Where does the limitation for higher current SEPICs come from, eg 40W 3A  - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 12:23:16 am
content: So I'm seeing with a lot of DCDC switching regulators/controllers the datasheet will have example schematics for using their chip for a 5A boost or buck regulator, but then the SEPIC example using the same chip is always only like 1.5A or maybe 2A.What is the core reason for this and where does the loss come from if you try and run a SEPIC at higher currents instead of boost/buck options?I assume you get lost heat in having 2 inductors instead of 1. And the AC coupling cap probably gets warm. but is that the only reasons?

author: Benta
author_rank: Super Contributor
timestamp: June 30, 2023, 12:37:48 am
content: There's nothing inherently "lossy" in the Sepic.Could you point to some of the "lots" you're talking about, please?

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 01:00:40 am
content: It's was just a pattern I was noticing last week when browsing datasheets looking for something that would easily do a 3A 40A sepic 12V to 12V. The sepic examples were often less current than the non-sepic examples.I would have to look them up again to find examples.Maybe I was reading too much into it.

author: jwet
author_rank: Frequent Contributor
timestamp: June 30, 2023, 03:37:14 am
content: Sepics are a nice topology for simple buck boost supplies but do tend to be less great at higher power (>30 or so Watts). There are just better ways to do things at these higher power levels including asking yourself why you need to buck and boost. One big issue is the output rectifier- it has to take to take a lot of current- basically the sum of the input and output current. Even with a synchronous setup, modern low voltage very high current supplies get messy. TI and Maxim have written decent app notes on Sepics. Like everything, you can do it but there may be better ways.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 30, 2023, 08:43:49 am
content: The main limitations in integrated switch controllers is dissipation and breakdown in the main switching device. Dissipation is a sum of switching and conduction loss, and for any switching technology there is a tradeoff between the two given by device sizing among other things.A large fraction of the switching losses depend on Vds^2, and the conduction losses go with Ids^2. So for a given switching controller, the power that can be processed depends on the relative scaling of voltages and currents for the topology used. These are simplifications assuming CCM mode and switching losses dominated by Vds*Qoss, but simulations or calculations can be done for DCM and real device losses to show that the conclusion stands.In a buck, the switch sees the input voltage and the output current. In a boost, the switch sees the output voltage and input current. In a SEPIC, the switch sees the sum of the output and input voltages, and the sum of the output and input currents. So for a given loss budget, you get less power processed with a SEPIC.

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 10:03:09 am
content: Are there any good synchronous buck-boost converters with the 4 mosfets integrated that will do say5A at 11-40V in and 13V out, and that don't cost $15 each?

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 30, 2023, 10:19:44 am
content: 40V input and 5 A output with two stage conversion in a single chip is a bit on the border of what's practical, so there are not that many cheap options on the market. The best candidates I found with a quick search were MAX25240 (4.9/3.6 USD qty 1/100) and LT8350 (8.6/6.5 USD qty 1/100).

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 10:39:12 am
content: The LT8350 I have looked at before, it's a nice chip but the switcher is 6A, which means it can really only regulated 3.5AKinda annoying that they always spec the amps as the switcher rating and not the regulated output current.Maybe I should investigate a Step up and then Step down entirely independent of each other.It does have the advantage that chips which can only do one at a time tend to be much more widely available and cheaper.I have not really looked into this because it seems so inefficient to have two totally separate regulators one after the other.But maybe its worth looking into.

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 11:42:07 am
content: Anyone familiar with the MAX25431https://nz.mouser.com/datasheet/2/609/MAX25431-3128174.pdfPrice looks good, it needs external fets but at that price I can probably do that.But the floating gates of high-side switches with diode/cap boost that recharges from the bottom side makes me nervous.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 30, 2023, 11:59:46 am
content: Quote from: Psi on June 30, 2023, 11:42:07 amWhat's wrong with that? Almost every switching DC-DC with NMOS high side do that.

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2023, 12:19:09 pm
content: I know, just makes me nervous, like it's only ever an inch away from a shoot-through explosion.I'd feel better if it drove the boost from a separate osc, rather than the switching of the lower fets.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2023, 01:32:53 pm
content: There is nothing wrong with bootstrapped high-side N-MOS drivers. They have full cross-conduction protection and UVLO logic. You would be correct to be nervous if that logic did not exist, it is pretty much necessary in the charge pump configuration, but that's also why they are integrated and well-tested.

author: CosteC
author_rank: Regular Contributor
timestamp: July 02, 2023, 07:03:49 am
content: It is all about "semiconductor utilisation" and "core utilisation" both relate to how much cost is associated with project. Beyond some point transformer based topologies are smaller and cheaper. SEPIC also needs very good capacitors. At higher powers they are not small, neither cheap.

author: jwet
author_rank: Frequent Contributor
timestamp: July 02, 2023, 06:17:28 pm
content: @PSI- don't worry about cap pumped high side drivers if this is what you mean. This is very standard- every notebook PC main PS is based on this topology. The alternative is Hi side PMOS or a real floating supply above the source of the high sides. The only thing you can't do is go to 100% duty cycle, which to me is a feature.