Dc to DC buck converter. Anyone use the Rohm BD9G341AEFJ? - Page 1
Forum section: electronics / projects

author: qrper
author_rank: Contributor
timestamp: July 08, 2017, 05:08:39 pm
content: Hi gang,I have a need for a dc to dc buck converter. 40-72 volts input, 12 volts output at about 1-1.5 A. Why the wide range input you ask? It's the voltage of my battery bank charged by my solar array. It's a 48 vdc system, but fully discharged, 40 V, when equalizing the lead acid batteries 60 V dc. The 12 V output will run a control board.I know there are plenty of them out on the market, and I've purchased several from eBay from the Chinese vendors. The preassembled modules are cheaper than trying to source the parts here. And, on top of that, the Linear Technologies PWM IC is not recommended for new designs. The build quality is somewhat iffy, depending on the Chinese vendor. I've had several explode after being subjected to 60 V. (the max voltage specification is 62 V)After a few nights of google searches, I came across the BD9G341AEFJ by Rohm. Seems simple enough. I might try and roll my own.Any one ever use this IC in a project?Mike

author: mariush
author_rank: Super Contributor
timestamp: July 08, 2017, 05:59:05 pm
content: 19$ : SSTW001A3B41Z GE Critical Power DC/DC CONVERTER 12V 15W (1.25A)Input : 36V.. 75VOutput : 12V at 1.3A at up to 87.5% efficiencyLINK :https://www.digikey.com/product-detail/en/ge-critical-power/SSTW001A3B41Z/555-1351-ND/4840127As a suggestion, you coulduse a linear regulatoras a sort of max voltage protection, to limit the input voltage to a level which would allow you to use better switching regulators that can only handle certain maximum voltage.For example, you could use a 2$TL783which supports up to 125v DC input, and up to 0.7A output and like most regulators, if the input voltage is not higher than the set output voltage it will output input voltage, minus the forward voltage drop of around 1.25v:Let's say you want to use this :11$ : OKI-T/36W-W40N-C Murata Power Solutions Inc.Input voltage : 19v..40vOutput voltage : 5.021 ~ 15.5 V 3A (max 36w) at up to 95% efficiency.LINK :https://www.digikey.com/product-detail/en/murata-power-solutions-inc/OKI-T-36W-W40N-C/811-2261-1-ND/2605212You set the linear regulator to output 38v no matter the input voltage.With 38v at the input, at 12v 1.5A, the DC-DC converter will be about 90% so let's say it uses (12v x 1.5A ) x 10/9 = 20 watts. At 38v input, that's 20w / 38v = 0.52A (well, within the regulator's 0.7A limit). Let's round it to 0.55A for simplicity.Scenario 1 : battery voltage is minimum 40vLDO In : 40v , OUT 38v and 0.55A - the regulator will dissipate 1.1 watts as heat.. no problem.Scenario 2 : battery voltage is 60vLDO In : 60v , OUT 38v and 0.55A - the regulator will dissipate (60-38) x 0.55 = 12w .. significant amount but still can be dissipated with a passive heatsink, easy to do with a to-220 part.Scenario 3 : voltage up to 75v (worst case scenario)LDO In : 75v , OUT 38v and 0.55A - dissipating (75-38v) x 0.55 = 20w a bit much but still not a problem.Would be fairly easy to add a 12v fan to blow over the dc-dc converter AND the ldo and act as a minimum load for the dc-dc converter and to improve the cooling of the ldo.May seem like a waste to throw away 12-20 watts, but you say 1.5a is peak currents, and that the nominal voltage is 48v and 60v is when equalizing, so not really a 24/7 thing.. so most of the time, the ldo would only waste at most 2-5 watts? Think of it like this ... you waste 2-5w in the ldo and then get 90% efficiency in the dc-dc converter, or you use a wide input voltage dc-dc converter and get only 80% efficiency at 60v+ .. so basically you're not really getting something more efficient.

author: qrper
author_rank: Contributor
timestamp: July 08, 2017, 06:38:42 pm
content: Thanks for your input.right now, that's basically what I'm doing in an attempt to keep those chinese dc/dc buck inverter in one piece.Since they rate them at 60 V max, I use a small preregulator that lowers the battery voltage down to a more tolerable 45 V to the dc/dc inverter.It's a simple series regulator using a TIP122 and a zener diode. Gets hot! But I have a sensor mounted on the heatsink and a fan will come on to cool things down.I wanted to eliminate the heatsink and fan, there's nothing wrong with using either, but something different like that BD9G ic. It's suppose to do upwards of several amps, and can tolerate the 72 V input.mike