Warning - DC-DC covertors - watch those minimum loads! - Page 1
Forum section: electronics / projects

author: splin
author_rank: Frequent Contributor
timestamp: June 22, 2015, 09:01:08 pm
content: Just been playing with a RECOM REC3.5-4815SRW/ER10/A, a 3.5W isolated convertor, 36V to 72V i/p, 15V o/p. The minimum specified minimum load is 20% but I only had 10%. As a result I expected that its regulation might be a bit out of spec, but it behaved fine, maintaining just over 15V over a wide input voltage range.Until I wound the input voltage down to 0V; at about 8V the output suddenly jumped to just over 30V!!! Repeating the test with 20% load, the ouput topped out at about 16.2V.Ok, 8V is out of spec, but lots of equipment with big power supply caps have quite slow decay rates when the power is switched off, so it wouldn't be unusual for a nominal 48V i/p to be in the 8V region for several hundreds of milli-seconds. Quite long enough to fry everything on the isolated 15V rail! And can you guarantee the 20% minimum load when the equipment is powering down and the circuit is likely to be in its reset state?So if you use one of these be careful - either provide a crowbar, be very careful with the minimum load or use the Ctrl input to the convertor to shut it off during power down (but is that guaranteed to work as the input voltage is well out of spec at 8V?) [EDIT: not an option in my case as I just realized mine don't have the Ctrl option].I tried another sample just to check it wasn't an isolated fault; it wasn't. I don't know if this is typical behaviour of such devices but I wouldn't be very proud of myself if I was responsible for this shockingly bad design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2015, 10:25:57 pm
content: DC-DC modules range from okay to crappy... minimum loads and destruction from momentary shorts are par for the course.I'd think the RECOMs are generally pretty good, but that's given you look at the bigger ones. Their small ones for instance are just regular two transistor choppers, like everyone else's, and die at the drop of a hat.3W might vary with series (that's an odd symptom, though, going nuts at least voltage). I hadn't noticed any problems with the 3W+ "medical grade" ones.I do have a question: what the heck are you using that's so sensitive that it'll blow for ~mA above 15.0V? Even CD4000 logic will handle a couple volts without damage (varies by part, oddly enough; some are rated up to 18V). Maybe not 30V (at unstated current) though.Unless you have a surge protection hazard to deal with*, an 18V zener diode should cover for small excursions, without wasting quiescent current as well.(*And if you did, 15V already being in the 'danger zone' is a non-starter for having enough headroom to handle surges. So you'd want to consider more changes, like better parts or lower system voltage.)Tim

author: splin
author_rank: Frequent Contributor
timestamp: June 23, 2015, 03:28:47 pm
content: It turns out these aren't quite as bad as I first thought - I just checked my results with a more controlled setup and it seems my 10% load couldn't have been connected properly.The spec states the minimum load is 20 % full load - but doesn't say if that is rated output power or rated load current.I/P voltage O/P voltage O/p current % Rated o/p current % Rated o/p power6.3V 30.0V 0mA 0% 0%7.1V 27.4V 11.6mA 5% 9.1%7.1V 24.1V 23.3mA 10% 16.0%7.3V 20.6V 34.0mA 14.6% 20.0%7.4V 17.5V 46.6mA 20% 23.3%Quote from: T3sl4co1l on June 22, 2015, 10:25:57 pmI'm sure they are usually - it seems this is one that got away. I wouldn't be at all surprised by cheap Chinese convertors behaving badly but didn't expect this from a reputable company.QuoteNot that surprising I'd have thought - there's obviously plenty of voltage and power to keep the oscillator and switching transistors going but the control circuitry has obviously lost it at 7V - it is designed to operate from 36V to 72V after all. The feedback circuitry on the o/p side would still be working fine though. It should be designed so it doesn't behave this way of course.QuoteYou'd hope they'd have been tested to death!QuoteI'm not using them at the moment I just got them and was trying them out. But I'm pretty sure that there are plenty of devices that can be damaged or destroyed by a 50 to 100% over-voltage without necessarily being accompanied by large currents. I'm no expert on the susceptibility of semiconductor devices to over-voltage stress but I wouldn't be happy applying 25 to 30V to the 15V rail of any of my equipment. Of course if you expect such over-voltages then you can design accordingly or add clamping or crowbar protection.I just wanted to warn others of the potentially damaging over-voltage behaviour of these convertors and possibly other similar devices. I expected some loss of regulation when operating outside normal conditions but 30V really shocked me - metaphorically speaking!

author: amyk
author_rank: Super Contributor
timestamp: June 24, 2015, 03:09:12 am
content: QuoteA 300 ohm 1W resistor across the output will do.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 24, 2015, 08:55:12 am
content: It's not just switched mode supplies you need to worry about. I once fried a rather expensive and difficult-to-obtain DSP with a linear regulator that had a minimum load requirement. Damn DSP came up in a low power mode and drew next to no current from its core voltage supply, which rose above the abs max for the device and killed it.

author: Delta
author_rank: Super Contributor
timestamp: June 24, 2015, 10:24:48 am
content: Quote from: splin on June 23, 2015, 03:28:47 pmWhat were you using as a load, and where were you measuring voltage? (across the load itself or at the converter's output terminals?)

author: Kalvin
author_rank: Super Contributor
timestamp: June 24, 2015, 10:35:21 am
content: I typically hook a low-dropout linear regulator after the DC/DC-converter which will reduce the output ripple but also would protect the circuitry connected from the possible overvoltage scenarios. Instead of regulator one may consider transient voltage supressor or hefty zener to clamp any voltage spikes.

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2015, 10:38:51 am
content: You think that's bad.Watch out for these things.With no load the 72v output is actually 170V(not a problem in some applications but definitely something you want to be aware of)http://nz.mouser.com/ProductDetail/Murata-Power-Solutions/NMT0572SC/?qs=sGAEpiMZZMvGsmoEFRKS8Nx4sdQZDGhImbKCgCYdOkc%3d

author: splin
author_rank: Frequent Contributor
timestamp: June 24, 2015, 09:53:15 pm
content: Quote from: Delta on June 24, 2015, 10:24:48 amThe load was a decade resistor. The voltage was measured at the ends of two 300mm leads connected to the convertor terminals.

author: splin
author_rank: Frequent Contributor
timestamp: June 24, 2015, 10:18:34 pm
content: Quote from: Psi on June 24, 2015, 10:38:51 am170V? Yikes! And you think that by paying the extra cost of a ready made module rather than rolling your own you'd avoid such problems. Really how much would it cost them to provide an under-voltage lockout - a zener, a transistor or two and a few resistors for 3 or 4 cents? Admittedly over-voltage protection for multiple outputs would be a bit more expensive, but such convertors are more expensive anyway.At the very least there should be a warning in the datasheet - 170V is potentially quite a safety hazard, and a load becoming disconnected is hardly an unusual failure mode.The moral is to never take anything for granted.

author: DanielS
author_rank: Frequent Contributor
timestamp: June 25, 2015, 04:52:23 am
content: Quote from: splin on June 24, 2015, 10:18:34 pmEven simpler would be to use a PWM controller that has built-in lock-out and soft-start.I haven't checked their datasheets but from how poorly regulated they are, I am guessing those isolated converters are non-regulated oscillators rather than PWM-regulated DC-DC converters.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 25, 2015, 06:02:04 am
content: Well, they have these graphs:And the table only shows the output voltage with a 100 mW load, and a 3W load.Typically, these black magic parts areunregulatedDC-DC magnetics.

author: ftransform
author_rank: Frequent Contributor
timestamp: June 25, 2015, 07:13:04 am
content: I would imagine a 300 ohm 1 watt resistor is a better solution then a zener right, since the diode will take a beating every time it starts up pretty much, especially if there are high voltage spikes. ? then again 0.75 watt of dissipation on a 15v rail for nothing is kinda lame.How do you handle fairly high power ones (15+ W )? combination of load power resistor, zener and linear regulator adds a fair bit of price.. I need to redesign something now, never considered it.Is this also the case for non isolated ones? (i.e. low dissipation LM317 replacement)?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2015, 02:19:50 pm
content: Quote from: splin on June 24, 2015, 10:18:34 pmNow, now;1. Roughly speaking, 170V is about as high above 72V as 30V+ is above 15V. It's no worse than the other, electrically speaking.2. These are almost certainly self-excited oscillator type designs, i.e., unregulated, and the output follows the input voltage. (Well, the OP's was regulated, if misbehaving.)3. 72V is above SELV to begin with, so it's not making any additional hazard. (I forget if the hazard level is supposed to increment through there, though.)4. It's still at low current, probably enough that you could drag it back down to 72V with your fingers (maybe with some spit required). You'd feel it, but you'd be feeling 72V.Tim