PMLKBUCKEVM: How to calculate the input current ripple? & Why the output voltage ripple is discontinuous?

Sidong,

1. I will try to find the derivation of these equations and get back to you soon.

2. During TON, IQ ramps up so ICIN ramps up (negative due to the defined direction of ICIN). The current from CIN causes CIN to lose some charge. During TOFF, all that lost charge is replenished so current must flow into CIN. Initially it's lots of current because VIN is steady and VCIN is low from the lost charge. As CIN charges, VCIN rises, and ICIN ramps down. The discrepancy you see between IIN and ICIN is a good catch. It looks like the ICIN waveform was shown using a high-ESR cap but the IIN waveform was shown using a low-ESR cap. I'll confirm.

3. This discontinuity is due to the ESR of the capacitor.

-Sam

Thanks for your prompt reply.

As to the third question 3) Why the output voltage ripple is discontinuous when I do the Test#1 in Experiment 2? 

My understanding is that the sudden drop of current through the ESR of the 220uF capacitor C16  causes the output ripple to be discontinuous. However, 

considering the continuity of the inductor current ripple and the load current ripple, why does the current flowing through the capacitor suddenly drop?

You can refer to the following information.

The schematic is as shown below.

I remeasure the load current ripple as shown in the purple curve below. Its peak-to-peak is about 20mA

The inductor current ripple as shown in the purple curve below. Its peak-to-peak is about 380mA. In this situation, the output voltage ripple drop is about 20mV.

Question 3 Update

I compared the output ripple measured in 2 places(A, B), and the shape is shown below.

1. Inductor current ripple - Purple curve
2. The voltage measured across the output capacitor C16 – Blue curve, test point B
3. The voltage measured across the output port that connected to the electronic load. – Yellow curve, test point A

There are significant differences between the output voltage results measured across the output port of the two boards. The result of board#1 is the third question I mentioned in my post. 

It seems that the ground plane fluctuated at the switching time in the output port of board#1, which cause the output voltage ripple measured across the output port to jump.

Hi Sidong,

Where and how the data is taken matters when measuring output voltage ripple. Using a test point with a ground clip creates an inductive loop which can cause the artificial data. We recommend using a tip and barrel across the output capacitor. You should AC couple and not bandwidth limit your waveform. 

As for the discrepancies between the boards, capacitor tolerance and component variance can occur to cause different output voltage ripple.

Regards,

Ethan