Buck vs. Boost and Selection of Rectifying Supply - Page 1
Forum section: electronics / projects

author: abalducci
author_rank: Newbie
timestamp: August 14, 2015, 09:13:14 am
content: Dear all,I have a question for a current project that even after much Googling and trying to look at reference texts I simply haven't been able to find an answer to or locate the relevant 'theory' for this problem.I am working on a project that (like many) requires a number of different VSS and current supplies for different components. As an example:* Motor 1 - 24V - 75VDC (typical current 1 - 4 Amp, 10 Amp Max)* Motor 2 - 24V - 75VDC (typical current 1 - 4 Amp, 10 Amp Max)* Motor 3 - 3V (2 Amp)* Peripheral 1 - 35VDC +/-.5 V(3 Amps)* Peripheral 2 - 35VDC, +/-.5(3 Amps)* Electronics - 5VDC, (500 mA)'Ideally' I would have all these run off a single (off the shelf AC/DC) supply and then supply the necessary buck/boost circuitry for regulation of the various voltages. In terms of current requirements, this seems more straight forward (or I have read of the '10%' rule of thumb of total current under load + 10% margin).However, when it comes to DC out voltage, as long as one can construct the proper buck/boost circuitry my question is does the supply voltage selected 'matter' ? If so how (and why) ?Obviously part of the issue here is that non standard voltage power switching supplies tend to cost more than those at standard voltage-- But are there other considerations here ? Is going high (and using only buck circuity) preferable to 'all boost' ? Or mixing the two is there any reasoning why some should fall under one category and others under another (here I am thinking perhaps of the tight VSS requirements on the peripherals).Any suggestions (or pointers to the relevant theory as to what to choose and how the decision is made) would be much appreciated as I am a bit lost on this one.-A

author: ajb
author_rank: Super Contributor
timestamp: August 14, 2015, 02:45:08 pm
content: It's all a balancing act of efficiency, size, development time, production qty, etc, so there's not really a hard and fast set of rules. Generally speaking, you'd want to have your largest loads loads running directly from the main power supply, since that reduces the total amount of power that you'll need to convert yourself. Building a 3-10W buck converter is a heck of a lot simpler and cheaper than building a 1500W boost converter.Will your first two motors run at any voltage between 24V and 75V? That's quite a range.Sometimes it's easier to look at it in terms of power.Motor 1&2: 240-750W/ea @ 24-75V, 480-1500W totalMotor 3: 6W @ 3VPeripheral 1&2: 105W/ea @ 35V, 210W totalElectronics: 2.5W @ 5VThat adds up to 700-1800W, which is on the higher side for a single off the shelf PSU, but there are suitable units out there. If you can run motors 1 & 2 at 35V, then you could use a single 1kW 36V PSU (most industrial power supplies can be trimmed up or down enough that you could get close enough to 35V from a nominal 36V unit), but if you have the option of running motors 1&2 at 24V, it may be cheaper to go with a 480W 24V PSU and a 240W 36V PSU, and buck the 36V down to get your 3V/5V supplies. Spreading the load out onto multiple power supplies likely also simplifies your wiring, and reduces the amount of current available at any one output terminal which means that fault protection becomes easier.

author: abalducci
author_rank: Newbie
timestamp: August 15, 2015, 12:44:46 am
content: Dear Blueskull, Ajb,Thanks both for your replies/advice. At first I was surprised it was so difficult to find PSU selection information on the web (including what factors to consider) -- with the exception of for computer PSU's, for which there is no *end* of info but mostly geared towards determining your current draw-- And I guess what I am concluding then is that it really 'depends'.In any case, in response to your question ajb I'm looking at using theTeknic Clearpath series of brushless servo motors. Personally I was surprised as well at the high viable voltage range but this is what is quoted in the datasheet.I also think I will go with your advice Blueskull for the buck-boost topology. I myself am primarily an 'embedded' designer (i.e. I spend most of my time under 5Vso I am presently learning more about power electronics, design and application.Thanks again.-A

author: ez24
author_rank: Super Contributor
timestamp: August 15, 2015, 02:13:29 am
content: https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/wizard/allresults/what part number are you looking at?

author: abalducci
author_rank: Newbie
timestamp: August 15, 2015, 02:55:51 am
content: Exact model TBD as the load profile is not yet fully finalized, but I quite like the integrated controller (the price is not bad either by comparison), but every motor supports a 24 - 75 V range (with of course, a different torque profile depending on the voltage).