Passive Components Passive Embedding for higher Performance and Reliability
The embedding and integration of passive components is making great advances. New miniaturized components designed specifically for embedding enable even more compact and reliable systems.
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The dimensions of passive components and their ruggedness for further processing often determine whether they are suitable for specific embedding and integration technologies. TDK has developed innovative capacitors and thermistors and employs state-of-the-art integration technologies that enable superior passive embedding solutions.
Embedding capacitors in IGBT modules
Traditionally, IGBT modules in the mid power range and based on Si and SiC technologies employ external snubber capacitors. Until now, it was not possible to embed these components and thus shorten the long leads that are afflicted with parasitic inductances. Irrespective of their dimensions, conventional capacitors are insufficiently resistant to the heat involved in the direct assembly of the IGBT module. In addition, some have only a low capacitance per volume and suffer considerable loss of capacitance at high rated voltages.
Now, with the EPCOS CeraLink, a completely new kind of capacitor has been developed that suffers none of these drawbacks. CeraLink technology is based on the ceramic material PLZT (lead lanthanum zirconate titanate). In contrast to conventional ceramic capacitors, CeraLink has its maximum capacitance at the application voltage, and this even increases proportionately to the share of the ripple voltage (Figure 1).
Another advantage is its high insulation resistance. The RC time constant τ is 70.000 ΩF at 25 °C and this value drops only slightly even at 150 °C. As a result, this prevents the feared uncontrolled thermal runaway from occurring. Its parasitic effects are also very low: ESR is only 50 mΩ at 100 kHz and drops to only 10 mΩ at 1 MHz, resulting in very low losses. The ESR declines even further as the temperature rises: at 85 °C it is already less than 20 percent of its original value at 25 °C. This results in charge and discharge times of between 25 ns and 30 ns. The ESL of CeraLink capacitors is below 5 nH, making this technology particularly suitable for fast-switching inverters.
CeraLink technology predestined as snubber capacitors
All these advantages make CeraLink technology predestined to be embedded in IGBT modules as snubber capacitors. Two SMD types with rated voltages of 500 V DC are available for this purpose (Figure 2). The low-profile 1 µF variant with dimensions of only 4.35 mm × 7.85 mm × 10.84 mm and the 5 µF type with dimensions of 13.25 mm × 14.26 mm × 9.35 mm are particularly compact and may be placed very close to the semiconductor with negligible ESL.
Embedding temperature protection in IGBT modules
IGBT modules in inverters achieve the highest possible efficiency when they are operated at their upper temperature limit. Thus, exact monitoring of the operating temperature is required in order to prevent damage to the semiconductors. The suitability of standard SMD NTC thermistors used for this purpose up until now, however, is rather limited because they are not compatible with all semiconductor assembly processes. In particular, these include high-temperature soldering and silver sintering under pressure.
In order to solve this problem, a wafer-based manufacturing process for EPCOS chip NTC thermistors was developed (Figure 3). The new components are now able to withstand the thermal and mechanical stresses encountered during assembly. Moreover, they save space because they need no special pads for soldering to the semiconductor substrate.