Selection and Application of DC/DC Module Power Supply
DC/DC module power supply is increasingly widely used in communication, networking, industrial control, railway, military and other fields due to its compact size, outstanding performance and convenient use. Many system designers have realized that selecting DC/DC module power supplies correctly and reasonably can save trouble in power supply design and debugging, and focus their main energy on their professional fields. This not only improves the overall reliability and design level of the system, but more importantly, shortens the entire product development cycle, and wins valuable business opportunities for leading and winning in the fierce market competition. So, how to choose DC/DC module power supply correctly and reasonably? The author will discuss this issue from the perspective of DC/DC module power supply development and design, combined with user feedback obtained during the promotion and use of module power supply by Delus company in recent years, for the reference of system designers.
Selection of DC/DC module power supply
In addition to the basic voltage conversion function, there are several aspects to consider when choosing to use a DC/DC module power supply:
1. Rated power
It is generally recommended that the actual power used be 30-80% of the rated power of the module power supply (the specific proportion size 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 and stable and reliable. A load that is too light causes resource waste, while a load that is too heavy is detrimental to temperature rise, reliability, and other aspects. All module power supplies have a certain overload capacity, for example, Delus products can reach 120-150%, but it is still not recommended to work under overload conditions for a long time, as this is a short-term emergency plan.
2. Packaging form
There are various packaging forms for module power supplies, including those that comply with international standards and those that are non-standard. For the same company's products, products with the same power have different packaging, and the same packaging has different power. So, how to choose the packaging form? There are mainly three aspects:
① Under certain power conditions, the volume should be as small as possible, so as to give more space and functions to other parts of the system;
② Try to choose products that comply with international standard packaging, as they have good compatibility and are not limited to one or two suppliers;
③ It should have scalability to facilitate system expansion and upgrading. Choosing a packaging, the system's power module packaging remains unchanged due to the increased power requirements of functional upgrades. The system's circuit board design does not need to be modified, greatly simplifying product upgrades and updates, saving time.
Taking Delus company's high-power module power supply products as an example: all comply with international standards and are widely used in the industry for half brick and full brick packaging. They are fully compatible with famous brands such as VICOR and LAMBDA, and the power range of half brick products covers 50-200W, while full brick products cover 100-300W.
3. Temperature range and derating use
Generally, manufacturers have several temperature range products to choose from for their module power supplies: commercial grade, industrial grade, military grade, etc. When selecting a module power supply, it is important to consider the actual required working temperature range, as different temperature levels, materials, and manufacturing processes can result in significant price differences. Improper selection can also affect usage, so careful consideration is necessary. There are two options:
one is to choose based on the usage power and packaging form. If the actual usage power is close to the rated power under certain volume (packaging form) conditions, then the nominal temperature range of the module must strictly meet the actual needs or even have a slight margin.
The second is to choose based on the temperature range. If a product with a smaller temperature range is chosen due to cost considerations, but sometimes there are situations where the temperature approaches the limit, what should be done? Reduced usage. By choosing products with higher power or larger packaging, the "big horse pulling small car" can lower the temperature rise and alleviate this contradiction to a certain extent. The derating ratio varies with different power levels, generally ranging from 3 to 10W/℃ for power levels above 50W. In short, either choose products with a wide temperature range, more efficient power utilization, smaller packaging, but higher prices; Either choose products with a general temperature range, lower prices, larger power margins, and packaging forms. We should consider a compromise.
4. Working frequency
Generally speaking, the higher the operating frequency, the smaller the output ripple noise and the better the dynamic response of the power supply. However, the requirements for components, especially magnetic materials, are also higher, and the cost will increase. Therefore, the switching frequency of domestic module power products is mostly below 300kHz, and some are only around 100kHz. This makes it difficult to meet the requirements of dynamic response under load changing conditions. Therefore, in high demand applications, high switching frequency products should be considered.
On the other hand, when the frequency of the module power switch is close to the signal operating frequency, it is easy to cause beat oscillations, and this should also be considered when selecting. The power switch frequency of Delus's module can reach up to 500kHz, with excellent output characteristics.
5. Isolation voltage
In general, the isolation voltage requirement for module power supply is not very high, but a higher isolation voltage can ensure that the module power supply has smaller leakage current, higher safety and reliability, and better EMC characteristics. Therefore, the commonly used isolation voltage level in the industry is above 1500VDC.
6. Fault protection function
According to statistical data, the main reason for module power failure within the expected effective time is damage caused by external fault conditions. The probability of failure during normal use is very low. Therefore, an important part of extending the lifespan of module power supplies and improving system reliability is to choose products with complete protection functions. That is, when the external circuit of the module power supply fails, the module power supply can automatically enter a protective state without permanent failure. After the external fault disappears, it should be able to automatically restore normal operation. The protection function of the module power supply should at least include input overvoltage, undervoltage, and soft start protection; Output overvoltage, overcurrent, short circuit protection, high-power products should also have over temperature protection, etc.
7. Power consumption and efficiency
According to the formula, Pin, Pout, and P losses are respectively the input and output power of the module power supply and its own power loss. From this, it can be seen that under certain output power conditions, the smaller the module loss P, the higher the efficiency, the lower the temperature rise, and the longer the lifespan. In addition to normal losses at full load, there are two losses worth noting: no-load losses and short-circuit losses (module power losses during output short circuits), because the smaller these two losses, the higher the module efficiency, especially in the case of a short circuit that may last for a long time if measures are not taken in a timely manner. The smaller the short-circuit losses, the lower the probability of failure. Of course, the smaller the loss, the more in line with energy-saving requirements.
Precautions for module power supply application
1. Extremely light load usage
Generally, module power supplies have a minimum load limit, which varies from manufacturer to manufacturer and is generally around 10%. This is because when the load is too light, it is difficult for the energy storage components to continue flowing, resulting in discontinuous current and unstable output voltage, which is determined by the working principle of the power supply itself. But what if the user does use it lightly or even without load? The most convenient and effective method is to add a certain amount of false load, about 2% of the output power, which 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 chosen, the module efficiency will be reduced. However, some circuit topologies do not have a minimum load limit, such as the E-series module power supply from Dinglixin Company, which can meet the normal use of users from no-load to full load.
2. Multi channel output power allocation
When selecting a power supply for a multi-channel output module, attention should be paid to the power distribution between different output channels. Taking dual channel products as an example, there are generally two types: one is dual channel balanced load, that is, the current size of both channels is the same; Another type is unbalanced load, where the load currents of the main and auxiliary circuits are not the same, with the main circuit being larger and the auxiliary circuit being smaller. For this product, it is recommended to choose a power ratio of 1/5 to 1/2 between the auxiliary and main circuits. 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 too high or too low. On the other hand, if the dual loads are already different, it is advisable not to choose a balanced load module power supply, as this type of power supply is specifically designed for symmetrical loads. If the load is unbalanced, the voltage accuracy of the auxiliary circuit is not high.
3. Try to reduce the temperature rise of the module power supply
The working temperature of the internal components of the module directly affects the lifespan of the module power supply, and the lower the temperature of the components, the longer the lifespan of the module. Under certain working conditions, the loss of module power supply is certain, but improving the heat dissipation conditions of module power supply can reduce its temperature rise and greatly extend its service life. For example, module power supplies above 50W must be equipped with a heat sink. The larger the surface area of the heat sink, the more conducive it is to heat dissipation. The installation direction of the heat sink should be as favorable as possible for natural air convection. For power supplies above 150W, in addition to installing the heat sink, a fan for forced air cooling can also be installed. In addition, in areas with high ambient temperatures or poor air circulation conditions, the module must be downgraded to reduce power consumption, thereby lowering temperature rise and extending its service life.
4. Reasonably install to reduce mechanical stress
The lead out method of the module power supply is all metal pins, and the module power supply is connected to the external circuit and the metal pins are connected to the internal circuit of the module power supply by welding. In some special situations where mechanical vibration intensity is high, especially when a heat sink is added to a high-power module power supply, this situation is even more serious. Although thermal insulation rubber is generally encapsulated inside the module power supply to provide good buffering and protection for the components, the solder joints may not withstand strong vibration stress and break, resulting in the failure of the module power supply. In this case, additional fixing and buffering measures must be taken on the basis of welding, such as using fixtures or bolts (for modules with screw holes) to fix the module to components with relatively good anti vibration performance such as chassis and large circuit boards, and placing some elastic materials in between to cushion the stress generated by vibration.
In short, module power supplies, like other components, can only maximize their performance and ensure their reliability through careful selection and reasonable application. Only then can module power supplies be more widely adopted!