What is a defendant's duty to instruct the jury on an affirmative defense?

A defendant is entitled to a jury instruction on an affirmative defense if the record contains substantial evidence evidence sufficient for a reasonable jury to find in favor of defendant to support the defense. "In determining whether the evidence is sufficient to warrant a jury instruction, the trial court [decides] only whether 'there was evidence which, if believed by the jury, was sufficient to raise a reasonable doubt . . . .'" (People v. Salas (2006) 37 Cal.4th 967, 982.) But if the evidence of the purported defense is minimal or insubstantial, there is no duty to instruct on it. (People v. Barnett (1998) 17 Cal.4th 1044, 1145.)