Industry standard CC-CV design for isolated power supply (LLC resonant conv.) - Page 1
Forum section: electronics / projects

author: Sud_09
author_rank: Newbie
timestamp: November 13, 2023, 12:45:34 pm
content: I am designing a 16V and 20A output power supply using the LLC resonant converter topology. Can somebody please suggest a topology (industry standard preferred) for Constant Current and Constant voltage circuit on the output side? I am operating at 100kHz and have an input of 375V.Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2023, 01:24:48 pm
content: You may find something like this handy,https://www.diodes.com/assets/Datasheets/AP4320.pdfMind that LLC doesn't work in CC down to much voltage ratio: at some point the controller likely faults out, giving a self-reset (hiccup) mode, or maybe it latches off. If you need a full CC/CV output down to 0V, you should probably consider a post-reg stage.There may also be controllers with enough smarts to limit current on the primary side. They're usually programmable, and somewhat tightly held by sales... NXP seems to be one of the leaders here.Tim

author: temperance
author_rank: Super Contributor
timestamp: November 13, 2023, 07:46:32 pm
content: QuoteIndeed some LLC controllers revert to skip mode or something but in that case the output ripple becomes excessive. An other option is to limit the upper switching frequency (which is an advantage when considering common mode noise currents) and revert to PWM mode. One controller capable of PWM mode is the NCP4390 but the chip itself requires quite a bit of external circuitry to make it work properly.-Under voltage lockout with respect to the primary voltage.-The controller has no shut down pin and the soft start input cannot be used as a shut down.-Circuitry for CC and CV control.You can also run an LLC converter in open loop and adjust the input voltage with a current mode controlled buck converter. That's pretty easy to implement but not very efficient.Other topologies might be more suitable.-Phase shifted full bridge.-Interleaved two switch forward which can be "made" in two different ways.-Interleaved two switch fly-back.The question is what do you need / want to achieve?

author: decook1110
author_rank: Newbie
timestamp: November 17, 2023, 12:27:00 am
content: This is something I've been looking into. Just curious why an LLC-FB instead of a PSFB.To get myself educated on the differences I watched this TI video.https://www.ti.com/video/5841918974001After that I'm currently planning a PSFB, but this is for 3 kW, 750 volts down to 72 volts. PSFB in my case is better, because each unit is one channel out of 6 in an interleaved system.This is an upgrade for a system that we're currently using six interleaved 2 switch forward converters for.This TI reference design is my starting pointhttps://www.ti.com/tool/TIDA-020031It uses this control chiphttps://www.ti.com/product/UCC28951-Q1.This sounds close to your current application requirement.Dave

author: decook1110
author_rank: Newbie
timestamp: November 17, 2023, 01:07:23 am
content: Tim recomended the chip belowQuote from: T3sl4co1l on November 13, 2023, 01:24:48 pmI tried absorbing the Onsemi datasheet and wasn't sure exactly how it worked or what its limits were, according to the 3 sample schematics its vcc pin is always connected to the V out of the power supply which means its only good to regulate up to 36 volts and we're planning something up to 72volts.I did find a similar chip that appears to be interchangablehttps://www.st.com/en/power-management/tsm1052.htmlIn the ST datasheet, the first schematic has the same vcc connection as the three schematics in the Onsemi datasheet, but the second example schematic indicates that it can operate down to vcc of 0 volts and appears to have its vcc externally supplied. In this case, can one of these simple analog cc-cv chips be used to control a power supply putting out 72 volts?Also, both chips appear to be incorporated into the secondary of an isolated system and provide feedback through an optocoupler to the primary side, am I saying that correctly and if so, what is the signal that goes back through the optocoupler?If there is a different application note I should read to understand what this is trying to do, I'd appreciate being pointed in that direction.Even if this isn't a chip I should consider using, I'd like to understand how it works.ThanksDave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2023, 02:20:32 am
content: The basic application is just the usual TL431 error amp, combined with an opto to a primary-side controller.For higher output voltages, depending on how much efficiency you need to eke out of the thing, you might run a (linear) regulator off the main output, or an aux winding to supply the error amp. Or have an entirely separate aux supply, which is also handy when the application has some standby loads, like for the common electronic (soft touch) power button these days, etc.Which is also why 5V family parts aren't a big deal, or, needn't be anyway. Not so much for this kind of part exactly, but TLV431 for instance is 7V abs max; granted there is a TLVH431 variant, among others with lower bias current and reasonable supply voltage rating compared to the basic TL431, but say you wanted to use a power monitor, you could just use a 7V-max reset generator for instance, and divide down the supply from 72V to, whatever close to its threshold, and get the supply monitor function that way.Anyway, it's basically that, a '431, but with two inputs and two references (give or take), so you can monitor two variables. Usually voltage and current. Thus making a square CV/CC operating curve. And yeah, if you want CC to go down below some Vout (including all the way to zero), the error amp itself will need to be supplied externally, forcing an aux supply.Regarding PSFB vs. LLC, you might want to ask that in another thread? Rather than drifting this one here.Tim