AN1541 A FINAL COMPARISON OF IGBTS BJTs AND Short Circuit Rated Devices POWER MOSFETS Using IGBTs in motor control environments requires the device to withstand short circuit current for a given period The conduction losses of BJTs and IGBTs is related to the Although this period varies with the application, a typical forward voltage drop of the device while MOSFETs determine value of ten microseconds is used for designing these conduction loss based on RDS(on). To get a relative comparison of turn-off time and conduction associated specialized IGBTs. Notice that this is only a typical value and it is suggested that the reader confirm the value given on the losses, data is presented in Table 1 where the on-resistances data sheet. IGBTs can be made to withstand short circuit of power MOSFET, IGBT and a BjT at junction temperatures of 25C and 150C are shown conditions by altering the device structure to include an Note that the devices in Table 1 have approximately the additional resistance(Re, in Figure 6)in the main current path The benefits associated with the additional series resistance same ratings. However, to achieve these ratings the chip size are twofold of the devices vary significantly. The bipolar transistor requires 1.2 times more silicon area than the igbt and the mosfet requires 2.2 times the area of the IGBT to achieve the same GATE ratings. This differences in die area directly impacts the cost EMITER f the product. At higher currents and at elevated temperatures, the IGBT offers low forward drop and a POLYSILICON GATE switching time similar to the BjT without the drive difficulties Table 1 confirms the findings offered earlier in Figure 1a and elaborates further to include a BJT comparison and temperature effects. The reduced power conduction losses offered by the IGBT lower power dissipation and heat sink Thermal Resistance An IGBT and power MOSFET produced from the same size die have similar junction-to-case thermal resistance because of their similar structures. The thermal resistance of a pow P+ SUBSTRATE MOSFET can be determined by testing for variations in temperature sensitive parameters(TSPs). These parameters are the source-to-drain diode on-voltage, the gate-to-source threshold voltage, and the drain-to-source on-resistance. All previous measurements of thermal resistance of power MOSFETs at Motorola were performe Figure 6. Cross Section and Equivalent Schemati using the source-to-drain diode as the TSP. Since an IGBT of a short Circuit Rated Insulated gate does not have an inverse parallel diode another TsP had to Bipolar Transistor Cell gate-to-emitter threshold voltage was used as the TSP to First, the voltage created across Re, by the large current measure the junction temperature of an IGBT to determine its passing through Re, increases the percentage of the gate thermal resistance. However before testing IGBTS, a voltage across Re, by the classic voltage divider equation correlation between the two test methods was established by Assuming the drive voltage applied to the gate-to-emitter comparing the test results of MOSFETs using both TSPs. By remains the same, the voltage actually applied across the testing for variations in threshold voltage, it was determined gate-to-source portion of the device is now lower, and the that the thermal resistance of MOSFETS and IGBTs are device is operating in an area of the transconductance curve essentially the same for devices with equivalent die size that reduces the gain and it will pass less current. Table 1. Advantages Offered by the IGBT When Comparing the MOSFET, IGBT and Bipolar Transistor On-Resistances (Over Junction Temperature )and Fall Times(Resistance Values at 10 Amps of Current) Characteristic TMOS Current Rating 20A 20A 20A Voltage Rating 500v 600V R(on)@ TJ=25C 0.29 0.24 0.18g Ron)@TJ=150°c 0.6g Fall Time(Typical 40 ns 200ns Indicates VcEo Rating MOTOROLA

MOTOROLA 5 A FINAL COMPARISON OF IGBTs, BJTs AND POWER MOSFETs The conduction losses of BJTs and IGBTs is related to the forward voltage drop of the device while MOSFETs determine conduction loss based on RDS(on). To get a relative comparison of turn–off time and conduction associated losses, data is presented in Table 1 where the on–resistances of a power MOSFET, an IGBT and a BJT at junction temperatures of 25°C and 150°C are shown. Note that the devices in Table 1 have approximately the same ratings. However, to achieve these ratings the chip size of the devices vary significantly. The bipolar transistor requires 1.2 times more silicon area than the IGBT and the MOSFET requires 2.2 times the area of the IGBT to achieve the same ratings. This differences in die area directly impacts the cost of the product. At higher currents and at elevated temperatures, the IGBT offers low forward drop and a switching time similar to the BJT without the drive difficulties. Table 1 confirms the findings offered earlier in Figure 1a and elaborates further to include a BJT comparison and temperature effects. The reduced power conduction losses offered by the IGBT lower power dissipation and heat sink size. Thermal Resistance An IGBT and power MOSFET produced from the same size die have similar junction–to–case thermal resistance because of their similar structures. The thermal resistance of a power MOSFET can be determined by testing for variations in temperature sensitive parameters (TSPs). These parameters are the source–to–drain diode on–voltage, the gate–to–source threshold voltage, and the drain–to–source on–resistance. All previous measurements of thermal resistance of power MOSFETs at Motorola were performed using the source–to–drain diode as the TSP. Since an IGBT does not have an inverse parallel diode, another TSP had to be used to determine the thermal resistance. The gate–to–emitter threshold voltage was used as the TSP to measure the junction temperature of an IGBT to determine its thermal resistance. However before testing IGBTs, a correlation between the two test methods was established by comparing the test results of MOSFETs using both TSPs. By testing for variations in threshold voltage, it was determined that the thermal resistance of MOSFETs and IGBTs are essentially the same for devices with equivalent die size . Short Circuit Rated Devices Using IGBTs in motor control environments requires the device to withstand short circuit current for a given period. Although this period varies with the application, a typical value of ten microseconds is used for designing these specialized IGBT’s. Notice that this is only a typical value and it is suggested that the reader confirm the value given on the data sheet. IGBTs can be made to withstand short circuit conditions by altering the device structure to include an additional resistance (Re, in Figure 6) in the main current path. The benefits associated with the additional series resistance are twofold. Figure 6. Cross Section and Equivalent Schematic of a Short Circuit Rated Insulated Gate Bipolar Transistor Cell POLYSILICON GATE N+ P+ N– EPI N+ BUFFER P+ SUBSTRATE P– Rmod EMITTER N+ P+ P– COLLECTOR GATE NPN MOSFET PNP KEY METAL SiO2 Rshorting Re First, the voltage created across Re, by the large current passing through Re, increases the percentage of the gate voltage across Re, by the classic voltage divider equation. Assuming the drive voltage applied to the gate–to–emitter remains the same, the voltage actually applied across the gate–to–source portion of the device is now lower, and the device is operating in an area of the transconductance curve that reduces the gain and it will pass less current. Table 1. Advantages Offered by the IGBT When Comparing the MOSFET, IGBT and Bipolar Transistor On–Resistances (Over Junction Temperature) and Fall Times (Resistance Values at 10 Amps of Current) ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Characteristic ÁÁÁÁÁÁÁÁ TMOS ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ IGBT ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ Bipolar ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Current Rating ÁÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Voltage Rating Á ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁ 500 V Á ÁÁÁÁÁÁÁ ÁÁÁÁÁ 600 V Á ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁ 500 V* Á ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ R(on) @ TJ = 25°C ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 0.2 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.24 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 0.18 Ω ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ R(on) @ TJ = 150°C ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 0.6 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.23 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.24 Ω** ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Fall Time (Typical) ÁÁÁÁÁÁÁÁ 40 ns ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 200 ns ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 200 ns * Indicates VCEO Rating ** BJT TJ = 100°C