy with PV inverter for residential, but also micro-inverters.
• All the industries using the medium and small range of motor drives (conveyors and/or belts)
• All small/medium power UPS, in data centers, SMEs, industries…
• And so many other applications…

The blurry boundaries between MOSFET and IGBT

MOSFETs are good because they are fast. Their switching frequency can go up to several 100MHz. And it’s their main advantage. You also have to think about and Include Super Junction MOSFET in the story. We will write an article about the history of Super Junction MOSFET. But remember that SJ MOSFET have even better characteristics compared to MOSFET. They are also more expensive.

On the other hand, IGBTs can handle more power, meaning that at a comparable voltage, they outperform MOSFETs in current handling capability.

This is the main idea and it comes from losses. All transistor have losses. If we highly simplify the problem, you end-up with two type of losses: Conduction losses and switching losses, respectively happening during conduction mode, or at switching on/off or off/on (you could guess that, right?).

So what happens is that:

• IGBT have lower conduction losses but higher switching losses compared to the best Super Junction MOSFETs
• MOSFETs in general and especially Super Junction MOSFETs, have low conduction losses (the famous RdsON they advertise everywhere) but have higher conduction losses compared to IGBT

And this is the key thing you have to keep in mind.

SiC MOSFET or GaN FET: With new generation, come new questions

Next generation compound semiconductor devices are not in the party yet. They are too expensive for you, and you better let another niche applications and adventurous company put Wide band gap semiconductor on the market. But you better watch and try the devices, validate them and get ready to launch.

Though the choice is still blurry for now: GaN FeT should be used for voltages up to 1200V. SiC is giving it’s best at high and very high voltage: 1700V but, based on R&D, it’s outstanding at voltages up to 10kV.

It’s still an unclear boundary. So far, GaN is available at 600V only (1200V are samples).

SiC begins to be used, for very specific applications.

MOSFETs and IGBTs are now becoming a commodity: More and more Chinese companies are able to manufacture IGBTs. Only early generation and simple device designs are available from them, but that’s a sign. On the other side, Infineon recently announced that they are moving some MOSFETs (40V OptiMOS) to their Dresden 12in. (300mm) wafer fab. It’s becoming common and cheap to have Power MOSFETs and IGBTs.

But the market of GaN and SiC is not at it’s breakeven point. Investment is what make manufacturers survive, not sales. This makes us tell it’s early.

How to choose between MOSFETs and IGBTs: frequency and voltage (but mostly losses…)

But mainly frequency. They are still king on their own voltage (low for MOSFET, and high for IGBT). but between 400V and 1700V, you have to choose. Look at the losses, the on and off time, the frequency of your applications, but also now you are looking at the footprint (MOSFET generally have small packages).

Keeping in mind these points:

• IGBT have lower conduction losses but higher switching losses compared to the best Super Junction MOSFETs
• MOSFETs in general and especially Super Junction MOSFETs, have low conduction losses (the famous RdsON they advertise everywhere) but have higher conduction losses compared to IGBT.