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Preventing Core Saturation in Parallel Power Transformers in a Switching Converter

IP.com Disclosure Number: IPCOM000043490D
Original Publication Date: 1984-Sep-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Spreen, JH: AUTHOR

Abstract

If a switched mode power supply contains two power transformers with the primary windings connected in parallel, then a circulating current will be established, as shown, for example, in the dual switch primary circuit of Fig. 1. Such a circulating current may be large enough to cause saturation of a transformer core. The circulating current is caused by an unintentional difference between voltages applied to the primary inductances Lp1 and Lp2 . Both primary windings are connected to a common source, vB, during Ton, the time interval when both transistors Q1 and Q2 are simultaneously conducting. Similarly, both windings fly back to a common reset voltage (-vB) when Q1 and Q2 turn off and reset diodes DR1 and DR2 conduct.

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Preventing Core Saturation in Parallel Power Transformers in a Switching Converter

If a switched mode power supply contains two power transformers with the primary windings connected in parallel, then a circulating current will be established, as shown, for example, in the dual switch primary circuit of Fig. 1. Such a circulating current may be large enough to cause saturation of a transformer core. The circulating current is caused by an unintentional difference between voltages applied to the primary inductances Lp1 and Lp2 . Both primary windings are connected to a common source, vB, during Ton, the time interval when both transistors Q1 and Q2 are simultaneously conducting. Similarly, both windings fly back to a common reset voltage (-vB) when Q1 and Q2 turn off and reset diodes DR1 and DR2 conduct. However, a difference between the voltage drops across R1 and R2, which represent wiring and winding resistance, causes the primary inductances to experience different voltages. If no core saturation occurs, an approximate value of the resulting steady-state circulating current is given by the expression:

(Image Omitted)

If the voltage drop across R1 is larger than that across R2, then the circulating current flows in the direction shown by the arrow in Fig. 1 and expression (1) yields a positive value for ic . A different set of load currents may reverse the order of the voltage drops, producing a negative value from expression (1), indicating that the circulating current flows in the direction opposite that indicated by the arrow in Fig. 1. For a fixed set of loads, it is always possible to reduce the circulating current to insignificance and, hence, to eliminate this potential cause of core saturation, by adding a series resistance, RA, as shown in Fig. 2. This resistance may be obtained by adding a discrete component or by adjusting the winding and wiring resistance. The value of RA is chosen so that expression 1, evaluated with RA + R2 substituted for R2, yields zero for ic . No fixed value of RA can be expected to produce ic = 0 for a range of load currents. However, it may be possible to...