The type of Schottky diode to be selected shall be determined according to the voltage Vo, Current IO, heat dissipation, load, installation requirements and required temperature rise to be output by the switching power supply.
1. In general design, we should leave a certain margin
For example, VR is only used below 80% of its rated value (under special circumstances, it can be controlled below 50%), and if is used below 40% of its rated value.
In the single end flyback switching power supply, assume a product: input voltage Vimax = 350vdc, output voltage Vo = 5V, Current IO = 1a. According to the calculation formula, the reverse voltage VR and the forward current if of the rectifier diode are required to meet the following conditions:
VR≥2VI × NS/NP
IF≥2IO/(1- θ MAX)
Wherein:
NS / NP is the ratio of secondary and primary turns of transformer
θ Max is the maximum duty cycle
Assuming that NS / NP = 1 / 20, θ MAX=0.35
Then VR ≥ 2 × 350/20=35(V)
IF≥2 × 1/(1-0.35)=3(A)
In this way, we can refer to SR340 or 1N5822. If the product is cooled by a fan, the tube can leave a small margin. The tubes of to220 and to3p can be fully encapsulated or semi encapsulated, which should be selected according to specific conditions. The heat dissipation of the semi enclosed tube is better than that of the fully enclosed tube, but it is necessary to pay attention that the radiator is connected with the middle pin. If the load is capacitive load, it is recommended that if reserve another 20% margin.
2. Correct selection of RC compensation network RC buffer for Schottky diode
Because the leakage inductance of the high-frequency transformer and the junction capacitance of the tube form a resonant circuit when it is cut off, it can cause transient overvoltage oscillation. Therefore, it is necessary to set RC buffer in the power output to protect the safety of the pipe. In addition, RC network can also reduce the output noise, reduce the heat consumption of pipes, and improve the efficiency and reliability of products.
The selection principle of buffer is to make the buffer effective and reduce the loss as much as possible. The following is the reference formula.
R=√(Li/Cj)/n
Where: Li is the leakage inductance of transformer（ μ H)
CJ is the junction capacitance (PF) of the tube
N is the ratio of primary and secondary side turns (NP / NS)
The capacitance C may be arbitrarily from 0.01 to 0.1 μ The specific value is determined by experiment.
For VO = 5V, r = 5.1 Ω, 0.5W, C = 0.01 μ F
The tube with small IR has small heat consumption, so in the same case, the tube with small IR can be selected.
When designing PCB, the tubes and radiators shall be kept away from the heat sensitive devices such as electrolytic capacitors as far as possible. To increase the life of the product.
The welding pad of the welded pipe shall be large enough to ensure reliable welding and avoid desoldering due to thermal stress.
Once the Schottky diode is selected, the temperature rise and working waveform of the Schottky diode shall be measured under the worst condition of the input and output of the product through simulation experiments, and the indexes shall not exceed the limit parameters.
