# Multiple DC Buck converter stages

Hi all,

I’m working on a design which requires three different voltage rails (3.3V, 5V, 12V) all driven from a 24V DC input. Total power is probably ~10W max, more or less evenly distributed across the rails.

My question is: is it preferable in general to run the three stages in parallel from the 24V input, or to chain them? Chaining them will obviously increase the loading on each higher stage, but that may be beneficial in terms of regulator efficiency. Are there any other trade-offs (particularly w.r.t. EMI) for either case?

Regards,
Chris

I usually chain the 3.3v one as there are a wider variety of parts to choose from with the lower input voltage, and I can usually just use an LDO hung off the 5v rail.
Efficiency won’t a problem if you have a constant load, but can get tricky as the in/out differential gets bigger due to inductor choice.
If you chain all three with 85% efficiency then you have 0.85 * 0.85 * 0.85 = 61% efficiency on the 3.3v rail. Might be ok, might not, depends on the specs you need to meet.

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Related to @PeeJay’s reply, it may help to calculate total power loss in both scenarios to see which is best. As you noted, the efficiency of the first and second modules in a daisy-chain arrangement should increase, so I think a daisy-chain is likely to be the best solution.

Besides efficiency, you may want to consider whether your circuitry can tolerate the ripple typical of switchmode converters. It may be wise to add some linear (LDO) post-regulation if noise on any of the nodes is a problem.

As others write it depends on the load, but generally better to have them all from 24V as efficiency will be better. If you don’t need tight regulation, a transformer could be used, but that is overcomplication if power levels are low

How i’ve dealt with this in the past is to use an isolated tracopower module which handles 9-36V input, 5V output, and then used a 3.3V LDO connected to the 5V output. Depending on which traco module you use, you may be able to achieve different EMI classes. Traco Power support are very helpful too. Of course, there is a serious price increase if that is a factor (in my project it wasn’t thankfully).

Thanks for all the useful replies. I’ll have a think.