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Posted on 08 April 2020

Effects of Humidity on Power Electronics Design

 

 

 

 

 

 

 

 

Air humidity is likely to condense at all parts of a device if the device temperature differs from the air temperature to a certain extent. In extreme cases, condensation causes the generation and accumulation of water droplets which might be drawn to moisture sensitive components such as power semiconductors. Anti-condensation design requirements to avoid moisture induced failure depend on the application specific climatic conditions which the device is developed for. Examples of anti-condensation measures are as follows:

  • Case with separate internal air circulationPower semiconductors integrated in such systems are usually liquid-cooled or cooled by heat sinks located outside the case and separated by a seal.
    • Closed system (min. protection degree IP65) with an air-to-air or water-to-air heat exchanger
    • Closed system with internal air conditioning (air temperature and air humidity regulation)
    • Open system with in-case forced ventilation
  • Heat sink temperature control using cooling-air intake control (fan speed or fan duty cycle)
  • Internal temperature control using interior heating

The diagram in Figure 1 illustrates the relevant design features with the example of a switch-gear cabinet with integrated SKiiP power modules.

Power electronic switchgear cabinet

Figure 1. Design details of a power electronic switchgear cabinet:

  1. Power module to be protected from moisture
  2. Critical zone: Heat sink, heat sink temperature T1, e.g. adjustable with a two-step controller to T1 ≥ T2 → Risk of condensation if T1 < T2
  3. Heating: Interior temperature is adjusted to T2 within the admissible operating temperature range of the components
  4. Cooling air intake to the switchgear cabinet (air flow and air humidity control)
  5. Air intake and outlet: Possible chimney effects must be attenuated by way of air channeling to make sure that the cooling air is evenly distributed
  6. Case with IP54 protection min.
  7. Deflection of water droplets from the case top
  8. Optional: Water-absorbing granulate for passive air humidity control
  9. Critical zones: Cabinet side panels

In a closed case, surfaces that are colder than the interior air temperature are likely to be affected by condensation or even ice formation. Condensation water is typically generated if the temperature of the interior environment drops to a point where the relative air humidity reaches 100%. The air humidity will then condensate on surfaces with temperatures below the interior temperature (e.g. air intake and outlet, case panels, heat sink). The requirements for ventilation inside the device depend on the respective climatic conditions of use.

The air inside closed interior systems without active air humidity control may absorb water up to saturation state. The absorbable amount of liquid is limited by the interior air volume. If the temperature changes, this process will either continue or be reversed, i.e. water condenses from the air. In such systems, water-absorbing granulate might be used to avoid condensation.

A change in temperature will cause air pressure compensation. Ventilation valves with a defined humidity compensation capability (e.g. Gore Prevent) may be installed in order to prevent moisture from penetrating the closed system as a result of temperature fluctuations.

Heat sink temperature control is recommended for closed systems. To avoid damage due to failure in the sealing system, the air humidity inside the system should be monitored. Systems used in tropical or other high humidity environments should be equipped with active air humidity control system.

Inside switchgear cabinets without closed interiors, areas of condensation exist as a result of the constant supply of fresh air. Provisions must be made to shift these areas to places where condensation, ice formation or water droplets do not have detrimental effects on the system. The temperature of critical live components (such as heat sink, power semiconductors, PC boards etc.) must always exceed the environment temperature. This can be achieved, for example, by way of suitable preheating, interior ventilation and air distribution measures. Condensation water must be collected and suitably deflected from critical components.

When controlling the heat sink temperature, transition states such as low-load operation and standby to operating mode switchover also have to be taken into account, since in such states the above-mentioned requirements can only be fulfilled by reducing the cooling temperature. In standby mode it might make sense to maintain the supply of auxiliary voltage and, if necessary, induce additional heating of the heat sinks and the interior to keep the temperature at a reasonable level.

 

For more information, please read:

Device Failure due to Electrical and Thermal Conditions

Causes of Failure of Power Semiconductor Devices (PSDs)

Reliability Engineering in Power Electronics

 

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One Response

  1. avatar Scott says:

    I am choosing new system now, and I am on the budget. According to reviews, these are good: http://pc24.de/ventilator/itho You have any ideas how this will work for closed systems?

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