How to select diodes in switching power supply?

As a hardware engineer, there are many component selections that need to be paid attention to in the design of switching power supplies. As one of the basic components, diodes play the role of rectification or freewheeling in switching power supplies. In DC DC non-synchronous switching power supplies, we generally use diodes for freewheeling. In the architecture of flyback switching power supplies, we generally use diodes for rectification. Whether it is freewheeling or rectification, there are several points that need to be paid attention to when selecting a diode. In this article, I will summarize these points.

Points that should be paid attention to when using or selecting diodes:

1. Forward conduction current. Forward conduction current is one of the important parameters of the diode. It determines how much current the diode can maintain normal life and function when it continues to flow for a long time. When selecting a diode, we must know how much current the load behind it requires, and then estimate how much forward current the diode needs. To give an example: In a power supply with an output of two amps, if a rectifier diode is selected at the output end, then I recommend choosing a diode with a forward current of 3.5 amps to 4.5 amps (leaving a certain margin of more than 1.5 times).

2. Forward conduction voltage When the diode is forward conduction, there is a certain forward conduction voltage, generally between 0.3 volts and 0.7 volts. The forward conduction voltage of Schottky diodes will be lower. Generally speaking, the forward conduction voltage The lower the pass voltage, the better, because it will bring less power consumption. When we design some circuits, its forward conduction voltage cannot be ignored. For example, when we design a power supply that outputs 3.3 volts, the voltage drop of the diode on the output side cannot be ignored. If you ignore it, you may design a power supply that is much lower than 3.3 volts (3.3-0.7). This is not allowed.

3. The maximum power of the diode. When the diode is conducting in the forward direction, current will flow, passing a certain current and maintaining a certain voltage, and this part of the power will be consumed in the form of heat on the diode package. This is something we should pay attention to. The power value we calculate must be less than the rated power value given in the diode data sheet, and a good margin must be left. Generally, an additional 50% to 100% of power must be left, so as to ensure that the circuit can work at high and low temperatures for a long time. stability in the situation.

4. Reverse breakdown voltage When we use ordinary diodes, we must try our best to ensure that the diode does not breakdown, otherwise problems may occur in the circuit. In the design process of switching power supply, the reverse withstand voltage value of the rectifier diode is a very important parameter, because on the output side or input side of the transformer, there is AC voltage. To put it bluntly, both sides of the transformer are positive for a while. It is negative for a while, and we usually rectify the positive half cycle and cut off the negative half cycle, so the reverse withstand voltage value of the diode is required, which is much larger than the actual reverse withstand voltage value.

5. Diode frequency When designing a switching power supply, we use diodes for rectification or freewheeling. We must pay attention to what is the maximum frequency of the diode? If the parameter frequency of our diode is lower than the switching frequency of the application, the diode is likely to overheat and eventually be damaged due to repeated switching. Therefore, when we select a diode, we must also pay attention to the frequency of the diode. This parameter must also leave a certain margin, generally 50% to 75%. For example, if I choose a diode in a 1MHz switching power supply, it is best to choose a diode with a switching frequency of 1.5MHz to 1.75MHz. These are generally Schottky diodes. Ordinary diodes cannot reach such high frequencies. .

6. The reverse leakage current of the diode. As we all know, the diode is not completely closed when reverse cut-off, but is leaked by a part of the leakage current, which is generally at the microampere to nanoampere level. When doing some delicate circuit design , the leakage current may need to be considered. When doing general circuit design, hardware engineers should be aware of it.

The above 6 points are what I have summarized. Several points that should be paid attention to when selecting diodes in switching power supply design. I believe that all readers have understood the content of this short article. If you pay attention to these 6 points of diode selection during actual design, there should not be any big problems in the diode circuit.

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