Power modules are favored by many engineers due to their high integration, reliability, and simplified design. However, even with the same power module, different usages can lead to vastly different system reliability. Improper usage not only fails to leverage the advantages of the module but may also reduce system reliability.
Designers often focus on the power parameters on the first page of the DC-DC isolation power module data sheet, such as power, input voltage, output voltage, efficiency, operating temperature, and voltage withstand level. However, in practical applications, the section on “Circuit Design and Applications” in the data sheet is equally important as it provides valuable reference circuit experience for users during the actual peripheral circuit design process. If the peripheral circuit design of the power module is improperly used, it not only fails to leverage the advantages of the module but may also reduce system reliability. This time, we will discuss some core points of the peripheral circuit design for power modules.
1. Two-stage Surge Protection Circuit: Improper Use Can Be Counterproductive
Power modules are compact, and in situations with high EMC requirements, additional surge protection circuits need to be added to enhance the system’s EMC performance. As shown in Figure 1, to improve the surge protection capability of the input stage, a varistor and TVS diode are added to the periphery. However, the circuits (a) and (b) in the figure aim to achieve two-stage protection but may be counterproductive. If the varistor MOV2 in (a) has a lower clamping voltage and current capacity than MOV1, in a strong interference scenario, MOV2 may fail to withstand the surge impact and damage prematurely, leading to a system failure. Similarly, in circuit (b), due to the faster response time of the TVS compared to MOV, often MOV does not act, and the TVS fails too early.
Figure 1: Two-stage Surge Protection
Adding an inductor forms a two-stage protection circuit. As shown in circuits (c) and (d), inserting an inductor separates the protective devices into two stages. For high-frequency surge pulses, the inductor has a large impedance, so the front-end varistor acts first, while the back-end varistor and TVS can further absorb the residual voltage to protect the module. Additionally, even for single-stage protection, adding an inductor can help avoid direct surge voltage at the module input.
2. Excessively Large Output Filter Capacitor Leading to Module Anomalies
It is usually recommended to add a certain amount of filter capacitance at the output of the power module, but during use, due to insufficient understanding, an excessively large output filter capacitor is used, which increases costs and reduces system stability.
Figure 2: Excessive Capacitive Load
As shown in circuit (a) of Figure 2, for a 3W module, a capacitor of 2000uF is used at the output, while the product manual indicates that the maximum output capacitance recommended for the module is 800uF. An excessively large output capacitor may lead to poor startup, and for micro-power DC-DC modules without short-circuit protection, an excessively large output capacitor may even permanently damage the module.
3. Connecting to Switching Power Supply Chip: Be Careful of Poor Startup
As shown in Figure 3, the output voltage of the power module builds up gradually. In circuit (a), the LM2576 does not have undervoltage lockout designed, and when the VIN voltage is low, it starts up. If the OUT load is too heavy, it may be misjudged as a short circuit or excessive capacitive load by the 24V module, leading to poor startup.
Figure 3: Adding Undervoltage Lockout
Therefore, it is recommended to use circuit (b), where a simple external undervoltage lockout is used, allowing the 24V module output voltage to reach the preset value before starting the external switching power supply chip like LM2576, which can greatly avoid poor startup issues.
Alternatively, a power module with a larger power margin can be used, and the ON/OFF pin can also be connected to an MCU for control.
4. Dual-Output Module: Pay Attention to Load Balancing
For dual-output modules, the load requirements for the two outputs differ. Such modules usually only perform voltage feedback on one output, while the other output reaches the required voltage through transformer coupling.
When the load on the regulated main output is too heavy and the auxiliary output is too light, the auxiliary output voltage may rise significantly. If the auxiliary output has strict voltage requirements, a three-terminal regulator should be added. Conversely, if the non-regulated auxiliary load is too heavy and the main output is too light, it may result in unstable output voltage or excessively low auxiliary output voltage, in which case a dummy load should be added to the main output.
Some modules from ZLG Zhiyuan Electronics have both main and auxiliary outputs regulated, such as the ZY0GD1212DI3-15W, which is a dual 12V regulated output product.
5. Parallel vs. Redundancy: Not the Same Thing
When there are two identical modules at hand, and the power of a single module is insufficient, it is natural to think of using two modules in parallel to meet the power requirements. However, paralleling ordinary power modules to increase power carries significant risks, as the module with a higher output voltage needs to provide excessive current, leading to module overloading.
Figure 4: Redundant Applications
As shown in circuit (a) above, if the load requires 5W of power, exceeding the load capacity of a single module, one of the modules may be overloaded. For this type of application, a single module with a power greater than 5W, such as ZY0JGB12P-10W, should be used. Circuit (b) is different; each module’s power can meet the load’s needs, and this is a redundant design.
6. Tantalum Capacitors: Use with Caution at Power Inputs and Outputs
Tantalum manganese capacitors are prone to breakdown and short circuits, with poor surge resistance. When powered on or when external power is connected, they can generate significant surge currents or voltages, leading to the burning out of tantalum capacitors due to short circuits or over-voltage breakdown. Unless strictly evaluated, it is recommended to use ceramic capacitors or electrolytic capacitors.
7. Choose High-Quality Isolation Power Modules for Efficient Circuit Design
ZLG Zhiyuan Electronics independently develops and produces isolation power modules with a wide input voltage range, isolation ratings of 1000VDC, 1500VDC, and 3000VDC across multiple series, and various packaging forms compatible with international standards such as SIP and DIP. They also provide tailored services based on project specifics to customize features and special packaging for customers. Additionally, ZLG Zhiyuan Electronics, with extensive power design and application experience, can provide professional reference for power peripheral application circuit design to enhance product reliability.
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