Over current shutdown of inverter - Page 1
Forum section: electronics / projects

author: theoldwizard1
author_rank: Regular Contributor
timestamp: January 01, 2016, 05:58:34 pm
content: So I have been studying DC-AC inverter designs and watching quite a few tear down/repair videos. They all seem to use some kind of PWM controller ASIC driving multiple power MOSFETs which, in turn, drive 1 or more transformers. Frequently the MOSFETs are placed in parallel to handle the high current. When one fails, the load shifts to the others and wipes them all out. Fusing ahead of this is a waste(fuses are much too slow to react).I have often wondered why(besides cost)they don't use a shunt resistor to sense incoming current and shutdown the PWM controller. Or would this still not be "fast" enough ?Where several MOSFETs are used in parallel, but sized to accommodate one pf them failing (for a brief time) how would you detect that one of the MOSFETs has died ?

author: diyaudio
author_rank: Frequent Contributor
timestamp: January 01, 2016, 07:42:33 pm
content: Quote from: theoldwizard1 on January 01, 2016, 05:58:34 pmShunts have advantages and disadvantages, there are tons of papers on this subject (use google), current transformers are almost always are used for high current detection, but even they are not fast enough have limited bandwidth and suffer from ageing, short circuit conditions can last a new nano seconds (and slip through trip detection in a matter of nS), these intervals are short enough to destroy the die, bond wires and melt the silicon. and you already mentioned cost, decent high speed hall effect current sensors are too expensive to use in a design as the objective is to keep costs low.its very rare to find a high resistance or open circuit path in a mosfet after death by high current, by the time you see or smell the magic smoke the silicon has already melted and shorted, this happens in nano seconds.some designs may use and I have only seen this in international rectifier class-d amp chip like the IRS2092, uses a feature to sense current directly using the mosfet RDS(on) and obtaining the current sample from V(sample) = I^2/Rds(on), however the the layout and circuit need will need a tight layout, any failure in snubbing or inductive paths will cause huge di/ti and result in false tripping.I would look look at Ti they have some nice modern designs, in the inverter space

author: nctnico
author_rank: Super Contributor
timestamp: January 01, 2016, 08:16:41 pm
content: A lot depends on how far the inductors are from saturation. If there is a reasonable margin then the current won't ramp up very fast and hence you won't need very fast overcurrent protection. In a 750W converter design from which only a few got produced I used Rds-on current sensing and huge (in size) inductors so there is a lot of margin before things go wrong badly. In this design I did not need to replace the MOSFETs on the prototype during testing!

author: station240
author_rank: Unknown
timestamp: January 02, 2016, 01:40:21 am
content: Quote from: blueskull on January 01, 2016, 06:34:09 pmThe big inverters usually have a single IGBT module, rather than many parallel IGBT mosfets. Hence there isn't the issue of cascade failures due to short circuit.Also due to the high costs of the entire inverter, it's got better surge protection, temp sensors etc, so you don't get overload conditions in the first place.the 20Kva UPS I've been experimenting with, has 4 current transformers (hall effect) +/- input rails to boost inverter, inverter output, UPS output.Some of the modern 6 IGBT modules (3 phase H bridge) have inbuilt temp sensors on the die itself run to external pins.By contrast, the cheap inverters often don't even have a MOV on the output. So things like power surges kill the output stage.