DC-DC program selection discussion

[Benjam]

I am trying to make a DC-DC converter circuit, the bus voltage is 48V, the circuit needs 24V, 12V, 5V, 3.3VD, 3.3VA, 1.2VD, which of the following is the best solution:

a: (step1) 48V -> 24V;
     (step2) 24V -> 12V;
     (step3) 12V -> 5V;
     (step4) 5V -> 3.3VD(3.3VD->3.3VA);
     (step5) 5V -> 1.2V;

b: (step1) 48V -> 5V;
      (step2) 5V -> 12V;
      (step3) 12V -> 24V;
      (step4) 5V -> 3.3VD(3.3VD->3.3VA);
      (step5) 5V -> 1.2V;

c: (step1) 48V -> 12V;
     (step2) 12V -> 5V;
     (step3) 12V -> 24V;
     (step4) 5V -> 3.3VD(3.3VD->3.3VA);
     (step5) 5V -> 1.2V;

Or have any better suggestions? The prerequisite is that only MLCC capacitors are allowed.
Regarding the current, I don't have a better idea yet.

[mariush]

How much current on each voltage?
What voltages need to be always present?

That's important.
If you only need a low current on 24v, then it may better to have a dc-dc converter circuit to boost 12v to 24v
It may also be a voltage that's not required all the time, in which case your dc-dc boost circuit could have an enable pin, and only produce 24v as needed.
For 3.3v and 1.2v again depending on current, you may want to consider using a linear regulator to get 3.3v from 5v.
If it's a fpga that may have a lot of current required on 1.2v, then it may make more sense to get 1.2v from 5v using a buck regulator.

[Benjam]

How much current on each voltage?
What voltages need to be always present?

That's important.
If you only need a low current on 24v, then it may better to have a dc-dc converter circuit to boost 12v to 24v
It may also be a voltage that's not required all the time, in which case your dc-dc boost circuit could have an enable pin, and only produce 24v as needed.
For 3.3v and 1.2v again depending on current, you may want to consider using a linear regulator to get 3.3v from 5v.
If it's a fpga that may have a lot of current required on 1.2v, then it may make more sense to get 1.2v from 5v using a buck regulator.

24V requires 2A current, 12V requires isolated type (12V low current, about 250mA is fine), others can be non-isolated, 5V requires 1.5A current, 3.3V is for DSP power supply, so There is not much current demand, just 600mA.

[Benjam]

The preliminary idea is to switch from 48V to 24V to get 24V (max 3A current), then use 24V to 12V (using flyback topology, isolated), and then use 24V to 5V (or use 12V to switch to 5V, right? would it be better?), then 5V to 3.3V, and 5V to 1.2V.

I really need some better advice, I am a poor beginner

[Benjam]

Looking forward to expert answers

[capt bullshot]

Isolated 12V: There are ready made modules that can do that for you. Two kinds are available: Cheap ones that do not have regulation and require a regulated input (e.g. 24V -> 12V or 12V -> 12V). Others have a suitable input range that can directly connected to your 48V and have a regulated 12V output, these will cost more.
If you want to roll your own, google and think about "flybuck" topology - this lets you easily create an isolated output from a "standard" buck converter.

Other voltages:
24V: 48V to 24V step down (buck converter)

Probably highest cost, maybe best efficiency: Create a non-isolated intermediate 12V rail from the 48V input using a buck converter, as there are multi-output buck / LDO chips available that can do 5V, 3V3 and 1V2 from 12V for you (but rather not from 24V).
Mediocre: 48V to 5V buck converter and use some small switchers (or LDOs) for 3V3 and 1V2. That high 48V to 5V ratio isn't optimum efficiency for a buck regulator. Often flybacks are used here, but don't expect better efficiency.
Maybe best overall: 24V to 5V buck, and 5V to 3V3 / 1V2.

[Benjam]

thanks very much. help a lot.