LED driver high current and voltage

Question

For a project I have to design a led beacon that will consist of +400 LEDs. These LEDs will be driven only a short amount of time maximum of 3% duty cycle (frequency: 100Hz, pulse: 300us). The LEDs have an individual Vf of 5V and an If of 5A for this short surge. I wonder if there are smarter ways to make a led driver because the way I'm thinking about now is to create a led driver from a current shunt, MOSFET, and opamp so that the current is adjustable and the surge duration is adjustable.

When I look into LED drivers the downside is normally that they have a fixed frequency and only PWM dimming. Is there a better way to do this since I'm now looking into combining 16 LEDS in series and 25 (or more) of these strings parallel? The flash frequency and duration must be adjustable by microcontroller.

Summary specs per led:

• If: 5A
• Vf: 5V
• Frequency: 20-100 Hz
• Pulse: 102-300 us

Powered by mains power and there will be a capacitor bank. Lifetime and durability is more important then cost and space.

Answer 1

400 LEDs, with each LED taking 5v and 5A (25W each) is a lot of power, 400 x 25 = 10kW of power when they're on. With a 3% duty cycle, that's 300W average.

Your LED driver therefore needs some form of energy storage.

The maximum pulse duration determines how much storage you need. If you want 1uS on and 30uS off, you can do that with a few small caps. One second on and 30 off is a rather different, 10kJ takes up a lot of room. I suspect your target is somewhere between those two.

Next you have to consider efficiency. Is the energy expensive and must be husbanded (portable battery) or cheap (mains, or vehicle generator)? A current shunt, mosfet and opamp will result in an inefficient driver, you will need switch mode to improve the efficiency. Even if you're only 33% efficient, 600W is not too much heat to get rid of in your electronics if it means a much cheaper circuit implementation, or is it?

Add pulse timing and energy costs to your specifications, and clarify whether you do really mean 10kW of LEDs, and then this or other answers can guide you better where to go.

Answer 2

Interesting project, I like it. Let me guess, its a NASA beacon for guiding astronauts back to Earth from Mars.

There are LED drivers specifically for flash but not at 5A. Usually used in mobile phones. The flash drivers commonly use a charge pump. It's a matter of being able to charge that much energy within 10 mS. You should use one charge pump per strip and synchronize.

Lifetime and durability is more important then cost and space.

I would use considerably more real estate and much less amperage per LED. Not so much for reliability but thermal management. Lower heat will increase lifespan.

Five Amp LEDs generally have lower efficacy. By using lower power LEDs you will need less energy due to improved efficacy.

With good optics you can improve the irradiance significantly. Most high power LEDs have a view angle of 120°. If you use a collimated beam of 10° you will increase the irradiance by over 100X.

---

Example CCT=5000K:

A Luxeon M with a max current of 4.8 A has an efficacy of 134 lm/W @ 2.8 A. At 4.8 A you get 40% more luminous intensity and significantly less efficacy.

A Cree XP-3G @ 350 mA (2 A max) has an efficacy of 185 lm/W. This is the highest efficacy high power LED available today.

I would use the maximum PCB real estate possible with the highest efficacy LEDs that would fit in the area available. I'd use multiple charge pumps.

Answer 3

If you are going to be using a microcontroller, then why do you need an LED driver? Connect the LED strings to MOSFETs, and connect the microcontroller's IO pins to the MOSFET gates, and you can control the LEDs in any way you wish.

That way, you can do PWM if you want, or simply do 300µs pulses times in software if that's what you need.