1) In some cases, additional EMI filters added between the AC input of the system and the power supply can cause the surge waveform to ring higher, causing the power supply to fail. In one case, I was doing Common-Mode surge testing on a system that had extra EMI filters. The surge voltage that the supply saw at it's input terminals was approximately 2x the applied test voltage! The power supply manufacturer may have done their testing with no additional filtering.
2) Be careful when measuring DC output voltages during Common-Mode testing. Oscilloscope grounding & stray capacity can cause the displayed waveform to look worse than it really is. I float the oscilloscope ground, and wrap the scope lead through a large ferrite core to reduce the stray effects. 3) The power supply manufacturer claimed the unit passed their testing. What was their Performance Criteria: A, B or C? The description for Criteria A states 'Normal performance with the specification limits'. What were the specification limits they used? On Tue, 13 Jun 2000 18:09:11 -0400, e...@itsqs.com wrote: >What I have been hearing from customers is that when they measure the >secondary voltage (3.3VDC,5.0VDC)on the power supply with a scope after the >surge, the voltage drops momentarily thus re-setting the processor. The >power supply when tested as a stand alone unit passes EN 61000-4-5 because >it continues to run. > >Usually the client puts the onus on the power supply manufacturer. >Best Regards > >Edward F. O'Toole >Intertek Testing Services > >-----Original Message----- >From: Rich Nute [mailto:ri...@sdd.hp.com] >Sent: Tuesday, June 13, 2000 4:52 PM >To: rbus...@es.com >Cc: emc-p...@majordomo.ieee.org >Subject: Re: Line/Neutral to Earth MOVS > >Hi Rick: > >> Somewhat related to this discussion is the opportunity I had last week to >> perform surge testing on an off-the-shelf, rack mounted PC. The power >> supply >> in question was from a reputable manufacturer and had safety, EMC and CE >> markings. The system (power supply) passed the "Heavy Industrial" limits >> when tested line to neutral but the computer would reset when tested L/N to >> ground. My rationale for this anomaly was the lack of a suppression device >> (MOV?) to ground. > >I can take some guesses as to what is happening: > >1) The surge current path within the PC developed an > EMI radiated signal within the PC that initiated a > reset. > >2) The PC was interconnected with other units; the > surge current path divided between the PC's ground > wire and the interconnected units' ground wires. > A voltage developed across the signal ground wires > (because they are smaller diameter and higher > resistance the protective ground wires) which > appeared to be a corrupt data signal. The PC reset. > > (However, I would have expected corrupt data, not a > consistent reset, from this hypthesis.) > >3) Same as 2, but an EMI radiated signal from the > grounding path caused the reset. > >Whether or not the PC has a discrete suppression device, >the surge current MUST return to its source (i.e., >complete the circuit). The suppression device limits >the voltage excursion, and probably actually increases >the current. So, I'm not at all sure that a suppression >device would prevent the reset > >In the absence of a suppression device, the current is >due to the Y capacitors and stray capacitance in the unit. >Initially, the surge current will be high, but then, >through the capacitance, it can decay faster than with a >discrete suppression device. But, the Y-capacitors will >cause a double differentiation and there will be a >negative voltage swing before the surge event is complete. > >A "suppression device" CAN be used to control the current >path through the PC such that the surge current does not >cause a reset. > >By the way, Y capacitors can also set the surge current >path. > > >Best regards, >Rich > > > > > > >------------------------------------------- >This message is from the IEEE EMC Society Product Safety >Technical Committee emc-pstc discussion list. > >To cancel your subscription, send mail to: > majord...@ieee.org >with the single line: > unsubscribe emc-pstc > >For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > >For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > >------------------------------------------- >This message is from the IEEE EMC Society Product Safety >Technical Committee emc-pstc discussion list. > >To cancel your subscription, send mail to: > majord...@ieee.org >with the single line: > unsubscribe emc-pstc > >For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > >For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > ---- Patrick Lawler plaw...@west.net ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
