π-type LC filter design problem!

燕园技术宅

π-type LC filter design problem! [Copy link]

       I'm working on a topic recently. The system is powered by a 12V switching power supply. I need to use the MP2225 step-down chip to reduce the 12V to 5V, with a maximum output current of 5A. I want to add a π-type LC filter to the 5V output, but after looking at it online, I still don't understand how to design the specific parameters. I hope someone can give me some guidance. The switching frequency of MP2225 is 500kHz. The system has a lot of loads and relatively high power consumption. The current needs to be between 3A and 5A. What size inductors and capacitors should I choose? Is it better to use electrolytic capacitors or can ordinary chip capacitors meet the requirements? ?

maychang

"I want to add a π-type LC filter at the 5V output end." I have not found the "step-down chip" of the model MP2225. What is the purpose of adding a π-type LC filter at the 5V output end?

燕园技术宅

maychang posted on 2018-7-18 21:49 "I want to add a π-type LC filter at the 5V output end" I did not find the "step-down chip" of the MP2225 model. Add a π-type LC filter at the 5V output end...

In order to get a purer 5V DC voltage, I want to design a π-type LC filter with a relatively low cutoff frequency. The attached is the data sheet of MP2225, please take a look! Thank you for your guidance!

maychang

Yanyuan Technology House published on 2018-7-18 22:21 In order to get a purer 5V DC voltage, I want to design a π-type LC filter with a relatively low cut-off frequency. The attachment is the data sheet of MP2225, please take a look...

The output end of the typical circuit of this chip must have a capacitor, that is, C2 in the figure, which is two 22uF in parallel. This is generally not to be moved. You only need to connect an inductor in series between the output end and the load, and then connect a capacitor to the ground.

maychang

Yanyuan Technology House published on 2018-7-18 22:21 In order to obtain a purer 5V DC voltage, I want to design a π-type LC filter with a relatively low cut-off frequency. The attached is the data sheet of MP2225, please see...

As for the selection of L inductance and the following C capacitance, since most of the electrical devices using 5V voltage are electronic devices (chips, etc.), almost every chip has a bypass capacitor from the 5V end to the ground, so the capacitance from the end of L to the ground does not need to be very strict, and a larger capacitance is fine. This capacitor can use an aluminum electrolytic capacitor. As for the inductor, the larger the inductance, the more expensive it is and the larger the volume. Your load current needs to be 5A, which is not a small current. The inductor that can withstand such a large DC current must be relatively large, so the inductance cannot be too large. On the other hand, the inductor always has a DC resistance. The larger the inductance, the greater the voltage drop caused by the DC resistance of the inductor, which will reduce the load stability (the voltage changes when the load changes). Therefore, the DC resistance of the inductor must be relatively small. Assuming that the minimum allowable reduction in the supply voltage is 0.1V, the DC resistance of the inductor must be less than 0.1V/5A=0.02Ω.

燕园技术宅

maychang posted on 2018-7-19 09:25 As for the selection of L inductance and C capacitance, since most of the electrical devices using 5V voltage are electronic devices (chips, etc.), almost every chip has a bypass...

Thank you for your advice! You mean that the output end of the MP2225 chip has a capacitor, so you can directly connect an inductor in series, and then add a capacitor with appropriate capacity to ground. Then when choosing the inductor, I mainly consider the three factors of DC resistance, volume and inductance!

maychang

Yanyuan Technology House published on 2018-7-19 14:19 Thank you for your advice! You mean that the output end of the MP2225 chip already has a capacitor, so you can directly connect an inductor in series, and then add a capacitor with appropriate capacity...

"Then I mainly consider the three factors of DC resistance, volume and inductance when choosing an inductor!" The choice of inductance is still secondary, as long as the resonant frequency of the LC loop formed by the inductance and the subsequent capacitance is much lower than the PWM frequency of MP2225, because the capacitance is quite large, this is easy to meet. In addition to the two factors of DC resistance and volume, because your load current is large, the ability of the inductor to withstand DC current must be evaluated (the core cannot enter saturation under 5A DC current).

燕园技术宅

maychang posted on 2018-7-19 15:02 "Then when choosing an inductor, I mainly consider three factors: DC resistance, volume and inductance!" The choice of inductance is secondary, as long as the inductance...

OK, thank you