IGBT vs MOSFET

This topic contains 8 replies, has 2 voices, and was last updated by  Paul Thomson 7 months, 2 weeks ago.

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  • #8402

    Paul Thomson
    Participant

    I watched your video on “What is a IGBT?” and it states that the IGBT switches at 12KHz. The MOSFET is much faster then that at 300KHZ, so why wouldn’t they use a MOSFET in the drive?


  • #8403
    John C
    John C
    Galco Representative

    Hello Paul, this is a very good question. So ok, in the drive the IGBT is being switched at 12KHz BY the drive. That is usually a parameter in the drive that lets you select the “Carrier Frequency” for the output of the drive. Most manufacturers will give you a range of Carrier Frequencies to choose based on the application. Actually the optimum CF is around 3.6Khz for best efficiency. However this lower frequency makes the motor audibly noisey, 12 to 16Khz will make the motor very quiet but compromises the drive max power availability. In other words you have to actually derate the drives power rating to use the higher Carrier Frequencies. Having said all that, the IGBT is capable of much higher switching frequencies. The “Slew” rates of these devices have them transitioning on and off in 0.1 to 0.05 micro seconds or 10 to 20 mega-hertz. In fact they switch so fast that they will actually create “Standing Waves” on the motor leads depending on their length. Basically, the longer the motor leads are, the higher the spikes will get at the motor end of the cable. Sometimes it is recommended to use a motor filter to suppress these Standing Waves to keep from damaging the motor.
    This help?


  • #8404

    Paul Thomson
    Participant

    Ok very interesting John.
    So if it can damage the motor why do they use them?


  • #8405
    John C
    John C
    Galco Representative

    Great question and understandable. There are probably several different reasons that the IGBT is used for drive applications. First of all it is a voltage controlled device that requires very little current to operate. So driver circuits can easily control a 10amp rated IGBT the same as a 1000amp device. So this is a design advantage. Because they switch so fast (very high speed Slew rates) this makes them more efficient too. However there is always a trade off, in this case it is the high speed switching rates that cause these Standing Waves on the motor leads dew in part to the inductive load of a motor, but also the inductance of the motor leads.
    Make sense?


  • #8406

    Paul Thomson
    Participant

    Ok well so what did drives use before IGBTs?


  • #8407
    John C
    John C
    Galco Representative

    Oh ok, very good. It was the “Power Darlington” bipolar transistor. Also available in the same current and voltage ranges as the IGBT, but this is a current controlled device. The IGBT is voltage controlled. This means that driver circuits that connect to the Base input control terminal of the PDBT have to increase in size with the size and amp rating of this type of device. They also switched about 20 times slower than the IGBT does, so no Standing Waves with the PDBT but their less efficient too. The other trade offs.
    OK?


  • #8408

    Paul Thomson
    Participant

    Ok wait so your saying that the PDBT doesn’t create Standing Waves but the IGBT does?
    And this is better why?


  • #8409
    John C
    John C
    Galco Representative

    Correct and because for 1 thing the Power Darlington has been obsoleted by most of it’s manufacturers due to the advent of the IGBT and it’s advantages. With smaller current demand to operate the device, the driver circuits can be very small and don’t require additional current for larger devices like the Power Darlingtons did. This allows for smaller circuit boards and a more compact drive chassis overall, which is kinda the way everything is going these days, eh? The IGBT is also very efficient in power transfer with on state saturation voltages very low and high Slew rates they don’t get very hot for the amount of amps they can handle.
    Now having said all this, can you see the differences between the MOSFET and the IGBT and why this device is so popular for Drives?


  • #8410

    Paul Thomson
    Participant

    Ok got it, thank you John


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