450V rated Linear regulator suffers high voltage spike to its input - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: March 15, 2018, 06:22:31 am
content: Hello,We are using a 450V rated LR8 linear regulator in our offline product. If the product happens to get switched on at mains peak then the LR8 gets a voltage of 600V peak at its input (as in the scope shot attached).However, if we put a 100nF X2 capacitor across live/neutral, then the peak voltage applied to the LR8 is 500V. …However, the peak duration is longer even though its less voltage (as shown in attached scope shot).Which is less harmful to the LR8?LR8 Linear regulator datasheet:http://www.microchip.com/wwwproducts/en/LR8Do you think these overvoltages could cause long term damage to the LR8?

author: Delta
author_rank: Super Contributor
timestamp: March 15, 2018, 06:31:41 am
content: Remember you are dealing with a mains voltage of 292v RMS for periods of several minutes.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 15, 2018, 06:56:14 am
content: Quote from: treez on March 15, 2018, 06:22:31 amYes.This is clearly stated in any datasheet I know of:-------------------Stresses above those listed under “Maximum Ratings” may cause permanent damage to the device. This isa stress rating only and functional operation of the device at those or any other conditions above those indicated in theoperational listings of this specification is not implied. Exposure to maximum rating conditions for extended periodsmay affect device reliability.--------------------From my experience, modern devices have really very little margin from their nominal ratings to destruction. Older ones often had large margins, and some designers made use of that. Absolutely no more recommended with today stuff.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 15, 2018, 06:57:31 am
content: What output voltage are you going for? If it's for a low voltage bias supply, it would make sense to use a resistor to drop a lot of the voltage. And it would also make it a lot easier to add additional protection.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 15, 2018, 09:02:16 am
content: Quote from: treez on March 15, 2018, 06:22:31 amHow is that even a question???

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2018, 09:13:50 am
content: Add a series resistor, say 10k, and a transient suppression diode, such as the P6KE400, before the regulator. That should limit the input voltage to a safe level.https://www.diodes.com/assets/Datasheets/ds21502.pdf

author: dmills
author_rank: Super Contributor
timestamp: March 15, 2018, 11:38:17 am
content: Concur, add some series resistance, the ringing is probably cable inductance interacting with the input cap, something of a classic killer of switchers (But you usually see it with DC supplies) the tell being the frequency change when you add the 100nF. What inrush current are you measuring?One does not get reliability by running anywhere close to (never mind exceeding) abs max ratingswhich seems to be something you guys keep on doing, and then being surprised when you see the same old problems.73 Dan.

author: ocset
author_rank: Super Contributor
timestamp: March 15, 2018, 12:22:38 pm
content: Quote3V

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2018, 02:04:55 pm
content: Quote from: treez on March 15, 2018, 12:22:38 pmHow much current?You definitely need a series resistor, to reduce the power dissipation in the regulator, perhaps even a low pass filter, might help. Try 10k and 100nF.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2018, 07:11:17 pm
content: Quote from: Ice-Tea on March 15, 2018, 09:02:16 amI lowkey suspect treez is an ex-pat Russian mechanic forced into electronics for some other reason.Yuo see Ivan, it not bad if regulator overvoltage a little. Puor vodka on top, it fix.Tim

author: BravoV
author_rank: Super Contributor
timestamp: March 15, 2018, 07:38:21 pm
content: Quote from: treez on March 15, 2018, 06:22:31 amWill you ask the same question again if it was struck by lightning ?

author: Red Squirrel
author_rank: Super Contributor
timestamp: March 15, 2018, 08:26:49 pm
content: Quote from: BravoV on March 15, 2018, 07:38:21 pmDepends on the duty cycle of the lightning.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 15, 2018, 08:59:52 pm
content: Treez, take a look at Figure 1 and table 2 of this document, 450V is low for a semiconductor to be connected to mainshttps://library.e.abb.com/public/6f03cdd0f2264ff48f2992e62497dd5a/Voltage%20ratings%20of%20high%20power%20_%205SYA%202051NLay.pdf

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2018, 11:13:01 pm
content: Quote from: MasterTech on March 15, 2018, 08:59:52 pmCome on, that document is targeted at industrial controls, not the domestic environment. This is evident from the fact that the lowest voltage in the table is 400VAC.If you take a look at any piece of equipment, designed for use in the home or office on 230V, you'll find the semiconductors are usually rated to 400V. 450V is also common and I've seen the odd 500V here and there, but 400V is sufficient for most applications. MOVs and spark gaps are used to protect against higher voltage surges and generally do their job quite well.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 16, 2018, 02:06:11 am
content: Of course it is, but any *well designed* system with semiconductors connected directly to mains, even for 220V should follow those guidelines. Ligthings transmit across power transformers, and inductive loads are present in every homeDo a study for example on offline switched buck converters, these are eminently low power and aimed at home appliances (viper family, Ucc28880, topswitch, ncp112x....). Their mosfets are rated 600V and upwards and I guess there is a reason.400V does not even cover a short term increase of 10% in line voltage over the maximum of 264Vrms as defined in EN 50160.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 16, 2018, 07:46:49 am
content: Quote from: MasterTech on March 16, 2018, 02:06:11 amWhich is why you have MOVs and the like.QuoteYes, the reason is called 'reflected voltage'.QuoteNo? Because that would give a peak voltage of 372V

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 16, 2018, 08:26:56 am
content: Quote from: Ice-Tea on March 16, 2018, 07:46:49 amYes, they are so effective that I wonder why not install a couple of these at the point of entrance of every home and voila, surges and lightings are gone for ever and for every equipment inside the house /sarc. MOVs and the like can only take so much, so the higher rated, in voltage terms they are the better, which means that the semiconductors will have to cope with more. Unfortunately few have read the life expectancy data on these devices. I did write *well designed* system for a reason.QuoteBuck converters don't suffer reflected voltage, but again refer to the previous answer, the higher rated a mains connected semiconductor is, the better.QuoteNope, and I know reading correctly the standards is a bit difficult, the 10% applies over the 10% of the nominal 240V

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 16, 2018, 08:56:49 am
content: Quote from: MasterTech on March 16, 2018, 08:26:56 amIf you design regular home appliances like you would industrial control gear (like ABB) you won't make a penny, ever. In this case, "well designed" is "over designed". You could also design according to MIL specs or space specs. Granted, the result might be better or more reliable but nobody will ever buy it. The art of electronics is not so much to design something that will never ever break and has the best possible specs imaginable. It's also about designing something that makes economical sense. It's perfectly possible to design a system that will handle surges and has a 450V semiconQuoteSure. Bring on the 2kV semiconductor. And what does a buck have to do with this?QuoteGee, thanks. Guess I should take those reading comprehension classes again. This is what the spec says:Under normal operating conditions the voltage variation should not exceed ± 10%: Overall experience has shown, that sustainedvoltage deviations of more than ±10% over a longer period of time are extremely unlikely, although they could theoretically be within the given statistical limits of 2.3.2. Therefore, in accordance with relevant product standards and application of IEC 60038 end users’ appliances are usually designed to tolerate supply terminal voltages of ±10% around the nominal system voltage, which is sufficient to cover an overwhelming majority of supply conditions. It is hence neither technically nor economically viable to generally give appliances the ability to handle supply terminal voltage tolerances broader than ±10%. If, in single cases, evidence is given, that the magnitude of the supply voltage could depart beyond this limit for a longer period of time, additional measures are to be taken in cooperation with the local network operator. The same applies in cases, where specific appliances have an increased sensitivity with respect to voltage variations. Situations like those arising from faults or voltage interruptions, the circumstances of which are eyond the reasonable control of the parties, are excluded.Must be me, but I can't find the 10% on top of the 10%. Again: maybe just me. Even then, the 450V device Microchip designedspecificallyto operate ofline would pass that requirement.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 16, 2018, 09:23:01 am
content: Depending on your input range you'll be dropping at least 150v across the regulator, so dropping a significant part of this across a resistor isn't going to hurt, will spread out the heat and reduce stress on the regulator.If you don't need universal input, so your input range is smaller, you may be able to lose the regulator and use a simple resistor+zener or shunt regulator.

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2018, 09:37:23 am
content: Quote from: Ice-Tea on March 16, 2018, 08:56:49 amI agree. It also wouldn't surprise me if industrial installations are more prone to higher surges, because the cables will be thicker and therefore lower impedance.Nearly every PC PSU I've seen has 400V rectifier diodes, a 400V electrolytic and 400V MOSFETs and surprisingly very few fail. MOVS and gas discharge tubes help, but semiconductors will also go into avalanche breakdown, absorbing the spike and resume normal operation, if the spike doesn't contain enough energy to damage them. I wouldn't be surprised if the rectifier diodes inside the PC I'm using at the moment have absorbed a few high voltage transients, yet still work perfectly.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 16, 2018, 10:36:39 am
content: Quote from: Hero999 on March 16, 2018, 09:37:23 amUsually, it's the rather large electrolytic capacitor that absorbs the transient energy. First, the transients energy must put enough charge into the cap to let the voltage rise, this takes care of a lot of transients, and second, they act like an zener diode if you exceed the rated voltage by some 10%.So the voltage across the cap is limited to maybe 440V by the capacitor itself, that helps to protect the semiconductors from too high a voltage. If you put enough energy into the transient, the rectifier diodes fail due to high single pulse current, and the electrolytic may vent. They do vent in a spectacular way if the energy is high enough.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 16, 2018, 04:51:02 pm
content: Google "overvoltage category" and check the needed impulse withstand voltage for 230 V rated appliancies. Electrolytic capacitor are quite slow and are almost transparent to fast transients. The EMI filter at the PSU's input reduces emissions and increases immunity.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2018, 02:09:51 pm
content: Quote from: treez on March 15, 2018, 06:22:31 amYes, an input protection circuit is going to be required. I would use some combination of a passive series and shunt protection network.If continuous faults are possible, then I would use a high voltage transistor cascode which can also disconnect the input. Foldback current limiting would be a good idea also. Of course if you implement all of this, then the LR8 may be superfluous.

author: fcb
author_rank: Super Contributor
timestamp: March 17, 2018, 02:27:03 pm
content: Cool - I was not aware of the LR8.*looks for a project to use it in*

author: ocset
author_rank: Super Contributor
timestamp: March 20, 2018, 06:34:41 am
content: Thanks, We cannot add anything on the PCB itself , as there's no room.We can only add stuff to the live and neutral wires that connect to the product. –Hence the X2 capacitor we added, as described in the top post.The "500v peak" scope shot in the top post was with a 100nF X2 capacitor connected between live and neutral. There was also 25uH (10 metres) of coiled up mains cable ahead of that so that we could simulate line inductance.When we changed the coiled up cable to 100metres of (coiled up) length, then the voltage peak goes up to 550V (as attached)…..youch! (100m of mains cable had an inductance of 73uH )Obviously, the more “line inductance” we add, the higher the peak voltage to the LR8 is going to go….youch again!So, the attached 550v peak scope shot is with 73uH of mains cable and then a 100nf x2 capacitor, then the productWe are wondering what is the typical maximum line inductance in the installation that our lamp will see?It’s a lamp that gets put in pub gardens, outdoors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2018, 06:53:48 am
content: Put (22R + 220nF) in parallel.Tim

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2018, 09:37:57 am
content: Quote from: treez on March 20, 2018, 06:34:41 amThat's not a very accurate test. It will probably show worse high voltage spike, than what you'd get in real life.Were both the live and neutral coiled up together? It will have more inductance if the coil is just on the live or neutral, than both conductors inside the same sheath.In any case, a coil of cable will have a lot more inductance, than a straight length of cable.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 20, 2018, 10:48:44 am
content: The actual inductance is not only depending on the length of tha cable, but also on the loop size. Two separate wires will have higher inductance than a cable where the individual wires are kept together by the insulating jacket.Sine shaped, damped oscillation at power on. Almost the same will happen with a DC step on the input with a ceramic capacitor. (Linear has an interesting appnote on this, google MLCC Linear). Compare it to a stepup regulator, where the wire is the coil and the capacitor is the switch transistor. At power on the capacitor is empty, which equals a closed switch. As the voltage over the capacitor reaches the input voltage, the switch "opens". V=L×di/dtYou would like to have a capacitor between L and N for filtering and protection of your regulator. To change the di/dt you can add a series resistor between L and the filter capacitor. The regulator sits in parallel with the C, but the 230 Vac sees an RC. The addition of a series resistor was mentioned earlier in this thread.