Faster switching in inverters

Wolfgang Dreipelcher, Senior Director, Global Reference Designs, EPCOS


New EPCOS CeraLink capacitors benefit power-inverters DC-link circuits for fast-switching IGBT modules

The power-semiconductor switches used in power supplies and inverters are based on two technologies, namely MOSFETs and IGBTs. MOSFETs can operate at relatively high switching frequencies significantly above 30 kHz but, unlike IGBTs, they have a very large chip surface area.

A new generation of IGBT modules from Infineon Technologies operates at frequencies of up to 100 kHz. Line-based and turn-off losses are of about the same magnitude in both systems. Fast IGBTs, whose manufacturing complexity is significantly lower and chip areas are often smaller than superjunction MOSFETs, are the basis of an IGBT3 technology with high switching frequencies and an excellent price-performance ratio.

Fast-switching systems require circuit designs with minimized ESR and ESL values. Accordingly, the passive components—inductors, but especially capacitors—must also keep pace with the high switching frequencies. These in turn permit the use of more compact and lightweight passive components, producing lower losses and increasing efficiency.

Capacitors are at the focus of these developments. They must combine high switching frequencies with low ESL and ESR values as well as an extremely compact design.

Conventional capacitor technologies are only partially able to meet these requirements. The EPCOS CeraLink represents a completely new approach: This advanced component is a ceramic multilayer ripple-current suppressor, also known as a link circuit or DC link capacitor; moreover, it functions as a snubber.

New inverter designs possible

EPCOS developed the CeraLink in the company’s Competence Center for Ceramic Components in Deutschlandsberg, Austria. The expertise that formed the basis for this advanced component was gained, among others, over many years in the volume manufacture of piezo actuators.

The EPCOS CeraLink offers the advantages of ceramic capacitors without their unfavorable characteristics. The patented multilayer component—based on antiferroelectric ceramic material with special copper internal electrodes—allows the use even more economically of both standard IGBTs and the new high-speed types with significantly higher switching frequencies. This naturally also applies to circuits with corresponding superjunction MOSFETs.

The innovative CeraLink combines high capacitance per volume with low ESL and ESR values, which thus enables significant improvements in efficiency, reliability, and space requirement for future IGBT and MOSFET inverter designs. Moreover, CeraLink is also available in a low-profile SMD design, making it suitable as a snubber for integration in power modules.

The EPCOS CeraLink components were adapted and continuously optimized for the first designs of special IGBT modules in close cooperation with Infineon Technologies, the market leader in IGBTs. This allowed the best results to be achieved in terms of performance and energy efficiency. For both the EASY automotive series from Infineon Technologies and the corresponding types in industrial applications, all the relevant capacitor parameters and properties were optimized towards more economy and efficiency (table 1).

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Table 1: Optimized parameters and properties of the EPCOS CeraLink

The first designs of an on-board inverter, the Infineon EASYKIT DCDC, were based on existing OEM specifications for rated voltages of about 400 V DC on the high-voltage side. The EPCOS CeraLink is currently available in several designs. Its capacitance range extends from 1 µF to 100 µF at rated voltages of 400 and 800 V DC.

CeraLink capacitors are available with various terminal. The SMD versions (LP and SMD) are designed for direct integration in semiconductor power modules in view of the restricted space available (table 2). They can be soldered, bonded, or sintered.

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Table 2: Various design versions of the EPCOS CeraLink

In close cooperation with EPCOS, Infineon Technologies has developed an HV/LV DC-DC demo board with an output of 2.7 kW. The requirements included a high-voltage input range from 200 to 400 V DC—depending on the HV battery used—and a low-voltage output range from 8 to 16 V DC that is typically standard in automobile electronics systems. Furthermore, the demo board had to cover a current range of up to 200 A DC.

Nearly 100 EPCOS and TDK components

Various types of circuit topologies are available on the DC-DC converter market. However, the most widely used is the full-bridge circuit with a ZVT (zero-voltage transition) based on MOSFET transistors.

Infineon Technologies has redesigned these circuits with various EPCOS components and adapted them to its EASY series of fast-switching IGBTs (figure 1). They employ a large number of diverse EPCOS and TDK components (Table 3).

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Figure 1: Infineon EASY 2.7 kW inverter

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Table 3: EPCOS and TDK components for fast-switching IGBTs from Infineon Technologies

The EPCOS CeraLink combines high capacitance per volume, low ESL and ESR values, and a minimum leakage current, and thus satisfies all the requirements of high-speed IGBT modules or MOSFETs. This system configuration also permits control of high current change rates (di/dt) of up to 10 kA/µs. Despite these extremely high potential rates, the generated voltage peaks (V = Ldi/dt) are extremely low thanks to the low ESL of the CeraLink.

The capacitor is not the only source of parasitic inductance. Noticeable stray inductances occur in a normal system configuration for several reasons, including the contacting inside the IGBT module and the feed line to the capacitor. The EPCOS CeraLink allows for dramatic reduction in the parasitic-inductance values for the feed line—to the same extent as the values for the capacitor itself, thanks to its compact design.

The compact link to the IGBT module simultaneously attenuates its over voltages, and a snubber capacitor is usually not necessary. Figure 2 shows the voltage curve at turn-off of the IGBTs with and without an EPCOS CeraLink. The voltage rise is thus only minimal and is within the safe range for the IGBTs. In this case, the switching frequency is 100 kHz, meaning the capacitor sees a ripple-current frequency of 200 kHz. Figure 3 shows the impedance and ESR curves as a function of the frequency.

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Figure 2: Voltage across the IGBT due to parasitic inductances when switching

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Figure 3: Impedance and ESR as a function of the frequency

Thanks to its very low ESR values, the EPCOS CeraLink attenuates the overvoltage peaks very effectively. As a rule, therefore, additional snubber capacitors are not necessary.

Although the capacitance of the EPCOS CeraLink is usually sufficient for pure DC-DC applications, it may be too low for motor operation, for example. This can be remedied by connecting aluminum electrolytic or film capacitors in parallel, as their high capacitance carries the low-frequency current component. The EPCOS CeraLink then handles the high-frequency component, including the snubber component.