Faster opamps may introduce instabilities. You need to slow it down for stability. But low noise opamps are very appreciated.
Polymer caps/MLCCs may also introduce instabilities. You need some feedback trick to introduce more output ripple into the loop for stability.
What happens to noise when you start messing with magnetic feedback paths? ...
After many years designing linear P/Supplies, mostly Intrinsic / mining etc, this was my goto design for several years -
(obviously not the full design, but main excerpts) - 0-36VDC op, 0-20A+ op: MAX loss - Bridge and regulator = 0.8V (best min was 0.6V, but I left headroom).
NO fan, very minimal heatsink ! ~95% efficiency (not including transformer) .. not bad for linear :-) 220-260V ACin
Original design had a multi-tap transformer (lots of taps), then played with magnetic feedback, resonant circuits etc .. but a completely linear circuit !
One day, I'll see if Daves capacitance multiplier works :-) ... oddly enough it has a micro, but just for the front end :-)
So, can Linear P/Supplies get better ?? Shit yeah !!
... any chance you share the whole schematic ? Including the tap switcher if you mind ... is that with adjustable current too ?
I can see you're still using that venerable LT4320 there.
Quote from: BravoV... any chance you share the whole schematic ? Including the tap switcher if you mind ... is that with adjustable current too ?Tricky situation. It was 1 of many contract designs ... then got sacked after several years :-) (long story) .... never paid for R+D or other "trades" ...
With all that, I don't want to risk any more animosity ... circuit operation theory, though, is common knowledge -
Run a small dual 5-8VAC, say 50mA transformer to power logic, REF 0V, and an isolated 6-12V high side FET driver rail
Use a small micro, with A/Ds to monitor AC V and I, and DC V and I. Then depending on Vop, switch in the respective winding, using the bridge type FETs.
Have this pre-regulator on a separate PCB, with it's OWN storage caps. Then, IF you want to get fancy, "active feed" the MAIN storage cap that supplies the
regulator PCB. At low current, it doesn't matter if Vin_Reg is a bit higher, as power dissipation will be low. Keeping the Vreg cap at an ideal, low ripple voltage
is the "tricky" part :-) .. but then, working that out, is the challenge :-) In one 10A design, I believe I got ~97% efficiency .. not bad for linear (-Trx).
I can see you're still using that venerable LT4320 there.
Ahhh yeah, didn't I send you a couple boards way back? Did you ever use them? Useful? It's a DAMN fine IC !!
How its perform, say with switcher pre-regulator instead of using step-down transformer ? Did you ever try this combination ?
How do you deal with inductance ? Like a high power rail that is long? Snubbers allowed or do you need to do something else?
For the really high frequency ripple of a switched mode pre-regulator it is a good idea to have some LC filtering before the linear stage. At least for the higher frequencies this is relatively simple, so that the LDO only has to deal with the base frequency at a reduced level. At high power it might be worth to have the switched mode part in a multi-phase circuit to reduce the ripple and keep the frequency high.
For the linear stage it depends on the type of circuit how good they suppress higher frequencies. Ready made LDOs like the LT3080 may not be the best solution here.
Lower is better ?
Lower is better ?
I'd say slower rise is better (for noise, not for efficiency). If we slow down frequency we can also decrease switching speed => less RF noise. The downside is the inductor needs to be bigger. It may also have worse parasitics, e.g., capacitance and series resistance. This may also affect size of the board and pcb parasitics.