Why dont Use IGBT in all applications.

[kushal4269]

Hello people,
 I am new in the field of power electronics, Basic knowledge says we should use IGBT when higher current and lower frequency is needed in application. I am using IGBT(bsm50gb 150 delay time info attached) which has low total delay time(which means it can be used in high frequency application). My question is why we even use MOSFETs if this IGBT has no drawback?

[dmills]

A modern small switcher for low voltage use might be going at over 2MHz, what does that do to efficiency when you have 100ns rise and fall times, and fully half a microsecond of turn off delay?

IGBTs are a win when I * Vce(sat) << I^2 * Rds(on), but that is not always the case, and for example if dealing with say a 5A supply with 12V across the top switch it is very hard to beat a modern mosfet.

Another fun one, a synchronous buck outputting say 0.75V @ 50A for some modern processor core, an IGBT will struggle here as the bottom switch because Vce(sat) is comparable to the output voltage so efficiency will suck.

The part an IGBT might be a sort of competitor for is the SiC mosfet, in high voltage applications at modest current and frequency, but even there the Vce(sat) is often not competitive with Rds(on) * Id.

Where IGBTs excel is high voltage high current applications in things like traction drives where the switching frequency is low enough to make the horrid rise and fall times unimportant, and the approximately constant Vce(sat) beats the equivalent mosfet ohmic drop. 

They are an ideal tool for a specific niche application, but there is a reason we have a whole box of tools.
 

[golden_labels]

Skipping technical details outlined above…

Not every switch needs to survive 1kV. And if you ignore the voltage, just compare the prices of MOSFETs and IGBTs with similar characteristics to get your answer.

[T3sl4co1l]

They're also lower cost, what's not to like!?!...

Well, Vce(sat) is nonzero, whereas MOSFETs can perform synchronous rectification to arbitrarily low voltage drops (simply Id * Rds(on), put enough in parallel and get Rds(on) as low as you want).  This is especially important at low voltages, where the IGBT's 1-2V drop might be utterly infeasible to begin with.  (I'm guessing you didn't mean ALL "all" applications, just, say at >300V or something.  But if so, why overgeneralize?  There are far more applications, than just whatever immediate project you're working on that brought this question to mind!)

IGBTs are also slower.  A MOSFET switching in ~20ns is pretty common (given a powerful gate driver and optimized layout), making switching frequencies over 400kHz feasible, even over 1MHz for resonant operation.  Part of this is the IGBT has a long tail of turn-off current, due to the bipolar current flow component, which acts like diode reverse recovery for similar reasons.  This current drops off exponentially over time, and is not well captured by the turn-off time characteristic (which measures the 90-10% current, or something like that -- a large part of which can be due to majority carriers (MOS current flow)).  Rather than precisely characterize the turn-off waveform, it's more common to simply give an aggregate property: turn-off energy.  With a figure of some ~mJ being common, you can clearly see the losses at 100s kHz can be a huge problem!

IGBTs are also less robust.  They typically show short-circuit / fault durations up to 10 or 20µs.  MOSFETs typically handle five times this -- the dies are much larger.  The same feature that makes IGBTs affordable (high current density) also makes them less robust.

IGBTs are also prone to avalanche failure; I think a few are available with this rating now, but by and large this is simply not available (or hasn't been, at least).  If nothing else, the smaller die area means less energy handling.  Not that you should be relying on avalanche in general, but it can save things in random cases.  As a consequence, IGBTs tend to have higher voltage ratings (600V vs 400-500V MOSFETs; 1200V vs. 800-1000V MOSFETs -- though the somewhat fixed voltage drop means they scale somewhat better to higher voltages, or did before SuperJunction MOS were introduced).

IGBTs also don't have an intrinsic body diode, though this is a mixed bag.  There are some applications where not having one is helpful; though (bare) IGBTs largely handle only -6V or so, making this a rather non-feature.  (There are some available with full reverse voltage ratings, which are useful for current-sourcing and multi-level type inverters.)  Co-packs (with FREDs or whatever) are readily available, and can perform better, as the FRED has less recovery than the MOSFET body diode (which tend to perform rather poorly, especially at high voltage ratings).

IGBTs are also nearly unsuitable for linear operation, not that they'd really be good candidates in the first place given their high power density -- that is, making them even more prone to 2nd breakdown, and having lower power ratings for the same V*I ratings -- though, bizarrely, there are a few IGBTs I've seen boasting a DC SOA curve(!).

Tim

[Benta]

The question is like "why not use self-tapping screws for everything?"