help about SMPS input inductor

[milad-k800]

hi to all..
I want to buld a smps that require 80A from 12V car battery...
but i dont know for input choke diameter???
if I use 100uH input choke what diameter of wire must use.pls help me

[acid_k2]

hi milad-k800

you have no answer because your question is incomplete, and nobody could answer correcly with so little information.

at least we need:
tipology of SMPS;
input and output voltages, and currents;
usage intermittent or continous...

for the inductor:
max losses admitted;
core type;
dimensions....

[milad-k800]

my smps have -/+70V-10A per rail.it request 80-100A current from car battery.my transf core is EDT type.and with push pull method....
i dont choise the input choke yet,may u choise that core for my design?

[Eva]

The input choke does nothing without some capacitance before it and after it. It must be designed as a single CLC filter system, whose target is to provide good attenuation above, say, 1Mhz.

In my last car amplifier I used 100nF and 10uF (0.3 ohm ESR) before the inductor and two 2200uF low ESR (.014 ohm) after the inductor. The inductor itself was made from 10.5 turns of 1.8mm magnet wire on a 35mm D x 10mm H ferrite rod, which gave 5.3uH and 40A saturation.

If you model this system with any circuit simulator, you will notice that it only has a little resonant bump at 20Khz (not much of a problem since it's not tall) and then it provides 40dB attenuation at 1Mhz (and more for higher frequencies).

This is not actually the whole story because the transformer leakage inductance and the secondary side capacitance should also be included in the system.

You will have to design a similar system tailored to your needs. I design rod inductors by measurement and trial and error, and by extrapolation of data from other models that I built previously. There are threads on measuring inductors, use the search engine.

Never throw random components at the circuit, learn to size them properly so that everything merges together nicely.

[acid_k2]

hi all

@milad-k800_
100uH 100A inductor is a BIG beast.
I haven't experiences at such dimensions.

A starting point here:
http://info.ee.surrey.ac.uk/Workshop/advice/coils/#ec

and ETD cores datasheets.

But as Eva suggested, you could use a smaller inductor in the CLC filter if you have more capacitance.

[zilog]

I am in a similar situation with my smps, just how important is the input filter in a car audio application? I suspect that the 5mOhm ESR capacitor bank I have will attenuate quite a lot of the 106kHz switching fundamental.

Is there any need for input filtering in a car audio smps other than to filter out altenator noise? Some amplifiers I have looked at have large inductors to keep out alternator noise, but these inductors go into full saturation already after a few amperes, and thus become useless for EMI filtering.

[Eva]

The purpose of the filter is quite obvious: To attenuate the harmonics of the switching frequency (and the ringing) before they have a chance to enter the electrical system of the vehicle, where they would get easily radiated.

Good attenuation is required above 1Mhz and up to 30Mhz (little things are likely to happen in a push-pull above that frequency).

[zilog]

Originally posted by Eva
The purpose of the filter is quite obvious: To attenuate the harmonics of the switching frequency (and the ringing) before they have a chance to enter the electrical system of the vehicle, where they would get easily radiated.

Good attenuation is required above 1Mhz and up to 30Mhz (little things are likely to happen in a push-pull above that frequency).

I have not simulated using your values yet, but in my older simulations including a pi-style input filter (I had a lot more inductande than you, maybe erroneously high values) with a simulated smps feeding a pulse-train load, I have observed that the input filter comes into resonance from time to time. How do you place the corner frequency and Q-factor of the filter with respect to the switching frequency, output filter corner frequency, control loop bandwidth and so on to avoid this? Have I aimed at a too low corner frequency of the input filter?

[Eva]

The trick is to use the ESR of the electrolytic capacitor before the inductor to damp the resonance (the ceramic one handles the RF). It helps too if the resonance happens at a high frequency so that little excitation current is available and a not too big capacitor value is required.

[zilog]

Originally posted by Eva
The trick is to use the ESR of the electrolytic capacitor before the inductor to damp the resonance (the ceramic one handles the RF). It helps too if the resonance happens at a high frequency so that little excitation current is available and a not too big capacitor value is required.

How do you model the battery/generator impedance, and how do you model the complex load the smps constitutes? The simulated damping factor and resonances in the system is quite dependent on this.