What obligations does a trial court have to instruct a defendant on a specific defense?

The trial court has a sua sponte duty to instruct on the "'"general principles of law relevant to the issues raised by the evidence."'" (People v. Breverman (1998) 19 Cal.4th 142, 154.) This includes a sua sponte obligation to instruct on a specific defense, "'"if it appears that the defendant is relying on such a defense, or if there is substantial evidence supportive of such defense and the defense is not inconsistent with the defendant's theory of the case."' [Citation.]" (People v. Maury (2003) 30 Cal.4th 342, 424.) Substantial evidence means evidence of a defense, which, if believed, would be sufficient for the jury to find a reasonable doubt as to the defendant's guilt. (People v. Salas (2006) 37 Cal.4th 967, 982.)