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#1 Oct 12, 2016
Volker_D
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Unexpected "Rated Values" in Smoke-Analysis

I am using smoke analysis on a circuit.

For some components the results-window shows some unexpected values for "Rated Value":

In this case V29A is a diode LL4148 from the EDIODE.olb / EDIODE.lib.

The smoke parameters for LL4148 in the Model Editor:

So I would expect rated value in smoke analysis should be 500m instead of 422.8571m. I am not using any deratings.

In Capture the part has no additional smoke related parameters defined:

Any ideas on this?

Thanks and best regards.

Volker.

 

Wed, 2016-10-12 12:42
alok
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PSpice SMOKE analysis applies appropriate deration (let's call it dynamic deration) based on operating condition of device. For semiconductor devices, this is due to current operating temperature of device. Curve shown below defines the deration curve for semiconductor devices. Rated power is available only at 25C and beyond that it is derated as per device specification.

 

Based on available details - I suspect this to be most likely reason for this observation. If you can share the exact operating condition - dissipated power and junction temperature etc., this can be further clarified. The key capability of SMOKE to apply automatic deration for devices as per current operating conditions.

 

Thu, 2016-10-13 05:02
Volker_D
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 @alok: Thanks for your reply.

I did some further investigation:

1) I used a simple circuit with the LL4148  from the EDIODE.olb / EDIODE.lib

 

2) I set the simulation temperature to 25°C

3) Smoke results:

-> The rated value for PDM is 428 mW where I would expect 500 mW!

3) I changed the simulation teperature to 10°C:

-> rated value for PDM ist still lower than 500 mW!

4) I changed the simulation temperature to 0°C:

-> Now the rated value is 500mW.

My conclusion:

The temperature dependent derating  for LL4148 in Pspice is: Decreasing from 500mW @0°C to 0mW @175°C (Tj).

It should be: Decreasing from 500mW @ 25°C to 0°C@ 175°C (Tj).

Now the question ist:
How can I change these ambient temperature deratings?
( In my opinion it should be adjustable for each part individual)

Thanks for further comments. Regards.

Volker.

Thu, 2016-10-13 05:06 (Reply to #3)
Volker_D
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Correction: ... It should be: Decreasing from 500mW @ 25°C to 0mW @175°C (Tj) ...

Thu, 2016-10-13 06:13
Volker_D
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OK, I think I got it:

LL4148 has per default from the lib the following Smoke-Parameters with PDM = 500mW:

PSpice now calculates a corrected "PDM rated value" for smoke-analysis depending on simulation temperature (TNOM) and the smoke-parameters TJ, RJC, RCA:

PDM (rated value) = (TJ - TNOM) / (RJC + RCA)

In this case at TNOM=25°C: PDM (rated value) = (175 - 25) / (300 + 50) =  428 mW (which ist different to the smoke parameter PDM = 500mW!).

If the PDM-value from the model (here: 500mW) ist smaller than the corrected value, then the lower value ist used.

PDM (model) larger than corrected value:

PDM (model) smaller than corrected value:

@Cadence: Some more detailed information in the documentation on this would be useful. Or didn't I find it?

Regards.

Volker.

 

Thu, 2016-10-13 06:25
alok
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Yes, You are correct. As mentioned earlier - derated power rating is available power and that is what used for stress calculation (and display). I did indicate that my note, however now I realize that I could have been more explicit.  In this case due to specific value set the available PDM at 25C is < rated power. Your analysis is very comprehensive. Thank You!

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Cadence