[SI-LIST] Re: Conducted Emissions and SMPSs help needed
- From: "Derek Walton" <lfresearch@xxxxxxx>
- To: npatel@xxxxxxxxxx
- Date: Fri, 9 Feb 2007 14:24:00 -0600
Erin,
Even though this is a power supply, and the CE spec applies to the
input, dont forget that the output produces emissions too. I have come
across many situations where the output emissions can be observed at the
LISN.
If you have access to a clamp on current probe, try clamping it on the
power wires coming to the LISN, then also leaving to your EUT. It could
be your input filter is fine and the noise is not going through it.
Hence changes you make have no effect.
You could also have a resonance issue, perhaps some small resistor value
in any "tank" network would help By small I mean .1 to .8 ohms or so.
Sincerely
Derek Walton
L F Research EMC Design and Test Facility
npatel@xxxxxxxxxx wrote on 2/9/2007, 1:43 PM:
> Erin,
>
> In the past I have had very good luck using common mode chokes right at
> the input of PS to pass conducted emissions. You can also try increasing
> the X and Y cap values. Try to find the source of the frequency in
> question. There are FET's switching at very high duty cycle on the
> primary and secondary side especially if you have some DC-DC converters
> in it that causes emissions. Try increasing the gate resistance of the
> FET's to slow they down. You might run into thermal issues if you do
> that. Ask the vendor what components, IC's are switching at what
> frequency. I would also start near field probing the PS and see if you
> can pick up anything there. Take the output of the spectrum analyzer and
> feed it into a Oscilloscope to see if you spot the frequency. Check the
> routing on the boards. Usually PS boards are single layer with big power
> and ground loops. Also you might try snapping torrid on cables in the PS
> including the input PS cable to see if one them is acting as an antenna.
>
> Hope it helps.
> Nikhil=20
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
> Sent: Friday, February 09, 2007 10:39 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Conducted Emissions and SMPSs help needed
>
> Hi
> Wondering if any of the Guru's out their might be able to provide some
> advice on how to proceed on a problem I am having with a switch mode
> power supply.
>
> The power supply is a 3 Phase supply (L1,L2,L3,N, Gnd) operates from=20
> 90Volts to 300 Volts AC. Currently, it has a very low margin at 200
> kHz=20
> for CISPR 22 Class B conducted levels. The mains filter consists of 5
> mH=20
> inductors and the standard capacitor values for L-GND and phase to phase
>
> (4n7 and 47n respectively). All my efforts have reduced everything
> over=20
> 400 kHz considerably but has not budged the two spikes at 200 kHz and
> 300 kHz. Using a splitter(0deg and 180deg) and two LISNs, the source of
> the conducted emissions is common mode.
>
> Any ideas on how to reduce this low frequency conducted emissions would
> be greatly appreciated.
>
> Erin
>
>
>
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