Hi
In response to a query yesterday, I tried to sketch from memory my old minimal sm reg circuit. This is a minimal parts sm regulator.
Its asynchronous, as that saves lotsa parts. In plain english, the timings are all over the place.
This version is a minimum parts switched mode regulator. Its optimised for build cost rather than best performance.
I'm running from distant memory here... cross fingers:
. c e . IN _________________ _ ___________O_O_O____________________ OUT . | (_) | | | . | | | L1 | | . R1 > | b | ___ | . > | | ZD /^\\ | . | | | | | . | | ___ R2 > ___ + . | | /^\\ > --- . |_________|_____ | ____________ | | . | c |_ | | . | (_)-+ | . | e | b | . | | | . 0v __________________________________________________________ 0v
Tr P_diss is not so easy to calculate, as it depends on switching slew rate, frequency, load, how hard it switches on and off, etc. Guess, allow margin, then check its not too hot.
Output reservoir cap should be as small as is consistent with satisfactory operation. Increasing the cap will increase the tr's max current, and make any startup overshoot last longer. Smaller cap also means less inductance needed. Realise the max tr i is only controlled by L1, C, i_load and the in and out Vs, and will peak during startup for regs without soft start.
Worst case V overshoot (during startup with zero load current) is apx V_out + 2x (Vin - Vout). IRL it'll be much less due to resistances and load.
Maybe that was it, the 2 tr version that worked well. But I really cant remember. Just 2 Trs, 2 Rs, L & C.
Feedback welcome...
NT
PS I dont remember the L details at all, so heres a diy one for the @home hobbeasts: 4" of 1mm insulated gardening soft iron wire can be folded and apx 50 or 100 of turns of enamelled copper wrapped round it for an inductor.