usb charger schematic?

[bob]

"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.

 

[hamilton]

"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.

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

 

[Jamie]

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

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

Jamie

 

[hamilton]

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

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

 

[Spehro Pefhany]

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

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

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

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.

Large AC transformers pulling a current with nothing attached.

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

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

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

 

[[email protected]]

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

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.

 

[[email protected]]

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.

Which shows how capricious it all is.

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.

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

 

[legg]

"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.

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