Design of a 50A@2V CW laser diode driver for a laser cutter. - Page 1
Forum section: electronics / beginners

author: Bigcheese
author_rank: Newbie
timestamp: February 03, 2014, 01:07:06 am
content: So I bought 3 ofhttp://www.dilas.com/pages/products.php?category=22&series=40(30W, probably 808nm. Not sure if 200um or 400um) at auction for $500. Now I need a way to drive them.I figured since CPUs/GPUs/etc use high amperage at low voltage I could use the same topology (sync buck) and ICs.I ended up picking the LM25117http://www.ti.com/product/lm25117because it had application notes for using it as a constant current driver. I modeled it up in TINA-TI and (after much effort) got a stable output current controlled via a pot with 500mA ripple. It was at this point that I remembered that I need to be able to control the current from a micro-controller to actually use this in a laser cutter. I also need to be able to change the power over the full 50A range quickly (< 100us). My current setup can do neither of these.Here's my current setup:https://drive.google.com/file/d/0B3IwI7ioPGAJTkZEeWpmTjZFVEU/edit?usp=sharingI have three questions. Is this the right general approach? How would I go about Controlling the feedback with a micro-controller, preferably an analog voltage or pwm signal? And how can I improve the rise and fall times?For the second question I have considered a digipot, but that requires an I2C (or similar) connection. Although this is probably what I'll go with if nobody has a better solution.I'm not sure about the 3rd. All of these chips have control loops designed to keep a constant voltage with rapidly changing load demands, which seems to not work well for my needs. If I lower the comp cap too much, it just starts ringing or has a giant overshoot. If I lower the output capacitance, the ripple goes up to 1-2 amps. However the commercial laser diode drivers I've seen out there are citing 5us response times at 400mA ripple. So clearly there's a way.

author: Bigcheese
author_rank: Newbie
timestamp: February 05, 2014, 06:48:21 pm
content: Ping

author: Marco
author_rank: Super Contributor
timestamp: February 05, 2014, 07:57:01 pm
content: "I also need to be able to change the power over the full 50A range quickly (< 100us)"Why exactly do you need to be able to do this? (This isn't a realistic application of a switcher's current limit.) Also why abuse a voltage supply converter? There are plenty of current source switchers with external MOSFETs out there (most LED drivers).That said, personally I'd just make a DC DC 2 Volt 50 A power supply and handle the current limiting separately rather than though the switcher's control loop ... very slightly less efficient, but can obviously modulate the current far faster.

author: Bigcheese
author_rank: Newbie
timestamp: February 05, 2014, 11:23:18 pm
content: Quote from: Marco on February 05, 2014, 07:57:01 pmWhen raster scanning the laser over the work piece I need to be able to precisely control when the laser is on. The faster I can turn it on and off the faster I can scan the laser for a given precision. For normal CNC operation I need to be able to keep a constant energy density on the work piece given a variable speed due to acceleration limiting (corners and curves).Quote from: Marco on February 05, 2014, 07:57:01 pmI was not able to find any that looked like they could handle this power level. Although it may not matter with external MOSFETs.Quote from: Marco on February 05, 2014, 07:57:01 pmWouldn't I still run into the same problems in this second stage? I'd like a stable output current, thus I would need a filter. Which is basically what a buck converter is.Thanks for the help.

author: scientist
author_rank: Frequent Contributor
timestamp: February 06, 2014, 05:19:23 am
content: This is why people buy pre-made laser cutters.