How do switching power supplies fail? - Page 1
Forum section: electronics / projects

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: August 06, 2015, 09:31:15 am
content: Hi everyoneNo i am not asking how they fail because i know that there are many causes but what i want to know is when they fail do they have a large voltage spike. I want to know this because i am using a very battered atx power supply that is running something that doesn't like sudden voltage spikes.Thanks

author: con-f-use
author_rank: Unknown
timestamp: August 06, 2015, 09:52:35 am
content: Then you should protect it with TVS diodes and/or mov(s) and/or PTC(s) and/or an electronic fuse. In principle there's an inductor and probably high input voltages involved so there can be voltage spikes, when the diode fails or the switching transistor goes short.

author: Kjelt
author_rank: Super Contributor
timestamp: August 06, 2015, 10:00:04 am
content: use a crowbar circuit at V+1V or something safe, to blow a fuse to protect the secondary circuit.

author: wraper
author_rank: Unknown
timestamp: August 06, 2015, 10:03:05 am
content: Depends on PSU, some of the worst ones can output 2x+ of the rated voltage when failing. If talking about ATX PSUs, JNC were famous about destroying all of the PC together with itself. Good PSUs usually have OVP (over voltage protection), if output voltages exceed limits, PSU will shut down.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 06, 2015, 10:47:01 am
content: Quote from: rickselectricalprojects on August 06, 2015, 09:31:15 amSwitch failure on the primary side of an offline SMPS inevitably results in a voltage spike on the secondary side because too many volt*seconds get applied to the transformer primary (or, if you prefer, too many amp*turns; same thing, just expressed differently).Depending on the amount of secondary side filtering, a traditional crowbar circuit (ie - using a thyristor [aka SCR]) will likely be way too slow to clamp the sharp spike that results from primary side switch failure. Conversely, a TVS or zener diode will almost certainly be fast enough, but the TVS will have a wide range of clamping voltage while the zener will exhibit a rather high dynamic resistance during conduction that may allow the spike voltage to go too high, anyway (and zeners tend to have much lower energy ratings per unit area than any other semiconductor device). Generally speaking, some combination of more filtering on the secondary plus a couple of TVS diodes with small ballasting resistors (to limit clamping current and therefore make clamping voltage more predictable) is sufficient to protect most loads.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 06, 2015, 06:55:56 pm
content: The most common failure mode for SMPS is the output caps' ESR going up and this can be fairly easily mitigated by replacing the existing caps with higher quality ones if applicable and using additional caps on top of those.For most failures on the primary side, adequate filtering on the output should eliminate most of whatever surge the primary might send.The most dangerous failure mode would be the feedback loop going open which would lead to outputs going as high as the transformer turn ratio will allow for a forward design or however high is necessary to dissipate the energy being pumped into the transformer for flyback designs. If you want your delicate analog bits to survive that, you will likely need an LDO to eat the voltage rise until something shuts down the PSU.