Switching Converter | PartQuest™ Explore

This design demonstrates the use of the TDFS (Time Domain Frequency Sweep) simulation method, to measure the open-loop frequency response of an operating closed-loop system containing switching elements.The stability of the "Buck DC to DC Converter - Switching" design is assessed. This is a switching circuit, it does not use a state-average model for the modulator, so the standard AC Analysis method cannot be used. Rather, the frequency response is generated from time-domain simulation results. The TDFS approach can also be used for systems that contain sampling or digital control aspects.This particular example is directly comparable to the design titled "TDFS Loop Stability for Buck DC to DC Converter - State Average". In that design, both the TDFS and AC Analysis methods are used to measure the open loop transfer function of an equivalent non-switching circuit.Note that the approach used to characterize the loop stability, by injecting a small sinusoidal stimulus signal in series with the loop and then measuring the complex ratio of the ground referenced return signal to the injected signal, is described in:D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries

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PWM Modulator - VCVS - corrected

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PWM Modulator - VCVS-------------------------------------------------------------------------------------This is a structural representation of the language-based behavioral model found in SMSSA_modulator_v6p4. It is constructed from delay lines, summing and subtraction points, spectral quantity generatots (i.e. AS signal sources, passive R-C elements and Voltage-controlled voltage sources acting as high gain amplifiers and an integrator. It implements the same modulation expressions as v6p4 . All components are available from LTspice or any other Spice derivative.All formulas have been developed rigorously except for the final dependence of the switch drive output on the control voltage. The assumptions of a periodic Vctl and the scaling applied to that voltage. These factors have been adjusted t o match results below 1/2 Fsw as simulated with the TDFS mechanism developed at Mentor by Mike Donnely.Voltage-Controlled Voltage Sources act as ideal hi-gain opamps with infinite bandwidth, infinite Rin and zero Rout. Fixed AC sources account for the nonlinear behavior of the modulator. They are phase-shifted to 180 degrees to produce an in-phase result after three stages of summing points.

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PWM Modulator - VCVS

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PWM Modulator - VCVS-------------------------------------------------------------------------------------This is a structural representation of the language-based behavioral model found in SMSSA_modulator_v6p4. It is constructed from delay lines, summing and subtraction points, spectral quantity generatots (i.e. AS signal sources, passive R-C elements and Voltage-controlled voltage sources acting as high gain amplifiers and an integrator. It implements the same modulation expressions as v6p4 . All components are available from LTspice or any other Spice derivative.All formulas have been developed rigorously except for the final dependence of the switch drive output on the control voltage. The assumptions of a periodic Vctl and the scaling applied to that voltage. These factors have been adjusted t o match results below 1/2 Fsw as simulated with the TDFS mechanism developed at Mentor by Mike Donnely.Voltage-Controlled Voltage Sources act as ideal hi-gain opamps with infinite bandwidth, infinite Rin and zero Rout. Fixed AC sources account for the nonlinear behavior of the modulator. They are phase-shifted to 180 degrees to produce an in-phase result after three stages of summing points.

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Nonlinear SMPS AC Analysis - Buck Converter - V5p2.2

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V5px is the culmination of my goal to construct a nonlinear SMPS that can be analyzed in the frequency domain without averaging out the switching cycle variations. The circuit modeling follows Vorperian's concept [1] of introducing equivalent circuits for the PWM modulator and the switch leaving the remainder of the converter as is. The PWM modulator and switch model nonlinearities by sourcing voltages at frequencies outside the external stimulae. With the restriction of averaging lifted the model becomes applicable at arbitrary frequencies across the spectrum up to and beyond the UHF band where EMI is studied, At frequencies below Fsw the model should produce the same results as the averaged switch.

This initial version of the modulator and switch is limited to voltage mode control and continuous conduction. It retains the assumption of small disturbances compared to steady state operation but the derivations make limited use of that assumption.

The test bench is the same buck converter Mike Donnelly used to demonstrate TDFS Loop Stability Analysis. I have replaced the switch assembly with an equivalent circuit derived by the methodology Vatche Vorperian documented in his May 1990 paper [1]. I drive the switch with a PWM modulator that sources the Fourier transform of the pulse sequence..

The key to coding the models is the use of VHDL-AMS spectral source quantities to formulate Fourier transforms involving products and ratios of waveforms. By this mechanism the models present all the characteristics of the non-linear components in the DC-DC converter.

swdrive continues to be derived as a closed form expression of the modulator output but now includes the combined steady state and transient drive signals. The switch model outputs a theoretically correct formulation of the switch outputs, the voltage at the common terminal and the current sourced out of the active terminal.

Potential applications:

1) Response at high frequencies to arbitrary disturbances including finite pulses, white noise, AM radio, X-Ray radiation...

2) Analyze susceptibility to electromagnetic interference (requires modeling of that interference).

3) Rad-Hard testing (requires modeling of that interference),

4) Generation of RF interference (e.g., by modeling the choke magnetics).

This version is under test to determine its accuracy and faithfulness to the actual switching characteristics.

[1] See Vorperian's "Simplified Analysis of PWM Converters Using Model of PWM Switch Part 1" in the May 1990 issue (Vol. 26, No. 3) of the IEEE Transactions on Aerospace and Electronic Systems.

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