High Speed IGBTs Take on the Super Junction MOSFET
In the last decade, the automotive electronics industry has been turned upside down. Twenty years ago, the power MOSFET was the predominant component in terms of both socket count and dollar value when comparing the different types of power semiconductor components on the standard vehicle. Almost all of the applications were running off a 12V battery and typical power semiconductor applications could be described as ‘heavy loads’ – fans, pumps and actuators. DC-DC converters, battery chargers and large inverters were unheard of, and as for other types of power switches besides the MOSFET, there was typically only one IGBT – in the ignition system.
By contrast, on the latest hybrid or electric vehicles, there can be several hundred dollars worth of IGBTs in some cases.
The main driver for this rapid adoption of the Insulated Gate Bipolar Junction Transistor (IGBT) has been the seismic shift in the applications on board a typical vehicle. The IGBT has long been the choice of component in slow switching applications requiring high voltage and high current, such as consumer and industrial off-line motor drive applications. The IGBT’s bipolar junction makes it inherently efficient at both blocking high voltages and conducting high currents. As shown in Fig. 1, the IGBT becomes more efficient than a MOSFET as the current increases.
Fig. 2 shows some of the typical emerging applications on a hybrid or electric vehicle, all of them require power semiconductor switches that are capable of blocking many hundreds of volts and in some cases conducting hundreds or even thousands of amps. The driver application and the one where the IGBT is most at home are in the motor drive applications – the main inverter and auxiliary motor drives. Sockets in the battery charger and DC-DC converter could benefit from the IGBT’s excellent high voltage and high current performance but require devices to switch in the range of 70 – 150kHz, depending on topology. Traditionally, the IGBT becomes inefficient at such high frequencies. This is why the Super Junction MOSFET (SJ MOSFET) has been the component of choice. The SJ MOSFET can block high voltages, conduct relatively high currents, and switch well into several hundred kilohertz. However the performance limitations of SJ MOSFETs, combined with advancements in the switching speed of the IGBT, has created the need for an alternative power switch for these applications.
International Rectifier has recently developed a new type of IGBT aimed at high frequency applications, CooliRIGBT™ Gen 1. By combining the planar IGBT structure with thin wafer technology, the device can achieve a good balance between switching and conduction losses, enabling the devices to switch up to 200kHz! This is considerably more than competitive IGBTs that at most can achieve switching speeds of up to 70kHz. Fig. 3 summarizes the relative merits of the two technologies.
One automotive market driver is operating temperature. The automobile is a box of environmental extremes; the same car must function equally well at -20˚F in an Alaskan winter as well as it would under the summer sun in Arizona at 115 ˚F! Furthermore, for added reliability and longevity, system manufacturers like ‘thermal headroom’ between the Tj max of a device in the application and the ambient temperature. Such headroom can act as a buffer to ensure good reliability as the thermal interface of a system degrades over the ~10 year lifetime of the typical car. This makes it almost a prerequisite that automotive power semiconductors have a TJ max of 175 ˚C. SJ MOSFETs have a maximum TJ of 150ºC, due to the complex construction.
As outlined in Fig. 1, the bipolar nature of the IGBT enables it to act more efficiently at higher currents. The SJ MOSFET does have an advantage at low currents, but as power levels rise above 1.5kW, the IGBT rapidly becomes more efficient in terms of overall losses. Fig. 4 compares the current handling capability across frequency for both the ultra-fast IGBT and a comparable SF MOSFET. The new device can deliver 50% higher current than a SJ MOSFET while operating at a 16% higher junction temperature.
We must also examine what seems to be an inherent weakness of the IGBTs- the lack of an in built body diode. With a MOSFET, the body diode is a convenient ‘parasitic’ component that is essential for the MOSFETs successful operation in most systems. The IGBT, however, does not have this in built body diode. To account for this power semiconductor, manufacturers frequently ‘co-pack’ a separate diode with every IGBT. This is inevitably an added process step and one that can add considerable cost depending on the type of diode and package used.
Though through careful design this can be tuned to an advantage as now an optimal diode can be selected – one that is both well suited to the application and the IGBT performance. With stringent EMC regulations applying to the confined space that it is the vehicle chassis, having a diode with good Trr performance can often make the difference to both system cost and design cycle time as well as overall performance and efficiency – particularly in high speed switching applications. This is a luxury that the SJ MOSFET simply does not have; the built-in diode means that performance often has to be traded off between the MOSFET and body diode as either one is tuned. In recent years SJ MOSFET manufacturers have made positive steps in reducing the snappy nature of the body diode and improving its robustness, but moving away from this fundamental tradeoff performance between MOSFET and diode is hard to achieve. So much so, that in recent months even leading manufacturers of SJ MOSFETs have started to promote the virtues of ultra-fast IGBTs.
The ultra-fast IGBT is set to give long-term automotive programs sustainable power semiconductor solutions in terms of price, reliability and performance.
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