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UC3845: Pulse Transformer with 2 Isolated Mosfets via Gate Transformer

Christian Schröder
Prodigy 20 points
Part Number: UC3845

Hello,

we want to implement with the driver IC UC3845 an flyback converter with isolated gate transformer to drive two mosfets of a big welding machine transformer.

The application is a welding machine with a transformator TR2.

I want to know what kind of driver IC I need to put on the primary side of the TR1 (gate drive transformer) in order to connect it to the UC3845:

Could you please also check if the secondary side of the Gate drive transformer is correct?

The Mosfets should be on at the same time.

regard Christian

LSQ_Draft.pdf

  • 0
    TI__Genius 9021 points

    Hi Christian,

    -UC3845 has a totem-pole output stage capable of sinking/sourcing current to drive N-channel MOSFETs (see block diagram). It means that the device is a gate driver itself and it can be connected to the gate of the main switch MOSFET. This gate driver has a peak current of 1A (check parameters in the product folder webpage).

    -You need to connect the Isense pin to the source of that MOSFET. The check more details on the gate driver capabilities, please check the Output Section of the Electrical Characteristics in the Datasheet.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Prodigy 20 points

    Hi Manuel,

    thanks for your answer.

    On this application we we need to drive two mosfets which are both on at the same time in a flyback transformer configuration.

    I have marked them here in red:

    How can this be done with the UC3845? I think with the upper mosfet there will be an issue with GND reference, that's the reason why I added the gate transformer.

    Can you please double check if it would work like this or suggest an alternative component?

    best regards

    Christian

  • 0 in reply to Christian Schröder
    TI__Genius 9021 points

    Hi Christian. 

    TI is in a long holiday and the answer might be delayed.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    TI__Genius 9021 points

    Hi Christian.

    You can use a PWM controller driver (like UC3845) to drive the PWM signals of the high side MOSFETs (low side too) at the secondary side. No need to use a switch. Also, no need to tie the primary winding to +12V rail. You can tie it to OUT pin of the driver and the other side to GND (see picture below). Please, keep in mind that the driver capabilities (UC3845 has a 1A peak current) depends on your switches at the secondary side and the transformer design.

    For more details I recommend reading the article attached, Section 7.1.

    Thank you.

    Fundamentals of MOSFET Gate Driver Circuits