I am an EE student at UIC (chicago) and am looking to design a 42V@2.8A max LED driver for Cree's new cxa3070 LED array as a side project.
The LED is not in TI's webench LED architect database, but I custom defined it and webench cannot come up with any design solutions for my given Vin range. Either topology not enabled, exceeds max duty cycle, or current too high for the integrated switch solutions.
I was first considering a boost, but the duty cycle ends up being around 80%. http://www.ti.com/lit/ds/symlink/tps92690-q1.pdf says max duty cycle is 90% so I'm not sure why this chip wouldn't work. is this not supported in webbench because it cannot calculate control loop compensation at this high duty cycle? or is it still a limitation of the boost converter in current mode feedback that I'm missing?
the tps40211 also looked good, but the datasheet's section on "Current Sense and Sub-Harmonic Instability" mentioned issues with current mode boost converter duty cycles above 50%. Is this device still usable with the right sense resistor and a few modifications? again, 80% duty cycle does fall on the charts and seems within the max range, although switching frequency is limited over 80%.
efficiency is much more important in this design than BOM count or footprint size. so I started to think another topology might be better, such as resonant or flyback?
Below are more detailed design preferences
efficiency highest priority (soft-switching? resonant topology?)
BOM count<60
magnetics easily available from digikey/mouser/etc or something that uses cheap standard cores I can wind on
smd and high frequency is fine. although no bga and avoid dfn/qfn and pitch <.5mm if possible
design does not need to be isolated
soft start needed, or I can build that in later I think.
Hopefully I've included all the info needed. I am not looking for someone to design this for me, but simply point me in the right direction as far as topology and controller. Maybe ideal switching frequency, and maybe some hints with magnetics. I have no issues following design guides, datasheet calculations and charts, and doing high freq prototyping and assembly with smd.
Thanks in advance.
Bill
