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AC FREEWHEELING DIODES

IP.com Disclosure Number: IPCOM000005460D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 67K

Publishing Venue

Motorola

Related People

Thomas L. Hopkins: AUTHOR

Abstract

STATEMENT 0F PROBLEM The circuit shown in the attached figure may be used to chop the AC mains to provide pulse width modulated control of an AC machine such as a universal motor or fluorescent lamp (the load). One of the .main problems with thls type of application is that some means must be provided to absorb the inductive energy stored in the load when the transistor turns off to prevent transistor failures. One method of doing this is to use snubber networks. This typically involves costly and bulky components. The other common- ly used method is to clamp the inductive energy in a freewheeling diode. Since the diode is inherently a unipolar device, it would clamp the energy in only one polarity and short circuit the load in the other. To effectively use this technique In AC circuits, some means must be devised to clamp the inductive energy and not short circuit the load half of each cycle.

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m MOTOROLA Technlcal Developments Volume 2 Januarv 1982

AC FREEWHEELING DIODES By Thomas L. Hopkins

STATEMENT 0F PROBLEM

The circuit shown in the attached figure may be used to chop the AC mains to provide pulse width modulated control of an AC machine such as a universal motor or fluorescent lamp (the load). One of the
.main problems with thls type of application is that some means must be provided to absorb the inductive energy stored in the load when the transistor turns off to prevent transistor failures. One method of doing this is to use snubber networks. This typically involves costly and bulky components. The other common- ly used method is to clamp the inductive energy in a freewheeling diode. Since the diode is inherently a unipolar device, it would clamp the energy in only one polarity and short circuit the load in the other. To effectively use this technique In AC circuits, some means must be devised to clamp the inductive energy and not short circuit the load half of each cycle.

SOLUTION

   Two SCR's are used in the circuit shown in the figure to clamp the inductive energy in the same man- ner a diode would be used in a DC application. In this application, however, the gate drive to the SCR's is synchronized to the AC mains in such a way that the gate is driven on wh:n the anode to cathode is nominally reverse biased. In this way the SCR will not conduct when the PWM transistor is on but will be forward biased by the inductive spike associated with turn...