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Selecting the appropriate IGBT

For many applications IGBTs are the power devices of choice, but with so many devices to choose from a rigorous process of selection is crucial, according to Andrea Gorgerino

Insulated Gate Bipolar Transistors (IGBT) are the power devices of choice for applications that use bus voltages ranging from a few hundred Volts up to close to a thousand Volts. Being minority carrier devices, IGBT have superior conduction characteristics to MOSFET in this voltage range while offering a very similar gate structure and hence ease of control. Furthermore, the absence of an integral reverse diode gives manufacturers the flexibility of choosing an application-optimised fast “co-pak” diode (IGBT and diode in the same package), as opposed to the inherent MOSFET diode which displays increasing Qrr and t¬rr¬ as the voltage rating increases. Of course the increased conduction efficiency comes at a price: IGBT typically have relatively high switching losses which decrease application switching frequencies. This main trade-off, as well as other application and manufacturing considerations have given rise to several generations of IGBT as well as different sub- classes. This multitude of products makes it important to use a rigorous process when selecting devices, as this can have a significant impact in electrical performance and cost. From a user perspective, the process of selecting an IGBT can be simplified. Since this process is iterative in nature it is well suited to be automated, with online selection tools available, such as International Rectifier’s which contains the electrical and thermal models for the entire IR IGBT catalogue of more than 200 devices.

Voltage selection

IGBT have traditionally been available in 600V rating for the 110-220V rectified bus applications, and 1200V for 3-phase 380-440V rectified bus applications. IR also offers a limited number of devices at 900V, as well as in recent years an expanding selection at 330-330V (not usually used in applications directly tied to the mains). Unlike MOSFET, IGBT are not rated in avalanche so it’s important to make sure that in the worst case condition the voltage the device sees stays below the BV rating. This worst case condition usually needs to take into consideration: • Maximum bus voltage using maximum line input voltage, as well as maximum bus overvoltage (for example when electrically braking in a motor drive application)

• Maximum over-shoot as seen by the device during turn-off using maximum switching speed (di/dt), maximum stray inductance, and minimum bus capacitance

• Lowest operating temperature as the breakdown voltage has a negative temperature coefficient

Short circuit SOA rating This characteristic refers to the capability of the device to withstand the full bus voltage across it’s terminals for a certain amount of time (measured

order to limit the dissipated power: this leads to the main trade-off with VCE(ON) as illustrated in the example in Table 1.

Short circuit rated IGBT The requirement for this type of devices is application driven as shown in Figure 1 when a short circuit occurs at the output of a motor drive inverter. The IGBT needs to survive enough time to allow the protection circuitry to safely turn the devices off.

For large industrial drives, the Figure 1: IGBT selection flow

presence of long cables between the output of the inverter and the motor and its associated parasitic capacitance force the designers to add some blanking time to the protection circuits to avoid false trips. This in turns increases the requirement on the IGBT and the industry has standardised on a value of 10µs for these applications. In some cases it is possible to

decrease the protection circuitry blanking time, for example in integrated motor drives where the motor is located directly at the output of the inverter and it is possible to optimise the device.

Figure 2: IR online IGBT selection tool Figure 3: Hard switching and soft switching waveforms

in µs) and being able to turn-off safely. In this condition the IGBT will reach its saturation current (depending on generation and device current rating) and effectively control the current in the system, while dissipating an enormous amount of power.

Although all IGBT have an inherent short circuit Safe Operating Area (SOA) capability, IGBT are mainly categorised as short circuit rated versus non short circuit rated. Short circuit rated devices are designed to limit the saturation current in

Non short circuit rated IGBT In some applications like power supplies there is an inductor between the devices and the output terminals. Here, a short happening at the output terminals places the output inductor in series with the DC bus so the current di/dt is controlled by the inductor as shown in Figure 2. In this case the devices themselves are not in short circuit condition, and the short circuit protection circuits have sufficient time to turn them off. Removing this requirement from the IGBT allows IR to offer a complete range of non-short circuit rated IGBT with very low VCE(ON) for use in welding, UPS, solar and similar applications.

Speed selection: turn-off behaviour For IGBT, the main parametric trade-off is between conduction and switching losses, in which the characteristic tail current is a major contributor. Silicon designers can optimize the balance between these two depending on the switching frequency of the application: an


16 CIE Power Supplement April 2012

Table 1: Example of short circuit SOA trade-off for a 1200V trench IGBT Short circuit rating 10µs 6µs


VCE(ON) @75A 1.9 V 1.8 V 1.6 V


12.3 mJ 12 mJ

11.5 mJ

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