The fan of a power supply driven by an electronic motor (12Vdc fan) is the main source of cooling for all types of sources, the power and control of the fan being part of the circuit of the source itself. Airflow is directly related to the power dissipation of the power supply. Normally a boxed power supply needs fan cooling when its power exceeds 300W.
Airflow direction
The information on the direction of the air flow is specified in the product data sheet, in the mechanical specifications part. The two options that we find are either flow directed towards the PSU or flow expelled from the power supply. Considering the system's ventilation design, customers may require changing the airflow direction of a standard power supply. In these cases we will have to take into account a series of requirements as shown in Table 1:
Fan control behavior
The fan control mainly has three functions depending on the power level: fan on/off control, speed control depending on load, and fan control based on thermal conditions. For high power power supply models like RST series and low-end products, there is no fan control, the fan is always ON.
1) Fan on/off
When the source is without load the fan is off. Under load conditions, when the load is higher than a certain value, the fan starts to rotate until it reaches its maximum speed. It has no fan lock protection function and is widely used for medium power supplies.
2) Fan speed control depending on the load
There is a direct relationship between load and fan rotation. If the load is higher, the fan speed is higher. This reduces noise and increases fan life under light load conditions.
3) Fan control depending on thermal condition
Inside the power supply there are one or several internal temperature sensors to detect critical components, depending on the temperature registered in them, the fan speed will increase or decrease. This form of fan control can reduce fan noise if the operating temperature is low. For medium power power supplies functionality is easy. If the detected temperature is higher than the set value, the fan will work at full speed; if it is less than this value, the fan will be off. In this type of sources we can see how the fan remains on for a certain period of time and then turns off due to this type of control.
fan life
The life expectancy of a fan depends on the bearing system and the ambient temperature. There are several calculation methods to determine the life of the fan, such as L1, L10, MTBF, MTTF. Many fan manufacturers use the L10 to determine fan life, this value can be found on the product data sheet. In general, for a ball bearing fan, an engineer can expect a life of about 70.000 hours under normal operating conditions (according to L10). Fan life is a factor to consider in calculating the MTBF of a power supply. In order to prolong the life of the system, select a power supply with fan control, which is of high quality (longer life) and use it at a lower ambient temperature.
Reduce fan noise
Reducing fan noise is a growing demand, especially for indoor applications. To achieve this fan noise reduction we will have to take into account the following aspects:
1) Reduce fan noise in standby or light load conditions
For applications such as data transmission, chargers, or dynamic charging, there is a possibility that the power supply is in standby mode for a long period of time. Users may be required to reduce fan speed under these conditions or have the fan turn off. The fan speed behavior can be modified by hardware or firmware depending on the models.
2) Reduce fan noise under all operating conditions
Acoustic Noise (dB) is a parameter to determine fan noise which is shown on the fan data sheet. In order to reduce noise, a fan should be selected that generates as little acoustic noise as possible. The fan is considered a critical component and must be recorded in the CB report. If the selected fan is not included in the CB report, the source needs to be re-certified.
3) Fanless Design
In harsh environment applications, the fundamental requirement to reduce fan noise is to change the model to a fanless design, as the dusty environment and high ambient temperature affect the life of the fan. For this type of system there are special designs such as products on DIN rail, products encapsulated with resins (HEP Series), most with special or very heavy formats.
