Listed below are
the papers published by Dr. Ray Ridley in the field of power
electronics. After the title and reference for the paper is
the abstract, taken directly from the official proceedings.
Following this, in
italics, is a commentary on the paper and its contributions
and important results.
The papers are in approximate order of their relative importance
to power systems designers. A Designers' Series appears quarterly
in Switching Power Magazine, an REI publication. Refer to the
magazine
website for more information.
Ridley, R. B., A New Continuous-Time
Model for Current-Mode Control
IEEE Transactions on Power Electronics, April, 1991, pp. 271-280.
(Special issue on Modeling for Power Electronic Circuits and
Systems.)
The accuracy
of sampled-data modeling is combined with the simplicity of
pole-zero representation to give a new current-mode control
model, accurate to half the switching frequency. All of the
small signal characteristics of current-mode control are predicted,
including high-frequency subharmonic oscillation which can
occur even at duty cycles of less than 0.5. The best representation
for the control-to-output transfer function is shown to be
third-order. Model predictions are confirmed with measurements
on a buck converter.
The
first model which simultaneously allows prediction of the
current-loop instability, transition from voltage-mode to
current-mode control, and a simple pole-zero representation.
The paper shows why you have to use a three-pole representation
of a current-mode system to get results which closely model
those seen in the lab. A fully expanded version of this paper
is to be found in Dr. Ridley's dissertation, for those interested
in the complete details and history of current-mode modeling.
Send an e-mail to RRidley@ridleyengineering.com
to get information on ordering this dissertation.
Ridley, R. B.,
A New Continuous-Time Model for Current-Mode Control with
Constant On-Time, Constant Off-Time, and Discontinuous Conduction
Mode; IEEE Power Electronics Specialists Conference
Record, San Antonio, Texas, June 1990, pp. 382-389.
A new small-signal
model for current-mode control of PWM
converters is extended for constant on-time control, constant
off-time control, and discontinuous conduction mode. Constant
on- or off-time converters in CCM have two complex right-half-plane
zeros in the current loop feedback, but reduced modulator
gain eliminates current-loop instability. The modulator gain
exhibits frequency-dependent phase which introduces an extra
gain term in the circuit model. The new model for DCM operation
does not have RHP zeros in the current feedback loop, but
the buck converter exhibits an instability for higher conversion
ratios.
This
paper completes the set of models for current-mode control.
Some interesting results were obtained for modulator gain
with the
variable frequency control schemes. The buck converter instability
is a little-known phenomenon that can cause problems for modern
power supplies converting 5 V down to 3.3 V, for example.
Ridley, R. B.,
Secondary LC-Filter Analysis and Design Techniques for
Current-Mode Controlled Converters; IEEE Transactions
on Power Electronics, Vol. 3, No. 4, October 1988, 499-507.
Small-signal
characteristics of current-mode-controlled PWM converters
with a second-stage LC filter are analyzed. A secondary filter
can be designed to provide good attenuation of the switching
ripple while maintaining adequate stability margins with capacitive
loading. Design guidelines for the filter are given.
This
paper shows you how to properly design and model a second-stage
filter. This is a very good and practical guide on the right
way to reduce the noise on the output of your power supply
without introducing instability. The results are contrary
to the design approach taken by many engineers.
Ridley, R. B., S. Kern, B. Fuld, Analysis and Design of
a Wide Input Range Power Factor Correction Circuit for Three-Phase
Applications, IEEE Applied Power Electronics Conference
Proceedings, San Diego, 1993, pp. 299-305.
A combined buck
and boost topology power factor correction circuit which can
operate with input voltages from 150 - 540 VAC
is presented. The design and analysis of the operation and
dominate losses of this circuit are given. The features of
the
topology are compared with those of a standard boost power
factor correction circuit.
This
topology is at the heart of one of the most advanced and reliable
power systems in use today in the mainframe computer industry.
The power cord for the system can literally be plugged into
either a low-line 208 VAC or a high-line 480 VAC feed, either
single phase or three phase, with no tap switches to be made.
The system can also survive an outage on one of three input
lines for extended periods of time, without any of the power
stages needing to be oversized. A very important technology
for the 2-20 kW power range in systems that need an isolated
output bus.
Ridley, R. B., New Simulation Techniques for PWM Converters,
IEEE Applied Power Electronics Conference Proceedings, San Diego,
1993, pp. 517-523.
New simulation
techniques are presented which take advantage
of the special structure of PWM converter power stages and
their
compensation circuits. These techniques provide reduction
of system order, and allow for the fastest possible simulation
without any iteration algorithms. This is achieved while retaining
accurate large-signal simulation with details of cycle-by-cycle
waveforms.
This
paper gives some of the theory behind the special and unique
simulator used in POWER 4-5-6,
demonstrating why this is the fastest method of simulation
that you can use and still get very accurate results.
Vlatkovic, Vlatko,
Juan A. Sabate, Raymond B. Ridley, and Fred C. Lee, Small
Signal Analysis of the Phase-Shifted PWM Converter,
IEEE Transactions on Power Electronics, Vol. 7, No. 1, January,
1992, pp. 128-135.
The specific
circuit effects in the phase-shifted pulse-width-modulated
(PS-PWM) converter and their impact on the converter dynamics
are analyzed. The small-signal model is derived incorporating
the effects of phase-shift control and the utilization of
transformer leakage inductance and power FET junction capacitances
to achieve zero-voltage resonant switching. The paper explains
the differences in the dynamic characteristics for the PS-PWM
converter and its PWM counterpart. Model predictions are confirmed
by experimental measurements.
The
phase-shifted full bridge is a circuit which has substantial
performance advantages over its PWM counterparts. If you are
going to use a full bridge, you should look very hard at implementing
the phase-shifted version, especially since Unitrode now has
a controller for driving this topology. This converter also
forms a part of the system mentioned above.
Ridley, R., M.
Reynell, and S. Kern, Thermal Considerations for Distributed
Power Converter Systems, IEEE Applied Power Electronics
Conference Proceedings, San Diego, 1993, pp. 615, 866-872.
Modern high-density
DC-DC power supplies present a significant challenge to the
system designer in cooling the package. In many applications,
cooling air is restricted and large heatsinks are needed to
prevent excessive temperature rise. This paper presents thermal
design examples for typical dc-dc systems, and uses a three-dimensional
thermal modeling tool (FLOTHERM) to predict temperature rise.
This
paper is high in the list to stress the importance of proper
thermal design. Even in a simple system with just one heat-dissipating
element, surprising results can be obtained, and some basic
thermal modeling can predict these results before hardware
is built.
Hsiao, C. J.,
R. B. Ridley, H. Naitoh, and F. C.. Lee, Circuit-Oriented
Discrete-Time Modeling and Simulation for Switching Converters,
IEEE Power Electronics Specialists Conference Record, Blacksburg,
Virginia, June 1987, pp. 167-176.
A generalized
discrete-time modeling and simulation program,
applicable to any PWM, resonant or quasi-resonant converter,
has
been developed. From a circuit description, this program automatically
generates state-space equations corresponding to
each switching interval and performs time-domain simulations
by
using state-transition equations with a fast-convergence algorithm
for topological change.
The
program this paper described, COSMIR, was one of several
computer programs developed around this time to address the
specific problems of PWM converter simulation. It ran very
fast, but the code was prone to logic errors on occasion.
Also, the circuit description for anything but a simple PWM
converter was rather cumbersome. The biggest lesson learned
from this was not to try and write a special purpose program
as a stand alone application. In today's computer software
market, the resources needed to create and maintain such a
complex program are formidable. This lesson has been applied
in developing POWER 4-5-6.
Ridley, R. B.,
and F. C. Lee, Practical Nonlinear Design Optimization
Tool for Power Converter Components, IEEE Power Electronics
Specialist Conference Record, 1987, pp. 314-323.
A computer-aided
design tool for power converter components is
described. This tool allows a designer with a minimum of programming
and optimization experience to interface with nonlinear optimization
routines. Realistic design values and available vendor components
can be incorporated in a design without using an extensive
data base program structure.
This
paper used a nonlinear optimization tool to design a power
supply from the basic design equations and specifications.
It worked OK, but the optimization tool was very cumbersome
and not recommended for this task. POWER 4-5-6 takes this work to a more sophisticated
level, incorporating many more design rules and substituting
experience for an optimization engine.
Ridley, R. B.,
C. Zhou, and F. C. Lee, Practical Nonlinear Design Optimization
for Power Converter Components, IEEE Transactions on
Power Electronics, Vol. 5, No. 1, January 1990, pp. 29-39.
A computer-aided
design approach for power converter components is described.
A designer with a minimum of programming and optimization
experience can interface with nonlinear optimization routines
to rapidly perform design trade-offs which would be impossible
by hand. A power converter design using MOSFET and bipolar
junction transistor (BJT) switches is shown to illustrate
the power of optimization routines in power electronics. Realistic
design values and available vendor components can be incorporated
in a design without using an extensive data base program structure.
A practical example is given with experimental data to verify
the accuracy and usefulness of optimization software.
Same
topic as the above paper, with some more details added for
this IEEE Power Electronics Transactions version.
Kelkar, S. S.,
R. B. Ridley, C.J. Hsiao, R. Ramkumar, and F. C. Lee, A
Computer-Aided Design and Simulation Tool for Switching
Converters, IEEE Power Electronics Specialists Conference
Record, Blacksburg, Virginia, June 1987, pp. 3-12.
A fully automated
computer-aided design approach is presented which results
in an optimal power stage and control circuit design. The
design meets all dc, small signal and large signal closed-loop
performance specifications. The proposed approach is very
efficient; it can help in reducing component and manufacturing
costs as well as design time and is successfully demonstrated
on a multiple-output flyback converter breadboard.
Gives
a practical demonstration of the use of the tools described
in the previous three papers. Is the result truly "optimal"?
Depends on what you use to measure this. The more reliable
and faster processes used in Power 4-5-6 improve greatly on
these methods.
Sable, D. M.,
R. B. Ridley, and B. H. Cho, Comparison of Performance
of Single-Loop and Current-Injection Control for PWM Converters
which Operate in Both Continuous and Discontinuous Modes of
Operation, IEEE Power Electronics Specialists Conference
Record, San Antonio, Texas, June 1990, pp. 74-79.
A comparison
of single-loop and current-mode controlled power converters
operating in the continuous and discontinuous mode is performed
using the PWM switch model and a new, continuous time model
of current-injection-control (CIC). The theoretical and experimental
results show a significant performance improvement that can
be realized when CIC is employed in converters which operate
over a wide load range.
Another
good practical paper. Bottom line: if you build a converter
which will operate in both CCM and DCM, you should use current-mode
control if you expect to get good transient performance. There
is no reason why a converter should NOT operate in both regions,
though you will find many designers who go to great pains
to avoid this.
The
lessons of this paper are being forgotten in the late 90's
as many semiconductor companies have started making voltage-mode
only controllers again.
Sable, Dan M., Raymond B. Ridley, and Bo H. Cho, Comparison
of Performance of Single-Loop and Current-Injection Control
for PWM Converters that Operate in Both Continuous and Discontinuous
Modes of Operation, IEEE Transactions on Power Electronics,
Vol. 7, No. 1, January, 1992, pp. 136-142.
A analysis of
current-injection-controlled (CIC) power converters operating
in both the continuous and discontinuous modes is performed
using the PWM switch model and a new, continuous-time model
of current-injection control. The stability, output impedance,
audio susceptibility, and transient response are compared
with single-loop control. The control of an example buck converter
is designed with CIC and single-loop control. It is shown
how single-loop controlled power converters exhibit a large
change in the dynamic performance when crossing the boundary
between continuous mode and discontinuous mode. This is especially
true for the output impedance and transient response. The
dynamic performance of current-injection-controlled power
converters remains relatively fixed when crossing this boundary.
A significant performance improvement that can be realized
when CIC is employed in converters that operate over a wide
load range.
IEEE
Transactions version of the previous paper. This is the preferred
version.
Vorperian V.,
and R. B. Ridley, A Simple Scheme for Unity Power Factor
Rectification for High-Frequency AC Buses, IEEE Transactions
on Power Electronics</I>, Vol. 5, No. 1, January 1990.
A simple scheme
is proposed for off-line unity power factor
rectifications for high frequency ac buses (20 kHz). In this
scheme, a bandpass filter of the series resonant type centered
at the line frequency is inserted between the line and the
full-wave rectified load. The Q = Zo/Rl formed by the load
and the characteristic impedance of the tank circuit determine
the power factor, the boundary between the continuous and
discontinuous conduction modes, the peak stresses and the
transient response of the rectifier. It is shown that for
Q < 2 the line current is nearly sinusoidal with less than
five percent third harmonic distortion while the power factor
is essentially unity. A increase in the value of Q causes
an increase in the peak voltages of the tank circuit and a
slower transient response of the rectifier circuit. The dc
small-signal and transient analyses of the rectifier circuit
are determined and the results are in good agreement with
simulation and experimental results.
Note:
this paper is not an endorsement of high-frequency ac versus
dc power distribution, a topic of very heated debate for the
space station in the late 1980s. However, if you are planning
on using a high-frequency ac bus for power distribution, you
should at least make sure that the power system operates with
low harmonics. This paper presented a simple scheme with a
series-resonant LC notch filter to greatly improve the harmonics
and power factor of the system.
This
system is practical, it really does work quite well in the
right application. A recent research project applied the techniques
here to a 20 kHz, 10 kW power system which needed to transmit
power over a 750 foot long, 1/4 inch diameter cable.
Some
of the analysis presented in this paper is not new, it was
originally done by Steve Freeland at Caltech. The small-signal
modeling is original, and has been verified experimentally.
Ridley,
Raymond B., Wojciech A. Tabisz, Fred C. Lee, and Vatche Vorperian,
Multi-Loop Control for Quasi-Resonant Converters,
IEEE Transactions on Power Electronics, Vol. 6, No. 1, January
1991, pp.28-38.
A new, multi-loop
control scheme for quasi-resonant converters is described.
Similar to current-mode control for PWM converters, this control
offers excellent transient response and replaces the voltage-controlled
oscillator with a simple comparator. A signal proportional
to the output-inductor current is compared with an error voltage
signal to modulate the switching frequency. The control can
be applied to either zero-voltage-switched or zero-current-switched
quasi-resonant converters. Computer simulation is used to
demonstrate the effectiveness of the control method applied
to a zero-current-switched buck quasi-resonant converter.
Experimental results are presented for zero-current flyback
and zero-voltage buck
quasi-resonant converters, operating up to 7 MHz.
Some
interesting experiments in pushing the loop gain of a converter
to its limits. Using some wide-bandwidth components,
and a passive current-mode sensor, crossover frequencies in
excess of 100 kHz were obtained. There are some techniques
used in this paper that are very relevant and applicable to
PWM converters operating at higher frequencies. The paper
is not an endorsement of zero-voltage quasi-resonant operation,
but if you are going to use this technology, this is a good
way to control it.
Ridley, R. B.,
A. Lotfi, V. Vorperian and F. C.. Lee, Design and Control
of a Full-Wave, Quasi-Resonant Flyback Converter, IEEE
Applied Power Electronics Conference Proceedings<, New Orleans,
1988, pp. 41-49.
Design considerations
for a quasi-resonant converter for a typical distributed power
system application are described. Different topologies are
compared in terms of the peak stresses on the power switch.
The design of a full-wave, zero-current-switched flyback quasi-resonant
converter with a novel, multi-loop control is described in
detail. Circuit waveforms, small-signal and large signal measurements
are compared with theoretical predictions.
If
you are going to build a full-wave, quasi-resonant flyback
converter, this is a good way to control it. Whether or not
you should use this topology is another matter-- generally
it is not recommended, though some very special applications
may be able to make use of it. A simple PWM circuit at significantly
lower frequency will often outperform this technology.
Ridley, R. B.,
W. A. Tabisz, and F. C. Lee, Multi-Loop Control for High-Frequency
Quasi-Resonant Converters, Intertec Communications Inc.,
High Frequency Power Conversion International, 1988, pp. 381-389.
Limitations
of multi-loop control for a 1MHz flyback quasi-resonant converter
are discussed. A new control circuit is presented which allows
operation up to 10MHz. Circuit waveforms and small-signal
characteristics for a 7MHz zero-voltage-switched quasi-resonant
converter are presented to demonstrate the effectiveness of
the new control circuit.
Same
comment as for the earlier paper: the control works well,
but this is not necessarily an endorsement for any kind of
resonant converter for most applications.
Sable D. M., and
R. B. Ridley, A High Frequency Multi-Module Spacecraft
Boost Regulator, International Telecommunications Energy
Conference, 1988.
Choi, B., B. H.
Cho, R. B. Ridley, and F. C.. Lee, Control Strategy for
Multi-Module Parallel Converter System, IEEE Power Electronics
Specialists Conference Record, San Antonio, Texas, June 1990,
pp. 225-234.
The control
strategy for a multi-module converter system for high-current
low-voltage applications is investigated. The system consists
of several converter modules in parallel to effectively deliver
a high-current output. A multi-stage output filter is employed
to efficiently attenuate the ripple and high-frequency noise.
In addition to output voltage and inductor current feedback,
a feedback from the intermediate filter stage is employed
to optimize the transient response in the event of failure
of a converter module, and also to improve the other closed-loop
performances of the system. Based on the small-signal analysis,
a systematic control-loop design procedure for optimal performance
of the system is presented.
A
complex power system used in high-power mainframe computers.
The simulation program Saber proved to be the most useful
in analyzing this system. A paper for the control design fanatic
who likes to think in the Laplace domain.
Zhou, C., R. B.
Ridley, and F. C.. Lee, Design and Analysis of a Hysteretic
Boost Power Factor Correction Circuit, IEEE Power Electronics
Specialists Conference Record, San Antonio, Texas, June 1990,
pp. 800-807.
The design of
an active unity power factor correction circuit with variable-hysteresis
control for off-line switching power supplies is described.
Design equations relating the boost inductor current ripple
to the boost inductor selection and circuit performances are
developed and are verified with measurements. A computer-aided
design program is developed to select the optimal circuit
components. Design guidelines for the low-frequency feedback
network are presented using the switch model for the power
factor correction circuit. Small-signal transfer functions
for open and closed-loop responses are derived.
Hysteretic
current-mode was very popular at this time for PFC applications.
Since the introduction of the Unitrode Average current-mode
chip, this popularity has faded. In general, the hysteretic
control is more problematic to implement, and more noise sensitive.
Sable, D. M.,
B. H. Cho and R. B. Ridley, Elimination of the Positive
Zero in Fixed Frequency Boost and Flyback Converters,
IEEE Applied Power Electronics Conference Proceedings, Los Angeles,
California, March 1990, pp.
205-211.
It is shown
how a fixed-frequency, leading-edge modulated PWM
can eliminate the undesirable positive zero in practical boost
and flyback converters. This allows a substantial improvement
in the closed-loop characteristics. Several techniques are
employed to predict this result. The design procedure for
elimination of the positive zero is presented. Experimental
verification is provided.
Note:
this is an analysis of an existing satellite power system
which had succeeded in eliminating the RHP zero by clever
and intuitive design. It is not recommended that you build
your boost or flyback converters this way, but you may run
into this phenomenon at some time. Interesting paper f you
are into control theory of converters. It's not practical
in most cases since it requires the use of high esr caps which
leads to elevated loss and noise.
Tang, W., F. C.
Lee, R. B. Ridley, I. Cohen, Charge Control: Modeling,
Analysis and Design, IEEE Power Electronics Specialists
Conference Record, Spain, 1992, pp. 503-511.
A new power
converter control method, charge control, is studied. A complete
small-signal analysis is performed for the control scheme.
Subharmonic oscillation similar to that of CIC control is
found and the relationship between the subharmonic oscillation
and line/load condition of charge control is defined. Based
on analysis, the design guidelines which guarantee the stability
of the control system under given line and load ranges are
proposed. The small-signal model was confirmed experimentally.
Tang, W., F. C.
Lee, and R. B. Ridley, Small-signal Modeling of Average
Current-Mode Control, IEEE Applied Power Electronics
Conference Proceedings, Boston, 1992, pp. 747-755.
A recently proposed
average current-mode control is analyzed.
A complete small-signal model for the control scheme is developed.
The model is accurate up to half the switching frequency.
By closing the current loop, a flat control-to-inductor current
transfer function, up to half the switching frequency, can
be achieved. This control scheme enables the converter to
behave as an ideal current source. The subharmonic oscillation,
as frequently reported in peak current-mode control, also
exists in this control. This subharmonic oscillation can be
eliminated by choosing a proper gain of the compensation network
in the current loop. Model
predications are confirmed experimentally.
Paper
shows the analysis of the average current-mode control, widespread
in the PFC field. It shows that this control scheme can also
have oscillations in the current loop.
Choi, Byungcho,
Bo H. Cho, Fred C. Lee, and Raymond B. Ridley, The Stacked
Power System: A New Power Conditioning Architecture for Mainframe
Computer Systems, IEEE Transactions on Power Electronics,
Vol. 9, No. 6, November, 1994, pp. 616-623.
The stacked
power system combines series and parallel connections of multimodule
power supplies to produce several low
voltage outputs at high current levels. The system could generate
ultra low voltage outputs with higher efficiencies than conventional
centralized or distributed approaches, using standard single-output
converters. This paper presents the architecture of the stacked
power system and establishes a design procedure for the system.
Chin, Shaoan A.,
John Tero, Milan M. Jovanovic, Raymond B. Ridley, and Fred C.
Lee, A New IC Controller for Resonant-Mode Power Supplies,
IEEE Applied Power Electronics Conference Proceedings, Los Angeles,
California, March 1990, pp. 459-466.
The NE5580 is
an all-purpose control IC providing control functions for
zero-current-switching (ZCS) or zero-voltage-switching (ZVS)
resonant or quasi-resonant power supplies. It features: frequency-modulated
constant on-time or constant off-time control with 10 MHz
voltage-controlled oscillator; 10 MHz error amplifier; dual
1A peak totem-pole output drivers with no cross conduction
current; and other functions as will be described.
The
controller worked well for a wide range of resonant converters.
Unfortunately, Signetics never solved the quiescent dissipation
and output driver shoot-through problems, and the chip was
never commercially successful.
Chin, S. A., J.
Tero, M. M. Jovanovic, R. B. Ridley, and F. C. Lee, A
10 MHz Universal Control IC for Resonant-Mode Power Supplies,
High-Frequency Power Conversion Conference, Santa Clara, California,
May 1990.
Ridley, R. B.,
B. H. Cho and F. C.. Lee, Analysis and Interpretation
of Loop Gains of Multi-Loop-Controlled Switching Regulators,
IEEE Transactions on Power Electronics, Vol. 3, No. 4, October
1988, pp. 489-498.
Two different
loop gains of power supplies with current-mode control are
considered. The relationships between the loop gains
and closed-loop characteristics are derived. Both loop gains
can
be used to aid design. A common problem with the implementation
of current-mode control is discussed.
This
is at the bottom of the list for a good reason. An interesting
paper, it seems to make a lot of sense. Unfortunately, for
current-mode control, most of it is wrong, due to an early
error in
modulator gain! See the later paper on a new small-signal
model
for current mode control for proper analysis of the current-mode
system. Don't read this paper unless you are particularly
curious.
The best lesson contained in this paper: don't believe everything
that you read, publication is not necessarily an indication
of accuracy.
Post Script: The
same error in modulator gain has appeared again in yet another
publication on current-mode control.
