For engineers, the selection of switching power supply is a step that needs to be completed every time they design power supply. On the surface, it is a single multiple-choice question, but in fact, before the final selection, engineers need to consider many factors. Of course, what we think of at the first time will be the cost. Today, this article wants to explain some internal factors that we need to pay attention to in order to select a more suitable power module in addition to the cost in the process of switching power supply selection.
For the selection of switching power supply module, we need to pay attention to and consider many rules. For example, the nominal value of fuse 1A refers to the index at 25 degrees, but if the equipment works at 50 degrees, the nominal value of fuse can be lower than 1a, and the design margin at this temperature should be larger. Similarly, the 1mH of inductance is not always 1mH. It is 1kHz. If you use it at 1MHz, the value of 1mH inductance sent by the processor is not 1mH, because the distributed capacitance in the inductance coil begins to have a great impact at 1m, which will offset part of the effect of inductance. The insertion loss of the filter IL = 25dB is when MHz RS / RL = 50 ohms (source impedance and load impedance). In fact, it is difficult for our filter to meet this requirement, so the insertion loss of 25dB will be greatly reduced. Magnetic beads, capacitors, diodes, resistors... All have similar rules. Let's talk about the rules of switching power supply module selection except cost. There are many topologies of power modules, including flyback, forward, push-pull, half bridge and full bridge. Each has its advantages in some characteristic indexes because of its different principles.
Here we will explain the usage rules of several typical topologies. The first is the flyback power supply, which does not discharge during the charging period in a switching cycle. Because of this characteristic, its time response characteristics and ripple characteristics are difficult to do well. Although it can be solved with the help of a large energy storage capacitor, the principle defect is hard injury after all, and the lack of intelligence can be compensated by diligence, But when we encounter critical problems, we can't cross a certain obstacle. But its advantages are simple circuit, low cost, small volume, no need to add magnetic reset winding, and wide input voltage range. Because of this, it has a share of more than 70% of the total power supply market.
Let's talk about the topology of other major switching power supplies in the power market. The output voltage of the forward power supply has good transient control characteristics and strong load capacity, but its disadvantages are also significant. It uses a large energy storage filter inductor and a freewheeling diode, has large volume, high back EMF voltage of the primary coil of the transformer, and has high requirements for the switch tube (easy to breakdown and damage). The push-pull power supply has high current transient response speed and good voltage output characteristics. In all topologies, it is a switching power supply with high utilization, no magnetic flux leakage and simple driving circuit. But its disadvantage is that two switching devices need high withstand voltage value; To have two groups of primary coils, push-pull switching power supply with low power output is a disadvantage. If the two forward converters are not completely symmetrical or balanced, the magnetic bias accumulated after several cycles will saturate the magnetic core, resulting in excessive excitation current of high-frequency transformer and even damage the switch tube. The output power of bridge switching power supply is very large, the working efficiency is very high, the withstand voltage value of switch tube is relatively low, and only one winding is required for the primary coil of transformer. The disadvantage is low efficiency, semi conduction region and large loss.
The above problems are caused by the inherent advantages and defects of its topology. Although we can regard the power module as a black box, this is also a point we should pay attention to when selecting the power supply. Because it can complete the design scheme with the same function, one can be completed easily, and the other can be completed with great effort.
In addition to the advantages and disadvantages of topology selection, we also need to determine according to the fluctuation of load. Some loads are stable, others fluctuate greatly, and even some will have no-load, full load, instantaneous load increase, or instantaneous load drop. If there is such a problem, it is better to explain with the power module manufacturer, Confirm that necessary protective design measures have been taken in its design, and not all power supplies can achieve this goal. Therefore, the type of load is also an influencing factor. For general modules, the output is designed as resistive load by default. If the load is inductive or capacitive load, it needs to be explained separately with the module manufacturer, so that the internal devices or parameters of the power module can be slightly adjusted when the manufacturer leaves the factory.
In addition to the above very important selection factors, we also need to consider the switching frequency, ripple, safety requirements, etc. during the selection of switching power supply module. The switching frequency of the power module also needs attention. It determines the selection of filtering parameters (cut-off frequency and order) of the external power filter. Ripple is related to topology, capacitance and inductance parameters and load conditions. For a 5V power supply, the ripple is 50mV, and the error of a single power supply is 1%. For circuits with high accuracy requirements, the error of power supply, amplification circuit, signal cable and ad rounding error. After multiple errors are accumulated and combined, the total error can be large. Whether there is filter design in the power module, whether there are safety requirements (leakage current, insulation withstand voltage, humidity requirements), temperature rise characteristics, conversion efficiency, fluctuation range of input voltage, load adjustment rate, etc. in the equipment where the power module is located, there are still many requirements. Therefore, we can understand that the selection of switching power supply module is not the only indicator of cost, which deserves attention.
