Delivering global DC convergence for large mixed-signal circuits via homotopy/continuation methods

Abstract

Homotopy/continuation methods are attractive for finding dc operating points of circuits because they offer theoretical guarantees of global convergence. Existing homotopy approaches for circuits are, however, often ineffective for large mixed-signal applications. In this paper, we describe a robust homotopy technique that is effective for solving large metal-oxide-semiconductor (MOS)-based mixed-signal circuits. We demonstrate how certain common circuit structures involving turning-point nesting can lead to extreme inefficiency, or failure, of conventional probability-one homotopy methods. We also find that such situations can lead to numerical ill-conditioning and homotopy paths that fold back upon themselves, leading to algorithm failure. Our new homotopy model for MOS devices, dubbed Arc-tangent Schichman-Hodges (ATANSH), features decoupled continuation parameters that are instrumental in avoiding these problems. ATANSH-based homotopy methods in production use have led to the routine solution of large previously hard-to-solve industrial circuits, several examples of which are presented.