What is the relationship between buck power supply ripple voltage and inductance?

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What is the relationship between buck power supply ripple voltage and inductance? [Copy link]

Ripple voltage refers to the small amount of voltage fluctuation contained in the output voltage. These voltage fluctuations occur synchronously with the switching frequency, and the ripple voltage is ideally zero. This is because if the ripple voltage fluctuates greatly and is lower than the system's minimum operating voltage at the load end, it will cause abnormal system operation. In recent years, the trend of low voltage and high current in DC-DC converters has been developing, so it is necessary to provide them with a more stable voltage.

Figure 3-3-1 What is ripple voltage?

As with the efficiency verification, we will now investigate the effect of changes in inductance on ripple voltage.

Fig. 3-3-2 Ripple voltage evaluation by simulation experiment

• Simulation software: LTSPICE
• DC-DC converter: Switch model
• Operating frequency: 4MHz
• Input voltage: 3.6V
• Output voltage: 1.8V
• Output current Iout: 2A
• Inductance: 0.47/1.0/2.2H

From Figure 3-3-3, it can be seen that the higher the inductance, the better the ripple voltage suppression effect. The higher the inductance, the more the ripple current flowing through the inductor can be suppressed, so the output ripple voltage will also be reduced. It should be noted that the inductance will decrease due to the DC superposition characteristics. Using components with poor DC superposition characteristics will result in a decrease in inductance and an increase in ripple voltage. Therefore, it is obvious that when making a selection, it is necessary to ensure that Isat is greater than the load current.

Fig. 3-3-3 Ripple voltage and inductance

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maychang

Regardless of the type of switching power supply, LC forms a second-order low-pass filter. When the switching frequency of the switching power supply is much higher than the LC resonant frequency, the output signal component is attenuated to 1/10 for every 10 times increase in L. Therefore, the larger L is, the smaller the output ripple is.

This is true for L and C. When the capacity of C is increased by 10 times, the ripple is attenuated to 1/10.

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When the switching frequency remains unchanged, increasing the inductance will increase the inductance.

Increasing the switching frequency, at the same inductance, increases the inductive reactance and improves the filtering effect.

Capacitors increase the high frequency and high voltage of small capacitors.

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The switching frequency of 4M is very high, much higher than the switching frequency of 50K and 100K.

From the graph, the peak-to-peak value of the ripple is about 15mV at 047uH, and the peak-to-peak value is one-fifth of that at 2.2uH, which is 3mV.

The ripple accounts for 1.5mV/1.8V *100%=0.083%, which is very good.