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Op amp for High slew rate DC electronic Load

Manjunath Inder
Prodigy 190 points
Other Parts Discussed in Thread: LMH6703

Hi folks,

I'm Designing a high slew rate DC electronic load in which a voltage feedback operational amplifier is driving a power MOSFET (Varying its resistance in the saturation mode and dissipating heat) .The main objective of this

work is to design a high slew rate load (current Sinking).

I have two doubts Regarding the above.

1.Will the Input Control Voltage Slew rate across Op-AMP determine the Output current slew rate Through MOSFET

2.Do we need a Saturation mode Gate driver to achieve the above objective.  

and is there any FET Drivers available(That operates MOSFET in Saturation region).

What I believe is  Driving the MOSFET with just Op Amp will not yield the required high slew rate (since the input current of the FET is not sufficient to Charge the Input capacitance of

the MOSFET .

Correct me If im wrong and thank you in Advance

Regards

Manjunath I  

  • 0
    TI__Genius 13935 points

    Hello Manjunath,

    Have you looked into using a GaN FET?  The gate capacitance is significantly smaller. 

    Our current feedback amplifiers will offer the best slew rate with large voltage swings.  Driving a large capacitance will require special care for stability. 

    I would not put the FET in the feedback loop, for example. 

    You may want to look at the LMH6703.  It is very fast. 

    Regards,

    Loren

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Prodigy 190 points

    Hi Loren,Thanks for the reply

    When I posted the same query in MOSFET Forum ,I got the following Reply. Can you Please Give more insight about this.

    Regards Manjunath I

  • 0 in reply to Manjunath Inder
    TI__Genius 13935 points
    Hello Manjunath,

    Another benefit of the GaN FETs is the PCB interface. Most GaN FETS are sold as bare die that solder directly to the PCB. This will reduce the inductance of the FET and PCB.

    None of our op amps are designed specifically as FET drivers, and I think the bulk of our application notes for FET drivers are with regards to switching power supplies.

    www.ti.com/.../snva723.pdf

    It looks like you require linear operation, is this correct?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 190 points
    Hello Loren,

    Thanks for replying.

    It is for Saturation mode (aka Linear mode ) of operation. The sole purpose of using the FET is to sink the load(or dissipate).The current through FET should rise as fast as possible so that I can achieve Max slew rate. Several factors decide this

    1. like .Qgs and Qgd. For the FET to reach desired Vgs ,Cgs should be charged quickly as possible. For this Qgs should be low as and Ig should be high. Qgd should also be low.The op amp output current decides this.

    2.The Inductance in the MOSFET path should be Low .So I have to use SMT package. And take care of thermal aspects also.

    My concern is whether I would be able to achieve this without using a Gate driver.Is there any gate drive that is available that is used in saturation mode of operation of FET's (as opposed to abundantly available switching Mode).

    And also, Do you think the above drawn circuit will work ?. Glad to hear any modifications/additions.


    Thank you
    Regards
    Manjunath I