Application circuit to improve power supply reliability

- Aug 28, 2018-

Power module is favored by many engineers due to its multiple advantages such as high integration, high reliability and simplified design. However, even if the same module is used, the reliability of the system will vary greatly due to different USES.Improper use may not only give play to the advantages of modules, but also reduce the system reliability. 

Application circuit to improve power supply reliability

1.Two - stage surge protective circuit

The power module is small in size. In the case of high EMC requirements, additional surge protection circuit is needed to improve the system EMC performance.As shown in figure 1, in order to improve the surge protection capacity of input level, the pressure sensitive resistor and TVS tube are added in the peripheral.However, the circuit (a) and (b) in the figure are intended to achieve two-level protection, but may be counterproductive.If (a) MOV2 has a lower pressure-sensitive voltage and flow capacity than MOV1, in the case of strong disturbance, MOV2 may not be able to withstand the surge impact and damage ahead of time, resulting in the system breakdown.Similarly, the circuit (b), because TVS respond faster than MOV, usually MOV does not work, and TVS is damaged prematurely. 

Application circuit to improve power supply reliability

Add an inductance to form a two - stage protective circuit.As shown in the circuit (c) and (d), an inductance is connected to separate the protective device into two stages. The inductance has a large impedance to high frequency surge pulse. Therefore, the piezoresistive resistance of the front end is the first to work, and the piezoresistive resistance of the back end and TVS can further absorb the residual voltage protection module.In addition, even with single-stage protection, increasing inductance can play a certain role in avoiding the surge voltage directly added to the module input terminal. 

2.The output filter capacitance is too large, which leads to abnormal module

The output end of the power module is usually recommended to add a certain amount of filter capacitance. However, in the process of use, due to lack of understanding, excessive output filter capacitance is used, which not only increases the cost but also reduces the stability of the system. 

Application circuit to improve power supply reliability

As shown in circuit (a) in figure 2, the output of a 3W module USES the capacity of 2000uF. However, according to the product manual, the maximum output capacity of the module is suggested to be 800uF.Too large output capacitance may lead to poor start up, while too large output capacitance may even cause permanent damage to micro-power dc-dc modules without short circuit protection. 

3.Connect the switch power chip, pay attention to bad start

As shown in figure 3, the output voltage of the power module is gradually established. The LM2576 of circuit (a) does not design undervoltage locking, which starts when the VIN voltage is low. 

Application circuit to improve power supply reliability

Therefore, it is recommended to use circuit (b) and external simple undervoltage lock, so that the 24V module output voltage is set to the preset value before starting the external switch power chip such as LM2576, which can largely avoid the problem of poor start. 

Or the power module with a larger power margin can be used, and the ON/OFF pin can be connected to the MCU for control. 

4.Dual path module, pay attention to load balancing

For the dual output module, the two outputs have different requirements on the load. Usually, such module can only provide the stabilized voltage feedback on one way, and the other way can achieve the required voltage through the transformer coupling. 

When the main road load is too heavy and the auxiliary road is too light, the auxiliary voltage will be higher. At this time, when the auxiliary road has strict requirements on voltage, the three-terminal voltage regulator shall be added.When the load of unsteady auxiliary road is too heavy and the main road is light, the output voltage may be unstable or the voltage of the auxiliary road is too low. At this point, it is necessary to add false load to the main road. 

Part of the modules of zhiyuan electronics are the main and auxiliary circuit stable voltage. For example, zy0gd1212di3-15w is the double-12v double-stabilized output product. 

5.Parallel is not the same thing as redundancy

When there are two identical modules in hand and the single one is short of power, it is natural to think of two modules being used in parallel to meet the power requirements. However, the method of using the ordinary power module in parallel to increase the power has great hidden risks. Modules with high output voltage need to provide excessive current, which leads to the over-power of the module. 

Application circuit to improve power supply reliability

As shown in circuit (a) above, the load requires 5W power, which exceeds the carrying capacity of a single module, so one of the modules may be overused.For such applications, a single module greater than 5W power is required, such as zy0jgb12p-10w.However, circuit (b) is different. The power of each module can meet the needs of load, which is a redundant design. 

6.Although tantalum capacitors are good, they should be carefully placed in the power input and output

Tantalum manganese dioxide capacitors are relatively easy to break short circuit and have poor resistance to surge. Large surge current or voltage may be formed when the capacitor is powered on or when external power supply is connected, resulting in burnt short circuit or over-voltage breakdown of tantalum capacitors. It is recommended to use ceramic capacitors or electrolytic capacitors without strict evaluation