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*To*: Yair Darshan <YairD@xxxxxxxxxxxxxx>*Subject*: Re: Potential heating problem with standard patch Cat5 panels*From*: Jack Andresen <jandresen@xxxxxxxxxx>*Date*: Mon, 07 May 2001 11:41:28 -0700*CC*: stds-802-3-pwrviamdi@xxxxxxxx*References*: <E41E8344567CD411A71A00D0B723EDA5AFE2EA@SUN>*Sender*: owner-stds-802-3-pwrviamdi@xxxxxxxx

Yair, Correction: A 0805 SMT resistor is 2 X 1.6 mm, which translates to 79 X 63 mils. If we assume that the effective convection/radiation area is only that area (i.e., no conduction to the board nor to the traces) you have .07/.005 or 14 watts/sq in. That would clearly burn up. Hence, you need to take into account nearby conduction. If we allow .25 in. spacing per device in an array and assume uniform conduction/radiation, we still get .07/0625 or 1.1 watta/sq in. That translates to over 100 degrees C rise. Still a problem. Concerning my numbers, I did make a mistake that I only realized when trying to rationalize your numbers. I forgot that my model was for a pair, yielding twice the wattage. Thus, my calculation should have shown a 19 degree C rise. Still not a problem. Note. At 25 degree C a .1W/sq.in generates 15 degree C temperature rise while 1W/sq.in. gives a 100 degree C rise Jack Andresen ****************************** Yair Darshan wrote: > > What is the worst case of the two cases: > > Case 1: PC board trace with 0.07W/inch generating 12C rise > Case 2: 0.07W dissipated on 0805 SMT resistor rated to 125mW and its area is > 8mils x 5 mils. > > I know that case 2 is being used without failure reporting. Since in case 2 > the power density/area is much higher than case 1, it seems that it is not a > real problem. > > What others think about it? > > Yair. > > > > > -----Original Message----- > > From: Jack Andresen [SMTP:jandresen@xxxxxxxxxx] > > Sent: ה, מאי 03, 2001 11:21 PM > > To: stds-802-3-pwrviamdi@xxxxxxxx > > Subject: Potential heating problem with standard patch Cat5 panels > > > > A customer mentioned panels burning up when one of his customers senr > > power down the network.. I did some canculations and found a potential > > problem. > > > > Jack Andresen << File: DTE power.doc >>

**References**:**RE: Potential heating problem with standard patch Cat5 panels***From:*Yair Darshan

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