Solution for DC/DC converter to reach 15-20mV Ripple?

[mr_76]

Hi all,

i'm not an expert in electronics:shock:, what i need is a ±15Vdc with max 15-20mV ripple amplitude, are perhaps
already available PCB modules with filters on them suitable to mount DC DC converters and further reduce their output ripple?
I have found an XPPOWER DC DC IT4815S with 50mV Ripple max ...
how can i create filter on its output to reduce its ripple to 15mV pp ?:shock:

Thanks you all:-D

 

[dick_freebird]

Imposing an additional filter LC will impose additional
series resistance and degrade load regulation. Maybe
this is tolerable, or not.

More shunt C from output to GND will further suppress
the ripple, but you want to have a good look at the
character of the ripple - baseband, or HF? - to pick
what type. And beware that the converter loop has
been compensated based on the present, plus perhaps
some range of added C and too much could be destabilizing
depending on the control loop design.

Offhand I'd say you might get to your goal by adding 2X
the present output filter C, of similar ESR/ESL, in shunt.

Or, you could get a +/- 16-18V setup with LDOs to clean
it up further. If your current requirement is modest at
least.

 

[mr_76]

Thanks for your answer dick_freebird ,

my current requirements are ±100mA on ±15V... so do you mean i have to add 1 LDO with +15Vin and +15Vout and 1 LDO with -15Vin and -15Vout ?

With a total of 3 components i can realize so a very low ripple PSU ±15Vdc@±100mA?

Thanks again

 

[FvM]

Imposing an additional filter LC will impose additional
series resistance and degrade load regulation. Maybe
this is tolerable, or not.

LC filter is the standard means to reduce ripple. Series resistance can be reduced as long as component size allows it. Transient response may be more critical.

Analog post regulator (LDO) is of course an option for very low ripple, at cost of overall efficiency. I don't think it's needed for 15 mV.

 

[mr_76]

Thank you FvM

I've seen that a TRACO TEL 3 2423 is capable to deliver ±15@±100mA with 1000uF max capacitive Load.

I've simulated a C-L-C 20uF - 10mH(600ohm) - 400uF network on TINA with 50mV ripple and seems that ripple is greatly reduced.

Can i create a simple circuit on PCB with this DC DC and 3 passive components and feed my device with it?

Thanks

MR

 

[FvM]

The Traco converter has 300 kHz switching frequency, so a 1uH/10uF low-pass gives already about factor 50 attenuation.

 

[mr_76]

Hello,

do you mean i can add simply an L-C on the two outputs with common GND as in attached Pic?

Thanks

 

[FvM]

Yes, presuming that the DC/DC doesn't require additional output capacitors to achieve the ripple specification. In the tel-3 datasheet, no additional capacitors are mentioned. I would provide an assembly option for additional ceramic capacitors on the filter input, just in case.

 

[mr_76]

Hello,

sorry for the delay in my answer, but i haven't understood exactly what you mean. When you say that datasheet does not mentione any capacitor, do you mean to reach the ripple reported in datasheet itself i suppose, so if i put external filtering in output i can lower that ripple.

Is there a formula for dimensioning this output filter for dual output?

Thanks

 

[FvM]

so if i put external filtering in output i can lower that ripple

Yes, that's my thought.

My design formulas are simple: choose impedance as low as possible with suitable components, calculate low-pass cut-off frequency, assume slope of 12 dB/octave above it.