SMPS Ripple Reduction on DC Side

[oddbudman]

smps ripple

Hey ppl.

Currently I am working on a project that uses a SMPS and we are seeing excessive ripple voltage on the DC side. The switchmode supply is based around the Fairchild FPS365R..

The ripple is at 100khz (the smps frequency).

Just wondering if there is any way to reduce its amplitude. Currently it is 640mV p2p on a 6.6V output (10%).

Any tips would be appreciated, we think it may have something to do with our snubber network, but are unsure.

Attached is a AC waveform capture showing the 100kHz 640mV noise.

 

[IanP]

ripple reduction smps

Try to implement additional output filter with >1mH inductors on both rails ..
See picture below ..
Regards,
IanP

 

[artem]

ripple in smps

have a look here :

 

[pisoiu]

ripple smps

I assume the ripple from the image is with the load connected. First, make sure the trnsformer is not saturated. Check this by measuring the current in the primary winding. I am not familiar with this IC, but if you have a low value resistor (<0,5 ohm) between the swiching element and the primary ground, you may check the voltage across it. It should be a ramp of triangular shape. The ramp of the triangle should be linear. If at some point it rises abruptly, then the core gets saturated, and this is the cause of your noise, at lease one of them. Check this for all the input voltages you want to use (use an autotransformer), and in a full load condition. Make sure to connect the osc. probe right across the resistor terminals, otherwise you will collect noise through ground traces. If the core gets saturated, then you will have to re-design it, and perhaps to use a bigger air gap between halves.
Secondly, check the transformer winding style, it is important. The reccomended order is primary, bias, shield, secondary1, secondary2... Span all the windings across all the width of the support, but keep the margin distance. Try to use a shield made from a copper foil connected to the plus side if the primary winding.
In the last, check the PCB, it is very important. Avoid stubs, use thick traces from the windings to the caps. Use several caps for each filtering group, do not use only one high value cap, that one will not cut the high freq. noise. Use also 100nF and 10nF in parralel. Try to cut the solder mask above those traces, the extra solder deposited on them at the PCB factory will lower the impedance of the traces.

/pisoiu

 

[ebenni]

reduce ripple voltage in smps

Best method,

regards.