The proposed smps LED driver circuit is extremely versatile and specifically suited for driving high watt LEDs, however being a non-isolated topology does not provide safety from electric shocks at the LED side of the circuit.
Apart from the above drawback, the circuit is flawless and is virtually protected from all possible mains surge related dangers.
Although a non-isolated configuration may look a bit undesirable, it relieves the constructor from winding complex primary/secondary sections on E-cores, since the transformer here is replaced with a couple of simple ferrite drum type of chokes.
The main component here responsible for the execution of all the features is the IC VIPer22A from ST microelectronics, which has been specifically designed for such small transformerless compact 1 watt LED driver applications.
The circuit functioning of this 1 watt to 12 watt LED driver can be understood as given under:
The input mains 220V or 120V AC is half wave rectified by D1 and C1.
C1 along with the inductor L0 and C2 constitute a pie filter network for cancelling EMI disturbances.
D1 should be preferably replaced with two diodes in series for sustaining the 2kv spikes bursts generated by C1 and C2.
R10 ensures some level of surge protection and acts like a fuse during catastrophic situations.
As can be seen in the above circuit diagram, the voltage across C2 is applied to the internal mosfet drain of the IC at pin5 to pin8.
An inbuilt constant current source of the VIPer IC delivers a 1mA current to pin4 of the IC which is also the Vdd pin of the IC.
At about 14.5V at Vdd, the current sources gets switched OFF and forces the IC circuitry into an oscillatory mode or initiates pulsing of the IC.
The components Dz, C4 and D8 become the circuit regulation network, where D8 charges C4 to the peak voltage in the freewheeling period and when D5 is forward biased.
During the above actions, the source or the reference of the IC is set to about 1V below ground.
For a comprehensive info about the circuit details of the 1 watt to 12 watt LED driver, please go through the following pdf document presented by ST microelectronics.