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Bridge rectifier dissipation in 60W offline flyback

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T

treez

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Hello,

We have designed a 60W offline 240VAC input Flyback LED driver with PFC.
We had to use a DBLS209G rectifier bridge because we couldn’t fit anything bigger on the PCB.
Each of the 4 diodes in the DBLS209G bridge package carries 124mA average , but each carries 341mA RMS.

In a thermal test, with a thermocouple glued to the case, the DBLS209G case temperature was 102degC. This was with the PCB in its plastic enclosure, which is also inside a metal enclosure….and the external ambient was 22degC. In the real application, the highest external ambient will be 25degC. –But the lamp will be on for 7 hours per night.

The DBLS209G datasheet says operating temperature can be up to 150degC. However, I cannot believe this gives long life….i remember when I was at Panasonic, the cases of semiconductors was never above 90degC.

Do you think we are running this diode too hot for long life?

Incidentally, there would actually be room to have two DBLS209G packages on the PCB, one on the top side, and one directly underneath it……we could then tie the “AC connection points” of each DBLS209G package together, and make up one more_thermally_capable bridge rectifier out of the two DBLS209G packages. Has anyone actually trIed this?.......i saw them doing this kind of thing on a reverse engineered 3Kw Electric car battery charger. The diode voltage increases significantly enough with current to make this work I would have thought?


DBLS209G rectifier:
**broken link removed**
 

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  • Bridge diode current.jpg
    Bridge diode current.jpg
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We can guess about 500 mW total bridge rectifier power dissipation, gives not more than 20K delta over internal "ambient" temperature. Means problem isn't the rectifier, it's total circuit power dissipation. Other components must be expected to be exposed to similar temperatures.
 

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