Power modules for fast switching motor drive applications

UK: Most of the current high power semiconductors are optimized for switching frequencies between 4 and 8 kHz. Today, however, more and more applications require frequencies of 10 kHz and higher. New power modules can now fill that gap.

According to current surveys, about 40 percent of the power consumption of motor driven systems in the European Union comes from fans and pumps. Most of these systems are outdated and not electronically controlled. A state-of-the-art system with a high frequency inverter can be up to 30 percent more efficient. While most of these systems are running day and night, the energy and cost saving potential is enormous. In times of increasing energy costs and the growing importance of environmentally conscious behaviour, energy efficiency is becoming increasingly important.

Reasons for higher switching frequencies
Fans and pumps are often seen in heating, ventilation, air conditioning and refrigeration applications, which are usually, installed in audible noise sensitive environments. Electromechanical effects from switching high power can cause audible noise. For that reason, a common feature in all these application areas is a switching frequency of >16 kHz, above the audible range.

Further application areas include motor drives with highly accurate torque control, e.g. when used for surface processing, or drives for ultra high dynamic operations. Other reasons for higher switching frequencies are the possibility to use smaller passive components, such as capacitors and coils. This leads to smaller packaging sizes and lower system costs.

Power semiconductors for higher switching frequencies
For a powerful, reliable and cost effective motor drive, the right choice of power semiconductors is very important. Most of the current high power motor inverters use IGBTs (Insulated Gate Bipolar Transistor) for the power switching.

IGBTs currently available in the market are optimized for different application areas. In most cases the optimization is a trade-off between static and dynamic losses, two very important parameters of an IGBT. The static losses are the losses while the IGBT is switched on, given by the Collector-Emitter-Voltage VCEsat. The dynamic losses are the losses during the switch-on and switch-off of the IGBT.

A special disadvantage of the IGBT is their current tail during switch-off. The reason for this is that during turn-off, the electron flow can be stopped rather abruptly by reducing the gate-emitter voltage below the threshold voltage. However, holes are left in the drift region, and there is no way to remove them except via a voltage gradient and recombination. The IGBT exhibits a tail current during turn-off until all the holes are swept out or recombined. A short current tail is therefore a very important feature of an IGBT with low dynamic losses.

Compared with 600V, there are only a few 1200V-rated IGBTs available for higher power applications, running with switching frequencies of more than 10 kHz. The most common components are built in Trench Field Stop technology and are optimized for switching frequencies below 8 kHz. The reason is that the main application field for IGBTs are industrial drives. And most of these applications currently work at switching frequencies between 4 and 8 kHz.

The standard Trench Field Stop IGBTs is optimized for VCEsat and therefore lower switching frequencies and exhibits due to the Trench technology a rather high gate capacity. The gate capacity is up to three times higher compared with devices using planar structures.

Another technology group, optimized for fast switching applications, are the Fast Non Punched (Fast-NPT) Trough IGBTs. They are typically used in applications with switching frequencies from 30 up to a few hundred kHz, e.g. in power supplies or welding equipment. The disadvantages of Fast-NPT are the very high static losses and missing of short circuit capabilities. This makes this technology unusable for motor drive applications.

A real alternative might be the Planar Field Stop technology. It promises low static losses, with much lower gate capacity and faster switching capabilities.