Electronics > Beginners
CCCV MOSFET Buck
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wraper:

--- Quote from: Leeroy on September 12, 2019, 05:24:15 pm ---This is ridiculous for a simple feature of current setting.
Sub 1Ohm resistors are only easily accessible here in standart some shunt values like R100, R050, R005 and so on..

--- End quote ---
Nope, you are ridiculous by calling that ridiculous. It's completely normal and even preferable to select shunt resistance according to intended current. Because in that way you can minimize losses in shunt while keeping circuit stable and simple. And if there is no standard value, connect two resistors in parallel or series to get right value, oh my.

--- Quote ---Ok let's say I went through all the trouble to calc and obtain the correct resistor for my 750mA. Then some time later I need to assemble myself a similar circuit for a 350mA LED and I should go through all that again every time? Fuck that..
--- End quote ---
:palm: On my, it's so difficult. For LM3404 calculations for standard circuit are more complicated because current through LED is not constant as there is no capacitor at all on the output (can be simplified as further if you can deal with a bit of imprecision by assuming current is constant). But if you add capacitor or use TPS92512HV calculations become extremely simple.  Also nobody prohibits complicating feedback circuit a little bit and make it adjustable. Though it that case it's a bit easier to use regular buck driver IC because of higher reference voltage. But it's not needed really as both LM3404 and TPS92512 have PWM input.


--- Quote ---Take a look at this thread:
https://www.eevblog.com/forum/beginners/adding-cc-to-a-cv-buck-to-make-it-cccv/
--- End quote ---
That's a crappy workaround for adding crude current limit to CV circuit as already mentioned in that thread. And already mentioned it will be oscillating (you complained about noise).
mariush:
I don't understand sometimes, are people allergic to "led driver ICs" ?

Just get some LED driver IC and control the brightness using a microcontroller (to send a pwm signal to chip)...

Get a couple LT3956 : https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LT3956EUHE-TRPBF/LT3956EUHE-TRPBFCT-ND/8126002

Parallel two leds at a time and make a chain of 12-13 pairs ... 13 x 3.25v = 42.25v , 26 leds x 3w = 78w , 2 drivers x78w =156w
~12$ x 2 for chip and all components in the schematic below, all parts are available on Digikey

You can basically copy the application circuit from the datasheet, just adapt some resistor values if needed to account for the lower voltage:



Leeroy:

--- Quote from: mariush on September 12, 2019, 07:37:22 pm ---Get a couple LT3956

--- End quote ---

This one is a total overkill. I won't have neither 4 layer PCB to properly trace this IC, nor solder mask to reliably solder QFN package. Not to mention that LT parts are the most rare in my country and price is not appealing at all.

On the other hand, I found the biggest problem with my circuit. It was a weak internal pullup resistor on the pin 2 (~HIN) of IR2102.
PWM signal was just above 1V peak to peak with -500mV offset... After adding 1k external pullup output frequency became as expected, inductor stopped overheating and Buck is now alive.



Moving current shunt to low side did not work. Seems like IC does not support this mode.

Switching noise is mostly gone. But in some narrow range of load current it comes back, see screenshot (ignore low frequency noise, it is a scope fan). This range lies somewhere near full load, just before CC mode is about transition in CV. Oddly enough oscillations go away before still in CC mode and not with the transition itself.



Silent gaps that you see on the screenshot are the result of random discovery:
While (and only while) my scope ground lead (which is not connected to the earth btw) is connected to source of the mosfet, if I touched or placed my finger near the exposed wires connected to some points (like pin 4 of TL494; "Vdiv" on my schematic; "5Vref"; probably GND too) oscillation was going away and noise too.
Any ideas how to stabilize?
mariush:

--- Quote from: Leeroy on September 13, 2019, 06:29:08 pm ---
--- Quote from: mariush on September 12, 2019, 07:37:22 pm ---Get a couple LT3956

--- End quote ---

This one is a total overkill. I won't have neither 4 layer PCB to properly trace this IC, nor solder mask to reliably solder QFN package. Not to mention that LT parts are the most rare in my country and price is not appealing at all.


--- End quote ---
Eh, you'd do it just fine with a 2 layer board, but fine... here's some simpler stuff...

AL8862 : <1$ each
Datasheet: https://www.diodes.com/assets/Datasheets/AL8862.pdf
Available in lots of places you could order from : https://www.findchips.com/search/AL8862
 
Up to 60v, up to 1A current , set maximum current through a resistor and then use a PWM signal or DC voltage to adjust output current up to set limit.  So you'd need to use one for each chain...

MP24833A : <1.2$ each
Datasheet: https://www.monolithicpower.com/pub/media/document/MP24833-A_r1.01.pdf
Available everywhere : https://www.findchips.com/search/MP24833  - also on eBay , 8-10$ for a 5 pack

up to 55v, up to 3A  (so you could have pairs of leds or maybe even groups of 3 leds)
same deal , enable, pwm or dc input to adjust current after setting maximum current through a resistor.

LED6000PHR  .. up to 61v 3A ...

and lots of others...

If you'd be willing to drop your maximum voltage to 24v, you'll find lots more chips that are cheap and simple to use. A lot of chips can handle maximum 36v or 40..42v.
24v power supplies are very common, and you could have 12-18-24 leds per driver easily (6 x (1..4) in series)


ex: PAM2863ECR (max 40v 2a) : https://www.diodes.com/assets/Datasheets/PAM2863.pdf  .. also on eBay something like 4..7$ for a 5 pack
 
ex : MP2840 ( max 36v 3A) : https://www.monolithicpower.com/pub/media/document/MP2480_r1.01.pdf   .. eBay has them as 10 for 8$, 50 for 20$ .. though who knows if they're genuine or counterfeit.

LED5000PHR (up to 48v 3A) (datasheet

 

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