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| Ultra Short, Ultra Fast LED Flash |
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| Zero999:
--- Quote from: ogden on March 06, 2019, 02:37:06 pm --- --- Quote from: Siwastaja on March 06, 2019, 01:28:04 pm ---to maintain some current regulation, you need to drop some meaningful voltage over the resistor, let's say at least 20-30% of the total supply voltage. --- End quote --- 20-30% of waste power? :) Please don't... Somebody may take your joke as an advise. Example: Osram 24V strips uses 7 white LED's in series. --- End quote --- For short pulses, at a low duty cycle, the efficiency is often unimportant, as the total power is very low. If the LED is being over-driven the resistance of the bond wires starts to become significant at high currents and an additional series resistor is unnecessary. I've recently tested a white COB LED, the Cree® XLamp® CMA3090, with a nominal forward voltage of 45V and maximum current rating of 3.6A, with 1µs 105V pulses, no resistor, and the bond wires limited the current to 43A: well over the absolute maximum rating but it ran with a 100Hz repetition rate for an hour or so, with no damage or measurable reduction in the light output. Note that although the instantaneous power was 4.5kW, the duty cycle was only 0.01% so the total power was just 0.45W. The efficiency is probably much lower than non-pulsed operation but who cares if it's only on for just enough time to take a picture? https://www.cree.com/led-components/media/documents/ds-CMA3090.pdf I agree about not using a conventional SMPS, as it will be too slow to regulate. If the delay isn't important or can be factored into the timing, one possibility is to charge up an inductor to the required current, then dump it through the LED. If the inductance and current are known variables, then the energy per pulse can be accurately controlled. |
| ajb:
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. |
| ogden:
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. |
| Zero999:
I agree 30% voltage drop is quite a lot. The big advantage of using an inductor is the voltage can be boosted, so a lower voltage can be used to drive a long string of LEDs. It's also good if the LED has a much lower forward voltage than the power supply, so a power wasting resistor can be avoided. Note that in boost configuration there needs to be something to protect against open circuits, otherwise the high voltage spike will destroy the switching transistor. I agree that it's better if the power supply voltage can be set to a suitable value and the current limited with a resistor. As I said above: it's very important to keep the grounds for the logic and power separate: use an isolated MOSFET driver or pulse transformer for the transistor and another transformer or Hall effect sensor to measure the current. I experimented with several different pulse and current transformers. Keep the burden, the secondary side resistor, as low as possible to avoid core saturation and improve accuracy. It's fine to overdrive current transformers, with low duty cycles, as long as the burden resistor is proportionally reduced to avoid core saturation. I've had quite a lot of success with winding my own on ferrite ring cores, but I found off the shelf ones work quite well. In the schematic I posted previously, I used the Murata 54100C with both primary windings in parallel and a 1R terminating resistor for a 10mV/A output. At 250A it worked perfectly fine and gave accurate measurements, when compared against a resistor. https://docs-emea.rs-online.com/webdocs/0eb1/0900766b80eb1ad9.pdf |
| Marco:
With RGB LEDs you'll need resistors for current limiting for white balance, even if they can take just dumping the full voltage in. Personally I'd try 74LVC1G123 to generate the pulse, MCP14A0301 gate driver, 12 V power supply, random low voltage low rds MOSFET, resistors for current limiting and balancing, a lot of LEDS&resistors in parallel, feed the power&signal to the gate driver/mosfet/flasher section through two common mode chokes. If it generates too much noise you can always go the GDT route later. |
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