In Part 1 of this two-part tutorial, IGBTs were reviewed and tradeoffs considered. We now take a look at an IGBT datasheet to give you an in-depth understanding of its characteristics.
Static Electrical Characteristics
BVCES &mdash Collector-Emitter Breakdown Voltage
Measuring the actual collector-emitter breakdown voltage is practically impossible without destroying the device. Therefore, BVCES is the collector-emitter voltage at which no more than the specified collector current will flow at the specified temperature. This tracks the actual breakdown voltage.
As shown in Figure 8, BVCES has a positive temperature coefficient. At a fixed leakage current, an IGBT can block more voltage when hot than when cold. In fact, when cold, the BVCES specification is less than the VCES rating. For the example shown in Figure 8, at -50°C, BVCES is about 93% of the nominal 25°C specification.
Figure 8 Normalized Breakdown Voltage vs. Junction Temperature
RBVCES — Reverse Collector-Emitter Breakdown Voltage
This is the reverse collector-emitter breakdown voltage specification, i.e., when the emitter voltage is positive with respect to the collector. As with BVCES, RBVCES is the emitter-collector voltage at which no more than the specified emitter current will flow at the specified temperature. A typical value is about 15 Volts, however RBVCES is often not specified since an IGBT is not designed for reverse voltage blocking. Even though in theory an NPT IGBT can block as much reverse voltage as forward voltage, in general it cannot due to the manufacturing process. A PT IGBT cannot block very much reverse voltage due to the n+ buffer layer.
VGE(th) — Gate Threshold Voltage
This is the gate-source voltage at which collector current begins to flow. Test conditions (collector current, collector-emitter voltage, junction temperature) are also specified. All MOS gated devices exhibit variation in VGE(th) between devices, which is normal. Therefore, a range of VGE(th) is specified, with the minimum and maximum representing the edges of the VGE(th) distribution. VGE(th) has a negative temperature coefficient, meaning that as the die heats up, the IGBT will turn on at a lower gate-emitter voltage. This temperature coefficient is typically about minus 12mV/C, the same as for a power MOSFET.
VCE(on) — Collector-Emitter On Voltage
This is the collector-emitter voltage across the IGBT at a specified collector current, gate-emitter voltage, and junction temperature. Since VCE(on) is temperature dependent, it is specified both at room temperature and hot.
Graphs are provided that show the relationships between typical (not maximum) collector-emitter voltage and collector current, temperature, and gate emitter voltage. From these graphs, a circuit designer can estimate conduction loss and the temperature coefficient of VCE(on). Conduction power loss is VCE(on) times collector current. The temperature coefficient is the slope of VCE(on) versus temperature. NPT IGBTs have a positive temperature coefficient, meaning that as the junction temperature increases, VCE(on) increases. PT IGBTs on the other hand tend to have a slightly negative temperature coefficient. For both types, the temperature coefficient tends to increase with increasing collector current. As current increases, the temperature coefficient of a PT IGBT can actually transition from negative to positive.
ICES — Collector Cutoff Current
This is the leakage current that flows from collector to emitter when the device is off, at a specified collector emitter and gate-emitter voltage. Since leakage current increases with temperature, ICES is specified both at room temperature and hot. Leakage power loss is ICES times collector-emitter voltage.
IGES — gate-emitter leakage current
This is the leakage current that flows through the gate terminal at a specified gate-emitter voltage.
