Designing a front-filter for a power supply.

Hi

Your noise will be mainly differential, so a LC filter with for example the sdr0805 from Bourns may be sufficient

But your input current of 1A is not enough information. You need to find the peak current to decide for the relevant inductor

Then you need to supply information of what standards you must uphold

Regards

Klaus

Use a common mode filter and a capacitor.

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Sorry to reply here. I can't see the OP.

You can use an LC filter similar to what you have on the output of the switcher.

I did once neglect to filter the input on a switcher, and noise did couple in to other boards through the power supply. It was basically square wave noise at the switching frequency. This was a 5V to 3.3V converter operating at around 2 amps output and 750 kHz.

With switchers, when the load current is steady, the duty cycle is a function only of the input and output Voltages. This means you can pretty easily figure out what the input current waveform will look like (approximate it with a square wave) and thus conceptually test out different values of L and C (also include the ESR) in a simulator or spreadsheet.

HTH!

--Mac

Is your +5V output's ground returned to the +12V ground anywhere but at the common connection of the power supply? If so your life is slightly more complicated.

L1 L2 ------- L4 +12V -((((--+---((((---+---! DC-DC !---+---((((--+--+5V load ! ! ------- ! ! --- --- ! --- --- C1 --- C2 --- ! --- C3 --- C4 ! ! ! ! ! ! -------+------- ! ---------+---((((-----------+-----------------+-- 5V return L3

Notice that the heart of the DC-DC converter is only connected directly to the outside world at one point. It the 5V load is isolated and inside a metal enclosure, it is likely that L3 and L4 can be deleted without a problem.

Now for the simplifications that make your life easier:

We can assume that the +12V input has zero impedance and that the ripple current into it is the specification that matters.

The allowed ripple current on the +12 in many times less than the input ripple current of the converter.

The impedance of L1 is many times the impedance of C1. This includes C1's ESR and ESL.

L3's impedance is similarly much larger than C2's.

Your DC input current on the DC-DC converter is 1A so I'd estimate that you have a 0.5A RMS ripple current there.

Your ratios of values are huge so you can ignore phase relationships and say things like:

The ripple in L2 = 0.5 * ZC2 / (ZL2 + ZL3)

The ripple in L1 = (ZC1/ZL1) * 0.5 * ZC2 / (ZL2 + ZL3)

Chances are C2 = (C1 + 0.1u)

The 0.1u is a high frequency bypass to help eat the higher harmonics.

Now you need to decide if this is thru hole or surface mount and a bunch of issues like that and get out your inductor and capacitor catalogs.

Try

and look for a 5022 series inductor that can handle the average current. Assume this for L1 and L2 and plug in the ZL1 and ZL2 and solve for (ZC1 = ZC2). Look for a capacitor with this or less impedance.

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