XHP70.2 6V Buck driver and mosfets advice? - Page 1
Forum section: electronics / projects

author: Wizzard_dr
author_rank: Newbie
timestamp: October 13, 2019, 06:44:40 pm
content: I am currently in the research and development phase of building my own quad Cree XHP70.2 (6V) flashlight. I will be running it of 2S 18650 but would like it to be 3S or 12V capable.I plan to build an I2C controllable LED driver so I can reuse the driver in a single LED flashlight in the future. The driver board will be controlled by an ATtiny85 for temp and current sensing. I plan to use a voltage controlled PWM modulator at 1Mhz for the Buck driver.I intend to run the LED’s at between 8a and 12a continuous depending on the cooling capability.Which is the better buck driver:NCP3420 :https://www.onsemi.com/pub/Collateral/NCP3420-D.PDFORTPS28226 :http://www.ti.com/lit/ds/symlink/tps28226.pdfThey both seem like similarly adequate drivers but I am aiming for max efficiency.Also, I am planning using SiR800ADP Mosfetshttps://www.vishay.com/docs/79335/sir800adp.pdfare these suitable/capable and is there a better option?Thanks in advance for any responses.P.S. lots of people have seen the post and I have no replies. this is my first time posting can someone tell me if I have done something wrong or posted it in the wrong place or something?

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 14, 2019, 12:58:50 am
content: I got nuthin'. Maybe the medium-power guys who deal in amps per microsecond on the daily haven't made it around yet?It seems like your design is logically complete enough to throw together on the bench. Prototype pc boards are cheap. Just build one of each design.If it were my design, and I respect that it's not, I'd try to get the analog oscillator out of there to save space and power. Instead I might use the Timer/Counter 1 that can do 64MHz, or pick another small micro with a faster ADC. I'd also look hard at the 1MHz requirement to see if the energy spent just pushing the gates back and forth could be easily reduced.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2019, 03:51:40 am
content: I'd have replied sooner but I've been out all day and you didn't send beer money, so...Both gate drivers look fine. Pick the cheaper one, I suppose. (I've got a TPS28225 in a recent project, which seems to be doing fine.)ATtiny85 is not fast enough to control current. You need more like a 10MSps ADC to do that at this Fsw. Use a purpose-made regulator or controller IC instead. Get one that controls output current internally. Vary output voltage by tweaking the feedback node with a DAC to a resistor to FB. (DAC can be PWM, in which case split the resistor in half and put a nice big filter cap in the middle.)Might as well implement temperature and UVLO in analog as well -- a dual op-amp or comparator takes up as much space as the ATtiny, though admittedly the resistors supporting them will take much more space, of course. (A few years ago, I designed an all-analog circuit of similar capability, with a boost power stage to run from a single cell at up to 10W.https://www.seventransistorlabs.com/Images/Flashlight2Sch.pnghttps://www.seventransistorlabs.com/Images/Flashlight2_Schematic.png) Only reason I see to bring out a MCU is if you're more fluent with it, or if you wish to add complex functionality like selectable blinking.Don't see a need for absurdly low Rds(on) -- that really invites more problems (switching loss, short circuit / fault / shoot-thru current) than it fixes. Optimize between switching frequency and Rds(on).Could maybe go for a two-phase interleaved design as well, but eh, not really a problem here. The advantage is easier switching -- each inverter switches half the current -- and less input and output ripple current, offering some savings on filter caps. If you are optimizing for size, hmm, not sure which one is better, you'd have to try both.If you want to run super fast, you may consider a resonant design as well. Kind of stupid for a non-isolated LV converter, but it could go at a few MHz, and you might consider GaN transistors as well (which would go with a similar gate driver, but designed for Vgs(on)=5V, and higher dV/dt). The problem then becomes, there aren't really any controllers yet that go that fast, they all top out at 1-2MHz or are integrated (regulators instead of controllers), so you pretty much have to roll your own. (Which isn't all that painful: the core is a gated VCO. Adjust frequency to control power over a modest range, then cut it off completely (hysteretic mode control) to cover the low range.)Keep in mind that you'll have a lot of reactive power in the filter inductor (5-30 VA, same or greater for resonant), so it needs a high Q (20-100+?) to not trash your efficiency budget. It can be hard to find inductors rated for Q at frequency.Lots of shopping around.I don't see much reason to optimize for efficiency -- you're burning ~20W of heat regardless, and another one or even two watts will make no appreciable difference in heat dissipation or run time. Don't sweat it.Tim

author: mrjoda
author_rank: Regular Contributor
timestamp: October 14, 2019, 08:00:12 am
content: Hi,i did similar thing, I put XHP70 CREE from farnell to chasis from Aliexpress. The original driver had some sad efficiency, around 58%. I design a new driver based on LED2000 driver, current is set to 2A, it works from 6,2V do 15V (because 16V X7R on input). It works like a charm. I can upload design files if you want.EDIT : Check the LT3762 it is a beast.But, i'm sorry, i didn't noticed that you want 12A current .

author: Wizzard_dr
author_rank: Newbie
timestamp: March 05, 2020, 05:03:37 pm
content: Thanks for the responses so far.After getting to spend some more time on this I have found the MP8869Whttps://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP8869W/document_id/3906/https://www.monolithicpower.com/en/products/dc-dc-power-conversion/digital-regulator/mp8869w.htmlCould this be made to run as a current control system? (With a shunt on the output so feeding into the Vout and FB such that the current is set via the I2C to be between 0a and 12a)Also, what would be the max voltage of this system (constant current)? Would the bootstrap system restrict it? Would it be possible to output a higher voltage i.e. 9v ish?

author: Wizzard_dr
author_rank: Newbie
timestamp: March 05, 2020, 06:28:53 pm
content: Also, Would a XAL7030-332 be suitable?https://www.coilcraft.com/xal7030.cfmThanks in advance