Electronics > Beginners
Linear PSU with modern components, will it matter ?
coppercone2:
if your switcher is making RF then it makes sense to do so before the LDO because the RF gets into the LDO and it ends up mixing and rectifying and messing with its control system and also becoming higher in frequency and more difficult to deal with.
I think it might manifest itself as a low frequency ripple / wandering offset voltage, especially with high impedance set LDO like a LT3080.
Kleinstein:
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.
BravoV:
--- Quote from: Kleinstein on December 19, 2018, 07:30:50 am ---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.
--- End quote ---
Cmiiw, the LC filtering is good at high freq spikes right ?
What interest me is you mentioned "base frequency" , with the current linear regulator, even designed from ground up with discrete components, does it matter at what frequency its the easiest to suppress ?
We're talking switching freq range from > 100Khz up to few MHz, and how about sub 100KHz ? Lower is better ?
--- Quote from: Kleinstein on December 19, 2018, 07:30:50 am ---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.
--- End quote ---
Any hint or direction which or what is the ideal circuit (and/or incl. chip) especially as the front end for the switcher ?
exe:
--- Quote from: BravoV on December 19, 2018, 07:38:30 am ---Lower is better ?
--- End quote ---
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.
BravoV:
--- Quote from: exe on December 30, 2018, 04:45:41 pm ---
--- Quote from: BravoV on December 19, 2018, 07:38:30 am ---Lower is better ?
--- End quote ---
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.
--- End quote ---
As the scope of this discussion is for bench top psu, I think size and efficiency are not big concern.
So what are you saying is the old switcher that work at sub 100K Hz switching actually are better in our case here ?
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