Split Rail Buck Converter Balance Question/Problem - Page 1
Forum section: electronics / beginners

author: Telefunkian
author_rank: Newbie
timestamp: June 16, 2023, 10:11:07 pm
content: :-//Greetings!Longish post but hopefully of value to others who (like me) are trying to learn about split rail buck converters?I am working on a device that has issues starting up (booting) and in my efforts to try to 1. understand the problem and 2. potentially fix it I have determined that one of the power supply sections in the device may not be working properly. FWIIW, the device is an Apogee Symphony MK1, with a 'Main Board', and two ancillary boards. There is absolutely no service literature or schematic available that I know of. The device is powered by a small single rail SMPS that provides 24v to the main board. This 24v is then distributed various places and other rails are all derived from the +24. The main board works fine, but when the 8x8 board is plugged in the problem manifests and the device attempts to start, cycling several times, and starts up only rarely. Device contains several FPGAs, DACS, ADCs, OP amps relays etc.The 8x8 board has what I will call a split rail buck converter that I believe should be producing a balanced +/-15v to supply op amps and the like (derived from 24v). It is unbalanced, producing +15, and -16.2, which I believe to be incorrect. (There is also a +/- 9V supply and several 5v, 3.3, 1.2 and 1 v supplies scattered on this board, they all seem to be fine).In my attempts to understand this I have begun to reverse engineer/draw out the schematic of the power section of the 8x8 board, and my draft is attached. The +/- 15v are produced by a LM3430 (normally used as a boost controller, here not) driving a single FR4105 MOSFET. There is a LTC2927 Tracking controller involved, which I believe controls the ramp up time, or sequencing of the rails relative to others. The circuit uses an Eaton Versa Pack coupled inductor/transformer with six 1:1 windings which appear to be wired in three pairs.My questions in no particular order....1. Notwithstanding my messy schematic (_draft_, please forgive me!), is it possible to discern the topology of the +/-15 supply? What would you call it? (some sort of Sepic/Cuk? hybrid?)2. Is it reasonable to assume that the design is such that it should provide balanced rails?3. What could contribute to this imbalance? All diodes, resistors and caps have been checked. All components appear to be either within tolerance (resistors), providing appropriate diode drops (diodes) and caps are not shorted. SMD electrolytics have been replaced as ESRs were all high. (device is ~10-12 years old and remains 'On' most of the time.) The resistances from the output bulk capacitors C2 (positive) and C1 (negative) to ground are ~840 and 117 ohms respectively.4. The LM3430 seems to be getting its feedback from the 431 monitoring the positive rail, so I assume the negative rail should track. My understanding is that rather than the negative rail being pulled further negative it would seem more likely that the positive 15 rail is being pulled down somewhere and accordingly requiring less 'bucking' (getting a longer duty cycle) and this is pushing the negative rail further negative. Is this a good way to look at this?

author: Telefunkian
author_rank: Newbie
timestamp: June 18, 2023, 03:07:43 am
content: Moderators please let me know if this should be in a different forum?!? Cheers!