Cell phone swichmode chargers safety and isolation? - Page 1
Forum section: electronics / beginners

author: 001
author_rank: Super Contributor
timestamp: November 05, 2018, 08:27:44 pm
content: HiCell phone USB swichmode chargers also have capasitor behind MAINS side and LO sideIs it necessary?Can I remove it to use it like my project power supply (isolated)?

author: ArthurDent
author_rank: Super Contributor
timestamp: November 05, 2018, 08:40:21 pm
content: You first have to ask yourself if the people who designed the charger would have added unnecessary parts to the charger that would increase the cost if they weren't actually necessary and/or required by safety certification agencies.As to you removing the capacitors, just don't. Either use a power supply/charger with a system ground connected through the power cord ground pin or use a properly designed bench supply with a 3 wire cord to ground it.

author: MiDi
author_rank: Frequent Contributor
timestamp: November 05, 2018, 08:41:18 pm
content: All SMPS I have seen so far have thiscapacitor.This cap does not affect safety or isolation, it is certified for this purpose.

author: 001
author_rank: Super Contributor
timestamp: November 05, 2018, 08:57:23 pm
content: Quote from: MiDi on November 05, 2018, 08:41:18 pmThank You!Can You say me how this capasitor works? What is his intedituse? Can I add ground lug to outlet and eliminate cap?I think what some AC current and about 80VAC can destroy my circuit if I try to connect it with my other grounded gear

author: ArthurDent
author_rank: Super Contributor
timestamp: November 05, 2018, 09:15:31 pm
content: Here is the way one person described it on line."Switched mode power supplies use what is known as a "flyback converter" to provide voltage conversion and galvanic isolation. A core component of this converter is a high frequency transformer.Practical transformers have some stray capacitance between primary and secondary windings. This capacitance interacts with the switching operation of the converter. If there is no other connection between input and output this will result in a high frequency voltage between the output and input.This is really bad from an EMC perspective. The cables from the power brick are now essentially acting as an antenna transmitting the high frequency generated by the switching process.To suppress the high frequency common mode it is necessary to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer. This effectively shorts out the high frequency and prevents it escaping from the device.When designing a class 2 (unearthed) PSU we have no choice but to connect these capacitors to the input "live" and/or "neutral". Since most of the world doesn't enforce polarity on unearthed sockets we have to assume that either or both of the "live" and "neutral" terminals may be at a significant voltage relative to earth and we usually end up with a symmetrical design as a "least bad option". That is why if you measure the output of a class 2 PSU relative to mains earth with a high impedance meter you will usually see around half the mains voltage.That means on a class 2 PSU we have a difficult tradeoff between safety and EMC. Making the capacitors bigger improves EMC but also results in higher "touch current" (the current that will flow through someone or something who touches the output of the PSU and mains earth). This tradeoff becomes more problematic as the PSU gets bigger (and hence the stray capacitance in the transformer gets bigger).On a class 1 (earthed) PSU we can use the mains earth as a barrier between input and output either by connecting the output to mains earth (as is common in desktop PC PSUs) or by using two capacitors, one from the output to mains earth and one from mains earth to the input (this is what most laptop power bricks do). This avoids the touch current problem while still providing a high frequency path to control EMC.Short circuit failure of these capacitors would be very bad. In a class 1 PSU failure of the capacitor between the mains supply and mains earth would mean a short to earth, (equivalent to a failure of "basic" insulation). This is bad but if the earthing system is functional it shouldn't be a major direct hazard to users. In a class 2 PSU a failure of the capacitor is much worse, it would mean a direct and serious safety hazard to the user (equivilent to a failure or "double" or "reinforced" insulation). To prevent hazards to the user the capacitors must be designed so that short circuit failure is very unlikely.So special capacitors are used for this purpose. These capacitors are known as "Y capacitors" (X capacitors on the other hand are used between mains live and mains neutral). There are two main subtypes of "Y capacitor", "Y1" and "Y2" (with Y1 being the higher rated type). In general Y1 capacitors are used in class 2 equipment while Y2 capacitors are used in class 1 equipment.So does that capacitor between the primary and secondary sides of the SMPS mean that the output is not isolated? I've seen lab supplies that can be connected in series to make double the voltage. How do they do that if it isn't isolated?Some power supplies have their outputs hard-connected to earth. Obviously you can't take a pair of power supplies that have the same output terminal hard-connected to earth and put them in series.Other power supplies only have capacitive coupling from the output to either the input or to mains earth. These can be connected in series since capacitors block DC."

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 05, 2018, 09:17:29 pm
content: 1) The purpose of that capacitor is to reduce the amount of noise (RFI) transmitted by the wall-wart charger.The charger was certified for all the dozens of legal requirements WITH that capacitor.If you remove that capacitor, you will likely be increasing the amount of trash thrown out by the charger.2) It is true that the presence of that capacitor will quite possibly create a path for unwanted voltage from the power mains.And, as you say, that is quite undesirable to allow that "sneak current" into sensitive loads.However, remember that those little wall-wart charger gadgets were designed for a "floating" load (your cell phone) which is connected to nothing else.3) Those little wall-wart chargers are NOT appropriate for circuits like yours which are sensitive to this leakage current through the capacitor.If you were to dissemble the little wall-wart charger and put it into a metal box with a proper 3-wire mains cord and plug, with the proper connection for the green-wire safety earth/ground that would eliminate the undesirable sneak-current.Note that even SOME wall-wart (or line-lump) chargers with proper 3-wire connection to the power mains (including the green-wire safety ground) do not connect the green-wire ground through to the DC output. This is especially a problem with portable computers when you try to connect them to a separate sound system, for example.

author: 001
author_rank: Super Contributor
timestamp: November 05, 2018, 09:35:01 pm
content: Quote from: Richard Crowley on November 05, 2018, 09:17:29 pmGreat answerInteresting thing: Capasitor will not work properly if I plug charger to outer isolation transformer, isnt it?So is no way to isolate it?

author: MiDi
author_rank: Frequent Contributor
timestamp: November 05, 2018, 10:04:48 pm
content: Quote from: 001 on November 05, 2018, 09:35:01 pmIf these wall warts would damage your equipment, then your euipment is not even ESD safe2 plug adapters will work correctly with isolation transformer, but they are already isolated. If not, they would not have certificationThe only problem is that this capacitor couples RF through - not good for sensitive measurements.If this is a concern, you have to go with linear PS, if your application is very sensitive you need a special shielded/guarded PS for this purpose.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 05, 2018, 10:15:15 pm
content: Quote from: 001 on November 05, 2018, 09:35:01 pmItISalready "isolated". But there are different meanings of "isolated".The wall-wart phone-chargers are "isolated" in terms of the SMPS radiating RF noise while connected to a floating load.But your application is NOT what the gadget was designed for and your definition of "isolated" is different than theirs.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 05, 2018, 10:22:17 pm
content: Go to YouTube and search for the channel of "BigClivedotcom"He has at least a dozen good videos of autopsy and analysis of various USB chargers.Some exemplary and good, and many terrible and downright dangerous.After viewing half a dozen of these, you may want to re-think your choice of power for your project.This is the most popular teardown of a terrible example:

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2018, 11:11:55 pm
content: There is a way to avoid the need for the Y capacitor, or at least minimise the leakage through it, without having an earth conductor. Add a screen between the primary and secondary of the flyback transformer and connect it directly or via a Y capacitor to the mains side. The coupling from the mains on the screen to the secondary, via the shield will be much less than a Y capacitor would cause and it will prevent the high switching frequency from being coupled to the secondary side.