Right way to calculate DESAT voltage

Patrik Resutik

Hi, i am using a ISO5500 in my current design of 3phase converter and i would like to ask how is proper way to calculate a DESAT voltage. I need lower a threshold for detecting DESAT fault, so i need to add a diodes. But there is my question. What forward voltage from diode datasheet should i choose ? Because i dont know what current is floating there. Should i pick a voltage around 0.65V for MURS160T3 diode ? I need lower a DESAT threshold around 3.5V so i need get a 7.2-3.5V=3.7V drop at diodes. When i take a 0.65V for MURS diode need i use a 6 diodes ? Is this right way to calculate this voltage? Thank you.

over 7 years ago

0 Don Dapkus over 7 years ago

TI__Guru**** 189401 points

Hi Patrick,

Welcome to e2e, and thanks for your interest in our products.

Section 9.2.2.9 DESAT Diode and DESAT Threshold of the ISO5500 data sheet discusses how to determine the DESAT trip point. Please review that section and let us know if it doesn't clarify the situation.

0 Patrik Resutik over 7 years ago in reply to Don Dapkus

Prodigy 60 points

Thank you for reply,

i checked that section, there is formula for DESAT voltage trip point i understand that. But i don´t understand a Vf voltage mentioned there, because when you look at Vf voltage of MURS160T3 recomended diode, this voltage is function of current. And i don´t know what value of Vf should i choose for calculation ?

0 Xiong Li over 7 years ago in reply to Patrik Resutik

TI__Intellectual 1665 points

Hi Patrick,

The current through the DESAT diode is only 0.5mA. You can treat the voltage as the turn-on threshold voltage. To avoid using too many diodes in series, you can add a Zener diode in series with the DESAT diode. The clamping voltage of the Zener diode can be set based on the DESAT protection threshold.

With Regards,

Xiong

0 Patrik Resutik over 7 years ago in reply to Xiong Li

Prodigy 60 points

Hi, thank you for answer. In datasheet they recommend a low capacitance diode, this can´t be problem with zener diodes as you suggesting ? Thank you for helping me.

0 Xiong Li over 7 years ago in reply to Patrik Resutik

TI__Intellectual 1665 points

Hi Patrick,

The capacitance of the DESAT diode would dominate the total capacitance.

With Regards,
Xiong

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Right way to calculate DESAT voltage