How to choose and use a power supply

　　Today, power users are faced with countless choices , the numerous performances of power products and the long product specifications of power suppliers make purchasing power 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 to ensure the safety of the power supply equipment from the dangers of high-voltage feeders, which is the most basic and often overlooked. This safety of the power supply equipment is achieved by the power transformer. Therefore, in order for the transformer to transmit enough power, it must have a corresponding size.

　　
Figure: Power supply equipment has become a key component of industrial control system architecture

A larger transformer is usually equipped with a heat sink to achieve good product life. In addition, double isolation is used　　between the primary and secondary windings of the transformer to ensure maximum safety.

　　reliability

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

　　Because power supply equipment manufacturers do not understand the end-user's usage conditions, the only life performance they can provide is the mean time between failures (MIBF) of the power supply product.

The MTBF value of a power supply is, in any case, determined by the MTBF value of the electrolytic capacitor inside the power supply. When the influence of the capacitor is excluded from the power supply device, the calculated MTBF may be 100,000 hours or longer. However, the typical MTBF value of electrolytic capacitors is only 30,000 hours.

　　Because some power supply equipment manufacturers have developed their own power supply MTBF calculation methods, and the calculated MTBF values ​​are relatively high, users are better off using the MTBF value defined in the MIL-HDBK-217E method to compare with the power supply MTBF value given by the manufacturer to correctly judge the performance of the product. Because the MTBF calculation method defined in MIL-HDBK-217E has been proven and is widely accepted, the calculated MTBF value is also verifiable.

　　When evaluating the nominal life of a power supply product, whether the power supply is running at the rated full load is another important consideration. If the power supply device is equipped with a suitable heat sink and has no thermal cycle, the power supply can have a longer life when it is working continuously at less than full load. Taking all the above factors into consideration, it is recommended that the selection engineer should rely on the MIL-HKBK-217E method to verify the MTBF value of the power supply product to ensure that the power supply is working under appropriate conditions. As long as this is done, there is no need to consider the short life of the electrolytic capacitor.

　　Power Factor Correction

　　Another key performance factor of a power supply is its power factor. The power factor defined in textbooks is the cosine of the phase angle between the voltage and current waveforms applied to the load (if the phase angle difference between the voltage waveform and the current waveform is φ, then cosφ is the power factor of the power supply). When the voltage and current waveforms applied to the load are in phase (i.e., the phase angle difference φ=0), the power factor cosφ=1 is an ideal situation; when the phase angle difference between the voltage and current waveforms applied to the load is 90° (i.e., φ=90°), the power factor is equal to zero (at the minimum value); usually, the power factor of a power supply is between 0 and 1, i.e., 0≤cosφ≤1, and can be expressed as a percentage.

　　One of the results of the phase difference between the voltage and current waveforms applied to the load is a reduction in power supply efficiency, that is, a larger power input is required to generate the required power; another result, which is a more serious consequence, is that the difference in voltage and current waveforms produces too many high-order harmonics. A large number of high-order harmonics are fed back to the main input line (grid), causing the grid to be polluted by high-order harmonics, which becomes a hidden danger of serious accidents; at the same time, such high-order harmonics will also disrupt sensitive low-voltage circuits in the control system.

　　There are two main methods of power factor correction (PFC): the first method uses a simple coil at the input end; the second method uses a special electronic power factor correction circuit. Using a coil is called "passive" PFC, and a power factor of 0.7 to 0.8 can usually be obtained using this method. The second method (also called "active" PFC) can generate the least amount of high-order harmonics and use the power provided by the power grid more efficiently. Active PFC can produce a power factor higher than 95%, which is most useful in large power supplies because the high-order harmonics generated are directly proportional to the load current. For example, it is most appropriate to use the active PFC method in a 24Vdc power supply with a load current of 10A or even higher.

　　Engineers should realize that the importance of power factor correction is not only to ensure that the power supply does not radiate or conduct unwanted electrical noise. Therefore, as a planning and selection engineer, you must look for power products that meet international standards, including the power emission standard EN55011-BtEN55022-B and the standard EN61000.3.2 for high-order harmonic emission pollution of the power grid.

　　Surge protection

　　Built-in surge protection in power supplies is an increasingly popular feature. Many power supplies already use separate surge protection devices to protect against high voltage spikes, such as lightning strikes.

　　Some switching power supplies now offer surge protection as defined in EN61000-4-4 and EN61000-4-5, which is built-in surge protection (providing up to 4kV surge protection), reducing precious panel space by eliminating the need for external suppressors. This new international standard makes it easier for engineers to select power supplies because standardized levels of surge protection have long been established.

       Overload and short circuit protection

　　An important feature of any power supply is that it provides a published continuous full load capability. Even more important is that the power supply has some built-in margin of error or tolerance for calculated (accounted for) overload conditions. A good power supply will provide at least 5% overload protection, and ideally 10% overload protection.

　　An overload condition is when an excessive amount of current is drawn from the power supply. The planning and selection engineer has two options. The first option is that when the power supply is subjected to an overload condition, the power supply device starts a hiccup circuit. With this design, the power supply device can suspend operation, and after the hiccup, the power supply attempts to restart and continue to work. When the overload condition disappears, the power supply restarts successfully and begins to work normally again. This design is suitable for low-current devices.

　　For larger power supplies, a method called "constant current" power supply is a good choice for overload protection. In this case, when the power supply is forced to supply a constant current, the power supply reduces its output voltage.

　　Short circuit protection is another safety feature of power supply equipment, which should not be ignored. Although the main purpose is safety, the biggest advantage is that the power supply has an automatic reset feature. The protection time provided by this feature can last until the short circuit fault has been detected.

　　Economy and size of power supplies

　　Power supply economics and size are related. Fortunately for the end user, both are improving. Some newer power supply products offer the full performance described above.

, it can achieve a 50% smaller footprint than older, less efficient designs at a lower cost than ever before.

　　Of the two characteristics of economy and geometry, geometry is often more important. This is because a lot of geometry tricks have been accumulated in the past, such as using smaller components and effective board size. Now, some of the most effective designs integrate the heat sink into the power supply housing assembly space, thus effectively reducing the space and cost of additional heat sinks and plastic housings.

　　Ease of use

　　An additional common requirement for power supply equipment is easy-to-assemble termination. Today, many power supply products offer a variety of design features, such as maximum assembly flexibility and lowest final installation and connection costs. To meet global applications, popular performance of power supplies include: sensitive and safe assembly double-row guide rail bracket (DIN-rail-mounting bracket), small shell design and universal width input voltage range. Other performance of power supply equipment includes the following: front panel assembly input and output connections, pluggable touch-reliable termination components, easy assembly/replacement of input fuses and output voltage adjustment.

　　Recently, a new type of power supply has been launched on the market, which directly connects to the three-phase 340-480Vac input voltage, eliminating the cost and space required for the voltage drop transformer. The end result is that this new power supply product is more efficient and less expensive than using a single-phase power supply with an additional transformer.