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I'm designing a fifth generation pcb for my project and yet I stumbled across another design problem. First of all, the circuit and all components are enclosed from the environment and users (because non-isolated) for safety.

My design would rectify the mains (European 230V AC) to a DC voltage and after that some buck-switching controller would do the job. The only problem here is, that there aren't such IC's, that would accept the ~350V DC voltage. After looking at DC-DC Converters from LCSC, I knew there are those Switching Controllers, but after a while - all the datasheets feature an isolation transformer to step-down the voltage.

For the previous designs I was using the HLK-PM01 PSU, but that's wasting a lot of space. HLK-PM01

I planning to use a non-isolated supply, because it doesn't require a custom transformer to build, just an inductor that I'll even find at LCSC.

Power output requirement: 5V at 150mA (maximum), to power an ATmega168P with some SPI/I2C chips.

In what direction should I search to find some information related to this kind of SMPS? I'll be really grateful for all advices I can get from others, more experienced engineers.

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Given you need it to power some controller with bus, if there is anything going out of your enclosure, like a I2C Line or SPI Line or anything like it, it is strongly suggested to do an isolated supply!

The risk is not about the power supply itself, but if the power supply fails, you might end-up with your electronic at 260V and whatever is connected to it downstream, which can kill.

To do so you don't need custom transformer, there are plenty of parts doing the job.


The chip itself doesn't need to be able to support the main's voltage, you are probably looking at the wrong category. Usually you would use "Controller" chip, which will control mosfets either with a single mosfet and flyback diode or two mosfets for better efficiency.

There are plenty of chip on the market for that purpose.


Non-Isolated

You can check here for reference design, there is a table that shows the input voltage.

There is also here a reference design page 8

There are really multiple possible design, it's mostly about a question of cost, efficiency and size.

This design is able to take 260VAC input to 22V output and is farily simple.

enter image description here


Isolated:

This one is similar but isolated, and all the parts are given, you can just order as the BOM and copy the design. You just need to adjust some resistor to have the voltage output you need.

enter image description here

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  • \$\begingroup\$ The I2C and SPI lines are inside the device casing (printed in ABS). Yes, of course I understand the potentially risk. I take a note of the first schematic and do some research about few Switching Controllers I'll find at LCSC. Thank You, really appreciate Your response. \$\endgroup\$ – Tomi Nov 23 '18 at 15:25
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It seems to me that you are against using a transformer because you believe it might be too large or need hand-winding yourself. The impact of this decision is: -

  • Non-isolated output voltage (potentially unsafe)
  • Problems finding a buck converter chip that will operate at over 300 volts DC

Let me try and put your mind at rest with this off-the-shelf design supported by Premier Magnetics: -

enter image description here

This is just one of dozens of designs that they make (and sell) the transformers for. The one above happens to produce 5 volts out at up to 0.6 amps. The transformer is here: -

enter image description here

It measures about 17mm x 16mm so it isn't massive and it gives you peace of mind. On page 2 of this document you get the full list of the transformers they sell.

For the previous designs I was using the HLK-PM01 PSU, but that's wasting a lot of space.

That device appears to be about 30mm x 20mm so the above circuit should compare favourably.

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