Every switch cycle in a flyback converter ends the same way: the switch turns off, the leakage energy in the transformer releases, and the output rectifier rings like a struck bell. The spike and the oscillation have a rhythm you can measure on a scope, and the snubber is the tuning that damps it. This article works the flyback behavior from the leakage spike through the ringing, designs the RCD clamp and the RC snubber with real component math, chooses the snubber diode by speed and rating, and closes with the before-and-after scope verification that proves the fix.
The Leakage Spike and Its Ringing: Where the Waveform Comes From
The flyback’s signature waveform is born in two places: the transformer leakage and the circuit’s parasitic ringing. Understanding the source is what makes the snubber design a calculation instead of a guess.
At the end of each switch on-time, the switch turns off and the transformer’s leakage inductance tries to sustain its current. That stored energy has nowhere to go but to push the voltage across the switch and the output rectifier up — the first spike. The spike is followed by ringing because the circuit is a resonant system: the leakage inductance pairs with the distributed capacitance of the windings and the parasitic capacitance of the parts, forming a lightly damped tank that oscillates after the spike. The oscillation rings at the resonance of that L-C pair, decaying with the circuit’s natural damping. The flyback snubber context and the gate drive design article document the leakage-spike mechanism, and the reverse recovery physics explains the rectifier’s own contribution to the waveform.
The design consequence is that the spike and the ringing are not two problems but one: both are the leakage energy finding its way through an under-damped path. The snubber’s job is to absorb the energy and add damping, which this article sizes with the RCD clamp and the RC snubber.
RCD Clamp Design: Resistor, Capacitor, Diode Choices
The RCD clamp is the workhorse snubber of the flyback: a resistor, a capacitor, and a diode placed to capture the leakage spike. Its three elements have three jobs, and each is sized from the energy budget.
The resistor sets the damping and the rate at which the captured energy leaves the capacitor. It is sized so that the capacitor discharges between events without wasting excessive power continuously — a trade between damping and loss. The capacitor stores the leakage energy and sets the voltage the spike reaches: a larger capacitance holds the spike lower but stores more energy, and the voltage across it is the clamp level the circuit sees. The diode is the one-way gate that lets the clamp capture the spike without re-injecting it; the diode choice is the subject of its own section below. The RCD math, worked from the leakage energy and the target clamp voltage, is the heart of the flyback snubber guide and the numeric anchor this article uses.
The sizing habit is to compute the leakage energy from the transformer’s leakage inductance and the turn-off current, then size the capacitor so that energy raises it to the target clamp voltage, and the resistor so that it discharges before the next event. The three choices are coupled, which is why the RCD is sized as a set rather than part by part.
A worked RCD example makes the sizing concrete. Suppose a flyback turns off at 5 A against a leakage inductance of 20 microhenries. The stored leakage energy is half of L times I-squared: 0.5 x 20 uH x 25 = 0.25 millijoules per event. If the target clamp voltage is 400 V, the clamp capacitor must be sized so that 0.25 mJ raises it from its nominal level to 400 V; a 10 nF capacitor absorbing that energy lands near the target, and a larger value holds the spike lower at the price of more stored energy. The resistor then discharges the capacitor between events: at 100 kHz there is one event every 10 microseconds, so the resistor must drain the capacitor in a fraction of that window without burning excessive continuous power. The numbers make the three choices coupled, exactly as the design method describes, and the flyback snubber sizing walks the same arithmetic from the transformer-side leakage.
The worked example also shows the sensitivity that the design must bracket. Doubling the turn-off current quadruples the stored energy, because the energy scales as current squared — so a converter whose output duty drifts upward needs a re-check of the clamp capacitor and resistor, not a confidence that the original sizing still holds. The snubber that was tuned for the nominal point absorbs the worst-case energy only if the design computed the worst-case corner; the verification section at the end is where the bracket is actually checked.
RC Snubber Across the Output Rectifier: The Fast Fix
The RC snubber is the simpler cousin of the RCD, placed directly across the output rectifier to damp its ringing without the RCD’s one-way diode. It is the fast fix when the ringing, not the spike, is the problem.
An RC snubber is just a resistor-capacitor pair across the rectifier: the capacitor absorbs the high-frequency energy of the ringing, and the resistor dissipates it. It has no diode, so it conducts both ways and is cheaper and simpler than the RCD — but it also dissipates power during the normal conduction, because the resistor sees the rectifier’s operating voltage, and its loss is continuous rather than event-limited. The RC snubber wins where the ringing dominates and the continuous loss is acceptable; the RCD wins where the spike energy is large and the continuous loss matters. The flyback snubber part-selection and the fast recovery selection cover when each snubber belongs.
The sizing math for the RC pair is the resonance of the ringing: the capacitor is chosen large enough to dominate the parasitic capacitance that the ringing uses, and the resistor is chosen to add enough damping to settle the oscillation within the budgeted time. The two snubber types, RCD and RC, are the complete dampening toolkit, and the scope verification at the end decides which one a specific flyback actually needs.
Choosing the Snubber Diode: Speed and Rating
The snubber diode is the part engineers treat as an afterthought, and it is the part that decides whether the snubber works. The RCD’s diode must do two things: be fast enough to capture the spike at the switching speed, and be rated to hold the clamp voltage and carry the clamp current.
The speed requirement comes from the timing. The leakage spike rises in a small fraction of the switch cycle; a diode that switches too slowly lets part of the spike pass before the clamp engages, defeating the snubber. A fast-recovery diode in the snubber role captures the event; a standard part may be too slow to matter. The rating requirement comes from the energy: the snubber diode must hold the clamp voltage with margin and carry the clamp current without overheating, and its reverse capability covers the rectified path. The fast recovery diode guide and the diode selection guide cover the speed and rating checks for the snubber role.
The selection habit is to size the snubber diode against the same spike the snubber is meant to clamp, not against the steady converter current. A diode chosen for the average output will be too small for the peak clamp duty; one chosen for the spike, with the derating margin, is the part that holds when the event arrives.
Verifying the Fix: Scope Before and After
The snubber design is not complete until the scope says so, and the before-and-after measurement is the honest verification. The before picture shows the problem; the after picture shows the fix; and the difference is the proof the design was needed and worked.
The before measurement captures the leakage spike and the ringing amplitude at the operating point: the peak voltage across the switch and the rectifier, and the oscillation frequency and settling time. The after measurement runs the same operating point with the snubber installed: the spike should be clamped to the target level, the oscillation should decay within the budgeted cycles, and the added loss should show up as a small, accounted-for temperature rise. The ripple and noise diagnosis guide and the EMI filter article describe the measurement readiness, and the field reliability checklist turns the after measurement into the release evidence.
The same discipline covers the tuning. If the after waveform still rings, the resistor is too small or the capacitor too small; if the loss is unacceptable, the RCD may be the wrong snubber and the part choice the real fix. The scope read closes the loop, and the design is complete when the before-and-after tells the same story the calculation predicted.
The flyback snubber is the tuning that makes the converter quiet. The leakage energy spikes at every turn-off, the parasitic resonance rings, and the RCD or RC snubber absorbs and damps both — sized from the energy, diode chosen for speed and rating, and verified on the scope before and after. A converter designed this way stops being a source of hidden voltage spikes and starts being a predictable power stage. The power MOSFET category and the fast recovery category supply the parts the snubber design lands on, and the rectifier selection guide is the selection reference that closes the chain.