Characteristics of Switching Power Adapter Transient Loads

　　In the process of working and running, the switching power adapter has the advantages of reliable operation and easy portability, which is also closely related to its integrated system and miniaturized package selection. As a widely used power supply conversion device, the switching power adapter also has a load capacity for transient currents. Let's explore the actual characteristics of this transient load in detail.

　　The transient load of the switching power adapter is also called di/dt dynamic load by engineers in daily work. This remarkable feature is that the load current often changes transiently, and the rate of change of the load current is very large. Taking an example to make a specific analysis, the current change rate of high-speed logic circuits and RF microwave transmitters may exceed 100A/ps during operation. The low-voltage microprocessors currently produced will also affect the switching power supply when rapidly switching between different operating modes, and the output current of the switching power supply can change by several orders of magnitude within a few nanoseconds.

　　In addition to the examples listed above, many current computer switching power adapters can also handle transient load problems. The voltage of many notebook computers in our country is +3.3V, and the switching power supply is required to respond to the load current jump of 30A/us when the data is retrieved from the database. Assume that the time it takes for the load current to change from zero to 5A is 1us. If the bandwidth of the switching power supply is 25kHz, the time required to complete the above changes is 1/25 kHz=40us. Assuming that the current rises linearly, the amount of missing charge is (5A/2) X40us=100uC. If the +3.3 voltage fluctuation is allowed to fluctuate by 50mV, and the instantaneous energy is provided by the output filter capacitor, a capacitance of 100 uC/50mV=2000uF is required to prevent the voltage from falling below the specified value.

　　However, when engineers take advantage of the transient load characteristics of switching power adapters for voltage processing, it is important to note that one can not expect to achieve the above-mentioned goals by using only one nominal capacitor. Engineers should choose to use several small-capacity capacitors in parallel to form 2200uF in combination with the actual situation, and then perform capacity calculation and transient current processing. Because the total output filter capacitor equivalent series resistance R’esr=50mV/5A=10mΩ, the equivalent series resistance of each small-capacity capacitor should be R’esr=10mΩ/n. When n=4, R'esr=40mΩ. This greatly reduces the requirements for output filter capacitors.

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