Large Noise from Bridge Rectifier; Conducted Emission Measurement

On Thu, 03 Jul 2014 15:33:21 -0700, Klaus Kragelund  
<klauskvik@hotmail.com> wrote:

>> ...snip....
>
> Tonight it was 10 PM :-)
>
> We have full fledges internal lab, really cool equipment
>
> I found the cause, it was the reverse recovery of the diodes. Putting a  
> monster 1uF cap over each diode solved the problem. Now I will se how to  
> pull back to smaller size and with resistors
>
> I also solved a resonance in the filter that aggregated the problem
>
> Cheers
>
> Klaus

THANKS for coming back and telling the progress.

I asked about the EMI Filter for the reason that poor EMI Filter once bit  
me! Some previous engineer had selected an absolutely WRONG filter. One of  
those that looks good on paper, but turned out to be the WRONG impedance  
match at EACH end!

I asked about the 'placement' of parts, because I've seen switching  
currents bypass ALL the filtering, like windings on a transformer, with  
the signals simply going around any attempt to EMI filter out stuff.  [a  
little copper tape shielding showed THAT weakness]

And, lastly, I asked about manufacturer BECAUSE FAIRCHILD has historically  
made extremely LOUD rectifiers [having worked there once, I have no idea  
why or how that happened but perhaps they wanted EFFICIENT snap OFF with  
no regard for that reverse recovery time shutoff being so abrupt that it  
almost produces noise out to MHz.] In our 'linear' supplies we simply  
removed Fairchild from our approved vendors list and bought Motorolas,  
General Radio(??) and such.

PS: it is really worth it to model your Test Setup, AC mains cabling,  
LISN's and whole power supply and let LTspice show you where the tendency  
towards ills are.  Matches VERY closely out to around 10MHz and roughly on  
up to 30MHz.

Klaus Kragelund wrote:
> Hi
>
> I am working on a high power frequency inverter (>1kW) and have a "funny" looking emission graph
>
>
> The converter is the typical inverter type, EMC filter->3 Phase Rectifier->DC Inductor->DC Link capacitor->6 switch Inverter->Motor
>
> Like this:
>
> www.electronicsdesign.dk/tmp/Inverter.jpeg
>
> We stripped down the circuits and isolated the noise to the EMC filter and bridge rectifier.
>
> The problem is the very high conducted emission noise at 150kHz:
>
> www.electronicsdesign.dk/tmp/NoisefromRectifier.pdf
>
> (Conducted emission plot narrowed in from 150kHz to 2MHz, conducted with a 2-stage EMC filter and a bridge rectifier with a capacitor on the output and a resistive load)
>
> www.electronicsdesign.dk/tmp/PowerModel_V0.pdf
>
> The bridge rectifier is a part of an intregrated power module, but that noise also comes from a standard single phase rectifier
>
> First thought would be that the noise is coming from the reverse recovery of the diodes, but the noise increases a less load (<  200W), and I would think the reverse recovery current glitch would be worse at higher currents running in the bridge rectifier
>
> I have tried with snubbers and that did not help.
>
> Any insights into the cause and the remedy?
>
> Thanks
>
> Klaus
   Try adding a small resistor in series with each diode; value TBD.