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How to select gate driver for MOSFET and IGBT?


 

Selecting a Gate Driver requires consideration of a number of factors.

  • How many inputs/output are provided for/by the Gate driver?
  1. For the inputs, It depends on the choice of the microcontroller and the control algorithm chosen by the designer 
  2. For 2 inputs, the choice is high-side / low-side gate driver 
  3. For 1 input, the choice is a half-bridge driver 
  4. The number of outputs depending on the number of half-bridges that require driving
  • How to select the voltage rating?
  1. A conservative rule is to pick a voltage rating 3 times the operating voltage, with 1.5 times being a recommended minimum. However, this depends purely on the system requirements and is usually set by the designer
  2. Gate drivers always work with MOSFET/IGBT, the best practice is to match the voltage rating of the chosen MOSFET/IGBT
  • How much drive current is required? 
  1. Information about the required gate charge to raise the gate voltage to the desired level is essential
  2. Gate charge information is provided by the MOSFET manufacturer in their datasheet, usually for a gate voltage of 10V
  3. Now that we know the required gate charge, we choose the drive current rating depending on the rise and fall times we are targeting. The equation to use is Qg =Igate * time Example: Qg = 50nc. Required Tr = 50ns and Tf = 25ns. Igate (source) = 50/50 = 1A of source current. Igate(sink) = 50/25 = 2A of sink current.
  4. The above calculation provides you with a minimum figure. Often it is not easy to find a tailored gate driver. The best practice is to choose a gate driver with higher than the required rating and use series gate resistors to limit the source and sink currents
  • Special functions
  1. Some applications need special functions like inbuilt and/or adjustable dead time, enable option, shoot through prevention logic, delay matching, etc. to ensure the selected gate driver comes with the required optional features.
      • Key external component selection
      1. Bootstrap capacitor selection: 
        • Where an external bootstrap diode is necessary, the designer should choose its voltage and current ratings appropriately. VR rating of the bootstrap diode should be >= the voltage rating of the gate driver OR the MOSFETs, whichever is lower. 
        • Though the average current flowing through the bootstrap diode under normal operation is very small, it is important to consider the start-up current. When the system is first powered, there will be an inrush current flowing into the bootstrap diode 
        • Inrush current is directly proportional to the size of the bootstrap capacitance. The larger the capacitance, the larger will be the inrush current. Hence it is important to follow the design recommendations in the previous slide while choosing the capacitor. Also, a series current limiting resistor is recommended almost every time 
        • Optimum capacitor size and appropriate series resistance combination is important to avoid any unnecessary stresses on the bootstrap diode
      2. Gate resistor selection 
        1. A typical gate drives current control circuit is shown here. 
        2.  By adjusting the RGon and RGoff resistors respectively, the rise and fall times can be controlled individually 
        3. The effect of the gate resistance on the switching time is shown in the below example, where the on-time is increased from 68ns to 86ns
      3. Layout recommendations for managing switch node noise

      Some more points need to consider: 

      • Types: 
        1. Full - Bridge
        2. Half- Bridge
        3. High Side
        4. Low Side
        5. Inverting
        6. Non-Inverting
      • Output Current 
      • Mounting style ( SMD/SMT or through-hole)
      • Supply Voltage min and max
      • Rise Time
      • Fall Time
      • Operating Temperature
      • Qualifications
      • Package
        Reference : 

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