Circuit for producing 10ns pulses of several amps

Fast-on only if you need a short delay... the transmission line provides the "off"

I've used mercury-wetted switches to generate nanosecond pulse _widths_.

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

What kind of MOSFETs are you using? The lower the RDSon the slower due to the enormous gate capacitance.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

So far I haven't used any, except for a IRF530 in LTspice.

The physical circuits I've used are just the MOSFET gate drivers mentioned.

But yes, in principle I would expect that some trade off btw RDSon/Qg and speed would apply.

The Directed Energy laser diode drivers are based on their RF MOSFETs and RF MOSFET gate driver ICs.

I have one board that is tuned to make 15ns pulses up to 50A, the PCO-7110. But it can only do a 0.01% duty cycle. This is worth playing with to learn about the circuit, but ultimately I'll need 50kHz rep. rate, so about 0.05% to 0.1% duty for 10-20ns pulses.

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_____________________
Mr.CRC
crobcBOGUS@REMOVETHISsbcglobal.net
SuSE 10.3 Linux 2.6.22.17

Don't you mean the rise time, rather than "short delay?"

It would be interesting indeed to rig that up to do 50kHz pulse repetition frequency.

--
_____________________
Mr.CRC
crobcBOGUS@REMOVETHISsbcglobal.net
SuSE 10.3 Linux 2.6.22.17

n

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A

e a

s y

remember it's been up before, did you ever try something like this?

-Lasse

I have all their datasheets. Some of those RF devices are difficult to source, except in quantities of 50, for a total of about $1500.

They do make off-the-shelf boards for fast laser diode pulsing. I have two of them, and will be testing them.

Probably the answer to my question is, to do what IXYS RF does. But I still want to see if there are other ways...

--
_____________________
Mr.CRC
crobcBOGUS@REMOVETHISsbcglobal.net
SuSE 10.3 Linux 2.6.22.17

That one is quite old.

It does apply. Last year I've designed a forward converter. A MOSFET with a 10 times lower RDSon was less efficient. IMHO the most efficient (and therefore fastest) MOSFET for a particular application barely meets the specs. It will have the lowest gate capacitance.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

That's nice, if you want to short out the LED quickly at the end of the pulse, and you don't mind the continuous power supply needed to keep the current flowing in the inductor.

Yes, it can. 10-20 ns is fairly slow. Layout will need to be tight, as the dI/dT will be big, and every nanohenry will hurt.

That depends on the gate capacitance. You can use paralleled TinyLogic gates as mosfet gate drivers, sub-ns edges with a few ohms equivalent source drive.

NL37WZ16US is three brutal buffers in a can, for 12 cents. I run them at 6.5 volts and they seem happy.

There are some multi-amp mesfets and PHEMTS and GaN fets, which have absurdly low gate capacitances compared to mosfets. The nice thing about these parts is that the source is usually the substrate, so you can get a low inductance source ground by soldering the tab to the ground plane. But you should be able to get a few ns rise/fall from mosfets driven by TinyLogic.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

How about a GaN FET with matching driver?

eg. EPC1014/LM5113

Rather prototyping-unfriendly package on the FET.

Yep. Barely making it is often the most efficient. Many junior designers fail to account for the drive power. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Yes, that is Ok. I don't have to market it. So a low voltage/high current CC lab supply would be fine for keeping the inductor full.

Yeah, it shouldn't be too hard. Not like your crazy ps stuff. Fat traces 8-10mils away from a sheet of copper are inherently helpful toward avoiding inductance.

I need to start stocking and playing with 2-sided bare FR4 material, and prototyping this sort of thing. But where I work, the cost of just running off a 4-layer manufactured proto is no big deal.

I will be able to learn a lot from studying the Directed Energy (IXYS) assemblies, b4 I try to make my own.

Hmm, I wonder how they can compete with the RF MOSFET gate drivers, when loaded with a real gate? Also, since they are lower voltage, will the lower threshold NMOS needed be inherently slower than a higher threshold device?

These new exotic devices (maybe not so new?) are something I'll have to study and see if they are suitable.

Thanks for the input. Or was it output? Nah, it was feedback!

--
_____________________
Mr.CRC
crobcBOGUS@REMOVETHISsbcglobal.net
SuSE 10.3 Linux 2.6.22.17

Yes. I once tried a simple buck converter with 5V logic supply (would've saved the bother of a 12V analog supply). Performance was terrible: "logic level" transistors aren't any different from regular devices. Silicon is silicon, and expecting them to have the same performance at half the gate drive is silly, even when doubled up, with double the drive. Changed to 12V analog supply, well worth the effort.

The only things that work well at 5V (or less) are very small geometry FETs (which might take 3V drive to switch the load at any current you want -- there are sub-miliohm devices available), but they only stand off 10V or so.

Better off rolling your own. I've made a discrete gate drive that does the same thing as one of those IXYS/DEI drivers, except with half the switching time, and orders of magnitude better availability. It's not hard to do, really; just think "how can I slam this device on really hard, then slam it off really hard too".

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

I use them to drive mosfet gates, switching a couple of amps with roughly 1 ns output edges. They are so cheap, you can use a bunch, like 2 chips/6 sections in parallel. 6 volts will enhance most low-voltage mosfets pretty well, and that's overkill for logic-level stuff.

A quick check of Onsemi came up with MCH6662, a dual fet, each section good for 2 amps, 4 total. Parallel the drains and drive each gate from

6 tiny-logic sections. Cg is 128 pF, so they will switch fast. Paralleling smaller fets keeps the inductances down.

There are probably better fets around... that was just a quick look.

Most gate drivers are pretty slow, and the Ixys things are expensive.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Some fairly short, high current pulses can be harvested as cross-conduction phenomena in (unloaded) gate drive circuits - their periods being equivalent to the rise or fall time of the pre-driver output.

Problems with this include stray inductance in the current path, minimum capacitive loading of the pre-driver, headroom/gate threshold relationships and techniques in tailoring the current fall time. Using discrete parts, the cross conduction can be intentionally manipulated.

RL

Avalanche transistor?

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Tektronix did a pulse generator that did just that.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Nice part, 10 amps and 280 pF, maybe a little big for CRC's LED driver requirement, but it would work fine. You could drive it from my TinyLogic gates, or from some paralleled 74AC gates... it only needs

2-3 volts of gate drive. EPC2012 might be a better fit.

There are PHEMTS with Idss in the amps, and they sometimes enhance about 2:1 better than that. They can switch at sub-ns speeds with fairly easy gate drive. Avago has some enhancement parts.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Charge up a capacitor, and use a triac to dump the charge through the load. You'll get fast rise time and a wimpy trailing edge. LEDs look like a resistor at high currents. It's amazing how much current they can take.

Triac?!? You think it can do