# Smoothing SMPS ripple

I am designing a Raspberry Pi shield and it needs to have two separate supplies: one for the RPi itself, and one for the rest of the circuitry (consisting of energy metering ADE7758 chip). I plan on using Mean Well switching mode power supplies. The ADE7758 part of the circuit needs 5V/100mA. The RPi supply needs to provide 2A at 5V.

The concern I have is about the power supply ripple and I am looking for advice on how to calculate the value for the smoothing capacitor or any other method for smoothing the output of the supply. I don't want to plug in a LDO regulator after the SMPS as I don't have enough space on the board. The datasheets of the power supplies specify 150mV and 200mV ripple but do not specify the frequency, so I assume it is 50Hz.

Using the equation for capacitor discharging is giving me unreasonable values if I use Vc = 5.000, starting voltage 5.075V and t = 0.01s: 13.43mF. That is if I want to reduce the 150mV ripple in half.

Edit 1 (as suggested by the comments): Acceptable ripple for the RPi supply is 120mVp-p, and for the ADE7758 power supply is 50mVp-p.

Edit 2: Adding a schematic.

• The concern I have is about the power supply ripple Why? Before assuming it is an issue you need to quantify it. What amount of supply ripple is acceptable? Just "having a concern" is OK but before you can fix it you need to know what you're up against. Suppose you decrease the ripple by a factor 10, is that enough? Or do you need a factor 100? Nov 28, 2019 at 9:40
• ...ripple but do not specify the frequency, so I assume it is 50Hz. Why? Because the mains frequency is 50 Hz? But the PSU is an SMPS, it rectifies the mains frequency and then uses switching and a high frequency transformer (usually 100 kHz or so) to convert the power to a lower voltage. So 50 Hz isn't an issue, the 100 kHz might be. For that you need low ESR capacitors, not capacitors that work at 50 Hz. Nov 28, 2019 at 9:43
• According to datasheet (meanwell.com/Upload/PDF/IRM-02/IRM-02-SPEC.PDF) switching mode PS operates at 130kHz. It also says: "Ripple & noise are measured at 20MHz of bandwidth by using a 12" twisted pair-wire terminated with a 0.1uf & 47uf parallel capacitor". With a reasonable output decoupling strategy, like @Bimpelrekkie mentioned, you could potentially get lower output ripple. Have you looked for similar drop-in packages with lower ripple specs? Nov 28, 2019 at 13:03
• I am stuck with the Mean Well supplies, but I do not have on at the moment for testing. The clarification about the output ripple frequency is really useful, that means I can use relatively large electorlytic capacitor with small ESR.
– Slav
Nov 28, 2019 at 14:02
• @Slav I wouldn't recommend just electrolytic caps, but a combination of an electrolytic and multiple MLCCs (MLCCs have much much lower ESR). Also looking again at your circuit, I would recommend moving the fuses downstream (MeanWell module > decoupling caps > fuse) and make sure it is rated to handle the inrush current of your downstream devices. For the LED circuit, you may want to use a higher value for the resistor, with only 2-3mA most green LEDs are already well visible in daylight. What is your intention behind adding inline ferrites and parallel fuse at the power input? Nov 29, 2019 at 16:39