Loop stability in DC-DC converter with wide range of operation - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: August 30, 2018, 09:04:31 pm
content: I have been working on a few DC-DC converters that will ultimately end up in a system with a broad operational range. I have no control of the loads and the power sources are also likely to vary considerably. My plan to attack this is to make the most stable and durable design I can and use that to write the specs for the final product. The more 'problems' it can handle, the better chance of success.General parameters:TI LM5175 boost/buck controller, but this thread is not specific to this particular controller.Input - 10-36vOutput - 16v / 10A (perhaps higher if I can go with multiple phases)The input power is typically sourced from a switched battery fed bus. The system will pick a priority source and has 1 or 2 backup sources that it switches to if the primary fails. This means that the voltage can rapidly rise and fall during a switch and there is generally some droop of a few volts for a 1-2ms or so.The output is whatever the end user plugs in, but commonly it is a system with a capacitive load with a DC-DC converter. Inrush can be considerable but it is ok to allow some voltage droop I have electronic fuses on the outputs, so overloads are dealt with rather quickly. Most of the loads are fairly dynamic with FPGA's to BLDC's ensuring nothing is ever stable.With that in mind - I am trying to figure out how to best approach the loop compensation to deal with such a wild system. I am inclined to sacrifice regulation and lean toward a slow bandwidth which I believe will help the system stay stable at the expense of regulation. In general, regulation is not particularly important. The output can range from 15-17v without any problems. My test system is a TI LM5175 boost/buck controller which so far is doing fine with relatively benign tests - fantastic efficiency. I have made some changes to the PCB layouts primarily to fit the final form factor and I will start testing late next week. Hoping have a strategy in place before I start the heavy tests. Will a low crossover frequency benefit stability? Is there anything that can help the system best deal with rapid changes in VIN?I have been using the TI supplied spreadsheet that allows me to see calculated bode plots based on the compensation values - not sure how much I should believe the results. I believe I finally have a functional FRA system in place to test the actual circuit for gain and phase margins which should help ease into the final compensation with smoking too many PCB's. I have been studying control theory as much as my brain can handle, but this is still out of my comfort zone. If anyone has self-education suggestions like white papers or app notes - that will be very helpful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2018, 03:30:39 pm
content: Eh, that's not really wide. Wide would be, hmm, more than 5:1 would be challenging for compensation purposes (compromising transient response, probably)?A feed-forward method is nice. Some controllers support this; LTC3810 comes to mind. The effect is to reduce on-time proportional to supply voltage, independent of the control loop: this improves ripple rejection and loop stability. PFC controllers usually do this, too (for good reason).Tim

author: ignator
author_rank: Regular Contributor
timestamp: August 31, 2018, 04:37:15 pm
content: Are you designing a power supply for avionics that uses +28VDC nominal power? That's what it appears for your input range, as well redundant power sources.I have nothing to add, other then I worked with the supply designers. They never seem to be able to design a supply that could provide power from light load to full load without putting a dummy power wasting resistor, to keep it stable.I can't help much here, other then I qualified them to RTCA DO-160, and preventing EMI clock noise from being conducted back to the supply source, and radiated off the wire. This was always a fun exercise in filter design. Our supply was a flyback design, with typically 9 different microprocessors running at different frequencies from 6MHz to 33MHz. Our switching frequency was 80KHz to stay under the conducted mode EMI emission plot minimum requirement (I recall the bode plot shows the start frequency at 150KHz). Keeping noise out of the COM band was the major task, as the test requirement had a notch that was close to the noise floor, for emissions.This is all moot if your using this in an experimental aircraft, but TSO is all I know.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 31, 2018, 05:17:42 pm
content: Quote from: T3sl4co1l on August 31, 2018, 03:30:39 pmBy industry norms, you are correct - it is not all that wide in terms of the voltage range. The part that I am worried about is the wide variety of loads, inrush currents, transients on top of having the input VIN drop rapidly from 26v to 12v and then back to 28v again. Maybe that is not as hard as my mind is dreaming right now.Quote from: ignator on August 31, 2018, 04:37:15 pmFortunately NOT for aviation. I have done mechanics for aviation and that was 1% engineering and 99% paperwork. I am totally unqualified to design electronics for aircraft.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 31, 2018, 06:23:05 pm
content: Some ideas to cope with widely varying loads:- Use a minimum load (either constant current or a simple resistor)- Use "hickup" mode in the converter, plus a linear postregulator- use two converters (isolated with diodes, plus a cap) in parallel. One for light loads, with current limit,the other for heavy lifting. Can be combined with linear postregulation

author: nctnico
author_rank: Super Contributor
timestamp: August 31, 2018, 06:54:54 pm
content: The easiest way to get a power supply stable is to add a boat load of capacitance on the output (like 10mf to 50mf) and have a relatively slow control loop. This will cause ripple at the output when a rush-in current occurs but this can be mitigated by adding a soft-start circuit if every load has a dedicated output. DC powered devices may already have a soft-start to meet requirements. Most 48V DC telecom grade equipment has some kind of soft-start and there are requirements on the current waveform as well in order not to upset the DC bus too much.

author: rx8pilot
author_rank: Super Contributor
timestamp: September 02, 2018, 02:03:36 am
content: This project is space limited so, unfortunately, I don't have the option to pile on massive output capacitance. At the moment, I believe I can manage about 1200uf of polymer caps which will force the lower limit of the loop crossover point (I think).At the moment, I have been trying to get a useful simulation setup. TI Webbench exports to a free version of TINA for simulation, but I have not yet been able to figure out how to get a bode plot. If I can get the simulation to work, it will certainly help me iterate through some possible solutions so I hope I can figure it out.Webbench offers simulation, but it is very basic.