UC3825 frustration and SMPS design - Page 1
Forum section: electronics / projects

author: John_Hofmeyr
author_rank: Contributor
timestamp: June 17, 2017, 04:45:20 am
content: Hello everyone, Im currently in the process of building my first SMP.I wanted to start off with the buck converter topology because the transformer I have only outputs 45V and I want a max output of 30V.Main design considerations:40 - 45V Input0 - 30V OutputTotal output power of around 90 - 120W (This kind of power will come later, for now just want the thing to work)Attiny85 and thermistor for temperature sensing and thermal shutdown if things get too hot.Things I want to add later:Constant current output controlMain parts I’m using:MOSFET - IRFP260N15A 20uH coilB1645 16A schottky DiodeHuge Full bridge rectifier for the AC to DC conversion71V 10,000uF capacitor for filtering after the Full bridge rectifier56V 5,000uF capacitor for final power filtering after the DC-DC conversion(I’m using such large capacitors because in the final design i will be pulling 10A at 12V)The largest annoyance for me sofar is choosing the right controller IC. At first I tried using the TL594 but I had major problems with having it switch the mosfets properly. So I switched to the SG3525 and I had good success with that but I had issues with maximum output power because the SG3525's max duty cycle is 49%. I decided to give the UC3825 a try but I'm having MAJOR difficulties getting it to work properly. I looked at its data sheet and apparently it can use either voltage mode control or current mode control. I would like to use voltage mode control because it seems to be easier to do that current mode control but I cannot figure out how to use the UC3825 in voltage mode control. If anyone knows how to use the UC3825 and how to use it in voltage mode control I would really appreciate it if you could help me out. Also if there is anything that I should add or charge on my parts list please let me know.Thanks!

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 17, 2017, 06:04:50 am
content: I think it would be easier for others to help you if you posted a schematic of your present circuitI have used this chip many times but more usually in current mode, any particular reason you want to use voltage mode ?

author: rx8pilot
author_rank: Super Contributor
timestamp: June 17, 2017, 06:31:47 am
content: The schematic is definitely useful, but physical construction of the circuit is the difference between not working at all and good performance. Measurments are critical and difficult in SMPS as well - lots of traps that create false rabbit holes.Rule 1 of beginning SMPS design :Take a deep breath.Rule 2:Expect some disappointment.Sent from my horrible mobile....

author: CarlG
author_rank: Regular Contributor
timestamp: June 17, 2017, 12:26:46 pm
content: It's been a while since I touched a UC3825, but anyway: I would also like to see a schematic, as well as your design calculations. How did you arrive at those component values?Right me if I'm wrong, but if you need more than 50% duty cycle with the SG3525 (given the other parameters unchanged), you'll still need it with the SG3825, and then you will have to add slope compensation. If this is your first SMPS design, I understand if you're frustratedYou have set up some tough requirements.I don't really see how you're gonna get 0 to 30V output with just the UC3825? Have you considered the minimum on time? And with low load you'll surely get an unstable output. Variable output adds even more complexity to the design analysis; if even acheivable (I'm sceptical...). (At the same time you say 12V out in the final design. What's your main goal? ) And why voltage mode? Current mode is usually easier to stabilize.It's also a bit unclear why you've gone for UC3825? You say "15A, 20uH coil"; that sounds like you're designing a single ended buck but the UC3825 is designed for push-pull.BR