How to Select DC/DC Module Power Supply Correctly and Reasonably
Time:2022-12-28
Views:1412
How to select the DC/DC module power supply correctly and reasonably? The author will talk about this problem from the perspective of DC/DC module power supply development and design, combined with the user information feedback obtained during the promotion and use of Dinglixin‘s module power supply in recent years, for the reference of the majority of system designers.
Selection of DC/DC module power supply
1. Rated power
It is generally recommended that the actual power used should be 30~80% of the rated power of the module power supply (the specific proportion is also related to other factors, which will be mentioned later.). Within this power range, the performance of the module power supply in all aspects is relatively sufficient, stable and reliable. Too light load will waste resources, while too heavy load will be detrimental to temperature rise and reliability. All module power supplies have a certain overload capacity. For example, Dinglixin‘s products can reach 120~150%, but it is still not recommended to work under overload conditions for a long time. After all, this is a short-term emergency plan.
2. Packaging form
There are various packaging forms of module power supply, including those conforming to international standards and non-standard ones. As for the products of the same company, there are different packages for the same power products, and the same package has different powers. How to choose the packaging form? There are mainly three aspects: ① The volume should be as small as possible under certain power conditions, so as to give more space and more functions to other parts of the system; ② Products packaged in accordance with international standards shall be selected as far as possible because of good compatibility, not limited to one or two suppliers; ③ It shall be expandable to facilitate system expansion and upgrading. Select a package. Due to the higher requirements of the system for power supply due to function upgrading, the package of the power module remains unchanged, and the design of the system circuit board does not need to be changed, which greatly simplifies product upgrading and saves time. Take Dinglixin‘s high-power module power supply products as an example: all of them comply with international standards, are half brick and full brick packages widely used in the industry, are fully compatible with VICOR, LAMBDA and other famous brands, and the power range of half brick products covers 50~200W, and that of full brick products covers 100~300W.
3. Temperature range and derating
General manufacturers have several temperature range products for module power supply: commercial, industrial, military, etc. When selecting module power supply, you must consider the actual required operating temperature range, because the price varies greatly with different temperature grades, materials and manufacturing processes. Improper selection will also affect the use, so you have to consider carefully. There are two options: one is based on the power used and the packaging form. If the actual power used is close to the rated power under a certain volume (packaging form), then the nominal temperature range of the module must strictly meet the actual needs or even have a slight margin. Second, it is selected according to the temperature range. What if a product with a smaller temperature range is selected due to cost considerations, but sometimes the temperature approaches the limit? Derate use. That is to say, select products with higher power or package. In this way, the temperature rise will be lower, which can alleviate this contradiction to a certain extent. The derating ratio varies with the power level, generally 3~10W/℃ above 50W. In short, either choose products with a wide temperature range, with more full power utilization and smaller package, but the price is higher; Or choose products with general temperature range. If the price is lower, the power margin and packaging form must be larger. A compromise should be considered.
4. Operating frequency
Generally speaking, the higher the operating frequency is, the smaller the output ripple noise will be, and the better the dynamic response of the power supply will be. However, the higher the requirements for components, especially magnetic materials, the higher the cost will be. Therefore, the switching frequency of domestic modular power supply products is mostly below 300 kHz, or even about 100 kHz, which is difficult to meet the dynamic response requirements under variable load conditions, Therefore, high switching frequency products should be considered for applications with high requirements. On the other hand, when the switching frequency of the module power supply is close to the working frequency of the signal, it is easy to cause beat oscillation, which should also be considered when selecting. The switching frequency of Dinglixin module power supply can reach up to 500kHz, with excellent output characteristics.
5. Isolation voltage
Generally, the isolation voltage of module power supply is not very high, but the higher isolation voltage can ensure that the module power supply has less leakage current, higher safety and reliability, and the EMC characteristics are better. Therefore, the isolation voltage level prevailing in the industry is above 1500VDC.
6. Fault protection function
Relevant statistical data show that the main reason for the failure of the module power supply within the expected effective time is the damage under external fault conditions. The probability of failure in normal use is very low. Therefore, an important link to prolong the service life of the module power supply and improve the system reliability is to select products with perfect protection functions, that is, when the external circuit of the module power supply fails, the module power supply can automatically enter the protection state without permanent failure, and should automatically recover to normal after the external failure disappears. The protection function of module power supply shall at least include input overvoltage, undervoltage and soft start protection; Output over-voltage, over-current, short circuit protection, high-power products should also have over temperature protection.
7. Power consumption and efficiency
According to the formula
, where Pin, Pout and P losses are the module power input, output power and self power losses respectively. It can be seen that under a certain output power, the smaller the module loss P, the higher the efficiency, the lower the temperature rise and the longer the service life. In addition to the full load normal loss, there are two losses worth noting: no-load loss and short circuit loss (module power loss in case of output short circuit), because the smaller the two losses, the higher the module efficiency. In particular, the short circuit may last for a long time if measures are not taken in time. The smaller the short circuit loss, the lower the probability of failure. Of course, the smaller the loss is, the better it meets the requirements of energy conservation.
Notes on application of module power supply
1. Extremely light load use
Generally, the module power supply has the minimum load limit, which varies from manufacturer to manufacturer, generally about 10%. Because when the load is too light, it is difficult for the energy storage element to continue the current, which will cause the output voltage to be unstable. This is determined by the working principle of the power supply itself. However, if the user does have light load or even no-load use, the most convenient and effective way is to add a certain false load, which is about 2% of the output power. It can be preset by the module manufacturer before leaving the factory, or the user can install an appropriate resistor outside the module as the load. It is worth noting that if the former is selected, the module efficiency will be reduced. However, some circuit topologies have no minimum load limit. For example, the E-series module power supply of Dinglixin Company can meet the normal use of users from no-load to full load
2. Multiple output power distribution
When selecting the power supply of multiple output modules, pay attention to the power distribution between different outputs. Take two-way products as an example, there are generally two types: one is two-way balanced load, that is, the two-way current is the same size; The other is unbalanced load, that is, the load currents of main and auxiliary circuits are different, the main circuit is large, and the auxiliary circuit is small. For this product, it is recommended to select a power ratio of 1/5~1/2 between the auxiliary circuit and the main circuit. Only within this range can the voltage stability of the auxiliary circuit be guaranteed (within 5%), otherwise the voltage of the auxiliary circuit will be higher or lower. On the other hand, if the dual load is not the same, try not to choose the balanced load module power supply, because this power supply is designed specifically for symmetrical load, and if the load is unbalanced, the voltage accuracy of the auxiliary circuit is not high.
3. Try to reduce the temperature rise of module power supply
The working temperature of the components inside the module directly affects the life of the module power supply. The lower the component temperature, the longer the module life. Under certain working conditions, the loss of the module power supply is certain, but the temperature rise can be reduced by improving the heat dissipation conditions of the module power supply, thus greatly extending its service life. For example, the module power supply above 50W must be equipped with a radiator. The larger the surface area of the radiator is, the more conducive to heat dissipation. The installation direction of the radiator should be as conducive to the natural convection of air as possible. In addition to the radiator, forced air cooling can also be installed when the power is above 150W. In addition, in places with high ambient temperature or poor air circulation conditions, the module must be derated to reduce power consumption so as to reduce temperature rise and prolong service life.
4. Reasonably install to reduce mechanical stress
The module power supply is led out by metal pin. The module power supply is connected to the external circuit, and the metal pin is connected to the internal circuit of the module power supply by welding. In some special occasions, the mechanical vibration intensity is relatively high, especially when a radiator is added to the power supply of a high-power module, which is more serious. Although the heat conductive insulating rubber generally filled in the module power supply can play a good role in cushioning and protecting the components, the welding point may break due to the failure of the module power supply due to the failure of withstanding strong vibration stress. In this case, additional fixing and buffering measures must be taken on the basis of welding. For example, clamps or bolts (for modules with screw holes) can be used to fix the module with chassis, large circuit boards and other components with relatively good vibration resistance, And some elastic materials are placed between them to cushion the stress generated by vibration.
In a word, just like other components, the module power supply can be used more widely only if its performance is maximized and its reliability is fully guaranteed through careful selection and reasonable application!
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