Thermal Junction Schottky Diode

[Winsu]

Hi All,

I have a Schottky diode in a LED driver design and it is getting hot. I have already increased the number of them ( 3 at the moment ) and I can't put more diodes because of a lack of space.

The case temperature reaches 105°C at worst case conditions and the Maximum Tj of the diode is 125°C. The datasheet of the diode doesn't include thermal resistance between case and junction so I can't calculate the Tj temperature.

Can I say safely that the Tj is below Tj maximun because in still air conditions Tj is just a few degrees above Tc?.

Thanks,
Winsu.

 

[KlausST]

Hi,

I don´t think it´s safe.

First: show your design: Schematic, PCB layout, waveform (inlcuding timing information), average current, exact diode type with link to manaufacturer site datasheet.

What does "I have already increased the number of them" mean?
In series, in parallel? --> None of both works sataisfactory.

and I can't put more diodes because of a lack of space.

There are some possible solutions:
* reduce the average current (maybe by changing the waveform)
* use a diode with less power dissipation (less forward voltage)
* use a bigger diode for better heat spreading
* use a better PCB layout for better heat spreading
* use a heatsink or improve air flow
...

Klaus

 

[Winsu]

Hi Klaus,

I can't change the wave form as it could impact EMC. I have already looked for others diodes that are better but I cant find anything better ( the current one is PN SK24A R3G). I have put 3 in parallel and I can't put a SMB or SMC because there is no more space. The best solution so far for temperature wise is with 3 x SMA ( PN SK24A R3G).

The layout will be improved adding more heat transfer pads and more vias but I won't be able to test that solution ( tight deadline). I am considering adding thermal jumpers ( PN THJP1225AST1) to transfer heat from diodes to a copper plane with vias, but I won't be able to reproduce the final solution ( just an approximation test on current prototype).

I have read this from a document from Cypress:

https://www.infineon.com/dgdl/Infineon-AN4017_Understanding_Temperature_Specifications_An_Introduction-ApplicationNotes-v11_00-EN.pdf?fileId=8ac78c8c7cdc391c017d071d497a2703

I think with layout improvement I will be able to reduce the heat something between 3-5 degrees so the maximum temperature could be 100°C on the case. Wouldn't that just be enough?


Regards,
Daniel.

 

[KlausST]

Hi,

"First"....


Klaus