Control of stored magnetic energy in power converter transformers

Abstract

Apparatus (and a method) useful in a switching power converter having a transformer and a primary switch for connecting a DC input source to a primary winding of the transformer during a portion of each of a succession of converter operating cycles, the apparatus includes a reset capacitor and a reset switch. Reset circuitry cooperates with the reset switch to connect and disconnect the reset capacitor in a manner which provides for resetting the transformer and which allows a current having a non-zero average value to flow in the reset switch.Another aspect is a method for limiting the slew rate in a switching power converter which includes a transformer and a reset circuit of the kind which non-dissipatively recycles the magnetizing energy stored in a transformer during each of a succession of converter operating cycles. The method includes sensing the magnetizing current which is flowing in said transformer, and allowing initiation of another converter operating cycle only if the magnetizing current meets a predefined criterion for safe converter operation. In embodiments of the invention the predefined criterion may be the magnetizing current passing through a predefined value, e.g., zero.

Description

BACKGROUND OF THE INVENTIONThis invention relates to control of stored magnetic energy in power converter transformers.In one class of power converters, called single-ended switching power converters (10, FIG. 1), a primary switching element 20 is used to repetitively connect an input source to the primary winding 329 of a power transformer 25 (FIG. 1) during a portion of each of a series of converter operating cycles. In some such converters, called "single-ended forward converters," forward energy transfer from the input source toward the load occurs during the time that the switch is closed. In other topologies, called "single-ended flyback converters", energy is stored in the transformer during the time that the switch is closed and is transferred forward to the load after the switch is opened. In yet other single-ended converters a portion of the energy may be transferred during both the on and off times of the switch. In general, practical converters of this kind must incorpor...

AI Technical Summary

Benefits of technology

In an aspect, the invention features apparatus (and a method) useful in a switching power converter having a transformer and a primary switch for connecting a DC input source to a primary winding of the transformer during a portion of each of a succession of converter operating cycles, the ap

Problems solved by technology

One of the advantages of this circuit is that it can maintain the minimal value of peak switch voltage consistent with converter input voltage and duty cycle; one of its disadvantages is that it is dissipative and therefore compromises both converter efficiency and power density.

While this provides a natural mechanism for reducing the magnetizing energy to zero it does not provide a mechanism for managing transformer 25 leakage energy, which can cause excessive switch 20 voltage when the switch 20 is opened.

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