PID Controller Tuning in Scalable Multiphase Buck Converters under Constant On-Time Control for Ultra-Fast Transient with Phase Current Balancing

Abstract

With the growing computational capability of modern cloud servers, multiphase buck converters need to meet ultra-fast transient requirement with increasing phase count. As per the latest industry trend, a combination of linear and nonlinear control methods is attempted. However, this requires more resources, including extra circuits for smooth mode transitions, which require more silicon area. This paper considers constant on-time voltage mode control with a PID voltage controller (or a type-III compensator) in a scalable multiphase buck converter. A large-signal based PID controller tuning method is developed with the objective to achieve near time optimal transient recovery for step load transients using a simple voltage feedback controller. Considering op-amp saturation limits, practical PID controller gains are formulated, which can be extended for increasing phase count. Under phase mismatches, the current balance is achieved using an adaptive on-time method by considering a slower current loop using low bandwidth sensed low-side phase currents, and the loop bandwidth is kept less than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{sw}}/20$</tex> to avoid any interaction with the voltage feedback loop. Large-signal based design case studies and effects of parameter variations are considered. MATLAB and SIMPLIS simulation results are presented to demonstrate ultra-fast transient performance using the proposed tuning.