Reply by Robert Baer July 4, 20142014-07-04
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.
Reply by RobertMacy July 3, 20142014-07-03
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.
Reply by Klaus Kragelund July 3, 20142014-07-03
On Thursday, July 3, 2014 12:25:06 AM UTC+2, Robert Macy wrote:
> On Wed, 02 Jul 2014 09:22:20 -0700, Klaus Kragelund > > <klauskvik@hotmail.com> wrote: > > > > >> ...snip.... > > > Any insights into the cause and the remedy? > > > > > > Thanks > > > > > > Klaus > > > > Interesting gets worse with lighter loads. > >
That's because the conduction angle get smaller
> > Where do I see what's inside your emi filter? >
I cannot disclose that, sorry
> > > What's the physical location of all these parts? > >
On a big 4 layer PCB board
> > What LISN are you using? > >
Standard 3 phase LISN as per CISPR something something
> > Who manufactured your 1N4007's? > >
We are not using the 1N4007, that's just to show what we are talking about. The diodes are integrated into a custom Fairchild power module
> > You seem like you've been through a lot of these battles and really are > > caught off guard here. Sympathies. At least you're not at the Test Lab at > > 2am trying to sort this out. >
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
Reply by josephkk July 3, 20142014-07-03
On Wed, 02 Jul 2014 13:54:09 -0500, Jon Elson <jmelson@wustl.edu> wrote:

>Klaus Kragelund wrote: > > >> One suspicion I have is interaction of the diode bridge with the EMC >> filter, causing undamped ringing. But it's hard to measure, when we =
are
>> talking 550V DC, to catch a small signal which the differential probe >> cannot resolve. >Make a pickup coil and hang it near the area, and also display current >from a current probe, and sync to that. Even syncing to a voltage >probe would probably be good enough. Then, correlate the pickup coil's >signal to the waveform to understand what part of the rectifier's cycle >is producing the output. it will almost certainly be pulsed at mains >frequency. > >Jon
That's a good tip. I will remember it. ?-) =20
Reply by Bill Sloman July 3, 20142014-07-03
On Thursday, 3 July 2014 02:22:20 UTC+10, Klaus Kragelund  wrote:
> Hi >=20 > I am working on a high power frequency inverter (>1kW) and have a "funny"=
looking emission graph
>=20 > The converter is the typical inverter type, EMC filter->3 Phase Rectifier=
->DC Inductor->DC Link capacitor->6 switch Inverter->Motor
>=20 > Like this: > =20 > www.electronicsdesign.dk/tmp/Inverter.jpeg > =20 > We stripped down the circuits and isolated the noise to the EMC filter an=
d bridge rectifier.
> =20 > The problem is the very high conducted emission noise at 150kHz: >=20 > www.electronicsdesign.dk/tmp/NoisefromRectifier.pdf > =20 > (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)
> =20 > www.electronicsdesign.dk/tmp/PowerModel_V0.pdf > =20 > The bridge rectifier is a part of an integrated power module, but that no=
ise also comes from a standard single phase rectifier
> =20 > 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 t= hink the reverse recovery current glitch would be worse at higher currents = running in the bridge rectifier
> =20 > I have tried with snubbers and that did not help.=20 > =20 > Any insights into the cause and the remedy?
No insight into the cause, but the remedy usually turns out to be working o= ut the path around which the high-frequency current circulates, and finding= a way of making the area included within that loop smaller, and the path i= tself shorter. It can help if you can twist the path, so that adjacent patc= hes of the included area are radiating in opposite senses.=20 Ralph Morrison's book is helpful in getting you to think about the relevant= physics http://www.amazon.com/Grounding-Shielding-Techniques-Ralph-Morrison-ebook/d= p/B000WCVEN8 I've got a copy of the 4th edition (which is what Amazon is selling), which= has been up-dated to "the digital age" but the good stuff dates from the f= irst edition, which I first read back in about 1967. --=20 Bill Sloman, Sydney
Reply by Joerg July 2, 20142014-07-02
John Larkin wrote:
> On Wed, 02 Jul 2014 14:25:10 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Wed, 2 Jul 2014 09:22:20 -0700 (PDT), Klaus Kragelund >>> <klauskvik@hotmail.com> 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 >>> Could still be reverse recovery spikes. Can you poke around with an >>> oscilloscope? >>> >> Aside from using current transformers or a current probe, I'd set a >> communications receiver to 150kHz, don the headphones, connect an EMCO >> I-probe (a smaller one) and sniff around. The human ear is much more >> sensitive and faster than even a fancy scope or analyzer. >> >> Of course, every time I do that at a new client some people think that I >> am off my rocker ... > > But there's nothing like an oscilloscope to tell you WHY things are > happening. Zoom in on the falling edge of the transformer secondary, > just past the AC peak when the diodes are coming out of conduction; > that can be horrific. >
Sure, in this case it probably works. Maybe. But there were cases where folks had the whole arsenal on the bench. Scopes, analyzers, all five-digit Dollar gear, and nothing could be seen. My first case was in the late 80's when they were sure the noise was coming out of one of the signal processing boards. It had to. It just had to. Out came the receiver and head phones. Turned out it leaked from board at the other end of the system, a RAM bank on a video board. Everybody sat there in disbelief but the fix to prove it took only minutes. -- Regards, Joerg http://www.analogconsultants.com/
Reply by John Larkin July 2, 20142014-07-02
On Wed, 02 Jul 2014 14:25:10 -0700, Joerg <invalid@invalid.invalid>
wrote:

>John Larkin wrote: >> On Wed, 2 Jul 2014 09:22:20 -0700 (PDT), Klaus Kragelund >> <klauskvik@hotmail.com> 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 >> >> Could still be reverse recovery spikes. Can you poke around with an >> oscilloscope? >> > >Aside from using current transformers or a current probe, I'd set a >communications receiver to 150kHz, don the headphones, connect an EMCO >I-probe (a smaller one) and sniff around. The human ear is much more >sensitive and faster than even a fancy scope or analyzer. > >Of course, every time I do that at a new client some people think that I >am off my rocker ...
But there's nothing like an oscilloscope to tell you WHY things are happening. Zoom in on the falling edge of the transformer secondary, just past the AC peak when the diodes are coming out of conduction; that can be horrific. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by Joerg July 2, 20142014-07-02
Klaus Kragelund wrote:
> On Wednesday, July 2, 2014 11:25:10 PM UTC+2, Joerg wrote: >> John Larkin wrote: >> >>> On Wed, 2 Jul 2014 09:22:20 -0700 (PDT), Klaus Kragelund >>> <klauskvik@hotmail.com> 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 >>> Could still be reverse recovery spikes. Can you poke around with >>> an oscilloscope? >> >> >> Aside from using current transformers or a current probe, I'd set a >> >> >> communications receiver to 150kHz, don the headphones, connect an >> EMCO >> >> I-probe (a smaller one) and sniff around. The human ear is much >> more >> >> sensitive and faster than even a fancy scope or analyzer. >> >> >> >> Of course, every time I do that at a new client some people think >> that I >> >> am off my rocker ... >> > > I all ready did that. > > I used the Rohde und Schwarz test receiver in manual mode at 150kHz, > listened to the speaker, even took out the signal to a scope. That's > how we found out it was related to the bridge in the first place, > nice 300Hz signal there, but difficult to see anything else that is > was mains related > > Another nice trick is to extend the normal range (150kHz and up), by > running at say 10kHz and up and at 200Hz bandwidth, that reveals if > the noise is broadband and the individual spikes comes out clearly >
What I often do is pick one of the nastier spikes in the conducted emissions plot, dial the receiver onto it and then turn on a 300Hz crystal filter. By 600Hz it's rolled off 60dB or so, so that allows me to really zero in on stuff and sniff with smaller magnetic field probes. Otherwise the din of many other sources can muffle the signal you are hunting. My hunch is that you'll probably need a better EMC filter, assuming the grounding at the EMC filter is ok. -- Regards, Joerg http://www.analogconsultants.com/
Reply by RobertMacy July 2, 20142014-07-02
On Wed, 02 Jul 2014 09:22:20 -0700, Klaus Kragelund  
<klauskvik@hotmail.com> wrote:

>> ...snip.... > Any insights into the cause and the remedy? > > Thanks > > Klaus
Interesting gets worse with lighter loads. Where do I see what's inside your emi filter? What's the physical location of all these parts? What LISN are you using? Who manufactured your 1N4007's? You seem like you've been through a lot of these battles and really are caught off guard here. Sympathies. At least you're not at the Test Lab at 2am trying to sort this out. Most SA's have a line sync option that let's you find a bit more information about the spikes and AC mains without having to delve into narrow bandwidth's, which also works. The loop and finding out EXACTLY where the spike occurs sounds useful.
Reply by Klaus Kragelund July 2, 20142014-07-02
On Wednesday, July 2, 2014 11:25:10 PM UTC+2, Joerg wrote:
> John Larkin wrote: >=20 > > On Wed, 2 Jul 2014 09:22:20 -0700 (PDT), Klaus Kragelund=20 >=20 > > <klauskvik@hotmail.com> wrote: >=20 > >=20 >=20 > >> Hi >=20 > >>=20 >=20 > >> I am working on a high power frequency inverter (>1kW) and have a >=20 > >> "funny" looking emission graph >=20 > >>=20 >=20 > >>=20 >=20 > >> The converter is the typical inverter type, EMC filter->3 Phase >=20 > >> Rectifier->DC Inductor->DC Link capacitor->6 switch Inverter->Motor >=20 > >>=20 >=20 > >>=20 >=20 > >> Like this: >=20 > >>=20 >=20 > >> www.electronicsdesign.dk/tmp/Inverter.jpeg >=20 > >>=20 >=20 > >> We stripped down the circuits and isolated the noise to the EMC >=20 > >> filter and bridge rectifier. >=20 > >>=20 >=20 > >> The problem is the very high conducted emission noise at 150kHz: >=20 > >>=20 >=20 > >> www.electronicsdesign.dk/tmp/NoisefromRectifier.pdf >=20 > >>=20 >=20 > >> (Conducted emission plot narrowed in from 150kHz to 2MHz, conducted >=20 > >> with a 2-stage EMC filter and a bridge rectifier with a capacitor >=20 > >> on the output and a resistive load) >=20 > >>=20 >=20 > >> www.electronicsdesign.dk/tmp/PowerModel_V0.pdf >=20 > >>=20 >=20 > >> The bridge rectifier is a part of an intregrated power module, but >=20 > >> that noise also comes from a standard single phase rectifier >=20 > >>=20 >=20 > >> First thought would be that the noise is coming from the reverse >=20 > >> recovery of the diodes, but the noise increases a less load (< >=20 > >> 200W), and I would think the reverse recovery current glitch would >=20 > >> be worse at higher currents running in the bridge rectifier >=20 > >>=20 >=20 > >> I have tried with snubbers and that did not help. >=20 > >>=20 >=20 > >> Any insights into the cause and the remedy? >=20 > >>=20 >=20 > >> Thanks >=20 > >>=20 >=20 > >> Klaus >=20 > >=20 >=20 > > Could still be reverse recovery spikes. Can you poke around with an=20 >=20 > > oscilloscope? >=20 > >=20 >=20 >=20 >=20 > Aside from using current transformers or a current probe, I'd set a >=20 > communications receiver to 150kHz, don the headphones, connect an EMCO >=20 > I-probe (a smaller one) and sniff around. The human ear is much more >=20 > sensitive and faster than even a fancy scope or analyzer. >=20 >=20 >=20 > Of course, every time I do that at a new client some people think that I >=20 > am off my rocker ... >=20
I all ready did that. I used the Rohde und Schwarz test receiver in manual mode at 150kHz, listen= ed to the speaker, even took out the signal to a scope. That's how we found= out it was related to the bridge in the first place, nice 300Hz signal the= re, but difficult to see anything else that is was mains related Another nice trick is to extend the normal range (150kHz and up), by runnin= g at say 10kHz and up and at 200Hz bandwidth, that reveals if the noise is = broadband and the individual spikes comes out clearly Cheers Klaus