The first thing is to define the peak load (in Ohms) and the frequencies your trying to filter. The peak load normally occurs at low input voltage & max load when the current is typically at its largest. The next step depends on whether you know how much attenuation you actually require and the type of noise your experiencing. A finger in the air approach would be to assume only differential mode noise is present and as a start point design a LC filter to give approx 40db Attenuation at the switching frequency. To do this you can choose a capacitor with an impedance of around 1/10 of the peak load impedance c = 1/(2*pi*f*(Zload/10)) and an inductor with an impedance of 10x the load at the switching frequency L =((10*ZLoad)/Ipk)/(2*pi*F). If you find you need more attenuation lower the filter frequency and you'll get more attenuation at the higher frequencies and to lower the attenuation you can increase the frequencyHow to design EMI input filter in dc-dc converter? any experience in this point?
Would the emi occur at 30khz frequency? Is it a must to design EMI input filter? and how to measure the noise dB in the circuit? andThe first thing is to define the peak load (in Ohms) and the frequencies your trying to filter. The peak load normally occurs at low
Would the emi occur at 30khz frequency? Is it a must to design EMI input filter? and how to measure the noise dB in the circuit? and
I couldn't get the same L from your formula L =1/ (2*pi*F*(10*ZLoad)) =1/(2*pi*100e3*10*6.5)=2.4e-8
Any frequency on a wire can be considered a conducted emission, whether or not you need to worry about it will depend on the type of product and the type of market its going into as this will define what the certification requirements will be. If high frequencies are on a long cable they can radiate also so best practice is to deal with the emissions early in the design lifecycle. As a side note whilst a filter is not always a must it can serve many other purposes such as protect the circuit from outside influence (such as voltage spikes and transients) not just prevent emissions from coming out from the product your designing.Would the emi occur at 30khz frequency? Is it a must to design EMI input filter? and how to measure the noise dB in the circuit? and
I couldn't get the same L from your formula L =1/ (2*pi*F*(10*ZLoad)) =1/(2*pi*100e3*10*6.5)=2.4e-8
Not really, its more a impedance scaling of L & C by rule of thumb. If the impedance of the filter is too large you starve the DC/DC converter so the 10x factor is a common impedance scaling factor to avoid this whist giving decent attenuation. TI do some decent application notes around filtering of DC/DC converters so you may find more detail there.Thank you , is there a reference or application note for these equations to read in depth?. As i want to know more about them.
Only if you have a FFT function, you can nomally get a dBV measurement which you have to convert to dBuV to get an idea of where you are but really you need the LISN and Ground plane to be representative.how to measure noise db on my digital oscilloscope?
Missed this, Change the Value of F to change the 40dB point. The resonant frequency will change also as the role off of a LC filter is 40dB/Decade. Try playing with the formulas and do a frequency plot to see the effect. Test it at full load and light load and you'll se the resonance liftThank you , how to change the resonant frequency?is there a relation between resonant and lc values and noise db. is there a reference or application note for these equations to read in depth?. As i want to know more about them.
by Luke James
by Luke James