Doubts about a project of DC/DC converter - Page 1
Forum section: electronics / projects

author: gabrielcassio
author_rank: Newbie
timestamp: June 02, 2017, 01:31:13 am
content: Hi everyone.In the last days, I've been searching some components for a project of a DC/DC converter. The requirements are: input voltage 44 up to 32 V (lithium-ion batteries will discharge through time), output voltage of 48V, maximum outup current of 18A. Is there any IC recommended for this?Any help will be welcome.Thank you.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 02, 2017, 01:37:11 am
content: I am on the road, but have a quick response.Have you ever designed an SMPS before? They are quite challenging and nuanced. This much power is no joke and requires substantial skill.Lastly, you specs will need to be expanded before any logical choice of system design can begin.Sent from my horrible mobile....

author: debininja
author_rank: Contributor
timestamp: June 02, 2017, 03:44:34 am
content: Quote from: rx8pilot on June 02, 2017, 01:37:11 amTell me about it...I've been at it for 2 weeks trying to get just a 5Watt step-down flyback SMPS working. (Well, it works, but it's not optimal).@gabriel, that's close to 900Watts of output powerAssuming 75% efficiency, you're gonna need ~1100Watts at the input. You will need really beefy mosfets, maybe even in parallel. I'm a noob myself at SMPS design, so I can't tell you what topology to use, but I do know that your LiPo/Ion battery packs will have to be heavy duty, rated for high discharge rates. A minimum of 34 Amps at 32V, not considering any sort of safety margins. What are the specs on your batteries? Are you making the pack yourself or buying it?

author: jbb
author_rank: Super Contributor
timestamp: June 02, 2017, 08:15:38 am
content: First up, I suggest you check the load. It might be possible to make it run straight off the battery (maybe add some more cells).If that won't work, a DCDC converter is definitely possible. But it's quite a beast.On the efficiency front, you could expect to achieve between 90% (poor effort, would not recommend) and 97%.48V * 18A out is 864W. At 90% efficiency, the input power will be 960W. Losses would be around 100W, which is a lot to dissipate.If you shoot for 95%, losses will be around 50W and the thermal engineering will be much easier. This means synchronous rectification and careful attention to switching losses.On the topology front, you need your output voltage to be higher than your input. That's a boost converter. They have a problem with control though; they can't gracefully deal with an output overload or short circuit.If you add some more cells to your Li pack, you can get your minimum output voltage above 48V. Then your output voltage is less than your input voltage, which is a buck converter. They can give you free short circuit protection.If you raise the battery voltage so it straddles the output voltage (sometimes higher, sometimes lower). That calls for a buck-boost converter. They need more switches and have lower efficiency.Finally, you should consider a multi-phase design. This splits the power across multiple channels, which is easier. It also helps with response time and filter design.