On Fri, 20 Jul 2012 20:24:31 GMT, nico@puntnl.niks (Nico Coesel) wrote:>"Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> wrote: > >>Nico Coesel wrote: >>> "Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> 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. >>> >>> What kind of MOSFETs are you using? The lower the RDSon the slower due >>> to the enormous gate capacitance. >> >> >>So far I haven't used any, except for a IRF530 in LTspice. > >That one is quite old. > >>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. > >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.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.
Circuit for producing 10ns pulses of several amps
Started by ●July 20, 2012
Reply by ●July 20, 20122012-07-20
Reply by ●July 20, 20122012-07-20
John Larkin wrote:> On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" > <crobcBOGUS@REMOVETHISsbcglobal.net> wrote: >> 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. > > 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, 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.>> 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. > > 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.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.>> 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! > > 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.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?> 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.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
Reply by ●July 20, 20122012-07-20
"Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> wrote in message news:juctsg01lgj@news6.newsguy.com...>> 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. > > 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?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
Reply by ●July 20, 20122012-07-20
On Fri, 20 Jul 2012 17:41:51 -0700, "Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> wrote:>John Larkin wrote: >> On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" >> <crobcBOGUS@REMOVETHISsbcglobal.net> wrote: >>> 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. >> >> 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, 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. > >>> 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. >> >> 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. > >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. > > >>> 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! >> >> 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. > >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?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
Reply by ●July 21, 20122012-07-21
On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> 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.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
Reply by ●July 21, 20122012-07-21
On Fri, 20 Jul 2012 10:52:32 -0700, Mr.CRC wrote:> 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.Avalanche transistor? -- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)
Reply by ●July 21, 20122012-07-21
On Fri, 20 Jul 2012 12:10:14 -0700, Mr.CRC wrote:>> I've used mercury-wetted switches to generate nanosecond pulse _widths_. > > > It would be interesting indeed to rig that up to do 50kHz pulse repetition > frequency.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)
Reply by ●July 21, 20122012-07-21
On Fri, 20 Jul 2012 16:51:33 -0400, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Fri, 20 Jul 2012 20:24:31 GMT, nico@puntnl.niks (Nico Coesel) >wrote: > >>"Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> wrote: >> >>>Nico Coesel wrote: >>>> "Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> 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. >>>> >>>> What kind of MOSFETs are you using? The lower the RDSon the slower due >>>> to the enormous gate capacitance. >>> >>> >>>So far I haven't used any, except for a IRF530 in LTspice. >> >>That one is quite old. >> >>>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. >> >>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. > >How about a GaN FET with matching driver? > >eg. EPC1014/LM5113 > >Rather prototyping-unfriendly package on the FET.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
Reply by ●July 21, 20122012-07-21
On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" <crobcBOGUS@REMOVETHISsbcglobal.net> 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.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.
Reply by ●July 21, 20122012-07-21
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. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
