Looking for SEPIC 9-36V board mount brick DCDC with current limit - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: September 12, 2020, 04:13:58 am
content: I'm designing a new product (volume ~100 per year) and I would really like to use a little brick DCDC module rather than rolling my own design.However i'm having trouble finding a unit that matches my needs.It must run from a 12V or 24V lead acid battery (so thinking 9-36V module)Output Current >=2AAdjustable output voltage that can do ~5V to 14V or can be fixed 13.8V if it has it's own adjustable current limit (most don't)Price under ~US$26Must be SEPIC for step up and down. (this is where i'm having trouble finding something)Cannot be a cheap china module, known brandnames only(i know lots of those exist and have used them for other projects but this thing needs to be reliableThe reason for the SEPIC is that it will need to charge a 12V SLA from a 12V or 24V SLA (universal input 12/24V SLA)I could add some sort of bypass electronics to bypass the reg if the input is 12V, but that gets messy.And I know that a DCDC module is not a battery charger. I plan to put a current sense on the output and feed that back to the reg voltage adjust to reduce output voltage.Any ideas welcome? or links to converters that might work.Alternatively, if i have to roll my own DCDC for this, can anyone can recommend a manufacture DCDC design tool that i can plug my specs into and it will spit out a design for rapid development. That would be great

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 12, 2020, 04:28:28 am
content: There are a bunch of cheap modules based on the LTC3780, those use a 4 switch buck/boost design that's actually superior to the SEPIC. I recall Julian Ilett has reviewed a few of those and the ones that have 3 trimmers offer the most flexibility in adjustment.

author: Psi
author_rank: Super Contributor
timestamp: September 12, 2020, 04:30:36 am
content: Are there any known brandname modules that use the LTC3780 ?For this application i cannot be putting aliexpress modules in the product.However, copying one of them with my own 'known legit' parts is an option.

author: Psi
author_rank: Super Contributor
timestamp: September 12, 2020, 05:03:34 am
content: Looks like the LTC3780 is a $6 chip at like 50-100 volumes. That's not a cheap IC.All the Aliexpress modules must be using counterfeit copies.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 12, 2020, 05:43:09 pm
content: You can also look at buck converters that can go to 100% duty cycle. If you can change the internal battery to 3S lithium ion (cheaper than sealed lead acid nowadays, especially in small sizes), you can then reduce the voltage to 12V (4V/cell) and be able to keep the battery charged with an even lower input voltage.

author: julian1
author_rank: Frequent Contributor
timestamp: September 12, 2020, 09:38:23 pm
content: You may not necessarily need sepic. Any topolology with a coupled inductor / transformer can generally do both step up and step down. So a DC to DC flyback or forward-converter with 1 to 1 winding ratio would match the criteria. Sepic often winds both coils on the same inductor, so the materials expense is comparable. I have no idea about particular manufacturer's modules though.

author: Psi
author_rank: Super Contributor
timestamp: September 13, 2020, 12:43:54 am
content: Quote from: NiHaoMike on September 12, 2020, 05:43:09 pmunfortunately both batteries are part of an existing system outside of my control, so can't change themBut yeah, would be nice if we could.

author: Psi
author_rank: Super Contributor
timestamp: September 13, 2020, 12:45:35 am
content: Looking at options, i think maybe a buck converter that can go to 100% duty cycle is the way to go.As suggested by NiHaoMike.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2020, 10:45:04 am
content: Quote from: julian1 on September 12, 2020, 09:38:23 pmExactly, SEPIC is not the common choice even when step-up and step-down operation is required except at low currents. More commonly a transformer flyback or transformer isolated buck converter is used.

author: Psi
author_rank: Super Contributor
timestamp: September 13, 2020, 10:59:43 am
content: Agreed, don't need sepic, just a buck reg that has a dropout under ~600mVThis is the current top contenderhttps://www.digikey.com/product-detail/en/texas-instruments/LMZM33606RLXR/296-52221-1-ND/9757456