re: spike at the flyback output voltage

re: spike at the flyback output voltage

hi sir,

i have designed flyback topology smps. In the output,i have selected Co capacitor ,this capacitor causes 4V thin spike at the output side. In order to reduce this spike,i have to go with LC filter or i have to increase the capacitance by putting number of capacitors in parallel. but i would like to go with LC filter.

Can any one suggest how to reduce thin thin spike using LC filter.

Pls provide equation to select suitable inductor.

Note: it not in the snubber part. its in the output voltage spike.

Re: spike at the flyback output voltage

Go with LC output filter. using two L both in + and - will reduce conduction RF noise at output too.(for example two 5uh instead of one 10uh)
Cap Paralleling won't eliminate these spikes.

Re: spike at the flyback output voltage

thank you for the reply.

I will put LC filter at the output. But can you explain me with the equation to calculate the LC filter value.

Re: spike at the flyback output voltage

Make sure the spike is 'really' there, if you're using a CRO to view the output the ground wire can act (and will act) as an antenna often showing spikes and muck that isn't really there. (Usually from the sharp snap off of the rectifier diode). If there really is a spike then an LC filter is superior over just capacitive filtering. When you say thin, what what's the spikes duration? as this will be the first indicator to the size of the LC filter you will need. Zoom right into the spike and see if it rings and note that frequency. Also, depending on what voltage and switching frequency you're using, the ideal output caps are large value ceramic types. (Very low ESR -> lower ripple and high freq filtering) However, these are generally only available up to 30V or so in the high capacitance range and are found on relatively high freq switchers. Basically the bigger the LC filter (outside of the feedback loop) the slower the response to transient loads. Generally I would design the filter around the inductor, as these tend to be the bulky part. Look at your output current, expected operating temp, size constraint (assuming cost is not an issue here) and look through that range of inductors, size capacitor accordingly.

Re: spike at the flyback output voltage

s.manikandan said:

thank you for the reply.

I will put LC filter at the output. But can you explain me with the equation to calculate the LC filter value.

Click to expand...

It depends to ESR and ESL of your CAPs and the voltage and current of the smps.
Its typically few uH because capacitor network ESL is around 20nH.