cooling water and power electronics corrosion
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Water cooling in power modules can be used for very high power inverters (MW range) as well as for low-power devices which already have a water cycle for operating reasons (e.g. car drives, galvanic installations, inductive heating). In most cases, the admission temperature of the coolant values is as much as 50...70°C when the heat of the coolant is directly dissipated to the atmosphere; in industrial plants with active heat exchangers, the temperature is about 15...25°C. The temperature difference between heatsink surface and coolant, which is lower than for air cooling, may be utilized in two ways:
increased power density, but with high dynamic ΔTj of chip temperature per load cycle, or
low chip temperature, long module life.
Figure 1 shows an example of water cooling of a 6-fold SKiiP on a water-cooled heatsink.
The following factors influence the thermal resistance in a liquid cooler:
the contact area to the coolant (e.g. number of cooling channels)
the volumetric flow rate as a function of the pressure drop
the heat storage capability of the coolant
turbulence in the water flow
heat conduction and spreading in the heatsink (heatsink material)
the coolant temperature (depending on viscosity and density)
Enlarging the contact area of heatsink/coolant will result in improved heat transfer. The traditional cooler is subject to limitations with regard to the number of cooling channels. The pin-fin cooler features small columns protruding into the coolant, which enlarges the contact area and also ensures sufficient turbulence.
The particular shape of the liquid cooler and a sufficiently high flow velocity creates a turbulent flow which substantially reduces the heat transfer resistance between heatsink and liquid (also see the spiral shaped inserts in Figure 6). Without turbulence, a liquid film is created on the cooler surface which impairs heat transfer.
Even more so than with air coolers, an even distribution of heat sources across the heatsink surface is important for low thermal resistance. Due to the high heat transfer coefficient of some 1000 W/(m2K), the heat flow is dissipated to the cooling liquid with only minor cross-conduction. This means that essentially only those areas on which power semiconductor modules are mounted are used for cooling. Copper rather than aluminium as heatsink material will reduce the volume resistance, increase cross-conduction, thus also increasing the effective cooling area. A cooler made of copper allows for a reduction in Rth(j-a) by approximately 20% for a standard IGBT module.
Especially in water-glycol mixtures, Rth(s-a) depends on the coolant temperature. This is due the glycol viscosity, as well as the changing density of the coolant, albeit to a lesser extent. For a mixture of 50% glycol and 50% water in the temperature range of 10°C to 70°C, it was found that Rth(r-a) was reduced by about 25% between the temperature sensor and coolant.
increased power density, but with high dynamic ΔTj of chip temperature per load cycle, or
low chip temperature, long module life.
Figure 1 shows an example of water cooling of a 6-fold SKiiP on a water-cooled heatsink.
The following factors influence the thermal resistance in a liquid cooler:
the contact area to the coolant (e.g. number of cooling channels)
the volumetric flow rate as a function of the pressure drop
the heat storage capability of the coolant
turbulence in the water flow
heat conduction and spreading in the heatsink (heatsink material)
the coolant temperature (depending on viscosity and density)
Enlarging the contact area of heatsink/coolant will result in improved heat transfer. The traditional cooler is subject to limitations with regard to the number of cooling channels. The pin-fin cooler features small columns protruding into the coolant, which enlarges the contact area and also ensures sufficient turbulence.
The particular shape of the liquid cooler and a sufficiently high flow velocity creates a turbulent flow which substantially reduces the heat transfer resistance between heatsink and liquid (also see the spiral shaped inserts in Figure 6). Without turbulence, a liquid film is created on the cooler surface which impairs heat transfer.
Even more so than with air coolers, an even distribution of heat sources across the heatsink surface is important for low thermal resistance. Due to the high heat transfer coefficient of some 1000 W/(m2K), the heat flow is dissipated to the cooling liquid with only minor cross-conduction. This means that essentially only those areas on which power semiconductor modules are mounted are used for cooling. Copper rather than aluminium as heatsink material will reduce the volume resistance, increase cross-conduction, thus also increasing the effective cooling area. A cooler made of copper allows for a reduction in Rth(j-a) by approximately 20% for a standard IGBT module.
Especially in water-glycol mixtures, Rth(s-a) depends on the coolant temperature. This is due the glycol viscosity, as well as the changing density of the coolant, albeit to a lesser extent. For a mixture of 50% glycol and 50% water in the temperature range of 10°C to 70°C, it was found that Rth(r-a) was reduced by about 25% between the temperature sensor and coolant.
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