Multiphase Buck Converter 120 A Capacitor Current Updated

Added: 2026-06-02

[00:00:01]In this short video, we want to investigate how much current is going to go through the output capacitor when we we design a converter for six phases, a total of 120 amps.

[00:00:14]As we uh might guess that the RMS current in the cap is drastically reduced by using multiple phases, but let's see how much.

[00:00:24]Start [snorts] our design.

[00:00:26]>> Click on specifications to start your design.

[00:00:29]>> Here we can see we have a buck converter ready to go. I actually like to change this to a synchronous rectifier buck converter.

[00:00:35]>> Designing new topology with Ridley IQ.

[00:00:39]Simulating to steady state.

[00:00:42]Design and simulation complete.

[00:00:44]>> Now we've replaced the diode with the synchronous rectifier.

[00:00:47]And let's look at our specifications.

[00:00:51]>> Enter your converter specifications on this form.

[00:00:54]>> You can see we're going to have 36 to 48 volts on the input. We're going to have six phases.

[00:01:00]12 volts, and then each phase is going to be 20 amps.

[00:01:07]>> Setting power levels.

[00:01:09]Design complete.

[00:01:11]>> So we have our design.

[00:01:14]>> Let's look at that simulation of the inductor current right here. So here we can see startup. It's going up to 32 amps or so, then it's coming down into steady state.

[00:01:23]You hit continue.

[00:01:26]We can see that our peak-to-peak ripple current here is a about 15 amps.

[00:01:33]And of course, if we had six times the power level of a single phase, it would be six times 15 amps. So that would be 90 90 amps peak-to-peak ripple current, which is quite large.

[00:01:45]What is the current through the capacitor in a Well, at 20 amps for this single phase, the cap would see 4.2 amps RMS.

[00:01:57]And of course, if we had six phases it would see six times that which would be about 25 amps RMS into that capacitor.

[00:02:05]Which is going to make our capacitor quite large.

[00:02:09]Now let's go see how that capacitor current is reduced from the multi-phase action. So we're going to kick this design out to LTSpice.

[00:02:17]>> Creating LTSpice schematics.

[00:02:19]>> That's all you have to do. Everything is done there. The complete LTSpice schematic is created.

[00:02:27]And it's optimized the control loop and it's even adjusted the gains of the control loop for the number of phases.

[00:02:33]That's fully automated here. So nothing for you to do to generate this except click that one button.

[00:02:38]So let's go look at the phase current see here.

[00:02:42]Run the simulation.

[00:02:44]And we'll click on the first inductor.

[00:02:47]Right there.

[00:02:49]And here we can see that 15 amp peak-to-peak ripple current coming in.

[00:02:54]That's the first phase.

[00:02:56]There's the second phase which is phase shifted.

[00:03:00]And the third.

[00:03:02]And the fourth.

[00:03:05]And the fifth.

[00:03:07]And the sixth. So now you see those multiple phases.

[00:03:14]The converter is probably pretty much in steady state at this point so we can stop that simulation.

[00:03:20]Let's zoom in on these currents, these phase currents.

[00:03:24]You can see them all there.

[00:03:26]And this nice display all shifted by 1/6 of a cycle.

[00:03:30]From one converter to the other.

[00:03:32]So now let's dig in and see what's happening on the output cap. That's what we wanted to find. What is the current in the output cap? Right here.

[00:03:42]Here we see it's way smaller than the individual phase currents. That's the beauty of the multi-phase. And then if we click on the RMS of that, we can see it's only 186 milliamps, so 200 milliamps essentially.

[00:03:59]If that had been a single-phase, this would have been I think 25 amps is what shows up. So, the reduction in ripple current is is just outstanding here.

[00:04:10]And that's the real power of the multi-phase.

[00:04:13]Let's try changing the input voltage because it's going to be affected by that. We'll take this up to 64 volts now.

[00:04:21]Run the simulation one more time.

[00:04:27]See that coming along. See it coming into regulation there.

[00:04:30]Green is the capacitor current here.

[00:04:34]And we can probably stop the simulation at that point.

[00:04:40]And zoom in again on that steady state.

[00:04:44]And now what's happened to our cap current? Now the cap capacitor current has gone from 200 milliamps up to 700 milliamps. So, it's it's quite variable with the input line, but this is going to be the worst case for this one.

[00:04:55]So, 700 milliamps here versus 25 amps if we had done this with a single-phase.

[00:05:02]And that's what gives us the drastic reduction in output capacitance when we build these multi-phase converters.

[00:05:09]Hope you've enjoyed this. This is all done with Ridley Works with the automated link to LT Spice.

[00:05:14]You can buy that program direct from Ridley Engineering.com.

[00:05:18]Or of course you get it when you come to one of our workshops. Next workshop is in September in Canberra.

[00:05:24]Hope you've enjoyed this. Hope it's instructional. Enjoy your multi-phase design.