Why resonant ringing occur in DCM mode ?  - Page 1
Forum section: electronics / beginners

author: Patrick66
author_rank: Regular Contributor
timestamp: January 03, 2024, 10:52:36 am
content: Hello everyone, may I ask what causes the resonant ringing to occur in DCM mode? The internet says it is due to the resonance occurring between the primary inductor and the switch node (not sure is MOSFET or diode at the secondary side). Hope that someone can help me with that.

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2024, 02:30:21 pm
content: In a manner of speaking, looking at your sim circuit you have a class C amplifier driving a resonant tank circuit with a load on the secondary side. The resonance is your inductor and the MOSFET internal capacitance plus all stray and coupled capacitance. It is 100% normal. Any energy pushed into the inductor and not delivered to the load is going to tend to ring (and do damage). You have no snubber circuit in your sim. SMPS topology in 99.9% of all cases WILL need a snubber circuit to clamp the ringing. Often can be as simple as a diode in series with a resistor from the drain of the MOSFET back to the positive DC supply rail.

author: Patrick66
author_rank: Regular Contributor
timestamp: January 03, 2024, 03:05:51 pm
content: Sorry, Sir I do not understand this sentence "Any energy pushed into the inductor and not delivered to the load is going to tend to ring". The coupled inductor didn't completely demagnetize in DCM mode? Where did the energy that wasn't delivered to the load when the inductor current fell to zero does it mean all the energy was delivered to the load?

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2024, 06:34:53 pm
content: I believe some of the energy is in the leakage inductance and parasitic inductances and those energies are not absorbed by the load. There is usually a snubber circuit of some type on the input side of the inductor. Read about SMPS snubber circuits and why they are required. Design guides have entire chapters dedicated to the subject. You have a SIM circuit and the real world has imperfections. SIM's may include to a much larger or even a much smaller degree than the real world unless every imaginable aspect is included in the model.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2024, 09:45:27 pm
content: Quote from: Patrick66 on January 03, 2024, 03:05:51 pmLook at the instant in time as diode current falls to zero.You would consider this "completely demagnetized".But the voltage does not instantly return to zero (or +V at the primary), nor can it. There are capacitances everywhere. Primarily, MOSFET Coss and diode Cj, which act in parallel (give or take turns ratio), holding the voltage up for the next instant. Inductor current continues to fall,reversing, and voltage begins to fall. Eventually voltage crosses zero, but now the inductor is remagnetized oppositely, to a current near I = Vpk / Zo, where Zo = sqrt(L/C). And so on. Eventually, losses (predominantly in the inductor) cause the ringing to decay, and this remaining energy is dissipated as heat rather than transferred to the load.Typically, this decay is hastened by adding an Rs + Cs snubber, Rs = Zo and Cs > 2.5 C, across one of the windings (using the respective L and C accounting for turns ratio).When you include leakage inductance (K(K) < 1), there is additionally ringing at the secondary (diode Cj + secondary-referred LL) due to MOSFET turn-on, and at the primary due to MOSFET turn-off (FET Coss + primary-referred LL). These can be dealt with using two additional RC snubbers, or often a dV/dt (rate) snubber on the primary side handles both, as well as improving turn-off losses a little.Tim