I have studied that there is a transformer which steps down the ac voltage after then bridge rectifier to convert into ac to dc and then given to filter, this is what I have learnt in books, but when I open a mobile charger I couldn't figure it out how it actually works?
Phone chargers are incorrectly named. Most are 5 V power supplies. The charge control is done inside the phone. The power supply will continue to give out 5 V when the phone is completely charged.
Figure 1. Switched-mode power-supply block diagram.
How it works:
- The mains voltage is rectified to provide a high voltage DC supply.
- A transistor "chopper" switches this on and off at high frequency.
- A small transformer steps this down to a low-voltage high-frequency AC.
- A rectifier converts this to low-voltage DC.
- The chopper controller feeds back to the chopper and adjusts the chopping cycle to maintain the required voltage on the output - usually 5 V.
Figure 2. Anatomy of an SMPS 'charger'. Image source: Analogic Tips.
- Plug pins.
- Four diodes for the bridge rectifier.
- Smoothing capacitor for high-voltage DC.
- Switching transistor.
- Switching transformer.
- 5 V supply indication LED.
- Output rectifier and filter.
- Opto-isolator for feedback from the output to the chopper controller.