usb charger schematic?

"Wall wart" charger used to be large because they contain an AC transformer.

Recently, cellphone and usb chargers have become smaller and lighter than a typical transformer, yet they usually work with world voltages (110 to 240). How do they do it?

I opened up a cheap no brand usb charger and found some discrete components (transistors, capacitors, resistors, ...), and a tiny transformer. I guess the AC input is first reduced to DC (perhaps with a zener diode to accomodate variable input voltages) to power a high frequency oscillator (much higher than 60Hz). This then passes through an isolation transformer (because of high frequency it is much smaller than an AC transformer), and then rectified and regulated into 5V DC output.

The charger for ipad2 is even smaller than those no brand usb charger and outputs 2A. Does it use a similiar design? I don't want to break it open to look (there is no screws on it for easy disassembly).

I googled for such schematic on the web and found some circuits that do not contain any transformer. This means the output is not really isolated from the AC input, which I think is a no-no for this type of application.

Reply to
bob
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No zener - which would dissipate too much power for that kind of compact package.

Search on switching power regulators - the variable input voltages are accomodated by varying the mark-to-space ratio of the chopped DC that goes into the - much smaller, and much higher frequency - AC transformer that does both the isolation and the voltage reduction.

r
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It is still an AC transformer - but the number of ampere turns its needs to accomodate is reduced in proportion to the switching frequency. Regular AC transformers work at 50 of 60Hz. The transformers in switching regulators work at anything from a few hundred kiloherz to a few MHz.

to

If you can't afford to break it open, you can't know for sure, but it seems very likely that it works the same way, if perhaps at an even higher frequency.

ot

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Correct.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Some 10-15 years ago the US instituted new power transformer rules for manufacturers.

All wall warts will use switching technology to reduce the "phantom load" on the nations power grid.

Large AC transformers pulling a current with nothing attached.

I don't remember the specification but it stated all remaining stock of old style wall warts could be used till depleated, any new wall warts would have to use the new switching technology.

The date of compliance was pushed up for companies that had lots of stock. That date has passed years ago and now all WW should look like what you now have.

Does anyone remember what the law was that changed this ?

hamilton

Reply to
hamilton

You better remind them then, because I still can buy newly manufactured wall warts with 50/60hz Xformers in them.

Jamie

Reply to
Jamie

I am sure you can.

The manufacture I was working for at the time had 10,000+ transformer based wall wart in stock.

We had to contact the government to say we had so much stock that we could not meet the deadline.

So I am not sure what the law stated, but I know we had a hard time getting a waver for compliance.

hamilton

Reply to
hamilton

The great thing about laws is that if you have enough of them, it's impossible to enforce many of them.

John

Reply to
John Larkin

The "US" as in the Federal government?- I don't think so.

Again, I don't think. I recall efficiency standards for power supplies that did not call out any specific technology. AFAIR, the only thing ruled out (indirectly) would be those using 60Hz transformers using the bottom-of-barrel crap steel laminations, and maybe some SMPS ones too. ood (smaller) transformers are fine. 0.5W standby loss for single digit max power outputs, IIRC.

Magnetizing current isn't so much of an issue, but power losses are.

Look at the California Energy Commission. But I suspect they can really only influence stuff being sold at physical retail locations in that state (which is more than enough to change the actions of many high volume manufacturers such as Dlink etc. because they don't want Best City Staples or whatever to not have their products on the shelf everywhere ).

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

But they don't have to. They can pick and choose which ones they want to enforce. You have to defend against all of them, though.

Reply to
krw

Not really. Just don't attract too much attention, and be friendly to The Powers That Be if they show up, and you won't be harassed much. It's not the theoretical ideal, but in practice it works.

If we occasionally get nabbed on some technicality (as we recently did, for having a few gallons of "hazardous" latex paint in the building) you just accommodate them (ie, pay their silly fee) and get on with life. It's not like Zimbabwe, for Pete's sake.

John

Reply to
John Larkin

Which shows how capricious it all is.

If you pay the "protection", even the mafia can be quite friendly.

Reply to
krw

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To a good first approximation, the governemnt is just the largest protection racket in town. If you want to work out were their priorities are, think protection racket, and you'll do a lot better than you will by reading the constitution.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

It's not hard to find references to single chip isolated converters below 5W. Power Integrations LNKxxx or ON Semi NCP1030 or similar all integrate the power switch.

At this power level, self oscillating flybacks are possible, even with bipolar transistors in SOT23 - commodities thanks to the CCFL lighting manufacturing volumes.

Branded chargers have only to function with associated hardware. Label output ratings are potentially meaningless in that case. A true USB charger needs a bit of intelligence built in to fully comply to the USB standard.

There's no preventing the use of 'USB' on labeling and there's no reason for dumb parts not to perform as required, where properly specified and tested. You can often get what you pay for.

Non-isolated parts might serve for automotive apps, but are unlikely to fly off-line. A transformer is not the only method of isolation provided in the industry, just the most versatile.

RL

Reply to
legg

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