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Generalized Macromodels for Input Filters in Switching Converters

IP.com Disclosure Number: IPCOM000099915D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 124K

Publishing Venue

IBM

Related People

Kelkar, RS: AUTHOR

Abstract

The invention is described with reference to Fig. 1 which shows the power stage of a typical full-bridge switching converter. The supply voltage is fed in through an input filter, comprised of L1 and C1, 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.

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Generalized Macromodels for Input Filters in Switching Converters

       The invention is described with reference to Fig. 1 which
shows the power stage of a typical full-bridge switching converter.
The supply voltage is fed in through an input filter, comprised of L1
and C1, 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 input filter is used attenuate the switching current
waveforms and ensure that the supply voltage carries DC current.
However, it is well known that the input filter can have serious
effects on converter performance as well as stability.  Cases have
been documented where a well-designed converter was paired with an
input filter and the complete supply had stability problems,
manifested as oscillations in the output.  Analyzing the effects of
the input filter is best done through simulation and the key
contribution described in this disclosure is a generalized macromodel
for the input filter that can be used easily in a simulation of the
complete circuit.

      Simulating a switching converter using analog models leads to a
very practical, accurate and general technique (1), as described
earlier.  The input filter macromodel is designed to be used with the
other models described so that the complete system can be easily
simulated.

      Using the well-known averaging technique (2), the model is
derived from the state space equations.  The equations are averaged
and the resulting single equivalent nonlinear equation is synthesized
in terms of a circuit.  The synthesis of a circuit from the
equivalent equation is the key step because of the need to increase
user friendliness by making the model both easily identifiable as
well as general.

      Fig. 2 shows the switching converter model with the analog
models included.  The inverter and rectifier portions are modeled in
terms of two analog models, DOWN and XFMR, as described earlier.  The
input filter model is identical to the actual filter topology for
large signal simulation (such as transient response and cross
regulation); the model in Fig. 2 is designed for large signal
simulation.  The close correspondence between the model and the input
filter circuit makes it very easy to use the model.  It is to be
noted that if the input filter topology is a more complicated one
than the one shown (perhaps a multi-stage filter), the model can
still be used because the same principle applies.  The model will
still be identical to the actual circuit topology.

      Fig. 3 sh...