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Description

This design is a detailed circuit implementation of the more abstract "state-average" buck converter model shown in the companion design example: “Step-Down (Buck) DC to DC Converter - Continuous”. This example includes the low-pass voltage sense circuit, an op-amp implementation of the difference amplifier and the lead-lag compensator, as well as PWM switching control of a power MOSFET. Simulation results for the line and load transients are very similar to the results from the continuous model.

This design uses a number of "datasheet characterized" components, including the power MOSFET (MCH6337), freewheel diode (NRVTS560EMFS) and op-amps (MC33272A), as well as the soft-saturation inductor (XAL6060-223) and capacitor (PEG127KA3110Q) of the power stage . The parameter values of these devices were entered directly from the datasheet for the corresponding part, including the "Maximum Ratings" information.

While the simulation time for this switching circuit is significantly longer than for the abstract model, more detailed information about the circuit’s signals and components is available. This includes the component stress levels, which are monitored within all the "datasheet" models.

The companion design, "TDFS Loop Stability for Step-Down (Buck) DC to DC Converter - Switching", demonstrates a method to directly assess the open-loop frequency response, and hence the stability margin, of this converter. The TDFS (Time Domain Frequency Sweep) method circumvents the need for state-average models of the switching elements.

Copy of Step-Down (Buck) DC to DC Converter - Switching - on Tue, 01/28/2020 - 11:53

Daniel Deitschel
Designer257

Daniel Deitschel

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9 years 6 months

43

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Description

This design is a detailed circuit implementation of the more abstract "state-average" buck converter model shown in the companion design example: “Step-Down (Buck) DC to DC Converter - Continuous”. This example includes the low-pass voltage sense circuit, an op-amp implementation of the difference amplifier and the lead-lag compensator, as well as PWM switching control of a power MOSFET. Simulation results for the line and load transients are very similar to the results from the continuous model.This design uses a number of "datasheet characterized" components, including the power MOSFET (MCH6337), freewheel diode (NRVTS560EMFS) and op-amps (MC33272A), as well as the soft-saturation inductor (XAL6060-223) and capacitor (PEG127KA3110Q) of the power stage . The parameter values of these devices were entered directly from the datasheet for the corresponding part, including the "Maximum Ratings" information.While the simulation time for this switching circuit is significantly longer than for the abstract model, more detailed information about the circuit’s signals and components is available. This includes the component stress levels, which are monitored within all the "datasheet" models.The companion design, "TDFS Loop Stability for Step-Down (Buck) DC to DC Converter - Switching", demonstrates a method to directly assess the open-loop frequency response, and hence the stability margin, of this converter. The TDFS (Time Domain Frequency Sweep) method circumvents the need for state-average models of the switching elements.

Multi Freq v0

Philippe_1
Designer130866

Philippe_1

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7 years 5 months

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TDFS Loop Stability for Step-Down DC to DC (Buck) Converter - Switching

TakayukiAtago
Designer223968

TakayukiAtago

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5 years 1 month

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Description

This example demonstrates the TDFS (Time Domain Frequency Sweep) Loop Stability Instrument Model. It is used to compute the open-loop transfer function of an operating (closed-loop) switching power converter. There is no need for state-average or continuous equivalent models for the modulator section of the design, as normally needed for frequency-domain (or "AC") analysis. Rather, the actual circuit component models can be used directly, because the open-loop transfer function is computed from time-domain simulation results.In this case, the converter is operating at 200kHz switching frequency, and is converting the 12V DC input to a regulated 5V output, while supplying a 5A current to the 1 Ohm load resistor. The TDFS measurement instrument indicates that the open-loop gain crossover frequency is at 26 kHz, and the phase margin is just under 60 degrees. This verifies that the opamp-based lead-lag compensator is providing adequate stability margin under these operating conditions.Note that the TDFS instrument model characterizes the open loop transfer function by injecting a small sinusoidal stimulus signal in series with the loop, and then measures the complex ratio of the return signal to the injected signal, is described in:D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries.The companion example, "Step-Down (Buck) DC to DC Converter - Switching", shows the line and load transient response of this converter design. Another companion example, "Step-Down (Buck) DC to DC Converter - Continuous", uses a state-average model of the switching (or modulator) section of the converter, so it supports traditional "AC" or frequency-domain analysis.

Nonlinear R-L-C

Norm
Designer43361

Norm

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8 years 7 months

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TDFS Loop Stability for Buck DC to DC Converter - Switching

ikguy
Designer7846

ikguy

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9 years

12

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Description

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

PWM Modulator v8

Norm
Designer43361

Norm

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8 years 7 months

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Buck Converter PWM Modulator Line Transient Demo

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Designer43361

Norm

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8 years 7 months

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PWM Modulator SMSSA_modulator_v7p4

Norm
Designer43361

Norm

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8 years 7 months

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Buck Converter, Structur of PWM Modulator - V7p2

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Designer43361

Norm

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8 years 7 months

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Description

V5px, V6px and beyond are working prototypes of my nonlinear SMPS model that that lifts the restriction of averaging from frequency domain SMPS simulations. These initial prototypes are restricted to Voltage Control applications. My intent is to lift that restriction in a subsequent model enhancement.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.