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26 Note that many IGBT circuits also need a reverse-blocking (anti-parallel) diode which cannot be fabricated with the IGBT, so the IGBT+diode combination is often co-packaged and offered as a single module. Single-ended topologies such as boost-PFC power supplies do not need this diode, and use an IGBT alone. The drive current needed at the gate to turn on either device varies, but is typically about 10% of the rated current of the device. Driving this current (sourcing) into the gate's capacitance fast enough for required turn-on speed, and pulling it out (sinking) for the turn-off cycle are two of the largest challenges in developing a complete motor-drive circuit, (Figure 4). In addition, for safety, electrical compatibility with low-voltage digital signals, or to "oat" the upper device's driver, the path must often include galvanic isolation between the digital output of the controller's processor and the driver circuit. Key parameters As with most electronic components, there are a few primary parameters and performance specications that determine the initial match between the device and the application. These are followed by a large number of secondary parameters and then by tertiary ones that, when taken as a group, point to a suitable choice. Of course, there is no single "best" choice, as any selection forces decisions with respect to the weighting of the many unavoidable tradeoffs of component selection (including cost, of course). For both devices, the top-level parameters are the current-handling and peak-voltage ratings, as these determine if a specic part can support the motor's load requirements. For MOSFETs, the next critical parameters are on-resistance (R DS(on) ) and gate capacitance. Lower on-resistance means reduced resistive loss and voltage drop when conducting, and thus reduced dissipation load and increased efciency. Advances in MOSFET design have reduced on-resistance to tens of milliohms—small, for sure, but still a potential problem when handling tens or hundreds of amps. Gate capacitance determines the current and slew rate needed turn the gate fully on and off with the desired transition time (which relates to switching speed). The amount of current to be injected or pulled out is based on the basic equation I = C dV/dt; there's no way around that. For IGBTs, the next critical specication is the on-state voltage drop Vdrop of about 2 V, which is the sum of the diode drop across the internal PN junction and its internal driving MOSFET; it never goes below a single-diode threshold value. If these parameters were static, selection would be easier. But the reality is that both MOSFET RDS(on) and IGBT Vdrop are affected by temperature and current level, and power devices such as these do have signicant self-heating. For MOSFETs, the voltage drop is resistive and proportional to current, and R DS(on) increases with temperature. For IGBTs, the drop is diode-like, increasing with the log of the current, and is relatively constant with temperature. In general, when comparing MOSFETs and IGBTs, the former offers higher switching speeds (MHz); higher peak current; and a wider SOA (safe operating area). But their conduction is strongly dependent on temperature and voltage rating; as the voltage rating goes up, the reverse recovery performance of their integral diode deteriorates, increasing switching losses. IGBTs are available with higher current ratings, and are rugged, but have slower switching speed; their lack of an internal reverse-recovery diode means you must nd an IGBT co-packaged with a diode matched to your application. For motor-drive applications, starting guidelines are that MOSFETs are a better choice at lower voltages and currents, and at higher switching frequencies; IGBTs are a better choice at higher voltage/current and lower frequencies. Caution: All guidelines have many exceptions, depending on the application's specics. Since most motors need only lower-frequency operation (a function of the number of poles and the maximum rpm), IGBTs are a viable option for this application. Thermal and packaging considerations No discussion of MOSFETs and IGBTs for motor drive is complete without discussion of dissipation and packaging. Since motors involve power control, the switching component must be able to dissipate the inevitable heat that results from internal losses. The industry has standardized on a relatively few package sizes and types for these devices (D2PAK, D-Pak, TO-220, TO-227, and TO-262 for small/moderate power levels), which simplies heat-sink and cooling options. Figure 4: The ST Microelectronics L6480 dual full bridge gate drive circuit controls four pairs of MOSFETs in a dual H-bridge from a 5Mbit/s SPI bus. (Souce: STMicroelectronics) CK SDO SDI SW STCK DGND VDD ADCIN VCC CP VBOOT PGND VS STBY/RESET FLAG CS BUSY/SYNC HVGA1 LVGA1 HVGA2 HVGB1 LVGA2 LVGB1 OUTA1 OUTB1 OUTA2 HVGB2 OUTB2 LVGB2 AGND VSREG VCCREG OSCIN OSCOUT VREG L6480 C FLY C VS C VCC C VCCREG C VDD C VSREG C BOOT C VSPOL C VREG C VREGPOL D1 Q1 Q2 Q4 Q3 Q5 Q6 Q8 Q7 Motor R B R A R PU R PU HOST

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