DC_DC module power supplies are increasingly used in communications, industrial automation, power control, rail transit, mining, military and other industries. The modular design can effectively simplify the customer’s circuit design and improve the reliability and maintenance efficiency of the system. So, how to improve the reliability of the power system based on DC_DC module? Most of the time, what we think of is to choose a supplier with a good brand to provide highly reliable power modules. However, does choosing a highly reliable power module mean that our power system is very reliable? This article briefly analyzes and discusses this topic.

1. Preface

DC_DC module power supplies are increasingly used in communications, industrial automation, power control, rail transit, mining, military and other industries. The modular design can effectively simplify the customer’s circuit design and improve the reliability and maintenance efficiency of the system. So, how to improve the reliability of the power system based on DC_DC module? Most of the time, what we think of is to choose a supplier with a good brand to provide highly reliable power modules. However, does choosing a highly reliable power module mean that our power system is very reliable? This article briefly analyzes and discusses this topic.

2. Why do you need DC_DC module power supply?

The DC-DC isolation module power supply is mainly used in distributed power supply systems to achieve isolation, noise reduction, voltage conversion, voltage regulation and protection functions for the power supply system. The four functions of using the DC-DC isolation module power supply are as follows:

First, the module power supply adopts an isolated design, which can effectively isolate the influence of the common mode interference from the primary side equipment on the system, so that the load can work stably.

Second, different loads require different power supply voltages, for example, the control IC requires 5V, 3.3V, 1.8V, etc.; the op amp for signal acquisition requires ±15V; the relay requires 12V, 24V. The bus voltage is mostly 24V, so voltage conversion is required.

Third, the bus voltage will have line loss during long-distance transmission, so the voltage is low when it reaches the PCB board level, and the load needs a stable voltage, so it needs wide-voltage input and regulated output.

Fourth, the power supply needs to protect the load of the system and itself is not bad under abnormal conditions.

So, how to choose a DC-DC module power supply?

3. How to choose a high-reliability DC-DC module power supply

1. Adopt mature power topology

The design of the power module should use mature power topologies as far as possible. These topologies have passed the test of time and are mature and reliable. For example, the 1-2W constant-voltage input DC-DC power supply module selects the Royer circuit, while the wide-voltage input series selects the Flyback topology and some Forward topology.

2. High efficiency in full load range

High efficiency means lower power loss and lower temperature rise, which can effectively improve reliability. In practical applications, the power supply will choose a certain degree of derating design, especially when the power consumption of the load IC is getting lower and lower today, the power supply is likely to work under light load conditions most of the time. Therefore, high efficiency in the full load range is a very critical parameter for power system reliability, but is often ignored by power supply manufacturers. In order to attract customers with the parameters in the technical manual, most manufacturers tend to achieve higher full-load efficiency, but the efficiency is lower at 5%-50% load.

Taking Jinshengyang’s 15W DC-DC module power supply VRB2412LD-15WR2 as an example, the efficiency of VRB2412LD-15WR2 at a rated voltage of 24V input at 10% light load is 15% higher than that of mainstream counterparts, as shown in Figure 1 and Figure 2.

The application design and quality of the power module are equally important

Through the improvement of efficiency, the temperature rise of the case of the product can also be effectively reduced. The temperature rise of VRB2412LD-15WR2 is 13.8 degrees lower when working under actual load.

3. Extreme temperature characteristics

The geographical area of ​​application of power modules is very broad, and there may be tropical heat and severe cold similar to Russian winter. Therefore, the minimum operating temperature range of the DC-DC module is required to be -40 degrees to 85 degrees, and there are also better ones. For example, the working temperature of Jinshengyang’s constant pressure R2 generation 1-2W can reach -40 degrees to 105 degrees. If it is used in automotive BMS and high-voltage bus monitoring applications, the operating temperature needs to be -40 degrees to 125 degrees. At present, only Jinshengyang’s CF0505XT-1WR2 product can achieve a working temperature of 125 degrees for DC-DC modules in the industry.

Extreme temperature test is the best way to verify the reliability of power modules, such as high temperature aging, high temperature & low temperature live performance test, high and low temperature cycle impact test, and long time high temperature and high humidity test. Formal power supply development will go through the above tests. Therefore, the existence of such test equipment has also become the basis for judging whether a power supply manufacturer is a copycat manufacturer.

4. High isolation, low isolation capacitance

Medical products require extremely low leakage current, and power Electronic products require as little parasitic capacitance between the primary and secondary as possible. These two industries have a common requirement, that is, the highest possible isolation withstand voltage and the lowest possible isolation capacitance to reduce the impact of common mode interference on the system. If it is used in medical or power electronics applications, it is recommended to select a power module with an isolation capacitance of less than 10pF for 1-2W DC_DC, and a power module with an isolation capacitance of less than 150pF as far as possible for wide-voltage products.

5. EMC characteristics

EMC performance is the guarantee for the normal and safe operation of the electronic system. At present, the electronic industry has put forward high requirements for the EMC performance of products. We often encounter customers complaining that the system is reset and restarted or even fails early due to poor EMC handling. Therefore, Excellent EMC characteristics are the core competitiveness of power modules.

Fourth, the reliability of power system application design

The reliability of the power supply itself is important, but in fact, due to the complexity of the working environment of the power supply system, if the most reliable power supply does not have a reliable system application design, the power supply will eventually fail. The following introduces several common power system application design methods and precautions.

1. Redundant Design Tips

In occasions with high reliability requirements, it is required that even if the power module is damaged, the system cannot be powered off. At this point, we can adopt redundant power supply to improve the reliability of the system. As shown in the figure below, when one power module is damaged, the other module can continue to supply power. Figure 3 shows one of the common redundant design schemes.

The application design and quality of the power module are equally important

Matters needing attention: D1 and D2 are recommended to use Schottky diodes with low voltage drop, so as to avoid the voltage drop of the diodes from affecting the work of the back-end system, and pay attention to selecting the withstand voltage value of the diodes to be higher than the output voltage. This method will generate additional ripple noise, and an external capacitor is required to reduce the ripple or add a filter circuit.

2. Derating design

As we all know, the derating design can effectively improve the working life of the power supply, but when the load is too light, the performance of the power supply cannot work in the best state. For example, Jinshengyang DC_DC module power supply is recommended to be used within 30%~80% of the load range, and the performance in all aspects is the best at this time.

3. Reasonable peripheral protection design

There are many application industries for power modules, and the application environment requirements are not the same. Because of its universal design, the DC-DC module power supply can only meet the general requirements. Therefore, when the customer’s application environment is demanding, it is necessary to add appropriate peripheral circuits to improve the reliability of the power supply.

Taking Jinshengyang’s 20W DC-DC railway power supply URB24XXLD-20WR2 as an example, a single module can only pass the 1S test of EN50155 1.4 times the input voltage Vin, but because of the size, there is no way to pass the RIA12 standard, by adding peripheral circuits (you can also choose Jinshengyang EMC auxiliary device FC-AX3D) can pass the test requirements of 3.5Vin/20mS required by RIA12. Therefore, a reasonable peripheral circuit design can make the module meet higher-level technical specifications, adapt it to a harsher application environment, and improve the reliability of the power module.

4. Thermal Design

About 15% of the damage of industrial-grade power modules is caused by poor heat dissipation. Power modules are developing towards miniaturization and integration. However, in many applications, the power supply works continuously in a closed environment. Dissipated, the devices inside the power supply may be damaged due to excessive thermal stress. The usual cooling methods include natural air cooling, heat sink cooling, and forced cooling fans. Some experiences of thermal design are shared as follows:

(1) Convection ventilation of the power module

For power modules that rely on natural convection and thermal radiation to dissipate heat, the surrounding environment must be convenient for convection ventilation, and there should be no large devices around to block, so as to facilitate air circulation.

(2) Placement of heat-generating devices

If there are multiple heat sources in the system, such as multiple power modules, they should be kept away from each other as far as possible to avoid the heat radiation transfer between them and the overheating of the power modules.

(3) Reasonable PCB board design

The PCB board provides a heat dissipation path, and the heat dissipation path should be considered more in the design. For example, increase the copper area of ​​the main circuit, reduce the density of components on the PCB board, etc., and improve the heat dissipation area and heat dissipation channel of the module. For example, the power module should be placed as vertically as possible as shown in Figure 4, so that the heat can be dissipated upwards as soon as possible; if the DC – If the DC module is placed at the bottom of the PCB, the heat dissipated upward will be blocked by the PCB, so that the accumulated heat of the product cannot be dissipated.

(4) Larger package size and heat dissipation area

For a power supply of the same power, if possible, choose a larger package and a heat sink with a larger heat dissipation surface, or use thermal adhesive to connect the power module housing to the chassis. In this way, the power module has a larger heat dissipation area, the heat dissipation will be faster, the internal temperature will be lower, and the reliability of the power supply will naturally be higher.

5. Matching design, safety design

The application design and quality of the power module are equally important

The input trace of the power supply should be kept as straight as possible to avoid the formation of a loop antenna to attract external radiation interference. At the same time, the input line and output line need to maintain a suitable distance according to the safety requirements of UL60950 to avoid failure of withstand voltage. Furthermore, wiring under the power supply base plate is prohibited, especially the signal line. The electromagnetic wire of the power transformer will interfere with the signal.

Another thing that designers should pay attention to is that they need to pay attention to the staggered frequency multiplication between the primary power supply and the secondary power supply, as well as the operating frequency of the power supply and the system, so as to avoid the problem of system matching between them.

V. Summary

The reliable application of the DC_DC power module requires the original power supply to provide high-quality power, and also requires the design engineer’s reasonable application design. Only from the bidirectional consideration of design and application can a reliable power system be finally obtained.

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