ADUM7234: how to realize the current and voltage protection of MOSFET by using ADUM7234?

Hello,

My task is to protect MOSFET from overcurrent and overvoltage in buck converter. I read  the application note of ADUM7234. I need to use this IC to drive MOSFET but I didn't find the circuit to protect MOSFET.  Does  ADUM7234 have the feature of protection of overcurrent and overvoltage ?  If not, could you give me some advice? Thank you very much

  • 0
    •  Analog Employees 
    on Jan 16, 2017 11:28 PM over 4 years ago

    Hello,

    The ADuM7234 provides a means for delivering a higher driver strength signal to the power MOSFET gates, as well as the ability to level shift the highside gate signal. This is done in an isolated fashion, allowing the primary and secondary sides of the ADuM7234 to remain at different isolation regions.

    When looking to protect against over voltage and over current, are you referring to the gate or the drain side of things?

    If you are referring to the power MOSFET gate, the ADuM7234 will provide a gate drive voltage that is equal to the voltage supplied to VDDA and/or VDDB. As long as an adequate external series gate resistor is used (based on parasitic inductance between the gate driver and the gate being driven) little to no voltage overshoot on the gate will be seen.

    If you are talking about over voltage and over current on the drain side of things, there are a few things to consider. What power are you looking to drive the buck at? What bus voltages are you targeting? For lower power applications, over current can often be detected by using a current sense resistor in the return path of the power loop. By setting a threshold, and using a comparator, one can set up a max current limit in the circuit. This information would then either be fed to a controller, or directly go to the enable/disable pin of the gate drivers for a fast shut off reaction. A current sense resistor is usually not desired in high power/high current applications since it leads to extra losses (>1 kW range). In that case, a current transformer can be used, or directly sensing the drain to source voltage of the power device while on can give a hint a the current levels in the system.

    Shutting a system down abruptly can sometimes cause an overvoltage at the drain of the power device. This occurs due to the current in the parasitic inductances in the power loop. Imagine a system with high current, switching between two switches, and then both switches turn off instantly. The current has nowhere to go, so there is a build up of voltage at the drain of the power device.

    Some isolated gate drivers that addresses a few of these issues is the ADuM4135 and/or ADuM4136. These gate drivers have desaturation detection built into the driver. This is basically sensing the current in the system by measuring the voltage of the power device drain to source while it is on. The current protection provided by the ADuM4135 and ADuM4136 has a single level, and is usually used to detect gross over current events. If the drain to source voltage of the power device is above 9 V while the device is supposed to be on, the ADuM4135/ADuM4136 shuts off and sends a fault signal to the controller. When these gate drivers shut off, they use what is called a soft-shutdown, where the internal resistance of the gate drive is purposely increased to cause the gate voltage of the power device to fall more slowly than normal. This allows less of a voltage overshoot on the drain of the device due to the parasitic inductances mentioned above.

    Is your system synchronous or asynchronous? In a buck converter, factors that affect the chance of over voltage of the power devices are the choice of the FET, choice of bus voltage, and parasitic inductances in the design. Higher parasitic inductances can lead to higher overshoot in synchronous half-bridge topologies. For silicon FETS, this number is often around 30%-60%. For this reason, it is common to choose a FET with a breakdown voltage approximately double the bus voltage.

    If you are looking to achieve overvoltage protection at the load, this would be more of a case where a comparator with a reference would be used. By sensing the output voltage through a sensing network, and feeding this to the controller, a parallel control path could be made where if the voltage goes above a preset threshold, either the PWM signal of the buck is reduced, or the system is brought into a shutdown mode. This depends on the power level being used again. This type of over voltage protection does not typically rely directly on the gate driver itself.

    Please let me know if any of this information pertains to your application. If so, we can talk in more detail about it.

    RSchnell

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:42 PM over 2 years ago
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin