Fullscreen HTML5Med Res (??MB)HTML4

An introduction to IGBTs When it comes to high power switching applications circuit designers generally choose between bipolar transistors or Mosfets. But there is an alternative which combines the best of both devices – the insulated gate bipolar transistor or IGBT. It can be thought as a bipolar transistor with a high impedance gate instead of a low impedance base. More and more we are seeing heavy duty switchmode power circuits – inverters, power supplies, induction motor control and so on. As the applications continue to become more stringent, semiconductor manufac­turers need to create products that approach the ideal switch. The ideal switch would have: (1) zero resist­ance or forward voltage drop in the on-state; (2) infinite re­sistance in the off-state; (3) switch on and off with infinite speed; and (4) would not require any input power to make it switch. Fig.1: reduced forward voltage drop of an IGBT compared to a Mosfet with similar ratings. 76  Silicon Chip Since we don’t yet have the ideal switch, designers must choose a device that best suits the application. The choice involves considerations such as voltage, current, switching speed, drive circuitry, load and temperature effects. There are a variety of solid state switch types available and they all have their strong and weak points. High voltage power Mosfets The characteristics that are most desirable in a solid-state switch are fast switching speed, simple drive requirements and low conduction loss. For low voltage applications, power Mosfets offer very low on-resistance [RDS(on)] and approach the desired ideal switch. But in high voltage applications, Mosfets exhibit increased RDS(on) which results in increased conduction losses. In a power Mosfet, the on-resistance is proportional to the breakdown voltage raised to approximately 2.7: RDS(on) = (VDS)2.7 Mosfet technology has now advanced to a point where RDS(on) is near the theoretical limit. A new approach is needed to obtain very low on-resistance without sacrificing switching speed. This is where the IGBT comes in. By combining the low conduction loss of a BJT (bipolar junction transistor) with the switching speed of a power Mosfet an optimal solid state switch would be obtained. In fact, the IGBT is a spin-off from power Mosfet technology and its structure closely resembles Fig.2: reduced die size of an IGBT compared to a Mosfet with similar ratings. Fig.3: reduced package size of an IGBT compared to a Mosfet with similar ratings. that of a power Mosfet. The IGBT has a high input impedance and fast turn-on like a Mosfet. And they have an on-voltage and current density comparable to a bipolar transis­tor. Compared to SCRs, the IGBT is faster, has better dv/dt immunity and above all, has better gate turn-off capability. While GTOs (gate turn-off SCRs) are capable of being turned off at the gate, substan­tial reverse gate current is required, whereas turning off an IGBT only requires the gate capacitance to be discharged. Against that, SCRs have a slightly lower forward voltage and a higher surge current capability than IGBTs. Many of today’s switching circuits use Mosfets because of their simple gate drive. Since the structure of both devices is similar, the change to IGBTs can be made without having to redes­ign the gate drive circuit. Like Mosfets, IGBTs are transconduc­tance devices and can remain fully on if the gate voltage is held above a certain threshold. As shown in Fig.1, using an IGBT in place of a power Mosfet dramatically reduces the forward voltage drop at currents above 12 amps. By reducing the forward drop, the conduction loss is decreased. The gradual rising slope of the Mosfet in Fig.1 can be attributed to the relationship of VDS to RDS(on). The IGBT curve has an offset due to an internal forward biased p-n junction and a fast rising slope typical of a minority carrier device. Replacing a Mosfet with an IGBT can improve the efficiency and/or reduce the cost. As shown in Fig.2, an IGBT has consider­ably less silicon area than a similarly rated Mosfet. The reduced silicon area makes the IGBT the lower cost solution. Fig.3 shows the package area reduction by using an IGBT. This suits it for designs where space is restricted. Speaking IGBT Before we go any further, perhaps we should tell you how to say IGBT. Instead of referring to them as “Iggbets” most design­ers call them by the initials, “eye gee bee tees” – more of a mouthful perhaps but that’s the way it is. IGBTs are replacing Mosfets in high voltage applications where conduction losses must be kept low. In fact, SILICON CHIP featured a 2kW sinewave inverter with IGBTs in the October 1992 to February 1993 issues. Four 1kV IGBTs were used in the high voltage H-pack section where 365V DC is converted to a 50Hz sinewave using pulse width modulation at around 4kHz. In this instance, we were forced to use IGBTs because no combination of currently available power Mosfets was sufficiently rugged for the job. With zero current switching or resonant switching tech­niques, IGBTs can be operated in the hundreds of kilohertz range. Typically though, although turn-on speeds are very fast, turn-off of the IGBT is slower than a Mosfet. It exhibits a significant current fall time or “tailing”. This tailing restricts IGBTs to operating at less than 50kHz in traditional “square wave” PWM switching applications. Up to 50kHz then, IGBTs are often a better solution than bipolar transistors, Mosfets or thyristors (SCRs). Fig.4: forward voltage drop (VCE(sat)) and fall time (tf) has improved since IGBTs were introduced. August 1996  77 Introduction to IGBTs –­ continued Fig.5: cross-section and equivalent schematic of an insulated gate bipolar transistor (IGBT) cell. When compared to BJTs, IGBTs have similar ratings in terms of voltage and current but the isolated gate in an IGBT makes it simpler to drive. BJTs used as switches require sufficient base current to maintain saturation. Typically, the base current needs to be at least 1/10th of the collector current. BJT drive circuits must therefore be sensitive to variable load conditions. In other words, base current for a BJT must be kept propor­tional to the collector current; otherwise the device will come out of saturation with high-current loads and will have excessive base drive under low-load conditions. Either way, it can lead to increased power dissipation. BJTs are minority carrier devices and charge storage Fig.6: cross-section and equivalent schematic of a metaloxide-semiconductor field-effect transistor (Mosfet) cell. 78  Silicon Chip effects including recombination slow the performance when compared to majority carrier devices such as Mosfets. IGBTs also experience recombination that accounts for the current “tailing”, yet IGBTs have been observed to switch faster than BJTs. Since the introduction of IGBTs in the early 1980s, semicon­ductor manufacturers have learned how to make the devices faster. As illustrated in Fig.4, some trade-offs in conduction loss versus switching speed exist. Lower frequency applications can tolerate slower switching devices. Because the loss period is a small percentage of the total on-time, slower switching is traded for lower conduction loss. In a higher frequency application, just the opposite would be true and the device would be made faster and have greater conduction losses. Notice that the curves in Fig.4 show reductions in both the forward drop VCE(sat) and the fall time tf of newer generation devices. These capabilities suit the IGBT for applications such as motor control, power supplies and inverters which require devices rated at 600-1200V. IGBT structure The structure of an IGBT is similar to that of a double diffused (DMOS) power Mosfet. One difference between a Mosfet and an IGBT is the substrate of the starting material. By varying the starting material and altering certain process steps, an IGBT may be produced from a power Mosfet mask; however, at Motorola, mask sets are designed specifically for IGBTs. In a Mosfet the sub­stance is P+ as shown in Fig.5. The n- epi resistivity determines the breakdown voltage of a Mosfet as mentioned earlier using the relationship: RDS(on) = (VDS)2.7 To increase the breakdown voltFig.7: the age of the Mosfet, the n- epi region symbols for thickness (vertical direction in the IGBTs (a) and diagram) is increased. Reducing Mosfets (b). the RDS(on) of a high voltage device requires a greater silicon area to make up for the increased n- epi region. The effects of the high resistive n-epi region were overcome by conductivity modulation. The n-epi was placed on the P+ sub­strate, forming a pn junction where conductivity modulation takes place. Because of conductivity modulation, the IGBT has a much greater current density than a power Mosfet and the forward voltage drop is reduced. Now the P+ substrate, n-epi layer and P+ “emitter” form a BJT transistor and the n-epi acts as a wide base region. Current tailing has been mentioned above. The device struc­ture shown in Fig.5 provides an insight into tailing. Minority carriers build up to form the basis for conductivity modulation. When the IGBT turns off, these carriers do not have a current path to exit the device. Recombination is the only way to elim­inate the stored charge resulting from the build-up of excess carriers. Additional recombination centres are formed Fig.8: IGBT current turn-off waveform. by placing an N+ buffer layer between the n-epi and P+ substrate. While the N+ buffer layer may speed up recombination, it also increases the forward voltage drop. Hence the tradeoff between switching speed and conduction loss becomes a factor in optimising performance. The N+ buffer layer also prevents thermal runaway and punch-through of the depletion region. This allows a thinner n- epi to be used which somewhat decreases forward vol­tage drop. Four layers The IGBT has a four layer (PNPN) structure, resembling that of an SCR. But unlike the SCR where the device latches on and gate control is lost, an IGBT is designed so that it does not latch on. Full gate control is available at all times. Because the IGBT is a four-layer structure, it does not have the inverse parallel diode inherent in power Mosfets. This is a disadvantage to motor control designers who use the anti-parallel diode to recover energy from the motor. Like a Mosfet, the gate of an IGBT is electrically isolated from the rest of the chip by a thin layer of silicon dioxide, SiO2. This gives it a high input impedance and excellent drive efficiency. a voltage across the base-emitter junction of the NPN. If the base-emitter voltage is above a certain threshold level, the NPN will begin to conduct causing the NPN and PNP to enhance each other’s current flow and both devices can become saturated. This results in the device latching on in a fashion similar to an SCR. Device pro­ cessing directs currents within the device and keeps the voltage across Rshorting low to avoid latching. The IGBT can be gated off, unlike the SCR which has to wait for the current to cease, allowing recombination to take place in order to turn off. IGBTs offer an advantage over the SCR by controlling the current with the device, not the device with the current. The internal Mosfet of the IGBT when gated off will stop current flow and at that point, the stored charges can only be dissipated through recombination. The IGBT’s on-voltage is represented by the sum of the offset voltage of the collector base junction of the PNP transistor, the voltage drop across the modulated resistance Rmod and the channel resistance of the internal Mosfet. Unlike the Mosfet where in­creased temperature results in increased RDS(on) and increased forward voltage drop, the forward drop of an IGBT stays relative­ly unchanged at increased temperatures. Switching speed Until recently, slow turn-off speed limited IGBTs from serving a wide variety of applications. While turn-on is fairly rapid, initial IGBTs had current fall times of around three microseconds. The turn-off time of an IGBT is slow because many minority carriers are stored in the n- epi region. When the gate is initially brought below the threshold voltage, the n- epi contains a very large concentration of electrons and there will be significant injection into the P+ substrate and a correspond­ing hole injection into the n- epi. As the electron concentration in the n-region decreases, electron injection decreases, leaving the rest of the electrons to recombine. Therefore, the turn-off of an IGBT has two phases: an injec­tion phase where the collector current falls very Equivalent circuit IGBT operation is best understood by again referring to the cross section of the device and its equivalent circuit shown in Fig.5. Current flowing from collector to emitter must pass through a pn junction formed by the P+ substrate and n- epi layer. This drop is similar to that seen in a forward biased pn junction diode and results in an offset voltage in the output characteristic. Current flow contributions are shown in Fig.5 using varying line thickness, with the thicker lines indicating a high current path. For a fast device, the N+ buffer layer is highly doped for recombination and speedy turn off. The additional doping keeps the gain of the PNP low and allows two-thirds of the current to flow through the base of the PNP (electron current) while one-third passes through the collector (hole current). Rshorting is the parasitic resistance of the P+ emitter region. Current flowing through Rshorting can result in Fig.9: cross-section and equivalent schematic of a short circuit rated IGBT cell. August 1996  79 Introduction to IGBTs –­ continued quickly and a recombination phase in which the collector current decreases more slowly. Fig.8 shows the switching waveform and the contributing factors to tail time of a “fast” IGBT designed for PWM motor control. In power Mosfets, the switching speed can be greatly affected by the impedance in the gate drive circuit and the same rules apply to IGBTs. Comparing IGBTs, BJTs & Mosfets The conduction loss of BJTs and IGBTs is related to the forward voltage drop of the device while a Mosfet’s conduction loss is based on RDS(on). Table 1 gives a comparison of turn-off and conduction losses at 10 amps for a power Mosfet, an IGBT and a BJT at junction temperatures of 25°C and 150°C. Note that while the devices in Table 1 have approximately the same ratings, their chip sizes vary significantly. The bipo­lar transistor requires 1.2 times more silicon area than the IGBT while the Mosfet requires 2.2 times the area of the IGBT. This difference in die area has a direct effect on the cost of the devices. At higher currents and high temperatures, the IGBT offers low forward drop and a switching time similar to the BJT without the drive difficulties. The lower conduction losses of the IGBT reduce power dissipation and heatsink size. Thermal resistance An IGBT and power Mosfet produced from the same size die have similar junction-to-case thermal resistance Fig.11: IGBTs offer performance advantages in PWM variable-speed induction motor drives. They can directly control 3-phase motors from a rectified mains supply. 80  Silicon Chip Fig.10: the waveforms associated with anti-parallel diode turn-off. because of their similar structures. Short circuit rated devices Using IGBTs in motor control circuit requires them to with­stand short circuit current for a given period. Although this varies with the application, a typical value of ten microseconds is used for designing these specialised IGBTs. Notice that this is only a typical value given on the data sheet. IGBTs can be made to withstand short circuit conditions by altering the device structure to include an additional resistance (Re, in Fig.9) in the main current path. The benefits associated with the addition­al series resistance are twofold. First, the voltage created across Characteristic Re, by the large current passing through Re, increases the percentCurrent Rating age of the gate voltage across Re, by Voltage Rating the classic voltage divider equation. R(on) <at> TJ = 25°C Assuming the drive voltage applied to the gate-to-emitter remains the R(on) <at> TJ = 150°C same, the voltage actually applied Fall Time (typical) across the gate-to-source portion of * Indicates VCEO rating the device is now lower. This causes the device to operate in an area of the transconductance curve that reduces the gain and it will pass less current. Second, the voltage developed across Re results in a similar division of voltage across Rshorting and VBE of the NPN transis­tor. The NPN will be less likely to attain a VBE high enough to turn the device on and cause a latch-up situation. These two situations work together to protect the device from catastrophic failure. The protection period is specified in the ratings, giving the circuit time to detect a fault and shut off the device. The introduction of the series resistance Re also results in additional power loss by slightly elevating the forward drop of the device. However, the magnitude of short circuit current is large enough to require a very low Re value. The additional conduction loss of the device due to the presence of Re is not excessive when comparing a short-circuit rated IGBT to a non-short circuit rated device. Anti-parallel diode When using IGBTs for motor control, designers have to place a diode in anti-parallel across each device in order to handle the regenerative or inductive currents of the motor. The optimal setup is to have the diode co-packaged with the device. A specific line of IGBTs has been created by Motorola to address this issue. These devices work very well in applications where energy is recovered to the source and are favoured by Table 1: Device Characteristics TMOS IGBT Bipolar 20A 20A 20A 500V 600V 500V* 0.2 ohms 0.24 ohms 0.18 ohms 0.6 ohms 0.23 ohms 0.24 ohms** 40ns 200ns 200ns ** BJT TJ = 100°C motor control design­ers. Like the switching device itself, the anti-parallel diode should exhibit low leakage current, low forward voltage drop and fast switching speed. As shown in Fig.10, the diode forward drop multiplied by the average current it passes is the total conduc­tion loss produced. In addition, large reverse recovery currents can escalate switching losses. A secondary effect caused by large reverse recovery currents is EMI at the switching frequency and the frequency of the re­sulting ringing waveform. This EMI requires additional filtering in the circuit. By co-packaging the IGBT with its anti-parallel diode, the parasitic inductances that contribute to ringing are greatly reduced. Induction motor drive Mains operated, PWM variable speed motor drives are an application well suited for IGBTs. As shown in Fig.11, IGBTs may be used to directly control the voltage supplied to a 3-phase motor to control its speed. Depending on the application, the IGBT may be required to operate from the full-wave rectified mains supply. Acknowledgement This article reproduced by arrangement from Motorola Semi­conductor Application Note AN1541. SC THE “HIGH” THAT LASTS IS MADE IN THE U.S.A. Model KSN 1141 The new Powerline series of Motorola’s 2kHz Horn speakers incorporate protection circuitry which allows them to be used safely with amplifiers rated as high as 400 watts. This results in a product that is practically blowout proof. Based upon extensive testing, Motorola is offering a 36 month money back guarantee on this product should it burn out. Frequency Response: 1.8kHz - 30kHz Av. Sens: 92dB <at> 1m/2.83v (1 watt <at> 8Ω) Max. Power Handling Capacity: 400W Max. Temperature: 80°C Typ. Imp: appears as a 0.3µF capacitor Typical Frequency Response MOTOROLA PIEZO TWEETERS AVAILABLE FROM: DICK SMITH, JAYCAR, ALTRONICS AND OTHER GOOD AUDIO OUTLETS. IMPORTING DISTRIBUTOR: Freedman Electronics Pty Ltd, PO Box 3, Rydalmere NSW 2116. Phone: (02) 9638 6666. August 1996  81