Filters for air conditioning

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DC-DC converter for filters

To effectively clean the air in large rooms from microparticles as small as 0.01 µm, conventional filter systems with paper or fabric filters are unsuitable.

New approaches are needed, such as electro-mechanical filter systems, to specifically improve air quality and thus guarantee passengers a clean environment in the carriages. For this purpose, the air to be filtered and cooled is fed past metal plates that are subjected to very high voltages of 8,000 V. Over 92% of the positively and negatively charged aerosols are attracted or repelled by the plates, settling on them. As soon as the distance between the plates becomes smaller and smaller, due to contamination, a rapid sparkover (short circuit) occurs between the plates, thus burning the particles. This process is repeated several times within seconds, depending on the degree of pollution in the air.

The requirement for AUTRONIC was to develop a completely new DC/DC converter for use in railroad technology, for worldwide use in new systems, as well as for existing air conditioning systems as a retrofit component. The complete electronics are installed close to the roof, where very high temperatures prevail in everyday use. There are also many cables running in the roof and sensitive electronic components, such as switches, repeaters, and PIS, which must not be disturbed by the new PSU. Conversely, the new DC/DC converter must be insensitive to external interference.
 
The basic standards used were EN 50155 and EN 50121-3-2, shock/vibration according to EN 61373, fire protection according to EN 45545-2, safety according to EN 62368-1, MTBF according to MIL-HDBK-217F (Ground Mobile)

Thermal load, Useful Life and Safety

The high efficiency over the entire input range ensures that the customer can use the DC/DC converter in thermally critical environments. The optimized PCB and package design ensures that heat is perfectly dissipated. The low heat dissipation to the ambient air simplifies installation, as all hotspots in the device are mounted on the mounting base and dissipate heat via short paths, provided that the PSU is thermally well connected to the chassis. This largely prevents heat buildup in the hermetically sealed unit.

8000 V can be a major hazard, so the focus had to be on the grounding concept. To safely ground the base, cover and basic housing, contact areas in the anodized housing are mechanically exposed from screws and cables in an elaborate manner.

Trains should also be equipped with as little maintenance as possible. Proof of reliability is an important point to be demonstrated during development. Mistakenly, an MTBF (Meantime Between To Failure) calculation is often used to determine the reliability of electrical equipment. However, AUTRONIC takes a better approach to determine the lifetime of failure-critical components (e.g. electrolytic capacitors), considers them under realistic environmental conditions with actual heat load, and calculates their lifetime. Here, the customer could be assured of a Useful-Life of at least 20 years.
To be sure that the DC/DC converter is stable enough against permanently occurring sparkovers, a test environment corresponding to reality was simulated in order to exactly reproduce the spark duration and length of the short circuits occurring in the field. The results of the extensive testing were incorporated into an extremely stable design of the device.


EMC requirement

The EMC delicate installation condition requires a lot of know-how in the development of power supplies to ensure safe use. EMV06, a strict EMC requirement of the railroad radio services of DB, SBB and ÖBB, is complied with. This ensures that important radio frequencies remain free of interference and thus represents an important safety feature.

Cleaning the air in passenger compartments is enormously important. To meet this, a filter system must not fail, even if the EN 50155 railroad standard permits conditions that allow the application to be switched off and automatically started up. Surge, burst and ESD from the new unit have therefore been tested and approved with criterion A, i.e. EMC disturbances will have no effect on the PSU.

Environmental conditions

Despite an IP54 housing, it must be prevented that condensation in the device can not cause a malfunction. For this reason, the module is provided with a protective coating using a dipping process. The double insulation, with correspondingly longer clearances and creepage distances, prevents additionally.

The characteristics of the new device are:

  • Ultra-wide input range from 14.4 V to 154 Vdc (one unit) with high efficiency of at least 85% over the entire supply range
  • regulated output voltage: 8000 V
  • continuous output power: 80 W
  • high thermal load due to critical installation location (Class OT1 +ST1, ST2)
  • long supply and discharge lines
  • IP54 housing
  • High EMC requirement
  • Very high electrical load due to rapid and repetitive short circuit frequencies (spark discharge)
  • Spark detection
  • Evaluation function of the spark detection, which leads to the safe disconnection of the filter, in case of abnormal behavior