How the Circuit Works
The proposed 12V, 5 amp smps battery charger circuit employs a flyback converter topology which results in the required smps based high current, compact, mains isolated converter design.
Here, the a high power mosfet becomes the main switching component and is used for triggering the ferrite primary winding with the set high frequency mains rectified Dc.
When switched ON, the 470k resistor charges the mosfet gate into conduction and initiates the switching action.
The above action induces a voltage across the auxiliary winding of the transformer which results in a feedback voltage to the mosfet gate via the 2n2/100V capacitor forcing the mosfet to conduct even harder.
As soon as this happens, the primary winding gets connected with the full 310V DC rectified voltage via the mosfet drain/source terminals.
During this process, the voltage across the 0.22 ohm resistor situated at the mosfet source tends to cross the 0.6V level, which instantly triggers the transistor BC546, which shorts the gate of the mosfet to ground, rendering it completely switched OFF.
This also ensures cutting-of the auxillary feedback voltage, restoring the entire primary section to its original switched OFF state.
The cycle now begins afresh and is switched continuously at around 60kHz rate which may be varied by increasing or decreasing the values of the 2n2 feed back capacitor and the 100pF base capacitor of BC546 NPN (it's not recommended though).
During the switched OFF periods of the primary winding, an induced equivalent back emf is transferred to the secondary winding which translates it into the specified stepped down low voltage, high current secondary output.
The above secondary output is appropriately rectified and filtered by the high current diode and a filter capacitor.
A feedback stage across the secondary and the primary stages is implemented via a optocoupler which determines the required fixed, regulated output voltage.
The zener associated with the optocoupler may be tweaked for getting different stabilized outputs for the desired applications.
Here it has been fixed to about 14.4V which becomes the optimal level for charging a 12V lead acid battery.
The current output of this transformerless 12V, 5 amp smps battery charger can also be changed by two methods.
Either by modifying the secondary wire thickness of the transformer or by tweaking the value of the 0.22 ohm resistor positioned across the source/ground terminals of the mosfet.
The input stage typically consists of a bridge rectifier stage, followed by an NTC and filter stage.
The input EMI coil is optional.
How to Wind the ferrite transformer
The ferrite transformer is wound over a 15mm EE ferrite core compatible plastic bobbin.
The one half primary is wound first, using a 0.4mm super enamelled copper wire (15 turns).
Secure the end of this on one of the primary side pins of the bobbin. Cover the winding with a layer of insulation tape.
Next wind the secondary winding (5 turns) using 0.6mm wire over it.
Terminate the ends on the secondary pins of the bobbin.
Apply insulation tape over this winding.
On this wind 3 turns of 0.4mm auxiliary winding, cover it with insulation tape.
Finally continue from the secured end of the first primary winding and wind 15 more turns over the above auxiliary wind to finish of the ferrite transformer coils.
Put a few layers of insulation tape to finalize the winding insulation.
Fix the EE cores and tape it yet again along its periphery.
Make sure the EE core edges are separated with an air gap through a piece of insulation tape or a paper, this will prevent core saturation and stalling of the desired smps induction.
THE CIRCUIT EXPLAINED ABOVE IS NOT ISOLATED FROM MAINS, AND THEREFORE IS EXTREMELY DANGEROUS TO TOUCH WHILE EXPERIMENTING IN POWERED CONDITION, AND ALSO THE DESIGN IS RECOMMENDED SPECIFICALLY FOR USERS HAVING ADVANCED KNOWLEDGE IN THE FIELD, NOT FOR THE NEWBIES..