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App note: Gate drive characteristics and requirements for HEXFET power MOSFETs

Posted on Sunday, August 6th, 2017 in app notes by DP

an_infineon_AN-937

App note from International Rectifier on driving their Power MOSFETs. Link here (PDF)

The conventional bipolar transistor is a current-driven device. A current must be applied between the base and emitter terminals to produce a flow of current in the collector. The amount of a drive required to produce a given output depends upon the gain, but invariably a current must be made to flow into the base terminal to produce a flow of current in the collector.

The HEXFET®is fundamentally different: it is a voltage-controlled power MOSFET device. A voltage must be applied between the gate and source terminals to produce a flow of current in the drain. The gate is isolated electrically from the source by a layer of silicon dioxide. Theoretically, therefore, no current flows into the gate when a DC voltage is applied to it though in practice there will be an extremely small current, in the order of nanoamperes. With no voltage applied between the gate and source electrodes, the impedance between the drain and source terminals is very high, and only the leakage current flows in the drain.

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4 Responses to “App note: Gate drive characteristics and requirements for HEXFET power MOSFETs”

  1. Drone says:

    This is a venerable App-Note…

    However I do not think it properly deals with the likes of a HEXFET structure in linear mode. If you look at the app-note’s “Figure 2. Basic HEXFET Structure” you will see that in essence, the device is multiple FET devices (domains) on the die itself. This can cause problems in linear applications, especially with high current. One domain may turn on before others do. This can result in on-die failure of individual domain FET structures.

    If you are designing for linear relatively high current applications, it is probably best to seek a “Linear Qualified” or “Linear Mode” MOSFET part.

    IXYS Corp. www[dot]ixys[dot]com has a range of nice linear qualified MOSFETs to examine. Unfortunately, the IXSYS web page is a nightmare to navigate, and there are almost NO LTspice compatible models available from IXSYS (that I can find anyway).

    Corrections please anyone. It’s been awhile since I looked at Linear MOSFETs. I’m working from memory here.

    • Drone says:

      Qualification on the LTspice compatible models… There should be at least level-2 SPICE models. Level-3 is welcome, but are a barrier to cross-tool use. Often thermal level-3 inclusions are a way to obscure electrical functionality.

  2. poorchava says:

    Well, this is not exactly accurate. In a MOSFET transient currents do flow when the gate is being switched on and off, as with any capacitor. Those currents are very small, but can make a difference in some applications.

  3. Drone says:

    @poorchava you said:

    “Well, this is not exactly accurate. In a MOSFET transient currents do flow when the gate is being switched on and off, as with any capacitor. Those currents are very small, but can make a difference in some applications.”

    How is my post as you said, “not exactly accurate [sic]”? There is no such thing as “exactly accurate”. Never mind the symantics…

    My post was about linear mode applications, something that the HEXFET devices are typically NOT designed for, but CAPABLE of in carefully designed circuits (to avoid runaway of single on-die transistors before others). This is what the AN misses IMO.

    Gate capacitance does have an impact when it comes to gate charge/capacitance differences in linear-qualified MOSFETs vs. switching parts. But I did not intentionally address this in my post which was focused on linear applications. The Devil is in the details. Linear qualified MOSFETs will have larger on-die transistor area, which results in higher gate capacitance.

    Some MOSFET parts include on-die Zeners (or similar) to protect against the effects of gate charge unwanted “parasitic” effects in switching applications. So careful part selection is needed. Unfortunately, parts with on-die gate protection do not typically simulate well in SPICE. at least unless the model takes careful account of the gate protection structures. A level of complexity to simulate such on-die structures is rare in models for general-purpose use MOSFET devices.

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