Thanks for the details, Brian! I have Pressman's 3rd edition and didn't see any significant discussion of initial transient/charging behaviors, but perhaps haven't read closely enough yet.
My experience w/50% FL de-rating has been different but seems related to your comments regarding component temperatures/airflow. One supplier has public qualification data that includes e-cap lifetimes versus load % and ambient temperature and has told me that e-cap stress is their biggest reliability concern. Another supplier has provided predicted reliability at different load %'s (but not demonstrated). I am fairly familiar w/HALT and SR322 (had a reliability engineer role at one time). For the in-rush behavior, I have thought it to be a function of the primary side circuit design (capacitors & current limiters) in the 1ms range. Did you mean AC bus or DC bus charging for the multiple cycles case? I am interested to do a bit more testing of an existing design under different load conditions based on your description. I have disassembled supplies from different suppliers in the past in the 100W to 300W load range in order to compare input/output capacitors and overall designs. On the primary side, all capacitances have been within a 2X range (80uF to 164uF @ 420/450V) and on the output side a 10X range (1000uF to 10000uF and with very different e-cap voltage de-rating from 1.5x to 7x of Vout). For the upstream protection, there are definitely differences in CB performance whether thermal or combined thermal/magnetic. Depending on the installation location, I have seen instances of local supplementary protection (UL1077 CB's) in addition to branch protection, so I am wanting to make sure the whole power distribution system is well understood for a range of system designs/sizes. Thanks again! -Adam On Mon, Mar 20, 2017 at 2:48 PM, Brian O'Connell <oconne...@tamuracorp.com> wrote: > *From:* Adam Dixon [mailto:lanterna.viri...@gmail.com] > *Sent:* Sunday, March 19, 2017 9:59 AM > *To:* EMC-PSTC@LISTSERV.IEEE.ORG > *Subject:* [PSES] AC/DC power conversion and system architecture (in-rush > limiting, reliability, cabling) > > > > Long post from this weekend's studies.... I have been thinking about power > distribution system tradeoffs for large systems where multiple AC/DC power > supplies are used. Surveying 5 or 6 suppliers, picking an arbitrary 100W - > 200W range for comparison, I see in-rush current specs with a very wide > range (14A to 80A) and a bit of variation in the specified voltages. Some > like to specify at 200VAC, others at 230VAC -- all are auto-switching > universal input, so the datasheet numbers must be scaled to make an > equivalent comparison. > > The first one or two cycles are mostly to fill up the DC bus caps. Some > PFC implementations could increase the period of inrush to three to ten > cycles. That said, the peak for the inrush current is (at least for my > employer’s stuff) is well under 1mSec for one or two cycles. Auto-switch > designs are not same as ‘universal’ input. Some auto-switch units will also > have another inrush condition during transition from 120V to 230V input. In > any case, the inrush number is useless unless for the least favorable > normal operating condition, which is typically 230V. > > Targeting a 50-70% of full load rating for improved reliability seems > reasonable from reviewing qualification data, as well as past discussions > with two suppliers. That will in some cases increase the number of power > supplies in the system based on mounting location, ease of manufacture and > cabling for a large physical structure. Voltage drop on the DC output is > another parameter that affects power supply location. > > > > Reduced FL will not necessarily increase MTBF; and for many SMPS designs, > output load does not necessarily affect the peak inrush current, but can > affect the period of initial high input current. Input V and source Z are > the dominate factors for inrush, but for power supplies that have a > de-rating for some operating conditions, the 50% load can be an interesting > test condition. > > > I'd appreciate feedback about in-rush current limiting hardware at the > system level. I've seen power supply specifications with block diagrams > that identify in-rush limiting circuitry which I expect are mostly either > NTC's or planar surge resistors. At the system level, it looks like three > main options: a hybrid surge resistor/bypass relay module (European > suppliers(?)), a softstart controller (targets motor applications) and > switched outlet PDU's for data center applications. I think the hybrid > module is best for a largely capacitive inputs and these modules' > datasheets give a capacitive load rating (1500uF up to 10000uF from what > I've seen so far). Network access for the smart switched outlet PDU is > probably not an option for the system design. > > ‘System’ level inrush limiters could cause problems for some edge cases. > If input current rise or voltage rise goes non-monotonic, some SMPS designs > will not be happy. While NTCs are typical solutions to SMPS inrush > limiting, there are obvious problems where input power can be cycled after > the unit has reached operating temperature, and for efficiency. The common > solution is a relay across the input NTC, so the NTC never stays warm, and > less power and less heat. > > Have seen a few soft-start functions of control ICs that resulted in weird > poles and zeros. And some were indeterminate given certain input > conditions. So depends on the design and how used > > Inrush-limited PDUs can be problematic for both EMI problems and safety > hazards where the inrush limiting solution is not closely mapped to > characteristics of the particular power conversion equipment. > > There also look to be moderate cost differences by technology > type/application. > > Any good reference material beyond supplier datasheets and application > notes? I've done some searching this weekend and have seen one general > lighting reference with estimates for rectifier/PFC topologies of being > 30-100x of operating current for in-rush, which doesn't mate well with how > the circuit breakers are spec'd (10x to 30x for the millisecond range > in-rush transient). I've also seen a few data center-oriented papers and > quite a few pages/papers for inductive motor in-rush applications which is > not what I am considering. > > > > The Pressman book on SMPS design is recommended. Many component power > supply mfrs have published guides for the end-use equipment designer. > > > > There are special considerations for motor power ≥ ½ HP in both the way > things are connected per NEC, and for power supply design considerations. > > > > Branch circuits typically use CBs for current interrupt, which are less > affected by short-interval overloads. > > > > Any suppliers of preference worth evaluating for in-rush limiting for a > 12-16A operating current application with common AC/DC open/closed frame > supplies? > > Is the 50-70% FL de-rating for improved reliability a common design > target? > > > > FL ‘de-rating’ has little direct effect on published reliability numbers > for a SMPS. The typical and heart-breaking tragedy of pre-mature component > power supply death is typically from input surges or inadequate air-flow or > constant overload. > > > > Other design attributes that jump to the forefront for you? > > Look for stuff where the mfr’s design process includes HALT and SR322. > > Thanks for reading the whole way through and giving it some thought! > > > > Cheers, > > Adam > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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