How to select and use power supply

  • Detail

How to choose and use power supply

today, power users are faced with countless choices. The numerous performance of power products and the long product specifications of power suppliers make the purchase of power supply a headache. Fortunately, there are many process standards and technical specifications that can help engineers choose reliable and safe power supplies

safety first

the power supply equipment needs to provide isolation function, so as to ensure the safety of the power supply equipment and avoid the danger from the high-voltage feeder, which is the most basic and often easy to be ignored. The security of power supply equipment is realized by the power transformer. Therefore, in order to transmit enough power, the transformer must have a corresponding scale

a large transformer is usually equipped with a radiator, which can obtain a good product life. In addition, double isolation shall be used between the primary and secondary coils of the transformer to ensure maximum safety


people often simply require the service life of power supply products. In fact, there are many factors that affect the service life of power supply, such as average load rate, vibration and ambient temperature. Among them, the ambient temperature is very important, so it is very important to discharge the total heat generated by the internal components of the power supply

because power equipment manufacturers do not know the service conditions of end users, the only service life performance they can provide is the mean time between failures (MIBF) of power products; In any case, the MTBF value of the power supply is determined by the MTBF value of the electrolytic capacitor inside the power supply. When the influence of capacitance is excluded from the power supply equipment, but the quality and quality of components and sensors of China's tension machine can not meet the demand factors of the domestic market, the calculated MTBF may be 100000 hours or more. However, the typical MTBF value of electrolytic capacitor is only 30000 hours

because some power supply equipment manufacturers have developed their own power supply MTBF calculation methods, and the calculated MTBF value is relatively high, users had better use the MTBF value defined in mil-hdbk-217e to compare it with the power supply MTBF value given by the manufacturer to correctly judge the performance of the product that can be tested. Because the calculation method of MTBF defined in mil-hdbk-217e has been proved and is widely acceptable, the calculated MTBF value is also verifiable

when evaluating the nominal life of power products, whether the power supply operates at the rated full load condition is another important consideration in evaluating the power supply. If the power supply equipment is equipped with a suitable radiator and has no thermal cycle, it can have a longer service life under the condition of less than full load and continuous operation. Considering the above factors, it is suggested that the model selection engineer should preferably rely on the mil-hkbk-217e method to verify the MTBF value of the power supply product to ensure that the power supply works under appropriate conditions. As long as this is achieved, there is no need to consider the short service life of the electrolytic capacitor

power factor correction

another key performance factor of the power supply is its power factor. The power factor defined in the textbook is the cosine of the phase angle between the voltage waveform and the current waveform applied to the load (if the phase angle difference between the voltage waveform and the current waveform is φ, Then cos φ Is the power factor of the power supply). When the phase of voltage and current waveform applied to the load is consistent (i.e. phase angle difference φ= 0 various physical and mechanical performance tests of plastic film, wire and cable, waterproof coiled material, metal wire, carton and other materials), then the power factor cos φ= 1 is the ideal situation; When the phase angle difference between the voltage and current waveform applied to the load is 90 ° (i.e φ= 90 °), then the power factor is equal to zero (at the minimum value); Generally, the power factor of the power supply is between 0 and 1, i.e. 0 ≤ cos φ ≤ 1, expressed as a percentage

one of the results caused by the phase difference between the voltage and current waveforms applied to the load is that the power supply efficiency is reduced, that is, it is necessary to input more power to generate the required power; Another result and more serious consequence is that the waveform difference between voltage and current produces too many high-order harmonics. A large number of high-order harmonics are fed back to the main input line (electricity), causing the electricity to be polluted by high-order harmonics and becoming a hidden danger of malignant accidents; At the same time, this high-order harmonic will also disturb the sensitive low-voltage circuit in the control system

there are two main power factor correction (PFC) methods: the first method is to use a simple coil at the input end; The second method is to use a special electronic power factor correction circuit. The coil is called "passive" PFC, and the power factor of 0.7 ~ 0.8 can usually be obtained by this method. The second method (also called "active" PFC) can generate the least amount of high-order harmonics, so as to make more effective use of the electric energy provided by electricity. Active PFC can generate a power factor higher than 95%, which is most useful in large power supply, because the higher harmonics generated are directly proportional to the load current. For example, the active PFC method is most suitable for 24VDC power supply with 10A or higher load current

the engineer should realize that the significance of power supply with power factor correction function is not only to ensure that the power supply does not radiate or conduct unwanted electrical noise. Therefore, as a planning and model selection engineer, we must look for power products that conform to international specifications, including the electric emission specification en55011-bten55022-b and the specification en61000.3.2 related to the electric pollution caused by high-order harmonic emission

surge protection

the built-in surge protection function in the power supply has become an increasingly popular performance. Many power supplies have used separate surge protection devices to prevent high-voltage peak surges, such as lightning surges

some switching power supplies now provide surge protection as defined in en and en, which is a built-in surge protection function (providing surge protection up to 4KV). Since no additional suppressor is required, the precious panel space is reduced. This new international standard makes it easy for engineers to choose power supply, because the standardized level of surge protection has long been established

overload and short circuit protection

an important feature of any power supply is to provide the declared continuous full load capacity. More importantly, the power supply has some built-in error margin or fault tolerance for calculating (considering) overload conditions. A good power supply can provide at least 5% overload protection, and ideally 10% overload protection

the so-called overload refers to taking out excessive current from the power supply. The planning and model selection engineer has two options. The first option is to start the hiccupcircuit when the power supply is overloaded. With this design, the power supply equipment can be suspended, and after the suspension, the power supply tries to restart for continuous operation. When the overload condition disappears, the power supply restarts successfully and starts to work normally again. This design is suitable for low current equipment

for large power supply equipment, a method called "constant current" power supply is a better choice for overload protection. In this case, when the power supply has been forced to supply a constant current, the power supply device reduces its output voltage

short circuit protection function is another safety performance of power supply equipment, which can not be ignored. Although the main purpose is safety, the biggest advantage is that the power supply has automatic reset feature at that time. The protection provided by this feature lasts until a short-circuit fault has been detected

economy and size of power supply

economy and geometric size of power supply are related. Fortunately for the end users, the economy and geometric dimensions of the power supply have been improved. Some newer power supply products provide the above full performance. Compared with the past, it can obtain 50% lower package size than the old low efficiency design products at a lower cost

among the two characteristics of economy and geometric dimension, the geometric dimension characteristic is often preferred. Because in the past, a large number of geometric dimension skills have been accumulated, such as using smaller components and effective plate dimensions. Now, some of the most effective designs combine the heat sink into the assembly space of the power supply housing, thus effectively reducing the space and cost of the additional heat sink and plastic housing

easy to use

an additional common requirement for power supply equipment is easy to assemble termination. Many power products today offer a wide variety of design features, such as maximum assembly flexibility and minimum final installation and connection costs. In order to meet the global application, the popular performance of power supply includes: sensitive and safe assembly of DIN rail mountingbracket, micro housing design and universal width input voltage range. Other performances of the power supply equipment include the following: the front panel installation has attracted many enterprises to seize the western market, with input and output connections, pluggable touch reliable termination parts, easy assembly/replacement of input fuses and output voltage adjustment

recently, a new power supply product has been launched, which is directly connected to the three-phase 340 ~ 480vac input voltage, eliminating the cost and space required by the voltage drop transformer. The final result is that this new power supply product is more effective and cheaper than adding a transformer with a single-phase power supply. (end)

Copyright © 2011 JIN SHI