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A Precision and High-Speed Behavioral Simulation Method for Transient Response and Frequency Characteristics of Switching Converters
Abstract
We propose a fast and precise transient response and frequency
characteristics simulation method for switching converters. This method
uses a behavioral simulation tool without using a SPICE-like analog
simulator. The nonlinear operation of the circuit is considered, and the
nonlinear function is realized by defining the nonlinear formula based
on the circuit operation and by applying feedback. To assess the
accuracy and simulation time of the proposed simulation method, we
designed current-mode buck and boost converters and fabricated them
using a 0.18-µm high-voltage CMOS process. The comparison in the
transient response and frequency characteristics among SPICE, the
proposed program on a behavioral simulation tool which we named NSTVR
(New Simulation Tool for Voltage Regulators) and experiments of
fabricated IC chips showed good agreement, while NSTVR was more than 22
times faster in transient response and 85 times faster in frequency
characteristics than SPICE in CPU time in a boost converter simulation.
We propose a fast and precise simulation method for replicating the transient responses and frequency characteristics of switching power converters using a mixture of real circuits and behavioral models. The method used a behavioral simulation tool (Verilog-A), which was supported by an analog simulator such as SPECTRE®. The time-step became variable, however, the nonlinear operation of the circuit was considered and state-space equations were formulated by applying the hybrid-dynamical-system control scheme in behavioral modeling [1]. Each macro block can have either the real schematic and behavioral model. The transient simulation operates in such a way that the analog simulator advances the simulation step by step while obtaining results from behavioral blocks. Further application of the periodic steady state (PSS) and periodic stability (PSTB) analyses, allowed us to obtain the frequency characteristics in short time. For verification, we simulated the circuits of the current-mode buck DC-DC converter IC, which was previously fabricated using a 0.18-um CMOS process. When we chose blocks of a buffer, the switching and output circuits and RS-FF real transistor circuits, while the rest behavioral models, we achieved transient response and frequency characteristics that were 13 and 75 times faster, respectively, than the analog simulator alone, while keeping less than 3.9% and 2.5% of errors compared with IC's evaluation results.
A method for modeling switching-converter power stages is developed, whose starting point is the unified state-space representation of the switched networks and whose end result is either a complete state-space description or its equivalent small-signal low-frequency linear circuit model. A new canonical circuit model is proposed, whose fixed topology contains all the essential input-output and control properties of any dc-to-dc switching converter, regardless of its detailed configuration, and by which different converters can be characterized in the form of a table conveniently stored in a computer data bank to provide a useful tool for computer aided design and optimization. The new canonical circuit model predicts that, in general, switching action introduces both zeros and poles into the duty ratio to output transfer function in addition to those from the effective filter network.
A current-mode control power convertor model that is accurate at
frequencies from DC to half the switching frequency is described for
constant-frequency operation. Using a simple pole-zero transfer
function, the model is able to predict subharmonic oscillation without
the need for discrete-time z -transform models. The accuracy of
sampled-data modeling is incorporated into the model by a second-order
representation of the sampled-data transfer function which is valid up
to half the switching frequency. Predictions of current loop gain;
control-to-output; output impedance; and audio susceptibility transfer
functions were confirmed with measurements on a buck converter. The
audio susceptibility of the buck converter can be nulled with the
appropriate value of external ramp. The modeling concentrates on
constant-frequency pulse-width modulation (PWM) converters, but the
methods can be applied to variable-frequency control and discontinuous
conduction mode
Stability is the key requirement in designing switching converter ASIC. Achieving this requires an accurate loop gain design. Since DC-DC switching converters are time-varying system, traditional small signal analysis in SPICE cannot be directly used to simulate the loop gain of this kind of system. A new method named periodic small signal analysis is proposed to analyze and simulate DC-DC switching converter inside a SPICE like simulator without the need for averaging. This general method is suitable for any switching regulators. The results are accurate comparing with average modeling and experiment results even beyond half switching frequency. A general procedure to design loop gain is demonstrated in a peak current control buck.