Electronics > Projects, Designs, and Technical Stuff

Ultra Short, Ultra Fast LED Flash

<< < (6/18) > >>

Zero999:
Some oscillograms.

A low current test: 3A, but the LXM2-PH01-0070 is only rated to 700mA continuous. The signal generator waveform isn't great because I used it to drive the gate drive transformer, without a MOSFET driver IC. The photodiode was about 100mm from the LED.


A high current test: 60A through the CMA3090, which is only rated to 3.6A continuously. The photodiode distance was increased to 200mm, otherwise it saturated. I managed to increase the voltage to 210V, giving a 120A pulses, before the LED failed open circuit.

Siwastaja:

--- Quote from: ogden on March 06, 2019, 09:07:09 pm ---I do not see inductor as better solution than simple resistor. I would just make 3-channel system with individually programmable voltages and timing for each color, obviously with (inrush) current limiting resistors to drop like 3..5% of voltage on each string. Not for god's sake 30%. It is not needed to increase voltage specs of whole system just because someone here in the forum said that ballast resistor shall drop 30% of the voltage which for high power LED's shall be considered as insanity.

--- End quote ---

I said: at least 20-30% for good current regulation (against Vf variations through temperature and unit variations). This may or may not be important. For a low-duty special effect equipment, a 20-30% efficiency drop might not be a problem at all. For a general purpose LED lighting fixture, it would be an environmental disaster IMHO, so I understand where you are coming from.

Yeah, don't follow my advice blindly, do your own analysis. 20-30% drop was based on my own work on time-of-flight imaging where exposure consistency does matter.

Of course, if current regulation is unimportant, a no-resistor-at-all, limited-by-bondwires approach is often just fine. Or as you say, a few percent of the supply voltage. If you just need light for some photographic trick photo and have enough dynamic range in the imager, and are not expecting predictable exposure within a few percent, and your duty cycle is low and you are confident you are not killing the LEDs on unpredictable overcurrent, by all means reduce the waste power in the series resistor.

You may even be able to choose the FET so that its Rds(on) temp coeff compensates (at least partially) for the LED Vf temp coeff, regulating the current! This can be a neat trick.

You may even want the current to increase with increasing temperature (within reason; kind of controlled/anticipated "thermal runaway"), because LEDs produce fewer photons per coulomb at high temperatures, so you may want to increase the current with temperature to keep the light output constant. If you want this, then use a resistor "too small".

Of course, all of this babble is just about finetuning a simplistic, easy approach which, by definition, will be inaccurate in light output. I'm assuming this is good enough for the OP.


As a general note, I have seen that the very short pulse handling capabilities of LEDs often tend to be better than specified, but the efficiency does go down, so increasing current a lot has diminishing returns.

ogden:

--- Quote from: Siwastaja on March 08, 2019, 12:22:57 pm ---You may even be able to choose the FET so that its Rds(on) temp coeff compensates (at least partially) for the LED Vf temp coeff, regulating the current! This can be a neat trick.

--- End quote ---

Good idea indeed, but you don't have any control over results *and* you shall thermally connect FET to LED. "Standard" and more controllable approach would be to measure LED temperature, set voltage accordingly (assuming that LED V/I curve is known, measured in advance).

ajb:

--- Quote from: ogden on March 08, 2019, 09:45:11 am ---
--- Quote from: Giaime on March 08, 2019, 08:24:06 am ---
--- Quote from: ajb on March 06, 2019, 07:31:19 pm ---Many switching constant current LED drivers support PWM dimming by using a transistor to short the LED(s), so you could do the reverse here.  Establish your target current in the inductor, then open the switch across the LEDs for your desired duration, then close it again.  If the pulse is long enough and the response of the switching converter is fast enough the converter will pick up the load to maintain the LED current, otherwise you can just use a big enough inductor to sustain your required pulse.

--- End quote ---

Been there, done that. This is the best way. Use Ti parts like LM3409 or TPS92641, TPS92515, etc, all support shunt PWM dimming. You can get 300-500ns pulses like that if you control the circuit inductance carefully.

--- End quote ---

Why, why do you people suggest such  :bullshit: ?

First one, LM3409. Datasheet states that buck regulator frequency above 1MHz is hard to achieve. This means that one buck regulator pulse alone is longer than required, not to mention that first pulse most likely will not reach nominal current. Following waveform from LM3409 DS clearly shows problem why buck regulator and inductor-based approach as such is not the best choice (to say it politely) for this application:

--- End quote ---

You've missed the boat.  The point is not to turn the switching regulator on at the start if the pulse and off at the end, that wouldn't work well at all, as you've mentioned.  The point is to turn the converter on ahead of time, while the LEDs are shorted by the PWM transistor.  This will establish the desired current in the inductor, then when you want a pulse of light, you turn the PWM transistor off, which will allow the inductor current to flow through the LEDs instead.  This way the current is directly controlled, rather than relying on a regulated voltage and a resistor (which becomes subject to all sorts of other variables that will affect the actual current through the LEDs). 

This is a fairly conventional topology for PWM dimming with constant current switching converters, and many have built-in support for it, like in the example below.  This shows the dimming transistor being controlled through the switching converter; this is often done to allow the converter to adapt its switching behavior or even apply power saving behaviors when the dimming transistor is active, but driving the dimming transistor externally may be better for this application.

Zero999:

--- Quote from: ajb on March 08, 2019, 03:42:50 pm ---
--- Quote from: ogden on March 08, 2019, 09:45:11 am ---
--- Quote from: Giaime on March 08, 2019, 08:24:06 am ---
--- Quote from: ajb on March 06, 2019, 07:31:19 pm ---Many switching constant current LED drivers support PWM dimming by using a transistor to short the LED(s), so you could do the reverse here.  Establish your target current in the inductor, then open the switch across the LEDs for your desired duration, then close it again.  If the pulse is long enough and the response of the switching converter is fast enough the converter will pick up the load to maintain the LED current, otherwise you can just use a big enough inductor to sustain your required pulse.

--- End quote ---

Been there, done that. This is the best way. Use Ti parts like LM3409 or TPS92641, TPS92515, etc, all support shunt PWM dimming. You can get 300-500ns pulses like that if you control the circuit inductance carefully.

--- End quote ---

Why, why do you people suggest such  :bullshit: ?

First one, LM3409. Datasheet states that buck regulator frequency above 1MHz is hard to achieve. This means that one buck regulator pulse alone is longer than required, not to mention that first pulse most likely will not reach nominal current. Following waveform from LM3409 DS clearly shows problem why buck regulator and inductor-based approach as such is not the best choice (to say it politely) for this application:

--- End quote ---

You've missed the boat.  The point is not to turn the switching regulator on at the start if the pulse and off at the end, that wouldn't work well at all, as you've mentioned.  The point is to turn the converter on ahead of time, while the LEDs are shorted by the PWM transistor.  This will establish the desired current in the inductor, then when you want a pulse of light, you turn the PWM transistor off, which will allow the inductor current to flow through the LEDs instead.  This way the current is directly controlled, rather than relying on a regulated voltage and a resistor (which becomes subject to all sorts of other variables that will affect the actual current through the LEDs). 
--- End quote ---
Yes, that will work and I did hint on using it in posts 10 and 13. The main downside is the inductor needs to be charged up before the pulse, which takes time. It's not a problem if there's an advance warning before flashing the LED, but it's no good if it has to trigger as quickly as possible.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod