Two DC-DC Converters same frequency?

[mrburnzie]

Hi!
I have a design where I have two DC-DC converters. One is 5V output other is 3.3V.

They are both adjustable switching frequency.

I haven't found much documentation on the fact that when two converters are close to each other, should the switching frequency be the same or be different (and by how much)? What is better for EMI?

[NiHaoMike]

Ideally, they should be the same frequency but 180 degrees out of phase.

[Zero999]

Ideally, they should be the same frequency but 180 degrees out of phase.

Which won't happen unless they're synchronized. If they're very close in frequency they'll probably phase lock and both run at exactly the same frequency.

[mrburnzie]

Okay, but what will happen with the functionality and the EMI?

[m98]

Spread the energy out a bit to reduce your peak envelope, run them at different frequencies, just don't have one be a harmonic of the other. Operating two converters in-phase at the same frequency would be pretty much the EMI worst-case, but in any case, some shielding and filtering will probably be required.

[David Hess]

Which won't happen unless they're synchronized. If they're very close in frequency they'll probably phase lock and both run at exactly the same frequency.

I have never seen them injection lock unless there was a circuit addition to force it.

Spread the energy out a bit to reduce your peak envelope, run them at different frequencies, just don't have one be a harmonic of the other. Operating two converters in-phase at the same frequency would be pretty much the EMI worst-case, but in any case, some shielding and filtering will probably be required.

Some sensitive DC circuits will be upset if the converters are close in frequency because of the low frequency mixing product, so it is better to be injection locked or operating on distant frequencies.