Flybuck Converter with XL6009 - Page 1
Forum section: electronics / projects

author: Stephen37
author_rank: Contributor
timestamp: November 10, 2016, 04:50:11 am
content: I want to make a step-up "Flybuck" isolated converter with anXL6009regulator due to low costs. A suitable coupled inductor might beSRF0703-470Mwhich is pretty cheap.My needs are 40V out from 5V at 50-100mA for two opamps isolated from each other.My question is whether I can simply replace the inductor with a coupled inductor and use the secondary with a diode and capacitor as presented inAN-2292 Designing an Isolated Buck (Flybuck) Converteror it will work only with the LM5017 (or another synchronous regulator).Do I have to be careful about anything else ? Thank you.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 11, 2016, 02:22:46 pm
content: If you need 40V out from 5V, then you require a boost converter, or a flyback if it has to be isolated. Do you require two isolated outputs? And if so, why do you need them isolated?

author: Stephen37
author_rank: Contributor
timestamp: November 12, 2016, 12:35:57 pm
content: The TI paper says that one can get away in case of small loads with only a coupled inductor because the secondary follows the primary and there is no need for an optocoupler for the feedback as in case of a normal flyback configuration.I need a dual output converter with one output isolated for a diy TENS unit.The XL6009 chip works fine as a step up converter for 5v to 40v.Now I want to convert it to run as a buck boost converter with two outputs, I am waiting for some inductor samples to arrive from WE and I will see what I can do.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 12, 2016, 09:29:42 pm
content: It was the 'flybuck' that confused me. Essentially you're making a flyback, which is just a boost converter with isolated output and the facility to change winding ratio to keep the switch node peaks at a sensible level.You won't get the best regulation if you just set it up as a boost and have the primary for the feedback voltage divider, but it'll give pretty much the same voltage.For two outputs you could just have one isolated, and one non-isolated, otherwise you would need three windings.

author: jashugan
author_rank: Regular Contributor
timestamp: November 14, 2016, 03:45:57 pm
content: In short, if you're doing a flyback, you can use this chip. If a flybuck, you can't. (need to use a synchronous buck with correct light load implementation)

author: Stephen37
author_rank: Contributor
timestamp: November 15, 2016, 06:52:26 pm
content: Quote from: jashugan on November 14, 2016, 03:45:57 pmI guess if I have the same load on both outputs then it should work without a synchronous buck converter to some extent. But if there is no load on the primary (which is not possible in my case) then there is no current on the secondary.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 17, 2016, 02:43:11 am
content: Stephen, I'm not sure you're getting what jsdhugan and myself are saying. It isn't a buck, its a boost converter, or rather, because you're using a coupled inductor - a flyback. You would only need a buck converter if you need the output to be lower than the input voltage. This can actually be the case for a flyback if you're secondary is the output because one node is at ground, where-as in the primary inductor, one node is held at the input voltage, so it can only go higher than this, not below.The XL6009 is incapable of being a buck converter because it has a low-side switch with current sensing. And using it with a coupled inductor (I keep saying that as opposed to transformer, because strictly speaking, transformers do not store energy, the output follows the input, where-as coupled inductors do store energy) forms a flyback for either an isolated supply (true isolation requires opto or primary side feedback) or for an output that is required to go above and below the input.However, for your purpose of isolated 40V from 5V in, then its fine. I think this whole thread is more about terminology than any technical problem. generally:Buck - step down, output can never be higher than the input. High side switch.Flybuck - isolated buck converter*Boost - step up, output can never be lower than the input (well it can, by about one diode drop, if the switch is turned off). Low side switch.Flyback - isolated boost converter**because different winding ratios can be used, these can both step up, or step down, although using a 'flybuck' for step up seems a bit silly, as it would require a greater turns ratio between inductors than a flyback. A flyback can be used for step down (just like most low power wall warts).

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 19, 2016, 04:02:52 pm
content: Has someone invented a new term "flybuck"?AFAIK an isolated buck converter is called a forward converter. Can be single or double switch but still a forward converter. Any other unusual buck converter is simply referred to as a buck derived topology.