Opinions needed - SMPS circuit with DC-DC module

[Gimlix]

Hi everybody!

I am designing an embedded system and I have some doubts with my power supply stage. The system is intendeed for industrial use and it will be powered always by batteries (it will be compatible with systems of 9V up to 90V)

As I am not going to design a whole SMPS for the main stage, I will use a Traco DC-DC converter. The converter IC1 is this one:

**broken link removed**

And the schematic I've made is this one:



Regarding this app. note I have some doubts. It says on sheet 49, C1+C2+C3+C4 need to be added for Class A compliance. I don't see the point for adding two 1uF capacitors in parallel, could I just use one capacitor instead of two?

On sheet 51 the app. note says an LC filter is recommended at the input, but I don't know if it should be before or after C1+C2. Any idea? What happens if I don't install this LC filter?

And at the output, as I want to have a clean 12V line, would it be right to add the L1+C5+C6 stage?

The second part of the circuit is a small SMPS build as stated in the datasheet from TI for the LM2574. Do you think the 3,3V output is enough filtered in order to power up a couple of processors and ADCs without big problems?

Any ideas, comments or advices would be awesome. Thanks!

 

[FvM]

The input voltage notation is somehow misleading, because no TRACO converter supports a 9 to 90 V range, you'll use three different types to achieve it.

The double capacitors point may be matter of AC current rating or ESL.

Regarding LC fílters, I have two comments:
- buck converters have their large ripple on the input. This means the 3.3V regulator will dwart the effect of the 12 V LC filter.
- LC filters involve a risk of oscillations with load transients. You should check if this may be a problem for your application.

 

[Gimlix]

Thanks for your answer FvM. You are right about the TRACO converter: I am going to use two different models depending on the input range voltage.

I see the problem with the reflected ripple at the input of the 3,3V SMPS so, would it be right if I connect this input, directly to the output of the TRACO converter, and I place the LC filter after this connecion? This way I would only filter the two outputs, but not the middle stage.

The difficult part is how to check the risk of oscillations with load transients. The only loads that will switch on and off will be some relays and a buzzer, but the biggest load will have a consumption of 10% over the total power output. How can I make some calculus about that subject? I never faced that problem. Danke schön for your help!