IGBT tutorial 1.docx

1、IGBT tutorial 1IGBT tutorial: Part 1 - SelectionJonathan Dodge, P.E., Senior Applications Engineer; John Hess, Vice President, Marketing, Microsemis Advanced Power Technology 3/8/2007 4:07 PM EST The insulated gate bipolar transistors (IGBTs) combines an easily driven MOS gate and low conduction los

2、s, and is quickly displacing power bipolar transistors as the device of choice for high current and high voltage applications. The balance in tradeoffs between switching speed, conduction loss, and ruggedness is now being finely tuned so that IGBTs are encroaching upon the high frequency, high effic

3、iency domain of power MOSFETs. In fact, the industry trend is for IGBTs to replace power MOSFETs except in very low current applications. Part 1 helps you understand the tradeoffs and helps with IGBT device selection, application and a relatively painless overview of IGBT technology. Part 2 provides

4、 an example walkthrough of IGBT datasheet information. How to select an IGBTThis section is intentionally placed before the technical discourse. Answers to the following set of burning questions will help determine which IGBT is appropriate for a particular application. The differences between Non P

5、unch-Through (NPT) and Punch-Through (PT) devices as well as terms and graphs will be explained later.1. What is the operating voltage? The highest voltage the IGBT has to block should be no more than 80% of the VCES rating. 2. Is it hard or soft switched? A PT device is better suited for soft switc

6、hing due to reduced tail current, however a NPT device will also work. 3. What is the current that will flow through the device? The first two numbers in the part number give a rough indication of the usable current. For hard switching applications, the usable frequency versus current graph is helpf

7、ul in determining whether a device will fit the application. Differences between datasheet test conditions and the application should be taken into account, and an example of how to do this will be given later. For soft switching applications, the IC2 rating could be used as a starting point. 4. Wha

8、t is the desired switching speed? If the answer is the higher, the better, then a PT device is the best choice. Again, the usable frequency versus current graph can help answer this question for hard switching applications. 5. Is short circuit withstand capability required? For applications such as

9、motor drives, the answer is yes, and the switching frequency also tends to be relatively low. An NPT device would be required. Switch mode power supplies often dont require short circuit capability. IGBT overviewAn N-channel IGBT is basically an N-channel power MOSFET constructed on a p-type substra

10、te, as illustrated by the generic IGBT cross section in Figure 1. (PT IGBTs have an additional n+ layer as well as will be explained.) Consequently, operation of an IGBT is very similar to a power MOSFET. A positive voltage applied from the emitter to gate terminals causes electrons to be drawn towa

11、rd the gate terminal in the body region. If the gate-emitter voltage is at or above what is called the threshold voltage, enough electrons are drawn toward the gate to form a conductive channel across the body region, allowing current to flow from the collector to the emitter. (To be precise, it all

12、ows electrons to flow from the emitter to the collector.) This flow of electrons draws positive ions, or holes, from the p-type substrate into the drift region toward the emitter. This leads to a couple of simplified equivalent circuits for an IGBT as shown in Figure 2. Figure 1 N-Channel IGBT Cross

13、 SectionFigure 2 IGBT Simplified Equivalent Circuits The first circuit shows an N-channel power MOSFET driving a wide base PNP bipolar transistor in a Darlington configuration. The second circuit simply shows a diode in series with the drain of an N-channel power MOSFET. At first glance, it would se

14、em that the on state voltage across the IGBT would be one diode drop higher than for the N-channel power MOSFET by itself. It is true in fact that the on state voltage across an IGBT is always at least one diode drop. However, compared to a power MOSFET of the same die size and operating at the same

15、 temperature and current, an IGBT can have significantly lower on state voltage. The reason for this is that a MOSFET is a majority carrier device only. In other words, in an Nchannel MOSFET only electrons flow. As mentioned before, the p-type substrate in an N-channel IGBT injects holes into the dr

16、ift region. Therefore, current flow in an IGBT is composed of both electrons and holes. This injection of holes (minority carriers) significantly reduces the effective resistance to current flow in the drift region. Stated otherwise, hole injection significantly increases the conductivity, or the co

17、nductivity is modulated. The resulting reduction in on state voltage is the main advantage of IGBTs over power MOSFETs. Nothing comes for free of course, and the price for lower on state voltage is slower switching speed, especially at turn-off. The reason for this is that during turn-off the electr

18、on flow can be stopped rather abruptly, just as in a power MOSFET, by reducing the gate-emitter voltage below the threshold voltage. However, holes are left in the drift region, and there is no way to remove them except by voltage gradient and recombination. The IGBT exhibits a tail current during t

19、urn-off until all the holes are swept out or recombined. The rate of recombination can be controlled, which is the purpose of the n+ buffer layer shown in Figure 1. This buffer layer quickly absorbs trapped holes during turn-off. Not all IGBTs incorporate an n+ buffer layer; those that do are called

20、 punch-through (PT), those that do not are called non punch-through (NPT). PT IGBTs are sometimes referred to as asymmetrical, and NPT as symmetrical. The other price for lower on state voltage is the possibility of latchup if the IGBT is operated well outside the datasheet ratings. Latchup is a fai

21、lure mode where the IGBT can no longer be turned off by the gate. Latchup can be induced in any IGBT through misuse. Thus the latchup failure mechanism in IGBTs warrants some explanation. Basic structureThe basic structure of an IGBT resembles a thyristor, namely a series of PNPN junctions. This can

22、 be explained by analyzing a more detailed equivalent circuit model for an IGBT shown in Figure 3. Figure 3 IGBT Model Showing Parasitic Thyristor A parasitic NPN bipolar transistor exists within all N channel power MOSFETS and consequently all N3channel IGBTs. The base of this transistor is the bod

23、y region, which is shorted to the emitter to prevent it from turning on. Note however that the body region has some resistance, called body region spreading resistance, as shown in Figure 3. The P-type substrate and drift and body regions form the PNP portion of the IGBT. The PNPN structure forms a

24、parasitic thyristor. If the parasitic NPN transistor ever turns on and the sum of the gains of the NPN and PNP transistors are greater than one, latchup occurs. Latchup is avoided through design of the IGBT by optimizing the doping levels and geometries of the various regions shown in Figure 1. The

25、gains of the PNP and NPN transistors are set so that their sum is less than one. As temperature increases, the PNP and NPN gains increase, as well as the body region spreading resistance. Very high collector current can cause sufficient voltage drop across the body region to turn on the parasitic NP

26、N transistor, and excessive localized heating of the die increases the parasitic transistor gains so their sum exceeds one. If this happens, the parasitic thyristor latches on, and the IGBT cannot be turned off by the gate and may be destroyed due to over-current heating. This is static latchup. Hig

27、h dv/dt during turn-off combined with excessive collector current can also effectively increase gains and turn on the parasitic NPN transistor. This is dynamic latchup, which is actually what limits the safe operating area since it can happen at a much lower collector current than static latchup, an

28、d it depends on the turn-off dv/dt. By staying within the maximum current and safe operating area ratings, static and dynamic latchup are avoided regardless of turn-off dv/dt. Note that turn-on and turn-off dv/dt, overshoot, and ringing can be set by an external gate resistor (as well as by stray in

29、ductance in the circuit layout). Punch through vs NPTPT versus NPT technologyConduction lossFor a given switching speed, NPT technology generally has a higher VCE(on) than PT technology. This difference is magnified further by fact that VCE(on) increases with temperature for NPT (positive temperatur

30、e coefficient), whereas VCE(on) decreases with temperature for PT (negative temperature coefficient). However, for any IGBT, whether PT or NPT, switching loss is traded off against VCE(on). Higher speed IGBTs have a higher VCE(on); lower speed IGBTs have a lower VCE(on). In fact, it is possible that

31、 a very fast PT device can have a higher VCE(on) than a NPT device of slower switching speed. Switching lossFor a given VCE(on), PT IGBTs have a higher speed switching capability with lower total switching energy. This is due to higher gain and minority carrier lifetime reduction, which quenches the

32、 tail current. RuggednessNPT IGBTs are typically short circuit rated while PT devices often are not, and NPT IGBTs can absorb more avalanche energy than PT IGBTs. NPT technology is more rugged due to the wider base and lower gain of the PNP bipolar transistor. This is the main advantage gained by tr

33、ading off switching speed with NPT technology. It is difficult to make a PT IGBT with greater than 600 Volt VCES whereas it is easily done with NPT technology. Advanced Power Technology does offer a series of very fast 1200 Volt PT IGBTs, the Power MOS 7IGBT series. Temperature effectsFor both PT and NPT IGBTs, turn-on