advice on SMPS topology - Page 1
Forum section: electronics / projects

author: JPortici
author_rank: Super Contributor
timestamp: April 10, 2017, 05:56:30 pm
content: I'm writing down notes for a project, i'd like to make a battery powered power supply for my eurorack synth case. Triple output, highly unbalanced loads.I'm trying to decide which topology use.Assume 7-15V in range (from lithium battery to car battery)Assume+12V 500mA-1A-12V some 100 mA+5V 500mA-1AI had a couple of ideas in mind.1) SEPIC for +12Vbuck from Vin for 6V + 5V LDObuck-boost for -12V1.b) SEPIC for +12Vbuck from 12V for 6V + 5V LDObuck-boost for -12V2) flyback for 12V and 5Vbuck-boost for -12V2.b) flyback for 12V and -12Vbuck for 6V + 5V LDO, either from Vin or 12V2.c) single flyback for everythingi'm reluctant to use a flyback configuration, because of the unpredictable and widely unbalanced loads, even though it would save me a switcher or twoi'm also reluctant to use a flyback configuration because i want to be as low profile as possible (~ 1cm), i think a transformer would be too high (but i havent had a look at catalogs yet)then i'm concerned about the buck boost start current, even though i would implement soft start.Altough my experience in power electronics is yet limited, in the end i would then want to replace the switcher controller(s) with an MCU because hard <=> fun and that would do a bunch of stuff anyway (switcher, current monitoring and logging, power sequencing, ..)having fun while learning, suggestions?

author: JPortici
author_rank: Super Contributor
timestamp: April 10, 2017, 08:15:29 pm
content: Quote from: evb149 on April 10, 2017, 07:12:39 pmwhich is why i was thinking SEPIC.. also to have one less stage, boosting to 15 would mean boost + buck + buck + buck/forward.. yuck! plus, to keep inductors small i was thingking to have the three stages in parallel, in the end using an MCU i would keep them syncronizedthinking loudly, this coupled inductor buck seems interesting, to have one less switcher, but wouldn't i have the same issues as with using an unbalanced flyback? the current consumption ratio between +12 and -12 is roughly 3:1 to 10:1 in any case

author: MagicSmoker
author_rank: Super Contributor
timestamp: April 11, 2017, 05:22:11 pm
content: Personally, I'd go with a DCM or BCM* flyback with a multi-winding secondary. You get good cross-regulation of the outputs, there are no inductors required (aside from the transformer, that is) and it is fairly easy to compensate (again, if operating in DCM or BCM).In contrast, the SEPIC is deceptively difficult to get working well, especially with coupled inductors, and it is also among the most difficult of all the hard-switched (ie- non-resonant) converters to compensate.* - Discontinuous or Boundary Conduction Modes, respectively.

author: znroot
author_rank: Contributor
timestamp: April 11, 2017, 06:01:43 pm
content: You have many options to achieve your target, as electronics usually has.IMHO you can separate the problem into two stages:1) Boost from battery to something like 16Voutput2) buck for 5Voutput, buck for 12Voutputand a buck-boost for the -12VoutputThe second solution yields something like:1) buck for 5Voutput2) buck-boost for -12Voutput3) non-inverting buck-boost for 12V outputMy choice goes to the first configuration I propose because you have only the boost converter which need to be designed to get a wide input operating range.Contoroller side: for this project I would go for analog control and an MCU which only monitor the system... Otherwise you need a very powerful DSP to get a decent digital controller (at least 3 indipendent DPWM pheriperals and you need a very fast clock to get calculus result in a reasonable time, you have the math for min 3 different control loops).

author: JPortici
author_rank: Super Contributor
timestamp: April 11, 2017, 06:07:17 pm
content: I already have the MCU laying around! (dspic33EP, GS series.. used for a very different purpose)I decided I'm going to proceed in various steps..first, a board which will have the MCU and a buck to 5V to test if it's doable. Only then i'll try to implement the other converters, one at a timeFIY: yes it is because of blueskull's thread. he mentioned this pic, i have it here, got me curious

author: sanwal209
author_rank: Regular Contributor
timestamp: April 11, 2017, 07:05:58 pm
content: For me Transformer with multiple secondary winding is good and its simple. Drive it with half bridge as @evb149 said in his comment. It will be also easy to regulate.

author: mariush
author_rank: Super Contributor
timestamp: April 11, 2017, 08:51:38 pm
content: If price is not an issue, use something like LT8582 :https://www.digikey.com/product-detail/en/linear-technology/LT8582EDKD-PBF/LT8582EDKD-PBF-ND/2835494Then simply use a voltage doubler IC in inverter configuration to get your -12v at up to 100mA, or a separate invert mode regulator to get -12 from 7..15v