[SOLVED] AC/DC Converter for low-power applications

[yolco]

Next post try to explain the doubts and requirements for designing a PSU which has to power some DC low-power components.

First, of all, the load (DC low-power components) works at 5VDC with a maximum of 1.5A input current.
For this purpose I will place a DC buck converter at the output of the rectifier.
Also, it will need to have reduced dimmensions in order to be able to mount on a PCB as small as possible.

The circuit I have thought for this transformer are next:


So, here are the doubts:

• Transformer. How to select the right transformer for this application? Voltage primary (N1), voltage secondary (N2), power or switching, etc
  • Input is known 220VAC (nom.).
  • Output is known too, 5VDC@1.5A, if it is possible. if not, which would the best option to accomplish the target (maybe, 12VDC).
    • I know this is an ambiguos question, but I'd prefer to ask for help to concrete.
• Rectifier.
  • Will 600V be enough for voltage peak max?
  • Which would be the optimum DC forward current? (0.5A, 1A, 1.5A)?
  • Ripple can be in the range of 100mVpp.
• Fuse. 1A or 1.5A, it will depends on the maximum transformer output current allowed.
• Smoothing Cap. C[F] = If[A] / (Vrp[V] * f[Hz])
• Regulator.
  • This step-down converter in case transformer voltage will be 12VDC.
  • Or another one buck/boost converter if circuit provides 5VDC.

And finally, which would be the best option in order to isolate the other circuit's parts?

• Develop it as a standalone solution?
• Or isolate ground mounting it in the same PCB of the load?

- - - Updated - - -

* Perphaps, there are another AC/DC converter topologies better than the one I have purposed for this kind of application (for example, flyback converters).
If it is so, it will be welcome your explanation about them.

 

[KlausST]

Hi,

Transformer. How to select the right transformer for this application? Voltage primary (N1), voltage secondary (N2), power or switching, etc

Input is known 220VAC (nom.).
Output is known too, 5VDC@1.5A, if it is possible. if not, which would the best option to accomplish the target (maybe, 12VDC).
I know this is an ambiguos question, but I'd prefer to ask for help to concrete.

you know AC voltage recitfied and filtered gives a larger DC voltage ( about U_DC = 1.4 x U_AC_RMS -1.4V)

so expect about 15.5V DC with a 12V AC input.

Current: You need 5V x 1.5A = 7.5W output. Expect a buck converter efficiency of at least 85% at max load. This means the converter input power is 7.5W/0.85 = 8.8W.
8.8W = transformer rating. (almost independent of transformer voltage)

a buck converter with 16V input and 5V output is OK.

Rectifier.

Will 600V be enough for voltage peak max?
Which would be the optimum DC forward current? (0.5A, 1A, 1.5A)?
Ripple can be in the range of 100mVpp.

with a 12V output transformer you have 12V x 2 x 1.4 = 33.9Vpp expect a higher output voltage at low load. I recommend to use at least 100V rectifier.
current: 8.8W /12V = 0.73A (pulsed, average). a 1A should do, better use a 1.5A one. (the rating is for pulsed load. They have a much higher single pulse current rating)

Ripple depends on capacitor.
you could handle much more tahn 100mV converter input ripple. 2V should be no problem.
But calculation with 100mVpp ripple: average current = 0.73A, frequency is 100Hz (doubled mains frequency by rectifier) = 10ms,
C = I x t / V = 0.73A * 10ms / 0.1V = 73mF = 73000uF.
(for 2Vpp it is 3600uF)

fuse: match it the transformer rating. Us a slow one.

Klaus

 

[yolco]

Hi KlausST,

thanks!

From your answers:

• Transformer. Are you referring to a power transformer with 12VAC output (secondary)?
  • Sorry, but I'm not understanding your explanation.
  • First part is the transformer (from 220VAC to 12VAC), then rectificates to 15.5VDC by the bridge. Is this right?
• Buck. It is fine!
• Rectifier.
  • 100V peak voltage and forward current between [1A - 1.5A]. For example, this **broken link removed** or this 1.5A If bridge.
  • Why have you doubled the frequency? Is it not enough to work with 50/60 Hz?

 

[KlausST]

Hi,

Transformer: In your description it is not clear wha transformer you want to use.
In my example i used 12V AC. (a transformer is specified with AC output)
But i gave all the formulas, so you can choose and calculate every other configuration.

Sorry, but I'm not understanding your explanation.

What exactely? Maybe you need to read some basics on transformers, rectifier, filtering.....

First part is the transformer (from 220VAC to 12VAC), then rectificates to 15.5VDC by the bridge. Is this right?

Yes, I tried to keep on your picture.

Why have you doubled the frequency? Is it not enough to work with 50/60 Hz?

it´s not me that doubles the frequency, it´s the rectifier.
You may find a lot of information and scope pictures in the internet where you can see it.

Klaus

 

[yolco]

Thanks KlausST!!

You're right I need to read more about this thread. I have too newbie questions.