I. Preface
DC-DC power supply modules are increasingly used in industries such as communications, industrial automation, power control, rail transportation, mining, and military. Their modular design effectively simplifies circuit design for customers, improving system reliability and maintenance efficiency. So, how can we improve the reliability of DC-DC module-based power systems? Most of the time, we think of choosing a reputable supplier to provide highly reliable power modules. However, does choosing a highly reliable power module necessarily mean that our power system is highly reliable? This article provides a brief analysis and discussion on this topic.
Keywords: power supply application design, DC-DC module power supply, reliability
II. Why is a DC-DC module power supply needed?
DC-DC isolation power supplies are primarily used in distributed power systems to provide isolation, noise reduction, voltage conversion, voltage regulation, and protection. The four main functions of DC-DC isolation power supplies are as follows:
Firstly, the modular power supply adopts an isolated design, which can effectively isolate the impact of common-mode interference from primary-side equipment on the system, enabling the load to work stably.
Secondly, different loads require different supply voltages. For example, control ICs require 5V, 3.3V, 1.8V, etc.; operational amplifiers used for signal acquisition require ±15V; and relays require 12V and 24V. However, the bus voltage is mostly 24V, so voltage conversion is necessary.
Third, the bus voltage will have line loss during long-distance transmission, so the voltage is low when it reaches the PCB board level. However, the load requires a stable voltage, so a wide input voltage and a regulated output voltage are required.
Fourth, the power supply needs to protect the system load and itself from damage in abnormal situations.
So, how do you choose a DC-DC module power supply?
III. How to Select a High-Reliability DC-DC Module Power Supply
1. Adopting a mature power supply topology
Power module design should prioritize mature power topologies that have stood the test of time and are reliable. For example, 1-2W constant input DC-DC power modules typically use Royer circuits, while wide input series often opt for Flyback topologies, with some using Forward topologies.
2. High efficiency across the entire load range
High efficiency means lower power loss and lower temperature rise, which can effectively improve reliability. In practical applications, power supplies are typically designed with a certain degree of derating, especially today when the power consumption of load ICs is getting lower and lower, meaning power supplies are likely to operate under light load conditions most of the time. Therefore, high efficiency across the entire load range is a critical parameter for the reliability of power supply systems, but it is often overlooked by power supply manufacturers. Most manufacturers, in order to attract customers with the parameters in their technical manuals, tend to achieve high full-load efficiency, but their efficiency is lower under 5%-50% load conditions.
Taking Mornsun's 15W DC-DC module power supply VRB2412LD-15WR2 as an example, the VRB2412LD-15WR2 has an efficiency of 15% higher than the mainstream peer level when the rated voltage is 24V input and the light load is 10%, as shown in Figure 1 and Figure 2.
Improving efficiency can also effectively reduce the temperature rise of the product's casing. The VRB2412LD-15WR2 has a temperature rise that is 13.8 degrees lower when operating under actual load.
3. Limiting temperature characteristics
Power supply modules are used in a wide geographical area, ranging from tropical heat to frigid conditions like a Russian winter. Therefore, the minimum operating temperature range for DC-DC modules is -40°C to 85°C, with some offering even better performance. For example, Mornsun's R2 generation 1-2W constant voltage module can operate from -40°C to 105°C. For automotive BMS and high-voltage bus monitoring applications, an operating temperature range of -40°C to 125°C is required. Currently, only Mornsun's CF0505XT-1WR2 DC-DC module in the industry can achieve an operating temperature of 125°C.
Extreme temperature testing is the most effective way to verify the reliability of power modules. Examples include high-temperature aging, high and low temperature live-line performance testing, high and low temperature cycling shock testing, and long-term high-temperature and high-humidity testing. Legitimate power supply manufacturers all undergo these tests. Therefore, the availability of such testing equipment has become a basis for determining whether a power supply manufacturer is a counterfeit or substandard manufacturer.
4. High isolation and low isolation capacitors
Medical products require extremely low leakage current, while power electronic products need minimal parasitic capacitance between the primary and secondary sides. Both industries share a common requirement: the highest possible isolation voltage and the lowest possible isolation capacitance to reduce the impact of common-mode interference on the system. For medical or power electronic applications, it is recommended to select power modules with isolation capacitance below approximately 10pF for 1-2W DC-DC converters, and below 150pF for wide-voltage products.
5. EMC characteristics
EMC performance is the guarantee for the normal and safe operation of electronic systems. Currently, the electronics industry has put forward very high requirements for the EMC performance of products. We often encounter customer complaints that poor EMC handling leads to system resets, restarts, or even premature failures. Therefore, excellent EMC characteristics are the core competitiveness of power modules.
IV. Reliability of Power System Application Design
While the reliability of the power supply itself is crucial, in reality, due to the complexity of the power system's operating environment, even the most reliable power supply will eventually fail without a reliable system application design. Below are some common methods and considerations for power system application design.
1. Redundancy Design Techniques
In applications with high reliability requirements, the system must not lose power even if a power module fails. In such cases, redundant power supplies can be used to improve system reliability. As shown in the figure below, when one power module fails, another module can continue to provide power. Figure 3 illustrates one common redundancy design scheme.
Precautions: It is recommended to use low-dropout Schottky diodes for D1 and D2 to avoid the diode voltage drop affecting the operation of the downstream system. Also, ensure that the diode's voltage rating is higher than the output voltage. This method will generate additional ripple noise, requiring an external capacitor to reduce the ripple or a filter circuit.
2. Reduced Ratio Design
As is well known, derating design can effectively improve the lifespan of a power supply, but if used under too light a load, the power supply's performance will not be optimal. For example, Mornsun DC-DC modular power supplies are recommended to be used within a load range of 30% to 80%, at which point their performance is at its best.
3. Reasonable external protection design
Power modules are used in a wide variety of industries, and the environmental requirements vary greatly. Due to their general design, DC-DC power modules can only meet common general needs. Therefore, when customers face demanding application environments, appropriate external circuitry needs 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 standalone module can only pass the EN50155 1.4 times input voltage Vin 1S test, but due to its size, it cannot pass the RIA12 standard. By adding external circuitry (or choosing Mornsun's EMC auxiliary circuitry FC-AX3D), it can pass the RIA12 requirement of 3.5Vin/20ms. Therefore, a reasonable external circuit design can enable the module to meet higher technical specifications, adapt to harsher application environments, and improve the reliability of the power supply module.
4. Heat dissipation design
Approximately 15% of industrial-grade power module failures are due to poor heat dissipation. While power modules are trending towards miniaturization and integration, many applications involve power supplies operating continuously in enclosed environments. If heat cannot dissipate, internal components may fail due to exceeding thermal stress. Common heat dissipation methods include natural air cooling, heat sinks, and reinforced cooling fans. Here are some key thermal design tips:
(1) Convection ventilation of the power module
For power modules that rely on natural convection and thermal radiation for heat dissipation, the surrounding environment must facilitate convection and ventilation, and there should be no large components obstructing the flow of air.
(2) Placement of heating element
If the system has multiple heat sources, such as multiple power modules, they should be kept as far apart as possible to avoid heat radiation transfer between them, which could cause the power modules to overheat.
(3) Reasonable PCB board design
PCB boards provide a heat dissipation pathway, so heat dissipation pathways should be considered during the design phase. For example, increasing the copper area of the main circuit and reducing the density of components on the PCB board can improve the heat dissipation area and channels of the module. For instance, the power module should be placed vertically as much as possible, as shown in Figure 4, so that heat can be dissipated upwards as quickly as possible. If the DC-DC module is placed at the bottom of the PCB, the upward dissipation of heat will be blocked by the PCB, causing the product to accumulate heat and not be able to dissipate.
(4) Larger package size and heat dissipation area: For power supplies of the same power, if possible, choose a larger package size and a heat sink with a larger heat dissipation area, or use thermally conductive adhesive to connect the power module casing to the chassis. This gives the power module a larger heat dissipation area, resulting in faster heat dissipation, lower internal temperature, and naturally higher power supply reliability.
5. Compatibility design and safety design
The power supply input traces should be kept as straight as possible to avoid creating loop antennas that attract external radiated interference. Simultaneously, input and output lines must maintain appropriate spacing according to UL60950 safety regulations to prevent withstand voltage failure. Furthermore, wiring under the power supply chassis is prohibited, especially signal lines, as the electromagnetic wires of the power transformer can interfere with signals.
Another point that designers need to pay attention to is ensuring that the primary and secondary power supplies, as well as the frequency multiplication factor between the power supply and the system operating frequency, are not misaligned to avoid system compatibility issues.
V. Summary
The reliable application of DC-DC power modules requires high-quality power supplies from the original power supply manufacturer, as well as reasonable application design by design engineers. Only by considering both design and application can a reliable power system be obtained.
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