Industrial SMPS input circuit - Page 1
Forum section: electronics / projects

author: Mad ID
author_rank: Regular Contributor
timestamp: October 29, 2014, 01:57:21 pm
content: I'm in a design process of an industrial system which has a buck converter at the input. My current design is as follows:1) 0.5A Slow blow fuse at the input (Input voltage 24V)2) Common mode filtering via X2Y capacitor. I'm note sure if a common mode choke is a better option?3) Bidirectional TVS diode with 28.2V minimum breakdown voltage (1mA current rating)4) Schottky reverse protection diode5) Resistor with two roles. First is to prevent the slow blow fuse from blowing when charging the 47uF capacitor. I have tested this multiple times and without some resistor in series slow blow fuses always blow. Alternative is to use a High I2T fuse which are less common. Second role is circuit protection.6) At least one big capacitor to absorb the surgeWould you do this differently? I'm note sure if unidirectional TVS can be used in this design, i.e. schottky diode can go before the unidirectional TVS. But in that case I presume the schottky will die when subject to reverse surge (e.q. -500V 8/20us pulse)?Thanks guys.

author: wraper
author_rank: Unknown
timestamp: October 29, 2014, 02:18:24 pm
content: Quote from: Mad ID on October 29, 2014, 01:57:21 pmDon't you confuse unidirectional and bidirectional? There is bidirectional in your schematic. If you using unidirectional in the same way, you are guaranteed to have small explosion on reverse polarity.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 29, 2014, 02:29:02 pm
content: Quote from: wraper on October 29, 2014, 02:18:24 pmIn my current design there is a bidirectional TVS as the symbol shows. What I'm wondering is if it's possible to use a unidirectional diode and make the circuit work simpler/better?

author: wraper
author_rank: Unknown
timestamp: October 29, 2014, 02:33:07 pm
content: Then I don't understand how schottky could go before unidirectional TVS as it will clamp smallest reverse voltage.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 29, 2014, 02:35:27 pm
content: Quote from: wraper on October 29, 2014, 02:33:07 pmMe neither, that's why I ask. I saw designs with schottky reverse polarity before TVS diode. I don't understand how it can survive negative surge in that case.

author: tszaboo
author_rank: Super Contributor
timestamp: October 29, 2014, 03:14:32 pm
content: You might need to add inductor and or a ferrite bead on the input, maybe common mode filter. I've seen very long cables...If the 47u alone blows the fuse, that is a bad sign. I would increase the fuse's capacity, because it should survive the inrush current without any resistors.And keep in mind it might be used in elevated temperatures.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 29, 2014, 03:21:03 pm
content: If you use an unipolar tvs, negative surges will forward trough the diode. While positive surges will reverse trough the tvs.You might want to put the resistor before the tvs, check the datasheets.You'll need filtering after your buck converter, put the chokes/ferrites there.It's also nice if there is a user serviceable fuse in industrial applications, such as typical 5x20 fast fuse (they are quite slow already), instead of a return-for-service fuse.

author: mux
author_rank: Regular Contributor
timestamp: October 29, 2014, 03:45:23 pm
content: Do you have board size and/or efficiency requirements? That makes a big difference on a design like this.For the best possible protection, all your protection mechanisms need to be orthogonal, i.e. they should all be very good at preventing one and no other failure modes. If board space is at a premium you may want to start combining protection mechanisms.For instance, on boards that I make that need some semblance of ruggedization, I put a PTC fuse (low current) or non-glass fuse (usually SMD ceramics) right on the input, right after that a unidirectional TVS or a MOV and antiparallel diode to clamp both overvoltage and reverse voltage events (and blow the fuse when that happens, thus isolating the circuit). After that, I put an LC filter that ensures any expected overload pulses don't propagate to the circuit in a destructive manner. This is not an ideal strategy, but it is very board space and energy efficient as you don't need a series diode.If this isn't important, you just want to do all the things:- Make sure power going in is being properly conditioned; reduce slew rates with LC filters- Make sure power going in is symmetrical, use common mode filters- Make sure your protection devices are situated in a place where they can actually protect your circuit. There is no use in putting a MOV or TVS in parallel with a 2200µF capacitor.- Make sure your circuit is always isolated in the case of an overload, that the isolation is sufficient (i.e. don't use 6V rated SMT fuses - they will have a chance to fail short in a 24V system)- Fuses are to prevent fire, not to prevent your circuit from being damaged. If ruggedization is necessary, you need an active overcurrent protection device in addition to a fuse.- Same goes for TVS/MOV overvoltage protection. These devices have very large differences between guaranteed minimum conduction voltage and maximum clamping voltage; make sure that either your circuit can handle this range of voltages OR add some way of sufficiently quickly turning off current actively, e.g. with an active load switch and voltage monitor IC- Make sure the traces on your board are sized to overload conditions, not to operational conditions. Otherwise your board may melt before the fuse does (mostly an issue for high-current designs).Forgot anything?

author: Mad ID
author_rank: Regular Contributor
timestamp: October 29, 2014, 06:39:25 pm
content: Quote from: NANDBlog on October 29, 2014, 03:14:32 pmI have a X2Y capacitor which should provide much better common mode filtering than the common mode choke. I will try to confirm this on my next visit to the EMC lab.Regarding the fuse and inrush, I agree with you, but.. 0.5A @ 24V = 12W. My design needs 5W. If I put 1A slow blow fuse it will still blow from inrush at 24V. Maybe 2A slow blow fuse could survive, but in that case 2*24V = 48W..and I need only 5W. It looks silly to me to use a 2A fuse just because of inrush when I only need 0.5A or less. That is why I have included a small series resistor. This design of course fails if one cannot allow for any voltage drop, i.e. high currents. I'm not sure if this is a good practice, that's why I ask

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2014, 11:48:05 pm
content: Filtering and protection requirements depend entirely upon the purpose:- What kind of equipment is this?Is it standalone equipment?Is it mounted inside an electrical panel?Is it expected to withstand faulty wiring, e.g. 240VAC (or more!) applied to the input (or other connections)?- What environmental conditions?Controlled temperature and humidity?Stuffy electrical panel?High particulates and contaminating fumes?Explosive environment (no kidding -- flammable fumes, etc.)?Electrical noise?Nuclear radiation?! (Alright, but hey, some things do have to deal with it...)- And under the range of conditions, how is it expected to perform?Short lifetime / continual maintenance cost / disposable?Long life (10khr?) under nominal conditions?Is it acceptable that it fail safely / passive / destructively / fireworks show when any of the above, uh, stimuli goes outside of safe limits?- Does the device, in turn, have to meet any environmental or safety requirements?Will people be touching it (SELV = safe extremely low voltage, usually grounded)?Can it radiate emissions, and how much?Much industrial equipment is FCC Part 18 compliant = ridiculously dirty. FCC class B requirements are very typical for complete [consumer / commercial] products. Will it be part of a system with an integral EMI filter, or..?I could go into detail about many of these, of course, but if you can narrow down a particular case, it'll probably be more useful. Or if just thinking about some of the goals is helpful, that's a good start too.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 02:06:56 am
content: And how does a "common mode filter" between power and ground .... return to ground? Shouldn't it be earthed?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2014, 02:23:44 am
content: ^ Which filters are used depends on the source of noise, what you're trying to do with it, what the load is (power lines or what), etc.And yeah, a common mode filter is used on both power and ground, not one at a time. Attempting to misuse such a component usually results in core saturation and severely degraded performance.Tim

author: a210210200
author_rank: Regular Contributor
timestamp: October 30, 2014, 02:41:26 am
content: 500mA max sounds like a very low level of current you could even just use off the shelf industrial power over Ethernet equipment and they can do 15W or more regulated, 3kV isolation, all nice and standardized too. You can get modules and converters too just don't use passive injectors those are not 802.3af and so on compliant. You get power, data, and isolation on both ends.But your application area is quite vague so it really depends on what your doing, the environment, and so on as others have said.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 30, 2014, 07:59:24 am
content: You can put in a bridge rectifier to have polarity free DC inputs.Maybe even low voltage AC input, if you're putting in the right capacitors.

author: tszaboo
author_rank: Super Contributor
timestamp: October 30, 2014, 10:25:25 am
content: Quote from: Mad ID on October 29, 2014, 06:39:25 pmI have to disagree with you. The X2Y (after looking up) is actually not a good practice to put on the power input .Why? Well, because first, you put it before the TVS protection, and even if you use the 500V device, end ESD event is easily 2-4KV, which can degrade or damage your capacitor.http://www.johansondielectrics.com/images/stories/surface-mount/x2y/JDI_X2Y_2014-10.pdfAlso, look at the "Measured Common Mode Rejection" diagram. At the frequencies an SMPS works, 100-3000Khz, the CM filter provides some 20dB more attenuation. Yes, they work better at GSM frequencies for example, and it might be a good idea to use them series with the CM filter.Just imagine that they put your device on a 10m cable, what will happen. Voltage drop on the cable, cable picking up noise, if there is communication, you will have ground potential differences for the transmitter/receiver.Now if that did not scare you enough, I worked with installation of 300m+ cables. It was cheaper to build a linear power supply for it, than filter all the crap from a switching one.Yes, a PTC sized to the actual current of the circuit is a good idea, but nothing can replace a good old UL recognized fuse. For industrial stuff it is not optional.And T3sl4co1l has good points. If you dont know the answers for it, ask, if they cannot tell you, build it like someones life depends on it.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 01:05:25 pm
content: Quote from: T3sl4co1l on October 30, 2014, 02:23:44 amIt's a capacitor, there's no core?Anyways, it's more like each end of the cap goes to Line and Neutral (ground for us electronics dudes), and the center pin goes to ground (earth).It's just an expensive capacitor the way it's used now.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 03:07:30 pm
content: Quote from: NANDBlog on October 30, 2014, 10:25:25 amBut my TVS is in parallel with the X2Y capacitor and hence it's safe..right?Regarding the attenuation, you are right. I will use a larger capacitor i.e. 1uF. The SMPS is not the only one which will cause common mode currents to flow in the cable, any digital logic will have an effect. That's why one of my design had peeks on the EMC test far away from the SMPS frequency (I doubt they were harmonics)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 03:12:47 pm
content: Quote from: Mad ID on October 30, 2014, 03:07:30 pmIt's safe but entirely useless.There is no common-mode filtering occurring here.This is how it's supposed to be connected.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 03:40:10 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 03:12:47 pmI appreciate your help and hope you will help some moreI only have 2 wires, + and - from DC input cable.How should I connect the middle terminal of the X2Y? In other words, what's the difference between "N" and ground symbol in your hand CAD?I do not have an earth input available and I don't have a metal case!Thanks.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 04:01:23 pm
content: Quote from: Mad ID on October 30, 2014, 03:40:10 pmI don't want to sound mean, but do you know what a common-mode current is?

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 04:06:02 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 04:01:23 pmYou don't sound meanFlowing in the same direction on the cable. In this case I want to reduce it because common mode current are No.1 reason for radiated emissions..that's what I know. I know how to use a common mode choke..but not sure if X2Y can replace it in this situation?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 04:11:07 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 04:01:23 pmHere's about as clear as I can be in Paint.The problem is that if you are talking about common mode noise, you have to be in a situation where it's a problem.You can have conducted (physical connections between boxes at different potentials, like in a factory) or radiated noise.The blue I load is how your current flows in a load, you can see that the current flows in a loop between your two conductors. One conductors flows, the other drains.Now common mode, the noise flows into both wires, and returns through a third path. This can be anything really, metal, or the air.So where is your circuit going to be used? Is it 2 feet away from the power supply in an enclosure?Then you don't need to worry IMO.I could be massively wrong though. But that's the way I see it.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 04:22:06 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 04:11:07 pmI can have 5 meter of cable from an AC/DC power supply to my system.A similar design I did (3 meters of power cable into an SMPS module inside metal box) failed the radiated emissions. A ferrite on the cable fixed the problem.Now, I don't want a ferrite but would like to filter the noise inside the module somehow..or reduce the "radiation efficiency". I how a common mode choke increases the common mode resistance and hence decreases the current.Can I achieve a similar effect with X2Y capacitor which is a lot smaller and cheaper, not to mention I don't have to watch for heating etc.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 04:36:31 pm
content: Quote from: Mad ID on October 30, 2014, 04:22:06 pmAh I see. It's a case by case issue. What is your module? Is it a metal box? It's gonna be a PCB?What was the source of the previous CM problem? Are you sure it was a CM problem? These system issues are hard to diagnose without seeing it in its entirety.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 05:05:05 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 04:36:31 pmIt is a PCB in a plastic enclosure.Previous CM problem was a PCB inside metal enclosure..the device failed the EMC radiated emissions. When we added a huge ferrite on the power cable close to the device it passed the emissions test.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: October 30, 2014, 06:39:59 pm
content: We normally use the following order:FuseMOVTVSNTC Thermistor (better efficiency than static resistor)Differential & common mode filteringRectifierThough that's for 90-264VAC input SMPS

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 06:43:48 pm
content: Quote from: Mad ID on October 30, 2014, 05:05:05 pmWhat else does the plastic enclosure go to? You need to identify that 3rd path the CM signal needs.If the CM problem is induced current from a radiated source, you can use twisted pair for your power cable.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 30, 2014, 06:47:23 pm
content: Quote from: Alex Eisenhut on October 30, 2014, 06:43:48 pmI will be mounted on a plastic enclosure for DIN rail. And the cabinet is grounded..the DIN rail with it.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 30, 2014, 06:51:20 pm
content: Quote from: Mad ID on October 30, 2014, 06:47:23 pmOh. Can you get one of those green DIN rail clips for earthing? Can you connect to that?This sounds industrial, DIN rails, 24VDC... Twisted pair power cable is not something I've come across in industrial settings.So you likely have two cabinets face to face on some machine, one has the PLC and power supply, the other side has the motor drives? Am I getting warm?

author: Mad ID
author_rank: Regular Contributor
timestamp: October 31, 2014, 12:15:10 am
content: Quote from: Alex Eisenhut on October 30, 2014, 06:51:20 pmI see where this is going. I don't think there will be a ground connection available. What I could do is use common mode choke which will function without ground connection (by reducing the common mode current).Anyway I found an excelent document which states that it's not possible to implement common mode filtering if you don't know the common mode current path as you say"UNDERSTANDING AND CONTROLLING COMMON-MODE EMISSIONS IN HIGH-POWER ELECTRONICS" by Henry W. Ott

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 31, 2014, 01:48:24 am
content: Quote from: Mad ID on October 31, 2014, 12:15:10 amGood, you'll find it's not that mysterious or complicated, it's just that each and every situation needs to be exhaustively described... then you show up like a hero with a capacitor or a common-mode choke and ta-dah!