how to reduce dc/dc full bridge output noice

[darla1]

how to reduce output of transformer

dear all

i am working on full bridge dc/dc coneverter...when i increase the load on ouput side the noice ripple increases like shown in fig...

i think that this is common mode noice...

i know to elimate this we can use common mode inductor...but i dont want to use common mode inductor for this.

can anybody tell me is there any way to reduce this.

thanks
darla

 

[svhb]

full bridge common mode

You have to be very carefull with these kind of measurments, the ground lead of your probe can pick up enough signal to show this kind of spikes. Try to measure again with an as short a possible ground connection.

A bit like in the attached drawing.

PS. I didn't say there can not be a problem, I just know a wrong measurement can show the problem to look much, much bigger

 

[max0412]

I agree with svhb .

My probe will pick up secondary waveforms off of a flyback SMPS when it’s a several inches away. Ideally you’re supposed to use shielded coax and BNC connectors. If you don’t want to do that Stick your probe with a small ground lead right in the output terminal block this provides some shielding ,or it does for me. I bet your spikes will get a lot smaller.

 

[darla1]

thank you mr svhb and max

i know that probe picks up noice,....but my problem is when i increase the load noice increases and my dc/dc transformer giving some sound.....i dont know why..

but i did some analysis here...when i loaded the converter upto 4A the o/p dc is pure dc with out any spikes/noice....here everything is fine i.e controlling and no sound at my transformer..

when current beyond 4A output voltage has the spikes...and transformer is giving some sound...

thanks
darla

 

[Kral]

darla1,
One simple way of reducing nois is to place a series RC network across the transformer secondary. Values of .001uf and 1.2K work well. It requires a bit of experimentation to determine the optimum values. Much of the noise is caused by momentary shorts across the secondary when one set of diodes turns on before the other set has completely turned off.
Regards,
Kral