AC to DC Adapters

Thread Starter

sg253545

Joined Dec 23, 2020
8
Hello,
I am trying to build an AC Power Adapter and have a few questions regarding its inner workings. At its simplest form I understand that the adapter is simply composed of a step-down transformer, bridge rectifier, and a voltage regulator or filter capacitor, and load resistor. However, I have seen the teardown of Apple and Samsung chargers' adapters and I don't understand why they are very complex. In particular, I am trying to understand the 110-240V input Voltage, 0.5A input current adapter with an output voltage of 5V and output current of 2A. I would truly appreciate it if someone could help me understand the Apple or Samsung or Google AC Adapter with a visual representation of a circuit schematic perhaps.

Thanks
 

MrChips

Joined Oct 2, 2009
23,531
Welcome to AAC!

AC Power Adapters come in different designs for different purposes and applications.

1) In the simplest case, an AC adapter is simply a step down transformer to produce a lower AC voltage than what the wall outlet supplies.

2) The next level up is to add a bridge rectifier and smoothing capacitor to supply unregulated DC voltage. These will rarely have a voltage regulator.

3) The next level is a transformerless regulated supply that is used for charging smart phones, computer laptops, etc. The output voltage and maximum supply current will depend on the specific device being charged.

Also there is your common 5V USB charging adapter which is a specific "intelligent" version of the above (3). It has built-in charge controller that requires USB interface and identification in order to set the charge current.

These circuits are not simple and the description will depend on actual circuit design.
The basic concept will follow along these building blocks:

1608742201615.png

Reference: https://www.monolithicpower.com/en/ac-dc-power-supply-basics

1608742110257.png
 

dl324

Joined Mar 30, 2015
12,871
Welcome to AAC!
I have seen the teardown of Apple and Samsung chargers' adapters and I don't understand why they are very complex. In particular, I am trying to understand the 110-240V input Voltage, 0.5A input current adapter with an output voltage of 5V and output current of 2A.
They're switching regulators and likely don't have transformers. They can accept a wide range of input voltages because power dissipation in the pass transistor isn't as much of an issue as it would be in a linear design.
 

Audioguru again

Joined Oct 21, 2019
3,193
Small lightweight AC to DC adapters use a high frequency oscillator driving a small high frequency transformer. They operate on North American 120VAC or Europe's 240VAC so they have many small parts in their circuit.
 

dl324

Joined Mar 30, 2015
12,871
Was he granted the Darwin Award?
When the issue was pointed out to him, he said line voltage didn't bother him.

I worked with another guy that had a heatsink connected to 50V. When he saw me grounding myself on it before handling some charge coupled device prototypes, he asked if it gave me a shock. It never bothered me.
 

ronsimpson

Joined Oct 7, 2019
1,230
I am trying to build an AC Power Adapter .......... At its simplest form
This one is just a transformer with no regulation. If the input increases a little the output will also increase. Will not work from 110 to 220!
1608745060102.png
I am trying to understand the 110-240V input Voltage
In a switching supply the output voltage is monitored and kept constant even if the input voltage changes form 100 to 230V.
1608745147710.png
 

MrChips

Joined Oct 2, 2009
23,531
I am referring to the line frequency transformer between AC mains and the first rectification stage, not the buck-boost high frequency output transformer.
 

AnalogKid

Joined Aug 1, 2013
9,256
They're switching regulators and likely don't have transformers.
Really? I don't think an offline switching power supply with full international safety certifications does not use a transformer as the isolation barrier between primary and secondary. AND, stepping 350 Vpeak down to 5 V is a long way to go with just a buck regulator. That's less than 1.5% duty cycle.

ak
 

dl324

Joined Mar 30, 2015
12,871
Last edited:

dl324

Joined Mar 30, 2015
12,871
Last edited:

Thread Starter

sg253545

Joined Dec 23, 2020
8
Welcome to AAC!

AC Power Adapters come in different designs for different purposes and applications.

1) In the simplest case, an AC adapter is simply a step down transformer to produce a lower AC voltage than what the wall outlet supplies.

2) The next level up is to add a bridge rectifier and smoothing capacitor to supply unregulated DC voltage. These will rarely have a voltage regulator.

3) The next level is a transformerless regulated supply that is used for charging smart phones, computer laptops, etc. The output voltage and maximum supply current will depend on the specific device being charged.

Also there is your common 5V USB charging adapter which is a specific "intelligent" version of the above (3). It has built-in charge controller that requires USB interface and identification in order to set the charge current.

These circuits are not simple and the description will depend on actual circuit design.
The basic concept will follow along these building blocks:

View attachment 225725

Reference: https://www.monolithicpower.com/en/ac-dc-power-supply-basics

View attachment 225724
Wouldn't we need a step-down transformer before the input rectifier/filter?
 

Thread Starter

sg253545

Joined Dec 23, 2020
8

dl324

Joined Mar 30, 2015
12,871
Yeah saw this one, but a fake isn't going to get me any closer to determining this circuit. I underestimated the complexity in such a small device.
If you're trying to get something as small as the commercially available adapters, you can't use the step down transformer you referenced in your original post.

Unless this is for the purpose of learning how to design one, a make vs buy analysis will show you that buying will be cheaper.
 

Thread Starter

sg253545

Joined Dec 23, 2020
8
In a switching supply the output voltage is monitored and kept constant even if the input voltage changes form 100 to 230V.
So is it safe to say that most common smart phone charging adapters these days use switching voltage regulators? Because I was reading up that generally linear regulators do the trick, however personally I was thinking a switching buck (step-down) regulator is a better fit since the linear regulators cause greater heat dissipation which can be harmful. And then to filter the noise generated from the switching, I could probably use a capacitor filter or a linear regulator. However, I wanted to compare my thoughts with most common charging adapters to be sure of things.
 
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