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I am using a UC1845A to control the switching element MOSFET (FCB20N60F) which is used for a typical Flyback Converter circuit. MOSFET's drain is connected to transformer. Rarely MOSFET is not working stable and broke down. It is usually works fine. Do you have a suggestion to overcome this FET problem? Can changing the MOSFET to TI csd19502q5b solve the problem (This FET has lower Vds voltage as 80 V although the one used on circuit has 600 V Vds voltage. Can you suggest a better FET instead of FCB20N60F)?

Thank you.

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    \$\begingroup\$ can you show the complete circuit \$\endgroup\$ – JonRB Apr 14 at 13:49
  • \$\begingroup\$ circuit is almost similar with the application note on UC1845A datasheet page 13. \$\endgroup\$ – layout789 Apr 14 at 14:42
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    \$\begingroup\$ The 650-V MOSFET you have stated first is a monster: 20 A for a 30-W or less power supply, this is way too big. Also, a 80-V MOSFET is not adapted for this application. Universal mains power supplies usually use BVDSS of 600 or 650-V. What is the maximum input voltage and what is the transformer turns ratio? \$\endgroup\$ – Verbal Kint Apr 14 at 17:52
  • \$\begingroup\$ Thank you Verbal. My flyback converter's input is about 28VDC nom, 23VDC min, 33VDC max instead of AC. Turn ratio is 0.42. For current FET FCB20N60F, you maan the Rds is too big to describe dissipate more power? It has 600 BVDSS and i could not find so many alternating FET that has a lower Rds resistance with other similar parameters. What will be your suggestion? \$\endgroup\$ – layout789 Apr 15 at 4:51
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    \$\begingroup\$ You can easily find 100-V MOSFETs but you need first to calculate the maximum drain voltage at the switch opening. It is the max input voltage plus the reflected voltage and the leakage inductance contribution. If you are interested in designing switching power supplies, you might consider reading this book first: amazon.com/Switch-Mode-Power-Supplies-Second-Edition/dp/… \$\endgroup\$ – Verbal Kint Apr 15 at 11:16

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