Cheap mini power supplies

Here, I want to tell a story about mini power supplies. Conventional power supplies, which use mains transformers, have the disadvantage that the product becomes heavy and large, which makes shipping by mail expensive. For this reason, I use the 3.3V NA01-T2S03 AC-DC module for my ready-made spot welder controller:

NA02 T2S05 tear down
NA02 T2S05 tear down

NA02 T2S05 re-engineered
NA02 T2S05 re-engineered

Primary side sensing

The NA02-T2S05 power supply module costs less than $2 and it contains just a few parts. Therefore, I assumed it would be of poor quality. But after some research, it turned out not to be the case. Despite the seemingly simple circuit, the properties are particularly good. To save costs, there is no feedback via an opto-coupler. However, the output voltage remains very constant while the output current varies from 0 to 800mA. That is amazing good, how is it possible? The cause is the use of primary side sensing, this was invented in the year 2000:

Primary side sensing regulation OB2535CP
Primary side sensing regulation OB2535CP

Reverse engineering the On-Bright OB2535CP converter

The AC-DC converter uses the chip OB2535CP from On-Bright, see the circuit that I have re-engineered:

AC-DC 2W converter NA02 T2S05 circuit
AC-DC 2W converter NA02 T2S05 circuit

OB2535CP features

  • Constant voltage regulation at universal AC input
  • Primary-side sensing and regulation without opto-coupler
  • Programmable CV and CC regulation
  • Adjustable constant current and output power setting
  • Built-in primary winding inductance compensation
  • Programmable cable drop compensation
  • Power on soft-start
  • Cycle-by-Cycle current limiting
  • VDD under voltage lockout with hysteresis

Change output voltage

For 3.3V instead of 5V, R7 should be 4.5k.

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