Posted on 07 March 2016

System Solution: “SiC-Inverter for Industrial Motor Drive”

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In many power electronics-based applications such as industrial motor control units, requirements like space, weight and efficiency play an increasing role. Product development and manufacturing expenses should remain low while the design effort should result in more compact systems and, at the same time, product quality and reliability should be guaranteed.

By Aly Mashaly and Fabrice Gringore, Rohm Semiconductor GmbH

This leads to more demanding design requirements on the system and component level and ultimately affects the overall consistence of power devices, passive components, cooling technologies and PCBs.

In order to achieve the required enhanced system properties, semiconductor devices have to cope with higher power density, higher efficiency and reliability. In consequence, silicon carbide (SiC) has become of higher interest in recent years, because it sets new standards in terms of temperature resistance and performance, leading to improved switching voltage and frequency, switching losses and size and in some applications leading to the reduction of total system costs.

Compared to silicon (Si), the electric breakdown field of SiC is higher by almost a factor of ten (2.8 MV/cm vs. 0.3 MV/cm). The higher dielectric field strength of the extremely hard substrate allows for a thinner layer structure and reduces the surface resistance. In combination with the high carrier mobility shorter switching times can be generated, which causes the energy loss in the switching to diminish significantly and to remain almost constant even at considerably higher ambient temperatures, compared with conventional Si semiconductors, as shown in Figure 1.

SiC shows better switching performance at higher tempera ture than Si Devices

Total power losses in switching applications consist of static and switching losses. Switching losses result from turning on and turning off the device and demand to be taken into particular consideration if high switching frequencies are required. The switching frequency in power electronic systems is often defined by application and system specific limits. For example, in motor drive application the switching frequency is determined by the required output frequency to the AC motor, resonance performance of the entire system, EMC requirements and thermal management.

Turn-off velocity is confined by the permissible switching over voltage and also EMC requirements such as common mode effects, however the turn-on velocity is confined by the permissible peak current and electromagnetic immunity (EMI), both setting the frame for the feasible switching speed.

Power semiconductors are taking on various static and dynamic states during switching operation. In any of these states, energy is dissipated, heating up the device and accumulating to the overall power loss of the switch. Therefore, suitable thermal management concepts have to be taken into account to avoid over-heating and to ensure the reliability of the device and the entire system as well.

The most popular power device for high-voltage, high-current applications used to be IGBT. Contrary to IGBTs, MOSFET do not have a threshold voltage for the on-state characteristic. IGBTs achieve lower on-resistance by injecting minority carriers into the drift region but these generate tail current when transistors are turned off. SiC devices do not need conductivity modulation to achieve low on-resistance due to their much lower drift-layer resistance and in consequence, do not generate tail current. Compared to silicon-based fast recovery diodes, SiC SBDs have much lower recovery loss and noise emission, at similar threshold voltage, and unlike silicon FRDs, these characteristics do not change significantly over current ranges (Figure 2).

SiC has overall better switching properties at higher currents  comparing to Si Devices

A Myriad of Possibilities - Application Examples

The greater switching speed allows for a higher switching frequency and makes SiC devices particularly suitable for use in many industrial applications, e.g. DC/ DC Converters, active front ends, energy recovery systems, solar inverters and UPS.

For motor drive applications the insulation materials of AC motors are a challenge when using high speed switches like IGBTs and even more when using SiC. For industrial motors with standard insulation, the switching speed is limited from 1 kV/μs up to 5 kV/μsin order to minimize stress on the insulating materials. By using SiC, switching speed in a range higher than 15 kV/μs is possible. This value is dependent on factors such as output signal of the inverter, coupling effects, cable length as well as the type of cable. The high dv/dt can lead to damage the motor insulation and thus to premature aging of the motors. Depending on the application and the length of the motor cable, an output filter like dv/dt filter or sinusoidal filter is needed in order to prevent this scenario (as shown in figure 3).

Industrial motor drive application with sinusoidal filter

By using this kind of output filters, cables without shielding can be used which leads to reduce costs dramatically. Another benefit of such filters is that the high frequency currents in the motor windings decrease which leads to reduced losses, heating and noise in the engine. As a result, the life time and the reliability of the entire system improve.

The benefit of SiC can be well seen in industrial applications where sinusoidal filters on the output of the inverter are needed, e.g. motor drives where power up to the double-digit Kilowatt range with cable length of 100 m between motor and inverter is required. These applications are commonly featuring motor frequencies in a range of 50 Hz and switching frequencies in a range of 10 kHz and are mainly solved with IGBT technology. Using IGBTs for these applications with higher switching frequencies is not feasible due to the high thermal stress on IGBTs which occurs when the switching frequency exceeds these values.

Considering the parasitic elements in this system, like commutation parts of the DC link, motor windings and cables and conceiving this system as a resonant circuit, the main and common challenge in this application remains that the resonant frequency is normally close to the low switching frequency. This means that the resonant circuit could be excited by the switching frequency of the IGBT, leading to high oscillation on the system and subsequently extremely thermal stress on the passive components and motor as well. To prevent this phenomenon, Power engineers have to make a trade-off between thermal stress and resonance behaviour of the system. To overcome this technical challenge engineers commonly used to connect a sinusoidal filter to the output of the inverter in order to reduce such stress.

SiC opens new doors in this market by giving engineers the possibility to define a higher switching frequency for such an application (>16kHz), which is not possible with IGBT. Using SiC leads to minimized thermal stress, shift the switching frequency away from the resonant frequency, ripple current becomes smaller, the output filter can be downsized and the reliability of the entire system increases.

With the new full SiC modules from ROHM, the switching losses can be reduced by 75%.

To illustrate the difference between Si-IGBT technology and 3rd Generation of SiC technology from ROHM, a simulation of an inverter has been performed.

Simulation parameters are as follows: Vdc= 600 V, Imotor=200 Arms, Fsw.=10 KHz.

Comparison between  IGBT, hybrid module and full  SiC technology

As shown in Figure 4, switching losses in inverter applications can be dramatically reduced by using 3rd generation SiC MOSFETs from ROHM. This excellent step allows the engineers to increase the switching frequency without dealing with thermal stress like when using IGBTs. This also results in significantly smaller and lighter inductors and capacitors. Smaller and lighter coils mean fewer required components e.g. for noise reduction, as well as smaller heat sinks. All in all, SiC helps to downsizing the system.

With its Powers Systems Application group located in the Headquarter near Düsseldorf ROHM can now support customers by thoroughly examining the application and customer requirements, investigating the advantages of the SiC technology on a system level and by finally identifying the best and most cost efficient solution. For example, in most of the industrial motor drive applications AC motors are driven with an output frequency of only 50 Hz. High switching frequency is normally not needed for this kind of applications. Therefore, for these applications a reduction of 30% of power losses is absolutely sufficient to find a perfect cost/benefit compromise. This can be achieved by using a hybrid configuration combining Si-IGBT and SiC Schottky Barrier Diode in the circuit instead of a Si-IGBT/Si FRD (Figure 4) – which results in significantly improved thermal management. Reverse recovery behaviour of the SiC SBD is almost completely eliminated even at high operating temperatures, see figure 1.

ROHM solutions for inverter applications

In the development phase of their commercial Silicon Carbide switches, ROHM always had in mind to deliver not only SiC MOSFETs and SiC Diodes on their own but an effective system solution, (Figure 5) ROHM offers many ways to reduce the BOM and production costs, i.e. a multitude of semiconductor devices on Si and SiC basis for tailor-made solutions of all kinds of power electronic requirements, from the DC/DC Converter and control units to the driver stage.

Solutions from ROHM for high power density inverter applications

Schottky diodes, Super Junction MOSFETs, hybrid MOS, IGBTs and FRDs cover voltage ranges from 300 to 1200V, SiC MOSFETs and SBDs cover voltage ranges up to 1700V.

In the case of a DC/DC converter for an auxiliary power supply, the use of the new 1700V SiC MOS (SCT2H12NY) in a TO268 package resulted in significantly improved Rdson (1.15 Ώ instead of 9Ώ by Si-MOS) as well as in higher current capability in the same package are a compared to Si-MOS. With its very low input capacitance (Ciss) the switching frequency can be higher than 100kHz, which leads to distinct volume reduction of magnetics and space on the PCB. By using a TO268package the possibility of automatic assembly is given now, which means a significant reduction of production costs and ultimately, the reduction of total costs. To get the best performance of this device ROHM developed a special driver (BD768xFJ-LB) with dedicated controller in a SOP-J8S package.

Further advantages become evident when examining the example of an optocouplerless isolated flyback converter for DC voltage conversion (BD7F100HFN-LB): In the market there are two conventional solutions. One solution is by using a third winding on the primary side. Disadvantage is that it leads to enlarge the transformer, increase the power consumption and it is inaccurate to control the output voltage. The other solution is to get the feedback signal from the secondary side by using an adjunct optocoupler. The disadvantage here is, that this concept requires a voltage divider, increases the power consumption and because of a coupling capacitance between the primary and secondary side the EMC performance is critical which makes an extra filter necessary to minimize EMC noises.

Compared to these conventional solutions, the new (BD7F100HFN- LB) does not require signal feedback from the secondary side, which means that an optocoupler or transformer with third winding becomes unnecessary. The module contains a 60-V MOS for currents up to 1.25 A and it operates with a constant switching frequency of 400 kHz. To ensure reliable operation, the flyback converter is protected against low input voltage, over-current, output short circuit and over-temperature. With this integrated solution, not only the design becomes smaller, the response times are faster as well.

Also, as ROHM proposes, closely associating the isolated gate driver to the power stage component choice and design consolidates this approach of a system solution. Indeed, driving Silicon carbide switches requires significantly higher performance than legacy drivers, and this in many areas.

The first key feature to bear in mind is the immunity to common mode transients (CMTI). As mentioned before the switching speed of SiC can be higher than 15 kV/μs, it could go well above 50 kV/μs.

Commonly, as featured in all ROHM’s isolated gate drivers (see figure 6), 100 kV/μs is the safest and minimum immunity that shall be guaranteed for a safe system drive.

Common mode transient immunity performance test beyond  the 100kV/μs limit

Still related to safely supporting significantly higher frequencies, the propagation delay, and particularly the matching of single channel isolated gate drivers, is critical. As such, a general rule of thumb for the propagation delay and device to device matching is to be kept respectively below 100ns and 50ns. For the latter, some applications are even tending to require around 20ns in the very next future. This tight timing reliability, over the whole temperature range, is enabled by the coreless transformer technology of isolation, unlike with conventionally-used optocouplers for legacy IGBT or MOSFET switches (Figure 7).

Benchmark of propagation delay performance and reproducibility

Since the gate capacitor needs to be charged more frequently, having a sufficient gate current capability may allow pushing out the limit where you need to add an external push-pull buffer, enabling few tens of cents savings as well as some propagation delay. A 3 Amp minimum gate current drive is already significantly higher than the majority of conventional solutions, particularly optocoupler-based solutions.

Last, since Silicon-Carbide switches do require higher drive voltage than IGBTs, a gate output voltage range of operation above 22V is available by the ROHM driver. It enables to drive agnostically any generation of Silicon Carbide switches.

Combining all of these drastic specification improvements in an isolated gate driver enables the Silicon Carbide-enabled system to reliably reach its optimal performance.

Summary

At first glance, the SiC technology is in fact somewhat complicated and, lacking larger numbers, currently more expensive than that of silicon. However, the use of SiC power semiconductors on a system level enables significant improvements in terms of efficiency, circuit complexity, size and weight as well as extended lifetime, particularly at high voltages and currents. The overall system efficiency is noticeably improved, the operation is possible in higher temperature ranges and it requires less passive circuit elements; not to forget the considerable benefits that can be leveraged not only in the design phase, but in the medium and long term.

All in all, ROHM is currently the sole Silicon carbide MOSFET supplier able to provide this complete and efficient power stage.

Finally, ROHM started to establish a power lab in its facility in Germa - ny in order to support the European customer technically and strongly on system level, circuit level as well as on device level.

 

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