Power film caps improve dc/dc converter performance in green energy

Power film caps improve dc/dc converter performance in green energy

Film capacitors offer superior life expectancy and environmental performance

BY GILLES TERZULLI
TPC, division of AVX
www.avx.com

Since 1980, great improvements have been made on dc filter capacitors using a combination of metallized plastic films and different segmentations of the metallization on those film dielectrics. Over the last several years, volume and weight have been reduced by a factor of three or four. Recently, film manufacturers have developed significantly thinner films and have improved segmentation techniques used on the metallization, which has helped immensely to improve the performance of such capacitors.

Using non-gas impregnated designs, the voltage ranges from 600 to 1,500 Vdc can be more economically covered by film capacitors rather than electrolytic. Depending on the application, over 1,500 Vdc, vegetable oil-filled versions are recommended.

With the strong semiconductor evolution, the exigencies for dc link filters moved in such a way that stray inductance has to decrease dramatically in order to limit over-voltage due to semiconductor commutations. Manufacturers have developed capacitors that can be directly mountable on to the new IGBT modules offering a major advantage that the bus bar between the IGBT module and capacitor is no longer required.

Fig. 1: Tri-phase IGBT inverter assembly by direct sandwiches: module — cooling plate — filter capacitor.

On the example, seen in Fig. 1, a cooling plate is used between the capacitor and the IGBT module with straps used to establish the electrical connection adding only a very low stray inductance in the circuit. The power film capacitor is directly screwed onto the IGBT module using its own terminals.

Fig. 2: Tri-phase IGBT inverter assembly by direct connection on air cooling exchanger.

Lifetime expectancy

Film technology allows for a long-lifetime expectancy, depending of voltage load conditions (working voltage) and hot-spot temperature. For dc filtering, lifetime meets the following curves:

Fig. 3: Voltage load conditions and hot-spot temperature curves for dc filtering.

As we can see on these curves, the film capacitors are design for a lifetime of 100,000 hours under the rated voltage and 70C hot spot. End of life criteria has a decrease of capacitance value of 2%. However, this is a theoretical end of life, because, the capacitor can still be used beyond this point. If the application can allows 5% capacitance decrease, lifetime will be widely increased.

Hot-spot temperature will be determined with the following expression:

with τmaxhotspot — the maximum hot-spot temperature:

tgδ0 — dielectric losses.

Rth — Thermal resistance.

Rs — serial resistance.

θ hot spot will be 85° or 105°C function of the application and the technology.

Based on traction applications (nominal electrical and environmental conditions), the lifetime of these capacitors is more than 100,000 hours. In order to achieve this lifetime, the capacitor uses a plastic or aluminium box, hermetically sealed with a polyurethane resin. This polyurethane resin allows not only a total protection against the environment, but also allows the capacitor to meet the NF F 16-101 and NF F 16-102 fire behavior standards for railway rolling stock, which is not possible for a non-encapsulated design.

In addition, this resin filled technology allows the use of different types of terminals, such as a large copper plate terminal separated by an insulating sheet. A metallic top could not allow such type of terminals to be used. Designs for lower lifetime expectancy can also be made using the same technology by increasing the gradient of voltage, which increases the energy density. Ageing laws have been established and software developed in order to be able to answer to any specific customer request.

Another important point is the innovative internal mounting that enables a very low stray inductance. Only a very specific technology can be used to achieve a stray inductance of lower than 10 nH, even for large capacitors. As a result it becomes no longer necessary to use decoupling IGBT capacitors, saving even more on cost.

The true accreditation has been its long-term use in high-speed trains with no degradation to the component.

The final auxiliary power inverter prototype is shown in the Fig. 4. It is possible to see the three components in the power inverter prototype. These are the capacitor, the IGBT module and the heat sink/ heat plate.

Some electrical connections were adapted for this new assembly. The mechanical frame is the TGV standard box for such functions; however, with this new solution only half of this box is required to install the new components.

Fig. 4: Photography of the prototype inverter for tests, results of the improvement with integrated technology (IGBT module and capacitor).

The biggest advantage is to reduce by a lot the number of power components, from 27 components per phase with two PCB drivers and wire connections for GTO thyristor technology, to only three power components for the three-phase inverter: integrated triphase inverter, power capacitor, air cooler.

The operational lifetime of the advanced power film capacitors not only significantly increase the reliability, but the devices also reduce the total number of components used for the inverter.

Future applications for this technology include hybrid and electric vehicles, as the advanced capacitors will improve and simplify the inverter circuitry.

Fig. 5: Simplified schematic for hybrid vehicles. fapo_EnSav_AVX06_may2011 Fig. 6: Example of integrated capacitor for HEV. fapo_EnSav_AVX07_may2011 Fig. 7: Simplified schematic for hybrid vehicles. fapo_EnSav_AVX08_may2011 Fig. 8: Example of integrated capacitor for EV.

Fig. 9: Example of integrated capacitor for HEV and EV.

The concept allows a design engineer to avoid the busbar to link the capacitor to the IGBT, thus avoiding decoupling capacitors in addition to simplifying the inverter.

With the trend of renewable energy, this technology can also be applied to other green energy applications, such as wind power. The power film’s life expectancy of more than 100,000 hours meets the performance and reliability requirements of wind turbines. In addition only one capacitor can replace several, which significantly increases mean time between failure.

Fig. 10: Simplified schematic for wind turbine.

Fig. 11: Example of integrated capacitors for wind turbine.

In recent years, great improvements have been made on dc filter capacitors using a combination of metallized plastic films and different segmentations of the metallization on those film dielectrics. Film capacitors offer significant technological advantages, including superior life expectancy and environmental performance, as well as the ability to handle the various types of “in-application” technical issues (overvoltage and reverse voltage) that can easily occur.

In addition, volume and weight have been reduced by a factor of three or four. Manufacturers such as AVX have expanded their product portfolio to include a wide range of sizes, capacitance values, voltages, and peak current capabilities. ■