Mextram Model Convergence Issue with Selfheating

1
Mextram Model Convergence Issue with Self-heating

• Cadence Design Systems, Inc.
• May, 2006

2
Outline

• History
• What we have done from Cadence
• Solution

3
Mextram convergence issue history

• Intersil reported the convergence issue with
  self-heating in Q1 2005
• As of April 13 2006 phone meeting, there are
  extensive testing, but no solid conclusion and
  clear solution found yet.

4
Check possible derivative bugs in derivative
calculation

• Inconsistency between derivative and equation is
  the most common reason for convergence issue.
• We tried ADMS-CMI flow to convert Verilog-A code
  to CMI model. This doesnt help the convergence
  either.
• This means bugs in derivatives calculation, even
  if there are any, are not the real reason for the
  convergence issue.

5
Model Property Checking in BsimProPlus

• BsimProPlus ( Cadence owned model parameter
  extraction tool ) is used to check the model
  behavior with temperature
• The Ie-temperature (left plot) and the derivative
  (right plot) look like unphysical

6
The solution and conclusion

• By adding a Gmin between thermal node and ground
  solved the convergence problem for Intersil case
• We will send the detailed fix to TU Delft/Philips
• This is not the final solution yet.
• We suspect that unphysical behavior of
  temperature model is the root cause of the
  convergence problem and improvement in
  temperature model is critical to improve
  convergence.
• We also noticed that very small base resistor
  between B1 and B2 makes convergence difficult.
  Clamping non-linear resistors in the model above
  a small value helps convergence.