If output power remains at 500 W and the switching frequency is raised to 134 kHz at the higher temperature, the IGBT will exhibit slightly worse losses (25.2 W) than the MOSFET (23.9 W). If the same measurements are taken at room temperature, losses are 17.8 and 15.1 W, respectively. The increase in switching losses at the higher frequency eliminates the advantage that the IGBT had at high temperature when the switching frequency was lower.
These examples illustrate that there is no iron-clad rule that can be used to determine which device will offer the best performance in a specific type of circuit. The choice of IGBT or MOSFET will vary from application to application, depending on the exact power level, the devices being considered, and the latest technology available for each type of transistor.
In the battle between MOSFETs and IGBTs, either device can be shown to provide an advantage in the same circuit, depending on operating conditions. Then how does a designer select the right device for his application? The best approach is to understand the relative performance of each device and realize that if the component looks too good to be true, it probably is.
There are a few simple things to keep in mind about specifications. Test data, supplier claims, or advertisements which select conditions at maximum current and temperature will favor the IGBT in a given application. Take, for example, a motor-control application where a forklift is lifting its maximum-rated load while moving up an inclined ramp in the desert at noon.
In this particular scenario, the IGBT appears to be the device of choice. But when the average power consumption during an entire workday is considered, the maximum torque of the forklift motor is needed only 15% of the time, and the average torque load of the motor is only 25% of the rated torque. Under average or typical conditions, a MOSFET provides the longest battery life while meeting all peak-performance lev els—and usually at a lower cost.
Data that are based on applications at the highest switching frequency, the shortest pulse width, or the lowest current will tend to favor the MOSFET over the IGBT. For instance, a power supply operating at room temperature with nominal load and line voltage will make the MOSFET appear to be better than the IGBT. Conversely, if the power supply is operated at the maximum case temperature, maximum load, and minimum line voltage, the IGBT will look better. Actual performance, however, is almost never under "nominal conditions." Variations in ambient temperature, line voltage, and load are more realistic, and they should be considered.
Presently, some of the newest IGBTs can offer competitive performance and cost advantages in ZVS PFCs at 1000 W and up, operating at switching frequencies of 100 kHz and above. Nevertheless, in all other power-supply applications, the MOSFET continues to reign supreme.
There seems to be an industry-wide perception that MOSFETs are a mature product category that will not offer significant performance improvements in applications, while IGBTs are a new technology that will replace MOSFETs in all applications above 300 V. Such generalizations aren't true.