Key points for selecting DC/DC module power supply

DC/DC 模块电源以其体积小巧、性能卓异、使用方便的显著特点，在通信、网络、工控、铁路、军事等领域日益得到广泛的应用。很多系统设计人员已经意识到：正确合理地选用DC/DC模块电源，可以省却电源设计、调试方面的麻烦，将主要精力集中在自己专业的领域，这样不仅可以提高整体系统的可靠性和设计水平，而且更重要的是缩短了整个产品的研发周期，为在激烈的市场竞争中领先致胜赢得了宝贵商机。那么，怎样正确合理地选用DC/DC模块电源呢，笔者将从DC/DC模块电源开发设计的角度，结合近年来鼎立信公司模块电源推广使用过程中得到的用户信息反馈，谈一谈这方面的问题，以供广大系统设计人员参考。

Selection of DC/DC module power supply

In addition to the most basic voltage conversion function, there are several other aspects to consider when choosing a DC/DC module power supply:

1. Rated power

It is generally recommended that the actual power used is 30-80% of the rated power of the module power supply (the specific ratio 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 full and stable and reliable. Too light a load will cause a waste of resources, and too heavy a 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 measure.

2. Packaging

There are various packaging forms for modular power supplies, some of which meet international standards, and some are non-standard. For the same company's products, the same power products have different packaging, and the same packaging has different powers. So how to choose the packaging form? There are three main aspects: ① The volume should be as small as possible under certain power conditions, so that more space and more functions can be given to other parts of the system; ② Try to choose products that meet international standard packaging, because they have good compatibility and are not limited to one or two suppliers; ③ They should be scalable to facilitate system expansion and upgrading. Choose a package. The system has higher requirements for power supply power due to functional upgrades, but the power module package remains unchanged, and the system circuit board design does not need to be changed, which greatly simplifies product upgrades and saves time. Take Dinglixin's high-power modular power supply products as an example: all meet international standards, and are widely used in the industry in 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, and the power range of full-brick products covers 100~300W.

3. Temperature range and derating

Generally, there are several temperature ranges for the power modules of manufacturers: commercial grade, industrial grade, military grade, etc. When choosing a power module, you must consider the actual required operating temperature range, because the prices of different materials and manufacturing processes vary greatly with different temperature grades. Improper selection will also affect the use, so you have to consider it carefully. There are two ways to choose: one is to choose according to the power used and the packaging form. If the actual power used is close to the rated power under the condition of 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. The second is to choose according to the temperature range. If a product with a smaller temperature range is selected due to cost considerations, but sometimes the temperature is close to the limit, what should I do? Use it at a reduced rating. That is, choose a product with a larger power or package, so that the "big horse pulls a small cart" and the temperature rise is lower, which can alleviate this contradiction to a certain extent. The derating ratio varies with the power level, generally 3 to 10W/℃ for more than 50W. In short, you can either choose a product with a wide temperature range, which has more power utilization and a smaller package, but a higher price; or choose a product with a general temperature range, which has a lower price, a larger power margin and a larger package. You should consider a compromise.

4. Operating 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 even only around 100kHz. This makes it difficult to meet the requirements of dynamic response under load variation conditions. Therefore, high-switching frequency products should be considered for applications in high-demand situations. On the other hand, when the switching frequency of the module power supply is close to the signal operating frequency, it is easy to cause beat oscillation, and this should also be considered when selecting. Dinglixin module power supply switching frequency can reach up to 500kHz, with excellent output characteristics.

5. Isolation voltage

The isolation voltage requirement for the module power supply is not very high in general occasions, but a higher isolation voltage can ensure that the module power supply has a smaller leakage current, higher safety and reliability, and better EMC characteristics. Therefore, the current general isolation voltage level in the industry is above 1500VDC.

6. Fault protection function

Relevant statistics show that the main reason for the failure of the module power supply within the expected effective time is damage under external fault conditions. The probability of failure under normal use is very low. Therefore, an important part of extending the life of the module power supply and improving the reliability of the system is to choose 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 it should be able to automatically return to normal after the external fault disappears. The protection function of the module power supply should at least include input overvoltage, undervoltage, soft start protection; output overvoltage, overcurrent, short circuit protection, and high-power products should also have overtemperature protection.

7. Power consumption and efficiency

According to the formula

Among them, Pin, Pout, and P consumption are the module power input, output power, and self-power loss respectively. It can be seen from this that under certain conditions of output power, the smaller the module loss P consumption, the higher the efficiency, the lower the temperature rise, and the longer the life. In addition to normal loss at full load, there are two losses worth noting: no-load loss and short-circuit loss (module power loss when the output is short-circuited), because the smaller these two losses are, the higher the module efficiency is, especially when the short circuit is not promptly taken, it may last for a long time, and the smaller the short-circuit loss is, the greater the probability of failure. Of course, the smaller the loss is, the more in line with the requirements of energy saving.

Module power application considerations

1. Extremely light load use

Generally, module power supplies have a minimum load limit, which varies from manufacturer to manufacturer, but is generally around 10%. This is because when the load is too light, the energy storage element will have difficulty in continuing the current, resulting in current discontinuity, which will lead to unstable output voltage. This is determined by the working principle of the power supply itself. But what if the user does have a light load or even no-load situation? The most convenient and effective method is to add a certain amount of dummy 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 a 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. For example, Dinglixin's E series module power supply can meet the user's normal use from no-load to full-load.

2. Multi-channel output power distribution

When choosing a multi-channel output module power supply, pay attention to the power distribution between different output channels. Taking dual-channel products as an example, there are generally two types: one is a dual-channel balanced load, that is, the dual-channel current is the same; the other is an unbalanced load, that is, the load current of the main and auxiliary channels is different, the main channel is large, and the auxiliary channel is small. For this type of product, it is recommended to choose a ratio of 1/5 to 1/2 between the power of the auxiliary channel and the main channel. Within this range, the voltage stability of the auxiliary channel is guaranteed (within 5%), otherwise the voltage of the auxiliary channel will be too high or too low. On the other hand, if the dual-channel loads are not the same, try not to choose a balanced load module power supply, because this type of power supply is specially designed for symmetrical loads. If the load is unbalanced, the voltage accuracy of the auxiliary channel is not high.

3. Try to reduce the temperature rise of the module power supply

The operating temperature of the internal components of 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, thereby greatly extending its service life. For example: module power supplies above 50W must be installed with a radiator. The larger the surface area of ​​the radiator, the more conducive to heat dissipation, and the installation direction of the radiator should be as conducive to natural convection of air as possible. In addition to installing a radiator, fans can also be installed for forced air cooling when the power is above 150W. In addition, in places where the ambient temperature is high or the air circulation conditions are poor, the module must be used at a reduced rating to reduce power consumption, thereby reducing temperature rise and extending service life.

4. Reasonable installation reduces mechanical stress

模块电源的引出方式均为金属针，模块电源与外接线路、金属针与模块电源内路电路均采用焊接方式连接。在一些特殊场合机械振动强度较大，尢其是大功率模块电源上还要加装散热器，这种情况更为严重。虽然模块电源内部一般灌封导热绝缘橡胶可以对元件起到较好的缓冲保护作用，但焊点有可能经受不住强烈振动应力而断裂，导致模块电源工作失效，这时必须在焊接的基础上再采取另外的固定和缓冲措施，比如可以用夹具或螺栓(对于有螺孔模块)将模块与机箱、大线路板等相对抗振性能好的部件固定，并且在它们中间垫一些弹性材料以缓冲振动产生的应力。

In short, module power supplies, like other components, can only maximize their performance and fully guarantee their reliability if they are carefully selected and reasonably applied, and module power supplies will be more widely adopted!