FET's in series

Internal gate zeners are fairly common these days, typically about

+-40 volts. And I've never seen a gate blow out below about 70!

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It's fear that drives gate protection.

--
 Thanks, 
    - Win

Thanks Win, I hadn't thought about what happens when someone hooks it up backwards. (or otherwise.)

I'm starting to go back to resistors and some slow PWM, roll off the edges hard... (life is all about time constants.)

George H.

Me too, His "age of fighting sail" is a wonderful historic read. I gave my copy away to a favorite uncle, I need to find another.

George H.

I hadn't seen that. It's on order from Amazon.

I have a couple of shelves of seafaring warfare books, from The Armada to a bunch of WWII stuff.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

On a sunny day (Sun, 6 Mar 2016 19:10:16 -0800 (PST)) it happened George Herold wrote in :

Put the whole heater thing in a bridge rectifier. And then it works for AC too. Done it.

The significant data sheet phrase is "ESD protected", which generally seems to mean a roughly 40 volt bidirectional zener. It can come in handy some times, but you have to test real parts to see how tough they are.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Hey, (unrelated to ESD protection) I was looking at Idss on spec sheets (zero gate voltage drain current.) listed as max 25 uA (no typical values.) I went to measure it on a IRF520 and was seeing ~30 pA at 50V DS... while not 100 V DS that's a lot less than 25 uA! is that typical?

George H.

Yes. Nobody wants to test cheap mosfets for pA leakages on a production line... it's too slow. Few users care.

Gate leakages are often electrons per second turf; a open gate voltage may hang pretty constant for hours.

Here's a 2N7000 with an open gate, halfway on

You can do all sorts of fun stuff with this rig, like moving charge here and there with fingers or a small insulated screwdriver.

Drain currents can be pretty low too. I got this somewhere, maybe from Win.

I wonder if negative gate voltage would make Id even lower.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Yes, my measurements and a plots in star office spreadsheet, 11 years ago.

Too bad I didn't think of reversing the voltage. But as you can see, the current seems to be made up from Ebers-Moll style Vgs-programmed current (yellow squares, 178mV/decade) and a fixed leakage current of about 1.8pA, which dominates at Vgs = 0. The leakage current, IIRC, depends on drain voltage, and looks like a fixed parallel resistor. If the measurements were made at Vds = 5V, the leakage resistor was about 2.8T at room temp.

--
 Thanks, 
    - Win

Jfets and phemts have an optimum negative gate voltage for min Id; more negative that that point, drain current increases. But they are very different animals than mosfets.

Some small-signal bipolar transistors have absurd leakage currents, like fAs.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

In defense of the 2n7000, it's really quite a big part, with a higher current capability than a typical small BJT, more than an amp.

--
 Thanks, 
    - Win

Indeed. To get any kind of linear bandwidth out of it, you need to push 100s of mA. Which means quite low voltages, which don't help with capacitances. I suspect a lot of 10W-tier RF FETs are nothing but 2N7000 dice, probably modified for larger die area (more power dissipation, plus better packaging) and lower Crss.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

A couple of paralleled 2N7002s, with good gate drive, can put 50 volts into 50 ohms in under 1 ns. It's ancient, but I haven't found a more modern mosfet that's any better. We pay 2 cents per.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

putting them in parallel would have the advantage that you can connect the taps to a plane

(if you can get them to share current)

-Lasse

Indeed, all the drains bolted to the same plate. The sharing can be relatively easy. We have observed that MOSFETs from the same batch tend to have closely matched Vgs characteristics. This means you can use an opamp with a source sense resistor to create the gate voltage for one MOSFET and add the rest with paralleled gates and drains, but with individual source resistors.

--
 Thanks, 
    - Win

Oh Shit! Right, I've got to bolt all the drains to the thing to be heated... (sil pads or some other insulation and low temperatures could get ugly.) Putting source resistors everywhere is not ideal.. they'll get hot too.

OK I'm trashing this idea... sorry for the interruption, back to our regular schedule of political rants*. :^)

George H.

*I don't know how long it will last, but I've made a deal with myself to skip the political stuff. (Else SED will turn into RCM)

There are insulated TO220 fets around. Three of them, with separate opamps, would share accurately with very small source resistors.

I've had bad experiences, lasting emotional scars, trying to get multiple mosfets to share current with common gate drive and reasonable source resistors, especially at the lower range of operating currents. In a power amp, bad sharing won't matter at low currents; in your heater, it would.

Politics affects business, and the right Prez could push policies (better yet, reduction of policies) to help small biz and create US jobs. But nobody is talking about that.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Hmm three with logic level drive at DK... I'll get some of these,

"Silence foul temptress". (or whatever the male equivalent of temptress is.... temperor?)

George H.