Ceska verze

3,4V 2,5A switching supply with optocoupler

     This switching power supply has been created to power the digital camera. Current onsumption of this camera is about 0.6 A and 1.3 A peak (flash charge). For this purpose, it certainly could use a classic linear supply with eg LM317, but the effectivity would be little over 30% with a heavy transformer and stabilizer with a considerably heating heatsink. This switching power supply is a much more elegant solution. On the Internet there are many schematics of switching power supplies with 3842, auxiliary transistors or at least countless unnecessary components. The diagram below shows a very simple schematic of a small switching power supply with a single transistor and optocoupler. Switching power supply without optocoupler with the indirect stabilization would be even more simple, but its output voltage is not stable enough. This switching power supply works as a flyback converter. The principle is simple: After connecting the power the 1M 1W base resistor partly opens the transistor. It induces the positive voltage on the auxiliary winding (8 turns) and the transistor opens completely. When the capacitor 2n2 is discharged, The transistor is turned off and the voltage induced to the secondary is charging the filter electrolytic capacitor. When the 2n2 capacitor gets charged again, the transistor re-opens and everything is repeated. When the desired voltage specified by resistive divider 3k3 and 10k opens the TL431 circuit, the LED in the optocoupler starts shining and a phototransistor limits the current to the transistor base. This reduces the PWM duty cycle and reduces energy delivered to the transformer. This method of stabilization is very effective, the full load voltage drops by no more than 0.01 V. This switching power supply can not work with no load, so the output loading load resistor 33R is connected to eliminate this problem. Zener diode protects the powered device before the overvoltage in case of failure of stabilization. Instead, you can use another way of overvoltage protection, such as the one with SCR. 68n capacitor ensures EMI interference suppression, resistance 10R reduces the inrush current when turned on. 2n2 capacity change can affect the operating frequency. Printed circuit board must be arranged so that the primary (mains) and the secondary section were apart far enough.
The transformer is wound on an EE ferrite core with an effective cross section 0.5 cm2. First, the first half of primary turns is wound, ie 40 turns. The wire has a diameter of about 0.2 - 0.3 mm. Then at least 8 layers of insulating tape are wound. Then the secondary coils. For safety I used wire with thick insulation, which with only 4 turns is not a problem. Then again followed by 8 layers of insulating tape. Furthermore, the auxiliary winding 8 turns is wound, using the same wire as the primary. Then again insulating layer, which may not be as strong. Finally, other 40 turns of primary are wound. Then again, a few layers of insulation. Between the core halves is placed one layer of insulating tape to form an air gap to prevent core saturation. Finally, the core of such sealed with glue.
This switching power supply can of course be modified for different output voltages, just change the number of secondary turns (approximately 1 turn corresponds to 1V) resistor 39R is changed by about 10R for each 1V and stabilized output voltage be adjusted by changing the resistor 3k3 so that at the required voltage the divider gives 2.5 V at input of TL431. Zener diode is chosen slightly greater than the output voltage. Rectifier diode must have a reverse voltage at least 8 times larger than the output voltage. For higher voltages, therefore, replace Schottky diode by fast diode because the Schottky diodes have always low rated reverse voltage. Of course, the output electrolyte must be rated for the sufficient voltage.

     Warning! Switching supply is not for beginners, because most of its circuits are connected to fatal mains voltage. When bad design, the mains voltage can reach the output! Capacitors can remain charged to dangerous voltage even after disconnected from mains. Everything you do at your own risk, for any injury to health or property I do not take responsibility.

schéma malého impulzního zdroje
The schematic of the 3,4V 2,5A switching supply with optocoupler.

Starting to wind the first half of primary winding.


Auxiliary winding.

The second half of the primary.

Completed transformer and the core.

Starting to make the board.

The finished PCB.

Operating SMPS.

And after built into the box from a broken universal wall outlet adapter. (The LED with a 150R resistor added)