I need to design a constant current driver for a filament LED bulb. The input is 230VAC and the output should be 120VDC, 20mA (constant current) for 4 filaments arranged in series-parallel structure. Can somebody please help me to find a simple transistor-based solution or some small linear circuit with low BOM cost.
You could consider a circuit like this one (one for each string, plus a bridge/capacitor to make +300VDC):
M1 will have to be rated appropriately and have an appropriate heatsink, perhaps of a type typically used in off-line switchers.
Or see if two resistors (one per string) plus the bridge + capacitor is good enough, because that will be cheaper and more reliable while using the same amount of power.
You can use a capacitor power supply where most of the voltage is dropped in a cap on the AC input side of the bridge rectifier.The losses can be very low .The cap should be a X rated type at least 250VAc .The DC side of the bridge rectifier should have a filter cap across it to prevent annoying flicker .Place a surge limiting resistor of say 1 Kohm 5W in the AC side to protect things at turn on .The Cap supply is nothing new and it is described elsewhere.In fact some mains LED string drivers use them .