Help me to handle a peak of 20A - 100ns - Page 1
Forum section: electronics / projects

author: ehilarioc
author_rank: Contributor
timestamp: July 23, 2015, 12:56:53 am
content: Hello everyone!I'm currently working with a hybrid pulsed laser diode (SPL LL90) to make a Rangefinder. I'm using the recommended circuit at the end of the application note here:http://www.osram-os.com/Graphics/XPic1/00018318_0.pdf/Operating%20the%20pulsed%20laser%20diode%20SPL%20LLxx.pdfThe circuit works good, the rise time and the pulse width of my application are the ones I need. However the voltage at theTriggerCharge Pin is giving me problems.Ideally the voltage should stay at 18V for the entire 100ns of the pulse, so I can get the maximum optical power of the Diode. I've made some measurements with my oscilloscope and, what begins at 18V drops to almost 5V. I've shortened the wires, I've shortened and reinforced the traces with solder and I've added a 15pf, 1nf, 10nf, 100nf, ceramic 1206 caps and 10uF,100uF electrolytic caps in parallel (110,111uF aprox in total) to theTriggerCharge pin of the Laser without success. If the voltage only dropped to 16-15v that would be awesome.I'm currently using a common power supply (like the one soldHere) to power all the circuit but in the end I want to make a portable rangefinder so the whole system will be powered with a Lipo Battery. What can I do keep the voltage at 18V. Anyone knows a regulator that I can use to achieve this? Or is there any other way to solve my problem?Any help will be useful.EdwinEdit: I edited my epic fail.

author: moffy
author_rank: Super Contributor
timestamp: July 23, 2015, 01:24:31 am
content: Use a separate supply to power your MOSFET driver chip.

author: nuno
author_rank: Frequent Contributor
timestamp: July 23, 2015, 01:32:35 am
content: You meant the voltage at Charge Voltage pin 2?

author: DanielS
author_rank: Frequent Contributor
timestamp: July 23, 2015, 12:54:54 pm
content: The specifications for the laser diode do not show any capacitors on the trigger pins and only recommends pulse width up to 30ns.It sounds like you put capacitance on the MOSFET driver's output. You should not be doing that since capacitively loading the driver will slow it down and make it waste power.

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2015, 01:39:59 pm
content: Look at Figure 3, anything over 30 ns is useless ... the built in capacitors inside the laser diode module are empty by then (and it has high inductance leads). Even if it could lase for longer you should have added the capacitance between Vs and GND of the MOSFET driver ... it's the MOSFET driver output which is dipping and the only reasonable cause for that would be if it's power supply is dipping, adding capacitance on the driver output is silly.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 23, 2015, 02:13:48 pm
content: Quote from: ehilarioc on July 23, 2015, 12:56:53 amYou should be connecting caps across the supply of the MOSFET driver, not the trigger pin. Also use 10u ceramic, not electrolytic.

author: ehilarioc
author_rank: Contributor
timestamp: July 23, 2015, 03:16:35 pm
content: Quote from: nuno on July 23, 2015, 01:32:35 amI've made a horrible mistake in my explanation.Maybe because I had slept only 3 hours.Obviously the capacitance I'm adding is at the Charge pin of the Laser. The reason I'm using a 100ns pulse is because that is the shortest I can get with my signal generator SDG1050. In the final design I will use a shorter pulse, between 50-60ns. The problem is that I'll have to make additonal effort on the design of the transimpedance amplifier... but that is not the issue now.I will try using two supplies, one for the driver and one for the laser. I hope I get an improvement with that.EdwinEdit: I've added a pic of my first board (proof that I added the capacitance to the pin 2). I did not solder the 100uf cap because it didn't fit.

author: owiecc
author_rank: Frequent Contributor
timestamp: July 23, 2015, 03:52:33 pm
content: Can you post the waveforms from pins 7 and 8 (out and Vs)?

author: nuno
author_rank: Frequent Contributor
timestamp: July 23, 2015, 04:18:28 pm
content: As already been said, electrolytics will do nothing considering the time lengths involved.Anyway, since the chip has internal capacitors, you don't know what's going on inside; you may be reading X at the pins, but the reality at the die be different; tens of A at nH at ns can create deceiving perceptions. I suggest you check actual light output power...

author: ehilarioc
author_rank: Contributor
timestamp: July 23, 2015, 04:41:31 pm
content: I've attached the signals at the Trigger (blue) and the Charge pins.I'm currently working on a board design with separate supply.Edwin

author: Kalvin
author_rank: Super Contributor
timestamp: July 23, 2015, 04:53:55 pm
content: I guess you shouldn't put any capacitors to the pin number 2. And the voltage of the pin 2 is supposed to be act like you have measured. The capacitors inside the chip will be charged to the voltage Vc through the "charging R", and when the trigger pulse is being applied to the MOSFET gate (pin 1), the MOSFET will become conducting, the capacitors will be discharged rapidly providing sufficient energy for the laser diode in order to create the 50ns pulse.

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2015, 05:03:33 pm
content: Quote from: ehilarioc on July 23, 2015, 03:16:35 pmTake a LED and attach it to a power supply set to 18V without current limit and from a safe distance turn on the power supply ... this is what you're doing to this poor laser diode module, if it's still working it's a bloody miracle. If you want to have a broader light pulse than 30 ns use a normal laser diode and current drive it.This module is made for up to 30 ns broad light pulses. It doesn't matter that your function gen creates a 100 ns broad drive pulse, the MOSFET is allowed to stay on for longer than 30 ns, the external charge resistor (you do have one right?) is there to limit the current to safe levels (and below lasing threshold).

author: ehilarioc
author_rank: Contributor
timestamp: July 23, 2015, 06:49:00 pm
content: Quote from: Marco on July 23, 2015, 05:03:33 pmI forgot to say I'm pulsing the laser diode with a duty cycle of 0.1%. The laser is still working, I've removed the IR filter of a webcam and I can see the laser glow. It is quite intense. However when I compare it with a commercial rangefinder, the brightness is not the same. When I touch the laser it stays at room temperature.The figure 3 of the application note shows a test with pulse widths that go up to 80ns, I think additional 20ns won't hurt too much if I keep the 0.1% duty.the laser i'm using is this:http://www.osram-os.com/Graphics/XPic1/00083640_0.pdf/SPL%20LL90_3.pdfHowever there is no information of max current.Here is another laser that says that max current is 40A and is rated for 30W optical power.http://www.osram-os.com/Graphics/XPic2/00115230_0.pdf/SPL%20PL90.pdfThe one I use says 75W.What value of resistance should I use? I started with 10 ohm, then 1 ohm, then 0.3 ohm and then just trace resistance. The brigthness grew every step. Like I said before, the laser is still working.Edwin

author: Kalvin
author_rank: Super Contributor
timestamp: July 23, 2015, 07:06:17 pm
content: Quote from: ehilarioc on July 23, 2015, 06:49:00 pmThe application note states that the internal capacitance is 47nF. The 10% to 90% rise time is approximately 2.2 * R * C. Using the time constant 5.0 * R * C will charge the internal capacitors a bit closer to the Vc. Using the 5.0 * R * C and 47nF for the capacitance C, you will get following charge times:1K: 235us100R: 24us10R: 2.4usThe application note states that the duty cycle should be limited to 0.1% ie. 1/1000. So, it seems that the 100R would be a quite safe choice.

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2015, 09:26:32 pm
content: Quote from: ehilarioc on July 23, 2015, 06:49:00 pmOptical pulse width stops going up at 30 ns in the figure, the graph goes horizontal. When it's being used as intended the current pulse across the laser diode is created by the RLC circuit inside the module consisting of the MOSFET, diode, capacitance and parasitic inductance ... after 30 ns the capacitors can no longer drive the laser. External current should play no role.QuoteThis is a laser diode which is meant to be directly current driven, if you want an optical pulse broader than 30 ns you need to use something like this (but with more complex drive circuitry). The SPL LL90 is meant to provide current to the laser diode through the internal MOSFET and capacitance. Any external current above lasing threshold is just going to distort the pulse shape.QuoteI'd say 100 Ohm, this will probably be below the lasing threshold so you won't get a long tail on your optical pulse if you're slow at turning the MOSFET off. I'd estimate the internal capacitance to be <100 nF, which would give you <10 us RC time with 100 Ohm, which would be enough to charge it up again with 100 uS between pulses.PS. you don't want an optical pulse broader than 30 ns, fix your transimpedance amplifier instead.

author: ehilarioc
author_rank: Contributor
timestamp: July 23, 2015, 11:34:50 pm
content: Ok,I will use the attached circuit after reading all your advice. I can't generate a 30ns pulse but I think that a 50ns will be lot better, with a repetition rate of 1Khz. I will stop using the signal gen and I'll use the microcontroller I have.I hope everything works better now. I will post results soon. Thank you all,Edwin

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2015, 11:56:04 pm
content: 5V is really low for the gate drive, 15V speeds up rise time.

author: nuno
author_rank: Frequent Contributor
timestamp: July 24, 2015, 12:01:55 am
content: If you want the FET to switch "really" fast, you'll need to short the distance between the die and the driver's output pin, or make it low inductance (such as connecting both with twisted pair, but connect to the laser near the package). That connection's inductance introduces a negative feedback to fast pulses, putting an upper limit on the FET gate's charging speed and therefore switching speed.Also, I don't know how are you measuring, but if you want to know what's happening at the laser "as much as possible", you'll have to probe, with the ground clip, at the laser's pins near the package (as close as you can physically get).Quote from: ehilarioc on July 23, 2015, 03:16:35 pm

author: ehilarioc
author_rank: Contributor
timestamp: July 24, 2015, 02:54:46 pm
content: Quote from: Marco on July 23, 2015, 11:56:04 pmYeah, I'm using the 7805 as an adjustable regulator. It outputs between 16-17 volts. I don't have lm317 in a small package size.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2015, 03:16:40 pm
content: Shouldn't you be going for the shortest pulse possible, to achieve the maximum positional certainty?Don't play the victim on signals here! Don't give us that! Don't have a signal generator that'll do it? Throw in an RC time constant that gets the pulse width you need. Pulse generators are easy to make, as simple as a few resistors, a capacitor or two, and some logic (74HC logic is even suitable here, though LVC at 3.3V would be preferable).Or just use the device's internal RC/RLC time constant. Why not? How much does it really matter that the pulse power be constant during the pulse? (It will matter to the characteristics of the laser beam, due to mode hopping and noise thresholds and weird stuff like that. But if you aren't doing interference optics as well, that's no big deal.)As for the detector, don't play the power game. That's dumb! Play the smart game instead! How? Well, what do you know about your optical pulse, that's very unlikely to be a property of ambient noise? Aha, pulse width... So let's say we keep the detector input completely blanked out (during which time we average the amplifier's baseline level instead), except for a tiny 10ns window, say, where we connect the amplifier to the detector. We repeat and average this measurement for many pulses, so we can determine with good confidence whether the pulse was in the window or not. Finally, we vary the delay between laser pulse and detection window, and stop where we find the maximum correlation.Now you'll find the detection is massively sensitive, in fact capable of resolving quite distant pulses, assuming the delay locked loop has sufficient range to time the pulse (and repeatability, since jitter is your enemy). You'll also want to check for multiple reflections, because a DLL will get stuck on a local maxima just fine. Maybe sweep the delay and find peaks -- an ideal application for software, doing the complex stuff -- best to leave all the fast, heavy lifting to the analog backend!Tim

author: Marco
author_rank: Super Contributor
timestamp: July 24, 2015, 05:22:28 pm
content: With a differentiator + zero crossing detector or constant fraction discriminator the absolute width of the pulse matters less than the derivative. From the datasheet it seems that up to 10 ns the peak of the pulse remains about equally sharp.