How to control DCDC output ripple and calculate the parameters of output LC filter

S3S4S5S6

How to control DCDC output ripple and calculate the parameters of output LC filter [Copy link]

Here are some examples:

Now there is a DCDC module with an input of 24V, an output of 5V/1A, a switching frequency of 500KHz, and an efficiency of 85%. The output ripple of this module is 50mV

Now we want to control the output ripple within 1mV, how to calculate the value of LC?

maychang

[Now we want to control the output ripple within 1mV, how to calculate the value of LC]

It is best not to change the LC value of the original typical circuit, but to add a stage of LC filtering at the output end of the original circuit.

maychang

It is difficult to control the ripple within 1mV, especially when the newly added LC inductor may have magnetic coupling with the original filter inductor. If there is no special requirement for efficiency, you can consider using a 9V output power module and adding a linear voltage regulator at the output end of the power module.

S3S4S5S6

maychang posted on 2024-4-18 07:20 [Now I want to control the output ripple within 1mV, how to calculate the LC value] It is best not to change the LC value of the original typical circuit, but in the original circuit...

If we want to reduce the ripple from 50mV to 1mV, how should we calculate the value of the LC in the next stage, or what is the calculation method?

S3S4S5S6

maychang posted on 2024-4-18 07:44 It is difficult to control the ripple within 1mV, especially when the newly added LC inductor and the original filter inductor may have magnetic coupling. If there is no special effect on efficiency...

It is best to add a linear voltage regulator. Sometimes the load current is large and the linear voltage regulator will heat up.

The other one goes back to your first reply, which is why you want to add a linear regulator and what is the basis. This goes back to the previous point, the calculation problem of LC.

Through LC calculation, it is found that if this condition is to be met, the LC value may be very large, resulting in the lack of such a device, or the device is too large to be used due to space limitations.

maychang

S3S4S5S6 Published on 2024-4-18 08:06 If the ripple is to be reduced from 50mV to 1mV, how should the value of the LC in the latter stage be calculated, or what is the calculation method

Your ripple frequency is 500kHz, so the added LC level only needs to attenuate 34dB (1/50) at the 500kHz frequency.

maychang

S3S4S5S6 Published on 2024-4-18 08:06 If the ripple is to be reduced from 50mV to 1mV, how should the value of the LC in the latter stage be calculated, or what is the calculation method

Of course, there are still many options for LC with a 34 dB attenuation. At this time, the DC resistance of the inductor should be as small as possible to avoid a large DC voltage drop on the inductor.

tagetage

The best way to control the ripple within 1mV is to use LDO instead of LC.

wangxiangtan2

Listen to your words, just listen to your words.

I don't understand. The 1mV ripple requirement is really killing me.