Until the MOSFET came along in the 1970s, the bipolar transistor was the only ‘real’ power transistor. Though it provided the benefits of a solid state solution for many applications, it had limitations and drawbacks too. It had a relatively slow turn-off characteristic, which is known as current tail, and it required a high base current to turn on and was susceptible to thermal runaway due to its negative temperature coefficient. Also, the lowest attainable on-state voltage or conduction loss was governed by the collector-emitter saturation voltage. In contrast, the MOSFET is a device controlled by voltage rather than current and has a positive temperature coefficient, preventing thermal runaway. The MOSFET also has a body-drain diode, which is particularly useful in dealing with limited freewheeling currents. Its on-state resistance has no theoretica limit, so its on-state losses can be far lower than those of a bipolar device. All these advantages and the comparative elimination of the current tail quickly made the MOSFET the device of choice for power switch designs.
In the 1980s, the Insulated-Gate Bipolar Transistor (IGBT) came along. This device is a cross between the bipolar and MOSFET transistors. It has the output switching and conduction characteristics of a bipolar transistor, but it’s voltage-controlled like a MOSFET. Generally, this means it combines the high-current-handlingcapability of a bipolar part with the ease-of-control of a MOSFET. Unfortunately, the IGBT still has the disadvantages of a comparatively large current tail and the lackof a body-drain diode. Another potential hazard with some IGBT types is the negative temperature coefficient, which can lead to thermal runaway.It also makes the paralleling of devices hard to effectively achieve. Currently, this problem is being addressed in the latest generations of IGBTs that are based on Non-Punch-Through (NPT) technology. This development maintains the same basic IGBT structure, but is based on bulk-diffused silicon, rather than the epitaxial material that both IGBTs and MOSFETs have historically used. The IGBT is a powerful competitor of the power-MOSFET, particularly at lower frequencies. The IGBT, with substantially higher current densities and lower operatingtemperature range, is a good alternative to MOSFET switches, which require high-voltage performance. Slowly, the IGBT has evolved into new-generation offerings with higher operating frequencies and lower saturation voltages. IGBTs with standard operating frequencies of 50 kHz at rated current are available and you can now control the bipolar fall-time tail.Comparisons of IGBT and power-MOSFET performance show that you can extend an IGBT’s operatingrange well beyond 50 kHz. The circuit configuration of thepower converter affects the maximum frequency at which an IGBTcan outperform a power MOSFET, but the maximum frequency is greater than 100 kHz in many cases.
In most applications of power switches to 100 kHz, the IGBT is a better choice. For applications with frequencies as high as 100 kHz, an IGBT can handle more current at high voltages than the same-size power MOSFET. Before directly comparing IGBTs and power-MOSFETs, however, you need to understand the difference in their physical structure and how these differences affect their performance and application. To perform comparison tests, you also need to make some underlying assumptions and choose comparable data-sheet values. MOSFET and IGBT structures look very similar, but there is one basic difference—the addition of a p-substrate beneath the n-substrate in the IGBT. This variation is sufficient to produce some clear distinctions as to which device serves which applications better. The IGBT requires just as thick a layer of highresistivity material as the MOSFET to provide voltage-breakdown resistance. However, because the substrate is p-type material, it causes the injection of minority carriers (electrons) into the substrate. Thisminority-carrier injection reduces the resistivity of the n-region just as if the device were constructed of low-resistivity silicon. This temporarily lowers the forward-voltage drop by as much as a factor of 10, and the conduction losses in any application all decrease proportionately. The IGBT is definitely the choice for breakdown voltages of above 1000 while the MOSFET is for device- breakdown voltages below 250 V. Device selection isn’t clear, though, when the breakdown voltage is between 250 and 1000 V. In this range, some component vendors advocate the use of MOSFETs. Others make a case for IGBTs. Choosing between them is a very application- specific task in which cost, size, speed, and thermal requirements should be considered. IGBTs have been the preferred devices under the conditions of low-duty cycle, low frequency (less than 20 kHz) and small line or load variations. These also have been the device of choice in applications that employ high voltages (greater than 1000 V), high allowable junction temperatures (greater than 100 degrees Celsius), and high output powers (greater than 5 kW). Some typical IGBT applications include — motor control, where the operating frequency is greater than 20 kHz and short circuit/in-rush limit protection is required; uninterruptible power supplies with constant load and typically low frequency; welding, which requires a high average current and low frequency (less than 50 kHz); Zero-Voltage-Switched (ZVS) circuitry and low-power lighting with operation at low frequencies less than 100 kHz. MOSFETs are preferred in applications with high-frequency operation greater than 200 kHz, wideline or load variations, long-duty cycles, low-voltage applications (less than 250 V), and lower output power (less than 500 W). Typical MOSFET applications include switch-mode power supplies that use hard switching above 200 kHz or ZVS below 1000 W. Battery charging is another common use for MOSFETs. Of course, nothing is as easy as it seems. Trade-offs and overlaps occur in many applications. The main purpose, however, is to examine the ‘crossover region’ that includes applications operating above 250 V, switching between 10 and 200 kHz, and power levels above 500 W. In these cases, final device selection is based on other factors such as thermal impedance, circuit topology, conduction performance, and packaging. A ZVS Power-Factor-Correction (PFC) circuit is one example of an application that falls into the crossover area between IGBTs and MOSFETs. When both device types are tested in hard-switching applications, measurements show that the MOSFETs exhibit lower losses. This could let a smaller IGBT replace the larger MOSFET in some applications.
Such was the state of technology in 1997, when IGBTs had a slight edge over MOSFETs at 50kHz and were making inroads into designs of up to 100 kHz. Recent advances have given the advantage back to MOSFETs. The lower-charge MOSFETs, which are now available, have reduced the losses at high frequency. These have, therefore, reasserted MOSFETs’ dominance in hard-switching applications above 50kHz. When the application uses zero-voltage switching, results vary with operating temperature. With 50kHz switching and a 500W output, the 9.5W IGBT losses are higher than the 7W MOSFET losses at room temperature. When the temperature is raised to reflect operating conditions, the MOSFET’s conduction losses rise more quickly than the IGBT’s switching losses. The losses at elevated temperatures increase by 60 per cent for the MOSFET, while the IGBT’s total losses increase only by 20 per cent. At 300W, this makes the power losses almost equal. At 500W, the advantage goes to the IGBT. If output power remains at 500W and the switching frequency is raised to 134kHz at the higher temperature, the IGBT will exhibit slightly worse losses (25.2W) than the MOSFET (23.9W). If the same measurements are taken at room temperature, losses are 17.8 and 15.1W for IGBTs and MOSFETs, 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 ironclad 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. Evaluating the performance of IGBTs and highvoltage power-MOSFETs for switching applications requires a common set of applications and assumptions. Comparisons of maximum current versus frequency reveal that IGBTs are competitive at frequencies as high as 100 kHz. In the battle between MOSFETs and IGBTs, either device can be shown to provide an advantage in the same circuit, depending on operating conditions. How then does a designer select the right device for his application? The best approach is to understand the relative performance of each device and realise 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 favour 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 per cent of the time, and the average torque load of the motor is only 25 per cent of the rated torque. Under average or typical conditions, aMOSFET provides the longest battery life while meeting all peak performance levels and usually at a lower cost. Data that’s based on applicationsat the highest switching frequency, the shortest pulse width, or the lowest current will tend to favour 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 kept in mind. There seems to be an industrywide perception, which is not quite true, that MOSFETs are a mature product category that do not offer significant performance improvements in applications, while IGBTs are a new technology that will replace MOSFETs in all applications above 300V. Presently, some of the newest IGBTs can offer competitive performance and cost advantages at 1000W and upwards, operating at switching frequencies of 100kHz and above. Nevertheless, in all other power-supply applications, the MOSFET continues to reign supreme.
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