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Analog Models for the Digital Subsystems in Switching Converters

IP.com Disclosure Number: IPCOM000035962D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 4 page(s) / 59K

Publishing Venue

IBM

Related People

Kelkar, SS: AUTHOR

Abstract

The invention is described with reference to Fig. 1 which shows a typical switching converter; only the key subsystems like the inverter, rectifier, output filter, etc., are included. The supply voltage is fed in through an input filter comprised of Lf and Cf to an inverter and rectifier combination. The inverter consists of four MOSFET or BJT transistors that are turned on by the control loop. The power transformer steps down the voltage, and two diodes are used for rectification of the AC voltage at the transformer output. The analog signal processor (ASP) compares the actual output voltage Vo with the desired value (the reference), and the processed error voltage is then fed in to the digital signal processor (DSP).

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Analog Models for the Digital Subsystems in Switching Converters

The invention is described with reference to Fig. 1 which shows a typical switching converter; only the key subsystems like the inverter, rectifier, output filter, etc., are included. The supply voltage is fed in through an input filter comprised of Lf and Cf to an inverter and rectifier combination. The inverter consists of four MOSFET or BJT transistors that are turned on by the control loop. The power transformer steps down the voltage, and two diodes are used for rectification of the AC voltage at the transformer output. The analog signal processor (ASP) compares the actual output voltage Vo with the desired value (the reference), and the processed error voltage is then fed in to the digital signal processor (DSP). The DSP produces a digital signal with the signal width TON controlled by the error voltage, and this signal is then used to turn on the inverter transistors. Output voltage regulation is thus maintained by controlling the conduction time of the transistors.

It is thus seen that a typical switching converter has a mixture of analog and digital technologies. The inverter transistors are not operated in the linear, active region, and thus the inverter is a part of the digital section of the converter. The input and output voltages of the inverter and rectifier (IR) digital section are both analog signals, while the DSP has an analog input but a digital output. Another complication introduced by this mix of technologies is that

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the eigenvalues of the system are far apart, with the digital portions operating at typically 100 KHz and the dominant eigenvalues of the analog portion at typically 300 Hz.

Simulating such a circuit using conventional simulators like SPICE or ASTAP is not easy. The wide range of eigenvalues causes storage, convergence and central processor unit (CPU) time problems. By replacing the digital portions of the system with analog models, as is described in this invention, it is possible to develop a suitable model that removes the storage, convergence and CPU time problems.

The averaging technique 1 is well known and forms the basis for the models. It is used to model the power stage of the converter in order to derive an analog model for the inverter and rectifier section; a model for the DSP is derived in a similar fashion. The state equations are first derived for the ON and OFF portions of the cycle. The equations are averaged, and the resulting single equivalent nonlinear equation is synthesized in terms of a circuit. The key condition that must be satisfied in order to apply the averaging technique is that the dominant resonant frequency of the output filter and the switching frequency must be far apart; this is easily satisfied in most switching converters. The synthesis of a circuit from the equivalent equation is a key step because of the need to increase user friendliness by making the relevant portions easily ...