Frequency control of resonant power converters (theory) - Page 1
Forum section: electronics / beginners

author: sean0118
author_rank: Frequent Contributor
timestamp: March 23, 2014, 07:48:38 am
content: Hi everyone,I'm pretty comfortable with how PWM control works, but when it comes to VFM I can't find anything. Maybe I'm just using the wrong keywords while searching?To be more specific I'm trying to find information on the control of LLC resonant converters. But really at the moment I would be happy with anything decent on VFM. So, does anyone know of any sites, papers, books etc that would act a a good introduction to the topic?edit:My understanding at the moment is that changing the switching frequency of the LLC converter effects the impedance of the circuit. Which can be used to set the voltage over the magnetising inductor that is in parallel with the load.I get that the converters output voltage needs to be sampled, then it goes into a black box that sets the switching frequency. But what is inside this black box? I guess there is probably an IC that does this in real life, but I really want to know the theory behind it.

author: teske
author_rank: Newbie
timestamp: March 23, 2014, 04:35:30 pm
content: have a look at:'Consideration of Operating Characteristics for Bi-directional LLC Resonant Converter'http://www.ijrer.org/index.php/ijrer/article/viewFile/493/pdfor google: 'texas instruments LLC', you get some IC, application notes and even evaluation boards. ST and other manufactures also makes ICs for LLC

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 27, 2014, 08:29:50 am
content: There is also quite a good app note from ST Microelectronics.www.stm.comI think. I can't remember the number though.

author: sean0118
author_rank: Frequent Contributor
timestamp: April 10, 2014, 03:00:21 am
content: Thanks guys, sorry for the delay in getting back to you. What you posted should be very helpful.I also found a good book that deals with the basic control schemes for most converters including resonant converters. In case anyone is interested its "Power-Switching Converters" by S. Ang and A. Oliva. It's definitely my favourite power electronics book atm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2014, 03:20:57 am
content: Quite simply, you raise the frequency (above resonance) to reduce power transfer, and drop it (no lower than resonance) to increase power. It's just a PID (or whatever compensation you prefer) into a limited range VCO.I've done this for induction heating, which is challenging because of the uncertainty of load (it could span, say, 50 to 100kHz in a given setup, let alone with different resonating capacitors and coils and all) and the high Q (which makes the loop very insensitive in that wide span away from resonance, and very sensitive riding the slope up to resonance). Also... the time constant of the feedback ("plant") corresponds to the difference between operating and resonant frequencies, which makes compensation....difficult at best. It varies nonlinearly with offset, and can very easily lead to chaotic instability. (The low Q, near-fixed-frequency setups typical of LLC converters fortunately aren't so finicky.)Tim

author: sean0118
author_rank: Frequent Contributor
timestamp: April 12, 2014, 05:30:04 am
content: Quote from: Richard Head on March 27, 2014, 08:29:50 amThisone right?Quote from: T3sl4co1l on April 10, 2014, 03:20:57 amThanks, that's given me a few ideas about how I'm going to simulate this.Btw a question that has been bugging me is whether variable-frequency control, variable-frequency modulation and pulse-frequency modulation are referring to the same thing?Also, is there a functional difference between control ICs that use current controlled oscillators (CCO) and ICs that use VCOs? TakeFAN7631and theL6599as an example.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 13, 2014, 02:19:52 pm
content: No, that looks like a datasheet only. The one I'm referring to is AN2644.I tried to attach it but it's 1.3Mb and the limit is 1Mb.If you have problems getting it from the ST website I can try to split it up into two smaller files.Dick

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 06:25:41 pm
content: Assuming the CCO/VCO are of typical design (a fixed or variable current charges a capacitor to a variable or fixed threshold, respectively), the difference is a proportional vs. reciprocal transfer curve (since I = C * dV/dt). Over a small range (an LLC wouldn't be expected to need more than maybe... 20% range?), this doesn't matter; over a wide range, it's much more important (you wouldn't want to use a CCO for a musical synth VCO).Tim

author: sean0118
author_rank: Frequent Contributor
timestamp: April 14, 2014, 12:37:47 am
content: Quote from: Richard Head on April 13, 2014, 02:19:52 pmyep it's one of the ones I tried to link, there are a couple of PDFs in the 'Design Resource' menu tab, but when I linked it defaulted back to 'Quick View'. Anyway, I have read some of the PDFs but not AN2644 so I'll give that a go.Thanks for your help guys