Coilcraft XAL 6060-223 22uH was finally used
Maximum DC Current: 5 A
Maximum DC Resistance: 60.63 mOhms
I build over two hundred Buck drivers so trust me 4.7uH is right there is no take 10 times the inductance
I managed to get up to 96% efficiency on 4S to 12V builds, so about 72W to the LED and 3W on the driver, the parts stay nice an cool
for a good regulation you need about 0.5-1A ripple depending on the use buck chip, this is what 4.7uH gives you
also such an inductor has less DC and far less AC resistance so the heat will be significant lower
usually I use coils that I use are at least 3-4 times higher rated than the output current to keep losses low,
you cant use a 5A max rated coil for 5A output
also noticed on your design the input caps are missing, no wonder the inductor has a hard time
same with the capacitors there is a calculation that defines minimum which with like 10uF at 500kHz for 3S—>6V,
you can simply calculate it fromm the offtime C=I*toff(/1000000000 for ns)/1,44****(2 minimum safety value, better x4)
I doubled itat least to be on the safe side,
the more input caps the better for efficiency as the voltage stays higher so we get less current losses
you can trust the tool, the results are almost identical to my LM3409 calculation
0.8A ripple current is perfect
also at 200kHz you need input caps
the thing is its dynamic switching frequency like most high side sensing resistor buck controllers
no way to modify it besides Inductor selection, this leads you down more or less to measure switching frequancy and then if its not around 300-500kHz change inductor value
I took your values and calculated the fwitching frequency
I get about 800kHz with 4.7uH, which is a bit on the high side
the thing is that switching frequency should be calculated by the tool but its not, very odd
you could even drop the LDO as the chip has a 5V 10mA LDO integrated you can put the MCU on it
I calculated the switching frequency now and would even drop the inductor to 3.3uH
this gives me 550kHz switching frequency
with 33uH that thing would run on 5.5MHz no wonder it cookes like crazy
Thank you very much.
I will try it with a 3.3uH inductor and add input capacitor 3x10uF.
Shall I also add a 10uF capacitor accross the LED terminals? Or is it only optional? No calculation of it in the datasheet.
I also calculated the switching frequency according to the formula that can be found in the manual of the 16820 chip.
According to this, if you increase the inductivity the frequency gets lower.
And if you change the switching frequency in the calculator of maximintegrated the min. inductivity do not change.
I am not sure with low side switched buck, likely no caps across the LEds
Can you have a look at the modified design?
Would you suggest more modifications?
(R4 or R3 is optional, only for flexibility, different sizes and power)
I have just had time to build the modified driver.
With the modification you suggested the driver got cooler but it is still hot. Especially the FET.
driver
I connected it to an adjustable power supply and measured the followings:
Input 8.4V 4.16A >> 34.9W
LED 6V 5.04A >> 30.24W
So the efficiency is about 86%
I tried out what happens if I use it without cooling (the driver was not attached to any cooling plate, only the led).
The input current increased and when the efficiency decreased to about 83%, the fet got smooking hot and died.
In this case the driver should have dissipated ~6.5W
So I think that my driver less efficient and much hotter than it should be.
And one thing that is also strange, I ajusted the input voltage from 6 to 12 V. i thought that the led current should be constant. But is changed parallel to the input voltage.
I thought that a buck driver provide constant output if the input voltage is bigger than the led voltage.
Which FET do you use, sounds it has way too high switching losses
You need one with low gate charge, usual decent fast N-Mosfets have 4-7mOhms but only about 5-10nC total gate charge
Has this regulator a fixed switsching frequency or do you need to set it with a resistor?
It likely switches way too fast
The only way to see whats going on is using a decent digital memory oscilloscope
Also looked in the datasheet and according to it the buck has no low dropout, so its not allowed to drive 6V LED with 2S battery configuration
Increasing voltage should stay constant current so something is buggy
I use PSMN013-30MLC115 powerfet.
FET
13.6 mOhm RdsOn
8nC Qg
The XAL7070-332ME is only 3.3 uH, this is a mistake on my grawing.
XAL7070-332ME
The buck chip says regulating voltage is 200mV
So 21.5mOhm would get you 9A
How did you calculated this 21.5 mOhm?
I do not understand it, can you explain it?
It’s a simple omhs law calculation
I know that 200mV/21.5mOhm= 9.3A
How did we get the 21.5 mOhm?
You do the opposite:
0,200V/9A = 22,2mOhms.