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Basic Knowledge Of Selecting And Using A Safe Power Source
- Dec 01, 2018 -

Today, power users face countless choices, the numerous features of power products and the long product specification of power suppliers, making the purchase of power supplies a headache. Fortunately, there are a lot of process standard specifications that can help engineers purchase reliable and safe power supplies.


★Safety first

Power supplies need to provide isolation to ensure the safety of power supplies from the danger of high voltage feeders, which is the most basic and often overlooked. This safety of the power supply is achieved by the power transformer, so that the transformer must be of a corresponding size in order to transmit sufficient power.

A larger transformer is usually equipped with a heat sink for 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 ask for the life of power products. In fact, there are many factors that affect the life of the 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 the power supply equipment manufacturer does not understand the end user's conditions of use, the only life performance they can provide is the average time between failures of the power supply (MIBF) (MeanTimeBetweenFailure); the MTBF of the power supply, in any case, by the power supply The value of the internal electrolytic capacitor MTBF is determined. When the factor of the capacitor is removed from the power supply, the calculated MTBF may be 100,000 hours or longer. However, the typical MTBF value of an electrolytic capacitor is only 30,000 hours.

Because some power supply equipment manufacturers have developed their own power supply MTBF calculation methods, and the calculated MTBF value is relatively high, so users should use the value of MTBF defined in MIL-HDBK-217E mode, the same The value of the MTBF given by the manufacturer is compared to correctly judge the performance of the product. Since the calculated MTBF method defined by MIL-HDBK-217E has been proven and widely accepted, the calculated MTBF value is also verifiable.

When evaluating the nominal life of a power product, whether the power supply is operating at a rated full load condition is another important consideration in evaluating the power supply. If the power supply unit is equipped with a suitable heat sink and there is no thermal cycling, the power supply will have a longer life at less than full load and continuous operation. Considering the above factors, it is recommended that the selection engineer should rely on the MIL-HKBK-217E method to verify the MTBF value of the power supply to ensure that the power supply works under the right conditions. As long as this is done, it is no longer necessary to consider the short life of the electrolytic capacitor. problem.


★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 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 (ie, the phase angle difference is φ = 0), the power factor cos φ = 1 is ideal; when the voltage and current waveforms applied to the load have a phase angle difference of 90° When (ie φ = 90°), the power factor is equal to zero (at a minimum); usually, the power factor of the power supply is between 0 and 1, ie 0 ≤ cos φ ≤ 1, which can be expressed as a percentage.

One of the consequences of the phase difference between the voltage and current waveforms applied to the load is that the power supply efficiency is reduced, ie, the required power is required to input more power; the other result is a more serious consequence, then That is, the waveform difference between voltage and current generates excessive high harmonics. A large number of higher harmonics are fed back to the main input line (grid), causing the grid to be contaminated by higher harmonics and becoming a hidden danger of malignant accidents. At the same time, such higher harmonics can also disturb sensitive low voltage circuits in the control system.

There are two main power factor correction PFC (PowerFactorCorrection) methods: the first method uses a simple coil at the input; the second method uses a special electronic power factor correction circuit. The coil is called a "passive" PFC, and a power factor of 0.7 to 0.8 is usually obtained by this method. Using the second method (also known as "active" PFC) produces the least amount of higher harmonics to make more efficient use of the power provided by the grid. Active PFCs can generate power factors greater than 95%, which is most useful in large power supplies because the higher harmonics produced are directly proportional to the load current. For example, an active PFC method is most suitable for use in a 24Vdc power supply of 10A or higher load current.

Engineers should be aware that the power factor of a power supply with power factor correction is not only to ensure that the power supply is not radiated or that it does not conduct unwanted electrical noise. Therefore, as a planning and selection engineer, it is necessary to find power products that meet the specifications. These specifications include the electrical emission specification EN55011-BtEN55022-B and the specification EN61000.3.2 on the high-order harmonic emission pollution grid.


★Surge protection

Built-in surge protection in the power supply is an increasingly popular feature. Many power supplies have used separate surge protection devices to protect against high voltage spikes such as lightning strikes.

Some switching power supplies now offer surge protection as defined by EN61000-4-4 and EN61000-4-5, which is built-in surge protection (provides surge protection up to 4kV), since no additional suppressors are required This reduces the precious panel space. This new international standard makes it easy for engineers to choose a power source because the standardization level of surge protection has long been established.


★Overload and short circuit protection

An important feature of any power supply is the ability to deliver a published continuous full load capability. More importantly, the power supply has some built-in margin of error or fault tolerance for calculating (considering) overload conditions. A good power supply provides a minimum of 5% overload protection, ideally providing 10% overload protection.

The so-called overload condition refers to the excessive current drawn from the power supply, and the planning and selection engineer has two options. The first option is to start the hiccup circuit when the power supply is overloaded. With this design, the power supply can suspend work, and after a pause, the power supply tries to restart and continue working. When the overload condition disappears, the power supply restarts successfully and starts normal operation again. This design is suitable for low current devices.

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

Short-circuit protection is another safety feature of power supplies, a feature that cannot 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 be sustained until a short circuit fault has been detected.


★Economics and size of the power supply

The economics and geometry of the power supply are related. Fortunately for the end user, both the economics and geometry of the power supply have improved. Some of the newer power products offer the above-mentioned full performance, which can be packaged at a lower cost than the old low-efficiency design products.


★The size is 50% lower.

Of the two characteristics of economy and geometry, it is often more of a geometric dimension. Because of the accumulated skills of a large number of geometries in the past, such as the use of smaller components and effective board size. Some of the most efficient designs now incorporate a heat sink into the power housing assembly space, effectively reducing the space and cost of the external heat sink and plastic housing.


★Easy to use

An additional common requirement for power supplies is the easy to assemble termination. Many power products today offer a wide range of design features such as maximum assembly flexibility and minimal final installation and connection costs. To meet a wide range of applications worldwide, the power supply's popular features include a sensitive and safe assembly of DIN-rail-mounting brackets, a small housing design and a universally wide input voltage range. Other features of the power supply include 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 introduced, which is directly connected 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 is more efficient and less expensive than using a single-phase power supply.

One of the consequences of the phase difference between the voltage and current waveforms applied to the load is that the power supply efficiency is reduced, ie, the required power is required to input more power; the other result is a more serious consequence, then That is, the waveform difference between voltage and current generates excessive high harmonics. A large number of higher harmonics are fed back to the main input line (grid), causing the grid to be contaminated by higher harmonics and becoming a hidden danger of malignant accidents. At the same time, such higher harmonics can also disturb sensitive low voltage circuits in the control system.

There are two main power factor correction PFC (PowerFactorCorrection) methods: the first method uses a simple coil at the input; the second method uses a special electronic power factor correction circuit. The coil is called a "passive" PFC, and a power factor of 0.7 to 0.8 is usually obtained by this method. Using the second method (also known as "active" PFC) produces the least amount of higher harmonics to make more efficient use of the power provided by the grid. Active PFCs can generate power factors greater than 95%, which is most useful in large power supplies because the higher harmonics produced are directly proportional to the load current. For example, an active PFC method is most suitable for use in a 24Vdc power supply of 10A or higher load current.


Engineers should be aware that the power factor of a power supply with power factor correction is not only to ensure that the power supply is not radiated or that it does not conduct unwanted electrical noise. Therefore, as a planning and selection engineer, it is necessary to find power products that meet the specifications. These specifications include the electrical emission specification EN55011-BtEN55022-B and the specification EN61000.3.2 on the high-order harmonic emission pollution grid.


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