Odd behavior of LTSpice for NMOS and PMOS in series with zero gate voltage

Doh! I shouldn't try to design and simulate circuits while watching TV or being otherwise distracted. It would have been obvious if the symbol included the body resistor. Now to figure out how to do this.

Thanks,

Paul

Makes perfect sense. The substrate diodes are in series and conducting.

It would help if you labeled the nodes.

I used two PMOS back to back, and that seems to work well for input voltages above about 2.4 volts. Here are simulations:

Thanks,

Paul

Resistor?

It does show the diode!

I guess nobody talks about what a MOSFET symbol actually means, anymore?

Just as in any other semiconductor symbol: it's the triangle in/out of the middle. That's the channel-substrate junction. What's the channel? There are two doped regions (source and drain), and the channel is between them. For an N-ch type, S and D are N doped, and the substrate is P doped. The one arrow does double duty, representing both of those junctions.

Which, incidentally, means an N-ch MOSFET is also an N-P-N transistor, just a crappy one, like the old lateral PNP hack. The wide base (equal to channel length) gives low hFE, probably in the ballpark of 1 for most designs.

Since source and substrate are strapped together, it's even a diode-strapped transistor. But with the low hFE, there's not much point in calling it that way, so a diode it is!

Indeed, any MOSFET symbol you see with the antiparallel diode is redundant*, and if they draw it as a zener diode, then, redundant and silly! ;-) (MOSFETs are usually rated for avalanche these days, so the device can behave like a zener/avalanche diode. But that's just controlling the drain junction to try to not destroy itself. It's not a separate diode.)

• "FETky" parts have an integrated schottky diode -- you can't make a FET with schottky body junctions; heh, well, there are PHEMTs I guess, but no, these are just the two parts, integrated together however they please.

Tim

This is one channel of my 32-channel VME SSR module.

The protected mosfets turned out to have too much personality, so we use regular mosfets with software overload protection.

The board is almost done with layout. I did one channel as a model for my layout guy

This is my preferred mosfet symbol

which visibly behaves like an NPN transistor. The usual symbol is too fussy.

Incidentally, I have characterized an enhancement-mode PHEMT as a diode. A very fast, 1 pF, 1 amp diode!

I made symbols for NMOS and PMOS, with the body *diode*.

Having that as part of the symbol makes it more obvious which way it will conduct current.

Paul

NPN transistor points in the direction of conventional current flow.

So, uh, you've shorted out the supply?

Tim

Ditto in my mosfet symbol. The mosfet is just an NPN with an insulated base. Very intuitive.

What would make you think that?

MOSFETs are majority not minority carrier devices. Minority carriers flow through the body diode, as indicated by the triangle in all semiconductor symbols......

No, that's an IGBT. Very slow for your things.

But anyway, we've trolled this before.

Tim

Nobody seems impressed with my analog multiplier thing.

When I was a little kid the arrow pointing in meant "iN" and the arrow pointing out meant "Pee"

I can count on one finger the number of people here who wish to massage your ego. Besides, you forgot to add the duct tape. -1.

It's a cool circuit. The transistors operate with bases at ground and collectors at ground. It works well with a cheap non-monolithic dual transistor. It lets you really push a mosfet safely, far better than a primitive current limiter, even a foldback limiter. Can save a ton of money on mosfets and heat sinks and fans.

This isn't about ego, it's about circuits. Really, my ego doesn't need any outside approval.

John L, Nobody commented on your circuit because 99.5% of the lurkers here have no clue as to how it works. Most can't even properly bias a transistor let alone tell you how it's working.

As for John S, he couldn't find his "S" even with the help of an assistant holding the mirror and flashlight >:-} ...Jim Thompson

On Sunday, November 12, 2017 at 11:01:30 AM UTC-8, John Larkin wrote: ..

...

It seems a good application of this circuit arrangement to use it to measure device dissipation in real-time.

I first saw the circuit about 30 years ago in the National Semi Linear Applications Handbook and Bob Pease described it in one his articles in 2002.

kevin

..

When I worked for EIL Instruments in the 70s and 80s we sold wattmeters and watt transducers that used a non-linear arrangement of resistors and diodes in a bridge circuit to produce a DC signal proportional to watts. Here is the patent showing how it works:

Paul

PNP = Piss iN Pot

That's pretty horrible.

The Jones multiplier dates to 1963. Gilbert reinvented and improved it in 1967.

The 2 uF calculation is obviously wrong.