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OPA2990: Using op-amp for MOSFET gate driving

Arttu Rasanen
Prodigy 180 points

Part Number: OPA2990

Dear TI,

I'm designing a driver circuit for driving a pair of MOSFET switches for switching a long communication line or cable between two power supply rails (36 V and 24 V). Currently the topology I'm working on consists of a dedicated gate driver IC and two N-channel switching transistors. For more details on the drive circuit, please see my previous discussion topic: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1581266/ucc21520-driving-long-cables

The high switching speed of the gate driver IC causes lots of oscillations when driving long cables at high speeds (2.4+ kHz). I have measured up to 46 V peak voltages seen by the load. 

I think the solution to the oscillation problem would be to use a suitably slower switching speed. Based on some simulations, an op-amp (such as OPA2990) slewing the gate voltage of a complementary pair of FETs between ca. 16V and 42V in a controlled "ramp" of ~100 µs should help with the oscillation. The sketch below shows op-amps scaling the 0/5V input signal up to 15..45V suitable for driving the gates of common-source N-/P-FETs. Op-amps are powered from +48V and +12V rails. "Scope" trace below shows the input signal and the oscillating output voltage.

Näyttökuva 2025-12-16 162321.png

Which kind of circuit would you recommend to achieve this kind of output voltage slope from an op-amp? So far I've found TIDU026 and SBOA218B, but they seem to concern with slower slew rates up to 1s. Given a "step input" into an OPAx990, how to slow down its output slewing between 15V and 45 V in, say, ~100 usec (or about 0.3V/µs)?

Looking forward to your suggestions.

  • 0
    TI__Mastermind 29338 points

    Arttu,

    The oscillations you are seeing are due to the capacitance gate-to-source, and gate-to-drain on the MOSFET transistors.  This constitutes a capacitive load for the OPA2990.  The way to solve the problem is to place a capacitor between the op amp and the load.  Fortunately, you already have a resistor here.  The only problem with that resistor is that it is too small.  The required size of the resistor will depend on the op amp open-loop output impedance (Ro), the op amp unity gain bandwidth (fugbw), the the load capacitance (FET input capacitance).  A rough estimate is that you will need about 376 ohms.  If you increase RG to something like 376 to 1k ohm the oscillation should be minimal.  I don't think you need to do any slew rate limiting.  Op Amp Stability Theory and Compensation Methods covers the topic of stability in detail in case you want the theoretical background.  

    Best regards, Art