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ISO5852S: desat soft turn-off problem

Milos Stojadinovic
Prodigy 20 points
Part Number: ISO5852S

To whom it may concern,

The ISO5852S IC is used to drive the SiC MOSFETs (1200V/523A) in the ongoing project we are currently working on. We are also using the desat detection option of the mentioned IC, unfortunatelly the soft turn-off function doesn't seem to work. In the attached .pdf file, I am providing the schematic of the gate driver circuit, together with the measurements obtained during the fault condition.

The nominal DC voltage that we have in the system is 800V. But, as you can see on the waveforms, due to non-existent soft turn-off the drain-to-source voltage shoots up to 750V with approximately 400V at the input.

Can you comment on why the soft turn-off function is not working? We are not using the inverting BJT pair on the output of the gate driver, since it is stated in the data sheet of the device that it is not compatible with desat logic.

Best regards,

Milos Stojadinovic

8182.desat_detection_attachment.pdf

  • 0
    TI__Intellectual 1665 points

    Hi Milos,

    Thanks for reaching out. An RC circuits need to be added to the base of the BJT circuit in order to have proper soft turn-off with external BJT buffer. 

    In ISO5852S, a weak current source of 130mA is turned on in case of the DESAT detection. The 130mA current would be the base current of the BJT buffer. The collector current would be huge with the current amplification, which would drive the BJT to operate in saturation region. By adding a capacitor on the base of the BJT buffer, the PNP transistor would operate like a emitter follower. The voltage of the capacitor at the base would ramp down slowly with the 130mA current source. You can size the capacitance based on the soft turn-off time needed. 

    Hope this clarifies your concern. Please  let us know of any further questions. 

    With Regards,

    Xiong

  • 0 in reply to Xiong Li
    Prodigy 20 points
    Hello Xiong,

    Thank you for the quick reply. I will try what you suggested and let you know how it went.

    Best,
    Milos