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DC-DC power supply design analysis and use attention

baijiahao | 05/05/2021

DC-DC converter refers to a device that converts electric energy of one voltage value into electric energy of another voltage value in a DC circuit. The layout of DC-DC is very important and will directly affect product stability and EMI effects.

Some experience and rules of DC-DC power supply

1. Handle the feedback loop properly. The feedback line should not go under the Schottky, don’t go under the inductor, don’t go under the large capacitor, and don’t be surrounded by a large current loop. If necessary, add a 100pF capacitor to the sampling resistor to increase stability ( But the transient will be affected a little);

2. The feedback line is rather thin than thick, because the wider the line, the more obvious the antenna effect, which affects the stability of the loop. Generally use 6-12mils line;

3. All capacitors are as close as possible to the IC;

4. The inductance is selected according to the capacity of 120-130% of the specifications in the specification, and it should not be too large, as it will affect the efficiency and transient;

5. The capacitance is selected according to 150% of the capacity of the specification. If you are using SMD ceramic capacitors, if you use 22uF, it is better to use two 10uF in parallel. If it is not sensitive to cost, the capacitor can be larger. Special note: If you use aluminum electrolytic capacitors for the output capacitors, remember to use high-frequency and low-resistance ones. Don't just put a low-frequency filter capacitor!

6. Minimize the surrounding area of ​​the high current loop as much as possible. If it is inconvenient to shrink, use copper to become a narrow gap.

7. Do not use thermal resistance pads on critical circuits, as they will introduce excess inductance characteristics.

8. When using the ground plane, try to maintain the integrity of the ground below the input switching circuit. Any cutting of the ground layer in this area will reduce the effectiveness of the ground layer, and even signal vias through the ground layer will increase its impedance.

9. The via hole can be used to connect the decoupling capacitor and the IC ground to the ground layer, which can minimize the loop. But it should be kept in mind that the inductance of the vias is about 0.1~0.5nH, which will vary according to the thickness and length of the vias, and they can increase the total loop inductance. For low-impedance connections, multiple vias should be used.

10. It should be noted that using the ground plane as a path for current return will introduce a lot of noise into the ground plane. For this reason, the local ground plane can be separated and connected to the main ground through a point with very low noise.

11. When the ground layer is very close to the radiation loop, its shielding effect on the loop will be effectively strengthened. Therefore, when designing a multi-layer PCB, a complete ground layer can be placed on the second layer so that it is directly below the top layer that carries a large current.

12. Unshielded inductors will generate a large amount of magnetic leakage, which will enter other loops and filter components. In noise-sensitive applications, semi-shielded or fully-shielded inductors should be used, and sensitive circuits and loops should be kept away from the inductors.

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DC-DC converter

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