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

Why do we need a DC-DC module power supply?

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

First, the module power supply adopts an isolated design, which can effectively isolate the impact of common-mode interference from primary-side devices on the system, allowing the load to 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 used for signal acquisition requires ±15V; the relay requires 12V, 24V; and 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 lower when it reaches the PCB board level, while the load requires a stable voltage, so a wide voltage input and a stable voltage output are required.

Fourth, the power supply needs to protect the system load and itself from damage under abnormal circumstances.

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

Uses proven power topology

When designing power modules, try to use mature power topologies. For example, 1W~2W constant voltage input DC-DC power modules use Royer circuits, while wide voltage input series mostly use Flyback topology, and some use Forward topology.

High efficiency over the entire load range

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

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

The improvement in efficiency can also effectively reduce the temperature rise of the product casing. The temperature rise of VRB2412LD-15WR2 is 13.8℃ lower when working under actual load.

Extreme temperature characteristics

The geographical area of ​​power module application is very wide, and there may be tropical heat or severe cold similar to the Russian winter. Therefore, the minimum operating temperature range of the DC-DC module is required to be -40℃～85℃, and there are also better ones, such as Jinshengyang's constant voltage R2 generation 1W～2W operating temperature can reach -40℃～105℃. If it is used in automotive BMS and high-voltage bus monitoring, the operating temperature needs to be -40℃～125℃, and Jinshengyang's CF0505XT-1WR2 can reach 125℃.

Extreme temperature tests are the best way to test the reliability of power modules, such as high temperature aging, high temperature & low temperature live working performance tests, high and low temperature cycle shock tests, and long-term high temperature and high humidity tests. Formal power supply development will undergo the above tests. Therefore, whether there is such testing equipment has become the basis for judging whether the power supply manufacturer is a copycat manufacturer. High isolation, low isolation capacitor

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

EMC characteristics

EMC performance is the guarantee for the normal and safe operation of electronic systems. Currently, the electronics industry has very high requirements for the EMC performance of products. Customers often complain that the system resets and restarts or even fails prematurely due to poor EMC handling. Therefore, excellent EMC characteristics are the core competitiveness of power modules.

Reliability of power system application design

The reliability of the power supply itself is certainly important, but in fact, due to the complexity of the working environment of the power supply system, even the most reliable power supply will eventually fail if there is no reliable system application design. The following introduces several common power supply system application design methods and precautions.

Redundancy Design Tips

In situations where reliability is high, the system must not be powered off even if the power module is damaged. In this case, redundant power supply can be used to improve system reliability. Figure 3 shows a common redundant design. When one power module is damaged, the other module can continue to supply power.

In the figure, it is recommended to use low-voltage Schottky diodes for D1 and D2 to avoid the voltage drop of the diode affecting the operation of the back-end system. In addition, the withstand voltage of the diode should 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.

Derating design

As we all know, derating design can effectively increase the working life of the power supply, but if the load is too light, the performance of the power supply cannot work at the best state. For example, Jinshengyang DC-DC module power supply is recommended to be used within the load range of 30% to 80%, at which time all aspects of performance are best.

Reasonable peripheral protection design

Power modules are used in many industries and their application environment requirements are also different. Due to its universal design, DCDC module power can only meet general common requirements. Therefore, when the customer's application environment requirements are demanding, appropriate peripheral circuits need to be added to improve the reliability of the power supply.

Taking Mornsun'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 due to its size, it cannot pass the RIA12 standard. By adding peripheral circuits (you can also choose Mornsun's EMC auxiliary device FC-AX3D), it can pass the 3.5Vin/20ms test requirements of RIA12.

Therefore, reasonable peripheral circuit design can make the module meet higher-level technical specifications, adapt to harsher application environments, and improve the reliability of the power module.

Cooling by design

About 15% of industrial power module damage is caused by poor heat dissipation. Power modules are developing towards miniaturization and integration, but in many applications, the power supply is continuously working in a closed environment. If the accumulated heat cannot be dissipated, the components inside the power supply may be damaged due to excessive thermal stress. Common heat dissipation methods include natural air cooling, heat sink cooling, and enhanced cooling fans. Some experience in thermal design is shared as follows:

Convection ventilation of power modules. For power modules that rely on natural convection and heat radiation to dissipate heat, the surrounding environment must be convenient for convection ventilation, and there must be no large components blocking the air flow.

Placement of heat generating devices. If there are multiple heat sources in the system, such as multiple power modules, they should be placed as far away from each other as possible to avoid heat radiation transfer between them that may cause overheating of the power modules.

Reasonable PCB board design. PCB board provides a heat dissipation path, so it is necessary to consider the heat dissipation path more during design. For example, increase the copper area of ​​the main circuit, reduce the density of components on the PCB board, etc., improve the heat dissipation area and heat dissipation channel of the module. For example, the power module should be placed vertically as much as possible to allow the heat to dissipate upward as quickly as possible; if the DC-DC module is placed at the bottom of the PCB, the heat dissipated upward will be blocked by the PCB, resulting in the product's accumulated heat being unable to dissipate.

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

Matching design and safety design. The input wiring of the power supply should be kept straight as much as possible to avoid forming a loop antenna that attracts external radiation interference. At the same time, the input and output lines need to maintain a suitable spacing in accordance with the safety requirements of UL60950 to avoid voltage failure. Furthermore, wiring is prohibited under the power supply bottom plate, especially signal lines and electromagnetic wires of the power transformer, which will interfere with the signal.

Another thing that designers need to pay attention to is that they need to pay attention to the frequency mismatch between the primary power supply and the secondary power supply, as well as the power supply and system operating frequency, to avoid system matching issues between them.