Design Feature: March 1, 1996

In an ever-increasing number of applications, power MOSFETs and insulated-gate bipolar transistors (IGBTs) are displacing traditional bipolar power transistors. The MOSFETs and IGBTs use less silicon for a given rating and require vastly lower drive power. To choose between the two devices, you must evaluate several criteria and consider how your design will drive and provide protection for the devices.

The processing for MOSFETs and IGBTs is similar, and the devices use identical processing steps, except for substrate polarity. This one-step difference yields the MOSFET and IGBT structures in Figures 1a and 1b. The processing for MOSFETs produces a source-to-drain body diode. In the more complex IGBT structure, an n-channel MOSFET, an npn transistor, and a junction FET drive the pnp output transistor. (The "collector" and "emitter" labels in Figure 1 are misnomers; those shown are the emitter and collector of the pnp transistor.)

The bipolar npn and pnp in Figure 1b form a silicon-controlled rectifier (SCR) (Reference 1). If the gains of these devices are high enough, the SCR can latch up. The base resistance, R, however, prevents the npn from turning on, and the IGBT's equivalent circuit reduces to that in Figure 1c. This structure offers the best of both worlds: the high input impedance of a power MOSFET and the low saturation voltage of a bipolar transistor.

Table 1, derived from Reference 2, provides a quick comparison of the switching performance of MOSFETs and IGBTs. For comparison purposes, the table includes bipolar devices and translates the usual V_CE(SAT) for IGBTs to R_ON. Two facts spring from this data: First, the bipolar devices are considerably slower than MOSFETs, principally because of the bipolar devices' long turn-off times. Second, the on-state resistance of the bipolars is relatively invariant with temperature, in comparison with the high-temperature coefficients inherent in MOSFETs. This temperature-coefficient information is an important consideration in designing thermal-safety margins into high-power systems.

Table 2 (Reference 3) presents a more general comparison of device characteristics. Thanks to their gate-input structure, MOSFETs and IGBTs are voltage-driven devices, which demand minimal drive power. A bipolar transistor is a current-driven device (output current divided by h_FE), so it demands hefty drive power. The input capacitance for MOSFETs and IGBTs, depending on rating, can be considerable, and the drive circuitry must be able to rapidly charge and discharge this high capacitance.

IGBTs win hands down on current-density ratings. The trade-off in Table 2 reflects a compromise in device selection. To maintain latch-up resistance, IGBT manufacturers mandate lower current densities for faster devices. International Rectifier, for example, offers three IGBT families: Standard, Fast, and UltraFast. The company specs these families' current densities to be inversely proportional to their switching speeds. V_CE(SAT) increases correspondingly: 1.3, 1.5, and 1.9V, respectively.

The last parameter in Table 2, switching losses, reflects the switching speeds of the various devices. For MOSFETs, both turn-on and -off transitions are very fast. For the IGBTs, again you face a trade-off: switching speed vs current density; hence, faster devices suffer higher conduction losses. The switching times of IGBTs, mostly comprising turn-off times, generally limit the use of these devices to systems operating at switching rates lower than 100 kHz.

Table 3 (Reference 2) shows the dramatic trade-off in conduction losses vs voltage ratings for MOSFETs. For equal current density per unit of silicon area, MOSFETs' on-state resistance increases exponentially with respect to voltage rating. IGBTs, specified in terms of V_CE(SAT) rather than on-state resistance, maintain their low conduction losses for all voltage ratings. The MOSFETs in Table 3 have slightly lower voltage ratings than those of the IGBTs; this disparity reflects the MOSFETs' hardier avalanche resistance.

Figure 2 from Reference 2 underscores the high current density, thus, the low on-state losses, IGBTs can offer. The graphs show the on-state voltage drops for a MOSFET and two IGBTs of the same die size, with 10A load current. You face a trade-off of IGBT switching speed vs conduction losses. The lowest loss device in Figure 2 is the IRGBC40S from International Rectifier's Standard family of IGBTs; the UltraFast model IRGBC40U offers faster switching and correspondingly higher on-state losses.

In static terms, the input impedance of both MOSFETs and IGBTs is practically infinite. Drive circuitry does not have to supply the significant base currents that bipolar devices require. For ac operation, however, the impedance is far from infinite. The input capacitance of the devices can range from hundreds or thousands of picofarads to double-digit nanofarads. The driving source for MOSFETs and IGBTs must be able to rapidly charge and discharge this high input capacitance to ensure fast switching of the power device.

Figure 3, derived from Reference 4, shows three techniques for driving an IRF320 power MOSFET. In Figure 3a, an open-collector TTL gate drives the device. In this configuration, the MOSFET turns on much more slowly (in about 2 µsec) than it turns off. In turn-on mode, the gate-to-source and -drain capacitances charge exponentially through the 680-ohm pullup resistor. In turn-on mode, the saturated output transistor of the TTL gate rapidly discharges these capacitances.

Two parallel high-voltage drivers in Figure 3b provide faster turn-on transitions (about 400 nsec), but this figure is still well below the capability of the MOSFET. To obtain the fastest possible switching (about 40 nsec), the driver circuit must provide active pullup and pulldown drive, as in Figure 3c. You can configure the MOSFET totem poles in either a source-follower or a common-drain connection.

Instead of designing your own driver, you could select from a variety of ICs designed for driving MOSFETs. Table 4 (Reference 5) gives a comparison between the bipolar and common-drain-FET totem poles in Figure 3c and several IC drivers available from Micrel Semiconductor and Unitrode. The power MOSFET is an APT5020BN from Advanced Power Technology, switching a 10.5-ohm load connected to a 250V supply. Several of the IC drivers equal the performance of the MOSFET totem pole, and the rest range in performance between the FET and the bipolar totem-pole drivers.

The drive requirements for IGBTs are similar to those for MOSFETs, in that both device types present high values of input capacitance. However, IGBTs and MOSFETs differ markedly in turn-on threshold voltages. Traditionally, n-channel MOSFETs require 10V gate-source voltage to ensure full enhancement. Now, a wide variety of logic-level MOSFETs is available, with guaranteed on-state resistance at gate-source voltage levels equal to the logic-one levels of common digital ICs.

IGBTs require 10V gate-emitter voltage to fully turn on and 0V to fully turn off. Large, high-voltage, and high-current IGBTs and IGBT modules work better with even higher input levels. Powerex, for example, recommends +15V and -10V turn-on and turn-off voltages, respectively, for its large IGBT modules. The company offers a number of hybrid gate drivers to provide those drive voltages to the big modules.

Protective measures

Both IGBTs and power MOSFETs are vastly more robust than bipolar transistors. Their safe operating areas are relatively square, with power limited only by the admissible junction-temperature rise. In contrast to early devices, modern IGBTs are highly resistant to latch-up when you operate them within data-sheet limits. However, with their thin oxide layers and megacell construction, you must take care to protect both IGBTs and MOSFETs from overvoltage conditions.

Figure 4 (Reference 6) shows various ways to protect power devices from overvoltage conditions arising from inductive effects. Figure 4a shows the result of trying to turn the MOSFET off; the overvoltage transient is an inductive "kick." The diode in Figure 4b clamps the main inductive-kick component, but stray inductances still produce an overvoltage transient. In Figure 4c, a zener diode or other clamping device clips the transient to a safe level.

Figure 4d shows an alternative clamping arrangement. Capacitor C is a "reservoir" capacitor that charges to a relatively constant voltage, and resistor R dissipates the clamping energy while maintaining the desired voltage across the capacitor. You must carefully select diode D such that its forward-recovery characteristic does not spoil the transient-clamping action of the circuit.

You could also use the simple RC snubber in Figure 4e. However, an RC snubber not only limits the peak voltage, but also lowers the effective switching speed. It absorbs energy during the entire switching period, not just at the end of it, as does a voltage clamp. A snubber is, therefore, less efficient than a true voltage-clamping device.

These points are only a few of the aspects involved in choosing and using a power MOSFET or an IGBT. Comprehensive treatments of the drive, protection, and applications aspects of these devices fill entire textbooks. Most MOSFET and IGBT manufacturers publish large amounts of application literature on their products and on power devices in general.

1. Mitter, CS, "Application Considerations Using Insulated Gate Bipolar Transistors (IGBTs)," Application Note AN1540, Motorola Inc.

2. Takesuye, Jack, and Scott Deuty, "Introduction to Insulated Gate Bipolar Transistors," Application Note AN1541, Motorola Inc.

3. Clemente, S, A Dubashni, and B Pelly, "IGBT Characteristics," Application Note AN-983A, International Rectifier.

4. Clemente, Steve, "Gate Drive Characteristics and Requirements for Power HEXFETs," Application Note AN-937B, International Rectifier.

5. Dierberger, Kenneth, "Gate Drive Design for Large Die MOSFETs," Application Note APT9302, Advanced Power Technology.

6. Pelly, Brian, "The Do's and Don't's of Using Power HEXFETs," Application Note 936A, International Rectifier.