Anyway, who need so much power for a single speaker ?
As for MOSFETS, they seem like a good idea in theory but have two major issues, one that the SOA is usually much moire limited -as in can handle a jawdropping 600V or 50A, but only 2.5A and 30V when applied at the same time.
I think you've got that backwards. MOSFETs generally have a wider safe operating area, than BJTs and are capable of handling far higher currents, at the full voltage specification. Check out a few data sheets for high voltage MOSFETs and BJTs and you'll see what I mean.
For obsolete lateral MOSFETs this is almost the case but not for vertical MOSFETs. At low currents, the temperature coefficient of the gate threshold voltage reverses so current sharing is not enforced even on a single die. At low drain voltages this is irrelevant because power dissipation and heating are low but at high drain voltages, it results in thermal instability and current hogging. Most MOSFET datasheets do not bother to show this in their safe operating curves because they are intended for saturated operation where this does not apply.
https://www.fairchildsemi.com/application-notes/AN/AN-4161.pdf
http://www.irf.com/technical-info/appnotes/an-1155.pdf
http://www.ixys.com/Documents/Articles/Article_Linear_Power_MOSFETs.pdf
I didn't say that the safe operating area is not a limitation with MOSFETs, just that it's generally better than BJTs, especially at higher voltages. Check out the data sheet for a typical HV MOSFET such as the IRF740B. Can you find a BJT with a safe operating area of 400V at over 300mA?
https://www.mouser.com/ds/2/308/FairchildSemiconductor_1614842276095-1191888.pdf
Some mosfet technologies are a bit limited regarding DC operation but, Infineon still makes fully DC operable high voltage mosfets.
For example IPW90R1K0C3. I have used those in my high voltage power supply.
Some manufacturers (like BK Precision) think it is okay to use non DC rated mosfets in linear mode. Just look up one of the last Shahriars video, where he repaired the blown HV PSU. There was a STW15NK90Z used.
Part of the problem is that DJs and mixing guys typically wear highly-insulating headphones so they don't suffer the pain of their own misdeeds. Maybe they need to insist that open headsets be used. At least then the problem would be self-limiting. In most cases, anyway.
I didn't say that the safe operating area is not a limitation with MOSFETs, just that it's generally better than BJTs, especially at higher voltages.
The chances that the Infineon MOSFET is Ok for DC operation is good. The modern very high voltage MOSFETs (e.g. > 600 V) use a special technique, that makes them less sensitive to 2 nd breakdown. Also Infineon in many cases includes the effect of thermal instability in there SOA curves - so there is a good chance that the SOA curve is trustworthy. Some other companies tend to calculate there SOA from measured/calculated thermal impedance and thus ignore possible thermal instability in there SOA.
One might still need individual testing of the chips for high power linear operation, as there is always a small chance to have something like small voids in the die attachment (or local contamination) that could cause premature failure for a few rare samples, even with other wise good types. This also applies to BJTs - here audio transistors with individual FBSOA testing are available.
But the Infineon datasheets and applications notes do not say anything about linear operation and safe operating area of Coolmos parts
Higher voltage parts have a much larger die size for lower junction to case thermal resistance for the same Rds(on) and greater resistivity which helps.
High voltage parts effectively include derating and source ballasting but that applies to any high voltage MOSFET and not just Infineon's Coolmos parts.
I think it much more likely that like most other manufacturers, they leave that part off of the safe operating area curve because it is irrelevant for the intended switching applications and difficult to quantify. It also makes their parts look worse to why advertise it?
FBSOA testing is pretty destructive. I think I read an application note which mentioned screening using x-ray imaging to look for die attachment and drain metalization problems in linear rated parts. It seems like a high resolution thermal camera would work to look for hot spots before encapsulation.
Integrated Darlingtons are rarely used in audio, instead the discrete Darlington or Sizlaki or triple emitter follower, so 2-3 discretes.
I think it's because there is no access to the middle node, at the base of the last transistor. This usually connects to a resistor on the opposite side, even in STK's.
But the Infineon datasheets and applications notes do not say anything about linear operation and safe operating area of Coolmos parts
They say something: if there's a DC SOA curve. Which most (all I've seen?) CoolMOS parts do.
QuoteHigher voltage parts have a much larger die size for lower junction to case thermal resistance for the same Rds(on) and greater resistivity which helps.
Compared to... lower voltage parts in a given generation? Compared to prior generations? Or what? Not sure what you're getting at here.
QuoteHigh voltage parts effectively include derating and source ballasting but that applies to any high voltage MOSFET and not just Infineon's Coolmos parts.
Hmm, could you explain this more? What do you mean by derating and ballasting?
I think the SOA test uses electrical properties -- watching d/dt (runaway) tendancies, or increasing pulse durations followed instantly by measuring die temp via body diode tempco (which is a useful method, as the hottest spot has the least voltage drop, so you're mostly measuring the peak temp of the die, rather than the average over the die area).
Most power MOSFET specifications include a DC SOA curve but leave out the effect of thermal instability. The cake curves are a lie. The thermal instability also applies under pulsed operation just like secondary breakdown in a bipolar transistor but they usually do not show that either. I linked a pair of examples where this is shown.
For the same Rds(on), the die size is roughly proportional to the voltage rating squared which is *not* the case with bipolar transistors and IGBTs.
Many of the characteristics which make a power MOSFET with a smaller thermal instablity region are shared with high voltage MOSFETs. A larger die decreases the junction to case thermal resistance helping to prevent hot spots. The higher voltage construction increases resistivity which compensates for some of the Vgs thermal coefficient.
There are some interesting comments in the Fairchild application note I linked. High current induced failures occur under the bond wires where the current is greatest while thermal instability failures occur away from the bond wires which act as a heat sink helping to prevent the formation of hot spots. Imperfections in the die attachment and metalization provide points for likely thermal instability failures.
Most power MOSFET specifications include a DC SOA curve but leave out the effect of thermal instability. The cake curves are a lie. The thermal instability also applies under pulsed operation just like secondary breakdown in a bipolar transistor but they usually do not show that either. I linked a pair of examples where this is shown.
Hmm, so you're saying manufacturers publish DC SOA, and are lying?
Then you must be accusing me of lying. I tested two devices to failure outside of their DC SOA, just as it should be. I don't much appreciate that.
QuoteFor the same Rds(on), the die size is roughly proportional to the voltage rating squared which is *not* the case with bipolar transistors and IGBTs.
Ah, previous generations were -- SuperJunction is proportional. Which is why it so thoroughly destroys the specs of earlier parts -- compare, say, a 600-1200V PolarHV part to a MDmesh M2 part of the same rating. (Or, IXYS licenses their own line of SJ now, IIRC, if you prefer to stay within the same brand. Speaking of IXYS, a lot of their PolarHV or HiPerFET or whatever parts, didn't even bother with an SOA at all -- you have no way to know if those beasts are even safe for a single switching event! Scary!)
And which makes it all the more remarkable that SJ has a full SOA, and that's no lie.
QuoteMany of the characteristics which make a power MOSFET with a smaller thermal instablity region are shared with high voltage MOSFETs. A larger die decreases the junction to case thermal resistance helping to prevent hot spots. The higher voltage construction increases resistivity which compensates for some of the Vgs thermal coefficient.
Ah, okay.
Why do SuperJunction FETs have full SOA?
QuoteThere are some interesting comments in the Fairchild application note I linked. High current induced failures occur under the bond wires where the current is greatest while thermal instability failures occur away from the bond wires which act as a heat sink helping to prevent the formation of hot spots. Imperfections in the die attachment and metalization provide points for likely thermal instability failures.
Sounds reasonable.
I didn't see anything about this "instability region" you're talking about, though. Is this an imaginary region on the SOA where thermal drift (increasingly, until failure) occurs?
All SOAs shy away from this region; I'm not sure how you got the idea that manufacturers lie about it.
Have you tested any parts to destruction that showed otherwise? I've shared my data.
We should be so lucky that everything in our domain can be proven by measurements!