Electronics > Beginners
CCCV MOSFET Buck
Leeroy:
Hello.
I am trying to make this project work for months now, but I am an amateur and it is not cooperating. Although I've consumed a lot of theory info about DC-DC converters, there are still so many variables that it is way above my expertise to even attempt to debug such circuits.
Idea: make ~150W passively cooled LED light source for home use and for educational purposes.
I am going to use 4 strings of 3W 750mA 3.24Vf "Cree" LEDs from Aliexpress. Each string consists of 14 LEDs in series. PSU: MEAN WELL LRS-200-48. Each string needs DC-DC Buck converter with constant current output controlled with MCU+DAC providing reference voltage for gm error amplifier (that last part is out of the scope of this thread).
So far I succeeded in designing a working circuit around TL494 using datasheet approach with BJTs. Tested with real LEDs and works flawlessly and what's most important dead silent. But of course after measuring input and output power I see that at low brightness levels efficiency is around 69%, which is terrible for LED lighting IMO.
I decided to try switching to FET approach. I modified working schematic to utilize high side n-channel switch with an integrated driver and something went wrong. I'm using mosfet driver for the first time, so there's that fulfillment on the educational purposes part and I probably do not understand what am I doing..
MOSFET circuit appears to be working in that it regulates output current, but either transistor or inductor is making awful audible noises, which follows me in any FET based buck circuit that I try (yes this is not the first one):
And moreover inductor is getting hot very fast, I haven't powered it for more than 30 sec and still started to smell the compound between the coil and shielding. Tried changing 47uH part to 220uH, hot as before.
Tried to scope output of driver IC and saw some waveforms with a frequencies always less then 1kHz, which is basically a K.O. for me:
Souldn't it be the same as input frequency that is an output of TL494 (and I verified that it produces ~110kHz square waves as it should) ??
Also you can see 4 channels on this scope screenshots, although it had only 1st one hooked up. DS1054Z turns off channels that are not hooked when pressing "AUTO" trigger mode button, but this time it turned them on and showed same frequency waveforms. That tells me that I made one hell of an EMI source if an open BNC connector is enough to see this. At this point I feel like my experiments are doing more harm than good..
What am I doung wrong?
You can find PDF schematics in my nextcloud folder along side with setup photos:
https://cloud.plox.uk/s/B4WKkgK3bawjwtd
I understand photos are not suitable for debugging, should just give an idea of conditions. Basically all the components are soldered to each other; switch, diode, inductor and caps connections are kept short and their ground is a 1mm diameter inch long piece of copper wire. My load for unstable circuit consists of automotive lamps (4x 24V5W in series + 4x 12V10W in series) that draws a bit more current than my LEDs (~800mA); required voltage is a slightly less than sum forward voltage of 14 LEDs (more like 11-13, I believe) but still comparable.
Partnumbers not specified in schematics:
D1 MBRS360T3G
L1 SCDS125T-470M-N (47uH), B82477G4224M (220uH)
C1 TKR221M2AJ20
C2, C3 2.2uF X7R 10% 1812 100V (Taiwan)
C4 TKR102M2AK35
C9 GRM319R71H104K
Leeroy:
Anyone? |O
hussamaldean:
get yourself this one
https://www.aliexpress.com/item/32639621677.html
this buck converter has CCCV modes which will help you to get started how to design
but if you are interested see this video from greatscott which explain how to add current limiting feature for buck-boost converter
wraper:
I wonder why do you even bother with TL494 instead of modern specialized IC? It's not meant for buck converter as it's main purpose and is outdated. Not to say maximum rated voltage is 40V but you run it at 48V. And BJT circuit is crap because you get voltage drop of two transistors because of pass transistor arrangement. Not that there is any reason to use BJT to begin with as it's guaranteed to be suboptimal. You also can use buck IC with integrated switch like LM2592HV. Or better specialized buck LED driver like TPS92512HV.
wraper:
BTW there is no good reason to use high side current sense.
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