I was looking at a pcb (which I cannot share) in which the 48V output of a power supply comes through a connector to the pcb board.
As you see in the picture, 48V input is filtered with a common mode filter and sent to an isolated DC-DC.
It is not in the picture, but the 5V output of the DC-DC will power a microcontroller




I have some practical questions that I would like to discuss with you, I have provided you with the datasheets and can give you all the information you need (and that I have available):

1) The cutoff frequency is 1 / [2*pi*sqrt(L*C1)] ??

2) How do I choose IF and HOW much to filter the 48V input?
Ideally the input is DC, but it obviously contains some ripple at higher frequencies. I can't find any information in the power supply datasheet about this.

3) How do I choose the parameters of L and C?
I am not talking about their values (Farad and Henry), but the others that I have to filter for example on DigiKey (see image below).
In the case of inductor, for example:
- maximum current (which I would say is at least twice the maximum drawn by the DC-DC)
- impedance (it is usually reported for MHz ... but in the picture the filter is a few kHz)
- AC and DC RATINGS
- etc.




I actually notice that since I choose 470uH@1kHz ... I no longer have the option to choose the other parameters/filters, but the choice between them remains:



These are necessary questions by generic design, which is why I am asking for help.

Thanks

 

What makes you think you need to filter the input to this isolated DC-DC converter? Does the manufacturer recommend it?

 

What makes you think you need to filter the input to this isolated DC-DC converter? Does the manufacturer recommend it?

Suppose I needed it, I'd like to get some tips on choosing L and C

 

Suppose I needed it, I'd like to get some tips on choosing L and C

There are plenty of sites on the web giving CLC design formulae. There's no point in putting two inductors as this is DC so there's no common mode signal to cancel out. Given the spec of the main 48v supply I don't see the need for it, but pay attention to good grounding practices.

Is there a specific reason for an isolated DC-DC supply?

 

There are plenty of sites on the web giving CLC design formulae. There's no point in putting two inductors as this is DC so there's no common mode signal to cancel out. Given the spec of the main 48v supply I don't see the need for it, but pay attention to good grounding practices.

Is there a specific reason for an isolated DC-DC supply?

thanks for the answer.

The isolated dc-dc supply is there because there will be a microcontroller after that that needs isolation.

As for the filter, I was also wondering about the sense of using a common choke.
Maybe because there are 5/6 other boards connected to the 48V and GND? (which are in other parts/pcb of the machine)
..but I don't think so..

Do you agree that an ordinary LC filter could have been used?
If the answer is positive, the question I asked in the main post remains:
how do I know just from the information I reported whether there is a need to filter?

 

The isolated dc-dc supply is there because there will be a microcontroller after that that needs isolation.

Really? In my experience its very rare for the microcontroller to need isolation... maybe a sensor or an output driver, but those are best handled in other ways else things become increasingly complex.

Do you agree that an ordinary LC filter could have been used?
If the answer is positive, the question I asked in the main post remains:
how do I know just from the information I reported whether there is a need to filter?

Yes, an ordinary π-filter could be used, keeping a common ground, but given the spec of the 48v supply and the use of a DC-DC converter, IMHO, its an unnecessary complication. The DC-DC converter will remove any impact of the 48v supply on the 5v output. If there is any impact of the DC-DC converter on other boards in the system or vice-versa, that begs questions about their design. Without more knowledge of the system and its components its hard to be more specific, but, in my experience, power supply issues often come back to poor grounding practice, such as creating ground loops, or ineffective ground bonding.

 

In post #5 we learn that the same 48 volt supply is also feeding other circuits that are part of the project. It may be that some of those other circuits are noise sensitive, or possibly contribute noise. THAT is an unknown variable.
An isolated DC/DC step down supply is probably immune to input noise, although any power interruptions will be an issue. So filtering the supply to the step down isolation supply is not mandatory, nor beneficial. BUT large capacitors to hold the 48 volts steady could be useful. The value of that choice depends on the other loads and how the system is physically arranged.