In this article we’ll show how a third way of defining average current leads to an infinite gain of the modulator gain for current-mode systems. Another simple derivation produces a fourth expression for the modulator gain, thus completing the set of gains to be found in the literature.
In this special article we’ll look on work resulting from Ridley Engineering’s magnetics presentations at APEC 2016. Among things shown is how a linear circuit model can be used inside any version of Spice (or other circuit simulator) to get accurate results for the ac proximity loss resistance of magnetics
In the third part of this series, we’ll see how Dr. Fred Lee’s analysis of current-mode control resulted in a high modulator gain, and also look at the consequences of this gain on current-loop predictions.
Current-Mode Control Circuit
Figure 1 shows a buck converter
In this second part of this series of articles, we show how Dr. David Middlebrook approached the analysis of current-mode control, and the consequences of the modeling approach on loop gain predictions.
Current-mode control circuit
Figure 1 shows a buck converter with peak
Current-mode control has been around in one form or another for almost 50 years now, but it is still a topic that creates much confusion in the minds of many engineers and researchers. In this series of articles, Dr. Ridley shows historical reasons for the confusion, and presents for the first time the source
The first two parts of this article showed how the core losses for real waveforms could be modeled better. In the first part, a continuous expression was used to model a wide range of excitations. In the second part, the effect of duty cycle on the loss of the core material was included in a single equation.
As modern power converters move towards higher efficiency, it is essential to have better models for all of the loss mechanisms in your components. In the first part of this article, we talked about the need for a single continuous equation to match the empirically measured losses. In this second part, the effect
As modern power converters move towards higher efficiencies, it is essential to have better models for all of the loss mechanisms in your components. We have talked about winding loss effects in several articles. Magnetics core loss modeling is an area that needs improvement.
Modern power converters are moving towards high efficiency, high density, and high switching frequencies. Vendors of power supplies are doing an outstanding job of meeting these goals with a range of new technologies. At the same time, despite the use of advanced control chip technology, many control loops have
Proper design of magnetics components is at the heart of any power converter. In this article, the flaws of some production magnetics are shown, clearly showing why it is essential that designers of power supplies should be intimately involved in magnetics design and characterization.
As power levels are increased and output voltages drop, it is common to have just a few turns in the windings of inductors and transformers. When this happens, standard round wires are usually not the best choice for building magnetic structures, and foil is often used. With a foil winding, the width of the
The latest generation of off-the-shelf inductors for dc-dc converters features flat windings with a helical structure. While this reduces AC losses in many cases, the AC resistance is still considerably higher than the DC resistance and must be considered carefully when selecting your inductor.
Let’s explore the measured control characteristics of a Cuk converter with separate inductors, and with a tightly-coupled inductor. The coupled-inductor Cuk has fewer diverse transfer functions, but it is still a challenge to control over a wide range.
Cuk Converter with Separate Inductors
It is always desirable to simplify the measurement process of power supplies, and characterize them without overly invasive testing. This can work for loop gains with limited success, and shows the pitfalls of trying to measure loops noninvasively.
Power supply output impedance measurements
Sepic Converter with Voltage-Mode Control
This article shows the measured characteristics of Sepic converter using current-mode control, either with a coupled inductor or separate inductors. In the last article of this series, the coupled-inductor Sepic was examined.
This article shows the measured characteristics of coupled-inductor Sepic converters and compares them with a separate inductor Sepic. The coupled-inductor Sepic has less diverse transfer functions, but it is still a challenge to control over a wide range. Publications in the past have discussed the coupled-inductor
Depending on the conditions under which you measure a Sepic converter, it can appear either easy to control, or extremely difficult. Make sure you always make an extended range of measurements on your Sepic design if you wish to ensure avoiding the troublesome regions.
Sepic converter with
Board-mount power supplies use multilayer ceramics for both their input and output capacitors. The inherently low ESR values lead to transfer functions with higher Q than converters using electrolytic capacitors, and this can lead to interesting effects when making measurements.
Many newcomers to
This article shows the link between loop gain Bode plots and Nyquist diagrams. The Nyquist diagram is a subset of the Bode Plot, omitting crucial design data. The linear scaling of the Nyquist diagram restricts its practicality, and omission of frequency as an explicit variable in the plot is a major drawback.
There have been many dramatic changes in power supply development over the last 20 years, but loop gain measurements remain the key to rugged and aggressive system performance. Understanding how to read a loop gain is important.
Loop gain measurements in modern systems
In this third article about power supply failures, the magnetics are examined for their contribution to the failure rate. These are usually the least understood of all components, and poor magnetics design can lead to many different failure mechanisms in power supplies.
In this second article about power supply failures, the capacitors are examined for their contribution to the failure rate. The causes of failure for different types of capacitors are discussed. In the last part of this article, the question was asked of group members “Why do power supplies fail?” and the
We here at Ridley Engineering recently conducted a survey with a group of almost 3,000 active power supply design engineers to discover the various reasons and circumstances they have encountered power supplies that failed. The experiences the engineers shared with us were very enlightening, and this article
Point-of-load converters commonly use multilayer ceramic capacitors instead of electrolytic capacitors. The effect of this change is strongly felt in the bulk storage capacitor value, and can lead to unexpected variations in control measurements. Instability can result if input filter capacitors become too small.
There has always been a debate about how essential it is to measure the loop of a switching power supply. Some power designers have always resisted this part of development and design validation, claiming they are able to tune a loop properly through step-load testing . While this may have worked with low
As mentioned in the last article of this series , Dr. VatchÃ© VorpÃ©rian developed the PWM switch model in 1986, which replaced the need for state-space averaging and greatly simplified the analysis process. It was a very elegant and intuitive modeling approach, which is easily grasped by new students
Soon after switching power supplies were implemented in industry in the 60s and 70s, it became apparent that they had unique issues with control that defied normal circuit analysis. This gave rise to state-space averaging, a technique with which the two distinct conditions of the power switch and diode could
This article is the fifth of a series in which Dr. Ridley shows the steps involved in designing and building an offline flyback converter. With full input voltage and full load applied, the control-to-output transfer function and loop gain is measured and compared to predictions.
This article is the fourth of a series in which Dr. Ridley shows the steps involved in designing and building an offline flyback converter. The power transformer is added to the circuit, and the current sensing and snubbers are designed to operate at full power without excessive stress on the components.
This article is the third of a series in which Dr. Ridley shows the steps involved in designing and building an offline flyback converter. The input filter rectifier is simulated to find the design range of the converter, and the transformer design is completed and measured.