How to choose and buy reliable and safe power
Faced with the choice, users still he was at a loss to know how to choose a safe and reliable power supply products. In many product specifications manual introduces, craft standard specification to engineer the choose and buy and safe reliable power supply.
Power equipment need to provide isolation function, so as to ensure the safety of the power equipment, high pressure from the risk of feeders, is the most basic is often easy to ignore. The power of the safety equipment is realized by power transformer, so, in order to make the transformer can send enough power requires it must have the corresponding scale.
A large transformer usually equipped with radiator, so that it can obtain the good product life. In addition, the edge in the transformer and deputy edge coil to use a double between isolation, so as to ensure the safety of the largest.
People often simply requirements of power source product life, in fact, many factors influence power life, such as like average load rate, vibration and the surrounding environment temperature, etc. Among them, the environmental temperature is important, so the emissions generated by power supply the internal components of the total quantity of heat is critical.
Because the power equipment manufacturers do not understand the end user of the conditions of use, so the only thing they can provide the life span of the performance test of the power supply products is average fault time interval MIBF (MeanTimeBetweenFailure); The power supply MTBF value, in any case, all is the electrolysis of internal by power supply electricity
Let the values of MTBF of the decision. When the power supply device to exclude the influence factors of capacitance, the calculation for 100000 hours or MTBF may be longer. However, typical of the electrolytic capacitor MTBF the values of only 30000 hours.
Some power equipment preparation manufacturers have developed their own power supply MTBF calculation method, and the calculated value is higher MTBF, so the user had better use in MIL-HDBK-217 E way defined in the values of MTBF, manufacturer of power with the values of a given MTBF, in comparison to judging correctly the performance of our products. Because MIL-HDBK-217 E of the definition of the calculation method is has been confirmed MTBF and is widely accepted that the calculated value is also can check MTBF.
When evaluation of nominal power source product life, the supply is running at full load condition is rated evaluation of another important power factor. If power equipment with appropriate radiator and no heat cycle, in less than full load cases and work continuously, the power supply can have a long life. Comprehensive consideration of the above factors, the proposal type selection engineers had better rely on MIL-HKBK-217 E methods validation power products of the values of MTBF, ensure that the power supply in the right conditions work, as long as do this, also don´t have to consider electrolytic short life.
Power factor correction
The power of another key element is the performance of the the power factor. The textbook definition of power factor is added to the load voltage and current waveform phase Angle between the cosine (if voltage waveform and current waveform phase Angle φ for the poor, the cos φ is the power supply power factor). When added to the load voltage and current waveform phase agree (that is poor phase Angle φ = 0), the power factor cos φ = 1 is the ideal situation; When added to the load voltage and current waveform phase is bad for 90 ° (i.e. φ = 90 °), the power factor is equal to zero (in a minimum value); Usually, the power supply power factor in between 0 to 1, 0 than cos φ than 1, can be expressed as percentage.
Add in the load voltage and current waveform phase difference between the result of power supply is one of low efficiency, which produce the required power need to enter more power; In another result and is more serious consequences, and that is the voltage and current waveform sent generate as much higher harmonic. A large number of high harmonic feedback to the main input line (network), creates the network was higher harmonic pollution become malignant accident; Also, the higher harmonic also would disrupt the control system of the sensitive low voltage circuit.
We have two major power factor correction PFC (PowerFactorCorrection) method: the first kind of method is in the input is simple to use coils of wire; The second approach is to use special electronic power factor correction circuit. Use the coil is called "passive" PFC, can be obtained by using this method usually to 0.7 ~ 0.8 power factor. Use the second method (also called "active" PFC) is the least amount of can produce higher harmonic and more effective utilization of power grids of electric power. Active PFC can produce more than 95% of the power factor, in large power supply most useful in, because produce higher harmonic and load current is directly proportional to the. For example, in 10 years and even higher 24 Vdc power supply load current using active method is the most appropriate PFC.
Engineers should be realized, power have power factor correction function is not only the important significance to ensure that the power supply not radiation or don´t have to conduct electrical noise. Therefore, as a planning selection engineers must be looking for interstate standard power products, including the launch of the standard electricity standard EN55011-BtEN55022-B and relevant higher harmonic pollution emission of power network EN61000.3.2 standard.
The built-in power surge protection function already is increasingly popular performance. Many power already use separate surge protection devices, in case of high voltage wave impact, such as lightning shock.
Some switching power supply is now provide EN61000-4-4 and EN61000-4-5 defined surge protection, it is the built-in surge protection function (provide up to 4 kV surge protection of), because do not need additional suppressor to narrow the precious panel space. The new international standards make easy to choose engineer power, because of the surge protection level standardization