Hi everyone,

i have a switcing regulator circuit using lm2576, at its output there is a ripple voltage with changing value change. i have tried to add filter capacitor but no change at all. can anyone give me some suggestions? thank you

 

The output should have some ripple, because its what the regulator generates as it decides to switch on or off. The regulator switches on, and the voltage rises, off, and it falls. You don't mention the level of ripple, but it should be millivolts. Its also unclear when you say "the ripple voltage with changing value change." Adding more capacitance usually just changes the frequency of the ripple but not the amplitude much. To get the output ripple down, the typical and most effective way is to add another series inductor after the output capacitor, and add a capacitor to ground after that. The inductor counteracts ripple at high frequency, and converts these variations to a more smooth steady voltage. The following capacitor then tries to smooth out these fluctuations further to reduce ripple even more. The inductor resistance, and inductance as well as the capacitance are what determines the ripple reduction and amount of dc drop (loss) across the inductor, which can be minimized. Example: use a 100mhy inductor and a ceramic 10uF capacitor paralleled with a 270 uf aluminum cap and you will significantly reduce ripple. The ceramic capacitor most efficiently takes care of the high frequency part of the ripple, the larger Aluminum cap the lower frequencies. The peak current capability of the inductor should be at least the peak load current, and the inductor resistance will lose (drop) voltage based on the load current and resistance (E+I*R), so minimize resistance to minimize voltage drop. Sorry for this simple explanation, but I am guessing from what little information you gave. A Inductor with 1.27 Ohms resistance will lose about 1.27 volts per ampere load.

 

so the resulting ripple is indeed worth mV and its value is always changing. The highest value produced is 600 mV and the lowest is about 80 mV.Oke, thank you for your suggestion, i will try it.

 

The other thing that is often done is to have the switching reg running a couple of volts higher than the required output than feeding it into a low dropout linear regulator for better noise results. This saves the linear reg from having to dissipate lots of power at higher current, and gives the cleaner output of a linear reg.
The best of both worlds.

 

Your 80 to 600mv ripple measurement appears to be far too high, and suggests some possible problems. One possibility is that the inductor, capacitor, and other circuit values may not be correct. Another possibility is your wiring and circuit arrangement may not be best. Also, measuring with an oscilloscope, especially with long lead wires for the ground, can give the appearance of having more ripple voltage than actually exists. I suggest reviewing the literature for this regulator and making any changes before adding the extra inductor and capacitors. You should be able to measure around 10 to 40mv peak ripple with this regulator using proper components and good layout of components before adding the extra filter (the inductor and capacitors added to the output that I describe). If the switching regulator is properly shielded and located away from sensitive circuits, adding the extra inductors and capacitors should get noise down to sub-millivolt levels. If the circuit is located near the load your are driving, the noise radiated from the circuit elements will get into the load circuit and filtering will not help at that point. I suggest reviewing and repairing the circuit first, get the ripple down to the low millivolt range, then start adding the shielding, and filter section I suggested earlier. Do first things first! Also, as you work on the circuit, you will learn a lot. Example: the main inductor in the switching regulator circuit will radiate less noise if it is a shielded type, and its position and orientation is important as well. Having filtering on the circuits input can help also. Have fun!