On 2012-07-21, Mr.CRC <crobcBOGUS@REMOVETHISsbcglobal.net> wrote:> qrk wrote: >> 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 <4ns risetime?closer to 4us I think. Thyratron perhaps? Spark gaps are fast, right? perhaps build your own thyristor from some UHF transistors? ⚂⚃ 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---
Circuit for producing 10ns pulses of several amps
Started by ●July 20, 2012
Reply by ●July 21, 20122012-07-21
Reply by ●July 21, 20122012-07-21
"Jasen Betts" <jasen@xnet.co.nz> wrote in message news:jufe7n$d0d$1@reversiblemaps.ath.cx...>> Triac?!? You think it can do <4ns risetime? > > closer to 4us I think. > > Thyratron perhaps? Spark gaps are fast, right? > > perhaps build your own thyristor from some UHF transistors?Run of the mill xenon or mercury thyratrons are typically slower than their silicon counterparts, with comparable risetime and deionization time in the ms. Hydrogen thyratrons are well renouned for their speed, however. As circuits go with high current, fast risetime, and dreadful tails, there's this one. You still need a pulse generator, but that's easier than the current gain. Some TinyLogic would take care of that. http://cds.linear.com/docs/Application%20Note/an122f.pdf 'Course, Jim would've been about the only person on Earth with a stash of stud type BJTs. YMMV. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Reply by ●July 22, 20122012-07-22
On 7/21/2012 2:01 PM, Mr.CRC wrote:> qrk wrote: >> 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<4ns risetime? > > Interesting concept. I'll keep it in mind. > > >What's the actual specification of the pulse you're trying to generate? This is the first mention I've seen of 4ns risetime. falltime? How variable does the width need to be. 50KHz rep rate was mentioned, but not sure if the context was relevant. Do you care about jitter? How about life of the apparatus. Both are relevant for relays. Is it a current pulse or a voltage pulse? And what is required of the other? voltage compliance for current pulse or peak current available for voltage pulse? Does delay matter? Maybe something like a distributed amplifier made from smaller/faster devices. I didn't catch whether this is a test fixture or a high volume device where cost matters a lot. What's the actual specification of the pulse you're trying to generate into exactly what range of loads. LED is an ambiguous definition. The devil is in the details. You can do some interesting things with avalanche transistors and/or snap diodes. I've never tried it at this power level, but you don't need much speed.
Reply by ●July 22, 20122012-07-22
On Saturday, July 21, 2012 2:01:07 PM UTC-7, Mr.CRC wrote:> qrk wrote: > > 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 <4ns risetime? > > Interesting concept. I'll keep it in mind.Common triacs won't do it, of course (the data sheet has a dI/dt limit, and there's a good reason for that). Phototriacs, on the other hand, DO turn on abruptly. They won't turn off so fast, but a Blumlein pulser might not care.
Reply by ●July 23, 20122012-07-23
On Sat, 21 Jul 2012 00:12:11 -0500, legg <legg@nospam.magma.ca> wrote:>On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" ><crobcBOGUS@REMOVETHISsbcglobal.net> wrote: ><snip>>>I wish to achieve 10-20ns pulses of 1-10 amps. >><snip>>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.Talking to myself, it seems. A quick LTspice simulation follows, using the cross-conduction alone. RL Version 4 SHEET 1 880 680 WIRE 192 64 64 64 WIRE 512 64 192 64 WIRE 16 80 -48 80 WIRE 512 160 512 64 WIRE -176 176 -192 176 WIRE -48 176 -48 80 WIRE -48 176 -96 176 WIRE 64 176 64 160 WIRE -192 240 -192 176 WIRE 64 256 64 240 WIRE 64 352 64 336 WIRE 192 352 192 128 WIRE 192 352 64 352 WIRE -48 432 -48 176 WIRE 16 432 -48 432 WIRE -192 480 -192 320 WIRE 64 480 64 448 WIRE 64 480 -192 480 WIRE 432 480 64 480 WIRE 512 480 512 240 WIRE 512 480 432 480 WIRE 432 496 432 480 FLAG 432 496 0 SYMBOL LED 48 176 R0 WINDOW 0 40 -8 Left 2 WINDOW 3 28 65 Left 2 SYMATTR InstName D1 SYMATTR Value QTLP690C SYMATTR Description Diode SYMATTR Type diode SYMBOL ind 48 240 R0 WINDOW 0 58 38 Left 2 WINDOW 3 48 70 Left 2 SYMATTR InstName L1 SYMATTR Value 5E-8 SYMBOL cap 176 64 R0 WINDOW 0 42 18 Left 2 WINDOW 3 23 53 Left 2 SYMATTR InstName C1 SYMATTR Value 300E-12 SYMBOL res -80 160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10R SYMBOL nmos 16 352 R0 WINDOW 0 87 27 Left 2 WINDOW 3 57 56 Left 2 SYMATTR InstName M1 SYMATTR Value FDS6961A SYMBOL pmos 16 160 M180 WINDOW 0 82 68 Left 2 WINDOW 3 53 40 Left 2 SYMATTR InstName M2 SYMATTR Value Si9400DY SYMBOL voltage 512 144 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=.01 SYMATTR InstName V1 SYMATTR Value 10 SYMBOL voltage -192 224 R0 WINDOW 0 -36 112 Left 2 WINDOW 3 -52 285 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(10V 0V 1E-6 1E-8 1E-8 1E-5 2E-5 10) TEXT -248 528 Left 2 !.tran 0 11.2E-6 11E-6 1E-9 uic
Reply by ●July 23, 20122012-07-23
Jim Thompson schrieb:> > I've used mercury-wetted switches to generate nanosecond pulse > _widths_.Hello, you closed and opened the same switch within some nanoseconds? Or did you close one switch and open another one within some nanoseconds? Bye
Reply by ●July 23, 20122012-07-23
Uwe Hercksen wrote:> > Jim Thompson schrieb: > > > > > I've used mercury-wetted switches to generate nanosecond pulse > > _widths_. > > Hello, > > you closed and opened the same switch within some nanoseconds? > Or did you close one switch and open another one within some nanoseconds? > > ByeYou do it by connecting a charged, open-circuited coax stub via the relay. The coax empties itself out in a time t = 2L/sqrt(epsilon). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 23, 20122012-07-23
On 20/07/2012 18:52, Mr.CRC wrote:> Hi: > > I have been using MOSFET drivers to pulse LEDs at currents of up to 21A > (for 100s of ns to several microsecond pulses) and down to about 22ns > for 1A pulses into 1mm^2 power LEDs. > > I can't get any faster with the drivers I've tried than about 20ns for > "parity" optical output power with the 1.0A CW max current typical of > blue 1mm^2 LEDs. (see note 1 below) > > > I wish to achieve 10-20ns pulses of 1-10 amps. > > > Three circuits that come to mind are: > > 1. Capacitive discharge by MOSFET switch such as the Directed Energy > PCO-7110 driver. > > This circuit has the drawback of a slow trailing edge. > > 2. Discontinuous current mode flyback circuit. The stored current in > the inductor is switched into the LED when the MOSFET turns off. > > This circuit also has a slower tail than a symmetric drive, but is > better than RC. I have gotten 30ns or so 3-4A pulses in a LTspice sim, > with 100-1000pF in parallel with diode loads. > > 3. Continuous current mode flyback circuit. The stored current in the > inductor is switched into the LED when the MOSFET turns off, then > shunted back through the FET when it turns back on. > > This circuit produces a nice sharp pulse. I have gotten 15ns or so 3-4A > pulses in a LTspice sim, with 100-1000pF in parallel with diode loads. > > At this point I have no idea if the simulated performance can be > realized with a physical circuit. > > Also, much of the challenge is in the MOSFET gate drive. Hence, I keep > coming back to the fact that if the gate driver is fast enough, just > hook the LED to it and be done! > > I did buy some Directed Energy (IXYS) laser diode driver assemblies to > test, but haven't gotten to spend much time with them. I still want to > be able to build my own, to meet custom mechanical requirements. > > Just wondering if there are any completely different approaches to think > about? > > I'm aware of transmission line pulse generation approaches, and would > consider them. But that should be a last resort. Those still require a > fast switch. So it seems all of this boils down to "how to switch > on/off several amps in 5ns or less?" > > > NOTES: > > 1. So far I've tried TC4422A and IXD630. The IXD630 is better on > paper, but with real LEDs, the TC4422A outperforms in the <100ns regime. > > The way to get it to work isn't very practical for anything but lab > experimentation anyway, since to get very short pulses, I have to just > "tickle" the switching threshold of the driver by varying the amplitude > of the input pulse. The actual input pulse duration hardly even matters > below about 60ns, so I set it to 40ns and then the output pulse width > becomes a function of the input amplitude. This also varies with supply > voltage, and horribly with temperature.Have you examined datasheet/appnotes for ZTX415 avalanch transistor - its all in the tuned co-ax line (apparently).
Reply by ●July 23, 20122012-07-23
Phil Hobbs schrieb:> You do it by connecting a charged, open-circuited coax stub via the > relay. The coax empties itself out in a time t = 2L/sqrt(epsilon).Hello, ah, the length of the pulse is determined by the coax stub only. Pulses of some 100 ps should be possible too. But if you want periodic pulses with a period of some ms I would expect a jitter of some 100 ns even when the relay is driven with a good XTAL based clock. Cheers
Reply by ●July 23, 20122012-07-23
On Mon, 23 Jul 2012 17:47:46 +0200, Uwe Hercksen <hercksen@mew.uni-erlangen.de> wrote:> > >Phil Hobbs schrieb: > >> You do it by connecting a charged, open-circuited coax stub via the >> relay. The coax empties itself out in a time t = 2L/sqrt(epsilon). > >Hello, > >ah, the length of the pulse is determined by the coax stub only. >Pulses of some 100 ps should be possible too. >But if you want periodic pulses with a period of some ms I would expect >a jitter of some 100 ns even when the relay is driven with a good XTAL >based clock. > >Cheers >Relays will jitter many microseconds. The mechanical bits ring in a zillion modes, like a church full of bells, after every hit. -- 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
