AoE x-Chapters, using power MOSFETs as linear transistors

These x-chapters look like they will be great, classic stuff for decades. A bit different audience than AoE probably.

Back when we were in the NMR gradient driver business, we ran switcher-type mosfets as constant-current linear amps, with PPM levels of settling and noise; one box had 20 KW peak output. We just wrapped an opamp around each fet to hide all the sub-threshold complexities.

What we couldn't hide was blowing up the fets in the far corners of the SOAR graph. That involved a lot of testing behind protective plexiglas sheets to absorb the shrapnel.

Some presumably switching fets were just a lot better as linear amps than most.

MRI gradient amps are usually switchers, but they are too slow and noisy for NMR.

Couldn't find the Ixis 4000V IXTx03N400 part on their site but they also don't seem to allow searching by Vds parameter.

In the list, footnote a says first sorted by Vds and then die size. Die size meaning power dissipation I presume which are relatable to each other...

I also notice the applications are strictly small signal and not power like for audio power amplifiers ?

Anything about paralleling mosfets in linear mode ? (I may just not have looked closely enough)

Thanks. The M1 mosfet symbol used in fig 3x.44 looks wrong.

piglet

Well, Win.... I would be interested in how well your mosfet data could be fitted to the DMOS extension that models subthreshold shown here

Also, the fit to the capacitances of the above model would be useful as well...and indeed the Quasi sat region as well. I don't really do any real physical work anymore, its all on that expensive Cadence suite...

-- Kevin Aylward

Do you have anything in the X or AoE III (sorry, don't have a copy yet, just my well-worn AoE II) on low-voltage current sources? JFETs in linear region, depletion MOSFETs, etc?

This old chestnut works okay in some situations down to ~2 volts it kind of sucks the lemon wrt tempco and output impedance, though:

For low currents, this works pretty well:

The tempcos track nicely, as long as the transistor doesn't self-heat much.

There are a bunch of IC current limiters around now.

If noise plays a role, don't use Osram, use Avago HLMP-6000. (as long as Broadcom does not kill it.)

I'm preparing an article "The Silence Of The Lamps" :-) The HLMP6000 is quite dark. I have the gut feeling: the brighter, the more noise.

cheers, Gerhard

But the Osram orange is so pretty.

Nice - thanks!

It might be well worthwhile to have a discussion of the thermal/SOA issues involved in using power V-MOSFETs in linear applications where they may be called upon to dissipate a significant amount of power (I can't recall if this is discussed in detail in the MOSFET section of the new AoE)... if so, recapping or expanding on it might be helpful.

The column "Part Number" had a footnote marker, "x".

The package columns had characters: P A Y U H, etc. Substitute the character in the IXTx03N400 and get, e.g., IXTH03N400 for the TO-247 package. That's a 4kV part, nice! Oops, Littelfuse has now taken over IXYS, and it appears that part has gone! But Google will still show you its datasheet.

Well, we do deal with high power, but as stated, these are HV amp, rather than audio amp oriented.

No, that's not an easy subject. Source resistors...

Thanks, you are correct!

Thanks, Kevin, I'll have to study your material.

Although we show one company's attempt at SPICE modeling capacitance, it's an area I've stayed away from. I've been happy to model HV amplifiers in the region well away from the supply rails, where I can simply use a fixed value appropriate to the region.

We don't go much into the region below a few volts. But note that a BJT works well down to Vce = 200mV.

BTW, you should really get AoE III, you'll enjoy it.

Yes, you are correct. That has been requested, in a rather coherent manner, and it's on my short list. I have three more weeks to add more stuff.

The Littelfuse website is a mess. But you can find the IXYS HV MOSFET parts here (2500 to 4700 volts):

They may have replaced 4kV parts with 4.5kV parts.

Just like bipolar transistor paralleling.

Thanks

uV should be uA

You did mention in the main AOE3 book some stuff about the DC SOA of power MOSFETs at high VDS. I mean the region where part of the die can hog current and melt, (due to negative temperature coefficient of Vth and consequent positive tempco of ID and power density, for each region of the die). In the main book I think you also mentioned the related issue of current hogging between MOSFETs in separate packages, due to the same tempco problems. Perhaps you'd consider expanding on that topic or at least referring to it in this X-chapter.

I have a client who wants low-voltage oscillator (~2 volts) with very low Q "meander" inductors, printed on a piece of flexible plastic film or something. The inductance varies as the film is compressed or deformed and hence the oscillator frequency.

Not an insurmountable problem but I'm a bit stumped for a temperature stable current source down there.

The self-resonant frequency of these is around 100MHz, at 10MHz probably like 4uH with 100 ohm ESR and 10p parallel capacitance