Cliff Jamerson

Megamp post regulators-practical design considerations to allow operation under extreme loading conditions

Practical design equations and guidelines are established to allow magnetic amplifier (magamp) operation under extreme loading conditions. Three conditions are considered: shutdown of output, foldback of the output current, and operation under discontinuous inductor current mode. Design equations are derived for the three extreme operations, and their use is illustrated by numerical examples. The design equations provide the designers with tools for parameter trade-offs and lend themselves to computer-search design procedures. >

Seven ways to parallel a magamp

Paralleling multiple-output switch-mode power supplies poses challenging design trade-offs. Seven different ways that can be used to parallel a magamp post regulator are described. Some practical design considerations are given.<<ETX>>

A simple model predicts small-signal control loop behavior of magamp post-regulator

A block diagram approach is used to provide an insight into magamp feedback control operation. The analysis reveals a pole-shifting phenomenon inherent in the type-A loop transfer function. A procedure for calculating the gain and open-loop poles in the control loop of each reset circuit is given. The procedure also shows how to compensate the error amplifier for stable operation. The detailed example of a 50 kHz magamp is given; however, the validity of the model has been demonstrated experimentally at 100 kHz. With a factor of two included in the Faradays law block, the model describes loop behavior for a magamp with two saturable reactors for symmetrical topology. >

Magamp post regulators-practical design considerations to allow operation under extreme loading conditions

Practical design equations and guidelines to allow magamp operation under extreme loading conditions are established. Three conditions are considered: shutdown of output, foldback of the output current, and operation under discontinuous-inductor-current mode. Several numerical examples are used to illustrate design considerations and the use of the design equations.<<ETX>>

Magamp post regulators for symmetrical topologies with emphasis on half-bridge configuration

The major differences between symmetrical topology magamps and the forward power converter magamp are discussed. In a forward power converter, a small portion of the magnetizing current of the power transformer is diverted to the secondary for reset of the magamps saturable reactor. In symmetrical topologies, the transformers magnetizing current is not used for reset of the saturable reactor(s). The magnetizing current shifts to the secondary where it causes a series of problems. The authors examine these problems, identify solutions, and explain why a half-bridge magamp power supply is inherently less versatile than the forward power converter.<<ETX>>

Modeling of control loop behavior of magamp post regulators

Small-signal control models are presented for a magamp switching-mode post regulator. Two commonly used current reset schemes are considered: an external reset scheme and a self-reset scheme. Models for both continuous and discontinuous modes of operation are presented. It is mathematically shown that the open-loop gain is a two-pole, single-zero transfer function for continuous-mode operation and a single-pole function for discontinuous-mode operation. The equations for predicting the loop gains for both types of reset circuits are derived and verified experimentally. It is shown that the loop gain characteristics of a magamp regulator depend on power circuit parameters, the reset circuit parameters, and the saturable reactor parameters. Therefore, in a normal design procedure of a magamp, if the design parameters of the saturable reactor are changed, a corresponding change of the compensation network must be made to optimize the control performance. The models presented provide the designer with a tool to facilitate the feedback loop design of a voltage-mode magamp post regulator.<<ETX>>

Applications of half-turn on E-core in switching power supplies

A turn around an outside leg of an E-core is called a half-turn because it encloses only one-half of the cross-sectional area of a turn around the center leg. It is well known that a half-turn in a secondary winding of a power transformer greatly increases the leakage inductance between windings, thus causing an adverse effect on cross-regulation. However, the increased leakage inductance of a half-turn can be very beneficial in tapped inductors for boost circuits and in coupled output chokes. This paper explains some of these little-known applications of a half-turn. The theory and formulae for prediction of leakage inductance added by such a half-turn are presented.

Stabilizing magamp control loop by using an inner-loop compensation

By using the proposed inner-loop compensation scheme the resonance peaking of the open-loop transfer function of a magamp circuit for post regulator applications is eliminated. This makes the loop compensation of magamp control much easier. If properly used, the proposed scheme is capable of making the resultant loop gain essentially independent of parameter variation of the saturable reactors. A review of the problems associated with a conventional compensation scheme is given. The compensation scheme is then proposed, analysed, and implemented in a circuit. Experimental verification is given, and a practical design procedure is illustrated in an example.<<ETX>>

Techniques for reduction of required headroom in high-frequency magamp postregulators

High headroom in a magamp postregulator increases circuit losses, including power transistor losses, saturable reactor core loss, and freewheeling diode snubber loss. Higher headroom also requires the use of higher voltage diodes. The authors describe common causes for large headroom requirement and discuss various techniques to reduce it. >

Leading-edge modulation voltage-mode control with flux unbalance correction for push-pull converter

This paper presents a leading-edge modulation voltage-mode control scheme for a 3 kW, 96 V battery-backed-up DC-DC converter with 400 V/ 800 V output, which is developed to power a half-bridge inverter in an on-line sinewave UPS system. A new circuit scheme is proposed to implement pulse-by-pulse current limit and flux unbalance correction functions with a leading-edge modulation voltage-mode PWM control IC, TL598. Design considerations for implementing voltage control loop for this application and experimental results are also presented.