DIN rail power supply units: How the service lifetime is determined by the temperature

Higher efficiency means less heat

An essential factor for low heat generation in the power supply is, a high efficiency of the unit.

The efficiency value describes the ratio of output power to input power of a power supply. The difference is converted into heat as loss. The advantage of high efficiency is obvious on closer examination. The lower energy losses mean that not much heat is generated in the first place, thus reducing the amount of cooling required. As a result, the electrolytic capacitors achieve the service lifetime specified by the manufacturer. The reliability of the entire unit is also increased (see graphic 2).

Graphic 2: A high efficiency leads to less energy loss in the form of heat. This is reflected in a longer service lifetime of the power supply units, a smaller design and higher reliability.

 

Optimized heat dissipation is essential for a long service lifetime

Heat puts a strain on the service lifetime of components. In addition to high efficiency, another factor for a long service lifetime is a good cooling concept.

Despite high efficiency, a small amount of energy is always lost. PULS units have a very high efficiency of up to 95.6 %. The remaining 4.4 % of the input power is lost. The heat generated by the losses is unavoidable and must be dissipated directly to the surroundings. The outer surfaces of the housing and the convection air flow that passes through the unit serve this purpose. The convection air flow should be able to flow without obstruction between the components.

In practice, however, this is difficult to achieve. Power supplies are becoming increasingly complex and smaller at the same time. Space inside the device is limited. This makes it even more important to install cooling ducts and consider them equal to the other components in the device. In addition to the installed cooling ducts, short and direct connections of the heat generators are also an option in order to keep as little heat as possible in the device. A smart cooling concept often eliminates the need for internal heat sinks. This benefits the low weight and also significantly reduces the cost of the power supply.

The right component arrangement is the key

The optimal arrangement of temperature-sensitive components is essential for long lifetime of units.

Circuit designers tend to optimize the arrangement of components according to electrical requirements. In doing so, they often compromise on thermal design. In order to ensure the longest possible service lifetime, life-determining components must be placed in the cooler areas. Temperature-sensitive components are primarily electrolytic capacitors, varistors and optocouplers. The electrolytic capacitors can be placed near the cool air flow.

The practical test: A unit under a thermal imaging camera

Thermography can be used to prove that these measures contribute to a cool power supply. Thermography makes it possible to look inside DIN-rail power supplies. They are especially informative in the case of convection-cooled switched mode power supplies.

The practical test with a high-tech thermal imaging camera shows the hot spots in the device. the heat development in certain areas and the position of the components that heat up the most. Due to the “Cool Design”, the unit can develop its full potential under load without overheating.

In convection cooling, the warm air is directed outwards by a flow. The effect of a functioning air flow can be proven by thermography.

The shown thermographic image of a PULS CP10 clearly demonstrates that sensitive components, such as the electrolytic capacitors (image labels: 1, 2, 3, 4, 5), have been optimally placed and stay cool (see graph 4 + 5).