What factors need to be considered in the selection of switching power supply
For engineers, the selection of switching power supply is a process that needs to be completed every time they plan a power supply. It is a single-choice question on the surface, but before the final selection, engineers need to consider many factors. Of course, we thought of it at the first moment It will be a question of cost. What I want to explain in today's article is that in the process of switching power supply selection, in addition to the cost, we need to pay attention to some internal factors in order to choose the most suitable power module.
Regarding the selection of switching power supply modules, we need to pay attention and consider many rules. For example, the nominal value of insurance wire is 1A, which refers to the target at 25 degrees, but if the equipment works at 50 degrees, the nominal value of insurance wire may be lower than 1A, and the design margin at this temperature must be selected Bigger. In the same way, the 1mH of the inductance is not always 1mH, it is at 1kHz, if you use it at 1MHz, the value of the 1mH inductance sent by the processor is not 1mH, because at 1M the inductance coil Distributed capacitance initially plays a big role, which will offset some of the inductance. The insertion loss of the filter IL=25dB is when MHz Rs/RL=50 ohms (source impedance and load impedance), but in practice, it is difficult to achieve the impedance to meet this requirement in our filter application, so 25dB The insertion loss will be greatly reduced. Beads, capacitors, diodes, resistors... all have similar rules. Let's talk about the rules of switching power supply module selection other than cost. There are many topologies of power modules, such as flyback, forward, push-pull, half-bridge, and full-bridge, each of which is superior in certain characteristic indicators because of its different principles.
Let's talk about the topological structure of other important switching power supplies in the power supply market. The output voltage transient control characteristics of the forward power supply are better, and the load capacity is stronger, but its disadvantages are also obvious. A large energy storage filter inductor and a freewheeling diode are used, the volume is large, and the back electromotive voltage of the primary coil of the transformer is high. The requirements for the switching tube are high (easy to breakdown and damage). The push-pull power supply current transient response speed is very high, and the voltage output characteristics are excellent. In all topological structures, it is a switching power supply with the highest utilization rate, no magnetic flux leakage, and a simple driving circuit. But its disadvantage is that the two switching devices need a high withstand voltage value; there are two sets of primary coils, and the push-pull switching power supply with small power output is a disadvantage. If the two forward converters are not completely symmetrical or balanced, the accumulated bias magnetization after several cycles will make the magnetic core full, resulting in excessive excitation current of the high-frequency transformer, and even damage the switch tube. The output power of the bridge switching power supply is very large, the working power is very high, the withstand voltage value of the switch tube is relatively low, and the primary coil of the transformer only needs one winding. The disadvantage is that the power is low, there will be a semi-conducting region, and the loss is large.
The above problems are caused by the inherent advantages and disadvantages of its topological structure. Although we can regard the power module as a black box, this is also a point we should pay attention to when choosing a power supply. Because of the solutions that can realize the same function, one can be realized easily, and the other can be realized with great effort.
In addition to considering its advantages and disadvantages when selecting a topology structure, we also need to judge according to the instability of the load. Some loads are relatively stable, while some loads are more unstable, and some even have no-load, or Full load, or momentary load increase, or momentary load drop occurs, if there is such a problem, it is best to clarify with the power module manufacturer and admit that there are necessary protective measures in its plan, not all power supply can achieve this goal. The type of load is thus also an influencing factor. Ordinary modules, whose output is designed for resistive load by default, if the load is rational or capacitive load, it needs to be separately explained 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. when selecting the switching power supply module. The switching frequency of the power module also needs to be paid attention to. He chose the selection of the filter parameters (cut-off frequency, order) of the external power filter. The ripple is related to the topological structure, the parameters of the capacitance and inductance, and the state of the load. For a 5v power supply, the ripple can reach 50mv, and the error of a single power supply is 1%. For circuits that require high precision, the error of the power supply and the expansion circuit The error of the signal cable, the error of the signal cable, the rounding error of AD, after multiple errors are accumulated and merged, the total error will be large. Whether there is a filtering scheme in the power module, whether there are safety requirements (leakage current, insulation withstand voltage, humidity requirements), temperature rise characteristics, switching power, input voltage fluctuation scheme, load regulation rate, etc. in the equipment where the power module is located , There are still many places to ask for. So we can understand that the selection of switching power supply modules is not only the cost, which is the only indicator that deserves attention.
