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Short Winding Protection of Flyback Converters

IP.com Disclosure Number: IPCOM000171346D
Published in the IP.com Journal: Volume 8 Issue 6B (2008-06-25)
Included in the Prior Art Database: 2008-Jun-25
Document File: 2 page(s) / 36K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

The Flyback converter is a DC to DC converter with a galvanic isolation between the input and the output. Such Converters feature extremely low stand-by power consumption making them the preferred choice in a large variety of power application, such as mobile handset travel adaptors. However, these electronic devices are not free from failure. For instance the primary winding in the flyback converter may be faulty, degraded by weather influences or damaged during transportation. As a result, when coupled to the mains, this may cause a short from the high input voltage to the drain of the power transistor, and a huge current may flow from the mains to the power transistor. This will easily damage the power transistor if it is subjected to prolonged high current stress. In a worst case scenario overheating leading to fire can occur. It is therefore desirable to provide protection circuits against short winding faults.

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Short Winding Protection of Flyback Converters

Idea: Gong Xiao Wu, SG-Singapore; Teo Yong Siang, SG-Singapore

The Flyback converter is a DC to DC converter with a galvanic isolation between the input and the output. Such Converters feature extremely low stand-by power consumption making them the preferred choice in a large variety of power application, such as mobile handset travel adaptors. However, these electronic devices are not free from failure. For instance the primary winding in the flyback converter may be faulty, degraded by weather influences or damaged during transportation. As a result, when coupled to the mains, this may cause a short from the high input voltage to the drain of the power transistor, and a huge current may flow from the mains to the power transistor. This will easily damage the power transistor if it is subjected to prolonged high current stress. In a worst case scenario overheating leading to fire can occur. It is therefore desirable to provide protection circuits against short winding faults.

In existing implementations, large currents caused by short winding faults are sensed through a shunt resistor. When the voltage across the shunt resistor, current sense (CS), exceeds a certain level after a blanking time (TBL), the system will denote that it has experienced a short winding fault. The integrated circuit (IC) will then shut down and block the power transistors gate. However, there is a disadvantage regarding this approach by relying upon one single blanking time event as an integral part of the protection function. If the blanking time for the short winding protection becomes longer than the leading edge blanking time, the circuit protection can not be activated. Robustness is thus low, as noise signals from switching as well as external sources can add to blanking time. The other disadvantage of this approach is that the blanking time is process and temperature dependent. At higher temperatures, the IC might not be able to detect short winding protection and at high temperatures, the power transistor is more susceptible to damages due to high current stress.

Apart from the shunt resistor, another approach is to place a resistor in series with the gate of the power transistor. This is to delay the switching off of the power transistor. The disadvantage of this approach is that the gate fall time will increase and this will affect the electromagnetic interference (EMI) performance of the system. Additionally the conduction loss of the power transistor will increase. Thus, this approach...