DCM is usually 10 to 20 times lower than parasitic ringing, and that makes sense because the leakage inductance is usually around five percent of the total inductance.Īnother way to tell these two oscillations apart is the power level where they occur. One way to distinguish DCM ringing from parasitic ringing is the frequency. That corresponds to a frequency of 4.5 MHz. This happens in standard, non-synchronous circuits of any typology, and it also happens with modern, more sophisticated synchronous control LCEs and regulators that control their synchronous switches to emulate a diode’s one quadrant behavior.ĭCM ringing is normal, since the parasitic capacitance of whatever switches are used, forms an LC tank with the full inductance of the power inductor itself when the current falls to zero. It falls to zero before the end of the switching cycle. The inductor current, in green, is discontinuous. DCM stands for discontinuous conduction mode, and this scope shot shows why. When “Ringing” is Normal Non-synchronous converters (and some synchronous converters with “diode emulation”) ring when DCMįor anyone who hasn’t seen parts one and two of this seminar series, it’s important to distinguish between undesirable parasitic ringing and normal DCM ringing. The body diode turns on before the MOSFET does, producing switching loses, and that’s wonderful, but the body diode also has a lot of reverse recovery charge, and that’s terrible for the transient on the switching node as it turns off. I put the word bad in quotations here because, like most anything, it really has pros and cons. Most of the parasitic capacitance is in the value labeled Coss, and it’s notably lower than the capacitance of the Schottky diode from the previous page, but there’s another problem, which is the parasitic diode or body diode of the fit. If your buck switcher is a synchronous device, then the power MOSFET replaces the diode as the low side switch. Schottky diodes are practically free reverse recovery charge, but are by no means perfect, because they still have plenty of junction capacitance, and that stores energy. It’s relevance here is that it’s a source of energy that excites LC tanks. Reverse recovery charge is a fascinating topic, but I won’t dig much deeper into its origin. That means up to 100 volts DC, with great pricing and plenty of pin compatible or clone products with the same part numbers from various manufacturers, and up to 200 volts with more limited selection. If you watched part two on buck regulator design, you heard me recommend Schottky diodes as the recirculating element or low side switch whenever possible. For the resonance that comes afterwards, several methods of damping the LC filters will be presented. One way to reduce the overshoot and undershoot voltage transience is to slow the switching edges. This resonance is the ringing I’ve referred to multiple times in the previous two parts of the seminar, and it falls firmly in the band of radiated emission limits for the majority of power supplies. The energy comes from the power switches, be they MOSFETs or BJTs, fast rectifier diode or Schottky as they turn on and turn off. Add a source of energy to an LC tank and you’ll get resonance, especially when the L and C are mainly parasitic because these elements have very little damping. These are unavoidable physical properties of any real circuit. Parasitic Capacitance and Stray Inductance Are UnavoidableĮven the very best layouts will have parasitic capacitance and parasitic, or so called stray inductance. The last part of section 4-1 introduces the R-C Snubber circuit for damping resonances, and presents two methods for picking the right values. With that cleared up, the webinar will go on to explain why and how gate resistors can help slow edges and how to select the best values. Before digging deeper, we’ll review a normal situation where an LC oscillation does occur, which is in discontinuous conduction mode or DCM. Combined with the energy stored in the junctions of the diodes or MOSFETs or BJT power switches, transients and oscillations called spikes or ringing are generated. This first section on switching edge control starts with the fact that even a perfect PCB has unwanted capacitance and inductance. Noise energy sources depending upon switch type.Parasitic capacitance and inductance cannot be avoided.This part of thePower Supply Design Tutorial is dedicated to edge control for optimum radiated EMI. Previous section: Step-by-Step Example for Practical PCB Design, Part 3
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