The post explains how to make a simple, cheap yet extremely reliable smps based 220V/120V mains operated cell phone charger circuit.

Why TNYxxx Tiny Switch is Used


The TNY series of tiny switch ICs provide us with an option of making perhaps the smallest possible smps circuits with high reliability. The tiny switch series includes the following ICs: TNY267P, TNY263, TNY264, TNY265, TNY266, TNY267, TNY268, TNY280.

The above ICs have an integrated in-built mosfet switching control circuit, protection against over current and thermal overshoot, along with rugged voltage and current specifications.

The IC comes in a DIP8 package that's exactly how a 555 is enclosed. The maximum tolerable voltage limit of the TNY series ICs is a massive 700V, a margin that's way beyond our normal household AC specs. The operating frequency is at about 132kHz.

The IC is specifically designed and built for implementing compact and reliable 120/220V mains operated SMPS flyback converters.

Although the application of the proposed simplest SMPS design could be huge, it could be best used as a mains operated 5V cell phone charger circuit.

The proposed cell phone charger design using the IC TY 267 can be visualized in the below shown diagram.

How the SMPS Circuit Works


The circuit can be understood as follows:

The mains input which could be anywhere between 100 and 280V is half wave rectified and filtered through shown 1N4007 diode and 10uF/400V input rectifier stage.

The 10 ohm/1watt resistor is included to provide some sort of restriction against the surge current inrush during power switch ONs and also forms like a fuse in case of a catastrophic situation.
The switching voltage is acquired via the BA159 diode at pin5 of the IC.

The IC instantly locks into the specified 132kHz switching frequency when switched ON across the input winding of the switching ferrite transformer.

The 180V zener diode safeguards the IC from peak switching voltages.

The above switching generates the calculated stepped down low voltage across the output winding of the transformer.

The BA159 diode at the output rectifies the 132kz pulsed DC while the 220uF capacitor filters the high frequency ripples to produce a clean DC.

The optocoupler acts like a feedback link between the output and the IC in order to ensure that the output never exceeds a certain predetermined voltage level.

This feedback limit is decided by the adjoining 4.7V zener diode, which ensures that the output stays well within the 5V range just suitable for charging any attached cell phone.


How to wind the ferrite transformer


The shown ferrite transformer along with the IC forms the heart of the circuit, however due to its simple configuration winding this transformer is much easier compared to other mains operated cell phone charger circuit topologies.

The input primary winding consists of around 140 turns of 36 SWG, while the output secondary winding is made up of 8 turns of 27SWG super enameled copper wire turns.

The core used can be a small E19 type ferrite core with bobbin having a central core area section measuring 4.5 by 4.5mm.

The primary is wound first. After winding it, it must be covered with a layer of insulation before winding the 8 number of secondary turns on top of the primary layer.

A copper or aluminum tape layer should be preferably included in between the primary and the secondary winding and a wire connected with this tape with the "cold" end of the primary winding (see the trafo in the figure), this provides guaranteed isolation between the winding as well as guards against interference issues.

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