USB Power supply.

[markm6164]

Hi i need some help understanding a circuit on a usb power supply i bought. Its only used to pull apart and learn how it works. Anyways below is some pictures of the board and i am confused why i don't measure a voltage on the output of the transformer as in the picture below. But there is positive voltage after the diodes. Maybe i'm measuring it wrong or there is something simple i have missed. Anyhelp would be great.

Here is a crude diagram i made in ms paint to show how i think the circuit goes after the transformer.

Also just so you know while testing this circuit i do cover all the mains side of the board and insulate myself from it. I am very aware of the dangers of the high side and i am careful.

 

[(*steve*)]

You've shown what is essentially the low voltage side of the SMPS, and this is also the boring stuff.

Essentially it rectifies the output (only half wave rectification you'll note) and filters it with a few relatively low capacitance (but also low ESR) capacitors. The resistors 11 to 14 provide a bias voltage on the data pins which cause the device attached (possibly an iPhone or iPad etc) to charge at a current appropriate for the charger.

What you don't show is a connection, often via a zener diode and a resistor to an optocoupler that is used to provide feedback on the output voltage.

Also unseen is the high voltage side which boils down to 5 main pieces:

1. Filtering (to prevent noise getting OUT!)
2. Rectification and filtering of the mains to get a 400VDC rail (or thereabouts)
3. An oscillator to create pulses used to switch the...
4. Power transistor (typically a MOSFET) to switch the DC on and off to the transformer
5. Some feedback to adjust (3) based on the output voltage.

 

[Theodor Iacob]

Identify the markings on IC1

Try and identify the markings on IC1 and look it up on the web. Find its datasheet and an application diagram. Learn how it works from there first. Then try to identify the corresponding parts in your circuit.
Do this without power applied -- you can learn a lot this way.

Be careful when measuring voltages when powered from AC!
Learn about working with high voltages. Protect yourself, use an isolation transformer.
You cannot easily measure the voltage at the output of the transformer because it is an AC signal at 50kHz to 300kHz. Many DVMs don't work well at these ranges. Besides that, it is AC not DC. You need a scope.
Good luck!

 

[markm6164]

Try and identify the markings on IC1 and look it up on the web. Find its datasheet and an application diagram. Learn how it works from there first. Then try to identify the corresponding parts in your circuit.
Do this without power applied -- you can learn a lot this way.

Be careful when measuring voltages when powered from AC!
Learn about working with high voltages. Protect yourself, use an isolation transformer.
You cannot easily measure the voltage at the output of the transformer because it is an AC signal at 50kHz to 300kHz. Many DVMs don't work well at these ranges. Besides that, it is AC not DC. You need a scope.
Good luck!

Thanks for your reply. I have googled the codes on the ic and can't find a related datasheet. Here is the markings... B32020 0B2535CP.

Also last night i did scope the output from the transformer and it was very intresting. I soldered some small wires onto the output of the transformer and connected the scope, all not being powered of course. I insulated the high volt stuff and kept hands off at all times then i powered the circuit.