Primary-Side Current Monitoring Won’t Stop Overcurrents In DCM-Operated Flybacks

by John Bottrill and Lisa Dinwoodie, Texas Instruments, Dallas, Texas, How2Power Today, Feb 28 2011

Focus:
Many designers assume that the use of the primary-side pulse-by-pulse current monitoring and control in a flyback power converter is sufficient to limit the secondary-side current to slightly more than the maximum load requirement. A review of this technique demonstrates that this is not the case and that the current out of a flyback converter can easily result in higher than expected currents, if using this method. In turn, this can result in damage to the components in the power converter. To explain why this is the case and why improved fault protection is needed, the authors review operation of the flyback converter and discuss the factors that leave the flyback converter vulnerable to an overcurrent event when operating in discontinuous mode. Finally, the authors describe a technique used by newer PWM controllers such as the UCC28610 to prevent damage in the event of a short circuit while operating in discontinuous mode.

What you’ll learn:

• How to understand why flyback converters operating in discontinuous conduction mode are vulnerable to short circuits on the output
• How to protect flyback converters converters operating in discontinuous conduction mode from short circuits on the output

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