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Are IGBTs Better Than MOSFETs?

I've done my fair share of power designs, both switching power supplies and H-bridge motor controllers. Back in the 1970s and 1980s, my designs were done with NPN bipolar power transistors. We rarely used PNP devices because they typically cost more and were not available in current ratings as high as their NPN counterparts (all else being equal).

We also shied away from Darlington transistors. They have high to very high gain, but their saturation voltage is also high, significantly increasing the power dissipated within the device.

NPN Darlington transistor

NPN Darlington transistor

Since the collector of the input transistor connects to the collector of the output transistor, once the output transistor starts to turn on, it robs drive current (or deprives a voltage source if you'd prefer) from the input transistor. The result is a saturation of the compound device of around 1 volt. At high collector currents, the power dissipated in the device makes it run pretty warm.

From the 1980s to the present, the transistor of choice was generally the N-channel MOSFET. As with the bipolar devices, the P-channel FETs were not available in power ratings as high, and were more expensive. With FETs, the extremely high input impedance makes driving the gate somewhat easier. Gate-to-source capacitance negates that advantage somewhat, especially at high switching frequencies.

I never used insulated gate bipolar transistors (IGBTs), partially because I never completely understood them. I initially thought of them as Darlington devices with an N-channel FET substituted for the input bipolar transistor. That would produce a device with extremely high input impedance and high overall gain, but of course there would still be the high saturation voltage and corresponding high power dissipation.

I am seeing more press releases recently that talk about IGBT devices, so I decided to take a closer look at what the devices really are. A quick look at Wikipedia shows that I was only partially correct regarding my assumptions of the inner workings.

The functional equivalent of an IGBT (Source: Wikipedia)

The functional equivalent of an IGBT
(Source: Wikipedia)

Aha! It does in fact use an N-channel FET as the input device, but the bipolar device is a PNP device. Now it becomes much more efficient, and the device could have a very high breakdown voltage capability. Turn on the FET with just a few volts, and you turn on the PNP transistor hard. There is that parasitic NPN transistor; combined with the PNP it makes the bipolar section look like an SCR. In fact, early IGBT devices suffered from latch-up: Sometimes, once you turned them on, you couldn't turn them off, unless you cut collector current flow (shut off the main power supply). That problem has been cured with modern devices.

By the way, you'll see different symbols for the IGBT; this one is semi-common:

A commonly used symbol for the IGBT (Source: Wikipedia)

A commonly used symbol for the IGBT
(Source: Wikipedia)

Note that the upper terminal is called the collector, but it connects to the PNP emitter. That's just to simplify everyone's understanding of how it's used, rather than what's going on inside.

These devices are not the solution for all applications. They have a lower forward voltage drop than a regular MOSFET as long as you compare them with comparable high-voltage, high-current devices (up into kilovolts and hundreds-of-amps range). At more moderate current levels, regular FETs are better. And if you need high switching speeds for PWM rates — into the hundreds of kHz or MHz range — again use conventional FETs.

Let me know if you've used these devices and how well they worked in your application.

13 comments on “Are IGBTs Better Than MOSFETs?

  1. goafrit2
    February 12, 2013

    Simply, there is no game here. Mosfets win IGBTs anytime and anyday. There are many reasons but the key ones are ease of integration and excellent low static power disspitation.

  2. Brad Albing
    February 12, 2013

    Not to dispute that (since I'm not an expert regarding IGBTs), but if that's true, why are companies making IGBTs?

  3. goafrit2
    February 12, 2013

    Except for producing bias voltages where I use BJTs, I have consistently used MOSFETS. In short, few colleges teach BJT these days. MOSFETs with minor changes in the doping structure seems to be doing everything.

  4. dleske
    February 16, 2013

    IGBTs are hard to beat in capability and value for money when you need to control high power. For 600V or 1200V rating, with >30 amps in a small SMD package, they offer a lot. Smaller die than MOSFET for the same rating.

    But when you try to apply them in place of MOSFETs in a high frequency switching converter, you discover they are not the same! Slower turn-off, and often need a co-packed reverse diode. Some resonant configurations also depend on the MOSFET capacitance.

  5. amrutah
    February 16, 2013

    dleske, Correct me if I am wrong…

      When it comes to high power handling or applications then IGBT's are more efficient, since the voltage drop is across the junction and not in the channel and the parasitic BJT's or junction formed can be turned off or controlled well…

    When we are talking about handling high speed or low voltage then we need fast switching devices like MOSFETS, FINFETS, Si-nanowire technology. These are the technologies of future which will enhance very high level of integration.

  6. goafrit2
    February 18, 2013

    >> IGBTs are hard to beat in capability and value for money when you need to control high power.

    That is true and not debatable. When it comes to control of high power as you see in most PWM systems, IGBT  is great. But in most other areas of modern electronics manufacturing. CMOS has eclipsed it.

  7. goafrit2
    February 18, 2013

    Companies use IGBT when it comes to power systems control. I think that will be the case for a very long time.

  8. goafrit2
    February 18, 2013

    >> When we are talking about handling high speed or low voltage then we need fast switching devices like MOSFETS, FINFETS, Si-nanowire technology.

    You are very correct. MOSFET will remain the driver of the semiconductor industry. I do not think any other one will come close. Yet, there are many things CMOS is not great at. High voltage control is better done with IGBT.

  9. dleske
    February 18, 2013

    Of course, asking “Are IGBT's Better Than MOSFETs?” is a bit like asking “Are Axes better than Shovels?”. Both are useful tools, with different purposes!

    IGBT's are a 100% specialised device, for high energy power electronics. My earlier comment relates speciifically to the pros and cons of IGBTs in power control compared to high power MOSFETs in power control.

    NB: Power electronics <> Digital electronics <> Analog electronics. It is a specialty field with its own distinct features.

    MOSFETs are a spectacularly successful technology, scalable both to very large Power devices and to incredibly small Logic devices (used in complementary pairs as CMOS logic), and also usable in linear mode as Analog amplifier devices. They can do just about anything, though for certain specialised purposes other niche devices sometimes perform better.

    Other very important device types are Bipolar Junction Transistors (NPN, PNP), signal and power Diode rectifiers, and Zener diodes. Our electronics world would be much more limited without the full set of them.

    IGBTs essentially are a hybrid of BJT and MOSFET, combining the high impedance drive and fast turn-on of a MOSFET in parallel with the low-drop high current conduction of a BJT, without the disadvantages of a Darlington Pair.

     

    By the way, I started in Power Electronics designing Arc Welders. The first one delivered 600A 60V modulated DC pulses using a giant (2000A) SCR force-commutated by a bank of (20x 50A) MOSFETs. It was a big step forward when we could change to 3x 300A 100V NPN Bipolars (which needed 20A peak base current  to fully saturate ON). It was a gigantic leap again to change to Switch-Mode technology using IGBTs switching at 50kHz.  Don't try that at home with your 50nm 1V CMOS logic gates!

     

  10. Brad Albing
    February 19, 2013

    That is a good summary of capabilities of the two devices – just the sort of info I was hoping to see. This clarifies my thinking on the uses for IGBTs.

    I did some work back in the 1980s on a spot welder/induction heater that used 4 hockey-puck, water cooled SCRs in an H-bridge configuration; the load was set up as a series resonant circuit which facilitated SCR commutation. At the time, we couldn't find sufficiently high-power, cost effective devices in any other technolgy.

  11. goafrit2
    February 21, 2013

    @dleske, that is an excellent comment. I think you just made everyone better. So it has to do with the premise of the “Are IGBT's Better Than MOSFETs?” and you simply nailed it. That is what it is. I am a MOSFET guy and have never worked with IGBT but I picked some directions from this comment.

  12. Vinca
    February 28, 2013

    IS there any application note about driving an IGBT and taking care for break down?

     

  13. Brad Albing
    February 28, 2013

    I came across quite a few by Googling; also by using our search engine (at top right of the page).

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