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. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
Circuit for producing 10ns pulses of several amps
Started by ●July 20, 2012
Reply by ●July 20, 20122012-07-20
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?"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_.> > >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....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.
Reply by ●July 20, 20122012-07-20
"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. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●July 20, 20122012-07-20
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. 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. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
Reply by ●July 20, 20122012-07-20
Jim Thompson wrote:> On Fri, 20 Jul 2012 10:52:32 -0700, "Mr.CRC" > <crobcBOGUS@REMOVETHISsbcglobal.net> wrote: >> 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?" > > Fast-on only if you need a short delay... the transmission line > provides the "off"Don't you mean the rise time, rather than "short delay?"> 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.>> >> 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. > > ...Jim Thompson-- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
Reply by ●July 20, 20122012-07-20
On 20 Jul., 19:52, "Mr.CRC" <crobcBO...@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. =A0(see note 1 below) > > I wish to achieve 10-20ns pulses of 1-10 amps. > > Three circuits that come to mind are: > > 1. =A0Capacitive discharge by MOSFET switch such as the Directed Energy > PCO-7110 driver. > > This circuit has the drawback of a slow trailing edge. > > 2. =A0Discontinuous current mode flyback circuit. =A0The stored current i=n> 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. =A0I have gotten 30ns or so 3-4A pulses in a LTspice sim, > with 100-1000pF in parallel with diode loads. > > 3. =A0Continuous current mode flyback circuit. =A0The stored current in t=he> 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. =A0I have gotten 15ns or so 3-4=A> 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. =A0Hence, 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. =A0I 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. =A0But that should be a last resort. =A0Those still requir=e a> fast switch. =A0So it seems all of this boils down to "how to switch > on/off several amps in 5ns or less?" > > NOTES: > > 1. =A0So far I've tried TC4422A and IXD630. =A0The 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. =A0The actual input pulse duration hardly even matter=s> below about 60ns, so I set it to 40ns and then the output pulse width > becomes a function of the input amplitude. =A0This also varies with suppl=y> voltage, and horribly with temperature. > > -- > _____________________ > Mr.CRC > crobcBO...@REMOVETHISsbcglobal.net > SuSE 10.3 Linux 2.6.22.17remember it's been up before, did you ever try something like this? http://www.ixysrf.com/pdf/driver_ics/deic420.pdf -Lasse
Reply by ●July 20, 20122012-07-20
langwadt@fonz.dk wrote:> > remember it's been up before, did you ever try something like this? > http://www.ixysrf.com/pdf/driver_ics/deic420.pdf > > -LasseI 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
Reply by ●July 20, 20122012-07-20
"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. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●July 20, 20122012-07-20
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.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.> >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.> >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. 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
Reply by ●July 20, 20122012-07-20
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.
