MOTOROLA Order this document SEMICONDUCTOR APPLICATION NOTE by AN1541/D AN1541 Introduction to Insulated Gate Bipolar Transistors Prepared by: Jack Takesuye and scott Deuty Motorola inc INTRODUCTION As power conversion relies more on switched applications The IGBT is, in fact, a spin-off from power MOSFET semiconductor manufacturers need to create products that technology and the structure of an iGBt closely resembles approach the ideal switch. The ideal switch would have: that of a power MOSFET. The IGBT has high input impedance 1)zero resistance or forward voltage drop in the on-state, and fast turn-on speed like a MOSFET IGB Ts exhibit an 2)infinite resistance in the off-state, 3)switch with infinite on-voltage and current density comparable to a bipolar speed, and 4)would not require any input power to make it transistor while switching much faster. IGBTs are replacing MOSFETs in high voltage applications where conduction When using existing solid-state switch technologies, the losses must be kept low. With zero current switching or designer must deviate from the ideal switch and choose a resonant switching techniques, the IGBT can be operated in device that best suits the application with a minimal loss of the hundreds of kilohertz range [1] efficiency. The choice involves considerations such as Although turn-on speeds are very fast, turn-off of the IGBT voltage, current, switching speed, drive circuitry, load, and is slower than a MOSFET. The IGBT exhibits a current fall time temperature effects. There are a variety of solid state switch or tailing The tailing restricts the devices to operating at technologies available to perform switching functions moderate frequencies(less than 50 kHz)in traditional"square however, all have strong and weak points. waveform"PWM, switching applications HIGH VOLTAGE POWER MOSFETS At operating frequencies between 1 and 50 kHZ, IGB Ts offe an attractive solution over the traditional bipolar transistor The primary characteristics that are most desirable in a MOSFETS and thyristors. Compared to thyristors, the IGBT is solid-state switch are fast switching speed, simple drive faster, has better dv/dt immunity and, above all, has better gate requirements and low conduction loss. For low voltage turn-off capability. While some thyristors such as GTOs are applications, power MOSFETs offer extremely low capable of being turned off at the gate, substantial reverse on-resistance, RDS(on), and approach the desired ideal gate current is required, whereas turning off an IGBT only switch. In high voltage applications, MOSFETs exhibit requires that the gate capacitance be discharged. A thyristor increased RDS(on) resulting in lower efficiency due to has a slightly lower forward-on voltage and higher surge capability than an IGBT. h-resistance is proportional to the breakdown voltage raised MOSFETs are often used because of their simple gate drive to approximately the 2.7 power(1) requirements. Since the structure of both devices are so MOSFET technology has advanced to a point similar, the change to IGB Ts can be made without having to densities are limited by manufacturing equipment capabilities redesign the gate drive circuit. IGBTS, like MOSFETS, are and geometries have been optimized to a point where the transconductance devices and can remain fully on by keeping g DS(on)is near the predicted theoretical limit. Since the cell the gate voltage above a certain threshold As shown in Figure 1a, using an IGBT in place of a play a major role, no significant reduction in the RDS(on)is MOSFET dramatically reduces the forward voltage drop at els above 12 amps. By reducing the forward drop the conduction loss of the device is decreased. The gradua problem of increased on-resistance without sacrificing rising slope of the MOSFET in Figure la can be attributed to witching speed the relationship of VDs to RDS(on ). The IGBT curve has ar offset due to an internal forward biased p-n junction and a fast DSS rising slope typical of a minority carrier device It is possible to replace the MOSFET with an IGBT and improve the efficiency and/or reduce the cost. As shown in ENTER THE IGBT Figure 1b, an iGBT has considerably less silicon area than a similarly rated MOSFET Device cost is related to silicon area; By combining the low conduction loss of a BjT with the therefore, the reduced silicon area makes the IGBT the lower witching speed of a power MOSFET an optimal solid state cost solution. Figure 1c shows the resulting package area switch would exist. The Insulated-Gate Bipolar Transistor reduction realized by using the IGBT. The IGBT is more space (IGBT)technology offers a combination of these attributes efficient than an equivalently rated MOSFET which makes it MOOROLA e Motorola Inc. 1995MOTOROLA 1        Prepared by: Jack Takesuye and Scott Deuty Motorola Inc. INTRODUCTION As power conversion relies more on switched applications, 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 with infinite speed, and 4) would not require any input power to make it switch. When using existing solid–state switch technologies, the designer must deviate from the ideal switch and choose a device that best suits the application with a minimal loss of efficiency. The choice involves considerations such as voltage, current, switching speed, drive circuitry, load, and temperature effects. There are a variety of solid state switch technologies available to perform switching functions; however, all have strong and weak points. HIGH VOLTAGE POWER MOSFETs The primary 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 extremely low on–resistance, RDS(on), and approach the desired ideal switch. In high voltage applications, MOSFETs exhibit increased RDS(on) resulting in lower efficiency due to increased conduction losses. In a power MOSFET, the on–resistance is proportional to the breakdown voltage raised to approximately the 2.7 power (1). MOSFET technology has advanced to a point where cell densities are limited by manufacturing equipment capabilities and geometries have been optimized to a point where the RDS(on) is near the predicted theoretical limit. Since the cell density, geometry and the resistivity of the device structure play a major role, no significant reduction in the RDS(on) is foreseen. New technologies are needed to circumvent the problem of increased on–resistance without sacrificing switching speed. RDS(on)
V 2.7 DSS (1) ENTER THE IGBT By combining the low conduction loss of a BJT with the switching speed of a power MOSFET an optimal solid state switch would exist. The Insulated–Gate Bipolar Transistor (IGBT) technology offers a combination of these attributes. The IGBT is, in fact, a spin–off from power MOSFET technology and the structure of an IGBT closely resembles that of a power MOSFET. The IGBT has high input impedance and fast turn–on speed like a MOSFET. IGBTs exhibit an on–voltage and current density comparable to a bipolar transistor while switching much faster. IGBTs are replacing MOSFETs in high voltage applications where conduction losses must be kept low. With zero current switching or resonant switching techniques, the IGBT can be operated in the hundreds of kilohertz range [1]. Although turn–on speeds are very fast, turn–off of the IGBT is slower than a MOSFET. The IGBT exhibits a current fall time or “tailing.” The tailing restricts the devices to operating at moderate frequencies (less than 50 kHz) in traditional “square waveform” PWM, switching applications. At operating frequencies between 1 and 50 kHz, IGBTs offer an attractive solution over the traditional bipolar transistors, MOSFETs and thyristors. Compared to thyristors, the IGBT is faster, has better dv/dt immunity and, above all, has better gate turn–off capability. While some thyristors such as GTOs are capable of being turned off at the gate, substantial reverse gate current is required, whereas turning off an IGBT only requires that the gate capacitance be discharged. A thyristor has a slightly lower forward–on voltage and higher surge capability than an IGBT. MOSFETs are often used because of their simple gate drive requirements. Since the structure of both devices are so similar, the change to IGBTs can be made without having to redesign the gate drive circuit. IGBTs, like MOSFETs, are transconductance devices and can remain fully on by keeping the gate voltage above a certain threshold. As shown in Figure 1a, using an IGBT in place of a power MOSFET dramatically reduces the forward voltage drop at current levels above 12 amps. By reducing the forward drop, the conduction loss of the device is decreased. The gradual rising slope of the MOSFET in Figure 1a 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. It is possible to replace the MOSFET with an IGBT and improve the efficiency and/or reduce the cost. As shown in Figure 1b, an IGBT has considerably less silicon area than a similarly rated MOSFET. Device cost is related to silicon area; therefore, the reduced silicon area makes the IGBT the lower cost solution. Figure 1c shows the resulting package area reduction realized by using the IGBT. The IGBT is more space efficient than an equivalently rated MOSFET which makes it perfect for space conscious designs. Order this document by AN1541/D    SEMICONDUCTOR APPLICATION NOTE  Motorola, Inc. 1995