Smoothing out power modules - Film capacitors for DC intermediate circuits

Capacitors on a DC intermediate circuit serve as buffers for power spikes in motor or generator actioned drive unit operation. They cushion system perturbations, i.e. line-conducted EMC, and as a snubber they function as filter components to protect the switching transistors from voltage peaks and high dU/dt (voltage gradient).

Significant characteristics of intermediate circuit capacitors

A low ESL (equivalent series inductance) and a small ESR (equivalent series resistance) are advantageous for the filter function against steep gradient impulses. In the data sheets, the quality is expressed as loss factor tan ♎︎. The power dissipation, which in the capacitor falls away on the ESR, is proportional to tan ♎︎ and results in undesired self-heating.

For the snubber function, due to parasitic line inductance, the capacitor is best positioned with a short cable length to the switching transistor. The unavoidable losses in the transistors at the final stage likewise cause an increase in temperature that shortens the service life of neighboring components. Here, film capacitors are robust and more long-lasting because, in contrast to capacitors with liquid electrolyte, they do not dry out.

Electrolytic capacitors were mainly used for power supplies with commutated line voltage. With the arrival of active power factor correction or battery power and faster switching IGBTs and MOSFETs, film capacitors or ceramic capacitors are being increasingly used - depending on the voltage situation. Film capacitors offer some advantages over electrolytic capacitors.

High dielectric strength, so no need to arrange the series connection of individual capacitors with additional voltage balancing.

High temperature resistance with solid state dielectric

Low ESR and thus high pulse power

Low self-inductance, effective at high frequencies, and suitable for a high di/dt (current gradient).

Self-healing in segmented metallized plastic film design, therefore robust and secure ("films fail open")

Long service life, calculable and gradual ageing with accompanying loss of capacitance. With the self-healing facility, no sudden failure caused by a short-circuit can occur, that could result in consequential damage.

Self-healing prevents a sudden failure

Self-healing is the capability of the capacitor to microscopically free burn small internal short-circuits, leaving behind a high impedance spot. To keep the internal short-circuit areas small, most manufacturers of power film capacitors turn to the controlled self-healing technology which has been proven for some decades now. The film metallization is segmented. Each metallized segment is connected to the neighboring segments by just a thin fusible link (Image 1). In the event of a local breakout in the dielectrics these links to the neighboring segments melt, thus isolating the damaged segment. So the damage remains local and the capacitor only loses a tiny proportion of its capacitance.

Polypropylene (PP) or polyester (PET) are often used as dielectrics. PP provides a low loss factor and is well suited to cope with pulse stress. PET has a high specific capacitance and low volume requirement.

Capacitors for smaller outputs

For smaller outputs, standard single capacitors are sufficient as intermediate circuit capacitors. They are interconnected on a PCB or via copper rails to the required capacitance. Due to the mechanical size and mass of the capacitors it is necessary to pay attention to the long-term stability of the soldering points, screws and the PCB. Manufacturers help here by making capacitor housings with more than just two solder pins (Image 2). The advantages of using standard components for PCB assembly is that they are easy to mount and users can scale the overall system.

Suitable semi-conductor modules for use in combination with PCBs are Infineon's EasyPACK 1B and 2B, and also CIPOS or modules such as in the ACEPACK housing by STMicroelectronics.

High currents require power capacitor modules

With the arrival of e-vehicles, Infineon has introduced semiconductor modules into their HybridPack 1, 2 and drive housings, as a power stage for traction inverters in vehicles. Due to the amount of load current flow contact is no longer made via pins, but via power rails. Made to fit to Infineon's housings, manufacturers of film capacitors have developed appropriate molded power capacitor modules; these include AVX, TDK Epcos, Rubycon and WIMA. Examples of suitable modules are shown in the table.

Internal design of the capacitor modules

Since the modules consist of individual windings that are parallel switched via rails, they provide good scalability of the overall capacitance, where individual elements from stable series production are used. By parallel switching the individual elements in the capacitor module (Image 3, Image 4) they simultaneously achieve low ESL and ESR values.

Besides the standard modules, customer-specific designs of capacitors and transistors are also possible, such as can be found in Tesla's inverter for the model 3 rear axle. So design, electrical values, and mechanical assembly can be optimized.

Engineering Support

But nowadays, the technical parameters taken from the data sheet and the manufacturing process cannot alone determine the quality of design-in components. For power components the manufacturer's support during the selection stage and during development play an important role. Because this prevents late changes thus reducing the time to market.

Thermal simulation

For rewarding projects AVX supplies thermal simulation of its power film capacitors as a special service. So, for example, with customer-specific capacitor modules, it is possible, at an early stage of the development process, to optimize internal material use and set out the measures to dissipate heat. The results of the thermal simulation (Image 5) can also be used to estimate its service life.

Where the project is appropriate and on request AVX also encapsulates temperature sensors inside the capacitor housing. The measurement results allow for verification and optimization of the thermal model.

Estimating service life by means of an application profile

In order to estimate the service life of the capacitor modules in the application, the manufacturer needs the anticipated operating data during the system's lifetime, in other words a histogram stating how long the capacitor will be exposed to specific current, voltage, ambient temperature combinations. From that, with the aid of the computed or measured temperature inside the capacity the manufacturer can calculate lifetime consumption. The service life model for the capacitor in question forms the basis of this, such as AVX, TDK Epcos, Rubycon and WIMA have for their products. The principles are well described, for example in Rubycon's technical notes - see <link aufwww.rubycon.com>www.rubycon.com</link>.

Equivalent electrical circuit diagram and 3D CAD data

Inserted into a simulation model, and with the equivalent electrical circuit diagram, it is possible to simulate the dynamic behavior in operation of the electrical quantities and in special situations with high di/dt, such as emergency inverter cut-outs. Developers can dimension their protection against voltage spikes with no hardware structure.

In the most straightforward case, a film capacitor is modelled as a series connection of ESR, ESL and the calculated end-of-life capacitance.

Where capacitor manufacturers supply CAD data for their products, the capacitor can simply be included into the CAD design drawing for the overall mechanical system. This also saves valuable development time.

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