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UCC21750QDWEVM-025: half-Bridge SiC/IGBT NTC connection to AIN of UCC21750

Part Number: UCC21750QDWEVM-025
Other Parts Discussed in Thread: UCC21750

My design is a 3-phase inverter with 3 Half-bridge devices. Each Half-bridge will be driven by 2 UCC21750 drivers. 

For the NTC conenction

a) I am assuming it will be better to use the AIN pin of the lower Driver ( i.e. the one with a connection to the DC Bus) and not the upper driver

whose common is connected to the connection of  the upper gate source and lower gate drain.

b) I would like clarification on the following:

Section 9.2.2.7.1 (Isolated Temperature Sensing ) of the UC21750  data sheet  SLUSDH9B  shows the NTC connected to an R-C lowpass filter,

with the R,C junction going to AIN.

However the EVM schematic (SLUUBX2B) shows the NTC should be connected to TR+ which is in parallel with C29 (0.01uF) and R42 ( 6.19kohm).

which means TR+ is the same as AIN .

Which option is the one to use? I will connect the other end of the NTC to the driver  COM pin.

I will use an R-C filter for the APWM output as in Fig 56 of  SLUSDH9B.

 Thanks

David Nyarko

  • David,

    a) I am assuming it will be better to use the AIN pin of the lower Driver ( i.e. the one with a connection to the DC Bus) and not the upper driver

    In normal operation i do not believe it would matter which driver NTC is connected to, as long as the tail is connected to the source of its corresponding driver. Unless the module mfr's datasheet says something specifically on that i would say there shouldn't be issues either way.

    b) I would like clarification on the following:

    Section 9.2.2.7.1 (Isolated Temperature Sensing ) of the UC21750  data sheet  SLUSDH9B  shows the NTC connected to an R-C lowpass filter,

    with the R,C junction going to AIN.

    However the EVM schematic (SLUUBX2B) shows the NTC should be connected to TR+ which is in parallel with C29 (0.01uF) and R42 ( 6.19kohm).

    which means TR+ is the same as AIN .

    Which option is the one to use? I will connect the other end of the NTC to the driver  COM pin.

    R42 main purpose is its used as part of the resistor divider stack which includes R33, R34, etc.

    You can depopulate R42 for NTC

    And yes You can Connect NTC to TR+ (its same node as  AIN, but node will be broken if you change R40 value to something nonzero) of course.

    R40 value selection is pretty straight forward. You can use it for "biasing" / bump up DC level presented at AIN, which changes from the DC point based on temp.

    If this answers your questions, please let me know by pressing the green button.
    Best

    DImitri

  • Hi Dimitri,

    Thanks for the clarification. I can go by the Section 9.2.2.7.1 (Isolated Temperature Sensing ) of the UC21750  data sheet  SLUSDH9B R-C connection.

    I am wondering if i am still missing information for using the NTC on the module.

    The datasheet mentions that AIN has 5V when floating and a voltage range of 0.5V to 4.5V. I can see how this can be established when performing a DC measurement by an appropriate choice of external resistors. 

    However for an NTC how is this voltage established? Is it by the internal 200uA current source?

    My module's NTC has a temperature of 50kohm (at 25C) to about 1kohm (at 150C). How can this 0.5V to 4.5V be maintained at the AIN pin  over this temperature range? A 200uA current source will result in a 10V to 1V voltage across the NTC device.

    Or do i have to provide an external bias source for the NTC to result in a 0.5V to 4.5V at the AIN similar to the a voltage divider network for the external DC measurement scheme.

    Thanks

    David

  • Hi David,

    You are correct, the voltage on AIN is biased by the internal current source and the resistance by the NTC. To fall withing the 0.5V to 4.5V operation range I would recommend to add a zener diode to clamp any over-voltage that would exceed the recommended operating range of the gate driver. Additional to the zener diode I would recommend adding a resistor network as shown below. R1 and R2 values can be sized in a way that will enable you to narrow the voltage range seen from the gate driver to get the highest accuracy on the temperatures of concern:

    Let me know if you there's any additional questions!

    Best regards,

    Andy Robles