Schottky Diode only for low output voltages.?

Hello,

This 150V rated Schottky diode (STPS3150) is said to be only good for Vout rails up to 24V.

(-according to datasheet page 1)

STPS3150 DATASHEET:
https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003323.pdf


Do you know why its only up to 24V?

(I am doing an offline flyback with Vout = 36V)

I would have thought that as long as the rated reverse voltage of 150V is not exceeded , then it matters not what is the output voltage?

I agree with you. Perhaps the 24 VDC rails specification and second order transient overshoot of 600% relates the PIV of 150 V to the rail supply?

Hi, It has more to do with thermal runaway because of the high reverse current in Schottky diodes.

More about this can be found in an app note from Diodes:

**broken link removed**

If you have time, connect a schottky with a small case size in reverse to a current limited power supply and set the voltage to 30V or more.(depending on the diode you are using) Slowly start to heat the device to 100°C and at some point it will go into thermal runaway, destroying itself.

Always check if you're circuit keeps functioning the way you designed it at high temperatures. In every semiconductor device, you will find leakage currents that will go up when the junction temperature rises.

MOSFETS are also tricky parts. The RDS-on (Drain source on resistance) of a MOSFET increases with increasing junction temperature causing even more I²R losses.

Inventor

Yes, the standing reverse voltage is certainly something to watch if you let your diode get overly hot (i.e. not enough heatsinking to allow for the rms forward current), but I believe the ST people who write the datasheets/app notes are probably thinking of a 24V o/p flyback offline supply, where the max reverse volts are applied when the primary side mosfet is ON, here at high line (265VAC for e.g.) the impressed voltage from the flyback transfomer + the o/p voltage could approach 120+ volts, and so a 150V part would just meet the requirement here.

Regards, Orson Cart

@ Orson, you are right. Wasn't thinking about it when I posted my answer.