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In a flyback converter, a MOSFET drain spike at turn-off is usually caused by energy stored in transformer leakage inductance interacting with parasitic capacitance. An RCD clamp limits the drain-voltage peak; an RC snubber is mainly used to damp ringing. The secondary rectifier can ring for related but distinct reasons, so it may need its own snubber.
Snubber values depend on the actual transformer, switching current, parasitics and operating frequency—not just the output voltage. The design goal is to reduce electrical stress and noise without adding excessive loss or heat.
Why a flyback converter produces turn-off spikes
A flyback transformer behaves as a coupled inductor. While the primary switch is on, energy is stored in the magnetic field; when it turns off, energy is transferred to the output. Because the windings are not perfectly coupled, some energy remains in primary leakage inductance rather than transferring to the secondary. At turn-off, that energy excites parasitic capacitances and produces an overshoot and ringing at the MOSFET drain.
Analog Devices’ November 12, 2001 application article describes these transients in multiple-output flyback supplies, a topology it says can save cost and space in high-voltage supplies up to 100 W. It identifies spikes at both the power-switch drain and the secondary rectifier. A snubber controls the effects of leakage inductance and can improve supply reliability.
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Estimating the primary drain peak
The article gives this relationship for the primary peak voltage:
VPEAK = IP × √(LLP / (CP + COSS)) + VIN + VOUT/N
- IP is the primary current at MOSFET turn-off.
- LLP is primary leakage inductance.
- CP is primary winding capacitance; COSS is the MOSFET output capacitance.
- VIN and VOUT are input and output voltages, and N is the secondary-to-primary turns ratio.
The equation shows why the nominal output voltage alone is not enough to select a clamp: leakage inductance, turn-off current and the effective capacitance also set the transient.
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Why the secondary rectifier rings
On the secondary, leakage inductance can resonate with the rectifier diode’s capacitance. Diode reverse-recovery current can contribute to the ringing. The resulting waveform may create conducted or radiated noise, disturb current sensing, or exceed the diode’s voltage rating. A primary drain clamp does not automatically solve this secondary-node problem.
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| Network | Primary purpose | Design implication |
|---|---|---|
| RCD clamp | Limits the MOSFET drain peak by absorbing leakage energy. | Size the clamp capacitor and resistor for clamp voltage, ripple and dissipation; use a fast, pulse-capable diode. |
| Rate-of-rise-control RCD | Controls the voltage rise by charging and discharging a capacitor each cycle. | Its RC time constant is much shorter than the switching period—about one tenth of a period is the cited order of magnitude. |
| RC snubber | Damps parasitic ringing and controls dv/dt. | It absorbs energy at transitions, so resistor loss and slower switching can reduce efficiency. |
These are not interchangeable labels for the same design. Use an RCD clamp when peak drain voltage is the main concern; use an RC network chiefly when ringing is the issue. A converter may need separate networks on the primary switch and secondary rectifier.
How to calculate a primary RCD clamp
Start with measured or estimated leakage inductance and primary current at turn-off. Choose a target clamp voltage with adequate margin to the MOSFET’s voltage rating, then account for the input voltage and reflected output term when checking the total drain peak. The following relationships, given in the Analog Devices article, use the article’s clamp-voltage notation:
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- PCLAMP = 0.5 × VCLAMP × ICLAMP × Δt × f
- Δt = LLP × IP / (VCLAMP − VOUT/N)
- RCLAMP = 2 × VCLAMP × (VCLAMP − VOUT/N) / (LLP × IP2 × f)
- CCLAMP = VCLAMP / (Vripple × RCLAMP × f)
Here, f is switching frequency and Vripple is the permitted clamp-capacitor ripple. ICLAMP is clamp current in the power expression. Keep the clamp capacitor’s RC time constant much longer than the switching period so its voltage does not substantially discharge between cycles. The capacitor should have low ESR and low inductance; the clamp diode should turn on quickly and withstand the peak current.
These equations are a starting point, not a substitute for checking the actual waveform and component stress. Reassess the design across relevant operating conditions and transformer/PCB variation; a nominal calculation alone does not establish the worst-case peak or thermal margin.
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For the rate-of-rise arrangement, the capacitor charges and discharges each cycle, so the intended RC time constant is much shorter than the switching period. The article gives about one tenth of a period as a typical order of magnitude. Its capacitor-current relationship is IP = C × (VC/tr), and its approximate resistor dissipation is P = C × VC2 × f / 2. Here VC is the capacitor voltage and tr is the targeted rise time. Calculate the resulting dissipation before choosing a resistor.
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How to size an RC snubber for ringing
For a simple RC snubber, first identify the parasitic resonant inductance and capacitance, Lres and Cres. The starting resistor value is near the resonant characteristic impedance:
R = √(Lres/Cres)
The snubber capacitor is generally at least three to four times the parasitic resonant capacitance, while still being small enough to keep resistor loss acceptable. Increasing capacitance can damp the ringing more strongly, but it also increases the energy dissipated at each transition. Excessive capacitance can slow MOSFET switching and lower efficiency.
Check the result on the node being snubbed: compare ringing amplitude and duration, voltage margin, resistor temperature and overall efficiency. The right balance depends on measured parasitics and operating conditions; the ratio is a design starting point, not a universal component prescription.
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Component selection and layout matter
- Use low-ESR, low-ESL ceramic or polymer-film capacitors with voltage, pulse, temperature and safety ratings appropriate to the converter. A material name alone does not establish suitability.
- Choose low-inductance resistors and avoid wirewound parts for the snubber path.
- For an RCD clamp, select a diode with fast turn-on and adequate peak-current capability.
- Place the network at the noisy node and keep high-current paths short. Minimize PCB stray inductance, which otherwise adds to the transient.
- Provide heat-spreading area where diode losses require it, and verify resistor, diode and capacitor temperatures under operating conditions.
What the MAX1856 example shows—and what it does not
In the MAX1856 flyback application circuit discussed by Analog Devices, D3/C11/R11 form a primary drain clamp, while R5/C10 form an RC snubber across secondary rectifier D2. The article reports R5 = 150 Ω and C10 = 330 pF for that secondary snubber and shows waveforms with and without it. Those values belong to that example circuit; they are not general recommendations for other transformers, switching conditions or layouts.
Validate the design against stress, loss and noise
Assess each candidate at the node it is meant to control. A useful comparison includes:
- Peak voltage and margin to the MOSFET or diode rating.
- Ringing amplitude and how long it persists.
- Snubber dissipation and component temperature rise.
- Switching loss and converter efficiency.
- Conducted and radiated EMI, including any effect on current sensing.
- Component voltage and pulse ratings, plus sensitivity to transformer and PCB parasitics.
A setting that suppresses a waveform at one operating point may impose needless loss or fail to provide adequate margin elsewhere. Select values from the measured or estimated circuit behavior, then check both electrical stress and thermal consequences.
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